Paving biodiesel’s way for greater use in advanced diesel systems

By Ron Kotrba | February 06, 2019

Biodiesel’s use in heavy-duty diesel applications continues to grow in California thanks in large part to the state’s Low Carbon Fuel Standard, but how will California’s Mobile Source Strategy—a plan to further cut NOx emissions by another 90 percent compared to the already low NOx-emitting diesel engines of today—affect the use of biodiesel?

Various approaches are being evaluated to cut NOx from California’s diesel engines by 90 percent, said Timothy Johnson, a Society of Automotive Engineers Fellow, consultant with Corning Inc., and one of the leading experts in the field. Johnson spoke at the National Biodiesel Conference & Expo in San Diego in late January, where he said close-coupled selective catalytic reduction (SCR) technologies are a frontrunner in NOx reduction approaches. Unlike today’s diesel aftertreatment systems that feature 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 as such the SCR catalyst would be exposed to soot and ash.

Johnson said high-porosity filters are being developed so the SCR catalyst can be imbedded directly on the filter as opposed to current systems, which employ separate DPF and SCR units. In this configuration, “virtually all ash will be on the SCR filter,” Johnson said. “There are concerns over the impact of ash on an unshielded [SCR filter]. Biodiesel soot is much more reactive, which helps diesel particulate filter regeneration, but the presence of metals and impurities might impact durability.”

In 2006, stringent limits were imposed on trace metals in ASTM D6751, the biodiesel quality specification, at a maximum of 5 parts per million (ppm) of calcium and magnesium, and 5 ppm of sodium and potassium. This was done to allay concerns from OEMs, which were launching a new generation of diesels equipped with sophisticated aftertreatment systems to significantly reduce particulate and NOx emissions per federal regulations. In 2007, many new diesels featured DPFs to meet particulate regulations, and by 2010, when NOx emissions regulations went into full swing for on-road trucks, most new trucks featured a diesel oxidation catalyst, DPF and SCR.

“The metals were previously limited by the sulfated ash test to relatively low levels,” Steve Howell, president and founder of M4 Consulting, former longtime technical director for the National Biodiesel Board and AOCS Fellow, told Biodiesel Magazine. “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. Prior to then, the only thing limiting those was the sulfated ash, and there were no separate specs for [those metals].”

These particular metals were identified as their residues can be present in improperly processed biodiesel, with sodium and potassium from the catalyst and calcium and magnesium from fuel washing.

“Most values we have 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 said. “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.”

The durability requirement for modern diesels is 435,000 miles, which is why OEMs are concerned over impurities in fuels, including trace metals in biodiesel. “In California, durability requirements and warranties will increase,” Johnson said during his presentation. Due to the long life of heavy-duty diesel trucks, he noted that pre-2010 models will continue to be the primary source of NOx emissions until 2023.

Howell, who also presented on the panel, said a 90 percent reduction in NOx from modern clean diesel engines would be a “step change.”

“Diesel engines are changing dramatically, and we have a lot of work to do,” Howell said, referring to ensuring that biodiesel quality specifications keep up with evolving diesel systems. “The changes we’ve made are working, [which is evidenced by the] quality of biodiesel we see in the market.” He said any fuel quality issues experienced with biodiesel in the market today are no different than “the normal issues we see with diesel fuel.”

Johnson said with the advent of close-coupled SCR filters, critical analyses must determine how biodiesel interacts with these devices. Howell and Johnson said NBB and the OEMs launched a major test program to investigate this. “The big focus is the metals levels,” Howell said. “It’s currently limited to 5 ppm, but [it will] probably [have to be set] lower. We are conducting research to allow biodiesel to grow through 2050. The future is bright, but only if we do this work.”

“The NOx from diesel exhaust will be so low in some conditions, there will be higher NOx levels in the air than what’s coming out of the tailpipe,” Johnson said.

The California Air Resources Board is aiming for implementation of the new ultra-low NOx standards by 2023-’24, Johnson said, adding that the main target is low-load vocational vehicles. According to CARB’s website on its Mobile Source Strategy, “staff … plans to develop regulatory amendments to improve the certification requirements to better reflect emission control under low-load urban driving operations, to improve engine and emission control system durability, and to expand and improve the in-use compliance testing program. These supplemental actions include adopting a new low-load certification test cycle…” Johnson said Southwest Research Institute is currently developing a low-load vocational test cycle.

“We are strapped with an obsolete approach vs. a work-based window method, with a moving window based on CO2,” Johnson said. “The window moves every 30 seconds,” he said, adding that a new NOx value will be calculated every time the window shifts, which is every 30 seconds. The time width of the window is between five and 15 minutes. “The window width is equal to the amount of CO2 emitted on the transient test cycle,” Johnson explained to Biodiesel Magazine after the event. “It will be longer under low load and shorter under high load.” He said while 90 percent of trucks pass the current “obsolete” method, only 12 percent meet the work-based test method.

CARB notes the need for additional federal action. “As vehicles purchased outside the state account for a majority of the heavy-duty vehicle miles travel in the South Coast Air Basin on any given day, federal NOx engine emission standards are necessary in order to achieve emission reductions from vehicles operating in California that were purchased outside the state,” the agency states on its website. “Thus, the U.S. EPA’s action to establish a new national low-NOx standard for heavy-duty trucks is critical. In response to petitions for a low NOx rulemaking from over 20 organizations including state and local air agencies from across the country, on Nov. 13, 2018, EPA announced the ‘Cleaner Truck Initiative’ to develop regulations to further reduce NOx emissions from on-road heavy-duty trucks and engines.”

While the purpose of NOx reduction is to improve air quality, the reduction of greenhouse gas emissions such as CO2 is aimed at mitigating climate change. If governments, the public and markets demand low-carbon transport, then biodiesel is a solution that is available today.

“Biodiesel makes the diesel engine a low-carbon engine,” Howell said.

Biodiesel for on-road transport has many specifications already: ASTM D6751 is the B100 spec for blendstock; ASTM D975, the diesel fuel spec, allows for up to 5 percent biodiesel; and ASTM D7467 was established for B6-B20. But to facilitate biodiesel’s role as an even greater carbon reducer in transport, specs for B21-B99 must be developed. The pathway to new high-level blends was discussed by Howell, who said although developing new biodiesel standards through ASTM has been the traditional route, he suspects a high-blend spec may develop as a trade association standard first before moving through ASTM committee.

“Modern diesel engines with SCR systems emit the lowest CO2 over highway driving,” Johnson said, but he added that ammonia slip, when unreacted diesel exhaust fluid slips passed the catalyst, can lead to unwanted nitrous oxide, or N20, a powerful greenhouse gas. “Natural gas engines, however, emit more CO2 under various test cycles,” he said. “And natural gas leakage of methane on pipelines and in engines can also occur. So natural gas engines are no better, and in many cases worse, than diesel engines regarding global warming emissions.”

Johnson talked about the federal SuperTruck II program to reduce fuel consumption and thereby reduce CO2 emissions from trucks. “Skirts pay for themselves in 9 months,” he said. “Dovetails are also very cost-effective. The goal of SuperTruck II is a 15 percent fuel reduction from the best engines today.” This can be done through a multidimensional approach.

“OEMs want higher-temperature exhaust under less fuel consumption, so insulation and waste-heat recovery are important,” Johnson said. The industry will meet upcoming regulations “by just doing engine stuff better,” he added.

Johnson mentioned new engine technology being developed in San Diego that utilizes two pistons in the same cylinder. He also discussed cylinder deactivation, which is employed on vehicles today to run more efficiently, depending on the load. Predictive GPS can play an important role too, Johnson said. If the system senses a hill coming, then preparations can be made such as providing enough cooling liquid to the engine or programming DPF regeneration to occur during the uphill climb. He said hybridization—a combination of diesel and electric power—is also a good option for reducing emissions, but it is a technology that is more applicable for city trucks vs. long-haul transport. Howell said while a lot of talk about electrification is dominating the vehicle landscape, it’s not easy to electrify heavy-duty trucks.

“Global warming will continue to be at forefront,” Johnson said. “It’s not going away.”


Array ( [REDIRECT_REDIRECT_STATUS] => 200 [REDIRECT_STATUS] => 200 [HTTP_USER_AGENT] => CCBot/2.0 ( [HTTP_ACCEPT] => text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 [HTTP_ACCEPT_LANGUAGE] => en-US,en;q=0.5 [HTTP_ACCEPT_ENCODING] => br,gzip [HTTP_HOST] => [HTTP_CONNECTION] => Keep-Alive [PATH] => /sbin:/usr/sbin:/bin:/usr/bin [SERVER_SIGNATURE] =>
Apache/2.2.15 (CentOS) Server at Port 80
[SERVER_SOFTWARE] => Apache/2.2.15 (CentOS) [SERVER_NAME] => [SERVER_ADDR] => [SERVER_PORT] => 80 [REMOTE_ADDR] => [DOCUMENT_ROOT] => /datadrive/websites/ [SERVER_ADMIN] => [SCRIPT_FILENAME] => /datadrive/websites/ [REMOTE_PORT] => 54060 [REDIRECT_QUERY_STRING] => url=articles/2516579/paving-biodieselundefineds-way-for-greater-use-in-advanced-diesel-systems [REDIRECT_URL] => /app/webroot/articles/2516579/paving-biodieselundefineds-way-for-greater-use-in-advanced-diesel-systems [GATEWAY_INTERFACE] => CGI/1.1 [SERVER_PROTOCOL] => HTTP/1.1 [REQUEST_METHOD] => GET [QUERY_STRING] => url=articles/2516579/paving-biodieselundefineds-way-for-greater-use-in-advanced-diesel-systems [REQUEST_URI] => /articles/2516579/paving-biodieselundefineds-way-for-greater-use-in-advanced-diesel-systems [SCRIPT_NAME] => /app/webroot/index.php [PHP_SELF] => /app/webroot/index.php [REQUEST_TIME_FLOAT] => 1618745145.556 [REQUEST_TIME] => 1618745145 )