| The main atmospheric pollutants in flue gas and exhaust gas are SO2,NOx and other substances.They are the main cause of acid rain and one of the pollutants causing haze,which causes serious harm to human living environment.Air pollutants are mainly derived from flue gas or exhaust emissions from coal-fired power generation,smelting,chemical,petrochemical and cement industries.Cement production is one of the important sources of atmospheric pollution,and its main atmospheric pollutants include dust,SO2 and NOx etc..Many researches on the emission reduction technologies for SO2 and NOx emitting from exhaust gases are conducted at home and abroad.A series of mature exhaust gas purification technologies have been developed,some of them have been industrialized and achieved relative good effects.However,the desulfurization technology and denitrification technology in China is developed unbalanced,which has led to many problems in the waste gas treatment technology of cement plants.For example,the purification of pollutants in exhaust gas is not complete and the concentration of SO2 and NOx after purification is still high.Treatment processes are often removed SO2 and NOx step by step.This traditional processing mode is called "one-to-one",as one system is designed to treat only one single pollutant,bringing with lots of problems such as poor matching,large occupied area,high system construction and operating costs,high energy consumption etc..With the constant increasing of environmental protection requirements and the types of pollutants required to be reduced,the environmental protection costs borne by companies are also increasing.Therefore,to develop a new,highly efficient,economical and green flue gas integrated clean technology has become the trend of smoke(tail)gas purification technology research.Industrial exhaust gas purification technology is undergoing a technical upgrading and transformation from step-by-step process to an integrated process,based on the simultaneous technologies of desulfurization and denitrification.Due to the excellent desulfurization and denitrification performances,sodium-based absorbents have been used in the simultaneous removal process.Therefore,based on the review of wet desulfurization process,denitrification catalytic process and ultrasonic technology,this paper firstly developed a new type of sodium-based simultaneous desulfurization and denitrification absorbent.According to the composition characteristics of cement kiln exhaust gas,a new type of alkali absorbent was developed by using NaClO2 and NaOH as the main components.Combined with ultrasonic technology,a new process,namely a new type of alkali-ultrasonic atomized exhaust gas simultaneous desulfurization and denitrification technology has been developed in order to realize simultaneous desulfurization and denitrification of cement kiln exhaust gasIn this paper,the main factors affecting the desulfurization and denitrification efficiency and the regeneration performance of the new alkali absorbent are studied in detail.Experiments showed that parameters in the desulfurization and denitrification reaction like pH value of the absorbent,NaClO2 concentration,reaction temperature and ultrasonic atomization had a significant effect on the removal rate.In the absorbent regeneration experiment,the pH value of the regeneration system,the regeneration temperature,the concentration of lime milk,and the amount of oxygen influent greatly affected the regeneration performance of the absorbent.The optimum process parameters of desulfurization and denitrification of the new alkali absorbent were studied.The best process parameters were as follows:NaClO2 concentration was 0.02mol/L,NaOH concentration was 0.1mmol/L,pH value was 10,oxygen content was 8%,and reaction temperature was 55?.Under this optimal condition,the desulfurization rate was 99.95%and the denitrification rate was 69.38%.In combination with the ultrasonic atomization reaction device,the denitrification rate of the novel alkali absorbent can be increased from 58.29%to 68.89%in the bubble reactor.The optimum process parameters for absorbent regeneration are as follows:the pH of the solution was 6,the temperature was 35?,the ratio of calcium to sulfur was 0.9,the aeration time was 90 minutes,and the proportion of calcium sulfate generated reached 69%.Through TEM analysis of the regenerated material,the results showed that the main components of the regenerated material were calcium sulfate and a small amount of calcium sulfite.The regeneration mechanism of a new alkali absorbent was discussed.The reaction process is as follows:2NaHSO3+Ca(OH)2?Na2SO3+CaS03+H2O Na2S03+Ca(OH)2?2NaOH+ CaS03 Na2SO4+Ca(OH)2?2NaOH+CaSO4Through the analysis and characterization of the reaction process of desulfu-rization and denitrification,the reaction mechanism of SO2 and NO removal by a new type of alkali absorbent was discussed,and the results were as follows:(1)Desulfurization reaction mechanism:? SO2 liquid phase absorption SO2(g)(?)SO2(aq)SO2+20H-(?)SO32-+H2O SO2(excess)+ OH-(?)HSO3-? SO2 liquid phase oxidation absorption 2SO32-+ C102-?2SO42-+ Cl-2HS03-+ C102-+ 20H-?2SO42-+ Cl-+ 2H2OThe total desulfurization reaction is:2SO2+ClO2-+40H-=2SO42-+ Cl-+2H20(2)Denitrification reaction mechanism:NO(g)(?)NO(aq)2NO+ ClO2-?2NO2+Cl-NO+NO2+20H-?2NO2-+ H202N02+2OH-?NO2-+NO3-+H202N02-+ ClO2-?2N03-+ Cl-The total denitrification reaction is:4NO+3ClO2-+ 40H-=4NO3-+3Cl-+2H2OIn this thesis,the thermodynamics and kinetics of the desulfurization and denitrification of the new alkali absorbent were studied.The results of thermodynamics showed that under isothermal and isobaric conditions,the changes in the Gibbs free energy of the desulfurization and denitrification reaction were-942.61kJ/mol and-1086.35kJ/mol,which were far less than zero,so the reaction proceeded in the positive direction and the limit was very deep.The equilibrium constant of the reaction was very great and the reaction was very completely preceded.The enthalpies of the two reactions were-2813.24kJ/mol and-2988.08kJ/mol,which were far less than zero.Both reactions were exothermic reactions.The increase of temperature was not conducive to the reaction.The results of kinetic studies showed that the desulfurization and denitrification reaction order in the process of desulfurization and denitrification were all one order,and the apparent activation energies of the reaction were 22.392kJ/mol and 8.726kJ/mol,respectively.As mentioned above,ultrasonic atomization technology was also introduced into the experiment of waste gas desulfurization and denitrification.Based on the principle of ultrasonic atomization technology,an ultrasonic atomization reaction device and a purification system were designed,and the ultrasonic and atomization effects on the physical and chemical aspects of the desulfurization and denitrification reaction were explored.Via CFD analysis software,a three-dimensional model of the flow field of the spray gun in the ultrasonic atomization system was established,and the velocity distribution,temperature distribution,particle distribution of the flow field under the initial and steady state conditions of the three different working conditions were simulated.By comparing the three simulation states,it was found that the velocity,temperature and particle distribution formed were optimal at an ultrasonic nozzle speed of 17 m/s and upwind conditions.On the basis of the above tests and CFD simulation results,the key technology of desulfurization and denitrification of waste gas by new alkali liquid-ultrasonic atomization was industrialized.and a provincial pilot project was constructed on a 5500t/d dry process cement production line of a cement plant in Guangdong Province,which has been running continuously for two years.Industrial trials were conducted in the dry cement production line.The provincial and ministerial level scientific and technical appraisal believed that the system had a reasonable technological process,compact layout,stable operation,good SO2 and NOx removal effect of exhaust gas,desulfurization rate of 96%,denitrification rate of 50%,comprehensive operating costs.As a whole,the pilot project has reached the leading level in China,which is of great significance to improve the development level of waste gas purification technology in non-electric enterprises.The technology has broad prospects for popularization.Based on the study of sodium-based sorbents and calcium-based regeneration reagents,we have exploratorily replaced calcium-based sorbents in briquette with sodium-based sorbents,which has improved the sulfur-fixing effect of briquette sorbents.Therefore,we have developed a new type of sorbent for briquette and successfully developed a new type of briquette sorbent for GCHTDS,and achieved industrial application.The new sulfur-fixing agent combined with special briquette molding technology and effectively solved the problem of secondary release of sulfur dioxide in high temperature area.While achieving ultra-low emission of SO2,it also effectively improves the thermal efficiency of the boiler.The thermal test results were as follows:Datong Coal thermal efficiency of 59.33%and a new type Datong coal for 78.02%.The provincial and ministerial scientific and technical appraisal believed that this technology has reached the domestic leading level and has important practical significance to reduce the air pollution caused by SO2 emission of coal-fired flue gas. |
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