Precision and accuracy of photoacoustic measurements of unburned carbon in fly ash

Abstract

This research investigates factors influencing precision and accuracy of photoacoustic measurements of unburned carbon in fly ash. This information is useful for the development of instruments to monitor the performance of coal-fired boilers. Important factors identified by this research include the protocols for sampling and preparing fly ash for measurements, the ambient conditions at which photoacoustic measurements are performed, and the wavelength of the excitation source. Since the quantity of fly ash interrogated by the photoacoustic technique is usually small, representative sampling is critically important. Most fly ash is extremely heterogeneous and grinding is an important precursor to sampling material for testing in the photoacoustic apparatus. Bulk properties of the fly ash also affected the photoacoustic signal. The effects of both ambient temperature and humidity on precision were evaluated through statistical testing. Temperature control of the photoacoustic cell to within a few tenths of a degree Centigrade proved to be important. The wavelength of the excitation source has two major effects on accuracy of measurements. Although mineral matter in fly ash is a relatively weak absorber of radiation compared to carbon, its concentration is typically so high that a pronounced background signal can arise from the mineral components. Careful selection of excitation wavelength can mitigate this effect on accuracy. Another factor influencing accuracy was signal saturation, which occurred at relatively low carbon concentrations when using 940 nm infrared radiation as an excitation source; this effect was avoided by diluting fly ash samples with optically non-absorbing powders.