DOI: http://dx.doi.org/10.1016/j.enconman.2012.03.005
ENERGY CONVERSION AND MANAGEMENT
DEVELOPMENTS TO AN EXISTING CITY-WIDE DISTRICT ENERGY NETWORK: PART II
– ANALYSIS OF ENVIRONMENTAL AND ECONOMIC IMPACTS1
Karen N. Finneya*, Qun Chen a†, Vida N. Sharifia, Jim Swithenbanka, Andy Nolanb, Simon Whitec and
Simon Ogdenb
a
Sheffield University Waste Incineration Centre (SUWIC), Department of Chemical and Biological
Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK
b
Sheffield City Council, Town Hall, Surrey Street, Sheffield, S1 2HH, UK
c
Creativesheffield, The Fountain Precinct, Balm Green, Sheffield, S1 2JA, UK
*Corresponding author. Tel: +44-114-2227563, Fax: +44-114-2227501, Email: [email protected]
†Present address: Department of Thermal Engineering, Tsinghua University, Beijing 100084, China
ABSTRACT
District heating can provide cost-effective and low-carbon energy to local populations.
Although this is rare in the UK, Sheffield already has an award-winning district energy
network. It has been previously determined that this could be expanded to incorporate new
heat sources and sinks. This paper determines the environmental and socio-economic impacts,
focussing on various fuels. Combined-heat-and-power generation in Sheffield coupled with
sustainable/renewable fuels, like waste, offer high efficiencies (>77%) and consistently lower
carbon emission factors (0.04-0.14 kg/MJ) than conventional energy generation using fossil
fuels, since up to 80% of the fuel-carbon is biogenic (CO2-neutral). Processing municipal waste
into a refuse-derived fuel prior to combustion or lowering the return-water temperature by
35°C in the district heating network could further improve efficiencies (81-93%) and reduce CO2
emission rates by 4 t/hr for the Sheffield plant, increasing avoided emissions from 69,000 t/a to
80,000-91,000 t/a. Moreover, ways in which the energy supply could be further decarbonised
were identified, as well as methods to minimise the impacts of responding to changes in
demand. Though initial costs of such schemes are high, they can be economically-viable for the
investor/operator and consequently offer competitive rates for customers. Financial support is
also available through government-backed schemes.
Abbreviations: CESP – Community Energy Saving Programme; CHP – combined heat and power; GIS –
geographical information systems; HESS – Heat and Energy Saving Strategy; RES – Renewable Energy
Strategy; RHI – Renewable Heat Incentive.
1
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Keywords: District heating; network expansion; environmental impact; economics; municipal
solid waste; solid recovered fuels.
1. INTRODUCTION
District/community heating systems based on combined-heat-and-power (CHP) generation can
have a range of benefits – offering cost-effective and low-carbon energy to local populations, in
terms of electricity, space heating/cooling and hot/cold water. The lower carbon emissions
from such schemes compared to conventional energy generation are due to both the
technologies and fuels used. Co-/tri-generation systems make use of ‘waste’ process heat and
operate at higher efficiencies, recovering significantly more energy. Not only does this reduce
fuel consumption to aid resource conservation, but it also decreases pollutant generation for
the same energy output – aiding the achievement of national/international CO2 emissionreduction targets. Carbon emissions can be lowered further through utilising renewable
and/or sustainable fuels, like biomass or wastes; this also means overall costs can be
considerably lower compared to energy generation from fossil fuels. Increasing the amount of
heat generated from distributed energy, namely district heating, can be both a sustainable and
secure means of meeting heat demands.
1.1 District Heating in Sheffield
1.1.1 The Current Situation
Although decentralised energy, and district heating in particular, are rare in the UK, Sheffield
already has an award-winning district energy network; currently the largest and most
successful in the UK [1]. The energy recovery facility (ERF), operated by Veolia Environmental
Services in conjunction with Sheffield City Council, combusts 28 t/hr of non-recyclable
municipal solid waste (MSW), providing heat and power to local populations and preventing
the release of ~21,000 t/a of CO2 [1]. There are presently over 140 buildings served by this, in
addition to ~3000 residential environments. Heat loss in the network is ~2%, but increases
with reduced heat demands.
1.1.2 Potential Future Expansions
The rationale for extending the network here includes providing sustainable, local energy and
reducing carbon emissions.
Expansion opportunities were consequently investigated;
domestic heat demands in the city are vast (>1500 MW), whilst other heat loads totalled 53 MW
for industrial, commercial, educational, health, council and leisure buildings [2]. Potential
suppliers of heat to meet these demands were also outlined, including a new E.ON biomass
facility, incorporating heat recovery [3]. Through identifying these heat sources and sinks,
areas where an extension to the existing network would be possible were delineated.
Additional heat pipelines could redistribute this heat throughout the city to meet the above
demands.
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1.2 Analysing the Impacts of District Energy
Environmental and financial aspects of distributed energy systems need to be assessed prior to
the planning, development and deployment of infrastructure. This section gives a background
to the analyses performed in Sections 2 and 3 below.
1.2.1 Environmental Analysis: Carbon Emissions, Carbon Savings and Carbon Footprints
The domestic-sector carbon footprint in Ireland was calculated by Kenny and Gray [4], who
determined the total annual household energy-use emissions for a home of three people. This
varied significantly (5735-11,515 kg/a), depending on the model employed. Carbon emissions
associated with energy use represent a considerable proportion of total household CO2
generation – around half to two thirds [4]. The total heat demand (space heating plus hot
water) accounts for ~82% of residential energy use [5]. Minimising carbon emissions from this
source, such as through renewably-fuelled district heating, would be an excellent way of
helping achieve national CO2 emission reduction targets.
A number of legislative policies (Carbon Emissions Reduction Target Scheme, Community
Energy Saving Programme and the Heat and Energy Saving Strategy – CERTs, CESP and HESS
respectively) have summarised the carbon savings that could be attained through district
heating. As shown from Tables 1 and 2, both annual and overall lifetime carbon savings can be
significant for CHP schemes and community/district heating, although abatement costs can
often be high (up to £195/t of CO2), depending on the energy source [6,7]. Although it is not
stated to what these values are compared for Table 1, it can be assumed that it would either be
the heat generating technologies used in the current housing stock (mainly gas-fired boilers) or
a baseline of gas-fired central heating. Table 2 in particular details carbon savings from district
heating compared to domestic-scale heat production. Schemes utilising renewable/sustainable
fuels, like biomass and the products of anaerobic digestion, result in the largest carbon savings.
Using wastes (MSW or biomass waste products) is especially attractive since this will also lower
fuel purchasing costs. Moreover, this will not compete with crops grown specifically for other
purposes, such as food production or for the transportation sector (e.g. bio-diesel).
RENEWABLE
HEATING
TECHNOLOGY
LIFETIME
(yrs)
Wood Chip CHP
Community GroundSource Heat Pump
Community Wood
Chip Heater
CARBON SAVINGS
HOUSEHOLD COSTS
ANNUAL
(kg of CO2)
LIFETIME
(t of CO2)
ENERGY COST INSTALLATION
SAVINGS (£/yr)
COST (£)
30
3438.12
96.5 - 107.1
443.17 - 491.80
9463 - 9579
40
545.61
20.4 - 22.7
7.87 - 8.74
4463 - 4486
30
3791.73
106.4 - 118.1
142.90 - 158.58
418 - 430
Table 1: Carbon savings and annual energy cost/bill savings for households and the total
household installation/administration costs of different heat technologies under the Carbon
Emissions Reduction Target Scheme.
Data Source: DECC [6]
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HEATING TECHNOLOGY
DOI: http://dx.doi.org/10.1016/j.enconman.2012.03.005
CO2 SAVINGS ANNUAL COST OF ABATEMENT
(kg/a/home)
HEAT (£/MWh) COSTS (£/tCO2)
Baseline (gas boilers and electric heating)
biomass CHP air turbine
natural gas engine
District Heating:
community natural gas boiler
Small-Scale
community biomass boiler
anaerobic digestion CHP
District Heating:
natural gas engine
Medium-Scale
CHP biomass steam turbine
small natural gas CCGTs
District Heating: medium natural gas CCGTs
Large-Scale
EfW incineration CHP
biomass steam turbine
4020
1000
~0
2890
5950
1785
3520
2245
2645
3325
4245
70
120
105
90
105
215
100
110
115
115
205
100
55
165
55
115
85
55
100
80
195
30
Table 2: Potential annual CO2 savings, annual cost of heat per household and abatement
2
costs for district heating in an area of medium heat density (~3000 kW/km ), compared to a
baseline of conventional heating systems (individual gas condensing boilers and electric
heating), as outlined in the Heat and Energy Saving Strategy.
Data Source: DECC [7]
Lund, et al. [8] also noted that substantial carbon emission reductions can be achieved through
district heating. Other studies [9-12] have quantified the environmental impacts associated
with decentralised energy. It has been reported that MSW-fuelled CHP schemes can reduce
emissions by up to 76% compared to conventional generation [9,10]. Eriksson, et al. [11]
investigated the life cycle assessments of different fuels used in CHP district heating schemes.
By comparing waste, biomass and natural gas, it was found that biofuel combustion
(waste/biomass) is most favourable for CHP applications in terms of environmental aspects,
such as the global warming potential (GWP) of emissions. Genon, et al. [12] found that
replacing domestic gas-/oil-fired boilers with natural gas-fired CHP schemes reduced the
overall environmental impacts. Global implications were significantly reduced (lowering CO2
emissions by ~36%), but local impacts were more complex. SOx (oxides of sulphur) were
completely minimised (often to <0.1 t/a) and particulates could be reduced by up to 50%; NOx
(oxides of nitrogen) however were likely to change very little, though levels would remain low
(up to 13.2 t/a for a system with a peak thermal output of 22 MW).
1.2.2 Economic Analysis
Several policies, such as CERTs, CESP and HESS, outlined the cost savings (on household
energy bills) that can be attained through community/district heating. Table 1 showed that
wood-chip CHPs would not only offer large carbon savings, but also noteworthy cost savings,
even though initial costs can be high (~£10,000) – this would still result in long-term savings of
nearly £4000 [13]. HESS also considered the cost of heat for these (Table 2). Many CHP options
used for district heating on various scales offer significant carbon savings, though often
resulting in high heat costs compared to the baseline (>£200 in some cases compared to ~£70
for the baseline) [7]. Despite this, the cost of CO2 abatement would not necessarily be high,
thus these can be cost-effective methods of reducing CO2.
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Østergaard, et al. [14] investigated how the energy needs for a Danish municipality could be met
with 100% renewable energy (geothermal, wind and biomass, incorporating district heating). It
was concluded that costs would be comparable to the reference fossil-fuel scenario, since
investment, operation and maintenance costs would be higher, but fuel costs significantly
lower (up to €22.4/GJ for fossil fuels, but €0-6/GJ for waste/biomass). This would have positive
local socio-economic impacts. Furthermore, energy systems that are sourced entirely from
renewables have the primary objective of reducing carbon emissions for climate change
mitigation; moreover, improved efficiencies and energy conversions can also result in socioeconomic benefits [15]. Li, et al. [16] investigated the integration of cooling cycles into CHP
systems (trigeneration), through the addition of absorption heat exchangers; this can enhance
overall economics. A greater temperature drop can be utilised to enhance efficiency –
increasing the heat delivery capacity and reducing investment costs for the pipe network; this
would offset the purchasing price of absorption heat exchanger units.
As more of the housing stock uses better insulation and other energy efficiency measures are
introduced (due to new European-wide legislation), the competitiveness of district heating may
be affected in the future, if heat demands from this sector decrease. Persson and Werner [17]
investigated this; future capital costs would be relatively low, especially in inner-city regions,
where there would be a lower risk of reduced competitiveness. A market share of 60% in
Belgium, Germany, France and the Netherlands, up from 21%, could be achieved with marginal
distribution capital costs (€2.1/GJ). It would not be competitive, however, in areas of low heat
density – e.g. sparsely-populated, rural regions. Patlitzianas [18] identified that the largest
constraint of implementing energy-efficient schemes in Greece is the lack of appropriate and
competitive financial support; this is likely to be the case in many European countries since
funding mechanisms are similar. The role of JESSICA instrumentation (Joint European
Support for Sustainable Investment in City Areas) was consequently investigated; it was
concluded that this scheme would be dependent on the willingness and capacity of the public
sector at all scales.
1.3 Research Objectives
This research was divided into two key areas. The first was to investigate the potential
expansions of Sheffield’s existing district energy network, detailed in Part I [2]. The second
aimed to assess the relative merits of extensions, in terms of the environmental and economic
impacts, looking specifically at carbon and cost savings.
2. ENVIRONMENTAL IMPACT ASSESSMENTS
A case-study analysis was conducted to quantify the efficiencies and emissions from district
heating, with a focus on the Sheffield ERF. The environmental impacts of responding to
changes in energy demand were also outlined, in addition to ways in which energy generation
can be further decarbonised.
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2.1 Potential Carbon Savings: Case-Study Analysis
2.1.1 Description of Cases
For the environmental evaluation, three primary cases were based on different scales of energy
generation (local/domestic, distributed and centralised) and different fuel types: fossil fuels
(gas and coal), MSW and a refuse-derived fuel (RDF) – solid recovered fuel (SRF). A summary of
each case is given in Table 3, including the feedrate and calorific value (CV) of the fuels;
electrical and thermal energy outputs are also detailed. A series of calculations were
performed to assess the environmental impacts of each.
CASE
Case A
I
B
A
Case B
II
C
D
Case III
ENERGY OUTPUT
NET CV
ELECTRICITY
HEAT
(MJ/kg)
(MWe)
(MWth)
1500 t/hr coal
28.9
4000
3
3.25 m /hr gas
44.7
28.4
19
9.4
60.3
30 t/hr MSW
(gross)
7.94
47.5
7.94
59.2
10.1 t/hr SRF
25.5
8.4
50.4
FUEL
FEEDRATE
CO2 EMISSION
RATE (t/hr)
3748
6.4
24.3
19.9
CO2 EMISSION
FACTOR
(kg/MJ)
0.26
0.06
0.355 (0.14)
0.112 (0.04)
-
Table 3: Overview of the different cases used in the environmental and economic analyses of
CHP district heating, including the fuel feedrate and energy content, as well as the results
for energy generation and carbon emissions. Emission factors in brackets are based on the
ratio of biogenic and non-renewable carbon in the fuel.
2.1.1.1 Case I – Fossil Fuel-Fired Systems
This ‘baseline’ involved the combustion of fossil fuels for the separate generation of heat and
power, widely used throughout the UK; two sub-cases were considered. Case I-A was for
electricity generation only, at a centralised coal-fired power station, based on the Drax Plant –
the largest in the UK, which generates 7% of our electrical power and has a capacity of 3960
MW [19]. Fossil fuels remain the primary resource for electricity generation, however coal is
the most carbon-intensive; CO2 emissions from electricity-generating power stations were
estimated to be 909 t/GWh for coal, but much lower for gas (398 t/GWh) and the total electricity
generating mix (458 t/GWh), for all fuels, including nuclear and renewables [20]. Case I-B, for
heat production only, was based on small (30 kW), domestic-scale heat production using a
natural gas-fired condensing boiler. Typical models offer efficiencies of ~90%, based on the
lower heating value of the fuel. Gas was chosen as the reference, as most (>83%) domestic
space/water heating comes from this source; <2% of domestic energy consumption for heating
comes from solid fuels, with 6.7% and 8.3% for electricity and oil respectively [21].
2.1.1.2 Case II – MSW-Fired CHP System
The second set of cases was based around MSW combustion at Sheffield’s ERF.
Thermodynamic calculations were performed to obtain mass flowrates, temperatures and
enthalpies for all streams, using heat and mass balances. The typical composition of MSW in
the UK was used to represent the fuel properties at this plant [22]. The MSW feedrate was 30
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t/hr in these computations, giving a net energy input of 72 MWth, based on the CV (Table 3).
Calculations were conducted for four sub-cases. Case II-A was for electricity production only,
whereas Case II-B was for district heating only. Cases II-C (representing the current operation
of this plant) and II-D were both for cogeneration; Case II-D has a lower return-water
temperature (30°C compared to 65°C). As MSW has a high moisture content (26-33% [22]), the
latent heat of water vapour in the flue gas is high; this portion of low-grade heat can be
recovered using flue gas condensers and utilised as an additional energy source for district
heating. The flue gases from the incinerator are used to preheat the return water and thus
some of the latent heat can be recovered.
2.1.1.3 Case III – SRF-Fired CHP System
For this case, the fuel for Sheffield’s ERF is assumed to be SRF (fuel-switching from MSW). SRF
has improved properties and composition compared to MSW, such as lower moisture and ash
contents, with more carbon and a higher CV (Table 4). Due to the lower moisture, the water
vapour fraction in the flue gas after combustion is low; as the dew point of the flue gas is ~36°C,
the latent heat of water vapour is difficult to recover.
COMPOSITION AND PROPERTIES
Proximate Analysis
(%, as received)
Calorific Value (MJ/kg)
moisture
volatile matter
ash
fixed carbon
gross
net
MSW
RDF
SRF
31.4
36.7
27.8
4.1
9.4
-
3.7
67.6
18.9
9.8
20.8
2.6
77.8
14.8
4.8
25.5
Table 4: Fuel properties and characteristics – comparison of municipal solid waste [22],
refuse-derived fuel [23] and solid recovered fuel [24].
2.1.2 Results for Electrical and Thermal Efficiencies
Table 3 details the heat and/or electricity outputs for all cases; the energy conversion or
utilisation efficiencies are summarised in Figure 1. Since energy losses are mainly due to the
flue gas released to the atmosphere without heat recovery, the thermal efficiencies for Cases I-B
and II-B are high (>80%). As the gas-fired boiler in Case I-B condenses the flue gases, the
efficiency of this case is particularly high (93%). In Case I-A, as the thermodynamic parameters
of superheated steam are set higher than those in Cases II and III, the electrical efficiency is
the highest (33%); the steam temperature for Case I-A was 563°C at 166 bar, compared to 400°C
at 40 bar for Cases II and III. For CHP generation, steam is taken off from the turbine under 5
bar, to reject heat at ~150°C for district heating purposes. This lowers the overall electrical
efficiency to 11% for Cases II-C and II-D from the high of 26.4% in Case II-A, where the plant was
only generating electricity. However, for electricity-only generation (Cases I-A and II-A), a large
amount of heat is wasted, thus a maximum electrical efficiency of 26-33% is achieved in these
cases. An important point to bear in mind however is that electricity is a ‘high-quality’ product
and thus is more valuable, whereas low-grade heat cannot be formed into a high-grade/quality
product (exergy content of the outputs).
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100
90
80
Efficiency (%)
Efficiency
(%)
70
60
thermal
thermal
50
electrical
electrical
40
30
20
10
0
Case I-A Case I-B Case II-A Case II-B Case II-C Case II-D Case III
Scenario
Scenario
Figure 1: Comparison of the electrical and thermal efficiencies for each case.
CHP technology captures a certain amount of thermal energy for heating purposes. Although
electrical efficiencies are inevitably reduced, thermal efficiencies are greatly increased, so
overall plant efficiencies can be much greater. The total efficiency of the plant in Cases II-C, IID and III are all above 75%, exceeding 90% for Case II-D. The Sheffield plant uses a backpressure system, designed to allow variations in the heat-to-power ratio; whilst these are often
used to improve electrical efficiencies, in this case, thermal energy recovery was maximised, as
the heat demand fluctuates much more than that for electricity.
2.1.3 Savings in CO2 Emissions
The calculated CO2 emission rates and emission factors are summarised in Table 3. Emission
factors are not provided for Cases II-C, II-D or III since these are for cogeneration and the
outputs of electricity and heat do not have the same exergy level. The emission factors from
MSW-fired generation (Cases II-A and II-B) appear to be higher than for Cases I-A and I-B.
However, various assessments have shown that only 20-40% (depending on the degree of
separate collection of paper and organic waste) of the carbon in MSW is derived from fossil
sources, such as plastics, and deemed to be non-renewable; the remainder is bio-derived and
can therefore be considered ‘renewable’ [25]. Consequently, the non-renewable portion of CO2
from MSW-fired power generation would be 0.07-0.14 kg/MJ for Case II-A (20-40% of the 0.36
kg/MJ emission factor). Similarly, the non-renewable CO2 from MSW-fired heat production
(Case II-B) would be 0.02-0.04 kg/MJ. These are lower than for the coal-fired plant (Case I-A)
and the gas-fired boiler (Case I-B). Emissions for the present UK electricity generation mix
have been estimated to be around half that of coal-fired plants [20]; emissions from Case II-A
would therefore be similar to this.
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As demonstrated then, energy recovery from MSW results in lower net CO2 emissions than
fossil fuels. Annual CO2 reductions for Cases II and III are shown in Figure 2, compared to the
Case I baseline; the avoided carbon emissions achievable would be 2.28 kg/s for Case II-A, 0.96
kg/s for II-B and 2.21 kg/s for II-C. These calculated reductions however by no means take into
account the avoided emissions from the use of MSW compared to landfilling. As the energy in
MSW is recovered for heat and power generation, the well-known emissions from traditional
landfills are negated; this however is not quantifiable at this stage. If the MSW was consigned
to landfill, then 50-100 kg of methane could be released per tonne of waste. Given the higher
GWP of methane (23, compared to the reference gas, CO2 = 1), this is equivalent to ~1600 kg of
CO2/t of MSW. In modern landfills about half the methane can be extracted and used for
energy production, reducing overall emissions from this source [25]. Mühle, et al. [26] report
the specific GWPs of energy-from-waste and landfill in the UK are 132 and 415.1 kg of CO2
equivalent per tonne of MSW respectively; the carbon footprints of these disposal options are
11.4 and 272.5 kg of CO2 equivalent correspondingly.
SRF gives a lower CO2 emission rate than MSW (comparison of Case III to its equivalent, Case
II-C, in Table 3), since the energy content is higher (Table 4), thus greater efficiencies can be
achieved (Figure 1). The renewable carbon content in SRF is reported to be 50-55% [27], thus
non-renewable CO2 emissions from Case III are ~9.55 t/hr. Consequently, carbon savings would
be 2.55 kg/s, with CO2 reductions of 80,000 t/a (Figure 2). Others [e.g. 28] have determined
higher values (68%) for the biogenic fraction in SRF, thus carbon savings could be greater.
Although only an assessment encompassing the whole energy lifecycle – from conversion to
delivery (including transport) – can give a realistic picture of carbon emissions, those from SRF
preparation (~2.4%) and transportation (<0.1%) are minor compared to combustion emissions
[29]. Moisture removal would be the most energy-intensive and costly process during
preparation, thus low-grade waste heat is often used for biofuel drying, to lower carbon
emissions and enhance energy savings [30].
2.1.4 Influence of Flue Gas Condensation on CO2 Emissions
The influence of low-grade latent heat recovery on CO2 reductions was also analysed. As shown
in Figure 1, heat recovery in Case II-D has a distinct advantage, since this improves the thermal
and thus the overall efficiency of the system, resulting in greater CO2 reductions than Case II-C.
In Case II-D, CO2 savings are high (2.92 kg/s), thus CO2 reductions are ~91,000 t/a (Figure 2). The
net CO2 reductions achievable by using a lower return-water temperature (Case II-D) or SRF
(Case III) are greater than for an equivalent MSW-fuelled system with a higher return-water
temperature (Case II-C). As flue gas condensation can recover a certain amount of latent heat,
it leads to significant reductions in CO2 emissions. The reason why emission savings are so low
for Case II-B compared to Case II-A (both single generation) is due to the differences to what
these are compared. As demonstrated in Figure 2, the use of waste (MSW/RDF) for energy
generation always results in considerable CO2 savings compared to the fossil-fuelled baseline.
Even if the UK electricity generation mix was used as a comparison, significant emissions
would still avoided.
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100
80
3
3
Avoided
(x 10
t/a)
AvoidedCO
CO
t/a)
2 Emissions (x10
2 Emissions
90
70
60
50
40
30
20
10
0
Case II-A
Case II-B
Case II-C
Case II-D
Case III
Scenario
Figure 2: Comparison of the results for the annual avoided CO2 emissions for Cases II and III,
compared to the Case I baselines.
2.1.5 Impacts on Other Flue Gas Emissions
In addition to reductions in CO2, another major benefit associated with energy recovery from
MSW is the reduction in a variety of other pollutants that have detrimental health and/or
environmental implications. Emission limited values for MSW incinerators are more stringent
than for coal-fired power stations. Using the best available techniques, such facilities have
reduced their emissions considerably over the last ten years, by a factor of 10 or more, due to
enhanced legislative controls [31]. In particular, dioxin emissions have been reduced to well
below those of other combustion processes under the regulation of the Waste Incineration
Directive [32]. However, the best available techniques for flue gas treatment installed in recent
industrial units built in Europe often have emissions that are significantly lower than those
imposed by law. Therefore, assuming that the emission factors must be equal to the emission
limits appears to be somewhat optimistic [33].
Table 5 compares direct measurements at state-of-the-art combustors (a coal-fired plant and
gas-fired boiler) with emission factors determined for MSW (Cases II-A and II-B herein). NOx,
PM10 (particulate matter <10 µm), non-methane volatile organic compounds (NMVOC), SOx and
HCl from the medium-scale combustion of MSW (for heat or electricity) have significantly
lower emission factors than large-scale, coal combustion. The results for Case II-B are
approximately half those for Case II-A, since the efficiency of energy conversion is that much
higher for heat production (Figure 1). As shown in Figure 2, carbon savings for Case II-A are
high (70,000 t/a). From Table 5, it can be seen that MSW combustion in Case II-A results in 88
g/kWh of CO2 being saved compared to electricity generation in coal-fired plants.
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POLLUTANT
COAL-FIRED
PLANT
DOMESTIC
GAS BOILER
MSW
CASE II-A
MSW
CASE II-B
CO2 (g/kWh)
CO (mg/kWh)
PM10 (mg/kWh)
SOx (as SO2, mg/kWh)
NOx (as NO2, mg/kWh)
N2O (mg/kWh)
NH3 (mg/kWh)
HCl (mg/kWh)
HF (mg/kWh)
Cd (µg/kWh)
Hg (µg/kWh)
Pb (µg/kWh)
NMVOC (mg/kWh)
Dioxin (I-TEQ, pg/kWh)
759
41
130
4399
1938
3.0
133
38
2.8
92
62
77
41
238
57.6
0.36
3.6
212.4
3.6
0.234
18
-
671
96
19
77
1350
158
19
68
7
96
96
963
32
489
334
48
9
38
672
79
9
34
3
48
48
479
16
244
Table 5: Emission factors for a coal-fired plant [34], a domestic gas boiler [34] and for MSW
combustion in Cases II-A and II-B.
When assessing MSW combustion for heat-only generation, the CO2 savings compared to gasfired boilers are less apparent. Some carbon in MSW however is biogenic, sometimes up to 80%
[25]; this bio-derived portion is carbon neutral, which means the net CO2 emissions from this
source would be lower than indicated in Table 5. It is worth noting though that the emissions
from different coal-fired plants for electricity generation can vary considerably; the same is
also true for different gas-fired boilers for residential heating.
2.2 Environmental Impacts of Responding to Changes in Demand
There are often temporal (and sometimes spatial) variations in the heat demand throughout
the network, both daily and seasonally. It is therefore necessary to carefully manage the heatto-power ratio in CHP plants in response to these. The heat-power balance can be changed to
ensure that excess heat is not produced when it is unwanted (off-peak/base load) and thus
electricity generation can be increased during these periods for transmission to the National
Grid. Although this can work to a certain extent, constantly switching the heat-to-power ratio
may not result in the most economic or environmental method of energy generation. In such
circumstances, other options can be used to generate the most efficient ratio, in terms of the
costs and environmental impacts.
One way of dealing with or mitigating the impacts of both short- and long-term fluctuations in
the energy supply and demand, specifically for changes in heat load (the most common demand
variation), is by using thermal energy storage. The main heat storage technologies are
classified as chemical storage, sensible heat storage and latent heat storage (through phase
change media). The full capacity of schemes is not always used, for example, overnight (base
load), when energy demand tends to be significantly lower than the morning/day peak load;
storage of surplus heat could meet later peak demands. This is something that will be integral
to meeting future energy demands – predominantly to enhance the benefits of CHP and
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decentralised energy [35]. Other countries, such as Denmark already utilise such technologies
in numerous schemes [36].
2.3 Decarbonising the Energy Supply
Heating contributes significantly to UK carbon emissions, as detailed in Section 2.1, thus
reducing emissions from this source is vital to meet highly-ambitious carbon emission
reduction targets. There are a variety of ways in which the energy supply here can be
decarbonised further; expanding district energy networks, integrating energy management
and/or efficiency measures and using lower-carbon fuels are considered briefly. Integrating
these into the current and expanded network in Sheffield would provide considerable
environmental benefits. Firstly, expanding the existing energy network in Sheffield could
further minimise carbon emissions from energy generation. Both the current and future
energy centres (considered in [2]) are based on high-efficiency cogeneration from renewable
and/or sustainable fuels, which can result in considerable avoided emissions (Table 2).
Secondly, energy management/efficiency measures could be introduced. Thermal storage could
be integrated into the existing/proposed systems to help meet the peak and off-peak (base load)
demands; this can also minimise overall costs [37]. Integrating some form of cooling system,
such as absorption chillers for cold water and/or air-conditioning, into the CHP process can be
beneficial. This ensures excess heat is not wasted and systems have superior operational
flexibility, as well as additional environmental, financial and energetic advantages compared to
CHP [37,38]. The lower energy costs are especially beneficial where fuel prices are high.
Thirdly, the case-study analysis above revealed that using processed MSW (RDF/SRF) would
provide additional environmental benefits, since the fuel properties are much improved. This
can achieve higher efficiencies whilst attaining lower CO2 emission factors and thus higher
avoided carbon levels (Figure 2). Moreover, this would minimise waste going to landfill.
Recovering low-grade heat from industrial processes could also significantly decarbonise
energy. The use of CHP and district heating can enable other renewables that often have
intermittent supply capabilities, like solar thermal or wind, to be integrated into the heat and
power supply more easily [39,40]. A number of other low-carbon fuels and renewable and/or
sustainable energy sources could be used at a range of spatial scales, such as ‘microgeneration’;
financial support is available for the installation of small-scale technologies [41-43]. Not only
would these lower carbon emissions, but also other harmful pollutants, like SOx and NOx.
3. ECONOMIC EVALUATION OF DISTRICT HEATING NETWORKS
3.1 Potential Cost Savings: Case-Study Analysis
3.1.1 Assumptions Used
A major drawback to waste combustion is that the fuel is notably heterogeneous, in both
composition and properties, particularly particle size. MSW also contains a significant amount
of moisture and ash, which reduce its energy content, negatively impacting combustion. The
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lack of homogeneity results in inconsistent combustion, causing fluctuations in the emissions,
adding to the difficulty of cleanup; this impedes designing for maximum efficiency. Turning
MSW into RDF however can reduce the volume of waste sent to landfill and simultaneously
recover embodied energy from the material [44]. Both SRF and RDF are produced from nonhazardous MSW and commercial/industrial wastes. SRF is a refined form of RDF, intended for
use in ERFs, produced to meet a published standard [45]. During SRF production,
recyclable/non-combustible materials are removed and the remainder is dried, shredded and
processed into a uniform fuel with an energy content much higher than the original MSW
(Table 4). Energy utilisation for this process has been reported to be 889 MJ/t of MSW for pretreatment and mechanical heat-treatment (including drying) and 81 MJ/t of SRF for
pelletisation [46]. Although the fuel quality of SRF/RDF is much improved compared to MSW
and makes for more efficient combustion, the overall cost of production is a major
disadvantage [47]; consequently, payback periods can be considerable. Sorting, drying and
pelletising MSW is capital-intensive, as well as time consuming, and thus needs to be done on a
large scale (in plants with feedrates >1000 t/d) to be economically-viable [22]. Revenue from the
sale of fuel has to be guaranteed – for example in the case of a district energy scheme, like that
in Sheffield. This necessitates an assured partnership with an incineration company (and a
network operator for heat distribution) for an extended period so the plant, and in fact all
involved partners, make a profit [48].
Herein, a simplified cost analysis is performed to evaluate the potential benefits for a wastefired CHP district heating system using SRF (fuel-switching). This analysis compares the costs
entailed with SRF production versus the financial benefits of recovering energy from SRF,
instead of from MSW directly. This is based on the assumption that the existing MSW
collection, transportation and incinerator systems do not need upgrading and thus no other
costs are entailed. The costs associated with fuel replacement include the initial purchase of
machinery, as well as the system operating and maintenance costs. As there is a negligible
difference in the renewable biomass content between MSW and SRF, the loss of Renewable
Obligation Certificates can be neglected.
3.1.2 Capital Costs of the Facility for SRF Production
Firstly, the capital costs of the SRF production facility were calculated. To transform MSW to
SRF, a mechanical-biological treatment (MBT) plant is needed to improve fuel characteristics.
The principle of MBT is to stabilise and separate waste into less harmful/more beneficial
output streams and is a generic term for the integration of several processes [31]. These are
designed to handle raw, ‘black bag’ waste, after source-segregation; for Sheffield, this includes
removing paper, glass and cans, but not ‘green’ waste. MBT involves a ‘recycle-recovery’
element and drying/partial composting, to produce a more stable residue. The remainder is
screened/sorted and homogenised to produce a feedstock for another treatment process, for
example, RDF/SRF for energy recovery.
The capital cost for a small MBT plant with a MSW treatment capacity of 88,000 t/a is
approximately €27m [49]. The capital cost of a larger facility treating 240,000 t/a of waste can
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be scaled with capacity/power by an exponent 0.75 [33]. Hence the proposed MBT facility
would have a capital cost of €57.3m (Table 6). Generally, the conversion rate for MSW to SRF is
about 50%, thus the output of SRF would be 120,000 t/a. For a facility of sufficient size to
process this amount of waste/fuel, the land requirement would be 3-4 ha [31].
ITEM
Costs
Revenue
capital costs
OPEX
CAPEX
gate fee
extra electricity
extra heat
SRF sale
REFERENCE COST
€57,300,000
€3,400,000/a
€29/t of SRF
€5,600,000/a
€49/t of SRF
€7,000,000/a
€30/t of SRF
€325,000/a
€0.031 kWh
€5,600,000/a
€0.23 kWh
€360,000/a
€11.6/t of SRF
Table 6: Summary of the cost-benefit analysis for the economic case-study analysis.
3.1.3 Annual Capital and Operation/Maintenance Expenditure for SRF Production
Secondly, the annual capital and operation expenditures (CAPEX and OPEX) for SRF
manufacture were determined. The operating and maintenance costs reported in the literature
for SRF production plants vary widely; Monson, et al. [49], for instance, estimated €35-55/t of
SRF; approximately half of this is for exhaust air treatment. Some technology suppliers’
figures show the operational costs to be €10-35/t of SRF. The total OPEX for this case would be
around €3.4m/a, based on a cost of €29/t of SRF (Table 6). Also shown in Table 6 is the CAPEX;
€49-116/t of SRF should be included for this, which totals ~€5.6m/a [50].
3.1.4 Cost-Benefit Analysis
Based on the above data, a cost-benefit analysis was completed and the payback period
calculated. Potentially, the gate fee to a MBT facility for SRF production is different from that
to an incinerator, often higher. Gate fees for existing incineration facilities are €37-91/t, with a
median value of €57/t [50]. The major potential revenue received from replacing MSW with
SRF is from the sale of increased amounts of electricity and heat generated (Table 3, comparing
Cases II-C and III), as well as through the sale of ‘recyclate’. Given that the CV of SRF is 25.5
MJ/kg and a fuel feedrate of 10.1 t/hr would be used in the combustion facility (Table 3), the
consumption of SRF would be in the region of 88,000 t/a under CHP operation. The other
32,000 t of SRF produced at the facility treating 240,000 t/a of waste could be sold at €11-12/t to
generate additional revenue.
As shown in Table 3 (for Cases II-C and III), replacing unprocessed MSW with SRF for
cogeneration increases both the electricity output (by 0.46 MWe) and the heat output (by 2.90
MWth). This is equivalent to ~4,000,000 kWh/a of electricity and ~25,000,000 kWh/a of heat.
Given a purchase price of €0.083/kWh for electricity [51], the profit from this would be
€325,000/a (Table 6). The price charged for district heating could be as high as €0.23/kWh; thus
the additional heat could increase revenue by ~€5.6m/a [52]. In the existing UK market, the
users of waste-derived fuels demand a gate fee in the range of €23-58/t, irrespective of its quality
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or energy value [53]. An informative price for SRF is -€1.70/GJ (-€35/t) [53]. However, there are
already signs that the market is changing, with continental users offering to pay a small cost
per tonne, and UK producers exporting SRF to continental users in the face of an increasing
demand [53]. In this case, the sale price of the remaining SRF product is assumed to be €11-12/t,
providing the transportation costs and gate fee outweighed the net benefits [51]. Annual
revenue from the sale of the remainder SRF would thus be €360,000.
Total expenditure is ~€9m/a (OPEX plus CAPEX), whereas total revenue from the sales of the
heat, power and excess fuel, as well as the gate fee, is ~€13.3m/a. Based on the differences
between these, the payback period would be ~13 years. After the plant has been paid for
outright, profits would accumulate at €4.3m/a, leading to an overall profit of ~€30m after 20
years operation.
It should be noted, however, that there are also a number of risks associated with SRF
production, such as those involving the planning stages and the risk that the technology will
not achieve the required performance levels. This could threaten the availability of the
products as they would not meet the specification for end-use and may be sent to landfill as a
result [49]. This would mean that the carbon reductions that could be accrued through the use
of MSW (either directly or through the use of the RDF produced from it) would not be achieved.
3.2 Customer/Socio-Economic Benefits
The main benefit to customers of distributed energy in general and district heating in
particular is the reduced energy bills. The use of waste as the fuel source will mean energy bills
can stay low when the cost of other fuels (like coal and gas) used for energy generation are
increasing. Expanding the district energy network in Sheffield will mean more people here can
have access to affordable heat and power – crucial for tackling fuel poverty in the city. In
addition to the economic benefits of cost savings on energy bills, there would also be savings on
the capital and maintenance costs. Additionally, there is likely to be socio-economic benefits
provided through increased employment, at least during the construction phase, but also for
the maintenance/operation of the network, energy facilitates and related infrastructure. The
expansion of the existing network in Sheffield could therefore provide jobs for local people at
several stages of the development.
Furthermore, a range of other advantages to customers can be achieved through district
heating, not just based on economic factors. Firstly, there will be benefits derived from the
reliability and security of supply; homes generally do not have their own back-up system,
whereas networked schemes have stand-by boilers for periods of maintenance. Secondly, space
savings could also be seen, since the heat exchangers utilised would be considerably smaller
than the gas boilers they would replace. Thirdly, there would be environmental benefits, such
as those outlined above for carbon savings primarily, but also for a range of other emissions
that cause health and environmental issues, such as global warming and acid rain. This will
benefit the entire local population – not just those connected to the district heating scheme.
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3.3 Potential Sources of Funding
There are several potential sources of funding available for distributed energy technologies
through government-backed schemes/initiatives, offering grants or financial assistance for
cogenerated heat and power. The Renewable Heat Incentive, for example, could financially aid
the expansion(s) in Sheffield [54]. Other potential sources of funding include FP7 – Smart
Cities: Heat and Cooling Systems, the European Regional Development Fund, Environmental
Transformation Fund, Yorkshire Forward and JESSICA, which provide finances for sustainable
urban renewal and development.
These schemes can make developments more financially attractive overall, although an indepth analysis of these complex issues is beyond the scope of this work at this early stage of the
investigation. The impacts of individual programmes change with location, due to the
variations in local circumstances. Such projects can generally still be feasible without these
funding mechanisms, which tend to be more influential and vital to the overall economicviability of smaller-scale, microgeneration systems.
4. CONCLUSIONS
District heating is well known in other countries for providing cost-effective and low-carbon
energy to local populations. Although Sheffield already has such a scheme in operation, it has
been determined that expansions here are possible and an excellent way to further the benefits
of decentralised energy in the city. This paper determined the environmental and economic
implications of decentralised energy, focussing on the use of various fuels.
The environmental analysis of the Sheffield plant showed that using MSW/SRF can offer high
efficiencies (>77%) and low net carbon emission factors (0.04-0.14 kg/MJ) compared to energy
generation from fossil fuels. Cogeneration, which recovers the ‘waste’ heat from electricity
production, would achieve the highest avoided CO2 emissions: 70,000 t/a using MSW. Fuelswitching (to SRF) or using a lower return-water temperature for district heating (30°C
compared to 65°C) would increase the avoided carbon emissions to 80,000 t/a and 90,000 t/a
respectively. Other pollutants, like NOx, SOx and particulates would also be emitted in smaller
quantities with waste-derived fuels than conventional energy generation. Other ways to
minimise the environmental impacts of energy production were also considered, including the
use of additional renewable energy sources and integrating thermal energy storage.
The economic evaluation revealed that the production of SRF fuels in Sheffield, via the
installation of an MBT facility, could be economically-viable, generating enough fuel for the
current ERF. Whilst the capital and annual costs would be high (€57.3m and €9m/a
respectively), based on the annual revenue from the gate fee and selling heat, power and excess
fuel (€13.3m/a), the payback period for a plant would be 13 years.
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In conclusion, Sheffield’s existing decentralised energy scheme is ideal for expansions to
redistribute the available/surplus heat in the city. The environmental and economic analyses
have shown that such developments can be hugely beneficial, producing low-carbon and lowcost energy for the surrounding populations. By increasing the size of this network, these
benefits can be more widespread and the scheme made more economically-viable through
financial support mechanisms offered by the government. This should again highlight to the
rest of the UK that such decentralised schemes are technically, environmentally and
financially feasible; the wider implications of this are the clear opportunities for similar
systems to be utilised elsewhere.
ACKNOWLEDGEMENTS
The authors would like to thank the UK Engineering and Physical Science Research Council
(EPSRC Thermal Management of Industrial Processes Consortium) for their financial support
of this project, as well as Veolia Environmental Services Ltd. and E.ON UK plc. for their
technical support of this work.
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