Carbon Capture

It has always seemed a long shot- capturing and storing carbon dioxide to reduce climate impacts. But a range of technologies now are being developed which some see as viable.  The most advanced are those that capture CO2 from power station exhausts. An early variant of this idea was tested by MIT and involved feeding captured CO2 to a bioreactor where it was absorbed by fast growing biomass- algae: http://web.mit.edu/erc/spotlights/alg-all.html. So that could be used as a fuel, in which case this was a pioneering version of what is now called Carbon Capture and Utilisation - CCU. More recent CCU developments have involved using captured CO2 and hydrogen gas to make syngases and fuels, the hydrogen being produced by electrolysis of water using surplus power from wind or PV. Power to Gas (P2G) and Power to Liquid (P2L) system are emerging in Germany, producing  methane for injection into the gas main, or synfuels for vehicle use: www.audi-mediaservices.com/publish/ms/content/en/public/pressemitteilungen/2014/11/14/new_audi_e-fuels_project.html  and : www.sciencealert.com/audi-have-successfully-made-diesel-fuel-from-air-and-water  Alternatively, the hydrogen/syngasses can be stored and then used for power production when there is a lull and /or high demand for power, this offering a way to balance the variable output from renewables: www.dw.de/power-to-gas-may-solve-renewables-storage-challenge/a-17754416/

The concentration of CO2 in power plant exhaust is very much higher than that in the air, but some think that the later can also be used, either for storage in some chemical form, or to make fuels, as with the ‘air capture’ system being developed with support from Bill Gates: www.sciencealert.com/scientists-are-building-a-system-that-could-turn-atmospheric-co2-into-fuel  The advantage of air capture is that it can be done anywhere there is room for the plants. Not everyone is convinced CCU for fuel production will be economically viable without more effort: http://pubs.rsc.org/en/content/articlelanding/2015/ee/c4ee04117h#!divAbstract  However there are obvious attractions in being able to make synfuels, and also some clever new ideas for producing other high value materials, like carbon nano-fibres: www.acs.org/content/acs/en/pressroom/newsreleases/2015/august/co2.html

While CCU is moving ahead, so is the more conventional approach- simple Carbon Capture and Storage(CCS) as discussed in an earlier post in this Renew Extra series. The attraction here is that CCS can allow for the continued use of fossil fuels for electricity production, with the resultant CO2 being stored deep underground, for example in empty oil and gas wells. There is some symmetry in this since that’s where the oil and gas came from, and in fact inert gas injection is already used in Enhanced Oil Recovery to squeeze out the last of the resource. In theory the compressed CO2 should stay safely stored since it simply replaces the geologically trapped fossil resources that had remained secure for millennia until the artesian well cap was breached for extraction. Some may also bond with rock strata to form new solid deposits. However, there are worries about storage in other locations, for example in open aquifers, particularly if these were on land rather than offshore. Sudden release of large amounts of CO2 could be very hazardous to health if the cloud spread to areas of human habitation: when cool CO2, is heavier than air so it could produced a suffocating blanket of gas. 

So far most of the new developments have focused on the capture phase. It is sometimes claimed that very high percentages of the power plant CO2 can be captured, but it has to be remembered that there are energy costs in doing this, as well as further energy costs associated with compression and then transmission to the storage site. As a result, since extra energy will have to be supplied to meet this needs, the net CO2 reduction will be reduced maybe to 60-70% depending on, amongst other things, the transmission distance.  And the cost, already high, will be increased.

So far progress has not been spectacular – despite there being some major programmes. The UKs £1bn CCS programme took a long time to get started, with two pilot projects being chosen. There had been disagreements about the focus. Some said that the more advanced pre-combustion carbon capture approach was the best option, and that gas CCS was the way ahead. In the event, for one of the chosen projects, the UK backed an oxyfuel power coal fired plant at the Drax site in Yorkshire- the White Rose project. http://www.whiteroseccs.co.uk/

Oxygen injection prior to combustion increases the proportion of CO2 in the flue gas, to improve capture efficiency. But it is expensive to do this, and, with the future of government support for CCS in doubt, last year the Drax company pulled out if it. The aim had been to transport the captured CO2 via the proposed Yorkshire and Humber CCS Cross Country Pipeline, a project being developed by National Grid, for storage offshore in deep saline formations under the southern North Sea. But now it all seems doubtful- given the governments sudden decision, last November, to abandon the £1bn competition entirely. That raised some protest- DRAX had already been hit by government cuts back for biomass conversion projects. http://www.theguardian.com/environment/2015/nov/25/uk-cancels-pioneering-1bn-carbon-capture-and-storage-competition

This decision will also impact on the second project chosen, a post-combustion carbon dioxide capture technology from a gas fired CCGT plant at the existing Peterhead gas power station in Aberdeenshire, led by Shell and SSE. The aim was use an amine based capture system, plus conditioning and compression of the captured CO2, which would then be transferred as dense phase CO2 by subsea pipeline to the existing Goldeneye gas platform in the central North Sea, where it would be injected into the depleted Goldeneye gas reservoir for permanent storage, at a depth of over 2.5 km below sea level. Once the required volume of CO2 had been injected (1 million tones p.a. over 15 year was expected) it was planned to monitor the reservoir pressure build-up for 3 years, and to leave the Goldeneye Platform in place. After that the platform would be decommissioned. Post-closure handover to a ‘competent management authority’ would follow the reservoir tests.

As can be seen the technology is complex and extended, and there are risks.  In the UK, CO2 is classed by the Health and Safety Executive (HSE) as a ‘substance hazardous to health’. The HSE says that: ‘In CCS operations it is likely that CO2 will be handled close to, or above, its critical pressure (73.82 bara) where many of its properties are similar to that of a liquid. In this state it is often referred to as ‘dense phase’, whereas above critical temperature (31.04°C) and pressure it is referred to as ‘supercritical’. Significant hazards associated with dense phase or supercritical CO2 arise when pressure falls suddenly or is lost completely.  However, the HSE says that where the risks are properly controlled the likelihood of a major hazard incident is expected to be very low, as in other similar processes in the energy, chemical and pipeline industries. In its insurance review, Shell say ‘many of the risks involved in CCS are not that different from a typical Upstream Oil & Gas project (although the process of extraction is reversed) and are closely aligned with those of Enhanced Oil Recovery (EOR) projects in which Shell and various partners have been involved over the past 20 years’, but note that, at Goldeneye, ‘due to integrity issues and CO2 phase behaviour management, it is not possible to use the wells without any modification.’ www.gov.uk/government/publications/carbon-capture-and-storage-knowledge-sharing-commercial-project-management-and-lessons-learned

For now, CCS seems to be stalled in the UK. It is possible that other companies will step in, but CCS is expensive and the money is evidently no longer there.  Some see CCS as vital and urgent, and so there may still be pressure to push ahead : www.eti.co.uk/carbon-capture-and-storage-building-the-uk-carbon-capture-and-storage-sector-by-2030/  New ideas have emerged which might improve its economics: www.chemengonline.com/modified-mofs-cut-carbon-capture-costs-half/  http://actu.epfl.ch/news/a-cost-effective-and-energy-efficient-approach-to-/  and www.myscience.org/news/2015/study_suggests_method_is_rock_solid_for_storing_carbon_dioxide_deep_underground-2015-imperial

However, there is clearly still a way to go, and in principle it might be argued that CCU is better than CCS, since it offers a fuel, and CCS does seem a little inelegant- stuffing a pollutant underground and hoping it will stay there, all so that we can continue to use fossil fuels for a while longer, while avoiding some of their emission impacts.  Some hope that the development of CCS for fossil fuel use will just be a first stage, to be followed by BECCS- biomass energy with Carbon Capture, which could give us a negative carbon option- sucking CO2 from the air. Direct Air Capture (with storage) would also do that, but some say what's wrong with reafforestation- growing more trees.  That could be the fastest, easiest and most cost-effective option for post-emission atmospheric CO2 reduction: www.smithschool.ox.ac.uk/research-programmes/stranded-assets/Stranded%20Carbon%20Assets%20and%20NETs%20-%2006.02.15.pdf

Though trees aren’t a permanent carbon store (they die, rot and can burn), and as mentioned in an earlier post in this Renew Extra series, pulling CO2 out of the air may not work- it may be replaced by excess CO2 outgassed from the seas. So we would also have to remove that. So carbon capture, by whatever means, does seem to be a limited option and the various technical fixes are not a long-term solution.  Even BECCs would need large areas of biomass (and CO2 storage) to make a significant impact. Instead we should deal with the problem at source and switch to using non-fossil fuels as fast as possible. 

*For a useful review of negative carbon options see: www.carbonbrief.org/in-depth-experts-assess-the-feasibility-of-negative-emissions