Biodiesel is a petroleum diesel fuel alternative produced from vegetable oils and animal fats most commonly through a process called transesterification. The primary product of this chemical reaction are fatty methyl esters (FAME), but it is more commonly referred to as biodiesel. Biodiesel is chemically different from renewable diesel (HVO) despite being produced from the same feedstocks. Although it is technically a suitable fuel for use in any diesel engine, no modifications necessary, there are significant concerns associated with operating engines on pure biodiesel and biodiesel blends. Thus, commercial biodiesel is most commonly blended in low concentrations with petroleum diesel fuel.
The official nomenclature for biodiesel blends is the letter "B" followed by the percentage of biodiesel in the fuel blend, by volume. "B5" refers to a 5% biodiesel, 95% petroleum diesel blend, "B10" refers to a 10% biodiesel, 90% petroleum diesel blend, and so on. The term B100 is used to denote pure, unblended biodiesel. Biodiesel can also be blended with renewable diesel, but the resulting fuel may have poor stability in storage. Such blends follow a similar nomenclature where "B" denotes the biodiesel content and "R" denotes the renewable diesel content. "R95B5" would therefore describe a 5% biodiesel, 95% renewable diesel blend.
A diesel engine could run on pure, untreated vegetable oil but it would have some undesirable results and many challenges to overcome. One of the biggest challenges with running an engine on straight vegetable oil is that it is significantly more viscous (thicker) than petroleum diesel fuel or biodiesel. For it to serve as a fuel, it must be heated to the point that its viscosity is close to that of diesel fuel because diesel fuel systems are not equipped to handle such thick liquids - the cooler the ambient temperatures, the more it must be heated.
How Biodiesel is Made
Biodiesel is surprisingly easy to produce and many people make biodiesel at home from waste cooking oil (WCO). The molecular structure of these oils is that of a triglyceride, which is an ester that contains a glycerol and three long fatty acid chains. A chemical reaction called transesterification is used to split the fatty acid chains from the glycerol molecule and convert them into esters - these esters are biodiesel. Once the reaction has concluded the glycerol is separated from the esters and discarded.
Transesterification
Transesterification is the chemical reaction by which glycerol is separated from the fatty acid chains of a triglyceride molecule and the fatty acids are converted into esters (FAME). The chemical reaction is initiated by combining the feedstock with an alcohol (typically methanol) and a catalyst. The most popular catalysts are strong bases such as potassium hydroxide or sodium hydroxide. It's worth noting that there are a number of other suitable catalysts and science has introduced some interesting methods for producing biodiesel. For the most part, all these catalysts serve the same function in the reaction.
The ingredients are mixed over a heat source and then set aside to rest. As the catalyst breaks apart the fatty acid chains, the denser glycerol molecules settle to the bottom of the vessel. After separating the glycerol, a second treatment is often performed with a smaller dosage of catalyst to ensure that all the triglycerides are broken down and removed. What is left is fatty methyl esters - biodiesel - and some impurities that includes unused catalyst.
Removing the impurities is accomplished through a process called washing. Distilled water is added to the biodiesel and gently agitated. Any remaining alcohol and catalyst is attracted to the water, which settles to the bottom of the vessel in what appears to be cloudy, white, soapy water. The water, impurities, and any sediment is drained off the bottom of the biodiesel. Washing is generally repeated several times over until the water that is removed is clear, which indicates that the impurities have been removed. As a final step, the biodiesel can be heated just above 212 °F so that any residual water or water suspended in the biodiesel boils off. The end result is purified, ready-to-use biodiesel.
Feedstocks
The raw material(s) from which biodiesel is produced are referred to as feedstocks. Biodiesel can be produced from a variety of vegetable oils and animal fats. Since glycerol is removed from the final fuel, biodiesel yields will always be lower than the original input. Different oils have been shown to have different yield rates, and thus economic factors play an important part of feedstock selection in commercial operations. Expensive feedstocks with lower yields, for example, may be less desirable than cheaper feedstocks with the same or higher yields.
WCO has been a popular feedstock for home brewers because restaurants need to cycle their fry oils. Although there was a time when a restaurant might jump at the opportunity to hand off their waste oil to anyone who would take it, commercial producers have disrupted this supply chain and restaurants now very often join programs for which fry oil is picked up by these entities. While some home brewers still obtain free waste oil, this seems to have become an increasingly rarer relationship.
Most of the feedstock used in commercial biodiesel production is virgin and has not previously been used. Soy bean oil is the most commonly used feedstock with canola oil coming in second. Different feedstocks produce fuels with different characteristics. This is due to differences in the fatty acid chain profiles, such as the length of the chains and their exact composition.
For example, biodiesel produced from canola oil has a significantly higher cetane rating than that produced from soy bean oil. On the contrary, canola based biodiesel is much more prone to oxidation than that derived from soy bean oil. Oxidation stability is the most important factor when it comes to fuel storage - fuels that are more resistant to oxidation can be stored longer.
Properties of Biodiesel
As previously mentioned, many of the properties of biodiesel are dependent on the feedstock blend. Vegetable and animal fats produce fuels with varying properties; certain classifications of animal fats and certain types of vegetable oils produce fuels with different characteristics. For this reason, the feedstocks are often blends that aim to maximize beneficial properties and minimize cost. The color of biodiesel varies from almost clear to golden brown, but it can also have a yellow or slightly amber appearance.
Biodiesel that meets ASTM D6751 standards must have a minimum cetane rating of 45, but biodiesel generally has a favorably high cetane rating in the 45 to 65 range, or even greater with certain feedstocks. On average, biodiesel is slightly more viscous than petroleum diesel and has a higher cloud point. This gives biodiesel poor cold weather performance when compared to traditional or renewable diesel fuels.
Biodiesel is an excellent lubricant and a strong solvent. Its advantageous lubricity results in reduced wear inside fuel system components such as injectors and fuel pumps. It is fairly well documented that even low concentration biodiesel blends can provide sufficient lubricity for ULSD to meet ASTM D975 standards, which requires a minimum 520 micron wear scar result from the HFRR test.
The solvency of biodiesel is one of its biggest challenges. Biodiesel can and will dissolve varnish and sediment accumulation in fuel tanks and within other parts of the fuel system. These essentially become contaminants traveling through the fuel system and often result in accelerated loading of the fuel filter(s). Continuous use of biodiesel will theoretically result in these deposits being removed, at which point concerns with fuel filter blockage should subside. More frequent fuel filter replacement is often recommended by engine manufacturers to circumvent problems with plugged fuel filters and the subsequent damage that low fuel flow can cause.
With regard to energy content, biodiesel is approximately 7% less energy dense than traditional diesel fuel [1]. Blending biodiesel at percentages of 20% or less helps to mitigate any performance or fuel economy losses. Using biodiesel results in reduced particulate matter, hydrocarbon, and greenhouse gas emissions at the expense of an increase in NOx emissions. Biodiesel is also more hygroscopic than traditional and renewable diesel fuels, so it will have a higher propensity to absorb water from the ambient air.
[1] - Based on United States Department of Energy average values; 128,488 btu/gal for petroleum diesel fuel, 119,550 for biodiesel. The actual energy content of both fuels can vary.
Advantages
- Excellent lubricity in its as-produced form, additives not required to meet minimum lubricity standards
- Significantly lower CO2, HC, and particulate emissions
- Can be made from a variety of feedstocks, including waste cooking oil
- Environmentally friendly in that it degrades quickly into basic organic compounds (considering spills and leakage)
- High cetane rating
- Can be blended with petroleum diesel fuels
Disadvantages
- Excellent solvent, breaks down deposits and build-up in fuel tanks and fuel system components which can result in filter plugging
- Poor cold weather performance, gelling occurs at a higher temperature than petroleum diesel
- Very limited storage life; FAME is an oxygenate and thus is highly susceptibility to oxidation
- Limited compatibility with many materials, tendency to cause deterioration in certain rubbers commonly found in seals and fuel hoses
- Increased NOx emissions
- High boiling point results in a higher propensity to condense on cylinder walls during DPF regeneration, higher potential for engine deposits and fuel dilution
- Absorbs water in ambient air more readily, heightened propensity for water contamination
- More prone to microbial activity and the resulting contamination
- Lower energy density (by volume)
- Questionable compatibility with other biofuels
- More expensive than petroleum diesel
Controversies
Biodiesel can be made from waste or virgin feedstocks. There are arguments that when it is made from vegetable oils grown solely for the purpose of producing biofuels, agricultural resources and precious farmland are exploited in a manner that could potentially disrupt supply chains and market prices. In this line of reasoning, farmland may be used to grow crops that offer the highest return on investment and this could contribute to shortages and price spikes.
Skeptics also add that an increase in biofuel demand could result in increased deforestation as a means to create suitable land for the cultivation of biofuel feedstocks. Feedstocks that are grown for fuel do not have to meet the same criteria or standards as those grown for consumption, thus quality is not a major concern and these crops do not have to be grown in prime conditions. A high demand for biofuels would require re-purposing existing farmland or finding new territory to expand cultivation.
The life-cycle carbon footprint and renewable nature of biodiesel has also sparked criticism. Some believe that environmental impacts and energy inputs during the production and transportation of the fuel are grossly misrepresented and that, all things considered, biodiesel is not as "environmentally friendly" as some proponents might suggest. This specific point of interest has been studied and analyzed ruthlessly. Reports that are published or accepted at the government level tend to lean in favor of biodiesel and biofuels like it, but they may rely on heavily opportunistic models. One could rebut that there are many incalculable variables at play and that these studies must make broad assumptions.
Domestic automakers have not made any efforts to certify nor approve their vehicles to run on B100. B20 is the current standard by which all modern diesel engines in the U.S. are approved to use. Older vehicles and equipment are often limited to a maximum B5 blend or are not compatible with biodiesel altogether. Modern vehicles were designed with the B20 standard consideration - older vehicles were not. As biodiesel finds its way into today's diesel fuels, it could be the source of expensive equipment failures and downtime, burdening small businesses and individuals alike.
There is also some controversy regarding the labeling of biodiesel. Federal law requires that pumping stations for biomass-based diesel fuels be labeled as such:
(1) Biomass-based diesel blends or biodiesel blends that contain less than or equal to 5 percent biomass-based diesel or biodiesel by volume and that meet ASTM D975 diesel specifications shall not require any additional labels.
(2) Biomass-based diesel blends or biodiesel blends that contain more than 5 percent biomass-based diesel or biodiesel by volume but not more than 20 percent by volume shall be labeled "contains biomass-based diesel or biodiesel in quantities between 5 percent and 20 percent".
(3) Biomass-based diesel or biodiesel blends that contain more than 20 percent biomass based or biodiesel by volume shall be labeled "contains more than 20 percent biomass-based diesel or biodiesel".Source: U.S. Title 42, chapter 152, subchapter II - 42 USC 17021: Biomass-based diesel and biodiesel labeling
The current labeling requirements do not distinguish between biodiesel and biomass-based diesel. Renewable diesel, a biomass-based diesel fuel, is drop-in compatible for any diesel engine and does not face the same compatibility or storage challenges as biodiesel. Biodiesel is not a drop-in replacement and is not necessarily safe to use in all engines, nor is it compatible with many materials. The current labeling standards therefore fail to establish the actual blending components in some diesel fuels.
