Research Development For Reducing NOx Emission On Two 75t/h CFB Boiler Burning Fujian Anthracite With Medium-Temperatue Cyclone

The NOx formation mechanism and its influencing factors of the coal-fired CFB boiler were analyzed and the current kinds of denitration technology solutions were compared under technical and economic comprehensive comparative analysis.On the basis of the domestic and international research results,the NOx emission reforms of the two 75t/h CFB boilers burning Fujian anthracite and using medium temperature separator were carried out by the retrofit of low nitrogen combustion?LNC?combined with the technique of selective non-catalytic reduction?SNCR?,then operation optimization was implemented to meet the NOx emission regulations.Before and after the modifications for the low nitrogen combustion,the Industrial thermal experiments were respectively carried out on the 75t/h CFB boiler burning Fujian anthracite and using medium temperature separator,with operation parameters of CFB boiler optimized and adjusted to reduce NOx emissions.Running results show that there is an optimal secondary air rate to minimize the NOx emission concentration for the above CFB boiler.From the aspects of NOx formation mechanism and the boiler's structure optimization,the low nitrogen combustion measures for the above CFB boiler were carried out.The effective cross-sectional area of ventilation board was reduced from 13.43m² to 11.38 m² by 15.26%,the secondary air nozzle height was increased by 0.4m and a layer of superior secondary air was added that has made the 75t/h CFB boiler become the domestic first with three layers of secondary air.besides,the rate of secondary air was increased from 40%to 50%and the speed of secondary air was increased by improving the nozzle design and the penetration of the secondary air.All of these have obtained good effects to adopt SNCR denitration technology on the tail flue gas of the boiler,such as the concentration of NOx emission reducing from 230mg/m³ before reform to 140 mg/m³ after reform which were measured under the condition of standard state and dry basis with oxygen concentration of 6%O₂.On the basis of the operating adjustment and low nitrogen combustion transform,the SNCR technology of simple modifica-tion,small initial investment and low operating cost was used and its CFD analysis for the above mentioned two 75t/h CFB boilers was done.What's more,the problems of spray gun denitration,residence time and mixing in the furnace were effectively solved and continuously improved.And then the most appropriate SNCR denitration project was carried out around the optimization designs of SNCR spray gun and its injection system.The results show that under the conditions of BMCR at 100%and 75%,the SNCR denitration efficiency is more than 60%,the concentration of NOx emission is less than 100 mg/m³ and the ammonia escape mass fraction is less than 8 mg/m³.So the NOx emission transformation is successful not only in the high denitration efficiency but also in the low ammonia escape rate.According to the statistic running data of these two remolded 75t/h CFB boiler in the second half of 2015,the influences of the boiler's load rate to the ammonia consumption capacity per unit evaporation capacity and per unit coal consumption capacity were studied.The denitration cost model of LNC plus SNCR was set and the effects of the cost or benefit to the denitration cost were analyzed,such as the investment depreciation expense,the running maintenance cost,the increased coal cost by influenced boiler efficiency,the feed-in tariff and environmental protection benefit due to denitration.Results show that after the deduction of the denitration electricity price subsidies and environmental profit,the denitration cost per feed-in 1MWH electricity is still RMB18.90 yuan in which RMB 18.01 is operation maintenance fee,that is,the denitration cost per removal 1 ton NOx is RMB 6112.07 in which running maintenance fee is RMB 5824.33.So the operation cost accounts for the most and the key to control the operation maintenance cost is saving ammonia consumption and optimizing compressed air.