Methane and carbon dioxide sorption studies on South African coals.
Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2009.
Sequestration of carbon dioxide, CO2, has received large interest as a viable option for mitigating the high atmospheric concentrations of this greenhouse gas. Each year 25 gigatons of anthropogenic CO2 (7.3 GtC/yr) are released into the earth’s atmosphere with the combustion of fossil fuels being the major contributing source. Research in the field of sequestration technology involves evaluating various geological structures as possible reservoirs, determining adsorption capacities of natural formations and developing methods for carbon dioxide injection and the monitoring thereof. Identified potential CO2 reservoirs for geological carbon sequestration (GCS) include saline formations, depleted oil and gas fields and deep coal seams. Carbon dioxide sequestration in coal seams provides the economic opportunity of enhanced coalbed methane (CH4) recovery (ECBM). In South Africa, some coal seams are considered a viable option for long term CO2 sequestration projects as they are abundant and closely situated to South Africa’s largest concentrated CO2 point sources. Many studies have been conducted to determine the sorption capacities for methane and carbon dioxide gases on various coals from around the world; however, similar data have not been recorded for South African coals. The objectives of this study are to determine the adsorption capacities for methane and carbon dioxide of three South African coals over a pressure range of 0 – 50 bar. In the study, single-component gas adsorption experiments were conducted and the absolute adsorption capacities are reported. Isothermal adsorption experiments were conducted using both the volumetric and gravimetric methods with the volumetric apparatus pressure range extending up to 50 bar and the gravimetric apparatus up to 20 bar. Carbon dioxide adsorption capacities are much higher than the methane adsorption capacities, which are expected. Gravimetric experiments produce greater adsorption capacities than the volumetric method. However, the relative CO2/CH4 ratios for each coal, as well as the relative CO2/CO2 ratios between coals, remain almost identical. The difference in adsorption capacity is attributed to the strength of the vacuum pump used on each apparatus. The gravimetric apparatus makes use of a much stronger vacuum pump which can thus evacuate the coal pores more adequately than in the volumetric apparatus. The methane and carbon dioxide adsorption capacities of the three moisture-free coals compare well with literature data. The adsorption isotherms fit conventional adsorption models (the Langmuir and Freundlich adsorption equations) extremely well thus indicating that monolayer adsorption takes place. Since no internationally recognised testing standards are in place regarding adsorption procedures on coal, it is very difficult to compare adsorption results presented in the literature. Respective researchers determine their own experimental conditions for the many variables in coal adsorption studies. It is recommended that international testing standards be set in place to make coal research comparable. Such efforts would aid the development of a coal adsorption database, another recommendation, which would advance sequestration technology exchange and eliminate duplication of research efforts. The objectives of the project were achieved by determining the absolute adsorption capacities for carbon dioxide and methane gas of the three South African coals within a pressure range of 0 – 50 bar. Further work is required to investigate adsorption capacities of South African coals under supercritical conditions (above 73 bar abs and 31.1 oC).