The Varying Characterization of Alkali Metals (Na, K) from Coal during the Initial Stage of Coal Combustion

The first step in controlling alkali gaseous phase concentrations in flue gas is understanding their behavior during coal combustion. The behavior of sodium and potassium in coal during the initial stage of combustion was investigated. A series of tests were conducted under two combustion temperatures (800 °C and 900 °C) and two atmospheres (21 vol % and 6.36 vol % O2). The data indicated that only a small portion of alkali metal in the coal was released during the initial stage of combustion process (e.g., devolatilization and volatile combustion stage). The varying behaviors of different occurrence modes of alkali metals were also studied, that are identified by sequential extraction with the solutions of water, 1 M ammonium acetate, and 1 M hydrochloric acid, respectively. In general, the water-soluble (WS) alkalies in the burning residue decreased with an increase in exposure time on high temperature combustion atmosphere, while the hydrochloric acid-soluble alkalies showed an opposite tendency. This result suggests there was conversion of the WS forms to the hydrochloric acid-soluble (HAS) forms through aluminosilicates as inter-media. The possible pathway in the process is that the WS alkali reacts with aluminosilicates to form chiefly alkali aluminosilicates that can be dissolved in 1 M hydrochloric acid solution or partly as ion exchangeable alkali metals.