摘要
煤炭气化是煤炭洁净转化利用的关键技术和龙头技术,实际的煤气化技术一般以水蒸气配比一定量的空气或氧气作气化剂。在气化剂中加入一定量的CO_2替代部分水蒸气作气化剂,生产出的浓度较高CO或者具有不同组成的煤气,可满足不同的煤气用途的需要;同时,可减少水蒸气的消耗量,也即减少污水的处理量,有着非常重要的现实意义。为此,煤炭科学研究总院青年创新基金项目(2006QN38)被批准立项,重点研究CO_2替代部分水蒸气作气化剂的气化特性。
本文结合该项目选取了三种不同变质程度的原煤——宝一褐煤、神东烟煤及王坡无烟煤制得的煤焦为研究对象。对三种原煤及其焦样进行了基础的煤质分析,对三种焦样还进行了比表面及孔径分布测试。在此基础上运用热分析技术,较系统且多角度地研究了三种煤焦在常压、1173～1323K,四种不同水蒸气及CO_2配比下的气化反应特性。用混合反应模型求取了气化反应动力参数,分析了动力学参数间的补偿效应,探讨了煤焦与水蒸气及CO_2共气化过程中的交互作用,最后进行了单一煤焦(褐煤焦)的常压管式炉实验,分析了所得煤气的煤气组成及煤气产率。
根据常压热天平实验数据,分别计算了三种煤焦的固定碳转化率、反应性指数及平均比气化速率,结果表明:
(1)三种煤焦的反应活性由高到低的顺序为神东煤焦＞宝一煤焦＞王坡煤焦,这与前人的研究结论——煤焦的反应性一般随煤化程度的提高而降低不相符合,分析原因认为可能是由于煤中矿物质的含量不同,所起的催化作用不同引起的。
(2)随着反应温度升高,在同一反应时间下,所有实验气化剂配比条件下,三种煤焦的固定碳转化率都呈现出增加的趋势;而且,随温度升高,煤焦达到最大转化率的时间缩短。
(3)随气化剂中水蒸气含量的增加、CO_2含量的减少,反应进行相同时间时,除个别实验外,所有实验条件下,三种煤焦碳转化率均呈现出增加的趋势,纯水蒸气气化的碳转化率均高于纯CO_2气化时的碳转化率。
(4)同一气化剂配比下,三种煤焦的反应性指数、平均比气化速率均随温度的升高而增大;除个别实验外,同一气化温度下三种煤焦的反应性指数、平均比气化速率,均随气化剂中水蒸气含量的增加、CO_2含量的减少而增大。
(5)混合模型对实验数据的线性拟合度高,得到的有关动力学参数n,k,Ea,lnk_0表明:三种煤焦的总反应级数介于0.7197～1.7846之间:三种煤焦与纯水蒸气反应的活化能介于134.8～187.5kJ/mol之间,与60%水蒸气+40%CO_2反应的活化能介于119.4～180.8kJ/mol之间,与30%水蒸气+70%CO_2反应的活化能介于136.9～199.7kJ/mol之间,与纯CO_2反应的活化能介于126.6～230.9kJ/mol之间。三种煤焦的活化能越高,对应的指前因子也越大,两者之间存在动力学补偿效应,其分析可由过渡态理论予以解释。
(6)两种配比的水蒸气及CO_2混合气体作气化剂时,宝一煤焦只在气化剂配比60%H_2O+40%CO_2、气化温度1323K时对气化速率产生了明显的交互促进作用,其它实验条件下对气化速率的交互作用不明显;神东煤焦及王坡煤焦在所有实验条件下对气化速率均产生了明显的交互促进作用。
通过对宝一褐煤焦1273K,常压管式炉实验所得煤气的气体组成分析,得到以下结论:
(1)在煤焦与水蒸气及CO_2共气化过程中,水蒸气及CO_2均参与了反应。
(2)在反应进行相同的时间时,随气化剂中水蒸气含量的降低,CO_2含量的增加,混合煤气中H_2的含量减少,CO的含量大致呈递增趋势,CO_2含量大致呈递增趋势,CH_4大致呈递减的趋势。
(3)随着水蒸气含量的降低,CO_2含量的增加,反应进行相同时间,所得混合煤气中H_2/CO的值逐渐降低,H_2+CO_2的含量逐渐减少。
(4)宝一煤焦与水蒸气及CO_2在管式炉共气化实验中对煤气产率产生了交互促进作用,提高了煤气产率;宝一煤焦与纯水蒸气气化的煤气产率高于与纯CO_2气化的煤气产率,而且宝一煤焦与纯CO_2气化的煤气产率也不低。
Coal gasification is the key and main technology of clean coal transformation and utilization technology. Generally, coal gasification technology practically use steam together with a certain ratio of air or oxygen as gasifying agent. If a certain amount of CO_2 was added in the gasifying agent to replace a part of steam, then higher-concentration CO gas or gas with different composition will be produced, which can meet needs of different gas utilization. At the same time, steam consumption can be reduced, that is, the treated volume of sewage can be reduced, so this technology has very important practical significance. Therefore, a project fund of youth innovation (2006QN38) supported by China Coal Research Institute has been approved, which priority research on the gasification characteristic of CO_2 substitute part of steam as gasifying agent.
Three coal char samples made from three different rank of raw coals—Baoyi lignite, Shendong bituminite and Wangpo anthracite, were selected as study objects in this paper combined with the project. Besides basic coal quality analysis of raw coal and coal chars, specific surface area and pore diameter distribution test of coal chars also have been done. On this basis, coal char reactivity at normal pressure, with temperature 1173～1323K and four different mixture ratio of steam and CO_2 were studied systematically by using thermal analysis technology. Mixture reaction model was used to calculate kinetic parameters. Compensation effect between kinetic parameters was analyzed and synergistic effect of co-gasification of coal char with steam and CO_2 were researched in this paper. At last, tube type furnace experiments for one kind coal char (Baoyi coal char) have been done on normal pressure; coal gas composition and gas productivity were analyzed.
Based on thermobalance experiment date carbon conversion, reactivity exponent (R) and mean specific rate were calculated, the result show that:
(1) The reactivity of three kind of coal chars from high to low are: Shendong coal char>Baoyi coal char>Wangpo coal char. This result did not agree with the conclusion of predecessors, which is the reactivity of coal char currently reduce along with the increase of coal rank; this may be caused by different content of minerals and then different catalysis in raw coal.
(2) The carbon conversion of three coal chars, at the same reaction time with all experiment mixture ratio of gasifying agent all increased, along with the increase of temperature, and the time needed to reach the lagest carbon conversion were shorten.
(3) Except one or two experiment, the carbon conversion of three coal chars, at the same reaction time under all experiment conditions all increased, along with the increase of steam content and the decrease of CO_2 content in gasifying agent. The reaction rates of char-steam were lager than that of char-CO_2 at all conditions.
(4) The reaction exponent and the mean specific reaction rate with the same mixture ratio of gasifying agent all increased along with the increase of temperature. Except one or two experiment, the reaction exponent and the mean specific reaction rate of three coal chars at the same temperature all increased along with the increase of steam content and the decrease of CO_2 content in gasifying agent.
(5) Mixture reaction model were used to deal with experimental data, and the kinetic parameters n, k, Ea, lnk_0 were gained. The total reaction order of three coal chars were 0.7197～1.7846; the activity energy of three coal chars with 100% steam were 134.8～187.5kJ/mol, with 60% steam and 40% CO_2 were 119.4～180.8 kJ/mol, with 30% steam and 70% CO_2 were 136.9～199.7kJ/mol , with 100%CO_2 werel26.6～230.9kJ/mol. The higger the activity the lager the pre-exponential, kinetic compensation effect were observed between them, which can be explained by transition state theory.
(6) When the mixture gas of steam and CO_2 were used as gasifying agent, synergistic effect on gasification reaction rate was observed for Baoyi coal char gasification with 60% steam and 40% CO_2 at 1323K, no synergistic effect on gasification reaction rate were found for other conditions. Obvious synergistic effects on gasification reaction rate were found for Shendong coal char and Wangpo coal char on all experimental conditions.
Based on the coal gas composition of Baoyi coal char gasificatin in a tube type furnace at 1273K and normal pressure, the follwing result were gained:
(1) Both steam and CO_2 participated in the co-gasification process of coal char with steam and CO_2.
(2) Along with the increase of steam content and the decrease of CO_2 content in gasifying agent, at the same reaction time, the content of H_2 in the mixed gas decreased, the content of CO approximately increased, the content of CO_2 approximately increased and the content of CH_4 approximately decresed.
(3) At the same reaction time, the value of H_2/CO reduced and the content of H_2+ CO_2 in the mixed gase decreased, along with the decrease of steam content and the increase of CO_2 content in gasifying agent.
(4) Synergistic effects on gas productivity were observed during Baoyi coal char gasification with the mixture gas of steam and CO_2 in tube type furnace, and gas productivity is increased. The gas productivity of Baoyi coal char gasification with 100% H_2O is higher than with 100% CO_2, moreover the gas productivity of Baoyi coal char gasification with 100% CO_2 is not low.
引文
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