Experimental Study On Sulfur-fixation Effects Of Cement Raw Meal During Its Blended Combustion With Pulverized Coal

At present,China's energy consumption is still dominated by coal,and the national requirements for sulfur dioxide emission limits for industrial boilers and cement kilns are becoming increasingly stringent.Aiming at the problem of SO₂ emission from small industrial boilers and cement kilns,the experimental study on sulfur-fixation effects of cement raw meal during pulverized coal combustion was carried out.The pyrolysis(combustion)characteristics and kinetics of cement raw meal pyrolysis,pulverized coal combustion and blended combustion(Ca/S=2)were analyzed by the thermogravimetric-Fourier infrared spectrometer(TG-FTIR)experimental platform.The fixed-bed reactor experimental platform was used to study the sulfur-fixation effects of cement raw meal during pulverized coal combustion under the atmosphere of O₂/N₂,O₂/N₂/CO₂ and O₂/CO₂ in 600^oC?1200?,and investigate the effects of five kinds of sulfur retention additives on the sulfur-fixation efficiency of cement raw meal.First of all,based on thermogravimetric-Fourier infrared spectrometer,the processes of cement raw meal pyrolysis,pulverized coal combustion and blended combustion were studied.The experimental results show that,the mechanism model of cement raw meal pyrolysis is a two-dimensional phase interface reaction model of shrinking cylinder(R2).And the activation energy and pre-exponential factor are 182.007 k J/mol and 3.70×10 ⁸ min^-1,respectively.The pulverized coal combustion reaction belongs to the first order reaction(F1),and the activation energy is 67.848 k J/mol,the pre-exponential factor is 3147.96 min^-1.There are two obvious weightless peaks in the blended combustion TG curve,which correspond to the pulverized coal combustion stage and the cement raw material pyrolysis stage in turn.For the two weightless stages,the mechanism functions F1 and R2 were selected using the Coast-Redfern method to obtain the activation energy and pre-exponential factor.The activation energy and pre-exponential factor of the pulverized coal combustion stage are 65.867 k J/mol and 3496.019 min^-1.Those are 295.512 k J/mol and 7.11×10¹⁷ min^-1in the cement raw meal pyrolysis stage.Fourier transform infrared spectrometer test shows that CO₂,H₂O and CO are mainly produced in the process of pulverized coal combustion and blended combustion.Then,with the help of the fixed-bed reactor experimental platform,the sulfur-fixation efficiency of cement raw meal during pulverized coal combustion was performed.It is found that,in the O₂/N₂ combustion atmosphere,the sulfur-fixation efficiency of cement raw meal shows a trend of first increase and then decrease with the increase of combustion temperature,and the sulfur-fixation efficiency reaches a peak at 800?,the sulfur-fixation efficiencies at each Ca/S molar ratio(0.5,1,2,3,4)are 66%,83%,89%,95%,and 97%.The recrystallization of the decomposition product Ca O of cement raw meal,the high temperature thermal decomposition of sulfur fixation product Ca SO₄ and the carbothermal reaction cause the sulfur-fixation efficiency of cement raw meal to decrease continuously at over 900?.When the Ca/S molar ratio is 2,the optimal sulfur-fixation efficiency of Ca CO₃ is also at 800^oC,which is 82%.But the sulfur-fixation efficiency is lower than that of cement raw meal in600??1200?.This is mainly because the alkali metal compounds contained in the cement raw material can improve the efficiency of the calcium-based sulfur-fixation reaction,and Fe₂O₃,Al₂O₃ and Si O₂ can increase the high-temperature stability of the sulfur-fixation products.In O₂/N₂/CO₂ combustion atmosphere,the sulfur-fixation efficiency of cement raw meal is higher than that in O₂/N₂ atmosphere in the temperature range of 1000??1200?,and the sulfur-fixation efficiency of cement raw meal increases with the increase of CO₂concentration.At 1200?,the sulfur-fixation efficiency in 21%O₂/79%CO₂ atmosphere is17%higher than that in 21%O₂/79%N₂ atmosphere.This is mainly because high CO₂concentration can inhibit the high temperature thermal decomposition of sulfur-fixation product Ca SO₄.Finally,the effects of Na₂CO₃,BaCO₃,Al₂O₃,Fe₂O₃ and CeO₂ on the sulfur-fixation efficiency of cement raw meal during pulverized coal combustion were experimentally investigated,the additive content is 2%of pulverized coal,the molar ratio of calcium to sulfur Ca/S is 2,and the experimental combustion atmosphere is 21%O₂/79%N₂.The experiment shows that,at 600??900?,the sulfur-fixation efficiencies of cement raw meal are improved the most by adding alkali metal compound Na₂CO₃,with the increases of 18.77%,14.81%,10.23%and 7.35%,respectively.While at 1000?,1100? and 1200?,the promoting effects on the sulfur-fixation efficiency of cement raw meal are relatively good after adding alkaline earth metal oxide CeO₂,which are 8.06%,8.64%and 5.99%,respectively.In summary,this thesis focused on the sulfur-fixation efficiency of cement raw meal during pulverized coal combustion under different temperatures,atmospheres and additives.The main conclusion is that,in 21%O₂/79%N₂,the optimal sulfur-fixation temperature of cement raw meal and Ca CO₃ is at 800?,but cement raw meal has better sulfur-fixation effect.At over 1000?,increasing the concentration of CO₂ helps to improve the sulfur-fixation efficiency of the cement raw meal.And the promoting effect of Na₂CO₃ on sulfur fixation of cement raw meal is very obvious in the temperature range of 600??900?,while CeO₂ is a relatively better choice in 1000??1200?.