Heavy Fuel Oil (HFO)
Heavy fuel oil is a thick, viscous byproduct of crude oil refining. It is widely used as a cost-effective marine fuel for large vessels like cargo ships and tankers. Due to its high viscosity, HFO requires pre-heating before use in low-speed marine engines to ensure efficient combustion. It is also known as residual fuel oil, bunker C or IFO 380/500/700 (if blended).
Key Characteristics:
- High Density – typically 0.95–1.01 kg/L at 15°C
- Very High Viscosity – must be heated to 100–150°C before use
- Variable Sulphur Content – Usually 2–3.5% m/m, but newer regulations (e.g. IMO 2020) limit sulphur content to 0.5% unless scrubbers (exhaust gas cleaners) are used
Environmental Concerns
HFO is considered highly polluting as it releases sulphur oxides (SOx), nitrogen oxides (NOx) and particulate matter. Spills are particularly damaging due to persistence in the marine environment and difficulties with cleanup.
Regulations:
- IMO 2020 Sulphur Cap: As of January 1, 2020, the International Maritime Organization (IMO) limits sulphur in marine fuels to 0.5% globally, down from the previous cap of 3.5%. Ships without exhaust gas cleaning systems (scrubbers) must switch to low-sulphur fuels or alternatives like marine gas oil (MGO) or very low sulphur fuel oil (VLSFO).
- Arctic Ban: The IMO is also moving toward banning HFO in Arctic waters due to its high risk to fragile ecosystems (MEPC 76 adopted such measures in 2021).
Intermediate Fuel Oil (IFO)
Intermediate Fuel Oil (IFO) is a blended marine fuel made by mixing heavy fuel oil with lighter distillates like marine diesel oil to achieve a specific viscosity. It is typically used in medium- and slow-speed marine engines and offers a balance between refinement and cost, being more refined than heavy fuel oil but less expensive than marine gas oil.
Key Characteristics
- Grades – Typically classified by viscosity, such as IFO 180 and IFO 380, indicating the fuel’s viscosity in centistokes (cSt) at 50°C (e.g., IFO 150 = 150 cSt)
- Medium Viscosity – Lower than HFO but still requires pre-heating (around 90–130°C) before injection into engines
- Variable Sulphur content – Depends on blending and regulation compliance. Historically up to 3.5% was allowed but shipping must now meet the IMO 2020 sulphur cap of 0.5% unless they have exhaust gas scrubbers
Environmental Concerns
Although cleaner than HFO, IFO still produces substantial emissions of SOx, NOx, CO₂, and particulates. Contamination or improper blending can impact engine performance and emissions. Its use has decreased in recent years due to tighter environmental regulations.
Regulations
- IMO 2020 Sulphur Cap: IFO must meet the global 0.5% sulphur limit unless a scrubber is installed onboard. As a result, compliant versions such as Very Low Sulphur Fuel Oil (VLSFO) are now more common.
- ISO 8217 Standard: IFOs must comply with fuel quality and safety standards outlined by ISO 8217 (e.g., RME180, RMG380 grades).
Low Sulphur Fuel Oil (LSFO)
Low Sulphur Fuel Oil (LSFO) is a marine fuel with a sulphur content of 0.5% or less, developed in response to the IMO 2020 sulphur cap designed to reduce shipping air pollution. It typically refers to Very Low Sulphur Fuel Oil (VLSFO), which is a blend of residual and distillate components, designed to comply with global sulphur limits while still being cost-effective compared to distillate-only fuels like marine gas oil (MGO).
Key Characteristics
- Sulphur content: ≤ 0.5% m/m (meets IMO 2020 global limit); some versions contain ≤ 0.1% for use in Emission Control Areas (ECAs)
- Viscosity: Varies widely depending on blend—often less viscous than traditional HFO but may still require heating (~40–80°C)
- Stability/Compatibility: Early blends had issues with fuel stability, compatibility between batches but refining quality has improved over time
Environmental Benefits
LSFO significantly reduces emissions of sulphur oxides (SOx), a major contributor to acid rain and respiratory issues. When used in place of high-sulphur HFO, it helps ships meet MARPOL Annex VI air pollution standards. While cleaner, LSFO still produces NOx, CO₂, and PM, though at lower levels than conventional residual fuels.
Regulations:
- IMO 2020 Sulphur Cap: Since January 1, 2020, all vessels operating outside ECAs must use fuels with ≤ 0.5% sulphur unless fitted with approved exhaust gas cleaning systems (scrubbers)
- ECAs: In designated areas (e.g., Baltic Sea, North Sea, North America, and U.S. Caribbean), ships must use fuels with ≤ 0.1% sulphur
- Fuel testing and certification: LSFO must meet ISO 8217 standards for fuel quality and safety, with special attention to stability, viscosity, and compatibility
Ultra-Low Sulphur Fuel Oil (ULSFO)
Ultra Low Sulphur Fuel Oil (ULSFO) is a marine fuel with a sulphur content of 0.1% m/m or less, designed specifically to comply with the strict emission standards in Emission Control Areas (ECAs).
It is typically a distillate-residual blend or a refined distillate, offering a cleaner alternative to conventional fuels like IFO and HFO. ULSFO is used primarily when ships operate in regions with tight sulphur emission limits, such as the North Sea, Baltic Sea, North American coast, and U.S. Caribbean Sea.
Key Characteristics:
- Sulphur content: ≤ 0.10% m/m (to comply with ECA limits under MARPOL Annex VI)
- Viscosity: Generally lower than HFO or VLSFO; usually ranges from 20–100 cSt at 50°C
- Flash point: >60°C (per IMO safety requirements)
- Cleaner burning: Produces significantly lower SOx, PM, and black carbon emissions than standard marine fuels
Environmental Benefits
ULSFO drastically reduces sulphur oxide emissions—up to 99% compared to HFO—and lowers particulate matter and smoke. This makes it critical for compliance in ECAs and for minimizing health and environmental impacts near coastlines and ports.
Regulations:
- MARPOL Annex VI – ECA Compliance: ULSFO is required when sailing in ECAs, unless the vessel uses an approved scrubber or alternative compliance method
- Fuel Standards: Must meet ISO 8217 specifications (typically DMB or DMA grades, or ECA-compliant proprietary blends)
- Fuel Management: ULSFO requires careful handling and system compatibility checks to avoid fuel pump or injector damage, especially when switching between fuel types
Marine Fuel Oil (MFO)
This is not a specific grade of fuel but an umbrella term for heavier bunker fuels (see heavy fuel oil, intermediate fuel oil or low sulphur fuel oil on this page).
Marine Gas Oil (MGO)
Marine Gas Oil (MGO) is a high-quality distillate marine fuel used in both main and auxiliary ship engines. Unlike residual fuels such as heavy fuel oil, gas oil contains little to no residual components.
Therefore MGO burns cleanly, and requires no pre-heating, making it ideal for operations in Emission Control Areas (ECAs). It is one of the cleanest conventional marine fuels available and is widely used for compliance with IMO sulphur regulations.
Key Characteristics:
- Sulphur content: Usually ≤ 0.1% m/m (especially in ECA-compliant grades)
- Viscosity: Low – typically around 2–6 cSt at 40°C
- Density: ~860–890 kg/m³ at 15°C
- Flash point: >60°C (compliant with SOLAS requirements)
- Grades: Defined by ISO 8217 (e.g., DMA, DMB)
Environmental Benefits
MGO produces significantly lower emissions of SOx, particulate matter, and black carbon than residual fuels like HFO or IFO. It’s frequently used during port operations, in coastal shipping, or when transiting ECAs, offering immediate compliance with IMO 2020 and ECA sulphur restrictions.
Regulations
- IMO 2020: MGO naturally meets the 0.5% global sulphur limit without blending or scrubbers
- ECAs: MGO with ≤ 0.1% sulphur is often the fuel of choice to comply with stricter ECA rules under MARPOL Annex VI
- ISO 8217 Compliance: MGO must meet specific fuel standards for cleanliness, stability, water content, and lubricity
Marine Diesel Oil (MDO)
Marine Diesel Oil (MDO) is a blend of marine distillates and a small amount of residual fuel, placing its properties and quality somewhere between Marine Gas Oil (MGO) and Heavy Fuel Oil (HFO).
It is less refined than MGO but cleaner and more manageable than HFO. It is often used in medium-speed and auxiliary ship engines, and is suitable for main engines not equipped to handle heavy fuel. It can also be used as a transition fuel when changing fuel types.
Key Characteristics:
- Sulphur content: Varies; traditionally up to 1.5%, but now it is often between 0.1% to 0.5% to meet IMO 2020 and ECA standards
- Viscosity: Higher than MGO (typically 6–12 cSt at 40°C), but still low enough to avoid pre-heating
- Density: Around 870–910 kg/m³ at 15°C
- Flash point: >60°C (per SOLAS safety requirements)
- Grades: Typically meets ISO 8217 standards (DMB grade)
Environmental & Operational Profile
Diesel oil emits fewer pollutants than residual fuels but more than pure distillates like MGO. It’s relatively easy to store and handle compared to heavy fuel oil, with better combustion efficiency and engine cleanliness, though it may contain trace contaminants such as catalytic fines and asphaltenes (if blended with residuals).
Regulations
- IMO 2020: Marine diesel oil must have ≤ 0.5% sulphur unless scrubbers are installed
- ECAs: To be used in ECA zones, diesel oil must meet the ≤ 0.1% sulphur limit
- ISO 8217 Compliance: Must conform to standards that specify fuel quality parameters (e.g., sulphur levels, viscosity, water content, density)
Operational Considerations
- May require filtration and centrifuging due to residual matter
- No pre-heating required prior to use
- Provides a balance of cost and cleanliness. Thus it’s often used in coastal shipping and for older engines not suited to heavy fuel oil
Liquefied Natural Gas (LNG)
Liquefied Natural Gas (LNG) is a cryogenic marine fuel made primarily of methane (CH₄) that is cooled to −162 °C to convert it into liquid form, reducing its volume by ~600 times.
LNG is used as a clean-burning alternative to conventional marine fuels and offers substantial reductions in sulphur oxides (SOx), nitrogen oxides (NOx), carbon dioxide (CO₂) and particulate matter (PM).
It is increasingly adopted by modern ships, including fuel carriers, cruise ships, container vessels, and ferries, mostly in response to tightening emission regulations.
Key Characteristics
- Sulphur content: Zero – LNG contains no sulphur, making it naturally compliant with IMO sulphur limits
- Viscosity: Very low (gas phase once vaporised); no pre-heating required but fuel systems must manage cryogenic temperatures
- Energy density: Lower than oil-based fuels (~45–50 MJ/kg), but clean combustion compensates via better efficiency
- Storage: Requires insulated cryogenic tanks (Type C or membrane tanks)
Environmental Benefits
- SOx reduction: ~100%
- NOx reduction: Up to 90% (when used in low-pressure dual-fuel engines)
- CO₂ reduction: ~20–25% compared to heavy fuel oil
- PM reduction: ~100% LNG enables compliance with IMO 2020, ECA standards, and Tier III NOx requirements without using scrubbers or exhaust treatment systems.
Regulations
- IMO 2020 and MARPOL Annex VI: LNG is fully compliant with global sulphur and ECA sulphur caps
- IMO IGF Code: Vessels using LNG must comply with the International Code of Safety for Ships using Gases or other Low-flashpoint Fuels (IGF Code)
- Bunkering protocols: LNG bunkering involves strict procedures and safety protocols due to its cryogenic and flammable nature
Operational Considerations
- Requires specialised engine systems: Dual-fuel (DF) or gas-only engines
- Higher upfront costs: Ship construction and fuel storage systems are more expensive
- Growing infrastructure: LNG bunkering ports and terminals are expanding globally
- Methane slip risk: Leakage of unburned methane (a potent greenhouse gas) remains an environmental concern, though modern engines manage to keep leakage to a minimum
Marine Fuels Comparison Table
| Fuel Type | Viscosity (cSt) | Sulphur Content | Density (kg/m³) | Heating Required | Emissions |
| Heavy Fuel Oil | 180–700 @50°C | 1.5–3.5% | ~990–1010 | Yes | High |
| Intermediate Fuel Oil | 180–380 @50°C | 1.0–3.5% | ~960–990 | Yes | High |
| Low Sulphur Fuel Oil | 30–380 @50°C | ≤0.5% | ~950–990 | Usually | Medium |
| Ultra Low Sulphur Fuel Oil | 2–6 @40°C | ≤0.1% | ~850–890 | No | Low |
| Marine Fuel Oil | 30–700 | Varies | 950–1010 | Varies | Varies |
| Marine Gas Oil | 1.5–6 @40°C | ≤0.1% | ~840–890 | No | Very Low |
| Marine Diesel Oil | 5–12 @40°C | ≤1.5% (or 0.1%) | ~890–920 | No | Low |
| Liquid Natural Gas | ~0.01 | ~0% | ~430–470 | Yes (cryogenic) | Very Low |
References
IMO – International Maritime Organization
DNV – Fuel Facts: Heavy Fuel Oil (HFO)
European Maritime Safety Agency (EMSA) – Sulphur Inspection Guidance
Intergovernmental Panel on Climate Change (IPCC) – Reports on emissions and marine fuel impacts.
ISO 8217:2017 – Specification for marine fuels
Bureau Veritas – Marine Fuel Factsheets
IMO – Sulphur 2020 FAQ