Emission Characteristics of NOx and Unburned Carbon in the Fly Ash of Sub-bituminous Coal Combustion

摘要

At present, coal-fired power stations in Japan use mainly bituminous coal as fuel. Since coal consumption is increasing worldwide and bituminous coal will be in short supply in the near future, low rank coal with lower calorific value will become an important fuel. Sub-bituminous coal, which contains more than 20% moisture, is looked to as a substitute fuel for bituminous coal, as the reserve is the most abundant among low rank coals. Problems involving the utilization of the sub-bituminous coal are the low ignitability and combustion efficiency caused by the moisture in the coal. Control of NOx emission will also be necessary. Before combustion, coal is carried to the furnace through the mill by hot air. When sub-bituminous coal is pulverized, part of moisture remains in the coal and part is vaporized. This report describes the influence of the remaining moisture in the coal and of the vaporized moisture on the combustion characteristics. This research was conducted using a pulverized coal combustion test furnace (coal combustion rate: 0.1 t/h). Furthermore, in order to optimize combustion conditions for sub-bituminous coal, we investigated the influence of the distribution ratio and the swirl force of secondary and tertiary air on the emission of NOx and unburned carbon in the fly ash. As a result, we clarified that, when the ratio of remaining moisture in the coal to that of the total amount of moisture was higher, ignition worsened and the combustion flame became widely diffused. Therefore, emissions of both NOx and unburned carbon concentration in the fly ash increased. When combustion air injection conditions, meaning Air/ Coal and swirl force, were optimized for sub-bituminous coal, ignition conditions improved and emissions of both NOx and unburned carbon in the fly ash decreased. Although ignition conditions worsened when the remaining moisture content in the coal was higher, adjustment of the combustion air injection conditions for higher moisture coal effectively improved ignition. Thus, the reduction ratio of NOx increased for higher moisture coal.