Numerical Simulation Of Down-Fired 350 MWe Utility Boiler On Operating Parameters

China will continue coal-based energy structure in a long term and development of clean and efficient coal combustion technologies and efficient use of low volatile coal in power generation are of great significance, Institute of Combustion Engineering in Harbin Institute of Technology studied down-fired boiler in a long-term, proposed multi-stage combustion with multi-ejection technology and desigined combustion system of a 350MW down-fired boiler. Before reality the new technology in the boiler, it is necessary to use numerical simulation to predict the effect and to guide the design work.Numerical simulation were performed on a down-fired 350 MWe supercritical utility boiler designed in multi-stage combustion with multi-ejection technology. The overall combustion performance inside the furnace was analysed and the influence of the operating parameters such as proportion of fuel-rich air and fuel-lean air the over fire air ratio on the combustion characteristics and NOx emission were studied, respectively.FLUENT 6.3.26 was used in the numerical simulation. Gas turbulence was specifically taken into account by the realizable k–εmodel. The Lagrangian stochastic tracking model was applied to analyze the gas/particle flow field. Radiation was described using the P-1 model. The combustion of volatiles was modeled by employing probability density function theory.From the numerical simulation of the overall combustion performance inside furnace, symmetrical combustion can be achieved in the furnace on multi-stage combustion with multi-ejection technology. At the same time the coal injection depth is deep, this is conducive to the burnout of pulverized coal. The oxygen concertration is high in the area near the front and rear wall of the lower furnace and area near the cool water tube wall of the hopper and this is conductive to prevent slagging.As the proportion of fuel-rich air and fuel-lean air gradually increases, temperature in the main combustion zone in the furnace center increases and the range of the zone progredssively increases, the O2 concertration in the zone increases. When the proportion of fuel-rich air and fuel-lean air increases from 2:8 to 5:5, the carbon content in fly ash increases linearly, NO emission decreases linearly. Considering the carbon content in fly ash and NO emissions together, an recommended optimum proportion of fuel rich air and fuel lean air is 5:5.As the over fire air ratio decreases, the penetration depth of the air decreases, the range of the main combustion zone increases, the position of the zone moves upward and the NO emission increases obviously. For the over fire air ratio 19.12%, the over fire air injects to the furnace center and when the over fire air ratio is lower than 10%, the penetration depth is very shallow and the over fire air turn upward shortly after over fire air injects into the upper furnace. As the over fire air ratio increases, the carbon content in the fly ash in the furnace outlet increases and NO emission decreases obviously. An recommended optimum over fire air ratio is 15%.