Potential for Increased Emissions from Vehicles Using Ethanol as Fuel

Most gas stations that I use in Massachusetts now carry fuel with up to 10% by vol alcohol (typically ethanol). Ethanol is used in reformulated gasoline (up to 10% can be used without requiring modification to the vehicles or fuel transport systems) to increase the octane rating. MTBE was the chemical used earlier but it was found to be seeping out of fuel storage tanks into ground water, and was gradually bannd after shown to be linked to adverse health effects.

I had posted earlier about a study that linked increase in cancer mortality risk in the case of wide use of ethanol as fuel. Here is a little more information on a similar topic, i.e. potential for increase in emissions as ethanol concentrations in the fuel are increased. This time, the source is ARB which just approved the use of 10% ethanol. The concentrations being discussed below are not E85 (i.e. 85% ethanol) but just E10 (i.e. 10% ethanol addition in reformulated gasoline). I guess CARB is saying to the refineries: you can use ethanol but come up with other ways to reduce the increased emissions (and fuel economy loss) incurred by the use of ethanol biofuel.

The California Air Resources Board (ARB) approved changes to its reformulated gasoline regulations to allow ethanol blends of up to 10% (E10). ARB also approved changes to the predictive model, a tool used by oil refining companies to formulate lower-emitting gasoline in California. (California’s predictive model is analogous to the EPA’s “Complex Model” for reformulated gasoline.) Refinements to the predictive model include the greater use of ethanol.

The predictive model is a set of mathematical equations that relate exhaust rates of hydrocarbons, nitrogen oxides, and toxics to values of eight regulated properties in California’s reformulated gasoline (CaRFG). This in turn is used by refiners to assure that their fuels obtain the required emissions reductions. This gives refiners flexibility in meeting emission limits defined in ARB regulations.
The new predictive model better accounts for widespread use of ethanol and will allow increased use of the biofuel in California’s gasoline as part of Governor Schwarzenegger’s Low Carbon Fuel Standard (LCFS) executive order.

By 2020, the LCFS is expected to produce at least a 10% reduction in the carbon content of on-road fuels, replace 20% of our on-road fuels with lower carbon alternatives, and more than triple the size of the state’s renewable fuels market. (Earlier post.)

Clean fuels are essential to reaching healthy air goals in California. This action helps fuel providers develop the optimum formula for the cleanest burning gasoline, and it allows ARB to forecast emissions from vehicles throughout the state. The greater use of ethanol in the formulas will also reduce global warming emissions.

—Dr. Robert Sawyer, ARB Chairman

California has been studying the effects of ethanol in fuel fairly intently since 1999, when it banned the use of MTBE as an oxygenate in its reformulated gasoline (RFG).

The Clean Air Act requires that RFG—which is blended to burn cleaner and reduce smog-forming and toxic pollutants—be used in cities with the worst smog pollution to reduce harmful emissions of ozone. It also specifies that RFG contain oxygen (2% by weight). The two prevalent oxygenating additives had been ethanol and MTBE. (Refiners have since stopped using MTBE.)

States, including California, can—and do—petition for waivers relieving them from the oxygen requirement. But the EPA has consistently denied those waivers, most recently in 2005 for California, Connecticut and New York. So with MTBE off of the table in 1999 in California, ARB began investigating the effects of ethanol to see what other fuel and fueling system parameters would have to be tweaked from a regulatory perspective to meet on-road emissions standards.

In other words, any increase in fuel emissions from increased ethanol use must be off-set with a similar reduction from other sources.

A study of E0, E5.7 and E10 blends by the University of California-Riverside—Effects of Ethanol and Volatility Parameters on Exhaust Emissions—published in 2006 found, for example, that:

  • NMHC emissions increased with increasing ethanol content combined with certain other blending properties. (T50 and T90, which correspond to the temperatures at which a given percentage of the gasoline sample enters a gaseous phase under specific experimental conditions. T50 thus is the 50% distillation point; and T90 the 90% distillation point.)
  • CO emissions decreased, were unaffected, or increased depending upon ethanol content and T50 levels.
  • NOx emissions decreased, were unaffected, or increased depending upon the T50 levels.
  • Fuel consumption increased by 1.4% when ethanol content was increased from E0 to E10. Fuel consumption decreased by 1.2% when T50 was increased from the low to the high level. Fuel consumption decreased by 0.6% when T90 was increased from the low to the high level.
  • With the T90 property held steady (T90=355°F) and the ethanol level increased from E0 to E10, NMOG emissions increased by 14%; acetaldehyde emissions increased by 73%; benzene emissions increased by 18%; and 1,3-butadiene emissions increased by 22%. (The results of the study do not permit any conclusions as to what effects ethanol might have had on NMOG or toxics emissions for fuels having low or mid-point T90 levels.)

The adjustments in the predictive model will help refiners determine what other fuel properties need to be altered to accommodate the higher ethanol blends.

Source: Green Car Congress



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