Influence of the Atmosphere on the Transformation of Alkali and Alkaline Earth Metallic Species during Rice Straw Thermal Conversion

The influence of the atmosphere on the transformation of alkali and alkaline earth metallic (AAEM) species during rice straw thermal conversion was investigated by a fixed-bed reactor at 1073, 1173, and 1273 K. Two types of atmospheres were considered: 100% (v/v) N-2 to simulate pyrolysis and 1% (v/v) O-2 in N-2 balance to simulate combustion. The release of the AAEM species during pyrolysis and combustion of the rice straw was quantified by a mass balance based on the weight measurements and inductively coupled plasma (ICP) analysis of the rice straw and the solid residue. Solid residues and released and condensed particles obtained in experiments were processed by water leaching and 1% HNO3 washing and analyzed by ion chromatography (IC) to determine the possible chemical form of the AAEM species contained in them. The experimental results showed that the oxidizing atmosphere facilitated the release of K at 1073, 1173, and 1273 K by enhancing the decomposition of K2CO3 and char K in the char during the char combustion stage of the rice straw combustion experiment. However, the oxidizing atmosphere suppressed the release of Na, Ca, and Mg at 1073, 1173, and 1273 K. For Na, the release suppression was caused by its promoted interaction with silicate in the oxidizing atmosphere. For Ca and Mg, such suppression was supposed to be caused by the suppressing effect of the oxidizing atmosphere on the release of Ca and Mg from char Ca and char Mg in the char of the rice straw.