On-Line Determination of Unburned Carbon in Airborne Fly Ash

Abstract

Knowledge of the carbon content of fly ash is important for calculating combustion efficiency in coal-fired boilers. However, present methods of measuring carbon content of fly ash are tedious and time consuming. Thus, we are investigating photoacoustic absorption spectroscopy (PAS) as a method for on-line monitoring of carbon in fly ash. Photoacustic absorption spectroscopy is capable of detecting very weak absorptions and is unaffected by light scattering in a particulate-laden gas flow. Accordingly, PAS has good potential for distinguishing small amounts of carbon from mineral matter suspended in flue gas. Experiments have been performed on fly ash samples of variable carbon content suspended in a gas flow. A 35-mW HeNe laser was able to detect carbon loadings of 0.75 g/m3. Order-of-magnitude improvements in detection sensitivity are expected with higher power lasers or improved PAS cell design.