Dynamic behaviors and heat recovery with hot gas withdrawal of flow reversal reactor for thermal oxidation of lean methane

Abstract

Lean methane from abandoned coal mines or drainage gas with methane concentration of 1%−3% is in general directly discharged into the atmosphere due to the lack of appropriate technology, which has caused serious environmental concerns due to its high global warming potential. While direct thermal oxidation of ultra-low methane in a flow reversal reactor offers an attractive solution, it poses challenges such as potential explosions and unstable combustion flames. Elucidating the dynamic behavior of thermal oxidation of ultra-low methane in a flow reversal reactor is the basis for practical application. To this end, autothermal operation boundary of a pilot-scale thermal flow reversal reactor was examined and the effects of hot gas withdrawal on the behavior of flow reversal reactor was deeply studied. It was found that autothermal operation can be achieved with a methane content of over 0.2% and heat can be recovered if methane content is over 0.5%. Withdrawal of hot air has a significant impact on the dynamic behavior of the reactor: maximum bed temperature at the pseudo-steady state without hot gas extraction keeps almost constant with methane concentration varying in 0.5%−3.0%; whereas for heat recovery by hot gas withdrawal, the maximum bed temperature increases with the increase of the amount of hot gas extracted, and the allowable hot gas exported from the reactor increases nearly linearly from 12.5% to 32% as the methane content increases from 0.5% to 3.0%. Furthermore, the appropriate switching time decreases with the increase of the amount of hot gas withdrawn; for most cases, reversing flow direction at a time interval of 30−50 s can ensure complete methane conversion and stable bed temperature. Thus, it may be concluded that lean methane (1%−3%) can be mitigated by thermal oxidation without worrying about the bed temperature runaway or explosion.