Development Of Selective Non-Catalytic Reduction For NO_x Emission Control In A Circulating Fluidized Bed Boiler

Nitric oxide (NO_x) is one of the important pollutants and the emission which isincreasing in China. Almost80%of the total NO_xemission is coming from industry,where about70%of NO_xgenerated by coal-firing power plants, which should be themost important area to control the NO_xpollutant. The NO_xcontrol technology for powerstation boilers includes combustion process control and flue gas denitrification. Amongall of the appealing de-NO_xtechnologies, Selective Non-Catalytic Reduction (SNCR) isbecoming popular due to its low operation cost. For a circulating fluidized bed (CFB)boiler, the operating temperature window and the residence time in the cyclone is veryfitful to use SNCR. The objective of dissertation is to study the key technologies ofSNCR and to develop a high efficient SNCR system for a CFB boiler.Therefore, a method of mass balance calculation was put forward for CFB boilerSNCR system, and the key parameters, such as consumption of reductant solution andflow rate of dilution water, were calculated. The effect of reductant solution on boilerthermal efficiency was also considered.Based on the mass balance calculation results, the characteristics of the flue gasflow field in the cyclone is analyzed by using Computational Fluid Dynamics (CFD)method. By comparing distribution of reductant residence time and mole ratio ofNH3/NO_x(NSR) under different arrangement of reductant nozzles, an optimized designof reductant nozzle arrangement was presented. Using this arrangement and reducedSNCR mechanism, the effects of gas temperature, NSR, injection parameters (injectionvelocity, atomized particle size, spray angle, etc.) and circulating ashon SNCRdenitrification performance were studied by CFD method. The best operatingtemperature window and NSR range were identified based on the above results.The different performance of SNCR systems using anhydrous ammonia, aqueousammonia and urea as reductants was compared and urea SNCR system, which is fitfulfor the demands of engineering project, was selected as the final design. The urea SNCRsystem was consist of following key units, including reductant storage, delivering,distribution, injection and system control unit, and they were designed for a CFB boiler.The best operation procedure was also provided as well as the maintenance procedure. As a test of the designed urea SNCR system, field tests were conducted on a100t/h CFB boiler. The results showed that the de-NO_xefficiency of the measured SNCRsystem is above60%, the response time120~180s. The operation of SNCR is verystable. The de-NO_xefficiency will increase as the increase of the amount of reductantinjection, while at the same time, the ammonia slip also increases correspondingly,which should be taken into consideration during pratical operation.