Biomass Gasifier Stove Dealers Mail

In Chapters 7 and 8, the thermal conversion of biomass to energy by combustion and to liquid fuels by pyrolysis and a few nonpyrolytic liquefaction processes was examined. In this chapter, the subject of thermal conversion will be expanded further by addressing biomass gasification. Biomass gasification processes are generally designed to produce low- to medium-energy fuel gases, synthesis gases for the manufacture of chemicals, or hydrogen. More than one million small-scale, airblown gasifiers for wood and biomass-derived charcoal feedstocks were built during World War II to manufacture low-energy gas to power vehicles and to generate steam and electric power. Units were available in many designs. Thousands were mounted on vehicles and many were retrofitted to gas-fired furnaces. Sweden alone had over 70,000 "GENGAS" trucks, buses, and cars in operation in mid-1945 (Swedish Academy of Engineering, 1950). Research continues to develop innovative biomass gasification processes in North America, and considerable research has also been conducted in Europe and Asia. The Swedish automobile manufacturers Volvo and Saab have ongoing programs to develop a standard gasifier design suitable for mass production for vehicles. Much effort has been devoted to the commercialization of biomass gasification technologies in the United States since the early 1970s. A significant number of biomass gasification plants have been built, but many have been closed down and dismantled or mothballed.

There is abundant literature on the thermal gasification of biomass. Information and data carefully chosen from this literature are discussed in this chapter. Information on coal gasification is also included because of its relevancy to the commercialization of biomass gasification; large-scale coal gasifiers have been in commercial operation for several years. This is not the case for most biomass gasifiers. Some of the coal gasification processes are also suitable for biomass feedstocks. Since the conditions required for coal gasification are more severe than those needed for biomass, some coal gasifiers can be operated on biomass or biomass-coal feedstock blends. Indeed, some gasifiers that were originally designed for coal gasification are currently in commercial use with biomass feedstocks.

The pyrolytic gasification of biomass has been interpreted to involve the decomposition of carbohydrates by depolymerization and dehydration followed by steam-carbon and steam-carbon fragment reactions. So the chemistries of coal and biomass gasification are quite similar in terms of the steam-carbon chemistry and are essentially identical after a certain point is reached in the gasification process. Note, however, that biomass is much more reactive than most coals. Biomass contains more volatile matter than coal, and the pyrolytic chars from biomass are more reactive than pyrolytic coal chars.

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