OCR Text |
Show struc~~ gue to inlet and boundary conditions were eliminated by careful tailoring the fuel gas mixtures'.' Furnace frring rate was held at a value which resulted in a fairly low exhaust temperature and therefore, presumably, low potential for post-flame oxidation of surviving species. In this study it was observed that: (1) The flame structure, although globally similar in tenns of wall attachments, recirculation zones, etc., did exhibit significant local changes with fuel gas composition. Those changes may be attributable to either partial premixing in the lifted flame jets or differential diffusion of hydrogen in the fuel mix. (2) None of the fuels produced benzene, ethylbenzene, or xylene emissions in excess of 0.5 ppbv under nonnal firing conditions. Toluene emissions were in the range from 0.2 to 2.7 ppbv. Since Major Source emission levels are of order 100 ppbv for these species, and the Residual Risk level for benzene is estimated to be of order 10 ppbv, the present measurements indicate that well-controlled, industrhu-style diffusion flames of these surrogate RFG fuels do not produce emissions of concern with respect to the 1990 Amendments to the Clean Air Act. (3) Under sub-stoichiometric conditions, all of the speCIes were detected, but the ethylbenzene and m-, and p-xylenes remained below 10 ppbv. Benzene, toluene, and 0- xylene increased by orders of magnitude in going to -25% excess air, with benzene replacing toluene as the dominant emission. (4) Increasing fuel hydrogen and propane contents at the expense of methane appears to cause a modest rise in toluene emission in the normal operating range of excess air, but emission variability prevents us from making this statement defmiti~ely. At -25% excess air, all emissions increase with use of the blended fuels, with the most dramatic rise-two orders of magnitude for benzene~curring between RFG6 and RFG3. (5) A preliminary investigation of burner failure modes indicated that only global substoichiometric frring resulted in emissions of sufficient magnitude to be of concern. - 11 - |