Char gasification, catalytic effects

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 ... 

Exchangeable cations play an important role in the behavior of lignite coals in coal conversion processes. It is, therefore, important that a fundamental understanding be attained, first of the possible interaction of exchangeable cations in lignites with each other, and, second, of the possible effects on the subsequent catalytic activity of the cations for lignite char gasification. 

Reactivity results for the 1273 K chars reacted in 0.1 MPa CO2 and 3.1 kPa steam at 1053 K are shown in Figures 3 and 4, respectively. The same trends evident for the 1273 K chars reacted in 0.1 MPa air are also seen in these reactant gases. Two regions of reactivity behavior are present in both cases. Calcium is seen to be a good char gasification catalyst, while Mg has very little catalytic effect. 

The addition of Mg to Ca does not have a significant effect on the subsequent activity of Ca for lignite char gasification under the slow heating conditions used in this study. Their catalytic effects were found to be additive, indicating that they did not interact in any significant way. This was supported by XRD results which showed that the sintering of CaO was not reduced by the presence of MgO. 

Zhang, L., Kudo, S., Tsubouchi, N., Hayashi, J., Ohtsuka, Y., Norinaga, K. (2013). Catalytic effects of Na and Ca from inexpensive materials on in-situ steam gasification of char from rapid pyrolysis of low rank coal in a drop-tube reactor. Fuel Process Technology, 113,1—1. 

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. 

Li, X., Hayashi, J., Li, C. Z. Volatilization and catalytic effects of atkah and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part Vll. Raman spectroscopic study on the changes in char structure during the catalytic gasification in air. Fuel 2006, 85, 1509-1517. 

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... 

Jiang, M., Hu, J., Wang, J. (2013). Calcium-promoted catalytic activity of potassium carbonate for steam gasification of coal char effect of hydrothermal pretreatment. Fuel, 109, 14-20. 

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