Cost-Efficient Nitrogen Oxides Control by a Hybrid Selective Noncatalytic Reduction and Selective Catalytic Reduction System on a Utility Boiler

Abstract

Abstract

The hybrid process of selective noncatalytic reduction (SNCR) followed by selective catalytic reduction (SCR) is an up-to-date nitrogen oxides control technique and can achieve a high efficiency (60%–95%) with moderate cost. The SNCR/SCR hybridization was investigated through experimentation on a 100 MW, coal-fired utility boiler in China. The baseline nitrogen oxides emission of the hybrid urea-based SNCR/SCR system was 170 ppm at 6% O₂ controlled by coal reburning technology. The hybrid SNCR/SCR system including only 89 m³ catalysts could control NO emission under 25 ppm with ammonia slip <4 ppm at 300–430 tons/h of boiler load. Total NO reduction efficiency was generally about 85% and could be beyond 90%, in which SCR's efficiency was 50%–70% individually and SNCRs was about 60%–75%. NO emission decreased from about 65 to 15 ppm when the flow rate of 35% urea solution increased from 0.15 to 0.47 tons/h. At higher boiler load, more urea solution had to be injected and higher layer of the agent injectors had to come into operation to hit the same emission target. The superheated steam was injected with high speed into flue gases at the inlet of reversing chamber and it was effective on enhancing both the total quantities and the distribution of SNCR's ammonia residue, which was used as reducing agent of SCR. In general there was more NH₃ where there was less NO at the inlet of catalysts and NO distribution was contrary to NH₃ distribution in some way. NO and NH₃ distribution at the outlet of catalysts was almost even compared with the inlet distribution. Average NH₃ slip at the outlet increased with NH₃ at the inlet and could be controlled below 4 ppm at 6% O₂ in any case. The difference between average NH₃ at the inlet and the outlet was smaller several times than the reduced NO by SCR catalysts. Some isocyanic acid (HNCO) as the product of urea solution thermohydrolysis might exist in the flue gas and it was hydrolyzed to NH₃ quickly on SCR catalysts, which then reduced NO on the surface of SCR catalysts.