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Show Further, they may reside in the baghouse for a significant residence time, during which they can react with gaseous compounds (e.g. sulfur dIoxide). 4.2. The Fate of Arsenic in a Fluidized Bed System Arsenic is generally considered to be among the most volatile of metals, with volatilization temperatures estimated to be 90°F (306°K), and with little need for chlorine presence to promote volatilization [1]. In conventional combustion systems, virtually all of the arsenic will volatilize. Some will condense on larger fuel particles in heterogeneous condensation reactions in cooler regions of the system. Some will condense by homogeneous nucleation reactions into fme particles or fume. Typically, some 90 percent of the arsenic which is emitted from conventional combustion systems can be captured in a baghouse either as larger particles where arsenic has been deposited by heterogeneous condensation or as fine fume particles [24]. 4.2.1. The General Reactions of Arsenic and Calcium Oxide The behavior of arsenic in combustion systems where calcium oxide is present is dramatically different from the behavior of arsenic in conventional combustion systems. The arsenic typically does not volatilize and escape in gaseous form from the combustor. If the fluidized bed particles become "sticky" on the surface, they physically accumulate trace metals in the feed [15]. This physical mechanism involves both solids-solids reactions and gas-solids reactions. A second mechanism also may operate, where the arsenic becomes mobile and reacts with the calcium oxide to form very stable compounds which evolve in ash products. This behavior of arsenic and calcium in combustion systems is well documented. S tumbar et. ale [20] demonstrated that the addition of lime to arsenic laden wastes being incinerated in the USEPA mobile kiln effectively bound up 90 percent of the arsenic as shown in Table Vil. In that system, only 10 percent of the arsenic volatilized and passed through as flyash to the scrubber, the air pollution control system. The non-volatile arsenic reacted with lime, did not become volatile, and was discharged with the kiln and cyclone ash [20]. Stumbar et. ale [20] report that similar results have been obtained by the EPA Combustion Research Facility in Jefferson, Arkansas. As documented above, Waterland et al. [25] document similar results with the incineration of arsenic contaminated soils in a pilot scale rotary kiln system [24]. Like other observers, Waterland and co-workers have found that the presence of calcium suppresses the volatility of arsenic, and its subsequent reactivity. Numerous others have observed similar phenomena. 4.22. The Mechanisms for Reactions Between Arsenic and Calcium Ho et. ale [11] tested this phenomena in a fluidized bed. Ho et. ale discussed the fact that arsenic capture by reaction with lime can occur by the following mechanisms: 1) upon particle heating up to I380°F (1023°K), the metal remains in the char and forms polymers, and then sticks to and remains within the ash matrix; 2) the arsenic reacts with surrounding compounds to form a polymer which is very |
