Green-Diesel school bus as clean or cleaner than CNG: study

Diesel Fuel News, Nov 25, 2002 by Jack Peckham

An emissions study at Southwest Research Institute (SwRI) shows that a diesel particle filter (DPF)-equipped school bus is just as "clean" or cleaner than commercially-sold compressed natural gas (CNG) school buses.

The study, unveiled at Society of Automotive Engineers (SAE) International Truck & Bus Meeting last week, compares International's DPF-equipped "GreenDiesel" school bus (running on a ConocoPhillips ultra-low sulfur diesel fuel) versus a similar CNG bus and a conventional diesel bus.

While the CNG bus wasn't equipped with an oxidation catalyst, it's logical to extrapolate the emissions results of the latest CNG transit buses equipped with oxi-cats. Reason: In recent California Air Resources Board (GARB) supervised tests, a CNG transit bus with oxi-cat was almost as good as a DPF-equipped diesel on particulate matter (PM) and "toxics" (see Diesel Fuel News 9/16/02, p6).

So, it's reasonable to assume that the latest CNG school bus with oxidation catalyst probably would be nearly as good -- perhaps virtually the same -- as the ultra-low PM and "toxics" of the "green diesel" bus.

The oxi-cat would reduce CNG's carbon monoxide (CO) and formaldehyde emissions, explains toxicology consultant Charles Lapin (formerly Arco's senior toxicologist), who presented the results of the SwRI study at SAE.

For the SwRI program, researchers considered testing the oxi-cat-equipped CNG bus, but chose to test the CNG and green-diesel buses that are actually sold in California. South Coast Air Quality Management District (metro Los Angeles) doesn't require oxi-cats on CNG buses, nor does California Air Resources Board (GARB).

Given that clean-diesel and oxi-cat-CNG school buses are almost identical on nearly all emissions, the big difference boils down to cost: CNG buses cost about 30% more than green-diesel, not including the huge extra cost of CNG refueling infrastructure.

What's more, the required ultra-low sulfur diesel (ULSD) for "green-diesel" is becoming more widely available each year, and becomes the main diesel fuel in 2006.

The, latest SwRI test results match up well with a host of recent studies by CARB, Sweden, BP, New York City Transit, Environment Canada, London Transport and others showing that DPF-equipped diesels match or beat CNG on PM and "toxics," and sometimes have lower nitrogen oxides (NOx) due to wide variability in field NOx performance of CNG engines.

Such was the case in the latest SwRI tests, where the CNG bus engine -- despite having a lower NOx certification level -- actually produced more NOx on the City-Suburban Heavy Vehicle Cycle, a standard test for school bus operations.

The only emissions that were higher with "green-diesel" were nitrogen dioxides ([NO.sub.2]) and carbon dioxide ([CO.sub.2]). [NO.sub.2] increase was expected due to the catalyzed DPF from Engelhard.

[CO.sub.2] difference is due to higher carbon content of diesel versus gas. CNG however has vastly higher methane emissions; methane has 20 times the "global warming" impact of [CO.sub.2].

CNG not only had higher PM and "toxics" emissions than "LED" (low-emissions diesel), but also had higher total hydrocarbon, non-methane hydrocarbon, NO and soluble organic fraction (SOF) PM emissions. CNG also had the highest emissions of aldehydes, ketones and other hydrocarbons not on California Air Resources Board's toxic air contaminant list. (Chart note: "CD" =conventional diesel):

TABLE 21

COMPARISON OF EMISSIONS AMONG THREE BUS CONFIGURATIONS

Relative Significance and Directions of Differences Between
Configurations

Criteria Pollutants and
Air Quality Emissions         CD vs. LED  CD vs. CNG  LED vs. CNG

[NO.sub.x]                       >>>          <<          <<
  NO                             >>>          =           <<
  [NO.sub.2]                     <<<          =           >>

PM                               >>>         >>>          <<<
  SOF                            >>>         >>>          <<
  [SO.sub.4]                     >>>         >>>         = (a)

THC                               >>         <<<          <<<
  NMHC                            >>          <            <
  Methane                         >          <<<          <<<

CO                               >>>         <<<          <<<

[CO.sub.2]                       <<<         >>>          >>>

Toxic Air Contaminants (TAC)

Bis[2-ethylhexyl]phthalate        =           =            =
Cyanide Compounds                 =           =            =
Dioxins and Furans                =           =            =
Hexane                            =           =            =
Phosphorus                        =           =            =

Biphenyl                         >>>         >>>           =
1,3-Butadiene                     =           <            =
Cresol isomers                    >           >            =
di-n-Butylphthalate               <           =            =
Methanol                          =           <            =
Naphthalene                      >>>         >>>           =
Phenol                           >>>          >>           =
POM (PAH   derivatives)           >>         >>>           =
Toluene                           >           =            =

Acetaldehyde                     >>>         <<<          <<<
Acrolein                          >>          <           <<<
Benzene                           >>          =           <<
Formaldehyde                     >>>         <<<          <<<
Methyl ethyl ketone               >>          >            <
Propionaldehyde                  >>>          <           <<<

(a)All data were 0

Symbol  Description (probability of opposite  "Mean A vs. Mean B"
Symbol  Description (probability of opposite  "Mean A vs. Mean B"
                    being true)

  =           p value > 0.05 (1 in 20)           No Difference
  <       p value [less than or equal to]        A less than B
                   0.05 (1 in 20)
  >       p value [less than or equal to]        A more than B
                   0.05 (1 in 20)
  <<      p value [less than or equal to]
                  0.01 (1 in 100)                A less than B
  >>      p value [less than or equal to]        A more than B
                  0.01 (1 in 100)
 <<<      p value [less than or equal to]        A less than B
                 0.001 (1 in 1000)
 >>>      p value [less than or equal to]
                 0.001 (1 in 1000)               A more than B

Source: Southwest Research Institute

DPF's also eliminate the "black carbon" recently associated with global warming, an issue first raised (but incorrectly as to DPF impact) by Stanford University scientist Mark Jacobson, then later clarified by University of Minnesota combustion particle scientist David Kittelson (see Diesel Fuel News 9/16/02, p5).

Ironically, 21 of the 41 "TACs" (toxic air contaminants) listed by CARB for diesel exhaust were completely absent from all three vehicles tested at SwRI: conventional diesel, clean-diesel and CNG.

Asked to speculate on this surprising finding, Lapin said CARB's diesel "TAC" emissions data probably aren't that robust, as they likely include the impact of now-obsolete lubes formulations, old engines, and unverified assumptions about engine emission contribution to ambient emissions monitors. It's also possible that the newer engines (both CNG and diesel) in the SwRI studies wouldn't have as much wear as some older engines, hence some "TAC" metals weren't found in the latest tests. All combustion engines have wear metals especially when they get older; CNG wouldn't be exempt.