Renewable diesel fuels represent an opportunity to decarbonize on- and off-highway diesel ICE vehicles with a minimal investment in infrastructure and little to no disruption of current fleet operations. However, market acceptance of these fuels has been slow in locations where legislation has not mandated their widespread use.

SAE Media recently interviewed Matt Leuck, technical manager at Neste, to discuss the benefits of renewable diesel for fleets and engine suppliers in terms of reducing greenhouse gases, the feasibility of supplying renewable fuel with the current infrastructure and the cost-offsets that renewable diesel offers in terms of engine performance and reduced maintenance.

Why do you see renewable diesel as the best alternative for a sustainable fuel source?
The future will need all solutions; nobody is going to solve the decarbonization puzzle with a silver bullet. Hydrogen is a hard molecule to work with and there are also transportation concerns. Then there’s the debate between hydrogen being used for combustion or just transfer of energy. There’s a lot of forks in the road for that fuel. Looking at the battery side, you’ve got not only the infrastructure needed, but also the total power demand. This is especially true when you get into medium- and heavy-duty machines. There are also obvious materials concerns. In my eyes, the biggest benefit to renewable diesel is that it’s available right now. There’s no massive infrastructure buildup, we don’t need colossal investments to move it across the country and we’re currently making it on three continents. It’s a drop-in solution.

What challenges do you see in terms of production, distribution and market acceptance of renewable diesel?
From the powertrain perspective, there’s really no concerns. It meets the same ASTM D975 spec that every diesel engine is built to meet. That means we can also work in the exact same infrastructure. In the bigger picture though, there are some things that could change. Right now, the core market for renewable fuels is the west coast of the U.S. because those states have low-carbon programs in place. The fuel is going to follow the legislation. The logistics of how you collect the feedstocks for renewable diesel also present some challenges. You can take oil from Texas and send it down a pipeline across the country and feed all those refineries. For renewable diesel we’re using waste and residues, such as used cooking oil and animal fat, so the aggregation points are more dispersed. It’s a completely different model and mindset.

What are the different feedstocks used to produce these fuels?
We want our feedstock to be a waste product that’s already gone through its first useful life. Cooking oil, animal fat, tallows, fish oils and technical corn oil are all things we can use. There’s also a lot of stuff out there that we’re not even using yet. Before the hydrotreating and isomerization production process, there’s also a pre-treatment. That’s where we could clean up some really nasty feedstocks. We’re really focusing on our pre-treatment process so that we can open up opportunities to increase volume and reduce overall cost.

What happens to the excess material once you’re done with the refining process?
There’s not much honestly. Once we break the molecules down, there’s three hydrocarbons that are kind of ‘stuck’ in the fat molecule. One of the byproducts of this process is renewable propane. At some of our facilities, the plant will take that renewable propane and put it back into the refinery for heat, because it does take a lot of heat to perform our processes. We can also capture that renewable propane and sell it to the market and commercialize that renewable product. Most of our feedstocks also have oxygen in them, so another byproduct of the hydrotreating process is water.

What is the chemical composition that you’re trying to build with these feedstocks?
Picture a bell curve with the X-axis being the carbon chain length and the Y-axis as the cetane rating. Carbon diesel is going to be around a C-10 to C-11 rating (10-11 carbons linked together). That curve is going to peak around 16 or so and then taper off out to around 20 or 22. That is the fossil diesel spectrum. What we shoot for the middle of that spectrum. For renewable diesel, the curve is going to be very flat and have a sharp peak from C-15 to -18, then flatten out. Those are the best diesel molecules. What we’re looking for is fats and residues that contain hydrocarbons of that length (15-18). Those triglycerides come in cooking and other oils. From a chemistry perspective, there’s three of those that are bonded together and all we want to do is find those ideal feedstocks to break apart.

What are the main differences between biodiesel and renewable diesel in terms of production and chemistry?
We use the same feedstocks as biodiesel, but renewable diesel and biodiesel are not the same product. Chemically biodiesel is not a hydrocarbon because it still contains oxygen. That changes the performance characteristics of the fuel as well as how much control the producers have over the final product. Once we perform our first step in the process (hydrotreating) where we break apart that molecule, we get a straight chain of paraffinic hydrocarbons. The only issue with that is the paraffins we make are a solid, so we do one more step called isomerization where we take that straight chain molecule and branch it. Meaning that we move a few molecules to different places. This keeps the same number of hydrogen molecules but reshapes it to make a liquid. The more we isomerize it, the lower we can get the cloud point of the fuel. By isomerizing, we can also guarantee winter performance characteristics that biodiesel cannot.

What are the combustion characteristics of renewable diesel compared to biodiesel?
Renewable diesel has a very high cetane number. Fossil diesel has a cetane of 41 or higher. Biodiesel cetane will usually come in around the mid-40s to lower 50s. Renewable diesel typically has a minimum of 70 and sometimes closer to 80. So that’s really going to promote quicker and cleaner combustion. Both are aromatic free which is good. However, biodiesel still has oxygen which means in combustion, that oxygen will create more NOx. Obviously, we want to reduce NOx, so if you’re looking at pure engine emissions (excluding aftertreatment systems) biodiesel will have an increase in NOx, whereas renewable diesel will have a decrease in NOx.

What is an example of a long-term benefit of running exclusively renewable diesel?
The fuel is incredibly clean. It looks and smells like water and is free of contaminants. Which means it will not start producing injector deposits that decrease the performance of the engine over time. Detroit Diesel did some testing on a DD15 where they artificially aged the fuel and ran it through an engine on a test bed. They were testing to see what would happen first: would the fuel break down and go off spec as time goes on, or would the parts physically fail because of deposits and corrosion? The B5 bio-blend lasted 13 days in the test before the injector seized. B0 pure fossil made it roughly 43 days. The fuel broke down enough that it had a lot of acids and the injector had deposits. After two months of testing with our fuel, they found that it didn’t break down when they pulled the injectors apart there were zero deposits.

What are some other net benefits of renewable diesel for engine OEMs and fleet owners?
Our fuel can help reduce NOx for 2006 and older off-road (pre tier 2) engines by roughly 10%. But post 2010/Tier 4 final engines will see reductions of up to 98%. While this fuel does not enable OEMs to remove SCR or DPF systems, it does offer benefits in overall maintenance because we’re putting 30-40% less soot into those systems. That means fewer regens, fewer ash cleanouts and less stationary regen. For fleet owners, it’s going to be about cost savings per mile. We have a customer in Oregon that told us when they switched to renewable diesel, they started saving two cents per mile over fossil diesel. According to their telematics data, 1.5-cents of that was from reduced DPF maintenance. They also do active oil sampling, which means they change it when it needs to be changed and not just on an interval. Because the oil was being contaminated less, they’re saving about a half-cent per mile.

What are the biggest challenges of market acceptance for renewable diesel?
I think the biggest challenge is education. Over the past five years, it’s become much more accepted. As of last year, more than half of California’s diesel consumption was renewable. We’re at a point now where in areas where we have renewable diesel, we’re not getting as many questions because the fleet owners are talking to each other. It really is about getting people to understand that it’s not biodiesel, that it really is a drop-in replacement and getting some pilot programs out there to show people that it can be done.