| linky USE The EPA regulates which oxygenates can be added to gasoline in the U.S. and in what amounts through the Substantially Similar rule and waivers (see U.S. Oxygenate Regulation and the Substantially Similar Rule).
The most widely used oxygenates in the U.S. are ethanol, methyl tertiary-butyl ether (MTBE) and tertiary-amyl methyl ether (TAME). Ethyl tertiary-butyl ether (ETBE) is another ether that could be used. Figure 4-1 summarizes some information about these oxygenates. Note that, on a volume basis, each ether has a different maximum concentration because each has a different oxygen content and relative density.
Figure 4-2 gives the ranges of oxygenate use — in terms of oxygen content of the gasoline — required by the U.S. emission regulations. fig 4-2
The regulations do not require a specific oxygenate. Any oxygenate use conforming with the EPA's Substantially Similar or Waiver conditions is acceptable. Different gasoline brands in the same area tend to contain the same oxygenate, but uniformity is not required. Here are a few of the many factors that determine a refiner's oxygenate choice: What are the economics? Ethanol can be more economical than the ethers because there is a federal excise tax exemption for its use, and about one-third of the states also offer a tax exemption or other incentive. These incentives are designed to encourage the development of an agriculturally based fuel ethanol industry.
What is the availability? The bulk of the ethanol is produced in the Midwest. Some ether plants are in or adjacent to refineries and use refinery streams as feed.
What is the best blending location? At refineries, ethers usually are used because ethanol blends cannot be shipped by pipeline. It is hard to keep pipelines dry, and water in the pipeline will reduce the ethanol content of the gasoline (see Chapter 4, Oxygenated Gasoline). The water tolerance of ethers is similar to that of gasoline.
Regulations limiting the use or banning of MTBE.
The adoption of a Renewable Fuels Standard.
At terminals, standard practice is to blend ethanol with gasoline in the tank truck that will deliver the blend to the service station. Terminal blending involves more operating and capital expense than refinery blending. Also, because oxygenates raise octane, a low-octane conventional gasoline is needed, if the resulting oxygenated gasoline is to have the AKI commonly available in the area. For RFG, the volatility of the base fuel must be adjusted so it will meet federal and state volatility requirements after the addition of ethanol. Such a base fuel is referred to as RBOB (reformulated gasoline blendstock for oxygenate blending). In California, it is referred to as CARBOB. Oxygenates may be used in areas of the U.S. where they are not required as long as EPA's concentration limits are observed. Because all oxygenates are more expensive than the conven-tional gasoline they displace, ethanol's tax incentives make it the only one likely to be used near its concentration limit. The other oxygenates will be used at lower concentrations (1–7 vol %) only when the octane of the gasoline needs enhancement and when adding oxygenates is the most economical solution. The information available to the retail consumer about the oxygenate composition of the gasoline varies. There is no federal labeling regulation; this is an area controlled by state regulations. Some states require dispenser labels for oxygenated gasolines in wintertime CO nonattainment areas and RFG gasolines. A typical label reads: The gasoline dispensed from this pump is oxygenated and will reduce carbon monoxide pollution from motor vehicles. Some labels, like this one, do not provide any information on which oxygenate(s) the blend contains; others do. Few, if any, states require labeling for gasolines containing low levels of ethers added to increase octane. A national association of state weights-and-measures agencies responsible for dispenser calibration and labeling issued a revised standard guideline for dispenser labeling in 1996. Several states recently have dropped ethanol labeling requirements and some are now requiring labeling for MTBE.
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