Experimental Study On The Combined Removal Of SO₃ And HCl From Coal-fired Flue Gas By Ca（OH）₂

In recent years,coal-fired power plants have achieved ultra-low emissions of SO2,NOx and PM in flue gas through the implementation of a series of modifications,making a contribution to the control of air pollutants in China.The control of flue gas pollutants from coal has entered a new stage,and the discharge and control of unconventional pollutants have been widely concerned.In addition to SO2 and NOx,the acidic gases in the flue gas of coal burning also contain trace amounts of SO3 and HCl.SO3 in flue gas leads to blockage of air preheater,resulting in increased resistance of flue gas system;HCl leads to a large amount of desulfurization wastewater discharge in wet desulphurization.Based on the research of literature,this paper puts forward the technical idea of the combined removal of SO3 and HCl on the basis of the removal of SO3 by calcium hydroxide injection process.However,a variety of acidic gases coexist in the flue gas of coal burning and the concentration of SO2 is much higher than that of HCl and SO3,so it is necessary to further study the interaction of various acidic gases and guide the application of the combined removal technology according to the reaction mechanism.In this paper,the priority of the reaction between different acidic gases and calcium hydroxide was obtained through thermodynamic calculation,and the reaction law and competitive reaction characteristics of calcium hydroxide with SO3 and HCl were studied experimentally.Finally,the removal performance of HCl was improved by modifying the absorbent.Firstly,the equilibrium constant and Gibbs free energy of the reaction involved were analyzed by using the thermodynamic software HSC.In order to facilitate the analysis,this paper defined the binding tendency of Ca(OH)2 and different acidic gases as the reaction priority,and analyzed the factors affecting the priority through thermodynamic data.The results show that the reaction of HCl and Ca(OH)2 is a step reaction,forming CaClOH first and then CaCl2.However,SO3 will react with CaClOH and CaCl2 to generate CaSO4 and HCl,resulting in slow dechlorination reaction.Then the influence factors of reaction priority are determined by using the free energy minimization method.At the same concentration,the priority of the reaction between these acidic gases and Ca(OH)2 is SO3>HCl>SO2>CO2.The priority of HC1 increases when the temperature decreases,while the priority of HCl decreases when the partial pressure of SO2 increases.In general,both SO3 and SO2 inhibit Ca(OH)2 and HCl reactions.In order to improve the combined removal effect,the kinetic mechanism of the gas-solid reaction of Ca(OH)2 and SO3/HCl was analyzed.Firstly,Ca(OH)2 alone absorbed SO3 and HCl,and determined the influence of concentration,temperature and particle size of absorbent on the removal effect.Then the gas-solid reaction mathematical model was combined with the obtained experimental data,and the internal and external diffusion equilibrium constants and chemical equilibrium constants were solved by Matlab.When the temperature is below 300℃,the reaction of SO3/HCl and Ca(OH)2 is controlled by external diffusion and chemical reaction.When the temperature rises above 300℃,the chemical equilibrium constant of SO3 and Ca(OH)2 reaction increases rapidly and is much larger than the external diffusion equilibrium constant.At this time,the reaction is only controlled by external diffusion.However,the equilibrium constants of HCl and Ca(OH)2 are close,and are always controlled by external diffusion and chemical reaction.From the analysis of reaction time,as the reaction progresses,the pore structure of absorbent is gradually blocked,leading to the rapid decline of internal diffusion absorption,and the two reactions are controlled by internal diffusion link.Based on the thermodynamic analysis,the inhibitory effect and inhibition mechanism of HCI removal from Ca(OH)2 by SO3 and SO2 were investigated separately using a fixed bed.The results of the study showed that the reaction between Ca(OH)2 and HC1 was severely inhibited by the SO3 and SO2 in the flue gas.The mechanism of SO3 inhibition of HCl removal was investigated experimentally,and the results showed that SO3 can react with CaClOH or CaC12 to form CaSO4 and HCl,and the reaction rate of this reaction will decrease when the temperature decreases;in addition,the formation of sulfate will also lead to the decrease of HCl removal effect.The mechanism of SO2 inhibition of HCl removal is that high concentrations of SO2 will compete with HCI for limited active sites at the same time,and the reaction rate of HCl and Ca(OH)2 is slow,resulting in the rapid consumption of Ca(OH)2 by SO2.Through thermodynamics,dynamics and competitive mechanism analysis,particle size and doping will be modified.Firstly,through literature research,ball milling was used to modify Ca(OH)2.By means of characterization,it was found that ball milling could reduce the particle size and grain size of Ca(OH)2 and prevent the aggregation of absorbent.Then the activity test was carried out by absorbing SO3,and the results showed that the conversion rate of Ca(OH)2 prepared by digestion increased by 40%after ball milling.However,in the experiment of HCI removal,it was found that SO2 and SO3 still inhibited the absorption of HCl.In order to improve the removal effect of HCl,metal Fe(NO3)3,Al(NO3)3,Cu(NO3)2 and organic solvent CTAB(cetyl trimethyl ammonium bromide)were selected to modify the calcium base.The experiment shows that Cu has the best doping effect.Further optimization of conditions shows that the removal effect of chlorine is the best when CuO-5%is doped,and Ca(OH)2 conversion rate increases by 11%.