Capture of carbon dioxide from flue gas using solid regenerable sorbents

Carbon dioxide emissions from the combustion of fossil fuels are a significant factor in global climate change. Large stationary sources such as coal-fired electric generating plants are likely to be the most cost-effective targets for carbon dioxide capture. At present, liquid amine-based scrubbing systems are the only processes available for this application. Processes based on regenerable solids that absorb carbon dioxide from flue gas and release it in concentrated form have the potential to be less expensive to operate. This paper summarises the results of studies conducted at RTI and Louisiana State University (LSU) to investigate the feasibility of using sodium or potassium carbonate as a sorbent. Upon reaction with carbon dioxide and water (also present in flue gas), this material is converted to sodium or potassium bicarbonate. Upon heating (ideally with low grade heat from the generating plant), carbon dioxide and water vapour are released and the solid carbonate can be reused. Work to date has focused on thermogravimetry (TG) and bench scale fluidised-bed testing, as well as characterisation of materials and thermodynamic and kinetic analyses. TG studies with sodium carbonate have indicated that the sorption reaction takes place rapidly at approximately 60°C and that the sorbent can be regenerated at temperatures less than 120°C. A five-cycle test conducted in a bench scale fluid bed reactor system indicated that the sorbent could be regenerated and reused. The process implications of compound salts and hydrates in the sodium carbonate system on the useful capacity of the sorbent and heat removal requirements were also investigated.