Assessing the environmental sustainability of energy recovery from municipal solid waste in the UK Academic research paper on "Environmental engineering"

Waste Management xxx (2016) xxx-xxx

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Waste Management

journal homepage: www.elsevier.com/locate/wasman

Assessing the environmental sustainability of energy recovery from municipal solid waste in the UK

H.K. Jeswani, A. Azapagic *

School of Chemical Engineering and Analytical Science, CIS, The Mill, Sackville Street, The University of Manchester, Manchester M13 9PL, UK

ARTICLE INFO

ABSTRACT

Article history: Received 10 August 2015 Revised 29 January 2016 Accepted 8 February 2016 Available online xxxx

Keywords: Energy from waste Fossil fuels Incineration Landfill biogas Life cycle assessment Municipal solid waste

Even though landfilling of waste is the least favourable option in the waste management hierarchy, the majority of municipal solid waste (MSW) in many countries is still landfilled. This represents waste of valuable resources and could lead to higher environmental impacts compared to energy recovered by incineration, even if the landfill gas is recovered. Using life cycle assessment (LCA) as a tool, this paper aims to find out which of the following two options for MSW disposal is more environmentally sustainable: incineration or recovery of biogas from landfills, each producing either electricity or co-generating heat and electricity. The systems are compared on a life cycle basis for two functional units: 'disposal of 1 tonne of MSW' and 'generation of 1 kWh of electricity'. The results indicate that, if both systems are credited for their respective recovered energy and recyclable materials, energy from incineration has much lower impacts than from landfill biogas across all impact categories, except for human toxicity. The impacts of incineration co-generating heat and electricity are negative for nine out of 11 categories as the avoided impacts for the recovered energy and materials are higher than those caused by incineration. By improving the recovery rate of biogas, some impacts of landfilling, such as global warming, depletion of fossil resources, acidification and photochemical smog, would be significantly reduced. However, most impacts of the landfill gas would still be higher than the impacts of incineration, except for global warming and human toxicity. The analysis on the basis of net electricity produced shows that the LCA impacts of electricity from incineration are several times lower in comparison to the impacts of electricity from landfill biogas. Electricity from incineration has significantly lower global warming and several other impacts than electricity from coal and oil but has higher impacts than electricity from natural gas or UK grid. At the UK level, diverting all MSW currently landfilled to incineration with energy recovery would not only avoid the environmental impacts associated with landfilling but, under the current assumptions, would also meet 2.3% of UK's electricity demand and save 2-2.6 million tonnes of greenhouse gas emissions per year.

© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://

creativecommons.org/licenses/by/4.0/).

1. Introduction

Sustainable management of municipal solid waste (MSW) is a critical issue for municipal authorities around the world. Traditional disposal method by landfill is considered to be the least favourable option in the waste management hierarchy, as that wastes valuable resources and gives rise to methane emissions (DEFRA, 2011). Therefore, policies and regulations in many countries, such as the Landfill Directive in Europe (EC, 1999), discourage landfilling and encourage recycling and resource recovery. With the drive towards circular economy gaining momentum, under current proposals, landfilling of all recyclables will be banned in

* Corresponding author. E-mail address: adisa.azapagic@manchester.ac.uk (A. Azapagic).

the EU by 2025, with all disposal by landfill virtually eliminated by 2030 (EC, 2014). In the UK, the landfill tax, which is intended to help the UK meet its targets for reducing the amount of waste being landfilled as stipulated by the EU Landfill Directive, has increased steadily from £7 per tonne of waste in 1996 to £82.6 in 2015, to make landfilling economically unattractive (HM Revenue and Customs, 2015). Owing to these policies, the proportion of MSW disposed of by landfill has decreased in the UK from 70% in 2004 to 34% in 2013 (EC, 2015). However, this is still very high compared to some other EU countries, such as Germany and the Netherlands, where less than 2% of waste is landfilled (EC, 2015). Similarly, the amount of MSW incinerated to recover energy is low: 21% compared to Germany and the Netherlands which incinerate 35% and 49% of their waste, respectively (EC, 2015). One of the main reasons for a low uptake of incineration in the UK is

http://dx.doi.org/10.1016/j.wasman.2016.02.010 0956-053X/© 2016 The Authors. Published by Elsevier Ltd.

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

the opposition of the public because of the perceived health risks from air emissions, increased local pollution and traffic, aesthetics and other concerns (Azapagic, 2011; DEFRA, 2013a; Nixon et al., 2013). Compared to landfills, incinerators also have higher capital and operational costs (Bozorgirad et al., 2013).

Environmental impacts of MSW management have been studied extensively, including a number of life cycle assessment (LCA) studies (for reviews, see e.g. Laurent et al. (2014) and Astrup et al. (2015)). Several of these focused on MSW management in European cities and elsewhere; for example, London (Al-Salem et al., 2014), Liège (Belboom et al., 2013), Rome (Cherubini et al., 2009), Macau (Song et al., 2013), Irkutsk (Tulokhonova and Ulanova, 2013) and Seoul (Yi et al., 2011). They considered various combinations of waste management options, such as landfilling, incineration, recycling, as well as aerobic and anaerobic digestion, to identify the optimal strategies for MSW management at a city level. In general, all of these studies recommend minimising land-filling, increasing recycling and maximising energy recovery from waste fractions with high calorific values.

A number of studies also compared life cycle impacts of waste incineration and landfilling in different countries. For example, Beylot and Villeneuve (2013) considered the environmental performance of 110 incinerators in France and found that, owing to a difference in energy recovery rates, the global warming potential (GWP) varied from -58 to 408 kg CO2 eq./t MSW. Kourkoumpas et al. (2015) also studied the GWP of MSW incineration in France but compared it to incineration in Greece, reporting that the impact of the latter is much lower (-326 kg CO2 eq./t MSW) than in France (172 kg CO2 eq./t MSW). This is due to the higher credits for the avoided greenhouse gas (GHG) emissions for the electricity mix in Greece, which is predominantly lignite based, than for the French grid, which has a high share of nuclear power and thus a lower GWP. In addition to the system credits, in their study of MSW incineration in Italy and Denmark, Turconi et al. (2011) found that factors such as waste composition and incineration technology also affect the environmental performance. Another study in Denmark (Damgaard et al., 2010) found that incineration is an attractive option because of significant developments in air pollution control technologies and energy recovery systems. Liamsanguan and Gheewala (2007) also concluded that the use of air pollution control, such as removal of nitrogen oxides and dioxins, could lead to incinerators in Thailand having comparable or lower impacts than conventional power plants. In a subsequent study, the authors compared the life cycle impacts of landfilling (without energy recovery) and incineration (with energy recovery) in Thailand, finding that incineration was superior to landfilling (Liamsanguan and Gheewala, 2008). However, the latter was a better option if methane was recovered and used for electricity generation. The study by Assamoi et al. (2012) also compared landfilling and incineration but in Canada, focusing on global warming, acidification and eutrophication, while Habib et al. (2013) and Wittmaier et al. (2009) compared the GWP of these two options in Denmark and Germany, respectively.

However, LCA studies of MSW management in the UK are scarce, with only four found in the literature. Two of these (Papageorgiou et al., 2009; Jeswani et al., 2013) focused on the GWP of energy recovery from incineration in a combined heat and power (CHP) plant; in addition, the latter study also considered heat and electricity generation from landfill biogas in comparison to incineration. The remaining two studies (Tunesi, 2011; Al-Salem et al., 2014) assessed LCA impacts of local waste management strategies in England and Greater London, respectively. In addition to the GWP, the former study considered only two other impacts (depletion of resources and acidification) and the latter three categories (acidification, eutrophication and photochemical smog). In this paper, we go beyond the previous studies to estimate

and compare 11 life cycle impacts of MSW incineration and land-filling in the UK, considering both CHP and electricity-only plants. Using the latest waste composition data, the study is first carried out at the level of different waste-to-energy technologies and then extrapolated to estimate the impacts at the national level. As far as we are aware, this is the first study of its kind for the UK.

2. Methods

The LCA has been carried out following the attributional approach and the ISO 14040/44 guidelines (ISO, 2006a,b). The goal of the study, data sources and the assumptions are detailed in the following sections.

2.1. Goal and scope of the study

The goal of the study is to estimate and compare the environmental impacts of MSW disposal by incineration and landfill for the UK conditions, with both systems recovering energy. Two options for energy recovery are considered for each system: generation of electricity only and co-generation of heat and power. To explore how the impacts may be affected by the definition of the functional unit, the options are compared for two units of analysis:

(i) disposal of 1 tonne of MSW; and

(ii) generation of 1 kWh of electricity from MSW.

The incineration and landfilling systems considered in the study are described in turn below.

2.1.1. Incineration

There are currently 25 MSW incinerators with energy recovery in the UK, 80% of which generate electricity and the rest recover both heat and electricity (DEFRA, 2013a; Nixon et al., 2013). Although CHP generation is the most efficient option for utilising energy recovered from waste, it requires infrastructure to supply the heat, such as district heating, which is not common in the UK.

The majority of MSW incinerators in the UK are moving-grate plants and are designed to handle large volumes of MSW without any pre-treatment (DEFRA, 2013a). Fig. 1 shows the life cycle diagram of a typical incineration plant with energy recovery. The system boundary considered here includes the following life cycle stages:

• transport of waste to the incinerator;

• construction of the incinerator;

• incineration of waste;

• flue gas treatment;

• transport and disposal of air pollution control (APC) residue, including fly ash;

• energy recovery and associated energy credits;

• recycling of ferrous metals and the related credit for the avoidance of virgin metals; and

• processing of bottom ash into a road aggregate and the credit for the avoidance of virgin aggregates.

The average composition of MSW in the UK is given in Table 1, with the average lower heating value of 9950 MJ/t (Veolia, 2014a). The waste is assumed to be transported for 45 km to the plant where it is stored in a bunker before being transferred to the incineration chamber. The waste is combusted at temperatures >850 °C; either natural gas or fuel oil is used for the initial start-up and to maintain the high combustion temperatures. To control the emissions of nitrogen oxides, acid gases, heavy metals and dioxins, urea or ammonia, hydrated lime and activated carbon are injected into

Fig. 1. System boundary for the incineration system. [MSW: municipal solid waste; APC: air pollution control (includes fly ash).]

Table 1

Composition of MSW (Harris, 2012).

MSW component MSW composition (WF) (% as wet weight) Fraction of dry matter (DM) Total carbon fraction (CF) in dry weight Fossil (FCF)

Paper 10.5 0.782 0.361 0.01

Card 8.4 0.770 0.395 0.01

Plastic film 8.9 0.642 0.692 1

Dense plastics 11.3 0.885 0.550 1

Sanitary waste 2.2 0.244 0.507 0.53

Wood 7.6 0.873 0.467 0

Textiles and shoes 5.7 0.923 0.521 0.46

Glass 2.2 1 0 0

Food waste 15 0.411 0.373 0

Garden waste 2.7 0.343 0.433 0

Other organic 1.7 0.568 0.445 0

Metals 3.5 1 0 0

Waste electrical and electronic equipment 1.5 1 0 0

Hazardous waste and batteries 1 1 0 0

Carpet, underlay & furniture 6 0.964 0.449 0.39

Other combustibles 2 0.705 0.049 1

Bricks, plaster and soil 5.9 0.740 0.047 1

Other non-combustible 1.6 0.740 0.047 1

Fines <10 mm 2.5 0.830 0.179 0.22

the flue gas; fly ash is removed by bag filters. Ferrous metals are recovered from the bottom ash and recycled locally. The APC residue, including fly ash, are disposed at a hazardous waste disposal facility.

Two types of incinerator are considered in this study with different energy recovery options:

(i) generation of electricity; and

(ii) co-generation of heat and power in a CHP plant.

Both types are based on the actual plants operated in the UK with the data obtained from the annual reports on the performance of the plants submitted by the operator (Veolia, 2014a,b) to the Environment Agency for England and Wales, as detailed below.

(i) Incinerator generating electricity: This is a moving-grate plant with a capacity of over 420,000 tonnes of MSW per year and can generate 29 MW of net electricity (Veolia,

2014b). On average, this translates to 519 kWh of net electricity per tonne of MSW, which is exported to the national grid (Veolia, 2014b). Table 2 lists the inputs and outputs from the incinerator, including air emissions, expressed per tonne of MSW incinerated; the equivalent data per kWh electricity produced are given in Table 3. The APC residue, including fly ash, is sent to a hazardous-waste landfill. The ferrous metals are recovered from the bottom ash and the remaining ash is used as a road-construction material (Veolia, 2014b).

(ii) CHP incinerator: This is also a moving-grate plant but it recovers both electricity and heat, which are exported to the national grid and district heating systems, respectively. The incinerator has an annual capacity of 225,000 tonnes of MSW, about half that of the incinerator generating electricity only. On average, it co-generates 449 kWh of net electricity and 1785 MJ of net heat per tonne of MSW (Veolia, 2014a). The data for this incinerator are summarised in

H.K. Jeswani, A. Azapagic/Waste Management xxxx (2016) xxx-xxx

Table 2

Inputs into and outputs from the incineration and landfill systems per tonne of MSW (Veolia, 2014a,b; ecoinvent, 2008).

Inputs/outputs per tonne of MSW

Incineration

Electricity

Electricity and heat

Landfill biogas

Electricity

Electricity and heat

Consumables Hydrated lime (kg/t) Activated carbon (kg/t) Ammonia/urea (kg/t) Auxiliary fuel (natural gas) (MJ/t) Auxiliary fuel (oil) (MJ/t)

Water (for steam generation and ash quenching) (l/t) Biogas

Total generated (MJ/t) Utilised (MJ/t) Vented (MJ/t) Flared (MJ/t)

Net energy generated (exported to the grid/district heating) Electricity (kWh/t) Heat (MJ/t)

Recovered materials Ferrous metals (kg/t) Bottom ash (kg/t)

Air pollution residue (kg/t) Landfill leachate (l/t)

Air emissions CO2 (fossil) (kg/t) CO (g/t) SO2 (g/t) NOx (kg/t) N2O (g/t) HCl (g/t) NH3 (g/t) HF (g/t)

Particulates, PM10 (g/t)

Dioxins/furans (ng/t)

Polyaromatic hydrocarbons, PAH (mg/t)

Cadmium and thallium (mg/t)

Mercury (mg/t)

Other heavy metals (mg/t)

CH4 (vented biogas) (kg/t)

45.3 288

22.1 219.4

3.0 17.9 92.3 1.7

5.1 7.7 102.9

7.31 0.25 0.53

449 1785

26.6 180.8

452a 23.9 75 0.6

23.1 32.9 0.4 1.1 4.9

36.2 1.0 2.0 6.9 99.6

1102 573 419 110

7.6b 7.1 20.2 5 x 10-

0.5 1.3 0.8 10.5

1102 573 419 110

54 226

7.6b 7.1 20.2 5 x 10-

0.5 1.3 0.8 10.5

a Estimated using waste composition in Table 1 and Eq. (1). The other emissions calculated from annual air monitoring reports for the respective incinerators (Veolia, 2014a,b).

b From biogas flaring and combustion.

Table 2 and Table 3. Like the electricity-only incinerator, the APC residue is disposed of in a hazardous-waste landfill and the bottom ash is processed into aggregates after the recovery of ferrous metals (Veolia, 2014a).

The lifetime of the incinerators is assumed at 20 years (ecoinvent, 2010). Note that, although both incinerators have a similar design using a moving grate, they differ in many other respects, including the capacity, age and pollution control systems, which is reflected in the data differences in Tables 2 and 3. Furthermore, they are located in two different regions within the UK with differing waste compositions, which in turn affects air emissions as well as the amount of ash and metals recovered.

2.1.2. Landfilling

In this study, MSW is assumed to be disposed of in managed sanitary landfills with recovery of biogas and a leachate collection system. Such types of landfills are common in the UK and in the EU. The life cycle of the system is shown in Fig. 2. The system boundary includes the following life cycle stages:

• transport of waste to landfill (45 km, as for incineration);

• landfill construction and operation;

• recovery of energy from a portion of biogas produced and associated energy credits;

• flaring and venting of the remaining biogas;

• landfill leachate management; and

• landfill closure and aftercare (covering and re-vegetating the site, cover repairs, safety, accessibility and other maintenance, and operation and maintenance of landfill gas extraction, utilisation and treatment systems, and wastewater treatment plant and/or discharge systems).

Note that in the EU, landfill aftercare is mandated by the Landfill Directive (EC, 1999). Similar regulations do not exist for incinerators and hence this stage is not considered for the incineration system.

To estimate the amount of biogas produced based on the MSW composition as well as the emissions to the environment, the method proposed by Doka (2009) and the ecoinvent tool for modelling MSW sanitary landfills (ecoinvent, 2008) have been used. Given the waste composition (Table 1), it is estimated that the landfill would generate 1102 MJ of biogas per tonne of MSW (Table 2). Based on the 2012 landfill gas recovery and utilisation data for the UK (EEA, 2014), it is estimated that 38% of landfill gas (419 MJ/t MSW) is vented to the atmosphere. The remaining

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Table 3

Inputs into and outputs from the incineration and landfill systems per kWh of electricity generated from MSW (Veolia, 2014a,b; ecoinvent, 2008).

Inputs/outputs per kWh of electricity

Incineration

Landfill biogas

Electricity

Electricity and heat

Electricity

Electricity and heat

Consumables Hydrated lime (g/kWh) Activated carbon (g/kWh) Ammonia/urea (g/kWh) Auxiliary fuel (natural gas) (MJ/kWh) Auxiliary fuel (oil) (MJ/kWh)

Water (for steam generation and ash quenching) (l/kWh) Biogas

Total generated (MJ/kWh) Utilised (MJ/kWh) Vented (MJ/kWh) Flared (MJ/kWh)

Net energy generated (exported for district heating Heat (MJ/kWh)

Recovered materials Ferrous metals (g/kWh) Bottom ash (g/kWh)

Air pollution residue (g/kWh) Landfill leachate (l/kWh)

Air emissions

CO2 (fossil) (g/kWh)

CO (mg/kWh)

SO2 (mg/kWh)

NOx (g/kWh)

N2O (mg/kWh)

HCl (mg/kWh)

NH3 (mg/kWh)

HF (mg/kWh)

PM10 (mg/kWh)

Dioxins/furans (ng/kWh)

Polyaromatic hydrocarbons, PAH (ig/kWh)

Cadmium and thallium (ig/kWh)

Mercury (ig/kWh)

Other heavy metals (ig/kWh)

CH4 (vented biogas) (g/kWh)

0.1 0.6

42.6 422.7

870.9a

1.7 3.3 62.2 34.5

5.8 34.5 0.2 3.3 9.8 14.8 198.3

16.3 0.6 1.2 0.02

59.2 402.7

1006.7a

53.2 167.0

1.3 51.4

73.3 0.9

2.4 10.9 0.1 2.2

15.4 221.8

19.6 10.2 7.5 2.0

135.2b 126.3 359.0 9 x 10-3

14.2 186.8

20.2 10.5 7.7 2.0

139.6b 130.4 371.0 9 x 10-3

9.2 23.9 14.7 192.8

a Estimated using waste composition in Table 1 and Eq. (1). The other emissions calculated from annual air monitoring reports for the respective incinerators (Veolia, 2014a,b).

From biogas flaring and combustion.

Credit for recovered energy

Fig. 2. System boundary for the landfill biogas system.

62% is recovered as follows: 52% (573 MJ/t MSW) is utilised for energy production and 10% (110 MJ/t MSW) is flared. The amount of leachate from the landfill is estimated at 2500 litres per tonne of MSW (Doka, 2009).

Although in the UK landfill biogas is typically used for electricity generation (EA, 2004), to make the landfill biogas system comparable to the CHP system, both options for energy recovery are considered here:

(i) Generation of electricity from landfill biogas: based on 38% efficiency of electricity generation for spark ignition engines used predominantly in the UK (EA, 2010) and the parasitic consumption of 7% (US EPA, 2014), the exported electricity to grid is estimated to be 56 kWh (202 MJ) per tonne of MSW from the landfill gas (see Table 2).

(ii) Heat and power generation from landfill biogas: it is assumed that the CHP plant has an electrical efficiency of

H.K. Jeswani, A. Azapagic/Waste Management xxxx (2016) xxx-xxx

Photovoltaics 0.5%

Heavy fuel oil 0.6%

Nuclear 18.9%

(a) Electricity mix (DECC, 2014)

Electricity ^ 9%

Solid fuel f 2%

^Gas 81%

(b) Heating fuel mix (DECC, 2012)

Fig. 3. UK electricity and heating fuel mix. (See above-mentioned references for further information.)

36.8% and thermal efficiency of 41.6% (US EPA, 2015). Considering parasitic consumption of 7% for electricity (US EPA, 2014) and 5% distribution losses for the heat (DEFRA, 2013b), 573 MJ per tonne of MSW of utilised biogas will generate 54 kWh of net electricity and 226 MJ of net heat per tonne of MSW (Table 2).

2.2. Data sources

As mentioned previously, the primary data for the operation of the incineration system have been obtained from the annual performance reports of the incinerators (Veolia, 2014a, 2014b). The typical MSW composition in the UK (Table 1) has been used as a basis for estimating the stack emissions of carbon dioxide (CO2) from incineration, as follows (IPCC, 2006):

Eco2 = MSW x R(WFj x DMj x CFj x FCFj x OF/) x 44/12 (t) (1) where

ECO2 CO2 emissions from MSW combustion (t)

MSW amount of municipal solid waste (as wet weight) (t)

WFj fraction of waste component j in the MSW (as wet

weight)

DMj fraction of dry matter content in the component j of

the MSW

CFj fraction of carbon in the dry matter of component j

FCFj fraction of fossil carbon in the total carbon of

component j

OFj oxidation factor (assumed equal to 1)

44/12 conversion from C to CO2.

As also mentioned earlier, the ecoinvent tool for MSW landfilling (ecoinvent, 2008) has been used to model the landfill system for the typical MSW composition in the UK. The life cycle data for the rest of the incineration system and for the landfill system have been sourced from the ecoinvent database (ecoinvent, 2010). The LCA data for the energy and material credits are also from ecoinvent.

2.3. System credits

For the analysis based on the functional unit of 'disposal of 1 tonne of MSW', all the systems have been credited for the outputs that they generate. The incineration systems have been credited for electricity, heat (where applicable) and recycling of steel and bottom ash (see Fig. 1). In the case of landfilling, credits are applied for electricity and heat generation (where applicable) from the recovered biogas. System expansion has been used for these

purposes, following the ISO 14040/44 guidelines (ISO, 2006a,b). The recovered electricity is credited for displacing the equivalent electricity from the UK grid, assuming the electricity mix in 2013 shown in Fig. 3a (DECC, 2014). The fuel mix used for heating in the UK (Fig. 3b) is used to credit the system for heat generation. The LCA impacts for both grid electricity and the heating fuel mix have been estimated using ecoinvent life cycle inventory data. System credits for other energy sources are considered in the sensitivity analysis (Section 3.1.12).

The incineration system has been credited for the amount of steel recovered for recycling by subtracting the impacts of the equivalent amount of virgin metal but adding the impacts of steel recycling, including the losses in the recycling process (9%), following the methodology developed by the World Steel Association (2011). A sensitivity analysis has been carried out to test this assumption using the method proposed by Gala et al. (2015) whereby the system is credited only for the proportion of virgin metal in the average market mix of steel. The system has also been credited for the use of bottom ash in construction by subtracting the impacts of a virgin aggregate (gravel). For the study based on the functional unit 'generation of 1 kWh of electricity', the credits are applied for all outputs except for electricity.

3. Results and discussion

GaBi V6.4 software (PE International, 2014) has been used to model the incineration and landfilling systems and to estimate the environmental impacts. Since the ISO standard (ISO, 2006b) does not specify a particular impact assessment method to be used, the impacts in this study have been estimated according to the CML 2001 method (Guinée et al., 2001), updated in April 2013. The results are presented first for the disposal of 1 tonne of MSW and then for 1 kWh of electricity produced from waste.

3.1. Impacts from the disposal of 1 tonne of MSW with energy recovery

These results are shown in Fig. 4a-k and are discussed for each impact in turn below. Overall, incineration has much lower impacts than landfilling for 10 out of 11 impact categories when both systems are credited for the recovery of energy and materials. One of the main reasons for this is the higher amount of energy recovered per tonne of MSW by incineration than from the biogas (Table 2). Incineration is also a better option for five out of 11 impacts considered, even without the credits for energy and materials recovery.

The contribution of different life cycle stages to the impacts from incineration and landfilling is summarised in Fig. 5. As can be seen from the figure, the main contributors to both systems are the operation and waste management stages. Transport is only

(a) Global Warming Potential (GWP)

□Without credit nWith credit

Incinerator Incinerator Landfill Landfill (electricity (CHP) (electricity (CHP) only) only)

(e) Eutrophication Potential (EP)

□ Without credit nWith credit

3,500 3,000 2,500 2,000 1,500 1,000 500 0

Incinerator Incinerator Landfill Landfill (electricity (CHP) (electricity (CHP) only) only)

(b) Abiotic Depletion Potential (ADP, fossil)

□Without credit nWith credit

17-71 . rrq

Incinerator Incinerator Landfill Landfill (electricity (CHP) (electricity (CHP) only) only)

(f) Freshwater Aquatic Ecotoxicity Potential (FAETP)

□ Without credit oWith credit

2,000 -I

Incinerator Incinerator Landfill Landfill (electricity (CHP) (electricity (CHP) only) only)

(c) Abiotic Depletion Potential (ADP, elements)

□Without credit oWith credit

Incinerator Incinerator Landfill Landfill (electricity (CHP) (electricity (CHP) only) only)

(g) Human Toxicity Potential (HTP)

□ Without credit oWith credit

Landfill Landfill (electricity (CHP)

(d) Acidification Potential (AP)

□Without credit DWith credit

Incinerator Incinerator Landfill Landfill

(electricity (CHP) (electricity (CHP)

only) only)

(h) Marine Aquatic Ecotoxicity Potential (MAETP)

□Without credit QWith credit

1,250 i

Incinerator Incinerator Landfill Landfill (electricity (CHP) (electricity (CHP) only) only)

(i) Ozone Layer Depletion Potential (ODP)

□ Without credit □With credit

15 10 5 0

ÈL k m

Incinerator Incinerator Landfill Landfill (electricity (CHP) (electricity (CHP) only) only)

(j) Photochemical Oxidant Creation Potential (POCP)

□Without credit oWith cred

Incinerator Incinerator Landfill Landfill (electricity (CHP) (electricity (CHP) only) only)

(k) Terrestrial Ecotoxicity

□Without credit

Potential (TETP)

With credit

Incinerator Indnerator (electricity (CHP) only)

Landfill Landfill (electricity (CHP) only)

Fig. 4. Environmental impacts of disposal of 1 tonne of MSW with energy recovery.

100% 80% 60% 440% -20% I 0%

□ Construction 0 Transport @ Operation aWaste management

Fig. 5. Contribution of different life cycle stages to the impacts from MSW incineration and landfilling. [The functional unit: disposal of 1 tonne of MSW with energy recovery. The results do not include system credits. I-elec.: Incineration (electricity only); I-CHP: Incineration (CHP). Landfill: both electricity-only and CHP systems (the contribution of different stages is equal for both systems). Incineration - Construction: construction of the incinerator; Transport: transport of MSW and air pollution control (APC) residue; Operation: incineration of waste and flue gas treatment; Waste management: disposal of APC residue. Landfilling - Construction: construction of the landfill; Transport: MSW transport; Operation: recovery of landfill gas and its combustion, flaring and venting and landfill aftercare; Waste management: treatment and disposal of landfill leachate. For the impacts nomenclature, see Fig. 4.]

significant for the depletion of fossil fuels, elements and the ozone layer. The contribution of construction is small across the impacts and the systems, except for landfilling for the above three impacts and terrestrial ecotoxicity. Note that, since the contributions to the impacts from the landfill with electricity-only and CHP options are the same, "Landfill" in Fig. 5 represents the results for both systems.

3.1.1. Global warming potential

As shown in Fig. 4a, the GWP of the electricity-only and CHP incinerators (without credits) are estimated at 496 and 487 kg CO2 eq./t MSW, respectively. With the credits for the recovered energy and materials, the GWP for the former reduces to 174 kg CO2 eq./t and to 58 kg CO2 eq./t for the latter. The operation stage contributes 96% to the impact (Fig. 5), mostly because of the stack

Fig. 6. Comparison of the global warming potential estimated in this and a previous study of incineration and landfilling

emissions, which are 453 kg CO2 eq./t for the electricity-only plant and 459 kg CO2 eq./t MSW for the CHP incinerator. This is due to the emissions of fossil-derived CO2 from the combustion of waste. Therefore, reducing fossil carbon in the waste would help to reduce the GWP of incinerators; however, this may also affect the calorific value of the waste (e.g. because of a lower amount of plastics) and consequently the amount of energy that can be recovered.

By comparison, the GWP of landfilling (with credits) for the electricity and CHP options is equal to 240 and 223 kg CO2 eq./t MSW, respectively (Fig. 4a). As shown in Fig. 5, the operation stage contributes 92% to the total impact, 95% of which is due to the biogas that is vented to the atmosphere from the landfill system. Thus, increasing the capture rate of biogas is an important parameter for reducing the GWP from this system.

As alluded to in the above discussion, both waste composition and the biogas capture rates are important parameters. For that reason, we compare these results with our previous study (Jeswani et al., 2013), which considered the same incineration and landfill systems with CHP generation but for a different waste composition and biogas capture. In the past decade, the composition of waste in the UK has changed significantly owing to increased recycling and composting, with the proportion of biodegradable waste in the residual MSW, after taking out the recyclables, decreasing from 68% to 51% and the fossil carbon fraction increasing from 30% to 45% (Parfitt, 2002; Harris, 2012). As a consequence, the GHG emissions from incineration have gone up by about 240 kg CO2 eq./t MSW, from -179 kg CO2 eq./t estimated previously to 58 kg CO2 eq./t for the present composition of waste (see Fig. 6). For the landfill system, the opposite trend is found, with the GWP now being lower than before, as indicated in Fig. 6 (223 vs 395 kg CO2 eq./t). Although the reduction of the biodegradable fraction in the waste has meant that 20% less of landfill gas is generated now, the recovery rate has increased from 53% to 62% (EEA, 2014). This means that more energy is recovered at present and the system credits are higher, reducing the overall impact.

We return to the discussion of the influence of the biogas recovery rates in the sensitivity analysis in Section 3.1.12 but prior to that, the results for the other impacts are discussed next.

3.1.2. Abiotic depletion potential (ADP, fossil and ADP, elements)

The depletion of fossil resources (without credits) for the electricity-only incinerator is slightly higher than that of the either of the landfill options: 0.49 vs 0.46 GJ/t (Fig. 4b). However, the impact from the CHP incinerator (0.29 GJ/t) is 67% lower than for the electricity-only plant and 57% lower than the landfill alternatives. This is mainly due to the lower amount of pollution abatement chemicals and natural gas used as an auxiliary fuel

compared to the electricity-only incinerator (Table 2). However, after applying the credits for energy and materials recovery, the impact for the electricity-only and CHP incinerators reduces to -3.07 and -4.88 GJ/t MSW, respectively, indicating a significant saving in fossil resources. Since less energy is recovered in the landfill systems in comparison to the incinerators, the depletion of fossil resources (with credits) for both landfill options is much higher than that of the incinerators: 95 MJ/t for the electricity-only and -150 MJ/t for the CHP option. Construction of the landfill is the main contributor (46%) to this impact owing to the use of diesel in excavation machinery, while the contributions from the operation stage and transport are about 20% each (Fig. 5). On the other hand, the main contributors for the incineration systems are transport and operation because of the fuel used in these stages.

As shown in Fig. 4c, the depletion of elements without system credits is relatively low for both the incinerator and landfill systems, with the CHP incinerator having the lowest impact (31 mg Sb eq./t) and the electricity-only plant the highest (43 mg Sb eq./t); the value for the landfill options is 39 mg Sb eq./t. However, with the credits for electricity generation and recovery of materials, the impact for the electricity and CHP incinerators reduces to -23 and -40 mg Sb eq./t, respectively.

Transport is the major contributor to the depletion of elements, ranging from 47% to 66% for the incinerators to 40% for the landfill (Fig. 5). For the latter, construction and waste management contribute a further 28% each, while for incineration, operation and construction add between 13-36% and 11-16%, respectively. For both systems, this impact is due to the use of resources for constructing the infrastructure.

3.1.3. Acidification potential (AP)

At 176 g SO2 eq./t MSW, the total AP from the landfill system (without the credits) is 3-4 times lower than that of the incineration systems (see Fig. 4d). However, with the credit for the recovery of energy and materials, incineration of MSW results in a net saving of 806 g SO2 eq./t for the CHP and 466 g SO2 eq./t for the electricity plant relative to the landfilling of waste.

As indicated in Fig. 5, the operation stage contributes 90% to the total impact from the incineration systems, 95% of which is due to the emissions of nitrogen and sulphur oxides during the incineration of MSW. By contrast, operation contributes only 44% to the AP from landfilling, with the rest split equally between construction, transport and waste management.

3.1.4. Eutrophication potential (EP)

As shown in Fig. 4e, without the system credits, the EP for energy from landfill biogas is between five and seven times higher than for incineration: 2691 vs 356-526 g PO4 eq./t MSW. This is due to the treatment and disposal of landfill leachate, which is responsible for 99% of the impact. In the case of incineration, the disposal of APC residue contributes to 70% of the impact, with the rest being from the operation stage. The difference in the impact between the two systems is increased further in favour of incineration when the energy and material recovery credits are considered. The best option is the CHP incinerator for which the EP is negative (-46 g PO4 eq./t), followed by the electricity-only incineration with 131 g PO4 eq./t. The effect of the system credits for the landfill systems is small, reducing the impact by only 2%, again because of the dominant role of the leachate for this impact.

3.1.5. Freshwater aquatic ecotoxicity potential (FAETP)

A similar trend can be noticed for this impact (Fig. 4f) as for the EP, with the electricity-only incinerator having 21 times and the CHP plant 31 times lower FAETP than the biogas options (without the system credits). With the credits, the FAETP from the CHP

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incinerator becomes negative (-17 kg DCB eq./t), making it the best option overall. Since the landfill leachate is the sole contributor to this impact (Fig. 5), the landfill systems do not benefit much from the credits, with the FAETP reducing only very slightly, from 1271 to 1264 kg DCB eq./t MSW for both energy recovery options. The leachate is also the reason why the electricity-only incinerator has a 33% higher impact (61 kg DCB eq./t without the credits) than the CHP incinerator: the amount of the APC residue, which is disposed of by landfill, is 34% higher for the former than the latter (Table 2).

3.1.6. Human toxicity potential (HTP)

Before the system credits, the HTP for electricity-only and CHP incinerators is 38% and 3% higher, respectively, than from landfill-ing (see Fig. 4g). The disposal of the APC residue is the major contributor (~90%) to this impact from incineration; see Fig. 5. The contribution of stack emissions, such as heavy metals and dioxins, is negligible since their emissions are low (see Table 2). However, with the system credits, the CHP incinerator has the lowest HTP, which is 2.4 times lower than for the landfill options. Most of these savings are due to the credits for electricity generation. By comparison, the impact from both landfill options remains similar before and after the credits (144 and 132 kg DCB eq./t, respectively) owing to the high contribution (87%) of the leachate. These values are comparable to the HTP of 141 kg DCB eq./t for the electricity-only incinerator, after applying the system credits.

3.1.7. Marine aquatic ecotoxicity potential (MAETP)

The MAETP of the landfill options is also several times higher than for the incinerators, both with and without system credit options (see Fig. 4h). With the credits, the impact is negative for both incinerators, with the savings of 120 and 216 t DCB eq./t for the electricity-only and CHP systems, respectively. Emissions of nickel and other heavy metals to water make the leachate the major contributor (82%) to the MAETP from the landfill options, estimated at 968 t DCB eq./t without and 9311 DCB eq./t with the system credits.

3.1.8. Ozone layer depletion potential (ODP)

As indicated in Fig. 4i, this impact is identical for the electricity-only incineration and landfilling systems before the system credits are taken into account. However, with the credits, the ODP becomes negative for the incinerators, with the CHP option having the greatest saving of 18 mg R11 eq./t MSW, mainly due to the credits for heat recovery. By comparison, the ODP for the landfill CHP option is estimated at 1.7 mg R11 eq./t (with the credits). Construction of landfill causes more than a half (56%) of the ODP from the landfill systems, while operation and transport contribute around a half each to the impact from the electricity-only and the CHP incinerators, respectively (Fig. 5).

3.1.9. Photochemical oxidant creation potential (POCP)

For this impact, landfilling is the worst option both before and after the system credits, with the POCP estimated at 79 g C2H4 eq./t for the electricity-only biogas facility and 75 g C2H4 eq./t for the CHP plant (Fig. 4j). The venting of biogas is the main cause of the POCP, contributing 73% to the total. Both incinerators have a negative POCP after the credits have been applied, with the CHP system saving 84 g C2H4 eq./t and the electricity-only 36 g C2H4 eq./t. The operation stage is responsible for more than 70% of the impact for all the systems, mainly due to the NOx and SO2 emissions.

3.1.10. Terrestrial ecotoxicity potential (TETP)

Before adding the system credits, the TETP for the MSW disposal by landfill is 5.5-9 times higher than for the incineration

options (see Fig. 4k). This is largely due to the heavy metals in the leachate. With the credits, the impact is further reduced for both incinerators: -0.3 kg DCB eq./t MSW for the CHP and 0.02 kg DCB eq./t for the other incinerator option. Most of these savings are due to the credits for electricity generation. The landfill options with lower energy recovery benefit little from the system credits because the TETP is dominated by the leachate, which is responsible for 97% of the impact (Fig. 5). In the case of incineration, the operation stage contributes 50-60% owing to the emissions of mercury and other heavy metals (Table 2).

3.1.11. Comparison of results with other studies

As mentioned in the introduction, quite a few LCA studies of MSW have been carried out, but comparison of the results is difficult because of different functional units, system boundaries, variation in waste composition and geographical regions as well as the assumptions and data sources. Nevertheless, an attempt is made to compare the GWP estimated here with some of the previous studies. Note that comparison of the other impacts is not possible as the studies that have reported them have either used different functional units and system boundaries (e.g., Tunesi, 2011; Al-Salem et al., 2014), or a different impact assessment method (Bozorgirad et al., 2013), or presented only normalised values (e.g., Damgaard et al., 2010; Turconi et al., 2011).

Several previous studies of CHP incinerators in different countries, such as Italy (Consonni et al., 2005; Rigamonti et al., 2009), Germany (Wittmaier et al., 2009) and the UK (Jeswani et al., 2013; Papageorgiou et al., 2009), have reported the net GHG savings ranging from -21 to -230 kg CO2 eq./t MSW. The GWP estimated in this study (58 kg CO2 eq./t MSW) is higher, mainly owing to the differences in the composition of waste. As discussed earlier, because of the increased recovery, recycling and composting, the ratio of fossil to biogenic carbon of the residual waste in the UK has increased, resulting in higher fossil CO2 emissions from incineration.

For the electricity-only incinerators, as mentioned in the introduction, the previous studies in Europe reported a large variation in the GWP, ranging from -326 to 408 kg CO2 eq./t MSW (Kourkoumpas et al., 2015; Beylot and Villeneuve, 2013; Papageorgiou et al., 2009; Cherubini et al., 2009). Thus, the result obtained in the current study (174 kg CO2 eq./t MSW) falls within the reported range. The GWP for landfilling with biogas recovery estimated in the other studies ranges even more broadly than for incineration, from -30 to 1030 kg CO2 eq./t MSW (Beylot et al., 2013; Belboom et al., 2013; Jeswani et al., 2013; Cherubini et al., 2009; Wittmaier et al., 2009) so that the value obtained here (223-240 kg CO2 eq./t MSW) is well within the range. The variation in the results is due to many factors, including different composition ofwaste and the assumptions on venting, recovery and utilisation of biogas. As shown in the sensitivity analysis below, the influence of these factors on the results could be significant.

3.1.12. Sensitivity analysis

It is apparent from the contribution analysis discussed in the previous sections that the credits for electricity and heat generation affect the impacts, particularly from incineration. For the disposal of waste by landfill, the impacts depend on the recovery rate of biogas and the rate of its utilisation for energy. Therefore, these parameters have been varied within the sensitivity analysis as discussed in the following sections. A further analysis has also been carried out to find out if and how the differences in the operational parameters between the two incineration systems affect the results.

3.1.12.1. Electricity credits. In this work, the incineration and landfill systems have been credited for displacing the equivalent

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amount of electricity from the UK grid. Here we consider instead the effect on the impacts if these systems were to replace the individual sources of electricity present in the electricity mix, such as natural gas, fuel oil, coal or nuclear power. As an illustration, only the incineration systems are considered, given that they produce more electricity per tonne of MSW (see Table 2) and are likely to be affected more by these assumptions than the landfill options.

These results are shown in Fig. 7 for the electricity-only incinerator and in Fig. 8 for the CHP plant. As can be seen from the figures, when the systems are credited for displacing electricity from natural gas instead of UK electricity, all the impacts but the ADP fossil and ODP are higher than in the base case. If, on the other hand, it is assumed that the systems displace heavy fuel oil, the impacts for both incinerators are further reduced in comparison to displacing the UK grid electricity; the exceptions to this are the ADP elements,

FAETP and MAETP which are higher than in the base case. Similarly, crediting the systems for displacing electricity from coal also results in lower impacts across all impact categories except for the ADP elements and ODP, compared to the base case. On the other hand, crediting the systems for displacing nuclear electricity, the overall credits for the impacts such as the GWP, AP, EP, ODP and POCP would be lower, leading to the higher overall values for these categories.

3.1.12.2. Heat credits. In the base case, the CHP systems have been credited for displacing the equivalent amount of heat assuming the UK fuel mix for heating. Here we consider how the results may be affected if the systems are credited for the heat from different sources used in the UK, such as natural gas, light fuel oil, coal and grid electricity (see Fig. 3b). Since the incinerator produces

100 80 60 40 20 0 -20 -40 -60 -80 -100

□ UK heating fuel mix (base case)

3 Natural gas

I Fuel oil □ Coal B Electricity

GWP [kg CO2 ADP, fossil ADP, elements AP [g SO2 EP |g PO4 FAETP |kg HTP [kg DCB MAETP [t DCB ODP [mg R11 POCP [g TETP [kg DCB eq./t MSW] [MJ/t MSW] x [mg Sb eq./t eq./t MSW] x eq./t MSW] x DCB eq./t eq./t MSW] x eq./t MSW] x eq./t MSW] C2H4 eq.t eq./t MSW] x 100 MSW] 100 10 MSW] 10 10 MSW] x10 0.1

Fig. 9. The effect of credit options for heat on the environmental impacts of CHP incineration. [The functional unit: disposal of 1 tonne of MSW with energy recovery. The results include system credits. For the impacts nomenclature, see Fig. 4.]

□ Incinerator (electricity only) s Incinerator (CHP)

ADP, fossil ADP, elements

Fig. 10. The effect of credit options for steel recovery on the environmental impacts of incineration. [The functional unit: disposal of 1 tonne of MSW with energy recovery. Systems credited for 50% of virgin steel according to the global market mix of steel, as opposed to 100% assumed in the base case. The results expressed relative to the base case values shown in Fig. 4. For the impacts nomenclature, see Fig. 4.]

more heat per tonne of waste (Table 2), it is likely that it will be affected more by the credits for heat than the landfill option; hence, only the incineration option is considered here. As can be seen in Fig. 9, crediting the systems for displacing heat from natural gas, leads to all the impacts, except the ODP, being higher than in the base case. On the other hand, crediting the systems for displacing heat from fuel oil, coal or electricity leads to further reductions in the impacts across all the categories. In the case of replacing heat from coal or grid electricity, all impacts become negative because of the higher credits for the avoided impacts.

3.1.12.3. Credits for recovered steel. In this work, the incineration systems have been credited for the amount of steel recovered for recycling by subtracting the impacts of the equivalent amount of virgin metal, following the methodology developed by the World Steel Association (2011). However, since the average market mix

is composed of 50% virgin and 50% recycled steel (EUROFER, 2015), this sensitivity analysis considers the credit for 50% of virgin steel as suggested by Gala et al. (2015), instead of 100% assumed in the base case. The results in Fig. 10 show that in that case the GWP would increase by 10% for the electricity-only incinerator and 35% for the CHP incinerator. There is also a significant increase in the POCP (15-32%). The AP and ADP elements would also be higher by 7-10% and 6-9%, respectively. On the other hand, the ODP would go down by 38% for the electricity-only incinerator. The variation in the other impact categories is below 5%.

3.1.12.4. Average operational data for incinerators. As shown in Table 2, there are some variations in consumables and emissions for the two incinerators assessed in this study. This sensitivity analysis is carried out to determine whether the difference in the environmental performance of the incinerators is due to the

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□ Incineration with electricity-only (base case) ■ CHP Incineration (base case)

s Incineration with electricity-only (average data) BCHP Incineration (average data)

GWP [kg CO2 ADP, fossil ADP, AP [g SO2 EP [g PO4 FAETP [kg HTP [kg DCB MAETP [t ODP [mg R11 POCP [g TETP [g DCB eq./t MSW] [MJ/t MSW] x elements [mg eq./t MSW] x eq./t MSW] DCB eq./t eq./t MSW] DCB eq./t eq./t MSW] x C2H4 eq.t eq./t MSW] 100 Sb eq./t 10 MSW] x.01 MSW] 0.1 MSW]

Fig. 11. The effect of operational data on the environmental impacts of incineration. [The functional unit: disposal of 1 tonne of MSW with energy recovery. The results include system credits. For the impacts nomenclature, see Fig. 4.1

Table 4

Sensitivity analysis for landfill gas recovery and utilisation rates.

Scenario

Recovered biogas (%)

Utilised for energy recovery

Flared

Vented biogas (%)

Base case

Scenario 1 (higher recovery

and utilisation) Scenario 2 (lower recovery and utilisation)

variations in operational parameters or due to the differences in energy recovery options. For this purpose, the operational data for the incinerators have been averaged for two incinerators while keeping the energy recovery the same as in the base case. The results in Fig. 11 suggest that there is a significant change in

eutrophication and the toxicity-related impacts for both incinerators compared to the base case, and a relatively small change in the other impacts, including the GWP, ADP fossil and POCP. However, the CHP option still outperforms the electricity-only incinerator for nine out of 11 impacts owing to a higher energy efficiency of CHP systems. For the remaining two impacts (FAETP and MAETP), the electricity-only incinerator is slightly better owing to the higher credits from the electricity. Despite these changes, both incineration systems still outperform landfilling, now across all the impact categories, including the HTP (which is slightly higher in the base case for the electricity-only incinerator than for the landfilling options).

3.1.12.5. Rate of landfill gas recovery and utilisation. The rate of landfill gas recovery and utilisation depends on the landfill design, operation and regulations. In the UK, the Environment Agency (EA, 2004) recommends that new landfills should recover 85% of

Fig. 12. The effect on the environmental impacts of landfill gas recovery and utilisation. [The functional unit: disposal of 1 tonne of MSW with energy recovery. The results include system credits. For the impacts nomenclature, see Fig. 4.]

□ Landfill (base case, electricity only)

0 Landfill (higher recovery of biogas, electricity only)

□ Landfill (lower recovery of biogas, electricity only) ■ Landfill (base case, CHP)

a Landfill (higher recovery of biogas, CHP) a Landfill (lower recovery of biogas, CHP)

GWP [kg CO2 ADP, fossil ADP, elements AP [g SO2 eq./t MSW] [MJ/t MSW] [mg Sb eq./t eq./t MSW] MSW]

EP [g PO4 eq./t MSW] x 10

FAETP [kg DCB eq./t MSW] x 10

HTP [kg DCB MAETP [t DCB ODP [mg R11 eq./t MSW] eq./t MSW] x eq./t MSW] x 10 0.1

POCP [g C2H4 eq.t MSW]

TETP [kg DCB eq./t MSW] x 0.1

H.K. Jeswani, A. Azapagic/Waste Management xxx (2016) xxx-xxx

80 70 60 50

□ Incinerator (electricity only) ■ Incinerator (CHP)

□ Landfill (electricity only) H Landfill (CHP)

^ CD CD О or

GWP [kg ADP, fossil ADP, AP [g SO2 EP [g PO4 CO2 [MJ/kWh] elements [mg eq./kWh] eq./kWh] eq./kWh] Sb eq./kWh]

FAETP [kg HTP [kg DCB MAETP [t

DCB eq./kWh] DCB eq./kWh] eq./kWh]

ODP [mg

R11 eq./kWh] x 0.01

POCP [g

C2H4 eq./kWh]

TETP [g

DCB eq./kWh]

Fig. 13. Environmental impacts of generation of 1 kWh of electricity from MSW. [The results include system credits. For the impacts nomenclature, see Fig. 4.]

landfill gas. By comparison, 62% has been considered in the base case (see Section 2.1.2). In some other studies, however (e.g. Doka, 2009), even lower recovery rates have been considered (53%). Therefore, as detailed in Table 4, two scenarios are considered here, one with a higher (85%) and another with a lower (53%) recovery rate. As in the base case, two scenarios are also considered for the recovery of energy:

1. generation of electricity: assuming the same net efficiency of 35% as in the base case, the utilised landfill gas in Scenarios 1 and 2 would respectively produce 76 kWh and 38 kWh of electricity per tonne of MSW, as opposed to 56 kWh in the base case; and

2. generation of heat and power: using the same electricity-to-heat ratio as in the base case (Section 2.1.2), the utilised landfill gas would generate 73 kWh of electricity and 305 MJ of heat in Scenario 1 and 37 kWh of electricity and 152 MJ of heat in Scenario 2 per tonne of waste. For the base case, these values are 54 kWh and 226 MJ, respectively.

The results in Fig. 12 suggest that, with a higher recovery and utilisation of landfill gas, the GWP and POCP for both the electricity-only and CHP options are reduced respectively by three and two times compared to the base case. The AP is also five times lower for the electricity-only while for the CHP plants, there is a net saving because of the credits for the avoided impact. Furthermore, owing to the high credits for energy recovery, there is also a net saving of ADP fossil of -32 MJ/t for the electricity-only and -362 MJ/t for the CHP. The remaining impacts are also lower than in the base case but the reductions are smaller (<10%). However, in comparison to the environmental impacts of incineration, the impacts of the landfill systems are still significantly higher.

If lower recovery and utilisation rates are assumed, the AP increases by two times on the base case, the GWP by 26-31% and POCP by 21-24%. Similarly, there is a significant increase in depletion of fossil resources because of the lower credits for the avoided burdens.

3.2. Impacts from generation of 1 kWh of electricity from MSW

The environmental impacts discussed so far were related to the functional unit of 'disposal of 1 tonne of MSW'. In this section, we

examine how the impacts of different energy recovery options change for the second functional unit, i.e., 'generation of 1 kWh of electricity'. For both the landfill and incineration systems, the credits for heat recovery (CHP) and recycling of steel (incinerators) have been applied. Since it is assumed that these are waste-to-energy systems with waste used as fuel, credits for waste disposal have not been applied. However, the effect of this assumption is evaluated in the sensitivity analysis in Section 3.2.2.

The results are summarised in Fig. 13. As indicated, in comparison to the electricity from landfill gas, the impacts of electricity from incineration are significantly lower across all the categories. For example, the GWP is five times lower for the electricity-only incinerator than the equivalent electricity production from the biogas; the difference between the two CHP options is seven times in favour of incineration. Some of the impacts are several times lower for the electricity-only incineration compared to biogas electricity, including FAETP (~200 times) EP, MAETP and TETP (~50 times each). This is due to several reasons. Firstly, as discussed in Section 3.1, most impacts of incineration per tonne of MSW are lower than for landfilling before applying the system credits. Secondly, the credits for the recovery of metals reduce further the impacts of incineration. Finally and most importantly, the net electricity produced from incineration is about 10 times higher than that from landfills (Table 2).

The comparison between the two types of incinerator systems reveals that electricity from the CHP plant is better across all impact categories, mainly because of the additional credits for heat recovery. On the other hand, the comparison between the two landfill options suggests that some of the impacts, such as the EP, FAETP, MAETP and TETP, are slightly higher for the CHP system. This is because of the slightly lower electricity production from the CHP plant which cancels out some of the benefits of heat credits.

3.2.1. Comparison with electricity from fossil fuels

This section compares the environmental impacts of electricity from MSW with that from the UK grid as well as the following individual fossil-based sources: coal, heavy fuel oil and natural gas. The life cycle inventory data for these sources are from ecoinvent (2010). As shown in Fig. 14, compared to the UK grid, the GWP of electricity from incineration is 37% higher for the electricity-only and 18% higher for the CHP incinerator. On the other hand, the GWP of electricity from incineration is lower than the impact

14 H.K. Jeswani, A. Azapagic/Waste Management xxx (2016) xxx-xxx

□ Incineration (electricity only) a Incineration (CHP) nUK grid «Coal electricity QOil electricity 03Natural gas electricity

eq./kWh] [MJ/kWh] x10 elements [mg eq./kWh] x 10 eq./kWh] DCB eq./kWh] eq./kWh] DCB eq./kWh] eq./kWh] x 0.1 C2H4 eq./kWh] x 10 Sb eq./kWh] eq./kWh]

Fig. 14. Comparison of environmental impacts of electricity from MSW incineration with electricity from the UK grid and fossil fuels. [Functional unit: generation of 1 kWh of electricity. The results include heat and material recovery credits for the MSW-based systems, where applicable. For the impacts nomenclature, see Fig. 4.]

□ Landfill (electricity only) B Landfill (CHP) 0 UK grid

■ Coal power plant aOil power plant ^Natural gas power plant

GWP [kg CO2 eq./kWh]

ADP, elements

[mg Sb eq./kWh] x 0.1

AP [g SO2 eq./kWh]

EP [g PO4 eq./kWh]

FAETP [kg HTP [kg DCB MAETP [t DCB ODP [mg R11 POCP [g C2H4 TETP [g DCB DCB eq./kWh] eq./kWh] x 0.1 eq./kWh] eq./kWh] x 0.01 eq./kWh] x 0.1 eq./kWh]

Fig. 15. Comparison of environmental impacts of electricity from landfill biogas with electricity from the UK grid and fossil fuels. [Functional unit: generation of 1 kWh of electricity. The results include heat and material recovery credits for the MSW-based systems, where applicable. For the impacts nomenclature, see Fig. 4.]

from coal and oil electricity by between 21-29% for the electricity-only and 58-68% for the CHP plant. Compared to the natural gas, however, incineration has 46% higher GHG emissions for the system which generates electricity only and 29% greater for the CHP.

The other environmental impacts are lower for CHP incineration than for the UK grid, coal and oil for all the categories except for the HTP (Fig. 14). A similar trend is found for the electricity-only incinerator in comparison to the UK grid, with the exception

of the EP, FAETP, HTP and TETP. In comparison to coal, it has higher ADP elements, HTP and ODP as well as the FAETP and MAETP than fuel oil. Compared to natural gas, both incineration options have lower depletion of fossil resources and ozone layer and higher impacts for all the other categories.

Therefore, if the electricity from incineration displaces the electricity from the grid, the ADP (fossil and elements), AP, MAETP, ODP and POCP would be reduced but other impacts, such as

H.K. Jeswani, A. Azapagic/Waste Management xxx (2016) xxx-xxx

□ Incinerator (electricity only) «Incinerator (CHP) □ Landfill (electricity only) sLandfill (CHP)

GWP [kg CO2 ADP, fossil ADP, AP [g SO2 EP [g PO4 FAETP [kg HTP [kg DCB MAETP [t ODP [pg R11 POCP [g TETP [g DCB

eq./kWh] [MJ/kWh] elements [mg eq./kWh] eq./kWh] DCB eq./kWh] eq./kWh] x DCB eq./kWh] eq./kWh] x 10 C2H4 eq./kWh] Sb eq./kWh] x 0.01 eq./kWh] x 0.1

Fig. 16. Environmental impacts of 1 kWh of electricity with the credits for avoiding MSW disposal. [MSW disposal refers to residual waste with no recyclables, 61% of which is landfilled and the rest incinerated. The results include system credits. For the impacts nomenclature, see Fig. 4.]

GWP and HTP, would increase. If it replaces coal or fuel oil within the UK grid, most of the environmental impacts of the national electricity mix would be reduced. However, if it displaces the electricity from natural gas, there will be savings for some impacts, but others would increase, including the GWP, AP, EP and the toxicity-related categories.

For the landfill biogas systems, the trends are different. The results in Fig. 15 indicate that the GWP of electricity from these systems is eight to 10 times higher than for the UK grid and natural gas, and four times higher than electricity from coal and fuel oil. Similarly, all other impacts are significantly higher compared to the UK grid and natural gas, except for the depletion of fossil resources which is lower for the CHP landfill option. Compared to coal or fuel oil, this impact is also lower, together with the AP. The electricity from landfill gas also has a lower ODP than fuel oil but all the other impacts are significantly higher. This suggests that, if the electricity produced from waste landfilling displaces electricity from the UK grid or natural gas within the grid, almost all of the environmental impacts of the national electricity mix would increase. On the other hand, if it replaces electricity from coal or fuel oil, there will be a decrease in the ADP fossil, AP and possibly ODP, but all other impacts would increase significantly.

3.2.2. Credits for MSW disposal

Considering that both the landfill and incineration systems also treat waste (thermally) in addition to providing energy, the effects of crediting the systems for the avoided impacts of waste disposal are assessed here. Residual MSW is considered for these purposes as the recyclables would be removed before disposal. Currently, 61% of residual MSW is landfilled in the UK and the rest is incinerated (EC, 2015). For each option, the credits are equivalent to the amount of MSW required to generate 1 kWh of electricity.

As indicated in Fig. 16, with the credits, CHP incineration has the lowest impacts across all the categories. Some impacts are also negative for both types of incinerator suggesting net savings, notably for the EP, POCP and all the toxicity categories. It can also be observed by comparing the results in Fig. 16 with those in Fig. 14 that electricity from incineration has now much lower impacts than that from the grid and fossil fuels. On the other hand, as in the base case, electricity from landfilling still has higher impacts than the grid and natural gas (see Fig. 15). However, compared

to the electricity from coal or oil, it has slightly lower impacts for some categories, such as the GWP and HTP.

3.3. Implications at the UK level

Based on the results discussed in the base case, this section considers the implications for the environmental impacts of energy recovery from the MSW generated annually in the UK. This is equivalent to 30.9 Mt, of which 13.8 Mt is recycled, composted or digested, 10.2 Mt landfilled, 6.4 Mt incinerated and 0.3 Mt disposed of in other (unspecified) ways (EC, 2015). Therefore, if all the waste was diverted from the landfill, the total amount that would be available for incineration, including the waste disposed of in other ways, is around 17.1 Mt. Only the incineration option is considered here, for two reasons: first, as the results of this study suggest, it is an environmentally more sustainable option than the energy from landfill gas, and secondly, landfilling will be phased out gradually in the future, as discussed in the introduction. Based on the current situation in the UK, it is assumed that 80% of incinerators generate electricity only and the rest are CHP plants (Nixon et al., 2013; DEFRA, 2013a). According to the assumptions in Section 2.1.1, 8760 GWh of electricity could be generated annually from 17.1 Mt of waste available for incineration. This would provide 2.3% of UK annual electricity generation, increasing the current energy recovery from MSW of 21% to 57%. However, it would require building 27 new large incinerators (for example, each with a capacity of 400,000 t MSW/yr) which would be difficult to achieve with the UK public being strongly opposed to incineration. Nevertheless, we consider this case, albeit as hypothetical, to explore if and by how much the impacts could be reduced by utilising MSW for energy recovery on a much larger scale.

The results in Fig. 17 suggest that the UK GHG emissions would increase by 2.6 million t CO2 eq./yr if electricity from incineration displaced the grid electricity; this is equivalent to 0.5% of UK's annual GHG emissions in 2013 (DECC, 2015). Furthermore, the EP and HTP would also go up by 25% and 144%, respectively. However, most other impacts would be reduced significantly, including a saving of 2.8 million GJ/yr of fossil resources. If, on the other hand, incineration displaced coal or oil, annual GHG emissions would be reduced by 2-2.6 million t CO2 eq./yr (0.3-0.45% of UK's emissions). Most other impacts would also be reduced, with

Fig. 17. Comparison of annual environmental impacts of electricity from MSW incineration with the UK grid and fossil fuels. [Functional unit: generation of 8760 GWh of electricity per year. The impacts of electricity from incineration are estimated using the results in Fig. 13 and assuming the national mix of 80:20 of electricity-only and CHP incinerators. The results include heat and material recovery credits for incineration. For the impacts nomenclature, see Fig. 4.]

the greatest savings found for the ADP fossil, AP, EP, FAETP and MAETP (against coal), ODP, POCP and TETP (against oil); the HTP would be comparable to that from coal electricity. Finally, if electricity from waste incineration displaced that from natural gas, all the impacts would increase, except for the depletion of fossil resources and the ozone layer.

4. Conclusions

In this work, the life cycle environmental impacts of energy recovery from MSW have been estimated for the UK conditions and compared to energy from the UK grid and fossil fuels. Two waste-to-energy options have been considered: incineration and biogas recovered from landfill, each generating either electricity only or both heat and power. The systems have been compared for two units of analysis: disposal of 1 tonne of waste and generation of 1 kWh of electricity. The results indicate that, under the conditions considered in this study, incineration of MSW has much lower impacts than landfilling across all the impact categories considered, when both systems are credited for their respective recovered energy and materials; the only exception to this is the human toxicity potential which is higher for incineration. This applies for both units of analysis. For the unit 'disposal of 1 tonne of waste', the impacts of incineration are negative for several categories as the avoided impacts for the recovered energy and materials are higher than the impacts caused by incineration. The environmental performance of CHP incineration is superior to generation of electricity alone, owing to a higher energy efficiency of the CHP systems. Similarly, the CHP using landfill biogas has lower environmental impacts than the electricity-only biogas system, but the difference between the two systems is not as high as in the case of incineration. This is due to the much higher heat recovery by CHP incineration compared to the biogas CHP plant (1785 vs. 226 MJ/t of MSW).

Most of the impacts from incineration would improve further if, instead of displacing the UK grid electricity, it led to a reduced consumption of heavy fuel oil or coal for electricity generation. However, the majority of impacts from incineration would increase if it displaced electricity from natural gas instead of electricity from the

UK grid. The impacts of CHP incineration would also be lower, albeit not as significantly, if the recovered heat displaced heat from fuel oil instead of natural gas because of the higher system credits. Similarly, by improving the recovery rate of biogas from landfills, the global warming, fossil resource depletion, acidification and photochemical smog potentials of landfill would be reduced significantly. However, all environmental impacts of landfilling would still be higher than the impacts from incineration, with the exceptions of the global warming and human toxicity potentials.

The analysis on the basis of '1 kWh of electricity generated' shows that the environmental impacts of electricity from incineration are several times lower in comparison to the impacts of electricity from landfill biogas. Electricity from incineration also compares well with coal and oil electricity with significantly lower global warming potential and some other impacts. However, it has significantly higher impacts than electricity from natural gas across all the impact categories, except for the depletion of fossil resources and the ozone layer. In comparison to the UK grid, incineration has higher global warming and toxicity-related potentials but lower other impacts. However, waste-to-energy systems provide an additional benefit of treating the waste (thermally) and therefore avoiding the impacts from landfilling. If the credits for this are considered, then electricity from incineration performs better than the UK grid and fossil fuels.

The results also suggest that, if the incinerators are considered as energy recovery rather than waste disposal plants, they could offer significant benefits over oil and coal power. At the UK level, diverting all 10.2 million tonnes of MSW currently landfilled to incineration with energy recovery would not only avoid the environmental impacts associated with landfilling but, under the assumptions made here, could also meet 2.3% of UK's electricity demand and save 2-2.6 million tonnes of GHG emissions per year, equivalent to 0.3-0.45% of national GHG emissions. Most other impacts would also be reduced, including fossil resource depletion, acidification, eutrophication, ozone layer depletion and photochemical smog. However, this would require constructing 27 additional large incinerators, or the equivalent number of smaller plants, which may be difficult because of the public opposition to incineration in the UK.

Acknowledgements

This work has been funded by the UK Engineering and Physical Sciences Research Council (EPSRC, Grant No. EP/K011820/1). The authors gratefully acknowledge this funding.

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