Knowledge Management System Of Institute of process engineering,CAS
|关键词||低阶煤 复合流化床 气化 热解 富甲烷合成气|
我国目前天然气供需矛盾严重，利用国内丰富的低阶煤资源，发展坑口煤制天然气 (SNG) 成为一种必然选择。构建高效、低成本煤气化制备SNG工艺要求煤气化过程生产的合成气中富含甲烷。因此，煤制天然气工厂一般采用Lurgi移动床块煤加压气化技术生产高甲烷含量的合成气作为制备SNG的原料气，同时副产高附加值的焦油，但这也造成了SNG工厂块煤资源紧缺而碎煤严重过剩的现象。为了解决这一难题，本研究提出了复合流化床碎煤气化工艺，通过耦合煤的底部流化床气化和上部输送床快速热解，提高合成气中的甲烷含量，同时联产煤焦油。针对工艺的技术特点，通过研究底部流化床气化和催化气化特性以及输送床快速热解特性获得了提高合成气甲烷含量和优化油品品质的操作参数，在复合流化床中通过耦合碎煤的热解和气化制备出了与移动床块煤加压气化甲烷含量相当的合成气和高品质焦油。本论文的主要研究内容和研究结果如下：1. 流化床煤气化特性。(1) 以次烟煤和褐煤为气化原料。增加过量氧气系数和水碳摩尔比，提高气化温度和停留时间，有利于煤的转化，增加气体产率。常压气化中甲烷主要来自煤的热解，因此提高碳转化率会导致合成气中甲烷含量的下降。过量氧气系数、气化温度和停留时间的增加与添加催化剂都会导致H2/CO比下降，而增加水蒸气的量会提高H2/CO体积比。褐煤中含有的钙能够催化碳的气化，因此在次烟煤中添加Ca(OH)2催化剂之后能显著提高次烟煤的气化性能。(2) 以添加10 wt.% Ca(OH)2催化剂的褐煤为催化气化原料。常压气化条件下，添加催化剂能够明显提高碳转化率和气体产率，对甲烷的产率影响不大。增加过量空气系数和水碳摩尔比，提高催化气化温度能够提高催化气化反应性，但是不利于热解甲烷的形成。常压下较低的气化温度和较高的水碳摩尔比能生产H2/CO比高达3的合成气，甲烷含量约为3~5 vol.%。增加流化床催化气化压力，能够提高碳转化率和甲烷产率，合成气中的甲烷含量和H2/CO比增大。在850℃，ER = 0.05，S/C = 1，气化压力为1.5 MPa下，碳转化率为70%左右，甲烷产率增至0.142 Nm3/kg-coal，合成气中甲烷含量高达10.1 vol.%，H2/CO体积比接近3。 2. 输送床煤快速热解特性。通过模拟底部流化床产生的含水蒸气的合成气气氛，在上部输送床中考察了热解温度和热解气氛对煤快速热解的影响。相比于N2气氛，在600℃以下水蒸气和合成气气氛对焦油的产率几乎没有影响；但是随着温度增加，水蒸气降低了焦油产率而合成气提高了焦油产率。合成气中的H2是提高焦油产率的关键组分，这是因为它能够作为大分子自由基的稳定剂和加氢剂。在模拟真实工况的含水蒸气的合成气气氛下，600℃下焦油产率达到最大值10.5 wt.%，高于格金分析焦油产率1.1 wt.%。之后，随着热解温度的增加，挥发分二次反应增强，焦油产率逐渐下降。焦油组成分析表明600℃以上含水蒸气合成气气氛结合了水蒸气和合成气的各自的优势，即：合成气气氛能够同时提高轻质焦油和重质焦油的产率而水蒸气能够降低重质焦油的产率。底部流化床气化产生的含水蒸气合成气在热解过程中有利于甲烷的生成。3. 复合流化床耦合煤热解和气化。(1) 以次烟煤为原料，复合流化床中较低的过量空气系数和流化床气化温度，较高的输送床热解温度有利于甲烷的形成。合成气中的甲烷含量随着压力的增大而增加，但是随着水碳质量比增加而减少。在常压，ER=0.1，S/C=0.1流化床温度900℃和输送床温度700℃下，耦合煤的输送床热解后合成气中甲烷含量从5.1 vol.%增加到7.1 vol.%。在1.4 MPa下，合成气甲烷含量高达11.2%，是普通流化床中2 vol.%甲烷含量的六倍，接近Lurgi气化炉中的甲烷含量。(2) 以褐煤和负载10 wt.% Ca(OH)2的褐煤为原料。煤中添加催化剂明显增强了碳的转化。1 MPa下相比于普通气化57.4%的碳转化率，催化气化的碳转化率高达78.9%。常压下，甲烷产率为0.045 Nm3/kg-coal，使用催化剂后甲烷产率几乎不发生变化。提高压力到1 MPa，对于普通气化和催化气化，甲烷产率分别增加到0.105 Nm3/kg-coal和0.167 Nm3/kg-coal，很明显催化气化产生了更多的甲烷。耦合输送床热解之后，合成气中甲烷的含量不同程度地增加。1 MPa下，普通气化合成气中甲烷含量从9.1 vol.%增加到11.3 vol.%；催化气化合成气甲烷含量从9.3 vol.%增加到10.8 vol.%。耦合上部热解会导致H2/CO比略有下降，基本保持在2左右。上部输送床热解产生了相当量的焦油，添加催化剂和增加热解压力降低了煤焦油产率，但是提高了油品品质。复合流化床耦合催化热解和气化能够实现高的碳转化率，生产富甲烷、高H2/CO合成气和副产品质较好的焦油，这是SNG工厂高度期待的气化技术。
The ever increasing demand of natural gas in China as well as the limited domestic supply provides a strong incentive to produce synthesis natural gas (SNG) from coal, especially low–rank coal. A high initial yield of CH4 produced from the gasifier is highly expected for the construction of high–effective and low–cost coal to SNG project since the reaction of H2 with CO to form CH4 (methanation) involves approximately 20% efficiency loss. Now, Lurgi moving bed gasifier has been widely used for SNG production as results of its producer gas with a high content of CH4. This gasifier also produces a considerable amount of coal tar which is highly demanded by chemical industry due to the limited availability of crude oil in our country. But Lurgi gasifier only adopts lump coal above 6 mm, there is a great need treat the more abundant powder coal, such as below 10mm via Lurgi–type gasifier. Accordingly, an integrated fluidized bed coupling an upper transport bed pyrolysis with a bottom fluidized bed gasification was proposed to process low–rank powder coal and co–produce coal tar and CH4–rich syngas. In this study, bottom fluidized bed gasification and upper transport bed pyrolysis were separatedly carried out to obtain the methods of increasing the CH4 content and the parameters of improving tar production. Finally, the CH4–rich syngas and coal tar were successfully prepared by coupling coal pyrolysis and gasification in an integrated fluidized bed.The main results from this study are summarized, as follows:1. Coal gasification in a fluidized bed. (1) The gasification characteristic of a subbituminous coal and two lignites with sizes of 0.5–1.0 mm was studied in a fluidized bed. Increasing excess oxygen ratio (ER) and steam to carbon mole ratio (S/C), and raising gasification temperature and residence time were beneficial to the conversion of carbon to gas. At atmosphere pressure, CH4 was mainly derived from coal pyrolysis and raising carbon conversion thus led to the reduction of CH4 content. Increasing ER, gasification temperature and residence time, and adding Ca(OH)2 catalyst resulted in the decline of H2/CO ratio while increasing S/C was help to increase H2/CO ratio. The presence of Calcium in lignite promoted the conversion of carbon to gas and adding Ca(OH)2 markedly improved the gasification reactivity. (2) The catalytic gasification characteristic of a lignite with sizes of 1–1.6 mm after impregnating 10 wt.% Ca(OH)2 was investigated in the same fluidized bed. At atmosphere pressure, adding catalyst increased carbon conversion and gas yield while CH4 yield kept no change. Increasing ER, S/C and gasification temperature could significantly raise carbon conversion but went against the production of CH4. Under conditions of appropriate temperature and S/C, H2/CO ratio in syngas was about 3. Elevating gasification pressure could enhance carbon conversion and CH4 production. At 1.5 MPa, catalytic gasification reached good aims with the carbon conversion of about 70% and CH4 yield of 0.15 Nm3/kg–coal. And its producer gas, with the best H2/CO ratio of about 3, was highly rich in CH4 was with its content of 10.1 vol.% that is well–matched in terms of SNG production.2. Coal rapid pyrolysis in a transport bed. An integrated fluidized bed (IFB) consisting of an upper transport bed section and a bottom fluidized bed section was adopted to investigate the transport bed pyrolysis using a kind of subbituminous coal with sizes of 0.3–0.4mm by varying its reaction temperature and reaction atmosphere adjusted to simulate steam–containing syngas produced by the bottom fluidized bed char gasification. Steam and syngas, in comparison with N2, as the reaction atmosphere little affected the tar yield below 600 oC but significantly decreased it for the former and increased it for the latter at rather higher temperatures. The presence of H2 in the syngas increased tar yield significantly because it could suppress polymerization and condensation reactions through providing H as radical stabilizer and hydrogenation agent. In the steam–containing syngas atmosphere, the tar yield obtained from transport bed rapid pyrolysis increased rapidly with raising temperature to a peak value of 10.5 wt.% (daf) at 600 oC, about 1.1 wt.% higher than the Gray–King assay yield, and then decreased due to the excessive secondary reactions. Analyzing tar composition further showed that steam–containing syngas combined their respective advantages that syngas improved the yields of both light and heavy tars while steam reduced the heavy tar yield, especially at temperatures above 600 oC. The steam–containing syngas atmosphere promoted CH4 production in comparison of the case in N2 atmosphere. 3. Coupling coal pyrolysis with gasification in an integrated fluidized bed. (1) This part investigated the effects of major operating parameters on CH4 content in the syngas produced from processing a kind of subbituminous coal with sizes of 0.2–0.5mm for upper pyrolysis and 1–2 mm for bottom gasification in an integrated fluidized bed. It was found that the formation of CH4 was facilitated under the conditions of lower overall excessive oxygen ratio (ER), lower temperature of gasification (830–970 oC), higher temperature of pyrolysis (500–700 oC) and higher holdup of coal in transport bed. The CH4 content in the produced syngas increased with elevating the operating pressure but decreased with increasing the steam to carbon mass ratio (S/C) for gasification. Coupling transport bed pyrolysis with fluidized bed gasification at atmospheric pressure caused the CH4 content in the syngas to be 7.1 vol.% under the conditions of ER = 0.1, S/C = 0.1, gasification temperature of 900 oC and pyrolysis temperature of 700 oC, which was higher than 5.1 vol.% for the case without coupling coal pyrolysis. At the operating pressure of 1.4 MPa the CH4 content reached about 11.2 vol.%, about six times higher than 2.0 vol.% for the usual fluidized bed gasification and close to 12.0 vol.% for the Lurgi gasifier. (2) A kind of lignite before and after impregnating 10 wt.% Ca(OH)2 with sizes of 0.2–0.3 mm for upper pyrolysis and 1.0–1.6 mm for bottom gasification was respectively investigated in an integrated fluidized bed. Catalytic coal gasification could significantly enhance carbon conversion. For example, at 1.0 MPa catalytic gasification resulted in a higher carbon conversion of 78.9% compared to 57.4% without catalyst in lignite. At atmosphere, CH4 yield with about 0.045 Nm3/kg–coal kept no change after using catalyst. Under elevated pressure, CH4 yield increased to 0.105 Nm3/kg–coal and 0.167 Nm3/kg–coal for both without and with catalyst, respectively. It is obvious that catalytic gasification gave rise to a higher CH4 yield. After coupling with upper transport bed pyrolysis, the content of CH4 in the syngas was raised to one degree or another. For example, at 1.0 MPa, CH4 content increased from 9.1 vol.% and 9.3 vol.% to 11.3 vol.% and 10.8 vol.% for before and after adding catalyst, respectively. Coupling upper pyrolysis led to a slight decline of H2/CO ratio while its value was still about 2. After adding transport bed pyrolysis, a considerable amount of tar was produced. Adding catalyst and elevating pressure was against the tar yield while beneficial to improve its quality. The integrated fluidized bed catalytic gasification using inexpensive Ca(OH)2 enjoyed CH4–enriched syngas with a better H2/CO and a higher quality tar at a higher carbon conversion of 78.9%, which is highly anticipated in the SNG plant.
|陈兆辉. 复合流化床低阶煤气化耦合热解制备富甲烷合成气和焦油[D]. 北京. 中国科学院研究生院,2016.|
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