Numerical Simulation Study On Combustion Characteristics Of Municipal Solid Wastes In Precalciner

With the rapid development of Chinese industry,the contradiction between urban development and municipal solid wastes is increasing,and how to deal with the problem of wastes has become more and more important.Traditional methods such as landfill,incineration and composting cannot solve the problem of a large amount of municipal solid wastes,and it is difficult to meet the increasingly stringent environmental standards.So the cement kiln co-processing waste system has attracted more interesting.This paper mainly uses the precalciner in a 3200t/d cement plant as a prototype,and feeds the refuse-derived fuel?RDF?which made from municipal solid wastes into precalciner for co-processing by using methods of numerical simulation,then the combustion characteristics of pulverized coal and RDF in the precalciner were gotten.The main tasks and results are as follows:?1?Aiming at the TTF-type precalciner,this paper builds the physical model in a 1:1 radio and simplifies the model according to its structural characteristics firstly.Then Gambit software is used to divide the simplified model by the idea of the overall structured grid and part of the model using unstructured grid.Secondly the appropriate CFD mathematical models were selected to achieve numerical simulation of physicochemical processes such as turbulence,raw meal decomposition,fuel combustion,and NO formation in the precalciner.?2?The FLUENT software is used to simulate the combustion characteristics of different fuels in the simplified precalciner.Firstly,the original working condition which is using coal as fuel is used to simulate the process and verify the model.The results show that the velocity distribution in the precalciner is reasonable,and the outlet flow rate of flue gas is 23.55m/s.Pulverized coal can burn out in the precalciner,the highest temperature appears near the main combustion chamber and the outlet average temperature is 1270K.The mass fraction of each species is conforms to the basic laws of thermodynamics.The mass fraction of CaCO₃ at the outlet is 4.5%,and its mass flow rate is 6.32kg/s.The decomposition rate is calculated as 91.7%.Error between the simulation results and data from cement plant is kept within 5%.It is verified that the selected model is reasonable and reliable,and the CFD can accurately simulate the combustion,decomposition and other reactions in the precalciner.?3?Based on the validated model,this paper numericallly simulates the RDF partially blended coal as fuel into the precalciner,which using five different blending ratios of RDF and coal.The simulation results show that velocity distribution as steady as the working conditions of coal.Temperature distribution is uniform,without local temperature too high or too low.Species distribution characteristics are consistent with the working condition of coal.The results show that RDF is suitable as an alternative fuel to burn in the precalciner.At last,this paper selects the raw meal decomposition,fuel burnout rate and NO decreasing rate as the most important evaluation indicators.The results showed that when the blending ratio was 20%,the raw meal decomposition?89.77%?and fuel burnout rate?98%?could be maintained highly in the precalciner.As well,NO decreasing rate can reach the highly rate of67.06%.So when the RDF blending ratio is about 20%,it is an ideal blending ratio.The data and results according to the simulation in this paper not only have certain guiding significance for the design of precalciner but also it is beneficial to cement kiln co-processing waste system.