The hydrogenation of CO to synthetic natural gas (SNG) needs a high molar ratio of H 2 /CO (usually large than 3.0 in industry), which consumes a large abundant of hydrogen. The reverse dry reforming reaction (RDR, 2H 2 ? 2CO $ CH 4 ? CO 2 ), combining CO methanation with water-gas-shift reaction, can significantly decrease the H 2 /CO molar ratio to 1 for SNG production. A detailed thermodynamic analysis of RDR reaction was carried out based on the Gibbs free energy minimization method. The effect of temperature, pressure, H 2 /CO ratio and the addition of H 2 O, CH 4 , CO 2 , O 2 and C 2 H 4 into the feed gas on CO conversion, CH 4 and CO 2 selectivity, as well as CH 4 and carbon yield, are discussed. Experimental results obtained on homemade impregnated Ni/Al 2 O 3 catalyst are compared with the calculations. The results demonstrate that low temperature (200-500°C), high pressure (1-5 MPa) and high H 2 /CO ratio (at least 1) promote CO conversion and CH 4 selectivity and decrease carbon yield. Steam and CO 2 in the feed gas decrease the CH 4 selectivity and carbon yield, and enhance the CO 2 content. Extra CH 4 elevates the CH 4 content in the products, but leads to more carbon formation at high temperatures. O 2 significantly decreases the CH 4 selectivity and C 2 H 4 results in the generation of carbon.Keywords Synthetic natural gas Á Reverse dry reforming of methane Á Gibbs free energy minimization Á Experimental study Á CO conversion List of symbols A k Total mass of k element in the feed f i H Standard-state fugacity of species i (Pa) f i

Biomass is quite abundant in the world, particularly in some countries like China. China has large quantities of straw and/or stalk-origin biomass resources and the attention is currently being paid to the exploitation of these resources to produce energy products via different technical solutions, among of which pyrolysis of biomass to produce hydrogen-rich gas is very promising as hydrogen is a very clear energy carrier. In this work, pyrolysis of rice straw, corn stalk and sawdust was carried out in a two-stage reactor (the first-stage reactor is a conventional fixed-bed pyrolyser, and the second-stage reactor is a catalytic fixed bed) to produce hydrogen-rich gas. The effect of catalytic bed on the pyrolysis behaviour have been investigated, with the emphasis on final product particularly hydrogen. The operation of the catalytic reactor appears significant in promoting biomass pyrolysis towards the production of gaseous products, especially hydrogen. At 750°C of the pyrolyser with rice straw as fuel, the use of the catalytic bed leads to the increases of gas yield from 0.41 Nm3/kg to 0.50 Nm3/kg, approximately 22% increase, and of H2 concentration from 33.79% to 50.80% in volume, approximately 50.3% increase, respectively. Compared with calcined dolomite, fresh nickel-based catalyst shows stronger catalytic effect on the pyrolysis of rice straw as its use in the catalytic bed results in the increase of gas yield from 0.41 Nm3/kg to 0.56 Nm3/kg, approximately 36.6% increase, and the increase of H2 concentration from 33.79% to 59.55% in volume, approximately 76.2% increase. Furthermore, two catalysts follow the same trend for the pyrolysis of corn stalk and sawdust. At temperature of 815°C, catalysts also follow the same trend. Catalytic bed can significantly reduce the level of tar which is carried out with the producer gas, to less than 1% of original level. Catalyst load or gas space velocity (hourly) has the influence on the gas yield and H2 concentration. 30% of load, i.e. gas space velocity (hourly) 0.9 × 104 h−1, appears reasonable. Beyond that, gas yield and H2 concentration remain almost unchanged.

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