Estimates of particle formation and growth in coal-fired boiler exhaust—II. Theory and model simulations

A pre-existing model of plume particles is examined to determine its usefulness for simulating particle formation and growth in coal-fired, cyclone boiler exhaust. Of greatest interest are fine particles (diameters of 0.2–2 μm) most likely to influence plume opacity. For the boiler examined, these particles consist primarily of water-sulfuric acid droplets formed from emitted SO3. Theoretical consideration is given to nucleation in both homo- and bimolecular systems to allow comparison with the nucleation and growth formulations used in the plume particle model. Observations in plumes from a non-scrubbed boiler, under various operating conditions, are used to evaluate model performance. Model simulations were found to overestimate particle formation and growth rates and inaccurately reproduced observed particle size distributions. Some of this bias is likely due to the particle nucleation formulation in the model, but modeled growth processes also appear to be too fast. Current binary nucleation theory offers little reason to be optimistic that substantial model improvements can be made at this time.