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

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).