Except during startup, wood pyrolysis is reported to have been carried out commercially in the 1920s and 1930s without an external heat source. For example, the Ford Motor Company’s continuous wood pyrolysis plant was operated on hogged hardwood dried to 0.5% moisture content and an external heat source was not needed (Chapter 8). Presuming oxygen is excluded in such processes and that exothermic partial oxidation is not a factor, several exothermic reactions can contribute to the self-sustained pyrolysis of wood— the conversion of carbon monoxide and carbon dioxide to methane or metha­nol, char formation, and the water gas shift (Table 8.2). Methanation has one of the highest exotherms per unit of carbon converted. These reactions or modifications and combinations of them seem to have occurred in the self­sustained process at a sufficient rate to make the overall process self-sustaining under the operating conditions used by Ford Motor Company.

When applied to biomass feedstocks, few steam gasification systems in which oxygen and air are excluded have been described or operated as autother­mal processes since this early work. Wright-Malta Corporation’s directly heated, pressurized steam gasification process for the production of medium — energy gas described earlier is one of these (Hooverman and Coffman, 1976; Coffman and Speicher, 1993). An external heat source is needed only during startup, and water is added as a cofeedstock if the biomass feedstock contains insufficient moisture (i. e., less than about 50 wt %). The process was described as follows (Coffman, 1981):

As the biomass moves through the kiln from the cool feed end, it is gradually heated and first partially dries, yielding steam; then pyrolyzes, yielding gas, liquids, tars, and char. These move co-currently down the kiln. The liquids and tars steam reform, yielding more gas; the char steam-gasifies, yielding still more gas. The hot gas moves back through coils in the auger and kiln wall, giving its heat to the process, and being discharged at the cool end. This regenerative heat and wood decomposition exotherm are sufficient to sustain the process after initial heat-up by an auxiliary boiler. Over-all energy efficiency, raw biomass to clean, dry product gas is estimated to be 88-90%.

As shown in Table 9.1, most of the steam gasification reactions listed are endothermic, but as noted in the discussion of the Wright-Malta process, substantial amounts of carbon dioxide and methane are formed. Many of the gasification reactions that yield these products are exothermic. Char formation and the water gas shift are also exothermic (Table 8.2). Estimated equilibrium gas compositions from the steam gasification of green biomass at different pressures and temperatures shown in Table 9.11 indicate that at the tem­perature and pressure ranges of the Wright-Malta process, about 2 MPa and 900 K, substantial quantities of carbon dioxide and methane are formed. Calculations show that the process can be exothermic under these conditions. The heat of the exotherm and the sensible heat of the exiting gases, which are passed through tubular heat exchangers in the kiln, and the enthalpy of methanation, which occurs in the kiln and the heat exchangers, apparently drive the process. The total heat released is apparently large enough under Wright-Malta’s operating conditions to sustain steam gasification.

It should be emphasized that many investigators who have specialized in biomass gasification have questioned the validity of the steam gasification of biomass without the application of external heat because only a few autothermal systems have been reported to be operable. It is important to develop additional data to establish whether such systems can be self-sustaining over long periods. If they are, adiabatic, autothermal steam gasification would have several advan­tages for both medium-energy gas production and synthesis gas production. These include acceptability of a wide range of green biomass feedstocks without pretreatment; lower process energy consumption; direct internal heating of the reactants and therefore more efficient energy utilization; elimination of the need for feedstock dryers, an oxygen plant, and more complex indirectly heated gasifiers and indirectly heated, dual, circulating, fluid-bed gasifiers; and lower overall operating costs because of process simplicity. Another advantage would involve environmental benefits; steam gasification is reported to avoid formation of dioxins and to convert any chlorinated compounds that may be present to salts and clean gas (Mansour, Durai-Swamy, and Voelker, 1995). The disadvantage may be the relatively long solids residence time in the gasifier compared to some of the other processes. This can increase the plant’s capital cost for a given throughput rate.