Oscillating coal and biomass flames: A spectral and digital imaging approach for air and oxyfuel conditions

The transient nature of a flame can be quantified by performing spectral and oscillatory analysis of its parameters, such as the flame's luminance and temperature. This paper presents an assessment of the effect of an oxyfuel environment on the combustion of two different solid fuels, a high volatile bituminous coal and a white wood biomass, in a 250 kWth pilot-scale combustion test facility. A digital flame monitoring system was fitted to the experimental furnace, and was used to record high speed videos of the flame. Transient signals for both digital luminance and temperature were obtained after the instantaneous frames were extracted from the original videos. Spectral analysis was performed over the transient signal in order to analyse the temporal coherence of the flame through a weighted oscillation frequency value. An additional parameter, the oscillation index, which accounts for the amplitude of the oscillation of the flame, was computed to complement the information recovered from the flame. The oscillation trends obtained from these experiments assess the dynamic response of the flame to different combustion environments within the furnace. In general, it was found that oxyfuel flames showed a discernible temporal repeatability and a lower magnitude of the oscillation of their flame parameters, and therefore are registered as being more stable than their counterpart under air combustion conditions. In addition, the biomass flames exhibit less sensitivity to the oxyfuel combustion environment than what was found with coal, which may allow future oxy-biomass regimes to operate under a much wider envelop of firing conditions.