Review of thermochemical conversion processes

The fuel bed (packed bed) is a two-phase system, also referred to as a porous medium [20]. Thermochemical conversion processes, such as drying, pyrolysis, char combustion and char gasification, take place simultaneously in the conversion zone of the fuel bed (Figure 16). They are extremely complex, and are reviewed more in detail in section B. 4. Review of thermochemical conversion processes. 

Helsen, L. and Van den Bulck, E. (2004) Review of thermochemical conversion processes as disposal technologies for chromated copper arsenate (CCA) treated wood waste, in Environmental Impacts of Preservative-Treated Wood, Florida Center for Environmental Solutions, Conference, Gainesville, Florida, February 8-11, Orlando, FL, pp. 277-94. 

Based on two reviews (1) literature review of experimental work (Appendix A) and (2) literature review and classification of thermochemical conversion of biofuels (Appendix B), it was revealed that textbooks, general theories, and methods to analyse the interdisciplinary and complex PBC process, especially with respect to the thermochemical conversion of the packed bed are scarce. To be able to carry out... 

Appendix B consists of a systematic classification and review of conceptual models (physical models) in the context of PBC technology and the three-step model. The overall aim is to present a systematic overview of the complex and the interdisciplinary physical models in the field of PBC. A second objective is to point out the practicability of developing an all-round bed model or CFSD (computational fluid-solid dynamics) code that can simulate thermochemical conversion process of an arbitrary conversion system. The idea of a CFSD code is analogue to the user-friendly CFD (computational fluid dynamics) codes on the market, which are very all-round and successful in simulating different kinds of fluid mechanic processes. A third objective of this appendix is to present interesting research topics in the field of packed-bed combustion in general and thermochemical conversion of biofuels in particular. 

This review defines the thermochemical conversion processes of solid fuels in general and biofuels in particular that is, what they are (drying, pyrolysis, char combustion and char gasification) and where they take place (in the conversion zone of the packed bed) in the context of the three-step model. 

The mass flow of the conversion gas, its molecular composition, temperature and stoichiometry, are a complex function of volume flux of primary air, primary air temperature, type of solid fuel, conversion concept, etc. Several workers have tried to mathematically model these relationships, which are commonly referred to as bed models [12,33,14,51,52]. It is an extremely difficult task to obtain a predictive bed model, which is discussed in the introduction of this ew. The review of the thermochemical conversion processes below will outline the complex relationships between these variables and their effect on the conversion gas in sections B 4.4-B 4.6. 

The aim of this section is to review the conceptual models used to describe the chemistry of the thermochemical conversion process of single particles in the scope of conversion of packed beds and the three-step model. The chemical kinetics are outside of the scope of this review. 

The success of any mathematical model, and in turn the computer code, depends completely on the clarity of the conceptual model (physical model). The authors have concluded from a comprehensive literature review on the subject of solid-fuel combustion, that there is a slight conceptual confusion in parts of this scientific domain. The first example of this is the lack of distinction between the thermochemical conversion of solid fuels and the actual gas-phase combustion process, which led these authors to the formulation of the three-step model. The thermochemical conversion of solid fuels is a two-phase phenomenon (fluid-solid phenomenon), whereas the gas-phase combustion is a one-phase phenomenon (fluid phenomenon). 

As will become apparent in what follows, some oxygenates are manufactured by established, commercial, microbial processes using biomass feedstocks. Others can be manufactured by microbial conversion of biomass, but are currently produced using thermochemical conversion methods, usually with fossil feedstocks because of economic or technical factors. A few other oxygenates must be manufactured by thermochemical conversion of fossil feedstocks because suitable microbial processes do not yet exist to produce the oxygenate from biomass. Still others can be produced by a combination of thermochemical and microbial conversion. Microbial conversion systems with biomass feedstocks are emphasized here, but thermochemical methods are briefly reviewed to present a perspective on the options available and what advancements are necessary to perfect suitable processes. 

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