When the International Maritime Organization established a compulsory limit on the sulfur level of maritime fuels in 2020, aimed at diminishing detrimental environmental and health effects, it presented shipping companies with three primary alternatives.
They could utilize low-sulfur fossil fuels, such as marine gas oil, or implement purification systems to eliminate sulfur from the exhaust gases generated by burning heavy fuel oil. Biofuels with a lower sulfur content offer another viable option, though their scarcity renders them a less practical choice.
Although the installation of exhaust gas cleaning systems, referred to as scrubbers, represents the most practical and cost-effective solution, there remains considerable uncertainty among enterprises, policymakers, and researchers regarding how “green” these scrubbers truly are.
Through an innovative lifecycle evaluation, scientists from MIT, Georgia Tech, and other institutions have discovered that burning heavy fuel oil with scrubbers in the open sea can either match or exceed using low-sulfur fuels when a broad range of environmental aspects are taken into account.
The researchers integrated data concerning the production and functioning of scrubbers and fuels with emissions measurements collected aboard an ocean-going cargo vessel.
They determined that, when the complete supply chain is assessed, burning heavy fuel oil with scrubbers emerged as the least damaging alternative concerning almost all 10 environmental impact factors examined, including greenhouse gas emissions, land acidification, and ozone creation.
“During our partnership with Oldendorff Carriers to widely investigate reducing the ecological impact of shipping, this examination of scrubbers proved to be an unexpectedly significant and intricate transitional issue,” states Neil Gershenfeld, an MIT professor, director of the Center for Bits and Atoms (CBA), and lead author of the research.
“Assertions regarding ecological risks and policies aimed at alleviating them ought to be supported by scientific evidence. One must observe the data, maintain objectivity, and devise studies that consider the complete scenario to adequately compare various options from a like-to-like perspective,” adds principal author Patricia Stathatou, an assistant professor at Georgia Tech, who initiated this research as a postdoctoral fellow at the CBA.
Stathatou is accompanied on the publication by Michael Triantafyllou, the Henry L. and Grace Doherty, and others from the National Technical University of Athens in Greece as well as the maritime shipping company Oldendorff Carriers. The findings are published today in Environmental Science and Technology.
Reducing sulfur emissions
Heavy fuel oil, conventionally burned by bulk carriers that constitute approximately 30 percent of the global maritime fleet, typically contains a sulfur concentration between 2 and 3 percent. This level greatly exceeds the International Maritime Organization’s 2020 limit of 0.5 percent in most ocean areas and 0.1 percent in regions adjacent to population centers or environmentally sensitive zones.
Sulfur oxide emissions are a contributor to air pollution and acid rain, posing threats to the human respiratory system.
In 2018, fewer than 1,000 vessels operated with scrubbers. Following the implementation of the cap, increased costs of low-sulfur fossil fuels and the limited availability of alternative fuels prompted numerous companies to install scrubbers to continue using heavy fuel oil.
Currently, over 5,800 vessels are equipped with scrubbers, the majority of which are wet, open-loop systems.
“Scrubbers are a well-established technology. They have historically been utilized for decades in land-based applications, such as power generation, to remove pollutants,” states Stathatou.
A wet, open-loop marine scrubber consists of a large, metal, vertical tank positioned in a ship’s exhaust stack, above the engines. Inside, seawater sourced from the ocean is sprayed through a series of nozzles downward to cleanse the hot exhaust gases as they exit the engines.
The seawater interacts with sulfur dioxide in the exhaust, transforming it into sulfates — water-soluble, environmentally safe compounds that naturally exist in seawater. The washwater is discharged back into the ocean, while the purified exhaust is released into the atmosphere with minimal sulfur dioxide emissions.
However, the acidic washwater may contain other combustion byproducts, such as heavy metals, leading scientists to question whether scrubbers are environmentally comparable to burning low-sulfur fuels from a holistic perspective.
Numerous studies have investigated the toxicity of washwater and pollution from fuel systems, yet none have presented a comprehensive perspective.
The researchers aimed to bridge that scientific gap.
A “well-to-wake” analysis
The team performed a lifecycle assessment using a global environmental database that covered the production and transportation of fossil fuels, including heavy fuel oil, marine gas oil, and very-low sulfur fuel oil. Comprehensive evaluation of each fuel’s lifecycle is vital, as producing low-sulfur fuel necessitates additional processing steps in the refinery, which leads to increased emissions of greenhouse gases and particulate matter.
“If we only consider everything that occurs before the fuel is bunkered onto the vessel, heavy fuel oil is considerably less impactful environmentally than low-sulfur fuels,” she remarks.
The researchers also collaborated with a scrubber manufacturer to gather detailed information regarding all materials, manufacturing processes, and transport phases involved in the construction and installation of marine scrubbers.
“Considering that the scrubber has an operational lifespan of about 20 years, the environmental impacts associated with producing the scrubber are negligible in comparison to the production of heavy fuel oil,” she notes.
For the final part of the study, Stathatou spent a week aboard a bulk carrier in China to measure emissions and collect seawater and washwater samples. The ship operated on heavy fuel oil with a scrubber and low-sulfur fuels under analogous ocean conditions and engine parameters.
Gathering these onboard data was the most demanding aspect of the research.
“The combination of all the safety equipment, along with the heat and noise from the engines on a moving vessel, was quite overwhelming,” she recalls.
Their findings indicated that scrubbers reduce sulfur dioxide emissions by 97 percent, placing heavy fuel oil on par with low-sulfur fuels concerning that measure. The researchers observed similar trends regarding emissions of other pollutants such as carbon monoxide and nitrous oxide.
Additionally, they tested washwater samples for more than 60 chemical parameters, including nitrogen, phosphorus, polycyclic aromatic hydrocarbons, and 23 metals.
The levels of chemicals regulated by the IMO were significantly below the organization’s standards. For unregulated substances, the researchers compared the concentrations to the most stringent limits for industrial discharges set by the U.S. Environmental Protection Agency and the European Union.
Most chemical concentrations were at least an order of magnitude lower than these regulations.
Moreover, since washwater is diluted thousands of times as it is dispersed by a moving vessel, the concentrations of these chemicals would be considerably reduced in the open ocean.
These results imply that employing scrubbers with heavy fuel oil can be deemed as environmentally equivalent to or even more advantageous than low-sulfur fuels across numerous impact categories explored by the researchers.
“This investigation reveals the scientific intricacies of the waste stream associated with scrubbers. Now, with the completion of a multi-year, thorough, and peer-reviewed study, commonly held apprehensions and beliefs have been addressed,” states Scott Bergeron, managing director at Oldendorff Carriers and co-author of the research.
“This pioneering study on a well-to-wake basis offers invaluable insights for ongoing discussions at the IMO,” adds Thomas Klenum, executive vice president for innovation and regulatory affairs at the Liberian Registry, underscoring the necessity “for regulatory choices to be based on scientific research that provides factual data and findings.”
In conclusion, this research highlights the significance of integrating lifecycle assessments into future strategies aimed at reducing environmental impacts, asserts Stathatou.
“There is much discourse surrounding the transition to alternative fuels in the future, but how sustainable are these fuels? We must conduct thorough comparisons with current solutions to evaluate their costs and advantages,” she emphasizes.
This research was partially funded by Oldendorff Carriers.