Kinetics thermal gasification

The following sections of this chapter describe the chemical kinetics of gasification processes and indicate that the reaction sequences are much more complex than given by the stoichiometric Reactions 5.1, 5.2. Subsequently, topographical kinetics, also called reaction anisotropy, is introduced and these explain how and why all carbon atoms of a carbonaceous material are not equally reactive, that is selective gasification occurs. Without such selective gasification, the process of thermal activation of carbon would not occur and there would be no removal of those carbon atoms which result in pore creation and pore widening. 

Flaming combustion of polymeric materials inevitably involves liberation of gaseous fuels. No wonder, therefore, that the interest in the thermal properties of polymers and the mechanisms and kinetics of polymer decomposition and gasification has been high. 

All the processes of biomass thermal eonversion (pyrolysis, gasification and combustion) begin with elementary steps of decompositions of each of the components of the starting material (cellulose, hemicellulose and lignin). It is hence necessary to well understand the kinetics of the corresponding fast elementary... 

The interest in the properties of the chars derived from cellulosic or biomass solid.s extends beyond those associated with thermal transport in the char. Insofar as the char residue from a pyrolysis process must typically be burned, gasified, or put to use as an activated carbon product, there is also a need to examine the porous nature of the char, bi acbvated carbons, the pore structure is key to adsorption performance. In combustion or gasification, the porosity can play a role in determining conversion kinetics in the intrinsic rate controlled or pore diffusion controlled regimes. 

All these factors explain why quenching of products of direct UFg decomposition in thermal plasma is such a challenging task. The condensed products should be separated from fluorine before heterogeneous reverse reactions take place. For example, the heterogeneous reverse reaction of tetrafluoride gasification, UF4 + F2 UFg, decreases the mass of a UF4 particulate in accordance with the following kinetic equation ... 

Quenching of Products of Direct UFe Decomposition in Thermal Plasma. Integrating kinetic equation (7-48), derive formula (7-49) for the mass decrease of uranium-containing product particles due to their gasification by fluorine. Analyze the effect of initial particulate size on the rate and characteristic time of the heterogeneous reverse reactions. Compare the efficiencies of quenching of UF4 and metallic uranium. 

As mentioned already in the previous chapter, time does not play a role in the formulation of equilibrium conditions. The mass and energy balances and the maximum conversion rate that can be achieved when the system is in thermal equilibrium are therefore not sufficient to define the dimensions of the best suited coal gasification equipment, which will here be termed the gas generator. A knowledge of how the gasification reactions proceed with time is therefore indispensable. To measure and, wherever possible, calculate this dependence on time is the object of reaction kinetics. Dealing with details of kinetic laws would go beyond the scope and purpose of this book and the reader should turn to the relevant literature [1.4]. 

Thermogravimetiy (TG) is one of the oldest thermal analytical procedures and has been used extensively to study pyrolysis, combustion, and gasification of coal, coke, and polymers. The technique involves monitoring the weight loss of a sample in a chosen atmosphere as a function of temperature, whereby the heating rate is kept constant. The usefulness of TG for analyzing kinetic... 

This Chapter, initially, provides a review of the kinetics and mechanisms of the extensively studied gasification reactions by molecular oxygen. This review provides a basis for an understanding of the mechanisms of thermal activation using carbon dioxide and water vapor. Relationships between carbon structure and reactivity are discussed and differences in activation mechanisms between carbon dioxide and steam, including inhibition reactions, are elaborated upon. 

Aj frequency factor for kinetic process, where i = initiation, branching, gasification, charring, etc a thermal diffusivity, = K/pc (m /s)... 

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