Design and security analysis for the liquefaction and distillation process of oxygen-bearing coal-bed methane

Abstract

Coal-bed methane (CBM) is a new clean energy. Because of the limitation of treating technologies, most of the oxygen-bearing coal-bed methane is not used efficiently and is evacuated immediately, which causes waste of resources and severe pollution of the environment. In order to use oxygen-bearing coal-bed methane efficiently, in this paper, a new liquefaction and distillation process has been proposed. However, the existing research shows that there may be an explosion hazard in the liquefaction and distillation process. Therefore, the formulas of the methane explosion limit proposed by Li et al., which are appropriate for low temperature conditions, are adopted to analyze the security of the process. Then, in order to guarantee the security of the process, nitrogen as a dilutant is added to the distillation column to reduce the oxygen content. At the same time, the flow rate and the inlet location of nitrogen are optimized. Research results indicate that this process can remove nitrogen and oxygen thoroughly. The purity of the LNG product can reach up to 99.91%, the recovery rate of methane is 97.21%, and the energy consumption per unit of LNG production is 0.94 kW h m⁻³. In the stages of compression, liquefaction and cold energy recovery, no explosion hazard exists. However, an explosion hazard may occur at the top of the distillation column. When a nitrogen flow rate of 50 000 N m³ h⁻¹ is added from the 7th plate of the distillation column, the mole fraction of oxygen in the whole column is less than 8% which is the minimum oxygen concentration. The product purity is 99.87% and the methane recovery rate is 91.35%. However, the energy consumption per unit of LNG production can increase by 38% at the same time.