Overview of ULSD and biodiesel properties contributing to filter plugging and engine problems, and overview of contaminants, including particulate, water and organic contaminants
- 3 simultaneous changes contributing to need for much higher level of fuel cleanliness: reduced sulfur (LSD, then ULSD), introduction of/increase in biofuels, and modern HPCR engine
- Refinery applies hydrotreating process which removes sulfur, but also naturally occurring lubricants
- Distillation temperature, fuel oxidation stability, conductivity, and aromatics contents are lowered
- Cetane number, cloud point and wax content are increased
- Changes are dependent on refinery feed stocks, temperature, pressure and catalysts used during desulfurization process
- Blends can improve engine performance by improving lubricity, providing higher cetane number and reducing emissions relative to ULSD
- Contaminants in Fuel
- Typical fuel cleanliness coming out of the pump are ISO 22/21/18 (20,000 to 40,000 per mm particles 4 µm and greater; 10,000 to 20,000 per mm for particles of 6 µm and greater; and 1300 to 2500 per milliliter for particles of 14 µm and greater).
- Can also enter fuel through tank vent or wear debris
- Require new filtration technology and systematic approach to remove them from fuel
- Water in Fuel
- Once amount exceeds maximum level for it to remain dissolved, water falls out of the fuel, forming an emulsion (small water droplets suspended in fuel) and then when more water is introduced during storage, shipping, pumping and condensation, free water will sink to tank bottom due to its density relative to fuel
- World Fuel Charters recommends max water content of less than 200 ppm
- Water content fluctuates with temperature changes and relative humidity
- Lab testing at Donaldson indicates ULSD has 50 ppm water saturation limit at
50°F and 200 ppm saturation limit at 100°F; 50 degree temperature swing from 50°F to 100°F at 100% RH increases dissolved water concentration from 50 ppm to 200 ppm, meaning 1.7 ounces of water per 100 gallons of fuel. When fuel in the tank and air space above it cools to 50°F the dissolved water holding capacity of the fuel is reduced to its starting value and the calculated amount of water is dropped out of solution and into the fuel tank as free or emulsified water.
- Lubricity additives increase fuel surfactancy, which increases stability of water droplets in fuel
- Biodiesel inherently more hygroscopic than ULSD due to its polar fatty acid and methyl ester composition and residual glycerin by-products created during transesterification process; emulsified water in biodiesel believed to have finer droplet distribution due to surfactant nature inherent in fatty acids and role of glycerin in stabilizing the emulsion
- Water filtration standard SAE J1488 no longer representative of today’s fuels
- Organic Contaminants
- Soft, sticky or slimy – occur naturally or as result of fuel degradation, additives, or water contamination
- Reduction of aromatics in ULSD lowers wax solubility and promotes wax precipitation when temperature is lowered
- Free radical reactions causing organic contaminants are accelerated for ULSD due to the removal of naturally occurring antioxidants during hydrotreating
- When fuel temperature is raised and in contact with metal surfaces, oxidation becomes increasingly problematic forming organic acids that may cause corrosion; with further oxidation, organic sediments can also form – fuel stabilizers are added to prevent this, but other fuel contaminants can coexist as oxidation sources and reduce the effectiveness of antioxidant additives
- Biodiesel blends more susceptible to oxidation - Fuel oxidation = rate of reaction of fuel molecules with oxygen through free-radical initiated mechanism; end products of oxidation may contain soft sticky particles that damage filters and engines; induction period of oxidation is used as standard to quantify fuel oxidation stability – longer the induction period, more resistant fuel is to oxidation when exposed to air; induction period decreases as blend ratio increases
- Lengthy discussion on fuel filtration systems
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