Water-diesel emulsions get big 'sip' boost from U.S. EPA

Diesel Fuel News, Jan 20, 2003 by Jack Peckham

A 20% water-in-diesel emulsion (Lubrizol's "PuriNOx" formula) conservatively should allow U.S. states to get credit for at least 10-12% reductions in nitrogen oxides (NOx) on highway diesels and about 17-20% NOx reduction for non-road diesel engines, according to a draft U.S. EPA analysis.

The draft "technical report" analysis, "Impacts of Lubrizol's PuriNOx Water/Diesel Emulsion on Exhaust Emissions from Heavy- Duty Engines," is open for comment (contact: Korotney.David@epamail.epa.gov).

Besides NOx cuts, the analysis also estimates that this 20% water emulsion should deliver about 51-58% reduction in particulate matter (PM) on highway diesels, and about 1623% PM reduction on non-road diesels. Greater PM reduction credits for non-road are likely once Lubrizol completes required transient-cycle tests for EPA verification.

Many states--especially California and Texas--are keen to speed-up NOx reduction credits possible with diesel emulsion fuel. Bonus: Diesel-water emulsions are vastly cheaper and much quicker to put into effect than alternative-fuel schemes such as compressed natural gas (CNG) conversions.

Emulsion-fuel combustion is likely to boost engine hydrocarbon (HG) emissions by about 30-120% (depending upon engine size), but HG emissions are very small from diesels anyway--and the increases won't push engines over certification limits for HG, EPA's analysis notes. Carbon monoxide (CO) might either increase or decrease slightly, but not enough to make any difference on certification emissions limits, the EPA analysis shows.

EPA cautions that its "technical report" won't by itself automatically grant "state implementation plan" (SIP) emissions reduction credits. "SIPs" are key market drivers for emulsions and other low-emissions technologies, especially for programs such as the EPA-supervised clean-diesel retrofit program (see Diesel Fuel News 11/11/02, p4; 6/10/02, p2). Sometimes, states will subsidize the extra cost of fleet retrofits (when tied to SIP compliance) in order to avoid enormous losses of federal highway funds.

"The SIP rulemaking would need to consider a variety of factors specific to the area such as fleet make-up, refueling patterns, program enforcement and ... additive evidence on emissions effects that might be available," EPA's draft analysis says. "While a technical report such as this may be a factor in such a rulemaking, the technical report is not intended to be a determination of SIP credits for a state fuel program."

Lubrizol eventually might claim even larger NOx/PM reduction benefits once it completes more tests under EPA's new fuel/additives retrofit test protocol, EPA explains.

Using a simple average of existing test data with "PuriNOx," EPA calculates that highway engines should realize about 13.7% NOx reduction and 58% PM reduction, while non-road diesels should get about 24.4% NOx reduction and 27.7% PM reduction.

Non-road diesel NOx reduction with emulsions is greater than that of highway diesels because highway engines already have tougher NOx controls, EPA points out.

Future diesel engines will have lower engine-out NOx, hence the benefit of emulsions will decrease over the next 20 years, the EPA analysis shows. The nearly 14% NOx reduction for today's highway diesels would slip to about 10% benefit by 2015, or about 7.5% by 2020. Likewise, non-road engine benefits wold dip from about 24% today to about 14% by 2020.

While Lubrizol has an impressive engine emissions database for PuriNOx, it still needs to test some more engines in certain engine groups in order to complete the EPA-supervised fuel/additives retrofit emissions reduction protocol requirements for key categories. That means more tests on engines with exhaust gas recirculation (EGR), more tests on light- and medium-heavy-duty highway engines, and more tests on non-road engines between 100-175 horsepower, EPA says.

Even with these data gaps, it's still possible today to calculate likely emissions impacts for SIP purposes, EPA finds. This can be accomplished via a "discount" factor for "missing" engines, EPA explains. "Any 'missing' engines or emission measurements would then be assigned the low end of the confidence limit" EPA found in analyzing the over-all database generated for "PuriNOx."

Example: EPA found that while an EGR-equipped engine tested with PuriNOx apparently achieves about 20% NOx reduction, that finding was based upon a single test. EPA would thus "discount" the benefit to a much lower confidence interval, until Lubrizol later fills-in the required data gaps to improve the confidence interval.

Meantime, for NOx SIP credit, EPA proposes to use a "prediction interval" that's even broader than a statistical "confidence interval" for PuriNOx.

"The prediction interval is necessarily broader than a confidence interval, as the confidence interval establishes the interval within which the true population mean is likely to reside," EPA says. "Thus the lower end of the prediction interval provides a very conservative estimate of the NOx benefits of PuriNOx. For our purposes, we chose a calendar year of 2007 [to calculate credits] since it is for this year that many [air pollution] non-attainment areas conduct their inventory analyses in support of efforts to reach attainment."