Q&A with Bob McCormick on Low-temperature Operability

No fuel is expected to perform below its cloud point, but a plugged filter above the cloud point is never quite expected. Bob McCormick discusses results from a recent study that investigated this phenomenon.
By Ron Kotrba | May 11, 2009
Bob McCormick, principal engineer with the National Renewable Energy Laboratory, presented at the National Biodiesel Conference in San Francisco, on a study the Coordinating Research Council conducted. The study, titled, "Biodiesel Blend Low-temperature Performance Validation," is CRC Report No. 650 and can be found at http://www.crcao.org.

Biodiesel Magazine talks with McCormick about the investigation and results.

Q: The objective of the low-temperature performance validation on biodiesel blends, which was conducted by the Coordinating Research Council in conjunction with NREL, was to validate a cold flow performance test for B100 to eliminate biodiesel that shows precipitation with B20 and lower blends above the blend cloud point, and to evaluate the correlation of bench test results to actual heavy-duty vehicle performance at low temperatures. Was the objective of this study met, and what was learned?

A: Yes, overall the study objective was met. What we did was test four B100s with a range of cloud points of zero degrees Celsius up to 12 C – zero, 5 C, 9 C, and one at 12 C. We tried to cover the full range of biodiesel cloud points, except for palm oil biodiesel. Out of the four B100s, we had one B100 that failed the cold soak filtration test; and the other three were all less than 200 seconds. So, for the B100s that had a cold soak filter time of less than 200 seconds – at both B5 and B20 blends – we had good operability for the vehicles down to the cloud point. We tested three different trucks and there was good operability down to the cloud point. Even though some people use other tests, such as the cold filter plugging point (CFPP) or the low temperature flow test (LTFT), the cloud point is really the most widely used diesel fuel measurement for low temperature operability limits.

ASTM D 975 has these charts that are based on historical weather data, which give the 10th percentile minimum temperature for every part of the country for the winter months.

What this means is that 90 percent of the time the temperature is going to be warmer than the 10th percentile minimum. So people tend to want to buy fuel in the winter with a cloud point that is below the 10th percentile minimum temperature – you don't want a fuel clouding up in a temperature that you can see more than 10 percent of the time. Some fleet users are [cautious enough] and feel if it is really critical that their vehicles operate when it is cold, they will buy fuel with an even lower cloud point.

Here in Denver, 6 degrees Fahrenheit is the minimum design temperature in December. One day this past December the temperature was minus 11 F, and a lot of diesel vehicles that have never even seen biodiesel did not run. That sort of thing happens with diesel vehicles. We were trying to see if the CFST was effective in ensuring that cloud point is a good predictor of the low-temperature operability limit for biodiesel blends, and we were able to operate down to cloud point with biodiesel that has less than 200 second cold soak filtration time. And, in some cases, we were able to even operate maybe a degree or two below the cloud points – including with the B20 blends. Of course, the B20 cloud points weren't anywhere near B5 cloud points. With very low cold soak filtration time B100, we didn't see any problem. On occasion, biodiesel blends can cause filter plugging at temperatures above the cloud point – no one expects that to happen. For low cold soak filter time biodiesel, that didn't happen. For the one high cold soak filter time material, the most severe vehicle design failed to operate at a temperature significantly above the cloud point – maybe four degrees above the cloud point. We didn't have the time or the budget to continue testing that vehicle to see how bad it was by going to even higher temperatures, but with that vehicle design, which can represent about a third of end use trucks in North America, we would predict there could be some significant problems operating at cool temperatures above the cloud point of the fuel, for biodiesel blends using B100 that failed the CSFT.

Q: You mention the severity of that particular truck design. What about that particular design makes it severe?

A: For the other fuels – the control petroleum fuel and the low filter time B20s – this design worked fine. It was only for the high cold soak filtration time material that it didn't work. Actually, for the B5 blend it worked okay, but it was clear to us that something was trying to plug the filter, but it didn't succeed. With the high cold soak filtration time B100 in a B20 blend, it definitely plugged and would stall out. I don't know why it's important for the OEMs to use this design. It's based on a lower fuel recirculation rate. And the fuel doesn't warm up as fast. But if the design caused problems with petroleum diesel, I am sure the OEMs would have changed designs by now. The design, even though it is a heavy-duty truck design, is more similar to the type of fuel systems used in light-duty diesels.

Q: So it sounds like the study was a success. Is that right?

A: Yes it was, but admittedly, the failure we saw was one fuel in one truck. And this type of study, where we have heavy-duty trucks tested in indoor laboratories where there are controlled cold temperatures, that is not cheap to do, so there was a limit to how much we could do. But, yes, the objectives were met. The study may be criticized for not being big enough. In a perfect world we would have had more vehicles and more fuels.

Q: I understand the tests were conducted at an indoor laboratory in Sarnia, Ontario, is that correct?

A: Yes, at a lab owned by Imperial Oil in Sarnia, Ontario. Imperial Oil is a division of ExxonMobil Corporation. The laboratory was very heavily utilized by oil industry and the fuel additive industry, and in a totally unrelated business decision ExxonMobil closed the lab down at the end of 2008, so we cannot go back there for additional testing.

We are, however, looking for another facility to do further testing. To satisfy the needs of the biodiesel industry, we are going to need to do some additional testing to determine things like whether the time requirement for cold soak filtration should be 360 seconds all the time; or is 200 seconds or less needed during the wintertime. The question is, do we need the 200-second limit or is a 360-second limit adequate? We want additional testing to try and answer that question. I don't think our study answers that.

Q: To be clear, the truck designs other than the truck with the most severe design, did not fail using high cold soak filtration time B20?

A: There were three truck designs. The design of each of the three trucks represent in some sense about a third of the market. The most severe design failed.

The intermediate design showed signs of potential issues for the high cold soak filtration material. As we measured pressure drop through fuel filter, we saw pressure build up, but it was never enough to stall the vehicle. What that means is it was right on the edge. In the real world, instead of in a controlled lab environment, it may have been worse. The third heavy-duty truck design incorporated an electric fuel filter heater, and it had no problem. We could have plugged the filter on that one with diesel or biodiesel with cold enough temperatures, but at cloud point of the fuel there was no problem.

Q: In the slides you presented at the National Biodiesel Conference in San Francisco, a phase two of this study was mentioned.

A: We are in the middle of that. We got some vehicle test data from the Sarnia Lab before it closed. We are aiming to get some additional test vehicle data to figure this all out. We may not have gotten enough data originally. So, we are looking at the data now, and we are doing additional analysis on the fuels. We are still looking for another test lab to do additional tests, but it's difficult to say when that will happen. We hoped to have results by now so that we could try to bring new proposals to the ASTM committee in the next six months but, in spite of the best laid plans, we just didn't get there.

Ron Kotrba is editor of Biodiesel Magazine. Reach him at (701) 738-4942, or [email protected].
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