Desulfurization kinetics of ZnO sorbent loaded on semi-coke support for hot coal gas

Availableonlineatwww.sciencedirect.comScienceDirectuRw群FNATURAL GASCHEMISTRYELSEVIERJournal of Natural Gas Chemistry 21(2012)556-562www.elsevier.com/ocate/jngcDesulfurization kinetics of zno sorbent loaded onsemi-coke support for hot coal gasZhiwei Ma, Xianrong Zheng, Liping Chang, Ruiyuan He, Weiren BaoKey Laboratory of Coal Science and Technology, Taiyuan Universiry of Technology Ministry of Education and Shanxi Province, Taiyuan 030024, Shanri, ChinaManuscript received November 18, 2011; revised January 12, 2012]abstractZn-based sorbent(Z20SC) prepared through semi-coke support in 20 wt% zinc nitrate solution by high-pressure impregnation presents anexcellent desulfurization capacity in hot coal gas, in which H2S can not be nearly detected in the outlet gas before 20 h breakthrough time.Theeffects of the main operational conditions and the particle size of Z20SC sorbent on its desulfurization performances sorbent were investigatedin a fixed-bed reactor and the desulfurization kinetics of Z20SC sorbent removing H2S from hot coal gas was calculated based on experimentaldata. Results showed that the conversion of Z20SC sorbent desulfurization reaction increased with the decrease of the particle size of thesorbent and the increases of gas volumetric flow rate, reaction temperature and h2S content in inlet gas. Z20SC sorbent obtained fromhydrothermal synthesis by high-pressure impregnation possessed much larger surface area and pore volume than semi-coke support, and theywere significantly reduced after the desulfurization reaction. The equivalent grain model was reasonably used to analyze experimental data, inwhich ks =4.382x10-exp(8.270x10IRgT)and Dep=1. 262x10-exp(-1. 522X10IRgT) It suggests that the desulfurization reaction ofthe Z20SC sorbent is mainly controlled by the chemical reaction in the initial stage and later by the diffusion through the reacted sorbent layerKey wordsdesulfurization kinetics; Zn-based sorbent; hot coal gas; medium-temperature desulfurization1 IntroductionThe early experimental results showed that Zn-based sorbent prepared by high-pressure impregnation method exhibited an excellent performance for removing H2S from hot coalliae. l ,s in coal gas undoubtedly affects the subsequent uti- gas in the conditions of 500 C, 500 ppmv H2S and balancelization of gas by its pollution[11, corrosion or poisoning cata- gas N2. Zn-based sorbent(Z20SC)prepared by impregnatlyst, so the removal of H2S from coal gas has been considered ing semi-coke support in 20 wt% zinc nitrate solution underas one of key techniques of coal cleaning conversion. Zno the high-pressure of 28 atm for 5 h has abundant pore structuresorbent is known as the fine sorbent for H2S removal from hot and big specific surface area [13], and we chose the equivalentcoal gas due to the high equilibrium constant for sulfidation grain model to study the desulfurization kinetics of Z20SCin[2]. The desulfurization reaction is typically a non-catalytic this paper. The result may provide theoretical basis for thegas-solid reaction [3]. This chemical reaction occurs not only industrial application of the sorbenton solid surface, but also into the interior of solid particles. Inthis reaction, grain size is variable and the reaction rate is notfixed, which is a kind of unsteady process. For non-catalytic2. Experimentagas-solid reaction, the dynamic models include the unreactedshrinking-core model [4-6], improved shrinking-core model,The desulfurization experiment was carried out in a fixedgrain model [7-10] and the equivalent grain model [11, 12], bed reactor. A vertical quartz reactor(2.0 cm i.d. )was placedin which the equivalent grain model mainly applies to loose in an extermal heating furnace. The feeding gases were com-or porous particlessed of H2S and N2. Schematic diagram of the fixed-bed re-Coresponding author. Tel/Fax: +86-351-6010482;baoweiren@tyut.du.cn(W.R.This work was supported by the National BasicProgram of China(2012CB7中国煤化工-nce Foundation of China20976117), Shanxi Province Natural Science Foundation1014-3)and Shanxi Province EHCN MH Gce and Technology Project(2010091015Copyright@2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserveddoi:10.1016S10039953(11)604040Journal of Natural Gas Chemistry VoL. 21 No 5 2012557actor for sorbent evaluation is shown in Figure 1. In order toThe results from Figure 2 show that desulfurization per-investigate the effect of support on the removal of H2S in hot formance of Z20SC sorbent was significantly better than thatcoal gas, the desulfurization experiments of Zn-based sorbent of ZOSC sorbent. Z20SC sorbent possessed 6.57% sulfur ca-ZoSC)prepared by impregnating semi-coke in the solution pacity and 20 h breakthrough time. Although semi-coke hadwithout zinc nitrate were also carried out under the same con- abundant pore structure and larger surface area, the capacditions as those of Z20SC. Experimental conditions are listed ity of desulfurization in a hot coal gas(500C)is negligi-in Table lble comparing with that of Z20SC sorbent. ZnO as the activecomponent of sorbent, reacted easily with H2S and the reaction rate was high, which made Zn-based sorbent show thehigh desulfurization precision [14]. Active components ZnOin Z20SC could be evenly dispersed on the semi-coke support through high-pressure impregnation, which increased thereaction probability of ZnO and H2S and showed good desulfurization performance. In the following discussion, desulfur-ization kinetics was studied for Z20SC sorbent0.25Figure 1. Schematic diagram of the fixed-bed reactor for sorbent evalu-ZOSCation. 1-high-pressure cylinder, 2-valve, 3-mass flow meter, 4-tubefurnace, 5--thermocouple, 6--temperature controller, 7---inlet gas sampling8--outlet gas sampling. 9-vent, 10-sorbent, Il-quartz tube0.l5Table 1. Operation conditions used in the experiments0.10Temperature(C)300-600Flow rate(mUmin)200-600H2s (ppmv)200-1000balance gasample weight (g)Particle size(mm009-0.25Time(h)Figure 2. Desulfurization performance curves of OsC and Z20SC sorbentsH2S concentration was analyzed by a gas chromatograph Co is the initial concentration of H2S in inlet gas; C is the concentration of(GC)equipped with a flame photometry detector(FPD)H2S at any time of experimental periodThe computational formulas for the conversion of sorbent(rs)is as follow3. 2. Elimination of the extermal diffusicUs. max -w0where, w is the weight of the sample at t time; wo is the initialIn view of dynamics research, the influence of the external diffusion should be eliminated for the desulfurizeweight of sample; ws, max is the theoretical weight of sample tion reaction of sorbent. Gas reactant through the externalwhen Zno is wholly changed into ZnSdiffusion resistance of gas film was generically related withthe gas volumetric flow rate. In this experiment, desulfur3. Results and discussionrization temperature and inlet H2S concentration were keptat 500C and 500 ppmv, respectively. The sorbent particle3. 1. Desulfurization performance of Z20SC sorbentsize of 0.. 150 mm was chosen. The change of conversion rates of Zno sorbent with gas volumetric flow rate wasThe desulfurization performance of ZoSC and Z20Sc tested and the experimental results are shown in Figure 3. Insorbent is shown in Figure 2. In the experiments, desulfurize- which, gas flow rate was controlled as 200, 300, 400, 500 andtion temperature and inlet H2s concentration were 500C and 600 mL/min. It can be seen that when the gas volumetric flow500 ppmv, respectively. The volume of ZoSc and Z20SC sobent(particle size in the range of 2.36-3. 35 mm) was 20 mL. same as thaYH中国煤化工 of Zno was almost thetal resultsWhen outlet H2S content was 0.1 ppmv, it was considered as that the extCNMHhinted under conditionthe breakthrough of sorbent bed and the desulfurization exper- of 500 mL/min, and it is considered as the optimal volumeiment stoppedflow rate of gas and used to the following experiments558Zhiwei Ma et al./ Journal of Natural Gas Chemistry VoL. 2I No. 5 20121003.4. Efect of H2S concentration on the conversion of sorbento-300 mL/minWhen the external diffusion and internal diffusion were如celiminated, the conversion rates of Zno sorbent were tested indifferent inlet H2S concentrations and the results are shownin Figure 5. The inlet H2S concentration was chosen as 200,300, 500 and 1000 ppmv. It can be seen that high concentra-tion H2S could promote the desulfurization reaction becauseit promoted the impetus of mass transfer when the gas volumetric flow rate was invariable. In this experiment, 500 ppmvH2S in inlet gas was relatively acceptableTime(m300 ppmFigure 3. Effect of gas flow rate on ZnO conversion in the desulfurization8-1000ppmprocess on sorbents3.3. Elimination of the intemal diffusionIn dynamics research, the influence of the internaldiffusion should also be eliminated for desulfurization reac.tion of sorbent. In this experiment, desulfurization temperature and inlet H2S concentration were also kept at 500C andTime( min)500 ppmv. The gas volumetric flow rate was 500 mL/minThe conversion rates of ZnO sorbent with different par-Figure 5. Effect of H2S concentration on sorbent conversionticle size were tested and the results are shown in Figure 4. In which, the particle sizes of sorbent were chosen as0.090-0.106, 0.106-0.125, 0.125-0.150, 0.150-0.180 and 3.5. Characterization of structure properties of fresh and sul0.180-0.250 mm, respectively. It can be seen that small size furized sorbentsorbent could promote the desulfurization reaction, because ithad a greater total surface area than that of the big size sorbent,The content of Zno in Z20SC sorbent pellet (Pfmwhich was beneficial to the interaction between gas and solid. obtained by AAS(atomic absorption spectrometry)wasThe experiment results showed that sorbent particle size of 1.055 molL and the porosity of sorbent is9.84%.BET sur-0. 125-0.150 mm for desulfurization dynamics research was face area and pore volume of the fresh and sulfurized Z20SCsuitablesorbent are listed in Table 2. For comparison, the results ofraw semi-coke are also listed. It can be seen that surface areaand pore volume significantly increased and the sorbent pre0.090mm-0.106pared by hydrothermal synthesis in the high-pressure impreg-0.125nation process possessed a developed pore structure, which80}→0.125m-0.150mmwas the basis to choose the equivalent grain model for study0.180mm-0.250mming the desulfurization kinetics of Z20SC sorbent. The sur-face area and pore volume of sorbent after the desulfurizationreaction reduced in some extent, suggesting that the reactionof Zno and H]S was"obturator process"and resulted in thedense structure of sorbent. This also revealed that the reactionof Z20SC sorbent removing H2S from hot coal gas should becontrolled by the diffusion through the reacted layers in the20later stageYH中国煤化工 e of different samplesCNMHG Pore volume(mLg)Figure 4. Effect of particle size on ZnO conversion in the desulfurizationFresh Z20SC262Sulfurized Z20SC2120.12Journal of Natural Gas Chemistry Vol. 2I No 5 2012Figure 6 and Figure 7 are the XRD patterns and SEM 3. 6. Desulfurization kinetics of znO sorbentphotographs of fresh and sulfurized Z20SC sorbent. The re-sults showed that fresh Z20SC sorbent possessed abundantChange of the conversion rates of ZnO sorbent with timepore structure and the active component Zno could be evenly was tested at different desulfurization reaction temperatures,dispersed on the semi-coke support. ZnO in sorbent after which was chosen as 300, 400, 500 and 600C. The resultsdesulfurization changed into ZnS and the phase composition are shown in Figure 8. It can be seen that high temperatureand morphology structure of sorbent showed great changes, could promote the desulfurization reactionin which the reaction products were heaped up together. ThiThe desulfurization reaction was typically a non-catalyticalso proved that the desulfurization reaction of the Z20SC sor- gas-solid reaction and there was a transfer of controlled area ibent and H2S was likely controlled by the diffusion through whole process. In the initial reaction stage, the product layerthe reacted sorbent layer in the later stagewas thin and the reaction gas passed easily through the productlayer to unreacted solid phase, in which the gas diffusion resis-tance was low. The desulfurization process was mainly con-trolled by chemical reaction. With the thickening of the product layer, the diffusion resistance increased and the desulfurization process was gradually controlled by the gas diffusionThe equivalent grain model suited to the loose or porousparticles was chosen for discussing the reaction kinetics ofFresh 220SCZ20SC sorbent. The results of relating the experimental datawith the equivalent grain model [ll, 12]are shown in Figures 913Sulfided Z20SCcc℃600℃Figure 6. XRD patterns of fresh and sulfurized Z20SC sorbent1030Time(min)Figure 8. Change of the sorbent conversion with sulfidation time at different600℃0.100051中国煤化工CNMHGFigure 7. SEM photographs of fresh and sulfurized Z20SC sorbents.(a) Figure 9. Relationship of Gfp(rh-t sorbent sulfidation in the entire experi-Fresh Z20SC, (b) sulfurized Z20SCmental period560Zhiwei Ma et al./ Journal of Natural Gas Chemistry Vol 21 No, 5201Theof the surface controlled area is shownIt can be seen from Figure 10 that Grp(a)and t had a goodfollowslinear relationship in the initial reaction stage. The desulfurt= Agrp(r)(2) ization process for ZnO sorbent loaded on semi-coke supportwas controlled by the chemical reaction. Desulfurization ki-netics parameters in the surface reaction controlled area areAks×C(3) listed in Table 3.For the reaction between ZnO and H2S, the reaction orderGp(x)=1-(1-x)3The relationship of ks Us. T follows the Arrhenius equa0.15600℃E0.10RoTAccording to Equations(2)-(5), Equation(6)can be ob-tainedksoCAo EaAPfm X Ro RgTFrom Figure 9, it can be seen that Gfp(r)-t curves afterTime(min)running the reaction for 20 min started to deviate straight lineFigure 12. Relationship of Pip(r)-t sorbent sulfidationand Pip(a)-t should be fitted in the later reaction stage. The re-sults running the reaction for 20 min are shown in Figures 10and 110.15300℃5.8●400℃500℃600℃6.20.051.50.00i00o/T(K68101214161820Figure 13. Relation of In(1/B)with IrFigure 10. Relationship of Gfp (z)-t sorbent sulfidation in the period of initialThe diffusion through the reacted layer can be described0 minas follows:B1+ Blip(r)By-5.2B2=mm(1-)(9)For the reaction between ZnO and H2S, the reaction orderis IR1_12/3中国煤化工1.21.61.8-ws the arrhenius equa-tonsCNMHGl000T(K)eaFigure ll. Relation of In(1/A)with 1/Tep= Depo expJoumal of Natural Gas Chemistry Vol. 21 No 5 2012561According to Equations(7H(11), Equation( 12)can be a good linear relationship in the later reaction stage afterobtained20 min. The desulfurization process for ZnO sorbent loaded6Denocedon semi-coke support was controlled by the diffusion through1B=x后(=9)-2了(12) the reacted layers. Desulfurization kinetics parameters in thiscontrolled area are listed in Table 4From Figure 12, it can be seen that Pip(a)and t haveTable 3. Desulfurization kinetics parameters in the surface reaction controlled area for Zno sorbent loaded on semi-coke supportTemperature(C) A(min) Correlation coefficient ks(mol-cm-2 min-l) kso(mol-cm-2-min-) E2(/mol) Correlation coefficient255.10209997725×10-44.382×108.270×1009989.997×1041650171210×10-3136986402×10-3Table 4. Desulfurization kinetics parameters in the diffusion through reacted layers for ZnO sorbent loaded on semi-coke supportS are (C) B)(min) B,(min)Corelation coefficient Dex (mol cm min Dexo( mol-" I, min) Ea U/ml) Correlationceficienfn-6.126740.7412224×101.522x10994400-8.974926110992879×10510.347330.0333.485×10-510.985249,3774038×1054. Conclusionsks Apparent chemical reaction rate constantZ20SC sorbent prepared by high-pressure impregnation kso Frequency factor(mol-cm-2-min"possesses much larger surface area and pore volume than raw DepDiffusion coefficient(mol-cm-I.min -semi-coke and presents an excellent desulfurization capacityDepo Frequency factor(mol-cm-minin hot coal gas. The desulfurization reaction kinetics of thisEa Activation energy of chemical reaction(k/mol)sorbent with developed pore structure can be reasonably de- Ed Activation energy of intrapellet diffusion (W/mol)scribed by equivalent grain model. The gas volumetric flowGr(r) Function of conversion of chemical reaction controldesulfurization kinetics research of Zn-based sorbent z20sc Fip(a) Function of conversion of the diffusion through theare appropriate, in which the extermal and internal diffusionreacted layershave been eliminated under the conditions of 500 C and 500 5 Porosityppmv H2S. The results of equivalent grain model correlatinRo Particle radius(mm)with the experimental data show that the desulfurization re. Rg Gaseous constant(8.314J-mol-IK-yaction of Z20Sc sorbent is respectively controlled by the ini- efm Zno concentration in pellet(molL)tial chemical reaction and later diffusion through the reacted tTime(min)layers,in which ks =4.382x10-exp(-8270x 10/R T)and T Temperature(K,C)Dep=1.262×10-exp(-1.522×10/R2Toefficient with B1AcknowledgementsThe authors gratefully acknowledge the financial supports of ReferencesNational Basic Research Program of China(2012CB723105), theNational Natural Science Foundation of China(20976117), Shanxi [1] Zhu F, LicH, Fan H L J Nat Gas Chem, 2010, 19(2): 169Province Natural Science Foundation(2010011014-3)and Shanxi [2] Liang M s, Xu h Y, Xie K C J Nat Gas Che, 2007, 16(2)Province Basic Conditions Platform for Science and Technology204Project(2010091015)[3] Guo H X Application of Chemical Kinetics. 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