What does the next generation of carbon capture technologies look like?

Posted on June 06, 2016
Posted By: Deborah Adams
 

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 60 US$/t. 

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 focussed 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. 

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 40 US$/tCO2. 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 focussed 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 30 US$/t. 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. 

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 30 US$/t. 

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 plant. 

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 30 US$/t are estimated at full scale. 

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 offer 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 and processes have impressive results at the laboratory and bench-scale, but overcoming the barriers to larger scale demonstration is notoriously challenging. Carbon capture at 30 US$/t CO2 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.

Next generation carbon capture technologies for coal by Toby Lockwood CCC/265, ISBN 978-92-9029-588-4, 127 pp, May 2016 is available for download from the IEA Clean Coal Centre Bookshop http://bookshop.iea-coal.org.uk/site/uk/clean-coal-technology-research-reports. Residents of member countries and employees of sponsoring organisations can download the report at no charge after a one-off registration. 

 
 
Authored By:
Deborah Adams is Studies Manager and Company Secretary at IEA Clean Coal Centre
 

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Comments

June, 14 2016

Richard Vesel says

Having looked at all these technologies over the past decade, there is only one logical conclusion to be drawn.

CCS may prove to be cost effective and workable in isolated cases, at a few plants in locations with just the right characteristics. However, as a broad solution to power generation from a traditional fossil source, it is an exercise in futility. There are too many competing sources for power, the competition is growing by leaps and bounds now, and here in the US, coal is rapidly becoming an albatross around the neck of many major utilities. They do not want to spend billions to replace their aging coal fleets with unproven technology which will garner no support from the investment sector.

Far better to abandon the topic to those who want to continue their research down this dead-end road, and spend the lion's share of the funds to solve the issues associated with bringing new generation power into the fore.

RWVesel

June, 16 2016

Richard Goodwin, Ph. D., P.E. says

Please see AAAS Peer Reviewed paper published in SCIENCE “Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions”; J’ M. Matter et. all; Science 10 Jun 2016: Vol. 352, Issue 6291, pp. 1312-1314; DOI: 10.1126/science.aad8132 “Dozens of pilot projects around the world have sought to test carbon capture and storage (CCS) as a way of curbing CO2 emissions from power plants. Very few have been scaled up, owing to prohibitive costs, estimated at $50 to $100 per ton of CO2 sequestered.”

DOE’s funding of research and demonstration projects to promote coal-fired power plants – achieving improved energy efficiency and carbon capture & sequestration [CCS] – should be considered as a part of an over-all energy strategy including nuclear, natural gas and renewable. Coal, Nuclear and Natural Gas comprise over 90% of base-load electrical generating capacity. The funding for CCS, however, could be questioned when compared to Engineering Procurement and Construction Costs [$2011] (“The Future of Coal Generation” 4/21/11): Nuclear = $5000/KW Supercritical Coal w/o CCS = $2800/KW; Supercritical Coal w/CCS = $3800/KW Natural Gas Combined Cycle = $700/KW Comparing current Operating Costs ($2011) for Coal vs. Natural Gas plants also favor Natural Gas {“Natural Gas Power Plants’ Fuel of Choice” July 2011). • Coal = $63/MWhr • Natural Gas = $57/MWhr If natural gas exceeds $6/MMBTU then a coal-fired plant, even with CCS, could be financially attractive i.e. achieving an equity pay-back within less than five years. So in the long run, investing in CCS could make financial sense. EIA has projected Natural Gas to remain below $3/MMBTU for next decade. So the economic case for CCS as a potential investment exceeds a five year horizon and would preclude investment community participation.

Richard W Goodwin West Palm Beach FL 6/16/16

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