Carbon dioxide (CO2) capture and storage (CCS) is a process designed to mitigate greenhouse gas (GHG) emissions. CCS consists of capturing CO2 from industrial generators, transportation to a site of storage and then long term isolation from the atmosphere. The aim is to achieve a net reduction in CO2 levels from the atmosphere ("negative emissions") and to stabilize CO2 levels to between 450 and 750 parts per million by volume (ppmv). Storage of captured CO2 may be in a range of media such as oceans, underground saline reservoirs, oil and gas fields and fixation into inorganic carbonates. Another storage medium is in un-mineable coal seams.
Coal deposits provide a potentially viable storage site for the long term capture of CO2 due to its global availability and through value added returns (methane, CH4). It has been estimated that 40% of the worlds CO2 emissions originate from facilities and plants which use fossil fuels (OECD/IEA, 2008). Hence, this particular sector has been identified as the prime candidate for CO2 capture technology due to the size of the industry and the expected growth required in sustaining future demand.
CCS includes, capture, transportation and storage. CO2 capture is the first step in this process. The focus for this initial stage has been on retrieving CO2 from power plants and in particular, pre-combustion and post-combustion CO2 capture. Depending on the location of the capture site and the selected storage medium, transportation to a storage medium is via vehicles, pipelines and seagoing vessels for offshore operations. However, situating the capture and storage sites within close proximity of each other limits the additional costs of transportation, providing a more financially effective operation.
The safe and secure storage of captured CO2 is considered the most crucial component in the success of this developing technology. With a number of different storage options from geological, oceanic and terrestrial, each medium has its advantages and disadvantages. However, geological storage and in particular, storage in un-mineable coal seams is considered a promising medium for safe storage. Un-mineable coal seams are those too deep or too thin to be mined economically, using current technologies.
Furthermore, each coal deposit contains different levels of methane, a natural gas that is a highly efficient source of energy. Methane is absorbed onto the pore surfaces of the coal seam. However, CO2 has a greater affinity to coal than methane and as a result, injection of CO2 causes the methane to desorb from the coal leaving methane gas that has the potential to be extracted (White, 2005). The process of CO2 injection and methane extraction is called Enhanced Coal Bed Methane Recovery (ECBM-R). Incorporating ECBM-R with CCS provides the opportunity of offsetting the costs of CCS. The use of carbon credits will also provide incentives for industry to adopt this technology.
Monitoring of stored CO2 will be an important aspect in the successful implementation of CCS in coal seams. Notwithstanding, such technology has great potential in achieving a reduction in GHG emissions.
This Thesis investigates the progression of this developing technology, specifically the storage of CO2 in un-mineable coal seams and investigates its potential as a GHG mitigation strategy.