Renewable Diesel Primer
Given the recent news that biodiesel has caused buses in Minnesota to malfunction in cold weather, I thought this would be a good time to review the differences between diesel, biodiesel, and green diesel. In order to explain the key issues, I am going to excerpt from the chapter on renewable diesel that I wrote for Biofuels, Solar and Wind as Renewable Energy Systems: Benefits and Risks.
First, what happened in Minnesota?
All schools in the Bloomington School District will be closed today after state-required biodiesel fuel clogged in school buses Thursday morning and left dozens of students stranded in frigid weather, the district said late Thursday.
Rick Kaufman, the district’s spokesman, said elements in the biodiesel fuel that turn into a gel-like substance at temperatures below 10 degrees clogged about a dozen district buses Thursday morning. Some buses weren’t able to operate at all and others experienced problems while picking up students, he said.
And in case you think this was an isolated incident:
The decision to close school today came after district officials consulted with several neighboring districts that were experiencing similar problems. Bloomington staffers tried to get a waiver to bypass the state requirement and use pure diesel fuel, but they weren’t able to do so in enough time, Kaufman said. They also decided against scheduling a two-hour delay because the temperatures weren’t expected to rise enough that the problem would be eliminated.
What is Biodiesel?
Biodiesel is defined as the mono-alkyl ester product derived from lipid1 feedstock like SVO or animal fats (Knothe 2001). The chemical structure is distinctly different from petroleum diesel, and biodiesel has somewhat different physical and chemical properties from petroleum diesel.
Biodiesel is normally produced by reacting triglycerides (long-chain fatty acids contained in the lipids) with an alcohol in a base-catalyzed reaction (Sheehan 1998) as shown in Figure 1. Methanol, ethanol, or even longer chain alcohols may be used as the alcohol, although lower-cost and faster-reacting methanol2 is typically preferred. The primary products of the reaction are the alkyl ester (e.g., methyl ester if methanol is used) and glycerol. The key advantage over straight vegetable oil (SVO) is that the viscosity is greatly reduced, albeit at the cost of additional processing and a glycerol byproduct.
The key thing to note here is that biodiesel contains oxygen atoms (the ‘O’ in the biodiesel structure above), but petroleum diesel and green diesel do not. This leads to different physical properties for biodiesel.
Biodiesel is reportedly nontoxic and biodegradable (Sheehan et al. 1998). An EPA study published in 2002 showed that the impact of biodiesel on exhaust emissions was mostly favorable (EPA 2002). Compared to petroleum diesel, a pure blend of biodiesel was estimated to increase the emission of NOx by 10%, but reduce emissions of carbon monoxide and particulate matter by almost 50%. Hydrocarbon emissions from biodiesel were reduced by almost 70% relative to petroleum diesel. However, other researchers have reached different conclusions. While confirming the NOx reduction observed in the EPA studies, Altin et al. determined that both biodiesel and SVO increase CO emissions over petroleum diesel (Altin et al. 2001). They also determined that the energy content of biodiesel and SVO was about 10% lower than for petroleum diesel. This means that a larger volume of biodiesel consumption is required per distance traveled, increasing the total emissions over what a comparison of the exhaust concentrations would imply.
The natural cetane3 number for biodiesel in the 2002 EPA study was found to be higher than for petroleum diesel (55 vs. 44). Altin et al. again reported a different result, finding that in most cases the natural cetane numbers were lower for biodiesel than for petroleum diesel. These discrepancies in cetane results have been attributed to the differences in the quality of the oil feedstock, and to whether the biodiesel had been distilled (Van Gerpen 1996).
A major attraction of biodiesel is that it is easy to produce. An individual with a minimal amount of equipment or expertise can learn to produce biodiesel. With the exception of SVO, production of renewable diesel by hobbyists is limited to biodiesel because a much larger capital expenditure is required for other renewable diesel technologies.
Biodiesel does have characteristics that make it problematic in cold weather conditions. The cloud and pour points4 of biodiesel can be 20° C or higher than for petroleum diesel (Kinast 2003). This is a severe disadvantage for the usage of biodiesel in cold climates, and limits the blending percentage with petroleum diesel in cold weather.
Another form of renewable diesel is ‘green diesel.’ Green diesel is chemically the same as petroleum diesel, but it is made from recently living biomass. Unlike biodiesel, which is an ester and has different chemical properties from petroleum diesel, green diesel is composed of long-chain hydrocarbons, and can be mixed with petroleum diesel in any proportion for use as transportation fuel. Green diesel technology is frequently referred to as second-generation renewable diesel technology.
There are two methods of making green diesel. One is to hydroprocess vegetable oil or animal fats. Hydroprocessing may occur in the same facilities used to process petroleum. The second method of making green diesel involves partially combusting a biomass source to produce carbon monoxide and hydrogen – syngas – and then utilizing the Fischer-Tropsch reaction to produce complex hydrocarbons. This process is commonly called the biomass-to-liquids, or BTL process.
Hydroprocessing is the process of reacting a feed stock with hydrogen under elevated temperature and pressure in order to change the chemical properties of the feed stock. The technology has long been used in the petroleum industry to ‘crack’, or convert very large organic molecules into smaller organic molecules, ranging from those suitable for liquid petroleum gas (LPG) applications through those suitable for use as distillate fuels.
In recent years, hydroprocessing technology has been used to convert lipid feed stocks into distillate fuels. The resulting products are a distillate fuel with properties very similar to petroleum diesel, and propane (Hodge 2006). The primary advantages over first-generation biodiesel technology are: 1). The cold weather properties are superior; 2). The propane byproduct is preferable over glycerol byproduct; 3). The heating content is greater; 4). The cetane number is greater; and 5). Capital costs and operating costs are lower (Arena et al. 2006).
A number of companies have announced renewable diesel projects based on hydroprocessing technology. In May 2007 Neste Oil Corporation in Finland inaugurated a plant that will produce 170,000 t/a of renewable diesel fuel from a mix of vegetable oil and animal fat (Neste 2007). Italy’s Eni has announced plans for a facility in Leghorn, Italy that will hydrotreat vegetable oil for supplying European markets. Brazil’s Petrobras is currently producing renewable diesel via their patented hydrocracking technology (NREL 2006). And in April 2007 ConocoPhillips, after testing their hydrocracking technology to make renewable diesel from rapeseed oil in Whitegate, Ireland, announced a partnership with Tyson Foods to convert waste animal fat into diesel (ConocoPhillips 2007).
Like biodiesel production, which normally utilizes fossil fuel-derived methanol, hydroprocessing requires fossil fuel-derived hydrogen5. No definitive life cycle analyses have been performed for diesel produced via hydroprocessing. Therefore, the energy return and overall environmental impact have yet to be quantified.
When an organic material is burned (e.g., natural gas, coal, biomass), it can be completely oxidized (gasified) to carbon dioxide and water, or it can be partially oxidized to carbon monoxide and hydrogen. The latter partial oxidation (POX), or gasification reaction, is accomplished by restricting the amount of oxygen during the combustion. The resulting mixture of carbon monoxide and hydrogen is called synthesis gas (syngas) and can be used as the starting material for a wide variety of organic compounds, including transportation fuels.
Syngas may be used to produce long-chain hydrocarbons via the Fischer-Tropsch (FT) reaction. The FT reaction, invented by German chemists Franz Fischer and Hans Tropsch in the 1920s, was used by Germany during World War II to produce synthetic fuels for their war effort. The FT reaction has received a great deal of interest lately because of the potential for converting natural gas, coal, or biomass into liquid transportation fuels. These processes are respectively referred to as gas-to-liquids (GTL), coal-to-liquids (CTL), and biomass-to-liquids (BTL), and the resulting fuels are ‘synthetic fuels’ or ‘XTL fuels’. Of the XTL processes, BTL produces the only renewable fuel, as it utilizes recently anthropogenic (atmospheric) carbon.
Renewable diesel produced via BTL technology has one substantial advantage over biodiesel and hydrocracking technologies: Any source of biomass may be converted via BTL. Biodiesel and hydrocracking processes are limited to lipids. This restricts their application to a feedstock that is very small in the context of the world’s available biomass. BTL is the only renewable diesel technology with the potential for converting a wide range of waste biomass.
Like GTL and CTL, development of BTL is presently hampered by high capital costs. According to the Energy Information Administration’s Annual Energy Outlook 2006, capital costs per daily barrel of production are $15,000-20,000 for a petroleum refinery, $20,000-$30,000 for an ethanol plant, $30,000 for GTL, $60,000 for CTL, and $120,000-$140,000 for BTL (EIA 2006).
While a great deal of research, development, and commercial experience has gone into FT technology in recent years6, biomass gasification biomass gasification technology is a relatively young field, which may partially explain the high capital costs. Nevertheless, the technology is progressing. Germany’s Choren is building a plant in Freiberg, Germany to produce 15,000 tons/yr of their SunDiesel® product starting in 2008 (Ledford 2006).
Straight Vegetable Oil
Unmodified vegetable-derived triglycerides, commonly known as vegetable oil, may also be used to fuel a diesel engine. Rudolf Diesel demonstrated the use of peanut oil as fuel for one of his diesel engines at the Paris Exposition in 1900 (Altin et al. 2001). Modern diesel engines are also capable of running on straight (unmodified) vegetable oil (SVO) or waste grease, with some loss of power over petroleum diesel (West 2004). Numerous engine performance and emission tests have been conducted with SVO derived from many different sources, either as a standalone fuel or as a mixture with petroleum diesel (Fort and Blumberg 1982, Schlick et al. 1988, Hemmerlein et al. 1991, Goering et al. 1982).
The advantage of SVO as fuel is that a minimal amount of processing is required, which lowers the production costs of the fuel. The energy return for SVO, defined as energy output over the energy required to produce the fuel, will also be higher due to the avoidance of energy intensive downstream processing steps.
There are several disadvantages of using SVO as fuel. The first is that researchers have found that engine performance suffers, and that hydrocarbon and carbon monoxide emissions increase relative to petroleum diesel. Particulate emissions were also observed to be higher with SVO. However, the same studies found that nitrogen oxide (NOx) emissions were lower for SVO (Altin et al. 2001). On long-term tests, carbon deposits have been found in the combustion chamber, and sticky gum deposits have occurred in the fuel lines (Fort and Blumberg 1982). SVO also has a very high viscosity relative to most diesel fuels. This reduces its ability to flow, especially in cold weather. This characteristic may be compensated for by heating up the SVO, or by blending it with larger volumes of lower viscosity diesel fuels.
In order to understand the potential problems with biodiesel under cold weather conditions, it is important to understand that biodiesel is chemically different from petroleum or green diesel – and thus should not be expected to have the same chemical properties. Biodiesel is an ester, while petroleum and green diesel are hydrocarbons. The only reason it is called ‘diesel’ is that it can fuel a diesel engine. Likewise vegetable oil, butanol, and even ethanol blends could be called ‘diesels’, as each of these can be used to fuel a diesel engine.
Finally, it should also be noted that petroleum diesel is not immune from cold weather gelling. It is just that these problems don’t begin to occur until the temperatures are much lower than those at which biodiesel begins to gel. If extremely cold weather conditions are likely, then petroleum diesel is blended differently. More kerosene is put into the mixture, which is a lighter diesel (and has a shorter carbon chain length and is just a little heavier than gasoline) and is referred to as #1 diesel.
1. Lipids are oils obtained from recently living biomass. Examples are soybean oil, rapeseed oil, palm oil, and animal fats. Petroleum is obtained from ancient biomass and will be specifically referred to as ‘crude oil’ or the corresponding product ‘petroleum diesel.’
2. Methanol is usually produced from natural gas, although some is commercially produced from light petroleum products or from coal. Methanol therefore represents a significant – but often overlooked – fossil fuel input into the biodiesel process.
3. The cetane number is a measure of the ignition quality of diesel fuel based on ignition delay in a compression ignition engine. The ignition delay is the time between the start of the injection and the ignition. Higher cetane numbers mean shorter ignition delays and better ignition quality.
4. The cloud point is the temperature at which the fuel becomes cloudy due to the precipitation of wax. The pour point is the lowest temperature at which the fuel will still freely flow.
5. Hydrogen is produced almost exclusively from natural gas.
6. Companies actively involved in developing Fischer-Tropsch technology include Shell, operating a GTL facility in Bintulu, Malaysia since 1993; Sasol, with CTL and GTL experience in South Africa; and ConocoPhillips and Syntroleum, both with GTL demonstration plants in Oklahoma.
Altin, R., Cetinkaya S., & Yucesu, H.S. (2001). The potential of using vegetable oil fuels as fuel for Diesel engines. Energy Convers. Manage. 42, 529–538.
Arena, B.; Holmgren, J.; Marinangeli, R.; Marker, T.; McCall, M.; Petri, J.; Czernik, S.; Elliot, D.; & Shonnard, D. (2006, September). Opportunities for Biorenewables in Petroleum Refineries (Paper presented at the Rio Oil & Gas Expo and Conference, Instituto Braserileiro de Petroleo e Gas).
ConocoPhillips. (2007). ConocoPhillips and Tyson Foods Announce Strategic
EIA, Energy Information Administration. (2006). Annual Energy Outlook 2006. DOE/EIA-0383, 57-58.
Fort, E. F. & Blumberg, P. N. (1982). Performance and durability of a turbocharged diesel fueled with cottonseed oil blends. (Paper presented at the International Conference on Plant and Vegetable Oils as Fuel, ASAE).
Goering C.E., Schwab, A. Dougherty, M. Pryde, M. & Heakin, A. (1981). Fuel properties of eleven vegetable oils. (Paper presented at the American Society of Agricultural Engineers meeting,
Hodge, C. (2006). Chemistry and Emissions of NExBTL. (Presented at the
Hemmerlein M., Korte V., & Richter HS. (1991). Performance, exhaust emission and durability of modern diesel engines running on rapeseed oil. SAE Paper 910848.
Kinast, J. NREL, National Renewable Energy Laboratory. (2003). Production of Biodiesels from Multiple Feed-stocks and Properties of Biodiesels and Biodiesel/Diesel Blends. NREL/SR-510-31460.
Knothe, G. (2001). Historical perspectives on vegetable oil-based diesel fuels. INFORM 12 (11), 1103–7.
Ledford, H. (2006). Liquid fuel synthesis: Making it up as you go along. Nature 444, 677 – 678.
Neste Oil Corporation. (2007). Neste Oil inaugurates new diesel line and biodiesel plant at Porvoo. Retrieved July 21, 2007 from http://www.nesteoil.com/default.asp?path=1,41,540,1259,1260,7439,8400
NREL, National Renewable Energy Laboratory. (2006). Biodiesel and Other Renewable Diesel Fuels, NREL/FS-510-40419 Sheehan, J. NREL, National Renewable Energy Laboratory. (1998). An Overview of Biodiesel and Petroleum Diesel Life Cycles, NREL/TP-580-24772.
Schlick M. L., Hanna, M. A., & Schinstock, J. L. (1988). Soybean and sunflower oil performance in diesel engine. ASAE 31 (5).
Van Gerpen, J. (1996). Cetane Number Testing of Biodiesel. (Paper presented at the Third Liquid Fuel Conference: Liquid Fuel and Industrial Products from Renewable Resources,
West, T. (2004). The Vegetable-Oil Alternative. [Electronic version]. Car and Driver. Retrieved June 28, 2007 from http://www.caranddriver.com/article.asp?section_id=4&article_id=7818
Book Chapter Outline
While I have posted extended excerpts from my book chapter, I covered quite a bit more material in there. Here is the full chapter outline:
Renewable Diesel by Robert Rapier
1. The Diesel Engine
2. Ecological Limits
3. Straight Vegetable Oil (SVO)
4.1.1. Definition/Production Process
4.1.2. Fuel Characteristics
4.1.3. Energy Return
4.1.4. Glycerin Byproduct
5. Green Diesel
22.214.171.124. BTL – Gasification/Fischer-Tropsch
6.1.1. Soybean Oil
6.1.2. Palm Oil
6.1.3. Rapeseed Oil
6.1.6. Animal Fats
6.1.7. Waste Biomass
8. Conversion Factors and Calculations
8.1. Conversion Factors