I. INTRODUCTION

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.

II. FUNDAMENTALS

A. Definition

Basically, there are three types of biomass gasification processes—pyrolysis, partial oxidation, and reforming. As discussed in Chapter 8, if the temperature is sufficient, the primary products from the pyrolysis of biomass are gases. At high temperatures, charcoal and liquids are either minor products or not present in the product mixture. Partial oxidation processes (direct oxidation, starved-air or starved-oxygen combustion) are those that utilize less than the stoichiometric amounts of oxygen needed for complete combustion, so partially oxidized products are formed. The term “reforming” was originally used to describe the thermal conversion of petroleum fractions to more volatile prod­ucts of higher octane number, and represented the total effect of many simulta­neous reactions, such as cracking, dehydrogenation, and isomerization. Exam­ples are hydroforming, in which the process takes place in the presence of hydrogen, and catalytic reforming. Reforming also refers to the conversion of hydrocarbon gases and vaporized organic compounds to hydrogen-containing gases such as synthesis gas, a mixture of carbon monoxide and hydrogen. Synthesis gas can be produced from natural gas, for example, by such processes as reforming in the presence of steam (steam reforming). For biomass feed­stocks, reforming refers to gasification in the presence of another reactant. Examples of biomass gasification by reforming are steam reforming (steam gasification, steam pyrolysis), and steam-oxygen and steam-air reforming. Steam reforming processes involve reactions of biomass and steam and of the secondary products formed from biomass and steam. Steam-oxygen or steam-air gasification of biomass often includes combustion of residual char from the gasifier, of a portion of the product gas, or of a portion of the biomass feedstock to supply heat. The processes can be carried out with or without catalysis.

Under idealized conditions, the primary products of biomass gasification by pyrolysis, partial oxidation, or reforming are essentially the same: The carbon oxides and hydrogen are formed. Methane and light hydrocarbon gases are also formed under certain conditions. Using cellulose as a representative feedstock, examples of some stoichiometries are illustrated by these equations:

Pyrolysis: C6Hw05 —> 5CO + 5H2 + C

Partial oxidation: C6Hi0O5 + 02^> 5CO + C02 + 5H2

Steam reforming: C6H10O5 + H20 —* 6CO + 6H2.

The energy content of the product gas from biomass gasification can be varied. Low-energy gases (3.92 to 11.78 MJ/m3(n), 100 to 300 Btu/SCF) are generally formed when there is direct contact of biomass feedstock and air. This is due to dilution of the product gases with nitrogen from air during the gasification process. Medium-energy gases (11.78 to 27.48 MJ/m3 (n), 300 to 700 Btu/SCF) can be obtained from directly heated biomass gasifiers when oxygen is used, and from indirectly heated biomass gasifiers when air is used and heat transfer occurs via an inert solid medium. Indirect heating of the gasifier eliminates dilution of the product gas with nitrogen in air and keeps it separated from the gasification products. High-energy product gases (27.48 to 39.26 MJ/m3 (n), 700 to 1000 Btu/SCF) can be formed when the gasification conditions promote the formation of methane and other light hydrocarbons, or processing subsequent to gasification is carried out to increase the concentra­tion of these fuel components in the product gas. Methane is the dominant fuel component in natural gas and has a higher heating value of 39.73 MJ/m3 (n) (1012 Btu/SCF).