Gasification rate equations

The addition of potassium to a nickel/ -alumina steam reforming catalyst provides resistance to the accumulation of carbonaceous deposits in two ways. First, the alkali reduces the rate of hydrocarbon cracking on the nickel component of the catalyst. Second, the promoter enhances the rate of the steam gasification of carbon on the catalyst. This is accomplished by increasing the surface coverage of water on the catalyst and hence supplementing the pre-exponential component of the gasification rate equation. 

The conversion of the parent hydrocarbon and the gasification rate is calculated from the following equation ... 

Gasification. The only study on gasification kinetics of Texas lignite has been performed by Bass (23). Using a differential reactor, he obtained rate data at 700°C and for pressures ranging from 61.6 to 225.9 kPa. Rate equations as a function of steam partial pressure and carbon conversion were developed. 

If Pi,i/Pi,e( d fhen equation (11) merely provides a small correction to equation (6). However, Pi,f/Pi,e(7]) may approach unity for some burning solids. In such cases, the gasification process, equation (1), no longer governs the overall burning rate some other process becomes rate controlling. In extreme cases it is sometimes reasonable and convenient to replace equation (11) by the simpler requirement of surface equilibrium, namely,... 

Bandyopadhyay, D. Ghosh, A. (1996). Validity of rate equation based on Langmuir-Hinshelwood mechanism for gasification of carbon - a reappraisal. 

The effect of the addition of a potassium promoter to a nickel steam reforming catalyst has been probed in terms of the propensity of the catalyst to resist carbon formation. It has been found that potassium facilitates a reduced accumulation of carbon by decreasing the rate of hydrocarbon decomposition on the catalyst and by increasing the rate of steam gasification of filamentary carbon from the catalyst. The effect of the promoter on the carbon removal reaction is evident in an enhancement of the pre-exponential factor in the rate equation by promotion of water adsorption on the catalyst surface. 

Elementary steps of coke formation from methane cracking and of the gasification of coke by H2O and Hz. Net rate equation for coke formation accounting also for diffusion of carbon through the Ni particles of catalyst. 

These rate equations describe the kinetics and mechanisms of the gasification reactions of carbons by carbon dioxide and steam. As such, they describe the processes of activation by carbon dioxide and steam giving insights into mechanisms of carbon removal (the activation process), together with differences between activations by different agents and the effects of inhibition by product gases. 

Attempts have been made to predict gasification rates using mathematical models. This area has been briefly reviewed by Rafsanjani et al. (2002) who discuss the use of (what are termed) the grain model, the random pore model, the simple particle model and the volume reaction model. They report that differential mass conservation equations are required for the oxidant gas and char particle. These authors use a simplified mathematical model (the quantise method (QM)) for activation of coal chars when both diffusion and kinetic effects have to be considered. Results are compared with other methods when it is found that QM predictions of rate are more accurate than predictions by the random pore model and the simple particle model. 

Therefore, the steam gasification reaction rate of the gingko nut shell-char can be represented by the following kinetic equation as ... 

This equation along with Equation (9.97) allows us to eliminate the convective flame heat flux to develop an equation for the flame temperature. This equation will still contain the burning rate in terms of the effective heat of gasification, Lm. From Equation (9.97), we define Lm as the modified heat of gasification by the following ... 

This shows that this modified heat of gasification includes all effects that augment or reduce the mass loss rate. Recall that the term in the [ ] becomes zero if the solid is thermally thick and the virgin solid equilibrates to the steady state. Equating Equations (9.107) and (9.108) gives an equation for the flame temperature ... 

Carbon-Carbon Dioxide Reaction. There is general agreement 6, 37-43) that experimental data on the rate of gasification of carbon by carbon dioxide fit an equation of the form... 

Equation (2) can now be shown to be consistent with at least two mechanisms where carbon monoxide retards the gasification reaction. Mechanism A applies where the rates of the back reactions of reaction (1) and (2) are negligible. 

A model Is presented for char gasification with simultaneous capture of sulfur In the ash minerals as CaS. This model encompasses the physicochemical rate processes In the boundary layer, In the porous char, and around the mineral matter. A description of the widening of the pores and the eventual collapse of the char structure Is Included. The modeling equations are solved analytically for two limiting cases. The results demonstrate that pore diffusion effects make It possible to capture sulfur as CaS In the pores of the char even when CaS formation Is not feasible at bulk gas conditions. The model predictions show good agreement with experimentally determined sulfur capture levels and reaction times necessary to complete gasification. 

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