In recent years, as control systems for boiler emissions have been upgraded, some utility sources have experienced increased plume opacity. Cases of plume opacity exceeding in-stack opacity are due to 1) the aerosol formed by condensation of primary sulfuric acid and water vapor onto polydisperse plume particles and 2) the presence of fine particles which grow into the visual size range by heterogeneous condensation and coagulation processes as the plume is cooled and diluted by mixing with the ambient air.

In order to better understand the factors leading up to acid plume formation, a computer simulation model has been developed. This plume opacity model has been utilized to simulate sulfuric acid aerosol formation and growth. These processes result from homogeneous nucleation, condensation and coagulation which substantially increase the concentration of submicrometer sized aerosols. These phenomena bring about significant increases in plume opacity.

Theoretical relationships have been derived and transformed into 21 computer model to predict plume opacity at various downwind distances resulting from pulverized coal combustion operations. This model consists of relatively independent components-such as an optics module, a bimodal particle size distribution module, a polydisperse coagulation module, a vapor condensation and nucleation module and a plume dispersion module-which are linked together to relate specific flue gas emissions and meterological conditions to plume opacity.

This unique, near-stack, plume-opacity-model approach provides an excellent tool for understanding and dealing with such complex issues as:

• increasing plume opacity observed for emissions containing sulfuric acid aerosols,

• explaining the correlation between primary particle size distribution and light—scattering effects,

• predicting the opacity level resulting from combustion of various coal types,

• predicting control equipment effects on plume opacity.