Battle Of The Biofuels: Renewable Diesel vs. Biodiesel

by Patrick O’Brien (Opportune LLP/Biofuels Digest) Find out why knowing the difference between renewable diesel and biodiesel can go a long way in understanding the benefits of each.

Fuel standards programs such as the federal Renewable Fuels Standard (RFS-2) and California’s Low-Carbon Fuels Standard (LCFS) have incentivized fuel producers to innovate and create new renewable fuels like biodiesel and renewable diesel. As new low-carbon diesel fuels and blends become more readily available, it’s important to understand the differences between them and how each impacts engine performance.

Diesel Engine Basics
Before jumping into the different fuel options available, we’ll discuss the basics of a diesel engine and how they differ from gasoline engines. In the U.S., diesel engines are commonly used by trains, boats, barges, and heavy-duty trucks, as well as public buses and farming, construction, and military equipment.

Diesel engines have the highest thermal efficiency of any internal combustion engine.

Biodiesel, or sometimes referred to as fatty acid methyl ester (or FAME), is typically produced by reacting vegetable oils or animal fats with methanol or ethanol in a transesterification process to create mono-alkyl esters of long-chain fatty acids. Biodiesel contains oxygen atoms, which makes it chemically distinct from regular ultra-low sulfur diesel (ULSD). Like ethanol blending limits in gasoline engines, most diesel engine manufacturers only support diesel blends up to B20, with higher blends requiring fuel system modifications.

Renewable diesel on the other hand uses the same feedstocks as biodiesel; however, it undergoes hydrotreating, thermal conversion, or biomass-to-liquid production processes. The end result is a fungible renewable fuel chemically identical to ULSD that can be used as a drop-in replacement for all current diesel engines.

The differences between ULSD, biodiesel, and renewable diesel is best illustrated in the table below, but there are a few properties worth discussion.

Unfortunately, neither renewable diesel nor biodiesel can match the energy density of ULSD. Renewable diesel only has a 4.65% decrease compared to biodiesel’s 8.53% decrease. This value is calculated by multiplying the energy density per unit of mass and the volumetric density and unit conversions. However, the energy density isn’t the only important property related to the energy and volumetric densities. Biodiesel’s higher volumetric density is also responsible for its higher cloud point, poorer cold flow properties, and improved lubricity compared to ULSD. Conversely, renewable diesel’s improved cloud point and cold flow properties can both be attributed to its lower volumetric density.

The different chemical properties of the three diesel fuel types significantly impact the resulting GHG, NOx, and particulate matter emissions.

Based on this information, California’s Air Resource Board (CARB) recently announced a change within their Alternative Diesel Fuels program, which would require biodiesel blends greater than 5% to be approved by CARB. To mitigate this increase in NOx emissions, CARB has approved diesel formulations with at least 55% or 75% renewable diesel and at most 20% biodiesel.

Although renewable diesel has rapidly grown in popularity because it’s a drop-in replacement of ULSD and favorable emissions reductions, some blenders have started to mix the two renewable fuels. REG has created a blend of both biodiesel and renewable diesel, coined “REG Ultra Clean Diesel”, to capture the best attributes of each. Ultra Clean Diesel can reduce emissions across the board while offering improved engine starting, lubrication, reliability, engine life, and more complete combustion.

These renewable fuels can reduce lifecycle GHG emissions even more than electric vehicles can since electric vehicles derive their electricity from natural gas, which produces 195% higher GHG emissions on a lifecycle basis than REG B100.

Federal and state governments have realized the potential benefits of biodiesel and renewable diesel and are incentivizing the production of both.

Renewable diesel accounts for 1.7 RINs, while biodiesel accounts for 1.5 RINs. READ MORE