Formation of Reductive and Corrosive Gases during Air-Staged Combustion of Blends of Anthracite/Sub-bituminous Coals

摘要

Air-staged combustion of blended coals is now a common choice for coal-fired boilers in China as a primary method to control NO emission. Reductive gases, CO and H-2, and corrosive gases, H2S and COS, are generated in the fuel-rich zone which leads to problems of high temperature corrosion, slagging and fouling under a reducing atmosphere. This study focuses on the generation properties of reductive and corrosive gases during blended coals air-staged combustion. Experiments of anthracite, sub-bituminous, and their blends with different mixing ratios were conducted in a drop tube furnace. Generation characteristics of reductive gases, CO and H-2, and corrosive gases, H2S and COS, in the fuel-rich zone were carefully studied at 1573 and 1673 K with different stoichiometric ratios in primary combustion zone. Blending effects on char burnout and NO,, emission were also reported. It was found concentrations of CO and H-2 increase obviously in the fuel-rich zone by blending sub bituminous into anthracite due to the fast gasification of sub-bituminous. For blended coals, extremely high concentrations of H2S and COS were observed under deeply air-staged condition due to part of SO2 released by anthracite of higher sulfur content was converted to H2S and COS by more reductive gases generated by sub-bituminous of higher char reactivity. Char reactivity during the burnout stage of the blends was dominated by the less reactive coal, anthracite, so that, blending had no big effect to improve the final char burnout. Blending sub-bituminous into anthracite was only of tiny effect to enhance the NOx reduction and the stoichiometric ratio in the primary combustion zone should be selected carefully, less than 0.8.