Experimental characterization of pulverized coal MILD flameless combustion from detailed measurements in a pilot-scale facility

Abstract

This paper presents a series of measurements on pulverized coal MILD flameless combustion in a pilot-scale facility for nitrogen oxides (NOx) emissions reduction. Local measurements of gaseous species concentrations, gas temperature and velocity associated to reaction zone imaging by OH* chemiluminescence highlight specific features of this MILD flameless combustion regime. The flameless burner used during the investigation could abate significantly NOx emission levels.Specific aerodynamics of the flow in the furnace induced by the burner geometry are discussed. High momentum turbulent air jets favor a large recirculation of hot flue gas in the combustion chamber. Such a recirculation induces a large diluted combustion regime from the entrainment in the air jets of devolatilized species from the pulverized coal jet and of recirculating hot inert combustion products. The main reaction zone is lifted from the burner exit as it starts in the mixing layers of separated pulverized coal and air jets. As is typical of a flameless combustion regime, such a large dilution of reactants induces low local heat release and temperature increase in the reaction zone. Nitrogen oxides are generated from the fuel NO route at quite a limited rate due to the low temperature environment.The effect of the carrier gas of the pulverized coal jet is also analyzed. The change from carbon dioxide to air as carrier gas generates a first reaction zone attached to the burner exit. This reduces the dilution and increases the heat release in the main lifted reaction zone, leading to higher NOx emissions.