Steam gasification, alkali carbonate

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. 

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. 

Subsequent graphite gasification research by White and his group (3,4) found potassium carbonate to be an excellent catalyst at an optimum proportion of 20 percent (by weight) of the carbonaceous fuel feed. White and Weiss (3J noted that alkali carbonates also catalyzed the water-gas shift reaction when steam was injected during gasification ... 

Cox and co-workers (7, 8, 9) catalyzed the gasification of wood, coal, paper, sludge, manure and municipal refuse with potassium carbonate. They found that alkali carbonates promoted the steam-carbon and steam-hydrocarbon reactions and concluded ... 

Additional literature by Cox, (8), Appell (11), Hooverman (12), Cavalier (13), Knight (14) and Love (15) serves to reinforce the evidence that alkali carbonates increase reaction rates and overall gas yields in the gasification of biomass, coal and refuse. The general consensus is that alkali carbonates catalyze the water-gas shift and carbon-steam reactions. 

Alkali Carbonate and Nickel Catalysis of Coal-Steam Gasification... 

Kroger (3) found that metallic oxides and alkali carbonates or mixtures catalyzed the carbon-steam reactions. Lewis and co-workers (4) stated that if reactive carbons are catalyzed with alkali carbonates, reasonable gasification rates are attainable at temperatures as low as 650 °C. A process which uses molten sodium carbonate to catalyze as well as to supply heat for the carbon-steam gasification has been described (5). 

The process is based on the concept that alkali metal salts (such as potassium carbonate, sodium carbonate, potassium sulfide, sodium sulfide, and the like) will catalyze the steam gasification of coal. In addition, tests with potassium carbonate showed that this material also acts as a catalyst for... 

C. Conversions increased with alkali content indicating that the potassium cation was involved in the active sites for the reaction. Carbon was deposited on the catalyst and the conversion increased in proportion to the amount of carbon deposited. The carbon was steamed off subsequently and the rate of gasification with steam increased with K content, confirming the promoting effect of K on the steam-carbon reaction (see Figure 10). A linear relationship... 

One method that is used to prevent carbon formation is to promote the steam-reforming catalyst with alkali, usually potassium.Alkali serves to increase the resistance to carbon formation on the catalyst and promotes the carbon (re)gasification. " ... 

Catalysts used for steam reforming are oftenbased on Ni/NiO or Co formulations supported on materials such as magnesium alumina spinel (Rostrap-Nielsen, 1993). These compositions can be combined with alkah or alkali-earth compounds to reduce methanation and facilitate coke gasification to avoid carbon deposition. 

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