Optimal utilization of waste-to-energy in an LCA perspective
Introduction
Renewable energy is becoming increasingly valuable today in order to shift from dependence on fossil fuels to more stable and local sources of energy. The considerable focus within recent years on emissions of greenhouse gases from the energy sector has also brought attention to alternative fuels. Municipal solid waste (MSW) has in a range of countries, especially in Northern Europe, played a significant role as a source for energy by means of waste incineration at very high conversion efficiencies. In Denmark, waste incineration currently supplies about 5% of the electricity demand and about 20% of the district heating demand. In Denmark, a ban on landfilling of combustible waste from 1997 has relocated combustible waste from landfills to incineration plants meaning that waste incineration with energy recovery is in practice the “reference” treatment technology for municipal solid waste. In Southern European countries, MSW is often pre-treated to produce solid recovered fuels (SRF) for incineration while the organic fraction is separated mechanically, biologically stabilized and landfilled (e.g. Consonni et al., 2005a).
The question is which technology should be preferred for energy production from waste, now and in the future? From an energy perspective, MSW can be grouped into three fractions: (1) mixed high calorific waste materials suitable for SRF production, (2) organic waste materials suitable for biological treatment, and (3) mixed waste materials not fitting into the former two fractions. Again from an energy perspective, currently the most important options available are: direct incineration (fractions 1–3), combustion or co-combustion of SRF after sorting/pre-treatment (fraction 1), and anaerobic digestion (AD) of the organic fraction after sorting/pre-treatment (fraction 2). For the remaining mixed fraction (fraction 3), mass burn incineration is in practice the only energy producing technology available. The thermal technologies can generate electricity and heat (co-combustion at power plants at higher electricity efficiencies than waste incinerators) while anaerobic digestion generates biogas which can be used either for electricity/heat production or as a transport fuel.
Selecting between these technologies on a strategic level for implementation or further development of waste-to-energy, a solid basis for comparing the environmental benefits and drawbacks of the technologies is required. Life cycle assessment (LCA) has proven a suitable tool for this. A range of studies has within the recent decade discussed energy production from waste. Most of these have focused on waste incineration as an individual technology (e.g. Liamsanguan and Gheewala, 2007, Riber et al., 2008, Morselli et al., 2008) or as part of a national waste system, in some cases also discussing other options such as recycling and landfilling (e.g. Eriksson et al., 2005, Finnveden et al., 2005, Björklund and Finnveden, 2007). Fewer studies have evaluated anaerobic digestion (e.g. Börjesson and Berglund, 2006, Börjesson and Berglund, 2007), and only a single study was found on LCA of co-combustion of SRF from MSW (Consonni et al., 2006). Astrup et al. (2009) reported accounts of CO2 emissions from co-combustion and incineration, and Garg et al. (2009) performed an environmental assessment of SRF treatment based on a few selected parameters. Only a few studies have compared several technologies with a dedicated focus on energy production (e.g. Consonni et al., 2005a, Consonni et al., 2005b, Azapagic, 2007). No studies provided systematic evaluations of the environmental performance of the waste-to-energy technologies in a future renewable energy system. Providing clear conclusions based on these individual studies is extremely difficult because assumptions about waste composition, energy substitution, conversion efficiencies, system boundaries, etc. are not comparable between the individual studies. To offer transparent recommendations suitable for strategic decisions regarding waste-to-energy, it is therefore necessary to provide an LCA including all three aforementioned technologies.
The overall purpose of the paper was to assess which technologies should be preferred for energy production from waste materials in Denmark, with the current Danish energy system as well as a potential future 100% renewable energy system. The paper focuses on comparing anaerobic digestion of organic waste materials and co-combustion of SRF relevant waste materials with dedicated waste incineration in the two energy system contexts. Specifically, the paper (i) evaluates the environmental impacts from the selected technologies and points out the most important emissions, (ii) discusses the importance of the time horizon, the energy system and fuel substitution, and the waste composition, and (iii) ranks the evaluated technologies to provide recommendations for selection.
Section snippets
Goal
The goal of the LCA was to compare two different waste-to-energy technologies with mass burn incineration with and without energy recovery. The two alternative technologies were:
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Co-combustion in coal-fired power plants
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Anaerobic digestion
Two different waste fractions were included in the life cycle assessment: (1) a high calorific fraction (SRF) suitable for co-combustion and (2) organic waste suitable for biological treatment. The modelled alternatives are outlined in Table 1. Waste
Results
The results of the assessment are shown in Fig. 2, Fig. 3, Fig. 4, Fig. 5 illustrating the total environmental load of the assessed waste-to-energy technologies for the considered waste fraction. Positive values represent environmental loads while negative values indicate savings. The results are shown as normalized impact potentials in milli-person equivalents (mPE) representing annual impacts from an average person in a given area (i.e. 100 mPE corresponds to 10% of the annual impact from an
Discussion
Based on the results of the LCA and the sensitivity analysis it was clear that none of the included alternatives could provide optimal solutions within all impact categories. For co-combustion of SRF it was basically a trade-off between the non-toxicity and the toxicity categories, whereas the results were less unambiguous for treatment of organic waste.
Regarding SRF treatment in a Danish present-day perspective, co-combustion provided a better result than incineration with energy recovery for
Conclusions
Co-combustion of SRF had benefits over waste incineration with energy recovery with respect to the non-toxicity impacts (GW, AC, NE and POF) but if the waste incinerators could utilize all the produced heat, the two technologies were comparable. Both alternatives caused environmental savings when the interacting energy system was based on fossil fuels. The opposite was the case with regard to the toxicity related categories (ETw, HTa, HTw and HTs). Waste incineration constituted a saving in all
Acknowledgements
This paper was partly funded by the Danish Council for Strategic Research as part of the research project “Environmentally Sustainable Utilization of Waste resources for Energy production (ENSUWE).
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