Impact of Reducing Particulate Emissions on Metals Emissions from Recovery Boilers and Lime Kilns, 1999 Environmental Conference Proceedings

James Frederick and Kristiina Iisa - The Institute of Paper Science and Technology

The discharge of metals in particulate emissions from recovery boilers and lime kilns is currently an important environmental issue to pulp and paper manufacturers. The objective of the work reported in this paper was to evaluate the impact of reducing particulate emissions from recovery boilers or lime kilns on emissions of metals from these sources. The metals in question were Sb, As, Be, Cd, Cr, Co, Cu, Pb, Mn, Hg, Ni, Ag, and Se.

Steady-state, input-dependent material balances for the elements Be, Cd, Cr, Co, Cu, Pb, Mn, Ni, and Ag, and Se were developed based on available data for the inputs, concentrations in process streams, and purges for kraft pulp mills. These balances were then used to calculate the impact reducing total particulate emissions from the recovery boiler and lime kiln. The data available for Sb, As, Co, Hg, and Se were too limited for material balances to be developed. However, the expected qualitative impact on these elements was inferred based on their solubility in green liquor, their emissions factors from recovery boilers, and their ionic character.

The mass balance results showed that metals emissions with particulates from recovery boilers and lime kilns are a small part of the total discharge of metals from kraft pulp mills. Most metals leave the mill with the pulp or dregs. The emissions of Cr, Cu, Mn, Ni, and Pb from recovery boilers and lime kilns together are less than 4% of the total metals leaving the mill. The metals emissions are very low, in part because of relatively low inputs of metals to the process and in part because they are much less volatile than sodium salts. The already high recovery of fume particles by the recovery boiler ESP ensures that metals emissions will be low.

The study showed that reducing particulate emissions from recovery boilers and lime kilns will reduce metals emissions as well. The reason is that most of the output of metals from the pulp mill is from other purges, and the particulate emissions are too small a metals purge to significantly influence the concentrations of metals in the liquor and lime cycle.