An analysis of extinction coefficients of particles and water moisture in the stack after flue gas desulfurization at a coal-fired power plant

Two important factors that affect in-stack opacity-light extinction by emitted particles and that by water moisture after a flue gas desulfurization (FGD) unit-are investigated. The mass light extinction coefficients for particles and water moisture, k_p and k_w, respectively, were determined using the Lambert-Beer law of opacity with a nonlinear least-squares regression method. The estimated k_p and k_w values vary from 0.199 to 0.316 m²/g and 0.000345 to 0.000426 m²/g, respectively, and the overall mean estimated values are 0.229 and 0.000397 m²/g, respectively. Although k_w is 3 orders of magnitude smaller than k_p, experimental results show that the effect on light extinction by water moisture was comparable to that by particles because of the existence of a considerable mass of water moisture after a FGD unit. The mass light extinction coefficient was also estimated using Mie theory with measured particle size distributions and a complex refractive index of 1.5-ni for fly ash particles. The k_p obtained using Mie theory ranges from 0.282 to 0.286 m²/g and is slightly greater than the averaged estimated k_p of 0.229 m²/g from measured opacity. The discrepancy may be partly due to a difference in the microstructure of the fly ash from the assumption of solid spheres because the fly ash may have been formed as spheres attached with smaller particles or as hollow spheres that contained solid spheres. Previously reported values of measured k_p obtained without considering the effects of water moisture are greater than that obtained in this study, which is reasonable because it reflects the effect of extinction by water moisture in the flue gas. Additionally, the moisture absorbed by particulate matter, corresponding to the effect of water moisture on the particulates, was clarified and found to be negligible.