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Combustion and heat release characteristics of biogas non-premixed flames under various hydrogen-enriched and oxygen-enriched conditions were investigated through chemical kinetics simulation using detailed chemical mechanisms. The heat release rates, chemical reaction rates, and molar fraction of all species of biogas at various methane contents (35.3-58.7%, mass fraction), hydrogen addition ratios (10-50%), and oxygen enrichment levels (21-35%) were calculated considering the GRI 3.0 mechanism and P1 radiation model. Results showed that the net reaction rate of biogas increases with increasing hydrogen addition ratio and oxygen levels, leading to a higher net heat release rate of biogas flame. Meanwhile, flame length was shortened with the increase in hydrogen addition ratio and oxygen levels. The formation of free radicals, such as H, O, and OH, are enhanced with increase in hydrogen addition ratio and oxygen levels. Higher reaction rates of exothermic elementary reactions, especially those with OH free radical are increased, are beneficial to the improvement in combustion and heat release characteristics of biogas in practical applications.

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An overpotential analysis is carried out in the range of 298 K-353 K to clarify the dominant overpotential factor above 333 K of the direct formic acid fuel cell, DFAFC using 2 mg cm -2 of Pd-black catalyst for the anode. It is found that the mass transport overpotential at the anode which is estimated at a specific current density showing the maximum current density increases from 45 mV to 196 mV when the cell temperature increased from 298 K to 353 K although the formic acid crossover flux through the membrane also increases. Moreover, the effect of the loading of microporous layer, MPL, in the range of 1-4 mg cm -2 at the anode fabricated using Nafion as a binder, which affects the mass transport, on the overpotential behavior is also investigated. It is found that the lower MPL loading is appropriate for the DFAFC operation due to the poor mass transport of the formic acid although the higher MPL loading is appropriate for the hydrogen-oxygen, PEFC, operation. Based on the comparisons of the overpotential distributions between the DFAFC and the PEFC, it is suggested that both the poor mass transport in the anode and the formic acid crossover should be simultaneously improved in order to obtain the higher power density of the DFAFC above 333 K.