Reactor modeling of sorption-enhanced autothermal reforming of methane. Part I: Performance study of hydrotalcite and lithium zirconate-based processes

This paper presents a performance analysis for the sorption-enhanced autothermal reforming of CH4 in a fixed bed reformer for pure H2 production with in situ CO2 capture. The process is analyzed for two candidate sorbents of K-promoted hydrotalcite and lithium zirconate in a fixed bed reactor using a conventional Ni/MgO steam reforming catalyst. A 1-D heterogeneous dynamic model is constructed to simulate the process, accounting for mass and thermal dispersion in the axial direction, pressure drop, and intraparticle and interfacial resistances. The process is found to be efficient and applicable even at a low temperature of 500 °C for steam reforming reactions and at pressure as low as 4.47 bar for CO2 adsorption. The hydrotalcite-based autothermal reforming process can provide CH4 conversion and H2 purity up to 85% and 96%, respectively, at operational conditions of 500 °C, 4.47 bar, steam/carbon ratio of 6, oxygen/carbon ratio of 0.45 and space velocity of 3071 h-1. This is associated with a low level of CO + CO2 impurities of less than 300 ppm. The lithium zirconate-based process demonstrated an enhanced CH4 conversion of 99.5% and dry basis H2 purity of 99.5% at similar conditions. The process is found to benefit from the high CO2 sorption capacity of lithium zirconate with respect to the CH4 conversion and the useful operational time. Nonetheless, lithium zirconate-based process shows a slip of CO + CO2 impurities up to 1000 ppm in the gas effluent during the transient sorption-enhanced regime before breakthrough. This is mainly attributed to the slow sorption kinetics of lithium zirconate.