Diesel back-up generator PM ~80% less than EPA thought; new technologies make difference

Diesel Fuel News, Sept 29, 2003 by Jack Peckham

Newport, R.I. -- A pioneering research project discovered that in the real world, diesel emergency-electric back-up generator ("bug") particulate matter (P[M.sub.2.5]) emissions are about 80% lower than the emissions rates assumed under U.S. EPA's admittedly shaky "AP-42" standard factor.

EPA has long known that its standard PM emissions factor assumption on diesel "bugs"--1.34 grams/kilowatt-hour--is out-of-date, sketchy and dubious. On a scale from "A" (high confidence) to "E" (poor), EPA gives its "bug" emissions confidence factor only a "D."

But it wasn't until this year--when California Energy Commission (CEC) awarded $1.5 million to University of California, Riverside (UCR) for a joint CEC/California Air Resources Board (CARB) study on "bugs"--that the extent of EPA's AP-42 error became clear.

Also sponsoring the UCR studies are EPA, South Coast Air Quality Management District and six diesel gen-set makers.

As UCR researcher Wayne Miller explained to Diesel Engine Emissions Reduction (DEER) conference here (sponsored by U.S. Department of Energy), real-world measurements from a wide variety of older and newer "bugs" (between 300-600 kilowatts) show P[M.sub.2.5], emissions in a range from about 0.1 g/kW-hr to about 0.3 g/kW-hr, not even close to the 1.34 g/kW-hr assumed in EPA "AP-42."

UCR employs a sophisticated, truck-trailer-mounted portable emissions laboratory to determine emissions on a wide variety of stationary and mobile engines. This system makes it possible to check real-world engines under actual operating conditions, without removing the engine from a vehicle or from a power-generation site.

--CARB Verifies Accuracy

CARB has verified the portable lab's measurement accuracy, giving air regulators more confidence in the significance of its findings.

For the field-test program, investigators initially chose 15 engines of 300-750 kW, plus three engines of 1,000 to 2,000-kW. Age ranges include pre-1987, 1987-1996 and post-1996 engines. Test results so far include engines of 300-600 kW, with future tests planned for uncontrolled, >1,000 kW engines, plus tests on older two-strokes using a fuel-borne catalyst/oxidation-catalyst combo, and a fuel-borne catalyst/bare filter combo. Another test will demonstrate the impact of a diesel particulate filter (DPF) combined with selective catalytic reduction (SCR).

Besides measuring engine-out emissions, the UCR investigators also measured the impact of engines running on water-emulsion fuel (Lubrizol "PuriNOx,") diesel oxidation catalysts, passive diesel particulate filters (DPFs) and an "active" DPF using supplemental electric heat for soot oxidation.

Even without exhaust aftertreatment, the newer diesel "bugs" have much lower PM emissions than the older engines, especially at lower loads. It's apparent that today's cleaner highway diesel engines are migrating to generator applications, even without specific regulatory mandates.

Surprisingly, tests of water-emulsion fuel showed a much bigger PM reduction benefit (over 70%) on the newer engines, compared to the older "bugs" (17% PM reduction). While investigators aren't exactly sure why, it's thought that since older "bugs" produce a relatively "wetter" soot (higher in organic carbon, OC) than newer diesels, the "wetter" soot may respond less to emulsions than "dryer" elemental carbon (EC).

Emulsion fuels also did better on PM reduction at the mid-to-high-load points rather than low-load/idle, researchers found. These higher loads are where EC predominates over OC.

This also could explain why diesel oxidation catalysts (DOCs) did far better in engines operated at low load and also in older, two-stroke "bugs" (42% PM reduction). By contrast, a newer "bug" with DOC got only 17% PM reduction benefit--probably not surprising, since newer engines produce less OC, the portion of PM oxidized by a DOC.

Meanwhile, "bugs" with "passive" DPFs achieved 91% PM reduction, while an "active" DPF got 99% PM reduction, researchers found. (Cleaire/Engine Control System's "BUGtrap"--see Diesel Fuel News 7/21/03, p9--is one such system employing electric heat for "off-line" soot oxidation.)

Following the research work on the larger "bugs," the UCR researchers are about to investigate <300-kW engines, starting with some non-road military engines, Miller told us.

Ultimately, all the data generated by the research project will be analyzed and reformatted with the help of various state and regional air-quality organizations including NESCAUM, MAP-AMA, NYSERDA and CARB, Miller said. Finally, EPA will help the researchers "shape it into AP-42 format" prior to EPA putting it out for public comment and eventual adoption, he said.