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Show 4) metals that are released from the ash and remain volatile. Metals that fall into the latter two categories are of particular concern because those are often the most difficult to control using conventional APCDs. These metals include mercury, selenium, and some radionuclides (235U, 238U, 226Ra, 210Pb). Boron, though not a HAP, is listed as a controlled metal by the California Air Resources Board (CARB) and was also found poorly contolled in conventional particulate control devices. Organic HAPs arising in coal combustion systems ultimately come from the organic matter of the coal itself. Their presence in stack gases is therefore a direct indication of combustion inefficiency-although that may be an artifact of other pollution control measures, such as internally-staged 10w-NOx burners. The precise mechanisms by which organic HAPs are formed, however, is a subject of many ongoing studies. Chlorinated dioxins and furans are a special case in point. Studies of waste incineration systems [1] revealed that they may be formed primarily downstream of the combustion zone through heterogeneous reactions with ash particles. In coal combustion systems, there is strong evidence that sulfur species may interfere with the formation mechanism [2]. The data presented here support that conclusion. Energy and Environmental Research Corporation (EER) has conducted several programs on the measurement and modeling of the behavior of air toxic compounds in coal combustion systems. This paper will describe the results of some of these tests, as well as work by others, and will contrast these results with modeling efforts that are used to aid in understanding the mechanisms behind the partitioning of volatile metals into the vapor phase. The results to date indicate that air pollution control devices designed to control particulate matter, such as electrostatic precipitators (ESPs) and fabric filter baghouses are effective for control of many low- and moderate-volatility metals. However, these data also show that metals that display high volatility at the ambi~nt conditions of the particulate control device can effectively escape with stack emissions. Interestingly, subtle changes in APCD operation, that might be caused by shift in boiler load, or other normal operating variabilities, can dramatically impact retention of volatile metals. AIR TOXICS MEASUREMENTS A summary of work conducted by EER and others under a DOE-sponsored program was published recently [3]. Under the DOE program EER tested a 615 MW Cell Burner power plant firing bituminous coal and equipped with an ESP. Metals were monitored using EPA Draft 3 |
