GTL diesel especially cuts nano-PM, boosts city fleets

Diesel Fuel News, Dec 9, 2002 by Jack Peckham

San Diego -- Fischer-Tropsch (PT) gas-to-liquids diesel combustion not only yields lower nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbon (HC) emissions than conventional diesel fuel, but also much lower numbers of the small "nano-particles" suspected of being the biggest health threat among various particulate matter (PM) emissions.

That's what GTL diesel producer Sasol found in tests on a heavy-duty engine and light-duty vehicle at University of Minnesota's Center for Diesel Research, headed by David Kittelson, one of the world's foremost authorities on diesel nanoparticle emissions.

Earlier this year, Kittelson reported that certain components of lube oils seem to be responsible for most of the nano-PM in engines burning ultra-low or very-low sulfur fuels (see Diesel Fuel News 9/2/02, p5), including FT diesel.

But comparing zero-sulfur FT diesel to conventional diesel combustion, "the most probable cause for the reduction in nucleation mode particles [nano-PM] is the negligible sulfur content of the Fischer-Tropsch fuel," according to a paper (SAE 2002-01-2 72 7) co-authored by Kittelson and U-Minn. colleagues Darrick Zarling, Robert Waytulonis and Sasol Oil researcher Paul Schaberg.

Sulfate from fuel- or lubes-borne sulfur is considered to be a primary precursor for nano-PM formation, combining with water and hydrocarbons in exhaust.

However, "nanoparticles may also form from the condensation and nucleation of gas-phase metals derived from fuel and/or lubricating oil additives," as Schaberg explained at the Society of Automotive Engineers Powertrain & Fluids conference here. "If fuel sulfur is removed, these may become the dominant source of nanoparticles."

For the tests, the researchers used Sasol's "slurry-phase distillate" (SPD, to be produced on commercial scale in Nigeria and Qatar in the next few years) with <1 ppm sulfur, <1% aromatics and 75 cetane number. (Cont. p11)

This was compared to burning a conventional diesel fuel of 404-ppm sulfur, 38% total aromatics and 41.5 cetane number.

That fuel was "close to the upper and lower limits likely to be encountered in commercially available crude-oil derived diesel," so the FT diesel benefits "are likely to be accentuated," the researchers said.

Heavy-duty tests were on a Cat C12 (430-hp, 12-liter, turbocharged/aftercooled) modified to 1998 post-consent-decree EPA emissions limits, but without any EGR system. This engine used a standard, manufacturer-recommended, 5w-40 synthetic lube oil.

Light-duty tests were on a Volkswagen TDI engine as used in Golf, Jetta and new Beetle vehicles. It's a 66-kilowatt, 1.9-liter turbocharged engine with a standard oxidation catalyst. A VW-recommended 5w-40 synthetic lube oil was used in the tests.

Heavy-duty steady-state emissions tests at four load/speed modes followed standard ISO 8178-C1 and US EPA procedures, while light-duty tests followed standard five-mode procedures.

* Large Help on Idle Emissions

Idle mode (mode 4) produced the highest emissions in the heavy-duty tests, and the FT diesel especially outshone conventional diesel on this mode. Idle mode (relatively cool exhaust) tends to produce the greatest amount of nano-PM, but FT diesel slashed these emissions by 81% compared to conventional diesel.

In the passenger-car engine tests, FT diesel likewise slashed nano-PM by large amounts (especially at idle mode).

"This finding has significance for vehicles operating in congested traffic, such as buses or taxis in city centers which spend a relatively large proportion of their operating time at either idle or low-load conditions," the researchers emphasized. "The results indicate that under these conditions, the use of FT fuel would result in significant reductions" in the most critical exhaust emissions.

Here are the nano-PM emissions results comparing the two fuels in light-duty engine tests:

Figure 11

Integrated Total Particle and Nanoparticle Number Emission Indices,
Passenger Car Engine

      Integrated Total     Integrated
      Particle Number    Particle Number
      (part./kg fuel)   < 50 nm (part./kg
                              fuel)
Mode      D2 & F-T          D2 & F-T

 1            2%               21%
 2          -93%              -96%
 3          -16%              -16%
 4           32%               62%
 5          -68%              -75%

Note: Table made from bar graph

Here are the nano-PM emissions results on the heavy-duty engine tests:

Figure 6

Integrated Total Particle Number and Nanoparticle Number Emission
Indices, Heavy-duty Engine

      Integrated Total     Integrated
      Particle Number   Particle Number
      (part./kg fuel)   < 50nm (part./kg
                             fuel)
Mode      D2 & F-T          D2 & F-T

1           -35%              -37%
2           -75%              -84%
3           -83%              -89%
4           -80%              -81%

Note: Table made from bar graph

Finally, here are the idle-mode composite results on both the heavy-duty and light-duty engines, showing all the emissions reductions with FT diesel versus conventional diesel: