R99 renewable diesel holds much less dissolved water than FAME biodiesel. Mixing these fuels changes how the finished blend handles water, cold temperatures, and storage.
Commercially formulated blends can perform well when they meet fuel specifications and engine requirements. However, mixing unknown fuels in a storage tank can create an unpredictable product—and blending should never be used as a substitute for removing water.
R99 and FAME Are Different Fuels
R99 renewable diesel is a hydrocarbon fuel commonly manufactured by hydrotreating renewable feedstocks. Its chemistry resembles petroleum diesel.
FAME—fatty acid methyl esters—is conventional biodiesel. It has a different chemical structure and is commonly sold in blends such as B5 or B20. B20 contains 20% FAME biodiesel; the remaining portion can include petroleum diesel or renewable diesel.
The U.S. Department of Energy distinguishes renewable diesel, which meets ASTM D975, from biodiesel blendstock, which meets ASTM D6751. The finished blend must meet the applicable specification.
Which Fuel Holds More Water?
FAME holds substantially more dissolved water than R99 at the same temperature. As FAME content increases, the blend generally gains the ability to keep more water dissolved.
One NREL study measured water-saturation values of approximately 85 ppm for the tested renewable diesel and 1,567 ppm for the tested B100 FAME. These are examples from specific samples, not universal limits. Fuel composition and temperature affect the actual saturation point.
| Fuel | General dissolved-water behavior |
| R99 renewable diesel | Low water-holding capacity |
| Petroleum diesel | Low water-holding capacity |
| FAME blends such as B5 and B20 | Generally increasing capacity as FAME content rises |
| B100 FAME biodiesel | Substantially greater water-holding capacity |
Greater water-holding capacity does not mean better fuel quality. It means more water can remain present without forming a visible layer.
Clear Fuel Can Still Contain Water
Water occurs in fuel in three forms:
- Dissolved water: Water distributed at the molecular level. The fuel may look clear.
- Suspended or emulsified water: Small droplets dispersed through the fuel, sometimes producing haze.
- Free water: Separated water that typically collects at the tank bottom.
A Karl Fischer test measures water in the tested sample. It does not, by itself, identify how much was dissolved and how much was present as droplets. Representative sampling is essential.
For example, 225 ppm by weight equals 0.0225% water. That result alone does not establish the FAME percentage, prove that all water is dissolved, or determine whether the fuel meets every applicable requirement.
Why Avoid Uncontrolled Blending?
1. Water Can Separate After Mixing
When a FAME-containing fuel is diluted with R99, the resulting blend may hold less dissolved water than the original fuel. If the water content exceeds the new blend’s saturation limit, some water can separate into droplets.
Cooling can also reduce water solubility. Fuel that appears clear during delivery may develop water haze or separated water after conditions change.
This separation is not inevitable. It depends on the starting water content, temperature, and final composition.
2. Adding FAME Does Not Remove Water
Adding FAME may allow more water to remain dissolved. The fuel could appear clearer while still containing the same water.
Making water less visible is not water removal. Adding dry fuel can lower the measured concentration through dilution, but it does not extract the water already in the tank.
3. Cold-Weather Performance Can Change
Adding FAME to a suitable winter-grade R99 can raise the blend’s cloud point. Depending on the feedstock and concentration, crystals may form at temperatures where the original R99 would remain usable.
The cold-weather performance of the finished mixture must be established rather than assumed from either component.
4. Storage Stability Can Change
FAME can reduce a blend’s resistance to oxidation. As fuel ages, oxidation can contribute to acids, gums, and deposits.
This matters for standby generators and other equipment where fuel remains in storage for extended periods. Mixing a fresh delivery with old fuel does not establish that the resulting mixture is suitable for continued storage.
5. The Final Blend May Exceed Equipment Requirements
FAME can improve lubricity, but it also changes density, viscosity, and evaporation characteristics. These changes become more significant at higher blend levels.
An engine approved for R99 is not automatically approved for every R99/FAME mixture. Confirm the allowable biodiesel percentage and finished-fuel requirements with the equipment manufacturer.
Can Fuel Polishing Fix the Problem?
Fuel polishing can remove solids and, with suitable equipment, free and dispersed water. Ordinary particle filtration, settling, and centrifuging do not directly remove truly dissolved water.
Reducing dissolved water requires an appropriate drying process, such as fuel-compatible vacuum dehydration or a properly selected adsorbent system. Polishing also cannot reverse chemical degradation caused by oxidation.
Water-block filters serve a different purpose. Cim-Tek describes Hydrosorb primarily as a water-alert filter that restricts flow when excess water is detected. It should not be assumed to dry fuel to a specified ppm level.
Know What Is Going Into Your Tank
Before combining deliveries, confirm the fuel type, FAME percentage, water condition, and equipment requirements. Check a separate tank-bottom sample for accumulated water, and use laboratory testing when precise measurements are needed.
Avoid blending unknown fuels to correct contamination. Identify the problem, remove the contamination, and verify the finished fuel. A controlled, documented blend is different from mixing whatever fuel happens to be available.
Dieselcraft helps operators assess contamination and select practical fuel-testing and polishing equipment.
Call 530-613-2150 or visit dieselcraft.com.
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