Grade 4 Diesel, commonly referred to as No. 4 Fuel Oil (FO-4), is a heavy distillate fuel positioned between lighter diesel fuels (No. 2) and residual fuel oils (No. 5 and No. 6). It is designed for industrial and commercial combustion systems where high heat output, stable combustion, and long operating cycles are required.
This fuel is significantly more viscous than automotive diesel and often requires fuel heating systems for proper pumping, atomization, and efficient combustion.
Physical & Chemical Properties
Density & Viscosity:
High density due to long-chain hydrocarbons
Kinematic viscosity substantially higher than Grade 2 diesel
Preheating required (typically 40–70°C)
Poor cold-flow behavior without heating
Energy Content:
Higher calorific value than lighter diesel grades
Typical gross calorific value: ~42–44 MJ/kg
Provides sustained heat output, ideal for continuous industrial use
Volatility & Flash Point:
Low volatility compared to automotive diesel
High flash point (>55–65°C) enhances storage and handling safety
Reduced evaporation losses
Sulfur Content:
Traditionally higher sulfur levels than Grade 2 diesel
Sulfur content varies based on refinery processing and regional regulations
May require desulfurization or flue-gas treatment in emission-controlled zones
Refining & Production
Fractional Distillation
Extracted from mid-to-heavy distillate fractions of crude oil
Typical boiling range: ~250°C to 350°C
Blending
Often blended from heavy gas oil and lighter residual streams to adjust:
Blending adjusts:
Viscosity
Pour point
Combustion stability
Upgrading & Treatment
May undergo:
Hydrotreating (to reduce sulfur and nitrogen compounds)
Thermal cracking (to improve flow and combustion properties)
Final product specifications depend on end-use and regulatory standards
Combustion Characteristics
Slow, steady burn rate, suitable for large burners
Requires:
Higher injection pressure
Preheated fuel lines
Robust atomization systems
Produces:
Higher flame temperature
Longer flame length
Improper combustion can lead to:
Soot formation
Slagging
Fouling of heat exchange surfaces
Improper combustion may lead to soot formation, slagging, and fouling of heat-exchange surfaces.
Storage & Handling Requirements
Storage
Stored in heated or insulated tanks
Long-term storage possible due to:
Low volatility
Minimal evaporation losses
Handling
Requires:Heated pipelines
Positive displacement pumps
Filtration to remove sediments
Periodic tank cleaning recommended due to asphaltene and sediment content
Industrial Applications
Industrial boilers and furnaces
Power generation plants (backup or base load in some regions)
Cement, steel, and glass manufacturing
Marine auxiliary engines (non-road use)
Large stationary diesel engines
Why Grade 4 Diesel is Preferred
High energy density
Cost-effective for bulk industrial consumption
Reliable under continuous high-load operation
Environmental & Regulatory Considerations
Higher sulfur and particulate emissions compared to light diesel
Requires:
Emission control systems (scrubbers, ESPs)
Compliance with regional fuel sulfur limits
In many regions, usage is restricted to:
Non-road industrial equipment
Permitted stationary installations
Comparison with Grade 2 Diesel
| Property | Grade 2 Diesel | Grade 4 Diesel |
|---|---|---|
| Viscosity | Low | High |
| Energy Density | Moderate | High |
| Sulfur Content | Low (ULSD) | Moderate to High |
| Preheating Required | No | Yes |
| Typical Use | Vehicles | Industrial Systems |
Advantages & Limitations
Advantages:
High thermal efficiency
Stable combustion for continuous operations
Lower cost per unit of heat
Limitations:
Requires heating infrastructure
Higher emissions without treatment
Not suitable for automotive engines