Fuel Oil

Fuel oil is a petroleum-derived liquid fuel used extensively in commercial and industrial heating systems. These fuels range from light distillates suitable for residential burners to heavy residual oils requiring preheating and specialized combustion equipment. Selection of fuel oil grade depends on burner type, storage capabilities, local availability, and environmental regulations.

Fuel Oil Grade Classification

The American Society for Testing and Materials (ASTM) D396 specification defines six grades of fuel oil, each with distinct properties and applications:

Grade No. 1 (Kerosene): Light distillate with low viscosity and pour point. Used in vaporizing burners and as a blending agent for heavier grades. Heating value approximately 135,000 BTU/gal. Flash point minimum 100°F. Suitable for systems without preheating capability.

Grade No. 2 (Heating Oil): Most common fuel oil for residential and light commercial applications. Heating value approximately 140,000 BTU/gal (138,000-141,000 BTU/gal typical range). Viscosity 1.9-3.4 centistokes at 100°F (32.6-45.8 Saybolt Universal Seconds). Pour point maximum 20°F. Used in pressure atomizing burners without preheating. Excellent atomization characteristics at ambient temperatures.

Grade No. 4 (Light Residual): Blend of distillate and residual stocks. Heating value approximately 145,000 BTU/gal. Requires preheating to 100-120°F for proper atomization and flow. Viscosity 5.8-26.4 centistokes at 100°F. Intermediate fuel for medium-sized commercial and industrial installations.

Grade No. 5 (Light and Heavy): Two subgrades: No. 5 Light and No. 5 Heavy. Heating values 148,000-150,000 BTU/gal. Preheating to 120-220°F required depending on grade. Higher sulfur content than lighter grades. Used in large commercial and industrial systems with preheating equipment.

Grade No. 6 (Residual or Bunker C): Heavy residual oil requiring extensive preheating (180-260°F for storage, handling, and atomization). Heating value approximately 152,000 BTU/gal. Highest energy density per gallon. Viscosity approximately 900-9000 centistokes at 100°F. Used exclusively in large industrial and utility boilers with specialized fuel handling systems.

Heating Value Characteristics

Heating value increases with fuel oil grade due to higher carbon-to-hydrogen ratio in heavier grades:

Higher BTU per gallon values in heavier grades offset by increased handling complexity and equipment requirements.

Viscosity and Flow Properties

Viscosity governs pumpability, atomization quality, and preheating requirements:

Viscosity Temperature Relationship: Fuel oil viscosity decreases exponentially with temperature increase. Atomizing burners require specific viscosity ranges (typically 45-150 SSU) at the burner nozzle for proper spray formation. Light grades achieve target viscosity at ambient temperature. Heavy grades require preheating to reduce viscosity to pumpable and atomizable ranges.

Kinematic Viscosity Specifications:

• No. 1: Maximum 2.5 centistokes at 100°F
• No. 2: 1.9-3.4 centistokes at 100°F
• No. 4: 5.8-26.4 centistokes at 100°F
• No. 5 Light: 26.4-54 centistokes at 100°F
• No. 5 Heavy: 54-125 centistokes at 100°F
• No. 6: 900-9000 centistokes at 100°F (125-300 centistokes at atomization temperature)

Pour point specifications ensure fuel remains pumpable at minimum storage temperature. No. 2 fuel oil pour point maximum 20°F; cloud point typically 10-15°F higher than pour point.

Atomization Requirements

Proper atomization is critical for complete combustion and efficiency:

Pressure Atomization: Used for lighter grades (No. 1, No. 2, No. 4). Fuel forced through small orifice at 85-300 psi, creating fine spray. Atomization quality depends on viscosity, pressure differential, and nozzle design. No. 2 fuel oil achieves droplet sizes of 20-80 microns at typical atomizing pressures.

Steam Atomization: Required for heavy residual oils (No. 6). Steam jets at 60-150 psi shear fuel film into droplets. Requires 3-10% fuel energy equivalent in steam. Fuel preheated to 180-240°F before atomization. Produces droplets 50-150 microns.

Air Atomization: Compressed air (5-15 psi) atomizes medium to heavy grades. More flexible than steam systems. Used in industrial applications with available compressed air.

Storage Tank Systems

Fuel oil storage design varies with grade and application:

Aboveground Storage Tanks: Steel construction for commercial installations. Single-wall or double-wall design. No. 2 fuel oil stored at ambient temperature. No. 4 and heavier grades require heated tanks maintaining 15-20°F above pour point. Insulation thickness 2-4 inches depending on climate and grade.

Underground Storage Tanks: Common for No. 2 fuel oil in residential and small commercial systems. Steel or fiberglass construction. Double-wall tanks with interstitial monitoring required in most jurisdictions. Protected from freezing by earth temperature. Cathodic protection for steel tanks in corrosive soils.

Tank Heating Systems: Heavy grades require electric immersion heaters, steam coils, or hot water coils. Heating capacity 5-10 watts per gallon for moderate climates. Temperature control maintains 100-150°F for No. 4, 120-180°F for No. 5, 180-220°F for No. 6. Thermostatic control prevents overheating and fuel degradation.

Venting and Pressure Relief: All tanks require pressure-vacuum relief venting. Vent capacity sized for maximum fill rate plus thermal breathing. Fill connections designed to prevent overfill. Leak detection required for underground and double-wall tanks.

Sulfur Content and Environmental Considerations

Sulfur content affects emissions, corrosion, and regulatory compliance:

Sulfur Specifications:

• No. 1 and No. 2: Maximum 0.5% sulfur (5000 ppm) per ASTM D396. Ultra-low sulfur heating oil (ULSHO) 15 ppm maximum increasingly mandated.
• No. 4 through No. 6: Sulfur content 0.5-3.0% depending on crude oil source and refining process.

Combustion Products: Sulfur oxidizes to SO₂ and SO₃ during combustion. SO₃ combines with water vapor forming sulfuric acid, causing low-temperature corrosion in flue gas passages when metal temperatures drop below acid dew point (typically 250-300°F for 2-3% sulfur fuel).

Emission Control: Low sulfur fuels reduce SO₂ emissions proportionally. Stack temperatures maintained above acid dew point prevent condensation. Air preheaters and economizers designed for corrosion resistance when burning high-sulfur fuels.

Combustion Characteristics

Complete combustion requires proper air-fuel ratio, atomization, and mixing:

Stoichiometric Air Requirements: No. 2 fuel oil requires approximately 1,500 standard cubic feet of air per gallon for complete combustion. Actual combustion air 10-20% excess (15% typical for modern burners) ensures complete oxidation while minimizing heat loss up stack.

Flame Temperature: Theoretical adiabatic flame temperature approximately 3800°F with stoichiometric air. Actual flame temperature 2000-2800°F depending on excess air and heat transfer. Lighter grades produce slightly hotter flames due to faster evaporation and mixing.

Combustion Products: Complete combustion of No. 2 fuel oil produces approximately 154 lb of CO₂ and 94 lb of H₂O per million BTU input. Flue gas volume approximately 1050 standard cubic feet per gallon with 15% excess air. CO₂ concentration 13-14% by volume indicates proper combustion efficiency.

Ignition Requirements: No. 2 fuel oil flash point 100°F minimum, ignition temperature approximately 700°F. Electric spark or gas pilot provides ignition energy. Flame establishing time 1-3 seconds depending on burner design and preheat temperature.