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Proceedings of Symposium on Energy Engineering in the 21<sup>st</sup> Century (SEE2000) Volume I-IV

ISSN:
1-56700-132-7 (Print)

LARGE UTILITY BOILERS BURNING LOW GRADE COALS WITH HIGH EFFICIENCY AND CLEAN ENVIRONMENT

Jim Zhou
Power Generation Group Babcock & Wilcox

Abstract

Low grade coals, such as anthracites and semi-anthracites as well as lean coals, contain lower volatile matter (VM), while the moisture and ash content are sometimes high. The conventional boilers and burners firing pulverized coal can not handle such fuels well due to the difficulties of ignition, flame stability and carbon conversion. The boiler efficiency generally is low and unburned carbon in the fly ash is high. The boiler load turn down range is poor while the minimum load without oil support is high. To successfully burn such fuels, enhancing ignition and flame stabilization and a longer residence time in a furnace are the key technical issues. These steps can be realized by burner design and its arrangement; furnace design and its configuration; as well as a combustion system type and its application. Depending on the coals as fired and combustion system adopted, the burner could be a straight jet or swirl type. The burner arrangement also could be corner or wall firing, even a roof downshot firing with W-shape flame. The downshot-firing furnace as approved is suitable for firing anthracite. The environmental concerns burning low VM coals are more serious compared to the case of burning high-grade coals (bituminous). Its NOx emission level is much higher than bituminous. The available technologies for De-NOx during the combustion stage are too sensitive to coal quality, and are sill under development and improvement. At the current stage in China, most of the downshot firing with W-shape furnaces are 300-350 MW in capacity. Most fuels as fired are blends of anthracites and lean coals. Larger capacity downshot W-flame furnaces are under development. Most of lean coals firing units in China have adopted corner firing technology. Performance on minimum load without oil support of such units should be improved.