Steam gasification of carbon

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 addition of steam to the CH4/C02 feedstock to avoid excessive carbon formation is a widely used technique in practical systems [3]. The resulting C02-steam gasification of methane process can be described by the following chemical equation ... 

Potassium carbonate is a well-known catalyst for the steam gasification of carbonaceous materials. We discuss the use of in situ high temperature X-ray diffraction to demonstrate that intercalation compound-like structures are not involved as stable intermediates. 

Although all the side reactions affect the selectivity, from the point of view of the catalyst, the production of carbon (coke) is the biggest problem as it remains in situ and acts as a poison. The use of superheated steam plays a vital role here, as it helps to clean the catalyst by gasification of carbon at the working temperature (Equation 10). 

The remaining three heterogeneous reactions involve gasification of carbon by steam, carbon dioxide and hydrogen (28- ). 

The carbonization of polypropylene is similar to that of polyethylene. During slow pyrolysis of polypropylene and for a temperature increase from 400 to 700°C, the yield in the liquid phase remains higher than 80% with a very small increase in the yield of gas phase (less than 20%). On the other hand, in flash pyrolysis of polypropylene, an increase of temperature from 550 to 700°C leads to a decrease of the yield in the liquid phase down to 40% with an increase of the gas phase up to 60%. As presented for PE (see Section 2.1), Sawagushi et al. [21] present the results of steam gasification of PP in a fixed bed reactor (Figure 10.8). 

Hansen, L.K., Rathmann, 0., Olsen, A. Poulsen, K. (1997). Steam gasification of wheat straw, barley straw, willow and giganteus, Risp National Laboratory, Optics and Fluid Dynamics Department, Project No. ENS-1323/95-0010. Cerfontain, M,B Meijer, R., Kapteijn, F, Moutijn, J.A.(1987). Alkali-catalyzed carbon gasification in CO/CO2 mixtures An extended model for the oxygen exchange and gasification reaction, Journal of Catalysis, vol. 107, pp, 173-180. 

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. 

Gasification of carbon by steam has also received much attention, but mainly in the context of the gasification of coals.By contrast, gasification by hydrogen or carbon dioxide has been relatively neglected, since these are of less industrial interest. 

Steam gasification of coal, followed by conversion of the synthesis gas obtained—a mixture of hydrogen and carbon monoxide—to methanol, which can subsequently by converted into other liquids, in particular hydrocarbons. 

This reaction is hardly directly involved in the steam reforming reactions on nickel catalysts (1), but it operates in regeneration of the catalyst. For H20/C Hm > ca. 10, there is affinity for gasification of carbon via reaction (2) depending on pressure. 

The solid product of Reaction 2 can also participate in the gasification of carbon by steam (Reaction 5). 

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