Experimental Study On Deep Reduction Characteristic Of NO_x With Coal-based Reductant In A Cement Kiln

As an energy-intensive and resorce-intensive industry,cement industry has not only made great contributions to the economic and social development of our country,but also brought environmental problems that cannot be ignored.At presenct,China’s cement production emissions of NOx accounts for about 10-12%of the country’s total industrial emission.And it is the third resource of NOx,behind thermal power generation and automobile exhaust.The NOx emission standard of cement kilns is becoming stricter,and it is a a severce challenge for cement industry to achieve ultra-low NOx emission.However,the currently used denitration technologies in cement industry are difficult to meet the increasingly stringent NOx emission standards.For the cement industry,the selective catalytic reduction technology(SCR)has low maturity and high economic costs,so it is not suitable for large-scale promotion at this stage.Taking pulverized coal as reducing agent to reduce NOx of high temperature flue gas as the breakthrough point,the Institute of Engineering Thermophysics,Chinese Academy of Sciences found that coal-based fuel with stronger reducing ability can be generated by modifying pulverized coal and creating a suitable reaction atmosphere,which can realize the deep removal of NOx.Based on this,this paper takes coal-based fuel as reduction medium,explores the deep reduction mechanism of NOx under cement kiln conditions,clarifies the homogeneous and heterogeneous NOx removel pathways,and realizes the directional concersion of nitrogen in the system.A theoretical system of coal-based deep denitrification technology suitable for the whole process of the new-type dry cement production is established.According to the characteristics of cement production process,this method sets up an external in-situ reduction zone during the combustion process to modify the characteristics of pulverized coal through heat treatment in situ(pre-removal of fuel nitrogen and strengthen reducing capacity of high temperature char).At the same time,the combustion organization in the precalciner is optimized to achieve NOx depath control;A median reduction zone with low oxygen concentration and appropriate temperature is set up to achieve deep reduction of the residual NOx in the flue gas through carbothermal reaction.The 2500 t/d project demonstration shows that the outlet NOx concentration can be reduced to below 80 mg/m3 under the condition that the same amount of ammonia injection.Through the experimental research on the reduction mechanism of NOx-coal under cement kiln conditions,NOx depth control during combustion process and deep reduction of residual NOx in flue gas,the theoretical foundation for the coal-based deep denitrification technology is laid.The conclusions are as follows:1.The law of nitrogen migration and transformation in the system with the low O2 concentration and high NO concentration is obtained.The reaction between NO and coal can be classified into two types(the homogeneous and the heterogeneous).Homogeneous reduction is the most significant mechanism during the earlier stage of reaction.Then,the char reduction mechanism becomes increasingly efficient.An appropriate concentration of oxygen(1%)can accelerate the release of volatiles and the formation of CHi radicalsm,thereby accelerating the homogeneous reaction of volatile-NO;At the same time,it promotes the regeneration of C*active sites on the surface of char and accelerates the heterogeneous reduction of char-NO.Excess oxygen in flue gas will consume volatile and char and results in a reduction in reducing media,thereby inhibiting the reduction of NOx.At the same time,it also promotes the generation of fuel NOx.The oxidation of fuel nitrogen and the reduction of the original NOx in the flue gas are two parallel reaction paths.And the most of fuel nitrogen does not participate in the reduction of the original NOx in the flue gas.2.The influence mechanism of cement raw meal on NOx reduction in the precalciner is revealed.The alkaline oxides decomposed from raw meal can reduce the activation energy of carbon atoms in char and promote the heterogeneous reduction of char-NO.The catalytic effect of raw meal on the heterogeneous of char-NO is the most obvious at the reaction temperature of 850℃.Alkaline oxides can change the pyrolysis path of pulverized coal and reduce the content of CHi free radicals,thereby inhibiting the homogeneous reduction of volatile-NO.Due to the high concentration of raw meal in the precalciner,the homogeneous reduction of volatile-NO is almost negligible.3.The principle of NOx depth control in the combustion process of cement kiln is clarified.The key to reducing NOx emissions during the combustion process in the precalciner lies in controlling the generation of fuel NOx and enhancing the reduction of high concentration NOx in the flue gas of the rotary kiln.By setting up an external in-situ reduction zone,the pulverized coal is subjected to in-situ modified heat treatment,and part of the fuel nitrogen is directionally converted into nitrogen outside the precalciner to realize the pre-removal of fuel nigtrogen and reduce the generation probability of fuel NOx in the precalciner;The reduction ability of the high temperature char-coal gas miture generated in the in-situ reduction zone is much stronger than that of conventional pulverized coal,among which the char-NOx heterogeneous reduction is the main one.The pore structure of char is developed,its specific surface area increases obviously,the proportion of active sites in the carbon frame structure increase,the degree of graphitization is reduced,and the reaction activity is enhanced,which can achieve the directional removal of high concentration thermal NOx in the kiln gas.4.The influencing factors of NOx depth control in the combustion process of cement kiln is obtained.Increasing the combustion ration and gasification intensity in the in-situ reduction zone can promote the release of fuel nitrogen and inhibit the generation of fuel NOx in the precalciner,but it will also affect the reduction characteristics of char;For coarse particles,the in-situ reduction is mainly achieved by greatly improving the reduction capacity of char,while taking into account the reduction of fuel NOx.For fine particles,the reduction of NOx emission is mainly achieved by greatly improving the pre-removal of fuel nitrogen,while taking intio account the improvement of char’s reduction capacity.In summary,the final NOx emission of pulverized coal with larger particle size is lower;A proper air coefficient(0.8)in the reduction zone at the bottom of the precalciner can promote the release of residual nitrogen in char and be reduced to stable N2 as much as possible.At the same time,it promots the renewal of carbon active sites in char,thereby strengthening the reduction of NOx in kiln gas;As the height of the reduction zone of the precalciner increase,the residence time of the kiln gas in the reduction zone increases,and the reduction of high concentration NOx in the kiln gas is strengthened.However,an excessively high height of the reducton zone will reduce the combustion efficiency and increase the loss of incomplete combustion;The decomposed raw meal can increase fuel NOx emission.Due to the low decomposition rate of the raw meal and the lower temperature in the reduction zone at the bottom of the precalciner,the catalytic ability of the raw meal on NOx reduction is limited.5.The deep reduction mechanism of residual NOx in the precalciner flue gas is revealed.When the temperature of suspension preheater C4 is below 900℃,the contact area of coal-NOx reaction can be increased by increasing the amount of reduced pulverized coal.Thus,the reduction rate of NOx can be improved in a certain range;When the oxygen concentration in the flue gas is higher than 1%,continuing to increase the flue gas oxygen content will promote the fuel nitrogen oxidation and increase the NOx produced by the combustion of reduced pulverized coal,which indirectly weakens the reduction effect of cold pulverized coal on the residual NOx in the flue gas;The flue gas temperature has an optimal temperature window for the reduction of flue gas residual NOx.When the flue gas temperature is within 900℃,as the flue gas temperature increases,the NOx reduction effection effect in the flue gas is enhanced.When the flue gas temperature exceeds 900℃,as the flue gas temperature increases,the amount of fuel-NOx generated will increases,which will weaken the final NOx reduction effect;The cold cement raw meal can adjust the temperature of the reduction window through physical heat absorption and endothermic decomposition.At the same time,the catalytic effect of raw cement on the deep reduction of residual NOx depends on the reaction atmosphere.When the oxygen concentration is higher than 1%,the raw meal mainly inhibits NOx reduction.When the oxygen concentration is lower than 1%,the raw meal mainly promots NOx reduction;In summary,the deep reduction of residual NOx in flue gas temperature can be achieved under the conditions of 900 flue gas temperature,large amount of reduced pulverized coal and suitable raw material introduction.