OCR Text |
Show center point of the test matrix, which was tested in triplicate (Tests 4, 5, and 6), is included in both groupings. The tabulated values represent the cumulative percent of the total mass of each metal found in a given particulate sample within the indicated range. The size ranges are less than 2 ~m, less than 4 ~m, less than 10 ~m, less than 30 ~m, and greater than 30 ~m, and correspond to the nominal size cuts resulting from the size fractionation described above. Metals find their way into flue gas particulate via two pathways. In one pathway, the metal remains in a condensed phase through the entire incinerator system and is carried out of the system with entrained ash in the combustion gas. In the second pathway, the metal vaporizes at some point in the incinerator, then recondenses when the flue gas cools. Both vaporization and condensation can occur locally under proper conditions. Vaporized metals can condense homogeneously into condensation nuclei that grow into a very fine fume, or they can condense heterogeneously onto existing flue gas particulate. In both mechanisms the tendency is to enrich (be found at higher per mass concentration) in fine particulate. In the former mechanism, fume particles are very fine (1 ~m or less). In the latter mechanism, the surface-to-mass ratio is higher for fine particles than for coarse particles. Since condensation onto an available surface is a per surface area event, this also leads to enrichment in fine particulate. Via the above mechanisms, the distribution of a given metal among flue gas particle size ranges is strongly influenced by the extent to which the metal vaporizes in the incineration system. Refractory metals that do not vaporize significantly tend to be relatively evenly distributed in the flue gas particulate size ranges on a per mass (mg/kg particulate) basis. Volatile metals tend to enrich in the fine particulate fractions, with enrichment tendency increasing with increasing volatility. To characterize a metal's volatility, equilibrium analyses can be performed to identify the metal's volatility temperature for a given set of incinerator conditions (4). The volatility temperature is the temperature at which the effective vapor pressure of a metal is 10-6 atm. The effective vapor pressure is the combined equilibrium vapor pressures of all species containing the metal. It reflects the quantity of metal that would vaporize under a given set of conditions. A vapor pressure of 10-6 atm is selected because it represents a measurable amount of vaporization. Volatility temperatures are a major parameter in the metal partitioning model used to predict metal behavior in an incinerator (4,8). Table 4 contains the volatility temperatures for the test metals, assuming an oxidative environment (4,8). However, the volatilities of some metals may be altered through reactions 9 |