Numerical Simulation Of Combustion Characteristic And NO_x Emission In Biomass-Fired Grate Boiler

With the depletion of fossil energy,the search for clean and renewable alternative energy is imminent.China is a large agricultural country with abundant available agricultural resource.Biomass plays an important role in the future renewable energy development.Biomass grate combustion technology is a significant means for the efficient utilization of biomass.However,due to the high volatile and moisture content of biomass materials,many problems,such as low combustion efficiency,unreasonable air distribution and high nitrogen oxide emissions are gradually exposed in biomass applications.Therefore,it is important to study the flow and combustion characteristics in a biomass-fired grate combustion system.Based on the computational fluid dynamics,integrated with the discrete element method and reaction kinetics,a simulation is carried out to describe the motion,pyrolysis,and combustion process of biomass particles on the grate.The distributions of gas flow,composition and temperature as well as particle motion,composition,temperature characteristics on the grate are obtained.The results show that the numerical model can give a continuous reasonable prediction on the combustion process compared to the empirical model.The change of moisture content inside the particle,temperature and density between the drying zone and the combustion zone can be recognized.In addition,the change of particle characteristics versus time and spatial location is analyzed.It is found that the correlation between particle density and position can be fitted by the Boltzmann curve.Based on a three-dimensional CFD biomass boiler model coupled with the gas flow,composition and temperature field above the grate,a simulation of a biomass boiler is carried out.The gas temperature,velocity,composition and NO_x distribution in the furnace are analyzed.It is found that the combustion of gas components such as CO as well as the generation of NO_x is mainly in the arch area.A few combustible components are burning with the oxygen provided by over-fire air(OFA).Two obvious vortex areas appear in the furnace,enhancing the disturbance of the air flow,which is beneficial to the uniform temperature distribution and prolongs gas residence time.In addition,the effect of the rear arch inclination is discussed.The results show that the uniform distribution of biomass combustible gas and sufficient combustion can be acquired by increasing the rear arch inclination with a minimum NO_x emission of 340 ppm at the exit.Besides,the arrangement of OFA is discussed.It is found that both staggered and parallel OFA arrangements can reduce the oxygen concentration compared with the traditional arrangement,resulting in a mitigation of NO_xemission.A minimum NO_x emission of 287 ppm is achieved when the OFA arrangement is staggered.