Partial oxidation of methanol over SOFC type reactor combined with MSR for simultaneous generation of electric power and hydrogen
(written by: Willy Yanto Wijaya)
When we notice the reaction in PEFC (polymer electrolyte fuel cell) and the hydrogen combustion, in overall, both reactions can be expressed “similarly” as:
H2 + 1/2O2 <–> H2O. (1)
However, in actual system, we know that these two reactions are actually different. While reactions in PEFC could directly produce electricity, the hydrogen combustion will only produce heat (which needs further steps if it is to be converted to electricity). Then, the question is that how come these two “similar” overall reactions can result in amazingly different actual characteristics? One of the plausible explanations is that these two reactions undergo different intermediary reaction paths. The hydrogen combustion goes through a highly disorder dissociation processes which generate huge amount of entropy, and thus abundant exothermic heat. On the other hand, through the help of catalytic reactions, PEFC undergoes different (more ordered, less chaotic) intermediary paths before summing up to become the “similar” overall reaction.
It was this intermediary path that enabled Prof. Ishihara from Oita University to realize a co-production of electricity, synthesis gas, and heat simultaneously. The intermediary paths were created by combining POx (partial oxidation) of methane with SOFC (Solid Oxide Fuel Cell) type reactor, where overall reaction can be written as:
CH4 + 1/2O2 <–> CO + 2H2. (2)
At temperature 1000°C, this reaction can have extremely huge exergy rate (dG/dH) about 10.2. Even at STP (1 atm, 25°C), the exergy rate still stands at 2.42. Suppose if the SOFC type reactor wasn’t used, the POx reaction (reaction 2) will certainly go through different intermediary paths, and electric power would have not been produced. It was by applying LaGaO3 based perovskite as electrolyte of the SOFC, then the oxide ion (O2-) conduction could be promoted, and thus large electric power was obtained. In thermodynamic point of view, the breath-taking idea lies at the value of the exergy rate (dG/dH) of the POx reaction, and how to use medium (such as SOFC type reactor) to extract the lure of this huge dG into useful work.
There must be many other chemical reactions, lurking somewhere, that possess this type of exergy rate characteristic. Nowadays, many researchers also perform POx of methanol, producing exothermic heat which is used for endothermic MSR (methanol steam reforming). This combined POx of methanol and MSR is called autothermal reforming of methanol. However, as we know, the POx of methanol
CH3OH + 1/2O2 <–> CO2 + 2H2, (3)
has the exergy rate (dG/dH) about 1.47 at STP (1 atm, 25°C). If some medium (such as SOFC type reactor) can be utilized to realize the intermediary paths:
1/2O2 + 2e– <–> O-2, (4)
CH3OH + O-2 <–> CO2 + 2H2 + 2e–, (5)
potential electricity generation might be achieved. Simultaneously, hydrogen and heat will also be produced. The heat produced by this (POx + SOFC), together with the unreacted methanol, will enter MSR (endothermic) reactor for further conversion into hydrogen. Thus, this combined system will not only co-produced electricity and hydrogen, but also exploit the exothermic-produced heat for further enhancement of the system efficiency.
However, what still remains a question is about the suitable and most optimum “intermediary paths” to extract the huge exergy rate (dG/dH) of the methanol partial oxidation, as well as what kind of material (and catalyst) that can best support this dG extraction.