Evaluating carbon dioxide storage in a variety of South African coals to estimate the potential for enhanced methane recovery

Premlall, Kasturie
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Due to the energy- and carbon-intensive economic structure of South Africa (SA), the country has become one of the biggest contributors to greenhouse gas emissions, emitting more CO2 than any other African country. The ratio of greenhouse gas emissions compared to per capita economic benefit, the so called carbon intensity of the economy, is amongst the highest in the world. Carbon capture and storage (CCS) seems to be the most immediate form of action that can be implemented with the possibility of instantaneous reduction of CO2. The injection of CO2 into deep-unmineable coal seams, although not commercially viable for coal production, is a possible mitigation option under CCS for permanent underground storage of CO2. As a spin-off, useful coal-bed CH4, referred to as enhanced coal bed CH4 (ECBM), could be extracted from the coal seam following CO2 injection. In SA it has been estimated that approximately 1.2 Gt of CO2 could be stored in the coalfields. Although not currently the preferred option for geological storage, coalfields provide the largest onshore CO2 storage possibility. The current research project aimed to study the fundamental differences in CO2 adsorption in a variety of SA coal samples in order to access the CO2 sorption capacities and secondly to evaluate the potential CH4 characteristics of SA coals. The investigation aimed to identify the fundamental differences around the effects of increased pressure under simulated in-seam conditions including super-critical pressures up to ~90 bar for gaseous and supercritical CO2 injection. The effects on CO2 adsorption with regard to the difference in coal moisture contents, simulated in the range from ~0.5 – 4.4% and the influence of increased temperatures in the range of 35 to 55 ˚C were carried out on ten (10) SA coals taking into consideration differences in coal properties, samples with varying rank, ash and maceral compositions were sourced for this research. Then secondly, to evaluate the desorption potential of CH4 for seven (7) selected SA coals. A High Pressure Volumetric adsorption system (HPVAS) was successfully designed and constructed in order to conduct experimental tests to generate the adsorption isotherms for the various parameters tested. Results presented show comparable results with published literature in terms of the degree of variance in coal properties (with respect to rank, maceral and mineral content, ash contents and the effects of moisture, and temperature variance) and the uptake of CO2. Higher rank coals have a greater CO2 absorption propensity, whereas lower rank bituminous coals tend to exhibit lesser CO2 uptake, however, this is dependent on the coals’ petrographic composition. It was clear that samples in the range greater than a vitrinite reflectance of 0.7% (RoVmr) exhibited increased CO2 uptake due to larger macro, increasing meso porosity and micro-pore volumes. Findings related to coal properties; revealed that coals with a higher ash content exhibited a negating effect with regard to enhanced CO2 adsorption. On average, for a 1% increase in ash content in HRC and MRC coals, a decrease of CO2 adsorption capacity of 1.1 mmol/g and 0.018 mmol/g is observed respectively. While for maceral composition these findings suggest that a specific or ideal ratio between only the maceral components, in similar rank coals, is the controlling factor for best CO2 adsorption required. In terms of addressing the adsorption parameters, such as super-critical pressure, temperature and moisture variations inherent in natural coal seams, etc., it was determined that with increased pressure, more adsorption takes place for most coal types. A very positive correlation was found to exist between adsorption of CO2 and desorption of CH4, with increased pressure injections, ranging from sub-critical to super critical pressures, exhibiting increased sorption results, irrespective of coal moisture or temperature effects. From these findings for simulated conditions regarding the effects of coal seam moisture and temperature variations, it has been concluded that results displayed an obvious decrease in CO2 sorption ranging from sub-critical to supercritical pressures overall. The decrease in CO2 sorption was as much as 77% from dry (0%) to the maximum moisture simulated value of ~4.4%. Sorption decreased almost linearly for every 1% of coal moisture increase, until the maximum coal saturation was approached at around 4%. Sorption results relating to increased temperature also displayed an inverse relationship, and hence lower overall CO2 sorption capacities were calculated. The heats of adsorption for these coals were found to be between 21.9 and 39.9 kJ/mol confirming the nature of adsorption to be physical. Results confirm that the calculated heat of adsorption (KJ/mol) and the adsorption capacity (mmol/g) are positively correlative. For investigations pertaining to CH4 desorption for CH4 saturated simulated coals (CH4 added to and then removed from coal samples due to the unavailability of freshly cored coal samples), it was observed that CO2 uptake by pressurized injection for low - high pressures certainly enhances CH4 desorption rate. Results revealed that incremental CO2 injection pressures yielded higher CH4 desorption rates, for both the HRC and MRC coals. Generally there was an observed increase in the rate of CH4 desorbed for all coals tested at 55 oC as compared to 35 oC. This can as well be attributed to the fact that the increase in temperature causes the adsorbed CH4 molecules to vibrate more due to the increased kinetic energy of the molecules. This consequently leads to ease of desorption when CO2 is pumped under pressure into the coal structure, which clearly favours ECBM potentials. Some very good findings have been highlighted in the thesis from a SA coal perspective, and certainly serve as a very good starting point for further investigations pertaining to CO2, CH4, and coal interactions. However, from the vast literature already published globally, it can be seen that much more needs to be done in terms of addressing coal-CO2-CH4 research from a SA perspective, and indeed CCS in SA in general. It is apparent that the results and sum of the key findings presented in this thesis, are of importance for the selectivity and technical modelling for CO2 onshore coalbed storage and ECBM projects to be implemented in SA in the near future so as to meet the demands required to reduce CO2 emissions in SA as part of the global community.
A Thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy in the School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, 2019
Premlall, Kasturie (2019) Evaluating carbon dioxide storage in a variety of South African coals to estimate the potential for enhanced methane recovery, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/29046>