Numerical Simulation Study On Combustion Characteristics Of Alternative Fuels In Cement Precalciners

Nowadays,the NSP cement production technology is widely used in the cement industry,as one of the important equipments in the process,the precalciners has become the main target of energy saving and emission reduction.In the context of energy shortages and environmental pollution,it is important to find clean alternative fuels for cement precalciners to reduce corporate costs and pollution.In this study,the numerical simulation method was used to study the combustion characteristics and pollutant emissions of alternative fuel biomass and RDF in the precalciners,which provided a reference for the cement industry to use alternative fuel technologies in decomposing furnaces.This article takes the TTF precalciner as the simulation object.Firstly,a three-dimensional physical model of precalciner is created by Gambit software,and the grids are meshed on this model.Secondly,the following mathematic models in the Fluent software were selected:the standard K-?two-equation model for simulating the turbulent flow of fluid in precalciner,the general limited rate model for simulating the transport and chemical reaction processes of fuels and raw meal,the DPM model for simulating individual particle trajectories,the single-rate model and kinetics/diffusion-limited for simulating fuel burning,the DO radiation model for simulating the heat exchange process in the precalciner.Finally,we will analyze the influence on the fuel combustion and raw meal decomposition in the precalciner of different alternative fuels according to the flow field,temperature field,concentration field of different species,CaCO₃ and CaO distribution and the difference of pollutant NO_x emissions.The results of simulation are as follows:?1?The velocity distribution under three conditions of coal,biomass and RDF in the flow field are stable and similar.There is a difference among the velocity magnitude in the main flow,the outlet flow rates of coal,biomass and RDF are:16.73 m/s,16.83 m/s and 17.68 m/s respectively.?2?The distribution of temperature field is similar under the three working conditions and there is an obvious difference between the cone portion of bottom and the first shrinkage of the precalciner.The average temperature of precalciner decreases from 1600K to 1200K along the height and the average temperature of the precalciner under coal,biomass and RDF conditions are 1476K,1442K,1433K.When the fuel is coal,the average temperature in the precalciners is the highest,and the RDF fuel is the lowest.The average temperatures of outlet are 1272K,1295K,1277K,and the flue gas temperature of outlet is the highest when biomass is used as fuel.?3?Due to the difference of the third air volumes,the distribution of oxygen in the precalciner differs greatly under the three conditions at the first furnace section.where the average content of O₂ is RDF,pulverized coal,biomass in turn.The content of O₂ in the second and third section on the top of the precalciner is basically unchanged.Due to the different degree of combustion,the outlet O₂ content of pulverized coal,biomass and RDF are 2.2%,1.0%,2.8%.The CO₂ content increase along the height of the furnace,and a large quantities of CO₂ are produced where combustion of fuels is intense and raw meal strongly decomposed.The CO₂ content of outlet in flue gas are 29.31%,31.20%,28.05%.CO mainly exists in the first furnace section,and it is produced by incomplete combustion in the bottom cone region and the combustion near the second burners.?4?The burnout rates of pulverized coal,biomass and RDF conditions are 96.7%, 98.3%,95.4%,respectively.It indicates that the burnout rate of biomass in the precalciner is the most complete among the three fuels.The burning rate increased when the excess air ratio is adjusted from 1.05 to 1.12.The decomposition rate of raw meal under three working conditions is 97.05%,97.72%,96.60%,which indicates that more raw meal can be decomposed when biomass is used as fuel,and the decomposition rate is proportional to the burnout rate of fuels.?5?In the absence of NO_x from the kiln,the NO_x emissions of coal,biomass and RDF are 303ppm,263ppm,109ppm.It shows that the most NO_x in the precalciner is produced by using coal as fuel.In the case of kiln NO_x,the average NO_x content of the outlet is 443ppm,412ppm,223ppm.Considering the NO_x generated by the fuel itself in the precalciner,the NO_x reduction rates under the three conditions are72.36%,73.64%,and 84.17%.The results show that the fuel grading structure of the TTF type precalciner has an obvious effect on the NO_x reduction,and the use of RDF instead of fuel in the precalciner has the greatest reduction effect on the NO_x reduction.