Diesel’s Fuel Quality Imperative

Advancements in diesel engine technology require a critical look at diesel fuel quality issues.
By Ron Kotrba | July 15, 2019

The pressure at which  diesel fuel is injected into combustion chambers directly affects combustion efficiency, performance, fuel mileage and emissions. High pressure common rail fuel injection systems are not new to diesel engine technology, but tightening emissions and fuel economy regulations are requiring greater injection pressures than ever before, which is dramatically lowering the tolerance for impurities in diesel fuel. Experts suggest much of the diesel fuel on the market today is not suitable for tomorrow’s diesel engines, but whether this is because fuel specifications are inadequate or because supply management techniques are insufficient depends on who you ask. But in a world where internal combustion engines and liquid fuels face the threat of extinction by regulators fixed on electrification, the value proposition for improving diesel fuel quality is the right to survive, says Paul Nazzaro, founder and president of Advanced Fuel Solutions. There is somewhat of a  consensus on what diesel fuel improvements are needed, but how to achieve these is less than clear. What is clear, however, is that no matter how high the quality of biodiesel produced, if it is blended with poor-quality diesel fuel and transported and stored in subpar conditions, then diesel fuel’s problems become visited upon the biodiesel industry.

“Diesel fuel quality has not kept up with engine changes,” says Rebecca Monroe, the fuel trademark program lead at General Motors. She says emissions regulations are driving diesel engine changes—again. To some, this may all seem like a bad case of déjà vu. In the early to mid-2000s, U.S. EPA regulations were put in place to ratchet down diesel particulate matter (PM) and NOx emissions by 2007 and 2010, when the standards went into effect. Today, negotiations between California Air Resources Board and EPA may lead to NOx emissions cuts by another 90 percent. According to Timothy Johnson, a Society of Automotive Engineers Fellow and consultant with Corning Inc., various approaches are being evaluated to reduce NOx from California’s diesel engines, but close-coupled selective catalytic reduction (SCR) technologies are a frontrunner. Unlike today’s diesel aftertreatment systems featuring an oxidation catalyst and particulate filter upstream of an SCR catalyst, a close-coupled SCR filter would be positioned as close to the exhaust manifold of the engine as possible to take advantage of higher exhaust temperatures for conversion, and the SCR catalyst would be exposed to soot and ash, bringing the metal  contents of diesel and biodiesel under additional scrutiny.

Not only must diesel engines further reduce tailpipe emissions of NOx, but there is serious discussion of significantly increasing the useful life of diesel engines and components from 435,000 to 1 million miles. On top of this, fuel economy standards are rising to satisfy greenhouse gas emissions reductions.

Whichever approaches the marketplace chooses to cut NOx emissions under new California or federal standards, one thing is certain: Fuel pressures in common rail injection systems will only increase to achieve more efficient combustion, better fuel economy and fewer emissions. “With ultra-low NOx standards and expected changes to the useful life of diesels, engines are using and will continue to use much more sensitive technologies for moving fuel through the system,” says John Eichberger, executive director of the Fuels Institute. “Now, instead of 5,000 [pounds per square inch (psi)], new systems inject fuel at 40,000 psi, and next-generation engines may be as high as 70,000 psi. This reduces the tolerance for fuel impurities down to 1 micron in the injector tip.”

The compounding nature of enhanced engine designs, new regulations and the threat of electrification is cumulatively putting significant, additional pressure on the market to ensure fuels delivered are the highest quality possible. “All OEMs say fuel quality is a top concern for maintaining warranties,” Monroe says.

Scott Fenwick, the National Biodiesel Board’s technical director, says the state of diesel fuel quality naturally comes up at ASTM meetings all the time. “It’s tough,” Fenwick says. “We’re trying to keep up with advancing regulations, advancing technologies—engine technologies specifically—but the fuels aren’t changing as fast.”

Fenwick says conversations inevitably turn to 2006, when ultra-low sulfur diesel (ULSD) was introduced as required on-road fuel for compatibility with diesel aftertreatment systems to reduce emissions. Nazzaro says reducing sulfur to 15 parts per million (ppm) changed critical areas of performance, lubricity, stability, conductivity, winter operability, solvency and density. “Although the environmental improvement associated with the reduction of sulfur was attained, these other critical performance criteria have created operational challenges associated with powering diesel vehicles nationwide,” he says. Fenwick points out that 2006 was about the same time as commercialization of biodiesel. “Engine technologies are changing fast,” Fenwick reiterates, “but it’s difficult when you get a room of 400 people in a Subcommittee E meeting and try to balance everyone’s opinions and agendas—OEMs, petroleum refiners, renewable fuel producers, an entire array of the fuel distribution system, all with their own opinions on what can and can’t be done to improve today’s fuels.” 

Despite differing opinions and agendas, much of the concern over diesel fuel quality focuses on water, particulate and other contaminations, metals and stability. However, it is up for debate whether fuel specifications must be modified, or downstream fuel handling and storage practices improved.

Is ASTM Enough?
“Engine manufacturers have told me that even if we take a 100 percent on-spec ASTM fuel and put it into high pressure common rail engines, we may have problems,” Eichberger says. “The engines are so much more sensitive.” ASTM D975, the specification for diesel fuel, contains a combined sediment and water limit of 500 ppm. “OEMs say that’s not sufficient,” he says. “In the EU, it’s 200 ppm. OEMs point to the Worldwide Fuel Charter. They want 24 ppm water/sediment content. They say ASTM is not enough.” Shailesh Lopes, a GM senior fuels engineer, says one of the most important changes needed in D975 relates to water and sediment. “There have been numerous ballots from the OEM side, both segregated and combined requirements,” he says. “They’ve tried and failed but haven’t given up. When you try for five years and keep failing though, there’s not the same level of enthusiasm.”

Nazzaro says the single best opportunity to improve diesel fuel quality would be to eliminate free water within the fuel. “Water contributes to fuel instability and allows microbial growth to thrive in that environment,” he says. “ASTM meetings can be a difficult place to reach consensus. Everyone throughout the fuel supply chain wants their supplier to address the issues so they don’t have to. There’ve been attempts to limit the ‘total’ allowable water content within diesel fuel, but the committee can’t agree on the proper allowable limit.” 

Engine manufacturers run tests to determine why engine components fail. “They mostly find sodium concentration in diesel fuel engines, and that sodium is causing problems—sodium and magnesium—but they talk about sodium most,” Eichberger says. “They don’t know where it’s coming from. There is no spec in ASTM D975 to control for metals.” While petroleum diesel fuel has no metals limit in D975, ASTM D6751, the spec for biodiesel, has 5 ppm limits for sodium/potassium and the same for calcium/magnesium—and sampling routinely shows market biodiesel is well below the limits for these.

Fenwick says if high sodium is found in a diesel sample, the first thought is often that the biodiesel is off-spec. “The biodiesel industry has broad shoulders and we know the blame is coming at times,” he says. “But biodiesel is not the only means for sodium to enter the diesel fuel stream. Sodium can come from rain water, salt used to melt ice at retail stations, crude oil and the refinery if something upsets their system, or certain additives if they’re inadvertently dosed too high. There are multiple ways fuels can be contaminated.”

Other efforts over the years at ASTM to change D975 have included measures to control deposit formations, oxidation and cold flow. “For deposit formations, these are specifically related to deposits in injectors,” Lopes says. “Until now there’s been no tool or method to characterize the propensity of the fuel to cause deposits, but numerous projects over the past decade have [made advances in this area].” He says there is still strong interest in putting stability requirements on finished diesel fuel. “Biodiesel has stability requirements, but when the finished fuel oxidizes, we’re not sure if it’s because of the biodiesel or petroleum diesel,” he says. Biodiesel’s oxidative stability is measured using the Rancimat test, but given the nature of the differences between diesel fuel and biodiesel, this would not be applicable for petroleum diesel, so a different test method and corresponding limits would have to be approved for D975. Lopes says there’s also been a parallel push to make nonmandatory cold flow guidelines mandatory in D975. “Diesel fuel has no mandatory requirements here in the U.S., unlike in the EU or other developed nations,” he says. 

Of the OEM’s greatest concerns over diesel fuel quality—moisture content, stability, metals, particulate contaminants, and cold flow—biodiesel actually has a specification and limits to address most of those, while diesel fuel does not. “Even though biodiesel is a relatively small component in the finished fuel, it’s the only component that has a filterability limit in the Cold Soak Filtration Test, a metals content limit, and a stability limit, which are some of the concerns OEMs have today,” Fenwick says. “Biodiesel is the only component that begins to address these.”

Biodiesel Specs
In 2006, as diesel aftertreatment devices were being introduced to reduce PM and NOx, limits were imposed on trace metals in D6751 at 5 ppm for calcium/magnesium, and 5 ppm for sodium/potassium. “The metals were previously limited by the sulfated ash test to relatively low levels,” says Steve Howell, president and founder of M4 Consulting, former longtime NBB technical director and AOCS Fellow. “With the advent of particulate traps coming in 2007 after ultra-low sulfur diesel was introduced in 2006, we added the separate specs for sodium and potassium, and calcium and magnesium for biodiesel.” These metals were identified since residues can be present in improperly processed biodiesel. “Most values we’ve seen for these in biodiesel are at or near the detection limit of 1 ppm, so it hasn’t really been a big industry focus,” Howell says. “This is especially so over the past few years, with most of the material being made from BQ-9000 companies that monitor closely to make sure their process is working properly.”

With more diesel technology changes coming, however, Howell says there’s always more work to do to make sure the biodiesel spec keeps up with evolving diesel technology. “The changes we’ve made are working, [evidenced by the] quality of biodiesel we see in the market.” Any fuel quality issues experienced with biodiesel today are no different than “the normal issues we see with diesel fuel,” he says.

Johnson says with the advent of close-coupled SCR filters, critical analyses must determine how biodiesel interacts with these devices. Howell and Johnson both say NBB and OEMs have launched a major test program to investigate this. “The big focus is the metals levels,” Howell says. “It’s currently limited to 5 ppm, but [it will] probably [have to be set] lower.”

The introduction of the CSFT 10 years ago was another milestone in the evolution of biodiesel fuel quality. “Biodiesel fuel that met the ASTM specification at the time was still plugging filters under certain conditions,” says Brian Hess, the technical service department manager with Evonik Oil Additives USA Inc. “The method was developed to help prevent these filter clogging issues in the field.” He says the test is performed using 300 milliliters (ml) of B100 cooled and held at 40 degrees Fahrenheit for 16 hours. “The B100 sample is then warmed to room temperature and subject to vacuum filtration with a 0.7-micron filter,” Hess says. “All 300 ml of B100 must pass through the 0.7-micron filter in less than 360 seconds. For use as Grade No. 1-B, the biodiesel must filter in less than 200 seconds.”

The method was initially a part of the D6751 annex, but it later got its own method—ASTM D7501. “One of the early changes to the method focused on the type of filter employed to improve test reproducibility,” Hess says, “so the current version calls for a specific brand of glass fiber filters.”

Evonik’s Viscoplex 10-340 cold flow improver (CFI) has been effective in taking B100 samples from failing the CSFT to passing. “Depending on the biodiesel quality, the CFI can also provide faster filtration times to allow a B100 stock to meet the No. 1-B requirement of 200 seconds,” Hess says. “Viscoplex 10-340 is effective in a wide variety of biodiesel types ranging from canola, soy, tallow and used cooking oil.”

The effectiveness of a CFI depends heavily on the type of feedstock. “Less saturated biodiesel feedstocks have better inherent low-temperature properties and will also respond best to cold flow additives,” Hess says. “More saturated feedstocks like soy and animal fat methyl esters have warmer starting cold filter plugging point (CFPP) and pour point values and require different additive compositions to achieve low-temperature improvement.” He says typical canola biodiesel can expect a CFPP improvement of 15 degrees Celsius with Viscoplex 10-340 whereas a soy biodiesel improvement in CFPP of 5 degrees C would be achievable.

Hess says Evonik can help producers select the proper CFI and optimize the treat rate for a given B100 or blend, noting that CFI dosages typically range between 250 and 1500 ppm. “Ensuring that B100 meets the CSFT is an important safeguard against potential filter blocking issues,” Hess says. “Viscoplex CFI technology can fix a B100 that fails the test and improve its performance to fulfill all ASTM D6751 quality requirements.”

Stability is another hot-button issue for biodiesel quality, with some arguing that the specification requirements are not stringent enough—even though petroleum diesel has none. In order to demonstrate adequate storage suitability (shelf life), B100 must meet a minimum three-hour induction period (IP) under the Rancimat test as described in D6751, while D7467—the spec for B6-B20 blends—must meet a six-hour IP.

Earl Christensen, a senior scientist at National Renewable Energy Laboratory, has overseen many biodiesel stability tests over the years, the most recent and comprehensive of which was completed in 2017. The test pulled 12 B20 samples from 12 different states, with one sample prepared in the lab, for a total of 13 B20s. The samples were aged at 110 degrees F per ASTM standard method D4625 to the IP thresholds, after which they were readditized with BHT, simulating nearly three years of long-term storage. After 32 weeks, simulating two and a half years, most samples were still on spec for six hours. While there is a point after which the fuel shouldn’t be used, if the fuel is readditized soon after it shows signs of degradation, a B20 could easily be stored successfully for three years and remain on-spec.

“B20 should be readditized before signs of degradation, which are indicated by peroxides and acids,” Christensen says. “Detectable increases in acids are preceded by the IP reaching a ‘threshold.’ The IP will go down, but the fuel will not change significantly in quality until the IP drops to low values, after which readditization is no longer viable. One happens before the other—IP goes down to a low level, peroxides spike, acids go up to an unacceptable level, and insoluble materials become apparent. The IP is a relative measure of the oxidation reserve. Once that reserve runs out, the IP gets very low and the fuel starts to oxidize. Making sure readditization is done well-above that low IP value allows you to significantly delay all the bad stuff from happening. Thresholds of about four hours appear to be pretty conservative as a readditization point.”

Christensen says one of the samples stood out to him. It arrived with only a two-hour IP, but when readditized it went 36 weeks before the acid number was out of spec. “This is really the more interesting sample to me, as this was not aged in the lab but was somehow aged in the field,” he says. “It’s a real-world example of B20 received out of spec. Acids and all other quality parameters were on spec so we still readditized it to see what would happen. Readditization increased the shelf life of this low-stability sample from only about three months to more than three years—quite an improvement with a few hundred ppm of BHT.” Overall, what Christensen and his team at NREL observed was samples that may have lasted slightly more than one year in storage could be extended to three or more years with readditization just up to the six-hour spec limit.

When asked whether current ASTM requirements on biodiesel stability are enough, Christensen says, “We are taking a very close look at these questions with our ongoing research. At this point our data from aging biodiesel blends inside vehicles and monitoring their stability in the lab indicate that with good, quality fuel the six-hour minimum is protective of the vehicle with fuel stored for one year or less. There’s plenty of evidence that off-spec fuels can be problematic, but on-spec appears to be doing its job. But keep in mind this is the bare minimum for what’s considered normal use.” He says if evidence demonstrates this limit is not high enough for normal fuel use and vehicles are at risk, then it must be raised to the appropriate level. “ASTM is a data-driven organization, so we need good evidence to argue for a spec change,” he says. “‘Everyone else is doing it’ is not a good enough reason to alter a fuel quality specification. If it were, ASTM might require a higher cetane number in D975 because the European spec is higher.”

Nearly two dozen revisions to D6751 have been implemented since its original publication less than 20 years ago, Nazzaro points out. “The NBB and its champions as the single voice representing the biodiesel industry through the years have driven those improvements,” he says. “Unfortunately, there isn’t a single champion amongst the diesel fuel industry, and each participant has a different opinion and agenda to contribute.”

While some argue that ASTM specs must be changed to improve the quality of diesel and biodiesel fuels leaving refineries and production facilities, others suggest improper fuel storage and handling downstream are to blame. “Refiners say, ‘We produce fuel to ASTM specifications, so it’s downstream—pipelines, terminals, distribution and retail establishments—where the contaminants are introduced,’” Eichberger says. “They argue that ASTM specs can’t be changed until downstream contamination is controlled. If we can control that, then we may not need to change the specs.”

Fenwick says people tend to overlook how much storage conditions impact fuel stability. “Not just biodiesel, but fuel in general,” he says. “Dirty tanks, rust, sediment, water—those have a big effect on fuel stability.”

Fuels produced on-spec can certainly become contaminated on the journey from refining and production to terminals and dispensers. “No testing takes place outside the bulk terminals prior to the dispenser, so here is where countless batches of fuel ultimately become comingled,” Nazzaro says.

“The end-of-the-line treatment and observation is where the fuel is most likely to degrade. Obviously high-turnover fuel is less susceptible to degradation, but time and temperature are drivers in the degradation process. Water, time and temperature drive microbial contamination, which then drives corrosion.” To protect the fuels, downstream processes must be tightened, Nazzaro advises.

Clearly a lot of finger-pointing is going on between many diverse stakeholders in the diesel fuel supply chain. As some try to hash it out at ASTM or the National Conference on Weights and Measures, others are working on alternative solutions to protect engine, vehicle and storage assets.

In late 2017 the Fuels Institute launched its Fuel Quality Council to evaluate the relationship between diesel fuel and modern high pressure common rail engines. “Our first charter is to get people with pointing fingers in a room and have them put their fingers away,” Eichberger says. “Let’s foster an environment of collaborative discussion. We don’t advocate, we research and study. We’re trying to quantify the scope of the problems in the market. There’s a lot of anecdotal data, but no quantifiable data set. We’ve been in the market for several months listening to fuel retailers, fleet operators and OEMs, gathering data on fuel quality. There are inherent costs associated with fuel quality problems, particularly with OEM warranty claims. If we can put a number on it, quantify the risk and narrow the items causing concern, that’d be a big deal—a huge step.” Gathering empirical data is step one.

Step two is developing best practices for the supply chain from rack to retailer, including comprehensive maintenance and housekeeping throughout the distribution chain. “The best practices will be an aggregation of knowledge in easy-to-follow steps with quantifiable risk-mitigation procedures,” Eichberger says. “A $500 investment this quarter may save you $200,000 in failed tanks in a year.” He expects the best practices guidance to be released this fall.

Finally, Eichberger says the FQC will study how best to monitor the quality of fuel going into vehicles, looking at sampling methodology and evaluating aspects that are intrinsically important to engines. “We’ll test mechanisms to measure metals at the tanks and in the dispensers—the overall health of the fuel as it’s introduced to vehicles, and compare that to refinery fuel,” he says. “What’s coming out of the nozzle, what’s different, how important the difference is, and how to pursue corrective action to get in line. If industry decides ASTM standards must change, then we can quantify how those changes will be reflected at the nozzle and measure the progress.”

While the FQC is gathering data and putting together best practices for the entire diesel fuel supply chain from retailer up through the distribution system and possibly to the terminal, another program focused on retail quality is making headway—the Top Tier Diesel Fuel program.

The Top Tier Detergent Gasoline program started in 2004 after federal lowest additive concentration (LAC) minimums significantly dropped the amount of detergents in retail gasoline, which led to increased fuel injector deposits and other issues in engines. The program was developed by a consortium of OEMs interested in engine longevity and reduced warranty claims. After more than a decade of success with its gasoline program, another consortium of diesel OEMs launched the Top Tier Diesel Fuel program in 2017 as a way to combat all the troubles with diesel fuel in the marketplace.

The diesel program began to take shape with feedback from a survey of OEMs, which identified five top factors of concern in diesel fuel: water, fuel stability, detergency/cleanliness, total contamination, and lubricity. This is the basis for the Top Tier Diesel Fuel program. Retailers wanting to participate in the volunteer program must demonstrate that their fuel meets specific requirements related to the issues identified above, which include parameters that are either tougher than ASTM specs or requirements that ASTM does not currently have, such as a stability requirement for diesel fuel.

Fuel stability of diesel fuels without biodiesel or up to 2 percent blends are determined within the Top Tier Diesel Fuel program by Rapid Small Scale Oxidation (the PetroOXY test) per the ASTM D7545 method. Acceptable fuel should have an IP of greater than 60 minutes. For B100 as a blend agent, Top Tier requires a minimum IP of eight hours as measured by the EN15751 test method vs. three hours required in ASTM D6751. “We love biodiesel,” Monroe says, “but we love very stable biodiesel.”

The diesel fuel supply chain is different from gasoline’s in many ways. For one, there are no federal additive requirements for diesel fuel as there are for gasoline in the LAC regulations for detergents. This means that while gasoline is most often additized at the terminal rack, not all diesel fuels are additized this far upstream. Thus, the Top Tier Detergent Gasoline program allows a retailer to enroll all of its stations in the program whereas the Top Tier Diesel Fuel program is site-specific. Thus, it may not be possible to qualify the stability performance of B100 used to make biodiesel blends, in which case for blends greater than 2 percent up to 5 percent—permissible in D975—Top Tier requires a minimum IP of 24 hours as measured by the EN15751 test method.

Detergent and lubricity additives to meet the requirements of Top Tier Diesel Fuel are likely dosed upstream from the retailer, so the only site-specific requirements are installation of water-absorbing 10-micron or lower filters on low-flow dispenser pumps and up to 30-micron filters for high-flow pumps typically found at truck stops and travel plazas. Naturally, filtration is a last defense, so Top Tier recommends good housekeeping practices and diligent water monitoring per the Coordinating Research Council Inc.’s Report No. 667, “Diesel Fuel Storage and Handling Guide.”

Once a retailer demonstrates its ability to meet Top Tier Diesel Fuel program requirements, an agreement laying out terms and conditions, such as displaying the program logo, is signed. Diesel truck drivers and vehicle owners can then easily identify where to buy the enhanced fuel. Participants are subject to random, routine monitoring to ensure they continue to meet the program’s requirements and pay a fee to participate.

Growmark is a major agriculture and fuel supply cooperative founded in Illinois in 1927. Today, the co-op owns five fuel terminals and has positions at additional terminals and pipelines owned by others. Growmark has the ability to additize diesel with its proprietary additives to make its Dieselex Gold brand of fuel, which meets or exceeds Top Tier Diesel Fuel standards, at bulk facilities or right on the tanker truck. In December 2017, Growmark announced it was supplying Top Tier Diesel Fuel blended with biodiesel. According to Curt Dunafin, Growmark’s energy services manager, the cooperative blends 18 to 20 MMgy of biodiesel. While the biodiesel is supplied by multiple sources, Growmark requires BQ-9000 certification from its suppliers.

“The things I see being brought up in meetings—problems with fuel contaminations—we’ve seen and battled for years,” Dunafin says. “We’ve adjusted our additive formulation and our [housekeeping] program and we know what to look for, how to find it and remove it. Our approach evolved over 65 years.”

With more than 20 sites certified under the Top Tier Diesel Fuel program, Dunafin says Growmark has long hoped OEMs would officially put statements behind what they want to see in fuel. Now that Top Tier has done this, he is glad to be a part of the program. “We see it as validating and catching up to what we’ve worked on for a very long time,” he says. “It gives credence to what we’ve been telling our customers, such as the use of detergents, the right filtration, water detection and removal, oxidative stability—with biodiesel and straight No. 2 diesel—and a lubricity spec. We’d like to see more of that. We want to see a fuel requirement with a minimum 47 cetane. That is something that’d be great for our industry. It would raise fuel quality for everyone.”

From Growmark’s standpoint, fuel quality is a combinative approach. “You can’t skip on any of it,” Dunafin says. “If you start with quality fuel and biofuel and add additives to protect it, then it makes the rest of the job easier. But you have to be proactive, transport it correctly, and keep things clean and dry.”

Author: Ron Kotrba
Editor, Biodiesel Magazine
[email protected]

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