The potential for biomass with CCS and direct air capture of CO2 should not distract from deployment of CCS on fossil fuel plants over the next few decades. Among other things, learning from CCS on fossil fuels will help make eventual deployment of CCS on biomass cheaper and more effective.
There appears to be increasing likelihood that atmospheric concentrations of greenhouse gases will grow to exceed levels consistent with the target specified in the recent UNFCCC Paris agreement of limiting temperature rises to “well below” two degrees. Such an outcome would require CO2 to be removed from the atmosphere faster than natural sinks allow, in order to restore concentrations to safe levels. Near zero net emissions in the latter part of this century will also be needed to stabilise concentrations.
Many models of future emissions pathways now show negative emissions technologies (technologies that result in a net decrease in CO2 in the atmosphere) needing to play a major role for in meeting climate goals. They have the potential to remove carbon dioxide from the atmosphere in the event of “overshoot” of target atmospheric concentrations, and to balance remaining emissions from sectors where abatement is difficult with a view to achieving net total emissions of close to zero. But the application of such technologies should not be considered in isolation.
Bio Energy with CCS (BECCS)
Bio energy with CCS (BECCS ) is the most widely discussed approach to negative emissions. BECCS is not a single technology but a combination of two major types of CO2 removal. Bioenergy from burning biomass to generate electricity or heat emits a substantial amount of CO2 on combustion – around as much as a coal plant. However much of this can be captured using CCS. More carbon dioxide is reabsorbed over time by the regrowth of the plants, trees – or perhaps algae – that have been harvested to produce the bioenergy. Together, the capture of the CO2 from the flue gas and the subsequent absorption of CO2 from the atmosphere by regrowth of biomass can lead to net removal of CO2 from the atmosphere, although this takes time. (The net amount of CO2, that is emitted on a lifecycle basis from burning biomass without CCS depends on the type of biomass and is subject to considerable variation and uncertainty – a controversial topic that would need another post to review.)
However at an energy system level, where the CO2 is captured – biomass plant or fossil fuel plant – is less relevant than the total amount that’s captured. Broadly speaking, if there is a biomass plant and fossil fuel plant both running unabated the same benefit can be achieved by capturing a tonne of CO2 from either.
Indeed there may be advantages to putting CCS on a conventional plant rather than biomass plant. It may be technically more tractable. Furthermore, CCS require a lot of energy to capture the CO2 and then to compress and pump it for permanent storage. Biomass is likely to be supply constrained (again, this is an issue that requires a post in itself), so using biomass rather than fossil fuels to provide this energy may limit other applications. Only when there are no fossil fuel plants from which to capture CO2 does biomass plant become unambiguously a priority for CCS. This is clearly some way off.
Furthermore biomass with CCS for electricity generation may not be the best use of bioenergy. Converting sunlight to bioenergy then turning that into electricity is a very inefficient process. Typically only 1-2% of the sunlight falling on an area of cropland ends up as useful chemical energy in the form of biofuels. There are various reasons for this, not least of which is that photosynthesis is a highly inefficient process. Burning biomass to make electricity adds a further layer of inefficiency, with only perhaps a third or less of the energy in the biomass turned into electricity. Sunlight therefore gets converted to electricity with an efficiency of perhaps 0.5%. This compares with around 15- 20% for solar cells, implying that scarce land is often likely to be best used for solar PV. (The calculation is closer if solar PV is used to create storable energy e.g. in the form of hydrogen). For similar reasons the use of biofuels rather than electricity in transport is inefficient, in part because internal combustion engines are less efficient than electric motors.
Limited available biomass may be better used for those applications where it is the only available lower carbon energy source, notably in aviation and (likely) heavy trucks, or for other applications such as district heating with CCS, where the ability to store energy seasonally is especially valuable.
At the very least, detailed system modelling will be required to determine the optimal use of biomass. Suggesting that BECCS should have a major role to play simply because in isolation it has negative emissions may lead to suboptimal choices.
Direct Air Capture (DAC)
Direct Air Capture (DAC), where carbon dioxide is chemically absorbed from the atmosphere and permanently sequestered, can also reduce the stock of CO2 in the atmosphere. However the typical concentration of CO2 in a flue gas of a power plant or industrial plant is several percent, about a hundred times as great as the concentration of 0.04% (400 ppm) found in the atmosphere. This makes the capture much easier. Furthermore, millions of tonnes can be captured and piped from a single compact site using CCS, generating economies of scale on transport and storage. In contrast DAC technology tends to be more diffuse. These considerations imply that CCS from power plants and industrial facilities is always likely to be preferable to direct air capture until almost all the opportunities for CCS have been implemented, which again is a very long way off.
Uncertainties and optionality
There are many uncertainties around negative emissions technologies, including the availability of biomass, cost, and the feasibility of reducing emissions in other sectors. For these reasons developing optionality remains valuable, and research to continue to develop these options, including early trial deployment, is needed, as others have argued[i].
One of the best ways of developing optionality is to deploy CCS at scale on fossil fuel plants. This will reduce the costs and enable the development of improved technologies through learning on projects. It will also help build infrastructure which can in turn benefit BECCS. This needs to run in parallel with ensuring that the lifecycle emissions from the bioenergy production chain are reduced and biodiversity is safeguarded.
Negative emissions technologies may well have a role to play in the latter part of the century. But they seem likely to make more sense when the economy is already largely decarbonised. In the meantime deployment of CCS, whether on industrial facilities or power plants, needs to be a much greater priority.
Adam Whitmore – 21st March 2016
[i] Investing in negative emissions, Guy Lomax, Timothy M. Lenton, Adepeju Adeosun and Mark Workman, Nature Climate Change 5, 498–500 (2015) http://www.nature.com/nclimate/journal/v5/n6/full/nclimate2627.html?WT.ec_id=NCLIMATE-201506