-4- - Thermal Decomposition and Conversion of Composite Pellets

Abstract

This chapter focuses on the process of thermal decomposition of inorganic and/or carbonaceous substances and reduction of inorganic compounds in large-scale rotary reactors. It reveals that in some cases a small amount of substance is eliminated from the product solids by heating them to an adequate temperature. Elimination of dioxins in fly ash and of silanol from ultra-pure silica are two examples of this case. The shrinking core model is applied for decomposition of limestone, dolomite, and magnesite. Heat conduction through the product blanket controls the shrinking rate of the core instead of a reaction with a gaseous reactant. Decomposition of limestone needs considerable thermal energy. When a particle of organic substance is suddenly fed into a region at high temperature, it is decomposed very quickly, issuing a variety of gaseous and vapor species. Thermal cracking of hydrocarbon solids finishes at high temperature within a minute. In contrast, cracking of coal and/or biomass at intermediate temperature, i.e., 400–500 °C, is of practical importance to produce char of suitable quality for further utilization. The slow rate of temperature rise is necessary to obtain suitable char for production of active carbon. The reduction of composite pellets, which are composed of ferro-chromium ore and coke, is important as a preliminary process for the production of ferro-chromium. At 1300 °C, it was found that the grain size of both the ore and coke did not affect the reduction rate very much. The existence of CO2 around the pellet deteriorates its conversion considerably.