Pre-combustion CO2 capture from biogas - the way forward?

Carbon Capture and Storage (CCS) systems are receiving a lot of attention nowadays. The system offers the potential to sequester the carbon dioxide - the most dangerous greenhouse gas - from fossil fuels into geological formations, so that the use of these primary energy sources contributes less to climate change. A lot of research is going on to study different techniques and sites to store the carbon dioxide safely so that it stays locked up for decades or centuries.

At the Biopact, we follow these developments, because CCS can be applied to biofuels as well. Whereas CCS applied to fossil fuels results in slightly positive or carbon neutral energy, 'Bio-energy with Carbon Storage' (BECS) systems are radically carbon negative. Scientists looked at BECS in the context of so-called 'Abrupt Climate Change' (ACC), which is basically an apocalyptic global warming scenario. In case ACC were to occur, BECS would be one of the only realistic mitigation options, because the system allows societies to reduce CO2 emissions radically while still using energy at the same time. BECS is the only carbon negative energy system in existence. But for BECS to work, CCS technologies must be reliable and commercially viable. And this is where a major problem arises.

CCS costs
The main obstacle to the commercial viability of CCS is the high cost of capturing the carbon dioxide gas. The other costs associated with integrated CCS-systems, such as geological assessments of potential storage sites, CO2 transport and injection, and monitoring and measuring stored carbon dioxide, are relatively low (see table, click to enlarge).

Several carbon capture technologies exist, only one of which stands out for being very simple, scaleable, tested and low-cost, namely CO2 capture from biogas fermenters. The Biopact will present this route, which is part of a broader concept, to the EU's public consultation on CCS.

Broadly speaking, there are two different stages at which the CO2 from fuels can be captured: either before the fuel is used in power plants (pre-combustion capture) or after burning it (post-combustion capture from flue gas).

The main problem with post-combustion capture is the low concentration of CO2 in the flue gas. Depending on which industry is concerned, this concentration can range between a few percent only to 15%. Other gases such as oxygen, water vapour or nitrogen also occur in flue gas. It would be out of the question to seek to compress them all for storage, from the standpoint of both the energy costs and the storage capacity. Separation methods are thus required so as to trap the CO2 preferentially, so that it can be compressed and make optimal use of the storage capacity of a sequestration site.

Within the post-combustion category the following CO2 capture techniques can be distinguished:

• absorption with solvents (generally amines)
• calcium cycle separation: quicklime-based capture that yields limestone, which is then heated, thereby releasing CO2 and producing quicklime again for recycling.
• cryogenic separation: based on solidifying CO2 by frosting it to separate it out; the low concentration of CO2 in the flue gas makes this uneconomical
• membrane separation: work is required on developing the membranes themselves, on their optimisation for large-scale generation conditions, and on minimising the energy required for separation
• adsorption: the fixation of CO2 molecules on a surface. The adsorbing material (mostly zeolites) undergoes a series of pressure or temperature variations to store/release CO2 as required
• Oxy-fuel combustion capture: not CO2 capture in the true sense of the term; the objective is to increase the CO2 fraction in the flue gas to 90% by performing combustion in the presence of pure oxygen. However, separating out the oxygen from air, performed mainly using the cryogenic principle, is both costly and energy-consuming.

The goal of pre-combustion capture techniques is to trap the carbon prior to combustion: the fuel is converted on entering the installation into synthesis gas – a mixture of carbon monoxide (CO) and hydrogen, through gasification, shift reaction or partial oxidation, after which the CO is separated mainly via:

• steam reforming in the presence of water: the CO present in the mixture reacts with the water during the shift conversion stage to form CO2 and hydrogen. The CO2 is then separated from the hydrogen, which can be used to produce energy (electricity or heat) without giving off CO2

CO2 capture from biogas
All the above technologies are currently too costly to make CCS commercially viable (see table). The alternative suggested by the Biopact is significantly lower-cost and consists of pre-combustion CO2 capture from anaerobically fermented biogas:
biomass :: bioenergy :: biofuels :: energy :: sustainability :: climate change :: carbon dioxide :: biogas :: biomethane :: natural gas :: carbon capture and storage :: CCS :: carbon negative ::

The advantage of biogas is the fact that the fermentation of biomass results in a gas the CO2 fraction of which is much larger than that of flue gas. Depending on the feedstocks and the production process, biogas contains between 35 and 45% of CO2. The remainder is methane (CH4), with some trace gases and elements. This large CO2 fraction makes pre-combustion CO2 capture technologies commercially viable. Comparisons show that CO2 capture from biogas is between 4 and 6 times less costly than other pre- and post-combustion separation techniques.

To put it in simple terms: biogas can be purified, the CO2 stored and the resulting high quality methane used as an ultra-clean and carbon-negative biofuel. The biomethane can be used either in power plants, or in CNG-capable vehicles.

So what would the ideal system of 'biogas with carbon capture and storage' (BCS - see illustration, click to enlarge) look like? It consists of creating biogas production zones close to carbon sequestration sites (such as deep saline aquifers or depleted oil and gas fields), and preferrably in the subtropics and the tropics, where biomass yields are high.

Contrary to other CCS strategies, our system is independent of power-plants (because the CO2 capture occurs before the combustion of the methane) and thus independent of heavily urbanised or populated regions (where power plants are located). The system can be located close to the sequestration site, so that CO2 transport costs are reduced significantly too. On the other hand, the carbon-negative biomethane resulting from the process would then have to be shipped to power plants. This can be done by (existing) pipelines or by LNG tankers.

The Biopact is researching possible sites for this system - even though our expertise on this front is quite limited. The ideal-type system would look like this:

1. a sequestration site close to an existing LNG facility (possibly nearby depleted natural gas fields or oil fields where the CO2 can be stored while enhancing oil recovery)
2. dedicated energy crop plantations would be established nearby
3. the biomass - which sucks up atmospheric CO2 - is anaerobically fermented into biogas
4. the CO2 fraction is separated, transported (piped) and injected into the sequestration site (the gas field)
5. the pure biomethane (99% CH4) is liquefied at the existing LNG facility, and exported to world markets
6. as a carbon negative gas, it would fetch premium prices, provided a global market for CO2 comes into existence

Alternatively - but this obviously remains a concept that would require serious investments - we start from scratch and build a new LNG facility close to a near-shore/on-shore sequestration site where our biogas system would be located. The ultra-clean, carbon negative biomethane would then be liquefied and shipped to world markets.

Take into consideration that biogas made from dedicated energy crops in large-scale production facilities in the tropics is expected to be competitive with natural gas (if natural gas prices stay as high as they are today, there would even be a serious margin, making biogas considerably cheaper).

To fill the largest LNG tanker currently on the market - with a capacity of 250,000 tons of liquefied natural gas, equivalent to around 300 million cubic meters of natural gas - with pure biomethane, one would need between 450 and 650 million cubic meters of biogas. This amount can be obtained from around 60,000 hectares of cassava, or 40,000 hectares of sugarcane.

Comparing different 'BECS' systems
'Biogas with carbon storage' can be considered to be a BECS system that results in a biofuel that can be used in power plants as well as in automotive applications (CNG-capable vehicles, fuel cell vehicles). But overall, the carbon capture stage would be considerably lower-cost than BECS relying on solid biofuels (wood co-fired in coal plants, or in dedicated biomass power plants). Because with solid biofuels, only post-combustion CO2 capture is feasible or, alternatively, the expensive pre-combustion capture techniques based on gasification.

Similarly, an alternative clean carbon-negative automotive biofuel would be obtained from biomass-to-liquids, the CO2 of which is captured in the pre-combustion stage; biomass would be gasified, the CO from this syngas would be removed by steam reforming, after which the remaning hydrogen-rich gas is synthesised via the Fischer-Tropsch process into so-called 'synthetic biofuels'. The problem is that this pre-combustion CO2 separation is far more expensive than CO2 removal from biogas.

Finally, a word on the potential of biogas. According to a recent study by the Institut für Energetik und Umwelt, based in Leipzig, and by the Öko-Instituts Darmstadt, the gas can be produced on a very large scale. The study shows that the EU can produce 500 billion cubic meters of natural gas equivalent biogas per annum by 2020, enough to displace all imports of Russian natural gas (earlier post).

In the tropics and subtropics, production would be more cost-effective and energy efficient. By feeding biomethane produced in the South into existing LNG export hubs (such as those in Nigeria, Malaysia, Indonesia, Papua New Guinea (planned), Brunei, Equatorial Guinea (planned), Venezuela, Bolivia (planned) or Angola (planned)), it can be shipped to LNG terminals in the North and fetch premium prices. Purification and liquefaction of biogas into renewable LNG is already a commercial reality in the US.

Conclusion
The system as we described it here, is only at the conceptual stage. One aspect of the carbon negative energy system, the cost-sensitive CO2 capture process, is relatively low-cost compared to other techniques associated with the use of fossil fuels or with solid biofuels. The Biopact is writing an introductory dossier on the concept, and will present it to the EU's public consultation on CCS.

More information:
On the low costs of geological assessments, see:
S. J. Friedmann, J. Dooley, H. Held, O. Edenhofer, "The low cost of geological assessment for underground CO2 storage: Policy and economic implications", [*.pdf] Lawrence Livermore National Lab, Energy & Conversion Management, February 15, 2005.

On the cost of different carbon capture techniques:
Mahasenan N, Brown DR. “Beyond the Big Picture: Characterization of CO2-laden Streams and Implications for Capture Technologies”. In: Proceedings of 7th International Conference on Greenhouse Gas Control Technologies. Volume 1: Peer-Reviewed Papers and Plenary Presentations, IEA Greenhouse Gas Programme, Cheltenham, UK, 2004

On CCS in general:
The IEA's CO2 Capture and Storage website, part of the IEA Greenhouse Gas R&D Programme.
UK Carbon Capture and Storage Consortium.
IEA's Clean Coal Center: Carbon Capture and Storage (Sequestration).
EurActiv dossier on CCS.

On the EU's public consultation round on CCS:
European Commission, DG Environment: "Capturing and storing CO2 underground - Should we be concerned?" (public consultation website).