Effect of the interactions between volatiles

Effect of the interactions between volatiles and char on sulfur transformation during brown coal upgrade by pyrolysisYulong Zhang a ,Meijun Wang a ,Zheng Qin a ,Yunlong Yang a ,Chunhui Fu a ,Li Feng b ,Liping Chang a ,⇑a Key Laboratory of Coal Science and Technology,Taiyuan University of Technology,Ministry of Education and Shanxi Province,Taiyuan 030024,People’s Republic of China bSchool of Chemical Engineering and Technology,China University of Mining and Technology,Xuzhou,Jiangsu 221116,People’s Republic of Chinah i g h l i g h t s"A modified two-stage quartz reactor has been designed."Sulfur distribution based on interactions between volatiles and char."H 2S formation combined with H radicals and organic sulfur in raw coal."COS formation combined with C–O radicals and organic sulfur in raw coal."Comprehensive mechanism of sulfur transformation in different forms.a r t i c l e i n f o Article history:Received 29April 2012Received in revised form 31August 2012Accepted 24September 2012Available online 6October 2012Keywords:CoalPyrolysisVolatiles–char interactions Sulfur transformationa b s t r a c tA modified two-stage quartz reactor was designed to further investigate the mechanism of sulfur trans-formation based on the interactions between volatiles and char during brown coal pyrolysis at different temperatures.With the two-stage reactor,the active volatiles generated from the coal fast pyrolysis can be in direct contact with the nascent char produced from the coal temperature-programmed pyrolysis.The results show that the interactions between volatiles and char have a pronounced effect on the sulfur transformation in the upgrading process of brown coal by pyrolysis,with the effect being a function of temperature.The interactions between volatiles and char promote more sulfur into the gas phase at lower temperatures of below 600°C,while remain more sulfur in char at higher temperatures of above 600°C.The evolutions of H 2,CH 4,CO and CO 2at different reaction stages play important roles for the for-mation of sulfur species in char,which are the main evidences to reveal the mechanism of sulfur trans-formation.The XPS data of chars obtained from the different types of experiments indicate that the distribution of sulfur forms on the char surface is closely related to sulfur transformation.A comprehen-sive mechanism of sulfur transformation due to the interactions between volatiles and char during brown coal pyrolysis was also explored.Ó2012Elsevier Ltd.All rights reserved.1.IntroductionLow grade coals with a high water and/or high sulfur contents have gradually become important energy resources because of their abundant minable reserves and the substantial consumption of high-quality coals.However,the sulfur and water contained in these coals largely inhibit its effective and extensive utilization.The research to improve inferior coal quality has been widely car-ried out,which mainly includes the drying,washing and pyrolysis techniques.The appropriate thermal decomposition of coal pre-sents a potentially low-cost alternative for yielding liquid fuels from condensed volatile matter and upgrading char as value-added carbon products.The pyrolysis process can make sulfur in the raw coal transfer to coal-based gas,tar,and char,forming different forms of sulfur and regulating the designed removal of sulfur according to the required destination.However,the previous research mainly considered coal pyrolysis as a whole and few resolved the whole pyrolysis into its elementary reactions.In the process of coal pyrolysis,the interactions between volatiles and char are the main factors influencing gaseous products,except for the interactions of volatiles itself and the change of the coal matrix itself [1].In addition to sulfur containing gases (H 2S,COS,CS 2),the vola-tiles contain a large amount of active groups,such as H radicals.Generally,20–50%of sulfur in raw coal is retained in the solid phase when coal is pyrolyzed,which leads to an increase of sulfur content in char [2,3]and limits the range of application for char0016-2361/$-see front matter Ó2012Elsevier Ltd.All rights reserved./10.1016/j.fuel.2012.09.061Corresponding author.Tel./fax:+863516010482.E-mail address:lpchang@ (L.Chang).after pyrolysis of brown coal.With increasing temperature,the sul-fur is gradually transferred from the bulk coal to the char surface during coal pyrolysis[4,5]and then the gaseous H radicals could easily react with the surface S to form H2S[4].During the fast pyro-lysis,the free radicals generated as a‘‘self-gasification’’process of char by the volatiles[6],would help to produce H2S and COS.At thesame time,on the surface of nascent char,there are considerable functional groups and active sites.Consequently,it is obvious that there are strong interactions between volatiles and char based on the highly reactive nature of volatiles and the vulnerable structure of nascent char.In fact,the competitive reactions of sulfur transformation exist in coal pyrolysis.For example,the active groups in volatiles can react with the nascent char and promote the sulfur in nascent char into gaseous phase;the sulfur-containing gases can react with the nascent char and are retained in the char.The effects of the interactions between volatiles and char on the volatilization of alkali(alkaline)metal and the reactivity of nascent char during coal pyrolysis have been studied[7,8],but their effects on sulfur transformation has not been presented.The effect of H2S on organic sulfur behavior and the kinetics and dynamic behavior of sulfur transformation have been investigated during heat treat-ments[9–11].However,sulfurs transformation during pyrolysis is also not very clearly understood.In order to achieve the effective and clean staged conversion of brown coal with high-sulfur content,it is necessary to obtain direct experimental evidence about the effects of interactions between volatiles and nascent char on sulfur transformation at different pyrolysis temperatures.A novel two-stage quartz reactor was designed and a series of 2.2.PyrolysisA modified two-stage quartz reactor was designed based on the research in relation to the effects of interactions between volatiles and char on volatilization of alkali(alkaline)and reactivity of nas-cent char in pyrolysis experiments on brown coal samples[7,8]. The schematic diagram of the two-stage reactor is shown in Fig.1.The modified two-stage reactor has some features:two frits were installed in the quartz tube for charging coal and/or the char sample,in the form of twofixed beds(i.d.24mm);the upper and lowerfixed bed are used to produce volatiles and prepare char, respectively.The distance between the twofixed beds is5cm. The most important feature of the two-stage reactor is that it al-lows the study of the interactions between volatiles and char and their effects on the sulfur transformation during coal pyrolysis. Two groups of symmetrical T-pipes,glass ellipsoids and stop valves constitute the coal reservoirs and were installed above the reac-tors.Before the coal is fed on the upper bed,the two stop valves are closed.The coal sample is dropped on the upperfixed bed by the carrier gas and its gravity in the experimental process.The coal sample was pre-loaded on the lowerfixed bed through the middle quartz glass tube before experiment was done.Ultra high purity argon was used as the pyrolysis carrier gas.A total gasflow rate Table2Ash composition analyses of raw coal sample used in the experiments(wt.%).SiO2Al2O3Fe2O3TiO2CaO MgO SO3K2O Na2O P2O530.314.69.7 1.218.2 4.918.60.60.90.6Fig.1.A schematic diagram of the two-stage quartz reactor.916Y.Zhang et al./Fuel103(2013)915–922actions between volatiles and char during coal pyrolysis is shown in Fig.2.In thefirst single experiment(denoted as B1),the reactor was first heated up to the desiredfinal temperatures of300,400,500, 600,700,800,and900°C,respectively.About0.5g of coal sample was then fed into the upperfixed bed and was held for a pre-set time of about30min.In this case,the amount of volatiles produced was greater than that in the experiment using a slow heating rate. This type of experiment was termed the‘‘producing volatiles’’pro-cess.As soon as the pre-set holding time was reached,the reactor was lifted out of the furnace to be cooled down naturally with a continuous Ar gasflow.The cooled chars are termed‘B1-char’and the H2S and COS released in this stage are denoted as ‘B1-H2S’and‘B1-COS’,respectively.In the second single experiment(denoted as B2),about0.5g of coal sample was pre-loaded into the lower bed.The coal was then heated up at a heating rate of10K minÀ1,from room temperature to the requiredfinal temperature,and held for the pre-set time of about30min.This type of experiment was termed the‘‘preparing char’’process.As soon as the pre-set holding time was reached,the reactor was lifted out of the furnace to be cooled down naturally with a continuous Ar gasflow.The cooled chars are termed ‘B2-char’.The H2S and COS released from chars generated in the holding stage at differentfinal temperature are denoted as ‘B2-H2S’and‘B2-COS’,respectively.It needs to be highlighted that the yields of H2S and COS for B2type of experiment do not include that from temperature-rise period,which is just the process of pre-paring char sample used in the experiments.In the comprehensive experiment(denoted as C),the two single experiments were integrated.When the requiredfinal temperature of B2experiment was reached,B1experiment was commenced. The comprehensive experiment was notfinished until B1experi-ment was accomplished.The volatiles generated in the upperfixed bed as a result of rapid heating can pass through the lowerfixed bed and react with the nascent char.As soon as the pre-set holding time was reached,the reactor was lifted out of the furnace to be cooled down naturally with a continuous Ar gasflow.In this case, the cooled chars of the upper and lowerfixed bed are termed ‘C1-char’and‘C2-Char’,respectively.The H2S and COS are denoted as‘C-H2S’or‘C-COS’,respectively.A large amount of work has been done to ensure the accuracy of the experimental results.Repeated experiments were also conducted and the results showed that the relative error is less than3%for all experimental results.2.3.Chromatographic analysis of sulfur-containing gasesIt needs to be highlighted that the product gas stream was col-lected by once per minute only at the holding stage using a gasbag. The concentrations of H2S and COS in the outlet gaseous products were directly measured by a GC-950gas chromatography equipped with a Flame Photometric Detector.The total amounts of these gases in pyrolysis were obtained by accumulating each da-tum in the whole period of the holding stage.The yields(Y i)of B1-H2S(B1-COS)and B2-H2S(B2-COS)were calculated by following equation:Y i¼XP=Qð1ÞP¼ðC1;iþC2;iÞÂVÂtÂ273:15ÂM S2Â22:4Âð273:15þTÞÂ1000=Q¼m oÂð1ÀM adÞÂð1ÀA dÞÂS dafwhere C1,i and C2,i are the release concentration of H2S or COS at the adjacent sampling time period(%);V is theflow rate of the carrier gas(ml minÀ1);t is the sampling time interval(min);M S is the mo-lar mass of the sulfur atom(g molÀ1);22.4is the volume of mole gas (L/mol);T is the room temperature(K);M ad is the moisture content in coal at air-dried basis(wt.%);A d is the ash content in coal at air-dried basis(wt.%);S daf is the sulfur content in coal or char at the dry and ash-free basis(wt.%).In the B1type of experiment,m o is the weight of coal sample on the upperfixed-bed in each experiment ($0.5g)and S daf is the sulfur content in raw coal at the dry and ash-free basis.In the B2type of experiment,m o is the weight of char on the lowerfixed-bed at the beginning of the holding stage in each experiment(<0.5g)and S daf is the sulfur content of the correspond-ingchar.Y.Zhang et al./Fuel103(2013)915–922917The yields (Y i )of C-H 2S (C-COS)and B1-H 2S +B2-H 2S (B1-COS +B2-COS)were calculated by following equation:Y i ¼XP =Q 0ð2ÞQ 0¼m o Âð1ÀM ad ÞÂð1ÀA d ÞÂS daf þm 0o ð1ÀM ad ÞÂð1ÀA d ÞÂS 0dafIn the C type of experiment,m o is the weight of the coal sample on the upper fixed-bed in each experiment and S daf is the sulfur content in raw coal on the dry and ash-free basis;m 0o is the weight of char on the lower fixed-bed at the begining of the holding stage in each experiment and S 0daf is the sulfur content of the correspond-ing char.In the calculation of B1-H 2S +B2-H 2S (B1-COS +B2-COS),m 0and S daf are the same as that in the B1type of experiment;m 0o and S 0daf are the same as that in the B2type of experiment.2.4.Analysis of bulk sulfur in the coal and charThe total sulfur content in the coal and char were analyzed according to GB/T 214-1996using a chemical method.Pyrite and sulfate contents were determined according to GB/T 215-2003,and organic sulfur content was obtained by the difference.The pro-portion of sulfur forms in total sulfur of coal sample are also shown in Table 1.The bulk sulfurs mean the majority of sulfur in a large mass compared to the sulfur on sample surface.The bulk sulfur content is approximately considered as the total sulfur content of the sample from chemical analysis method,an average sulfur con-tent in coal or char.2.5.Py-MS experimentThe aforementioned three types of pyrolysis experiments were repeated under the same operational conditions.The on-line mea-surements of H 2,CH 4,CO,CO 2,H 2S,COS and SO 2gaseous product were carried out by MS in order to discuss the effect of ambient atmosphere on sulfur transformation during coal pyrolysis.The MS system (Hiden QIC-20)automatically samples and analyzes the gases evolved from the reactor.2.6.XPS test of sulfur forms on the surface of coal and charXPS permits analysis of the sample surface (20atomic layers).Usually the S 2p signal can be resolved into peaks corresponding to pyrite,sulfides,thiophenes,sulphoxides,sulphones and sulpho-nates/sulfates on the surface of sample.XPS analyses with a V.G.Scientific ESCALAB250using focused (diameter of the irradiated area was approximately 500l m)monochromated Al K a (hv =1486.6eV,150W)were performed.The residual pressure in-side the analysis chamber was 6Â10À10mbar.The relative con-centration of sulfur forms was based in the peak values of 162.5±0.5,163.3±0.4,164.1±0.2,166.0±0.5,168.0±0.5and 170.0±0.5eV binding energy for pyrite,sulfide,thiophene,sulfox-ides,sulfones and sulfates,respectively [12,13].The samples were placed in a sealed bag,cooled in liquid nitrogen and then placed in the refrigerator for characterizing really by XPS method.3.Results and discussion3.1.Effect of interactions between volatiles and char on H 2S formation Fig.3presents the yields of H 2S from B1,B2,and C types of experiments during coal pyrolysis at different final temperature.The yields of H 2S from the three types of experiments present the similar trend—increasing to a maximum and then decreasing with the increase of pyrolysis temperature.The peak temperatureis not entirely consistent for the three types of experiments,but it can be seen that the release of H 2S in the C experiment is domi-nated by the B1experiment.The yields of H 2S present the peak va-lue at 600°C in the B1experiment,which is consistent with the instantaneous evolution profile by MS (shown in Fig.4).The yields of C-H 2S are higher than the sum of two single exper-iments (B1-H 2S +B2-H 2S)at temperature below 550°C,suggesting that the volatiles promote the sulfur in nascent char to H 2S in the lower temperature.Fig.4presents the evolution profiles of H 2S,CH 4and H 2in the process of the B1experiment.Generally,the coal pyrolysis presents in the thermal cracking stage.The breaking of the bridged linkage lead to the formation of the H-containing rad-icals,such as ÅCH 2ÀR ÀCH 2Å;R ÀCH 2Å;CH 3Å;ÅCH 2Å;H Åand so on at 300–500°C,and further react to form CH 4[14–16],which has been proved by the results from Fig.4B.H 2S is produced by a combina-tion of hydrogen radicals and sulfur-containing species formed by decomposition of sulfur containing compounds in the coal [17].When the volatiles pass through the char on the lower fixed-bed,these H-containing radicals can react with the sulfur functional groups or sulfur radicals (HS Å,S Å)in char to form H 2S and are there-by released as gaseous pared with Figs.3and 4B,it can be found that the yields of H 2S in the C experiment show the same tendency with the evolution profiles of CH 4.The result shows that the active H radicals related to the formation of CH 4can pro-mote the formation of H 2S.The yields of C-H 2S are lower than that of B1-H 2S +B2-H 2S at the temperature above 550°C.This indicates that the interactions between volatiles and nascent char retain sulfur species in char and decrease the release of H 2S at higher temperatures when the H 2S produced in the upper fixed bed passes through the lower fixed bed.The H 2S was captured by organic matrix in char to form organic sulfur with complex molecular structures,which can be attributed to the increase of thiophenes and sulfides in the char [9,18].When the temperature is risen to above 550°C,the dehy-drogenation of aromatic structures and condensation of the aro-matic nuclei into coal char are the main sources of the molecular hydrogen [19]and H radicals may penetrate into the char matrix and favor the condensation of aromatic rings [20].H 2S produced from the upper fixed-bed can react with the nascent char on the lower fixed-bed and form sulfur species in the char matrix.The hydrogen free radicals,in related to the formation of H 2in volatiles,would favor the condensation of the aromatic ring,and accelerate the nascent sulfur species ‘‘aging’’,which is a very stable form and hard to decompose.These should explain the decrease of H 2S yields from the C experiment at a high temperature of above 550°C caused by the interactions between volatiles and char.The distinction of the H 2S yields between C-H 2S and B1-H 2S +B2-H 2S becomes larger with the increase of temperature in the final tem-918Y.Zhang et al./Fuel 103(2013)915–922perature range of600–900°C in Fig.3,which is related to the hydrogen free radicals in relation to the formation of H2during coal pyrolysis(shown in Fig.4C).The amount of active H radicals formed from indigenous hydrogen in coal during pyrolysis have been getting larger with the increase of temperature,which can be sufficient to accelerate the nascent sulfur species‘‘aging’’. Therefore,the distinction of H2S yields between C-H2S and B1-H2S+B2-H2S becomes larger at the tail section of Fig.3.Formation of H2S in coal pyrolysis can be considered as a heter-ogeneous reaction,which is controlled by chemical reaction and/or gases diffusion.In the B1experiment,with increasing temperature from300to600°C,the yields and instantaneous content of H2S in-crease rapidly,which show that the sulfur release is dominated by chemical reaction control in this region.While when the tempera-ture is above600°C,the decrease of H2S yield and instantaneous content with the increase of temperature,which shows that the sulfur transformation may be dominated by gas diffusion control. Fig.5presents the effects of carrier gasflow rate on the yields of H2S in the B1experiment.In the temperature range of300–600°C,the change caused by gasflow rate is small,which indirectly proves that the formation of H2S is a reaction controlled process in this region.When the temperature is higher than600°C, the yields of H2S in the condition of high gasflow rate are obvi-ously bigger than that under the low gasflow rate.The appreciable variation in H2S yields from different gasflow rate suggests that the sulfur release is dominated by diffusion control at600–900°C.The effects of the interactions between volatiles and char on sulfur transformation in different carrier gasflow rate have also been studied and the results are shown in Fig.5.For the gasflow rate of600ml minÀ1,the volatiles in nitrogen gas stream have a superficial velocity of0.02m sÀ1.The effect of the gas diffusion resistance can be neglected when the gas velocity exceeded 0.15m sÀ1at900°C or less[9].This suggests the interaction be-tween H2S and nascent char is dominated by gas diffusion control. The apparent residence time of tar and gaseous products in the lowerfixed bed is0.12s.H2S appears to have been remarkably adsorbed by the char through the formation of thermally stable organic forms of sulfur species at the initial stage of the interaction at800°C[9,10].Therefore,the appreciable difference in H2S yields was observed between C-H2S and B1-H2S+B2-H2S.Fig.5also shows that the difference of H2S evolution profiles under different carrier gas velocities in the C experiment is larger than that in the B1experiment over the whole temperature range.This means that the effects of carrier gasflow rate on the C experiment are larger than that on the B1experiment.By increasingflow rate,H2S can be quickly brought out from the reaction zone,which also mini-mizes the occurrence of secondary reactions.3.2.Effect of interactions between volatiles and char on COS formationFig.6presents the changes of yields of COS from different types of coal pyrolysis as a function of temperature.The formation of COS presents a similar trend with that of H2S,but the amounts of COS yields are one order of magnitude lower than that of H2S.ThisY.Zhang et al./Fuel103(2013)915–922919suggests that H2S is the main component of sulfur containing gas during coal fast pyrolysis.The yields of C-COS are higher than that of B1-COS+B2-COS at the temperature of below500°C and are lower at the temperature of above500°C,which show that the interactions promote the sulfur in nascent char or other sulfur spe-cies in volatiles to transform into COS at the lower temperature and retain the gaseous COS into nascent char at the higher temper-ature.This phenomenon is similar to the trend of H2S release but with a more obvious variation.This means that the effects of the interactions between volatiles and char on the formation of COS are bigger than H2S,which suggests that COS can be formed from other route besides the secondary reaction of H2S.COS during coal pyrolysis is mainly produced from pyrite, organic sulfur or secondary reactions of sulfur-containing groups [21].From the data in Table1,it can be seen that there is no pyrite in the coal sample,so the COS is mainly produced from organic sul-fur and secondary reactions of sulfur species.The formation of COS is related to the formation of CO2and CO by recombining C–S and C–O[21,22].CO2and CO in volatiles can react with the sulfur species producing COS,such as H2S+CO2=COS+H2O and H2S+CO=COS+H2.When the volatiles from fast pyrolysis of coal in the upperfixed-bed pass through the nascent char on the lower fixed-bed,the equilibrium relationship between hydrogenolysis/ hydrolysis reactions of COS and its reverse reactions at different temperatures are the main factors influencing the COS release, which is very sensitive to the interactions of volatiles and char.Fig.7presents the instantaneous release profiles of COS,CO and CO2from the B1experiment at different pyrolysis temperature with time.The evolution profiles of COS and CO2show a similar trend during fast coal pyrolysis,which are different from that of CO.The release amounts of COS and CO2present peak values at 600°C,but that of CO increases gradually with increasing pyrolysis temperature.This indicates that the C–O groups related to the for-mation of CO2are much easier to bond sulfur radicals than that for CO.At300–500°C,the volatiles containing much C–O groups related to the formation of CO2pass through the nascent char, and promote the sulfur species in nascent char and/or H2S in gases to transform into COS.Although the volatiles contain abundant C–O groups related to the formation of CO at600–900°C,the C–O groups show weak activity to promote the evolution of COS. On the contrary,COS can react with the active char and becomea part of the char-S.3.3.Effect of interactions between volatiles and char on sulfur distribution in charFig.8shows the sulfur content in chars at different temperature from the three types of experiments.Corresponding to the change trends of S-containing gases at different pyrolysis temperatures, the amounts of char-S from three types of experiments present the trend of decreasing to a minimum and then increase with the increase of pyrolysis temperature.The sulfur content in C1-char920Y.Zhang et al./Fuel103(2013)915–922and B1-char at different temperature is nearly identical,while there is a significant difference between B2-char and C2-char due to the interactions of volatiles and char in the C experiment.The sulfur on the surface of the chars obtained from B2-char and C-char and raw coal were studied by XPS and the analysis re-sults are presented in Fig.9.The sulfur on char surface mainly pres-ent in the forms of sulfide,thiophene,and pared with the total sulfur content of2.9wt.%in raw coal by chemical analysis method,the sulfur content of0.55at.%on the surface of raw coal is much lower,which suggests that sulfur is rich in the bulk and poor on the surface of coal.When coal is heated in the Ar gas,the sulfur content on the char surface is greater than the raw coal and in-creases gradually with increasing pyrolysis temperature,which can be mainly attributed to the increase in the sulfide and thio-phene on the char surface.The result suggests that the sulfur could transfer from the coal matrix to the char surface during pyrolysis and there exists a dynamic equilibrium of the sulfur content in bulk and on the surface of pared with the B2char,the total sulfur content in the C2char is lower at below600°C and higher at above600°C,which presents the corresponding relation with the release of sulfur-containing gases.The content of sulfide in C2char is lower than that in B2char over the whole experimental temperature range,which suggests that the interactions of volatiles and char decrease the content of sulfide in char over the whole experimental temperature range. The effects of interactions of volatiles and char on the transforma-tion of sulfide are mainly shown in two aspects.Firstly,the resi-dence time of volatiles is increased,which increase the chance for the interaction between sulfide in char of lowerfixed-bed and volatiles containing extensive H radicals from upper fixed-bed.Secondly,the alkali metallic species(e.g.potassium and sodium)contained in the volatiles increase the decomposition of sulfide in char.The alkaline earth metallic species,volatilized from the upperfixed bed during the coal fast pyrolysis[7,8],would be trapped by the char on lowerfixed bed.The trapped alkaline earth metallic species can transform the thermally stable organic sulfur(mainly sulfide)to more labile species[23],and enhance the decrease of the sulfide content in char.The change of thiophene content in C2char with temperature compared with B2char is lower at below500°C and higher at above500°C,which is consis-tent with the trend of H2S release.This result indicates that the ef-fect mechanisms of the interactions between volatiles and char on the transformation of sulfide and thiophene are different.At high temperature,the volatiles containing extensive quantities of H rad-icals related to formation of H2age labile sulfur compounds to more complex and stable species such as thiophene.Sulfone content is low and the effect of interactions between volatiles and char on the transformation of sulfone is insignificant at higher temperatures of above500°C.The content of sulfate on the char surface shows no change between C and B2experiments at400°C,while sulfide,thiophene and sulfone appear to decrease significantly,which show that the decrease of sulfur content on char surface owes to the decomposition of the organic sulfur.The decrease of the sulfate content in C2char at500and600°C impliesY.Zhang et al./Fuel103(2013)915–922921。