First generation BECCS power plants could have significantly lower power efficiencies than assumed. Inefficient BECCS would remove more CO₂ for an equivalent generating capacity, but would likely require a greater carbon removal subsidy.
3.1 Maximum power generation or CO₂ capture
An often-overlooked consideration is that to achieve the targeted 90 per cent, or higher, capture rates in BECCS-to-power plants, there is a significant energy requirement from the CCS equipment. Post-combustion capture requires heat to release the CO₂ molecules captured by the solvent, and additional energy is required to compress the captured CO₂ so that it can be piped to storage sites. This ‘energy penalty’ has the consequence of reducing the efficiency of the facility in converting the embodied energy of the biomass into electricity. As such, the capture rate and energy efficiency of the BECCS-to-power facility are intrinsically and inversely connected, creating a trade-off between power production and CO₂ capture. Or in other words, the more efficient at producing power a BECCS facility is, the less CO₂ that is captured.
Another way of looking at this is to start with the nameplate generating capacity of a BECCS power plant, and ask – how would a reduction in power efficiency impact the CO₂ capture potential? Given that Drax is seeking to become one of the UK’s first BECCS power plants, it is interesting to start with its current bioenergy power plant and play through the thought experiment in this context. Drax’s Selby biomass facility has a capacity of 2.6 gigawatts (GW), producing around 14.1 terawatt hours per year (TWh/yr) of power from 7,374 kilotonnes (kt) of wood pellets, which equates to around 38.9 TWh of embodied energy and 13.3 MtCO₂ of embodied CO₂ within the wood pellets. As such, the efficiency of the wood pellet power plant is around 36.2 per cent, and the load factor is around 62 per cent, meaning power was generated for nearly two-thirds of the year. It is the embodied CO₂ of the wood pellets that could, in the future, be captured by the CCS equipment.
Assuming a 90 per cent capture rate, there are two ways in which the volume of captured CO₂ could potentially be increased. Firstly, the power plant could run for a greater proportion of the year – increasing the load factor. This would require the dispatch protocol of the power plant to change, which – if the government chose to give BECCS-to-power plants priority dispatch to the grid – would be perfectly feasible, in effect turning BECCS-to-power plants into baseload power generators, rather than load following on the grid. This would, however, lower system flexibility and hence reduce the amount of variable renewables that could be integrated into the network. Secondly, the BECCS-to-power plant could decrease its power efficiency, in doing so it would combust more wood pellets to generate the same amount of power, hence the CO₂ available to potentially capture increases. In both instances, the limiting constraint is the generating capacity of the facility (currently 2.6 GW at Selby).
As a result of this trade-off it would be reasonable to suggest that a future UK BECCS removal target should define a fixed amount of biomass to be used within BECCS facilities, either domestically grown within the UK or imported. And as such, the efficiency of future BECCS power plants should simply be as high as possible to provide maximum power generation, as this would in turn increase revenues and hence decrease the subsidy that BECCS facilities would require to capture CO₂. This is a valid argument, but there are still downsides, principally that the number of BECCS facilities (or to be more accurate turbines) would need to increase. This is because the nameplate generating capacity of BECCS facilities running at maximum load factor limits the amount of biomass a given turbine can process. Figure 3 illustrates this highly efficient BECCS fleet future (scenario 1), as compared to a fleet of inefficient BECCS power plants (scenario 2). As can be seen, the efficient facilities generate more power and hence revenues, lowering the CO₂ capture subsidy, at the expense of more turbines being required. As each BECCS turbine has an associated capital expenditure (CAPEX), the cost to build the infrastructure is relatively high. In scenario 2, the lower efficiency facilities each combust a greater volume of feedstock, capture more CO₂ per facility, but generate less power revenues. Meaning the aggregate CAPEX is lower, but the subsidy requirement would be relatively high.
