【作者】 牛慧昌；

【导师】 刘乃安；

【作者基本信息】 中国科学技术大学 ， 安全科学与工程， 2014， 博士

【摘要】 森林火灾是一种频繁发生的自然灾害现象,并且在特定的条件下会造成严重的财产损失和人员伤亡。在城镇-森林交界域发生的火灾会通过多种传播途径危及城市公共安全。森林火灾的发生和蔓延不仅与可燃物的种类及存在形态有关,而且与可燃物的热分解机理密切相关。掌握森林可燃物的热解和燃烧机理,有助于开发森林火灾着火和火蔓延模型,评估森林火灾风险,预防和控制森林火灾的发生。本文的研究目标是,建立用于综合模拟木质纤维素材料热解和燃烧过程,同时考虑固相和气相反应的统一的动力学机理；采用改进的混合型遗传算法优化动力学参数；分析热解失重过程对实验条件如升温速率、颗粒粒径、样品量的敏感性,讨论升温速率对热解失重过程的影响,分析颗粒粒径对热解失重进程及其动力学的影响；采用本文提出的统一动力学模型,对樟子松的松针、松枝和松皮的热解和燃烧过程进行模拟,计算动力学参数,从动力学的角度讨论三种森林可燃物的燃烧性。本文的具体工作包括：建立了综合热解和燃烧过程，考虑固相和气想反应的，化学意义上正确的统一动力学机制。当前建立的热解和燃烧模型中存在多方面的不一致性,而且大多仅仅是基于燃料的固相质量变化,忽略了气相反应。本文基于热分析联用系统(TG-FTIR-MS),测量了三种森林可燃物在热解和燃烧过程中的质量损失特征和气相产物生成规律,提出了一个新的反应动力学模型,该模型基于传统的三步平行反应模型,将热解考虑为三种伪组分中可挥发部分的平行反应过程,将空气气氛下的燃烧过程考虑为四步平行反应,其中包括与热解类似的三步独立脱挥发份过程和一步炭氧化反应。该模型的主要特征在于对热解和燃烧的主要分解反应考虑了相同的化学组成,并通过主动控制各组分的含量,保证了该模型不仅可以准确地捕捉热解和燃烧失重特征,而且在化学意义上是正确的。通过改进的混合型遗传算法,该模型以较快的收敛速度获得了最佳的动力学参数。采用Kissinger方程解释了升温速率引起的失重曲线发生偏移的现象,并对基他升温速率下的峰值温度进行了预测。前人的研究仅仅从表观上讨论升温速率的影响,或者采用线性外推的方法预测不同升温速率下的失重速率曲线的峰值温度。本文利用Kissinger方程,从热分解反应的内在动力学角度出发解释了升温速率引发的失重曲线发生偏移的现象,并对不同升温速率下的峰值温度进行了预测。讨论了动力学控制机制下题粒粒径对三种森林可燃物热解失重过程及其动力学的影响。分析了颗粒热解和燃烧过程的反应速率控制机制,通过计算相关参数发现本研究所用的粒径范围内(<1500μm)的颗粒都处于动力学控制机制下。对不同粒径的松针、松枝和松皮颗粒进行了升温速率2℃/min下的热重实验,发现粒径在不同的温度区间以不同的方式影响三种可燃物的失重速率。研究认为在动力学控制机制下,粒径影响热解失重过程和动力学参数是通过改变颗粒的化学组成实现的。对不同粒径颗粒的工业分析结果也验证了该结论。丛动力学的角度对不同森林可燃物的燃烧性进行了研究。通过分析樟子松松针、松枝和松皮的热解和燃烧失重特征,结合燃料热解和燃烧动力学模拟结果,比较了三种森林可燃物的燃烧性。结果认为,松针由于具有较低的分解起始温度和反应活化能,因此具有较高的着火性能。松皮的整体反应速率偏低并且在整个反应进程中活化能都较高因此具有较强的燃烧持续性。而松枝的燃烧过程的两个阶段都具有较低的活化能,反应速率较快,维持时间短,因此松枝的燃烧强度大,但是维持燃烧的能力不强。更多还原

【Abstract】 Forest fire is a natural disaster that frequently happens worldwide. Under specific conditions, forest fires may cause great losses of properties and casualties. Especially forest fires may also threaten the safety of Wildland-Urban Interface areas. The ignition and spread of forest fires, are not only related to the properties of plant species, but also closely related to the mechanisms of pyrolysis and combustion. Therefore, research on the mechanisms of pyrolysis and combustion of forest fuels is valuable for the development of ignition and fire spread models, the assessment of forest fire risks, and the prevention and control of forest fires.The aim of this paper is to develop a unified kinetic mechanism that combines solid and gas phase reactions for simulating both the pyrolysis and combustion behaviors of lignocellulosic materials. A modified hybrid genetic algorithm is used for the optimization of kinetic parameters. The sensitivity of mass loss characteristics of pyrolysis to the experimental conditions is analyzed. The influence of heating rates, particle size and sample mass on the processes and kinetics of pyrolysis is discussed. The flammability of different parts of a plant is evaluated from the aspects of kinetics.The work and results of the thesis are summarized as follows.A unified kinetic scheme combining the solid and gas phase reactions is developed for simulating the pyrolysis and combustion of forest fuels. Previous pyrolysis and combustion models were inconsistent with each other, and most of them were developed only based on the mass loss in solid phase, for which reactions in gas phase were ignored. Using the TG-FTIR-MS thermal analysis systems, the mass loss rate in solid phase and evolution of products in gas phase of three forest fuels under inert and oxidative atmospheres were recorded. A unified kinetic scheme is proposed for simulating the pyrolysis and combustion reactions. Based on the traditional three parallel reaction model, the pyrolysis of forest fuels is considered to be the sum of devolatilization processes of three pseudo-components, and the forth reaction, oxidation of char, is added for the combustion mechanism. The presented scheme focuses on the consistent compositions for the major pyrolytic reactions of pyrolysis and combustion. The unified scheme not only reproduces the mass loss curves and captures most of pyrolysis and combustion features, but also is reasonable in chemical sense. A modified hybrid genetic algorithm is presented for optimization of kinetic parameter with high convergence speeds.Kissinger equation is used to explain deviation of mass loss rate curves caused bv the increasing of heating rates, and the peak temperatures of mass loss rate is predicted under different heating rates. Previous studies were mainly focused on the apparent temperature deviation, and the peak temperatures of mass loss rate curves were predicted by linear extrapolation. From the intrinsic kinetics aspect, this work explained the effects of heating rate on the reaction processes, and predicted the peak temperatures of mass loss rate under different heating rates.The effects of particle size on the mass loss processes and kinetics of pyrolysis of three forest fuels under regimes of kinetics is discussed. The regime of kinetics is verified for all of the particles (<1500μm) under study. Particles of three forest fuels, Pine Needle (PN), Pine Branch (PBr), and Pine Bark (PB) with different dimensions were pyrolyzed under heating rate of2℃/min to investigate the effect of particle sizes. It is concluded that the difference of mass loss rates and kinetics is mainly due to the variation of chemical composition, which is verified by the industrial analysis results.The flammability of forest fuels is evaluated from the aspect of kinetics. The flammability of PN, PBr and PB of Pinus Sylvestris is compared by the thermal stability and the kinetics of the pyrolysis and combustion processes. It is concluded that PN holds the highest ignitability due to the lowest initial degradation temperatures and activation energies during the initial stage. PB is considered to be most sustainable in fire due to the lowest reaction rates and the highest activation energies during the whole process. PBr holds the highest combustibility due to the two highest peak of mass loss rates.更多还原