OCR Text |
Show correla!ed,..with methane and ethane content. Hydrocarbons heavier than methane are present in significant quantities due both to the nature of the refining process and to the fact that it is desirable to have a near constant heating value per unit volume of RFG. In fact, it is common practice when using high-hydrogen RFG to help balance the dilution effect of the hydrogen by injecting C4-rich gases to achieve a constant specific gravity (and thereby energy content) fuel stream. When taken in combination, the size of petroleum refining operations, the fact that HAP species are often present in RFG (in the C6+ category and possibly butadiene), and the fact that they are present in trace amounts in the combustion effluent make it apparent that air toxics regulations will pose a significant challenge to the U.S. petroleum industry. Table I helps to frame this problem by summarizing some air toxic concerns for the combustion effluent from a typical 250,000 barrel per day refmery. Listed here, in order of increasing molecular weight, are species from the HAP list which have been identified as potential contributors to the air toxics problem. Note that these species range from those which are light (M - 30) and easily oxidized, to those which are quite large (M - 276) and generally considered refractory. Also note that neither chlorinated nor metal-containing species are considered to playa significant role in combustion-generated HAP emissions for this industry given that neither chlorine nor metals are present in RFG. The last two columns of. the table provide some perspective on the relative importance of these species. The fIrst of these gives the concentration of each species required to make it, -individually, a Major Source emission (Le., to achieve 10 tons/year of that species from a 250,000 bpd refinery). The concentrations range from about 300 ppbv for light species down to 30 ppbv for the heaviest. Since the concentration of species usually drops off rapidly with molecular weight, it is the low molecular weight end of this table (above the dashed line) which is expected to cause a problem with respect to the abundance of emissions. In contrast to these are the high molecular weight P AH species listed at the bottom of the table. These species are expected to be present in concentrations of order 1 ppbv (or less) but are -3- Table I |