低阶煤中镉的赋存对其在热解中释放的影响
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摘要
为了探明重金属镉元素在低阶煤热解过程中的释放机制,在铝甑反应器中考察了不同热解终温下(400～800℃)新疆淖毛湖煤和宝日褐煤热解过程中镉的迁移转化行为。提出了"原煤热解行为-镉在煤中的赋存形式-重金属镉的释放行为-FactSage模拟"这一技术路线,首先在TGA反应器中考察了2种低阶煤的热解特性,其次利用逐级化学提取法和电子扫描电镜研究了重金属镉在煤中的赋存形式,然后在铝甑反应器中研究重金属镉在煤热解过程中的释放行为,最后结合FactSage热力学模拟计算,分层次系统地揭示重金属镉元素在热解过程中的释放机制。结果表明,镉在煤中的赋存形式决定了其在煤热解中的挥发性:2种低阶煤中有机结合态的镉元素比例均较高,导致低温时镉元素即可随着有机质的热分解而逸出;宝日褐煤中铁锰结合态镉元素较多,且大多分布于煤颗粒表面,受热容易分解从而导致其挥发;高温时宝日褐煤由于其SiO2含量较高,高温下熔融硅酸盐可与气态的镉元素发生反应,使镉元素的挥发率降低,而淖毛湖煤由于其含有的碱金属和碱土金属较多,可与矿物形成低温共熔物而导致镉元素释放率持续增加。另外,FactSage化学热力学模拟的结果与实验现象相符。计算表明镉是一种易挥发的元素,低温时煤中的镉元素几乎全部以气态单质镉的形式挥发,镉不与煤中Si,Al,Cl,Fe和碱金属等元素反应,但是这些矿物对镉元素进行吸附或包裹会影响镉元素在热解中的释放。
In order to reveal the release mechanism of heavy metal cadmium(Cd) in low rank coal during pyrolysis process,two low rank coals from Xinjiang and Neimeng(Baori) province was pyrolyzed at different temperature from400 to 800 ℃ in an aluminum crucible reactor to investigate the migration behavior of Cd during pyrolysis process.Thetechnical route of"coal pyrolysis behavior-the occurrence mode of Cd in coal-the release behavior of Cd-FactSage simulation"was proposed.Firstly,the pyrolysis characteristics of two low rank coals were investigated in the TGA reactor,the occurrence mode of Cd was studied by stepwise chemical extraction and scanning electron microscopy.Then,the release behavior of Cd in coal pyrolysis process was studied in a fixed aluminum crucible reactor.Finally,the FactSage thermodynamic simulation calculation was used to systematically reveal the release mechanism of Cd during pyrolysis process.The results showed that the occurrence mode of Cd was crucial for its volatility during coal pyrolysis process.The main occurrence mode for these two rank coals was organic bound state Cd,which could be released with the decomposition of organic matter in coals at low temperature. Baori coal contained more iron-manganese bound Cd,and most of them were distributed on the surface of coal particles,which was easily decomposed by heat and cause it to volatilize.At high temperature,the volatility of Cd in Baori coal decreased due to its more silicate,which can react with gaseous cadmium.For Xinjiang coal,it contained more alkali and alkaline earth metal,which can form low-temperature eutectic with other minerals,leading to the release of mineral-rated Cd.In addition,the results of the FactSage chemical thermodynamic simulation were consistent with the experimental phenomena.The calculation showed that Cd was a volatile element.At low temperature,almost all Cd in coal was volatilized in the form of gaseous elemental cadmium.Cadmium does not react with elements such as Si,Al,Cl,Fe and alkali metals in coal,but these minerals would adsorb or encapsulate Cd which affected the release of cadmium in pyrolysis process.
In order to reveal the release mechanism of heavy metal cadmium(Cd) in low rank coal during pyrolysis process,two low rank coals from Xinjiang and Neimeng(Baori) province was pyrolyzed at different temperature from400 to 800 ℃ in an aluminum crucible reactor to investigate the migration behavior of Cd during pyrolysis process.Thetechnical route of"coal pyrolysis behavior-the occurrence mode of Cd in coal-the release behavior of Cd-FactSage simulation"was proposed.Firstly,the pyrolysis characteristics of two low rank coals were investigated in the TGA reactor,the occurrence mode of Cd was studied by stepwise chemical extraction and scanning electron microscopy.Then,the release behavior of Cd in coal pyrolysis process was studied in a fixed aluminum crucible reactor.Finally,the FactSage thermodynamic simulation calculation was used to systematically reveal the release mechanism of Cd during pyrolysis process.The results showed that the occurrence mode of Cd was crucial for its volatility during coal pyrolysis process.The main occurrence mode for these two rank coals was organic bound state Cd,which could be released with the decomposition of organic matter in coals at low temperature. Baori coal contained more iron-manganese bound Cd,and most of them were distributed on the surface of coal particles,which was easily decomposed by heat and cause it to volatilize.At high temperature,the volatility of Cd in Baori coal decreased due to its more silicate,which can react with gaseous cadmium.For Xinjiang coal,it contained more alkali and alkaline earth metal,which can form low-temperature eutectic with other minerals,leading to the release of mineral-rated Cd.In addition,the results of the FactSage chemical thermodynamic simulation were consistent with the experimental phenomena.The calculation showed that Cd was a volatile element.At low temperature,almost all Cd in coal was volatilized in the form of gaseous elemental cadmium.Cadmium does not react with elements such as Si,Al,Cl,Fe and alkali metals in coal,but these minerals would adsorb or encapsulate Cd which affected the release of cadmium in pyrolysis process.
引文
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[20]吕海亮,陈皓侃,李文,等.义马煤中几种微量污染元素的赋存形态研究[J].燃料化学学报,2003,31(1):31-34.LHailiang,CHEN Haokan,LI Wen,et al. Mode of occurrence of several harmful trace elements in yima coal[J]. Journal of Fuel Chemistry and Technology,2003,31(1):31-34.
[21] ZHANG Guanjun. Mineral matter behavior during co-gasification of coal and biomass[D]. Freiberg:Technischen Universitt Bergakademie Freiberg,2014.
[2] YILGIN M,DURANAY N D,PEHLI VAN D.Co-pyrolysis of lignite and sugar beet pulp[J]. Energy Conversion&Management,2010,51(5):1060-1064.
[3] SWAINE D J,GOODARZI F. Environmental aspects of trace elements in coal[J].Energy&Environment,1995,2(1):1-2.
[4]党钾涛,解强.煤中有害微量元素及其在加工转化中的行为研究进展[J].现代化工,2016(7):59-63.DANG Jiatao,XIE Qiang.Hazardous trace elements in coal and their behaviors during processing and utilization[J].Modern Chemical Industry,2016(7):59-63.
[5] ZAJUSZ-ZUBEK E,KONIECZYNSKI J.Dynamics of trace elements release in a coal pyrolysis process[J]. Fuel,2003,82(10):1281-1290.
[6] GUO R,YANG J,LIU Z.Behavior of trace elements during pyrolysis of coal in a simulated drop-tube reactor[J]. Fuel,2004,83(6):639-643.
[7] WEI X,ZHANG G,CAI Y,et al. The volatilization of trace elements during oxidative pyrolysis of a coal from an endemic arsenosis area in southwest Guizhou,China[J]. Journal of Analytical&Applied Pyrolysis,2012,98(98):184-193.
[8] GUO R,YANG J,LIU D,et al.Transformation behavior of trace elements during coal pyrolysis[J]. Fuel Processing Technology,2002:77-78(25):137-143.
[9] SCACCIA S,MECOZZI R.TRACE C D,Co,and Pb elements distribution during Sulcis coal pyrolysis:GFAAS determination with slurry sampling technique[J]. Microchemical Journal,2012,100(100):48-54.
[10] JIE Wang,TOMITA A.A Chemistry on the volatility of some trace elements during coal combustion and pyrolysis[J].Energy&Fuels,2003,17(4):954-960.
[11]畅志兵,初茉,张超,等.颗粒粒径对油页岩热解产油率的影响[J].燃料化学学报,2015,43(6):663-668.CHANG Zhibing,CHU Mo,ZHANG Chao,et al.Influence of particle size on oil yield from pyrolysis of oil shale[J]. Journal of Fuel Chemistry and Technology,2015,43(6):663-668.
[12] LIU Q,HU H,ZHOU Q,et al.Effect of inorganic matter on reactivity and kinetics of coal pyrolysis[J].Fuel,2004,83(6):713-718.
[13]于敦喜,姚洪,刘小伟,等.煤燃烧中无机矿物向颗粒物的转化规律[A].燃烧学学术会议[C].2007:507-510.
[14] FINKELMAN R B.Modes of occurrence of potentially hazardous elements in coal:Levels of confidence[J].Fuel Processing Technology,1994,39(1-3):21-34.
[15] DAI S,WANG P,WARD C R,et al.Elemental and mineralogical anomalies in the coal-hosted Ge ore deposit of Lincang,Yunnan,southwestern China:Key role of N2-CO2-mixed hydrothermal solutions[J].International Journal of Coal Geology,2014,152:19-46.
[16] DAI S,SEREDIN V V,WARD C R,et al.Enrichment of U-Se-MoRe-V in coals preserved within marine carbonate successions:Geochemical and mineralogical data from the Late Permian Guiding Coalfield,Guizhou,China[J]. Mineralium Deposita,2015,50(2):159-186.
[17] LIU J,YANG Z,YAN X,et al.Modes of occurrence of highly-elevated trace elements in superhigh-organic-sulfur coals[J]. Fuel,2015,156(4):190-197.
[18] BUNT J R,WAANDERS F B. Trace element behaviour in the Sasol-Lurgi MK IV FBDB gasifier. Part 1-The volatile elements:Hg,As,Se,Cd and Pb[J].Fuel,2008,87(12):2374-2387.
[19] LU H,CHEN H,LI W,et al.Occurrence and volatilization behavior of Pb,Cd,Cr in Yima coal during fluidized-bed pyrolysis[J].Fuel,2004,83(1):39-45.
[20]吕海亮,陈皓侃,李文,等.义马煤中几种微量污染元素的赋存形态研究[J].燃料化学学报,2003,31(1):31-34.LHailiang,CHEN Haokan,LI Wen,et al. Mode of occurrence of several harmful trace elements in yima coal[J]. Journal of Fuel Chemistry and Technology,2003,31(1):31-34.
[21] ZHANG Guanjun. Mineral matter behavior during co-gasification of coal and biomass[D]. Freiberg:Technischen Universitt Bergakademie Freiberg,2014.