Separation And Enrichment Of Coal Mine Methane By Vpsa

China has become the second largest country for energy consumption, of which beyond70%is supplied from the coal. Every year, there are about20billion cubic meters of methane emitted from underground coal mining. Due to large amount of air mixed into coal mine methane (CMM) drainage, the CMM are diluted to a low CH4concentration and can’t be utilized directly, of which the utilization is only10%. Due to nature of CH4, there are several disadvantages for CMM to be discharged directly. The emission of CMM can cause the air pollution due to the greenhouse effect. On the other hand, CMM is a clean and useful energy resource, so the emission can result in the waste of energy. Thus, the study of the separation and enrichment of CMM within low CH4content is of a practical significance and theoretical value.O2, N2and CH4are the main components in the CMM. The enrichment of ventilation air methane (VAM,0.52%CH4and N2balance gas), the oxygen removal from CMM (10-15%O2,30-40%CH4and N2balance gas) and the upgrading of CMM without oxygen (40-50%CH4and N2balance gas) were preliminary investigated, respectively.Due to the similar physical properties for N2and CH4, it is difficult to seek a suitable adsorbent for the separation of N2and CH4by pressure swing adsorption (PSA). Nine kinds of commercial activated carbons, an activated carbon fibre and a13X zeolite molecular sieve were used to investigate the adsorption thermodynamics and adsorption kinetics of N2and CH4in the adsorbents. With the nine commercial activated carbons and other five carbon adsorbents prepared in the lab, the effects of specific surface area, microporous volume, microposity and micropore size distribution of the adsorbents on their CH4/N2separation factors were studied to confirm which factor played a key role on CH4/N2separation, which could provide a guide for activated carbons prepared.With coconut shell-based activated carbons as adsorbents, the enrichment of VAM was carried out by vacuum pressure swing adsorption (VPSA). The coconut shell-based charcoals were prepared from the pyrolysis of the coconut shells. To obtain the optimum pyrolysis temperature, the charcoal grindability was inspected. With the powder charcoals of a certain size milled from granular charcoals as precursor and phenolic resin as binders, the shaped activated carbons were manufactured by water vapor as activating agents. With as-prepared activated carbons as adsorbent, the enrichment experiments of VAM contained about0.5%CH4were carried out by a two-column VPSA unit. The results shown that the VAM could be enriched upto1.5%, and methane recovery was above96%with CH4concentration of below200ppm in the off-gas. A two-column VPSA and a three-column VPSA were used to evaluate the performance of oxygen removal from CMM. With the two-column VPSA, two commercial carbon molecular sieves (CMS), CMS1and CMS2, were selected to remove oxygen. with CMS1as adsorbents, there was a potential safety hazard for oxygen removal from CMM by a basic Skartorm VPSA and a low CH4recovery. The oxygen could be sefaly removed from CMM with a VPSA, in which the rinse step was introduced after adsorption step. With the VPSA within rinse step, the compartive experiments for CMS1and CMS2were performed to remove oxygen from CMM. It was found that the performance was ineffective with using either CMS1or CMS2alone as adsorbent. Compared with CMS1, the CMM can be enriched to higher methane concentration in the product, but there is a lower methane recovery with higher oxygen concentration in the product. In order to safely and effectively remove oxygen and improve CH4recovery, a layered VPSA with N2as rinse gas was proposed, for which both CMS1and CMS2were filled into the adsorption vessel along the direction of feedstock adsorbed with the volume ratio of1:1. After the oxygen was removed from the CMM containing37.62%CH4/13.10%O2/49.28%N2, the CH4and O2contents in the product were50.78%and0.44%with CH4concentration of2.62%in off-gas and the CH4recovery of95.9%under optimized condition. There were several disadvantages for oxygen removal by the two-column VPSA, such as the low feedsock throughput, the incontinuous process for feedsock treated and product generated. Thus A three-column VPSA unit was designed and built to evaluate the oxygen removal performance with CMS2. The result shown that the oxygen content in CMM could be safely reduced to0.5%in the product from10%. There were much more10%CH4in product than that of feedstock with methane recovery of above95%and CH4content of below5%in the off-gas.Though there were no oxygen in the CMM separated, the CH4concentration in product is not enough to meet the pipeline quality. A three-column VPSA contained N2reverse press urization, a high CH4purity gas reverse rinse step and reverse pressure equalization was used to refine the CMM with a commercial activated carbon. According to the results, it was found that with an increase of CH4purity from40%to50%, the CMM could be more easily concentrated but the CH4recovery will be decreased. The CMM could be enriched to over95%in product with methane recovery of above90%.