Method of oxygen-enriched two-stage underground coal gasification

Mining Science and Technology( China)21(2011)191-196Contents lists available at Science DirectMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcMethod of oxygen-enriched two-stage underground coal gasificationLiu Hongtao.", Chen Fenga, Pan Xia Yao Kai, Liu Shuqinrory of Coal-Based Low Carbon Energy, ENN Group Co. Ltd, Langfang 065001, ChinaI of Chemical and Emvironmental Engineering China Universiy of Mining E Technology. Bed/ing 100083, ChinaARTICLE INF OABSTRACTTwo-stage underground coal gasificatas studied to improve the caloric value of the syngas and toAugust 2010extend gas production times. A model test using the oxygen-enriched two-stage coal gasification methodwas carried out. The composition of the gas produced, the time ratio of the two stages, and the role of thetemperature fieldthe time of the first stage and prolongs the time of the second stage. Feed oxygen concentrations of 30%35*, 40, 45% 60%, or 80% gave time ratios(first stage to second stage)of 1: 0. 12, 1: 0. 21, 1: 0.51, 1: 0.64.1: 0.90, and 1: 4.0 respectively. Cooling rates of the temperature field after steam injection decreased withUnderground coal gasificationtime from about 19 1-27, C/min to 23-6. Cmin. But this rate increased with increasing oxygenEnriched axygenconcentrations in the first stage. The caloric value of the syngas improves with increased axygenTwo-stage gasificatione first stage. Injection of 80% oxygen-enriched air gave gas with the highest caloricvalue and also gave theroduction time The caloric value of the gas obtained from the oxygen-enriched two-stage asifmethod lies in the range from 5.31 MJ/Nmto 10.54 MJ/Nmt o 2011 china University of Mining Technology. All rights reserved.Suncun and Feicheng Mines in Xinwen where syngas of a muchhigher caloric value was obtained than before[7-9). In the past,Underground Coal Gasification(UCG)is a process of producing tests used the two-stage air UCG method where the time of the firstcombustible gases by controlled coal combustion. It involves both stage, which increases the underground zone gasification temperheating and chemical effects on the coal and combines the ature, is very long and the ratio between the first and second stageprocesses of building coal mines, coal exploitation, and above. times was about 1.8: 1-2. 7: 1(9-18]ground gasification. This is a multi-disciplinary, new technology foThe heating time of the first stage is shortened, and the time ofobtaining energy and chemicals. UCG has the advantages of safety. the second stage that produces gas is extended by using oxygen-low investment, high efficiency, environmental soundness, and enriched air in our two-stage UCG model testswith first suggesting underground coal gasification in 1868, Since to simulate the underground coal seam[11-14]. Fissures betweenthen many outstanding works have been completed by scientists the coal blocks are excessive and allow blow by of gases, andworldwide [1]. Dozens of underground coal gasification stations fluctuations of gas composition, during the tests. This makes itthe Angren UCG station is still running today [2]. Research on Now large scale coal blocks(dimensions 0. 4 m x 0.8 m x 0.8 m)underground coal gasification technology has also been carried out have been used to simulate the underground coal seam Model testsin China but the caloric value of the syngas produced by using of the oxygen-enriched two-stage UCG method have been carriedconventional UCG methods is very low[3-6] Improving the caloric out and variations of timing of the two stages have been explored.value of the syngas was the goal of Yu Liet al who conducted "Long Expansion of the temperature field has also been studied.Tunnel, Large Section, and Two Stages"underground coal gasincation method research. Similar tests were conducted at the xinheNo. 2 Mine in Xuzhou, the Liuzhuang Mine in Tangshan, and the 2. ExpeCorresponding author. TeL: +86 316 2595070E-mailaddressliuliute126.com(lOrdossee front matter Copyright o 2011, China University of Mining TechnologyYH中国煤化工 m the lignite opencastCNMHGdo:lo016/jmsc201102018L Hongtao et aL /Mining Science and Technology(China)21(2011)191-196Table 1Proximate and ultimate analysis of Ordos lignite( by weight 3)Proximate analysisUltimate analysisA Val FCduf Cdaf Har Oaf Nur Saaf100944144575543845116.7101the experiment were 0. 4 m x 0.8 m x 0.8 m. The ultimate(C, H, ON, and S content) and proximate(total moisture, ash, and volatilematter) analysis of the lignite sample gave the results shown inTable 1( Fig. 1).2.2. Experimental reactorThe UCG model test reactor was designed in the shape ofcylinder with the external dimensions 7. 4 m(length)and 3.5 m(diameter)which is shown in Fig. 2. The hearth of the reactor ispoured refractory material and its outer wall is made of a special2 A schematic diagram of the underground coal gasification test system.steel used for pressure vessels. The reactor can work at hightemperatures, up to 1800"C, for long terms. The hearth of the the air is injected. Combustion in the coal seam is initiated, whichreactor is in the shape of a rectangular prism 5.0 m x 1.6 m x 1.6 m. produces heat that is stored in the coal seam. The temperatureFive inlet or outlet pipes, thirty-three temperature and pressurincreases within the gasifier in a stepwise way and forms an idealeasurement ports, and four peep holes for a closed circuit temperature field. In the second stage steam is injected. The reacndustrial television(CCTV)were installed in the reactor. Data is tion between steam and the incandescent coal seam producesmeasured online during the test and collected using a Distributed water gas with a high content of H2 and CO. Because the decom-Control System(DCS)(Honeywell)and saved to a hard disk. Theosition reaction is endothermic, so the temperature in the gasifierarious curves can be exported online from the DCS.The coal blocks were piled up to simulate an underground coal value the gasification feed is changed to air again and the process isthe simulated coal seam were 4.0 m x 0.8 m x 0.8 m. Pulverized repeated. The more the heat produced in the first stage the morel and loess mixed with water was used to bond the coal blocks. a coal combustion in the first stage mainly affects the rate at whichtotal of 96 Nickel-Chromium/Nickel-Silicon(K type)armored heat is produced [21thermocouples were uniformly located in the seam in eight rows at We studied an oxygen-enriched two-stage UCG method. In thehree(upper, middle and lower)levels to measure temperatures model tests oxygen enriched air was injected during the first stageduring the test. The upper and lower spacing between the ther- and the temperature increase in the gasifier was faster than thanocouples was 0. 2 m. The roof was simulated by cement oI m seen with the conventional two-stage UCG method. In the seconthick Gasification channels 0.025 m in diameter were drilled in the stage steam was injected. According to Refs. [11-14]. when theite blocks. The principle of similitude was used to obtain average temperature of the coal seam has increased to 1000.Cthethe dimensions of this simulated coal seam. The dimensions of the second stage begins. As steam is injected the average temperaturesimulated coal seam were in the ratio of 1: 12.5 to model an of the coal seam declines. After the temperature declines to 700Cunderground coal seam 10 m thick having a length between the the gasification environment is changed back to oxygen-enrichedinlet and outlet tubes 50 m long( Fig 3).air. This process is repeatedAfter debugging the UCG model test system it is prepared for2.3. Experimental procedureniting. An electric igniter placed inside the gasification channelwas used to ignite the coal. At first oxygen mixed with little air waTwo-stage UCG is an underground gasification method that injected into the coal seam and the electricity was switched on.quires supplying air and steam stage-by-stage. In the first stage Then real time video from the CCTV, temperatures measured by thethermocouples, and the gas component at the outlet were used中国煤化工CNMHGFig. 1. Lignite samples used in the test(dimensions: 0. 4 m x 0.8 m x 0.8 m).Fig 3. Schematic diagram of the simulated coal seam.L Hongtao et aL/ Mining Science and Technology( China)21 (2011)191-196determine if ignition was successful. If the terdually times foroxygen-enriched air and for injerises to 600 oC and the concentration of carbon dioxide in the were reconanalysis. This studyeffluent gas exceeds 10% the ignition was successful. When the using 35%temperature of the entire burning area has increased to 1000c gasification data is顺d 80% oxygen concentrations. TheTable 2and the downstream temperature is also distributed in a stepwiseay the three zones(the oxidation zone, the reduction zone, and 3. Results and discussionthe dry distillation zone)have been formed in the gasificationchannel. After this the test was officially started3. 1. Timing of the two stagesFirst, oxygen enriched air containingxygen was injeinto the heated loess coal seam. After the temperature of the coalThe cycle time of the two-stage undergseam rose to 1000C the oxygen enriched air was replaced by technique depends on the temperature. A long producing time andteam at a flow rate of 10 m /h. The gas being produced was a high calorific value of the syngas are beneficial for the gasificationimultaneously monitored at 5 min intervals. When the average technique. It is necessary to consider the cost factors, however,temperature in the coal seam had decreased to 700C steam was because the gasification condition uses oxygen-enrichereplaced by oxygen-enriched air again to re-heat the coal seam. The heating process. According to the calculation method proposed byTable 2Gasification results from oxygen-enriched two-stage UCG model tests.Gas composition(by volume)(%)loric valueconcentration(%)First stageSecond stagtime(min11,113.7224324956503296721010737200793112714535.592932221401193428127159.7849.35282193720230.0040021.2524011.74264223.026000000890025.55013216YH中国煤化工198520CNMHGNote: The sampling time(minutes)was counted from the time steam was injected. The gasification parameters at time 0 min represent the steady parameters of the first stage.L Hongtao et aL/ Mining Science and Technology(China)21(2011)191-196the Soviet expert KpeHHHHHthe, cycle time ratio of the first stage tothe second stage is about 2.5: 1 1 19)And the cycle time ratio of thefirst stage to the second stage is about 1.8: 1-2.7: 1 for single steamnjected conditions in a"long-channel, big-section, two-stage"UCGFitting curvemethod discussed in Refs. 19-18 During these studies, the testswere carried out under single component(steam) conditions.When oxygen concentrations of30‰,35‰40%45‰60%or80%were used the resulting ratios of the first stage to the second stagetmes were1:012.1:021,1:051.1:064.1:090.and1:0 respectively( Fig. 4). As the oxygen concentration is increased the cycletime ratio decreased. The main reason is that the oxygen concenration is inversely proportional to the time needed for establishingthe temperature required for the second stage. The temperatureOxygen concentration( %)field is smaller with low gasification intensity, with a narrowerheat-affected region, so less heat is stored in the coal. Then wheFig. 5. Fit of time ratios(second stage to first stage)steam was injected the heat was quickly consumed in the reactionconcentration of oxygen the shorter the time of the second stcen: temperature plots that temperatures in the gasifier drop rapidiy chefitting formula isThe rate of temperature drop for points above 1000C is pre-sented in Table 3. After steam injection begins the cooling rate overthe temperature field gradually decreased with time from abouty=0.00788e012946+0.18814191-27.4C/min to about 23-6.8C/min. the cooling rateincreases with increasing oxygen concentration in the first stagewhere y is the cycle time ratio from the secorto the first feed. The main reason for this phenomenon may be that whenstage and x is the concentration of oxygenthe heating steam was injected into the gasifier nitrogen had not beenprocess. The correlation coefficient(R)of the fit is 0.99078. The completely purged from the cavity. The concentration of steam oncycle time ratio using air is 0.228 calculated from this formulathe surface of the incandescent coal is determined by the amount ofThe cycle time ratio is reported to be from 0.37 to 0.56 in the nitrogen in the first stage. This could affect the reaction intensityiterature I9) These reports are related to single component The results suggested that a highly positive correlation existedsteam)conditions in two-stage gasification tests. The difference between decomposition rate of steam and oxygen concentrationwe observe between calculated and the theoretical cycle time ratios The temperature decline in low oxygen concentrations is alsomay be caused by the scope of the high temperature field, which is slower than in high oxygen concentrations. However, the rate ofrelatively smaller in the initial stage. The heat-affected region is temperature drop is higher for a 60% oxygen concentration in thenarrower than in Reference resulting in a shorter time of the second first stage feed than it is for an 80% oxygen concentration. Thestage[9]. Besides, this shows that it is necessary to study the cycle reason for this is that the cavity expanded to a degree as the testtime ratio of two-stage gasification within different gasifiers before time passed and coal was consumed. Hence, the concentration ofconclusions about the cycle time ratios may be obtained.steam injected into the cavity became lower and lower as timepassed. The reaction intensity with the steam dropped for this3.-2. Temperature field transformationreason so the rate of temperature drop is hightThe changes in the temperature field before and after3. Gas compositiin Fig. 6. The temperature field changes of the 30The gas composition 10 min after steam was applied. for variousniched, two-stage UCG as shown in Fig. 6a-d areoxygen-enriched two-stage UCG processes, is shown in Table 4. Theresults show that the caloric value of the produced gas is between5.31 and 10.54 MJ/Nm. Note that H2 and Co are both majorFirst stagecomponents in the product stream. The total H2, CO, and CHaSecond stagecontent during the first stage heating changed. this shows that thereaction between steam and the incandescent coal seam followshe equation150C+H20=CO+H2 AH=-1315kJ/molThis is an endothermic reaction and, therefore, the temperaturein the gasifier declines gradually with continued injection of steam.The caloric value of the gas declines constantly. when the averagetemperature in the gasifier declines to 700 C the caloric value ofthe gas has declined to 4 MJ/Nm. Therefore, the caloric value of the中国煤化工 he caloric value is theOxygen concentration(%)CN MH Milar hydrogen content.Fg 4 cyde times of the oxygen-ennched two-stage UCG process.But the main difference here is the methane content. Ref [5 ] showsL Hongtao et aL /Mining Soience and Technology(China)21(2011)191-1951000a Temperature field 5 min before steam injectionb Temperature field 5 min after steam injectioneld 10 min after steam injectiond Temperature field 20 miFt 6 Changes of in the temperature field during a two-stage UCG process.that carbonization gas produced by a long gasification channel in Table 3the second stage was the principal gas. And, hydrogenation of Rate of temperature drop during the second stagecarbon and the methanation reaction occur in the presence of OxygenDuration ofe of drop('C/min)metal oxides in the ash that act as a catalyst. Hence the methane Concentration(m) second stage (min)5min 10 min 20 min 30 miThe natural structure of coal in a shaft-less underground coal 3025gasification scheme was simulated by using coal of a size largethan in previous model tests [11-14. Gaps between the coals were 45smaller, which creates a smaller high temperature field and a rela274170322tively shorter heating stage. Thus the contact area between thedistillation gas and coal seam is relatively small. On the other hand,because of the relatively smaller gasification channel the gas in the Table 4model gasifier has a shorter residence time which caused the Gas composition 10 min after steam injection begins.hydrogenation of carbon, and the methanation, reactions to be less oygen tion H? H, co CO, N, Co+Ha+ CHa(MI/Nm)Gas composition(by volume)(than in previous model tests [11-14. This creates the lowermethane content in the gas28.78164836222738143878354326538425641761991815203219832169method was studied by model tests. Our experimental resultssupport the following conclusions:1)0中国煤化工CNMHFe 7. Rate of temperature drop in a two-stage UCG process.=njected feed stream gave time ratios( first stage toL Hongtao et aL/ Mining Science and Technology(China)21(2011)191-196tage)of1:012.1:021,1:0.51.1:0.64,1:090.and1:40r13) Sun JL Lou YE, Xi JF, Li YL Liang xx Field test of underground coal gasificationtively. The time ratio is related to oxygen concentration in the14] Liu YH, Guo SR, Liang J, Sun JL Study on field test of underground coal gasial of liaoning Project Technologyniversity 2006: 25(5): 658-61 (in Chinesely=000788e0.12946+0.18814[5] Du F,ound coal gasification. Shanxi Coal 2where y is the time ratio(first stage to second stage)and x is 6l tion se Aew clean i a Mthzitio s echaiayae mor dide rgnergd 2007: 3210102:2)The cooling rate of the temperature field after steam injection [7 Liang J, Yu L"Long-large-section" UCG method. China Coaldecreased over time, from about 19 1-27. 4 C/min to23-6.8C/min. But this rate increased with increasing oxygen[8]Liang J, Yu L Theory and practice of generating hydrogenconcentration in the first stage feed.3)The caloric value of the product gas from the oxygen-enriched今m的 Science Technology Re92two-stage underground coal gasification model fell in the rangefrom 5.31 MJ/Nmto 10.54 MJ/Nm. The caloric value increased (10) Yu L R&D of coal underground pneumatolysis Science Technology Reviewwith increased oxygen in the first stage feed. An 80% oxygen-enriched air stream gave the highest caloric value and the [l yan ci journal of cho nawo-stage undergroun 24(2): 111-4 in Chinesel.tIon time4)The total amount of Co, H2, and CHa in the second stage gasdirections. Journal of China Coal Society 1996: 21(1): 68-71 lin Chineselincreased from 38.8% to 73. 11% as the oxygen levels increased (131 Yang LH, Liang J Model test two-stage underground coalcation inin the first stage feed streamniversity 2003: 26(2): 47-55 in Chinese[14 Yangtwo-stage underground coal gasification(TSUCG)in steep coal seam. Journal of Southeast University 2001: 31(5): 72-5The authors gratefully acknowledge the financial support of the [151 Yang LH Field test study of double-fireChemistry andNational Natural Science Foundation of China(No. 50876112)the fundamental Research Funds for the central universitie[16 Yang LH, Liang J. Yu L Double fire- two stage method and parameter calcu-(No. 2009QH13)and the Program of International S&T Cooperation(No.2009DFR60180,No.2010DFR60610)[171 Yangment and numerical simulationunderground coal gasification Fuel 2004: 83(4-5): 573-84I1] Stan,czyk K, Smolin ski A, Kapusta K, Wiatowski M, S'wiadrowski ]. Kotyrb ARenditions 1992: 12(2):20-1 in Chinese108911)3307-14[2] Tang FL Duan LC. shaftless underground coal gasification has great future1211Yang LH, Zhang X, Liu so yu nzhngnwring 2001:523). 273-6( in Chineselscale hydroge1-13 in chineselJournal of Hydrogen Energy 2008: 33(4): 1275-85中国煤化工CNMHG