Gasification reactions surfaces, reactivity

In the pore model developed by Bhatia and Perlmutter, the rate of the gasification reaction per unit pore surface area is characterised by the reaction rate constant, K,. As the original work addresses structurally based effects only, Kj may well be assumed constant throughout the gasification stage and, under kinetic control, the char reactivity is then a direct measure of the available surface area. To allow the description of additional (i.e., non-porous) phenomena, we follow a semi-empirical approach in which we assume that Kj can vary with time, the cause of which can either be structural or catalytic in nature. Accordingly, we define Ks(t) = KsoucnirtCt) Strictly... 

With uncatalysed gasification of charcoal, the functionality [l+(bt) ] appearing in Eq, (9) basically simulates the gradual emergence of new surface area (i.e., other than that already constituted by the charcoal pores) by the particle disintegration process, whereas the description of its decline with time by the gasification reaction is implicitly handled by the boundary conditions already set in the original derivation. The volumetric excess reactive surface area involved at time t can be estimated from... 

In the literature it has been suggested that the late reactivity maximum around XssO.7 (see Figure 4A) results from the saturation of the carbon surface area with catalytically active alkali species, (See, e,g., Hamilton et al. ) This explanation, however, is not supported by the catalyst accumulation factors (= [l+(bt) ]) derived by us as we find them to rise steadily with increasing conversion degree (See Figure 4C). Catalyst saturation may be defined as a state where the charcoal surface area is covered entirely by a mono-layer of catalytic species. If we assume the extreme case that carbon, but not the added alkali species, is being removed from the charcoal, then, from the initial atom ratio it follows that saturation effects may be encountered, but not before ca. 88% of the carbon has been consumed by the gasification reaction. 

More recently, Rosen (R3), Spalding (S5), and Johnson and Nachbar (J4) have considered a simplified approach using the analysis of laminar-flame propagation velocities. According to these investigators, the principal exothermic reactions occur in the gas phase. Some of the heat liberated by these reactions is then transferred back to the solid surface to sustain the endothermic surface-gasification processes. Thus, the temperature profile within the reactive zone is quite similar to that of Rice and Crawford. However, gasification of the solid surface is assumed to be endothermic, while exothermic reactions were considered in the studies discussed previously. 

The practical motivation for understanding the microscopic details of char reaction stem from questions such as How does the variability in reactivity from particle to particle and with extent of reaction affect overall carbon conversion What is the interdependence of mineral matter evolution and char reactivity, which arises from the catalytic effect of mineral matter on carbon gasification and the effects of carbon surface recession, pitting, and fragmentation on ash distribution How are sulfur capture by alkaline earth additives, nitric oxide formation from organically bound nitrogen, vaporization of mineral constituents, and carbon monoxide oxidation influenced by the localized surface and gas chemistry within pores ... 

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