HIGH-TEMPERATURE REACTIONS OF STRAW ASH AND THE ANTI-SINTERING ADDITIVES KAOLIN AND DOLOMITE

Straw of various types of rape, wheat and barley have been studied wit h respect to the formation of crystalline compounds and high-temperatu re reactions in ash, as well as sintering and melting behaviour. Durin g the low-temperature ashing process simple, crystalline compounds suc h as carbonates, sulphates and chlorides were formed. A significant pa rt of the ash from wheat and barley straw was amorphous whereas rape a sh was found to be mainly crystalline. The large content of potassium compounds present in wheat and barley straw ash contributes to their l ow melting points. The ash components primarily formed are reactive. S olid state reactions at temperatures above 800 degrees C lead to the f ormation of secondary products such as oxides and silicates. Minerals such as kaolin and dolomite have been suggested as fuel additives to g ive the ash a higher melting point. High-temperature reactions between straw ash and kaolin, Al2Si2O5(OH)(4), or dolomite, CaMg(CO3)(2), res pectively, were therefore investigated. Kaolin was found to be the mor e effective additive. The reaction between kaolin and potassium salts in straw ash gave KAlSiO4 and KAlSi2O6. A laboratory study of reaction s involving K2SO4 or KCl and kaolin showed that several products are p ossible, one of which is KAlSiO4. The potassium capture by kaolin part ly explains the higher melting point of the ash-additive mixture. Dolo mite added to wheat and barley ash reacted with silica to form silicat es. No reaction between dolomite and potassium compounds could be dete cted. The observed enhancement of the melting point caused by dolomite is probably an effect of dilution or adsorption. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.