A. Introduction

The effort to develop and commercialize advanced biomass gasification systems is not nearly as extensive as the effort to develop coal gasification. However, considerable research and pilot plant studies have been carried out since about 1970 on biomass gasification for the production of fuel gases and synthesis gases (с/. Stevens, 1994). Several processes have been commercialized. Basic studies on the effects of various operating conditions and reactor configurations have been performed in the laboratory and at the PDU (process development unit) and pilot scales on pyrolytic, air-blown, oxygen-blown, steam, steam — oxygen, and steam-air gasification, and on hydrogasification. The thermal gasification of biomass in liquid water slurries has also been studied.

The chemistry of biomass gasification is very similar to that of coal gasifica­tion in the sense that thermal decomposition of both solids occurs to yield a mixture of essentially the same gases. But as pointed out in the Introduction, biomass is much more reactive than most coals. Biomass contains more volatile matter than coal, gasification occurs under much less severe operating condi­tions for biomass than for coal feedstocks, and the pyrolytic chars from biomass are more reactive than pyrolytic coal chars. The thermodynamic equilibrium concentrations of specific gases in the mixture depend on the abundance of carbon, hydrogen, and oxygen, the temperature, and the pressure. As in the case of coal feedstocks, increasing pressures tend to lower the equilibrium concentrations of hydrogen and carbon monoxide, and increase the methane and carbon dioxide concentrations. Also, as in the case of coal feedstocks, methane formation is favored at lower temperatures, and carbon monoxide and hydrogen are dominant at high temperatures. Biomass is gasified at lower temperatures than coal because its main constituents, the high-oxygen cellulos — ics and hemicellulosics, have higher reactivities than the oxygen-deficient, carbonaceous materials in coal. The addition of coreactants to the biomass system, such as oxygen and steam, can result in large changes in reaction rates, product gas compositions and yields, and selectivities as in coal conver­sion.

Biomass feedstocks contain a high proportion of volatile material, 70 to 90% for wood compared to 30 to 45% for typical coals. A relatively large fraction of most biomass feedstocks can be devolatilized rapidly at low to moderate temperatures, and the organic volatiles can be rapidly converted to gaseous products. The chars formed on pyrolytic gasification of most biomass feedstocks have high reactivity and gasify rapidly. Heat for pyrolysis is usually generated by combusting fuel gas either in a firebox surrounding the reaction chamber or in fire tubes inserted into the reaction chamber. As discussed in Chapter 8, chars, tars and oily liquids, gases, and water vapor are formed in varying amounts, depending particularly on the feedstock composition, heating rate, pyrolysis temperature, and residence time in the reactor. For biomass and waste biomass, steam gasification generally starts at temperatures near 300 to 375°C.

Undesirable emissions and by-products from the thermal gasification of biomass can include particulates, alkali and heavy metals, oils, tars, and aque­ous condensates. One of the high-priority research efforts is aimed at the development of hot-gas-cleanup methods that will permit biomass gasification to supply suitable fuel gas for advanced power cycles that employ gas turbines without cooling the gas after it leaves the gasifiers (International Energy Agency, 1991, 1992). It is important to avoid gas turbine blade erosion and corrosion by removing undesirable particulates that may be present. The re­moval of tars and condensables may also be necessary. Furthermore, utilization of the sensible heat in the product gas improves the overall thermal operating efficiencies. Nonturbine applications of the gas may also be able to take advan­tage of processes that provide clean, pressurized hot gas, such as certain down­stream chemical syntheses and fuel uses. Special filtration and catalytic systems are being developed for hot-gas cleanup. Some of the other research needs that have been identified include versatile feed-handling systems for a wide range of biomass feedstocks; biomass feeding systems for high-pressure gasifi­ers; determination of the effects of additives, including catalysts for minimizing tar production and materials that capture the contaminants; and suitable ash disposal and wastewater treatment technologies. Research on thermal biomass gasification in North America has tended to concentrate on medium-energy gas production, scale-up of advanced process concepts that have been evaluated at the PDU scale, and the problems that need to be solved to permit large — scale thermal biomass gasifiers to be operated in a reliable fashion for power production, especially for advanced power cycles. Research to develop biomass gasification processes for chemical production via synthesis gas waned in the mid-1980s because of low petroleum and natural gas prices. More attention was given to the subject in the 1990s when the market prices for these fossil fuels began to increase.

Examples of the various types of biomass gasification processes are reviewed in the next few sections before commercial and near-commercial processes are described.