Assessment of waste derived gases as a renewable energy source – Part 2

Assessment of waste derived gases as a renewable energy source – Part 2

Sustainable Energy Technologies and Assessments 10 (2015) 114–124 Contents lists available at ScienceDirect Sustainable Energy Technologies and Asse...

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Sustainable Energy Technologies and Assessments 10 (2015) 114–124

Contents lists available at ScienceDirect

Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta

Original Research Article

Assessment of waste derived gases as a renewable energy source – Part 2 Pete Watkins a, Peter McKendry b,⇑ a b

SLR Consulting Ltd, Treenwood House, Rowden Lane, Bradford on Avon, Wilts BA15 2AU, UK SLR Consulting Ltd, Langford Lodge, 109 Pembroke Rd, Bristol BS83EU, UK

a r t i c l e

i n f o

Article history: Received 10 July 2014 Accepted 5 March 2015

Keywords: Energy GHG Wastes Gasification AD Efficiency

a b s t r a c t UK government requires local authorities to develop Municipal Waste Management Strategies, to put in place policies to reduce greenhouse gas emissions and contribute to meeting both the EU Landfill Directive and UK government recycling targets. To assist the decision making process, a study was commissioned to compare the energy produced by treating 100,000 tpa residual municipal solid waste to provide a fuel derived from treating the waste using anaerobic digestion (AD) and gasification processes used on-site, with the equivalent off-site use. Comparison of technology scenarios was based on a number of parameters, comprising efficiency of energy production, energy capacity, avoided CO2 emissions and capital and operating costs. The study examined a total of 41 technology combinations that produce/use waste-derived gases based on gasification and AD, against a base case of on-site power production only. Scenarios considered for using the gases derived included on/off-site use in CHP units based on gas engines and fuel cells and use as a transport fuel for vehicles. The relative performance of on/off site options was influenced by a trade-off between a reduction in efficiency caused by supply chain losses and parasitic requirements that tend to favour on-site options and improvements in efficiency achieved by delivering a greater quantity of recovered heat to end users. Based on the assumptions applied to the study, all scenarios involving gasification and on-site fuel use achieved higher overall energy efficiencies than the equivalent off-site use, despite the greater use of waste heat assumed for off-site scenarios. In contrast to gasification, off-site scenarios using biogas from AD via injection into the gas grid, had the highest efficiency. Off-site use as a liquid fuel in a fuel cell gave a lower energy efficiency than the base case of power only for MBT AD, while for source segregated (SS) AD the base case of power only had the marginally lowest energy efficiency. The magnitude of the energy produced by gasification/combustion gave the highest outputs, ranging from 44 GW h/y to 94 GW h/y. AD technologies gave net energy outputs between 10 GW h/y to 22 GW h/y for MBT AD and 40 GW h/y to 53 GW h/y for SS AD. The energy from waste power only/ CHP cases produced energy outputs of 65 and 73 GW h/y respectively.

Abbreviations: ABPR, animal by-products regulations; ACT, advanced conversion technology; AD, anaerobic digestion; APCR, air pollution control residues; APP, Advanced Plasma Power; ATT, advanced treatment technology; BERR, Department for Business, Enterprise & Regulatory Reform (now Department for Business, Innovation and Skills); CAPEX, capital cost expenditure; CCA, climate change agreement; CCAP, climate change action plan; CCL, climate change levy; CHP, combined heat and power; CHPQA, combined heat and power quality assurance scheme; CH3OH, methanol; CLO, compost like output; CO, carbon monoxide; CO2, carbon dioxide; CV, calorific value; DEFRA, Department for the Environment Food and Rural Affairs; DME, dimethyl ether; DNO, district network operator; DTI, Department for Trade and Industry; EfW, energy from waste; EU, European Union; FC, fuel cell; FT, Fischer Tropsch; GCV, gross CV; GHG, greenhouse gas; GW, gigawatt; GW h, gigawatt hour; H2, hydrogen; H2O, water; HC, hydrocarbons; HCl, hydrogen chloride; HDPE, high density polyethylene; IBA, incinerator bottom ash; IPPC, integrated pollution prevention and control; kg, kilogram; km, kilometre; ktpa, kilo tonnes per annum; kW, kilowatt; kW h, kilowatt hour; LBM, liquid biomethane; LDPE, low density polyethylene; LEC, levy exemption certificates; LBM, liquid biomethane; LPG, liquid petroleum gas; M, million; MCFC, molten carbonate fuel cell; MBT, mechanical and biological treatment; MJ, megajoule; MSW, municipal solid waste; MW, megawatt; MW h, megawatt hour; NCV, net CV; Nm3, normal cubic metre; NOX, generic term for mono-nitrogen oxides NO and NO2; OPEX, operating cost expenditure; PAS110, publically available specification number 110; PE, polyethylene; PET, polyethylene teraphtalate; PEMFC, proton exchange membrane fuel cell; PP, Polypropylene; PPA, power purchase agreement; PSA, pressure swing absorption; rMSW, residual municipal solid waste; ROCS, renewables obligations certificates; ROO, renewables obligation order; RTFO, road transport fuel obligation; SOFC, solid oxide fuel cell; SOX, generic term for sulphur mono, di and trioxides; Syngas, synthetic gas; TRIAD, average demand on the national grid during three peak half hours between November and February each year; WRATE, waste resources and assessment tool. DOI of original article: http://dx.doi.org/10.1016/j.seta.2015.03.001 ⇑ Corresponding author. E-mail addresses: [email protected] (P. Watkins), [email protected] (P. McKendry). http://dx.doi.org/10.1016/j.seta.2015.03.004 2213-1388/Ó 2015 Elsevier Ltd. All rights reserved.

P. Watkins, P. McKendry / Sustainable Energy Technologies and Assessments 10 (2015) 114–124

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Part 1 of the paper set out the background to the study, assumptions, methodology used and references for the 41 scenarios assessed, while in Part 2 the outputs generated by the assessment process are presented and discussed. Ó 2015 Elsevier Ltd. All rights reserved.

Introduction

Technology risk assessment

Waste disposal authorities have had to develop Municipal Waste Management Strategies (MSWS) to put in place policies to reduce greenhouse gas emissions and contribute to meeting both the Landfill Directive and UK government recycling targets as set out in Waste Strategy 2007. Simultaneously such strategies address the problems faced by high-density urban environments dealing with residual MSW (rMSW) and source segregated (SS) food wastes. An integral part of developing such strategies is delivering technology-driven waste management facilities, focussed on using waste as a renewable resource and targeting specifically those scenarios that have the capacity to deliver a range of gases and fuels to the commercial marketplace. To aid the decision making process, a study was commissioned by a local government body1 to compare the energy produced by on-site use of fuels derived from treating wastes using anaerobic digestion (AD) and gasification processes, with the equivalent offsite use. Comparison of the various technology scenarios was based on a number of parameters, comprising: efficiency of energy production; energy capacity; avoided CO2 emissions; and capital and operating costs. The findings of the study, including an assessment of the commercial and environmental impacts of waste derived gases as renewable energy, are presented as a two-part paper. In the first paper the background to the study is described, outlining the technology scenarios, assumptions and methodology used. This second paper presents the results of the assessment and discusses the major conclusions. The objective of the study was to provide a means of assessing potential options that generate energy from wastes using a combination of different technologies in terms of energy efficiency and potential environmental impacts. The base case of on-site power production is assessed against the 40 variations identified for the study. The combination of technologies is split into waste pre-treatment and gas production – using biological or thermal treatments – processing of the waste gas into a fuel and use both on-site and off-site in different energy producing technologies. While technologies such as incineration and anaerobic digestion treating rMSW are well-proven, gasification using rMSW as a fuel has a more limited track record. End-use technologies based on gas engines have greater experience than FC in producing power and heat. In this section the performance of the technology combinations is assessed based on the key determinants of avoided CO2 emissions; costs and revenues; efficiency for vehicle fuel options as km/te of waste; net energy efficiency and; energy generation and energy capacity for the power producing options. In Part 1 the 41 scenarios assessed were described in terms of the outline process configuration and the assumptions used in the assessment process defined.

Technology risk assessment refers to the maturity of the technology and its proven ability to provide the specified outputs in a reliable and cost effective manner. The assessment is undertaken as a high-level view only and does not take account of any planning risks. In Part 1 the technology combinations to be assessed were developed as 41 scenarios. The scenarios are presented in Appendix 1 as the output of this paper.

1

London Climate Change Agency, now disbanded.

Energy from waste (EfW) The base case of EfW is as a combustion process, configured as electricity only, or as combined heat and power (CHP). EfW represents a well-proven and robust technology with an established track record treating rMSW and is rated as low risk. Anaerobic digestion Use of anaerobic digestion to treat organic wastes is over 100 years old in Europe in terms of its application to sewage treatment. Many process variations have been developed successfully based on treating both high and low solids-content wastes. The application to treating organic residues from rMSW is relatively new. Digestion systems use a front-end separation stage to recover recyclables and the organic fraction. Separation processes can be ‘dry’ – based on shredding/screening – or ‘wet’ – based on hydropulping of the waste to produce a high solids content effluent for digestion. Both types are in commercial use. For treating source segregated organic wastes or organics derived from the processing of rMSW, biogas is a low-risk technology. Thermal gasification An established and proven process used widely in the chemical and energy industries. Feedstocks are typically homogeneous materials such as coal and biomass sources i.e. wood. Application to a heterogeneous feedstock such as rMSW is recent and currently few if any gasification plants using unprocessed MSW are believed to be operational. Pre-processing of rMSW to produce a consistent, high-biomass content feedstock is currently proposed at a number of sites in the UK but as yet none are operational. Gasification of rMSW poses a medium/high level of technology risk, as it is associated with the use of a heterogeneous feedstock. Pre-processing of rMSW to provide a high biomass content feedstock e.g. autoclaving, reduces the technology risk but is still likely to be deemed a medium risk technology. Plasma gasification Application of gas plasma gasification to MSW is undertaken commercially in Japan at a number of sites. Gas plasma can be used to both gasify the waste and clean-up the syngas produced. Syngas as-produced contains oils/tars from the thermal gasification stage that need to be removed or further processed to avoid damage to

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end-use equipment such as gas engines. Cleaning syngas using wet scrubbing methods reduces the calorific content of the syngas and generates hazardous wastes for treatment/disposal. Using gas plasma cracks the oils/tars into gaseous hydrocarbons that become part of the syngas. Unlike conventional plasma arc gasification, the system proposed by Advanced Plasma Power2 thermally gasifies pre-processed waste feedstock to produce a syngas and then passes the syngas through a gas plasma unit. The gas plasma removes hydrocarbons that exist as tars and oils, which cause problems when used as a fuel in gas engines and FC. The application of gas plasma to produce a clean syngas is considered a moderate/high risk, in view of the lack of an operational, commercial-scale plant. Autoclaving A widely used process in certain industries use but use as a pretreatment process does not yet have any significant track record on the scale proposed for treating rMSW. The technology has not previously been applied to rMSW and on this basis is considered a moderate/high risk technology. Gas engines A well-proven low risk technology tried and tested over many years on the both biogas and syngas: rated low risk. FC Still a comparatively new technology but with a proven track record in stationery applications using liquid and gaseous fuels e.g. the MTU Hot Module operating on biogas at Leonberg3, Germany. Vehicle or transport applications are still under development, with many claims made but with no/limited commercial systems available for general use. Many development programs are underway and significant progress is forecast in the next 3–5 years. Based on current developments, MCFC/SOFC are considered low/moderate risk technologies and PEMFC vehicle units a moderate/high risk. Biogas upgrading and liquefication Both are proven technologies used widely in the chemical and energy sectors. Application of these technologies to biogas and syngas has been demonstrated. A number of systems to upgrade biogas to pipeline quality are available e.g. Malmberg and are in use in Sweden, Denmark, the Netherlands and Germany. A plant built by Gasrec Ltd. to produce liquid biomethane (LBM) from biogas is understood to have recently been commissioned at the SITA Albury landfill site, Guildford. The LBM is to be used in vehicles operated by Guildford Borough Council. Upgrading to biomethane and LBM are considered low risk technologies. Pipelines Piping gas is a well proven, low risk technology, with natural gas from the North Sea being piped onshore and across most of the UK. 2 3

http://www.advancedplasmapower.com/ http://www.mtu-cfc.com/en/pres/pres_060614_01.htm

Comparison of scenario performance data The output of the detailed assessment of each technology combination considered is presented in the Appendix. The data presents the results for each technology combination assessed, in terms of net/gross power and energy output; avoided CO2 for all scenarios; capital and operating costs and revenues; fuel energy values and energy capacity. The data in the Appendix was used to produce the summary Tables 1–7:  Table 1 ranks the power producing scenarios in terms of the overall net efficiency (excluding vehicle fuel options);  Table 2 ranks the power producing scenarios in terms of the overall net efficiency within each technology group (excluding vehicle fuel options);  Table 3 ranks efficiency of the vehicle scenarios as km/te of waste;  Table 4 ranks all scenarios in terms of avoided CO2 emissions (tonnes of CO2 avoided per tonne of waste);  Table 5 ranks all scenarios in terms of avoided CO2 emissions (tonnes of CO2 avoided per tonne of waste);  Table 6 lists the capital and operating costs and revenues for each technology scenario;  Table 7 ranks the power producing scenarios by net energy output. It should be noted that all data presented in the following tables is subject to a degree of uncertainty. Some of the processes have yet to be proven on a large scale and the parameters used are therefore estimates derived from limited operational experience. Furthermore, for a given process type, performance may vary dramatically for different technology providers and operators. Given these limitations, variations of 10–20% between metrics for competing scenarios should not be taken to indicate that one scenario is necessarily superior to another. The ranking exercise does however provide at least a qualitative indication of which technology combinations might be expected to perform best, based on the assumptions and indicator metrics specified. Ranking as net CHP energy efficiency Tables 1 and 2 show net power, heat output and CHP efficiencies for all scenarios, other than those using waste derived fuel as a vehicle fuel. Scenarios involving treatment of residual waste (MBT and the three gasification options) have been ranked separately from the case of AD of source-segregated organics, on the basis that the two sets of scenarios cannot be helpfully compared. In Table 1 options are ranked in descending order of net CHP efficiency (power-only scenarios are ranked as CHP based on their power output). To illustrate the distribution of on and off site options within the overall ranking, those scenarios where wastederived gases are used off-site have been shaded in grey. Table 2 presents data identical to that in Table 1, with scenarios grouped according to the waste treatment technology. Table 2 shows clearly that within all waste treatment technology groups the base case of on-site power-only mode using gas engines ranks the lowest of all options. As such, the following discussion focuses on those options other than on-site power only using gas engines. The relative ranking of remaining on- and off-site options in Table 1 depends directly on the trade-off between two key factors:  reduction in efficiency caused by supply chain losses and parasitic requirements, (tending to favour on-site options); and

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P. Watkins, P. McKendry / Sustainable Energy Technologies and Assessments 10 (2015) 114–124 Table 1 Ranking of power, heat and CHP efficiencies.*

Residual waste

Refs.

Scenario

Net power efficiency (%)

Net heat efficiency (%)

Net CHP efficiency (%)

On/off site

Rank

28 27 21 20 35 34 41 29 40 22

29 22 24 19 23 17 23 12 25 11

8 14 6 11 7 12 5 15 0 14

36 36 30 30 29 29 28 28 25 25

On On On On On On On Off On Off

1 2 3 4 5 6 7 8 9 10

16 8 22 14

8 15 0 8

24 23 22 22

Off Off On Off

11 12 13 14

37 19 33 8 3 2 9 1 4 5

3. Gasification with autoclave – H2 FC CHP 2. Gasification with autoclave – gas engine CHP 3. Gasification with min pre treatment – H2 FC CHP 2. Gasification with min pre treatment – gas engine CHP 3. Gas plasma – H2 FC CHP 2. Gas plasma – gas engine CHP 2. Incineration with CHP 4. Gasification with autoclave – liquefied – road transport – gas engine CHP 1. Incineration 4. Gasification with min pre treatment – liquefied – road transport – gas engine CHP 5. Gasification with autoclave – liquefied – road transport – H2 FC CHP 4. Gas plasma – liquefied – road transport – gas engine CHP 1. Gasification with autoclave – gas engine (electricity only) 5. Gasification with min pre treatment – liquefied – road transport – H2 FC CHP 5. Gas plasma – liquefied – road transport – H2 FC CHP 1. Gasification with min pre treatment – gas engine (electricity only) 1. Gas plasma – gas engine (electricity only) 8. MBT AD – grid injection – gas engine CHP 3. MBT AD – biogas FC CHP 2. MBT AD – gas engine CHP 9. MBT AD – grid injection – Biogas FC CHP 1. MBT AD – gas engine (electricity only) 4. MBT AD – liquefied – road transport – gas engine CHP 5. MBT AD – liquefied – road transport – biogas FC CHP

11 19 17 5 8 6 6 6 2 4

8 0 0 4 0 2 0 0 4 0

20 19 17 9 8 8 7 6 6 4

Off On On Off On On Off On Off Off

15 16 17 18 19 20 21 22 23 24

17 18 12 11 13 14 10

8. 9. 3. 2. 4. 5. 1.

15 20 22 18 9 13 18

14 6 3 7 13 5 0

29 26 25 25 22 19 18

Off Off On On Off Off On

1 2 3 4 5 6 7

30 36 26 23

Sourcesegregated organics

SS SS SS SS SS SS SS

AD AD AD AD AD AD AD

– – – – – – –

grid injection – gas engine CHP grid injection – biogas FC CHP biogas FC CHP gas engine CHP liquefied – road transport – gas engine CHP liquefied – road transport – biogas FC CHP gas engine (electricity only)

* Net energy efficiency expressed as CHP is calculated from the energy in the input waste to the net output of all energy i.e. power and heat. Heat utilisation is calculated in accordance with protocol described in Section 7.5.

 improvements in efficiency achieved by delivering a greater quantity of recovered heat to end users – 90% of recovered heat is assumed to be utilised for off-site cases, compared to 45% for on-site (favouring off-site options). The rankings in Table 1 show that for gasification scenarios the trade-off favours on-site options (excluding those producing electricity only). This is attributed largely to the low energy efficiency of conversion of syngas to syndiesel (63%). For gasification options the supply chain losses and parasitic burdens tend to outweigh the gain in efficiency due to increased use of recovered heat. For the biological scenarios i.e. MBT and AD, the off-site options using grid injection emerge with higher efficiencies. Here the conversion efficiency from biogas to biomethane is high, estimated as 98%. As such increased delivery of heat to the end user is sufficient to favour off-site options. Conversely however, biological scenarios involving road transport of LBM to off-site locations rank lowest, due to the losses and parasitic loads incurred in producing the fuel. In Table 2 the rankings are presented each primary waste treatment group, illustrating those technology combinations that deliver the highest net energy efficiency. For all waste technology groups the on-site scenario using a stationary FC gives an electrical efficiency of 47%, significantly higher than the 38% efficiency achieved using a gas engine. While gas engine heat recovery is high – 38%, compared to 23% for the FC – this has a lower bearing on the net CHP efficiency, due to the assumption that only 45% of recovered heat is utilised by the on-site end-user. Changing this assumption would impact on the CHP efficiency achieved.

Conversely the off-site scenarios rank the gas engine option above a FC; off-site 90% of recovered heat is assumed to be used. As such the higher recovered heat fraction has a greater bearing on the CHP efficiency and dominates the electrical efficiency difference in favour of the gas engine. Ranking of vehicle fuel production options Each of the scenarios involving the use of waste-derived fuels in vehicles has been characterised in terms of the estimated distance a vehicle travels per tonne of waste treated. The basis of this calculated distance is estimated for a generic passenger vehicle and how far it could travel per unit of energy ‘deployed at the road’ i.e. the energy ultimately resulting in propulsion of the vehicle after internal combustion (or electrochemical conversion) and transmission losses (note that this approach assumes the internal combustion and proton exchange membrane fuel cell (PEMFC) vehicles have identical weight and size). As an indicative estimate a petrol-driven vehicle travels approximately 11 km per litre of fuel used4, or 1.2 km/kW h. Based on an assumed 18% petrol engine powerchain efficiency (Ref 33), the distance travelled per unit energy deployed at the road is 6.5 km/kW h. Powerchain efficiencies are known for both internal combustion (IC) and PEMFC vehicles (18% and 23% respectively). For a given scenario these data can be used to calculate energy deployed at the road, given the fuel produced per tonne of waste input. 4 Swedish Nation Road Administration (2001). Well-to-wheel efficiency for alternative fuels from natural gas or biogas, p. 29.

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Table 2 Power, heat and CHP efficiencies ranked within technology group.*

Residual waste

Refs.

Scenario

Net power efficiency (%)

Net heat efficiency (%)

Net CHP efficiency (%)

On/off site

Overall rank

Subgroup rank

1 2 3 4

6 6 8 2

0 2 0 4

6 8 8 6

On On On Off

22 20 19 23

5 3 2 6

4

0

4

Off

24

7

5 6 19

4 0 0

9 7 19

Off Off On

18 21 16

1 4 5

19

11

30

On

4

2

24 11

6 14

30 25

On Off

3 10

1 3

14

8

22

Off

14

4

22

0

22

On

13

5

22 29 12

14 8 15

36 36 28

On On Off

2 1 8

2 1 3

16

8

24

Off

11

4

17 17 23 8

0 12 7 15

17 29 29 23

On On On Off

17 6 5 12

5 2 1 3

11

8

20

Off

15

4

40 41

1. MBT AD – gas engine (electricity only) 2. MBT AD – gas engine CHP 3. MBT AD – biogas FC CHP 4. MBT AD – liquefied – road transport – gas engine CHP 5. MBT AD – liquefied – road transport – biogas FC CHP 8. MBT AD – grid injection – gas engine CHP 9. MBT AD – grid injection – biogas FC CHP 1. Gasification with min pre treatment – gas engine (electricity only) 2. Gasification with min pre treatment – gas engine CHP 3. Gasification with min pre treatment – H2 FC CHP 4. Gasification with min pre treatment – liquefied – road transport – gas engine CHP 5. Gasification with min pre treatment – liquefied – road transport – H2 FC CHP 1. Gasification with autoclave – gas engine (electricity only) 2. Gasification with autoclave – gas engine CHP 3. Gasification with autoclave – H2 FC CHP 4. Gasification with autoclave – liquefied – road transport – gas engine CHP 5. Gasification with autoclave – liquefied – road transport – H2 FC CHP 1. Gas plasma – gas engine (electricity only) 2. Gas plasma – gas engine CHP 3. Gas plasma – H2 FC CHP 4. Gas plasma – liquefied – road transport – gas engine CHP 5. Gas plasma – liquefied – road transport – H2 FC CHP 1. Incineration 2. Incineration with CHP

25 23

0 5

25 28

On On

9 7

2 1

10 11 12 13 14 17 18

1. SS AD – gas engine (electricity only) 2. SS AD – gas engine CHP 3. SS AD – biogas FC CHP 4. SS AD – liquefied – road transport – gas engine CHP 5. SS AD – liquefied – road transport – biogas FC CHP 8. SS AD – grid injection – gas engine CHP 9. SS AD – grid injection – biogas FC CHP

18 18 22 9 13 15 20

0 7 3 13 5 14 6

18 25 25 22 19 29 26

On On On Off Off Off Off

7 4 3 5 6 1 2

7 4 3 5 6 1 2

5 8 9 19 20 21 22 23 26 27 28 29 30 33 34 35 36 37

Sourcesegregated organics

* Net energy efficiency expressed as CHP is calculated from the energy in the input waste to the net output of all energy i.e. power and heat. Heat utilisation is calculated in accordance with protocol described in Section 7.5.

Table 3 Vehicle fuel scenarios – ranking of distance travelled per tonne of waste. Refs.

Scenario

km/t

32

7. Gasification with autoclave – liquefied – used directly in vehicle – PEMFC: vehicle FC 7. Gas plasma – liquefied – used directly in vehicle – PEMFC: vehicle FC 7. Gasification, min pre-treatment – liquefied – used directly in vehicle – PEMFC: vehicle FC 6. Gasification with autoclave – liquefied – used directly in vehicle – syndiesel: vehicle IC 6. Gas plasma – liquefied – used directly in vehicle – syndiesel: vehicle IC 6. Gasification, min pre-treatment – liquefied – used directly in vehicle – syndiesel: vehicle IC 7. SS AD – compressed hydrogen – used directly in vehicle – PEMFC: vehicle FC 6. SS AD – liquefied – used directly in vehicle – LBM IC: vehicle IC 7. MBT AD – compressed hydrogen – used directly in vehicle – PEMFC: vehicle FC 6. MBT AD – liquefied – used directly in vehicle – LBM IC: vehicle IC

1,997

39 25 31 38 24 16 15 7 6

1,811 1,671 1,317 1,194 1,102 1,017

Multiplying these rates by the above factor allows the potential distance travelled to be estimated. Table 3 below sets out distances travelled per tonne of waste for each of the vehicle fuel scenarios considered. As for the case of net CHP efficiency, gasification options achieve higher overall energy efficiencies. While the efficiency of conversion from biogas to vehicle fuel is higher than with gasification, the lower initial conversion efficiency of waste-to-gas for AD is low; giving gasification options the overall higher energy efficiency. For all primary waste treatment technologies, PEMFC cell vehicles travel a greater distance per tonne waste treated i.e. have a higher efficiency than internal combustion engines, due to the higher power chain efficiency. Ranking of scenario options by avoided CO2 burden

970 619 591

For each of the scenarios considered, a CO2 burden has been calculated encompassing the following elements:  direct emissions of CO2 resulting from the combustion of the fossil-fuel derived component of waste-derived gases;

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P. Watkins, P. McKendry / Sustainable Energy Technologies and Assessments 10 (2015) 114–124 Table 4 Scenario CO2 burdens – overall ranking.

Residual waste

Refs.

Scenario

28 27 32 26 30 29 31 21 20 25

39 33 37 36 41 38 40

3. Gasification with autoclave – H2 FC CHP 2. Gasification with autoclave – gas engine CHP 7. Gasification with autoclave – liquefied – used directly in vehicle – PEMFC 1. Gasification with autoclave – gas engine (electricity only) 5. Gasification with autoclave – liquefied – road transport – H2 FC CHP 4. Gasification with autoclave – liquefied – road transport – gas engine CHP 6. Gasification with autoclave – liquefied – used directly in vehicle – syndiesel 3. Gasification with min pre treatment – H2 FC CHP 2. Gasification with min pre treatment – gas engine CHP 7. Gasification with min pre treatment – liquefied – used directly in vehicle – PEMFC 3. MBT AD – biogas FC CHP 1. Gasification with min pre treatment – gas engine (electricity only) 3. Gas plasma – H2 FC CHP 9. MBT AD – grid injection – biogas FC CHP 2. MBT AD – gas engine CHP 8. MBT AD – grid injection – gas engine CHP 1. MBT AD – gas engine (electricity only) 5. Gasification with min pre treatment – liquefied – road transport – H2 FC CHP 4. Gasification with min pre treatment – liquefied – road transport – gas engine CHP 5. MBT AD – liquefied – road transport – biogas FC CHP 4. MBT AD – liquefied – road transport – gas engine CHP 2. Gas plasma – gas engine CHP 7. MBT AD – compressed hydrogen – used directly in vehicle – PEMFC 6. MBT AD – liquefied – used directly in vehicle – LBM IC 6. Gasification with min pre treatment – liquefied – used directly in vehicle – syndiesel 7. Gas plasma – liquefied – used directly in vehicle – PEMFC 1. Gas plasma – gas engine (electricity only) 5. Gas plasma – liquefied – road transport – H2 FC CHP 4. Gas plasma – liquefied – road transport – gas engine CHP 2. Incineration with CHP 6. Gas plasma – liquefied – used directly in vehicle – syndiesel 1. Incineration

12 18 17 11 10 14 13 16 15

3. 9. 8. 2. 1. 5. 4. 7. 6.

3 19 35 9 2 8 1 23 22 5 4 34 7 6 24

Source-segregated organics

SS SS SS SS SS SS SS SS SS

AD AD AD AD AD AD AD AD AD

t CO2/t waste

– – – – – – – – –

biogas FC CHP grid injection – biogas FC CHP grid injection – gas engine CHP gas engine CHP gas engine (electricity only) liquefied – road transport – biogas FC CHP liquefied – road transport – gas engine CHP compressed hydrogen – used directly in vehicle – PEMFC liquefied – used directly in vehicle – LBM IC

 emissions associated with power and heat usage for each process;  CO2 equivalent emission resulting from the disposal of any solid waste outputs.  CO2 emissions avoided by substituting power and heat derived from the UK national electricity and gas grids;  for the vehicle fuel options, CO2 avoided by substituting the fossil-diesel required to travel an equivalent distance; and  CO2 emissions avoided by substituting recovered recyclables for virgin materials, thereby avoiding fossil fuel combustion. The latter three components are taken as negative, with a negative overall calculated CO2 emission indicating an overall avoided CO2 burden. Table 4 following ranks all options in ascending order of CO2 burden. Again, results for source segregated AD options have been shown separately, being normalised with respect to a different waste input. As above, off-site options have been shaded grey to indicate the relative spread of on- and off-site options within the ranking. Table 5 presents the same data, with scenarios grouped by the primary waste treatment process.

On/off site

Rank

CHP efficiency rank

0.68 0.63 0.56 0.55 0.52 0.51 0.45 0.32 0.27 0.21

On On Off On Off Off Off On On Off

1 2 3 4 5 6 7 8 9 10

1 2 – 13 11 8 – 3 4 –

0.21 0.21 0.20 0.20 0.19 0.19 0.18 0.18

On On On Off On Off On Off

11 12 13 14 15 16 17 18

19 16 5 21 20 18 22 14

0.16

Off

19

10

0.16 0.16 0.15 0.14 0.13 0.12

Off Off On Off Off Off

20 21 22 23 24 25

24 23 6 – – –

0.08 0.08 0.05 0.03 0.01 0.02 0.04

Off On Off Off On Off On

26 27 28 29 30 31 32

– 17 15 12 7 – 9

0.24 0.23 0.22 0.21 0.18 0.18 0.17 0.15 0.13

On Off Off On On Off Off Off Off

1 2 3 4 5 6 7 8 9

3 2 1 4 7 6 5 – –

As might be expected, the order of the options to some extent mirrors that shown in Table 4 for net CHP efficiency. Most notably, the two best performing options remain gasification with autoclave with a stationary on-site fuel cell and gasification with autoclave with stationary on-site gas engine. In contrast to the case of net CHP efficiency, no obvious differentiation exists between the performances of on- and off-site options. Table 5 does however demonstrate that for all waste treatment technologies the on-site CHP options rank highest. As for net CHP efficiency, the determining factor for the relative ranking of on- and off-site options by CO2 burden is a trade-off between two factors: firstly, a reduction in supply chain losses achieved by combining processes on-site; and secondly, the increased utilisation of heat by end users for the off-site case. For the case of CO2 burden however, the factors tend to balance out, resulting in no strong distinction between the on- and off-site cases. In interpreting the ranking of options with respect to CO2 burden, it is important to note that the avoided CO2 achieved at waste treatment facilities by recycling contributes significantly to the avoided burden. This contributes significantly to the high ranking of the autoclave options, which have a higher recycling rate

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Table 5 Scenario CO2 burdens – ranking by waste technology.

Residual waste

Refs.

Scenario

1 2 3 4 5 6 7 8 9 19 20 21 22

32 33 34 35 36 37 38 39 40 41

1. MBT AD – gas engine (electricity only) 2. MBT AD – gas engine CHP 3. MBT AD – biogas FC CHP 4. MBT AD – liquefied – road transport – gas engine CHP 5. MBT AD – liquefied – road transport – Biogas FC CHP 6. MBT AD – liquefied – used directly in vehicle – LBM IC 7. MBT AD – compressed hydrogen – used directly in vehicle – PEMFC 8. MBT AD – grid injection – gas engine CHP 9. MBT AD – grid injection – biogas FC CHP 1. Gasification with min pre treatment – gas engine (electricity only) 2. Gasification with min pre treatment – gas engine CHP 3. Gasification with min pre treatment – H2 FC CHP 4. Gasification with min pre treatment – liquefied – road transport – gas engine CHP 5. Gasification with min pre treatment – liquefied – road transport – H2 FC CHP 6. Gasification with min pre treatment – liquefied – used directly in vehicle – syndiesel 7. Gasification with min pre treatment – liquefied – used directly in vehicle – PEMFC 1. Gasification with autoclave – gas engine (electricity only) 2. Gasification with autoclave – gas engine CHP 3. Gasification with autoclave – H2 FC CHP 4. Gasification with autoclave – liquefied – road transport – gas engine CHP 5. Gasification with autoclave – liquefied – road transport – H2 FC CHP 6. Gasification with autoclave – liquefied – used directly in vehicle – syndiesel 7. Gasification with autoclave – liquefied – used directly in vehicle – PEMFC 1. Gas plasma – gas engine (electricity only) 2. Gas plasma – gas engine CHP 3. Gas plasma – H2 FC CHP 4. Gas plasma – liquefied – road transport – gas engine CHP 5. Gas plasma – liquefied – road transport – H2 FC CHP 6. Gas plasma – liquefied – used directly in vehicle – syndiesel 7. Gas plasma – liquefied – used directly in vehicle – PEMFC 1. Incineration 2. Incineration with CHP

12 18 17 11 10 14 16 13 15

3. 9. 8. 2. 1. 5. 7. 4. 6.

23 24 25 26 27 28 29 30 31

Source-segregated organics

SS SS SS SS SS SS SS SS SS

AD AD AD AD AD AD AD AD AD

t CO2/t waste

– – – – – – – – –

biogas FC CHP grid injection – biogas FC CHP grid injection – gas engine CHP gas engine CHP gas engine (electricity only) liquefied – road transport – biogas FC CHP compressed hydrogen – used directly in vehicle – PEMFC liquefied – road transport – gas engine CHP liquefied – used directly in vehicle – LBM IC

compared with the thermal options. In contrast, with incineration, where recycling is limited to metals recovery from bottom ash, the avoided CO2 burden is small. An even greater factor in the high ranking achieved with autoclave options is the avoidance of production of fossil CO2. As in this case fuels are derived from a high-biomass fibre, fossil CO2 emissions are minimised. Heat demands for the autoclave process are high but as these are largely accounted for by waste heat from the gasification process, they contribute minimally to the overall CO2 burden.

Capital and operating costs and revenues Estimated scenario capital costs (CAPEX), operating costs (OPEX) and revenues are presented in Table 6. Table 6 separately sets out these three financial metrics for on-site facilities and for off-site facilities, to which waste derived fuels are exported. It should be noted that for off-site scenarios, the stated CAPEX and OPEX account for fuel storage only but exclude the cost of the gas engine/FC and associated grid connection etc. For scenarios

On/off site

Overall rank

Subgroup rank

0.18 0.19 0.21 0.16 0.16 0.13 0.14 0.19 0.20 0.21 0.27 0.32 0.16

On On On Off Off Off Off Off Off On On On Off

17 15 11 21 20 24 23 16 14 12 9 8 19

5 3 1 7 6 9 8 4 2 4 2 1 6

0.18

Off

18

5

0.12

Off

25

7

0.21

Off

10

3

0.55 0.63 0.68 0.51 0.52 0.45

On On On Off Off Off

4 2 1 6 5 7

4 2 1 6 5 7

0.56 0.08 0.15 0.20 0.03 0.05 0.02 0.08 0.04 0.01

Off On On On Off Off Off Off On On

3 27 22 13 29 28 31 26 32 30

3 4 2 1 6 5 7 3 2 1

0.24 0.23 0.22 0.21 0.18 0.18 0.15 0.17 0.13

On Off Off On On Off Off Off Off

1 2 3 4 5 6 8 7 9

1 2 3 4 5 6 8 7 9

involving off-site use of the waste derived fuel for power generation the on-site revenue does not include revenue from fuel sales and the off-site OPEX does not include the cost of purchasing the waste derived fuel. Users can determine potential fuel prices using the figures below and by applying project specific requirements e.g. required rate of return. While off-site options demand additional CAPEX and OPEX for conversion of the waste derived gas into a liquid fuel, these requirements are offset to some extent by reduced expenditure at the end of the supply chain. For gasification options the net result of this interplay is that for a given off-site scenario the CAPEX and OPEX remain relatively similar to the on-site counterpart. For SS AD/MBT options, in contrast LBM production adds considerably to the overall cost, overwhelming the saving on the power generation equipment. This is particularly the case for SS AD/MBT OPEX, which rise considerably for the off-site case. Full assessment of the financial viability is beyond the scope of this study. However, it should be noted that those scenarios demanding a high CAPEX and OPEX do not necessarily generate proportionally high revenues, thereby extending the payback

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P. Watkins, P. McKendry / Sustainable Energy Technologies and Assessments 10 (2015) 114–124 Table 6 Capital costs, operating costs and revenues for technology scenarios. Refs.

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 35 36 37 38 39 40 41

Scenario

1. MBT AD – gas engine (electricity only) 2. MBT AD – gas engine CHP 3. MBT AD – biogas FC CHP 4. MBT AD – liquefied – road transport – gas engine CHP 5. MBT AD – liquefied – road transport – biogas FC CHP 6. MBT AD – liquefied – used directly in vehicle – LBM IC 7. MBT AD – Compressed hydrogen – used directly in vehicle – PEMFC 8. MBT AD – grid injection – gas engine CHP 9. MBT AD – grid injection – biogas FC CHP 1. SS AD – gas engine (electricity only) 2. SS AD – gas engine CHP 3. SS AD – biogas FC CHP 4. SS AD – liquefied – road transport – gas engine CHP 5. SS AD – liquefied – road transport – Biogas FC CHP 6. SS AD – liquefied – used directly in vehicle – LBM IC 7. SS AD – compressed hydrogen – used directly in vehicle – PEMFC 8. SS AD – grid injection – gas engine CHP 9. SS AD – grid injection – biogas FC CHP 1. Gasification with min pre treatment – gas engine (electricity only) 2. Gasification with min pre treatment – gas engine CHP 3. Gasification with min pre treatment – H2 FC CHP 4. Gasification with min pre treatment – liquefied – road transport – gas engine CHP 5. Gasification with min pre treatment – liquefied – road transport – H2 FC CHP 6. Gasification with min pre treatment – liquefied – used directly in vehicle – syndiesel 7. Gasification with min pre treatment – liquefied – used directly in vehicle – PEMFC 1. Gasification with autoclave – gas engine (electricity only) 2. Gasification with autoclave – gas engine CHP 3. Gasification with autoclave – H2 FC CHP 4. Gasification with autoclave – liquefied – road transport – gas engine CHP 5. Gasification with autoclave – liquefied – road transport – H2 FC CHP 6. Gasification with autoclave – liquefied – used directly in vehicle – syndiesel 7. Gasification with autoclave – liquefied – Used directly in vehicle – PEMFC 1. Gas plasma – gas engine (electricity only) 2. Gas plasma – gas engine CHP 3. Gas plasma – H2 FC CHP 4. Gas plasma – liquefied – road transport – gas engine CHP 5. Gas plasma – liquefied – road transport – H2 FC CHP 6. Gas plasma – liquefied – used directly in vehicle – syndiesel 7. Gas plasma – liquefied – used directly in vehicle – PEMFC 1. Incineration 2. Incineration with CHP

On site

Off site

CAPEX (£M)

OPEX (£M/ y)

Revenue (£M/y)

CAPEX (£M)

OPEX (£M/ y)

Revenue (£M/y)

30.0 30.0 28.4 30.4 30.4 31.4 29.0 30.3 30.3 9.0 9.0 7.4 9.4 9.4 10.9 7.4 10.1 10.1 66.3 66.3 62.1 66.0

3.7 3.7 4.1 5.6 5.6 5.8 4.2 4.1 4.1 1.0 1.0 1.6 4.0 4.0 4.3 1.6 1.6 1.6 5.8 5.8 6.9 5.4

14.6 14.9 15.3 11.9 11.9 16.5 16.7 11.9 11.9 9.7 10.5 11.1 5.3 5.3 12.8 13.2 5.3 5.3 17.2 18.9 19.5 11.2

– – – 0.17 0.14 – – 0.17 0.14 – – – 0.17 0.14 – – 0.17 0.14 – – – 0.17

– – – 0.02 0.02 – – 0.00 0.00 – – – 0.03 0.03 – – 0.00 0.00 – – – 0.04

– – – 3.7 3.8 – – 3.8 3.9 – – – 6.0 6.2 – – 6.3 6.4 – – – 5.8

66.0

5.4

11.2

0.14

0.04

5.7

67.5

5.8

19.8







73.7

6.3

24.3







71.2 71.2 67.9 70.4

8.1 8.1 9.4 7.8

20.7 22.8 24.0 11.9

– – – 0.17

– – – 0.04

– – – 8.0

70.4 72.1

7.8 8.2

11.9 22.2

0.14 –

0.04 –

8.2 –

78.7

9.0

27.5







54.2 54.2 49.6 52.3 52.3 53.9 58.8 50.0 55.0

8.1 8.1 9.2 7.8 7.8 8.2 9.0 5.5 5.5

17.7 19.5 20.2 11.2 11.2 20.6 25.4 13.3 13.7

– – – 0.17 0.14 – – – –

– – – 0.04 0.04 – – – –

– – – 6.3 6.2 – – – –



Off-site option CAPEX and OPEX accounts for fuel storage only, excluding the cost of the end-use equipment, grid connection etc. For scenarios involving off-site use of the fuel the on-site revenue excludes revenue from the sale of the waste derived fuel and off-site OPEX excludes the cost of purchasing the waste derived fuel. Users can determine potential fuel prices using the figures above and by applying project specific requirements.

period for those scenarios (which tend to be those with higher energy efficiency). Ranking by energy generation Table 7 presents the annual energy output of each scenario as electricity, heat and CHP. The SS AD scenarios have been listed separately from those treating rMSW in order to ensure valid comparison. When biogas is processed to produce a liquid fuel for off-site use, the net CHP efficiency is lower than the comparable on-site CHP options. Off-site use via the gas grid produces the highest energy output while on-site power only scenarios produce the lowest energy output. Gasification scenarios produced larger energy outputs then the equivalent scenario based on biogas. Within the three gasification

technologies, gasification with autoclave produced the largest energy output, with gasification and minimal pre-treatment and gas plasma at comparable outputs. In all cases with gasification, on-site CHP use of the fuel provided more energy than the comparable off-site use. However, as for the biogas options, on-site power only scenarios produce the least energy. Amongst thermal treatments, incineration power-only produces more energy than the base case of gas engine power-only. The energy output of incineration-CHP was similar to gas plasma and gasification with minimal pre-treatment but less than gasification and autoclaving for both on- and off-site options. The high energy production of incineration is due partly to a greater quantity of waste entering the facility, as no material is removed for recycling. For the AD and gasification scenarios, the waste is pre-treated to remove material for recycling, before entering the treatment technology plant. Much of this material has a

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Table 7 Ranking of power producing scenarios by net energy output. Refs.

1 2 3 4 5 8 9 10 11 12 13 14 17 18 19 20 21 22 23 26 27 28 29 30 33 34 35 36 37 40 41

Scenario

1. MBT AD – gas engine (electricity only) 2. MBT AD – gas engine CHP 3. MBT AD – biogas FC CHP 4. MBT AD – liquefied – road transport – gas engine CHP 5. MBT AD – liquefied – road transport – biogas FC CHP 8. MBT AD – grid injection – gas engine CHP 9. MBT AD – grid injection – biogas FC CHP 1. SS AD – gas engine (electricity only) 2. SS AD – gas engine CHP 3. SS AD – biogas FC CHP 4. SS AD – liquefied – road transport – gas engine CHP 5. SS AD – liquefied – road transport – biogas FC CHP 8. SS AD – grid injection – gas engine CHP 9. SS AD – grid injection – biogas FC CHP 1. Gasification with min pre treatment – gas engine (electricity only) 2. Gasification with min pre treatment – gas engine CHP 3. Gasification with min pre treatment – H2 FC CHP 4. Gasification with min pre treatment – liquefied – road transport – gas engine CHP 5. Gasification with min pre treatment – liquefied – road transport – H2 FC CHP 1. Gasification with autoclave – gas engine (electricity only) 2. Gasification with autoclave – gas engine CHP 3. Gasification with autoclave – H2 FC CHP 4. Gasification with autoclave – liquefied – road transport – gas engine CHP 5. Gasification with autoclave – liquefied – road transport – H2 FC CHP 1. Gas plasma – gas engine (electricity only) 2. Gas plasma – gas engine CHP 3. Gas plasma – H2 FC CHP 4. Gas plasma – liquefied – road transport – gas engine CHP 5. Gas plasma – liquefied – road transport – H2 FC CHP 1. Incineration 2. Incineration with CHP

On site

Off site

Scenario total

Overall rank

Subgroup rank

Net Power (GW h/y)

Net Heat (GW h/y)

Net Power (GW h/y)

Net Heat (GW h/y)

Net Power (GW h/y)

Net Heat (GW h/y)

Net CHP (GW h/y)

14.8 14.8 19.7 14.0

0.0 5.8 1.2 9.6

– – – 19.1

– – – 19.0

14.8 14.8 19.7 5.1

0.0 5.8 1.2 9.4

14.8 20.6 20.9 14.4

29 27 26 30

5 3 2 6

14.0

9.6

23.6

10.4

9.6

0.8

10.4

31

7

7.9 7.9 32.2 32.2 40.1 15.1

9.6 9.6 0.0 13.3 5.8 7.2

19.9 24.6 – – – 31.3

19.8 10.8 – – – 31.2

12.0 16.7 32.2 32.2 40.1 16.2

10.2 1.2 0.0 13.3 5.8 24.0

22.2 17.9 32.2 45.6 46.0 40.2

25 28 24 20 19 22

1 4 7 4 3 5

15.1

7.2

38.8

17.1

23.6

9.9

33.5

23

6

4.8 4.8 49.4

7.2 7.2 0.0

32.7 40.4 –

32.5 17.8 –

27.9 35.6 49.4

25.3 10.6 0.0

53.2 46.2 49.4

15 18 17

1 2 5

49.4

28.4





49.4

28.4

77.8

4

2

62.9

15.5





62.9

15.5

78.4

3

1

7.6

0.0

35.6

35.4

28.0

35.4

63.5

10

3

7.6

0.0

44.1

19.4

36.5

19.4

55.9

14

4

57.5

0.0





57.5

0.0

57.5

13

5

57.5

35.3





57.5

35.3

92.8

2

2

73.7 10.6

20.0 3.2

– 42.6

– 42.3

73.7 32.0

20.0 39.2

93.6 71.2

1 8

1 3

10.6

3.2

52.6

23.2

42.1

20.0

62.1

11

4

43.6 43.6 58.2 18.2

0.0 30.7 16.8 0.0

– – – 38.6

– – – 38.4

43.6 43.6 58.2 20.4

0.0 30.7 16.8 38.4

43.6 74.3 75.0 58.8

21 6 5 12

5 2 1 3

0.0

47.7

21.0

29.5

21.0

50.6

16

4

0.0 14.0

– –

– –

64.6 59.4

0.0 14.0

64.6 73.4

9 7

2 1

18.2 64.6 59.4

high energy value e.g. plastics and paper and therefore the total quantity of energy entering the gasification plant is lower than that entering the incinerator. The situation is less pronounced with AD, as only organic materials e.g. paper and biomass, are of for digestion, with plastics not providing any contribution to energy production. Discussion Two sets of headline conclusions are discussed below. The first pertain specifically to the relative performance of on- and off-site options, while the second provide more in-depth information on the relative performance of specific scenarios. Overall comparison of on-site and off-site options The base case of on-site electrical power generation almost exclusively ranks lower than all other scenarios. Setting aside this

base case, the relative performance of on- and off-site options is determined by a trade-off between two key factors:  reduction in efficiency caused by supply chain losses and parasitic requirements, (tending to favour on-site options); and  improvement in efficiency achieved by delivering a greater quantity of recovered heat to end users – 90% of recovered heat is assumed to be utilised for off-site cases, compared to 45% for on-site (favouring off-site options). For the three gasification scenarios considered, the former of these factors has the greater impact, leading to a favouring (in terms of the indicators of CHP efficiency, and avoided CO2 burden) of the on-site cases. Conversely, for source-segregated AD and MBT, off-site options employing grid injection of biomethane surface as the most energy efficient. For grid injection, supply chain losses and parasitic loads are relatively small, such that increased delivery of heat to the end user is sufficient to favour the off-site cases. However it is also

P. Watkins, P. McKendry / Sustainable Energy Technologies and Assessments 10 (2015) 114–124

notable that as for gasification, the biological scenarios involving delivery of a liquid fuel for off-site power generation rank last in terms of energy efficiency. When considering cases employing CHP, given the delivery of liquid fuels by road, off-site use emerges as broadly less preferable in terms of energy efficiency despite the assumed high deliver of heat to end users for off-site cases. While this is the case, it should be emphasised that in practice, opportunities for development of a waste treatment facility in proximity to a heat user are likely to be rare. Given that off-site options outperform the base case of on-site electrical power only in virtually all cases, a strong impetus still exists for use of waste derived fuels at off-site facilities. These findings are contingent on the assumption that for offsite cases, a large fraction of recovered heat (90%) can be delivered to end users. Furthermore they are also the consequence assumed supply chain efficiencies and parasitic loads for fuel production. Such assumptions should be reviewed as the technologies considered are further developed and gain market acceptance. Comparison of scenarios Based on the specified performance parameters, comparison of the performance data of the various technology combinations and their energy outputs identifies clear performance groupings, indicating the preference of one technology combination over another. Comparison of the same technology combinations used on- and off-site results in different overall efficiencies. The differences arise from the assumptions used in for the on/off-site scenarios, which reflect the different practical and financial circumstances of each scenario. Net energy efficiency In all instances the base case for comparison is on-site electricity generation, which in almost all cases is the least energy efficient scenario. In nearly all cases on-site CHP production of energy compared to off-site CHP production achieves the highest energy efficiency. Comparing scenarios based on their net CHP efficiency (Table 1) highlights that using rMSW as the waste source, biogas based scenarios give the lowest efficiencies compared to gasification scenarios. This reflects the lower conversion efficiency of the primary waste to energy conversion process. Within the thermal treatment technologies, the ranking follows decreasing efficiency from gasification with autoclaving to gasification with min. pre-treatment to gas plasma, for on-site FC and gas engine CHP. The next lowest order of overall CHP efficiency involves the production of a liquid fuel for off-site use in a FC/gas engine CHP unit, again reducing from gasification with autoclave through to gas plasma. The range of efficiencies for the on-site CHP options is 29–36%. The range for production of a liquid fuel for use in the same CHP options off-site is 20–28%. For the same technology combinations, on-site electricity only production ranges from 17% to 22%, again the highest being for autoclaving and the lowest gas plasma. Gas plasma options tend to have the lowest efficiencies compared with autoclave and minimal pre-treatment options. Gas plasma has a relatively high electrical parasitic load, which reduces the overall efficiency. Autoclaving produces a high quality, energy dense feedstock that results in higher efficiency figures for autoclave options. If the gas plasma feedstock was preceded by autoclave pre-treatment, it would improve both the efficiency and energy production of the scenarios MBT AD technology scenarios gave the lowest overall efficiencies. Both the value and range of efficiencies was small, 4–9% for all technology options, on- and off-site and conversion to a liquid

123

fuel. Unlike the gasification options, biogas was upgraded to biomethane and injected into the national gas grid. In this case the off-site CHP options scored a similar value to on-site CHP use. As noted however the range of efficiencies is narrow and allowing for the accuracy of the source data, the difference between the options is small. For SS AD the comparison with MBT AD and even gasification options, shows a significant improvement. The range of energy efficiencies is 18–29%, lowest for the base case of on-site electricity only but at the higher end comparable with the gas plasma options. The increase is due to the greater proportion of waste used and the quality of the source segregated waste, as the primary conversion efficiency of waste to biogas remains low compared with the thermal treatment options. Any additional processing of the waste prior to gasification/digestion reduces the overall energy conversion efficiency of a scenario, due to both the loss of material in preparing the process feedstock and the energy consumed at the pre-treatment stage. Processing of waste gas to either enable transport to off-site locations, or to enhance its use as vehicle fuel, results in a lower efficiency than the equivalent on-site option. As each processing stage has an efficiency <100%, increasing the number of processing stages only reduces the overall efficiency of the technology scenario. Producing a fuel to allow transport to off-site locations results in lower overall energy efficiency, compared to the same end-use on-site. The electricity only on-site scenario always scores the lowest energy efficiency, even when compared with producing a fuel for use off-site, due to the energy wasted as heat and not recovered as with the CHP options. The exception here is producing biomethane for injection into the gas grid, which is comparable with on-site CHP use. In this instance the energy used to produce biomethane is minimal compared with producing a liquid fuel from syngas or biogas. Vehicle scenarios are excluded from the energy efficiency rankings and instead are ranked amongst themselves, using the parameter km/tonne of waste. The km/te range for gasification was 1997 km/te for scenario 32 (numbered per Table 2), to 1102 km/ te for scenario 24. The highest biogas mileage was achieved with scenario 16, 1017 km/te and the lowest at 591 km/te with scenario 6. Comparing on- and off-site use of fuels depends on the parameters of interest. On the basis of overall efficiency of conversion of waste to energy expressed as total energy or CHP, for the same thermal conversion technology, on-site use always provides the higher efficiency compared with the equivalent off-site use. If the quantity of energy produced is of interest, then thermal treatments also provide the highest quantity of energy, with on-site scenarios again producing more than the equivalent off-site option. MBT AD scores both the lowest CHP energy conversion efficiencies and also the smallest magnitude of energy output for the same quantity of rMSW treated; biological energy conversion is a much less efficient energy conversion technology than thermal treatments. For SS AD, the energy efficiencies are more than double the equivalent MBT AD scenarios and consequently, the energy output is also more than double; this is despite the different CVs of the two feedstocks. For both SS AD and MBT AD the highest energy efficiencies are achieved with grid injection and use off-site in CHP mode (26–29% and 7–9% respectively) compared with the equivalent on-site CHP mode (25% and 8% respectively); the base case in each scenario is less at 18% for SS AD and 6% for MBT AD. Conversion to a fuel for equivalent use off-site gives energy efficiencies for SS AD between 19–22% and 4–6% for MBT AD for offsite CHP.

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P. Watkins, P. McKendry / Sustainable Energy Technologies and Assessments 10 (2015) 114–124

CO2 burden Rankings based on avoided CO2 emissions show that the gasification scenario based on gasification with autoclaving (scenarios 26–30) achieved the largest avoided CO2 emissions for all scenarios, compared with the other technologies. The variation between on- and off-site uses as expected generally favours on-site uses. For autoclave scenarios the range of 0.45 to 0.68 te CO2/te waste is significantly higher than the next technology, gasification with minimal pre-treatment (scenarios 19–23), which ranges from an avoided burden of 0.32 te CO2/te waste down to 0.12 te CO2/ te waste. Autoclave scenarios score highly for avoided CO2 due to the high level of recycling achieved with this technology. The CO2 burdens of the MBT AD biogas options (scenarios 1–6 and 9) were between 0.13 and 0.21 te CO2/te waste and fell amongst the range for the other gasification options based on minimal pre-treatment and gas plasma. Incineration electricity only, with a net positive burden of 0.04 te CO2/te waste, and the gas plasma scenario producing a vehicle fuel, with a net positive burden of 0.02 te CO2/te waste, were the only scenarios that generated a positive burden; incineration CHP was approximately CO2 neutral at 0.01 te CO2/te waste. The SS AD scenarios were generally comparable to the equivalent MBT AD scenarios. The high avoided CO2 burden scored by the gasification and autoclaving scenarios is due to the high level of recycling achieved. Under WRATE the avoided CO2 burdens ascribed to recycling are high and any scenario that achieves a high level of recycling will accrue high avoided CO2 burdens. CAPEX and OPEX In terms of CAPEX perhaps not surprisingly those technologies and scenarios occupying the upper half of the CHP energy efficiency also have the higher capital costs. The thermal treatment scenarios are generally considerably more capital intensive than the biological treatment scenarios. The CAPEX range for gasification scenarios is from around £50 M to approximately £108 M, with incineration at the lower end at £50 M–£55 M. The biological treatment scenarios range from just over £7 M to around £30 M. As noted in Section 6, considerable difficulty was found in trying to obtain capital costs for these types of plant at this scale and also to identify what costs were and were not included. On this basis the data should be used accordingly, providing only a relative ranking of potential scenario costs, rather than any definitive value. Until tendered prices have obtained for a specific project, the CAPEX will not be known with any degree of certainty. The OPEX costs varied considerably according to the particular scenario. In £/y terms the costs ranged from £1 M for the low technology option of SS AD and on-site electricity generation (scenario 10), to over £9 M/y (scenario 32). In percentage terms, operating costs are generally in the range 10–15% of the capital cost but could be significantly higher for low CAPEX technologies involving multiple gas processing stages. Energy output As the tonnage of waste treated is the same for all scenarios, the energy generated by a given scenario is determined by the primary waste to waste gas conversion technology and the efficiency of the subsequent technology used to convert the fuel to energy as power and heat. The highest energy generation outputs are therefore

achieved with the thermal treatment technologies, gasification and incineration. The highest energy output as CHP is about 94 GW h/y (scenario 28). On-site gasification scenarios are mostly in the range 44 GW h/y to 78 GW h/y, while the off-site gasification scenarios are in the range 51 GW h/y to 71 GW h/y. The base case of electricity only ranges from 44 GW h/y for scenario 33 to 57 GW h/y (scenario 26). The top seven of the 24 rMSW scenarios for energy efficiency are for on-site CHP use of the waste gas as a fuel. The biological treatment scenarios have an on-site range from 15 GW h/y (scenario 1) to 46 GW h/y (scenarios 11–12); off-site scenarios range from 10 GW h/y (scenario 5) to 53 GW h/y (scenario 17). The base case of electricity only ranges from 15 GW h/ y for scenario 1 to 32 GW h/y for scenario 10. Incineration output is between 65 GW h/y for electricity only (scenario 40) and 73 GW h/y for scenario 41, on-site CHP. The top two scoring scenarios for avoided CO2 burdens are the same as for energy efficiency (scenarios 28 and 29). In terms of avoided burdens for other scenarios, the differences between technologies arise from the combination of primary conversion technology and the degree of recycling achieved (Table 4). With gasification, the on-site scenarios always achieve a higher avoided burden than the equivalent off-site option, as any avoided burden gained by recycling is credited to the on-site scenario. With biological conversion using AD, off-site use via grid injection of biomethane for CHP achieves the highest avoided burden, as a greater proportion of the waste heat is assumed to be used for displacement of fossil fuel sourced heat. Environmental impacts Environmental impacts are generated at each stage of the waste treatment process i.e. pre-treatment, production of waste gas, recovery of recyclates, gas processing, energy conversion technology and residues to landfill. The impacts produced are on air quality, land as wastes to landfill and potential emissions to water. The major impact on air quality arises from gasification and incineration, which are much greater than for biogas options. Exhaust gas emissions from these thermal processes will be treated in accordance with the permit requirements to comply with the Waste Incineration Directive. Both thermal processes produce residues from the exhaust gas clean-up stage and some ashes produced by the gasification/combustion of the waste that will require landfilling. Digestion produces a solid and a liquid digestate or residue. The solid digestate from source segregated digestion can be treated in accordance with PAS 110 to produce a saleable bio-fertiliser product for use on agricultural land. The CLO from digesting rMSW cannot be used on agricultural land but can be used for landfill cover, on land regeneration sites and for use on land used for energy crops. All impacts on air, land and water will be minimal if the processes are operated in accordance with their permit conditions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seta.2015.03.004.