Lube contribution to nano-PM might be verified by isotopic tracing

Diesel Fuel News, Dec 9, 2002 by Jack Peckham

Isotopic tracing potentially could verify lubricant contribution to nano-particle emissions from combustion engines, according to Lawrence Livermore National Laboratory (LLNL) investigator Bruce Buchholz.

Such a study potentially could help resolve any doubts about the rather alarming findings on lube oil nano-PM presented by University of Minnesota combustion particle scientist David Kittelson at the recent Diesel Engine Emissions Reduction (DEER) workshop (see Diesel Fuel News 11/14/02, p14; 9/2/02, p5).

What's more, it's possible to imagine lubes and additives formulators using isotopic tracing to develop the next generation of "ultra-clean" lubricants. Such advanced lubes perhaps could minimize or eliminate nano-PM emissions -- the tiny particulate matter suspected as perhaps the most troublesome among the PM for human health effects.

All combustion engines -- whether diesel, gasoline, or compressed natural gas (CNG)-- emit nano-PM According to recent studies at University of Minnesota, C28-C32 alkanes (normally associated with lube oil) seem to predominate in the nano-PM of engines burning very-low or ultra-low sulfur diesel fuel.

Several studies at West Virginia University also hypothesize that much of the "toxic" PM emissions from CNG engines are in fact derived from lube oil (see Diesel Fuel News 6/10/02, p7).

"It is certainly possible to deteimine the contribution of lube oil to nanoparticle production," LLNL's Buchholz told Diesel Fuel News following his presentation to Advanced Petroleum Based Fuels-Diesel Emissions Control (APBF-DEC) workshop at U.S. Department of Energy this fall.

Buchholz's APBF-DEC presentation focused mostly on recent LLNL tests using isotopic tracing of oxygenated fuel components in diesel combustion. Notably, these tests also had to account for lube oil effects.

LLNL's tests indicate that fuel or lube components can be labeled with "non radioactive" levels of the carbon-14 isotope using a laboratory-safe technique first developed decades ago for carbon dating of ancient materials.

"The biggest obstacle is producing universally labeled lube oil," Buchholz explains "Good organic synthesis chemists can do the re quired labeling. Blending PAOs [poly-alpha olefins] used for lube oils is a good option.

"The individual components of the PAGs can be labeled and serial tests with different components labeled could determine which molecules are the bad actors" for nano-PM formation.

To date, U.S. EPA hasn't indicated to the "PC-10" committee charged with developing North America 2007 diesel lubricants specifications any interest in lubes-caused nano-PM However, California Air Resources Board (CARB) is mulling possible sulfur limits on lube oils, in order to help protect advance catalysts for ultra-low particulate matter (PM and nitrogen oxides (NOx) emissions.

Sulfur species in fuel/lubes combustion seem to serve as promoters of the formation a nano-PM, with "volatile" hydrocarbons, metallic ash and water vapor attaching to sulfates. So, ultra-low sulfur diesel (UL SD) 'fuel -- perhaps combined with an advanced low-sulfur, low ash-forming lubricant -- should mitigate nano PM formation.

If lubricant chemistry can be optimized further, it's possible to imagine suppressing nano-PM formation even more, some experts suspect.

Isotopic tracing also can help determine the combustion fate of carbon from major fuel components such as aromatics, alkylated aromatics, and cyclo-alkanes, Buchholz points out. So, it's possible that such techniques eventually could be employed to create cleaner-burning fuels as well.