低碳活性组分介入的氨燃料着火、火焰传播与反应动力学研究

Ammonia, as carbon-free, hydrogen-carrier fuel, has attracted much attention recently. However, there are still some challenges in the application of ammonia, such as difficulty in ignition, low flame propagation speed, and high NOx emission. Ammonia combustion intervened by active promoters provide a solution for ammonia utilization. However, limited understanding of combustion characteristics and kinetic mechanism of ammonia binary fuel leads to the unsatisfactory results of combustion control in application. In this study, ammonia is considered as the major fuel, while hydrogen, natural gas, and DME are blended as intervention species. Experimental measurements will be carried out to understand the ignition delay and flame propagation characteristics of ammonia with active intervention species over a wide range of conditions. The flame instability and self-accelerating mechanism will be revealed. An accurate and reliable kinetic model will be proposed based on CWCR theory and modeling process cycle. The pressure dependent characteristics of combustion of ammonia and binary blends will be explored. The migration pathway of fuel-based nitrogen will be revealed, and the control methods for reducing the NOx pollutants will be proposed. This research has guiding significance for the optimization and control of ammonia-fueled engine and has important academic value for clarifying the key scientific problems in the combustion of ammonia intervened by active components.

氨作为无碳“氢基”燃料备受关注，但氨在应用中存在点火难、火焰传播速率低和NOx排放高的瓶颈问题有待解决。采用活性燃料介入的燃烧调控是解决氨利用的有效技术途径，但燃烧基础研究和动力学机理认识的不足导致氨燃烧调控难以达到满意效果。本项目以氨为燃料，以氢气、天然气和二甲醚为化学活性调节组分，通过激波管、快速压缩机和燃烧弹等实验平台开展高参数和宽工况条件下基础燃烧实验研究，获得活性调节组分介入的着火延迟期和层流燃烧速率随初始条件的演变规律和原因；揭示火焰不稳定性和自加速机理；通过反应级理论和模型循环优化法构建经多参数、宽工况验证的氨-低碳燃料动力学模型；揭示活性组分介入的燃料氮迁移路径，提出降低NOx污染物的调控方法；从总反应级数和总活化能的宏观角度，探索氨-低碳燃料燃烧的压力依赖特性。本研究对氨燃料发动机的燃烧优化与控制具有指导意义，对阐明氨-低碳燃料燃烧调控中的关键科学问题具有重要学术价值。

本项目以氨燃料在发动机上的应用为背景，采用实验测量、数值模拟和理论分析相结合的手段对氢气、甲烷和二甲醚等不同活性组分介入的氨燃料基础燃烧及化学反应动力学特性进行了深入研究。通过实验获得了宽广初始条件下纯氨及活性组分介入后氨燃料的着火延迟期等表征数据，阐明了温度、压力、掺混比等关键因素对燃烧特性的影响规律和机理。利用多元线性回归方法拟合了可体现燃烧物理边界参数对氨着火延迟期影响的Arrhennius关系式，该关系式可用于工程应用和模拟仿真。研究发现，压力和温度变化对纯氨的活化能没有明显影响。氢气、甲烷和二甲醚添加对氨的着火延迟期都起到了非线性促进作用，少量活性组分掺混后着火延迟期显著缩短。研究发现不同活性组分添加对氨着火的化学反应动力学影响机制存在差异。5%氢气添加后与氨发生了明显的化学交互作用，氢气在着火初期产生大量H原子导致氨燃料氢提取反应NH3+H<=>NH2+H2的化学平衡逆向，使原本对纯氨着火抑制作用最强的反应通道变成了H原子生成通道，因此显著缩短了着火延迟期。揭示了高温下5%甲烷添加可显著促进CH3+O2<=>CH2O+OH和NH2+HO2<=>H2NO+OH等OH自由基生成通道从而加速着火。发现二甲醚添加可促进反应NH2+HO2<=>H2NO+OH和H2O2(+M)<=>OH+OH(+M)等，在着火初期提高OH生成产率并缩短着火延迟期。研究发现，已有文献中模型无法对氨/二甲醚在中低温NTC条件下的着火延迟数据给出准确预测，本研究构建了氨/二甲醚的化学反应动力学模型，该模型对着火延迟期、层流燃烧速度和物种浓度实验数据均有较好的预测表现。此外，本项目还开展了活性组分添加对氨及典型烷烃着火延迟和化学动力学影响的对比研究。本研究结果对于认识氨燃料的燃烧特性以及指导氨燃料发动机的设计优化具有重要意义。