Analysis of heavy metal emission data from municipal waste combustion

Heavy metals contained in municipal solid waste (MSW), after combustion in modern waste-to-energy facilities, are collected in bottom and fly ash, only a small quantity being discharged from the stack as particulate or vapor. These metals are found to be broadly distributed throughout the constituents, limiting the potential for reducing them by targeting specific components. The many factors which determine metals partitioning to bottom ash, boiler hopper and emission control flyash, and stack emissions, shows that the complex relationship between feed composition and emissions makes it difficult if not impossible to assign cause and effect on their quantities and concentrations in these discharges. Data showing the relationship between particulate matter, emission controls, and emission factors for the heavy metals is examined. A finding that substantial spiking of lead and cadmium in the feed resulted in only marginal changes in stack emissions indicates that efforts to remove these metals from the waste would not produce a significant change in stack emissions. The range of trace metal emissions from a single waste-to-energy (WTE) facility over a period of three to four years is compared with the range reported from individual tests of about twenty facilities also having acid gas controls and fabric filters, indicating that the waste composition and the combustion and emission control technology employed all contribute to the variability of metals and particulate emissions. The relationship between annual averages and probable maximum values which may be anticipated from periodic testing is examined. Special attention is given to mercury, its various species, chemical reactions, and the effectiveness of various carbon-based reagents used for emission control. Emissions from WTE facilities are compared with those from oil and coal-fired utility boilers on a mass per k Wh generated.