Pyrolysis gas valorization through MILD combustion: A comprehensive experimental analysis

Gaseous biofuels unquestionably occupy a relevant role as CO2-neutral energy sources in heat and power sectors. However, their direct utilization through traditional combustion technologies suffers the lower Low Heating Values (LHVs) compared with fossil fuels. In this respect, MILD Combustion was already demonstrated to be suitable for biofuels valorization, with high tolerance to fuel composition, while respecting pollutant emission and process efficiency requirements. Given this background, this study analyzes the resilience of MILD combustion to pyrolysis gases through dedicated experimental tests in a cyclonic-flow burner, varying fuel composition and LHVs. Combustion stability and pollutant emissions were analyzed with respect to the most influent operating parameters (biofuel composition, equivalence ratio, preheating level, thermal power). The minimum operating temperature of 1100 K was identified as unique threshold for combustion process stabilization. It allows to totally oxidize pyrolysis gases for LHVs up to about 4 MJ/kg in a wide range of equivalence ratio and thermal power levels by an effective preheating strategy. Neither NOx nor H2 emissions were detected for the investigated cases, testifying the effectiveness of MILD combustion, whereas CO emissions depend on both CO-CO2 ratio in the fuel mixture and the specific LHV. Furthermore, the H2 content of the pyrolysis gas showed covering a key role with respect to CO emissions reduction since it increases the system reactivity without affecting the operating temperatures, but boosting the CO conversion to CO2. Finally, independently of the considered fuel mixture, a LHV equal to about 1.5 MJ/kg was identified as minimum level to stabilize MILD combustion conditions without any extra thermal input, assuming to use the sensible enthalpy content of the exhaust gases for the inlet mixture preheating.