What’s the Difference between Biodiesel and Renewable (Green) Diesel by Jesse Jin Yoon for Advanced Biofuels USA Before addressing the comparison between biodiesel and renewable diesel, it is With the intensifying search for a sufficient important to first define petrodiesel, more alternative to oil-based energy, the commonly known as “diesel fuel” or simply development of alternative energy sources “diesel.” becomes increasingly relevant. Fuels derived from biomass or biological sources Diesel fuel is a petroleum distillate rich in have received much attention. Among the paraffinic hydrocarbons. various alternative fuel options, biodiesel and renewable diesel have been gaining “Petrodiesel is produced from fractional traction in popularity. distillation of crude oil between 200C (392F) and 350C (662F) at atmospheric Although both fuels can be derived from pressure, resulting in a mixture of carbon biomass, they are two distinctly different chains that typically contain between 8 and fuels. The purpose of this paper is to define 21 carbon atoms per molecule. [1] ” both fuels and provide a general comparison between the two fuels. Petrodiesel falls under the specifications outlined by ASTM D975 in the United States and EN 590 in Europe. DEFINITIONS The distillation process of crude oil into its Petroleum diesel (Petrodiesel) fractions, including diesel, is depicted below in Figure 1. Figure 1. Crude oil is separated into fractions by fractional distillation. The fractions at the top of the fractionating column have lower boiling points than the fractions at the bottom. The heavy bottom fractions are often cracked into lighter, more useful products. All of the fractions are processed further in other refining units. [2] Biodiesel Biodiesel is defined under the standard of Biodiesel is produced using a ASTM D6751 as “a fuel comprised of transesterification process, “reacting mono-alkyl esters of long-chain fatty acids vegetable oils or animal fats catalytically derived from vegetable oils or animal fats.” with a short-chained aliphatic alcohol Biodiesel is also referred to as FAME (fatty (typically methanol or ethanol).” Glycerol is acid methyl ester) or RME (rape seed a by-product of this transesterification methyl ester) in Europe. process. [3] Figure 2 Figure 2. Transesterification of triglycerides from animal fats or plant oil (1) with methanol (2) to yield biodiesel (3) and glycerol (4). [3] Biodiesel is chemically different from • Animal fats including tallow, lard, yello petrodiesel and renewable diesel because it w grease, chicken fat, and the by- contains oxygen atoms (note the “O” in the products of the production of Omega-3 biodiesel (3) structure above). This leads to fatty acids from fish oil. different physical properties for biodiesel. • Algae, which can be grown using waste [13] materials such as sewage and without displacing land currently used for food Biodiesel is created using a large variety of production. feed stocks. [4] • Oil from halophytes such as salicornia bigelovii • Virgin oil feedstock; rapeseed and soybean oils are most commonly Biodiesel can be used in its pure form, or used, soybean oil alone accounting for blended with petrodiesel as an additive. about ninety percent of all fuel stocks in Biodiesel in its pure form is designated the US. It also can be obtained B100 where the “100” refers to 100% from field pennycress and jatropha and biodiesel. Biodiesels blended with other crops such petrodiesel follow a similar nomenclature. as mustard, flax, sunflower, palm For instance, a blended fuel comprised of oil, coconut, and hemp. 20% biodiesel and 80% petrodiesel is called • Waste vegetable oil (WVO); B20. Renewable (Green) Diesel There are three primary methods for creating renewable diesel, hydrotreating, thermal Renewable Diesel, often called “green conversion, and Biomass-to-Liquid. [8] diesel” or “second generation diesel,” refers to petrodiesel-like fuels derived from Renewable diesel can be made from the biological sources that are chemically not same feed stocks as biodiesel since both esters and thus distinct from biodiesel. require the tricylglycerol containing material Renewable diesel is chemically the same as from biomass. petrodiesel, but it is made of recently living biomass. [5] Renewable diesel blends follow the same nomenclature as biodiesel. Renewable The definition of renewable diesel is not as diesel in its pure form is designated R100 straight forward as that of biodiesel. The while a blend comprised of 20% renewable term “renewable diesel” has been defined diesel and 80% petrodiesel is called R20. separately by the Department of Energy Because renewable diesel is chemically the (DOE) with the Internal Revenue Service same as petrodiesel, it can be mixed with (IRS) and the Environmental Protection petrodiesel in any proportion but users may Agency (EPA). The specifics of the need to add an additive to address lubricity definitions can be found in the appendix. issue associated with compounds with no [6] oxygen. In addition, the terms renewable diesel and FUEL PRODUCTION green diesel have been further distinguished based on the processing method to create the Biodiesel fuel with petrodiesel-like chemical composition. A brief description of this can Transesterification be found in the appendix. [7] Transesterification is a chemical process where an ester is reacted with an alcohol to For the purpose of this discussion, the term form another ester and another alcohol. For “renewable diesel” will refer to all diesel the creation of biodiesel, triglyceride oils fuels derived from biomass that meet the (esters) are reacted with methanol (alcohol) standards of ASTM D975 and are not mono- to produce biodiesel (fatty acid alkyl esters) alkyl esters. and glycerin (alcohol). The process can be seen below in figure 3 where R1, R2, and R3 are long hydrocarbon chains, often called fatty acid chains. [9] Figure 3 As shown in the diagram above (Figure 3), transesterification process. Feedstocks with the triglyceride contains three separate ester more than 4% free fatty acids, which include functional groups and can react with three inedible animal fats and recycled greases, molecules of methanol to form three methyl must be pretreated in an acid esterification esters (fatty esters) and glycerol (glyceride). process. This process reacts the feedstock The catalyst for this reaction is sodium with an alcohol such as methanol in the hydroxide or another strong base such as presence of a strong acid catalyst such as potassium hydroxide. These hydroxides sulfuric acid in order to convert the free fatty cause the methanol to dissociate and acids into biodiesel. The remaining produce the methoxide ion, which is the trigylcerides are converted to biodiesel using actual catalytic agent that drives the reaction the transesterification process. forward to create biodiesel. The overall biodiesel production process is Some feedstocks require a pretreatment outlined in Figure 4 below. [10] reaction before they can go through the Biodiesel Production Process (Figure 4) dewatered and then fed to the • Acid Esterification. Oil feedstocks transesterification process. containing more than 4% free fatty acids go • Transesterification. Oil feedstocks through an acid esterification process to containing less than 4% free fatty acids are increase the yield of biodiesel. These filtered and preprocessed to remove water feedstocks are filtered and preprocessed to and contaminants and then fed directly to remove water and contaminants, and then the transesterification process along with fed to the acid esterification process. The any products of the acid esterification catalyst, sulfuric acid, is dissolved in process. The catalyst, potassium hydroxide, methanol and then mixed with the pretreated is dissolved in methanol and then mixed oil. The mixture is heated and stirred, and with and the pretreated oil. If an acid the free fatty acids are converted to esterification process is used, then extra base biodiesel. Once the reaction is complete, it is catalyst must be added to neutralize the acid added in that step. Once the reaction is complete, the major co-products, biodiesel Renewable Diesel and glycerin, are separated into two layers. • Methanol recovery. The methanol is Hydrotreating (hydroprocessing or typically removed after the biodiesel and hydrodeoxygenation) “Green Diesel” glycerin have been separated, to prevent the Process reaction from reversing itself. The methanol The hydrotreating process is a process is cleaned and recycled back to the utilized by petroleum refineries today to beginning of the process. remove contaminants such as sulfur, • Biodiesel refining. Once separated from nitrogen, condensed ring aromatics, or the glycerin, the biodiesel goes through a metals. In this process, feedstock is reacted clean-up or purification process to remove with hydrogen under elevated temperature excess alcohol, residual catalyst and soaps. and pressure to change the chemical This consists of one or more washings with composition of the feedstock. In the case of clean water. It is then dried and sent to renewable diesel, hydrogen is introduced to storage. Sometimes the biodiesel goes the feedstock in the presence of a catalyst to through an additional distillation step to remove other atoms such as sulfur, oxygen produce a colorless, odorless, zero-sulfur and nitrogen to convert the triglyceride biodiesel. molecules into paraffinic hydrocarbons. In • Glycerin refining. The glycerin by- addition to creating