assessing the feasibility of the thermal valorisation of ... · assessing its benefits. however,...

XXIV Summer School “Francesco Turco” – Industrial Systems Engineering

Assessing the feasibility of the thermal valorisation of spent coffee grounds converted into pellet

Volpi A.*, Bottani E.*, Montanari R.*, Rizzi A.*, Bonanzinga C.*

* Department of Engineering and Architecture, University of Parma Viale delle Scienze, 181/A 43124 – Parma – Italy ([email protected]; [email protected]; [email protected], [email protected],

[email protected])

Abstract: Millions of people worldwide enjoy their coffee drinks during the day and many studies are focused on assessing its benefits. However, the production of coffee generates also big amount of waste, spent coffee grounds (SCG), which have many potentials and thus are being investigated by scientific community. Many studies, in the last years, addressed the potential exploitation of SCG, especially as a source for the extraction of bio-oils and for the production of biodiesel; recent works also pointed out the potential thermal valorisation of SCG for industrial processes. The aim of the present work is to validate a new sustainable exploitation of SCG, transformed in combustible pellets; this source of thermal energy can then be used for heating public or private buildings, or for industrial processes. To this extent, a feasibility study of a pellet production plant is presented, which is fed by SCG collected by vending companies operating in the north of Italy. Moreover, since coffee grounds sources are very dispersed and require a transportation system, a logistics model is presented for gathering SCG from the different vending companies and collecting them into the pellet production facility. The study shows that the main costs related to the production of SCG pellet are transportation and manpower, which account for approximately 50% of the total production cost.

Keywords: spent coffee grounds, biopellet, logistics, feasibility study.

1. Introduction

Many studies, in the last years, addressed the potential exploitation of spent coffee grounds (SCG), especially as a source for the extraction of bio-oils and for the production of biodiesel; recent works also pointed out the potential thermal valorisation of SCG for industrial processes. The reason of such interest in this special waste has to be found in its market volumes: in fact, coffee is the second biggest traded commodity after petroleum and, moreover, Italy has a very diffused and appreciated culture for this drink. ICO (International Coffee Organization) data showed that the Italian coffee consumption in 2015 exceeded the large number of 340,000 tons of roasted beans (ICO, 2017). Therefore, almost the same amount represents the limit for the Italian market in terms of SCG that could potentially be converted into energy, as an alternative to the usage of natural gas. Although it is a good fuel, during the combustion, a substantial quantity of VOCs is generated (Bianchi et al., 2016; Ghermandi et al., 2015).

In order to consider the possible exploitation of SCG as a source of energy, a business plan of a project involving gathering and collection of SCG from vending machines or other sources (logistics), processing and transformation them into combustible pellet (production), and technical consideration for combustion in domestic or industrial stoves (heating) should be created and analyzed. This is the scope of the current work.

2. Literature Review

Today, SCG are treated as waste in Italy and, depending on the local administration, they end up in the undifferentiated garbage or in the organic fraction of municipal waste (Allesina et al., 2015). In the first case, SCG will end up in an incinerator plant, while in the second case the coffee is directly sent to a composting process. On the other hand, from an analysis of the scientific literature different ways to use this waste better can be identified. Numerous studies promote re-use of SCG in a non-energetic way: from geopolymers, to adsorption of pollutants for water treatments, to the aid of mushrooms growing, to ruminant feed (Kua et al., 2016; Franca et al., 2009; Murthy and Naidu, 2012; Givens and Barber, 1986). All those reutilizations are interesting but they don’t consider energy exploitation for mass purposes. SCG could be used also used for agricultural soil, but it can be dangerous for the plant growth (Hardgrove and Livesley, 2016).

On the other hand, bio-mass valorization, combustion and oil production are trendy topics for SCG recovery. Kondamudi et al. (2008) proposed an energy re-use of SCG, through oil extraction and making pellets from the SCG. The oil derived from the SCG and the solid residue after extraction can be used to produce biodiesel and electric energy using the system proposed by Allesina et al. (2014). In addition, the solid residue can be pelletized in order to use it as a more flexible fuel in a downdraft stratified gasifier (Pedrazzi et al., 2012; Allesina et al., 2013).

Literature reports several studies about thermal uses of coffee residues (Oliveira et al., 2015; Ismail et al., 2016; Law et al, 2013; Allesina et al., 2017), although no one analyzes

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the integration of these ideas with a feasibility study. Nonetheless, the energy demand and the disposal of spent coffee, could bring to the community a combined solution: the collection and the reuse of SCG shaped into combustion pellets for heating private and public buildings. A logistics model for collecting them form vending companies, or other sources, and processing them into combustion pellets shall be considered in order to evaluate the technical and economic feasibility of such solution. This transition to a renewable source leads to three main goals: significant savings for the end users, reduction of CO2 emissions in atmosphere and valorization of a valued bio-waste.

The model presented in this paper suggests transforming SCGs, produced by vending machines and collected by operators dedicated to their refilling, into bio-pellets (agri-pellet) to satisfy the energy demand for heating, and computes the resulting costs and the achievable revenues.

3. Material and methods

3.1. Feasibility study procedure

The feasibility study for the SCG pellet production was structured according to the following steps, which will be detailed in the subsection that follows:

1. Logistics model: as a first step, a logistic model was built to estimate the cost in order to collect the required SCG quantities from the different vending machines companies. Relevant data about the amount of SCG produced and of its collection procedure were retrieved thanks to direct contacts with these companies as well as from a questionnaire survey involving approx. 200 of them. Further data useful to model the transport activities were retrieved from Italian Ministry of Infrastructures and Transport;

2. Plant design: details about pellet production plants were retrieved from Smartwood S.r.l. (www.smartwoodsrl.com), a leading Italian company in pellet production plant engineering. The number and location of the plants was defined according to the outcomes of the logistics model;

3. Pellet usage: as far as the pellet destination is concerned, two possible scenarios were considered. In the first one, it is hypothesized to produce pellet composed of 50% SCG and 50% pine sawdust; the relating usage is feeding of domestic stoves. In the second scenario, the alternative pellet composition (98% SCG and 2% corn starch) was considered. This kind of pellet cannot be used for domestic purposes, but could be used for industrial or communitarian purposes. Hence, in this scenario, the feasibility of pellet production was evaluated with reference to a real sport facility, i.e. the sports plant “Giulio Onesti” in Rome (Italy), which was taken as an example of a facility that could benefit from the usage of this pellet for heating.

3.2. Experimental procedure

The third step of the feasibility study was supported by an evaluation of the heating value of the pellet that could be

produced from SCGs. To this end, about 5 kg of SCG were collected from vending machines sited in the University of Parma. Once dried, the SCG is mixed, in different ratios, with pine sawdust in order to obtain pellets with different compositions. The resulting products are shown in Figure 1. The Lower Heating Value (LHV) of pellet increases proportionally with the quantity of SCG; hence. two different pellet compositions were studied:

(i) pellet composed of 50% SCG and 50% pine sawdust. This kind of pellet is suitable for usage in feeding domestic stoves (having a LHV of 18.8 MJ/kg compared to 17.25 MJ/kg of pure pine sawdust pellets). This composition is suitable for combustion in domestic stoves;

(ii) pellet composed of 98% SCG and 2% corn starch as ligand (having a LHV of 20.6 MJ/kg). This composition exploits the recycling potential of SCGs.

Both compositions can be certified in accordance to EN 17225-6 regulation about non-woody pellets; it should be mentioned that local restrictions in some Italian regions allow the usage of class A1 pellet in accordance to EN 17225-2 only.

Figure 1: (a) SCG (50%) and pine sawdust (50%) pellet; (b)

SCG (98%) and corn starch (2%) pellet.

4. Feasibility Analysis

The distribution of vending machines companies over the Italian territory is reported in Table 1.

The proposed scenario is assumed to have four production plants, sited in four cities in the four mostly dense populated regions in North Italy; the selected cities are Milan, Turin, Padua and Bologna. The choice of using four production plants is a compromise solution, which takes into account different factors: (i) the need for reducing the distances during the SCG collection process; (ii) the production capability of an industrial pellet production plant, covering the SCG production of a region, and (iii) economies of scale.

In each abovementioned city, a pellet production plant is considered, whose production performance is summarized in Table 2, according to different work schedule.

SCGs collected by the vending companies are gathered by means of a truck and brought to the closest pellet production plant. In order to identify the vending companies operating with coffee drinks, their SCG production capabilities and their location, the national database has been checked and a questionnaire survey sent by e-mail to them. The main question was related to the capability of the company to collect SCG from each vending machine and bring them to the local headquarter.

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col raffreddamento. Così facendo si è arginata la sgretolabilità del prodotto e si è

ottenuto un pellet utilizzabile.

Fig.8 Pellet con caffè e ammendanti a circa il 2%

iii) Il terzo tentativo è stato motivato dall’idea di “arricchire” il pellet aggiungendogli

una componente legnosa, al fine di migliorarne le caratteristiche da un lato ancora

una volta di coesione, dal momento che la lignina come si è ripetuto fa da legante

nel pellet classico, dall’altro anche quelle inerenti alla combustione ma non dal

punto di vista del potere calorifico che è certamente confrontabile, quanto più da

quello dei residui di combustione, odori ed altri fattori. Tutto ciò con l’idea di

impiegarlo anche in stufe domestiche in cui anche l’aspetto della pulizia è

importante. Non è stata però abbandonato l’utilizzo di amido di mais, comunque

inserito in questo pellet.

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Analyzing the feedback from the companies (about 30 answers over more than 200 sent questionnaire) it is possible to estimate that about 71% of them is performing collection.

Table 1: vending companies’ distribution in Italy.

Table 2: characteristics of the pellet production plants

In order to produce a valid and realistic logistics model, different means of transport have been considered, having different full load truck (FLT) capabilities summarized in Table 3.

The typical journey considered consists of 64% of the path on high-speed roads (route to reach the various cities, i.e. highways), while the remaining 36% on low speed roads (secondary or urban routes to reach, in the specific city, the various companies). The toll cost has been taken into account for the quota of the trip covered on high-speed roads.

Knowing the speed, the working time per day, the full load mass of each vehicle it is possible to estimate the maximum number of stops allowed per day to collect SCG from the different companies. The assumptions for the development of the logistic model are summarized in Table 5.

Table 3: characteristics of the transport means.

Table 4: characteristics of the journeys.

Table 5: assumptions for the logistics model.

The withdrawal time of 20 minutes was justified as the time needed to collect the SCG, stored in big bag, from each visited vending company. The total travel time T, according to the kind of vehicle and the number of stops, can be easily calculated as:

! = #$%&'

+ !)*%+ ∗ -.

where d = total traveled distance [km], $%&'= average speed for the specific vehicle [km/h], !)*%+= loading time for each site (20 [min]), -.= number of stops (visited companies, as per Table 3).

As an example, Table 6 describes the different options to cover the route between Milan and Brescia, where 20 vending companies should be visited. It is estimated that each company produces about 3.5 tons of coffee grounds per month, but it collects an average of 63%, amounting to 2.2 tons; each company is visited once per month.

902No. % in Italy

Emilia-Romagna 88 9.8%Friuli-Venezia Giulia 24 2.7%Liguria 21 2.3%Lombardia 190 21.1%Piemonte 88 9.8%Trentino-Alto Adige 10 1.1%Valle D'Aosta 3 0.3%Veneto 92 10.2%North total 516 57.2%Abruzzo 16 1.8%Lazio 82 9.1%Marche 22 2.4%Molise 0 0.0%Sardegna 10 1.1%Toscana 70 7.8%Umbria 18 2.0%Centre total 218 24.2%Basilicata 6 0.7%Calabria 23 2.5%Campania 35 3.9%Puglia 48 5.3%Sicilia 67 7.4%South total 179 19.8%

RegionVending companies in Italy

NO

RT

HC

EN

TR

ESO

UT

H

Productivity 1 shift 2 shifts 3 shiftsTotal working time [h] 8 16 24 Pellet productivity [kg/h] 500 500 500 Pellet productivity [ton/day] 4 8 12 Pellet productivity [ton/month] 88 176 264 Pellet productivity [ton/year] 940 1,880 2,820

Category N2 N2 N3Type Van Truck Truck

Reference exampleFCA Ducato

4035 XL

Iveco Eurocargo

110

Iveco Eurocargo

180

Gross vehicle weight range [ton]

3.5 - 7.5 7.5 - 11.5 11.5 - 26

Gross vehicle weight [kg]

4,250 11,000 18,000

Maximum payload [ton]

2.5 6.7 12.5

Companies visited for a FLT

1 3 5

Highways (64%) average speed

[km/h]

Urban and suburban (36%) average speed

[km/h]Weighted average

speed [km/h]N2 Van 100 65 88N2 Truck 100 65 88N3 Truck 80 60 73

Daily working time [h/day] 8Working days [days/year] 235Working days [days/month] 22Time to load [min/site] 20Time to load [h/site] 0.33SCG per site [ton/month] 2.1

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Table 6: example of route (Milan-Brescia).

Referring to the previous Table 6, a N2 van can visit 1 company, load the monthly amount of SCG (2.2 tons) which saturates the vehicle, and go back to the production plant, requiring 2.4 h for completing the task. This simple activity can be performed 1, 2, or 3 times per day, obviously visiting different companies and with respect of 8 working hours. A N2 Truck can visit 1 company, load the monthly amount of SCG (2.2 tons) which does not saturate the vehicle, and thus visit a second and a third company (within 50 km distance from the first one, being both sited in the same city, e.g. Brescia), the latter saturating the vehicle. Then the truck will go back to the production plant (in Milan), requiring 3.6 h for completing the task. This simple process can be performed 1 or 2 times per day, obviously visiting different companies and with respect of 8 working hours. A N3 Truck can visit 1 company, load the monthly amount of SCG (2.2 tons) which does not saturate the vehicle, and thus visit a second, a third, a fourth and a fifth company (within 50 km distance from the first one in Brescia), the latter saturating the vehicle. Then the truck will go back to the production plant, requiring 5.3 h for completing the task.

Data obtained for all possible routes of the Lombardy region – as well as for other regions – were elaborated to determine the time and the routes to visit all the companies, for each city and for each vehicle type.

For all the considered production plants (Milan, Turin, Bologna and Padua), a single N2 or N3 truck vehicle is sufficient to ensure the operation of the plant on three work shifts with a reasonable saturation of the transportation system. For this reason, only the purchase option has been considered, whose economic data is shown in Table 7. The table reports the operational costs for a third party logistics (3PL) in accordance to data provided by the Italian Ministry of Infrastructures and Transport, updated to May 2019.

Table 7: cost for 3PL (May 2019).

The overall transportation costs for the four different production plants are summarized in Table 8; it can be

noticed that for all scenarios a single N2 or N3 Truck is suitable, although the latter shows lower operational costs.

Table 8: overall transportation costs.

Concerning Milan pellet production plant, work schedule is hypothesized as it follows: a single operator controls and manages the whole operations of the plant, including dryer and bagging station, being the plant highly automated. Another person, an accountant, is a shared resource among the four plants and deals with administrative and commercial tasks.

Table 9: economic data – manpower cost.

The hourly cost of the employees can change as a function of the number of shifts, as night work costs much more compared to the daytime work. Hence, when considering three shifts, the average employee cost changes as well.

Regarding the resale price of the 15 kg pellet bag, the price of a bag sold in a retail store is assumed as benchmark, and a retail model is assumed.

Table 10: economic data – retail sale prices.

The cost of the facilities needed to host each production plant is estimated assuming a unitary cost of 55 € per square meter per year, derived from ISTAT data on industrial facility rental in the Province of Milan; this is one of the most expensive cities of Italy, therefore the total cost (including the remaining three facilities) is probably overestimated, as a precaution. The layout and size of the plant, and thus the hosting building, have been provided by Smartwood S.r.l.; a scheme is proposed in Figure 2.

The production system is quite compact, covering a total area of about 100 square meters; this area must be increased because of storage areas (for SCG and pine sawdust) and sales areas, thus assuming a total amount of 300 square meters. Related costs are summarized in Table 11.

20

N2 Van N2 Truck N3 Truck

visi

ted

com

pani

es

dist

ance

[km

]

time

[h]

visi

ted

com

pani

es

dist

ance

[km

]

time

[h]

visi

ted

com

pani

es

dist

ance

[km

]

time

[h]

1 184 2.4 1 184 2.4 1 184 2.9 2 368 4.9 2 204 3.0 2 204 3.5 3 552 7.3 3 224 3.6 3 224 4.1 4 736 9.7 4 408 6.0 4 244 4.7 5 920 12.2 5 428 6.5 5 264 5.3 6 1,104 14.6 6 448 7.1 6 448 8.1 7 1,288 17.0 7 632 9.5 7 468 8.7

Milan - Brescia (20 companies)

[101-150 km] [151-250 km] [251-350 km] [351-500 km] [>500 km]

N2 Van 1.238 1.025 0.943 0.811 0.733N2 Truck 1.367 1.138 1.032 0.891 0.818N3 Truck 1.574 1.307 1.197 1.050 0.973

Minimum transportation costs for 3PLs [€/km]

SiteN2 Truck [€/month]

N3 Truck [€/month] Delta

Milan 6,706.40€ 6,462.28€ 3.8%Turin 2,757.27€ 2,668.02€ 3.3%Padua 3,401.15€ 2,858.54€ 19.0%Bologna 3,495.00€ 3,284.87€ 6.4%

1 shift 2 shifts 3 shiftsCost per hour per operator [Euro/h] 15 17 21 Daily cost of the operator [Euro/day] 120 136 168 Monthly cost of the operator [Euro/month] 2,640 2,992 3,696 Annual cost of the operator [Euro/year] 28,200 31,960 39,480 Operators per facility 1 2 3 Total cost for four facilities [Euro/year] 112,800 255,680 473,760

Manpower cost

Retail price of the pellet bag [Euro/bag] 3.50Retail price of the pellet bag [Euro/kg] 0.23Bag content [kg] 15SCG content [%] 50%

Retail market

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Figure 2: (a) plant 3D rendering and (b) layout

Table 11: economic data – hall rental.

Table 12: economic data – energy consumption.

Table 12 reports the electrical power consumption of the equipment for the pellet production plant; the energy bill is then evaluated assuming such machines working at full power 24 hours per day and 235 days per year, with an energy cost of 0.17 €/kWh. The most heat-consuming machine, the dryer for SCG, burns part of the produced pellets; in particular its energy requirement is equal to 10% of the pellet production.

The overall economic data is described in Table 13. From this table it is immediate to notice that the efficiency of the logistics system is an important aspect, as the transport activities affect the total cost of pellet production by 19%. The purchase of pine sawdust is a further relevant cost component (accounting for 25% of the total cost).

Manpower cost affects the total cost to the greatest extent (31%).

Table 13: economic data – Milan plant costs.

The monthly revenue from sales is about 23,000 € for this plant; each cost item was rounded up in order to avoid too optimistic results. It is possible to point out how each production plant can be very profitable; on the other side it is important to stress that the high sales incomes are due to the retail model adopted, which could be better investigated in terms of its acceptance by the customers and real feasibility.

To complement the analysis, the net present value (NPV) of the investment over 10 years was also evaluated. To this end:

• an average increase in transportation cost of +0.5% per year was assumed, taking into account a possible increase in highways tolls and fuel;

• similarly, the facility rental was increased by 0.5% every 4 years, taking into account the typical duration of leases in Italy;

• the manpower cost was updated yearly applying the salary adjustment suggested by ISTAT (0.3%);

• the amount of pellet produced from SCG was always assumed to be completely sold; however, the average selling price was varied from 0.10 €/kg (wholesale price – pessimistic scenario) to 0.23 €/kg (retail price – optimistic scenario), considering different mix of sold products;

• an interest rate of 0.96% was assumed.

Results of the NPV evaluation are proposed in Table 14. From this table it is easy to see that the investment is profitable over the time span considered, despite the presence of some years in which the annual result is not positive (mainly because of the low selling price).

Hall Each facility Four facilitiesSurface area [square meters] 300 -Monthly rental cost [Euro/month] 1,375 5,500 Annual rental cost [Euro/year] 16,500 66,000

Machineelectrical power [kW]

Pellet machine 45.00Dryer pellet poweredDoser 0.25Feeder 0.80Extruder 5.20Grinder 18.50Conveyors 3.00Bags filler 1.55Cooler 2.95Installed power 77.25

[€/month] [€/year] QuotaCosts 35,393€ 424,715€

Transportation 6,706€ 80,477€ 18.9%Facility (hall) 1,375€ 16,500€ 3.9%

Labour (manpower) 11,088€ 133,056€ 31.3%Energy (electricity) 6,172€ 74,067€ 17.4%Raw materials (pine

sawdust and SCG)8,864€ 106,369€ 25.0%

Amortisation 1,187€ 14,246€ 3.4%Sales 58,250€ 698,998€

Profit/loss 22,857€ 274,283€

Milan production plant

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Table 14: NPV of the investment.

In an attempt to increase the quota of SCG recovered, an alternative pellet composition (i.e. 98% SCG and 2% corn starch, as mentioned previously) has been considered and analyzed. With this composition, production costs would be lower since the pine sawdust purchase is no longer required; on the contrary, logistics costs would be a bit higher because of the greater amount of SCG in the pellet. Moreover, the resulting pellet could not be used in domestic stoves (below 300 m above sea level) because of the missing class A1 certifications; nevertheless, it can be used in industrial heaters with dedicated smoke treatment. The higher LHV makes this choice particularly interesting for industrial or communitarian purposes. To evaluate the effectiveness of this usage of pellets, the case of the sports plant “Giulio Onesti” in Rome is taken as an example. This facility is currently powered by compressed natural gas (CNG), but could potentially be heated by pelletized SCG. The relevant data for the evaluation is reported in the Table 15, which also details the amount of SCG pellets required to meet the same requirements guaranteed by CNG combustion.

Table 15: current and simulated scenario for the “Giulio Onesti” sport facility (Rome).

A SCG pellet quantity of about 140 tons/month is required to satisfy the thermal requirements of the abovementioned sport center; this amount is lower than the pellet production capacity of any considered plant, meaning that feeding this facility with the pellet would never be a problem.

5. Conclusions

This study has provided a preliminary evaluation of the production of pellets from SCGs and its potential usage for domestic or industrial purposes. The evaluation has dealt, in particular, with the determination of the logistics cost required to collect the SCGs from vending machines

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CNG scenarioMethane annual consumption [m3/year] 799,669 Methane cost [Euro/m3] 0.31€ Annual methane cost [Euro/year] 247,897€ Heater specific power [kWh/m3] 9.94 Gross thermal energy [kWh/year] 7,948,710 Efficiency 0.942 Net thermal power [kWh/year] 7,487,685

98% SCG pellet scenarioPellet LHV [MJ/kg] 17Pellet LHV [kWh/kg] 4.72SCG collected in Lombardy [ton/month] 281Pellet quantity per month [ton/month] 287Theorical thermal energy [kWh/month] 1,355,952 Theorical thermal energy [kWh/year] 16,271,429 Efficiency 0.942 Net thermal energy [kWh/year] 15,327,686

Quantity of SCG required [ton/year] 1,683 Quantity of SCG required [ton/month] 140

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companies and with the design of the pellet production plant. The former aspect has been somehow neglected in literature: most of the available studies have focused on the thermal use of SCGs and coffee residues in general, without, however, dealing with the logistics activities required to collect the SCGs. The analysis carried out in this paper also shows that the logistics cost for collecting SCGs cannot be neglected, as its impact on the total cost of pellet production is quite relevant (19%); this highlights the relevance of the results proposed in this paper for companies interested in collecting and reusing SCGs. Further interesting results of this study concern the possible usage of SCGs. More precisely, two possible usages of pellets from SCGs have been considered, i.e. the domestic one and the industrial/communitarian one. Both destinations were evaluated with respect to real scenarios; therefore, the outcomes obtained are realistic and are expected to be useful to make practical decisions. Again from a practical perspective, it should be mentioned that the total cost for collecting SCG and producing pellets from it is quite high and should be balanced by the revenue from sales. This aspect needs to be investigated in greater detail: in particular, the customer willingness to purchase SCG-based pellet should be evaluated. This point is left for future studies. Moreover, the economic feasibility of the SCG-based pellet production, which was the focus of this paper, is obviously only a part of the overall sustainability evaluation of the process. The environmental aspects would also need to be taken into account in a dedicated evaluation. This aspect will be deepened in future research activities.

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