| The energy consumption of china is increasing with rapid economic development which brought in serious environment problems. The reduction of sulfur dioxide emission will be offset by the rapid increase of nitrogen oxides (NOx) in the atmosphere. Acid rain still remains a serious problem. Considering most of the NOx emission comes from the coal-fired power boiler, SNCR (Selective Non-catalytic Reduction) as one of the common used flue gas denitrification technology was studied in this paper. SNCR and hybrid SNCR/SCR (Selective Catalytic Reduction) could get middle denitrification with low investment, easy retrofit, low operation cost compared with SCR system. SNCR or Hybrid SNCR/SCR could meet the NOx emission standard.First, this paper studied the decomposition and hydrolysis of urea which was commonly used as green reductant. When the temperature was higher than250℃. the main composition of residue were cyanuric acid and other macromolecular polymers, no urea was found in the residue. The decomposition would finish when the temperature was over500℃. The final products form urea decomposition was NH3and HNCO. Due to the oxidation, the yield of NH3and HNCO would decline sharply when the temperature rose to850℃with O2existence. A small amount of NO and N2O was also generated. HNCO was quite stable in the homogeneous condition but easily hydrolyzed over metal oxide to yield NH3even at low temperature. γ-Al2O3was most suitable as catalytic material due to its high catalytic activity, as well as good abrasion resistance and stability.Then, the study of Thermal DeNOx and NOxOUT process was done through experiments combined with CHEMKIN simulation. The impact of different additive on the NOxOUT process was also examined. The results showed that the Thermal DeNOx would get higher denitrification, lower favorable temperature, wider emperature window than NOxOUT on the same condition. N2O would be generated during SNCR process. About15%of NO was converted to N2O during NOxOUT process which was only3%during Thermal DeNOx process. The increase in water vapor content would make the denitrification curve offset to the high temperature side, while the oxygen had the opposite effect. The SNCR process could be carried out at lower temperature due to the addition of oxygenated liquid additives, but the best denitrification decreased. Compared with oxygenated liquid additives, a small amount of sodium salts would get very good effect. The addition of sodium salts could catalyze the HNCO hydrolysis to yield NH3. In addition the sodium salts didn’t participate in the SNCR reaction but generated OH radicals to promote the SNCR reaction at low temperature.Then the SNCR wall spray and porous nozzle were tested on the atomization test-bed. The two channel solid cone atomization spray and Y type atomization spray were researched including the effect of mixing chamber size, flow rate, air consumption on the atomization performance. The air consumption rate had great effect on the size of droplet size.Finally, particular experimental research on hybrid SNCR/SCR applied in a410t/h power plant boiler was carried out. It was found that more than40%of NOx was eliminated and the NH3slip was below10μl/L for the SNCR system while the NSR was1.5. The denitrification was effected by the temperature of the injection layer, spray flow and particle size of spray droplets. The best way to improve the denitrification was to make the urea droplet to distribute at the right temperature. The addition of complement of urea solution injection layer and high speed air turbulence could make the NH3even distribution before the SCR catalysts. The hybrid SNCR/SCR could get85%denitrification while the NSR was1.86. NH3slip increased the ammonia content in the fly ash and would react with SO3/HCl to form ammonium salts. The ammonium salt would have bad impact on the downstream equipment. In addition the urea solution should be avoided to scour the heat exchangers which might led to the corrosion. |
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