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题名	碎煤低焦油两段气化技术基础实验研究
作者	曾玺
学位类别	博士
答辩日期	2012-05-24
授予单位	中国科学院研究生院
导师	许光文
关键词	煤炭 两段气化 半焦气化 有氧热解 焦油重整
其他题名	Fundamentals of two-stage gasification process of crushed coal for fuel gas with low tar content
学位专业	化学工程
中文摘要	两段气化能有效脱除焦油，是一种生产洁净燃气的气化技术。目前，该技术主要用来处理块状生物质颗粒，存在放大困难、原料局限等问题。我国的工业煤气主要利用固定床气化优质块煤生产，不能利用低成本碎煤。利用两段气化的原理，开发出可以处理廉价而丰富的碎煤、且更易大型化、燃气中焦油含量低的气化技术具有重要的经济、环境和社会意义。 结合流化床和下吸式固定床反应器的优点，过程所创造性地提出通过将流化床和下吸式固定床反应器合理组合而形成的新型碎煤低焦油两段气化技术。煤在流化床热解器中充分热解，热解产物全部进入下吸式固定床气化炉中。半焦在气化炉中完全气化，同时利用半焦层强化焦油的高温裂解、部分氧化及催化重整而脱除焦油。本论文围绕该技术首次开展系统基础研究和工艺中试，首先在实验室开展了含氧/蒸汽气氛的流化床高温煤热解、固定床内半焦对焦油脱除特性和半焦二氧化碳气化动力学研究，优化确立了流化床热解和固定床气化及焦油裂解的匹配条件。在此基础上，设计建造了煤处理量50 kg/h的碎煤两段气化中试装置，并开展了连续性试验，对创新的两段气化工艺进行了验证，为技术放大与产业化提供了理论与数据支撑。本论文的主要研究内容和结果如下： (1) 碎煤流化床热解基础研究。重点考察了热解温度和热解气氛对新疆次烟煤在流化床内热解产物分布规律的影响。N2气氛中，随热解温度的升高，气体产率明显增加、半焦和焦油产率降低；含氧气氛中，半焦和焦油产率显著降低、气体产率增加，CO和CO2增加显著；水蒸汽的加入使得半焦和焦油产率进一步降低，气体、尤其是H2产率升高。纯N2气氛中，半焦的比表面积小，微孔数目少；在含氧和含蒸汽气氛中，半焦的比表面积大幅增加，尤其是微孔的比表面积增加。半焦气化活性与比表面积密切相关，纯N2气氛下生成半焦的气化活性最低、含蒸汽气氛下生成的半焦活性最高。然而，过高温度(900 ºC)和过量空气系数(ER，试验时单位质量煤实际消耗的空气质量与单位质量煤理论上完全燃烧所需的空气量之比，如0.22)会引起半焦出现石墨化倾向，造成气化活性降低。与纯N2气氛中煤热解产生的焦油相比，含氧气氛下的焦油重质组分多，热裂解活性差；而含蒸汽气氛下的焦油轻质组分增多，热裂解活性较高，有利于固定床对焦油的进一步重整。对于实际的自热式两段气化工艺，其上游流化床热解的最佳条件确定为：850 ºC、ER=0.15及水蒸汽通入量与煤的质量比S/C=0.15。 (2) 固定床脱除焦油基础研究。在下吸式固定床反应器内考察了高温裂解、部分氧化、半焦催化重整对流化床热解生成焦油的脱除效果的影响。结果表明，部分氧化和半焦催化重整在脱除焦油和气体改质方面较热裂解更有效，且对焦油的脱除具有一定的选择性，可有效抑制焦油转化过程生成积碳的反应。半焦的微孔结构及其所含金属氧化物的种类和含量都对焦油的脱除具有重要影响。比表面积越大，对焦油的催化重整作用越强。重整后，半焦的微孔比表面积下降显著，而介孔的比表面积大幅增加。通过在脱灰半焦中负载金属氧化物发现，不同种类的金属氧化物对焦油的脱除效果不同，依次为CaO > Fe2O3 > Na2O > MgO。两段气化工艺的下吸式固定床焦油裂解的最佳操作条件确定为：温度1100 °C、ER=0.04、气体停留时间大于1.3 s (对应的半焦床层的高度大于11 cm)。 (3) 半焦气化动力学研究。利用微型流化床反应分析仪(MFBRA)和热重分析仪(TGA)研究半焦与CO2气化反应动力学，通过对比发现，两种分析仪器求取的低温段半焦CO2气化活化能非常接近(MFBRA：282.82 kJ/mol；TGA：285.46 kJ/mol)，说明了实验求取的动力学数据可靠性。然而两者求取的频率因子差别很大，造成了气化反应速率的差异，充分说明利用热重求取的动力学数据不能直接指导流化床反应器的设计。同时，MFBRA能有效避免TGA中因切换气体造成的气化剂分压对气化反应过程的影响。 (4) 两段气化中试研究。对煤处理量50 kg/h的自热式两段气化中试平台的实验表明，随着流化床热解器温度的升高(900 ºC)，热解气的有效组分下降，半焦气化速率增加，产品气中CO和H2的含量明显增加，气体热值增加。试验过程中流化床热解器内的压降小于2 kPa，随着固定床气化炉内半焦床层的增加，气化炉压降不断增加。当流化床热解温度为900 ºC、气化炉内温度约为1100 ºC时，气化炉出口燃气中的焦油含量为94 mg/Nm3，充分说明了两段气化工艺能够有效脱除焦油的技术特征，为放大设计提供了重要保障。
英文摘要	The so-called two-stage gasification is featured by physically separating fuel pyrolysis and char gasification, and has been received more and more attentions for its effectiveness in removing tar inside the gasifier. Nonetheless, the technology has so far only applied to biomass so that the realized fuel treatment capacity is low and its fuel adaptability is limited. In China, coal is the major fuel for fuel gas production, and the existing fixed gasification technology is adaptive only to lump coal so that the fuel gas production can not use the low-cost powder coal. The development of new two-stage gasification technology to gasify powder coal and produce the clean fuel gas with low tar content is thus significant in both economy and environment. Institute of Process Engineering, Chinese Cademy of Sciences proposed a new two-stage gasification process consisting of a fluidized bed pyrolyzer and a downdraft fixed bed gasifier to gasifiy powder coal to produce clean fuel gas with low tar content. Coal is first autothermally pyrolyzed in a fluidized bed (FB) reactor, and its products, including pyrolysis gas, tar and char are all forwarded to a downdraft fixed- bed reactor to implement char gasification and also pyrolysis gas upgrading via char-catalyzed thermal cracking, partial oxidation and reforming. This study is about this new gasification technology, and it conducted first the laboratory studies on fluidized bed coal pyrolysis, tar removal in fixed bed and char gasification kinetics. On these fundamental studies, a pilot two-stage gasification apparatus treating 50 kg/h coal was built and operated to verify the process feature of the new technology. The results of these studies would provide great support to the technology scale-up. The following summarizes the major research contents and their results. (1) Fundamentals of coal pyrolysis in fluidized bed Experimental pyrolysis of a kind of subbituminous coal was conducted in a laboratory fluidized bed at different temperature and in varied atmospheres to study the product distribution, tar composition and char gasification reactivity. The yield of pyrolysis gas, especially of H2 and CO, increased with elevating the temperature and mass ratio of steam to coal (S/C). Adding O2 to the reaction atmosphere promoted the formation of CO and CO2 but decreased that of H2. The inclusion of O2 and steam into the atmosphere resulted in char with larger surface area and more micropores. Slight graphitization of char was observed for the prolysis at 900 ºC in N2 atmosphere or at 850 ºCin the atmosphere with oxygen (e.g., at an excessive air ratio of 0.22) so that the generated char had reduced char gasification reactivity. Analyzing the produced tar via TG-FTIR spectroscopy clarified that the presence of steam affected greatly the tar composition by forming more aliphatic hydrocarbons and fewer single-ring aromatics, phenols, and ketonic species. Consequently, the pyrolysis gas product from a steam-containing atmosphere would be easier to crack and be reformed in the downstream char gasifier. (2) Tar removal in downdraft fixed bed gasifier The removal of tar and in turn the upgrading of the pyrolysis gas generated in a fluidized bed pyrolyzer were investigated in a downstream laboratory fixed-bed reactor through the means of thermal cracking, partial oxidation and char catalytic reforming. The addition of oxygen to the fixed-bed reactor and the adoption of a char bed inside the reactor facilitated evidently the tar removal performance and improved the fuel gas quality. Analyzing the tar sample collected at the outlet of the tar removal reactor (fixed bed) with GC-MS clarified that both tar oxidation and char-catalyzed reforming exhibited certain selectivity to the chemical species present in the tar. The properties of char played important roles on the catalytic activity of the char. The higher the specific surface area, the better the activity of the char for removing tar was. The spent char had much reduced specific surface area of micropores but evidently increased specific surface area of mesopores, and the latter was even by up to 3 times. The tar-removal tests with metal oxides impregnated on the demineralized char particles demonstrated that both Ca and Fe oxides enabled better catalytic activity in comparison with Na and Mg oxides. On these studies, the viable operating conditions for tar removal over a char bed were at 1100 ºC with an excessive air ratio (ER) of 0.04, and the required gas residence time was above 1.3 s. (3) Char gasification kinetics via MFBRA and TGA The char gasification in CO2 was studied by isothermal method at different temperatures in both the micro fluidized bed reaction analyzer (MFBRA) and thermogravimetric analyzer (TGA). The adopted amount and size of quartz sand aprticles and fluidizing gas flow rate for MFBRA and the suitable amount and size of char, gas flow rate and crucible height for TGA were optimized to minimize the heat and mass transport limitations for gasification reactions. The shrinking core model was adopted to depict the char gasification reaction rate, finding that it expressed well the experimental data. The measurements in TGA and MFBRA resulted in the similar reaction activation energy for the gasification reaction, which was 282.82 kJ/mol for MFBRA and 285.46 kJ/mol for TGA. Nonetheless, the frequency factor was greatly different, much larger and thus much higher gasification rate in MFBRA. It is shown also that MFBRA can avoid the unsteady partial pressure of gasification reagent (CO2) caused in the gas switching method applied to TGA. (4) Pilot test of two-stage gasification process On the basis of the preceding results of fundamental studies, a pilot plant treating 50 kg/h coal was designed, built to tested to verify the the technology feasibility of the proposed two-stage gasification process. A kind of lignite from inner Mongolia was gasified in the pilot test, and the effects of operating temperature in the pyrolysis (first) stage and gasification (second) stage were examined. The results showed that the higher temperatures in both the reactors were benifitial to the upgrading of the fuel gas by leading lower tar content. At 900 ºC for fluidized bed and 1100 ºC for fixed bed the tar content in the produced fuel gas was found to be 94 mg/Nm3, verifying thus technical feasibility and feature of the two-stage gasification process.
语种	中文
公开日期	2013-09-25
内容类型	学位论文
源URL	[http://ir.ipe.ac.cn/handle/122111/1829]
专题	过程工程研究所_研究所（批量导入）
推荐引用方式 GB/T 7714	曾玺. 碎煤低焦油两段气化技术基础实验研究[D]. 中国科学院研究生院. 2012.

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