Role of Inorganics During Fluidised-Bed Combustion of Low-Rank Coals

Abstract

Inorganic matter in coal continues to be one of the major sources of operational problems during coal combustion. The role of inorganic matter during coal combustion is governed by its nature and occurrence in the coal matrix [Raask‚ 1985]. In pulverised coal combustion processes‚ slagging‚ fouling and corrosion are the main problems which arise from the coal mineral matter. These problems can be even more severe when firing low-rank coals with high alkaline and transition metal ions. In contrast‚ fluidised bed combustion processes operating at much lower tempearures ( 800–900 °C) offer better prospects for reducing ash-related problems associated with the firing of low-rank coals. However‚ low-rank coals do not present trouble-free operation in fluidised beds. A major problem is presented by particle agglomeration‚ bed defluidization and ash deposition on heat exchanger surfaces. The inorganic constituents in low-rank coals (mainly sodium‚ calcium and organically bound sulphur) transform to low-melting eutectics at fluid bed operating temperatures [Manzoori and Agarwal‚ 1993; Hodges and Richards‚ 1989; Manzoori and Agarwal‚ 1992; Souto et al.‚ 1996]. These eutectics are transferred to the surface of inert bed particles leading to agglomeration and defluidization of the bed. This phenomenon is dependant to a great extent on the operating temperature of the fluid bed combustor. The present work is aimed at understanding the effects of bed temperature on ash characteristics during fluid bed combustion of an Australian and an Indonesian low-rank coal. Data is presented to demonstrate the effect of bed temperature on the distribution of inorganic matter into variuos streams such as bed ash and cyclone ash. Comparisons of the characteristics of ash deposition on bed material and bed defluidization for Australian and Indonesian low-rank coals are presented. In addition‚ the chemical nature of the ash deposits is discussed‚ along with the chemical interactions that might have occurred during the combustion experiments to cause ash-related problems.