Leaf Concept

MILD combustion (moderate or intense low oxygen dilution) is a proven combustion technique in the furnaces and natural gas utilisation sectors and recently, it has been extended for gas turbines but only at very low TRL. There is no practical gas turbine combustor yet operating in this mode, although there is intense activity in this area. In particular, the fundamental features of MILD combustion, such as strong mixing with hot combustion products, detached flames, widened reaction zones due to the intense strain rates, and local reaction zones being amalgams of normal thin flames and wide regions of autoigniting fluid, result in (i) ultra-low NOx; (ii) quiet operation (i.e. very low propensity for thermoacoustic oscillations); and (iii) virtually no soot even from heavy liquid fuels like kerosene or SAFs. The third advantage has been amply demonstrated in the LEAFINNOX project, where a combustor concept based on MILD combustion principles, namely the “Lean Azimuthal Flame”, has been developed to operate on liquid fuels for the first time. The results demonstrate simultaneously: (i) very low NOx, at the single-digit ppm level, translating to Emission Indices (gNOx/kg fuel) significantly lower than current and future ACARE regulations (for example, of order 0.5 gNOx/kg fuel), and (ii) soot-free operation. In this combustor concept, a few azimuthally-arranged fuel injectors (small holes for gaseous fuels, pressure or air-assist atomisers for liquid fuels) are installed at the bottom plate of a cylindrical combustion chamber. The air is tangentially introduced from the top of the chamber, resulting in a spiral motion that makes the flame of one injector igniting the other. At some conditions, the individual flames merge and form a toroidal reaction zone and then the LEAF burner gives very low NOx and very low PM. In addition to the ultra-low emissions, the LEAF concept has proved to be very flexible with the fuel used, i.e. it burns a wide range of fuels, including hydrogen. Ultra-low NOx with H2 has also been demonstrated. The multi-point fuel injection, the very low NOx from a range of fuels including kerosene and H2 and the unique stabilisation method where one flame ignites the other, suggests that the concept can also be used with only minor alterations for SAF, H2 and their blends, hence promoting a single combustor architecture for many different, low-NOx, low-PM decarbonisation options. This capability is vital for industry and is explored further in this proposal.