April 2021

Special Focus: Clean Fuels

A solution to the IMO 2020 MARPOL Annex VI requirement

On January 1, 2020, a new requirement limiting the sulfur content of marine fuel to a maximum of 0.5 wt% went into effect.

Klussmann, B., Rigby Refining LLC

On January 1, 2020, a new requirement limiting the sulfur content of marine fuel to a maximum of 0.5 wt% went into effect. The International Maritime Organization’s (IMO’s) 2020 MARPOL Annex VI rule globally prohibits ships from operating while using a fuel with more than 0.5 wt% sulfur without an exhaust gas scrubber.

Most ship engines in service were designed to operate with ISO 8217-compliant high-sulfur, heavy residual fuel oil. Historically, ISO 8217-compliant fuel oil is generated by combining various high-sulfur refinery residues with varying percentages of cutters, vacuum gasoil (VGO) and gasoil until the fuel specification is met.

From the perspective of marine engine design firms and shipowners, the most desirable way to comply with the new regulation would be to use a heavy residual fuel oil that meets the ISO 8217 fuel specification and has a sulfur content of less than 0.5 wt% sulfur. An early recognition for such a fuel oil led to the development of a proprietary upgrading processa several years before the IMO 2020 rule took effect. Since the patented very-low-sulfur fuel oilb (VLSFOb), which was made using this proprietary process, maintains ISO 8217 bulk properties, it reduces the risk of engine performance problems related to fuel oil blending.

Due to the COVID-19 pandemic, the decline in demand of transportation fuel resulted in an excess of distillate materials—and in the refinery streams used to produce those same distillate materials. The excess of distillate and intermediate refinery products drove the industry toward blending distillate fuel oils or blending distillate materials with residual materials to adhere to the IMO marine fuels regulation. A wide range of problems related to such VLSFO blends available in the market are being reported in literature.1,2,3 From sludge formation, engine damage, safety concerns and pollution to increased soot and dumping of sludge, the new VLSFO formulations are creating significant safety and performance concerns for the shipping industry. Distillate blends may cause wear and tear to ships’ engine systems, as their bulk properties can be adversely affected when distillates are mixed with residual materials. Because the proprietary VLSFO is created solely through hydroprocessing, the resultant product is compliant and homogenous.

While installation of scrubbers on ships is an alternative to using VLSFO, scrubbers present significant issues, including complex operation, costly maintenance, emissions violations, bans on open-loop scrubbers in some ports, and disposal issues for toxic byproducts and sludge.4,5,6,7,8,9

The patented upgrading process

Refining processes have historically focused on breaking down and upgrading residual material to create higher-value distillate products. Counter to traditional residue upgrading technologies, the proprietary upgrading process intentionally maintains the bulk properties of residual fuel that make it the preferred fuel of the shipping industry. The patented technology is rooted in commercially proven refining processes, utilizing an alternate approach counter to traditional refinery residual material upgrading processes such as hydrocracking, deasphalting and coking. Because the proprietary upgrading process does not focus on cracking, hydrogen consumption is significantly lower than hydrocracking, and no low-value residual products remain to be processed further or sold into the high-sulfur fuel oil (HSFO) market.

The proprietary upgrading process was designed with a focus on three primary principles. These include:

  1. Feeding the process an ISO 8217-compliant heavy marine HSFO and removing the sulfur and other environmental contaminants, while maintaining the energy density and other bulk properties of residual fuel oils for which most ships’ main engines are designed.
  2. Addressing the overabundance of HSFO and high-sulfur residual components in the market by developing a straightforward, robust process to produce 0.5 wt% sulfur ISO 8217 residual fuel oil with a minimal yield of byproducts. Using 1 mass unit of HSFO feed, the proprietary upgrading process produces approximately 0.96 mass units of ISO 8217-compliant VLSFO, with the balance being about 0.025 mass units of sulfur and minor amounts of wild naphtha and light ends (less than 0.015 mass units).
  3. Positioning the process to be installed on a compact footprint within a refinery or near/adjacent to points of aggregation (i.e., fuel oil terminals). A preferred location will have excess hydrogen or pipeline hydrogen available and will utilize existing systems (e.g., tanks and utilities) of the current logistics flow in the existing HSFO supply chain. The addition of the proprietary upgrading process unit should cause little or no disruption to existing refinery infrastructure.

Commercially, the proprietary upgrading process is a lower-priced alternative to hydrocracking. Capital costs for estimated inside battery limits for implementation of an ebullated-bed hydrocracker are more than three times higher than the capital costs for implementation of the proprietary upgrading process unit of the same capacity. Outside battery limit integration costs are also significantly lower for a proprietary upgrading process unit. On a per-barrel operating cost basis, the proprietary upgrading process unit is about one-fourth the operating costs of an ebullated-bed hydrocracker. The proprietary upgrading process is much less operationally complex and is safer than residue upgrading alternatives.

In addition to the benefits outlined, a modular proprietary upgrading process unit allows much faster entry into the market, with the lowest need for integration and disruption to existing infrastructure at either a refinery or marine terminal. A modular version of the proprietary upgrading process can be fully operational at a greenfield site in less than 2 yr from the date of placing an order—or even sooner, if located inside an existing refinery, which is half of the schedule for a comparable ebullated-bed hydrocracker or coker. Beyond price and speed-to-market considerations, both coking and hydrocracking produce some amount of low-value residual material that must be sold into the market at a value lower than the feedstock and generally lower than HSFO. Compared to available information on competitive technologies, the proprietary upgrading process is the fastest to market—as either a fully modular or stick-built unit—because it has a lower capital expenditure and operational expenditure and is rooted in process technologies broadly understood and accepted by refiners.

As shown in FIG. 1, feeding the proprietary upgrading process is ISO 8217-compliant HSFO, which is mixed with hydrogen and heated against the reactor effluent. It is then heated to the reactor inlet temperature in a furnace and fed to the reactor system. The reactor system utilizes commercially available hydrotreating catalysts in its design. Effluent from the reactor system flows through the feed/effluent exchanger and to a series of vessels to separate liquid product from recycled vapor. Recycled hydrogen is contacted against amine to remove hydrogen sulfide (H2S) and is recycled to the reactor system via a recycle hydrogen compressor. Liquid effluents from the separator drums feed the product stabilizer. Hydrogen, H2S and light components are stripped from the liquid effluents to produce ISO 8217-compliant VLSFO or ultra-low-sulfur fuel oil (ULSFO).

FIG. 1. Process flow diagram of the proprietary upgrading process.

The proprietary upgrading process was successfully piloted at two separate pilot plant facilities, using three different commercial marine fuel oils (not laboratory blends): two RMG-380 feeds with 2.5 wt% and 2.9 wt% sulfur, and an RMK-500 feed with 3.3 wt% sulfur, to produce an ISO 8217-compliant VLSFO with 0.5 wt% sulfur. Production of an ULSFO with 0.1 wt% sulfur, which is a compliant fuel for MARPOL Annex VI designated emissions control areas, was also demonstrated during the pilot testing.

Proprietary fuelb properties

The proprietary fuel has been well received by the shipping industry and engine manufacturers, as it maintains energy density and remains compliant on all ISO 8217 specifications. As shown in TABLE 1, the ISO 8217 properties of the RMG-380 HSFO (i.e., the feed to the proprietary upgrading process) are compared to those of the proprietary VLSFO and to the two other widely available alternative bunker fuels used for compliance with IMO 2020 requirements: a marine gasoil and a blended VLSFO (80:20 blend). Of the three IMO 2020-compliant fuels, the properties of the proprietary fuel most closely align with HSFO, the fuel for which most ship engines were designed to use. In addition to producing an ISO 8217-compliant fuel with lower sulfur content, metals, catalyst fines and other environmental contaminants are nearly completely removed by the proprietary upgrading process. Metals reduction of 80%–90% is achieved in the proprietary upgrading process, and catalyst fines (aluminum and silicon) are reduced by more than 90%.

Compatibility and miscibility

The author’s position is that blending HSFO with distillates is not a commercially acceptable and sustainable solution to the IMO 2020 regulatory requirements. In the short term, blending a VLSFO requires the use of higher-value (and thereby higher-cost) ultra-low-sulfur distillates that are only more readily available during 2020–2021 due to decreased demand for transportation fuels because of the global pandemic. However, the asphaltenes present in residual streams are often not compatible with paraffinic distillates, which may cause serious problems.

Commercial experience, and several prior and recent industry surveys, show that blending to produce a low-sulfur fuel oil can result in a fuel that is incompatible with ship engines.10 In addition, blends “loaded on top” of residual fuels, distillate fuels or other blends in storage tanks or on ships can cause precipitation of asphaltenes due to incompatibility of these blends with other fuels. With these unstable blends, precipitated asphaltenes form sludge in tanks that can plug filters, purifiers, fuel injection equipment and even fuel lines. Precipitated asphaltenes cannot be brought back into solution, meaning that this sludge formed by a blended VLSFO will also need to be cleaned from tanks and fuel systems before being safely disposed of on land.

The author’s company has carried out compatibility and miscibility tests to demonstrate that the proprietary fuel does not create or experience these issues. The proprietary VLSFO was tested with RMG, RMK and distillate grades of marine fuels, and has proven to be miscible with residual fuels, as well as MGO and ultra-low-sulfur diesel (ULSD) in mixtures of 50/50, 30/70, 20/80, 10/90 and 5/95. Compatibility test samples and results are shown in FIGS. 2 and 3.

FIG. 2. Fuels for compatibility testing.
FIG. 3. Fuels for miscibility testing: (A) 20% RMG 380/80% distillate, (B) 20% proprietary fuel/80% distillate, and (C) 30% proprietary fuel/ 70% distillate.

Shelf life

Shelf life refers to the length of time that fuel oil remains stable and in solution in storage. The reserve stability number (RSN)—as measured by ASTM D7061—is an industry-accepted measurement of the stability of marine fuels. Fuels with an RSN of less than 5 are considered to pass and have a high stability reserve. Asphaltenes are not likely to flocculate, and the fuel is stable with a commercially reasonable shelf life. Fuels with an RSN of 5–10 have a much lower stability reserve, with limited shelf life and may flocculate. Fuels with an RSN greater than 15 are considered unstable. Testing has shown that, after more than a year in storage, the proprietary VLSFO had an RSN of 1.2—thereby demonstrating that the proprietary fuel is stable and has no issues around shelf life. Blends and other VLSFO/ULSFO products on the market are reported to have shelf-life problems and may be stable for just a matter of days or weeks in storage. This can create major issues with sludge formation over time in storage tanks on land and onboard ships. Costly and time-consuming cleaning of tanks and maintenance of ship fuel systems are required when unstable fuels have been bunkered.

Ignition and combustion properties

Ignition delay is the time that elapses from the start of fuel injection to the point of combustion. A long ignition delay results in an accumulation of unburned fuel in the combustion chamber, which can cause knocking, poor engine performance and, eventually, engine damage. The proprietary fuel demonstrated superior ignition properties when tested using the combustion pressure trace test.

FIG. 4 illustrates the proprietary fuel’s combustion and ignition properties vs. “normal” fuel (ECN=29) and a “problem” fuel (ECN=8), as taken from the International Council on Combustion Engines’ (CIMAC’s) 2011 “Fuel Quality Guide–Ignition and Combustion.”

FIG. 4. The proprietary fuel has ignition and combustion properties superior to a CIMAC good fuel (green line) based on the IP 541/06 fuel ignition and combustion test.

Poor combustion performance is normally characterized by an extended combustion period, along with low rates of pressure increase and low maximum pressure, resulting in incomplete fuel combustion. In contrast, good combustion exhibits a minimal ignition delay, a rapid combustion period, and high rates of pressure increase and high maximum pressure. The proprietary fuel possesses superior combustion performance, as demonstrated by the rate of heat release (ROHR) curve in FIG. 4.


Lubricity is commonly defined as the ability of a fluid to minimize friction between surfaces in relative motion under load conditions. A fuel oil with poor lubricity can rapidly cause severe wear to liners and piston rings, create turbocharger issues and ultimately result in engine failure. The ISO 12156 test method is commonly used to determine the lubricity of marine fuels. When tested in accordance with ISO 12156, the proprietary fuel resulted in lubricity of less than 100 µm, showing its superior lubricity properties vs. distillate-based fuels. For purposes of comparison, the lubricity specification for distillate fuels is less than 520 µm, meaning that anything less than 520 µm meets the requirement for lubricity.

Another issue related to lubricity recently identified and associated with low-sulfur fuel oil blends is FCC catalyst fines, which are present in small quantities. The catalyst fines, which are present in HSFO base material, become abrasive when blended with low-viscosity, ultra-low-sulfur distillates and can cause significant cylinder wear in ships’ engines. Because the proprietary VLSFO exceeds the lubricity specification and does not contain abrasive catalyst fines, these are not a concern for ships using the proprietary fuel.


The proprietary fuel meets the ISO 8217 specification for the residual marine fuel oil preferred by the shipping industry and is a robust solution to the environmental regulations specified in IMO 2020 MARPOL Annex VI, thus producing significantly lower sulfur oxide and nitrogen oxide emissions than HSFO. The proprietary upgrading process is rooted in commercially proven hydroprocessing technology—removing sulfur, metals and other contaminants from ISO 8217 high-sulfur residual marine fuel oil. With its lower hydrogen consumption and energy demand, the proprietary process has a smaller greenhouse gas impact than other residue upgrading options. HP


         a The Rigby Process®
         b Rigby Fuel (VLSFO)


  1. Wingrove, M., “Handling issues and engine damage—Are VLSFOs to blame?” December 2020, online: https://www.rivieramm.com/news-content-hub/handling-issues-and-engine-damage-ndash-are-vlsfos-to-blame-62133
  2. Degnarain, N., “Shipping-Gate: Why toxic VLSFO ‘Frankenstein fuel’ is such a danger for the planet,” Forbes, December 2020.
  3. “Chevron reports VLSFO causing abnormal liner wear,” Ship & Bunker, January 2021, online: https://shipandbunker.com/news/world/449689-chevron-reports-vlsfo-causing-abnormal-liner-wear
  4. Agarwal, S., “Exhaust gas scrubbers of ships—Boon or bane?” Marine Insight, January 2021, online: https://www.marineinsight.com/tech/exhaust-gas-scrubbers-of-ships/
  5. Bockmann, M. W., “Marine insurers investigate scrubber incidents amid risk-elevation concerns,” Lloyd’s List, September 2019, online: https://lloydslist.maritimeintelligence.informa.com/LL1129236/Marine-insurers-investigate-scrubber-incidents-amid-risk-elevation-concerns
  6. Lange, D. B., et al., “Impacts of scrubbers on the environmental situation in ports and coastal waters,” July 2015, online: https://www.umweltbundesamt.de/sites/default/files/medien/378/publikationen/texte_65_2915_impacts_of_scubbers_on_the_envoronmental_situation_in_ports_and_coastal_waters.pdf
  7. Comer, B., “Scrubbers on ships: Time to close the open loop(hole),” The International Council on Clean Transportation, June 2020, online: https://theicct.org/blog/staff/scrubbers-open-loophole-062020
  8. Agren, C., “Environmental impacts of ship scrubbers, AirClim, October 2019, online: https://www.airclim.org/acidnews/environmental-impacts-ship-scrubbers
  9. Mahajan, S., “Learning as we go: Challenges with the use of exhaust gas scrubbers,” Gard blog, October 2019, online: https://www.gard.no/web/updates/content/28519122/learning-as-we-go-challenges-with-the-use-of-exhaust-gas-scrubbers
  10.  “2020 Fuel Oil Quality and Safety Survey,” BIMCO, INTERCARGO, the International Chamber of Shipping (ICS) and INTERTANKO, 2020, online: https://www.bimco.org/-/media/bimco/news-and-trends/news/priority-news/2020/2020-fuel-oil-quality-and-safety-survey—-report.ashx#:~:text=On%2024%20February%202020%20BIMCO%2C%20The%20International%20Chamber,safety%20implications%20of%20the%20IMO%202020%20sulphur%20regulation 


The Author

Related Articles

From the Archive



{{ error }}
{{ comment.comment.Name }} • {{ comment.timeAgo }}
{{ comment.comment.Text }}