Numerical Simulation on Pulverized Coal Combustion Process in a 670T/H Tangential Firing Furnace
|School||Harbin Institute of Technology|
|Course||Thermal Power Engineering|
|Keywords||numerical simulation stereo-staged combustion Over Fire Air NO_x tangentially fired boiler|
A stereo-staged low NOx combustion system is consist of Horizontal Bias Combustion (HBC) burners and Over Fire Air (OFA). Numerical simulation of the combustion process and NOx emission in a 670t/h tangentially fired furnace with stereo-staged combustion system was performed on CFD (Computational Fluid Dynamics) software platform. In the numerical simulation of furnace process, 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. The devolatilization process was simulated by two competing reaction model. The combustion of volatiles was modeled by the Probability Density Function (PDF). Char combustion was simulated by diffusion-kinetics model.The temperature field and the distribution of combustion species’concentration in the furnace were gained. The comparison between the simulation and the in situ test shows a good agreement. The result also shows that the stereo-staged combustion system can keep good flame stability and low NOx emission.Several cases with different secondary air arrangements, excess air ratio and arrangement mode of OFA were studied. The Comparison of normal and postponed secondary air arrangements on combustion process and NOx emission shows that the normal secondary air arrangements reduces more NOx. The excess air ratio has an important effect on the emission of NOx. With the decrease of the excess air ratio , the concentration of O2 at the furnace exit reduces,and the emission of NOx also reduces. The effect of OFA was also studied by the comparison of two cases one with no air through the nozzles of OFA and the other with OFA working normally. Results show that the application of OFA reduces the emission of NOx greatly. The position of OFA in the furnace also has influence on the emission of NOx. While the residence time of coal in the primary combustion zone varys form 0.35s to 0.65s, the emission of NOx lessens when the elevation of OFA increases. When the residence time is more the 0.65s, there is a limit elevation above which the increase of the height of OFA has little impact on the diminishment of the emission of NOx.