Climate engineering — carbon capture and storage

Michael Stephenson
British Geological Survey
mhste@bgs.ac.uk
ECB Bulletin July 2016

Carbon capture and storage (CCS) is a technology to reduce the amount of anthropogenic CO₂ in the atmosphere. CCS tends to be associated with fossil fuel power stations, particularly those that burn coal, but could be used on any large point source of CO₂ in power generation or manufacturing, such as ammonia and cement factories and coal-to-liquid plants. The separated CO₂ can then be sequestered (stored) in deep rock layers. More recently, Bioenergy with Carbon Capture and Storage (BECCS) has received attention, but the environmental implications of large-scale biomass burning for energy remain unclear.

Long before Britain generated electricity from coal, it was using this organic-rich mineral in large quantities. Before 1700, fossil fuels had already overtaken wood as the leading provider of heat in peoples’ homes. Plentiful coal in the north of England enabled the natural supply of non-fossil energy to be bypassed. So began what the historian Andreas Malm has called the ‘fossil economy’ (1), which relies on burning carbon that comes not from local growing sources (trees), but from plant sources that are 330 million years old. From 1781, cotton manufacture, previously based near fast-flowing streams, became independent of water when the rotative steam engines of Boulton and Watt led to the growth of large, steam-powered mills concentrated in towns like Manchester and Salford. Steam engines for winding gear and pumps followed and meant that even more coal could be mined. This start of the fossil economy might also be seen as the start of the latest of the geological epochs, the Anthropocene (2), marked geochemically by, amongst others, the rise in CO₂, as recorded in ice cores.

Miscanthus sinensis. This perennial energy crop is one of the main candidates for BECCS. Credit: fuujin/Shutterstock

That relationship with coal is weaker in Britain today, as most of our electrical power is now generated by burning gas. But coal continues to be used elsewhere in the world. Predictions, such as those of the International Energy Agency (IEA), suggest that coal will continue to be used heavily in the future, and will probably be important for global electricity generation for many years to come. According to the most recent IEA forecast (3), coal demand will grow to 5814 million tonnes of coal-equivalent per year through 2020, a rate of 0.8% per year on average. Half of the growth will be in India. It is difficult to see India reducing CO2 emissions without CCS.

The EU has committed to cutting its greenhouse gas emissions to 20% below 1990 levels by 2020, and further cuts are being decided for 2050. This commitment is one of the headline targets of the Europe 2020 growth strategy and is being implemented through binding legislation. Power generation will have to take a particularly large part in emissions reductions, mainly by focussing on increasing surface renewables (wind, tidal and solar), nuclear and geothermal power, but also by reducing emissions from existing fossil fuel power plants, partly by CCS.

CCS has received new interest recently because it may be an important part of achieving the 2 °C limit set at the 2015 United Nations Climate Change Conference, COP 21, concluded in Paris in December 2015. Many of the Intergovernmental Panel on Climate Change (IPCC) climate scenarios include some form of “negative emissions”—that is, net permanent removal of greenhouse gas emissions from the atmosphere. Of the 400 IPCC climate scenarios that have a 50% or better chance of less than 2 °C warming, more than 300 assume the successful and large-scale uptake of negative-emission technologies.

The most popular of these technologies is Bioenergy with Carbon Capture and Storage (BECCS). The idea behind BECCS is fairly simple: grow energy crops and burn them in power stations for electricity, scrub out the CO₂ and sequester it permanently in the subsurface. Some commentators see two approaches to staying below 2 °C warming: the ‘pay early’ and ‘pay late’ plans. In the first, countries have to slash greenhouse gas emissions immediately; the second allows a slower phase-out by developing negative emissions, particularly BECCS. To some, BECCS is the last resort in the latter part of the century if all else fails to cut emissions.

One important constraint on BECCS is how much land and resource can be devoted to biofuel crops. In a world where population is growing and land and other resources are at a premium, can space be devoted to crops that we simply burn? Many think not. As noted by Smith and Torn (4), very high sequestration potentials for BECCS have been reported, but there has been no systematic analysis of the potential ecological limits to, and environmental impacts of, implementation at a scale consistent with climate change mitigation. Modern fossil fuel use emits about 8 Pg C y⁻¹ (petagrams of carbon per year). Using a simple model, Smith and Torn estimated that to remove just 1 Pg C y⁻¹ by burning biofuel would require at least 2×10⁸ ha of land (20 times the area currently used for bioethanol production in the USA), 20% of today’s global fertiliser nitrogen production, and 4×10¹² m³ y⁻¹ of water. No one really knows if this is possible.

The Sleipner oilfield. Carbon capture and storage at this site has been in operation for 20 years. Credit: Kjetil Alsvik, Statoil ASA

The UK Energy Technologies Institute (ETI) has worked intensively on what a British BECCS industry might look like: the crops that might be burned, quantitative models of biomass growth, soil chemistry and greenhouse gas emissions (5). ETI projects and other work suggest that the most attractive biomass feedstocks today are short rotation forestry species (such as Scot’s pine and poplar) and perennial energy crops (miscanthus, see the first photo, and short rotation coppice willow or poplar). The ETI sees BECCS as the “only credible route to significantly reduce atmospheric carbon (negative emissions) – unlocking the ability to meet national carbon targets at a much lower cost” (5).

In contrast to the uncertainties over the environmental sustainability of BECCS, subsurface CO₂ storage is well understood and has been operating successfully in a number of large pilot test sites for several years. For example, every year since 1996, about 1 million tonnes of CO₂ have been stored in a rock layer more than 800 metres below the seabed in the North Sea at Sleipner (see the second photo) (6). It seems likely that CCS on industrial point sources such as coal (and gas) power stations, ammonia and cement factories, and coal-to-liquid plants, will be part of the mix of technologies required to keep CO₂ emissions down.

References
1. Andreas Malm, Fossil Capital: The Rise of Steampower and the Roots of Global Warming, Verso, 2016.
2. C. N. Waters et al., Science 351, aad2622 (2016).
3.  International Energy Agency, Medium-term coal market report 2015, IEA 2015.
4. L. J. Smith, M. S. Torn, Climatic Change 118, 89 (2013).
5. Energy Technologies Institute, Bioenergy: Enabling UK biomass, ETI, 2015.
6.  See www.statoil.com/en/technologyinnovation/newenergy/co2capturestorage/pages/sleipnervest.aspx.