Citgo Microemulsion Cuts Nox 25-27%, Pm By 40-52% In Test

Diesel Fuel News, Oct 15, 2001 by Jack Peckham

A microemulsion of water-in-diesel-fuel invented by Citgo's parent Petroleos de Venezuela SA (PDVSA) achieved 25-to-27.7% average reductions in nitrogen oxides (NOx) in a special test program with five City of Houston diesel vehicles, Diesel Fuel News has learned.

The same emulsion also slashed particulate matter (PM) emissions by 40-to-52%, according to the joint test program between Citgo, Houston and Environment Canada (EC).

In the test, hydrocarbons (HC) emissions increased by an average 17.8% and carbon monoxide (CO) increased by between 1.5-9.8%. Still, HC/CO emissions are less of a concern with diesels (unlike NOx/PM, the biggest concerns) since engine-out HC/CO levels are usually so low. It's possible to imagine adding an oxidation catalyst that, if necessary, could more than compensate for any HC/CO increases.

Houston and Citgo split the cost of the test program at Ellington Air Force Base, with city funds coming from a special low-emissions diesel retrofit project that the city is aggressively pursuing in order to comply with tough limits on ambient ozone by 2005 (see Diesel Fuel News 2/19/10, p7). The city hopes emulsion fuels can play a role in NOx reduction for existing fleets, perhaps until introduction of new engines or new technologies with ultra-low-NOx levels become commercially viable.

For the tests, the researchers started with an ordinary highway diesel fuel meeting today's U.S. EPA sulfur limits (500 ppm). Citgo didn't reveal total water content, but suggested it was at least 10%, according to Steve Domaic, staff analyst for Houston Health Department, who supervised the test program.

Microemulsions are as clear as water and have long (up to 2 years) storage stability, unlike the milky-colored macroemulsions (such as Lubrizol "PuriNOx" or TotalFinaElf's "Aquazole"), as Houston found in its emulsion test programs earlier this year, Dornak explained.

During tests, Houston accidentally allowed PuriNOx macroemulsions (up to 20% water content in tests) to sit unused in some vehicle storage tanks for weeks at a time, with water phase-separation resulting. While tests were successful on NOx reductions (around 25%,) vehicles left in storage for weeks at a time suffered in-tank water phase separation, as did a refueling storage tank that inadvertently lacked the required electrical hookup for mixing the fuel, he said.

Given that Houston has a fair portion of its diesel equipment that sits in storage for lengthy periods, this makes macroemulsions a bigger management and storage challenge, he said. "We'd need separate fuel storage at a lot of our sites," requiring investments in new tanks, he said. "We'd probably have to switch to 100% emulsion fuel or buy new above-ground tanks for storing regular diesel fuel."

* PDVSA: Nezv Test Results

Meantime, PDVSA has found that a special formulation can reduce the cost of microemulsions (normally rather pricey, compared to macroemulsions) "to be compatible to tax incentives" that are emerging for all diesel emulsions in the U.S. and Europe.

In a research paper (SAE 2001-01-3525) that was to have been presented to Society of Automotive Engineers fuels & lubes conference last month, PDVSA found that certain microemulsions (with up to 17 vol% water) can cut NOx+HC by about 18%, PM by 43-55%, and CO by 27-50%.

Steady-state tests on two engines used baseline fuels of between 8-25% aromatics and various emulsions with water content of between 10-17%, the researchers said.

The PDVSA investigators not only studied emissions differences between emulsion fuels, but also compared these to ordinary high-sulfur diesel fuels meeting Venezuelan standards, as well as a fuel representative of low-sulfur, low-aromatics California Air Resources Board (CARB) diesel standards.

In transient tests with emulsions, hydrocarbon emissions increased, but were still 80% below the regulated limits, PDVSA found.

One test microemulsion "shows a preferential PM reducing capacity," while optimization work can change the Nox/PM trade-off, they found. Microemulsions tend to increase soluble organic fraction of PM as well as increase [SO.sub.4].

In tests, a 12 vol% water emulsion caused a 5% increase in formaldehydes but a 4% cut in acetaldehye and a 10% cut in 1,3 butadiene.

Other key PDVSA findings:

* Power reductions can be optimized to be lower than the water content, as emulsions can improve fuel combustion efficiency. Torque penalties diminish at higher engine speeds.

* Fuel lubricity, stability, engine protection and driveability "can be all completely satisfactory for end-users." The emulsion surfactant itself boosts lubricity.

* Microemulsion cost can be reduced by a "synergetic combination of low concentrations of surfactant and adequate mixing energy," with a water/surfactant ratio "higher than 10."

* Even after one year of storage at temperatures up to 60[degrees]C, microemulsions showed no observable property changes.

* Cetane improver addition can compensate for reduced cetane due to water incorporation.