SAF Technology Deep-dive #1: HEFA

We’re starting a new series at Fueling the Future where we will be doing deep-dives into the main technologies powering the SAF industry. For each technology, we will present the following:

• A technical overview of how the process works 

• An economic overview of the major cost drivers

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HEFA (Hydroprocessing Esters and Fatty Acids)

HEFA (Hydroprocessing Esters and Fatty Acids) is the process of producing high-value biofuels from feedstocks such as natural oils and fats through the chemical reactions of hydrocracking and hydroisomerization.

Conversion Process: Turning Biomass into Fuel

Step #0. Feedstock Selection and Preparation

HEFA feedstocks include animal fats, waste cooking oils, and oils from various biomass sources, including vegetable (soybeans, corn) and algal oils. These materials first undergo a cleaning process to remove impurities, leaving primarily triglycerides and free fatty acids. In certain cases, further pretreatment steps are needed such as degumming to remove phospholipids, bleaching to eliminate pigments and metals, and deodorization to extract volatile compounds. All of these are essential to maintaining feedstock purity, which if not done, can reduce conversion efficiency or even poison the reactor. 

The transformation of feedstock into usable biofuels then involves several key stages.

Step #1. Saturation of Oils

The first step in our process is to saturate our oil through catalytic hydrogenation. Saturated oils release more energy per molecule of fuel, improve stability, and increase combustion speed. At a high-level we want to break the double & triple bonds in the oil and replace them with single bonds to new hydrogen molecules. 

Catalytic hydrogenation is achieved through two distinct approaches, differentiated by reaction temperatures, pressures, and catalysts. Higher temperatures use more energy but typically involve cheaper catalysts.

1. High Temperature (150-220°C) with a nickel catalyst.

2. Low Temperature (80-120°C) with a palladium or platinum catalyst under 0.7-4 bar pressure.

For a generic unsaturated fatty acid, the hydrogenation process can be represented as:

R-CH=CH-R' + H₂ → R-CH₂-CH₂-R'

This reaction shows how, in the presence of molecular hydrogen (H₂) and the appropriate catalyst, the double bond (CH=CH) is converted into a single bond (CH₂-CH₂), resulting in a fully saturated fatty acid.

Step #2. Hydrolysis of Triglycerides

Next, we need to break down the triglycerides in the oils through hydrolysis. Hydrolysis splits triglycerides into glycerol and free fatty acids by cleaving the ester bonds through interaction with water molecules. As a byproduct, this process may create propane, which can be utilized as an additional fuel source or sold separately.

Step #3. Oxygen Removal

Our next step is to remove oxygen from the molecules to create long-chain hydrocarbons. There are three standard processes for removing oxygen - each process yielding a different hydrocarbon (octadecane, heptadecane, etc.) as well as a different secondary molecule (water, carbon dioxide, or carbon monoxide) 

1. Hydrodeoxygenation (HDO) RCOOH + 3H₂ → RCH₃ + 2H₂O: Converts carboxylic acids into alkanes, like octadecane, with the addition of hydrogen at high temperatures (300-600°C) using NiMo or CoMo catalysts.

2. Decarboxylation (DCO) RCOOH → RH + CO₂: Removes oxygen by releasing CO₂, producing heptadecane at lower temperatures (250-300°C) and pressures with Pd or Ni catalyst.

3. Decarbonylation RCOOH → R'CH₃ + CO + H₂O: Produces a mix of alkanes, carbon monoxide, and water using Pd and Ni catalysts, similar to decarboxylation.

Step #4. Hydrocracking and Hydroisomerization

The long carbon chains (17-18 carbons) we have produced are now broken down through hydrocracking to produce shorter chains (9-15 carbons), forming synthetic paraffin kerosene (SPK). We discussed in a previous post how the number of carbon atoms in the chain determines the type of fuel you are producing (gasoline vs. diesel vs. SAF). Hydroisomerization, a chemical process that rearranges straight carbon chains into branched-chain paraffins, is then used to enhance cold flow properties by lowering the freezing point due to changes in molecular structure

Step #5. Separation

The resulting hydrocarbons are separated through fractional distillation, which divides lighter hydrocarbons from heavier ones. 

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Economic Considerations: CAPEX, OPEX, and Cost Trends

Creating SAF using the HEFA pathway requires balancing capital expenditures and operating expenditures. Feedstock-related costs account for 51-69%, CAPEX ranges from 22-40%, and OPEX is 8-10%. The primary cost components of non-feedstock OPEX include labor costs, maintenance costs, energy costs, and hydrogen costs.¹

CAPEX

Facility size is a significant CAPEX factor in HEFA SAF production. Larger plants demand higher initial investments but provide greater output potential. For instance, a facility producing 119 million gallons annually, may cost between $229 million and $742 million. Boosting production to 165 million gallons annually increases the cost to between $277 million and $830 million. Smaller facilities, such as those producing 58 million liters annually, cost approximately $110 million. 

OPEX

Feedstock Costs

Feedstock costs constitute the majority of OPEX, representing over 50% of the levelized production cost. Although second-generation feedstocks, such as used cooking oil, can lower costs, their availability is limited. The production of HEFA SAF using waste oil costs approximately $2.75 per gallon, whereas using virgin oils costs about $5.05 per gallon. Even still, both costs are higher than the average price of Jet-A fuel, which is roughly $2.13 per gallon.²

The table above shows the different feedstocks and their respective OPEX, Jet fuel yield, and the Minimum Jet Selling Price (MJSP) which is the lowest price at which renewable jet fuel must be sold to cover all production costs and earn a profit.³

Hydrogen Costs

In the hydrogenation and hydrodeoxygenation (HDO) stages, which eliminate oxygen from fatty acids, hydrogen serves as the main reactant essential for HEFA production. The amount of hydrogen required fluctuates based on the feedstock's saturation requirements and the chosen production technique. Oils such as linoleic and linolenic acids demand more hydrogen to fully saturate the carbon atoms, whereas oils with monounsaturated fats, like olive oil, need less hydrogen. For HEFA production using cooking oil, approximately 0.13 to 0.15 gallons of hydrogen are needed to produce 1 gallon of jet fuel.⁴ In other words, for every 7 cents hydrogen costs go down, HEFA costs go down by approximately 1 cent. Hydrogen costs also vary depending on whether it is generated on-site through hydrolysis or acquired through other methods. For example, steam methane reforming costs range from $0.26 to $1.32 per gallon of hydrogen, electrolysis costs between $0.79 to $1.85 per gallon, and natural gas reforming with carbon capture is about $0.26 per gallon.⁵⁶⁷

Catalyst Costs

Catalysts are used in catalytic hydrogenation, HDO unit, and hydrocracking. While nickel-based catalysts are common, more expensive palladium and platinum catalysts offer higher efficiency and stability. These precious metal catalysts allow for lower operating temperatures and longer lifespans, which can offset their initial cost over time. Different factors that affect the cost are the type of catalyst used, the life cycle of the catalyst, and the amount of catalyst used. 

Cost Trends Over Time:

We believe that the HEFA pathway has limited cost reduction potential over the next decade, for the following reasons. 

1. HEFA as a pathway for diesel has existed for the last 2 decades, meaning that a lot of the process & CAPEX improvements have already been optimized. 

2. The imbalance in supply & demand due to competition for feedstock is driving up prices not bringing them down.

Over the next 30 years, only a modest 23% reduction in production costs is anticipated⁸. These are the areas of improvement we foresee as having the greatest impact.

• New supply opportunities

• Hydrogen production advancements

• Energy efficiency improvements

• Catalyst Improvements

1. https://www.researchgate.net/publication/368555938_Recent_Advances_on_Alternative_Aviation_FuelsPathways_A_Critical_Review

2. https://www.eia.gov/dnav/pet/hist/eer_epjk_pf4_rgc_dpgD.htm

3. https://biotechnologyforbiofuels.biomedcentral.com/articles/10.1186/s13068-017-0945-3

4. https://www.nrel.gov/docs/fy24osti/87803.pdf

5. https://www.sciencedirect.com/topics/engineering/hydrogen-production-cost

6. https://www.catf.us/resource/hydrogen-production-via-electrolysis/

7. https://www.sciencedirect.com/science/article/pii/S2666790822001574

8. https://www.researchgate.net/publication/368555938_Recent_Advances_on_Alternative_Aviation_FuelsPathways_A_Critical_Review