The primary objective of this study is to refine estimates of the mass emissions on a per-vehicle-mile basis, for selected vehicle fuels, for the fuel production and energy conversion portions of the total fuel-cycle, including fuel acquisition and refining, distribution, and refueling. The selected vehicle fuels are diesel fuel and liquefied petroleum gas (LPG) for internal combustion vehicles, and methanol for fuel-cell powered vehicles. Fuel-cycle emissions are compared to those from electricity generation for electric vehicles. The mass emissions of NOx, NMOG, methane, CO, CO2, and air toxics are quantified for each fuel and for each phase of the fuel cycle. Emission estimates were made for 1996 as a base year and for the year 2010 based on two different projection scenarios for each fuel, one pessimistic and one optimistic. The uncertainty associated with emissions from every step of each fuel cycle is estimated and those uncertainties are propagated to develop an overall uncertainty for each fuel.
This study provides a consistent basis for comparing the fuel-cycle emissions from a variety of fuels. LPG- and diesel-fueled IC engine vehicles, as well as methanol-powered fuel cell vehicles, have the potential for meeting SULEV requirements and also operating with low fuel-cycle emissions. Fuel-cycle emissions were determined for conventional vehicles in a 1996 study performed for ARB (Unnasch 1996). However, advanced vehicle technologies will have improved fuel economy, which will affect fuel-cycle emissions. Furthermore, uncertainties exist for some fuel-cycle emission sources, such as venting from LPG distribution trucks or breathing losses from bulk methanol storage tanks. These are potentially significant sources that require close analysis. Determining the emissions for each step in the fuel cycle requires a careful engineering analysis, and when possible, was based on actual fuel processing equipment experience. Obtaining speciated emissions to quantify the toxic components of hydrocarbon emissions also posed a challenge. The most significant source of hydrocarbon emissions is fuel spillage, refueling vapor losses, and storage tank venting. These emissions and others were analyzed in the context of a significant number of both SULEV and other vehicles operating in the year 2010.
The following sections discuss and review the methods used in this report to estimate and calculate the fuel-cycle emissions.
1.3 Project Approach
The fuel-cycle emissions associated with production and distribution of diesel, methanol, liquefied petroleum gas (LPG), and electricity were evaluated. Each fuel was evaluated based upon production from one or more feedstocks. Diesel, and LPG are considered for use in internal combustion engine vehicles. Electricity is assessed for pure ZEVs (battery-only electric vehicles), and methanol is considered for use in fuel cell vehicles with on-board methanol reformers.
The following outline summarizes the steps used in this project: