Sulfur Compounded in Ethanol Regulations

As the Tier 3 deadline approaches, it's important to understand the difference between EPA and ASTM treatments of the element.
By Susanne Retka Schill | July 15, 2016

The U.S. EPA’s Tier 3 rules are not new—they were finalized two years ago—but a compliance deadline is approaching that may catch a few ethanol producers unprepared. Come Jan. 1, ethanol must prove it meets the lower 10 parts per million (ppm) standard for sulfur content.

Sulfur levels are important for air quality measures, primarily because sulfur compounds kill the catalyst used to clean up automobile exhaust. Tier 2 rules finalized in 2000 required 30 ppm limits on sulfur, Tier 3 has tightened it further. Regulators expect an immediate benefit from the cleaner fuel, and future benefits as the tighter emissions enable more fuel-efficient engine designs that are incompatible with current emissions control systems. 

It’s a more difficult rule for the oil refiners to meet, Kelly Davis, Renewable Fuels Association director of regulatory affairs, explains. “They have to remove the sulfur during processing.” Pure ethanol is not a known source of sulfur, she adds. “We typically do not see sulfur above 10 ppm in our ethanol, but as part of the fuel supply chain, we have to prove that we comply, so it’s more of bureaucratic issue—paperwork.”

The industry actually has been meeting the standard for years. It first was adopted in California in 2003 and, due to the fungibility of the ethanol supply, the RFA has encouraged all ethanol be under 10 ppm ever since. What’s different, Davis says, is that it is now an EPA rule with reporting requirements. “One of my concerns is if producers don’t have to do anything except paperwork, it’s put off until the end,” she says.

While ethanol itself has little sulfur, plants can have low levels, Davis says. “Every ethanol plant will have a different base sulfur content,  because we’re all using different sulfur chemistries in our process.” Most of the sulfur comes from the denaturant used to make sure the fuel ethanol won’t end up in a glass, in which case it should be taxed. Natural gasoline is the most economical denaturant, and thus the most commonly used. Also called wellhead gas, drip gas or casing head gas, it is a coproduct of natural gas production. Petroleum-based gasoline can also be used, but generally isn’t due to price.

There should not be a problem for the ethanol industry to meet the new 10 ppm standard, Davis says. The industry has demonstrated it can for more than a decade of compliance with the California regulation. Moreover, with sulfur levels in natural gasoline typically ranging between 50 to 120 ppm, using it in a 2 percent blend brings those levels down to well below the allowable maximum, assuming the ethanol is near zero. 

Ethanol producers have two ways to demonstrate compliance, Davis says. “They can buy a sulfur analyzer and take on the regulatory responsibility, certifying all their batch loads and making sure their sulfur analyses are valid.  There’s also details like product transfer documents and annual EPA reporting.” Sulfur analyzers cost around $70,000, she says, and will require trained operators.

The second option is a calculation method. “EPA is allowing us to certify that we are 10 ppm on a batch level basis, if we know our denatured amount and take a certification of analysis from the supplier and do the calculation.” She expects most ethanol producers will use the calculation method, although the new requirement that the natural gasoline supplier be EPA certified may be an issue in some cases.
While the new rule takes effect Jan. 1, the system for reporting compliance to the EPA has not been revealed yet.

Beyond Tier 3  
Meeting sulfur tolerances goes beyond the new EPA Tier 3 rules, explains Monty McCoy, director of technology in the ethanol process group at U.S. Water Services. The Tier 3 rules are looking at reducing total sulfur in fuels with the goal of improving air emissions from gasoline engines. The new standards cover organic sulfur that is in the gasoline, he explains. “That is determined with a separate method and behaves in completely different ways than the sulfate that’s in the ethanol, which is inorganic. There are separate methods and separate limits for that.”

The sulfate requirement to meet ASTM specifications has been around since the mid-2000s, and with producers occasionally running into issues meeting those specifications, U.S. Water has been working to reduce the use of sulfur compounds in the process, McCoy says. “Sulfuric acid is used for pH control.  There normally is some pH control to make the enzymes work well and to suppress bacterial growth.” The majority of sulfuric acid is used after fermentation. Controlling pH in the beer minimizes mineral deposits that drop out when the beer hits hot heat exchanger plates or in the beer column or in the evaporators. “Plants use sulfuric acid in that case to keep equipment operational by keeping those mineral deposits from forming.” U.S. Water introduced a unique technology several years ago to break up the anionic portion of the minerals before they form deposits, reducing sulfuric acid needs by 60 to 80 percent.

Sulfites are another issue.

“Sulfites are really nasty from the standpoint of final product sulfur,” McCoy says.  Bisulfites are used for air emission compliance.  “There’s a small amount of some aldehydes in fermentation gas that are classified as hazardous air pollutants by the EPA and tightly regulated,” he explains. Bisulfite compounds react with those aldehydes, enabling their removal in the scrubber system.   

“The reason the chemistry is important is that sulfates, like those you get from sulfuric acid, are nonvolatile, so most of the sulfuric acid that gets used drops out in the coproduct,” McCoy says. Early on, the ethanol industry learned those levels need to be controlled closely to avoid problems when distillers are fed. “Sulfites are different because those are weak acids,” he continues, “essentially sulfur dioxide in water.” Sulfur dioxide is volatile and will come off the beer and work its way through the distillation and dehydration system to end up in the ethanol. “That’s your real enemy,” McCoy says. A separation unit is available that can remove sulfates, ideally from the 200 proof. U.S. Water has used it to treat denatured fuel in tanks and railcars.

There may be some changes coming with ASTM standards, McCoy says, and it’s important the ethanol industry gets involved, as some suggestions would make the sulfate specifications even tighter in higher blends. “I would like to see more people join,” he says. “It’s $75 a year and you can get in the queue to become an official voting member. Then these plants can start having some impact on the way these standards are written.”   Right now, he adds, the members from the auto and oil industries heavily influence the process.

Author: Susanne Retka Schill
Managing Editor, Ethanol Producer Magazine