Ethane as a cleaner transportation fuel.

Abstract

T recent shale gas revolution has increased the supply of ethane to an unprecedented level. The unexpected surplus of ethane has led to exceedingly low prices and a waste of a resource. Currently, the principal use for ethane is ethylene production. The surplus of ethane is turning into an excess of ethylene. The recent collapse of oil prices has greatly lowered the costs of petroleum-based ethylene, and limits the demand for ethane from ethylene producers. A small proportion of ethane may be blended into natural gas, but the heat value specifications limit the amount of ethane allowed. An increasing amount of ethane will likely be flared, which is a controlled burning of the gas only to get rid of it. Some projections suggest that U.S. ethane production may outgrow demand by hundreds of thousands barrels per day in the coming years. Due to the lack of infrastructure to utilize ethane, it is considered a nuisance in shale gas development. The physical and chemical properties of ethane make it a good a transportation fuel. For the same volume, ethane carries slightly more energy than liquefied natural gas (LNG), but is free from the evaporation loss problem in cryogenic LNG systems. The infrastructure required for ethane transportation are similar to those for compressed natural gas (CNG) vehicles, where the same cylinder can carry more than twice amount of energy in ethane than in CNG. A typical welding cylinder designed for CNG with 16.5 MPa pressure rating can hold liquid ethane safely. Even in hot summer days, when the liquid ethane completely evaporates, the pressure will not rise above the pressure rating. The promotion of natural gas vehicles (NGVs) is faced with several challenges. The CNG vehicles have significantly shorter driving ranges per refill than their gasoline or diesel counterparts. The LNG vehicles have similar driving ranges to the conventional vehicles, but require expensive cryogenic supply chain. Ethane vehicles offer longer driving range than LNG vehicles without cryogenic systems. Utilizing ethane to replace natural gas in transportation could potentially lower the market barriers to a clean alternative fuel and accelerate the adoption of gas vehicles in the United States. The end-use carbon intensity of ethane fuel is slightly higher than natural gas, but significantly lower than gasoline and diesel. We were unable to locate any assessments on upstream emissions for ethane. However, because ethane is a byproduct in natural gas production, its upstream emissions should be similar to that of natural gas. Figure 1 shows the well-to-wheel