The Combustion Model And Control Method Of Twin Burn Annular Shaft Limekiln

Recently,annular shaft kiln is widely applied to product active lime in calcium carbide industry since its products have good quality and high activity degree which can be used to diminish the production cost of calcium carbide and make sure the yield and quality.Closed calcium carbide furnace off-gas(CCFG)consists of a large number of CO which can be used as energy medium.Reasonable recovery and reuse of CCFG can save energy,and decrease productivity cost,improve economic performance.In the present research,mathematical model for twin burn annular shaft lime kiln which fuel is CCFG has been established with simulation method to study the fuel combustion characteristic and production process control for providing the forecast and theoretical basis for the production of the new kiln.The present study divided twin burn annular shaft kiln model into its combustion chamber model and kiln body model.Combustion chamber geometry model was build using its practical size.Based on finite volume method,realizable turbulence model has been selected to consider fluid flow in combustor chamber.Fuel and fuel gas were through different paths into the combustion chamber,so fuel combustion in combustion chamber was typical non-premixed combustion and non-premixed combustion model has been used as combustion model.The radiation heat transfer becomes more and more important with the increase of the combustion temperature.The present work used DO model to consider gas radiation.The predicted temperature profiles of the methane flame using geometry model are in good agreement with the experimental results in the literature,so the accuracy of the calculation model was verified.For the calculation of combustion chamber model,comparison between CCFG and traditional fuel for annular shaft kiln like natural gas and blast furnace gas was firstly be analyzed,moreover,the effects of excess air coefficient and air preheated temperature on the combustion of CCFG were discussed.The results are as follows:The calorific value of CCFG was between the two traditional fuels,so with the same heat consumption,its combustion temperature was higher than that of blast furnace gas and lower than that of natural gas.Due to the complex structure of combustion chamber,when the excess air coefficient was more than 1.0,fuel can't be used totally.The temperature distribution of CCFG become more uniform in the cross section of combustor chamber with air preheated temperature increasing under the condition of constant fuel preheating temperature.Moreover with the increase of air preheated temperature,the average temperature of different height axial section of the combustion chamber was gradually increased.In order to improve the calculation efficiency,the mathematical model of the 1/12 kiln body was set up.A porous medium model was adopted to consider the flow process of limestone in the flue gas,and the convective heat transfer between flue gas and limestone,the decomposition of limestone in the kiln and the release of carbon dioxide were considered by self-programming.In order to better understand the characteristics of flue gas flow field distribution,the present work compared fluid flow in kiln chamber with and without limestone filled.Additionally,excess air coefficient and limestone particle size were analyzed to study their effects on flow field and temperature field of the kiln chamber.The results are as follows:The flue gas maximum velocity in kiln chamber with and without limestone filled was 8.54 m/s and 5.16 m/s,respectively.The flow field had obvious vortices in kiln chamber without limestone filled which was different from that in the kiln chamber with limestone filled.The velocity of gas was increasing significantly when air excess number was larger than 1.0.The velocity was close when air excess number was 1.1 and 1.2.When the excess air coefficient is 1.0,1.1,1.2 and 1.3,respectively,the velocity is 5.16 m/s,6.04 m/s,6.18 m/s and 6.35 m/s.The limestone particle size was analyzed with constant flue gas inlet condition and fixed void fraction.The flow field distribution in typical position of kiln chamber was almost unaffected.Limestone particle size has little effect on the temperature distribution of flue gas in the kiln chamber,but has obvious effect on the temperature of solid.As the particle size increases,the solid temperature decreases.