چكيده به لاتين
In the present study, using the CFD modeling method, the combustion process in a 250 MW gas-fired boiler with opposed firing combustion has been investigated. The natural gas combustion process has modeled with the 4-step JL mechanism. The chemical and turbulence interaction in the reaction stream are modeled with the EDC model. In the created model for the burners, the exact dimensions and location of the gas nozzles, the inlet path of air streams, as well as the effect of the swirling vanes, have been considered. The thermal, prompt, and N2O-intermediate reactions are used for more accurate simulations of NOx generation. After validating the CFD model, the effect of changing various parameters such as air-to-fuel ratio and swirl ratio of burners flow on boiler combustion quality and pollutant emissions has been analyzed to optimize the combustion process. Finally, a new method has been proposed to optimize the combustion process based on the combination of OFA and FGR systems. Results show that for the air-fuel ratio of 17.6368, the combustion efficiency of the furnace reaches the highest value (96.32%), but the NOx concentration at the furnace outlet increases to 273.83 ppm. Also, it is concluded that by reducing the air-to-fuel ratio to 17.122, NOx emissions can be reduced by 45.96% with a little reduction in furnace efficiency (0.95% reduction). Based on the CFD results, the optimum swirl ratio is calculated to be 0.6 to improve flame stability, reduce overheating damage, and reduce NOx production by 4%. CFD results show that if the total injected flow from OFA and FGR nozzles contains 35.95% RFG and 64.05% air, the optimum condition for reducing NOx emissions, without decreasing the combustion efficiency and temperature of the exhaust gases at the furnace outlet is obtained. Under this condition, the amount of NOx is reduced by 57.43% compared to the initial design of the boiler.
