Abstract
Steam reforming is a highly endothermic process of hydrogen or synthesis gas production from methane or other hydrocarbon fuels. Metals from group VIII of the Periodic table are catalysts for steam reforming; nickel is the most commonly used among them. Platinoid catalysts: rhodium, ruthenium and palladium are more active, but less frequently used due to their high cost. Nickel catalysts have proven their effectiveness due to the simplicity of their production, stability and chemical activity [1].
It is known that the heat required for the endothermic process of steam reforming can be provided by an electrochemical reaction in a stack of high-temperature solid oxide fuel cells [2,3].
In the case of internal reforming, the process of steam reforming of methane proceeds directly at the anode of a solid oxide fuel cell (SOFC) due to the high operating temperature and the presence of nickel in the composition [4,5]. The advantage of internal reforming is not only thermal, but also chemical integration of reforming agent and generator – the water vapor produced during the SOFC anodic electrochemical reaction can be used for reforming without the need for anode recycling organization. Also, the endothermic effect of steam reforming can be used to control the temperature of the stack.
As it was shown in work [6] feeding of the mixture containing 30% of methane into the electrolyte-supported SOFC stack to the input leads to 22% of methane observed at the output. Thus, based on the literature data, we cannot expect a high conversion rate in the process of internal methane reforming.
Therefore, studies of the internal methane conversion process were carried out on an experimental short stack of two 100x100 mm electrolyte-supported SOFCs. A whole series of experiments were carried out on H2+CH4+H2O mixtures with different sets of conditions – an increase in the ratio and consumption of methane, and then a decrease in temperature.
The ranges of methane consumption and other key parameters of the experiment were based on the results of the analysis of available literature data on the experimental study of the kinetics of methane steam conversion on cermet SOFC anodes. Prior to the experiments, discussions were mainly caused by the probable carbon deposition at the anodes and the associated rapid performance degradation, the capabilities of the anodes for internal conversion were upper estimated according to the fuel consumption of SOFCs, so it was assumed that, at worst case, the conversion provides only its internal consumption needs.
The results of experiments at an operating temperature of 850°C showed that the kinetics of internal conversion was largely underestimated. The assembly of two SOFCs converts methane up to concentrations not exceeding the calculated equilibrium ones, up to the maximum available flow rate on the equipment used – 187 ml/min – even without current passing. This fuel flow rate is capable of providing a current of up to 53 A, while the rated operating current of these SOFCs is about 20 A, up to 30 A on methane-rich fuel. This means, that the conversion capabilities of SOFC even in the absence of current are at least twice higher than their own needs. The current further stimulates the conversion by generating additional steam. In order to discover the limits of conversion capabilities, the temperature was lowered to 750°C. The result of the conversion at maximum flow rate proceeding up to equilibrium values was repeated.
Thus, in the course of the experiments, it was not possible to obtain methane concentrations in the exhaust gas that significantly exceed the equilibrium ones. This result turned out to be extremely unexpected and undoubtedly positive, since it demonstrates an extremely high potential of internal conversion and opens the way to a sharp decrease in the requirements for the degree of conversion of fuel supplied to SOFC, abandonment of a bulky fuel processor, reduction of costs for SOFC cooling and, thereby, an increase in efficiency and reducing the mass and dimensions of SOFC power plants.
This work was carried out with financial support from the Russian Scientific Foundation, grant no. 17-79-30071.
References
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