| Incineration of municipal solid waste (MSW) is one of the key areas in the global cleaner energy strategy. In order to realize non-hazardous treatment technology of MSW incineration, understanding the generation mechanism and controlling technology are very important during MSW non-hazardous treatment. In this paper, systemic simulation and experimental study on combustion process in a fixed bed are investigated, and the base is established on the study of combustion process in a fixed bed and reducing pollutants emission in MSW incineration. The experiment and simulation results provide direction for design and optimization of the moving grate of MSW.The thermal gravimetric experiments are done to analysis thermal decomposition and combustion characteristics of MSW. The components of pyrolysis gas production are tested with a Fourier Transform Infrared spectrometry (FTIR) to get the fixed quantify Curve of pyrolysis gas(CO2,CO,CH4) emission. The quality of pyrolysis gas is calculated by integral method. According to TG data, kinetics parameters E,A for combustion of MSW samples are calculated by Coats-Redfern method to get the apparent kinetics model for combustion of MSW.In order to reveal the features of the combustion process in the porous bed of waste incinerator, a fixed-bed experimental reactor is employed to reveal the combustion characteristics in simulated municipal solid waste (MSW) beds. Simulated municipal solid waste consists of vegetable, sand, and paper card, with the similar ash, moisture, fixed carbon contents as actual municipal solid waste in China. The reactor shown is a vertical cylindrical combustion chamber suspended from a weighing scale. The effects of primary air, primary air temperature and material characteristics on the burning process of simulated MSW are investigated. Temperature distributions, ignition front velocity, bed weight, characteristics of NO release are measured during the combustion process. At the same time, the combustion process of MSW and characteristics of NO release on the moving grate are studied and simulated by CFD codes. In this paper, one dimensional unsteady batch mathmetics model is also established to simulate combustion process in bed. The mass, momentum, energy conservation equations of two phases in the waste bed are proposed, and the models of moisture evaporation, volatile matter devolatilizaton, char combustion, NOx production and reduction and dioxin formation are simplified and established. Velocity, temperature and gas species distribution in the bed are obtained together with rates of moisture evaporation, volatiles devolatilization and carbon burnout and ignition front position.It is found that the average concentration of CO and the conversion ratio of C to CO are inversely proportional to primary air flow rate; the conversion ratio of N to NO rises as the primary air flow increases and reaches a peak point at the critical air flow rate, after that the conversion ratio of N to NO declines as the air flow further increases; with the increase of primary air temperature, the average concentration of CO and NO, but the highest concentration of NO changes hardly; the average emission concentration of CO and CO2 is inversely proportional to the moisture and the ash level in the fuel; the conversion ratio of N in MSW to NO and average emission concentration NO decrease with the increase of moisture content; with the increase of ash content, the average emission concentration of NO increases firstly, then decreases. At a certain moisture level, the average flame propagation speed and the burning rate increase as the airflow rate increases until a peak point is reached, beyond which further increase in the air flow results in a fall in the average flame propagation speed, but when the flame propagation speed decreases to a certain extent, it keeps constant; when primary air temperature is lower than 100°C, the average flame front increases only a little, but the flame front increases faster near the grate; the total mass loss rate and moisture evaporation rate increase only a little with the increase of primary air temperature; with the increase of moisture and ash content, the flame propagation speed and the burning rate decrease.The simulation results are compared with experimental data, which shows that the incineration process of waste in the fixed bed is reasonably simulated. During the middle of combustion period, bed-mass decreases linear, the front of flame propagates down at a steady velocity. Rate of moisture evaporation and devolatilation are steady at a constant. The simulation results of weight loss, flame front, solid maximum temperature in bed are accordant with experimental data well, which shows that waste burning rate is approximately constant in the middle of incineration process, and moisture evaporation occupies the dominant time fraction of overall incineration experiment. The residual char reacts with O2 slowly so that the crest of bed mass is level approximately. The simulation results are compared with experimental data, which shows that bed temperature versus time at different height from the grate in the fixed bed is reasonably simulated. The simulation can predict the trend about combustion, heat transfer in bed and the propagation of flame front. With heat transferring, fuel goes through the drying, pyrolysis, char combustion and burnout. The combustion process of municipal solid waste is simulated accurately by the model. Due to the high water content of fuel, moisture evaporation consumes a great deal of heat and evaporation takes up the mostly combustion time (about 2/3 of the whole combustion process) so that it has an important effect on the time of the combustion, and evaporation of moisture mainly occurs in the bed temperature below 100℃. The gradient of temperature is steep in the pyrolysis zone and a small thickness of pyrolysis front is found. The results of simulations and experiments are compared and analyzed, which shows that simulation and experiment can reflect the process of waste incineration in bed. At the same time, the simulating results of O2 and CO2 are accordant with experiment results. In the process of NO formation, only fuel NO is considered and modeled by overall reaction mechanism in which HCN is used as intermediate product, which is suggested by De'Soete. With different primary air, the simulating results of NO are accordant with experiment results. Fuel NO is main source of NO in MSW combustion,and the N content is the key to NO formation.Through this experimental investigation, the combustion process of simulated MSW in bed can be better understood and the experiment results can be used to amend the mathematics model and be consulted by the application in project. The results from modeling can show the combustion process, and make us deeply know how the heat transfers in the fuel, gas yields from fuel. At the same time, the simulation can predict the maximum temperature of waste incineration and the trend about combustion. |
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