By Ara Barsamian
Three distinct types of biodiesels form the bulk of what is available today:
Biodiesels are also categorized by “generations,” where FAME-type biodiesels are the 1st generation, while the hydrogenated biomass oil and Fischer-Tropsch diesels are part of the 2nd generation.
Today, FAME is defined as the only “official Biodiesel.” Its pure properties (B100) are specified in ASTM D6751 and EN14214. It is made by a trans-esterification process, similar to making soap: vegetable oil + sodium hydroxide + methanol = FAME + glycerin. In the USA, according to ASTM D975-09, FAME can be blended in petroleum-based diesel up to 5 vol%. The resulting B5 diesel does not need to be identified as containing biodiesel. In the EU, according to EN590-2009, FAME can be blended in petroleum-based diesel up to 7 vol%. The resulting B7 diesel needs to be identified as containing biodiesel. FAME generally has poor cold flow properties (i.e., Cloud, CFPP, Pour). So in general, the equipment (e.g., tanks, piping, pumps, and blender manifolds) must be heat traced. FAME has poor oxidation and stability properties. Therefore, it must generally be used within six months of when it was produced before possibly becoming rancid (i.e., “going bad”) and unusable! FAME in B5 or B7s has negligible GHG lifecycle emissions reduction, if any.
This process hydrogenates biomass-origin (i.e., vegetable, wood byproducts, or animal fat) oil in a hydrotreater, possibly followed by isomerization of high paraffins to improve the cold flow properties. The resulting biodiesel is indistinguishable from the petroleum-based diesel. Thus, it can be blended in any amount (not limited to 5 or 7 vol%). Because it cannot be distinguished, there are no special specifications for it. Hydrogenated Biodiesel has very good GHG lifecycle emissions reduction. Several trade names for this type of biodiesel are NexBTL (Neste), Green Diesel (UOP, ENI, Topsoe), etc.
This type of biodiesel is produced using a two-step process:
This process has been used successfully for over 50 years by Sasol in South Africa, albeit with coal instead of biomass for Step 1.
Fischer-Tropsch Biodiesel properties are excellent. Like the Green/NexBTL diesel, it is indistinguishable from petroleum diesel, and thus can be blended in any amount into the petroleum-based diesel. It also has the lowest GHG carbon intensity from all biodiesels.
FAME requires the least capital investment; green diesel is the next higher expense (hydrotreater, H2 plant, maybe isomerization plant), and Fischer-Tropsch diesel is the most expensive (O2 plants, gasifiers, Fischer-Tropsch reactors, etc.). Nevertheless, the advantages of 2nd generation biodiesels in terms of properties, blending, handling, and low carbon/GHG footprint suggests that the future belongs to them.
ASTM approved a new B6 to B20 FAME-type biodiesel blend specification using between 6 to 20 vol% B100 FAME. It is the ASTM D7467-09 standard. Note, however, that B6 to B20 blends must be used with engines specifically approved to work with it, and the tank farm equipment and blenders must be properly segregated against contamination. The B100 FAME specification update is described in ASTM D6751-09.
ASTM D975-09 (automotive diesel) has been updated to explicitly say that it can include up to 5 vol% FAME. ASTM D396 (marine and heating fuel oils) has been updated to explicitly say that it can include up to 5 vol% FAME. Note that all specifications mention 5 vol% (or 7 vol%) biodiesel, not 5.0 or 7.0. This means that you can blend 5.99 vol% biodiesel and still be compliant with B5 specifications!
The following are several references related to biodiesels that are available free on the Internet: