Next generation carbon capture technologies show promise

Carbon capture and storage (CCS) technologies have been hailed as the solution to the problem of reducing emissions from fossil fuel power plants. Their widespread implementation has been included in numerous projections regarding how best to reduce global temperatures to 2ºC or lower if possible. The reality is that there are very few ongoing CCS projects for which there are several reasons, including cost and energy consumption. The answer may lie in the next generation of CCS technologies, which look to support a competitive form of low-carbon, transmittable energy. Technologies that incorporate this with power generation, such as chemical looping, have the potential to improve the efficiency and cost of CCS and are scaleable over the next five years.

The widespread rollout of carbon capture and storage (CCS) is included in many projections of how to limit the temperature increase to 2°C. But to date there are very few operational projects. There are a number of reasons for the slow spread of these technologies primarily designed to reduce emissions of CO2 from fossil fuel fired power plants. A major one is the cost: for example, the cost of electricity can increase by up to 80% when applying commercial capture technologies to coal-fired power plant. This means that CCS is only likely to proceed in the most favourable economic and legislative environments, with a CO2 price as high as US$60 per tonne.

The main reason the existing technologies are so costly is because of the large amount of energy they use – up to a third of the plant’s gross power output. So much research has focused on raising the efficiency of the gas separation processes which are fundamental to nearly all forms of carbon capture.

Toby Lockwood of the IEA Clean Coal Centre has studied the next generation of carbon capture technologies, which aim to support a more competitive form of low carbon, dispatchable energy.

Post-combustion capture

In post-combustion capture processes, CO2 is separated from standard coal flue gases, traditionally with amine solvents. Recently, novel liquid solvents, dry sorbents and membranes have been investigated as alternatives. Most new solvent systems aim to limit the energy consumption of the CO2 desorption step by reducing the strength of the interaction with CO2 or using a phase separation or precipitation step to reduce the thermal mass of the CO2-rich product. These systems tend to offer only incremental cost reductions and have not yet been tested at a large pilot scale.

The use of engineered forms of the enzyme carbonic anhydrase to accelerate the reaction of CO2 with environmentally benign carbonate solvents shows promise. It means lower-grade heat can be used and brings capture costs down to below US$40 per tonne. Solid sorbents also have potential, although scale up to reactor dimensions is technically challenging.

A number of medium-scale pilot plants have demonstrated effective capture using vacuum pressure swing sorbent processes and low-cost sorbents, but most current research has focused on temperature swing adsorption with more CO2-selective materials as a cheaper prospect for high CO2 capture rates at large scales. However, the challenge of achieving efficient heat transfer in solid systems requires novel reactor designs.

Harnessing some of the heat released in CO2 adsorption for the desorption step is a key strategy which has promised capture costs approaching US$30 per tonne. CO2-selective polymer membranes avoid steam extraction or chemical waste, and could be scaled up in a straight-forward modular fashion. However, the capture rate is limited by practically achievable pressure gradients and two separation stages are therefore required.

One approach is to use a flow of combustion air to help drive one separation stage, which provides CO2-enriched flue gas for the principal stage. Nevertheless, the cost of most membrane systems is likely to exceed 40 US$/t where 90% CO2 capture is required, but could become much more competitive at lower capture rates.

Some notable post-combustion capture concepts allow the capture plant to generate its own power, which mitigates the energy consumption of the gas separation step. Currently demonstrated at 1 ? 2 MWth, calcium looping is a form of sorbent-based capture where sorbent regeneration takes place in its own oxyfuel-fired boiler. At a similar scale, gas-fuelled molten carbonate fuel cells can act as CO2 separating devices, while generating electrical power.

Pre-combustion capture

In pre-combustion capture, CO2 is removed from a high-pressure mixture of CO2 and hydrogen obtained from coal-derived syngas, leaving the hydrogen to power a gas turbine. The established process is complex, but the high partial pressures of CO2 offer greater potential for the efficient use of sorbent- and membrane-based separations at high temperatures. In particular, the CO2 capture step can be effectively combined with the prior water-gas-shift necessary to convert CO to CO2, helping drive the reaction to completion and reduce consumption of steam reagent. These systems remain limited to small-scale trials, but could bring capture costs down to the region of US$30 per tonne.

Oxyfuel combustion

By substituting combustion air for oxygen, oxyfuel combustion produces a relatively pure stream of CO2 for sequestration. Current research exploits characteristics of the process for higher efficiency power generation. In particular, pressurised reactor concepts with minimal flue gas recycle have been developed which could reduce the efficiency penalty to below 5% points. Even higher efficiencies are possible by firing coal syngas in high-pressure oxyfuel gas turbine cycles such as the Allam Cycle, which has estimated lower power generation costs than unabated coal plants.

Chemical looping combustion instead delivers oxygen to the fuel in the form of a solid oxide ‘carrier’ material, so avoids any gas separation step and enables high efficiencies and low costs. For solid fuel applications in particular, a low cost carrier material is essential. Pilots beyond the current 3 MWth level are being actively pursued, and costs below US$30 per tonne are estimated at full scale.

Conclusion

For new coal plant applications, technologies which inherently incorporate CO2 capture with power generation, such as chemical looping and the Allam Cycle, are the most promising routes towards a step change in CCS efficiency and cost, and could feasibly be scaled up in the next five years.

For retrofit to the world’s substantial existing coal fleet, a range of post-combustion capture technologies offers potential gains over existing amine-based technologies, but as these are relatively proven and established, they may be hard to displace. However, considerable variation in site-specific factors means a range of technologies will have a role. Many novel materials processes have impressive results at the laboratory and bench-scale, but overcoming the barriers to larger scale demonstration is notoriously challenging. Carbon capture at US$30 per tonne appears to be fundamentally achievable by a variety of methods and could provide a major impetus to widespread adoption of CCS, but significant government support will be required if the most promising technologies are to progress to the large scales necessary for coal power application.

Edited by Jonathan Rowland.