Combustion features of a bio-fuelled micro-gas turbine

The authors examine in this paper the response of a micro gas turbine (MGT) combustor when supplied with gaseous fuels from biomass treatment or solid waste pyrolysis or from an anaerobic digestion process. Actually, a sort of off-design operation is induced by the employment of low calorific value fuels both in the combustor and in the whole micro turbine system. The objective is to optimize the combustor behaviour under the point of view of combustion efficiency and pollutant control. The first part of the paper discusses preliminary pollutant estimation, basing on a non-dimensional, time-dependent model that solves the kinetic equations for nitric oxides formation in the residence time domain. The initial conditions are derived from a thermo-fluid dynamic analysis of the MGT system under several conditions induced by the activation of a recuperator by-pass valve. In the second part of the paper, a CFD study employs as boundary conditions those obtained from the thermo-fluid dynamic MGT simulation and it relies on different methods for approaching the fuel oxidation process. In particular, the partially stirred reactor hypothesis combined with a flamelet model is able to describe the mechanisms of both primary oxidation and pollutant formation. Several solutions are also examined in order to improve the combustion efficiency with poor calorific value fuels. The simultaneous objective of nitric oxide reduction is attained through a proper choice of an alternate location of the pilot injector aiming at exploiting a sort of internal EGR.