Reduced Carbon Intensity of Ethanol Blend Gasoline

Abstract

<div>Tank-to-wheels (TTW) CO₂ reduction for ethanol blends is determined from either gasoline composition or vehicle exhaust measurements. Fuels are characterized using a carbon intensity (CI), which is the ratio of carbon (as CO₂ mass) in the fuel to the net heating value. Our objective is to assess changes in CI of market gasoline with varying ethanol content that can be used to appreciate change in vehicle tailpipe greenhouse gases (GHG) in response to policy controlling the ethanol level in market fuels. Ethanol has both a reduced carbon content and a reduced net (lower) heating value relative to petroleum species, with a CI slightly lower than that of typical petroleum gasoline. However, ethanol blending offers additional CI reduction because it enables a reduction of aromatics in the petroleum blendstock for oxygenate blending (BOB) while maintaining octane rating of the blend. Aromatics have a CI about 20% higher than paraffins. The primary refinery option for aromatic reduction is through lower severity or throughput for the gasoline reformer, which ultimately reduces CI in the BOB and the finished blend. Expected gasoline market blends were projected by developing a model that addressed US refining and blending in response to octane requirements. A TTW blending CI, or BCI, for ethanol is proposed to describe the total CI reduction in the finished blend enabled by the ethanol. The ethanol BCI was found to average at 59 gCO₂/MJ for E10, E15, and E20 (10%, 15%, and 20% ethanol by volume) market fuels in this study. This is substantially below the ethanol chemical CI of 71.0 gCO₂/MJ and petroleum CI of 73.5 g CO₂/MJ due to the enabling of aromatic reduction. E10 in comparison to E0 (purely petroleum) is estimated to offer a national US tailpipe CO₂ reduction of 16.6 billion kg annually.</div>