Investigations On Low NOx Combustion Systems For Large Capacity Supercritical And Ultra-supercritical Boilers

Coal is the predominant primary energy in China and it is determined thatcoal-based power constitutes the largest proportion in the power mix of China. Manykinds of pollutants are emitted during coal combustion in the power station, which hasbeen the biggest pollution source in China.The supercritical and ultra-supercritical thermal power units, with high efficiencyand low emissions, is the preferential choice for China to develop clean coalpower-generation technology. This work was involved in the study on the low NO_xcombustion systems equipped respectively in the 600MW capacity supercritical and1000MW capacity ultra-supercritical boilers.It is demanded that the primary air and coal powder can be evenly distributedamong the burners to prevent slag, high temperature corrosion and incompletecombustion, it is also the precondition of low NO_x emission. Pulverized coaldistributor is used for the distribution of the air and coal powder in the direct-firedpulverizing system. In this work, the distribution characteristics of the pulverized coaldistributor equipped in a 1000MW capacity ultra-supercritical boiler was studied indetail by means of experiments and numerical simulation. Experiments were carriedout to evaluate the performance of the distributor when the length of the straight entryduct was altered. The effect of the installation manner of the prefixed curved duct wasalso investigated. The tests results show that the distributor worked better when fixedvertically, with the curved duct installed at right angle to it.In this work, numerical simulation technology was applied to simulate theturbulent flow, coal combustion and NO_x formation process in the furnace of a1000MW capacity boiler. The carbon content of flyash under different condition wasalso evaluated during the simulation. The 1000MW capacity boiler studied here is adual circle tangential firing single chamber boiler, with the low NO_x, concentric firingsystem (LNCFS) is applied to reduce NO_x emission. From the numerical simulationthe following conclusions was obtained. The aerodynamic field is nearly symmetricalin the furnace and the heat load is uniformly distributed, which conduces to reduce theoutlet velocity and temperature deviation of flue gas. The NO_x emission can bereduced to 300 mg/Nm~3 with the use of air staging in the LNCFS system. Such deepair staging may lead to the dangerous high temperature corrosion and serious furnaceslag. In order to get a higher carbon bum-out and higher NO_x reduction efficiency, theflux distribution between the primary air and SOFA should be optimized.This work introduce the combustion commissioning process and performancetests of two domestic 600MW capacity supercritical boilers. The low NO_x axial swirlburner technology and LNCFS system are applied in the two boilers respectively. Thetests results show that this two advanced combustion technology has advantages ofgood low load flame stability, high combustion efficiency and low NO_x emissions.The optimal operation schemes of the combustion systems were determined duringthe tests. The combustion and NO_x emissions characteristics of one of the boilers wasalso simulated and the results were consistent with the performance tests well.