I. INTRODUCTION

The simple act of burning biomass to obtain heat, and often light, is one of the oldest biomass conversion processes known to mankind. The basic stoichoimetric equation for the combustion of wood, represented by the empiri­cal formula of cellulose, (C6H505)n, is illustrated by

(C6H10O5)n + 6n02—» 6nC02 + 5nH20.

Carbon dioxide (C02) and water are the final products along with energy. If most biomass did not sustain its own combustion to make heat readily available in preindustrial times when and where it was needed, our historical develop­ment would not have reached its present state, and would probably have taken a different course. Up to the early 1900s, much of industrialized society utilized biomass combustion and a few related thermal processes for a wide range of applications including heating, cooking, chemical and charcoal production, and the generation of steam and mechanical and electric power.

The science of combustion has advanced a great deal since then and im­proved our understanding of the chemical mechanisms involved. Improved

combustion processes are available for conversion of virgin biomass and com­plex waste biomass feedstocks to heat, steam, and electric power in advanced combustion systems and in co-combustion systems supplied with both biomass and fossil fuels. Small-scale catalytic woodstoves have been developed that operate at higher overall thermal efficiencies with low emissions. Medium — to large-scale incinerators have been designed with heat recovery capability for efficient combustion and disposal of municipal solid wastes (MSW) with mini­mal emissions. And modern boiler systems are available for wood, municipal solid waste, refuse-derived fuel (RDF or the combustible fraction of MSW), and other biomass fuels for municipal and utility use. The technology has progressed far beyond the basic idea of just using biomass as a solid, combusti­ble fuel.

It is noteworthy that of all the processes that can be used to convert biomass to energy or fuels, combustion is still the dominant technology. More than 95% of all biomass energy utilized today is obtained by direct combustion.

In this chapter, the basic chemistry of direct biomass combustion, develop­ments that have made it possible to improve operating efficiencies and environ­mental performance, and state-of-the-art systems that have been or are expected to be commercialized are examined. Improvements needed to overcome some of the operating problems and advancements that are expected from ongoing research are also discussed.