A. Ditsch, Oleo-X, Pascagoula, Mississippi
Throughout the past decade, biofuels production has significantly increased. Some of the largest growth in the sector has been with fuels that rely on lipids, such as vegetable oils and animal fats, as feedstock. This includes biodiesel, renewable diesel and sustainable aviation fuel (SAF).
This growth is projected to continue over the next decade, as both producers and users increasingly rely on more sustainable fuel alternatives. Renewable diesel production is expected to more than double by 2025, according to the U.S. Energy Information Administration (EIA),1 while SAF production is expected to grow more than 30 fold by 2027 compared to a 2021 baseline, according to the International Energy Agency (IEA).2 This growth is already putting significant pressure on feedstock availability, with > 40% of soy oil and > 50% of inedible tallow in the U.S. currently going to biofuels uses.3,4 Projections are that > 65% of all inedible and waste animal fats will be required to meet biofuel demand through 2027, and this demand is forecast to rise to 95% if no new sources of waste fats are utilized.2
While many animal and waste fats have been increasingly utilized for biofuels, one source that has not been heavily utilized is poultry fat. In 2022, more than 1 MMt of poultry fat was produced, essentially all of it an inedible byproduct of poultry meat production (FIG. 1).3 This is enough to make > 250 MM gallons (gal) of renewable diesel. However, only 8% of this was actually used to produce biofuels—a percentage that is much lower than that of inedible beef tallow or yellow grease. The remaining poultry fat was largely used as an additive to animal feed. In fact, during the past few years, the use of other waste feedstocks has more than doubled, while poultry fat use has declined (FIG. 2).4 This decline in usage is likely caused by perceived difficulties in handling and processing poultry fat into renewable fuels. However, important advancements are now proving those assumptions wrong.
Prior chemistry hurdles in processing poultry fat. While many potential properties are relevant to processing fats and oils in renewable diesel, two that are especially critical are gum content and flow properties. Crude lipids often contain gums, which can cause storage instability and deposition, as well as flavor and color issues in finished oils. The gums are predominantly composed of phospholipids and are typically removed before a fat or oil is used for human consumption. Since phosphorus is also a catalyst poison in hydrotreater units, a complete degumming is necessary to use this feedstock in renewable diesel facilities. However, there are a wide range of potential gums in crude fats and oils, with some vegetable oils like crude soy oil having exceedingly high gum content, with more than 500 parts per million (ppm) phosphorus.5
Since the gums are about 1/30th phosphorus, concentrations of 500 ppm phosphorus contain approximately 1.5% gums, indicating that a substantial portion of the oil is gums. For these oils to be used in cooking, the phosphorus is reduced to less than 10 ppm, typically around 3 ppm. For vegetable oils, these gums have traditionally been removed by methods such as acid refining and alkali refining.6 Most edible animal fats, such as beef tallow, have much lower gum content. This leads to much simpler gums removal, which can be accomplished by dry degumming that only relies on an adsorptive step to remove gums.7 These processes are much easier to perform on fats that are solid at room temperature and do not flow well. Unfortunately, poultry fat has historically not been degummed since it is inedible. Additionally, its phosphorus content is similar to that found in crude soybean oils, making dry degumming impractical.
As such, poultry fat would seem to present the challenges of both soy and tallow: high gum content and a high melting point. Since all potential feedstocks are critical to meet the demands of the growing renewable diesel market, the author’s company has developed a proprietary process to reduce phosphorus content from nearly 500 ppm to < 1 ppm. Through this process, it has been observed that the flow properties of the oil have improved substantially. This has yielded significant improvements in both pour point—the lowest temperature at which an oil will flow freely—and viscosity, making the oil much easier to handle than inedible tallow.
While crude poultry fat typically has a lower melting point than beef tallow8 (23°C–40°C vs. 40°C–50°C), it is often treated similarly with heat tracing for flowlines and heated storage. The crude poultry fat that the author’s company has treated typically has a slightly lower melting temperature, with a pour point of about 15°C and a phosphorus content of 486 ppm, indicating about 1.5% of the material is composed of gums.
Testing cold flow. When crude poultry fat is degummed, reducing phosphorus from 486 ppm to less than 1 ppm, the flow properties improve substantially. The following materials were subsequently tested for pour point (via ASTM D97) and viscosity (via ASTM 7042) to quantify what was seen in the processing facility:
Pour point. Pour point is the lowest temperature at which an oil will flow freely. The results of the pour point testing are summarized in FIG. 3. Tests revealed that the crude poultry fat had a pour point of 15°C, which means it is very close to solid at room temperature. However, the partially and fully degummed poultry fat has a much lower pour point of 3°C. This means that in all but near-freezing conditions, the material will flow freely. Soy oil has an even lower pour point of –9°C, and a 50/50 mixture of soy and refined poultry fat has a pour point of 0°C, which means that the pour point is the same as water.
Viscosity. While pour point provides information about the minimum temperature at which a material will flow, it does not indicate how difficult it will be to pump that material at the given temperature. So, the same samples examined during cold flow testing were tested for viscosity at a range of temperatures, and this data is summarized in FIG. 4.
At 40°C, where a typical inedible tallow would barely be near its melting temperature, all tested oils were highly flowable. Soy had the lowest viscosity of about 21 centistokes (cSt), while crude poultry had the highest at 39 cSt. Degummed poultry was slightly lower at 37 cSt, and a 50/50 mixture was 34 cSt. Any processing of the material in a hydrotreater will be conducted at this temperature or higher, where the liquid is only slightly more viscous than soy oil.
At lower temperatures, the impact of degumming becomes more apparent. At 20°C, the crude poultry fat is significantly more viscous (109 cSt), while refined poultry fat is only slightly higher than soy oil (85 cSt vs 65 cSt). At 10°C, crude poultry fat is below the pour point and does not flow, while refined poultry fat is a thick liquid (361 cSt). By blending the poultry fat with soy oil, this becomes much thinner at 175 cSt.
Implications for the biofuel industry. While crude poultry fat may be a difficult material to handle and process due to high melting temperatures and high gum content, fully refined poultry oil is a much easier feedstock to handle and process.
With < 1 ppm phosphorus and < 10 ppm total metals, this processed feedstock can be handled similarly to edible vegetable oils. When the gums are removed, the material exhibits pour points and viscosity that is more like vegetable oils than typical animal fats. The resulting material has roughly half the viscosity of a typical refinery feed, such as vacuum gasoil, and can flow at nearly a 40°C lower temperature than beef tallow and nearly 15°C lower than crude poultry fat.
As renewable diesel production continues to increase and existing feedstocks become fully utilized, refined poultry fat should be considered as an alternative to meet the needs of the industry. HP
LITERATURE CITED
Andre Ditsch is a Technical Advisor and co-inventor of the process at Oleo-X. Dr. Ditsch has been a technical and strategic advisor for a range of firms in the energy industry, including upstream oil and gas, refining, petrochemicals, electrical power and biofuels businesses over the past 20 yr. He earned a PhD in chemical engineering from MIT. For more information, contact oleox@backbaycommunications.com.