【摘要】：Air Quality Standards are requiring increasingly stringent ozone emissions from numerous furnaces in the chemical process industry. NOx emissions have been identified as a major contributor to ground-level ozone. Available technical solutions range from Co-firing pulverized coal combustion technology and addition of Separated Overfire Air or (overfire air) systems to post-combustion control methods such as selective non-catalytic Reduction SNCR. This dissertation describes (Chinese Power Station), approach to NOx control and its use of CFD modeling as an integral tool in the design and implementation of NOx reduction technologies in a200MW Tangentially coal-fired utility boiler applications. Several technologies were presented on how (Chinese Power Station), has used CFD modeling and measuring data to develop the combustion performance upgrades and modifications for reducing NOx emissions on real size tangentially fired utility boiler. They involve the staging of furnace combustion with co-firing of coal combustion (two fuel systems and three fuel systems), separated overfire air (SOFA) and selective non-catalytic Reduction (SNCR) to reduce NOx emissions from the tangential-fired utility furnace. Furnace simulations were used to optimize the amount of flow rate of co-firing fuels, also to optimize the OFA port placement as well as to identify locations of highest emission concentration. The co-combustion consequence of coal and blast furnace gas BFG (two fuel system), on furnace combustion'performance, and NOx destruction, was investigated. The overall furnace temperature distribution was decreased with the maximum range1601-1722k, that cause to reduce the NOx formed in the main combustion zone by37.5%. In addition, the concentration of02decreased along the furnace with maximum exit percentage of2.0%.Finally for the two-fuel system. The two methods probability density function method, and eddy dissipation method were investigated. The results show a qualitatively agrees well with each other, with negligible variation on all measured parameters.
For advance co-combustion technique of coal, BFG, and coke oven gas (COG), the eddy dissipation model was used to perform the numerical simulation of three-fuel combustion in a utility boiler, and to simulate the turbulent gas-phase reaction. The validated CFD model is then applied to investigate the effects of different BFG and COG flow rates on the boiler performance. It is found that increasing the BFG flow rate brings negative effects on the ignition of primary air and pulverized-coal mixture, pulverized-coal burnout, and heat transfer in the furnace and, consequently, decreases the thermal efficiency. However, increasing the COG flow rate can increase the thermal efficiency via improving the pulverized-coal burnout and heat transfer. Increasing both the BFG and COG flow rates is favorable to reducing NO emissions. The results also indicate that co-firing pulverized coal with BFG of about20%heat input and COG of about10%heat input is an optimal operating condition for improving the boiler performance at180MW load. The validated CFD model is then applied to investigate the effects on the temperature and velocity deviations along the upper furnace width. It is found that the maximum temperature deviation is about200K along the furnace width of the boiler load of180MW. The temperature in the right side is higher than that in the left side. With the increase of BFG flow rate, both the temperature and velocity deviations decrease. It indicates increasing the BFG flow rate have little effect on the temperature and velocity deviation along the furnace width. The COG was conducted as reburning fuel, because of its content hydrocarbons. When the COG flow rate increased100%, the in-furnace temperature decreased, and has more effect on NOx reduction. For the flue gas's treatment (flue gases washing out), the separated overfire air (SOFA) and selective non-catalytic reduction techniques were used to evaluate the NOx destruction through boiler. Firstly, the boiler was simulated under full load180MW and20%overfire air. The result shows for the exiting tangentially fired boiler with SOFA group location (2and4meter) upper the reburning zone, the NOx destruction about63%.
Finally. the urea was used as SNCR reagent. Numerical simulation was conducted using CFD code to investigate the SNCR technology on NOx reduction, for three types of coal under three different loads. The results obtained the average NOx emissions at full. medium, and lower load range from290-491,226-480, and166-350ppm respectively. The NOx reductions for these three loads ranged from39.69%, and61.59%percent. While the results varied from coal type-to-coal type for the same boiler load. In addition. the urea (NH3) slip for these different operation conditions (coal type and loads) was ranged35-90ppm.