Alcohols as energy carriers in MILD Combustion

Among the liquid fuels supporting the decarbonization of the energy conversion chain, alcohols play a key role. Mainly considered for engine application, their use in stationary systems designed for power generation is receiving considerable attention but requires further investigation. This work aims at demonstrating the feasibility of thermochemical conversion of lowmolecular- weight alcohols, methanol, ethanol, and 1-butanol, in a small-scale unit exercised under moderate or intense low-oxygen dilution combustion conditions. The highly recirculated flow field configuration allows for the stabilization of the process over a wide range of reactor temperatures. The experimental campaign is carried out by varying the mixture equivalence ratio and the thermal power. The burner was exercised with different gas feeding configurations, namely, premixed and non-premixed. Experimental results are reported in terms of operational temperatures and pollutant emissions (CO and NOx). For all of the fuels and thermal power, it was possible to reach NOx levels lower than 20 ppm and CO below 40 ppm for a wider range of the mixture equivalence ratio than hydrocarbon fuels. Despite similarities in the temperature profiles and CO emissions, NOx levels increase with the complexity of the alcohol molecules and their distribution is also a function of the injection strategy. Simulations in a perfectly stirred reactor and in a counterflow diffusion flame were performed to provide insights into the key factors controlling the NOx emission levels and distribution. Numerical results with a perfectly stirred reactor model show the role of NOx chemistry in determining the different emission levels of the three alcohols. On the other hand, simulations with a counterflow diffusion flame suggest that the separate reactant supply to the combustion chamber represents the key parameter in determining the experimental NOx distribution in the non-premixed mode.