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Ethanol emissions

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Interesting C/P selections from Ethanol Producer mag article on California evaporative and permeation emissions circa ’07.

 

-Start-

 

Tailpipe-out emissions are produced by the internal combustion of fuel, but permeation and vapor pressure emissions of unburned fuel are big concerns for air-quality regulators, too. “Evaporative emissions are growing as a percent of the total emissions.

 

“Evaporative emissions are inherently higher at the E10 blend,” Wuebben tells EPM. “But at the higher blends like E40 and up, testing shows evap emissions are lower than gasoline.” Van Amburg says E10 is the worst blend-area for evap emissions, calling it the “dogleg” of testing. The reason is the azeotropic reaction that results from mixing ethanol and gasoline at varying levels. “You can have ethanol and gasoline—each with the same vapor pressure—but when you mix them together, you’ll get an increase in vapor pressure,” Wuebben says. “Evaporative emissions are derived from the volatility characteristics of fuel—and the solubility.”

 

Evaporative emissions are typically discussed in terms of vapor pressure and permeation emissions. Vapor pressure is driven by molecular weight and density. Unlike petroleum gasoline, ethanol has an oxygen molecule. “That oxygen molecule makes ethanol an inherently lighter fuel,” Wuebben says. “The difference in polarity of the fuels results in an increase in Reid vapor pressure (RVP).” RVP is measured in units of pressure (pounds per square inch) created in a closed-loop system when a fuel is heated to 100 degrees Fahrenheit. Results are used by the fuel industries and regulators to categorize the volatility of a particular fuel. “It’s a physical phenomenon that ethanol blends reach their peak vapor pressure at E10,” Wuebben says.

 

Permeation emissions, or a fuel’s ability to give-off emissions of unburned hydrocarbons through materials like rubber, are driven by a different characteristic. “Permeation emissions have to do with the solubility of the fuel, and has nothing to do with vapor pressure,” Wuebben says. “Both contribute to evap emissions. E10 has higher permeation emissions and volatile vapor pressure than gasoline. Reformulated Blendstock for Oxygenate Blending (RBOB) is adjusted for aromatics and olefins prior to blending to accommodate for the physical effects of mixing gas and ethanol at these levels.

 

E85 Emissions Homestretch

Wuebben says low-level blends of ethanol may produce slightly lower NOx emissions, but at high blends like E85, ethanol’s lower Btu value produces lower flame and combustion temperatures, helping lower the production of NOx during combustion. “It’s important to note that technologies have already advanced to a point that emissions of NOx and hydrocarbons from base tests are already very low,” Wuebben tells EPM. The production of toxic emissions, such as benzene and “1, 3 butadiene,” are also lower from E85, he adds, while acetaldehyde emissions can increase. “However, catalysts on vehicles are so effective these days that acetaldehyde emissions from E85 are virtually the same as with gas,” he says.

 

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Note the point on ethanol a light fuel due to oxygen content or putting it another way in theory if ethanol fuel did not contain the oxygen molecule it would be heavier and more energy dense. The minute water content of ethanol also lowers energy density of real world ethanol fuel. But, let’s not forget these two constituents desirable within the fuel. They may drag down the energy density a bit, but improve the fuel to ideal status and enable more efficient ICEs.   

 

Note the point on E10 concentration within BOB gas, the two have achieve the highest RVP. This combination best for cold weather engine startup.

 

Another strong point of ethanol is the purity of fuel that simplifies the pollution control chemistry.  Compare the simple ethanol combustion chemistry to unleaded fuel.

 

C2H5OH + 3O2 → 2CO2 + 3H2O + heat

 

VS

 

The combustion of gasoline is a very complex chemical process for there are over 500 different hydrocarbons in gasoline plus many other elements and additives.

Gasoline hydrocarbons may have between 3 to 12 carbons, and gasoline used to have a boiling range from 30C to 220C at atmospheric pressure. In past years the boiling range has been narrowing as the initial boiling point is increasing, and the final boiling point is decreasing, both changes are for environmental reasons.

Note: the emissions of brewing process, the beer stage, produces simple pure CO2 that can easily harvested for food and weld quality (simple yet still expensive). Compare this to emissions of cracking hydrocarbons, etc.

 

 

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