(45a) Dry Reforming of Biogas to Syngas And Hydrogen for Fuel Cell Applications

Biogas (BG) is an important renewable resource for H2 production (here, the term BG also includes landfill gas, LFG, and digester gas). The advantages of using BG as a feedstock for H2 production are four-fold: (i) it is a domestic energy resource, (ii) it is a local resource, thus, there would be no need for its delivery by expensive pipelines, (iii) it can be obtained at little or no cost, and (iv) if not utilized, BG could potentially create significant environmental damage (since methane is more potent greenhouse gas than CO2). Although the resources of BG are vast and widely available throughout the country, it remains mostly unused. Direct catalytic reforming of BG is complicated by two factors: (i) the presence of sulfur-containing compounds that could easily poison catalysts used in the reforming process and (ii) a feedstock non-uniformity. Due to the fact that methane is a predominant component in BG (typically, [CH4]= 50-70 vol.%), an undesirable methane decomposition reaction may occur at the operating temperatures of the reforming process, which would result in catalyst deactivation.

In this paper, the authors investigate syngas and H2 production via catalytic dry reforming of a biogas-mimicking gaseous mixture containing CH4:CO2=1.3:1 (molar) and 0.1 vol.% H2S. The objectives are to determine the catalytic activity, selectivity and stability of a number of transition metal catalysts for CO2 reforming of CH4 at the conditions favorable for carbon lay down. H2S was removed from the gaseous mixture by a selective adsorbent prior to the dry reforming reaction. Two types of catalysts were tested: noble metal (NM) catalysts (e.g., alumina-supported Ru, Pt, Rh, Ir, Pd) and Ni-catalysts. NN catalysts, especially, Ru, Ir and Pt catalysts, demonstrated high activity and selectivity for CO2 reforming of methane and good stability (i.e., no methane decomposition and carbon deposition occurred during the reforming reaction). The H2:CO molar ratio in the syngas was 1, and it contained about 7 vol.% of unreacted CH4, according to the following reaction:

(1.3CH4) + CO2 = 2H2 + 2CO + (0.3CH4)

The resulting syngas is suitable as a fuel for molten carbonate and solid oxide fuel cells. Ni-based catalysts demonstrated poor selectivity since they also catalyzed methane decomposition reaction (leading to the catalyst deactivation). The deposition of carbon on the Ni catalyst surface could be prevented by adding relatively small quantities of an oxidant (CO2, O2 or steam) to the feedstock. The effect of different oxidants on the Ni-catalyst stability is reported. The resulting syngas was free of unconverted methane, and it was further processed via water gas shift reaction to H2-CO2 mixtures. Hydrogen was subsequently recovered from these mixtures by cryogenic adsorption method using an activated carbon based adsorbent. The use of a pressure-swing adsorption system for the separation of the H2-CO2 mixture would produce H2 with the purity of >99.999 % that is suitable for the operation of a polymer membrane electrolyte (PEM) fuel cell.