CO2热物理化学效应对富氧燃烧煤粉着火及炭粒燃烧影响机理研究

Oxy-coal combustion with flue gas recirculation is one of the most promising techniques to control carbon dioxide (CO2) emission as well as nitrogen oxide and sulfur dioxide in the near term. In oxy-coal combustion processes, the thermophysical and chemical properties of CO2 such as heat capacity, heat and mass diffusivity and CO2-char gasification reaction is significantly different from those of N2 in conventional pulverized coal combustion, moreover the CO2 effects (including dilute effect, thermal effect, direct chemical effect and diffusion effect) on pulverized coal ignition and char combustion occur simultaneously and are intimately coupled. Although many studies have been devoted to the effects of CO2 on coal ignition, char combustion and NOx formation, their presented results are totally or partially the combination of these effects because it is very difficult to separate CO2 effects from one to another. Up to now, their impacts on oxy-coal combustion have not yet been clearly understood and no fine descriptions of the mechanisms are available.. The applicants propose a novel method of adjusting the flame temperature in CO2/O2 condition back to the value corresponding to the N2/O2 condition without changing the oxygen concentration by replacing an appropriate portion of CO2 with argon (Ar) in the oxidizer, and use it to separate the different effects of CO2 on oxy-coal ignition and combustion. An optical laminar entrained flow reactor coupled with FTIR emission-transmission particle temperature measurements is constructed to provide the well-controlled combustion environments. The coal particle ignition, char combustion characteristics and the evolution of burning particle temperatures are measured and investigated at comparable CO2/O2, CO2/Ar/O2 and N2/O2 environments with the given oxygen concentrations and combustion temperatures, and the separating effects of CO2 on coal particle ignition and char combustion characteristics are quantitatively identified and compared. The motivation of the project is to firstly elucidate the separating CO2 effects on underlying mechanisms of coal particle ignition delay and char combustion reactivity for oxy-coal combustion.

富氧煤粉燃烧是一种有效实现CO2减排并控制NOx和SO2等多种污染物排放的煤粉燃烧新技术。尽管目前国内外对富氧燃烧煤粉着火及燃烧进行了较多研究，但由于CO2热物理化学特性与N2明显不同，CO2的多种热物理化学效应（稀释效应、热效应、化学效应、扩散效应等）同时存在且相互影响，也难以有效分离。已有的研究结果是这些效应全部或部分耦合的影响，不能准确地反映富氧煤粉燃烧的内在机理和物理化学实质。本项目提出了一种采用合适比例CO2/Ar混合气来调节燃烧火焰温度并有效分离CO2热物理化学效应的方法，在可控热氛围的透光式微量煤粉燃烧与FTIR发射-透射测温联用实验装置上，研究不同氧浓度和燃烧温度下CO2/O2、（CO2+Ar）/O2和N2/O2气氛煤粉着火、炭粒燃烧及颗粒温度的变化，定量确定分离的CO2热物理化学效应对煤粉着火和炭粒燃烧的影响，从而揭示CO2气氛对富氧燃烧煤粉着火和炭粒燃烧影响的内在机理。

富氧煤粉燃烧是一种有效实现CO2减排并控制NOx和SO2等多种污染物排放的煤粉燃烧新技术。尽管目前国内外对富氧燃烧煤粉着火及燃烧进行了较多研究，但由于CO2热物理化学特性与N2 明显不同，CO2 的多种热物理化学效应（稀释效应、热效应、化学效应等）同时存在且相互影响，也难以有效分离。已有的研究结果都是这些效应全部或部分耦合的影响，不能准确地反映富氧煤粉燃烧的内在机理和物理化学实质。本项目提出了一种采用合适比例CO2/Ar混合气来调节燃烧火焰温度并有效分离CO2 热物理化学效应的方法，在可控热氛围的透光式微量煤粉燃烧实验装置上，研究不同氧浓度和燃烧温度下CO2/O2、（CO2+Ar）/O2和N2/O2气氛煤粉着火、炭粒燃烧及颗粒温度的变化，定量确定分离的CO2热物理化学效应对煤粉着火和炭粒燃烧的影响，从而揭示CO2气氛对富氧燃烧煤粉着火和炭粒燃烧影响的内在机理。研究结果表明：（1）O2/CO2气氛煤粉着火延迟，着火温度更高。为了达到与空气气氛相匹配的神华烟煤着火温度，O2/CO2气氛下的氧浓度要比空气气氛21%氧浓度高，对于不同煤种O2/CO2气氛下氧浓度提高至26～44%可得到与空气气氛相匹配的煤粉着火温度。（2）O2/CO2气氛下煤焦颗粒表面总反应速率略高于O2/N2气氛，这是由于高浓度的CO2导致C-CO2反应速率明显提高，使得而煤焦总反应速率增加。CO火焰的存在会消耗大量由环境扩散到颗粒表面的O2，使得颗粒表面O2浓度降低，因此间接的改变颗粒表面的氧化和气化反应的比重，使得颗粒表面氧化反应减弱并且气化反应增强。（3）21%O2/79%N2气氛中煤粉颗粒燃烧温度比21%O2/79%CO2气氛中煤粉颗粒燃烧温度高380K。21%O2/79%CO2气氛下煤粉颗粒燃烧初期的发射率比较大，随着煤粉颗粒逐渐燃烧和燃尽，颗粒的发射率逐渐减小。（4）建立考虑炭粒表面CO空间反应、炭粒表面氧化还原反应以及热质传递等全耦合的富氧煤粉/炭粒燃烧数学模型及数值模拟方法，详细预报并分离CO2不同热物理化学效应对煤粉着火、挥发分燃烧以及炭粒燃烧的影响，O2浓度效应、热效应以及CO2化学效应对煤焦燃烧速率的影响分别为82.1%、11.2%和6.7%。

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