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Feasibility Report

Driving Innovation in AD

Optimisation – Uses for Digestate

This feasibility study examined the potential for linking anaerobic digestion (AD) with biomass fuel production in order to get the best value from the AD process by-products.

Project code: OIN001-407 Research date: March – June 2012 Date: October 2012

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Written by: Dr Arrash Shirani & Meirion Evans

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Optimisation – Uses for Digestate 1

Abstract

A feasibility study has been carried out as part of the WRAP ‘Driving Innovation in AD – Optimisation’ programme. The feasibility study examined the potential for linking anaerobic digestion (AD) with biomass fuel production in order to get the best value from the by-products of AD. A fibrous solid material is screened out of the process after a pasteurisation and primary digestion stage, but before the final digestion stage. The separated fibres were dried, milled and pelleted. Various mixes of fibres and softwood or hardwood waste were trialled as part of the phase 1 feasibility project. The pellet samples were characterised and combustion tests carried out on a 150 kW commercial pellet boiler. The results of the feasibility project have shown that high quality pellets can be produced. Gross calorific values ranged from 16.994 MJ/kg to 21.606 MJ/kg, comparable to or better than the softwood pellets produced (17.856 MJ/kg). The mechanical durability of the pellets was also good and ranged from 99.6 % to 99.8 %, better than the softwood pellets produced. Combustion trials indicate good performance with net boiler efficiency over 90 %. Exhaust stack emissions from the digestate pellets were comparable to stack emissions from combustion of ‘clean’ wood pellets, with the exception of particulates which were higher during combustion of digestate pellets. The energy required to dry, mill and pellet the materials was monitored. It was estimated that the total energy requirement for production of the pellets was 3.5 kWh/kg, the total energy potential of the pellets was 5.6 kWh/kg. The energy required to produce the pellet was 63 % of the energy potential of the pellet hence there is a net energy gain from producing the pellets. An economic model was proposed for drying, milling and pelleting 500 tonnes of fibres/year, followed by combustion and use of the heat for displacing fossil fuels. A simplified cost benefit analysis suggests a payback of 2.61 years is possible. Based on the pellet production rate for the proposed scenario (151 tonnes/year) it was estimated that it would be possible to produce the equivalent of 79,297 m3 of natural gas or 81,521 L of diesel oil, equivalent to heating 48 homes (Ofgem figures) or driving 512.3 miles in an average car.

http://www.ofgem.gov.uk/Media/FactSheets/Documents1/domestic%20energy%20consump%20fig%20FS.pdf


Executive summary

AWS Burdens Environmental Ltd (AWSBE) is a joint venture company set up specifically to design, build and commission smaller scale anaerobic digesters. AWSBE has designed and built a demonstration AD plant to treat up to 1,500 tonnes/year of waste in Llangadog, Carmarthenshire. The plant is built as an ‘add on’ to an existing civic amenity and waste transfer station. The aim of the company is to use the Llangadog facility as a demonstrator and to build confidence in the technology. This will enable us to sell our 1,000 to 6,000 tonnes per year AD technology to other users who require smaller scale solutions that are not readily available in the UK at this time. Because of the potential problems associated with spreading digestate, AWSBE have recognised that there need to be alternative methods for handling the outputs of an AD plant. One such method is using the separated solid fraction as a biomass fuel. The potential advantages for AD plant operators of converting their separated solids into biomass fuel include:

guaranteed use for heat from combined heat and power unit (CHP);

potential to claim renewable heat incentive (RHI) for the drying process;

additional income from sale of the biomass pellet as a fuel;

diversification of income stream for the anaerobic digester operator;

alternative disposal route for solids if there is not a suitable land bank near the digester;

reduced transport costs due to lower volume of pellet; and

pellets can also be used on site to provide extra heat by combustion in a biomass boiler if there is a higher heat demand locally.

Using waste heat from a biogas driven CHP to dry wet biomass in this way can interlink biomass and biogas systems and provide complete energy recovery from the feed stocks. Alternatively the dried and pelleted digestate can be used as a concentrated pellet fertiliser. A feasibility study has been designed to answer a number of key issues related to the drying and pelleting of the AD fibres. These issues are in relation to the technical, economic and environmental feasibility of converting the separated solid fraction into pellets and comparing this option with the existing operation, which is to mature the fibres for use as a soil enhancer. The process for producing pellets involves drying, particle size reduction and finally pelleting. Existing site equipment was modified into a dryer. This was a trailer with a hot water jacket. The hot water was provided through a biogas boiler. The results showed that the drying method used in this study required 3.5 kWh of thermal energy to remove 1 kg of water from the separated fibres. This is the equivalent of using 7.8 kWh to produce 1 kg dry material ready for pelleting. This is very high when compared with other data presented in the available literature; this shows that the method used in this study was not particularly efficient. However, the aim of the feasibility study was not to demonstrate or prove the efficiency of different drying techniques but simply to produce enough dry material to carry out the trials. For the purpose of making further calculations in this feasibility study a more realistic figure of 1.5 kWh/kg water removed was used as a guide to the amount of thermal energy required to dry the separated fibres. This is based on the publically available literature on biomass drying. Following drying, the materials were milled to a particle size of < 6 mm before being pelleted. Various mixtures of wood and fibres were pelleted on a small scale laboratory pelleting machine. The results showed that good quality pellets can be produced using only the dried separated fibre or by mixing it with different types of wood. This includes hardwoods which


are notoriously difficult to pellet due to the low lignin content. The pellets produced were very consistent, produced very little dust and did not have a strong odour. The results are compared with softwood pellets which were fairly inconsistent, produced a lot of dust and were quite crumbly. The pellets were sent for analysis, and the results showed that high quality pellets can be produced. Gross calorific values ranged from 16.994 MJ/kg to 21.606 MJ/kg, comparable with or better than the softwood pellets produced (17.856 MJ/kg). Mechanical durability of the pellets ranged from 99.6 % to 99.8 %, better than softwood pellets produced (table 6). Digestate pellets were also produced on a full scale commercial pelleting system and the results obtained were also of a very high quality (comparable to trial). The pellet samples produced were transported to a commercial pellet boiler manufacturer, Ashwell Engineering Ltd. The aim of the trials was to test the pellets produced from a mixture of separated fibre and softwood chip (mixed in a ratio of 1.3:1 on a volume basis) including testing of emissions from the boiler. The results of the trials showed that the pellets produced more energy than wood pellets and so the fuel feed rate could be significantly reduced. No problems with fuel ignition or ‘slagging’ were observed, however the mixed pellets did produce more ash than typical wood pellets which would require more frequent removal. The emissions from the boiler exhaust stack were monitored and they compared favourably with clean wood pellets with the exception of having higher particulate emissions (Table 11). The data collected through the feasibility project was used to determine a process mass and energy balance. The example determined was based on an AD plant producing 500 tonnes of separated fibres each year. The results show that to dry the material to a suitable dry matter 344 tonnes of water would need to be removed. This would produce enough dry material for 156 tonnes of pellets. The results of the energy balance show that the total energy used to produce the pellets is equivalent to 3.5 kWh/kg, this includes the energy used for drying, milling and pelleting. The energy potential of the pellets is 5.6 kWh/kg. The ratio of energy used to potential energy generated is 0.63, therefore there is a potential energy gain. The potential economic benefits of the proposed technology were assessed by determining the typical capital expenditure (CAPEX), operating expenditure (OPEX), potential revenues, the annual return and a simplified payback time. The economic assessment has been based on an example scenario and is specifically looking at the economics of the pelleting process, not the whole AD plant. The scenario examined could be a waste processor, a food processor or cooperative who their have their own waste material and a large heat load that could be met/partially met by using a biomass fuel. This assumes that the material is not transported and the pellets are not sold but used on site. The potential payback for the scenario examined was 2.61 years. Some key factors that allow for this quick payback time include the ability to claim the renewable heat incentive (RHI) for drying, sharing of infrastructure and labour with existing site operations, and saving on purchase of fossil fuels such as natural gas or fuel oil. It was calculated that the energy value of the pellets produced would be equivalent to 79,297 m3 of natural gas or 81,521 L of diesel oil. The key conclusions of the phase 1 feasibility study include:

High quality pellets can be produced using separated fibres from the AD process. Calorific values and mechanical durability were as good as, or better than those of softwood pellets.

No additional binders or lubricants were required to form the pellets. Hence it may be possible to use the dried fibres as a binder.

The precise chemical characteristics of the pellets could be adjusted by mixing the fibres with other feed stocks such as softwood or hardwood.


Using the waste heat from AD can create a storable and transportable fuel.

The processing and pelleting plant is flexible – alternative biomass fuels could be dried in addition to the fibres e.g. waste wood.

The key challenges facing the technology will be to prove the performance of the equipment and combustion of the pellets over a longer period, in order to develop a track-record. This will ensure that an end of waste specification can be developed for the material making it possible to sell pellets to a much wider market. This will rely on a more involved full scale demonstration project being undertaken as has been proposed in the phase 2 application section of the report.


Contents

1.0 Phase 1 – feasibility report ........................................................................... 8 1.1 Introduction and background ...................................................................... 8

1.1.1 AWS Burdens Environmental Ltd ....................................................... 8 1.1.2 The AWSBE demonstration plant ....................................................... 8 1.1.3 Background to feasibility project ........................................................ 9 1.1.4 Current and future application for combining AD and biomass ........... 11

1.2 Feasibility project objectives ...................................................................... 12 1.2.1 Aims and objectives of feasibility study and full scale demo project .... 12 1.2.2 Meeting the outcomes of the DIAD optimisation program .................. 12

1.3 State of technology .................................................................................. 13 1.3.1 Background to pelleting technology ................................................. 13 1.3.2 AWSBE’s previous experience of pelleting digestate fibres ................. 13

1.4 Detailed Technical appraisal ...................................................................... 14 1.4.1 Feasibility trial methodology ............................................................ 14 1.4.2 Materials used in feasibility trials ..................................................... 15 1.4.3 Drying method and results .............................................................. 18 1.4.4 . Milling method and results ............................................................ 20 1.4.5 Pelleting ........................................................................................ 21

1.5 Combustion trials and emissions monitoring ............................................... 26 1.1 Example process – mass and energy balance ............................................. 26 1.6 Economic/cost benefit appraisal ................................................................ 28

1.6.1 CAPEX ........................................................................................... 28 1.6.2 OPEX ............................................................................................ 29 1.6.3 Potential revenues ......................................................................... 30 1.6.4 Cost benefit analysis ...................................................................... 31 1.6.5 Alternative options ......................................................................... 32 1.6.6 Comparison with ‘business as usual’ ................................................ 32

1.7 Environmental impact ............................................................................... 33 1.8 Legislation ............................................................................................... 33 1.9 Conclusions from phase 1 feasibility trials ................................................... 34

2.0 Phase 2 – demonstration project................................................................ 35 2.1 Introduction ............................................................................................ 35 2.2 Demonstration project methodology .......................................................... 35 2.3 Demonstration project details .................................................................... 36

2.3.1 Permitting and approvals ................................................................ 38 2.3.2 External contracts .......................................................................... 38 2.3.3 Monitoring and evaluation............................................................... 38

2.4 Project timescale ...................................................................................... 39 2.5 Cost breakdown and milestones ................................................................ 41 2.6 Key personnel .......................................................................................... 42 2.7 Commercialisation plan ............................................................................. 42 2.8 Overall conclusions ................................................................................... 43 2.9 References .............................................................................................. 44


Figures

Figure 1: AWSBE demonstration AD plant ....................................................................... 8

Figure 2: Process flow diagram for AWSBE process .......................................................... 9

Figure 3: Potential model for linking AD with biomass production .................................... 11

Figure 4: Photograph of pellet burner with digestate pellets (a) camera flash on (b) camera

flash off ....................................................................................................................... 14

Figure 5: Project methodology ..................................................................................... 15

Figure 6: Softwood chip used in feasibility trials ............................................................ 16

Figure 7: Oak hardwood shavings and sawdust used in feasibility trials ........................... 16

Figure 8: Example of separated fibres used in feasibility trials ......................................... 17

Figure 9: Schematic diagram of the mobile trailer adapted to drying fibres for the feasibility

study ........................................................................................................................... 19

Figure 10: Graph showing the change in dry matter and density of digestate fibres against

the input of thermal energy for drying trial 3 (digestate only)........................................... 20

Figure 11: Example mass balance for drying, milling and pelleting separated fibres from AD

process ....................................................................................................................... 27

Figure 12: Outline project plan .................................................................................... 40

Tables

Table 1: Initial results from testing of pellets ................................................................. 13

Table 2: Analysis of separated fibres and comparison with common soil enhancers (results

presented on a fresh weight basis) ................................................................................ 17

Table 3: Analysis of separated fibres and comparison with the PAS 110 standard limits ..... 18

Table 4: Example of drying calculation based on results of drying trial 3 .......................... 20

Table 5: Calculated energy consumption by milling equipment ........................................ 21

Table 6: Various pellets produced in trial batches as part of the feasibility project ............ 23

Table 7: Analysis of characteristics of various pellets produced during trial runs (results

presented on as received basis) ..................................................................................... 24

Table 8: Chemical analysis of various pellets produced and comparison with various pellet

standards .................................................................................................................... 24

Table 9: Ash fusion behaviour of pellets under oxidising conditions (°C) ........................... 25

Table 10: Calculated energy consumption by pelleting and ancillary equipment ................ 25

Table 11: Boiler Exhaust emissions for digestate and softwood mixed pellets and wood only

pellets ......................................................................................................................... 26

Table 12: Energy balance for pellet production .............................................................. 27

Table 13: Summary of CAPEX ...................................................................................... 29

Table 14: Summary of OPEX ........................................................................................ 30

Table 15: Summary of potential revenues/savings ......................................................... 31

Table 16: Summary of economic information and cost benefit analysis ............................ 31

Table 17: Assumptions used for organic fertilisers ......................................................... 32

Table 18: Simplified assessment of fertiliser value of separated fibres ............................. 32

Table 19: Expected outcomes of the proposed Llangadog biomass pelleting plant ............ 36

Table 20: Summary of costs for pelleting plant equipment at Llangadog AD plant ............ 41


Table 21: Assessment of project labour and management costs ...................................... 41

Table 22: Estimate of sub-contractor costs .................................................................... 42

Glossary

AWS-BE LTD The Project Special Purpose Vehicle (All Waste Services Ltd/ Burdens

Environmental Ltd EA Environment Agency AVHL Animal Health Veterinary Laboratories CAPEX Capital Expenditure OPEX Operational Expenses AD Anaerobic Digestion ABPR Animal By-Products Regulations GCV Gross Calorific Value NCV Nett Calorific Value NVZ Nitrate Vulnerable Zone PAS 110 Publically Available Specification 110 RHI Renewable Heat Incentive j joule (Unit of Heat energy) kj joule x 103 (kilojoule) Mj joule x 106 (Megajoule) Gj joule x 109 (Gigajoule) Kg kilogramme kWhe kiloWatthour (unit of electrical power) kWhth kiloWatthour (unit of thermal power equivalent) CHP Combined Heat and Power


1.0 Phase 1 – feasibility report 1.1 Introduction and background 1.1.1 AWS Burdens Environmental Ltd AWS Burdens Environmental Ltd (AWSBE) is a joint venture company set up specifically to design, build and commission smaller scale anaerobic digestion (AD) plants. One half of the joint venture company is All Waste Services Ltd (AWS); a local waste management company that has expertise in waste collection, management, recycling and processing. AWS also has the relevant permits and licences required to run an AD plant. The other half of the joint venture is Burdens Environmental; part of the Burdens family of companies. Burdens is one of the UK’s leading suppliers of civil engineering and building materials to the infrastructure, environmental and industrial markets with a product range of more than 18,000 lines delivered from a national network of over 50 branches. Burdens has funded the AWSBE project through its environmental division and is committed to supporting marketing and distribution of the AD product developed through the joint venture. AWSBE has designed and built a demonstration AD plant to treat up to 1,500 tonnes/year of waste in Llangadog, Carmarthenshire. The plant is built as an ‘add on’ to an existing civic amenity and waste transfer station in the village. The aim of the company is to use the Llangadog facility as a demonstrator and to build confidence in the technology. This will enable us to sell our 1,000 to 6,000 tonnes/year AD technology to other users who require smaller scale solutions that are not readily available in the UK at this time. A photograph of the demonstration plant at Llangadog is shown in Figure 1.

Figure 1: AWSBE demonstration AD plant

1.1.2 The AWSBE demonstration plant The AD plant designed and built by AWSBE Ltd makes use of a patented three stage anaerobic digestion system to recover energy from the waste in the form of biogas, as well as producing quality fertiliser from the anaerobic digestate. The plant is designed to take a wide range of potential feed stocks and is currently processing source segregated commercial/municipal food waste, green waste, fat sludge and abattoir waste.


The plant is fully licenced with an Environmental Agency permit, planning permission and a licence to process animal by-products from the State Veterinary Service. The plant was constructed principally to demonstrate the concept of smaller scale AD. It was designed with support from Cardiff University Engineering Department and we have continuing development projects with Glamorgan University’s Centre of Excellence for Anaerobic Digestion. The AD process used by AWSBE has separate pasteurisation and primary digestion stages. After this point the material is passed through a screening stage. This stage removes any contamination plus fibrous and/or woody materials, preventing them from being transferred to the final digestion stage. These materials are high in lignin and are either impossible to digest or do not readily digest in a suitable time. This approach gives us a more robust final digestion stage with shorter residence times and fewer problems with floating and sinking layers in the reactor. A process flow diagram of the AWSBE AD process is presented in Figure 2.

Figure 2: Process flow diagram for AWSBE process

1.1.3 Background to feasibility project The planned rise in the number of anaerobic digestion plants in the UK will inevitably result in increased volumes of both liquid and solid digestate being produced. Acceptable routes for using this material and recovering the best value from it need to be developed. At present the most common route for use of the digestate is land application for agricultural

Waste reception

Maceration to >12 mm

Pasteurisation

Primary digestion

Separation Solids

Liquids

Final digestion Biogas

Excess digestate applied as fertiliser

CHP or Boiler


benefit. Despite the benefits from recycling the nutrients there can be a number of problems associated with spreading digestate, particularly if the digestion process uses waste feed stocks or if the site is not located near suitable agricultural land. For example:

if digestate is classified as waste, a permit and land deployment must be obtained from the Environment Agency (EA) before spreading is allowed;

the end of waste criteria (PAS 110 and Quality Protocol for Digestate (QP)) limits digestate use to agriculture, commercial horticulture, forestry, and land reclamation and restoration;

the planned increase in anaerobic digesters may result in increased competition for land that is suitable for spreading, especially in more urban areas and nitrate vulnerable zones (NVZs);

transport costs and distances – waste is created in urban areas but digestate is spread in rural areas – where should plants be?; and

both solid and liquid fractions of digestate have high moisture content – hence fairly dilute concentrations of macro/micro nutrients.

Because of the above potential problems AWSBE have recognised that there need to be alternative methods for handling the outputs of an AD plant. One such method is drying and pelleting the digestate for use as a biomass fuel. The UK’s use of biomass fuels such as wood pellets has started to expand in the past few years. This is being driven by a number of factors, including the recent implementation of the renewable heat incentive (RHI) scheme. This is expected to result in a large rise in the number of biomass installations to provide heat and power from renewable sources, in both domestic and industrial sectors. The production of a biomass pellet fuel from the separated fibres from an anaerobic digester could provide an alternative or supplement to fuels such as municipal waste derived solid recovered fuels (SRF) or traditional biomass such as wood chip and wood pellets. Academic research has shown that it is viable to produce solid fuel from residual digestate (Kratzeisen et. al., 2010). A WRAP report on new markets for digestate (WRAP, 2010) highlights that using the fibrous fraction of digestate as a solid fuel is a developed and commercialised technology but that further work is required to establish the technology on a wider scale. The potential advantages for AD plant operators of converting their separated solids into biomass fuel include:

guaranteed use for heat from combined heat and power (CHP) unit or boiler. Parasitic heat loads at AD plants are usually variable e.g. there is a higher parasitic load in winter due to greater heat losses;

potential to claim RHIs for the drying process if using heat recovered from a biogas combusting CHP or boiler;

additional income from sale of the biomass pellet as a fuel or a concentrated fertiliser;

diversification of income stream for the anaerobic digester operator;

alternative disposal route for solids if there is not a suitable land bank near the digester;

reduced transport costs due to higher density of pellets; and

pellets can also be used on site to provide extra heat by combustion in a biomass boiler if there is a higher heat demand locally.

Using waste heat from a biogas driven CHP to dry wet biomass in this way can interlink biomass and biogas systems and provide complete energy recovery from the feed stocks. This is illustrated by Figure 3. The result of the drying and pelleting process is an easily transportable and storable fuel.

http://www.wrap.org.uk/content/new-markets-digestate


Figure 3: Potential model for linking AD with biomass production

The quality of the biomass fuel product produced, its combustion performance and the energy required to produce it will all be dependent on the AD process feed stocks. These are issues that need to be addressed before the process can be fully adopted by AD plant operators. The cost and environmental implications of converting the solid fraction of digestate into a fuel also need to be considered. 1.1.4 Current and future application for combining AD and biomass The potential take up of AD currently remains somewhat uncertain but clearly the technology forms a major component of the UK’s renewable energy strategy for at least the next 10 to 20 years. We envisage that there are numerous opportunities for small scale AD plants. This is partly due to the fact that they are easier to get through permitting and planning. There is less social opposition to small scale solutions as they are less visible and the financial risks are smaller. In addition the waste can be treated where it is created instead of transporting it long distances to larger AD plants and in vessel composters, as is currently the case. This reduces the cost of transport – something that is often ignored when considering AD solutions. The market for small scale AD is typically around waste

AD process

Waste

Waste

producer

Biogas

CHP

Electricity

Heat

Separated

fibres

Drying and

pelleting

Pellets

Biomass

boiler

Heat

Sale of pellets

Other biomass

e.g. wood

waste


management for local authorities in particularly rural areas, food manufacturers/processors with their own waste arisings such as abattoirs, and smaller farm units. Applications such as food processors typically have high heat demands, so coupling the pellet production with an AD solution offers an alternative to fossil fuels to meet all or part of that heat demand. The heat demand is usually variable so pellets can be produced and used at peak demand to top up heat exported from the AD plant or non-renewable heat sources. In addition, biomass fuel pellets are an easily transportable and storable source of energy. 1.2 Feasibility project objectives 1.2.1 Aims and objectives of feasibility study and full scale demonstration project The aim of the feasibility study and demonstration is to address a number of key issues in relation to the drying and pelleting of the AD fibres. These issues are in relation to the technical, economic and environmental feasibility of converting the separated solid fraction into pellets and comparing this option with the existing operation, which is to mature the fibres for use as a soil enhancer.

Technical feasibility. Can the separated solids be converted to a pellet? Will the process scale up from small scale bench top tests to larger scale industrial equipment? Will the cost of energy used to create the pellets be more than the energy value associated with the pellets? Do the pellets work with existing pellet boilers available on the market?

Economic feasibility. What scale drying, grinding and pelleting equipment is available to use with the smaller scale AD facility operated by AWSBE? Do the potential revenues from the pellets give a reasonable payback on the capital and operating costs of the drying, grinding and pelleting plant? Is the solid material more valuable to the operator to sell as a soil enhancer or as a fuel?

Environmental feasibility. What emissions are associated with drying the solids and combusting biomass pellets. Is material more useful as fertiliser (displacing inorganic fertilisers) or as biomass fuel (displacing non-renewable fuels)?

1.2.2 Meeting the outcomes of the DIAD optimisation program According to the WRAP supporting documentation for the DIAD project, the goal of the program is ‘to make AD work better, quicker or more cheaply resulting in more profitable plants’. The feasibility report addresses the scope of the program in the following ways:

Better AD processes. The method we use in our process to ensure that the digester performance and reliability is robust is to screen out the fraction of solids that will not readily digest after primary digestion and pasteurisation; as a result there are fewer problems with settling floating layers in the main digestion tank. Therefore the improvements in the AD process are made more feasible if a beneficial route with added value is identified for the solid fraction. In this case the route being investigated is conversion of the solids into a biomass pellet.

Quicker AD processes. Screening the solid fraction prior to the final digestion stage can significantly reduce the residence time required in the digester. We estimate that digester residence times of less than 20 days are possible based on performance of our three stage demonstration plant. This is significantly shorter than the residence times at one stage digestion facilities where residence times can be anything from 25 to 50 days depending on feed stocks.

Cheaper AD processes. Development of a cost effective drying and pelleting process for the solid fraction will result in several cost benefits for the operator including a guaranteed heat demand for excess CHP heat that is frequently wasted, which includes the potential to claim the RHI for the drying process. The current experience in the UK


seems to be that farmers are not willing to pay for digestate fertiliser and compost and it is frequently given away for free. Production of a biomass fuel from the solid fraction would potentially add value for the AD plant operator and could help reduce the payback on capital equipment costs. For example, prices for wood pellets in 2008 were fairly stable at around £150/tonne (Hayes, 2009).

1.3 State of technology 1.3.1 Background to pelleting technology Biomass pellets are generally a superior fuel when compared to their raw feedstock. Not only are the pellets more energy dense, they are also easier to handle and use in automated feed systems. These advantages, when combined with the sustainable and ecologically sound properties of the fuel, make it very attractive for use. The standard shape of a fuel pellet is cylindrical, with a diameter of 6 to 8 millimetres and a length of no more than 38 millimetres. Larger pellets are also occasionally manufactured; if they are more than 25 millimetres in diameter, they are usually referred to as briquettes. Densification processes can increase the density of the material from around 180 kg/m3 for dry chip or loose cake to over 700 kg/m3 for pellets/briquettes, a volume reduction of nearly 4:1. The benefits include reduced transport costs, greater fuel energy density, significantly smaller boiler footprint and fuel storage, lower boiler installation costs and simpler automation of the feeding system. The process of manufacturing fuel pellets involves placing ground biomass under high pressure and forcing it through a round opening called a die. When exposed to the appropriate conditions, the biomass fuses together, forming a solid mass. This process is known as extrusion. Some biomass naturally forms high-quality fuel pellets, while other types of biomass may need additives to serve as a binder that holds the pellet together. However, the creation of the pellets is only a small step in the overall process of manufacturing fuel pellets. These steps involve feedstock grinding, moisture control, extrusion, cooling, and packaging. Each step must be carried out with care if the final product is to be of acceptable quality. 1.3.2 AWSBE’s previous experience of pelleting digestate fibres AWSBE have previously carried out small scale trials on pelleting of separate fibres when commissioning the AD plant using cattle slurry. Existing laboratory scale equipment was used to mill and pellet the fibres. A basic sample analysis was carried out by Cardiff University School of Engineering, a summary of the results is shown in Table 1.

Table 1: Initial results from testing of pellets

Parameter Units Result

Moisture content % 9.88

Volatile matter % 74.55

Ash content % 5.43

Fixed carbon % 10.14

Gross calorific value MJ/kg 17.60

A trial was carried by Twinheat UK to observe the burning characteristics of the pellet. 60 kg of pellets were combusted in a Twinheat M20i boiler over a period of approximately 3 days. Some photographs were taken during the trial and are presented in Figure 4.


Figure 4: Photograph of pellet burner with digestate pellets (a) camera flash on (b) camera flash off

The boiler was run initially at its maximum capacity, in this case 25kW (or 7kg per hour) then allowed to modulate down to its minimum, 1.5kW. The burn at maximum capacity produced an output that would be expected from that of a wood pellet; in fact the boiler was running on its default setting for wood pellets for the duration of the trial. No smoke was evident and a very slight, but not unpleasant, odour was noted. The burn at a minimum rate, again performed similar to a wood pellet and although smoke was present it was actually less than would be expected from a wood pellet, there appeared to be no odour on this minimum burn. The formation of slag or “clinker” was quite prevalent at this level, which had to be manually removed from the furnace. Approximately 3kg of ash was produced; this is additional to the clinker (which totalled 1kg). There was also a level of fly ash which is difficult to measure, but was noted as a fine coating in the heat exchangers. Based on these positive initial findings it was decided we should look for opportunities to take this further and link anaerobic digestion with biomass production. 1.4 Detailed Technical appraisal This section of the report gives details of how the feasibility study was carried out and the results obtained. The first section describes the study methodology and the materials used. This is followed by details of the methods used and results obtained for drying, milling, pelleting and finally combustion of the pelleted material. At each stage we have attempted to quantify the relevant parameters to enable mass and energy balances to be determined. 1.4.1 Feasibility trial methodology The method used to meet the aims and objectives of the project is explained in the flow diagram in Figure 5. At each stage of the process e.g. drying, milling and pelleting we have tried to make an assessment of the process. The ‘business as usual’ case for the project is collection and maturation of the separated fibres followed by land application under an EA mobile plant licence for land application of waste. The alternative examined by the feasibility study is drying, milling and pelleting the separated fibres. Options such as mixing other biomass materials with the solids are also considered.

a b


Figure 5: Project methodology

1.4.2 Materials used in feasibility trials This section gives details of the raw materials used in this study. Softwood chip The softwood material used in the feasibility trial is produced by AWS at their licenced recycling centre. A Vermeer HG 6000 is used to grind clean waste softwood into a 40 mm chip. The waste wood (for example broken pallets) is received at the waste transfer station and civic amenity. It is pre-sorted to ensure no contaminated (painted or treated) waste wood is present. The end result of the grinding process is passed over a drum magnet to remove any nails, screws, staples of other magnetic metals that could be present with the wood. AWS typically sell the chipped wood as a biomass fuel or processes it further for animal bedding. A photograph of the softwood used in the feasibility project is shown in Figure 6.

Separated Fibres from AD process

Mixing

Drying

Particle size reduction

Pelletising

Maturation and spreading as waste

“business as usual”

“option investigated”


Figure 6: Softwood chip used in feasibility trials

Hardwood A small amount of hardwood has been used during the feasibility trials. Hardwood is notoriously difficult to pellet and it was decided to see if using a combination of hardwood and separated fibres would make pelleting feasible. The hardwood used was oak shavings and sawdust from a local furniture manufacturer. A photograph of the oak hardwood used in the project is shown in Figure 7.

Figure 7: Oak hardwood shavings and sawdust used in feasibility trials

AD fibres The AD process introduced in Section 1 and Figure 2 produces a fibrous solid by-product after the pasteurisation and primary digestion stage. The separating equipment is a mechanical screen with a series of brushes and rollers that scrape and squeeze the solids to remove as much of the moisture as possible. The material fed to the separator has been through a macerator prior to the primary digester. This ensures that the material being pasteurised has a maximum particle size of 12 mm in one direction, as required in the animal by-products regulations (ABPR). The separator screen has 3 mm diameter apertures, hence the separated fibre is typically between 3 and 12 mm particle size range. The type and quantity of material separated out of the process depends on the feed stocks used.


Figure 8: Example of separated fibres used in feasibility trials

The solid material that has been screened out of the AD process between the primary and secondary digestion stages was subjected to a chemical analysis by a UKAS accredited laboratory. The testing suite carried out was to the standard required by the PAS 110 end of waste criteria. The analysis was carried out on a sample of freshly separated solids. The results are shown in Table 2; the data has been presented as the amount per fresh tonne to enable comparison between the separated solids and other materials typically used for agricultural application such as cattle manure and green/food waste compost. The results show that the material has 295 kg/t of dry matter (DM) in the fresh fibres. This is equivalent to 29.5 % DM, similar to cattle farmyard manure. The N content of the fibres is 8.53 kg/t but of this only 0.97 kg/t is in the form of NH4, this is the form of N that is readily available to crops. The separated fibres have 8.04 kg/t P2O5, a valuable crop fertiliser. This is higher than the P2O5 present in typical green/food waste compost and cattle farmyard manure. However the K2O levels in the separated fibres is 3.96 kg/t, lower than the levels typically present in green/food waste compost and cattle farmyard manure. The fibres also have reasonable levels of MgO and SO3, these are also valuable crop fertilisers. The concentrations are 0.96 kg/t and 4.08 kg/t respectively. The MgO concentration in the fibres is slightly lower than the typical green/food waste compost and cattle farmyard manure but the SO3 concentration is slightly higher than the typical green/food waste compost and cattle farmyard manure. Overall it can be seen that the separated fibres compare quite favourably with the typical green/food waste compost and cattle farmyard manure analysis presented.

Table 2: Analysis of separated fibres and comparison with common soil enhancers (results presented on a fresh weight basis) Parameter Units Separated fibres Green/food composta Cattle farmyard manure –

storeda

DM Kg/t 295 600 250

LOI Kg/t 253.99

N kg N/t 8.53 11 6

NH4 kg NH4/t 0.97 0.6 0.6

P kg P2O5/t 8.04 3.8 3.2

K kg K2O/t 3.96 8 8

Mg kg MgO/t 0.96 3.4 1.8

S kg SO3/t 4.08 3.4 2.4 aFigures from DEFRA, 2010


In addition to the data presented in Table 2 the separated solids were analysed for metals and contaminants and compared with the limits in the PAS 110 standard. This is shown in Table 3. The heavy metals results are presented as mg/kg on a dry basis (db). The results show that the heavy metals in the fibres are lower than the PAS 110 limit in all cases with the exception of Zn. The Zn level in the separated fibres was 762 mg/kg in comparison to the PAS 110 limit of 400 mg/kg. This result is slightly surprising as we would not expect high levels of Zn in the AD plant feed stock, source segregated food waste. Previous work carried out by AWSBE examining the chemical characteristics of food waste showed that Zn concentrations were 75 mg/kg on a dry basis. Similarly Zhang et al (2007) analysed chemical characteristics of food waste and found an average Zn concentration of 76 kg/kg. It is possible that the high Zn concentration is a result of the use of sewage sludge digestate to seed the AD plant, or the cattle slurry that was used to commission and test the plant before food waste was processed. The separated fibres were also analysed for the contamination levels. The results show that the contaminants > 2 mm made up 0.18 % of the fibres (on a dry basis). The laboratory results show that this contamination was in the form of plastic bags and packaging. The sample contamination was lower than the PAS 110 limit of 0.5 %. However the plastic contamination in the sample is visible and if AWSBE wanted to sell this material this level of visible plastic contamination would not be acceptable. This means another stage of processing would be required e.g. trommel screening or wind sifting. The separated fibres were also tested for pathogens. Salmonella and E.Coli microbiological tests were carried out. The results show that salmonella was absent in the 5 sub samples tested. The results also show that E.Coli was under 10 CFU/g for the 5 sub-samples tested. This meets the PAS 110 standard in both cases.

Table 3: Analysis of separated fibres and comparison with the PAS 110 standard limits Parameter Units Separated fibres PAS 110 limit

Cd

mg/kg (db)

0.17 1.5

Cr 19.6 100

Cu 44.1 200

Pb 13.2 200

Hg <0.05 1

Ni 14.7 50

Zn 762 400

Total contaminants (> 2 mm)

% (db) 0.18 0.5

Salmonella Absent Absent in 5 sub samples

E. Coli CFU/g <10 in all samples <1000 CFU/g in 5 sub samples

1.4.3 Drying method and results Drying is an essential stage of producing a good quality biomass fuel; the reasons for this are highlighted in Section 3.1. There are a wide variety of driers available for processing biomass, but in order to keep the costs of the feasibility study to a minimum we decided to make use of some existing equipment readily available on site. The trailer used has a hot water jacket for heating the contents, insulated outer layer and a grid to suspend the solids above the floor with a plenum beneath. The trailer was connected to the hot water supply from a boiler. The boiler burner used biogas produced by the AWSBE AD plant. The trailer


also included a hydraulic ram which could be used to position the body at an angle and tip out the dry contents. After testing the drying trailer we realised that it was quite a slow method and we added two small extractor fans to increase the air flow. Both fans were capable of 85 m3/hour flow rates and were used to blow fresh air into the base of the trailer and extract the warm moist air from the top. The thermal energy supplied by the biogas boiler and used in the drying process was measured and calculated by a SVM F22 heat meter. The heat meter measures the flow and return water temperatures and the flow rate of the heating liquid; and calculates the thermal energy used. A simple schematic diagram of the trailer to explain the drying method is shown in Figure 9. The material was dried by placing a layer across the grid in the trailer, typically 300 mm thick. In order to monitor the drying progress 5 grab samples were taken from different points in the trailer every day. There were manually homogenised into one sample and the moisture content and bulk density were measured in triplicate.

Figure 9: Schematic diagram of the mobile trailer adapted to drying fibres for the feasibility study

Three different drying trials were carried out during the feasibility phase. The first trial used softwood chip alone. The second drying trial used a mixture of 1.3 parts separated fibres to 1 part softwood chip (calculated on volume of wet material). The third and final trial used separated fibres – without any additions. In all of the trials the material was spread out over the grid in the trailer and the material was agitated on a daily basis to prevent crusting. Figure 10 shows the results of trial 3; drying of the separated fibres only. The x-axis of the graph shows the thermal energy used by the process. In this case the energy was supplied by hot water being circulated around the trailer. The left y-axis shows the dry matter (as % on a weight for weight basis) in the trailer and the right y-axis shows the bulk density in kg/m3. The results show that the fibres were dried from an initial DM of 28.1 % up to a final DM of 95.2 %. The bulk density of the material decreased from 617 kg/m3 to 189 kg/m3 due to the loss of water during the drying process. It can be seen that in order to dry the volume of material (1.3 m3) it required 1,927 kWh of thermal energy.

Warm moist air extracted from

trailer

ambient air blown into trailer

Steel support

grid

Hot water flow

Hot water return

Drying solids


Figure 10: Graph showing the change in dry matter and density of digestate fibres against the input of thermal energy for drying trial 3 (digestate only)

Table 4 shows that using the drying method employed in this feasibility study required 3.5 kWh of thermal energy to remove 1 kg of water from the separated fibres. This is the equivalent of using 7.8 kWh to produce 1 kg dry material ready for pelleting. This figure seems to be very high when compared to the available literature. Kratzeisen et. al. (2009) examined the drying and pelleting of two digestate samples from 25 % dry matter up to 80 – 85 % dry matter. They used a drum drier and found that the thermal energy consumption was 2.97 kWh/kg of dry material produced. Similarly Li et al (2012) reviewed various biomass drying techniques and the typical thermal energy demands ranged from 1.26 GJ/tonne of water removed up to 4.0 GJ/tonne of water removed. The specific thermal energy demand depended on the type of drier used. The higher limit of 4.0 GJ/tonne reported by Li et al (2012) equates to approximately 1.1 kWh/kg of water removed. The aim of the feasibility study was not to demonstrate or prove the efficiency of different drying techniques but simply to produce enough dry material to carry out the trials. If AWSBE are successful with the application for the second phase demonstration project a more efficient and faster drying technique would be sought out. For the purpose of making calculations in this feasibility study more realistic a figure of 1.5 kWh/kg water removed will be used as a guide to the amount of thermal energy required to dry the separated fibres.

Table 4: Example of drying calculation based on results of drying trial 3

Parameter Unit Result

Initial mass of fibres kg 802

Final mass after drying kg 246

Water removed Kg 556

Thermal energy used kWh thermal 1,927

Typical energy rating kW thermal 5.1

Typical trailer temperature °C 40 – 50

Thermal energy used kWh/kg water 3.5

kWh/kg dry matter 7.8

1.4.4 . Milling method and results After drying, the material was further processed to reduce the particle size to a level that is suitable for pelleting. AWS have a milling facility at the recycling centre which is used for processing wood; this was also used for processing the dried solids. The Christy and Norris

0

100

200

300

400

500

600

700

0

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60

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80

90

100

0 500 1000 1500 2000

Den

sity

(kg

/m3

)

Dry

mat

ter

(%)

Thermal energy used (kWh)

Dry matter (%) density (kg/m3)


Ltd X Mill Extra 3000/3/A hammer mill is a 300 horse power (approx. 225 kW) machine and was fitted with a 6 mm screen. The material to be milled was loaded into a hopper with a vibratory feeder. From the hopper the material was fed over a rotary drum magnet and into the mill via a conveyor. The mill has a full dust extraction system as well as a fire protection system. The mill set-up used has a throughput of 10 - 15 m3/hour depending on the material. In the feasibility trials the milling time was recorded and calculated to be 12 m3/hour. The energy requirement of the milling process has been calculated based on full load conditions and the power ratings set out in Table 5. It should be noted that this is likely to be overestimated as the figures are calculated based on full load conditions rather than the amount of energy used.

Table 5: Calculated energy consumption by milling equipment

Item Units Rating

Conveyor kW 4

Elevator KW 7

Mill feed auger kW 3

Hammer mill kW 225

Dust extraction fans kW 15

Total energy required kW 254

Estimated throughput m3/hour 12

tonnes/hour 2.3

Energy used for milling kWh/m3 21.2

kWh/tonne 112.6

1.4.5 Pelleting Initial pelleting trials were carried out on a California Pellet Mill (CPM) laboratory pellet mill. The mill is powered by a 2 horse power motor (approx. 1.5 kW) and also has a hopper with a variable speed auger to feed the dried, milled material into the pelleting mill. The CPM pelleting machine used has a rotating die with 9.6 mm diameter holes which are 40 mm deep. An adjustable knife is set into the body of the mill to break the pellets away from the die when they reach a suitable length; in this case it was set to approx. 20 mm. Various mixtures of wood and fibres were pelleted on the laboratory pelleting machine. The various pellets produced are described, along with their code names in Table 6. This shows that good quality pellets can be produced using the dried separated fibre by itself or mixed with different types of wood. This includes hardwoods which are notoriously difficult to pellet due to the low lignin content. The pellets produced with the fibres were generally very consistent, produced very little dust and did not have a strong odour. This is shown in comparison with the softwood pellets which were fairly inconsistent, produced a lot of dust and were quite crumbly. Samples of these pellets were sent away to a UKAS accredited laboratory for analysis. The results presented in


Table 7, Table 8 and Table 9.


Table 7 shows the results of the analysis of the key parameters of the pellet samples. The results show that the gross calorific value (GCV) of the samples ranges from 16.944 MJ/kg to 21.606 MJ/kg. The lowest GCV was for the CS-FWHP sample. This might be expected as the sample includes the fibres produced using cattle slurry as the AD plant feed stock. The energy content of slurry and muck is quite low as it has already been through the animal’s digestive system. The highest GCV was for the FW-DFP. In all cases where food waste was used as the AD plant feed stock the subsequent pellets produced from the separated fibres have a higher GCV then the softwood only pellets (SWP). Kratzeisen et al (2010) pelleted and tested two samples of dried digestate. The first digester processed maize silage, grass and grass silage, and potatoes. The second digester processes maize silage, grass silage, poulty manure and corn cob mix. They found that the GCV of the pellets produced were 17.3 MJ/kg and 16.4 MJ/kg, within the range of results found in this feasibility study. The ash content of all the samples ranged from 1.6 % to 13.4 %. The lowest value was for the SWP sample. The pellets that contain separated fibres had ash contents ranging from 5.7 % to 13.4 %. The highest value was for the FW-DFP at 13.4 %. Blending the separated fibres with wood helped to decrease the ash content of the pellets. Kratzeisen et al (2010) found the ash contents of the two digestate pellets were 18.3 % and 14.6 %, higher than the values found in this study. The mechanical durability of all the pellets produced was good, ranging from 99.5 % up to 99.8 %. The pellet samples that contained the separated fibres had improved durability when compared to the SWP sample. Table 8 shows the chemical analysis of the pellets produced as part of the project. The results have been compared to those reported by Kratzeisen et al (2010). The chemical characteristics of the pellets will affect the emissions during combustion. Of particular importance are the concentrations of N, S, Cl, Fl and the heavy metals Zn, As and Hg. The results show that the N content of the pellets that include separated fibre ranges from 1.03 % to 2.20 %. This is comparable with the results of Kratzeisen et al (2010) who found N concentrations of 1.5 % and 2.9 %. This is higher than the SWP sample which contains 0.31 % N. It can be seen that blending the separated fibres with wood can reduce the N concentration in the pellets, potentially reducing the subsequent emissions of NOx when combusting the pellets. The results show that the S content of the pellets that include separated fibre ranges from 0.16 % to 0.37 %. This is comparable with the results of Kratzeisen et al (2010) who found S concentrations of 0.3 % and 0.9 %. The results from the fibre pellet testing are fairly close to the SWP sample which contains 0.14 % S. It can be seen that blending the separated fibres with wood can reduce the S concentration in the pellets, potentially reducing the subsequent emissions of SOx when combusting the pellets. Another important issue is the concentration of Cl as it can also form noxious emissions during combustion. The results show that the Cl content of the pellets that include separated fibre ranges from 0.19 % to 0.43 %. This is comparable with the results of Kratzeisen et al (2010) who found Cl concentrations of 0.27 % and 0.84 %. The results from the fibre pellet testing are much higher than the SWP sample which contains 0.03 % Cl. The Heavy metal contents in the pellets have also been quantified. The Zn content of the pellets that contain separated fibres is particularly high in comparison with the SWP sample. High Zn concentrations were also found during the analysis of the fresh fibres, as presented in Table 3. This is potentially due to the use of sewage sludge digestate as a digester seeding material and/or the use of cattle slurry to commission and test the plant before food waste


was taken in. As levels were lower in the pellet samples that contained separated fibres when compared with the SWP sample. Hg levels were low in all of the samples tested.


Table 6: Various pellets produced in trial batches as part of the feasibility project Sample

#

Code

name

Description Sample photograph

1 SWP Soft wood pellets made from clean waste wood after drying and milling. The pellets produced were fairly inconsistent in their length. They were fairly soft and friable and

produced quite a lot of dust in the pelleting process and after handling and bagging. The trial pelleting machine could only pellet the material at slow rate – producing approximately 15

kg/hour

2 FW-DFP Food waste – digestate fibre pellets. Pellets made from the separated fibre when using source

segregated food waste as the plant feed stock. The pellets produced were fairly consistant in their size and appearance. They are a dull, dark

brown colour with a slight odour that is not unpleasent. The pelleting machine processed the

material easily and the mill could be fed at the highest feed rate – producing approximately 30 kg/hour

3 FW-FSWP Food waste – digestate fibre and softwood pellets. Pellets made from the separated fibre when

using source segregated food waste as the plant feed stock mixed with softwood chip.

The pellets produced were fairly consistant in their size and appearance. They are brown with

flakes of wood chip visiable and have a slightly shiny surface. They have a slight woody odour that is not unpleasent. The pelleting machine processed the material easily and the mill could be

fed at the highest feed rate – producing approximately 30 kg/hour

4 FW-FHWP Food waste – digestate fibre and hardwood pellets. Pellets made from the separated fibre when using source segregated food waste as the plant feed stock mixed with hardwood sawdust

The pellets produced were fairly consistant and similar in appearance to the FW-FSWP sample. They are brown with flakes of wood chip visiable and have a slightly shiny surface. They have a

slight woody odour that is not unpleasent. The pelleting machine processed the material easily

and the mill could be fed at the highest feed rate – producing approximately 30 kg/hour

5 CS-FHWP Cattle slurry – digestate fibre and hardwood pellets. Pellets made from the separated fibre when

using cattle slurry as the plant feed stock mixed with hardwood sawdust. The pellets produced were very consistant and are very hard. They are brown and have a shiny

surface. They have a very little odour. The pelleting machine processed the material fairly easily

and the mill could be fed at a high feed rate – producing approximately 20-25 kg/hour


Table 7: Analysis of characteristics of various pellets produced during trial runs (results presented on as received basis) Moisture (%) Ash (%) Volatiles (%) GCV (MJ/kg) NCV (MJ/kg) Bulk density

(kg/m3)

Mechanical

durability (%)

SWP 8.7 1.6 74.0 17.856 16.476 641 99.5

FW-DFP 6.9 13.4 68.5 21.606 20.115 701 99.8

FW-FSWP 9.6 10.2 64.1 17.923 16.556 714 99.7

FW-FHWP 7.7 8.9 69.0 20.529 19.038 698 99.7

CS-FHWP 12.3 5.7 64.2 16.944 15.596 708 99.6

Table 8: Chemical analysis of various pellets produced and comparison with various pellet standards C H N O S Cl Fl Cd Zn V Pb Cu Ni Sb Co Mn Tl As Hg

% dry basis mg/kg dry basis

SWP 50.8 5.87 0.31 41 0.14 0.03 64 0.13 50.08 0.55 12.84 48.33 4.6 0.44 0.34 59.72 <0.1 41.09 0.01

FW-DFP 51.6 6.59 2.20 24.6 0.37 0.28 95 0.23 613.88 4.6 53.49 33.82 18.97 2.2 1.89 219.03 <0.1 4.66 0.04

FW-FSWP 49.6 5.80 1.94 30.8 0.18 0.43 52 0.19 578.27 3.33 19.92 45.35 18.39 1.34 1.48 199.12 <0.1 17.9 0.05

FW-FHWP 52.1 6.53 1.55 29.8 0.16 0.21 16 0.15 426.2 3.1 32.72 23.75 10.08 1.54 1.2 148.59 <0.1 3.01 0.03

CS-FHWP 50.0 5.49 1.03 36.6 0.20 0.19 53 0.13 521.89 2.73 9.21 14.15 9.98 0.62 0.75 114.45 <0.1 0.84 0.02

Kratzeisen

1a

45.3 5.2 2.9 28.4 0.90 0.84 0.29 301 4.4 58.8 0.93 0.07

Kratzeisen 2a

43.2 5.5 1.5 35.9 0.30 0.27 0.15 125 0.78 18.2 0.54 0.04

aKratzeisen et al (2010)


Table 9 shows the results of the ash fusion behaviour testing of the pellets. The ash fusion behaviour affects how the ash behaves during combustion conditions. If the combustion temperature approaches the ash flow temperature there could be problems with slagging and clinker formation in the boiler. The results show that the pellets that contain separated digestate fibres have similar ash fusion temperatures to the SWP sample. The exception to this is the CS-FHWP sample which has a slightly lower flow temperature (1180 °C).

Table 9: Ash fusion behaviour of pellets under oxidising conditions (°C)

Initial

deformation Softening

temperature Hemispherical temperature

Flow temperature

SWP 1090 1270 1280 1320

FW-DFP 1040 1270 1280 1290

FW-FSWP 1020 1220 1260 1300

FW-FHWP 1080 1300 1340 1450

CS-FHWP 1150 1160 1170 1180

Kratzeisen 1a 1090 1290 1320

Kratzeisen 2a 1110 1150 1390 aKratzeisen et al (2010) Based on the performance of the laboratory scale equipment and the results presented in


Table 7, Table 8, and Table 9 we decided to take one mix for testing on a larger scale pelleting machine. The mix selected was FW – FSWP; a mixture of separated solids and softwood chip. Approximately 2 m3 of the dried and milled fibres were transported for pelleting. The pelleting machine used was driven by a 22 kW motor and had two roller assemblies and a rotating die. The die holes were 6 mm diameter and 30 mm deep. The pelleting mill was fed via a variable speed auger and a turbulator; used to mix any additives with the material, but in this case it was not necessary to use any. The throughput of the pelleting plant was 250 kg pellets/hour. An assessment of the total energy that would be required for a dedicated pelleting plant is shown in Table 10. This includes feed augers, the pelletiser, post pelleting conveyor and a cooling fan.

Table 10: Calculated energy consumption by pelleting and ancillary equipment

Item Units Rating

Feed auger kW 1

Turbulator kW 1

Pellet mill kW 22

Exit conveyor kW 3

Cooling fan kW 5

Total kW 32

Throughput of pellets kg/hour 250

m3/hour 0.350

Energy use kWh/kg 0.128

kWh/m3 91.4

The trials carried out on the larger scale pelleting milled confirmed the results of the small scale trials carried out on site. It was possible to produce good quality uniform pellets using the dried and milled fibres alone or mixed with woodchip. The equipment manufacturer who carried out the trials stated that “the digestate acted as an extremely effective binding agent”. 1.5 Combustion trials and emissions monitoring The pellet samples produced were transported to Ashwell Engineering Ltd, a commercial pellet boiler manufacturer based in Leicester. The aim of the trials was to test the pellets produced from a mixture of separated fibre and softwood chip (mixed in a ratio of 1.3:1 on a volume basis) including testing of emissions from the boiler. The boiler used during the tests was a 150 kW D’Alessandro CS100 wood pellet boiler. The following observations were made by Ashwell:

The calorific value of the fuel was found to be considerably higher than that of a normal wood pellet. The feed rate was reduced accordingly;

Primary and secondary air was adjusted to achieve the correct burning in the appliance;

The pellet fuel ignition was achieved without any problems;

During tests the ash produced was found to be considerably heavier than normal wood pellet ash and ash content was greater;

During the testing period this did not create any issues but ash removal would be more frequent; and

The fly ash produced was again more than would be found on wood pellet but this would be expected due to the high ash content.

Boiler exhaust emissions testing were carried out by Environmental Scientifics Group (ESG) and a summary of the results is presented in Table 11. These results are compared with emissions from clean wood pellets combusted on the same test boiler.


Table 11: Boiler Exhaust emissions for digestate and softwood mixed pellets and wood only pellets Parameter Units Digestate:softwood pellets Wood only pellets

Particulate matter mg/m3 441 36.07

Oxides of N (as NO2) mg/m3 451 596.04

Sulphur dioxide (SO2) mg/m3 80

Carbon monoxide (CO) mg/m3 331 737.15

Carbon dioxide (CO2) % 11.1 11.25

Moisture % 13.4

Note: all results normalised to 273 K, 101.3 kPa, dry gas and 10 % Oxygen The results show that the pellets containing digestate compare favourably with the wood only pellets on the amount of N and CO released during combustion in the test boiler. However the emissions of particulates are higher from the pellets that contain digestate. This would be expected due to the higher ash content of the digestate material. The implications of the higher particulate concentration in the exhaust are dependent on the final use of the material and whether it is classified as a waste or not. Further work on the material classification will be carried out as part of any follow on DIAD project. Very little information on digestate pellet combustion is available. The main reference that has been found and referred to in this report is by Kratzeisen et al (2010). The results of their combustion trials on a 44 kW pellet boiler show that no disturbance on the feeding system was observed (e.g. blocking of feed augers) and the combustion process proceeded without disturbance. They observed marginal ash melting and slight slag creation but this did not impact the flow of ash out of the combustion area.

1.1 Example process – mass and energy balance

Figure 11 shows an example mass balance for a drying, milling and pelleting process. This is based on the data collected through the feasibility project. The example used has been based on an AD plant taking in around 4,000 tonnes of food waste per year and producing 500 tonnes of separated fibres per year. It can be seen that in order to dry the material to a suitable dry matter 344 tonnes would need to be removed. This would produce enough dry material to produce 156 tonnes of pellets. Table 12 shows an example energy balance for the pelleting process. This has been determined from the mass balance shown in Figure 11 and the energy use for different processes calculated in Section 1.4. The results show that the total energy used to produce the pellets is equivalent to 3.5 kWh/kg while the energy potential of the pellets is 5.6 kWh/kg (based on a net calorific value of 20.116 MJ/kg). Therefore the ratio of energy used to potential energy generated is 0.63.

Figure 11: Example mass balance for drying, milling and pelleting separated fibres from AD process


Table 12: Energy balance for pellet production Units

Input quantity of wet fibres Tonnes 500

Drying process kWhth 515,833

kWhth/kg pellets produced 3.3

Milling (based on 0.103 kWhel/kg) kWhel 16,119

Pelleting (based on 0.128 kWhel/kg) kWhel 19,982

Total energy consumption kWh 551,934

kWh/kg pellets produced 3.5

Net calorific value of pellets MJ/kg 20.115

kWh/kg pellets 5.6

Ratio of energy used to energy generated

0.63

1.6 Economic/cost benefit appraisal This section of the report examines the potential economic benefits of the proposed technology by determining the typical capital expenditure (CAPEX), operating expenditure (OPEX), potential revenues, the annual return and a simplified payback time. The economic assessment has been based on an example scenario and is specifically looking at the economics of the pelleting process, not the whole AD plant. The scenario examined could be a waste processor, a food processor or cooperative who have their own waste material and a large heat load that could be met/partially met by using a biomass fuel. This assumes that the material is not transported and the pellets are not sold but used on site in an existing boiler. The economic analysis is based on the data collected during the feasibility trials and the mass and energy balances presented in Figure 11 and Table 12. This has been based on the outputs of a ‘typical’ AD plant that could process 4,000 tonnes of food waste per year. The precise outputs of such a plant are heavily dependent on the quality of the feed stocks,

500 tonnes of separated fibres

Drying 515,838 kWh thermal energy

344 tonnes of

water removed

156 tonnes of dried material

Milling 16,119 kWh electrical energy

Pelleting 19,982 kWh electrical energy

156 tonnes of pellets


the performance of the plant and the CHP system used. However, based on our experience and knowledge of operating the AWSBE AD plant, we feel that this could generate 200 kW thermal energy; equivalent to 1,600,000 kWh per year. The AD plant, including pasteurisation and a two phase digestion system would typically use up to 40 % of this heat as a parasitic load – leaving the other 60 % available for drying biomass or other applications. 1.6.1 CAPEX The capital costs of procuring a plant to produce pellets from the wet separated fibres from an AD process have been investigated based on the results of the pelleting trials carried out as part of this feasibility project. Wood and waste pelleting operations in the UK tend to be on quite a large scale. However there are several equipment manufacturers who are able to produce cost effective small scale machines. Quotes from manufacturers were obtained based on trials on a test machine. The following equipment is one option for a small scale automated drying, grinding, pelleting and bagging plant:

2.5 m3 wet material hopper including low and high level switches; with auger to feed drying system.

Drying system including boiler, fan blower, steel pipe, cyclone to separate dried material from exhaust gases, outlet and airlock, exhaust stack.

Conveyor/buffer hopper for dried material with high and low level switches; with auger to feed hammer mill.

11 kW hammer mill with 6 mm screen and dust extraction system.

Milled material hopper with high and low level switches and variable speed auger to transfer pellet mill.

Feed hoppers for additive, lubricant and water with augers and dosing pumps;

22 kW pelleting mill with turbulator to mix additives with feed material and 6 or 8 mm die.

Vibratory sieve to separate pellets and dust.

Variable speed cooling conveyor to collect pellets, fitted with bulk bagging gantry.

Control panel including starters and overloads for every motor plus inverter drives, speed controller, ammeters, manual override, switches and audible warnings/lights.

The total cost for the equipment outlined above was £129,600 exc. VAT. In addition to this there would be a small cost for electrical connection to the plant distribution board. Installation and commissioning of the plant are included in the price. It has been assumed that the equipment will be installed within another facility with existing site infrastructure and buildings, and materials handling equipment will be available e.g. forklift, front end loader or mini diggers are available for use. Hence these elements do not add to the capital cost. Alternative methods for drying and processing are also available. It may be possible to use low tech, low cost drying solutions that make use of the waste heat from the AD plant CHP. One example is an under floor heating system that makes use of the hot water supplied by the CHP. Another option would be to use the exhaust heat from the CHP to dry the material in a rotary drum drier. The minimum expected throughput of the pelleting machine specified is 250 kg/hour. Based on discussions with the manufacturer the pelleting plant would be expected to produce 420 tonnes of pellets/year. This is based on an 8 hour shift, 5 shifts per week.


The mass balance presented in Figure 11 shows that the process would produce 156 tonnes pellets. This only equates to producing 37 % of the capacity of the machine. In addition to the pelleting equipment, for the scenario examined, a biomass pellet boiler and hot water storage tank would also be required. Prices for biomass boilers are variable depending on the boiler type and fuel type. Discussions with potential suppliers and our experience with boilers would suggest an approximate price of £200/kW installed for a biomass boiler. Our own wood biomass installation in Llangadog has an 8,000 L thermal storage tank for the hot water. Based on our experience with this an estimated price of £10,000 has been used for the thermal storage tank and pipe work installation. It has been assumed in the calculations that existing process pipe work is already in place. A summary of the expected capital costs is presented in Table 13.

Table 13: Summary of CAPEX

Complete drying, milling and pelleting plant £ 129,600

Biomass boiler (254 kW) £ 50,800

Thermal store and pipework £ 10,000

Total CAPEX £ 190,400

1.6.2 OPEX Energy costs The main energy consumption of the pelleting process will be during the drying of the biomass. However it is assumed in these calculations that the thermal energy for drying will be provided by the AD plant CHP unit and no charge will be applied. In fact the energy used for drying will attract the RHI at a rate of 0.071 £/kWh (<200kWth CHP only), as detailed in the revenues section. This leaves the electrical energy that is used for milling and pelleting the dried material. The electricity costs have been set at 0.12 £/kWh to reflect the site prices for electricity. Labour The calculations of labour costs assume that there is existing labour available on site that can spend a small proportion of their time on the pelleting plant. For example the operators of the associated AD plant, waste transfer station, food processor or farm. Based on discussion with the manufacturer we estimate that the operators would spend approximately 3 hours per shift to oversee the process. This includes setting up and performing controlled shut downs of the plant, basic maintenance of the pelleting equipment such as greasing bearings, checking and cleaning out the drying system, and basic maintenance of the hammer mill. The number of times the pelleting plant is operated would depend on the throughput. Based on the mass balance in Figure 11 the plant would be expected to produce 156 tonnes of pellets per year. The assumption of an 8 hour shift and a labour cost of £15/hour have been assumed in the calculation. Equipment maintenance The main maintenance costs of the plant will include replacement of the hammer mill hammers, replacement of the pellet mill rollers, replacement of the pellet mill die and servicing of motors. Based on AWS Ltd experience of operating a hammer mill and discussions with suppliers the wearable components could conceivably last for 5 years before replacement (depending on the utilisation of the equipment). Replacement of all of


these components for a small hammer mill and small pellet mill would be expected to cost approximately £10,000. Averaged out the maintenance costs would be £2,000 per year. Consumables and other costs Consumables and other costs have been estimated at £2,500 per year. This includes diesel for equipment handling machines, protective equipment and other consumables such as lubricants. An allowance for laboratory analysis of samples has also been made. Summary of OPEX Based on the assumptions previously discussed a summary of the expected operating costs is presented in Table 14.

Table 14: Summary of OPEX

Cost

Electricity £4,332

Labour £ 3,513

Maintenance £ 2,000

Consumables and other costs £ 2,500

Total OPEX £ 12,345

OPEX/tonne pellets produced £ 79.08/tonne

1.6.3 Potential revenues There are several sources of potential revenue from the pelleting process. These include the RHI tariff that can be claimed for drying biomass and use of pellets on site to reduce the use of fossil fuels such as natural gas. Using biomass pellets would also attract an RHI payment Alternatively the pellets could be sold by the energy they produce or by the tonne to small commercial/industrial users. Typical pellet prices range from £37.7/MWh up to £59.8/MWh depending on whether they are supplied bagged or in bulk quantities (E4tech, 2010). Based on the pellet production costs calculated in Table 14 and the high quality of pellets produced from the raw materials we feel it would be reasonable to charge £120/tonne - £140/tonne. At the higher end of £140/tonne the energy would cost £25.06/MWh. If this model was used then the potential costs of transport would need to be included in the economic model. Renewable heat incentive (RHI) As previously detailed, we would expect the biomass drying stage of the process to attract the RHI. If using heat from the biogas powered CHP then the RHI rate for biogas combustion would be used. As of April 2012 this stands at 0.071 £/kWh for <200kWth CHP capacity. The amount of heat used and the subsequent RHI payments will depend on the drying method used. However the heat demand has been estimated in Table 12 for drying of separated fibres; the estimated heat requirement is 515,833 kWh. This could be provided by flue gases or by hot water. The RHI tariff is supported for 20 years and is expected to increase in line with the retail price index (RPI). Use of pellets on site Rather than selling the pellets to an end user, some operators may have a significant heat load on their site which they could supply through the pellets produced. This approach could have a significant effect on the revenues as the operator would be able to save on buying in non-renewable fuels such as gas, typically at £0.031/kWh. They would also be able to claim the RHI on the renewable heat they generate and use from combusting the biomass pellet fuel.


In order to calculate the potential revenue from this model of pellet use then some assumptions about the end user have had to be made. Boiler efficiency has been assumed at 85 %. It has also been assumed that the biomass boiler is operating at full load for one third of the available time to provide process heat. This is 2,920 hours per year. Based on the potential energy of the pellets produced and the boiler running time it has been estimated that the pellets could run a 254 kW boiler. RHI payments on heat generated by a boiler of this capacity are based on the medium scale biomass tariff. This is split into two tiers where tier 1 is £0.051/kWh and tier 2 is £0.021 kWh. The tier 1 payment can be claimed for a certain portion of the production, this is equivalent to 1,314 hours multiplied by the installed capacity of the boiler. The remaining heat use can be claimed at tier 2.

Table 15: Summary of potential revenues/savings

Units

RHI for biomass drying from AD CHP £/kWh 0.071

£ 36,624

Potential saving on non renewable fuels by biomass

combustion (natural gas at £0.031/kWh)

£ 22,984

Potential RHI for medium scale biomass combustion £ 25,579

Total revenues £ 85,187

1.6.4 Cost benefit analysis Based on the information presented in Section 1.6 a summary of the CAPEX, OPEX, revenues and annual returns is shown in Table 16. A simplified earnings before interest, taxes, depreciation and amortization (EBITDA) payback has also been shown. The payback on the scenario presented is 2.61 years.

Table 16: Summary of economic information and cost benefit analysis

CAPEX £ 190,400

OPEX £ 12,345

Revenue/savings £ 85,187

Annual returns £ 72,842

Simplified EBITDA payback 2.61 years

There are several key aspects that enable such a quick payback to be made, these include:

supply of heat available from AD plant;

sharing of resources with existing operations;

sharing of labour with existing operations; and

ready use of pellets on site or alternatively a ready market for sale of the pellets.

1.6.5 Alternative options The economics of the model presented are based on certain assumptions about capital costs, operating costs and revenues. This does not exclude alternative scenarios from being viable; but in many instances the assessment of whether a project is worthwhile is based on site specific elements. Some sites may have some suitable equipment already. Some sites may have outlets to sell the pellets directly to end users. Some site operators may have additional feed materials that they want to mix with the separated fibres to increase the pellet production. We have shown through the feasibility project that the separated fibres can be mixed with both clean waste softwood and hardwood and high quality pellets can be produced. Other


materials may also be used; for example straw or miscanthus, forestry residues or road verge cuttings. There may be several potential benefits to adding these materials to the process including:

additional mass of pellets produced;

further RHI can be claimed by drying;

potential gate fees for taking in waste materials; and

potential saving by diverting waste from landfill.

1.6.6 Comparison with ‘business as usual’ At present the ‘business as usual’ case is spreading digestate on agricultural land. Despite the existence of a PAS 110 standard and the QP for digestate as an end of waste criteria to convert waste digestate into a product it is still frequently given away rather than sold. Hence the base line condition with which we would compare the proposed technology is a value of zero. Despite this there is nutrient value in the digestate. By comparing the digestate nutrient concentration with those of inorganic fertilisers it is possible to assign a monetary value to the digestate. The NH4, P2O and K2O content of the separated fibres has been compared to the equivalent inorganic fertilisers. In this case the inorganic fertilisers are ammonium nitrate, triple super phosphate and muriate of potash. They have active ingredient concentrations of 34.5 %, 46 % and 60 % respectively. The assumptions on inorganic fertilisers are shown in Table 17. These prices are from May 2012 but it should be noted that fertiliser prices can change from day to day.

Table 17: Assumptions used for organic fertilisers Active ingredient

concentration (%) Typical cost

(£/t) Cost of active ingredient (£/kg)

Ammonium nitrate 34.5 330 0.96

Triple super phosphate

46 390 0.85

Muriate of potash 60 345 0.58

Based on the analysis of the nutrient content of the separated fibres shown in Table 2 and fertiliser prices, the value of the NH4, P2O5 and K20 in the fibres has been estimated. Table 18 shows that 1 tonne of fibres is worth approximately £10/tonne. This is a simplistic valuation as it does not take into account several factors such as the availability of the nutrients and the contribution of micro nutrients in the fibres. It also does not take into account any subsequent effects on the growth of crops.

Table 18: Simplified assessment of fertiliser value of separated fibres Separated fibre (kg/fresh tonne) Value of separated fibres

(£/tonne)

NH4 0.97 0.9 P2O5 8.04 6.8 K2O 3.96 2.3 Total 10.0

The example used for the mass and energy balances, shown in Figure 11 and Table 12 respectively, is based on processing 500 tonnes/year of separated fibre. Based on the simplified assessment in Table 18 the value of this material to the operator is £5,000 per year. However the costs of handling and transporting the digestate to suitable agricultural land followed by spreading are likely to be comparable or exceed the value of £10/tonne, hence it is unlikely that there is a margin to make profit from the material as it stands.


1.7 Environmental impact Linking AD and biomass in the way proposed by this feasibility study can have several environmental benefits. These include:

production of a biomass fuel which can displace fossil fuels; and

reduction in material volumes resulting in lower transport costs.

This has to be balanced against the potential negative environmental impacts, for example the loss of potential fertiliser resulting in continued use of inorganic fertilisers. Production of pellets from the separated fibres and subsequent use as a fuel is likely to lead to displacement of fossil fuels. The energy potential of the pellets has been measured as 5.6 kWh/kg. Making 151 tonnes of pellets would result in 872,271 kWh of potential energy. According to Carbon Trust guidelines on energy and carbon conversions (Carbon Trust, 2006) natural gas has an energy content of 11 kWh/m3 and diesel oil has an energy content of 10.7 kWh/L. Hence the energy value of the pellets produced would be equivalent to 79,297 m3 of natural gas or 81,521 L of diesel oil. The carbon emissions factors reported for the use of fossil fuels in energy production are 0.19 kg CO2/kWh for natural gas and 0.25 kg CO2/kWh for diesel oil. To produce the equivalent amount of energy that is available in the digestate pellets the carbon emissions would be 165,731 kg CO2 for natural gas and 218,068 kg CO2 for diesel oil. 1.8 Legislation The legislation that will affect the proposed technology depends on the nature of the site and the types of material being processed. For AD plants treating food waste the legislative requirements would likely be planning, an EA permit and animal by-products regulations approval to treat category 3 wastes. Further activities such as drying and pelleting could fit within these licences and permissions. Suitable provisions would have to be made to manage potential emissions from the process. This would include dust, noise and odours. Other operations treating non-waste, such as farm based systems, may be exempt from planning and licencing requirements. The pellets produced by the process may still be classified as a waste by the EA as they have yet to meet any end of waste criteria. The end of waste criteria for digestate is the PAS 110 standard and the QP; however the QP not currently include use of digestate as fuel as an allowable market. Because of this, part of the demonstration project would involve developing a specification for the pellets and making an end of waste submission to the EA. The three main factors that affect whether end of waste submissions are successful are listed by the EA in their guidance on end of waste submissions for fuel derived products (EA website guidance on ‘end of waste criteria’) and include:

the waste has been converted into a distinct and marketable product;

the processed substance can be used in exactly the same way as a non-waste, and

the processed substance can be stored and used with no worse environmental effects when compared to the raw material it is intended to replace.

Assuming that the pellets could meet an agreed specification they would no longer be classified as waste but a product, which would make sale of the pellets significantly easier. Currently the AD plant is operating on waste derived feed stocks. Hence for the time being the process outputs are still classified as waste. Depending on the types of wastes used and the boiler throughput the combustion of the pellets may be subject to the Waste Incineration Directive (WID). The rules under the WID apply to most activities that involve burning waste, including burning waste for fuel. The aim of the directive is to prevent or limit, as far as practicable, negative effects on the environment, in particular pollution by

http://www.environment-agency.gov.uk/business/sectors/124299.aspx

http://www.environment-agency.gov.uk/business/sectors/124299.aspx


emissions into air, soil, surface and groundwater and the resulting risks to human health, from the incineration and co-incineration of waste1. For the purposes of DIAD the project will be considered an experimental plant and is being undertaken on a R&D basis. 1.9 Conclusions from phase 1 feasibility trials The feasibility trials carried out have produced some interesting results and point to the technical, economic and environmental feasibility of linking AD with biomass pellet production. The following key conclusions can be drawn from the results:

High quality pellets can be produced using separated fibres from the AD process;

Calorific values and mechanical durability were as good as, or better than those of

softwood pellets.

No additional binders or lubricants were required to form the pellets.

The precise chemical characteristics of the pellets could be adjusted by mixing the

fibres with other feed stocks such as softwood or hardwood.

The total energy required to form the pellets was approximately 63 % of the energy

that could be generated by the pellets (table 12).

Using the waste heat from AD can create a storable and transportable fuel.

The potential paybacks for a biomass pelleting plant linked with an AD plant could be

as low as 2.61 years.

The processing and pelleting plant is flexible – alternative biomass fuels could be

dried in addition to the fibres.

The basic economic assessment of the value of the fibres has shown that production

of pellets may be a viable alternative to land spreading the material.

A biomass pelleting plant capable of producing 151 tonnes of pellets could produce

the equivalent energy from 79,297 m3 of natural gas or 81,521 L of diesel oil.

1 Defra Waste Incineration Directive http://www.defra.gov.uk/industrial-emissions/eu-international/wid/

http://www.defra.gov.uk/industrial-emissions/eu-international/wid/


2.0 Phase 2 – demonstration project 2.1 Introduction The work carried out for the phase 1 feasibility study has demonstrated that it is possible to produce a high quality pellet from the separated solids from an AD process. The original aims of the project were to address issues over the technical, economic and environmental feasibility of the proposed technology. The following points highlight how the phase 1 report has addressed these issues:

Technical feasibility. The results of the report show that a variety of mixtures of fibres and wood can be converted into high quality pellets. Tests on a commercial scale pelleting machine have also confirmed the viability of the material for producing pellets. Calculation of the energy balance has shown that there is a net energy gain from the process. Early results suggest that combustion performance in a pellet boiler is good but further work needs to be done to confirm.

Economic feasibility. Through the project AWSBE have located several sources for smaller scale drying, milling and pelleting equipment and have shown through an example economic model that it is possible to have short paybacks on the capital equipment.

Environmental feasibility. The results show that the pellets produced have the potential to displace a significant amount of fossil fuels; this could be particularly relevant for small to medium industrial and commercial businesses such as food processers. Further work has to be done to determine the emissions from the material during processing and combustion to see if it confirms to relevant emissions standards.

2.2 Demonstration project methodology Now that the feasibility of the proposed technology has been assessed a follow up demonstration project is required. The demonstration project would be a full scale drying, milling and pelleting plant complete with a biomass pellet boiler. The aims of the demonstration plant would be as follows:

To build up a track record of drying, milling, pelleting and combustion of AD fibres. The lack of available information available on pelleting of digestate would suggest that it is not yet an established process. A track record would help give confidence in the reliability of the process.

To use the demonstration trial results to develop an end of waste specification for the materials generated.

To trial drying and pelleting of other residues and wastes with AD fibres.

To have a working demonstration plant to show future potential clients.

The demonstration site will be located alongside the existing anaerobic digestion plant at AWS Ltd in Llangadog. The site will be ideally located to demonstrate the potential benefits of further processing AD residues in biomass pellets for several reasons:

There is already a suitably sized building available to house the drying and pelleting plant.

The AWS waste transfer station and recycling centre also operates a hammer mill that was tested during the feasibility stage and found to be suitable for milling the dried fibres.

The site has an existing permits and licences required.

Materials handling equipment and labour is available on site.

A variety of additional feed materials which could be mixed with the separated fibres are available on site.


2.3 Demonstration project details The proposal for the phase 2 demonstration project is to install a full scale plant to produce pellets from the separated fibres from the AWSBE AD plant. In order to produce pellets the key stages are drying, milling and densification. As previously discussed in the phase 1 feasibility report AWS has a hammer mill which was tested and was found to be suitable for the materials. The current utilisation of the mill means it will be available to process the material produced by the AD plant following drying. This means that a drying facility and pelleting mill would be required to complete the plant. In order to size the dryer capacity we have determined the likely outputs from the process, these are summarised in Table 19. The data used is based on our experience of operating the AD plant and on the results of the phase 1 feasibility trials.

Table 19: Expected outcomes of the proposed Llangadog biomass pelleting plant Separated fibres Tonnes/year 216

Water to be evaporated Tonnes/year 148.6

Projected energy requirement kWh/year 222,840

Pellets produced Tonnes/year 67.4

In addition to the separated fibres we will also examine drying and pelleting other biomass materials, separately or mixed with the fibres. These could include:

woody green waste from the civic amenity that are unsuitable for digestion e.g. twigs and branches;

clean waste wood – hardwood and softwood; and

road side hedgerow trimmings.

In addition to the expected 216 tonnes of fibres we would potentially have a further 100 tonnes of waste wood and green waste. We would also have over 800 tonnes of liquid digestate that we could use a portion of. With these additional materials we could produce over 100 tonnes of pellets and dry biomass. Drying system The drying system used in the phase 1 feasibility trials was shown to be ineffective in that the energy use was very high and it was quite slow. This is likely due to the poor heat transfer within the trailer; the typical thermal energy supplied to the trailer was only 5.1 kW/h. Based on our observations during the trial we predict that the annual throughput using this method would only be 20 tonnes of wet solids. This would only produce about 6 tonnes of pellets which we do not feel is enough to develop a meaningful track record. Despite the inefficiency of the drying method used it did give us some valuable information on the importance of air flow and agitation for material drying. An alternative drying method is being proposed for the demonstration phase of the project. This involves a two stage drying process to get down to the required dry matter. The first stage would involve the construction of several drying bays with under floor heating. This would enable us to dry very wet materials such as liquid digestate. Hot water from the AD plant CHP would be circulated underneath the bays and the heat distributed to the material on top. We envisage three bays being constructed in the building that has been made available next to the AD plant. Each bay would have an individually controlled hot water supply to ensure the process is flexible. The proposed bays would have dimensions of approximately 3 m by 6 m and a surface area of 18 m2. This would give a total drying surface of 54 m2. The drying bays would be used to reduce the moisture content of the fibres, liquid digestate and other biomass materials to around 50 %. At this point the


material would be dry enough to be processed by a drum drier. The drum drier would use hot flue gases to further dry the material to 90 % dry matter, as which point it could be successfully pelleted. The hot gases could be provided by combusting pellets or other biomass fuels. Most of the heat load needed for drying will be met by the hot water recovered from the AD plant CHP, which should have 280,000 kWh of thermal energy available. If we find that the CHP heat is insufficient to meet the needs of the drying facility we also have the option of connecting to the hot water supply from the 120 kW log boiler and 8,000 L thermal storage tank located on the nearby Burdens resource and renewable energy site. We will also be able to combust some of the pellets we are producing. Detailed design and sizing of the drying facility will form part of the phase 2 project and any installation work will be done by an MCS approved contractor who has been involved with the Llangadog AD plant and also has experience of biomass systems and under floor heating. The material drying time will be dependent on a number of factors other than the amount of heat supplied. This includes agitation to ensure fresh material comes into contact with air and also the heat source. This will encourage even drying. The drying bays will be designed to allow for a small machine, such as a mini digger, to have access to turn the material periodically. The small volumes being processed should mean that this takes about 15 minutes each day. Another important factor that affects the drying rate is the flow of air across the material. Removing the moist, saturated air and replacing it with fresh air will speed up the moisture transfer process. This will be achieved by using several fans. One or two fans will be used to extract air from the process building, the displaced air will be pumped outside while fresh air will be allowed into the building through vents. The sizing of the fans required will be part of the detailed design that is carried out as part of the phase 2 project. Several smaller fans will also be used to blow air across the surface of the drying material. This creates turbulence which helps with the moisture transfer and speeds up the drying process. The drum drier will be fed by a variable speed auger which will enable us to adjust the volume of material being dried and the amount of time it spends in the drier. An extractor fan is used to draw hot air through the system with the drying biomass, a cyclone will be used to separate the dry biomass from the hot air stream. The drum dryer, feed system and extraction system comes as a complete packaged unit. Odour emissions from the drying plant will be monitored in line with the existing AD plant odour management plan. This involves regular site walk overs and ‘sniff tests’ at different boundary points. We also intend to carry out full emissions testing from the drying process; for example we will test for odours, dust and bio-aerosols. If we determine at any time during the demonstration project that the drying or pelleting process is causing odour or dust problems we will implement corrective actions which could include temporarily stopping the process and installing an odour filter. To date the AD plant has received no complaints from the site users or members of the public. Our odour monitoring has not indicated any problems and we feel we have a good record for odour and nuisance management. Pelleting Pelleting equipment for the project will be supplied by one of two UK equipment manufactures. Both have experience of pelleting a wide range of biomass materials and offer smaller scale pelleting equipment. Both companies have multiple plants working across


the UK. A decision on who will supply the pellet mill will be made at the start of the phase 2 project and will be based on the technical solution offered, delivery times and price. Biomass boiler As part of the demonstration project a biomass pellet boiler will be installed. The boiler will be a test unit and will be installed at the Burdens resource and efficiency centre alongside the existing 120 kW log boiler. The pellet boiler will be connected to the existing 8,000 L thermal storage tank and the heat generated will be used to heat the offices and also to provide heat for the drying process if it is required. This will give the plant great flexibility and will mean additional biomass materials could be dried if required. The test boiler will be sized at 50 kW to 100 kW. This will give us a suitable facility to do longer term combustion trials on the pellets. The pellet boiler will be supplied by either Ashwells Biomass Ltd or Twinheat UK. Both companies have trialled the digestate pellets on their respective pellet boilers with good results. 2.3.1 Permitting and approvals The AWS site already operates under a waste management licence and EA permit. The AD plant is included within this permit and drying is one of the activities included within the permit. Based on discussions with the EA on a local level, before we can start processing material we would have to update our site documentation including the Facility Management Plan to include the drying operations. This would have to address potential issues such as noise and odour emissions. Once the documentation is in place it needs to be signed off by our local EA officer but we do not need to vary our existing permit. We envisage that modifying the site management plans, including updating odour management plans, and getting local EA approval would take 4 weeks. Based on the results of the first phase of the feasibility study AWSBE feel that it will be possible to generate an end of waste criteria of the biomass pellets produced. However, the demonstration project will be needed to build up a body of evidence at full scale to prove the requirements of the end of waste criteria and ensure a repeatable specification can be achieved. We would also have to consult with the Animal Health department as the feed stock for the AD plant includes category 3 animal by-products. Once the evidence required has been gathered an application will be completed and submitted. Following submission of the application it can take several weeks for the EA end of waste panel to make a decision. 2.3.2 External contracts The proposed demonstration project will involve AWSBE taking separated fibres from the AD plant, also operated by AWSBE. Hence there are no conflicts around contracts for raw materials supply to the demonstration project. The feed stocks for the AD plant come from the AWS waste transfer station and also from collections run by AWS. A suitable electricity supply for the pelleting plant is already in place in the proposed building and heat for the drying process will be supplied by the AWSBE AD plant CHP in the form of hot water or exhaust gases. Any pellets produced during the demonstration trials will be combusted on our own site and the heat generated will also be used on site. 2.3.3 Monitoring and evaluation The key aspects of the project monitoring and evaluation will be put in place to ensure we achieve the project aims set out in the introduction. This includes:

Regular sampling of the input feed stocks and process outputs.

How repeatable/consistent are pellets produced.

Performance of key equipment such as the dryer and pellet mill, including; reliability, maintenance requirements, how much energy do they use? What are the running costs?


How much time is needed to run them? Could design improvements be included for the next project?

Performance of the pellet boiler including; how much energy do the pellets produce? What are longer term emissions, how much ash is produced and what is the cost of disposal? What maintenance is required and how frequently?

Assessment of the environmental costs/benefits of using the separated fibre as a fertiliser compared to as a fuel.

Following the completion of a trial period sufficient data should be available to reassess the assumptions and measurements made during the phase 1 feasibility project. This should enable us to very accurately quantify costs and benefits based on data from a live demonstration plant. 2.4 Project timescale A project timeline has been developed to show the key activities and project milestones. This is presented in Figure 12. It can be seen that the overall project is planned to last for 34 weeks with the trials taking up the final 16 weeks of the project. Following the end of the demonstration project it is envisaged that the equipment will be continued to be used on site.


Figure 12: Outline project plan

Week number 1 2 3 4 5 6 7 8 9 10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

Project Evaluation

Draw up Project Plan and detailed designs

Pre-consultation with EA & AHVL

Research into equipment

Place orders and wait for dlivery

On site construction

Installation

Commissioning

Project Demonstration

Develop end of waste application

Wait for EA feed back

Project evaluation & Reporting


2.5 Cost breakdown and milestones The main cost element of the demonstration project will include the capital costs of the equipment required (including installation and commissioning). These have been summarised in Table 20. The estimated equipment capital cost of the project is £75,000.

Table 20: Summary of costs for pelleting plant equipment at Llangadog AD plant Specification Cost (£) Potential suppliers

CAPEX

Pellet mill 250 kg/hour mill with feed hopper, feed auger, turbulator, additive hoppers for

binder, oil and water

25,000 Farm Feed Systems or Treenergy Biomass

Products and Services Ltd

Drying system 54 m2 Underfloor heating drying bays

with air extraction to reduce moisture content to ~ 50 %

10,000 Designed by AWSBE,

installed by local contractors

Drum drier with feed auger, fuel stove,

rotating drum and extraction cyclone to reduce moisture to ~ 10 %. Expected

output of 200-500 kg/hr

25,000 Treenergy Biomass

Products and Services Ltd

Pellet boiler 50 kW – 100 kW pellet boiler for demonstration of pellet combustion

characteristics

15,000 Ashwell biomass or Twinheat UK

Total CAPEX 75,000

Note: prices include an allowance for commissioning equipment An assessment of the expected labour and management costs is presented in Table 21. This includes the costs for a waste technician to assist in carrying out trials and maintenance and project engineer and consultant to manage the day to day running of the project, develop designs and equipment specifications, liaise with suppliers, over-see subcontractors and carry out project monitoring and reporting. The total labour costs have been estimated at £11,270.

Table 21: Assessment of project labour and management costs Hourly rate Hourly rate (inc.

15% overheads)

Hours effort

required

Total cost

Project Engineer 20 23 300 6,900

Consultant

Engineer

20 23 150 3,450

Waste Technician 10 11.5 80 920

Total 11,270

Table 22 gives an estimate of sub-contractor costs for the demonstration project. These costs are significant but are necessary in order to get enough information to develop the end of waste submission.


Table 22: Estimate of sub-contractor costs Item Cost per unit (£) # required Cost exc. VAT (£)

Lab analysis of samples 250 20 5,000

Pellet production on large scale pelleting

plant

250 (per day) 3 (days) 600

Odour monitoring 2,650 1 2,650

Stack emissions monitoring 1,795 3 5,385

Total 13,035

The total project costs including capital equipment, labour and project management and sub-contractor costs is £99,305. 2.6 Key personnel AWSBE has a team with a variety of skills, the key people for the project are detailed below with their relevant experience. Arrash Shirani – Project Engineer from Burdens. Arrash is an environmental engineer with a background in academic and industrial research and development work. Arrash has worked in the waste management industry for the past 5 years. Arrash has been involved in the design, build, commission and operation of the Llangadog AD plant and was also the project Engineer on the first phase feasibility study for the WRAP DIAD project. Meirion Evans – Consultant Engineer. Meirion is a very experienced mechanical engineer and has spent many years working in the waste industry. He also has long experience of renewable energy from waste including gasification, autoclaving, biomass and AD. Meirion has been involved in the design, build and operation of the Llangadog AD plant and acted as Consultant Engineer for the feasibility stage of the DIAD project. Hefin Roberts – AWS owner and director of AWSBE. Hefin has been involved in the day to day operation of the AWS waste transfer station and recycling centre since it opened 7 years ago. He also holds the relevant waste management licences and qualifications such as WAMITAB and has experience of wood recycling and biomass fuels. Meirion Thomas – Burdens representative. Meirion is director of Burdens Agricultural division and has many years of commercial experience. Meirion will bring this knowledge to the project to help ensure we get the best commercial exploitation from the product developed.

2.7 Commercialisation plan Based on the successful demonstration and operation of the solids processing and pelleting plant it is envisaged that AWSBE will then be able to offer this option to all potential clients who are interested in purchasing a small scale AD plant. The decision of whether to take this route for treatment of the solids will depend on the end users requirements. We envisage that there are numerous opportunities for small scale AD plants. This is partly due to the fact that they are easier to get through permitting and planning. There is less social opposition to small scale solutions as they are less visible and the financial risks are smaller. In addition the waste can be treated where it is created instead of transporting it long distances to larger AD plants and in vessel composters as is currently the case. This reduces the cost of transport – something that is often ignored when considering AD solutions. The market for small scale AD is typically around waste management for local


authorities in particularly rural areas, food manufacturers/processors with their own waste arisings such as abattoirs, and smaller farm units. Applications such as food processors typically have high heat demands so coupling the pellet production with an AD solution offers an alternative to fossil fuels to meet part of that heat demand. The heat demand is usually variable so pellets can be produced and used at peak demand to top up heat exported from the AD plant or non-renewable heat sources. Other applications can be in built up areas such as industrial estates, with no easy access to agricultural land for application of digestate. Diverting part of the AD process output to solid fuel production also decreases the subsequent amount of material that has to be spread as fertiliser – reducing transport costs. As noted above the existing AWSBE demonstrator plant has received funding support from Burdens Ltd the technology has been developed as part of Burden’s strategy focussed on offering renewable energy products in addition to its traditional construction related core business. Burdens has a National and International profile and will market AWSBE small scale plants as part of its full range of large scale and small scale AD/Biogas technologies. We envisage that the pelleting process described above will be offered in the marketplace as an option for both large scale and small scale plants. As AD technology is relatively new to the UK the potential take up still remains somewhat uncertain in early years, but clearly the technology forms a major component of the UK’s renewable energy strategy for at least the next 10 to 20 years. Burdens strategy is formulated around establishing a sizeable market share of this emerging market. We predict therefore that the technology described above could be a very attractive and viable option for users of all sizes of biogas plants and might appeal to at least 20% of users, amounting to 25 to 50 plants depending on the uptake of AD in the marketplace. 2.8 Overall conclusions The key conclusions of the phase 1 feasibility study show that it is possible to produce high quality biomass pellets from separated AD fibres. The results show that the process is technically, economically and environmentally feasible. The key challenges facing the technology will be to prove the performance of the equipment and combustion of the pellets over a longer period, in order to develop a track-record. This will ensure that an end of waste specification can be developed for the material making it possible to sell pellets to a much wider market. This will rely on a more involved full scale demonstration project being undertaken. By processing a mixture of materials including; fibrous digestate, liquid digestate, wood waste and green waste AWSBE can potentially offer a complete recycling solution with energy recovery for several types of wet and dry municipal wastes.


2.9 References Carbon Trust, 2006, Energy and Carbon Conversions DEFRA, 2010, Fertiliser Manual (RB209), 8th Edition, ISBN 978 0 11 243286 9 Hayes S., 2009, Pellet Market Country Report UK, The National Energy Foundation, part of the Pellets@las project (www.pelletsatlas.info) KratzeisenM., Starcevic N., Martinov M., Maurer C., Müller J., 2010 Applicability of biogas digestate as solid fuel, Fuel, 89, 2544–2548 Li H., Chen Q., Zhang X., Finney K. N., Sharifi V. N., and Swithenbank J., 2012, Applied Thermal Engineering, 35, 71 – 90 WRAP, 2010, New Markets for Digestate from Anaerobic Digestion, Project code ISS001-001 Zhang R., El-Mashad H. M., Hartman K., Wang F., Liu G., Choate C., Gamble P., 2007, Characterization of food waste as feedstock for anaerobic digestion, Bioresource Technology

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