Is BECCS energy-supply-positive?
Many of the IAMs reviewed by the IPCC in successive reports project a significant reliance on BECCS not just for carbon removal but also for energy supply – with projected annual contributions of 150–300 exajoules (EJ), or 14–20 per cent of global primary energy supply, by 2100, alongside 8–16.5 GtCO2/year of atmospheric carbon dioxide removal. Implicit in these scenarios is the assumption that BECCS is an efficient energy vector (in the sense of converting stored biomass into power, heat or transport fuels). Even in the absence of CCS, the quantity of bioenergy feedstocks used in these IAMs (and the assumed land requirement) is generally unchanged, or even increases, compared with IAMs that include BECCS, due to bioenergy’s assumed potential to displace fossil fuels for power, heat or transport.52, 53
Biomass feedstocks inherently deliver less energy per unit of mass than do fossil fuels; biomass, whether from wood or herbaceous crops, is less dense and contains more moisture. Energy is also used in growing, harvesting, collecting, processing (including drying, pelletizing, etc.) and transporting the feedstock; the energy needed for collection is probably highest for diffuse residues such as those from harvests or forests, and lowest for processing residues such as sawmill or sugar mill waste. BECCS imposes no additional energy demand on these processes relative to bioenergy without CCS, but the energy efficiency of the supply chain does have a bearing on the energy and carbon break-even points. The CCS process itself imposes a ‘parasitic’ load on the biomass plant (i.e. as additional energy is required to power the CCS equipment), and energy is also needed to condense the captured carbon dioxide and move it to storage sites, whether by pipeline or surface transport.
Calculations of the overall energy balance suggest that BECCS projects may in fact deliver relatively little net energy as usable heat, power or fuel: indeed far less than the models project, once the energy requirements of full bioenergy supply chains are accounted for. A 2018 study concluded that energy output was strongly case-specific, with the energy return on investment (EROI) varying between a ratio of 0.5 (i.e. implying that more energy was consumed than produced) and 5.7 (roughly comparable to solar PV and, possibly, coal with CCS).54 The main energy leakages stemmed from biomass conversion into combustible fuel sources and from the parasitic load of CCS, followed by road transport, drying and farming (including inputs). While improving power plant efficiency reduced energy leakages, it also reduced the amount of carbon sequestered, in effect creating a trade-off between carbon capture and energy delivery. In general, biofuel fermentation captures less carbon but delivers more usable energy per unit of feedstock.
Exploring other ways of sequestering carbon from biomass that avoid the need to build potentially expensive or possibly unnecessary biomass plants should be a priority.
On top of this, the rapidly falling costs of other renewable technologies, notably wind and solar PV, are making BECCS – along with biomass-based electricity, liquid fuel, biogas and hydrogen production more generally – less competitive. IAMs place increasing value on the capture (i.e. ‘CCS’) component of BECCS rather than the bioenergy (‘BE’) component, as rising carbon prices are assumed to make carbon sequestration increasingly valuable over the course of the century. Thus, to the extent that BECCS plays a role in future climate mitigation pathways, it might be better seen primarily as a means of capturing carbon rather than as a useful energy vector relative to other carbon-neutral renewable energy alternatives. It should be compared – particularly in terms of cost, feasibility at different carbon prices and other impacts – with other carbon-capturing (but not energy-delivering) approaches such as DACCS, nature-based climate solutions (forest ecosystem restoration, reducing deforestation, etc.), and alternative uses for biomass that keep carbon out of the atmosphere, such as using wood for construction. Thus, given ongoing power-sector transformations, exploring other ways of sequestering carbon from biomass that avoid the need to build potentially expensive or possibly unnecessary biomass plants should be a priority.
52 Bauer, N., Rose, S. K., Fujimori, S., van Vuuren, D. P., Weyant, J., Wise, M., Cui, Y., Daioglou, V., Gidden, M. J., Kato, E., Kitous, A., Leblanc, F., Sands, R., Sano, F., Strefler, J., Tsutsui, J., Bibas, R., Fricko, O., Hasegawa, T., Klein, D., Kurosawa, A., Mima, S. and Muratori, M. (2018), ‘Global energy sector emission reductions and bioenergy use: Overview of the bioenergy demand phase of the EMF-33 model comparison’, Climatic Change, doi: 10.1007/s10584-018-2226-y.
53 IPCC (2019), Climate Change and Land, Chapter 6, cross-chapter box 7.
54 Fajardy, M. and Mac Dowell, N. (2018), ‘The energy return on investment of BECCS: is BECCS a threat to energy security?’, Energy and Environmental Science, 2018, 11, p. 1581, doi: 10.1039/C7EE03610H.