Gasification reactions diffusion control

Since a char particle typically contains < 2% Ca (w/w), while the char surface is > 200 m /g, the Thiele moduli for the calcium reactions are likely to be much smaller than those associated with char gasification even when the turnover numbers for the reactions are of the same order of magnitude. Thus, we will assume that the sulfur reactions are kinetically controlled while the gasification is diffusion limited. In that case HjS and 00S concentrations... 

If the pore diameter and process conditions are well defined, the rates of internal and film diffusion can be calculated. The temperature dependency of the rate can be presented in the form of an Arrhenius plot, that is, log rate vs. reciprocal absolute temperature. Gasification rates can be divided into three zones, depending on whether reaction rate is controlling,... 

The theory advanced by De Ris belongs to the first group. In his model of flame spread along a horizontal surface it is assumed that the diffusion flame contacts the surface at the point where the polymer vaporization (gasification) begins. Reactant diffusion to the narrow zone of chemical reaction controls the heat generation process. If heat is transferred from the laminar diffusion flame to the surface by conduction, then the flame spread rate follows the Equations a) for thermally thin materials... 

Carbon Gasification Rates. Because the reforming rates we observed during this work were often controlled by diffusion, it was not possible to. determine individual reaction rates and rate constants. However, from the TPSR measurements we were able to estimate rate constants for the gasification of catalyst and noncatalyst carbons. These rates are listed in Table VII along with selected results taken from the literature (29, 30, 31). We found that the catalyst carbon gasification rates were first order in carbon amounts up to equivalent (CO adsorption) monolayer... 

The char combustion is always controlled by external mass transfer for technically relevant temperatures (>800 °C), whereas for gasification with H2O and CO2 pore diffusion and the intrinsic rate of the reaction may also play an important role, until at temperatures above 1200 °C the effective rate is completely determined by external mass transfer. 

Figure 6.5.16 shows a comparison of the effective rate constants for coke combustion and CO2 gasification, indicating that the rate constant of the chemical reaction of combustion is by several orders of magnitude faster than the Boudouard reaction. For a particle diameter of 4 cm, the combustion rate is then already controlled by film diffusion for temperatures above about 900 °C. This confirms that at the much higher temperatures reached in the tuyeres (about 2000 °C) the combustion is almost instantaneous, that is, only a length of ten particle diameters are needed for complete oxygen conversion. 

The coal or carbon conversion in a gasification process is governed by complex mechanisms that are dependent on the quahty of the coal used (coal structure or rank, particle size, evolving pore structure, catalytic effects of char minerals content, changes in surfece area, char fi acturing, and coal moisture) and the physical and chemical conditions around the particle (temperature, pressure, concentration of reactants such as O2, H2O, CO2, H2 and their diffusion properties). Because of these numerous influences, a theoretical prediction of coal reactivity is nearly impossible without laboratory data [4]. One important aspect of heterogeneous reactions is whether rate is controlled by diffusion limitations in the boundary layer around the particle, so-called bulk surface diffusion, or by diffusion inside the pores of the particle. 

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