Environ. Sci. Technol. 2001, 35, 3002-3007
Dioxin and Furan Formation in FBC Boilers E. J. ANTHONY,* L. JIA, AND D. L. GRANATSTEIN CETC, Natural Resources Canada, 1 Haanel Drive, Nepean, Ontario, Canada K1A 1M1
Fluidized bed combustion (FBC) is increasingly being used for municipal solid waste incineration and for various applications in which dioxin and furan (PCDD/F) formation are concerns. Despite the fact that fluidized bed combustion is a relatively low temperature system, current evidence shows that they perform relatively well. One hypothesis is that sulfur, which is contained in some of the fuels used in FBC (e.g. coal in cofiring situations), helps to reduce dioxin production. This paper endeavors to examine this speculation and finds that such benefits depend on the Cl/S ratio and that at higher sulfur addition levels dioxins, furans, and polycyclic aromatic hydrocarbon (PAH) production may actually increase. This and the reasons for it are discussed.
Introduction Fluidized bed combustion (FBC) can be used as a means of energy production or incineration for almost any material containing carbon, hydrogen, and sulfur in a combustible form, be it a solid, liquid, slurry, or gas. FBCs commercially available vary in size from 1 MWt to 250 MWe (1-4). Increasingly, FBCs are finding applications for municipal solid waste (MSW) combustion in the 20-100 MWe range (5, 6). As FBCs operate at 800-950 °C they produce low NOx emissions, and SO2 emissions can be controlled by in-bed reaction with added limestone (4). However, other emissions such as HCl are less well controlled since there is negligible HCl capture at typical FBC temperatures (7). Based on their low combustion temperature one might also expect that FBCs would have high CO, PAH, and/or PCDD/F emission for fuels containing significant Cl contents. In practice this has not proven to be the case. Fuels with low Cl content such as most wood products have been shown to produce low polyaromatic hydrocarbons (PAH) production, and coal combustion using FBC has been associated with negligible dioxins and furans production (PCDD/F) (5). FBC technology has also been successfully employed to destroy polychlorinated biphenyls (PCB)-containing materials (8-10). In fact a mobile circulating FBC unit has destroyed PCB wastes with destruction and removal efficiencies of better than “eight nines” (i.e., 99.999999%) despite the fact the materials contained over 20-30% PCBs. In these tests, stack concentrations of PCBs averaged 107 ng/Nm3 (regulation is 36 000 ng/Nm3), while levels in the ash were consistently below 0.1 ppmw (regulation 0.5 ppmw). PCDD/F levels in stack gas and ash were also consistently below regulated values, by 1 or 2 orders of magnitude (10). That FBCs can burn wastes more effectively than conventional technologies at lower temperatures has also been acknowledged by the U.S. Environmental Protection Agency (EPA) for MSW combustion (11). Equally, studies done on organic emissions * Corresponding author phone: (613)996-2868; fax: (613)992-9335; e-mail:
[email protected]. 3002
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 14, 2001
(both PAHs and PCDD/F) from coal burning in FBCs have suggested extremely low emissions (12-14). The implication of these results is that high temperatures (1000 °C) are not essential to achieve low emissions of products of incomplete combustion, and suggested reasons for this will be given below. In the case of FBC plants burning MSW, one of the more promising new applications for FBC technology, these can achieve dioxins/furans emissions in the range of several ng/ Nm3 expressed as a toxic equivalent (TEQ) (15). However, to meet the strict German standards of
