【作者】 张成；

【导师】 陈刚； Xu Zhenghe；

【作者基本信息】 华中科技大学 ， 热能工程， 2009， 博士

【摘要】 煤是目前最常见的一次常规能源,由燃煤引起的汞和硫的排放已对生态环境和人类健康造成了极大的危害,其相关研究已成为当今国际的热点课题。本文以煤燃烧前汞/硫的脱除为主线,把几种典型的中国、加拿大的动力煤和高汞/硫煤为研究对象,从煤中汞/硫的赋存形态入手,通过系统的实验研究和理论分析,探索和研究了温和热解技术和空气重介质流化床干法洗煤两种不同的煤燃烧前汞/硫的脱除方法,揭示了煤中不同形态汞、硫在燃烧前的脱除规律,建立了温和热解汞/硫释放的动力学模型,为开发经济有效的新型燃煤汞/硫排放控制技术,实现燃煤污染物减排奠定了科学的基础。论文首先利用煤浮沉试验、逐级化学提取、低温灰化-X射线衍射(LTA-XRD)分析、X射线荧光(XRF)分析等多种分析手段,详细研究了痕量元素汞以及硫在煤中的赋存形态及其和矿物之间的相关特性。发现了在汞在加拿大AB煤与一种罕见的矿物Chaoite有较强的相关特性,在中国贵州两种高汞/硫煤LZ和ZY煤中大部分汞则分别富集在煤中的黄铁矿和高岭石中。利用水平管式加热炉进行了煤燃烧前温和热解脱汞脱硫的试验,研究了不同气氛、升温速率、热解温度、停留时间等众多因素对温和热解过程中汞硫联合脱除效率的影响及不同形态硫和汞的释放行为。实验结果表明,煤中90%以上的汞可通过温和热解实现燃烧前的脱除。按照汞在煤中的结合形式不同可将煤中汞分为“可挥发态汞”、“分子结合态汞”和“共价结合态汞”。含氧4%的微氧热解气氛较氮气气氛相比可在350-400℃的低温范围内提高汞的脱除率,同时对煤中黄铁矿硫的脱除也有明显的促进作用;快速升温热解对煤中的汞的释放比程序升温热解有明显的促进作用。动力学计算表明,的微氧气氛下热解400℃时汞释放的反应速率较氮气气氛明显提高,同时不同形态硫释放的活化能也相应降低。热解半焦的微观形态研究表明,快速升温热解时煤的膨胀和裂解更加剧烈,半焦产生了更多的微孔和更大而宽的裂隙,而微氧热解气氛使得热解半焦孔隙结构变得更为复杂,表面粗糙度增加,更有利于煤中汞在热解过程中的释放和逸出。本文对温和热解过程的能耗分析表明,含氧4%微氧气氛、400℃温度下快速热解为燃烧前温和热解汞/硫脱除的最佳工况。不同形态的汞和硫在热解过程中释放的温度不同。研究结果表明,富集于煤中有机组分中间的汞的释放温度区间在120-300℃;富集与黄铁矿组分中的汞在温和热解过程的释放温度区间为430-590℃;富集与煤中粘土矿物中的汞则极有可能的释放温度区间为250-350℃。煤中不稳定有机硫在热解过程中最易分解释放,其释放峰在370℃附近;黄铁矿硫分解释放峰在550℃附近,微氧气氛下可使黄铁矿的分解温度降低;而煤中难分解的有机硫在10%氧浓度下在800℃以上的温度才能分解。热解过程中氧分子对煤中化学键的断裂具有选择性,在4%的氧浓度下,氧分子首先断裂煤中C-S键,其次断裂C-C键,而当氧气浓度进一步增大,热解过程中氧分子对煤分子化学键断裂的选择性发生变化,大量的C-C键在10%的氧气浓度下会优于C-S键断裂,从而体现出半焦的热值损失增大。在空气重介质流化床上研究了煤中汞和硫在燃烧前干法选洗时的脱除特性。分析研究了空气流速、重介质粒径和密度、煤的粒径、煤的加载量等众多因素对空气重介质流化床干法选煤效率的影响,实现了1-3.66mm细粒径段煤的分选。考察了煤中灰分、硫分、以及重金属汞在燃烧前空气重介质流化床干法选煤中的脱除效率以及经济性。研究表明,煤质特性对空气重介质流化床燃烧前分选过程中的灰分及污染物脱除行为有较大影响。本文研究的煤种在最佳分选条件下的精煤产率达到75.31%,灰分脱除率为45.7%,煤中汞和硫的脱除率达到47.2%和49.3%,可燃基收率为89.4%。全文较为系统的研究了汞和硫在煤中的赋存形态,利用空气重介质流化床和温和热解两种方法对煤中汞/硫在燃烧前的脱除机制和行为进行了研究,并利用动力学计算和热力学模拟进行了计算分析,研究结果为开发廉价的燃烧前燃煤污染物联合脱除技术奠定了基础。更多还原

【Abstract】 Coal is the most popular source of primary energy and the coal-fired power plants are still the main source of the demand for electricity in the world especially in China.With the wide exploitation of coal and the rapid progress of society,clean and efficient utilization of coal has become the most important task during coal utilization.The objective of the thesis is to investigate the methods and mechanisms for Hg/S removal before coal combustion.Several typical high Mercury/Sulfur（Hg/S）containing coals and steam coals in China and Canada were selected in this study.The occurrence of Hg/S in these coals were firstly investigate.Then mild thermal upgrading and Air Dense Medium Fludized Bed（ADMFB） were used for the pre-combustion removal of Hg/S.The thesis revealed the mechanisms and rules for the removal of different forms of Hg/S and also obtained the relative kinetics data by systemic experiments and theoretical analysis. The overall research can provide scientific knowledges for the exploitation and development of pollutants controlling technologies.The occurrence of Hg and S as well as the correlation between those elements and the minerals in different coals were investigated by using variable methods such as float-sink test,sequential leaching,LTA-XRD analysis and XRF.A strong correlation between Hg and an infrequent mineral Chaoite was found in AB coal from Canada.For the other two Chinese coals,Hg was found to be associated with Pyrite and Kaolinite,respectively.The removal and releasing behavior of Hg and S during mild thermal upgrading were studied in the horizontal tube furnace.The results showed that over 90%Hg in coal could be removed by mild thermal upgrading.Mercury in coal can be defined as "volatilizable mercury","molecule bound mercury" and "covalent bond mercury" based on different forms of combination.The study also showed that rapid thermal upgrading and a mild oxidizing atmosphere between 350℃and 400℃can facilitate the release of mercury and pyrite surfur.The kinetics calculation showed that the activation energy of sulfur releasing was decreasd and reaction rate constant k was increased under mild oxidizing atmosphere. It indicated that the mild oxidiazing atmosphere was benifical to the removal of Hg and S for the concern of energy consumption.The formation of more and large sizes of cracks course by rapid thermal upgrading and more complicated pore structure and coarser surface of the char under mild oxidizing atmosphere could be responsible for the increase of Hg,and S removal during thermal upgrading.The thesis indicated that rapid thermal upgrading conducted at 400℃under 4%O₂ oxidizing atmosphere was an optimal working condition for the pre-combustion removal of Hg and S by mild thermal upgrading.Different forms of mercury and sulfur were found to release at different temperatures during thermal upgrading.It was detected that Hg associated with pyrite could be released from 400℃up to 550℃while Hg associated with clay minerals could be released from 250℃up to 350℃.The study also indicated that the unstabilized organic sulfur in coal was the easiest to decomposed at 370℃,and the SO₂ released by the decomposition of pyrite was found at 550℃,the mild oxidizing atmosphere can lower the releasing temperature of sulfide sulfur about 50℃,the most stable thiophene sulfur can only be decomposed when the temperature reached to over 800℃under 10%O₂ atmosphere.The determination of gases released from thermal upgrading indicated that oxygen would selectively break C-S bound than C-C bound under 4%O₂ concentration.With the increasing of oxygen content,more C-C bounds were broken together with C-S bond which will result in the significant loss of heat value of the char.The dry coal cleaning experiments for Hg/Sulfur removal were conducted on ADMFB.Various factors which could influence the efficiencies of the separation of ADMFB were investigated in this study.Under the optimal separation conditions,the fine coal particles with the particle size range from 1 to 3.66mm were separated successfully.The study showed that the characteristics of coal effected theseparation efficiencies distinctly,the best separation for dry coal cleaning could obtain about 75.31% yield of clean coal,45.7%ash rejection,47.2%and 49.3%at the highest Hg and S rejection and also the combustible recovery at about 89.4%.The thesis has systematically studied the occurrence of Hg and S in different coals, and investigated the mechanisms and behaviours of Hg/S removal by using two different methods,and also provided the thermodynamic and kinetic data.The conclusions could be useful for the exploitation of low-cost emission controlling technologies before coal combustion.更多还原