CO2 is the main greenhouse gas. CO2 emissions have an impact on global climate change. Global concentrations of CO2 in the atmosphere have increased from pre-industrialisation levels of approximately 280 parts per million by volume (ppmv) in around 1860 to approximately 316 ppmv in 1958 and rapidly to approximately 369 ppmv today (UNEP, 2005). Global CO2 concentration is predicted to rise to above 750 ppmv by 2100 if no action is taken to address the current situation.
With growing concerns over the increasing atmospheric concentration of anthropogenic greenhouse gases, effective CO2 emission abatement strategies such as Carbon Capture and Storage (CCS) are required to combat this trend. CCS is a “process consisting of the separation of CO2 from industrial and energy-related sources, transport to a storage location and long-term isolation from the atmosphere” (IPCC, 2005). From this definition, CCS consists of three basic stages:
(a) separation of CO2; (b) transportation and (c) storage. There are three major approaches for CCS: post-combustion capture, pre-combustion capture and oxyfuel process (IPCC, 2005).
In post-combustion capture CO2 is separated from the flue gas after fuel is burnt. This process can be added, or retro-fitted, to existing power stations, either coal or natural gas-fired.
During pre-combustion capture the fossil fuel is reacted with steam and oxygen, producing a synthetic gas (syngas) which is made up of mostly carbon monoxide (CO), carbon dioxide and hydrogen (H2). An additional reaction with water (known as a water gas shift) can be used to convert the residual carbon monoxide to CO2 and additional hydrogen. The CO2 is removed and the hydrogen can then be burned in gas turbines to produce electricity.
Oxy-firing combustion capture involves the combustion of fuel (coal or gas) in pure oxygen or oxygen-enriched air. The process can produce about 75 per cent less flue gas than air-fueled combustion and the exhaust consists of between 80 and 90 per
cent CO2. The remaining gas is water vapour, which simplifies the CO2 separation step. An air separation plant is required to produce pure oxygen for the process from air.
M. Wanga, A. Lawal, P. Stephenson, J. Sidders, C. Ramshaw. “Post-combustion CO2 capture with chemical absorption: A state-of-the-art review”. Chemical Engineering Research and Design 89 ( 2011 ) 1609–1624.
Capturing Carbon Dioxide. http://www.co2crc.com.au/