Sorption enhanced reaction process for direct production of fuel-cell grade hydrogen by low temperature catalytic steam-methane reforming

摘要

New experimental data are reported to demonstrate that a sorption enhanced reaction (SER) concept can be used to directly produce fuel-cell grade H_2 (<20 ppm CO) by carrying out the catalytic, endother-mic, steam-methane reforming (SMR) reaction (CH_4 + 2H_2O↔CO_2+4H_2) in presence of a CO_2 selective chemisorbent such as K_2CO_3 promoted hydrotalcite at reaction temperatures of 520 and 550℃, which are substantially lower than the conventional SMR reaction temperatures of 700-800℃. The H_2 productivity of the sorption enhanced reactor can be large, and the conversion of CH_4 to H_2 can be very high circumventing the thermodynamic limitations of the SMR reaction due to the application of the Le Chetalier's principle in the SER concept. Mathematical simulations of a cyclic two-step SER concept showed that the H_2 productivity of the process (moles of essentially pure H_2 produced per kg of catalyst-chemisorbent admixture in the reactor per cycle) is much higher at a reaction temperature of 590℃ than that at 550 or 520℃. On the other hand, the conversion of feed CH_4 to high purity H_2 product is relatively high (>99+%) at all three temperatures. The conversion is much higher than that in a conventional catalyst-alone reactor at these temperatures, and it increases only moderately (<1%) as the reaction temperature is increased from 520 to 590℃. These results are caused by complex interactions of four phenomena. They are (a) favorable thermodynamic equilibrium of the highly endothermic SMR reaction at the higher reaction temperature, (b) faster kinetics of SMR reaction at higher temperatures, (c) favorable removal of CO_2 from the reaction zone at lower temperatures, and (d) higher cyclic working capacity for CO_2 chemisorption at higher temperature.