| The process of activated coke(AC)dry flue gas desulfurization(FGD),with basically no water consumption,no waste water and waste residue generation,no secondary pollution problems,and can realize the recycling and of sulfur,is a kind of green and sustainable FGD technology.At present,the desulfurization AC commonly used in this process is the columnar AC(with diameter of about 5?9 mm),which has many problems such as complicated preparation process,high price,high strength requirements,uneven activation of the inner and outer surfaces,and low utilization of the inner surface,which seriously limit its large-scale and multi-field application.In response to this,our research team proposed an AC desulfurization process(powdered AC circulating fluidized adsorption desulfurization)using powdered AC(PAC)as an adsorbent.In order to meet its demand for PAC,it is necessary to prepare the PAC with low cost and high performance.So,in this paper,based on the rapid preparation process of PAC under flue gas atmosphere,the research is carried out on the aspects of the influence of coal rank,reaction mechanism of the preparation process,preparation parameters optimization,pilot test and other aspects,for realizing the industrial application of PAC preparation for desulfurization as soon as possible to accumulate experience and lay the foundation.Based on the different degrees of coal metamorphism,nine kinds of coal including lignite,sub-bituminous coal,bituminous coal,and anthracite were used as raw materials to prepare nine PAC samples.The effect of coal rank on the physical-chemical properties and the SO2 adsorption properties of PACs were studied,and the difference in pore formation and evolution mechanism in the preparation of PACs based on different kinds of coal was clarified,which provided a reference for the selection of raw materials for the rapid preparation of desulfurized PAC under flue gas atmosphere.Among them,the PACs prepared from low-rank coal(include lignite and sub-bituminous coal)had ideal structural characteristic for SO2 adsorption,and the corresponding SO2 adsorption capacities were relatively high,about 91.36?122.85 mg/g;but their yields were relatively low,about 45.56?51.18%.When PACs were prepared with bituminous coal of medium degree of coalification is mainly,the caking property and colloidal layer of the bituminous coal are disadvantageous to the rapid preparation of PAC,and the stronger the caking property was,the greater the negative effect was.The changes such as softening,melting,expansion,condensation and solidification of the colloidal layer lead to pore plugging and the decrease of the number of surface functional groups.Although the yields of the PACs were slightly higher,about 49.24?53.67%,their SO2 adsorption capacity were poor,about 17.01?55.54 mg/g.When preparing PACs from anthracite(with high degree of coalification),due to their low volatile content,less primary pores,and relatively stable physical and chemical properties,it was difficult to activate.Although the yields of PACs were high,about 72.17?85.37%,their SO2 adsorption capacities were also poor,about 26.49?32.25 mg/g.Taking SL-coal and JJ-coal,which were suitable for preparing desulfurization PAC,as the research subjects,the mechanism and contribution of carbonization and activation process during the rapid preparation of PAC under flue gas atmosphere were studied by using the difference method decouples.The carbonization process was mainly the precipitation of volatiles,accompanied by a small amount of carbon ablation,mainly forming carbon structures and initial pores;the activation process was mainly carbon ablation to form and develop pores.When SL-coal was used to prepare PAC,the contribution of the carbonization process was dominant,and the contribution ratios of the carbonization process and the activation process to the burn off,total specific surface area(SBET),total pore volume(VTot)and SO2 adsorption capacity were 4:1,1.7:11.5:1 and 1.7:1,respectively;when JJ-coal was used to prepare PAC,the presence of O2 in the flue gas had a significant effect on improving the activation process,resulting in a greater contribution from the activation process,in other words,the activation process was dominant.For example,the contribution ratios to SBET and SO2 adsorption capacity were 0.18:1 and 0.8:1,respectively.However,for the burn off,the contribution of the carbonization process was still dominant,and the contribution ratios to burn off was 2.4:1.In order to explore the reaction mechanism of the rapid preparation process of PAC,the properties of PACs prepared with different reaction time(0.5?4 s)were studied.During the rapid preparation process of PAC,the carbonization and activation processes started at the same time,and the O2 component in the flue gas was consumed in the initial stage of reaction,and the subsequent activation reactions were mainly the activation of CO2 and H2O(g).The carbonization and activation processes accompanied the whole PAC preparation process.With the development of the PAC preparation process,the carbonization was gradually weakened and the activation was gradually strengthened.The reaction process of the rapid preparation of PAC could be roughly divided into three stages:the rapid release of volatiles and the rapid consumption of O2(?),the process coupling reaction stage dominated by carbonization(?)and the process coupling reaction stage dominated by activation(?).In order to obtain the optimum condition of the preparation of PAC(satisfying the maximization of SO2 adsorption capacity and the minimization of burn off),and to provide the reference and basis for the process system design and operation control,combined with the factorial design method,response surface method and desirability function method,the SO2 adsorption capacity and the burn off were taken as target responses,and six parameters affecting the preparation of PAC were screened and optimized.Results showed that the reaction temperature,O2 concentration and H2O(g)concentration were the major parameters,and the oxygen coal equivalent ratio,reaction time and CO2 concentration were the minor parameters.The predicted values of the second-order polynomial models between the SO2 adsorption capacity,the burn off and the three major parameters were in good agreement with the experimental values,which could well predict the SO2 adsorption capacity of PAC prepared under specific preparation condition.Based on these second-order models,under the given optimization criteria,the optimum PAC preparation conditions were optimized.Among them,the optimum conditions for SL-coal were reaction temperature of 923.97?,O2 concentration of 5.89%and H2O(g)concentration of 20.04%;and for JJ-coal,they were 870.87?,4.65%and 26.17%,respectively.Besides,the properties of PACs prepared under the optimum conditions were verified by experiments.Based on the Aspen Plus software,the process balance of matching combustion control and PAC preparation conditions was established.Calculation results showed that it was possible to prepare PAC under optimum conditions through the coordination control of combustion,flue gas regulating and PAC preparation,and it could be achieved by the most simple and economical way of water spraying and air spraying.With 1 kg of fuel burned in the combustion section as the benchmark,when SL-coal was used to prepare PAC,if preparing PAC under the optimum conditions,it was necessary to spray air at room temperature with 1.04 Nm3 and water at room temperature with 0.80 kg,so as to control the temperature and composition of flue gas;meanwhile,the amount of pulverized coal for preparing PAC should be adjusted to 4.15 kg,and the theoretical yield of PAC was 1.83 kg.When using JJ-coal,they were 0.40 Nm3,2.00 kg,3.30 kg and 1.59 kg,respectively.Based on the above research,the design method for the rapid preparation of desulfurization PAC using pulverized coal under the flue gas atmosphere was summarized and formed,including the selection of coal types,the determination of the PAC preparation conditions and the regulation and matching strategy of the PAC preparation condition.According to the design method,with JJ-coal as the design coal,the complete design and construction of a pilot test system with the U type two-stage integrated PAC preparation furnace as the main body had been completed,and the construction and test of the pilot test system had been completed.Although the pilot test preparation of PAC was not carried out in the optimum conditions,its operating parameters basically accord with the design parameters,which verified that the system design was reasonable,the U type two-stage integrated PAC preparation furnace was feasible and the design method was correct and reliable.The continuous and stable preparation of PAC in pilot scale test directly verified the I reaction stage of its reaction mechanism,and indirectly verified the II and III reaction stage.At the same time,according to the feedback of pilot test,it was more appropriate to take the average temperature distribution in the PAC preparation section as the PAC preparation temperature.So the modified conditions of pilot test were as follows:reaction temperature was 1050?,O2 concentration was 5.76%and H2O(g)concentration was 15.03%.The SO2 adsorption capacity of the PAC prepared in pilot test was 57.47 mg/g,and the burn off was 51.17%.And feedback to the system design calculation,it was appropriate to take the average temperature of the inlet and outlet of the PAC preparation section as the design temperature.Through the pilot test,it was found that the inlet temperature of the PAC preparation section was the main factor affecting the average temperature of the PAC preparation section,it was necessary to control the inlet temperature of the PAC preparation section not to be too high for the control of the performance of the PAC,and some suggestions for temperature control measures of the inlet of the PAC preparation section were given. |
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