Experimental investigation of 40 kWth methane-assisted and self-sustained pulverized biomass flames

This paper presents an assessment of the effects of methane assistance on pulverized biomass swirl flames specifically regarding flow fields and flame structure. Experiments are carried out using a pilot-scale down-fired cylindrical combustion chamber equipped with a swirl burner and biomass/methane fuel mixtures. Studied conditions have an identical thermal output of 40 kW, with the thermal output share of methane gradually decreasing from 50% to 0% while the biomass share (walnut shells) increased from 50% to 100% (self-sustained condition). A detailed flow field characterization of the respective flames is conducted by in-flame, two-dimensional laser Doppler velocimetry measurements. These measurements are complemented by narrow-band flame imaging conducted at two different wavelengths (OH* and CH* radical band heads). Results show that the methane flames have a significant influence on the ignition and the determination of the flame flow field structure, including higher peak and overall velocities, as well as major changes in the ratio of tangential over axial velocity component. Further on, flame attachment of the self-sustained flame can be permanently achieved by the initial, short-term assistance of a methane flame with comparatively low thermal output. These observations are analysed and discussed, where higher measured velocities and overall changes in the flame structure between the self-sustained and the methane-assisted flames are attributed to important local expansion and momentum changes of the combustion gases introduced by the combustion of methane.