Life cycle assessment of conventional and two-stage advanced energy-from-waste technologies for municipal solid waste treatment

Journal of Cleaner Production xxx (2015) 1e12

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Life cycle assessment of conventional and two-stage advanced energyfrom-waste technologies for municipal solid waste treatment Sara Evangelisti a, Carla Tagliaferri a, b, Roland Clift c, Paola Lettieri a, *, Richard Taylor b, Chris Chapman b a b c

Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK Advance Plasma Power (APP), Unit B2, Marston Gate, South Marston Business Park, Swindon SN3 4DE, UK Centre for Environmental Strategy, The University of Surrey, Guildford, Surrey GU2 7XH, UK

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 September 2014 Received in revised form 20 March 2015 Accepted 20 March 2015 Available online xxx

The EU landfill and Waste Framework directives are driving new approaches to waste management in the UK, away from landfilling and mass-burn incineration, which has been regarded as the main alternative to landfilling. The objective of this study is to compare the environmental impacts of three dual-stage advanced energy-from-waste technologies, i.e. gasification and plasma gas cleaning, fast pyrolysis and combustion and gasification with syngas combustion, with those associated with conventional treatments for municipal solid waste, i.e. landfill with electricity production and incineration with electricity production. Results show that the two-stage gasification and plasma process has a significantly better overall environmental performance than the conventional waste treatment technologies and somewhat better than a more modern incineration plant, exemplified by a plant under commissioning in Lincolnshire in the UK. The benefits of the gasification and plasma process arise primarily from its higher net electrical efficiency. © 2015 Published by Elsevier Ltd.

Keywords: Life cycle assessment Gasification Pyrolysis Combustion Energy-from-waste technology

1. Introduction In 2008, 53% of the household waste produced in the UK was sent to landfill, while only 1% was treated by incineration. By 2012, the proportion of household waste treated by incineration had risen to 17%, while 37% was still sent to landfill (Eurostat, 2014). The drivers of this change have been the need to produce cleaner and affordable energy and to divert the waste from landfill as required by the European Landfill and Waste Framework Directives (European Commission, 1999, 2008). Until recently, the main alternative to landfill considered in the UK for treatment of municipal solid waste (MSW) is incineration (Arafat et al., 2013; Song et al., 2013; Ning et al., 2013). However, local authorities have started considering other thermochemical treatment options

* Corresponding author. Department of Chemical Engineering, UCL, Torrington Place, Roberts Building, Room 312, London WC1E 7JE, UK. Tel.: þ44 (0) 20 7679 7867; fax: þ44 (0) 20 7383 2348. E-mail addresses: [email protected] (S. Evangelisti), carla.tagliaferri. [email protected] (C. Tagliaferri), [email protected] (R. Clift), [email protected] (P. Lettieri), [email protected] (R. Taylor), [email protected] (C. Chapman).

to deal with MSW, including pyrolysis and gasification technologies, driven by public environmental concerns and fierce opposition to new incineration plants. Waste gasification or pyrolysis is not a new concept. Although pyrolysis and gasification have been used widely in the past to produce charcoal, coke or other fuels, it is only recently that these technologies have received increasing attention mainly due to the possibility to obtain a syngas suited for use in different applications with potentially higher energy efficiencies, lower costs, smaller footprints and reduced visual impact (Arena, 2012; Materazzi et al., 2013). In particular, fluidised beds are recognised as one of the most interesting technologies for gasification, because of their good process flexibility (Arena and Di Gregorio, 2014). Even so, the majority of existing energy-fromwaste plants are grate-fired boilers (i.e. incinerators) (Leckner, 2014). In the UK, public investments are supporting the design, installation and operation of advanced waste-to-energy technologies to achieve high recovery efficiency and flexibility and to demonstrate the improved efficiencies offered by gasification over other technologies (DEFRA, 2013a). Possible processes include a number of multi-stage thermochemical treatments, including fast pyrolysis with combustion (FPeC) and gasification,

http://dx.doi.org/10.1016/j.jclepro.2015.03.062 0959-6526/© 2015 Published by Elsevier Ltd.

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2. LCA methodology Nomenclature APC APP FPeC GePl GeSC IBA IWM LCA MSW RDF

air pollution control advanced plasma power fast pyrolysis with combustion gasificationeplasma process gasification with secondary high temperature oxidation of the syngas incineration bottom ash integrated waste management life cycle assessment municipal solid waste refuse-derived fuel

usually in a fluidised bed, with the resulting syngas cleaned by secondary high temperature oxidation (GeSC) or by some other process. An example of a new multi-stage process has been developed by Advanced Plasma Power (APP), which uses a plasma converter to clean and condition the gas. APP has developed a pilot refuse-derived fuel (RDF) plant for trials and experimental purposes and several design studies are ongoing for a 20 MWe plant. The plasma produces a sufficiently high quality syngas to be used in a range of applications from direct power generation to the production of substitute natural gas, hydrogen and/or liquid biofuels including methanol. However, the study reported here focuses on the use of a similar process for energy recovery from waste. Life cycle assessment (LCA) is a tool that can be used to compare such technologies and to evaluate their environmental performances, allowing decision makers to be correctly informed (Moberg et al., 2005). LCA has previously been used to assess waste to energy treatments of MSW, accounting for all processes from collection to material recovery and electricity generation (Consonni et al., 2005; Evangelisti et al., 2014). However, relatively few studies have been published using LCA to evaluate advanced thermal treatments other than incineration. Zaman (2013) analysed a pyrolysis-gasification process for MSW, revealing better environmental performance compared with conventional incineration; however, inventory data for the process were based on a plant in the UK although the study was based in Sweden. Khoo (2009) assessed eight different advanced treatment technologies; however the analysis was not consistent in terms of feedstock treated by each plant (from MSW to tyres). Pressley et al. (2014) recently published a study on gasification of MSW with a FischereTropsch process for production of liquid transport fuel from the resultant gas but the production and consumption of chemicals used were not included in the system. Al-Salem et al. (2014) analysed a low-temperature pyrolysis process for waste treatment; however the study considered plastic residue only. Finally, Arena et al. (2014) presented a comparative attributional LCA of a moving grate combustor and a vertical gas shaft gasifier coupled with direct melting, a technology mostly used in Japan. Results from this paper are compared with the findings of our study in Section 5. The goal of this paper is to assess the environmental performance of three two-stage thermochemical processes for the treatment of MSW: a gasificationeplasma process (GePl), fast pyrolysis with combustion (FPeC), and gasification with secondary high temperature oxidation of the syngas (GeSC). The processes are compared against conventional waste treatment technologies of the same scale, specifically incineration with energy recovery and landfill with electricity recovery from the landfill gas.

Life cycle assessment is one of the most developed and widely used environmental assessment tools for comparing alternative technologies on the basis of their resource consumptions and potentially harmful emissions. Although efforts are being made to give LCA some site specificity, it is still a tool for comparing technologies in general, whereas other tools like Environmental Impact Assessment are appropriate for siting decisions (Clift et al., 2000; Clift, 2013). LCA quantifies the amount of materials and energy used and the emissions and waste over the complete supply chain (i.e. life cycles) of goods and services (Baumann and Tillman, 2004). Moreover, it helps to determine the “hot spots” in the system, i.e. those activities that have the most significant environmental impact and should be improved as the first priority, thus enabling identification of more environmentally sustainable options (Clift, 2006). In LCA, a multifunctional process is defined as an activity that fulfils more than one function, such as a waste management process dealing with waste and generating energy (Ekvall and Finnveden, 2001). It is then necessary to find a rational basis for allocating the environmental burdens between the functions. The problem of allocation in LCA has been the topic of much debate (e.g. e, 2007). The ISO standards Clift et al., 2000; Heijungs and Guine (ISO, 2006a,b) recommend that the environmental benefits of recovered resources should be accounted for by broadening the system boundaries to include the avoided burdens of conventional production (Eriksson et al., 2007). The same approach is recommended by the UK product labelling standard provided that it can be proved that the recovered material or energy is actually put to the use claimed (BSI, 2011). This approach is applied in this study. Following the methodological approach of Clift et al. (2000) for Integrated Waste Management (IWM), a pragmatic distinction is made between Foreground and Background, considering the former as ‘the set of processes whose selection or mode of operation is affected directly by decisions based on the study’ and the latter as ‘all other processes which interact with the Foreground, usually by supplying or receiving material or energy’. The burdens evaluated here are considered under three categories (Clift et al., 2000): direct burdens, associated with the use phase of the process/service; indirect burdens, due to upstream and downstream processes (e.g. energy provision for electricity or diesel for transportation); and avoided burdens associated with products or services supplied by the process (e.g. energy or secondary material produced by the system). Following conventional practices (BSI, 2011), secondary data for the indirect and avoided burdens are taken as the averages for the background system, while primary data are used for the Foreground operations. Carbon dioxide from biogenic carbon is sometimes excluded from the comparison (Christensen et al., 2009) because it forms part of the renewable carbon cycle, theoretically removed from the atmosphere in succeeding products. However, in this study carbon dioxide emissions from biogenic carbon are included in the estimates for the Global Warming Potential (GWP) because the assessment is based on existing waste streams with defined carbon content so that the production of the materials in the waste does not enter the analysis. Therefore the total carbon content of the waste is considered, with no distinction between biogenic and nonbiogenic carbon, as shown in Table 3. Currently more than thirty software packages exist to perform LCA analysis, with differing scope and capacity: some are specific for certain applications, while others have been directly developed by industrial organisations (Manfredi and Pant, 2012). In this study GaBi 6 has been used (PE International, 2013). GaBi 6 contains databases developed by PE International, it incorporates industry

Please cite this article in press as: Evangelisti, S., et al., Life cycle assessment of conventional and two-stage advanced energy-from-waste technologies for municipal solid waste treatment, Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/j.jclepro.2015.03.062

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organisations' databases (e.g. Plastics Europe, Aluminium producers, etc.) and also regional and national databases (e.g. Ecoinvent, Japan database, US database, etc.). 2.1. Functional unit and system boundary The functional unit used for this comparative LCA is 1 kg of municipal solid waste as received at the plant. Fig. 1 shows the boundary of the system considered in this paper. The black boxes represent the Foreground system while the grey boxes form the background. The Foreground data used are site-specific wherever possible and based on experimental and modelling results, mainly derived from modelling studies validated against experimental results from pilot plants. Otherwise, average data from the literature, specific datasets, and general models are used. In this work, the background data are regionalised in the sense that they refer to the UK rather than the European average (PE International, 2013). Neither transportation of waste from the generation point to the plant nor the generation of the waste are considered in the system. The valuable outputs considered by system expansion are: the electricity generated from the waste; a vitrified product called “Plasmarok” generated by the gasificationeplasma process; the incineration bottom ash (IBA) which can eventually be reprocessed as secondary aggregate; and the metals recovered from the waste in the three advanced thermochemical processes and from the incineration bottom ash (IBA). The electricity produced is assumed to substitute electricity from the UK grid, which is based on an average mix of generating technologies described by data from PE International (2013). In the UK, the electricity is mainly produced from natural gas (44%), hard coal (28%) and nuclear fission (18%). This is reflected in the environmental impact associated with production of 1 MJ of electric energy from the grid, and in particular in the carbon footprint which is equal to 0.155 kg CO2eq for the UK (PE International, 2013). Plasmarok from the gasification and plasma process as well as incineration bottom ash from the incineration

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plant are assumed to avoid production of crushed rock for the primary aggregate industry, as suggested by Mankelow et al. (2008). For the metals recovered from the waste in the front end of the three advanced thermochemical processes and from the incineration bottom ash, the assumptions are: ferrous material e mainly steel e is assumed to be substituted at a 1 to 1 rate and the recovered material is assumed to be recycled by electric arc furnace processing, as reported in the GaBi database (PE International, 2013) and taken from the Worldsteel LCA Methodology report (World Steel Association, 2011). Non-ferrous metal e mainly aluminium e is assumed to be substituted at a 1 to 0.99 rate (PE International, 2013; Koffler and Florin, 2013). The recovered aluminium is assumed to be recycled by clean scrap melting and casting, as reported in the GaBi database (PE International, 2013) and taken from the Environmental profile report for the Aluminium Industry (European Aluminium Association, 2013). According to the ILCD methodology the average market mix of primary and secondary material is assumed for both ferrous and non-ferrous material (PE International, 2013). If the metal recovery included only virgin materials substitution the comparison among the processes analysed would have been strengthen. The scale of the plants analysed is 20 MWe. 2.2. Life cycle impact categories In the Impact Assessment phase, the emissions and inputs quantified in the Inventory phase are translated into a smaller number of impacts. Two general approaches are available, using socalled mid-points or end-points (Clift, 2013). This study has used the mid-point approach, in which the emissions and inputs are expressed in terms of their contribution to a set of impact categories; the alternative end-point approach expresses the impacts in terms of social and economic concerns. The standard mid-point e et al. (2001). In impacts used here are those defined by Guine this study the problem-oriented (or midpoint) approach is applied.

Fig. 1. System boundary.

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e et al. have defined a list of categories, based on the SETAC Guine working group studies. The study focusses on six impact categories which are found to be most significant for the comparison between the different processes, as shown in the normalisation results presented in the Supplementary information. Table 1 shows the impact categories analysed here. The GWP characterises and calculates the impact of greenhouse gases based on the extent to which these gases enhance radiative forcing. GWP values for specific gases, developed by the Intergovernmental Panel on Climate Change (IPCC), express the cumulative radiative forcing over a given time period following emission in terms of the quantity of carbon dioxide giving the same effect (IPCC, 2006). Following common convention, for example in the Kyoto Protocol, the 100year values have been used here. The Acidification Potential quantifies the impact of acid substances and precursors such as SO2, NOx and HCl. Rain, fog and snow trap the atmospheric pollutants and lead to environmental damage such as fish mortality, leaching of toxic metals from soil and rocks, and damage to forests and to buildings and monuments. Abiotic Depletion addresses the problem of the diminishing pool of resources, focussing on the depletion of nonliving resources such as iron ore, crude oil, etc. The measurement unit of abiotic depletion is MJ as the majority of non-renewable resources represent energy sources. The Eutrophication Potential includes all pollutants that promote microbiological growth leading to oxygen consumption, such as “algal blooms”. Nitrogen and phosphorus are the two main nutrients implicated in eutrophication: they can cause shifts of species composition and increased biological productivity. Photochemical Ozone Creation Potential (POCP) is an indicator of potential to create tropospheric ozone, expressed in equivalents to ethene as the reference species. Finally, the Human Toxicity Potential reflects the potential harm of chemical species released into the environment, based on both the inherent toxicity of , 2002). a compound and the potential human exposure (Guinee 3. Life cycle inventory In this study, the mass and energy balances of the different technologies analysed are based on a set of input data generated from a variety of sources including research reports, personal communication with experts, literature, and recent environmental reports of the world's leading companies in the related fields. Some data were taken directly from the GaBi 6.0 database (PE International, 2013). The LCA model of the three two-stage thermochemical processes were mainly based on data from the process plant design using Aspen Plus software and validated, where possible, through several experimental results by industrial developers (APP, 2013). The engine efficiency was taken from the engine manufacturer. An inventory table of the elementary input and output flows of the processes analysed is presented in the Supplementary information.

feedstock. However, Municipal Solid Waste is a very heterogeneous feedstock, in particular in ash and moisture contents, and it has a high variability depending on its source and location, the collection methods involved and seasonal effects (Larsen et al., 2013). As the basis for the process comparison, a common MSW composition received at the plant was assumed in this work, shown in Table 2; this reflects the average waste composition in the UK and it derives from samples of MSW collected in south-west England (APP, 2013). The composition of the waste is not particularly tailored for any of the processes analysed. Table 3 shows the ultimate analysis, moisture content and net calorific value of the MSW (APP, 2013). While landfill and incineration plants can process MSW directly, the three two-stage advanced processes need to convert the MSW into an RDF fluff, although no palletisation is required. 3.2. Conventional waste management treatments The two conventional waste management treatments compared with the advanced technologies are landfill with electricity production and two incineration processes. 3.2.1. Landfill with electricity recovery The inventory data for landfilling with electricity recovery are taken from the GaBi database (PE International, 2013) representing a typical MSW landfill with surface and basic sealing, meeting European limits for emissions. The site operations include landfill gas treatment, leachate treatment, sludge treatment and deposition. Part of the landfill gas is assumed to be flared (22%), part of it to be used for electricity production (28%) and the rest emitted to the environment (50%). All manufacturing processes of the sealing materials, as well energy requirements for the site, are included within the system. 3.2.2. Incineration As reported by the England's Waste Infrastructure Report (Environment Agency, 2010), in 2010 there were 73 permitted Incinerators and co-Incinerators in England, of which 18 processed MSW. The total MSW treated was 4,521,600 tpa, with a range in annual throughput of waste at these facilities from 3500 tpa up to 675,000 tpa. The comparison is based on two incineration plants: one currently operating in Sheffield, South Yorkshire since 2006 (Veolia, 2012); the other a new plant still under commissioning in North Hykeham, Lincolnshire (FCC Environment, 2014). The Sheffield plant is considered as the best established incineration technology in England, with moving grate combustor, emissions control using urea, hydrated lime and activated carbon injected into the flue gas, and particulate removal with filter bags (Jeswani et al., 2013). Ferrous metals are assumed to be recovered from the bottom ash as shown in Table 4. The gross electrical efficiency of the process

3.1. Waste composition Thermochemical processes such as gasification and pyrolysis are well established technologies when used with fossil fuel or biomass Table 1 Impact categories used in this study. Impact category

Acronym

Units

Global warming potential Acidification potential Abiotic depletion Eutrophication potential Photochemical ozone creation potential Human toxicity potential

GWP AP AD EP POCP HTP

kg CO2eq kg SO2eq MJ kg phosphate eq kg ethane eq kg DCBeq

Table 2 Composition of the MSW as received at the plant. Waste fractions (%)

MSW (as received at the plant)

Paper and cardboard Wood Metals Glass Textile WEEE Plastics Inert/agg/solid Organic fines Misc. comb

22.7 3.7 4.3 6.6 2.8 2.2 10 5.3 35.3 7.1

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S. Evangelisti et al. / Journal of Cleaner Production xxx (2015) 1e12 Table 3 Ultimate analysis, moisture content and net calorific value of MSW received at the plant and final RDF entering the three thermochemical processes (GePl, FPeC and GeSC) on a dry-basis. Ultimate analysis

MSW (as received at the plant)-dry basis

RDF (fuel entering the thermochemical process)-dry basis

%Ash content %C %H %O %N %S %Cl

15.8 41.8 5.1 35.3 1 0.25 0.59

16 42 5 35 1 0.3 0.6

% Moisture content Net calorific value (MJ/kg)

40 9

12 15

(allowing for the electrical efficiency of the steam turbine) is assumed to be 17% (Jeswani et al., 2013). The North Hykeham plant is new; it has a reverse acting grate with ammonia injection to reduce the NOx and meet the required limits. The high level diagram for the incineration processes is shown in Fig. 2. Bottom ash is recovered on site, and ferrous and non-ferrous metals are recovered in the IBA facility as shown in Table 4. Air pollution control (APC) residues are produced in the gas cleaning section, for treatment as landfilled hazardous waste. The gross electrical efficiency of the process is assumed equal to 26%, while the net electrical1 efficiency is 23% (FCC Environment, 2011a). 3.3. Advanced energy-from-waste technologies The characteristics of the three two-stage advanced thermochemical processes are summarised in Table 5: they are based on cutting edge technologies currently under development. For consistency, the LCA study conducted on all of the advanced thermochemical processes is performed with the same boundaries (see Fig. 1). The same amount of metals recovery has also been assumed for the three two-stage processes, as shown in Table 4. 3.3.1. Two-stage gasification and plasma process (GePl) The two-stage gasification and plasma process (GePl) can treat different waste feedstocks to produce electricity, steam and a vitrified product. The core of the system is the two stage process treating the waste by gasification followed by plasma cleaning to produce a low impurity, high energy syngas with high carbon conversion efficiencies (Taylor et al., 2013). Fig. 3 shows the GePl process. Four main sections are identified: solid fuel preparation unit; syngas generator unit; syngas refining unit; and power production unit. In the solid fuel preparation unit the received waste is pretreated and transformed to RDF by shredding and drying. Ferrous and non-ferrous metals are recovered by mechanical sorting, to be reprocessed for final sale as recycled metals. Table 4 shows the quantities of ferrous and non-ferrous metals recovered in the GePl process. The electricity required for pre-treatment includes consumption in the conveyors, shredders, fans, pumps and separator. In the syngas generator unit, the waste is thermally decomposed in a bubbling fluidised bed gasifier and transformed into a high temperature raw syngas. Oxygen and steam are used as oxidising

1

The “net electrical efficiency” is defined in this paper as the ratio of the net electricity generated by the plant (gross electricity generated minus parasitic loads) to the energy input (energy of the waste plus other energy into the process minus energy content of recovered materials).

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agents; the oxygen is assumed to be supplied by a cryogenic air separation process. The flows of the oxidising agents are controlled to maintain the bed temperature (850  C) and the required syngas quality and an inert gas (nitrogen) is supplied to the gasifier as a purge. Two main streams are distinguished going from the gasifier to the plasma converter: raw syngas and ash. The raw gas produced in the gasifier still contains entrained ash particles, unconverted char and residual tars and therefore further processing is required in the plasma converter (Ray et al., 2012). This unit produces a high purity syngas due to the cracking of the tars exposed to the high plasma temperature: the tars are almost completely converted in to H2 and CO, resulting in high syngas yield, little by-products and nearly 100% carbon conversion (Materazzi et al., 2014). Particles entrained in the gas are captured in the plasma converter and, together with the ash coming from the gasifier, vitrified into the socalled Plasmarok product. Unlike common incineration plants that produce bottom ash and fly ash which must be treated before use or disposal, this vitrified product is stabilised and can be used directly as an aggregate in road construction, without further reprocessing. In the syngas refining section the syngas is cooled and cleaned. The finest ash still contained in the syngas is collected and removed in the dry filter and in the scrubbers where APC residues are produced. Based on Astrup (2008), we assume physico-chemical treatment with acidic wastes in order to partially stabilise the APC residues. The energy required for this process, from Fruergaard et al. (2010), is taken as 0.6 L of diesel and 13 kWhe (46.8 MJ) per tonne of APC residue. The steam required by the drier is internally supplied by the waste heat boiler in the syngas refining unit which recovers the high thermal energy content of the syngas at the exit from the plasma unit, reducing the moisture content of the feedstock from 40% to 12%, as shown in Table 3. Further cooling is achieved in the quench and water scrubbing systems (such as acid and alkali scrubbers) used to remove contaminant compounds, i.e. phenol, sulphur dioxide, hydrogen sulphide and ammonia. The European Waste Incineration Directive e which includes gasification plant such as the two-stage GePl e defines the acceptable amounts of polluting species in aqueous effluents that can be discharged without further treatment to the public sewer system (European Commission, 2008; UK Government, 2003). However, these limits can be further constrained by local discharge limits embedded in Discharge Consents. In this model we assume that effluents from the quench and scrubber units are treated in standard waste water treatment plants. Finally, the last section includes the generation of electricity using a gas engine and cleaning of the flue gas. The steam produced by cooling the flue gas is fed to a steam turbine to produce an additional 0.5 MWe of power. In this study, the electrical efficiency of the gas engine is taken from the manufacturer's specification as 39e41% (Taylor et al., 2013) and the oxidising agent is air. The catalytic reactor cleans the flue gas leaving the gas engine, decreasing the amount of carbon monoxide and nitrogen oxides emitted to atmosphere. The exhaust gas is finally released to the atmosphere through a stack at almost 200  C. All energy and chemical consumptions have been taken into account to calculate the indirect environmental burdens. The energy required for the start-up of the process is nugatory as it contributes less than 0.1% to the total energy requirement. 3.3.2. Two stage fast pyrolysis and combustion process (FPeC) Fast pyrolysisecombustion process (FPeC) is a two stage process, deploying a pyrolyser and a combustor as shown in Fig. 4. As reported by Lombardi et al. (2015), pyrolysis application to waste for energy recovery is limited to few specific waste flows. In

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Table 4 Metals recovered in the incineration process and in the three two-stage advanced thermochemical processes (referred to the MSW composition reported in Table 3).

Ferrous metals (kg per kg of MSW as received) Non-ferrous metals (kg per kg of MSW as received)

GePl; FPeC; GeCS

Incineration (Sheffield)

Incineration (North Hykeham)

0.0289 0.0098

0.023 0

0.019 0.004

particular pure and homogeneous waste streams are required to produce good quality oil, which can be used in highly efficient energy conversion devices. The pyrolyser considered in this study converts the waste in the syngas generator unit using an internally circulating fluidised bed; the char produced by pyrolysis is converted in the bubbling fluidised bed combustor. After cooling and partial dust removal by a cyclone, the tars and remaining dust are removed from the syngas by multistage scrubbing with a hydrocarbons scrubbing oil. Very limited data are available for this type of cleaning technology because of the novelty of the system and its limited application to waste treatment. The syngas cleaning process assumed in this study consists of a collector, an absorber, and a stripper. The collector quenches the syngas with oil cooling the gas to a temperature above the water dew point. This allows part of the tar to condense and mix with the scrubbing oil. The absorber further removes the tar vapour by absorption in oil at a constant temperature. Finally, the stripper operates at higher temperature to drive the absorbed hydrocarbons from the absorber oil (Rabou et al., 2009). The recovered mix of hydrocarbons is then recycled to the combustor. A further scrubbing system is then used to remove most of the water, chloride and ammonia from the gas. The oil cleaning system and the scrubbers form the syngas refining unit. The cleaned syngas is finally used in the gas engine to produce electricity. More electricity is then recovered in a steam turbine.

3.3.3. Two stage gasification and syngas combustion process (GeSC) The gasificationesyngas combustor process (GeSC), shown in Fig. 5, uses a moving grate gasifier to produce a synthesis gas which is then oxidised at high temperature in a secondary chamber. The system incorporates a dry flue-gas cleaning system, which involves the injection of lime and activated carbon. Hot gas from the secondary chamber is recovered for steam production and a steam turbine is then used to produce electricity. Bottom ash produced in the gasifier as well as APC residues from the flue-gas cleaning system are treated and sent to landfill (Knowsley Energy Recovery, 2012). The net efficiency of the GeSC process considered here is 8% allowing for power use in pre-treatment to transform the residual MSW to RDF and recover the ferrous and non-ferrous materials; excluding the pre-treatment section, the net efficiency of the GeSC process itself is higher at 15%.

4. Results Fig. 6 shows the environmental impact of the three two-stage advanced thermochemical processes compared with conventional treatments for MSW. The results are expressed per functional unit, i.e. 1 kg of MSW received at the plant. The burdens are presented as

Fig. 2. High level diagram of the incineration processes. The schematic is valid for both Sheffield and North Hykeham plants. Non-ferrous materials and bottom ash as secondary aggregate are recovered only in the North Hykeham process. Table 5 Summary of the alternative thermochemical processes assessed in this study.

Acronym Feedstock Main technology Type of reactor Oxidising agent Cleaning technology Cleaning stage Energy recovery system Bottom ash post-treatment APC residues treatment Net electrical efficiency (based on MSW) a b

GePl

FPeC

GeSC

Gasification and plasma MSW Gasification and plasma Fluidised bed Steam/oxygen Wet and dry cleaning Pre-combustion Gas engine N/A Inertisation and then landfill 28%

Fast pyrolysis and combustion MSW Pyrolysis and combustion Internal circulation fluidised bed Air Oil scrubber Pre-combustion Gas engine Ageing process and landfill Inertisation and then landfill 26%a

Gasification and syngas combustion MSW Gasification and combustion Moving grate Air Dry cleaning Post-combustion Steam turbine Ageing process and landfill Inertisation and then landfill 8%b

This figure does not take into account the energy content of the oil used in the cleaning section; if it is included in the LCA, the net electrical efficiency will be lower. This value includes the pre-treatment section, which is considered the same for all the three dual-stage processes.

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Fig. 3. High level diagram of the two-stage gasification and plasma process (GePl). The four main sections of the process are highlighted in bold. System expansion is represented by rhombus.

Fig. 4. High level diagram of the fast pyrolyser-combustor process (FPeC).

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Fig. 5. High level diagram of the gasifieresyngas combustor process (GeSC).

direct þ indirect, avoided and total to show the contribution of the system expansion. As shown in Fig. 6a, the two-stage gasification and plasma process shows lower impact for the global warming potential compared to any of the alternative technologies. For the landfill plant, the main contributor to GWP comes from the landfill gas released to the atmosphere (50% of the total, as described in Section 3.3.1), which is primarily methane and carbon dioxide. Per kg, methane has a GWP 25 times that of carbon dioxide (IPCC, 2006). For the incineration process and the three two-stage advanced processes, the main contribution to GWP comes, instead, from the flue gases released from the stack. The incineration plant in Sheffield shows a higher GWP impact compared with the two-stage advanced processes, mainly due to its lower net electrical efficiency. The incineration plant at North Hykeham has a GWP slightly higher than that of the GePl process, i.e. 0.576 kg CO2eq per kg of waste compared with 0.458 kg for the GePl. The GePl process also shows better performance than the two other thermochemical processes considered. This is again mainly due to the higher net electrical efficiency of the GePl process leading to higher avoided burdens compared with the thermochemical processes analysed here. In fact, although the parasitic loads of the GePl process e defined here as the total power consumed by the equipments operating in the plant e are slightly higher compared with the FPeC and GeSC processes, the gross electrical efficiency more than compensates, as shown in Fig. 7. Fig. 6b shows the acidification potential of the GePl plant, compared with the alternative waste management scenarios investigated. The gasification and plasma process shows lower AP impacts than any other technologies. In fact, it shows a negative value for AP, meaning that it avoids the impacts of emissions from conventional production of electricity (90% of the avoided burdens), metals (10%) and aggregates (less than 1%). The AP impacts for the incineration plants are negative as well, but show less benefit than the GePl process, i.e. 2.76  104 kg SO2eq, 9  104 kg SO2eq and 9.71  104 kg SO2eq per kg of waste, for the Sheffield, North Hykeham and GePl plant, respectively. For the Sheffield plant, this is mainly due to impacts from the production of chemicals supplied to the cleaning system in the incineration plant, and to the high content of NOx and HCl in the flue gas at the stack, as shown in Table 6 (see Jeswani et al. (2013) for further information). The Sheffield incineration plant is assumed to produce electricity only. However, even if we consider the combined production of heat and power, which reflects the current best established technology in the UK where some existing incineration plants are operated for CHP, the resultant acidification potential impact of the incineration plant is ten times higher than

the gasification and plasma process (0.9  104 kg SO2eq and 10  104 kg SO2eq for the incineration with CHP and GePl process, respectively). Although the stack emissions of HCl, SO2, NOx and HF (which are the main contributors to AP impact) are higher for the North Hykeham plant compared with the GePl process, the overall acidification potential of this incineration plant is close to that of the two-stage GePl plant. This is because of the avoided burdens due to the large amount of IBA which can be recycled as secondary aggregates from the North Hykeham plant, as shown in Table 7. This incineration plant is in fact provided with an advanced IBA treatment facility which can recover up to 0.2 kg of IBA as secondary aggregates per kg of waste, which is more than one and half times the amount of Plasmarok produced in the GePl process (FCC Environment, 2011b). The acidification potentials of the FPeC and GeSC processes are equal to 7  104 kg SO2eq and to 3.8  104 kg SO2eq, respectively. The worse impact of the GeSC process is mainly due to two reasons: first, the gasifieresyngas combustion process has a lower electrical efficiency compared to the GePl process (see Table 5); and second, the secondary combustion stage leads to higher SO2 emissions (see Table 6). The abiotic depletion category shown in Fig. 6c is an indicator of fossil energy consumption. All the processes analysed give a negative AD impact (i.e. an improvement), due to the system expansion methodology applied in this paper. However, the two stage gasification and plasma process gives the greatest saving, corresponding to a larger saving of fossil fuel resources, because of its higher net electrical efficiency. Fig. 6d, e shows two further environmental impact categories: photochemical ozone creation potential and eutrophication potential. For POCP, the landfill scenario has a high impact due to the higher production of certain primary air pollutants, such as VOCs, CO and NOx emitted in the landfill gas, as shown in Table 6 (Evangelisti et al., 2014). The North Hykeham incineration process shows a lower POCP impact compared with the GePl process, i.e. 1.5  104 kg ethane eq and 8.6  105 kg ethane eq per kg of waste for the North Hykeham and GePl process, respectively. This is again mainly due to the larger amount of secondary aggregates which can be recovered from the incineration plant; in fact the IBA produced by the North Hykeham plant allows an avoided burden in terms of POCP of 6.8  104 of kg ethane eq per kg of IBA produced, based on the assumption that the recycled IBA substitutes primary aggregates from crushed rock (Korre and Durucan, 2009). For the EP impact category, the GePl process shows the best performance, although the difference with the FPeC process is very small (i.e. 1.8  104 kg phosphate eq and 1.4  104 kg phosphate eq for the FPeC and GePl process, respectively). This is mainly due to

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Fig. 6. Environmental impacts of the three two-stage advance thermochemical processes against conventional waste treatment technologies: a. global warming potential; b. acidification potential; c. abiotic depletion; d. photochemical ozone creation potential; e. eutrophication potential; and f. human toxicity potential.

the potentiality of the plasma torch which allows the conversion of almost all the ash and tars created during the gasification stage to Plasmarok (Materazzi et al., 2013). This results in the production of an amount of APC residues per kg of waste treated which is 40% less than the APC residue produced in the FPeC process, as shown in Table 7.

Finally, Fig. 6f compares the Human Toxicity Potential of the processes. Again, the two-stage gasification and plasma process shows the best result, being the only advanced technology with a negative HTP impact. This category is in fact mainly related to the capacity of the process to convert the ash and tars which are

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thermochemical processes analysed here are lower compared with the gasification with direct melting technology analysed by Arena et al. (2014). In the systems analysed in this study, the direct burdens that contribute to greenhouse warming correspond mainly to the CO2 emissions from the stack and are therefore directly proportional to the carbon content of the waste oxidised in the plant. The differences in contribution to global climate change between the technologies therefore result from the indirect and avoided burdens, of which the avoided burdens are expected to dominate. This means that, for climate forcing, the life cycle comparison between different technologies depends not only on the energy produced from the waste, but also on the valuable materials (i.e. metals) which can be recovered through the process. Fig. 8 shows the Global Warming Potential from the two-stage advanced technologies and from conventional MSW treatments expressed per kWh of electrical output. As expected, the trend mirrors the results shown in Fig. 6a. The relatively poor performance of the GeSC process is due to the low net electricity generation, as shown in Fig. 7. On this basis, the different impacts between the technologies are amplified. For the GePl process the GWP is 0.18 kg CO2eq/kWh el, only 30% that of the Sheffield incineration plant; 75% of the North Hykeham incineration plant; 40% the impact of the air blown gasifier system (GeSC); and 45% of the fast pyrolyser with gasifier (FPeC). We have already highlighted the main role currently played by incineration technologies compared with the other thermochemical processes, such as gasification or pyrolysis. However the results of this study show that the two-stage gasification and plasma process is a better environmental solution for the treatment of municipal solid waste compared with incineration technologies, for all the impact categories analysed here. The comparison can alternatively be framed to show that an incineration plant, based on the North Hykeham technology, must achieve at least 30% net electrical efficiency to display the same GWP impact (i.e. kg CO2eq per kg of MSW treated) as the two-stage GePl process. None of the incineration plants currently in operation in the UK reports a net electrical efficiency higher than 27% (DEFRA, 2013b). This is mainly due to the nature of the technologies involved and their achievable efficiencies: in the two-stage gasification and plasma process, electricity is produced from combustion of the syngas e a high calorific value fuel, mainly composed by CO and H2 e to give a gas at high temperature which is passed to a gas engine whereas in an incineration plant, electricity is produced in a steam turbine fed with lower temperature steam raised from the heat recovered from a completely oxidised flue gas.

Fig. 7. Parasitic loads e defined as the total power consumed by the equipments operating in the plant, gross and net electricity production for the processes analysed in this study per kg of waste treated.

formed in the first stage and to the amount of toxic substances released to the ecosphere.

5. Discussion High temperature gasification with direct melting technology is a wide used technology for the treatment of unsorted MSW in Japan (Tanigaki et al., 2012). In this type of plant, MSW is directly charged into a gasification and melting furnace from the top with coke and limestone using oxygen-rich air as reduction agent. The syngas is then injected into a combustion chamber to produce the steam to run a steam turbine (Arena, 2012). A vitrified slag is obtained as solid residues form the process, which can be directly be used as secondary aggregate for road construction (Arena, 2012). Tanigaki et al. (2012) reported gross power generation efficiency of 23% for gasification with direct melting technology, referred to a low calorific value of the MSW of 9 MJ/ kg. In the LCA study performed by Arena et al. (2014) on MSW gasification with direct melting plant, the thermochemical process shows an overall GWP impact of 960 kg CO2eq per ton of MSW, mainly due to the utilisation of metallurgical coke to obtain stable high temperatures in the molten section. For the same reason, Arena et al. (2014) reported a high consumption of fossil fuel resources for the gasification with direct melting technology in the abiotic resource depletion category. Overall, the environmental impacts of the advanced two-stage

Table 6 Emissions to the atmosphere at the stack in kg per kg of waste treated in each plant. Only the main chemical compounds are listed. Landfill HCl Particulate CO SO2 NOx HF

8.00 2.89 1.50 9.68 2.78 3.13

     

Incineration (Sheffield) 7

10 104 103 105 104 108

4.00 4.00 2.00 8.00 7.00 3.00

     

Incineration (North Hykeham)

5

10 106 106 105 104 107

8.70 8.37 4.30 4.30 8.70 1.37

     

5

10 103 104 104 104 102

FPeC 1.28 n.a. 2.96 3.00 5.00 1.15

 10    

6

104 104 104 105

GeSC

GePl

n.a. 6.14 n.a. 9.21 5.25 3.07

5.74 n.a. 1.33 1.49 2.26 1.03

 104  104  103.  104.

 107    

104 104 104 102

Table 7 APC residue and IBA production in the processes analysed in this study.

APC residue (kg per kg of MSW as received at the plant) IBA as secondary aggregates IBA residue (kg per kg of MSW as received at the plant)a a

Incineration (Sheffield)

Incineration (North Hykeham)

GePl

FPeC

GeSC

0.028 0 0.197

0.032 0.218 0.011

0.0151 0.079 0

0.02619 0 0.00748

0.0228 0 0.0532

After metals and secondary aggregates are recovered.

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Fig. 8. Global warming potential: comparison between three two-stage advanced thermochemical treatment and alternative waste management technologies, for 1 kWh of net electrical output.

Finally, the results show that the North Hykeham process has slightly higher environmental impacts compared with the twostage GePl. However, neither of these plants is fully commercialised; the inventory data used in this study is based on preliminary data from pilot plants and feasibility studies, so that the comparison should be revisited once data are available from operating plants.

of operation, they show that the GePl process may be regarded as a benchmark compared to other two-stage gasification technologies, when developing high efficiency advanced technologies for the treatment of municipal solid waste in future.

6. Conclusions

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jclepro.2015.03.062.

A life cycle assessment of three different two-stage advanced thermochemical processes has been conducted to evaluate their environmental performances in terms of carbon footprint and other environmental impacts against conventional waste treatment technologies. The two-stage advanced thermochemical processes considered in this study, specifically gasification and plasma torch (GePl), a fast pyrolyser with combustor (FPeC) and air blown gasifier system (GeSC), have a scale of 20 MWe net output. They have been compared with two conventional waste treatment technologies, i.e. incineration with energy recovery based on two different existing plants and landfill with electricity recovery. Overall, the results show that the environmental impact of the processes analysed is not only related to the electrical efficiency of the plant, but several other key factors determine the burden associated with the single waste treatment analysed. For example, treatment of MSW in the two-stage gasification and plasma process has substantially lower GWP and AP impact than landfilling (given the same amount of waste), this is primarily due to avoiding emissions of methane from landfilled organic material. Hence, the nature of the treatment involved e i.e thermochemical versus biological, determines the environmental impact of the process itself. For the same reason, the two-stage GePl process shows a much lower acidification impact compared to incineration, mainly due to the higher SO2 emissions when incineration technology is used. Moreover, the amount of metals recovered in the process can also play an important role in determining the overall environmental impact of a process plant. In this study, it is shown that the amount of metals recovered in the two-stage advanced processes is higher than in the incineration plants; this is because, while in the former, metals are recovered directly from the waste in the Solid Fuel Preparation unit, in an incineration plant, metals are recovered solely from the bottom ash, following combustion. Although the results presented in this paper are mainly based on pilot scale experiments and process simulations for the larger scale

Appendix A. Supplementary data

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