Study On Decomposition Characteristics And Reaction Kinetics Of Limestone

China is the largest cement producer and consumer in the world.Pollutant emissions of cement industry are second only to electric power industry and transportation.With the requirement to further improve environment nowadays,the new dry cement production line needs to be further optimized to increase output while reducing energy consumption and pollutant emissions.In the optimization process,the reaction in the precalciner needs to be studied.At present,most of the research is carried out on different platforms to study the decomposition reaction of calcium carbonate and raw meal.The effects of temperature,pressure,atmosphere and other factors on the reaction and the kinetic characteristics of the reaction under different conditions are obtained.However,as the reaction kinetic model and parameters in the precalciner are limited by the reaction conditions in the furnace,scholars have not reached a consensus.In this paper,an isothermal multi-atmosphere vertical tube combustion test platform was adopted.The temperature of furnace kept constant so that the sample can be heated up rapidly when pushed into the furnace.The reaction atmosphere was controllable.As a result it can simulate the process of material entering the furnace in the actual production process.The electronic balance can measure the weight loss of the sample online and record it.Through the experimental study,we can obtain the change of calcination quality with time under certain reaction atmosphere and temperature,the complete reaction time and reaction rate.Further we gained the kinetic characteristics and parameters of the reaction.The pore size and specific surface area of calcined products under different reaction conditions were obtained by BET test of nitrogen adsorption,and they revealed the variation rule of product activity with reaction conditions.It has guiding significance for engineering application.By calcining calcium carbonate,limestone and the mixture of different proportions of pulverized coal and calcium carbonate at high temperature,the law of reaction changing with temperature in the operating range of the precalciner was obtained-the increase of temperature was beneficial to the positive reaction and shortened the complete reaction time.Limestone can decompose at lower temperature than calcium carbonate.For the mixture of pulverized coal and calcium carbonate,the temperature has a great influence on the reaction rate in the lower temperature range.With the increase of the proportion of calcium carbonate in the mixture,the mixture is more sensitive to the temperature change.Through BET analysis of nitrogen adsorption on calcined products,the optimum reaction temperature was 950 ?.The decomposition of calcium carbonate and limestone in the precalciner atmosphere(25%CO2/75%N2)and pure N2 atmosphere was tested.The results showed that the decomposition of calcium carbonate accorded with the three-dimensional random nucleation and growth model(A3),and the decomposition of limestone accorded with the two-dimensional random nucleation and growth model(A2).The apparent activation energies E of the decomposition reaction of calcium carbonate at 25%CO2/75%N2 and 100%N2 were 207.7 kJ/mol and 203.8 kJ/mol,respectively.The apparent activation energies E of the decomposition reaction of limestone are 208.2 kJ/mol and 199.3 kJ/mol,respectively.CO2 inhibited the decomposition reaction of calcium carbonate and limestone to some extent.Limestone has lower activation energy so that it is easier to decompose than calcium carbonate in the same atmosphere.Through calcination experiments of mixtures with different pulverized coal ratios in air,the reaction kinetic model of mixtures changed with different ratio of coal/CaCO3.When the ratio of coal/CaCO3 was 1:1 and 1:3,the reaction mechanism conformed to the two-phase interface model(R2),and when the ratio of coal/CaC03 was 1:5 and 1:7,the reaction mechanism was between the two-phase interface model(R2)and the two-dimensional random nucleation and growth model(A2).When the ratio of coal/CaCO3 was 1:9 and 1:11,the reaction mechanism conformed to the two-dimensional random nucleation growth model(A2).The apparent activation energy under each ratio was obtained through subsequent calculation,which not only proved that the reasonable ratio of coal/CaCO3 should be controlled in the process of precalciner production and operation to improve the activity of the product while saving energy,but also provided data support for the later simulation calculation of precalciner.