Numerical Simulation Of Mbel Down-Fired With The High Efficient And Low NO_X Combustion Technology

Mitsui Babcock Energy Limited down-fired using a new high efficient and low NO_x combustion technology can effectively solve the problem of asymmetric combustion and particularly high NO_x emission. Based on the platform of CFD software Fluent 6.3, the combustion processes and high NO_x emission in 350MW MBEL down-fired using the existing structures and the new high efficient and low NO_x combustion technology. The realizable k-εtwo equations turbulent model was used to simulate the turbulence. The stochastic tracking model was applied to analyze the gas-solid flow field. The P-1 model was applied to simulate radiation. Two competing reaction model was used to simulate the devolatilization process. Probability Density Function (PDF) was used to model the combustion of volatile. Diffusion-kinetics model was used to simulate char combustion.In the first part, numerical simulation of the MBEL down-fired using the existing structures. Compared simulation results with experimental date in industry, the results show that the error in a reasonable range. This proved that the selected model is accurate. Through analysis the numerical simulation results and experimental date found that there had severe asymmetric combustion, and particularly high NO_x emissions using the existing structures.In the second part, numerical simulation of the high efficient and low NO_x MBEL down-fired combustion technology using the same model and grid painting with the existing structures. Study the impact of the declination angle of staged-air on combustion when the angle is 15°, 25°, 35°, 45°and 55°. When the declination angle of staged-air is 15°and 25°there had severe asymmetric combustion. The downward coal/airflow near the front wall penetrates deep into the middle and lower parts of the dry bottom hopper region ,from where it deflects upward along the right side of the hopper ,filling most of the area in the hopper.When the declination angle of staged-air is 35°, 45°and 55°, the phenomenon of asymmetric combustion is disappears. When the declination angle of staged-air is 35°, the unburned combustible is minimum about 5.86%. When the declination angle of staged-air is 55°, the NO_x emission is minimum about 903.35 mg/m~3. Study the impact when the ratio of staged-air and OFA is 0.5:1, 0.8:1, 1.25:1 and 2:1 on combustion. When the ratio of staged-air and OFA is 1.25:1, the unburned combustible is minimum about 5.51%. When the ratio of staged-air and OFA is 0.5:1, the NO_x emission is minimum about 855.5 mg/m~3. Study the impact when the ratio of secondary air and staged-air is 0.91:1, 1.23:1, 1.67:1, 2.23:1 and 3.46:1 on combustion. When the ratio of secondary air and staged-air is 3.46:1, the unburned combustible is minimum about 2.81%. When the ratio of secondary air and staged-air is 1.67:1, the NOx emission is minimum about 923 mg/m~3.