Methane thermo-catalytic decomposition has been recognized as an advanced process for hydrogen (H2) production/natural gas decarburization in the carbon constrained world due to the low CO2 formation with respect to energy supply. As an alternative to costly transition metal catalysts, it is believed that carbon materials are promising catalysts for this process, of which activated carbon (AC) provides reasonable reaction kinetics.
The application of a chemical looping process to methane thermo-catalytic decomposition using AC as a catalyst for continuous high purity H2 production has been studied. This process includes two interconnected reactors, a decomposer and a regenerator. In the decomposer, with the aid of AC catalyst, methane is continuously converted to solid carbon and H2 via methane thermo-catalytic decomposition, and a nearly pure H2stream can be produced under appropriate conditions. The spent AC catalyst which loses the activity to some extent, is regenerated in the regenerator for looping operation. Two major technical barriers need to be overcome in the regenerator to implement this process. First, the catalytic activity of the deactivated AC catalyst should be completely recovered, and second, the regeneration process should provide enough heat to support the endothermic methane decomposition reaction in the decomposer. Heat can be transferred from the regenerator to the decomposer via the AC catalyst in the form of sensible heat.
The regeneration agents, such as O2 and steam, play a detrimental role. O2 regenerated AC catalyst can provide enough heat, however, the activity can’t be completely recovered. AC catalyst regenerated by steam can completely recover the activity, but requires an extra heat for the endothermic gasification. The use of steam with O2 as regeneration agents was studied on TGA apparatus and a fixed bed reactor, and the results show that both requirements can be satisfied under certain mixing ratios. The product syngas from the regenerator has a H2 to CO ratio of about 0.7. The addition of CH4 to the regeneration agents to further increase the H2 to CO ratio to greater than 2 by methane partial oxidation is also investigated, hence the product syngas can be directly used for chemical production. Heat balance analysis and catalytic activity comparisons were performed to ensure the two processing barriers were successfully overcome.
The fresh, spent and regenerated AC catalysts were characterized by physical properties (morphology, N2 adsorption and desorption, mechanical strength) and surface chemistry (oxygen functional groups) studies. Additionally, the evolution of the catalyst with successive decomposition and regeneration cycles was also investigated. Some insights into the performance of the regenerated AC are provided.
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