Ethers, renewable 'SuperCetane,' FT diesel all eyed as candidates for oil-sands diesel cetane upgrading

Diesel Fuel News, Sept 29, 2003 by Jack Peckham

Newport, R.I. -- Canada's immense oil-sands bitumen resource (over 2 trillion barrels) not only offers North American refiners an attractive, growing source of high-quality syncrude but also helps them avoid risky over-dependence on crudes from far less-friendly parts of the world.

But while oil-sands derived diesel has excellent cold-flow and very low sulfur content, it has had rather low cetane (as low as 33, but above 40 in near future) and relatively high aromatics and density, a concern for meeting upcoming diesel nitrogen oxides (NOx) and particulate matter (PM) limits.

So, a group of interested parties including Suncor, Shell Canada, Imperial Oil, Canadian Petroleum Products Institute, Syncrude Canada, Natural Resources Canada (NRCan), U.S. Department of Energy (DOE) and National Research Council-Canada (NRC) investigated how oil-sands diesel properties affect emissions. In a second, related study, DOE and NRCan are investigating various options for boosting cetane and reducing PM emissions with alternative components and additives.

Among test programs completed, still under way, or under consideration:

--Researchers just compared engine emissions results from diesel fuels derived from both conventional crude and oil-sands syncrude. These tests confirmed that oil-sands derived diesel's finished properties (not its crude source) affected relative NOx/PM emissions, compared to a crude-based diesel.

Still unknown however is whether the relatively high cycloparaffin content of hydrotreated oil-sands-derived diesel has any significant impact on relative emissions.

Tests showed that relatively higher total aromatics and sulfur content boosted PM emissions, while higher total aromatics and density caused higher NOx emissions.

Today, analytical methods for measuring cycloparaffins aren't as well developed as test methods for aromatics, as NRC researcher Stuart Neill explained to the Diesel Engine Emissions Reduction (DEER) workshop here, sponsored by U.S. Department of Energy. So, researchers are now characterizing Canadian refinery streams for a future study on cycloparaffin relative impact on engine emissions.

While the final answer isn't known, it's suspected that cycloparaffins aren't expected to have the same effect as aromatics, because cycloparaffins are saturated with hydrogen. So there's probably less NOx/PM emissions effect than found with aromatics, or so the theory goes.

--In separate tests on fuel property impacts on emissions, sponsored by NRCan and DOE, researchers employed an engine with cooled exhaust gas recirculation (EGR) to achieve U.S. EPA 2.5 grams/brake horsepower-hour NOx HC heavy-duty limits (in effect since October 2002).

For the tests, researchers chose a "baseline" hydrotreated ultra-low sulfur diesel (ULSD) derived from oil-sands crude. This fuel had 15% total aromatics, 10-ppm sulfur, 44 cetane number, and 838 kg/m3 density.

To this "baseline" fuel, they added varying amounts of three different ethers, all to achieve a 10-cetane number boost: 8 wt% dipentyl ether (mono-ether); 15.4 wt% diethoxy butane (diether); 15 wt% ethoxy ethyl ether (tri-ether) and a mix of 11.1 % tri-ether plus EHN, to achieve the 10-cetane boost. The latter two blends employ ethers of different molecular structure to deliver the same oxygen and cetane levels.

Tests in progress are also evaluating the addition of a Fischer-Tropsch diesel that contains both normal and iso-paraffins (to improve cold-flow) as well as "SuperCetane," a hydrotreated (rather than esterified) version of yellow-grease-based biodiesel, without any oxygen content. With further processing (isomerization) of normal- paraffin "SuperCetane" component, this "renewable" cetane improver could achieve better cold-flow, researchers explain.

Also "in process" are tests of the addition of soy biodiesel esters plus EHN, or the addition of pure EHN or DTBP, to achieve the 10-cetane number boost.

In tests so far, the various diesel-ether blends show that PM and "soot" emissions decline with increasing oxygen content, but NOx goes up with higher oxygen.

--NOx/PM Tradeoffs Between EGR, Oxygenates

Still, despite the higher NOx penalty with the ether oxygenates, EGR was able to compensate enough to keep the engine just at or under 2.5 g/bhp-hr NOx. Meantime, the oxygenates decreased soot--below baseline level--compensating for the higher PM emissions typical of EGR engines.

While these results look promising, it's important to note the U.S. refiners are especially wary of ether blending, because of the water quality taste-and-smell problems earlier caused by MTBE/gasoline leaks into groundwater. Refiners today are anxious to avoid any repeat of ether-blend problems like MTBE.

The three selected test ethers had been screened and chosen in part because of their relatively low water solubility, although a more thorough screening would be required before any possible commercialization. Such an investigation on potential water-quality taste/smell issues from possible diesel-ether blend spill is "not finalized yet," as Natural Resources Canada research manager Rene Pigeon explained to us in a post-DEER-conference interview. "However, we have found that while the di-ether blend smells like an ether, it is light and fruity," he said.