Fuel and thermal load flexibility of a MILD burner

The strategy to stabilize distributed combustion regimes adopting cyclonic flow fields has been proven to be challenging. In fact, the establishment of a toroidal flow field within a combustion chamber may en- sure the recirculation of mass and sensible enthalpy required to simultaneously dilute the fresh reactants and increase the temperature above the autoignition one. The combination of reactants dilution and preheat- ing may greatly increase system energy efficiency and lower pollutants production producing very peculiar combustion regime (named MILD Combustion). At the same time this strategy can be compromised if the sensible enthalpy is not high enough to promote the auto-ignition process of diluted mixtures. This may hap- pen either because of an inefficient recirculation system or due to a too low calorific fuel. The paper aims at exploiting the performance of a small-size cyclonic burner for a conventional fuel (CH4) and a low calorific fuel (synthetic biogas) through the characterization of the process stabilization and pollutant emissions as a function of the mixture equivalence ratio and the nominal thermal power of the inlet mixture (from 2 to 10 kW), with the aim of identifying the optimal operating condition of the system. Results suggest that the system has to be exercised with mixtures with compositions slightly under the stoichiometric conditions and in a well identified temperature range to minimize both NOx and CO emission. The burner can be easily ex- ercised also with low calorific fuels for higher thermal powers according to the low LHV. However, it results that an efficient recirculation of the exhausts produces a robust MILD combustion condition also when low calorific fuels are used.