OCR Text |
Show still of .c09cem because they are carcinogenic. The last column of the table reflects the relative importance of these species via the Toxic Equivalency Factor (a measure of relative potency) nonnalized to naphthalene (Nisbet and LaGoy, 1992). Species with no entry in the TEF column .are not considered to impact the Residual Risk assessment. Note the importance of benzene as it has the ability to impact both the abundance and risk aspects of the problem.1 From the combustion chemistry point of view, the challenge lies with the C6 and higher hydrocarbons; mechanisms for these species are at the state of the art. Likewise, in practice, oxidation of the lightest species (e.g., formaldehyde) is fairly easy to accomplish, reducing the role that these species are likely to play in constraining the comoustion process. From the combustion science point of view, the challenge is then to understand and affect the combustion chemistry of these species unqer the real-world conditions and constraints of refmery process heaters. These conditions and constraints include: ftring of complex compositions of ~G, use of complex real-world turbulent diffusion flame burners, fIring at power densities or furnace temperatures characteristic of refmery process heaters, and fIring in such a way as ·to satisfy other pollutant constraints (particularly NOx). In addition, it must be recognized that, unlike NOx, the chemistry of air toxics does not freeze shortly after exiting the high temperature flame zone; downstream oxidation in, for example, the convective pass of a process heater must also be understood in order to adequately address the air toXics issue. It was with .these objectives in mind that the present study of air toxic emissions burning refmery fuel gas was undertaken. The objective of the study was to establish, under controlled conditions, the furnace-out emissions of the volatile organic compounds (VOC) Benzene, Toluene, Ethylbenzene, and the Xylenes (BTEX) from a well-crafted, industrial-style diffusion flame burner fIring various fuel mixtures at low power density (low furnace temperature and therefore low oxidation potential). Effects of post-furnace oxidation were not included in this study. 1 The TEF assignment for benzene is indicated as being approximate since it was not included in the data of Nisbet and LaGoy, but was instead made by comparison with benzo(a)pyrene data from Clement Associates (1988). -4- |