waste to energy: the new frontier?

Waste to energy: The new frontier?

by G. W. Rae Each year the South East ofEngland, including London, generates some 27.5 million tonnes o f municipal, industrial and construction wastes. With a fiw exceptions this waste is disposed ofat landjills, often some distancefrom the origin ofthe waste. However these landjlls are nearing completion and, because ofplanning and environmental constraints, are not being replaced at a rate suicient toguarantee demand is met. Indeed it is now recognised that by the turn ofthe century a substantial sho$all in landjll capacity will

exist. It is against this backcloth that several companies have sought to develop waste-to- energy plants. This papev, using a proposed plant at Belvedere as an example, looks at the waste-to-eneugy solution in the context ofthe evblving disposal crisis. It reviews the role it can play and considers whether this well proven technology truly represents a newjontier

for waste management in the UK.

Introduction

he South East remains the most populous region in the country. Though growth has slowed during the recession, the pick-up in T the economy, and initiatives such as the East

Thames Corridor, are set to see the South East resume its role as a major centre of economic growth. However, as an inevitable by-product of economic growth, waste arisings are also set to grow, and ths d pose a sigtuficant problem for the counties that make up the South East, and particularly London.

With the exception ofHampshxe, and the waste-to- energy plants at Edmonton and Lewisham, nearly all the wastes arising in the South East go dIrectly to landfill. Yet landfill is a diminishing resource as existing sites, whch were ofien of regional significance, near completion and new sites fail to come forward to replace them. Planning and environmental policies in the South East, notably Green Belt and aquifer protection policies, as well as increasing urbanisation, are serving to restrict severely sites that might be considered suitable for landfill. Indeed the London and South East Regional Planning Conference (SERPLAN)’ estimated in 1991 that landfill in the region was likely to be exhausted around 1998. As legislation, notably h m Europe, serves to make landfill less attractive to waste management companies, this

estimate could well prove optimistic. Iflandfill is unable to meet the region’s waste d~sposal

needs then alternative waste disposal facilities are required. The most attractive alternative to landfill, given the large volumes of waste to be dealt with, is incineration and particularly incineration with energy recovery. One major waste management company, Cory Environmental, with substantial interests in the South East, has evaluated a range of technologies that might provide a long-term solution and has determined that waste-to-energy plans offer the best solution. Accordingly the company is developing a waste-to-energy plant at Belvedere, in the London Borough ofBexley, to deal with 1-2 million tonnes per annum of London’s municipal waste. Ths represents about 30% of the capital‘s municipal waste arising and as such the Belvedere plant will go a long way to providmg security of waste d~sposal. Although permission to construct and operate the plant is still awaited from the Department of Trade and Industry, all other permissions are in place. It is anticipated that the plant will be operational towards the end of 1998.

T h s paper, by reference to the Belvedere waste-to- energy plant, examines the technology behind such plants and their acceptability within the urban environment. The role of such plants within the Government’s strategy for waste management is assessed.

ENGINEERING SCIENCE AND EDUCATION JOURNAL JUNE 1994

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Table 1: Qualitative assessment of available void space in the South East Region, 1990/1-200011

Waste disposals by landfill 1990/91* Total Net Approx.

Type A Type B&C Total waste void space void space landfill waste, waste, deposited, 19w1, 2Mxu1, sites million m3 million m3 million m3 million m3 million m3

available available life of

TOTALS 8,008 19,694 27.702 221.6 -55 42 1997B

*Source SERPLAN. RPC 2090 Table 14 Waste disposal in South East England Results of the 1991 waste monitoring survey, March 1992)

Waste arisings and disposal in the South East

The most up-to-date assesment of waste arisings and dlsposals in the South East is given in SERPLAN’s Waste Monitoring Survey for 1991’. (This survey has recently been repeated, though the results are not yet available.) The results ofthis 1991 survey are set out in Table 1. This table is a simple exercise which seeks to calculate the life of landfill sites by county if existing volumes of waste dlsposal in each county continue unchanged throughout the 1990s-a possibly conserva- tive estimate. The analysis concentrates on currently operational or approved and licensed sites. O n ths basis, the position is of great concern. By the year 2001 all counties in the South East, with the exception of Buckmghamshire and Surrey would have run out of landfill sites. As the Table shows, most authorities would, in fact, exhaust their existing approved sites by 1995.

This analysis can be taken one stage further by attempting to assess the cumulative impact of the exhaustion of landfills in the dlfferent counties. As counties exhausted their landfill capacity they would need to export their waste to other counties. As Table 2 shows, the effect would be that all existing approved capacity in the South East would be exhausted by 1998. Indeed, by that time, there would be a deficit in the South East ofmore than 5 million cubic metres. This is a conclusion shared by SERPLAN’.

Also since the 1991 Survey, the National Rivers Authority’ (NRA) have published their Aquifer Protection Policy This policy seeks to protect and conserve groundwater resources. To implement this policy the N U has recognised three groundwater

Source Protection Zones: Zone I (Inner Source Protection) is the zone located immedlately adjacent to the groundwater source. Its area is defined by a 50 day travel time fiom any point below the water table to the source and, as a minimum, of 50 metres radlus from the source. Zone I1 (Outer Source Protection) is defined genedy by d 400 day travel h i e &om any point below the water table to the source. It is accordmgly considerably larger than Zone I .

0 Zone 111 (Source Catchment) covers the complete catchment area of the groundwater source.

In terms of new landfill sites the NFU wd normally object to all sites within Zone I . It will equally normally object to landfill sites within Zone I1 unless it is satisfied that the m t e material to be landfilled is inert and that the site wd have acceptable operational safeguards. They will not normally object to landfillmg in Zone 111 ifit is satisfied that the risk of pollution of groundwater can be mitigated by engineering measures and operational management controls.

In addition the EC is moving towards finahsing a directive on the landfill of was&. T h s directive recognises that landfilling is the final resort in waste management and is aimed at harmonising the environmental and technical standards for the landfill of waste in all EC countries to ensure a high level of protection for the environment and particularly soil and water resources.

Clearly by the turn of the century the South East, and particularly London, will be facing a crisis in terms ofproviding suitable sites for landfill. New sites wd not be forthcoming, and alternative disposal routes will have to be found.


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Waste to energy It is against this background that the development of a large, state-of-the-art, waste-to-energy station at Belvedere in the London Borough of Bedey is being pursued. The station consists of four streanis each of a design capacity of 38.5 tonnes per hour which together will handle 1.2 d i o n tonnes of municipal waste per annum. This represents about 30% of London’s municipal waste arising. The station will produce 103 MW of electricity for export to the National Distribution Network. The full output was included in the 1992 tranche of the Non Fossil Fuel Obligation; this is the Government’s fiscal instrument aimed at encouraging the development of renewable energy

Ths will avoid vehicles having to run, as they do now, to landfills in Essex.

Ash and other residuals will be removed in sealed 20 foot I S 0 containers and transported by river for disposal at a landfill site in Essex.

Plant operation

A schematic dngram ofthe plant is shown in Fig. 1. All waste is drawn from the storage bunker (11) by one of two grab cranes (12) and fed into one of four feed hoppers serving the four lines of the plant. The wastes are pushed by hydraulic ram at the appropriate rate onto a grate (15) where the wastes are burned under

sources, includmg waste-to- energy conversion.

Delivery systems deficit of landfill void required by each space, million swce exhausted countv. m3 w r annum

Three types of waste delivery are proposed. First, waste in 20 foot (6 rn) I S 0 compaction containers (average 12.5 tonnes of waste per container) will be delivered by barge via a new jetty. As much waste as possible, probably 850 thousmd tonnes per annum m d possibly as much as 1 nullion tonnes per annum will be delivered by this system. The development of the Bel- vedere plant d ensure the continuance of the river .IS a nieans of transporting wastes arid thus maxiniise the investment local government has made in the five riverside transfer stations that serve central and east London. Without the station, the river system \d cease operation early next century when the two riverside landfills are complete.

Secondly, between 200 and 400 thousand tonnes per annum of waste will be delivered by road, either in sealed containers or in covered bulk vehicles. These vehicles will makr use of existing road-based transfer stations.

Finally provision has been made for refuse collection vehicles operating in the London Borough of Bedey to deliver directly to the site.

Table 2 Cumulative impact of regional landfill exhaustion

Year County experiencing Amount of void space Total required void


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10 keption building 11 Bunker 12 Crane 13 Crane conhol cabin 14 Feedhopper 15 Combustion grate 16 Slagconveyor 17 Slag handling

18 Secondary combustion chamber 19 Heat recovery boiler 20 Precipitator for recirculaqed

combustion gas 29 Control room 21 sprayrlryerlabsortKr 30 Magneticbett 22 Baghocaefilter 31 Ashremoval 23 IDfan 24 Chimneys

25 Addiiiinjection 26 Turbine generator 27 Turbine condenser

52 Ash handling unit

Fig. 1

controlled conditions to release the maximum heat ventlonal mrblnes to produce electricity The turbines value Sohd residues from the burnmg process, known are cooled by water from the Rwer Thames, as chnker, fall mto a sealed water bath (16) where they augmented in the summer by auxhary cooling art cooled before being conveyed, ma a ferrous metal

Waste is not an ideal fuel, being very heterogeneous and with a calorific value about one third that of The plant has been registered under Integrated coal A typical analysis of London's mumcipal refuse 1s Pollutlon Control wth HM Inspectorate of Pollution. given in Table 3 Accordmgly the design of the grate The emssion limits set by the Inspectorate are given in is crihcd For the Belvedere station, a forward Table 4. As can be seen very high standards of emssion reciprocahng design budt by Von Roll of Switzerland control, much tighter than required from conventional has been selected Von Roll is one of the foremost coal powered stations, are applied To achieve them a designers of waste-to-energy plants and this grate sophisticated g a 5 cleaning plant is employed

The heterogeneous over 200 applications (average lower calorific heating value = 10260 kllkg) nature Of mumclPal world-wde. means that the raw gases

The hot gases pass, wa 'yContent ?h By component given off during com- a secondary combustion bustion are heavlly con- chamber (18), to a boiler tmnated with dust and (19) This hghly efficient pollutants The expected boller converts the energy raw gas composihon for of the hot gases into steam the Belvedere plant is given through a series of heat in Table 5 exchangers and super- To achieve the high heaters. The steam is lead standards of emssion off from the boder to a control required by turbine house (26) where it the Inspectorate, a three IF used to power con- stage gas cleaning/control

Schematic diagram of proposed waste-to-energy plant at Belvedere, Bexley

recovery plant (30), to the ash boxes Gas cleaning

has been proven at Table 3: 'Average' composition of London's waste


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system will be employed. Some 17% of the exhaust gases are recirculated

through an electrostatic precipitator and reinjected as secondary air into the furnace. (The electrostatic precipitator is used to dedust the gas down to 100 mg/ Nm3 [the N indlcates a concentration in dry air at a temperature of 273 K, at a pressure of 101.3 kPa and with an oxygen content of 11% dry] set in order to protect the combustion air fans agamst corrosion.) The aim of t h s reinjection is to control the emission of oxides of nitrogen.

Oxides of nitrogen (NOx) can be formed either by the oxygenation ofnitrogen in the waste (known as fuel NO,) or by high-temperature fixation of nitrogen in the combustion air. The formation of fuel NO, is determined by the nitmgen content of the waste, total excess air rates and the relative distribution of primary and secondary combustion air. The formation of thermal NO,, on the other hand, depends on oxygen availabdity and the temperature, pressure and residence time of the gas in the combustion unit. The relative percentages of fuel and thermal NO, are determined by the characteristics of the waste and by the design of the combustion unit. At the Belvedere station up to 80% of the NO, in the raw gases may be fuel NOx.

The recirculation of the flue gases d allow combustion to take place at lower peak temperatures and in condltions of reduced oxygen availability The former reduces the amount ofthermal NO, whereas the latter limits the amount of oxygen available to react with the nitrogen. The amount of recirculation is limited to around 17% by flame instability and the decrease in thermal net output of the boiler.

In addition, to ensure consistent low NO, emissions, ammonia will be injected into the boiler. The net effect of t h s treatment system will be a reduction of NO, of approximately 20% and an improvement in boiler e5ciency of between 2 to 3%. This will result in the gases being exhausted to atmosphere always having an NO, content below 200 mg/Nm3.

The remaining gases pass through a spray drier/ absorber (21) where water, mixed with lime slurry, is injected into the gas stream. This technology is particularly effective at removing acidic gases and particulates. The main acidic gases arising from the burning of municipal wastes are hydrochloric acid, hydrofluoric acid and sulphuric acid.

The flue gases leave the boiler at approximately 230°C and enter the spray driedabsorber through a top-mounted hot-air inlet box. This will be equipped with profiled baffle plates and adjustable vane rings whch will create the condltions necessary for the intimate mixing of the flue gases with the lime slurry The HCl and SO2 concentrations of the clean gas will be used to control the rate of hydrate-of-lime suspension added when the pollutant concentration in the flue gas is below the base load. For normal to maximum loads the hydrate-of-lime suspension d be added at a constant rate.

Table 4 Emission limits set by HM Inspectorate of Pollution

Relea% Concentration Annual Release limit, m g h 3 limit (tonnes)

The followng reactions occur

Gaseous pollutant Sorbent Reaction products

2HC1 + Ca(0H)z + CaCl2 + 2H20 2HF +Ca(OH)z + CaFz+2H20 so3 +Ca(OH)2 -* Cas04 + H20 SO2 + Ca(0H)z + %Oz -+ Cas04 + H2O

The following emssion limts are guaranteed by the process equipment supplier (Lurg AG of Frankfurt)

sulphur dlomde 70 mg/Nm3 hydrogen chloride 30 mg/Nm’ hydrogen fluoride 1 mg/Nm3

In addlaon, activated carbon wdl be injected into the flue gases at this stage to ensure the almost complete removal of dlomdes and furans, as well as heavy metals such as mercury and cadmum Table 5: Raw gas prior to cleaning

Component YO (vol) wet

CO2 1 0 8 H a 0 16.2 Na 67.0 0 2 6-0

100% Pollutants in gas prior to treatment, rnglNm3*

Dust 2500 HCI 1wO HF 10 5 0 2 3M) Hg 0-8 NO* 350 CO 50 PCDD, K D F 5.0

* Nm3 refers to cubic metres under standard conditions: 273 K, 101.3 kPa, 11% 02,drygas


1 ns

Fig. 2 Artist's impression of Coy Environmental's proposed waste-to-eneqy plant at Belvedere

Dioxins have been shown to c h g to particulate matter, and studies by Lurgi have shown activated carbons to be particularly efficient at scavenging dioxins and brans h m the gas stream. As a side effect, the injection of activated carbon is also helpful in controlling the emissions of mercury and cadmmm. Mercury and cadmium are relatively volatile compared with other heavy metals and hence have a tendency to pass through conventional gas-cleaning equipment in the vapour phase.

The final component of the gas-cleaning train is the bag house filter (22), aimed primarily at removing particulates h m the gas stream. Bag houses are capable of maintaining mass collection efficiencies of greater than 99% generally, and greater than 95% for particle sizes of 0.2 pn or less in most applications. These efficiencies are largely insensitive to the physical characteristics of the gas and dust, but depend on the fabric cleaning method, the inlet dust loading, the temperature of the flue gases and the condensation point of the metals.

The fabric filter has been designed to deal with an inlet dust burden of some 5800 mg/Nm3 (dry) at a temperature of up to 220OC. The fabric material, whch WIII be PTFE, Ryton or glass fibre, will be selected with reference to its resistance to chemicals and moisture. Removal efficiency is very dependent on the temperature of the flue gas as t h relates to the condensation point for metals. The condensation point for most metals (such as compounds oflead, cadmium, chromium and zinc) is above 300°C. Hence at the 22OOC temperature at which the filter operates, metals will be in a particulate form and hence relatively easy to collect. Simdarly, heavy metal compounds, particularly chlorides, have condensation points below 300°C and thus d be in particulate form and hence again relatively easy to collect.

Fabric filters can remove a wide range of particle sizes, down to submicron sizes, and will continue to work in the event of a power fdure as they are passive devices. The process plant suppliers are prepared to guarantee a total particulate emission level of no more than 10 mg/Nm3 for the Belvedere station.

Residues

The residues from the electrostatic precipitator are conveyed to the ash-handling system (32), where they are mixed with the clinker from the furnace. The residues from the gas-cleaning system are first pressed into briquettes to minimise dust and to ease handhng. These are then conveyed to the main ash-handling system where they are mixed with the other residues. Provision has been made to deal with the residues from the gas-cleaning system separately should this prove necessary in the future.

The new frontier?

As has been noted above, waste-to-energy has been practised widely on the continent for a number of years. Indeed the fir** Von Roll plant was commissioned at Bern in 1954, with plants in Brussels (1957), Hamburg (1959) and Helsinki (1961). to name but a few, following shortly thereafter. By 1993 Von Roll alone had installed 486 incineration trains in 231 plants world-wide. The technology is well understood and widely applied. As such can it be reasonable in the UK context to refer to it as 'the new frontier?'

In a s i d a r vein the Government" in its white paper, 'This Common Inheritance', set out a four-fold strategy for waste management. This was to: 0 encourage the minimisation of waste


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0 promote the recycling of as much waste as possible- including the recovery of materials and energy

0 tighten controls over waste d~~posal standards and 0 take tough action to curb litter.

Though figuring hghly, waste-to-energy conversion is only one aspect of the waste recycling strategy. Again it may be thought as somewhat overgenerous to single it out as representing a ‘new bntier’.

The Royal Commission on Environmental Pollution5 has also published its study on the incineration of waste. The Commission’s general approach to waste management can be presented as a four-stage decision procedure:

0 1: wherever possible avoid creating wastes 0 2: where wastes are unavoidable recycle them if

possible 0 3: where wastes cannot be recycled in the form of

materials, recover energy b m them 0 4: when the foregoing options have been exhausted,

utilise the best practicable environmental option to dispose of the wastes.

However they also note (page 87):

‘Wholehearted adoption of this approach, through careful control of waste creation at the point of production and vigorous promotion ofthe recycling of unavoidable wastes, wiu eliminate some wastes, but not all. We conclude that there will std l remain, for the foreseeable future, substantial quantities of wastes for disposal.’

and conclude (page 88):

‘Incineration, followed by lan-g of solid residues, d in our view prove to be the best practicable environmental option for municipal waste.’

Waste minimisation and waste recycling represent very long-term strateges. Considerable changes in attitudes to packaging, reuse of materials and materials selection will have to occur. Adhtionally considerable changes in the economics of waste recycling wdl have to occur to make the recycled product competitive against virgin products. The considerable ditticulties faced even in the glass recychg industry demonstrate graphcally just how precarious the markets for recycled materials are.

Nor is it clear if materials recycling per se has a net environmental benefit. Recently, concern has been voiced regadng the impact on the environment that certain materials recycling activities have. For example, some forms of glass recychg are not as effective at saving energy when compared to washing and re-using appropriately designed, returnable and refillable bottles. Similarly it has been shown that it is more beneficial to the environment to use wood-pulp h m managed

forests than to recycle waste paper. Materials recycling should be done to save resources and should be carried out in an efficient manner so as not to waste money or energy.

It is against this backcloth of uncertainty of how quickly, and to what extent, recycling can play a major role in waste management, and the crisis facing waste management in the South East that waste-to-energy conversion does represent the new hn t i e r in waste management. It is a tried and trusted technology capable of deahng with substantial quantities of waste. Its environmental impacts are clearly understood and judged acceptable. Equally the economics of waste- to-energy conversion are clear. The technology is aordable and does allow for long-term waste disposal contracts to be entered into. In that way it provides a proven long-term solution to the ever increasing problem of disposing of wastes.

Conclusions

The South East is facing a crisis as its traditional method of waste disposal, landfilling, is increasingly stretched to meet demand. Sites are being completed at a rate faster than new sites are being brought on line thanks to a combination of planning policies, tougher environmental legislation and an unwillingness by companies to take on the long-term financial commitments inherent in l a n m n g .

A new form of waste hsposal is required whch can reliably deal with large volumes of waste over a prolonged period. When the available technologies are assessed, mass burn waste-to-energy offers the only secure route out of the crisis. Accordingly plans are in motion to build a waste-to-energy station at Belvedere (Fig. 2) capable of handhng around 30% of London’s waste annually whilst generating some 103 MW of electricity for export to the grid. The plant will set new standards for emission contml, showing clearly the acceptability ofsuch technology even within the urban environment.

References

1 The London and South East Planning Conference: ‘Waste disposal in South East England Results of the 1991 waste monitoring survey’ (RPC 2090, London, 1992)

2 National Rivers Authority: ‘Policy and practice for the protection of groundwater’ (NRA, Bristol, 1992)

3 Commission of the European Communities: ‘Amended proposal for a Council Directive on the landm of waste’. Com (3) 275 final syn 335, Brussels, 10 June 1993

4 HM Government: ‘This common inheritance’. CM1200 (HMSO, London, 1990)

5 Royal Commission on Environmental Pollution: ‘Incinera- tion ofwaste’. CM2181 (HMSO, London, 1993)

0 IEE: 1994

Dr. George Rae is Director of Technical Serivces for Cory Environmental Ltd., 25 Wellington Street, London WC2E 7DA, UK.


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waste to energy: the new frontier?

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