褐煤自燃特性热重实验及动力学分析

第43卷第6期热力发电Vol 43 No 62014年6月THERMAL POWER GENERATIONJun.2014褐堞旬燃特性热重实验及动力学分析张斌1,刘建忠,赵卫东2,袁绍1,王智化,周俊虎1,岑可法1(1.浙江大学能源清洁利用国家重点实验室,淅江杭州310027;2.西安热工研究院有限公司,陕西西安710032)[摘要]对锡盟褐煤和印尼褐煤进行热重实验,并与大同烟煤、平頂山烟煤、神华低灰烟煤、阳泉无烟煤等煤样进行了对比。通过分析热重曲线,将煤从吸氧氧化到燃烧的整个过程分为物理吸附、水分蒸发失重、吸氧增重、煤受热分解、燃烧和燃尽6个阶段,并得到各阶段的特征温度点。结果表明,2种褐煤的自燃倾向明显大于其他4种烟煤和无烟煤。通过 Coats-Redfern积分法研究煤样的氧化燃烧动力学特性,计算煤样的活化能,得到6种煤的氧化过程属于一级化学反应,且褐煤自燃傾向性较强;煤样粒度越小,褐煤自燃傾向性越强;随着氧气浓度的增大,煤样更易自燃。[关键词]褐煤;热重实验;特征温度;自燃倾向性;动力学分析;活化能[中图分类号]TQ530[文献标识码]A[文章编号]1002-3364(2014)06-007106[DOⅠ编号]10.3969/issn.1002-3364.2014.06.071Thermogravimetric experiments and dynamic analysison spontaneous combustion characteristics of ligniteZHANG Bin, LIU Jianzhong, ZHAO Weidong, YUAN Shao'WANG Zhihua, ZHOU Junhu, CEN Kefal(1. State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027,China2. Xian Thermal Power Research Institute Co. Ltd Xian 710032, China)Abstract: Thermogravimetric experiments on Ximeng lignite and Indonesia lignite were carriedout, and the results were compared with that of the Datong bituminous coal, Pingdingshan bituminous coal, Shenhua low ash bituminous coal and Yangquan anthracite. By analyzing the Tg andDTG curves, the whole process from coal oxidation to coal combustion could be divided into sixstages: physical adsorption, water evaporation with losing weight, absorption of oxygen with in-creasing weight, thermal decomposition, combustion and burning out. The characteristic temperatures of each stage were obtained. The results show that, the tendency of spontaneous combustionf the above two lignite was significantly obvious than that of the other four bituminous coal andanthracite Coats-Redfern integral method was used to analyze the samples oxidative combustiondynamic characteristics. Moreover, activation energy of the six coal samples was calculated to ana-lyze the tendency of spontaneous combustion of coal. The calculation results show that, the oxida-tion process of the six coal samples belonged to the first-order reaction, and the tendency of spontaneous combustion of the two lignite coal was greater. The tendency of spontaneous combustionof Ximeng lignite was affected by particle size and oxygen density. analysis shows that: the smal-ler the particle size, the stronger the tendency of spontaneous combustion; the greater the oxygendensity, the stronger the tendency of spontaneous combustion in gener收稿日期:201306-05基金项目:国家重点基础研究发展计划基金资助项目(973计划)(2012CB214906)作者简介:张斌(1989—),男,浙江温州人,硕士研究生,研读方向为热能工程H中国煤化工CNMHGE-mail: zhangbin8959@163. cor热力发电2014年Key words: lignite; thermogravimetric analysis; characteristic temperature; the tendency of spontaneous combustion; dynamic analysis; activation energy目前,低价煤褐煤由于碳化程度低,反应活性算、煤氧复合活化能、井下自燃指标气体分布预警以好,而被广泛作为高效清洁燃烧能源,但是褐煤易自及热分析、红外、色谱、SEM、XRD、XPS多种角度和燃。迄今为止,煤自燃的理论有黄铁矿导因说、细菌手段研究煤在低温氧化过程中结构和性质的变导因说、酚基导因说、煤氧复合学说、自由基作用学化4。本文采用热重分析法研究锡盟褐煤、印尼褐说、电化学作用学说、氢原子作用学说、基团作用理煤、大同烟煤、平顶山烟煤、神华低灰烟煤、阳泉无烟论等。但对于煤自燃的原因、机理和反应历程仍未煤的热重曲线和数据,并进行动力学计算,得到各煤见定论[13。被广泛接受的煤氧复合理论认为,煤的种的自燃倾向性自热与自燃是多因素作用的复杂物理化学过程。煤自燃的主要原因是低温下煤与空气中的氧相互作1实验条件用,导致煤缓慢氧化并释放热量,在适宜蓄热环境采用锡盟褐煤、印尼褐煤、大同烟煤、平顶山烟中,热量积累使煤温不断上升至着火点后自发燃烧。煤、神华低灰烟煤、阳泉无烟煤δ种煤样,其原煤煤对此,可通过煤的显微岩相组分低温氧化特性、低温质分析见表1。在实验室空气氛围中将煤样粉碎,氧化过程热力学、计算机辅助分子模型量子模拟计筛分后得到煤粉粒度为100~200m样品,密封保存。表1原煤煤质分析Table 1 Coal quality analysis of the raw coal工业分析/%煤种/(J·g-1)锡盟褐煤14.9915.6633.5735.781903947.33.350.810.4117.48印尼褐煤9.9117.742.5729.8246.773.67大同烟煤8.550.41平顶山烟煤9048.1625409神华低灰烟煤9.0928.7257.552783068.990.790.4412.05阳泉无烟煤2.0014.1676.2676.871.161.37采用瑞士 METTLER-TOLEDO公司生产的1—大同烟煤2—平顶山烟煤TGA/SDTA851e型热天平,温度准确度可达1003—神华特低灰±0.25℃,灵敏度为0.1g。以10℃/min的升温4一锡盟褐煤3--5—阳泉无烟煤6—印尼褐煤速率将样品由20℃加热到800C,样品用量约5mg。反应气氛为空气,流量为50mL/min;保护气为氮气,流量为40mL/min。2实验结果分析200400600800温度/℃图1煤样TG曲线2.1基于 TG-DTG曲线的自燃倾向分析Fig. 1 TG curves of different coals将煤从吸氧氧化到燃烧的整个过程分为物理吸附、水分蒸发失重、吸氧增重、煤受热分解、燃烧和燃1—大同烟煤2—早顶山烟煤尽6个阶段煤分子是以碳氢氧及氮原子为叶m3—神华特低灰主体的结构复杂的大分子,其不同结构部位活性不5—阳泉无烟煤一印尼褐煤同。而它们在不同的特定温度下均能参与煤氧间的化学吸附和化学反应,这些温度即为煤氧化自燃过程中的特征温度,因此可以通过研究特征温度来探4006讨煤的自燃特性121。6种煤样的TG和DTG曲线如图1、图2所示。中国煤化工CNMHGast coalshttp:www.rlfd.comcnhttp://rlfd.periodicals.net.cn第6期张斌等褐煤自燃特性热重实验及动力学分析由图1、图2可见,6种煤样受热氧化分解曲线到饱和。随着温度的升高,TG曲线开始下降,失重( TG-DTG曲线)的趋势大致相同;不同煤种各个阶速率也开始增大。这是由于煤样中的水分和部分甲段的特征温度存在差异,锡盟褐煤和印尼褐煤偏向烷、一氧化碳、水汽等气体挥发所致。图2中锡盟褐低温区,阳泉无烟煤最靠近高温区,其他3种烟煤处煤DTG曲线在48.3℃时达到峰谷,该温度为临界于中间位置。可见,锡盟褐煤和印尼褐煤热分解反温度,失重速率为-1.21%/min。临界温度越低,应所需温度较低,较易自燃,而阳泉无烟煤的反应温煤脱除水分等物质的速度越快,煤氧复合开始越早,度相对较高,最不易自燃。煤的自燃性越强。继续升温,失重速率减小,TG曲图3为锡盟褐煤温度低于30℃的TG曲线。线逐渐趋于平缓,在153.3℃左右,锡盟褐煤煤样剩101.6余质量比达到最小值95.3%。此后,小分子裂解速度加快并出现大分子结构断裂,煤表面出现许多孔隙吸附了大量氧气,此时为吸氧增重阶段。分析热重数据得到,在192.5℃左右,增重速率达到最大值0.033%/min,该温度被称为增速温度。加热温度100.0约为22.5℃时,煤样开始再次失重,到298.3℃时温度/C开始受热分解。受热分解温度越低,说明煤越容易图3锡盟福煤TG曲线被氧化分解,其自燃倾向性也越大。加热温度约为Fig 3 TG curve of the Ximeng lignite345℃达到着火点开始燃烧,煤样的失重速率开始急剧增加。加热温度约为390℃时,反应全过程中由图3可见,在初始阶段锡盟褐煤TG曲线处失重速率达到最大值一5.25%/min,此时煤分子内于上升趋势,煤样质量不断增加在23℃左右物理部发生剧烈的化学反应,燃烧速度大大增加。加热吸附阶段结束,质量达到最大,此时质量比为温度约为454.2℃时,失重过程基本停止,失重曲线101.5%。这是因为在受热温度的初始阶段,即物理趋于平缓,达到了煤的燃尽温度,标志着煤样氧化自吸附阶段,煤氧复合主要以物理吸附为主,煤的物理燃着火过程结束。表2为不同煤种的特征温度。吸附是一个可逆放热过程吸附速度相对较快,易达表2不同煤种的特征温度Table 2 The characteristic temperatures of different coals C煤种物理吸附受热分结束温度临界温度增速温度解温度着火点最大失重速率点温度燃尽点锡盟褐煤23.3298.3345.0390.0454.2印尼褐煤245.8288.3330.8405.8大同烟煤50.0232.5336.7401.7453.3500.8平顶山烟煤47.5236.7374.2441.7505.8551.7神华低灰烟煤43.3237.5345.8424.2465.0512.5阳泉无烟煤70,0524.2567.5609.0由表2可见,锡盟褐煤和印尼褐煤的临界温度、力学参数。某一时刻反应的转化率为:受热分解温度、着火点都明显低于烟煤和无烟煤(1)因此,褐煤的自燃倾向性高于其他煤种。式中,mo为失水结束时样品的质量,m1为反应结束2.2自燃动力学分析通过热重分析获得的反应动力学参数可以宏观时样品的质量,m为某一时刻样品的质量。动力学基本方程为地表征煤的整个化学反应过程,其中活化能可以定da量地表征煤的自燃倾向性,活化能越低,自燃倾向性d=k·f(a)(2)越强6,14式中,t为时间做÷为动力学机采用单个扫描速率法的 Coats-Redfern积分理函数法m,对 TG-DTG曲线进行分析,计算得出相关动CNMHG由 ArrhenIus在知http:/www.rlfd.com.cnhttp://rlfd.periodicalsnet.cn热力发电2014年k= AexpC-E/RT(3)温速率将煤样由20℃加热至800℃。反应气氛为式中:A为指前因子;E为表观活化能,kJ/mol;R为空气,流量为50mL/min;保护气为氮气,流量为普适气体常数,8.314J/(mol·K);T为热力学温40mL/min。图4为不同粒径煤样的TG曲线表4度为氧化过程中的主要特征温度。由升温速率B=dT/dt可得:—粒径60100"m2一粒径>100~120umdadT=xp(=E/RT)·f(a)(4)3一粒径>120-200um80假设反应符合反应级数模型,f(a)=(1-a)n,由Coats- Redfern一级近似整理可得:In(l-a)RT(n=1)(5)200400600800温度/℃(1-a)1图4不同粒度锡盟褐煤的TG曲线T2(1-n)Fig 4 TG curves of the Ximeng lignitenrF(1--RTwith different particle sizesE表4不同粒度锡盟褐煤的特征温度因为,一般活化能E远大于温度T,所以(1Table 4 Characteristic temperatures of the Ximeng lignite2RT/E)≈1。将热重实验数据代人得到式(5)和式with different particle sizes(6)左端的值,并绘制1/T曲线,根据拟合直线的斜临界温度/℃受热分解温度/℃率得出活化能E,再由直线截距得到指前因子A粒径/p43.3335.0计算中温度范围取自煤样吸氧增重结束开始受热分>100~12039.2332.5解时的温度到着火点温度,n取1。计算结果见表3。330.0表3 Coats-Redfern法求解煤样动力学参数由图4和表4可见,临界温度和受热分解温度Table 3 The samples dynamic parameters obtained均随着粒径的减小而减小,表明粒径越小反应越容by Coats-Redfern integral method易进行,即更容易自燃。煤种活化能E相关性系数/(kJ·mol-1)指前因子A2.3.2氧浓度盟褐煤119.580.9859尼褐煤分别采用含氧体积分数为5%、9%、13%、大同烟煤122.910.986217%、21%的氮氧混合气体,锡盟褐煤煤样粒度为平顶山烟煤127.730.9825120~200m,升温速率为20℃/min,样品由20℃神华低灰烟煤135.420.9889阳泉无烟煤173.250.9879加热到800℃,气体流量为50mL/min。图5为不同含氧浓度下的TG曲线,表5为不同氧浓度下锡由表3可知,6种煤在氧化热解反应中一级反盟褐煤的特征温度应的A均大于0.98,因此认为其氧化热解均为一级含氢量5%化学反应,比较E得到6种煤的自燃倾向性为:印1102—含氧量91003—含氧量13%尼褐煤>锡盟褐煤>大同烟煤>平顶山烟煤>神华-4含氧量17%65—含氧量21%低灰烟煤>阳泉无烟煤。这与上述特征温度值及实际应用中褐煤自燃倾向明显的特征基本吻合,因此基于热重分析的煤的活化能可以作为鉴定煤自燃倾向性的指标之一。2.3褐煤自燃的影响因素200400600800温度/r2.3.1煤样粒度tH中国煤化工曲线分别取粒径为60~100、>100~120、>120CNMHG200μm的锡盟褐煤煤样约5mg,以10℃/min的升trationshttp:lwww.rlfd.comcnhttp:/rlfd.periodicalsnet.cn第6期张斌等褐煤自燃特性热重实验及动力学分析衰5不同氧浓度下锡盟褐煤的特征温度progress on the mechanism and predictionof coal sponTable 5 Characteristic temperatures of the Ximeng lignitetaneous combustion[J]. 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