Study of Vitiated Turbulent Combustion for Low-Emission High-Efficiency Hybrid Energy Systems

随着能源安全和可持续发展成为21世纪的首要任务之一，寻求零排放或低排放、高效的能源系统正变得越来越重要和紧迫。一项结合了固体氧化物燃料电池(SOFC)和燃气轮机(GT)系统的新兴技术有望显著提高整体能源效率并显著降低有害排放水平。此外，还可以实现更低的总体成本。同样重要的是，由于SOFC系统在高温范围内运行，各种碳氢燃料无需预转化即可直接使用，从而提高了这种混合系统的生命周期效率和通用性。在混合SOFC-GT系统中，未用过的废气燃料和来自SOFC系统的高级热量被用于燃气轮机燃烧室，通常与新的燃料-氧化剂混合物结合使用。主要的挑战是当反应物混合物被水蒸汽(H2O)和二氧化碳(CO2)高度稀释时，如何有效地燃烧废气燃料。由于混合气的组成在空间和时间上都不同，燃烧可以在燃烧室的不同部分以非预混和预混两种模式进行。它还可能具有混合模式，称为三重火焰或边缘火焰，其特征介于非预混合模式和预混合模式之间。局部火焰熄灭和自燃预计将是重要的。这种情况带来了理论和模型方面的挑战，也给燃烧室的设计带来了困难。类似于SOFC-GT混合系统的湍流条件下的稀释燃烧必须进行系统的研究，以使下一代低排放高效电力系统的建设有更坚实的科学基础。鉴于整个问题的复杂性，拟议的研究将集中在与GT燃烧室湍流燃烧相关的共性基本问题上。对同向流动的污染甲烷-空气火焰进行了直接数值模拟、理论模拟和分析。选择这种基本配置是为了与加州大学伯克利分校一个小组在燃气轮机燃烧室中提升火焰稳定性的一系列实验研究中使用的配置相匹配，这样就可以获得有用的实验数据进行比较。这项研究将包括四个主要部分：(1)使用多步骤系统简化的化学动力学对污染的甲烷-空气火焰进行解析，极大地扩展了域名系统作为预测工具的能力；(2)不同水和/或二氧化碳稀释以及不同程度预混的参数研究；(3)部分预混火焰理论和模型的评价和进一步发展；以及(4)与SOFC-GT混合系统相关的性能和操作问题的关键评估。域名系统将利用国家高端计算(HEC)设施HPCx和Hector。这项研究将与美国的三个研究小组合作。结果将在国际会议和期刊以及工程应用计算燃烧联盟的会议和网站上传播，该联盟由EPSRC第EP/D080223/1(2006-2009)。

The search for zero- or low-emission, high-efficiency energy systems is becoming increasingly important and urgent, as energy security and sustainable development have become one of the top priorities of the 21st century. An emerging technology combining a solid oxide fuel cell (SOFC) and a gas turbine (GT) systems promises to dramatically increase the overall energy efficiency and significantly reduce the level of harmful emissions. Moreover, a lower overall cost can be achieved. Equally importantly, as an SOFC system operates within a high temperature range, a wide variety of hydrocarbon fuels can be utilized directly without pre-reforming, increasing the life-cycle efficiency and versatility of such a hybrid system. In a hybrid SOFC-GT system, the unspent exhaust fuel and high-grade heat from an SOFC system are utilized in the GT combustor, often in combination with a fresh stream of fuel-oxidizer mixture. The main challenge is to burn the exhaust fuel efficiently, when the reactant mixture is highly diluted with water steam (H2O) and carbon dioxide (CO2). As the mixture composition varies both spatially and temporally, combustion can take place in non-premixed and premixed modes in various parts of the combustor. It is also likely to have a mixed mode, called triple flames or edge flames, which exhibits features in between non-premixed and premixed modes. Local flame extinction and auto-ignition are expected to be significant. Such a scenario presents theoretical and modelling challenges, as well as difficulties for the design of the combustor. Diluted combustion under turbulent conditions similar to those in a hybrid SOFC-GT system must be systematically studied to put the construction of the next generation low-emission high-efficiency power systems on a firmer scientific foundation.Given the complexity of the complete problem under concern, the proposed research will focus on generic, fundamental issues related to turbulent combustion in the GT combustor. Direct numerical simulation (DNS), theoretical modelling and analysis will be conducted on a vitiated methane-air flame with a coflow. This base configuration is chosen to match that used in a series of experimental studies by a group at UC Berkeley on lifted flame stabilization in gas turbine combustors, so that useful experimental data is available for comparison. The study will consists of four main parts: (1) DNS of vitiated methane-air flames using multi-step systematically reduced chemical kinetics, greatly extending the capability of DNS as a predictive tool; (2) Parametric studies with various combinations of H2O and/or CO2 dilution and a varying degree of premixing; (3) Evaluation and further development of theories and models for partially premixed flames; and (4) Critical assessment of performance and operational issues related to hybrid SOFC-GT systems. The DNS will utilize national high-end computing (HEC) facilities HPCx and HECToR. The research will involve collaboration with three research groups in the USA. The results will be disseminated in international conferences and journals as well as at meetings and the website of the Consortium on Computational Combustion for Engineering Applications, funded by the EPSRC grant No. EP/D080223/1 (2006-2009).