Energy from Waste Potential in Scotland

Energy from Waste Potential in
Scotland
Quantifying the contribution Energy from Waste could make
to Scotland‟s energy needs
Report to the Scottish Government
January 2010
This report was prepared by Charlie Blair with support from
Phil Matthews, Valerie Caldwell and Maf Smith
Sustainable Development Commission Scotland
Osborne House
Osborne Terrace
Haymarket
Edinburgh
EH12 5HG
0131 625 1880
Scotland@sd-commission.org.uk
www.sd-commission.org.uk/scotland
Contents
Contents ......................................................................................................................................................................................................................... 3
Abbreviations ................................................................................................................................................................................................................ 4
1.
Executive Summary ............................................................................................................................................................................................ 5
1.1
1.2
2.
Methodology....................................................................................................................................................................................................... 7
2.1
2.2
2.3
3.
Information Sources.................................................................................................................................................................................................................... 7
Technologies ................................................................................................................................................................................................................................ 7
Assumptions ................................................................................................................................................................................................................................. 7
Energy Available ............................................................................................................................................................................................... 10
3.1
3.2
3.3
3.4
4.
Project Summary ......................................................................................................................................................................................................................... 5
Summary of Key Findings .......................................................................................................................................................................................................... 5
Base Case..................................................................................................................................................................................................................................... 10
Maximum Energy Case ............................................................................................................................................................................................................. 11
Electricity-only Case .................................................................................................................................................................................................................. 12
Sensitivity.................................................................................................................................................................................................................................... 12
Context .............................................................................................................................................................................................................. 14
4.1
4.2
4.3
4.4
Comparisons ............................................................................................................................................................................................................................... 14
Existing diversions ..................................................................................................................................................................................................................... 14
Future change? .......................................................................................................................................................................................................................... 14
Alternative uses of biogas ........................................................................................................................................................................................................ 14
Appendix One: Technical Assumptions .................................................................................................................................................................... 15
References & Endnotes ............................................................................................................................................................................................... 16
Sustainable Development Commission Scotland
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Energy from Waste Potential in Scotland (2010)
Abbreviations
AD
Anaerobic Digestion
CHP
Combined Heat and Power
C&I
Commercial and Industrial (waste)
CV
Calorific Value
EfW
Energy from Waste – generic term to include all technologies for recovering energy from waste streams. In this report:
Combustion technologies and anaerobic digestion
LCA
Life Cycle Assessment
MBT
Mechanical and Biological Treatment
MSW
Municipal Solid Waste
MW
See Watt (MWe = Megawatt electric; MWth = Megawatt thermal)
ODT
Oven Dried Tonne (woodfuel)
ROC
Renewables Obligation Certificate
SEPA
Scottish Environment Protection Agency
SDC
Sustainable Development Commission (Scotland)
SG
Scottish Government
W
Watt – a unit of capacity. 1,000W = 1kW. 1,000kW = 1MW. 1,000MW = 1GW. 1,000GW = 1TW. In this report we have sought
to display figures using the most appropriate form to avoid large number strings
Wh
Watt Hour – a unit of energy. 1,000Wh = 1kWh. 1,000kWh = 1MWh. 1,000MWh = 1GWh. 1,000GWh = 1TWh. In this report
we have sought to display figures using the most appropriate form to avoid large number strings
Sustainable Development Commission Scotland
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Energy from Waste Potential in Scotland (2010)
1.
Executive Summary
1.1
Project Summary
The Sustainable Development Commission Scotland (SDC), on
behalf of the Scottish Government, has been investigating the
potential for energy from waste (EfW) to provide for electricity
and heat demand in Scotland.
This work follows on from our 2009 study Renewable Heat in
Scotland,1 which provided background data to the Scottish
Government‟s Renewable Heat Action Plan.2
Renewable Heat in Scotland also drew on A Burning Issue; the
SDC‟s earlier advice to Government on the sustainability of
energy from waste.3 A Burning Issue concluded that EfW could
be considered a part of a sustainable waste policy for Scotland,
but set conditions for how it should be developed. These
conditions included setting caps on the level of municipal waste
that should be treated through EfW, setting minimum thermal
performance standards, and doing more to support and
encourage anaerobic digestion.
1.2
Renewable Heat in Scotland highlighted that currently use of
renewable heat equated to 1.4% of Scotland‟s forecast heat
demand, but that this was set to rise significantly over time. We
estimated that projects then in construction would double the
level of renewable heat, and that there were sufficient projects
in development to take renewable heat output to 4.7% of the
total. Included within this total are domestic and commercial
plants that use or plan to use renewable waste sources to
generate heat.
Renewable Heat in Scotland also assessed the potential for
thermal treatment of municipal waste to contribute to
renewable heat needs. However, these initial estimates
highlighted that further study was needed to provide
Government with clear recommendations on what the likely
energy generation would be per tonne of available waste.
The Scottish Government has therefore commissioned the SDC
to provide clear guidance on the potential of waste sources to
contribute to renewable heat and renewable electricity targets.
Summary of Key Findings
Scotland has significant medium term targets for renewable
energy: 11% of all heat by 2020, and 50% of all electricity. This
study analyses existing data on controlled waste streams in
Scotland to assess how much energy (heat and/or electricity)
might be available from waste-to-energy technologies.
The findings demonstrate that energy from waste could make a
contribution to these targets, though it should be noted that
not all energy produced by energy from waste plants would
necessarily be classified as renewable.
Our study shows that EfW in Scotland could contribute
approximately 2.0 TWh of useful heat and 0.90 TWh of
electricity per year. This is equivalent to approximately 3% of
Scotland‟s total heat demand and total electricity demand.
The study takes existing and planned waste targets (Scotland‟s
„Zero Waste Plan‟, currently under consultation) as a baseline.
Thus a 25% cap on combustion of Municipal Solid Waste (MSW)
is taken as a given. Waste diversion scenarios from the Scottish
Government Waste Team are used to estimate proportions of all
waste streams that might potentially be used to recover useful
energy.
from waste streams identified as potentially suitable for
combustion or AD would add up to around 3.5 TWh of useful
heat per year, which equates to around 440 MW of thermal
capacity.4 This is around 6% of Scotland‟s existing heat demand.
Without market support, most large EfW plants are likely to
generate electricity as this is more valuable commodity.
Typically two to three times as much fuel is used to produce a
kWh of electricity than a kWh of useful heat. The 60% overall
efficiency minimum recommended in the Zero Waste Plan
consultation5 effectively ensures that all plant is CHP. The
potential CHP output from identified controlled waste streams
amounts to around 2.0 TWh of useful heat and 0.90 TWh of
electricity per year (245 MWth and 112 MWe capacity). This
electrical output corresponds to around 7% of Scotland‟s
current renewable electricity output; and the thermal output is
two and a half times Scotland‟s current renewable heat output
(note that energy from combustion of waste is not 100%
renewable). Potential CHP output would contribute
approximately 3% of Scotland‟s total heat demand and total
electricity demand, as shown in Figure 1.
.
Direct combustion of solid wastes (aka „incineration‟) and
anaerobic digestion (AD) with biogas capture are the main
energy from waste (EfW) technologies considered.
This report contains outputs from the modelling (energy and
capacity) and a discussion of these findings.
The highest energy output could be achieved from EfW plants if
all thermal output is used for heat production because overall
efficiency is potentially 80% or more. The thermal-only output
Sustainable Development Commission Scotland
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Energy from Waste Potential in Scotland (2010)
Figure 1: Potential CHP Energy from Waste in Scotland
Total Capacity/
Output
CHP thermal
capacity MWth
CHP thermal
output pa MWh
CHP electrical
capacity Mwe
CHP electrical
output pa MWh
CHP overall
capacity MW
CHP Overall
output MWh
245
1,960,224
112
897,434
357
2,857,658
Note: CHP overall output is heat output plus electricity output
Existing landfill gas sites in Scotland are estimated by Scottish
Renewables to have a capacity of 90 MWe (the vast majority is
electric only generation), which equates to around 0.7 TWh per
year.
Sustainable Development Commission Scotland
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This is expected to tail off with a 15 year lag as landfilling of
organic matter ends. EfW could, therefore, more than double all
current generation from landfill gas, while also contributing
significant quantities of low carbon heat.
Energy from Waste Potential in Scotland (2010)
2.
Methodology
Baseline controlled waste figures for 2007/8 and 2008/9 have
been received from the Scottish Government Zero Waste Team.
These form the basis for this report. The basic methodology
involves making assumptions on a) the quantities and energy
content of different waste streams that could potentially be
used for EfW (informed by the Zero Waste Plan currently under
2.1
consultation); and b) the technologies that could be used to
recover useful energy from those waste streams. This enables us
to assess how much energy (MWh) could be generated each
year from EfW in the future, and what generation or heat-raising
capacity (MW) this equates to.
Information Sources
Various datasets on waste in Scotland were received from the
Scottish Government Waste Team. These included:
 Total waste arisings, municipal solid waste (MSW) and
commercial and industrial waste (C&I Waste) (original
data from SEPA)
 Report from Jacobs to SEPA on Development of a policy
framework for the tertiary treatment of commercial and
industrial wastes (Jacobs 2008)
 Modelling data for Annex J of the Zero Waste Plan for
Scotland consultation6
 Landfill Allowance Scheme data for MSW, 2008-9 (original
data from SEPA)
 Compositional analysis of household waste in England
(Scottish equivalent work not yet completed)
 Unpublished SEPA lifecycle assessment of waste disposal
opportunities - includes breakdown of waste arisings
(SEPA 2009).
 Report from ReMade Scotland to SEPA on waste wood
arisings in Scotland: Arisings of Waste Wood from the
Scottish Waste Management Industry (Remade 2009)
2.2
Technologies
Two technologies have been modelled: direct combustion and
anaerobic digestion. Other technologies - such as various
Mechanical Biological Treatments (MBT) - could be used to
recover energy from waste, but the overall energy output is
likely to be lower than from direct combustion. (For example,
when Refuse Derived Fuel from an MBT process is combusted in
an EfW plant total energy output is lower than for direct
combustion of waste – although MBT may be more desirable for
other reasons.)
Direct combustion of waste (also known as incineration with
energy recovery) involves incineration of waste materials on a
fluidized bed or grate, with energy recovery from steam boilers.
Mixed wastes can be burned, but combustion of segregated
waste streams, or residual waste after separation, is more likely
under current plans. We assume a combustion efficiency of 80%
for all waste streams except wood, for which we assume 85%.
Energy from combustion is considered renewable in proportion
to the organic component of the waste stream burned. For
2.3
reference the minimally sorted MSW burned in the Lerwick
incinerator is 71% organic.7 In practise in the UK, EfW plants
generating electricity are eligible for ROCs if the plant is
considered good quality CHP – as modelled here – or if
advanced conversion techniques (such as pyrolysis gasification)
are used, or if solid wastes are converted to liquid form before
combustion.
Anaerobic Digestion of waste streams in an enclosed tank
creates a methane rich biogas as a primary output, as well as
various solid and/or liquid secondary outputs. The biogas can be
burned onsite in boilers to raise heat, or, once cleaned, in
reciprocating engines or gas turbines. Cleaned biogas could be
used offsite in vehicles to meet transport renewable targets; or
fed into the gas pipeline network to displace fossil gas for
heating and cooking. Onsite electricity generation and export to
the grid is assumed in this report.
All energy from anaerobic digestion is considered renewable.
Assumptions
Assumptions on proportions of each waste stream were
developed from Scottish Government Waste Team and SEPA
literature,8 and via discussion with a reference group. A draft of
this report has also been circulated to the reference group for
comment. The group contains individuals from:
 Scottish Government Waste and Pollution Reduction
Division, and Renewables Strategy and Onshore
Renewables Team
 SEPA
 WRAP Scotland
 Scottish Enterprise
Sustainable Development Commission Scotland
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Energy from Waste Potential in Scotland (2010)
 Scottish Renewables
stream considered as potentially suitable for EfW – given
expected statutory and logistical constraints - are shown in Figure
2. (Note, for MSW the base case assumes 25% of the total is
suitable for combustion and 11.9% is suitable for AD).
 Highland Council
 Keep Scotland Beautiful
 Remade Scotland.
Of 15.5 million tonnes 5.5 million tonnes is theoretically
available because it contains either organic or otherwise
combustible matter. Significant proportions of these waste
streams could (and should, according to the Zero Waste Plan) be
re-used, recycled or composted. Quantities of each waste
Figure 2: Waste Streams potentially suitable for EfW (Base Case)
Tonnes
Waste pa
Total MSW Scotland
% incineration
25.0%
3,400,000
Total Construction Waste
% AD
Tonnes Waste for EFW
850,000
11.9%
404,600
9,500,000
Of which: wood
0.7%
66,500
Residual C&IW Scotland
Unspecified paper
496,000
50.0%
248,000
Packaging card
437,000
40.0%
174,800
Unspecified dense plastic
480,250
0.0%
Unspecified wood
68,500
0.0%
Furniture
5,250
0.0%
Garden waste
12,000
90.0%
10,800
Food waste
418,000
90.0%
376,200
A number of technical assumptions needed to be made. These
were developed from discussions with technology experts, and
reference examples of each technology (where possible sites in
Scotland, such as the combustion facilities in Lerwick, Dundee
and Stevenscroft, and Anaerobic Digestion in Stornoway).
alternative potential energy outputs (MWh thermal or electric)
have been calculated:
 Thermal only output, or
 CHP thermal output and CHP electrical output, or
 Electric-only output.
Main technological assumptions are shown in Appendix 1.
Assumptions on efficiency and energy content of fuels have
been taken where feasible from existing plant. This means that
there is certainly scope for technology improvements to
increase the total outputs, particularly of electricity. (Although
basic sensitivity analysis suggests that changing the
assumptions on quantities of waste used for EfW has more
impact on results than assuming technical improvements.) All
assumptions and waste inputs can be changed within the
spreadsheet that accompanies this report.
Thermal-only output will generate a higher total MWh of energy
from waste, but the energy will be less valuable from an
environmental and economic point of view than CHP heat and
electricity.9 Three scenarios have been modelled, reflecting
different proportions of thermal-only, CHP and electric-only EfW
plants, as well as altered assumptions on quantity and quality of
waste going to EfW plants.
Total potential energy from each waste stream can be
calculated by using these assumptions. For each waste stream
and technology (combustion or anaerobic digestion) three
The base case assumes that all waste streams will be sorted,
with proportions of materials being diverted to re-use, recycling
or composting according to the proposed Zero Waste Plan for
Sustainable Development Commission Scotland
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Energy from Waste Potential in Scotland (2010)
Scotland. Remaining proportions of suitable wastes are shown
in Figure 2. In this case the 25% of MSW used in combustion
plant is assumed to have a calorific value of 10MJ/Kg (this,
coincidentally, is the CV of both unsorted MSW and sorted waste
paper) and the 11.9% of MSW used for AD is assumed to be food
waste (which outputs more biogas than unsorted MSW). The
base case assumes that all EfW plants will be CHP – this reflects
the 60% minimum overall efficiency threshold proposed in the
Zero Waste Plan, and the higher value of electricity compared to
heat.
The maximum energy case assumes that combustion plant will
be thermal-only plant (like the Lerwick combustion plant, which
feeds a community heating scheme). Overall efficiency of a
thermal-only incinerator supplying low-grade heat suitable for
district heating schemes can be over 80%, compared to around
65% for CHP or 23% for electric only. The maximum energy case
also assumes that 50% of „unspecified dense plastic‟,
„unspecified wood‟, and „furniture‟ from the C&I waste stream is
combusted (these are not used for EfW in the base case).
The electricity-only case assumes that all EfW plants will be
electricity only (this could be taken as a business-as-usual case,
if the 60% overall efficiency limit is not preserved). This case
keeps all other assumptions the same as the base case. It is
included to give an indication of the very significant resourceuse efficiency advantage of CHP over electricity-only generation.
Sustainable Development Commission Scotland
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Energy from Waste Potential in Scotland (2010)
3.
Energy Available
Energy output per waste stream is given for the base case (CHP
thermal output and CHP electrical output), and for the
maximum energy case and electricity-only energy case.
3.1
Base Case
Figure 3 shows the total energy available from each controlled
waste stream considered. Total energy potentially available per
year from these waste streams is around 1.96 TWh of CHP
thermal output, and 0.90 TWh of electrical output. The
combined total (heat and electricity) is around 2.86 TWh per
year.
Figure 3: Total EfW available under Base Case
Tonnes Waste pa
Total MSW
Total
Construction
Waste
3,400,000
CHP
thermal
capacity
MWth
CHP
thermal
output pa
MWh
CHP
electrical
capacity
Mwe
CHP
electrical
output pa
MWh
CHP
overall
capacity
MW
CHP Overall
output MWh
133
1,062,203
53
424,881
186
1,487,084
13
105,331
13
100,064
26
205,395
0.7%
20
163,590
8
65,436
29
229,026
% incineration
% AD
25.0%
11.9%
9,500,000
wood
C&I Waste
Unspecified
paper
496,000
50.0%
39
309,913
15
123,965
54
433,878
Packaging card
437,000
40.0%
27
218,493
11
87,376
38
305,814
Unspecified
dense plastic
480,250
0.0%
Unspecified
wood
68,500
0.0%
Furniture
5,250
0.0%
Garden waste
12,000
90.0%
0.4
2,812
0.3
2,671
0.7
5,483
Food waste
418,000
90.0%
12
97,937
12
93,041
24
190,978
TOTALS
14,817,000
245
1,960,224
112
897,434
357
2,857,658
Note: The table is a replication of the spreadsheet provided to Government for the calculation of EfW potential. Light Green denotes variables that can
be changed by the user (total tonnes of waste, and percentages used for EfW; as well as all assumptions). Pink denotes thermal output, yellow electrical
output, and orange overall output (the sum of thermal and electrical outputs)).
Landfill gas
Scottish Renewables maintains a database of all renewable
electricity generation in Scotland. As of October 2009, this
includes 90.035 MWe of landfill gas electricity generating
capacity.10 Assuming these plants run for 8,000 hours (this is
probably a slight underestimate, as gas buildup can cause
problems if there is any downtime at landfill gas sites) this
equates to 720,000 MWh of electricity per year.
Existing landfill gas sites have a planned life of at least 15 years,
so something close to this level of generation is likely to
continue to 2020 before tailing off as the level of organic waste
being landfilled declines substantially. Operators report,
however, that as the organic content of waste falls landfill gas
sites are already seeing less gas output per tonne of waste
landfilled.
Sustainable Development Commission Scotland
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Agricultural and Forestry Wastes
Only controlled waste streams, which come under the auspices
of the Environmental Quality Directorate of the Scottish
Government, have been considered in this work. There is
undoubtedly potential for some electricity and/or heat
production from other „waste‟ organic matter, particularly
wood-processing (in-forest residues and processing residues)
and agriculture.
Agricultural wastes – manures, straw, and unmarketable food
produce – are generally disposed of on-farm,11 ensuring that at
least some of the nutrients are returned to the soil. For some
farms it is feasible to recover useful energy from these wastes
via anaerobic digestion while still returning nutrients to the soil
via the digestate sludge. On-farm digesters are common in
Energy from Waste Potential in Scotland (2010)
Germany (4,700 installed up to 2008, with total capacity of
12,000 MWe12 – more or less four times Scotland‟s total installed
renewable electricity capacity), and Scottish Enterprise reports
some interest from farms in Scotland. On-farm anaerobic
digester plants would in most cases combine uncontrolled farm
wastes with controlled food wastes, perhaps brought in from
outside. Farm wastes could therefore add to the 200GWh per
annum of anaerobic digestion electricity identified in this
work.13
Remade Scotland‟s (2009) report for the Scottish Government
(Arisings of Waste Wood from the Scottish Waste Management
Industry) suggests a theoretical maximum of 602,000 tonnes of
3.2
wood waste could be recovered by the waste management
industry in Scotland per year. This compares to a total of
305,000 tonnes of wood waste in Figure 2 (assuming wood is
5% of MSW, as in England), suggesting there may be
significantly more potential for EfW from wood wastes than this
modelling suggests.
A significant amount of uncontrolled wood-waste (mostly
sawmill and papermill wastes) is already used for energy
production; indeed boilers at wood processing sites deliver
around 75% of renewable heat used in Scotland, and E.ON‟s
Locekerbie power station takes waste wood streams as well as
virgin wood.14
Maximum Energy Case
Figure 4 shows that the maximum energy potentially available
from EfW (still broadly following Zero Waste Plan objectives)
could be as much as 4.86 TWh of thermal energy. This is around
70% higher than total (thermal and electric output) of 2.86 TWh
under the base case.
Base case total CHP output is shown for comparison.
Figure 4: Total EfW available under Maximum Energy and Base Cases
Maximum Energy
Thermal only
capacity MWth
Thermal only
output MWh
607
Base Case
4,857,941
357
The increase is explained by the overall efficiency improvement
of using thermal plant, rather than CHP plant (this change on its
own would equate to around 25% increase in output); and the
addition of plastic, wood and furniture waste streams from C&I
Waste (this would equate to around 35% increase).
Heat-only plant is simpler and cheaper to build than CHP plant.
The output is less valuable, however, both financially and
Sustainable Development Commission Scotland
CHP overall
capacity MW
11
CHP Overall
output MWh
2,857,658
environmentally, than CHP output. Significant heat-use from
EfW plants will only be a realistic possibility if there is a stepchange in the way heat is planned for and regulated. For
example, EfW plants would need to be sited near to long-term
heat loads (homes, swimming pools, some industry) and
significant investment and intervention would be needed to
build heat delivery infrastructure.
Energy from Waste Potential in Scotland (2010)
3.3
Electricity-only Case
Figure 5 shows that the total energy available if electric-only
plants are used to recover energy is a little more than a TWh per
year. The Electricity-only Case is identical to the base case,
except that electricity-only plant is used instead of CHP.
The electrical output is around 20% higher in the electric-only
case than for the base case. But the total (CHP) output in the
base case is nearly three times the total output in the electricityonly case. This is as expected, due to the much higher overall
efficiency of CHP compared to electric-only generation. Figure 5
compares the total energy output of the electricity-only case
and the base case.
It is worth noting that the difference in output between CHP
and electricity-only plant is more marked for combustion than
for anaerobic digestion: combustion CHP plant recovers around
2.8x more energy in CHP mode (with around 20% reduction in
electricity output), while anaerobic digestion recovers around
2.0x as much energy in CHP mode (with marginal reductions in
electricity output). This is primarily because the anaerobic
digestion plant uses almost half of the output heat from the gas
engine to maintain digestion temperatures.
Figure 5: Total EfW available under Electricity-only and Base Cases
Electricity Only
Electrical only
capacity MWth
Electricity only
output MWh
134
3.4
Base Case
1,073,502
CHP overall
capacity MW
357
CHP Overall
output MWh
2,857,658
Sensitivity
Composition of MSW
We have assumed in the base case that MSW combusted has an
energy content of 10 MJ/kg. This is a mid case from various
references. Sorted wood or paper/card streams have a similar
calorific value, and would produce around the same amount of
energy as minimally-sorted MSW if burned in boilers designed
for mixed waste. Sorted waste streams could be burned in
custom designed boilers, though, which might recover up to
10% more energy – and all the heat and electricity produced
would be considered renewable for organic waste streams.15
Sorted plastics have a significantly higher CV than MSW (or
paper) – SEPA‟s life cycle assessment of options for C&I waste
disposal suggests 22 MJ/kg. If the 25% of MSW used for
combustion EfW was 100% plastic then the total output (under
otherwise base case assumptions) would be around 4.6 TWh of
heat and electricity (580 MW combined CHP capacity). This is
more than 1.5x the base case CHP output of 2.9 TWh.16
Assessments of household waste composition in England,
however, suggest that plastics make up just 7% of MSW – and
none of the additional heat or electricity would be considered
renewable.
Proportions of waste
Figure 2 showed the size of the waste streams technically
suitable for EfW, and our estimates (developed with the Scottish
Government) of the proportions that might be politically and
logistically suitable. Obviously, increasing the total tonnage or
the percentages available for EfW would increase the total
energy output per year.
Sustainable Development Commission Scotland
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Assessments of household waste stream content suggest that
17% of all MSW is food waste. If all of this was processed in
anaerobic digestion plants (instead of the 70% assumed in the
base case) the total CHP output would be 2.95 TWh instead of
2.86 TWh. In fact assessments in England suggest that another
20% of MSW is garden waste. Including this in the anaerobic
digestion line could take the total to 3.29 TWh. Doing so would,
of course, change the likely energy content of the MSW allowed
for combustion, possibly reducing the combustion output, and
almost certainly reducing its renewable proportion.
Similarly, increasing the proportion of „packaging card‟ from
40% to 90% and „unspecified paper‟ from the C&I Waste stream
from 50% to 90% would increase the total energy output per
year from 2.86 TWh to 3.59 TWh (this calculation is included for
demonstration only – Scotland is committed to recovering and
recycling 60% of C&I packaging card).
Technical Efficiency assumptions
Assumptions on technical efficiency were taken from existing
plant in the UK, and from discussions with experts about the
current state of best practise. We do not expect the electrical or
total efficiency of combustion plants to increase by more than
5% before 2020. Increasing the electrical efficiency of
combustion plant by 5% (keeping parasitic energy consumption
and thermal efficiency constant, thereby increasing overall
efficiency by 5%) would increase CHP electrical output from 0.90
to 1.07 TWh. Total CHP output would increase from 2.86 to 3.03
TWh.
Efficiencies of biogas generators could increase significantly if
fuel cell generators become more efficient and more affordable.
If electrical efficiencies of anaerobic digestion plant increased
Energy from Waste Potential in Scotland (2010)
from 33% to 50%17 - with a corresponding reduction in thermal
efficiency from 35% to 15% - then CHP electrical output would
increase to from 0.90 TWh to 1.00 TWh, and overall CHP output
drop slightly from 2.86 to 2.84 TWh (with combustion plant
efficiencies remaining at base case level). If higher proportions
of waste streams were processed through anaerobic digestion
(or a MBT that produced a biogas output) then the electricity
output increase would be correspondingly higher.
Sustainable Development Commission Scotland
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Energy from Waste Potential in Scotland (2010)
4.
Context
4.1
Comparisons
Baseline energy from waste that could be available is around 3
TWh: 1 TWh electric and 2 TWh thermal.
Taking electricity output and demand figures from the 2008
Scottish Energy Study,18 and Scottish Government figures on
total heat demand, proportions of total energy demand which
could be met by base case EfW output are as follows:
Demand
Potential EfW
output
Electricity
35 TWh
0.90 TWh
2.6%
Heat
60.1 TWh
1.96 TWh
3.3%
% EfW
contribution
comes from SDC Scotland‟s work for the Scottish Government
on Renewable Heat (2009). The comparison highlights that the
potential level of EfW from heat is 2.3 times the current
renewable heat output.
Output
Potential EfW
output
% EfW
contribution
Renewable
Electricity19
(2005)
12.4 TWh
0.90 TWh
7.3%
Renewable
Heat (2008)
0.85 TWh
1.96 TWh
230%
Comparisons with current renewable energy output may also be
interesting, and suggest that EfW‟s contribution could be far
from insignificant. (Note: combustion EfW is not 100%
renewable.) The current renewable heat figure of 0.85 TWh
4.2
Existing diversions
To give some context to the feasibility of diverting this much
waste into sorted streams for EfW, only 254,000 tonnes of card
and paper and 54,000 tonnes of wood are currently recycled in
Scotland, compared to a theoretical maximum identified in
4.3
Future change?
Reducing the total quantities of waste (while keeping the
proportions used for EfW steady) would reduce the yearly
energy output very nearly proportionally. We assume that as
4.4
Table 2 of over a million tonnes. Even if EfW does become a
preferred waste disposal method for all wastes up to the
planned Zero Waste Plan levels the logistical challenges should
not be underestimated.
Scotland moves towards a Zero Waste society in the wider
sense, waste streams available for EfW will therefore reduce.
Alternative uses of biogas
Biogas from anaerobic digestion, if cleaned, is a high-quality
fuel and may in future be more valuable as a transport fuel than
as a CHP fuel. Basecase anaerobic digestion biogas production
could displace some 0.5 TWh worth (equivalent to around 50
Gigalitres of petrol, assuming petrol has an energy content of 10
kWh/litre) of fossil transport fuel. For comparison, Scotland‟s
total oil demand in 2010 is estimated at 4.2 TWh.20
Similarly cleaned biogas might be used in fuel cells with
electrical efficiencies of 50% or more (perhaps having been fed
into the gas network to enable maximum use of CHP heat). This
would double the amount of electricity available from basecase
anaerobic digestion, as mentioned in section 3.4.
Sustainable Development Commission Scotland
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Energy from Waste Potential in Scotland (2010)
Appendix One: Technical Assumptions
Technical Assumptions:
Source
Notes
Running time per year hrs
8000
assumed
Thermal plant efficiency COMBUSTION
80%
Adrian Judge, Juniper/ other literature /
estimate
CHP plant efficiency, heat COMBUSTION
45%
Adrian Judge, Juniper/ other literature /
estimate
CHP plant efficiency, electric COMBUSTION
18%
Adrian Judge, Juniper/ other literature /
estimate
Includes 10% parasitic
Electric plant efficiency COMBUSTION
23%
Adrian Judge, Juniper/ other literature /
estimate
Includes 10% parasitic
MWh per tonne MSW/paper COMBUSTION
2.78
Adrian Judge, Juniper/ other literature /
estimate
from CV of 10MJ/kg
MWh per tonne plastic COMBUSTION
6.11
SEPA LCA of C&I W options / derived
From CV of 22MJ/kg
Biogas yield (60% methane) m3 /
tonne food waste
125
Jacobs report.
CV of methane 35.7MJ/m3.
Biogas yield (60% methane) m3 /
tonne unsorted MSW
75
As above, with AJ proportions between food
waste ouput and MSW output
MWh per tonne food waste - AD
0.74
From above, CV of methane 35.7 MJ/m3
MWh per tonne MSW - AD
0.45
From above, CV of methane 35.7 MJ/m3
Biogas plant efficiency, heat – AD
72%
Assumed 90% with 20% parasitics
Biogas CHP plant efficiency, heat AD
35%
Jacobs report.
Includes 30% parasitics (digester)
Biogas CHP plant efficiency,
electric - AD
33%
Jacobs report.
Includes 5% parasitics
Biogas plant efficiency, electric
only - AD
33%
Jacobs report.
Includes 5% parasitics
Thermal plant efficiency - WOOD
COMBUSTION
85%
Renewable heat work; checked Jacobs
CHP plant efficiency, heat - WOOD
COMBUSTION
50%
Renewable heat work; checked Jacobs
CHP plant efficiency, electric WOOD COMBUSTION
20%
Renewable heat work; checked Jacobs
Electric only plant efficiency WOOD COMBUSTION
25%
Stevens Croft reference: 28% when burning
waste wood
Assumes 10% parasitic
MWh per tonne wood/cardboard WOOD COMBUSTION
4.92
Renewable heat work
Assumes wood waste has same CV as
ODT wood
Sustainable Development Commission Scotland
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Energy from Waste Potential in Scotland (2010)
References & Endnotes
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Sustainable Development Commission (2009) Renewable Heat in Scotland. Report to the Scottish Government.
Scottish Government (2009) Renewable Heat Action Plan: a plan for the promotion of the use of heat from renewable sources.
Sustainable Development Commission (2007) A Burning Issue. Report to the Scottish Government.
This result is the heat-only output of modeling the base case scenario. The „maximum energy‟ case also adds additional quantities of
waste (see the Methodology section for explanation of these scenarios).
See footnote 6 of the Zero Waste Plan consultation:
Annex II to the Waste Framework Directive then gives a non-exhaustive list of recovery operations. This “includes incineration
facilities dedicated to the processing of municipal solid waste only where their energy efficiency is equal to or above:
0,60 for installations in operation and permitted in accordance with applicable Community legislation before 1 January 2009.
0,65 for installations permitted after 31 December 2008”.
See: www.scotland.gov.uk/Publications/2009/08/19141153/55.
A published figure, but sourced from personal communications with Jim Grant of Lerwick Heat Energy & Power Ltd
Based on discussions with Scottish Government on EfW potential.
Until we have a 100% renewable electricity supply, with over-supply, electricity from fuel will always be more valuable than heat
from fuel. This is because heat raising boilers use fuel at around 90% efficiency, while electricity-only generators use fuel at around
30% efficiency. This explains why electricity prices in the UK are typically 3x gas prices; and why CHP is all other things being equal a better solution for EfW than heat-only plant. However, the market is complex and access to the Renewables Obligation (Electricity),
Renewable Transport Fuels Obligation and the forthcoming Renewable Heat Incentive, mean that developers must also consider
what stimulus to seek support under.
Scottish Government Waste Team has quoted a SEPA estimate of 67.5 MWe, but this appears to be out of date
Farms are exempt from certain restrictions on waste disposal – including restrictions on burning waste with energy recovery.
Numbers from the German Biogas Association, reported by Delta Energy & Environment, personal communications
This number for AD electricity is the electric output of AD CHP under the base-case (this would be very similar if all AD plant was
electric-only).
Remade Scotland (1999), and Forestry Commission Scotland woodfuel usage surveys.
We initially assumed that paper and card had energy content similar to oven-dried wood. This would have doubled the output from
the „unspecified paper‟ and „packaging card‟ streams of C&I Waste, and increased the total yearly energy output in the base case
from 2.86 TWh to 3.57 TWh.
The output from just the 25% MSW output (row 3 of the spreadsheet) is 3.27 TWh, compared to 1.49 under the base case.
At least one CHP engine developer is targeting electrical efficiency of 50% for an SOFC fuel cell: Wärtsilä with Topsoe Fuel Cells.
See: www.scotland.gov.uk/Publications/2008/11/14093227/7 Table 13.
Renewable electricity includes large hydro and does not include nuclear.
See: www.scotland.gov.uk/Publications/2008/11/14093227/6.
ISBN 978 0 7559 9319 2 (Web only publication)
Sustainable Development Commission Scotland
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Energy from Waste Potential in Scotland (2010)
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