Simulation and validation of NO emission characteristics of biomass pellet fuel combustion based on chemical kinetics

Abstract: In order to study NO emission laws and its formation mechanism of biomass pellet fuel combustion, a CFD (computational fluid dynamics) and Chemkin co-simulation method was used. The simulation was based on an experiment which was conducted on a biomass pellets combustion platform. Three kinds of biomass pellets, which were cotton stalk, corn stalk and wood, respectively, were chosen for the combustion test. Inlet air volume was controlled by changing the wind speed of the blower to study NO emissions under different conditions. Then a CFD model of the test boiler combustion cylinder was established and then divided into grids for the simulation, and a simplified chemical reaction mechanism composed of 17 components and 58 primitive reactions was imported using the interface to Chemkin and adopting the method of sensitivity analysis. The EDC (eddy dissipation concept) eddy dissipation finite rate chemical reaction model was set for Fluent solving process. The maximum temperature of 1320 K simulated by the Fluent was in good agreement with the experimental result of 1293 K, and the deviation was mainly caused by the model assumptions and settings, which signified that the simulated results to a certain extent could reflect the real combustion situation. Therefore, the results of the Fluent such as mixing zone volume and residence time could be used as inlet parameters for Chemkin. Then a Chemkin-PSR (perfectly stirred reactor) reaction simulation network was established, which was composed of 6 PSR reactors and one PFR (plug flow reactor) reactor, corresponding to the 6 reaction zones of the combustion cylinder and the exhaust pipe, respectively. The C2_NOx detailed mechanism developed by Reaction Design was chosen for NO emissions simulation of 3 kinds of biomass pellet fuels. The simulation results showed that cotton stalk produced the most amount of NO, corn stalk took the second place, while wood had the lowest NO emission. Simulation of corn straw also indicated that NO emissions increased apparently with the excess air ratio in the beginning, and then gradually peaked when the excess air ratio reached 1.7. In addition, the NO concentration distribution and temperature distribution in the reactor had a direct relationship with the distribution of CO, O2, and CO2 components. NO was mainly produced since Reactor 4. Finally the simulation results were compared with the experimental results. The deviation of cotton stalk was calculated to be 6%, which was the greatest of the 3 biomass pellets but still quite acceptable. Consequently, the correctness of the model and the chemical reaction mechanism has been proved, which provide a reference for the prediction and control of NO emissions of biomass fuel combustion. Besides, the shape and size of the combustor have big influence on NO emissions, so the study on that aspect should be given more attention. This paper gives the related chemical reaction mechanism, which is the basis of an accurate simulation.