Upgrading of Yi'an gas coal by low temperature pyrolysis under different atmospheres

Mining Science and Technology( China)21(2011)401-405Contents lists available at Science Direct氵Mining Science and Technology( china)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcUpgrading of Yi'an gas coal by low temperature pyrolysis underdifferent atmospheresWei Xiaomei, Zhou Min Zhang Chun, Lei Jiali a, Song liqiangLaboratory of Coal Processing 8 Effcient Utilization, School of Chemical Engineering G Technology, China University of Mining 6 Technology, Xuzhou 221008, chinaitute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, ChinaARTICLE INFOA BSTRACTThe quality of Yi'an gas coal before and after low temperature upgrading under either a n2 or H2 atmo-Received 15 October 2010Received in revised form 13 November 2010sphere was examined by thermogravimetric and infrared analyses the effect of upgrading on the pre-Accepted 10 December 2010pared coke quality was analyzed. The results show that the carboxyl and phenolic hydroxyls in theAvailable online 12 June 2011coal molecular structure are removed after upgrading by low temperature pyrolysis under either n2 orH2 atmospheres. This improves coal caking properties to a certain extent. the upgrading effect under aH2 atmosphere is remarkably better than the effect observed after upgrading under Nz. Compared to cokeobtained from raw coal, the compressive- and micro-strength of the cokes obtained from upgraded coalare greatly improved. The effect on coke reactivity with CO2 is not significant. the best upgrading tem-Upgradingperature for Yi'an gas coal under either a N2 or H2 atmosphere is 250 or 275C respectively.Coke qualitye 2011 Published by Elsevier B V on behalf of China University of Mining technology1 Introductionstudy. the anthracite was used as an inert component to estimatethe coke quality by the carbonization test. All these samples wereThe modification and upgrading of coal by low temperature air-dried prior to use: their properties are listed in Table 1pyrolysis is of great practical importance because it creates newopportunities for enlarging coal resources usable for coking. The 2.2. Coal upgrading at different temperaturestechnology functions by controlling the carbonization process toobtain coke having desirable properties. during the modificationYi'an gas coal was pyrolyzed at different temperatures, all lowerand upgrading process most oxygen-containing groups of the coal than the softening temperature. The chosen upgrading tempera-are removed, which improves the coking and caking property tures are listed in Table 2. In this test the electric tube furnace[1-3]. The authors have studied the modification of thermoplastic was heated to the assigned temperature and then N2, or Hz, wereproperties of coals, and the coke qualityobtained, by pyrolysisIned intoan inert atmosphere 4-7.the tube at a rate of 0.5 L/min. After that a 25 g samplin a steel vessel was quickly pushed into the constant temperatureBlending some gas coals for a conventional coking oven is zone of the tube and a rubber stopper was tightly screwed onto therestricted by the high volatiles and weak caking properties of these tube portal. the coal samples were cooled to room temperature.coals. In this paper we discuss the coal quality of Yi'an gas coals be- under an inert atmosphere, after being pyrolyzed for a fixed time.fore and after upgrading under n2 or Hz atmospheres. Thermogravi- They were then crushed mechanically to-0.2 mm and the remainsmetric and infrared analyses are used to analyze the coals. The effect were devoted to the carbonization test. Table 2 also lists the corre-on the individual indexes of coke quality after the upgrading isexamined by a carbonization test.sponding coke names for the samples obtained from upgradingsamples by carbonization.The upgrading time was determined by the change in oxygen2. Experimentalcontaining functional groups. Samples were pyrolyzed for differenttimes at a heat treatment temperature of 250C. chemical2.1. Coal samplesmethods were used to measure the oxygen containing functionalgroups [8 The total acidic and carboxylic content was, respecYi'an Gas Coal from the Huanyu Coking Plant, Co, Ltd, located tively, measured by the exchange method using barium ion andXuzhou, and anthracite from Taixi in Ningxia were used for this calcium acetate the difference between these was taken as thecontent of phenolic hydroxyl. Fig. 1 shows the results.Corresponding author. Tel. +86 13852138866Asin IE-mailaddress:zm@cumt.edu.cn(M.Zhou).ylic oxygeen co中国煤化工 y under either a1674-5264/s- see front matter o 2011 Published by Elsevier B V on behalf of China University of MiningCNMHGdoi:10.1016 mstc201105027lef et aL/ Mining Science and Technology( China)21(2011)and ultimate analyses of the coals(AYi'an gas coal3731626984.195.59826131065Taixi anthracite1.69725006Table 2Upgrading temperatures and corresponding coke namesUpgrading temperature(oc)Raw coal275Y200Y225YQ275YQ-300Q-350K200JK-275nitrogen or hydrogen atmosphere. After 20 min the loss in oxygen with the anthracite in the proportion of 1: 1 and then charged to abecame relatively slower so the upgrading time chosen for the fol- steel cup(height 70 mm and 57 x 4 mm) and carbonized bylowing work was 20 min.heating at 3C min from 200 C up to 900C. Several 5 mm diameter holes were punched in the bottom of the cup so that gas re2,3 Thermogravimetric analysisleased during carbonization could pass freely from the sampleThe cokes obtained were allowed to cool naturally and stored forhermogravimetric analysis was carried out using a STA409c further use.thermogravimetric analyzer Samples weighing 10*0. 1 mg wereheated to 9ooC at a 3C/min rate under a nitrogen flow of 2.6. Compressive -and micro-strength measurements of the coke100 mL/minThe cokes were analyzed with a TYE-20 compression testing2. 4. FTIR spectroscopymachine to determine the compressive strength index. then thecokes were broken and 5 g of coke 3-6 mm in size were put into%L All Fourier Transform Infrared(FTiR)experiments were con- a steel vessel (diameter 64 mm and height 69 mm)with five steelcted using a Nicolet 380 spectrometer. The dried samples were balls( diameter 28 mm ). the weight percentage of particles greatermilled with potassium bromide in the proportion of 1: 200(by than 0.2 mm, compared to the total mass, after 800 rotations in theweight)to enable quantitative analyses Each sample was scanned drum over 3 min was taken as the micro strength index, MSp2.The32 times and the scans averaged to give the final spectrumdata reported are the average of two identical tests2.5. Carbonization tests2.7. Coke reactivity measurementsThe 50g co-carbonization tests of Yi'an gas coal and taixiThe cokes were obtained from the carbonization tests. aboutanthracite were carried out in an electric resistance furnace20 g of a 3-6 mm particle size coke were placed in a reaction tubecause of the high volatility of the Yi'an coal samples, the anthracite and heated from room temperature to 800 C at a rate of 20-25Clwas used to provide an inert component during carbonization. This min and then kept at 800C for 5 min Then carbon dioxide gas wascaused the fluidity and swelling of the resulting plastic mass to de- introduced into the reaction tube for 2.5 min at a flow rate ofcrease Samples before and after upgrading were mixed uniformly500 mL/min. Then the effluent gas was collected with a gas ana-lyzer at 1 min intervals and the COz flow was stopped. This processwas repeated at increasingly higher temperatures every 50C untilUnder N2 atmosphere→Othe temperature of 1100C was reached. the data obtained were1.4N Under H2 atthe average value of two duplicate tests3. Results and discussion3. 1. Coal quality before and after upgradingThe changes in coal quality after upgrading under a n or Hatmosphere are shown in Tables 3 and 4. As the upgrading temperature increases, the carbon content rises and the content of hydro-gen, nitrogen, and sulfur changes irregularly. In addition, theoxygen content gradually drops, which agreed with changes in0.0the oxygen/carbon ratio that revealed the distribution and varia-tion of oxygen. This proved that the objective of de-oxidationUpgrading time(min)upgrading中国煤化工 e upgraded coal alsodecreaseThe cakingCN MH Gent under a n,atmoFig- 1. Effect of upgrading time on OcooH and Oph-oH of coals.sphere when the upgrading temperature was lower than 275CX wei et al/ Mining Science and Technology( China) 21(2011)401-405Tabke 3Coal quality after treatment under a Nz atmosphere at different upgrading temperatures.Ultimate analysis(‰)3731626984.198.2631YQ2001.30359584645521371.2084.705.5113705635.1964817.691.3605984.90.789622209485.12YQ-350853257310086Table 4Coal quality after treatment under a Hz atmosphere at different upgrading temperaturesProximate analysis(%)Ultimate analysis (%)haaf149373162695984.195.598.261310.6500987.6235.39YQ2251.157.1735.186482YQ-25035.16648484.90YQ275652685.04YQ32534.20YQ-3500917.4233.5966416185415.397130.57The highest enhancement was 10.17% when a H2 atmosphere was sistent with the study of Zheng [14]. the temperature range fromused improvements in the caking index of different degrees hap- 300 to 450C corresponds with the plastic phase of the coal, whosepened for different upgrading temperatures. this was especially mass and quality had an important effect on the coal caking prop-true when the upgrading temperature was below 300C. The max- erty. this further indicates that some change occurred in theimum increase was 16.95%. This shows that high upgrading tem- molecular structure of the upgraded coal. These changes causedperatures are unsuitable. the caking property of the coking coal variations in the pyrolysis process along with the system fluiditymproved because of the removing of the side chain oxygen- and thermoplastic propertiescontaining groups and some low molecular compounds with highchemical activity during pyrolysis process, which made the oxygen 3.3. Infrared spectral analysisand hydrogen react difficultly and give more active hydrogen free-radical to form plastic mass [9-11 However, more hydrogenousaliphatic side chains would release along with deoxidation with Fig. 2. The absorption peaks of hydroxyl, ycloalkane, or aliphaticthe upgrading temperature increasing, so the caking property hydrocarbons and oxygen-containing functional groups are locatedat3000-3600cm-1,2700-3000cm-,and1000-1800cm-1At the upgrading temperature of 250C, under a Nz atmosphere, respectively [15]. The infrared spectrum of the upgraded coal wasthe carbon content and caking index increased by 0.72% and 5.08 very similar to that of the raw coal and only the absorption intensitywhile the oxygen/carbon ratio and the oxygen content decreased of characteristic absorption peaks differ. note especially that theby 6.12% and 5.69%. However, under a H2 atmosphere in this exam- absorption peak of oxygen groups is weakened by upgrading. Theseple, the carbon content and caking index increased by 0.84% and results reveal that some of the oxygen groups are removed by15.25%, the oxygen/carbon ratio and the oxygen content decreased upgrading but the main structure of the samples remain unchangedby 9.18% and 8.84%. Hence, the effect on sample quality under a H2atmosphere was clearly better than under a n2 atmosphere at thesame upgrading temperature. It is important to point out that H2 3.4. Compressive strength of the cokessupplies a reducing atmosphere and also takes part in the reactionas a hydrogen donor This favors de-oxidation upgrading and playsThe compressive strength of the cokes obtained from the coalsa positive effect on upgrading.after upgrading treatment at different pyrolysis temperatures un-der either N2 or H2 are shown in Fig 3. Compared to coke obtainedfrom raw coal, the compressive strength of upgrading coals is3.2. Thermogravimetric studymarkedly improved over a certain temperature range. Note thathe compressive strength first increases but then declines as theTable 5 shows weight loss data obtained from thermogravimet- modification temperature is increased. Compressive strength dete-ric analysis that reflects the changes in coal pyrolysis over various riorates for upgrading temperatures higher than 300C.temperature ranges. Coals begin to produce free radicals at 300CWhen the upgrading atmosphere was N2, the compressiveand form a semi-coke through polycondensation between 550 andtrength reached a maximum at an upgrading temperature of600C. Therefore, the data in Table 5 are tabulated from 300 to 250C where it improved by 21.9%. Compressive strength was en600hanced by 51.7%中国煤化工275 C under a h2As shown in Table 5. compared to raw coal, the cumulative atmosphere. Thesopressive strengthweight loss of upgraded coals is significantly increased above under a H2 atmoCNMHGxtent than under300oC under both N2 and H2 atmospheres. these results are con- a N2 atmosphere. the reason for this is the hydrogen donor abilityX Wei et aL/Mining Science and Technoiogy(China)21(2011)401-405Percentage weight loss during pyrolysis of samples before and after upgrading.Under Nz atmosphereUnder H2 atmosphere(oC) Weight CumulativeYQ-250YQ-300YQ250Y300Weight Cumulativeweight lossweight loss3001051.621.793.593.212.173.861.984504076.99766108773611.2272311.00500721138915.42503.417.367.8518.2560025120873.1121.363.08216120.85N2 or H2 atmospheres are shown in Fig 4. Note that cokes preparedfrom samples with a low upgrading temperature vary in their microstrength depending upon the different atmosphere used Under a N20.040atmosphere, the Msr value of the cokes was enhanced 10.9% whenthe upgrading temperature was below 250C. This value then sharply declined as the upgrading temperature was raised. In addition,0.030under an H2 atmosphere, the Msrfirst increases but then decreasesas the upgrading temperature was increased. a maximum value oc-0020YQ250.N2curs at the upgrading temperature of 275C where a 12.6% increasecompared to coke from raw coal, was observed At higher tempera-YQ250H2y、tures the Msr drastically declines. This change is similar to the0.010changes in compressive strength of the coke made from upgradedgrade temperature should be moderate e outstandingly but the up-coal The latter effect was improve3300Wave number(cm)3.6. An analysis of coke reactivityFlg. 2. Infrared spectra of the raw and upgraded coalsSome changes also occurred to the Coz reactivity of the cokesobtained from raw coal and upgraded samples, as shown inFig. 5. Fig. 5 shows the co z reactivity of cokes prepared from Yi'angas coal under different upgrade temperatures. the reducing rate--umdar H, atmosphedecreased slightly before the reactive temperature of 950C afterwhich it insignificantly increased inversely as the reactive temperature was increased. Note that the effect of upgrading temperatureon coke reactivity was slight and irregular and was weaker forsamples prepared under a H2 atmosphere than for those preparedThe activation energy at certain temperatures, and the cokepore structure, are the primary factors affecting coke reactivity闭40[16]. At lower reactive temperatures, the activation energy dominates coke reactivity. However, transfer and diffusion between-under N, atmospher050100150200250300350Flg 3. Effect of upgrading on compressive strength.乱of the Hz atmosphere. This caused upgraded samples to have in-creased plastics mass and also improved plastic quality whilebeing carbonized. The gas pores are filled by colloid then, so theshrinkage of the semi-coke decreased and the porosity was im- 245proved, which enhanced the strength of the coke.3.5. Micro strength analysis of the cokes中国煤化工30030The micro strength indexes of the cokes obtained from differentCNMHGupgraded samples at different pyrolysis temperatures under eitherFig 4 Effect of upgrading on micro strength.Wei et aL/ Mining Science and Technology(China) 21 (2011)401-405890000000JK-200JK-250JK-300800850900950100010501100800850900950100010501100Reactive temperature (C)Reactive temperature(C)(a)Upgraded under a N2 atmosphere(b)Upgraded under a H, atmosphereig. 5. Effect of upgrading on coke COz reactivity.the gas and the solid play an important role in the process when Referencesreaction occurs at higher temperatures. The measurements suggestthat upgrading by low temperature pyrolysis pushed the activation [] Dai ZS, Liang YH, Zhen YH. 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Research on coal structure using FTIR J China Univ Min(4)The best upgrading temperature for Yi'an gas coal under a nTechnol 2002: 31(5): 362-6[in Chinese).tudy of factorsmicro-strengths are improved by 21.9% and 10.93 (17) Wu l Gan js, Li Q, study on reformatipreheating and blending coal torespectively. the best upgrading temperature under a Hcoking of Dongsheng coal. Coal Convers 1998: 21(4): 86-90 [in Chineseatmosphere is 275C where the coke compressive- andmicro-strength are improved by 51.7% and 12.6%中国煤化工CNMHG