Author: K.A.Morrison
Air & Waste Management Association Annual Meeting in Portland (OR) June 24-27 2008
Abstract
There are two widely-used models for examination of impacts due to water vapour emissions- SACTI and CALPUFF. The latter is widely use for air quality evaluations, and with the release of fog processing utilities is being used more fogging. However, a severe limitation in its usefulness for fog modeling is that it automatically assumes that 100% relative humidity means there is already fog present. In reality the greatest impairments to visibility due to cooling tower or other vapour emissions occur when the background humidity is 100%. This was brought home when evaluating the impacts of a new ethanol plant beside a major highway.
The model used as intended found few hours of problems, but during the first winter of operations there were many noted hours when there was visibility impairment on the highway and these corresponded to hours of 100% humidity, when the model simply considered that there was background fog. A work-around was done when all humidity was reduced by 1% for calculations, the water vapour being re-introduced during a post-post-processing stage. The results were then analyzed based on the extinction coefficient rather than humidity as such. The number of hours of calculated impairment corresponded well with observations, and the client moved their cooling towers as a result to reduce impacts.
