馬東民,李來新,李小平,白懷東,王 杰,劉厚寧,李方晴
(1.西安科技大學,陜西西安 710054;2.國家能源煤與煤層氣共采技術(shù)重點實驗室,山西 晉城 048204;3.陜西省煤層氣開發(fā)利用有限公司,陜西西安 710065;4.陜西省煤田地質(zhì)局131隊,陜西韓城 715400;5.中國煤炭地質(zhì)總局航測遙感局,陜西西安 710054)
大佛寺井田4號煤CH4與CO2吸附解吸實驗比較
馬東民1,2,李來新3,李小平4,白懷東5,王 杰1,劉厚寧1,李方晴1
(1.西安科技大學,陜西西安 710054;2.國家能源煤與煤層氣共采技術(shù)重點實驗室,山西 晉城 048204;3.陜西省煤層氣開發(fā)利用有限公司,陜西西安 710065;4.陜西省煤田地質(zhì)局131隊,陜西韓城 715400;5.中國煤炭地質(zhì)總局航測遙感局,陜西西安 710054)
以迅速降低大佛寺4號煤含氣量,提高地面煤層氣井采收率為目標,進行CO2驅(qū)替CH4技術(shù)的實驗研究。對采自大佛寺礦井40114工作面的樣品,進行多個溫度點柱體原煤與60~80目平衡水樣的CH4與CO2吸附解吸對比實驗。結(jié)果表明:CO2在煤孔隙表面與CH4一致,吸附過程符合Langmuir方程,解吸過程可用解吸式描述;由熱力學計算可知,柱體原煤升壓過程CO2吸附熱為56.827 kJ/mol,CH4吸附熱為12.662 kJ/mol,降壓過程CO2吸附熱為115.030 kJ/mol,CH4吸附熱為23.602 kJ/mol,無論升壓過程還是降壓過程CO2吸附熱遠大于CH4吸附熱,兩種氣體在煤孔隙表面競爭吸附時CO2占據(jù)優(yōu)勢,導致置換解吸;吸附勢、吸附空間計算驗證了這個結(jié)論;利用CO2驅(qū)替CH4技術(shù),提高煤層氣采收率,理論依據(jù)充分可行。
吸附;解吸;吸附熱;吸附勢;吸附空間;大佛寺井田
煤層氣多以吸附狀態(tài)賦存于煤儲層孔隙表面,煤層氣排采多采用降壓解吸的方式,目前雖然取得一定成效,但是現(xiàn)場排采發(fā)現(xiàn),即使井底壓力降的很低,煤層中依然存在未能解吸出來的CH4。這說明煤層氣是無法通過降壓全部解吸出來的。美國、加拿大有關(guān)公司所開展的注入CO2提高煤層氣采收率的現(xiàn)場試驗,從實踐的角度進一步證明了煤層氣置換解吸現(xiàn)象的存在。我國煤層氣產(chǎn)業(yè)界在煤層氣開發(fā)方面對CO2置換CH4可行性做了大量工作[1-6],2005年,張遂安等利用沁水盆地高變質(zhì)無煙煤和其他地區(qū)中變質(zhì)程度焦煤進行CO2置換CH4的實驗研究,證明了CO2置換CH4在我國中高階煤地區(qū)應(yīng)用的可行性,但對低階煤研究較少。
目前國內(nèi)對低階煤地區(qū)CO2置換CH4與產(chǎn)生置換的熱力學特征相關(guān)研究甚少。陜西彬長礦區(qū)大佛寺井田屬于低階煤高瓦斯礦井,目前地面煤層氣抽采井皆為高產(chǎn)井,但是產(chǎn)氣井處于3~5 a煤炭開采規(guī)劃區(qū)。僅靠降壓排采很難在短期內(nèi)降低煤儲層含氣量,為保障煤炭安全開采和地面煤層氣抽采方法,進行CO2置換CH4初步研究工作。
1.1 試樣來源與加工
實驗試樣采自彬長礦區(qū)大佛寺井田40114工作面。目前國內(nèi)煤層氣吸附解吸實驗樣品的制作均采用GB/T 19560—2008《煤的高壓等溫吸附實驗方法——容量法》60~80目平衡水分煤樣,但是經(jīng)過長期實驗研究發(fā)現(xiàn),60~80目煤樣主要為鏡煤,孔隙以微孔為主,比表面積偏大,不能完全代表煤儲層固有的吸附解吸特性,為了進一步驗證此觀點,同時更加真實的模擬煤儲層特性,加工以下兩種樣品進行對比試驗研究:其一,順層理方向鉆取?10 cm×13 cm順層柱體原煤樣,密封保存?zhèn)溆脤嶒?圖1);其二,參考ASTM(美國實驗材料學會)樣品制作,制取符合GB/ T 19560—2008《煤的高壓等溫吸附實驗方法——容量法》的60~80目平衡水分煤樣2 kg供實驗用。
1.2 煤的組成分析
根據(jù)GB/T 212—2008做煤的工業(yè)分析與煤巖分析,由表1,2可以看出,大佛寺4號煤為低灰長焰煤,其中鏡質(zhì)體反射率=0.43%,小于0.65%,表明大佛寺4號煤屬于低階煤。
圖1 柱體原煤樣試件Fig.1 Test specimen of coal cylinder raw coal sample
(1)試樣工業(yè)分析結(jié)果見表1。
表1 試樣工業(yè)分析結(jié)果Table 1 Proximate analysis results of sample %
(2)試樣煤巖分析結(jié)果見表2。
表2 試樣煤巖分析結(jié)果Table 2 Petrographic analysis results of coal sample %
1.3 實驗?zāi)康?/p>
(1)比較CO2與CH4的吸附、解吸數(shù)學模型;
(2)計算并比較CO2與CH4升壓過程與解吸過程的吸附熱、吸附勢;
(3)比較柱體原煤樣與60~80目平衡水樣實驗差異。
1.4 實驗設(shè)計
在25,30,35,40,45℃五個溫度點進行CH4吸附解吸實驗,在35,40,45℃三個溫度點進行CO2的吸附解吸實驗。
實驗儀器使用AST-2000Ⅲ型煤層氣吸附解吸仿真試驗儀。
2.1 吸附解吸實驗結(jié)果
吸附解吸實驗結(jié)果如圖2所示。
圖2 柱體原煤樣、平衡水煤樣吸附、解吸實驗曲線Fig.2 Test curves of adsorption and desorption about cylinder raw coal sample and cylinder equilibrium water coal sample
2.2 實驗結(jié)果分析
增壓吸附過程實驗數(shù)據(jù)用 Langmuir方程[7-11](式(1)),降壓解吸過程實驗數(shù)據(jù)用解吸式[12](式(2))進行擬合。
式中,Vads為煤層氣吸附到壓力 p時煤層氣吸附量,mL/g;a為煤樣最大吸附量,mL/g;b為吸附、解吸速度與吸附熱綜合參數(shù)。
式中,Vdes為煤層氣解吸到壓力 p時煤層氣吸附量,mL/g;c為常數(shù)。
數(shù)據(jù)擬合結(jié)果見表3,4。
平衡時間監(jiān)測表明,柱體原煤等溫吸附/解吸與標樣一致,在24 h內(nèi)能夠達到完全平衡,平衡壓力穩(wěn)定,實驗測得吸附量均為該壓力點的最大吸附量。
由實驗結(jié)果可以看出:
(1)CO2與CH4增壓吸附過程皆可以用Langmuir方程進行描述,降壓解吸過程皆可以用解吸式進行描述,差值較小,擬合度高;
表3 大佛寺4號煤柱體原煤煤樣吸附解吸實驗擬合結(jié)果Table 3 Fitted results of adsorption and desorption experiments of No.4 raw coal sample in Dafosi Mine
表4 大佛寺4號煤平衡水煤樣吸附解吸實驗擬合結(jié)果Table 4 Fitted results of adsorption and desorption experiments of No.4 cylinder equilibrium water coal sample in Dafosi Mine
(2)相同溫度,煤對CO2的飽和吸附量遠大于煤對CH4的吸附量,CO2較CH4更易于吸附在煤的孔隙表面,競爭吸附優(yōu)勢大,CO2置換解吸CH4可行;
(3)相同溫度,60~80目的平衡水煤樣對CH4, CO2的吸附量均大于柱體原煤樣對CH4,CO2的吸附量;
(4)煤樣對CO2吸附解吸過程也存在“解吸滯后”現(xiàn)象,且CO2的滯后環(huán)更為明顯,即便如此從解吸曲線上同樣可以看出相同的壓降CO2解吸量遠大于CH4。
根據(jù)實驗數(shù)據(jù),利用Clausius-Clapeyron方程[13]間接計算等量吸附熱。
極限吸附熱為壓力趨于0時的等量吸附熱,通過Virial方程[14]來進行計算。在壓力無限趨于0的情況下,等溫吸附曲線應(yīng)符合Henry定律[15],即
式中,n為吸附量,mmol/g;P為平衡壓力,kPa;K′為Henry常數(shù),mmol/(g·kPa)。
實際計算時,用Virial方程來描述等溫吸附線,并在低壓區(qū)域向零壓下外推求得不同溫度下的Henry常數(shù)K′,根據(jù)Henry常數(shù)K′與溫度T所遵守的VantHoff方程就可以計算出極限吸附熱qst0。VantHoff方程為
其中,ΔH0為吸附熱。計算結(jié)果如表5,6和圖3所示。
表5 CH4增壓(吸附)與降壓(解吸)過程吸附熱計算結(jié)果Table 5 The calculation results of adsorption heat about the process of pressurizing(adsorbing)and depressurizing(desorbing)CH4
表6 CO2增壓(吸附)與降壓(解吸)過程吸附熱計算結(jié)果Table 6 The calculation results of adsorption heat about the process of pressurizing(adsorbing)and depressurizing(desorbing)CO2
由計算結(jié)果可以看出:
(1)升壓吸附過程與降壓解吸過程,等量吸附熱與吸附量線性相關(guān);
(2)柱體原煤樣與標樣,無論升壓吸附還是降壓解吸,CH4與CO2的吸附熱都有交點,說明CH4與CO2競爭吸附時,達到一定的壓力(吸附量)后,CO2吸附放熱提供CH4解吸需要熱量,競爭吸附促進CH4持續(xù)解吸。
4.1 吸附勢計算
吸附勢是指吸附質(zhì)在界面進行物理吸附時,每1 mol吸附質(zhì)的自由能變化。利用吸附勢理論建立吸附勢與壓力之間的關(guān)系[16]為
圖3 CH4與CO2吸附熱對比Fig.3 Adsorption heat contrast of CH4and CO2
式中,ε為吸附勢,J/mol;P0為氣體飽和蒸汽壓力, MPa;Pi為理想氣體在恒溫下的平衡壓力,MPa;P為平衡壓力,式(5)計算中換算為MPa;R為普適氣體常數(shù),取值8.314 4 J/(mol·K);T為絕對溫度,K。
實際應(yīng)用中煤吸附(解吸)甲烷和二氧化碳都是在臨界溫度以上,因此,臨界條件下的飽和蒸汽壓力P0應(yīng)當采用Dubinin提出的超臨界條件下虛擬飽和蒸汽壓力的經(jīng)驗計算公式[17]進行計算,即
其中,Pc為氣體臨界壓力,MPa;Tc為氣體臨界溫度, K。本文中CH4的臨界溫度Tc為190.6 K,臨界壓力Pc為4.62 MPa;CO2的臨界溫度Tc為304.2 K,臨界壓力Pc為7.39 MPa。
4.2 吸附空間的計算
吸附空間是指煤中可供氣體吸附的場所[18],根據(jù)煤對單一組分氣體的等溫吸附/解吸數(shù)據(jù)和式(7)可以計算[19],即
式中,w為吸附空間,cm3/g;M為氣體的分子量, g/mol;Vad為實測吸附量,mol/g;ρad為氣體吸附相密度,g/cm3。
氣體吸附相密度ρad可根據(jù)經(jīng)驗公式計算[20],即
式中,R為普適氣體常數(shù),取值為8.205 cm3·MPa/ (mol·K)。
4.3 計算結(jié)果
計算結(jié)果與擬合曲線如圖4,5所示。
圖4 大佛寺4號煤柱體原煤增壓(吸附)降壓(解吸)過程吸附勢特征曲線Fig.4 Adsorption characteristic curves about the process of pressurization(adsorption)and depressurization (desorption)on No.4 coal cylinder raw coal sample in Dafosi
Polanyi理論指出,吸附質(zhì)分子在固體表面吸引場中的吸附勢,是將該分子由吸附空間中它所處的位置上移到無限遠時所需之功[21]。
由吸附勢與吸附空間的計算結(jié)果可以看出:
(1)兩種煤樣,無論增壓吸附還是降壓解吸過程,CO2與CH4的吸附勢各有其特征曲線,但相同壓力下CH4的吸附勢小于CO2的吸附勢,CO2的吸附空間比CH4大的多;因此競爭吸附時,CO2處于絕對優(yōu)勢;
(2)柱體原煤煤樣CH4與CO2的吸附特性曲線相交于A點,對應(yīng)的壓力為0.38 MPa,表明壓力在大于0.38 MPa階段,煤對CH4與CO2吸附的分餾效應(yīng)明顯,在小于0.38 MPa階段分餾效果較弱。若對大佛寺4號煤儲層進行注入CO2提高CH4采收率作業(yè),在井底壓力大于0.38 MPa時,效果會更佳。
圖5 大佛寺4號煤平衡水樣增壓(吸附)、降壓(解吸)過程吸附勢特征曲線Fig.5 Adsorption characteristic curves about the process of pressurization(adsorption)and depressurization (desorption)on No.4 coal equilibrium water coal sample in Dafosi
(1)CO2在煤孔隙表面的吸附行為模型與CH4一致,吸附過程符合Langmuir方程,解吸過程可用解吸式描述,R20最小為0.994,可以應(yīng)用。
(2)從熱力學計算結(jié)果看,柱狀原煤的實驗結(jié)果較標準煤樣更能夠反映煤層氣吸附解吸特征,臨界解吸壓力與現(xiàn)場煤層氣井排采實際接近。
(3)熱力學計算結(jié)果表明,柱體原煤升壓過程CO2吸附熱為 56.827kJ/mol,CH4吸附熱為12.662 kJ/mol,降 壓 過 程 CO2吸 附 熱 為115.030 kJ/mol,CH4吸附熱為23.602 kJ/mol,無論升壓過程還是降壓過程煤孔隙表面CO2吸附熱遠大于CH4吸附熱,低階煤孔隙表面氣體的競爭吸附最終導致CO2置換CH4。吸附勢、吸附空間計算所得特征曲線亦驗證了此結(jié)論。
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Contrastive experiment of adsorption-desorption between CH4and CO2in Coal Seam 4 of Dafosi Coal Mine
MA Dong-min1,2,LI Lai-xin3,LI Xiao-ping4,BAI Huai-dong5,WANG Jie1,LIU Hou-ning1,LI Fang-qing1
(1.Xi’an University of Science and Technology,Xi’an 710054,China;2.State Energy Key Laboratory of Joint Exploitation of Coal and Coal-bed Methane, Jincheng 048204,China;3.Shaanxi Coalbed Methane Development Corp.Ltd.,Xi’an 710065,China;4.Team 131 of Shaanxi Coalfield Geology Bureau, Hancheng 715400,China;5.Aerial Photogrammetry and Remote Sensing Bureau,China National Administration of Coal Geology,Xi’an 710054,China)
The study investigated the technology of the replacement of CH4by CO2in order to rapidly reduce the coalbed methane content in coal seam 4 at Dafosi Coal Mine and improve the recovery factor of the ground coalbed methane wells.Contrastive experiments on adsorption and desorption of CH4and CO2of cylinder raw coal and 60-80 mesh equilibrium water samples at different temperatures are conducted using the samples collected from the working face 40114 at Dafosi Coal Mine.The results indicate:the adsorption and desorption of CO2on coal pore surface is consistent with that of CH4,the pressure-rising process abiding by Langmuir equation and the pressure-dropping process being described by desorption expression.Based on thermodynamic calculation,in cylinder raw coal during pressure-rising process,the adsorption heat of CO2is 56.827 kJ/mol while that of CH4is 12.662 kJ/mol.During pressure-dropping process,the adsorption heat of CO2is 115.030 kJ/mol while that of CH4is 23.602 kJ/mol.In both processes,the adsorption heat of CO2is far greater than that of CH4,which proves that CO2is more competitive over CH4in their adsorption on coal pore surfaces,leading to replacement desorption.The conclusion is verified by the calculations of ad-sorption potential and adsorption space.The technology of replacing CH4by CO2is feasible with strong theoretical ground,and useful for improving the recovery factor of coalbed methane.
adsorption;desorption;adsorption heat;adsorption potential;adsorption space;afosi coal mine
P618.11
A
0253-9993(2014)09-1938-07
2014-05-06 責任編輯:畢永華
山西省煤層氣聯(lián)合研究基金資助項目(2013012009);國家科技重大專項資助項目(2011ZX05061-005-002)
馬東民(1967—),男,陜西合陽人,教授,博士。Tel:029-83858063,E-mail:mdm6757@126.com
馬東民,李來新,李小平,等.大佛寺井田4號煤CH4與CO2吸附解吸實驗比較[J].煤炭學報,2014,39(9):1938-1944.
10.13225/j.cnki.jccs.2014.8023
Ma Dongmin,Li Laixin,Li Xiaoping,et al.Contrastive experiment of adsorption-desorption between CH4and CO2in Coal Seam 4 of Dafosi Coal Mine[J].Journal of China Coal Society,2014,39(9):1938-1944.doi:10.13225/j.cnki.jccs.2014.8023