A 2013 life-cycle study of UK municipal solid waste (Jeswani, Smith and Azapagic) found that the net carbon footprint of incineration with energy recovery depends strongly on which electricity generation the recovered energy is credited against: crediting against the UK grid average gave −0.179 t CO2 eq. per tonne of waste, while crediting against coal or heavy fuel oil gave −0.35 or −0.51, and the study explicitly left the choice of the appropriate displacement assumption for policy appraisal unresolved. Later work engaged parts of this question: an empirical UK short-run marginal mix (about 35% coal, 65% combined-cycle gas turbine) was estimated from 2014–15 dispatch data, substitution-reporting frameworks and prospective energy-scenario methods were developed, and UK studies showed displacement credits eroding as the grid decarbonises, with reviews as late as 2025 confirming that the avoided-grid assumption still decides whether incineration or landfilling compares better on greenhouse gases. No study, however, establishes a settled selection rule for crediting recovered energy, and the UK empirical marginal-mix estimates predate the completed phase-out of coal. The surviving open problem is to determine, and justify on stated criteria, the displaced-electricity assumption appropriate for policy appraisal of UK energy-from-waste, and to quantify its consequences for the incineration-versus-landfill comparison under the current and projected net-zero grid, including transferability across plant sizes, waste compositions and country grids.

Background

Municipal solid waste (MSW) that is not recycled must be treated, and the two dominant residual-treatment routes in the UK are landfilling and energy-from-waste (EfW) incineration, i.e. combustion of mixed residual waste in a mass-burn plant that recovers electricity and heat. Landfills generate landfill gas (biogas) as biodegradable waste decomposes; part of it is captured and burnt for energy. Incineration emits fossil carbon dioxide from the plastic-derived fraction of the waste. Life cycle assessment (LCA, standardised in ISO 14040/44) compares such routes on a common functional unit, for example the disposal of one tonne of MSW, and sums all greenhouse-gas emissions and savings as a carbon footprint in CO2-equivalents. Because both routes are multifunctional (waste disposal plus energy delivery), LCA practice handles the energy outputs by system expansion with substitution credits: the recovered electricity is credited with the emissions of the generation it is assumed to displace, which are subtracted from the system's footprint. The credit is therefore fully determined by the displacement assumption — the average grid mix, a specific marginal source that would actually respond to the additional supply (in the short run, the plants near the top of the merit-order dispatch curve that vary output with demand; in the long run, the technology of which capacity would be built or retired), or a projected future mix. Attributional LCA uses average data, while consequential LCA, used for decision support, requires the affected marginal technology, whose identification is notoriously uncertain (Mathiesen, Münster and Fruergaard 2009). Jeswani, Smith and Azapagic (2013) compared UK incineration with landfill biogas recovery following the ISO 14040/44 life-cycle-assessment standards and PAS 2050, the UK specification for assessing the life-cycle greenhouse-gas emissions of goods and services, crediting both against the UK grid average: incineration scored −0.179 t CO2 eq. per tonne of MSW (a net saving) versus +0.395 for landfilling with biogas recovery. But crediting incineration against coal or heavy fuel oil instead deepened its footprint to −0.35 or −0.51 t CO2 eq. per tonne, higher paper recycling raised its fossil emissions, and the authors flagged the choice of displaced generation as an unresolved methodological decision for policy appraisal, with transferability to other plant sizes, waste compositions, country grids and future energy systems untested. Subsequent work sharpened rather than settled the question. Burnley, Coleman and Peirce (2015) estimated an empirical short-run marginal mix for the UK grid from half-hourly generation data for 2014–15 (annual average roughly 35% coal and 65% combined-cycle gas turbine) and showed the climate benefit of EfW over landfill shrinks by about an order of magnitude between coal-displacement and gas-displacement crediting; Vadenbo, Hellweg and Astrup (2017) proposed a reporting framework that makes substitution assumptions transparent without prescribing them; Meylan, Haupt, Duygan, Hellweg and Stauffacher (2018) developed prospective methods in which the marginal technologies change with the energy scenario; and Jeswani and Azapagic (2016) and Slorach, Jeswani, Cuéllar-Franca and Azapagic (2020) found that electricity from incineration already carries a higher global-warming burden than the UK grid average, so displacement credits erode as the grid decarbonises — a process since culminated in the UK's last coal-fired power station closing in 2024, which leaves the earlier coal-containing marginal mixes outdated. Reviews by Anshassi, Sackles and Townsend (2021) and Anshassi (2025) confirm that the avoided-grid-emissions assumption remains among the parameters that decide whether landfilling or incineration compares better. The unresolved target is a defensible convention for the displacement assumption — and its quantified consequences for the UK waste-treatment ranking under current and future grids.

Problem Statement

Determine which displaced-electricity assumption should be used to credit recovered energy in life-cycle greenhouse-gas appraisal of UK energy-from-waste, and quantify what the choice implies for the comparison of MSW incineration with landfill biogas recovery. The open problem is (1) to state and defend a selection rule for the displaced generation — UK grid average, an empirically estimated short-run marginal mix derived from electricity dispatch data, a long-run marginal technology, a scenario-dependent marginal technology, or a projected future mix — with explicit criteria tied to the policy-appraisal decision context (attributional versus consequential framing, i.e. crediting against average grid data versus the affected marginal technology; time horizon; treatment of heat as well as electricity credits); (2) to apply the rule to quantify the net carbon footprints of UK MSW incineration and landfilling with biogas recovery, at plant scale and at national scale, reporting how the magnitude of credited savings and the ranking of the two options change across the admissible displacement assumptions, under the current post-coal UK grid (the last UK coal-fired power station closed in 2024) and under projected net-zero grid trajectories, i.e. scenario pathways to a net-zero-emissions electricity system; and (3) to demonstrate the transferability of the rule and conclusions across the contexts the source study flagged — different plant sizes and electrical efficiencies, changing waste composition (including the feedback of higher recycling on calorific value, the energy content per unit mass of waste, and hence on recovered energy), and other countries' grid mixes. The unresolved target is a defensible, empirically grounded convention for crediting recovered energy from UK energy-from-waste, together with its quantified consequences for the incineration-versus-landfill decision under current and future grids and its transfer across plant sizes, waste compositions and country grids.

References

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