Optimizing hydrogen production by methane conversion in biomass pyrolysis/gasification processes

Biomass gasification presents a high potential for 'green' gas production and especially for hydrogen (H₂) production. Methane (CH₄) conversion into H₂ is the main key point to increase the mass yield of H₂ production from biomass gasification. CH₄ is formed during wood pyrolysis. According to mass balances, comprised between 92 and 102%, some mechanisms concerning the H₂ and CH₄ productions during wood pyrolysis have been stated. CH₄ and C₂H₄ could be considered as indicators of the tar content of a pyrolysis gas. The homogeneous and heterogeneous CH₄ conversion were studied. In a homogeneous phase and a model pyrolysis gas, the CH₄ conversion rate varies from 5 to 95%, for a temperature ranging from 1000 to 1400 deg. C and for a residence time of 2 s. Some radical mechanisms showed satisfying previsions of CH₄ conversion rate. CH₄ oxidation principally occurs by OH radicals addition onto C₂ compounds. For heterogeneous CH₄ conversion over a wood char, in a fixed bed reactor, the conversion rate reached 70% for a contact time of 120 ms at 1000 deg. C. A stable conversion rate was achieved thanks to the concurrent CO₂ and H₂O activation of the char. The active sites of CH₄ chemisorption could be carbon atoms with unpaired electrons. Mass/energy balances of H₂ production from wood pyrolysis coupling methane conversion were investigated. The two experimented ways of CH₄ conversion were thus compared. (author)