大佛寺井田4號(hào)煤CH4與CO2吸附解吸實(shí)驗(yàn)比較

馬東民,李來(lái)新,李小平,白懷東,王 杰,劉厚寧,李方晴

(1.西安科技大學(xué),陜西西安 710054;2.國(guó)家能源煤與煤層氣共采技術(shù)重點(diǎn)實(shí)驗(yàn)室,山西 晉城 048204;3.陜西省煤層氣開(kāi)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(duì),陜西韓城 715400;5.中國(guó)煤炭地質(zhì)總局航測(cè)遙感局,陜西西安 710054)

大佛寺井田4號(hào)煤CH4與CO2吸附解吸實(shí)驗(yàn)比較

馬東民1,2,李來(lái)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(xué),陜西西安 710054;2.國(guó)家能源煤與煤層氣共采技術(shù)重點(diǎn)實(shí)驗(yàn)室,山西 晉城 048204;3.陜西省煤層氣開(kāi)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(duì),陜西韓城 715400;5.中國(guó)煤炭地質(zhì)總局航測(cè)遙感局,陜西西安 710054)

以迅速降低大佛寺4號(hào)煤含氣量,提高地面煤層氣井采收率為目標(biāo),進(jìn)行CO2驅(qū)替CH4技術(shù)的實(shí)驗(yàn)研究。對(duì)采自大佛寺礦井40114工作面的樣品,進(jìn)行多個(gè)溫度點(diǎn)柱體原煤與60～80目平衡水樣的CH4與CO2吸附解吸對(duì)比實(shí)驗(yàn)。結(jié)果表明:CO2在煤孔隙表面與CH4一致,吸附過(guò)程符合Langmuir方程,解吸過(guò)程可用解吸式描述;由熱力學(xué)計(jì)算可知,柱體原煤升壓過(guò)程CO2吸附熱為56.827 kJ/mol,CH4吸附熱為12.662 kJ/mol,降壓過(guò)程CO2吸附熱為115.030 kJ/mol,CH4吸附熱為23.602 kJ/mol,無(wú)論升壓過(guò)程還是降壓過(guò)程CO2吸附熱遠(yuǎn)大于CH4吸附熱,兩種氣體在煤孔隙表面競(jìng)爭(zhēng)吸附時(shí)CO2占據(jù)優(yōu)勢(shì),導(dǎo)致置換解吸;吸附勢(shì)、吸附空間計(jì)算驗(yàn)證了這個(gè)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(shì);吸附空間;大佛寺井田

煤層氣多以吸附狀態(tài)賦存于煤儲(chǔ)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(chǎng)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(lái)的CH4。這說(shuō)明煤層氣是無(wú)法通過(guò)降壓全部解吸出來(lái)的。美國(guó)、加拿大有關(guān)公司所開(kāi)展的注入CO2提高煤層氣采收率的現(xiàn)場(chǎng)試驗(yàn),從實(shí)踐的角度進(jìn)一步證明了煤層氣置換解吸現(xiàn)象的存在。我國(guó)煤層氣產(chǎn)業(yè)界在煤層氣開(kāi)發(fā)方面對(duì)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(wú)煙煤和其他地區(qū)中變質(zhì)程度焦煤進(jìn)行CO2置換CH4的實(shí)驗(yàn)研究,證明了CO2置換CH4在我國(guó)中高階煤地區(qū)應(yīng)用的可行性,但對(duì)低階煤研究較少。

目前國(guó)內(nèi)對(duì)低階煤地區(qū)CO2置換CH4與產(chǎn)生置換的熱力學(xué)特征相關(guān)研究甚少。陜西彬長(zhǎng)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(kāi)采規(guī)劃區(qū)。僅靠降壓排采很難在短期內(nèi)降低煤儲(chǔ)層含氣量,為保障煤炭安全開(kāi)采和地面煤層氣抽采方法,進(jìn)行CO2置換CH4初步研究工作。

1 試樣與實(shí)驗(yàn)設(shè)計(jì)

1.1 試樣來(lái)源與加工

實(shí)驗(yàn)試樣采自彬長(zhǎng)礦區(qū)大佛寺井田40114工作面。目前國(guó)內(nèi)煤層氣吸附解吸實(shí)驗(yàn)樣品的制作均采用GB/T 19560—2008《煤的高壓等溫吸附實(shí)驗(yàn)方法——容量法》60～80目平衡水分煤樣,但是經(jīng)過(guò)長(zhǎng)期實(shí)驗(yàn)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(chǔ)層固有的吸附解吸特性,為了進(jìn)一步驗(yàn)證此觀點(diǎn),同時(shí)更加真實(shí)的模擬煤儲(chǔ)層特性,加工以下兩種樣品進(jìn)行對(duì)比試驗(yàn)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?shí)驗(yàn)(圖1);其二,參考ASTM(美國(guó)實(shí)驗(yàn)材料學(xué)會(huì))樣品制作,制取符合GB/ T 19560—2008《煤的高壓等溫吸附實(shí)驗(yàn)方法——容量法》的60～80目平衡水分煤樣2 kg供實(shí)驗(yàn)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(hào)煤為低灰長(zhǎng)焰煤,其中鏡質(zhì)體反射率=0.43%,小于0.65%,表明大佛寺4號(hào)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(jiàn)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(jiàn)表2。

表2 試樣煤巖分析結(jié)果Table 2 Petrographic analysis results of coal sample %

1.3 實(shí)驗(yàn)?zāi)康?/p>

(1)比較CO2與CH4的吸附、解吸數(shù)學(xué)模型;

(2)計(jì)算并比較CO2與CH4升壓過(guò)程與解吸過(guò)程的吸附熱、吸附勢(shì);

(3)比較柱體原煤樣與60～80目平衡水樣實(shí)驗(yàn)差異。

1.4 實(shí)驗(yàn)設(shè)計(jì)

在25,30,35,40,45℃五個(gè)溫度點(diǎn)進(jìn)行CH4吸附解吸實(shí)驗(yàn),在35,40,45℃三個(gè)溫度點(diǎn)進(jìn)行CO2的吸附解吸實(shí)驗(yàn)。

實(shí)驗(yàn)儀器使用AST-2000Ⅲ型煤層氣吸附解吸仿真試驗(yàn)儀。

2 吸附解吸實(shí)驗(yàn)結(jié)果與分析

2.1 吸附解吸實(shí)驗(yàn)結(jié)果

吸附解吸實(shí)驗(yàn)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(shí)驗(yàn)曲線Fig.2 Test curves of adsorption and desorption about cylinder raw coal sample and cylinder equilibrium water coal sample

2.2 實(shí)驗(yàn)結(jié)果分析

增壓吸附過(guò)程實(shí)驗(yàn)數(shù)據(jù)用 Langmuir方程[7-11](式(1)),降壓解吸過(guò)程實(shí)驗(yàn)數(shù)據(jù)用解吸式[12](式(2))進(jìn)行擬合。

式中,Vads為煤層氣吸附到壓力 p時(shí)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(shí)煤層氣吸附量,mL/g;c為常數(shù)。

數(shù)據(jù)擬合結(jié)果見(jiàn)表3,4。

平衡時(shí)間監(jiān)測(cè)表明,柱體原煤等溫吸附/解吸與標(biāo)樣一致,在24 h內(nèi)能夠達(dá)到完全平衡,平衡壓力穩(wěn)定,實(shí)驗(yàn)測(cè)得吸附量均為該壓力點(diǎn)的最大吸附量。

由實(shí)驗(yàn)結(jié)果可以看出:

(1)CO2與CH4增壓吸附過(guò)程皆可以用Langmuir方程進(jìn)行描述,降壓解吸過(guò)程皆可以用解吸式進(jìn)行描述,差值較小,擬合度高;

表3 大佛寺4號(hào)煤柱體原煤煤樣吸附解吸實(shí)驗(yàn)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(hào)煤平衡水煤樣吸附解吸實(shí)驗(yàn)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(duì)CO2的飽和吸附量遠(yuǎn)大于煤對(duì)CH4的吸附量,CO2較CH4更易于吸附在煤的孔隙表面,競(jìng)爭(zhēng)吸附優(yōu)勢(shì)大,CO2置換解吸CH4可行;

(3)相同溫度,60～80目的平衡水煤樣對(duì)CH4, CO2的吸附量均大于柱體原煤樣對(duì)CH4,CO2的吸附量;

(4)煤樣對(duì)CO2吸附解吸過(guò)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(yuǎn)大于CH4。

3 熱力學(xué)特征

根據(jù)實(shí)驗(yàn)數(shù)據(jù),利用Clausius-Clapeyron方程[13]間接計(jì)算等量吸附熱。

極限吸附熱為壓力趨于0時(shí)的等量吸附熱,通過(guò)Virial方程[14]來(lái)進(jìn)行計(jì)算。在壓力無(wú)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(shí)際計(jì)算時(shí),用Virial方程來(lái)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(jì)算出極限吸附熱qst0。VantHoff方程為

其中,ΔH0為吸附熱。計(jì)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(guò)程吸附熱計(jì)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(guò)程吸附熱計(jì)算結(jié)果Table 6 The calculation results of adsorption heat about the process of pressurizing(adsorbing)and depressurizing(desorbing)CO2

由計(jì)算結(jié)果可以看出:

(1)升壓吸附過(guò)程與降壓解吸過(guò)程,等量吸附熱與吸附量線性相關(guān);

(2)柱體原煤樣與標(biāo)樣,無(wú)論升壓吸附還是降壓解吸,CH4與CO2的吸附熱都有交點(diǎn),說(shuō)明CH4與CO2競(jìng)爭(zhēng)吸附時(shí),達(dá)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(jìng)爭(zhēng)吸附促進(jìn)CH4持續(xù)解吸。

4 吸附勢(shì)與吸附空間計(jì)算

4.1 吸附勢(shì)計(jì)算

吸附勢(shì)是指吸附質(zhì)在界面進(jìn)行物理吸附時(shí),每1 mol吸附質(zhì)的自由能變化。利用吸附勢(shì)理論建立吸附勢(shì)與壓力之間的關(guān)系[16]為

圖3 CH4與CO2吸附熱對(duì)比Fig.3 Adsorption heat contrast of CH4and CO2

式中,ε為吸附勢(shì),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(jì)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(duì)溫度,K。

實(shí)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(dāng)采用Dubinin提出的超臨界條件下虛擬飽和蒸汽壓力的經(jīng)驗(yàn)計(jì)算公式[17]進(jìn)行計(jì)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(jì)算

吸附空間是指煤中可供氣體吸附的場(chǎng)所[18],根據(jù)煤對(duì)單一組分氣體的等溫吸附/解吸數(shù)據(jù)和式(7)可以計(jì)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(shí)測(cè)吸附量,mol/g;ρa(bǔ)d為氣體吸附相密度,g/cm3。

氣體吸附相密度ρa(bǔ)d可根據(jù)經(jīng)驗(yàn)公式計(jì)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(jì)算結(jié)果

計(jì)算結(jié)果與擬合曲線如圖4,5所示。

圖4 大佛寺4號(hào)煤柱體原煤增壓(吸附)降壓(解吸)過(guò)程吸附勢(shì)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(chǎng)中的吸附勢(shì),是將該分子由吸附空間中它所處的位置上移到無(wú)限遠(yuǎn)時(shí)所需之功[21]。

由吸附勢(shì)與吸附空間的計(jì)算結(jié)果可以看出:

(1)兩種煤樣,無(wú)論增壓吸附還是降壓解吸過(guò)程,CO2與CH4的吸附勢(shì)各有其特征曲線,但相同壓力下CH4的吸附勢(shì)小于CO2的吸附勢(shì),CO2的吸附空間比CH4大的多;因此競(jìng)爭(zhēng)吸附時(shí),CO2處于絕對(duì)優(yōu)勢(shì);

(2)柱體原煤煤樣CH4與CO2的吸附特性曲線相交于A點(diǎn),對(duì)應(yīng)的壓力為0.38 MPa,表明壓力在大于0.38 MPa階段,煤對(duì)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(duì)大佛寺4號(hào)煤儲(chǔ)層進(jìn)行注入CO2提高CH4采收率作業(yè),在井底壓力大于0.38 MPa時(shí),效果會(huì)更佳。

圖5 大佛寺4號(hào)煤平衡水樣增壓(吸附)、降壓(解吸)過(guò)程吸附勢(shì)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

5 結(jié) 論

(1)CO2在煤孔隙表面的吸附行為模型與CH4一致,吸附過(guò)程符合Langmuir方程,解吸過(guò)程可用解吸式描述,R20最小為0.994,可以應(yīng)用。

(2)從熱力學(xué)計(jì)算結(jié)果看,柱狀原煤的實(shí)驗(yàn)結(jié)果較標(biāo)準(zhǔn)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(chǎng)煤層氣井排采實(shí)際接近。

(3)熱力學(xué)計(jì)算結(jié)果表明,柱體原煤升壓過(guò)程CO2吸附熱為 56.827kJ/mol,CH4吸附熱為12.662 kJ/mol,降 壓 過(guò) 程 CO2吸 附 熱 為115.030 kJ/mol,CH4吸附熱為23.602 kJ/mol,無(wú)論升壓過(guò)程還是降壓過(guò)程煤孔隙表面CO2吸附熱遠(yuǎn)大于CH4吸附熱,低階煤孔隙表面氣體的競(jìng)爭(zhēng)吸附最終導(dǎo)致CO2置換CH4。吸附勢(shì)、吸附空間計(jì)算所得特征曲線亦驗(yàn)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(zé)任編輯:畢永華

山西省煤層氣聯(lián)合研究基金資助項(xiàng)目(2013012009);國(guó)家科技重大專(zhuān)項(xiàng)資助項(xiàng)目(2011ZX05061-005-002)

馬東民(1967—),男,陜西合陽(yáng)人,教授,博士。Tel:029-83858063,E-mail:mdm6757@126.com

馬東民,李來(lái)新,李小平,等.大佛寺井田4號(hào)煤CH4與CO2吸附解吸實(shí)驗(yàn)比較[J].煤炭學(xué)報(bào),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