Fuel oil blends produced to meet the IMO 2020 sulphur limit, referred to as very low sulphur fuel oil (VLSFO), have had a lot of bad press. Most of it has been based on alarmist predictions about the quality and other characteristics of these fuels, which have later proven to be unjustified or only partially accurate.

Earlier this year, IBIA addressed claims that the shift from high sulphur fuel oil (HSFO) with up to 3.50% sulphur to VLSO blends meeting the new 0.50% sulphur limit for marine fuels would increase black carbon (BC) emissions. The claims, which proliferated in the press and on social media, were based on a study submitted to the IMO indicating that the new VLSFO blends could potentially be highly aromatic, and therefore increase black carbon (BC) emissions.

IBIA and others explained to an IMO meeting (PPR 7) in February 2020 that the fuel specimens used in the BC measurement study were not representative of most VLSFOs that were actually in the market. IBIA made a statement at PPR 7, and interventions by ISO and IMarEST are on record in an annex to the IMO’s report from PPR 7. VLSFOs delivered to ships have so far generally been more paraffinic and less aromatic than the HSFOs they have replaced.

Now, a new theory has been circulated in the press based on an article published on LinkedIn by Francisco Malta of VM Industrials, a distributor for additive maker Aderco. The article, originally* published with the headline “Why new VLSFO 0.5% Sulphur fuels emit higher Black Carbon Emissions” claimed that it isn’t a high aromatics content, but rather paraffinic hydrocarbons in VLSFO that are to blame for BC emissions.

IBIA finds claims made in the article questionable. Without getting into too much detail, let’s explain some of the issues.

First of all, the article gave the impression that the introduction of VLSFO has led to an increase in black carbon (BC) emissions.* However, the article did not present or reference any independently validated data from actual measurements of actual VLSFOs in use so far to support this observation.

It is important to note that the discussion at IMO in February was about a theoretical increase in BC emissions based on the BC measurement study, not an actual observed increase since ships began to use VLSFOs. The BC measurement study, which had been submitted to the IMO in November 2019, showed that an increase in aromatic content was associated with increased BC emissions. This was well understood by industry, which welcomed the study as an important contribution to build better knowledge about factors contributing to BC emissions. It was the assumption that VLSFOs would be more aromatic than the HSFOs they were replacing that was disputed.

Understanding all the factors behind black carbon emissions is a complex science and many people struggle to get a good understanding of it, including experts on chemistry and combustion processes. Formation of BC depends on multiple and variable factors and how they interact, including the engine type, engine load, engine condition, the makeup of the fuel and the pre-conditioning of the fuel prior to injection.

While the article claimed that an it was an increased level of paraffinic hydrocarbons in VLSFO that were causing an increase in BC emissions, we should not forget that a large portion of marine distillates are mainly paraffinic in nature. Yet there have not been any reports of an increase in BC emissions in emission control areas (ECAs), where most ships have been using marine gasoil to meet the 0.10% sulphur limit since 2015. We do note, however, that even clear and bright distillates can cause very visible black smoke when the engine is not in an optimum condition/and or setting.

The article stated that the asphaltenes in HFO 3.5% S “drop from suspension and end up as sludge in tanks” hence “they rarely ever make their way to combustion” whereas in the new VLSFO scenario, they “do make their way to the combustion chamber.” This isn’t what we have seen in practice. Conventional high sulphur fuel oils (HSFOs) are typically less prone to forming asphaltenic sludge prior to combustion than we have seen with VLSFOs so far. HSFO is typically more aromatic than VLSFO, and this helps keep the asphaltenes in suspension and stable. Hence, there would be more asphaltenes reaching the combustion chamber when using HSFO than VLSFOs. Also, VLSFOs typically contain less asphaltenes than HSFOs so it seems counterintuitive to suggest that more asphaltenes reach the combustion chamber when using VLSFO compared to HSFO.

Simply put, black carbon, or soot, is a result of incomplete combustion. From what we have heard, fuels with higher paraffinic content are associated with improved combustion compared to highly aromatic fuels. So far, we have not heard about VLSFOs being particularly prone to poor and/or incomplete combustion compared to HSFOs (which are typically more aromatic). Hence the claims in the article appear to run counter to real world experience.

We learnt from the discussions about VLSFO and BC emissions prior to the IMO meeting in February that misunderstandings can take hold when theories are taken as evidence of fact, when the reality may be quite different.

IBIA believes it is important to have proper data and empirical evidence to back up theories. When pointing to a certain fuel characteristic as the cause of an increase in BC emissions, it needs to be backed by independently validated data and specifics on the measurement methodology used to define black carbon.

VLSFO is currently the most widely adopted solution to meeting the 0.50% sulphur limit and for the most part it has performed better than predicted in the run-up to IMO 2020. As such, VLSFO blends have confounded much of the bad press. As VLSFOs are still quite new, we are still learning. We should, however, ensure that any conclusions drawn about the performance and environmental impact of VLSFOs are based on solid evidence.

Source: ibia