Photoacoustic response of unburnt carbon in fly ash to infrared radiation

Abstract

The photoacoustic response of unburnt carbon in fly ash to infrared radiation was investigated. The photoacoustic effect is the conversion of modulated electromagnetic radiation absorbed by gaseous, liquid or solid media into an acoustic wave, which can be detected by a sensitive microphone. The goal of this work was to develop an off-line instrument to measure unburnt carbon in fly ash. Twenty-four fly ashes from several pulverized coal and fluidized bed boilers burning a variety of coals were used. The photoacoustic effect generated by 1.315 μm infrared radiation from a 1.5 mW diode laser proved capable of measuring unburnt carbon in fly ash in the range 0.1–7.0 wt%. Compression and grinding of fly ash samples prior to measurement proved effective in improving the correlation between photoacoustic signal and carbon content. Such sample preparation reduces differences in powder voidage and particle size, both of which can affect photoacoustic response. The photoacoustic response was inversely proportional to the excitation modulation frequency, a result expected for photoacoustic saturation; however, the relatively large linear range observed for the photoacoustic signal as a function of fly ash carbon content is inconsistent with saturation. Therefore the photoacoustic response of unburnt carbon in bulk fly ash appears to be a blend of both solid-sample and single-particle effects.