Numerical Simulation Of Combustion And Optimization Of SNCR System Design In Grate Waste Incinerator

Nitrogen oxide is a major greenhouse gas emitted from combustion of fossil fuels and other nitrogen containing matters including the manucipal solid wastes (MSW). Incineration is currently an effective technology for the MSW treatment, but the control the nitrogen oxide emission is still a chanllege issue due to increased emission standards and strigent regulations. Selective non-catalytic reduction (SNCR) process is considered as an efficient and cost-effective technology for small to midiem scale industrial furnaces, and proposed for the MSW incinerators as well. However, the SNCR reactions are sensitive to temperature and reaction time, hence, an optimal design of the operating conditions for the reductant (urea or anmonia) injection is critally important for an industrial SNCR system.In this paper, a computational fluid dynamics (CFD) approach is appied in the description of MSW combustion and the SNCR process for a 750t/d moving grate incinerator. The bed combustion was simulated by using FLIC program, and the gaseous combustion was simulated with FLUENT software. The distribution of gas temperature, flow and concentration were obtained from CFD simulations, and used as reference in the design of a SNCR system. Optimal combination of process parameters, including the injection speed, the NSR and the secondary air scheme were investigated computationally.The simulation results show that:the lower heating value (LHV) played a critical role for the ignition and the subsequent burnout of waste solids on the grate bed. The position of the over bed flame center will affct the flow field and the gas temperature distributions in the combustor, which will affect SNCR reactions. It was found that an optimal SNCR reduction rate 31%was achieved when uear solution was injected with a spray angle 45°, an injection speed 50 m/s and a NSR 1.5 through a combination of two nozzles located in the front wall. By adjusting the angles of the secondary air nozzles, the gas temperature distribution is more uniform, and SNCR reduction efficiency can be increased by nearly 20%.