SCR Engines May Emit Toxins, CARB Staffer’s Letter Warns

This story appears in the Aug. 10 print edition of Transport Topics.

The selective catalytic reduction process that most engine manufacturers will use to comply with 2010 federal diesel emission standards could emit toxic byproducts, the California Air Resources Board’s top researcher wrote in a recent letter filed in federal court by Navistar Inc.

Navistar, which is suing the U.S. Environmental Protection Agency in federal appeals court, filed the letter July 30 and said it bolsters the truck maker’s claim that EPA could be endangering public health by skipping the federal rule-making process in its approval of SCR.

Written to “alert” a Boston nonprofit organization planning tests on 2010 engines, the letter said that SCR technology represents a “large departure from conventional emission controls by introducing a liquid additive containing an organic form of nitrogen.”

Navistar has asked the court to determine whether EPA violated federal rulemaking requirements by issuing a guidance memorandum on SCR (6-15, p. 1; click here for previous story). It said the letter helps show that “skipping the rigors of rulemaking means EPA and CARB have put the public at risk by allowing SCR on the highway now, when it may turn out that the cure is worse than the disease.”

The June 12 letter, written by Bart Croes, chief of CARB research, to the Health Effects Institute, said that a comprehensive literature search showed that the “primary concern is the release of organonitrogen compounds, many of which are carcinogenic or toxic in other ways.”

“Some toxic air contaminants that have been identified with SCR technology include hydrogen cyanide, cyanic acid, nitromethane, hydrazine, acrylonitrile, acrylamide, acetonitrile, and acetamide,” Croes wrote in his letter. “It is hoped that any exotic substances emitted from SCR technology will be at levels insignificant to exposure health effects.”

“Although we are encouraged by findings to date, which suggest that the technology can deliver significant reductions of many species of toxicological relevance, this work has also documented the increase in some emissions such as some metals, nitrous oxide and nanoparticles,” CARB’s letter said.

SCR systems reduce NOx in the aftertreatment by mixing the exhaust output with diesel exhaust fluid, a blend of pure urea and demineralized water, through a catalyst that converts NOx into harmless nitrogen and water vapor.

Navistar, Warrenville, Ill., manufacturer of International trucks, is the only engine maker planning to use exhaust gas recirculation to meet EPA’s 2010 emission standard.

The manufacturers using SCR include independent truck engine manufacturer Cummins Inc.; Daimler Trucks North America and its engine subsidiary, Detroit Diesel Corp.; Volvo Group America and Volvo subsidiary Mack Trucks.

An EPA spokeswoman and a Navistar spokesman declined comment on the letter. Several other SCR manufacturers did not return phone messages.

David Siler, director of marketing for Detroit Diesel, one of the SCR manufacturers, said he was not aware of the letter, but he defended SCR as safe.

“We’ve been operating all along on the understanding that this technology is safe and produces no present or downstream effects on public safety or health,” Siler said. “This has been validated by not only our North American experience but in Europe, where SCR has been used for several years and is the undisputed Euro 5 standard and also the upcoming Euro 6 standard.”

Spokesman Jim McNamara of Volvo Trucks North America said, “It is difficult to speculate on what could be possible under some unforeseeable combination of conditions, and there are no cases of health issues with hundreds of thousands of SCR systems deployed around the world.”

Daniel Greenbaum, president of the Health Effects Institute, said the letter was sent for a study of emissions and the safety of new advanced engine systems and fuels.

“In that context, we’re taking input ideas from a variety of the sponsors who are involved in the project, CARB being one of them,” Greenbaum said.

The CARB letter suggests that a number of chemicals be added to the 700 chemicals that HEI already plans to test on the 2010 engines to evaluate for potential health effects, Greenbaum said.

Hector Maldonado, a CARB air pollution specialist who worked on the CARB study, downplayed the significance of the letter, but he told Transport Topics, “There

is a possibility, at least in a theoretical sense, that some of these compounds could be formed. However, based on the experience we have to date, there are no alarm bells being rung, but at the same time, we are aware that there is a possibility.”

Maldonado said CARB is not “second-guessing” EPA’s 2010 standard, and it does support the idea of aftertreatment control of NOx emissions.

“The irony is that I think, if anything, SCR is incredibly effective in terms of NOx reduction,” Maldonado said. “It’s some of the exotic compounds, unregulated emissions that might be formed, that give us some cause for concern.”

However, he said using the letter to buttress a court argument against SCR is, “in my opinion, a blatant misrepresentation.”