NREL/Chevron team characterizes chemical composition and properties of renewable diesels derived from FT, hydrotreating, and fermentation of sugar
A team from the US National Renewable Energy Laboratory (NREL) and Chevron Corporation has examined the chemical composition and properties of several diesel fuels and blendstocks derived from Fischer−Tropsch (FT) synthesis, hydroisomerization of lipids, and fermentation of sugar via the terpenoid metabolic pathway.
In a paper published in the ACS journal Energy & Fuels, they report that the fuels consisted almost entirely of normal and iso-paraffins, with very low levels of residual oxygen impurities (below 0.1 mass %). All of the renewable and synthetic diesel fuels have significantly lower density than typical for a petroleum-derived diesel fuel. As a result, they have slightly higher net heat of combustion on a mass basis (2%−3% higher), but lower heat of combustion on a volume basis (3%−7% lower). Two critical diesel performance properties—cetane number and cloud point—were correlated with iso-paraffin content and chain length.
The use of renewable fuels is growing in many parts of the world for a variety of reasons, including government mandates. Some of these fuels are vastly different from petroleum-derived fuels, and the potential impact on the durability of engine components is a major concern to the manufacturers of engines and vehicles. Petroleum products have historically been the preferred transportation fuels because they offer the best combination of energy density, performance, availability, ease of handling, and price. As a result, engines and vehicles as well as the entire distribution system that transports fuels from the refineries to the end-users, have been designed around petroleum-derived fuels. Thus, there is a practical necessity for alternative fuel blend components to be as close as possible to matching the properties and composition of fuels from petroleum.
...Despite the vastly different feedstocks and processes used to produce these synthetic and renewable diesel fuels, the resulting products are composed of predominantly iso-paraffins and n-paraffins of various carbon numbers in the diesel boiling range. Thus, these fuels should have more similarities than differences regardless of feedstock or manufacturing process. In addition, such paraffins are already the most abundant bulk components of petroleum-derived diesel fuel, and thus are more suitable as “drop-in” blend components than oxygen-containing species. In this work we will focus on how the paraffinic compositions of these renewable and synthetic diesel fuels help explain their chemical and physical property trends.—Smagala et al.
|Why hydrotreating fatty acids and triglycerides produces good diesel blendstock|
|In their paper, Smagala et al. explain that while natural fatty acid chains in triglyceride molecules generally have even carbon numbers, the hydrotreating process converts those fatty acids to n-paraffins with either the same carbon number or one less compared to the parent fatty acid. The preference for even or odd n-paraffins in the products depends upon the reaction conditions, especially temperature and catalyst.|
|Because most oils and fats tend to be rich in fatty acid chains with 18 carbons, the most common products of hydrotreating such feedstocks are n-octadecane (n-C18) and n-heptadecane (n-C17). The next most common fatty acid chain length is 16 carbons, resulting in products from hydrotreating of n-hexadecane (n-C16) and n-pentadecane (n-C15).|
|“Paraffins around these carbon numbers are common bulk components of petroleum-derived diesel. Thus, the products from hydrotreating fatty acids and triglycerides are ideal for use as diesel fuel blendstock. However, the n-paraffins in this range have relatively high melting points, which greatly restrict their low-temperature operability. To counter this drawback, n-paraffins can be subsequently isomerized to form branched alkanes, which have lower melting points than straight-chain paraffins of the same carbon number.”|
—Smagala et al.
For the study, the team examined nine different renewable diesels from commercial producers and pilot plants:
- hydroisomerized vegetable oil/fat
- terpenoid hydrocarbon from sugar fermentation
- hydroisomerized camelina oil
- hydroisomerized algal oil
- hydroisomerized animal fat
- coal to liquid by Fischer−Tropsch
- hydroisomerized algal oil
- hydroisomerized vegetable oil/fat
- hydrogenated animal fat, not isomerized
The team used comprehensive two-dimensional gas chromatographic analysis with nonpolar and polar columns, 13C NMR, GC-MS, and elemental analysis to assess fuel chemistry.
They found that all of the samples were essentially pure saturated hydrocarbons, with either very low or below-detection levels of aromatics, olefins, sulfur, and oxygen. The carbon content of the renewable hydrocarbon fuels was slightly lower than that of the certification ULSD, and the hydrogen content was correspondingly higher, due to the lack of aromatics in the fuels.
Among their other conclusions were:
The subtle differences in properties of hydrotreated renewable and synthetic diesel fuels follow from differences in average carbon number and degree of branching, which vary with feedstock and process conditions.
The trade-off between cetane number and cloud point is a critical feature of these blendstocks, with cloud point and cetane number decreasing with increasing degree of isomerization. Despite this trade-oﬀ, all of the fuels from hydroisomerized fats and oils, as well as the FT diesel, have cetane numbers above 70 with cloud points in the range of −27 °C through −4 °C. The relatively low density—and thus low volumetric net heat of combustion (3%−7% lower) of these exclusively paraffinic fuels—will likely result in lower fuel economies when compared to S15 diesel from petroleum when used at high blend levels.
The many similarities between synthetic FT diesel and isomerized renewable diesel allow one to leverage previous extensive studies with FT diesel in engines and vehicles for the determination of fuel economy and emissions.
The low density explicitly limits the acceptable blend level of such fuels in certain parts of the world with specifications for minimum density.
The large density difference between the petroleum-derived component and either the renewable or synthetic components requires ratio blending or good mixing to avoid stratification.
Certain properties of these renewable and synthetic diesel fuels can be leveraged at refineries and terminals when preparing blends. The high cetane number of these fuels could reduce or eliminate the use of cetane number improver additive, especially for the higher cetane number requirements of European and California diesel. Similarly, these fuels are excellent blend stock for California diesel due their very low aromatics content.
Thomas G. Smagala, Earl Christensen, Krege M. Christison, Rachel E. Mohler, Erica Gjersing, and Robert L. McCormick (2012) Hydrocarbon Renewable and Synthetic Diesel Fuel Blendstocks: Composition and Properties. Energy & Fuels doi: 10.1021/ef3012849