Post Combustion Carbon Capture from Coal Fired Plants – Solvent Scrubbing

Publication Overview

The potential use of solvents for carbon dioxide capture from the flue gas from coal fired power plants is reviewed. After an introduction to solvent absorption of CO2, the use of alkanolamine solvents, particularly monoethanaloamine (MEA) is considered. The degradation of solvents in the flue gas environment and the consequent corrosion problems associated with the degradation products is then examined. The energy consumption for regeneration of the solvents is a key feature in determining the overall costs of solvent scrubbing. There is considerable research on alternative solvents to MEA which have higher capacity for CO2 capture and lower energy consumption among other attributes. The design of the absorption contactors which facilitate the contact and interaction of the gas and liquid phases can also contribute to lowering the energy consumption of the overall process. Techno-economic studies, process modelling and simulation are also reviewed. Some details of existing demonstration and pilot plants and current national and international R&D programmes are given. Finally, the potential environmental aspects of the solvent scrubbing processes are briefly examined.

Publication Summary

If we consider that there are over 2000 coal fired power plants in existence then, other than shutting them down, or substituting natural gas for coal, the best option for CO2 emission abatement is to retrofit as many as possible with carbon capture systems. At present, the lead contending technology would appear to be solvent scrubbing using amine solvents. Some of the advantages of amine scrubbing for postcombustion capture can be listed, the process:

• can be retrofitted to some existing power plants insuitable locations;
• has been proven on a small scale;
• may offer flexibility if it can be switched betweencapture and no capture,
• offers potential cost reductions if it proves to beanalogous to other technologies; and
• has a strong research base which should lead to bettersolvents and processes.

Nevertheless, the difficulties in retrofitting post-combustion capture must not be underestimated. There is little point in retrofitting plants of low thermal efficiency since the efficiency losses would render the plant uneconomic. Also, if the views of Ducroux and Jean-Baptiste (2005) that only limited evolution is expected in solvent development are correct, then CO2 capture by solvents may be superseded by other processes. These, such as solid sorbents and membranes, will be the topic of a future IEA Clean Coal Centre report. However, flue gas from coal fired power plant should be viewed as a fairly difficult material for amine solvents to deal with, so some challenges remain:

• the cost of electricity will increase;
• more coal is needed for the same power generationcapacity;
• it is likely that the FGD systems will need to beimproved; and
• there is no experience with CO2 capture in coal firedplant at full power plant scale.

There are other problems, such as the degradation products formed by solvent reaction with flue gas from coal and the associated corrosion problems. It is likely that these could be avoided by the use of stainless steel components in the CO2 capture equipment but this would increase the capital cost. The reason that the cost of electricity increases and more coal is needed is the reboiler heat duty, the energy that is needed to regenerate the solvent by driving off the captured CO2. Efforts to reduce this heat duty are at the heart of reducing the total costs of solvent scrubbing. There has been much work directed at finding alternative solvents to MEA that could result in the overall energy demand being reduced. Some of the energy demand can be reduced by improved design of the contactors which provide the area on which the gas and liquid phases interact. There is hope that the economics of CO2 capture will improve; analogous technologies have seen considerable cost reductions as a result of ‘learning by doing’ and many experts believe that significant improvements are possible in the next decade. Certainly, if new power plants are to be built, it makes sense to build them ‘capture ready’ if possible. Despite many well planned national and international R&D programmes and the existence of some pilot and demonstration plants, as yet there is no full-scale demonstration plant. One is sorely needed. A potential problem is that, in capturing CO2 with solvents, environmental problems are shifted elsewhere. It is likely that any environmental impacts can be properly mitigated but they certainly should not be ignored. Finally, the main challenge still remains: how to reduce the costs of capture.