钾元素对生物质及其三组分热解的影响

第43卷第1期燃料化学学报Vol 43 No. 12015年1月Journal of Fuel Chemistry and TechnologyJan.2015文章编号:0253-2409(2015)014003408Effect of potassium on pyrolysis of rice husk and its componentsHAO Qing-lan, LI Bo-lun, LIU Lei, ZHANG Zheng-biao, DOU Bao-juan, WANG Chang(1. College of Material Science& Chemical Engineering, Tianjin University of Science Technology, Tianyin 300457, China2. College of Marine Science Engineering, Tianyin University of Science Technology, Tianjin 300457, China)Abstract Different rice husk samples and their components ( cellulose, hemicellulose and lignin) were investigated with emphasison the influence of potassium on their pyrolysis behaviors by using thermogravimetric(TG)analysis. The results indicate that themaximum weight loss rate of cellulose decreases with the addition of KCl. However, no significant differences are observed for thepyrolysis behavior of hemicellulose and lignin. The TG/DTG curve of a model rice husk( a mixture of cellulose, hemicellulose andlignin)could be obtained by superposition of that for each component. However, during pyrolysis the raw stable structure of basiccomponents in the rice husk results in a change from a sharp peak for the model rice husk to a shoulder peak for the aw rice huskpretreated with HCl to remove K and the other mineral matters)at around 300 C. In addition the effect of KCl addition onpyrolysis of the Aw rice husks was also studied. The results show that potassium has a remarkable catalytic effect on pyrolysis of therice husk samples. The pyrolysis characteristics vary depending on the addition methods of KCl. While char yields decrease with theaddition of KCl using mechanical method except for the cellulose ) the char yield and the maximum weight loss rate ofimpregnated Aw rice husk increase gradually with the increase of KCl contKeywords: TG; rice husk; mechanical mixing; impregnation; KCICLC number. TK6Document code. ABiomasses are commonly accepted as one of the with product char, or dispersed on char surface.sustainable and renewable energy resources. They Knudsen et all7 reported that most K was releasedcould be converted to gas, liquid and solid products from the original binding sites when the pyrolysisthrough various thermo-chemical technologies such as temperature of straw was above 700 C. Some of thegasification and pyrolysis. The commercial potassium was transformed from the original bindingutilization of biomass energy is an alternative to fossil sites to gas phase, and the remaining presented inenergy to certain extent. In general, biomasses vary product char in the forms of KCI and K, CO,n its composition, depending on the species. The potassium silicates, or being bonded to organicmain components of biomass are cellulose(40%manx50%), hemicellulose (15 %-20%), lignin(20%Inherent K in biomass has catalytic effect on its30%),and small amount of mineral matters, such thermo-chemical conversion. Additionally, extermallyas K, Na, Ca and Mg, based on the dry base[3).Thedded K might also act as catalyst during biomassproduct yield and distributions are affected by the pyrolysis. The intrinsic k could restrain the formationpresence of mineral matters, especially K, during of volatile, and lower the initial pyrolysis temperaturebiomass pyrolysis(4,5)here are various forms of Kand the weight loss rate[8-101Nowakowski et alexistence during biomass growth process, and this reported that K could reduce the average apparentcomplicates its effect on the thermo-chemical activation energy for willow coppice pyrolysis by upconversion of biomassto 50 k/mol, and enhance the yields of char andK and other metallic elements exist in biomass in CH,. In the case of K in a synthetic biomass samplevarious forms, including ion state, interacting with the formation of levoglucosan was promoted duringorganic matrix, and mineral particles 6) Water- pyrolysis of cellulose. It is believed that external orsoluble K mainly presents in biomass in ion bonded added K promoted the formation of low molecularstate and minerals, while the K on the organic weight products and char during pyrolysis of poplaroxygen-containing functional groups is not soluble in wood, and the pyrolysis peak was shifted towards thewater.Moreover, the form of K may change during lower temperature rangeHowever, addition ofpyrolysis of biomass, and k on the original binding excessive K2CO, could promote pyrolysis of peanutsites of biomass may be deposited as fine potassium shell at higher temperature, and hinder the volatilealt( mainly KCl and K 2 CO,)particles, or mixed release[ 12中国煤化工inherent andReceived date: 2014-09-15: Received in revised form: 2014-12-03CNMHGFoundation item: Supported by the National Natural Science Foundation of China(21176191)CorrespondingauthorWangChang,Tel:+862260601433;e-mail;wangc88@163.com本文的英文电子版由Elsevier出版社在ScienceDirect上出版(hp:/w.sciencedirect.com/science/joumal/18725813)第1HAo Qing-lan et al: Effect ofium on pyrolysis of rice husk and its componentsadded K has not been understood well. And their impregnation method were also explored. The resultseffects on the pyrolysis of biomass and its three will provide a better understanding of the effect of Kcomponents( cellulose, hemicellulose and lignin) on the biomass pyrolysis for desired productsneed to be further studiedCellulose, hemicellulose, lignin, rice husk1 Experimentalmodel rice husk(mixture of cellulose, hemicellulose 1.1 Biomass samplesand lignin), and Aw rice husk( pretreated with HCIThe rice husk sample, used as the rawto remove K and the other mineral matters)were materials, was ground and sieved. Thefractionselected as the samples in this study. The effect of K 0. 12-0. 15 mm(120- 100 mesh was dried aton the pyrolysis characteristics of the samples were 105 C for 4 h before use. The ultimate, proximateinvestigated by thermogravimetric (TG)analysis. and chemical composition analyses of rice husk areThe addition of KCl by mechanical mixing and listed in Table I and Table 2, respectivelyTable 1 Ultimate and proximate analyses of rice huskUltimate analysis w/%Proximate analysis w/%SampleCHNA39.825.1015.8by differenceTable 2 Chemical composition analysis of rice huskSamplehemicelluloseextractives40.2.317.2dry ash free basisThe commercial cellulose. hemicellulose and 105 C in air for 24 h before uselignin( alkaline)were dried at 105 C for 4 h before 1.4 Thermogravimetric(TG )teststhe experiment. Xylan obtained from bagasse wasTG analysis was carried out using a Q50chosen as the substitute for hemicellulose. The model instrument( TA Instrument, USA ). Typically,arice husk with a certain amount of basic biomass 10 mg sample was treated in 60 mL/min N2 forcomponents: cellulose ( 40. 2%), hemicellulose 10 min, then heated from room temperature to 900 C(24. 3%),lignin (18. 1%)was obtained by at 20 K/ minmechanical mixing for 12 h. The sample was then 1.5 Analysis methoddried at 105C for 24 h before useThe content of K in different biomass sample1.2 Biomass pretreatmentwas measured by atom absorption spectrometryPortion of the rice husk sample was firstly treated (AAS)using WFX-120 Elemental analyzer( Beijingwith 7% HCI( mass ratio of biomass to HCl solution Beifen-Ruili analytical instrument, China).0.2 gwas 1: 50) for 2 h and filtered, while magnetic stiming biomass sample was wet-digested with 10 mL HNOwas kept. The filter cake, treated rice husk using ( A.R)and 4 mL H, 02(A. R)beforeHCl, was washed with ultrapure water to ensure the determinationremoval of trace HCL. Finally, it was dried at 105C 1.6 Scanning electron microscopy(SEM)for 24 h before use and labeled as aw rice huskThe morphology of the biomass samples was1. 3 Addition of Kclobtained using an SU-1510( Hitachi, Japan) scanning1. 3.1 Mechanical mixing of biomass samples electron microscopy SEM). The samples wereKCl was added to the tested samples using coated with gold for 80 s in 20 s intervals before themechanically mixing method for 12 h. The mass ratio measurements to avoid charging problemsof k to the samples was 1%, 3% and 5%0respectively. Then the mixed samples were dried at 2 Results and discussion105℃for24 h before use2.1 TG studies of three components of1. 3.2 Impregnation of Aw rice huskbiomassAbout 3 g Aw rice husk was immersed inDuring"V凵中国煤化工 allic elements150 mL of KCl solution of 1%, 5% and 10%, K, Na mayrespectively. The magnetic stirring was maintained for NaC/13] ToCNMHGas KCl and10 h during the impregnation processing. The pyrolysis of biomass components, different contents ofimpregnated samples were then filtrated, and dried at KCl( the mass ratio of KCl, measured by K element)燃料化学学报第43卷at 1%, 3% and 5%, respectively, was mixed with pyrolysis was monitored. The maximum rate ofthe tested cellulose, hemicellulose and lignin weight loss decreased with the increase of the amountmechanically. The effect of KCl on TG performance of KCl, which decreased from 2. 36%/C for the rawof cellulose, hemicelluloses, and lignin are shown in cellulose to 1. 45%/C. However, the correspondingFigure 1temperature at the maximum weight loss rate increasedto the maximum (370 C) when the addition of KClamount was 3%, and then declined to 367 C whenKCl content reached to 5%. At low temperature(25960312 C), the cellulose could be transformed toactive cellulose. With temperature increasing, theactive cellulose was converted to non-condensablegas, liquid tar and solid char 4. The added KCl inthe cellulose inhibited formation of the activecellulose, and this is adverse for the pyrolysis ofcellulose. At the meantime the heat and mass transferDo 200 300 400 500 600 700 during the cellulose pyrolysis were suppressed with theTemperature I/Cddition of KclFigure 1(b) indicates the performances of thehemicellulose+1%KClhemicellulose pyrolysis were similar to those of thehemicellulose + 5%cellulose. However, the weight loss was almostcomplete up to 550C, and reached to 82.0%.Themaximum rate of weight loss for hemicellulose wasonly0.73%/℃at296℃, which were obviouslylower than that of the cellulose. These results could beattributed to the different structures between cellulosed hemicellulose. Richness in branched chainstructure in hemicellulose could be responsible for200 300 400 500 600 700 these results. The more branched chain structure wasTemperature I/℃the worse thermal stability was for the hemicellulosewhich is prone to decompose into small molecules atthe lower temperatureComparing the behavior of celrolysIsbefore and after the addition of Kclre were nogin+3% KCIevident differences during the hemicellulose pyrolysiswhen different content of KCl was added. Only aslow decreasing trend of the maximum weight loss rateFigure 1 Effect of KCl on TG/DTG curves of cellulose(a), mass and heat transfer(is]esq(from0.74%/℃for1%- KCI sample to0.6%/℃三0.10for 5%-KCI sample) for the hemicellulose wasobserved. The addition of Kcl makes the branched0100200300400500600700800chain structure break easier during the pyrolysis ofhemicellulose but too mucKCl might inhibit thehemicellulose(b)and lignin(c)Figure 1(c)shows the TG curves of lignin. TheAs shown in Figure 1(a), the removal of surfacetemperature of the lignin pyrolysis experienced abound water resulted in the weight loss of celluloseweight loss range from 105 to 800 C.Thebefore 105 C. When the temperature increased fromresults of lignin showed that there were two pyrolysis200 to 400 C, the release of volatiles from the peaks at about 335 and 671 C. The weight loss atellulose almost completed with a weight loss of about 335TV凵中国煤化工 vage of aliphaticapproximately 89. 6%. The maximum weight loss rateCNMHGproducingfor cellulose was 2. 36%/C at 362 Cwater andThe influence of KCl addition on cellulose 671 C, the ether bond in the lignin structure is likelygenerating various phenolic第1期HAo Qing-lan et al Effect of potassium on pyrolysis of rice husk and its componentscompounds16.17during pyrolysis of the model rice husk.However, no obvious changes on the maximum 2.2.2 TG studies of Aw rice huskweight loss rate and its corresponding temperatureK plays a critical role in pyrolysis characteristicswere observed when the different amount of KCl was of biomass samples, but there is no K in the modeladded to the lignin samples. During the lignin rice husk because of the high purity of the threepyrolysis,a large number of benzene ring could components. In order to investigate the effect of K ongenerate much free radicals containing benzene 8, pyrolysis of rice husk, K was removed bywhich was further converted to gas, liquid tar and demineralization pretreatment. The rice husk was firstsolid char. The addition of KCl has little influence on treated by 7% HCl for 2 h before TG test. Thethe free radicals generationcontent of K in rice husk was 0. 7669%, and that in2.2 TG studies of model rice huskthe aw rice husk was 0.002 3%. This means 99% of2.2.1 TG analyses of model rice huskK could be washed awayThe model rice husk was made from theThe TG/dTG curves of Aw rice husk and modelmechanical mixture of cellulose, hemicellulose and rice husk are shown in Figure 3. Although inorganiclignin, based on the components of raw rice husk. components, especially Ked. the TG/The TG/DtG theoretical curves of model rice husk dtg curves of the model rice husk were still obtainedcould be obtained from the fitness of the individual by superposition of those of cellulose, hemicellulosecomponents. Figure 2 shows the theoretical and and lignin. The TG curve of model rice husk deviatedexperimental TG/DTG curves of the model rice husk. from that of Aw rice husk below 350 C. A sharpThe theoretical weight loss was lower than that of the peak can be observed at about 300 C in DTG curve ofexperimental values in the whole pyrolysis process, model rice husk. On the contrary, the pyrolysisespecially above 400 C. At 600C, the residue yields behavior of Aw rice husk was complicated and afrom theoretical calculation and experiment were shoulder peak appears at about 300 C. The strong23. 26%and 27. 37%, respectively. The crosslinking chemical bonds among cellulose, hemicellulose andinteraction between lignin and cellulose contributes to lignin in Aw rice husk, for example, hemicellulosethe more residue yield of the model rice husk. On links with cellulose by hydrogen bond and Van derthe other hand, the high residue yield was also Waals force, hemicellulose and lignin form the ligninresulted from reduction of the gas productscarbohydrate complexes by the chemical bondsmay contribute to the complication of the behavior fortheoreticalAw rice husk100Aw rice husk- model rice husk2018000100200300400500600700800900I pyrolysis results of model rice huskTemperature I/℃There are almost no differences for theFigure 3 TG/DTG curves oftemperatures of two pyrolysis peaks. However, theAW rice husk and model rice huskrice husk pretreatment conditions: washed by 7%hemicellulose pyrolysis could be promoted by theHCl for 2 h at room temperatureincrease of experimental pyrolysis2.3 Effect of KCl addition by mechanical300 C. On the contrary, the pyrolysis peak valuemIxIng中国煤化工367C decreased from 0. 96%/C( theoretical )to 2.3.1 EifelCNMHGel rice husk0. 90%/C(experimental), which might be attributedThe testesnIce lusk was mechanicallyto the increase of resistance for heat mass transfer mixed with KCl at mass ratio of a K to samples 1%3%and 5%,respectively. The38燃料化学学报第43卷addition on TG of model rice husk are shown in 369 C. The enlarged pore size and the increased poreFigure 4quantities in the Aw rice husk may explain themaximum weight loss rate of 0.89%/C.Therice huskincreased temperature might be attributed to therice husk+1% KO0000rice husk +3% KCIcatalytic activity of the minerals, especially K, duringrice husk +5% KCIthe biomass pyrolysis 2JAw rice huskAW rice husk+1%KClAw rice husk+3% KCIAW rice husk+5%KCl00010000405006000000Figure 4 Effect of KCI addition on画02TG/DTG curves of model rice huskKCl addition had a significant effect on pyrolysis0100200300400500600700800Temperature I/℃of the model rice husk. The sharp peak graduallyFigure 5 Effect of KCIchanged to the shoulder one at 300 C with theddition on TG/DTG curves of Aw rice huskincrease of K content. The weight loss rate, due tothe hemicellulose pyrolysis in the tested samplesFor the aw rice husk samples added with KCl, aclimbed from 0.28%/c to 0.34%/C shoulder and a sharp peak were observed at 300 andsimultaneously with the increasing of KCI content 369 C, respectively. However, the pyrolysisfrom 1% to 5%. On the other hand, the pyrolysistmperature of the sharp peak descended from 369 topeak at about 367 c gradually shifted to low 348 t with the increase of KCI content,and thetemperature, the maximum weight loss rate due to the corresponding maximum weight loss rate decreasedcellulose pyrolysis in the samples reducedfrom0.89%/℃to0.75%/℃.Thes0.90%/℃to0.57%/℃ with Kcl content from1%demonstrated that the effect of KCl on Aw rice huskto 5%0was almost the same as that of model rice huskSimilar to the pyrolysis properties of the model 2.4 Effect of KCl addition via impregnationrice husk, the pyrolysis of hemicellulose and ligninThe effect of KCI addition by impregnation onalso resulted in the two pyrolysis peaks of model rice pyrolysis of the Aw rice husk was studied. The Awhusk at 300 and 367 C when different amount KCl nce husk was immersed in KCl solution with a Kwas added. From the discussion in section 2.1.when concentration of 1%,5% and 10%,respectivelythe K content was more than 3%, the pyrolysis peakThe K contents of the impregnated Aw rice huskof cellulose markedly shifted to low temperature. This samples were 0. 107 2%, 0.401 8%and.5492%should explain the similar trends during pyrolysis of respectively. And the samples were labeled as KClthe model rice husk. Although different amount of lI, KCI-2I and KC1-31, separately. The pyrolysisKCl was added into the model rice husk, the pyrolysisesults were shown in Figure 6, including the TG/resperformances still depended on the superposition of DTG curves of the Aw rice husk(0.023%)cellulose, hemicellulose and lignin pyrolysisThe temperature at the maximum weight loss ratepropertiesdecreased from 369 to 342 C for the aw rice husk2.3.2 Effect of KCl on TG of Aw rice huskand KCl-31, respectively. However, the oppositeThe effect of KCl addition on TG of Aw rice trends for the maximum weight loss rate washusk are shown in Figure 5. The pyrolysis propertiesobserved, which accordingly increased fromof raw rice husk with K content of 0. 7669% were 0.89%/C to 0. 95%/C. Compared with the Awdifferent from those of the model rice husk( Figurerice husk. the K contentin the impregnated samples2). The weight loss was observed at about 200 C. a increased ma中国煤化工 bvious catalyticnoticeable shoulder peak in the DTG curve appeared at effect on thCNMHG Based on theabout 300 C. The maximum of weight loss ratediscussion in secuon 2.3.2, sImular catalytic effect ofincreased from0.84%/℃at339℃to0.89%/℃K element, no matter the occurrence of inherent K第1期HAO Qing-lan et al: Eect of potassium on pyrolysis of rice husk and its componentsand added K, on the pyrolysis of the different biomass husk samples after mechanical mixing treatmentsamples was observedsection 2. 3. 2), the maximum weight loss rate ofthe impregnated samples increased with the increase of=豆AW rice huskKCI content. During mechanical mixing, KCI wasadhered to the surface of the biomass particlesNevertheless, it is possible that the KCI also entersinto the internal pores of the biomass particles throughthe diffusion during the impregnation process. On the0.8other hand, compared with the mechanical mixingmethod, the contact area between biomass and Kclduring impregnation was larger. Therefore, the addedKCl in the Aw rice husk via impregnation had more200300400500600catalytic effect on the pyrolysis of Aw rice husk℃Although the K content in raw rice husk was thFigure 6 Effect of KCl ( impregnation)highest(0. 766 9%)on TG/DTG curves of aw rice huswas the lowest. This is because the pore size of theimpregnated samples was augmented to a certain extentHowever, different from pyrolysis of the rice during the impregnation process( Figure7),whichfacilitated the mass and heat transfer during pyrolysism×200kFigure 7 SEM micrographs of rice husk(a)and KC1-3I sample(b)Generally the main elemental constituents of rice 2.5 Effect of KCl on char yieldshusk minerals are Ca, Fe, Mg, Na and K. Among 2.5.1 Addition KCI by mechanical mixingthem K content is the highest in the rice husk, whichThe char yields of rice husk are relevant tois more than twice of that of the others. The influence pyrolysis temperature. Above 600 C the weight loss wasof the mineral matter on the rice husk pyrolysis very slow, and its variation could be ignored. Theharacteristics should be attributed to the role of residue amount above 600 c was regard as the charpotassium 2. The inherent and added K is beneficial yield, which have eliminated the influence of additivefor pyrolysis of the rice husk, which varied depending KCl. The effect of KCl addition by mechanical mixingn the addition methods of kclon the pyrolysis char yields are shown in Table 3Table 3 Effect of addition KCl on char yields of biomass pyrolysisChar yields w/%No. K addition w/%hemicellulosenodel rice husk Aw rice husk59.727.410.958.827.124.08.023.059.0YHS中国煤化工2.1CNMHG230eliminate the influence of addition KCl on char yieldThe char yields of cellulose, hemicellulose and lignin were 2. 4%, 24. 49 and 59. 7%, respectively燃料化学学报第43卷In the presence of KCl, the cellulose pyrolysisThe char yields gradually increased from 24.9%inclined to produce more char, and the formation of to 27. 6% with the increase of K addition. Thechar from the hemicellulose and lignin pyrolysis was reduction of the volatile during pyrolysis of therestricted. The addition of alkali metal, especially K, impregnated sample was beneficial for the charwas beneficial to char production during the biomass formation. These results also verified the catalyticpyrolysis[13. 21. The total content of hemicellulose and effect on solid production during the biomass pyrolysislignin was higher than that of cellulose, which in the presence of KClinclined to produce solid product in the present of KClin the model rice husk with lower char yield at higher 3 ConclusionsKCI contentThe pyrolysis behaviors of different biomassThe pyrolysis char yields of model rice husk and samples were studied using TG technology. The roleAW rice husk decreased with the increase of kcl of K was also explored. The pyrolysis of cellulose andcontent, but the former was slightly higher than the hemicellulose were restricted by the mechanicallylater. The low char yield during pyrolysis of Aw rice mixed KCl. However, KCl exhibited little influencehusk should be attributed to the original stable on lignin pyrolysis. The pyrolysis of model rice huskstructure of three components in the rice husk. These could be characterized as the simple superposition ofresults are verified in section 2. 2. 2those of cellulose, hemicellulose and lignin. Acid2.5.2 Addition KCI via impregnationwashing pretreatment could basically wash away allThe effect of KCl addition via impregnation on the metallic elements especially K) in rice huskthe char yields of Aw rice husk pyrolysis were shown The marked differences of pyrolysis behavior betweenin Table 4Aw rice husk and model rice husk were observed dueTable 4 Effect of addition KCl viato their different structures The different additionimpregnation on char yields of biomass pyrolysismethods of Kcl caused the differences of contactAmount of K inmanner and contact area between Kcl and the ricesample w/%Char yield·w/%husk, which resulted in the higher char yield of theAw rice husk0.0023Aw rice husk with KCI addition via impregnationKCI-lI0.107224.9The detailed product distributions of the rice huskKCI-2I0.401826.5pyrolysis due to the occurrence of inherent K andKCI-3I0.549227.6added K in the biomass should be further studiedeliminate the influence of addition KCl on char yieldReferences[1] SHUTTLEWORTH P, BUDARIN V, GRONNOW M, CLARK J H, LUQUE R. 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Thermochim Acta, 2006, 444(1):110-114钾元素对生物质及其三组分热解的影响郝庆兰,李博仑1,刘垒,张征标,豆宝娟2,王昶2(1.天津科技大学材料科学与化学工程学院,天津300457;2.天津科技大学海洋科学与工程学院,天津300457)摘要:采用机械混合法将KC1加入到纤维素、半纤维素、木质素以及稻壳和稻壳模拟物等生物质中,得到了一系列不同K含量的生物质样品,通过热重(TG)实验考察了K元素对生物质热解特性的影响。结果表明,K元素对生物质三组分热解特性的影响比较复杂纤维素的最大热解失重速率随着KC1添加量的增加而降低,但KCl对半纤维素和木质素热解特性的影响不显著。无论是否添加KCl,模拟生物质的热解特性均可以认为是三组分热解的简单叠加。但酸预处理稻壳三组分间的稳定结构,导致其DTG曲线在300℃左右的热解峰由稻壳模拟物的尖峰变为肩峰,其热解焦炭收率也比稻壳模拟物的略低。此外,实验还采用浸渍法向酸预处理稻壳中添加了KC。TG实验结果表明K元素的存在对生物质热解具有一定的催化作用,但KCl的添加方式不同,生物质的热解特性有明显差别,生物质样品经机械混合添加KC后,其热解焦炭收率呈下降趋势(纤维素除外),浸渍法添加的KC1导致酸预处理稻壳的最大热解失重速率和焦炭收率升高。关键词:热重;稻壳;机械混合;浸渍;KCl中图分类号:TK6文献标识码:A中国煤化工CNMHG