In this process, gasification is carried out in the presence of hydrogen. Most of the research on hydrogasification has targeted methane as the final product. One approach involves the sequential production of synthesis gas and then methanation of the carbon monoxide with hydrogen to yield methane. Another route involves the direct reaction of the feed with hydrogen (Feldmann et ah,
1981). In this process (Fig. 9.7), shredded feed is converted with hydrogen — containing gas to a gas containing relatively high methane concentrations in the first-stage reactor. The product char from the first stage is used in a second — stage reactor to generate the hydrogen-rich synthesis gas for the first stage. From experimental results obtained with the first-stage hydrogasifier operated in the free-fall and moving-bed modes at 1.72 MPa and 870°C with pure hydrogen, calculations shown in Table 9.8 were made to estimate the composi­tion and yield of the high-methane gas produced when the first stage is inte­grated with an entrained-char gasifier as the second stage. Note that although the methane content of the raw product gas is projected to be higher in the moving-bed reactor than in the falling-bed reactor, the gas from the first stage must still be reacted in a shift converter to adjust the H2/CO ratio, scrubbed to remove C02, and methanated to obtain SNG.

Other research shows that internally generated hydrogen for hydroconver­sion can be obtained in a single-stage, noncatalytic, fluidized bed reactor (Babu, Tran, and Singh, 1980). In this work, hydroconversion was envisaged to occur in a series of steps: nearly instantaneous thermal decomposition of biomass followed by gas-phase hydrogenation of volatile products to yield hydrocarbon

gases, hydrogen, carbon oxides, water, and hydrocarbon liquids; rapid conver­sion of a part of the devolatilized biomass char to methane at appropriate gasification conditions; slow residual biomass char gasification with hydrogen and steam to yield methane, hydrogen, and the carbon oxides; and combustion of residual biomass char, which supplies the energy for the endothermic char gasification reactions. Examination of hydroconversion under a variety of pressure and temperature conditions with woody biomass and hydrogen, steam, and hydrogen-steam mixtures and study of the kinetics of the slower steam-char reactions led to a conceptual process called RENUGAS®, which will be described in more detail later. Biomass is converted in the reactor, which is operated at about 2.2 MPa, 800°C, and residence times of a few minutes with steam-oxygen injection. About 95% carbon conversion is anticipated to produce a medium-energy gas which can be subjected to the shift reaction, scrubbing, and methanation to form SNG. The cold gas thermal efficiencies are estimated to be about 60%. Since this initial work, RENUGAS has been tested at the pilot, PDU, and demonstration scales, and is being commercialized.

Comparative studies on the gasification of wood in the presence of steam and hydrogen have shown that steam gasification proceeds at a much higher rate than hydrogasification (Feldmann et al, 1981). Carbon conversions 30 to 40% higher than those achieved with hydrogen can be achieved with steam at comparable residence times. Steam/wood weight ratios up to 0.45 promoted increased carbon conversion, but had little effect on methane concentration. Other experiments show that potassium carbonate-catalyzed steam gasification of wood in combination with commercial methanation and cracking catalysts can yield gas mixtures containing essentially equal volumes of methane and

carbon dioxide at steam/wood weight ratios below 0.25 and atmospheric pres­sure and temperatures near 700°C (Mudge et al, 1979). Other catalyst combina­tions produced high yields of product gas containing about 2:1 hydrogen/ carbon monoxide and little methane at steam/wood weight ratios of about 0.75 and a temperature of 750°C. Typical results for both of these studies are shown in Table 9.9. The steam/wood ratios and the catalysts used can have major effects on the product gas compositions. The composition of the product gas can also be manipulated depending on whether a synthesis gas or a fuel gas is desired.