Air Pollution Emissions Associated with Using Ethanol

摘要

The use of ethanol in gasoline blends and as a neat fuel can reduce harmful emissions from mobile sources. The principal air quality concerns are ozone, toxic air pollutants, and carbon monoxide. Ozone is produced from photochemical reactions of volatile organic compounds (VOC), nitric oxides (NOx), and carbon monoxide (CO) and is most pronounced in hot weather. Carbon monoxide emissions from mobile sources are greater in cold weather, and unhealthy levels of CO are primarily a wintertime problem. Adding ethanol to gasoline decreases tailpipe emissions of CO, a product of incomplete combustion, and VOCs. Although neat ethanol has a lower RVP than gasoline, blending ethanol into gasoline at low concentrations raises the RVP of the mixture, which increases evaporative emissions. The RVP increase can be compensated for by producing a subgrade gasoline with a lower RVP for ethanol blending. Changes in emissions are more complicated and depend on the type of emission control systemused. The U.S. EPA has determined that overall NOx emissions decrease slightly for ethanol blends, because of significant NOx emission reductions in high emitters. The State of California has arrived at an opposite conclusion. However, some recent tests by automakers on new model cars have concluded NOx emissions do decrease. Emissions of toxic air pollutants except acetaldehyde and formaldehyde decrease when ethanol is added to gasoline, primarily through dilution. Reductions in benzene and 1,3-butadiene emissions more than outweigh increases in aldehyde emissions because their toxicities are much greater. Formaldehyde emissions are lower for ethanol blends than for MTBE blends. Most of the aldehydes in an urban air shed come from atmospheric transformations of other hydrocarbons, and a California study concluded their atmospheric levels would increase only slightly for complying California gasoline containing 10 percent ethanol. Like all gasoline blending components, ethanol has its advantages and disadvantages. Besides its emission's benefits, ethanol is a high octane, virtually sulfur-free blendstock. Reducing sulfur in gasoline is crucial, because its presence degrades the effectiveness of catalytic converters. Ethanol's major disadvantage in summer gasoline is its high blending RVP. Refiners must determine the most economic way of producing gasoline meeting environmental requirements. With limitations placed on the use of MTBE because of its water quality problems, U.S. refinery analyses have shown the refiner value of ethanol increases even if the RFG oxygen requirement is removed. For China, there is an additional incentive for using ethanol to displace some of the need for adding very expensive refinery expansion units to produce high-octane blendstocks. Ethanol produced from corn is currently estimated to reduce GHG emissions between 12 and 19 percent relative to gasoline. With near-term technology improvements, the reductions are projected to be between 24 and 26 percent. For cellulosic ethanol, reductions in GHG emissions are projected to range from 84 and to more than 100 percent.