A Clean Coal Option

Coal will continue to be the largest energy source in most developed and emerging countries. Efficiency improvements alone, setting aside the difficulties of implementation, are unlikely to slow the increase in world greenhouse gas emissions. If the emission reductions sought by governments, and world climate bodies are to be met, more dramatic technical solutions are required, and most reviews are pointing to CO2 capture and storage (CCS) as the most promising technology for fossil fuel emissions.

UCG in combination with CCS is recognised as a potential route to carbon abatement from coal. Reports suggest that UCG in combination with CO2 injection into adjacent coal seams to enhance coal bed methane (ECBM) is a potentially attractive option, particularly under river estuaries near-shore and eventually offshore coal.

UCG has synergies which may be important to CCS. First and foremost it is a gasification technology with all the benefits that accrue in terms of CO2 capture from pre-combustion gas. These include smaller more efficient separation plant, lower capture costs and the ability to produce hydrogen in bulk quantities. Furthermore, the composition of UCG clean gas, and the high pressure available, suggests that a range of CO2 capture options could be considered.

The scope for optimising the capture process is considerable and emissions already low for coal can be reduced by partial CO2 capture, to those of natural gas combined cycle (CCGT) or taken to a level approaching renewable energy. The process is particularly well suited to the production of low carbon gas mixtures of hydrogen and methane. These have higher energy densities and are safer to handle than pure hydrogen. Such mixtures may be suitable for existing gas transmission lines and could make an effective transport fuel for some vehicles.

Close Coupled UCG with CO2 storage

The coal seam in the vicinity of the UCG gasification seam is a potential storage area for CO2 with several advantages. It can be accessed with the same drilling equipment as UCG and the pipeline distances are short. In addition, the gas may not need to be purified to the same extent as a CO2 gas gathering pipeline

CO2 injection into coal seams displaces methane and itself is adsorbed onto the coal in a physical bond. The use of CO2 to sweep the methane out of the coal is a process known as enhanced coal bed methane (ECBM), which offers considerable potential both as a method of extracting more methane from the coal and as a permanent store for the CO2. Tests have been made of ECBM in China, Japan, United States and Poland (REPOCOL) and by and large, the results have been encouraging, in spite of potential permeability and coal swelling problems in some cases.