(9g) A Life-Cycle Assessment of Greenhouse Gas Emissions for Transportation Fuels from North American Conventional Crudes

The life-cycle analysis for transportation fuels includes the well-to-tank (WTT) and tank-to-wheel (TTW) stages. The WTT stage includes crude recovery, transportation of crudes to the refinery, the refining of crudes to produce transportation fuels, and the transportation of these fuels to local refueling stations. The crude recovery part of the WTT stage consists of oil well drilling, crude extraction from the reservoir, processing of crude, associated gas and water, and oil field venting and flaring. Fuel consumption in engines is considered as TTW stage. Energy is consumed, and a significant amount of greenhouse gases (GHGs) are emitted in all of these stages. The purpose of this study was to quantify the energy used and GHGs emitted in all the stages in transforming different North American conventional crudes to transportation fuels, mainly gasoline and diesel. A spreadsheet-based model was developed based on fundamental scientific equations to calculate the WTW energy use and GHG emissions. Emissions from crude recovery depend on crude and reservoir, depth of well, extraction methodology, gas-to-oil ratio, water-to-oil ratio, and oilfield venting, flaring, and fugitives. Recovery emissions vary from 3.94 g-CO_2eq/MJ of crude to 23.85 g-CO_2eq/MJ of crude. Transportation from the feedstock to the refinery and the refinery to refueling stations contributes only 0.51%-1.87% of the total WTW life-cycle emissions, depending on the transportation methods and total distance transported. The HYSYS refinery model was used to calculate energy used during refining. Energy consumption in a refinery depends on the crude quality (e.g., API gravity and sulfur content), distillation curves, and refinery configuration. GHG emissions in refining were calculated from the amount of energy used in the refining of crudes for conversion into gasoline and diesel. Refining emissions vary from 12.40 g-CO_2eq/MJ of gasoline to 14.83 g-CO_2eq/MJ of gasoline and 5.37 g-CO_2eq/MJ of diesel to 8.75 g-CO_2eq/MJ of diesel. Emissions from fuel consumption in vehicles (passenger cars) were taken from the literature. This stage contributes 66%-79% of total life-cycle GHG emissions in the case of gasoline. Total WTW GHG emissions range from 91.03 g-CO_2eq/MJ of gasoline to 109.37g-CO_2eq/MJ of gasoline and 86.63 g-CO_2eq/MJ of diesel to 107.92g-CO_2eq/MJ of diesel.