Reactions between sodium and kaolin during gasification of a low-rank coal

Abstract

The experimental results from the reactions between a low-rank coal containing sodium either in the form of ion-exchanged carboxylate or physical impregnation with NaCl, with kaolin during pyrolysis in nitrogen and gasification with steam or carbon dioxide in conditions corresponding to atmospheric FBG are presented. Sodium and kaolin react to form sodium aluminosilicates. Thermodynamic equilibrium predictions have indicated the formation of either a solid sodium aluminosilicate nepheline, Na 2O·Al 2O 3·2SiO 2 or liquid albite, Na 2O·Al 2O 3·6SiO 2 during coal gasification. The reaction between kaolin and either forms of sodium present in the coal is more effective in steam than in nitrogen or carbon dioxide. This results principally in the preservation of meta-kaolinite hexagonal crystal structure and the formation of a high melting point sodium aluminosilicate nepheline, Na 2O·Al 2O 3·2SiO 2 or its polymorph carnegietite. It appears that the catalytic effect of carboxylate sodium has been reduced by the binding of sodium readily through the reaction with kaolin, resulting in lower char conversion. During gasification with steam at least 20 wt.% of sodium present in coal as sodium chloride reacted into insoluble aluminosilicates. No sodium silicate or liquid albite formation was detected in any of the samples. It can be expected that during gasification of coal containing sodium in the presence of kaolin, the formation of solid aluminosilicates, principally nepheline, should help to avoid the formation of liquid silicates and the problems associated with agglomeration and potentially defluidisation of a fluidised bed.