Grid plate flame stabilizers for low NOx emissions have renewed interest in recent years due to their use in low NOx hydrogen gas turbine combustors. For non-premixed grid plate combustion, the difference in flame stabilizer design is in how the grid plate air flow is fueled. In the present work a simple four hole grid plate is investigated using CFD with three methods of fueling the air holes: radially inward fuel injection using 8 fuel nozzles per air hole (Grid Mix, GM 1 and Micromix); central fuel injection (FLOX method); and through a fuel annulus around each air hole (GM2). ANSYS FLUENT CFD predictions for GM2 are compared with axial gas composition traverses inside the combustor and with the mean combustor exit plane emissions. The three methods of fuel injection are also compared using isothermal CFD to determine which of the three methods offer the best mixing quality, which controls the relative NOx emissions. The predictions were for an equivalence ratio of 0.624 for the combustion stage and 0.5 for the isothermal study, using industrial propane. CFD modelling used RANS simulation with Realizable k-epsilon turbulence model, non-premixed combustion with the Steady Laminar Flamelet model. The temperature and mixing profiles obtained for GM2 were in reasonable agreement with the experiments and the other two fuel injection methods were then compared with GM2.

The present work investigates one of the lowest NOx design concepts using directly fueled grid plate flame stabilisers [10] termed Grid Mix (GM). The work uses CFD to predict experimental results for the GM low NOx flame stabilisers operated with propane. An additional study was undertaken to analyse their performance on hydrogen, a zero-carbon fuel at the heart of the UK zero-carbon power generation policy. Combustors are in production using similar technology to GM, and at least two manufacturers have low NOx hydrogen combustors using similar flame stabilisers [9, 18, 21–24]. Also, as the tests were at atmospheric pressure, the results are relevant to process burners, and the test condition for the CFD was a 140 kW thermal input process burner. The Grid Mix technique for non-premixed combustion, with fuel injected into the airflow at the periphery of the air holes, allows rapid fuel and air mixing for lean non-premixed ultra-low NOx combustion. The technology has been investigated at high entry temperatures of modern industrial gas turbines with all the combustion air passing through the flame stabiliser at a typical reference Mach number for this condition of M = 0.047, at an overall pressure loss of ΔP/P = 2.4%, for a heat release of 28MW/m2. These conditions are well above most experimental and CFD publications on low NOx gas turbine emissions. The combustion and NOx predictions for hydrogen show that Grid Mix flame stabilisers offer a viable solution to low NOx with hydrogen combustors. In addition, the advantage of adding fuel and air mixing passage downstream of the Grid Mix fuel injector is also demonstrated, as used by Yorke et al. [18].