I. INTRODUCTION

As discussed in Chapter 7, the final products of biomass combustion are C02, water, and energy. This is the case, of course, for the combustion of all organic matter. It is not known how much time passed after the utility of biomass combustion was discovered by man until biomass pyrolysis was discovered. But when it was, it is probable that a new era in biomass usage evolved, and quite rapidly. Biomass pyrolysis can be described as the direct thermal decomposition of the organic components in biomass in the absence of oxygen to yield an array of useful products—liquid and solid derivatives and fuel gases. Eventually, pyrolysis processes were utilized for the commercial production of a wide range of fuels, solvents, chemicals, and other products from biomass feedstocks. Improvements continue to be made today to perfect the technology. (It is important to note at the outset that any organic material can be pyrolyzed. Indeed, the pyrolysis of coal has been in commercial use for many years, and still is in several areas of the world for the production of fuel gases, cokes, tars, and chemicals.)

Knowledge of the effects of various independent parameters such as biomass feedstock type and composition, reaction temperature and pressure, residence time, and catalysts on reaction rates, product selectivities, and product yields has led to development of advanced biomass pyrolysis processes. The accumu­lation of considerable experimental data on these parameters has resulted in advanced pyrolysis methods for the direct thermal conversion of biomass to liquid fuels and various chemicals in higher yields than those obtained by the traditional long-residence-time pyrolysis methods. Thermal conversion processes have also been developed for producing high yields of charcoals from biomass.

In this chapter, the basic chemistry of the direct pyrolysis of biomass and the state-of-the-art systems that have been or are expected to be commercialized are discussed. Pyrolysis in the presence of hydrogen (hydropyrolysis) and methane (methanolysis) are also addressed. Energy recovery in the form of liquid fuels and chars is emphasized. Another group of processes for direct thermal conversion of biomass employs a liquid medium for conversion of biomass to liquid fuels. For convenience, these processes are grouped together in this chapter as miscellaneous thermal liquefaction methods. Some discussion of biomass gasification is included in this chapter when needed to clarify pyrolysis chemistry. High-temperature pyrolysis, which yields gaseous fuels and feedstocks such as low — and medium-energy fuel gases, hydrogen, and synthesis gases, is discussed in more detail in Chapter 9.

II. FUNDAMENTALS