Fuel Chemistry in Storage and Handling

NREL researchers seek to reveal underlying reaction mechanisms and details of how the fuel storage environment impacts stability.

Fuels must be handled and stored during distribution without significant degradation that could damage engine fuel systems.

Prolonging Biodiesel and Biodiesel Blend Oxidation Stability

Based on research performed at NREL between 2005 and 2010, the ASTM standards for 100% biodiesel and for biodiesel blends include minimum stability requirements to ensure biodiesel is adequately stable in storage. Also, NREL has focused on determining whether biodiesel and biodiesel blends stored for a long period of time, such that much of the antioxidant is consumed and their stability rating is reduced, can be reset by adding more antioxidant. This situation might occur, for example, in fuels stored for use by emergency generators or by utilities storing large volumes of fuel for emergencies such as hurricanes.

B100 and B20 blends were stored under standard accelerated storage conditions of 43°C under air (one week of accelerated storage simulates one month in actual storage). The stability measured as induction time (also known as oxidation reserve) decreases over several weeks (simulated months) in storage but can be re-set to the starting oxidation reserve by adding antioxidant.

However, if oxidation proceeds too far such that acids and gums begin to form, this strategy is no longer viable. Those storing fuels for long periods of time are encouraged to monitor the stability and re-additize as needed.

Biodiesel Fuel Stability Challenges

Biodiesel is the highest production volume biomass-based diesel fuel. It is produced from vegetable oils, animal fats, and waste cooking oil and consists of fatty acid methyl esters. A fraction of the fatty acid chains in biodiesel are polyunsaturated. Thus, they contain a structural feature called bis-allylic C-H bonds.

These bonds are relatively weak. They can break producing carbon radical species that can react directly with oxygen, initiating a low-temperature oxidation sequence (autoxidation) that results in the formation of acids and gums that can harm engine fuel systems. Fuel producers use antioxidant additives to improve the stability of these fuels.

Also, NREL has focused on determining whether biodiesel and biodiesel blends stored for a long period of time—such that much of the antioxidant is consumed and their stability rating is reduced—can be reset by adding more antioxidant. This situation might occur, for example, in fuels stored for use by emergency generators or by utilities storing large volumes of fuel for emergencies such as hurricanes.

B100 and B20 blends were stored under standard accelerated storage conditions of 43°C under air (one week of accelerated storage simulates one month in actual storage). The stability measured as induction time (also known as oxidation reserve) decreases over several weeks (simulated months) in storage, but can be reset to the starting oxidation reserve by adding antioxidant. However, if oxidation proceeds too far, such that acids and gums begin to form, this strategy is no longer viable. Those storing fuels for long periods of time are encouraged to monitor the stability and re-additize as needed.

Related Publications

Storage Stability of Biodiesel and Biodiesel Blends  

Long-Term Storage Stability of Biodiesel and Biodiesel Blends

Storage Stability of High-Octane Alkyl Furans

The potential biofuels 2-methylfuran and 2,5 dimethylfuran exhibit among the highest blending research octane numbers measured, making them highly desirable as fuel blending components. However, when blended with petroleum refinery gasoline, these fuels failed the gasoline oxidation stability test.

NREL conducted a combined theoretical and experimental study to understand the reactions occurring to cause poor stability. Quantum mechanics calculations revealed that the bond dissociation energy of the carbon-hydrogen bonds of the methyl groups found in the methyl furans was only 85 kcal/mole. These bonds relatively easily break on the several-hour timescale of the oxidation stability test.

The peroxide radical species formed reacts yielding a highly reactive hydroxyl radical that adds to the aromatic furan ring, leading to ring opening and the formation of carbonyl species that can polymerize to form gums, or react further in a catalytic cycle to regenerate the hydroxyl radical. Common gasoline antioxidants could be effective at shutting down this reaction, but high additive addition rates were required. The potential of these high-octane components to be commercial fuels is therefore in doubt.

Related Publications

Properties of Oxygenates Found in Upgraded Biomass Pyrolysis Oil as Components of Spark and Compression Ignition Engine Fuels  

Selection Criteria and Screening of Potential Biomass-Derived Streams as Fuel Blendstocks for Advanced Spark-Ignition Engines  

Experimental and Theoretical Study of Oxidative Stability of Alkylated Furans Used as Gasoline Blend Components

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