|US Department of Energy
Released Date: September 2002
Ethanol in Gasoline
Ethanol, or gasoline containing ethanol, cannot be moved practically through today’s pipeline system, because it tends to pull water that usually exists in petroleum pipelines and tanks. Instead, ethanol is blended into gasoline at terminals near the end users (monitoring varies by state). Splash blending, in which ethanol is added directly to a tanker truck along with the base gasoline, is commonly used. Ethanol-blended product must be kept separate from product not containing ethanol, necessitating separate handling all the way to the gasoline pump. The separation is needed because movement of a small amount of ethanol (from the ethanol-blended mixture) to gasoline without ethanol can increase the vapor pressure of that gasoline mixture substantially, potentially pushing it above required VOC limits. Thus, ethanol must be moved through an independent distribution system until it is close to the end user, where it then is added before being delivered to retail stations.
Ethanol’s high blending RVP affects gasoline production as well as distribution and storage. Most summer conventional gasoline has an RVP of 9.0 psi. Adding 10 percent ethanol to 9.0-psi conventional gasoline increases the RVP by about 1 pound. However, a Federal waiver of 1 psi RVP (sometimes called the “1-pound waiver”) exists for conventional gasoline that is blended with ethanol. The waiver allows terminal operators to add 10 percent ethanol to finished 9.0-psi conventional gasoline with no penalty for the RVP increase. However, lower-RVP conventional gasolines will experience a larger RVP increase than 1 pound when 10 percent ethanol is added. For example, 7.8-psi gasoline might see a 1.3-psi increase. Because this RVP gain exceeds the 1-pound waiver, refiners cannot add 10 percent ethanol to a finished 7.8-psi gasoline. They must produce an unfinished blend with RVP of 7.3 psi that, when combined with 10 percent ethanol, creates a finished gasoline with RVP of 8.8 psi (7.8 psi plus the 1 psi waiver). This unfinished conventional gasoline base is sometimes called a conventional gasoline blendstock for oxygenate blending, or CBOB.
Producing reformulated gasoline using ethanol to replace MTBE presents a larger problem for refiners. Ethanol affects nitrogen oxides (NOx) and toxics emissions as well as VOC’s. In order for RFG to meet its VOC requirements, the finished blend must be at a fairly low RVP, generally less than 7.0 psi during the summer. To create a base unfinished reformulated gasoline mixture to which the ethanol will be added (referred to as reformulated gasoline blendstock for oxygenate blending, or RBOB), the RVP must be reduced to very low levels (6.0 psi or less). Light molecular weight, high RVP components must be removed to reduce the base mixture’s RVP. When these light components are removed, RFG’s distillation characteristics are changed, which may require removing some heavy molecular weight, high boiling point components as well. (The distillation characteristics, as measured by how much material evaporates at different temperatures, affect driveability.7) Ethanol also increases NOx emissions, and in the case of California reformulated gasoline, which has more stringent emission requirements than Federal reformulated gasoline, the NOx limitations will likely prevent refiners from being able to use much more than 6 percent ethanol. Ethanol, while clean on a relative basis, has a higher toxic impact than MTBE. Thus, refiners with very low Mobil Source Air Toxics baselines based on MTBE usage may have great difficulty switching from MTBE to ethanol.
When CBOB’s or RBOB’s are produced, these unfinished products travel through the petroleum distribution system to terminals where ethanol is added to create the finished gasoline. The ethanol will have traveled through a separate distribution system to the terminals.