【作者】 刘晶；

【导师】 郑楚光；

【作者基本信息】 华中科技大学 ， 热能工程， 2007， 博士

【摘要】 燃烧装置中排放的痕量元素对环境和人类健康具有很大的危害,而人们对痕量元素的排放规律和抑制机理的探索和认识却非常浅薄。目前,有关痕量元素排放已经成为燃烧污染中的一个新兴而前沿的领域,成为愈来愈关切的热点。本文对煤燃烧过程中痕量元素的释放、反应机理、在线监测及动力学模型等方面进行了较为系统的研究。将量子化学理论和研究方法引入到燃烧学领域,应用量子化学不同方法和典型基组分别计算含汞燃烧烟气中分子的几何结构、反应焓变、反应熵变及汞化合物的振动频率值,以验证量子化学计算所用的理论水平和基组的有效性。理论计算值与美国国家标准技术局(NIST)的实验数据进行了比较。结果以QCISD方法/Stevens基组的组合最好,其次为B3PW91方法/Stevens基组、B3LYP方法/Stuttgart基组。对金属原子Hg、非金属原子分开指定基组可使计算精度提高。在原子和分子的水平上对含汞燃烧烟气中汞的10个氧化反应的机理进行理论研究,建立较为全面的汞的均相氧化模型。应用量子化学从头计算QCISD方法,对于Hg,选用Stevens基组,对于非金属元素Cl、H、O、N,选用6-311++G(3df,3pd)基组,优化得到反应途径上各稳定点(反应物、产物、过渡态和中间体)的几何构型。不同稳定点的性质通过对其几何构型进行振动频率分析来确认。在此基础上进行热能(包括零点能)校正,并以此能量计算活化能,同时计算指前因子。采用过渡态理论计算得到了293-2000K温度范围内各基元反应的反应速率常数,建立了较为全面的汞的均相氧化模型。从本质上揭示了汞的生成及其与煤中其它元素相互作用的反应机理,可为寻求廉价、实用的汞控制方法奠定理论基础。将密度泛函理论应用于气固吸附机理研究,对烟气中不同形态的汞(Hg0、HgCl2和HgCl)在飞灰未燃尽炭固体表面的吸附进行理论计算。构建了较好表征未燃尽炭固体表面的锯齿形(zigzag)、扶手椅形(armchair)和尖端形(tip)的簇模型。以模型中边缘未饱和的碳原子模拟吸附的活性位,对其它碳原子使用氢原子加以封闭。计算表明,元素态汞在未燃尽炭固体表面的吸附属于较弱的物理吸附。氯原子会增加未燃尽炭对汞的吸附能力,使其更倾向于化学吸附。其它卤素原子的存在也增强了未燃尽炭对元素态汞的吸附,吸附强度顺序为F>Cl>Br>I。具有含氧官能团(内酯、羰基和半醌)的未燃尽炭固体表面对元素态汞的吸附属于化学吸附。HgCl2是以平行位稳定的吸附在未燃尽炭固体表面上,属于强的化学吸附。HgCl是以平行位及Hg端垂直位形式稳定的吸附在未燃尽炭表面上,属于强的化学吸附。在实际烟气中,氧化态汞更容易被飞灰中的未燃尽炭所脱除。与相关的煤燃烧实验进行比较,结果一致。从物质性质与其结构之间的关系以及微观相互作用出发定量描述吸附体系的特征和行为,对飞灰未燃尽炭作为汞吸附剂的性能,吸附产物的稳定性等问题进行较为系统深入的描述。发展一种基于电感耦合等离子体发射光谱(ICP-AES)的气相痕量元素在线测量方法来测定烟气中痕量元素的瞬时相对浓度,从而全面跟踪痕量元素的挥发释放过程。实验得到优化的ICP(功率1200 W)进气流速为:样品气体,0.1 L/min;氩气,0.2 L/min。对不同基体(矿物质基体、煤和城市固体垃圾)在流化床内(850°C)进行热处理,研究痕量元素的释放行为。Cd及Pb较易挥发释放,而Zn释放程度较低。痕量元素与飞灰颗粒在高温下反应生成ZnO·Al2O3、CdO·Al2O3等稳定化合物,从而抑制了痕量元素的释放。在进行烟气痕量元素在线监测实验研究的同时,发展直接模型和反模型来研究痕量元素蒸发释放的动力学模型。首先将模型应用于矿物质基体氧化铝来验证模型的有效性,然后应用于有机基体煤和城市固体废弃物。根据痕量元素的蒸发率(r=dq/dt)和固体颗粒中的痕量元素的浓度(q)的关系得到其蒸发的动力学规律。对于氧化铝得到1级反应动力学规律;煤样和城市固体废弃物得到2级反应动力学规律。对燃烧中痕量元素的挥发释放行为作出较为深刻的描述和预报。更多还原

【Abstract】 Trace elements emitted from coal-fired boiler would do great harm to human health, global agricultural and social sustainability, but the mechanism of release, emission and control is still not clear. Trace elements emitted from coal combustion have become an increasingly important environmental concern. In this work, we focused on describing the evaporation, reaction mechanism, on-line analysis and kinetic modeling of trace elements during coal combustion with computational and experimental methods.The quantum chemistry theories were introduced into the field of combustion. The geometry optimizations of molecular, heat of reactions, change of entropy and vibration frequencies were calculated by different levels of ab initio, DFT theory of quantum chemistry and typical ECP basis sets for combustion flue gas containing mercury system. The calculating results were compared with the NIST experimental results in order to validate the quantum mechanical method and basis set combination. The results show that the QCISD/Stevens combination is the most accurate, and than is the B3PW91/Stevens combination, B3LYP/Stuttgart combination. It improves the calculation results by appointing different basis set for metal atom and nonmetal atoms. The results provide a base for investigating kinetic mechanism of mercury interaction with combustion-generated flue gas by quantum chemistry.Theoretical exploration on mercury oxidation reaction mechanism in flue gas was conducted on the level of atoms and molecules. The geometry optimizations of reactant, transition state, intermediate and product were made at QCISD level by ab initio calculations of quantum chemistry. The basis set of Stevens was used for Hg, and the basis set of 6-311++G(3df,3pd) was used for nonmetal atoms (Cl, H, O and N). The property of stable minimums were validated by vibration frequencies analysis. The activation energies were calculated by thermal energy calibration (including zero point energy calibration). The reaction rate constants in the temperature scale of 293-2000K were calculated from transition state theory. The calculated parameters can provide new foundation for emission model of trace elements during coal combustion.The adsorptions of different species of mercury (Hg0、HgCl2 and HgCl) on the unburned carbon (UBC) surface were investigated by the density functional theory (DFT). The cluster models of zigzag, armchair and tip were set up to well representing the UBC surface. The unsaturated carbon atoms in the edge of the cluster model were used to simulate the active sites of the UBC surface, and other carbon atoms were closed by H atom. The present calculations show that UBC could substantially reducing gaseous mercury chloride (HgCl2 and HgCl), but have no apparent effect on Hg0, which is compatible with the available experimental results. Cl atom and surface functionality (lactone, carbonyl and semiquinone)increase the adsorption of mercury on UBC surface. The research method will provide the valuable information for the optimizing and selecting sorbent of the trace element in flue gas.An inductively coupled plasma atomic emission spectrometer (ICP-AES) was developed to continuously measure the heavy metal concentrations in order to track the metal release process. The system is devoted to the thermal treatment of metal-spiked mineral matrix, coal and municipal solid wastes in fluidized bed (850°C). This method was used to study the kinetics of heavy metal vaporization. The optimum values of the gas flows for the 1200 W power generator are 0.1 L/min for the sample gas and 0.2 L/min for Ar. During the thermal treatment of coal and municipal solid waste, the release process of Cd and Pb is short; Zn vaporizes lower than Cd and Pb. The formation of stable compounds such as ZnO·Al2O3 and CdO·Al2O3 could decrease the metals vaporization. In all cases, the experimental setup was successfully used to monitor the metal evaporation process during coal and solid waste thermal treatment. A study was carried out to investigate the kinetic law of toxic metal release during thermal treatment in a fluidized bed. Both direct and inverse models were developed in transient conditions. The direct model predicts the time course of the metal concentration in the gas from the vaporization rate profile, based on the Kunii and Levenspiel’s 2-phase flow model for Geldart Group B particles. The inverse model was developed and validated to predict the metal’s vaporization rate from its concentration in the outlet gas. A method to derive the kinetic law of heavy metal vaporization during fluidized bed thermal treatment of coal from the global model and the experimental measurements is derived and illustrated. A first order law was fitted for the mineral matrix and a second order law (simplified) was fitted for coal. This method can be applied to any matrix, whatever mineral matrix or organic matrix.更多还原