Simulation of stack plume opacity

The visual impact of primary particles emitted from stacks is regulated acc ording to stack opacity criteria. In-stack monitoring of the flue gas opaci ty allows plant operators to ensure that the plant meets U.S. Environmental Protection Agency opacity regulations. However, the emission of condensabl e gases such as SO3 (that hydrolyzes to H2SO4), HCl, and NH3, which may lea d to particle formation after their release from the stack, makes the predi ction of stack plume opacity more difficult. We present here a computer simulation model that calculates the opacity due to both primary particles emitted from the stack and secondary particles f ormed in the atmosphere after the release of condensable gases from the sta ck. A comprehensive treatment of the plume rise due to buoyancy and momentu m is used to calculate the location at which the condensed water plume has evaporated (i.e., where opacity regulations apply). Conversion of H2SO4 to particulate sulfate occurs through nucleation and co ndensation on primary particles. A thermodynamic aerosol equilibrium model is used to calculate the amount of ammonium, chloride, and water present in the particulate phase with the condensed sulfate. The model calculates the stack plume opacity due to both primary and secondary particles. Examples of model simulations are presented for three scenarios that differ by the e mission control equipment installed at the power plant: (1) electrostatic p recipitators (ESP), (2) ESP and flue gas desulfurization, and (3) ESP and s elective catalytic reduction. The calculated opacity is most sensitive to t he primary particulate emissions. For the conditions considered here, SO3 e missions showed only a small effect, except if one assumes that most H2SO4 condenses on primary particles. Condensation of NH4Cl occurs only at high N H3 emission rates (about 25 ppm stack concentration).