Thermochemical conversion process

Thermochemical conversion processes use heat in an oxygen controlled environment that produce chemical changes in the biomass. The process can produce electricity, gas, methanol and other products. Gasification, pyrolysis, and liquefaction are thermochemical methods for converting biomass into energy. 

Hydrogen can be produced from biorenewable feedstocks via thermochemical conversion processes such as pyrolysis, gasification, steam gasification, steam reforming of bio-oils, and supercritical water gasification (SWG) of biomass. 

Thermochemical conversion processes have met resistance from the environmental community and the public. An essential difference between direct combustion, pyrolysis, and gasification is that the latter two are intermediate processes for producing gaseous, liquid, and solid products that can be used in a wide variety of applications. Pyrolysis processes can be optimized for the production of biocrade oils. In the case of chemical and fuel production, the emissions from a direct process effluent can be avoided, although consideration must be given to emissions from the ultimate use of these products as they are used or combusted downstream. 

While exhaust gas cleanup of non-combustion thermochemical conversion processes may be easier than that associated with direct combustion, proper design of the process and emissions control systems is necessary to ensure that health and safety requirements are met. The output products of pyrolysis and gasification reactors can contain a variety of potential process and air pollutants that must be controlled prior to discharge into the ambient air. These include particulate matter... 

PM), aerosols or tars, oxides of nitrogen (NO ), oxides of sulfur (SO ), dioxins and furans, hydrocarbon (HC) gases, multiple metals, and carbon monoxide (CO). There are many strategies for controlling emissions from thermochemical conversion processes, and they are highly dependent on the process requirements of each individual facility. 

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. 

Thermochemical conversion processes on macro and micro scale... 

To be able to mathematically simulate and really understand the thermochemical conversion process of a packed bed both a single particle model and a bed model must be included in the overall bed model (CFSD code ... 

The thermochemical conversion process of a packed bed is too complex to mathematically model and verify within the scope of a PhD project. It requires huge resources to challenge and realise this vision. 

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. 

It is emphasized by several authors that an all-round mathematical model describing the thermochemical conversion process in the conversion zone needs to take both the micro- and the macro-perspective into account [25,26]. The micro-scale perspective in this context will refer to the single particle scale, whereas the macro-scale corresponds to an overall fuel-bed perspective. 

The conversion technology is a mechanical system, which is actually outside the scope of this. However, this section will give a short background to different conversion technologies because it is the conversion technology which supports or/and transports the solid fuel through the conversion system. Consequently, the choice of conversion technology will in practice control the fuel-bed movement (see subsection 0), which is crucial for the behaviour of the thermochemical conversion process of the fuel bed. 

The four characteristic thermochemical conversion processes taking place in the conversion zone are ... 

The preheating of solid fuel and the ash cooling are not included in the thermochemical conversion process. The basic criteria for these four thermochemical conversion reactions are that the solid-fuel convertibles (or moisture, char, volatiles) are converted from the solid phase into the interstitial gas phase and finally to the offgases (Figure 16 and Figure 19). The part of the solid-fuel convertibles that is converted into the interstitial gas phase is defined as the conversion gas [3]. The conversion gas is associated with two important physical properties, namely the empirical stoichiometry [CxHyOz] and the mass flux [kg/m s]. 

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 long-term goal in the science of thermochemical conversion of a solid fuel is to develop comprehensive computer codes, herein referred to as a bed model or CFSD (computational fluid-solid dynamics). Firstly, this CFSD code must be able to simulate basic conversion concepts, with respect to the mode, movement, composition and configuration of the fuel bed. The conversion concept has a great effect on the behaviour of the thermochemical conversion process variables, such as the molecular composition and mass flow of conversion gas. Secondly, the bed model must also consider the fuel-bed structure on both micro- and macro-scale. This classification refers to three structures, namely interstitial gas phase, intraparticle gas phase, and intraparticle solid phase. Commonly, a packed bed is referred to as a two-phase system. 

When authors illustrate the subject of thermochemical conversion of solid fuels in the literature, the conversion zone in a packed bed is divided into different process zones (drying zone, pyrolysis zone, char combustion zone, and char gasification zone), one for each thermochemical conversion process. The spatial order of this process zones is herein referred to as the bed process structure or conversion process structure. The conversion process structure is a function of conversion concept. Even more important, the bed process structure can only exist in the diffusion controlled conversion regime when the conversion zone has a significant thickness. 

The most important design variables of the conversion system are the mass flow and the empirical stoichiometry of the conversion gas. The conversion gas is the primary product of the thermochemical conversion process in the conversion system. The... 

One of the most important features of the heat and mass transport of the thermochemical conversion processes is the char combustion process, which can be divided into three oxidation regimes. The prevalent regime in PBC systems, labeled Regime III, is equivalent to conversion regime I and is controlled by interstitial gas diffusion of oxygen to the surface of the particle phase. 

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. 

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