Progress on black carbon (BC) and solutions for combatting it

Late in March, IMO’s Pollution Prevention and Response (PPR) sub-committee held its 8th meeting. This was yet another crossroads for the discussion on black carbon (BC), hinging on the question of what to do next. For a long while, it seemed that no decisions could be made given the prevailing view, namely that BC would be controlled through the HFO ban in the Arctic. It looked as if the last 11 years of discussions and investigations had been in vain.



However, a last attempt, brought about through excellent cooperation among Arctic countries, came to the rescue. PPR arrived at an agreement built on a phase-in, goal-based approach. IMO is to:

  • “… develop, as a starting point, guidelines on recommendatory goal-based control measures to reduce the impact on the Arctic of Black Carbon emissions from international shipping, taking into account the identified candidate control measures…,” and
  • “… further consider regulating or otherwise directly control Black Carbon emissions from marine diesel engines (exhaust gas) to reduce the impact on the Arctic of Black Carbon emissions from international shipping, taking into account the identified candidate control measures…” [PPR 8/13]

So, what does this mean in practice? Presumably, the idea is to start with voluntary (or recommendatory) BC control measures, a term I will translate in a bit. This means, with some expectations on the part of regulators, that ships – with focus on those operating in the Arctic – should look to reduce BC to a certain level. At a later stage, if so agreed, these voluntary control measures can be made mandatory based on the experience gathered.

The term “goal-based” is important, because it indicates that the means of reducing BC is of lesser importance. No means is prescribed and none is rejected, so long as the BC pollution isn’t transformed into another type of pollution as a result. A “control measure” could thus be a fixed limit or sets of limits, but it could also be the utilization of “best available technology” (BAT).

At this point, it isn’t clear what expectations regulators have on a “control measure”. My guess is that various nations have different ideas of such. The problem is that we don’t have any existing regime to lean on. From the regulation of trucks and cars, the shipping industry adopted a direct parallel: reducing SOx via the fuel and NOx through after-treatment. Even the emission levels were available as a reference. For BC we have nothing – IMO needs to invent it.

Alfa Laval has long wished that the BC agenda item had instead been a particulate matter (PM) and/or particulate number (PN) discussion. Had this been the case, the issue would have been more easily resolved with regard to monitoring, limit setting, reduction technologies, etc. When PM/PN is reduced, so is BC. To some extent, this view is embedded in the new product portfolio Alfa Laval is exploring and developing.

When it comes to technologies for reducing BC (and PM), the application benefits are much more dependent on ship type and operational pattern than those for SOx and NOx. This means that a broad product portfolio is needed, but the common denominator for all the technologies is that BC should be reduced by at least 90%. Likewise, they should all be suitable for marine use.

A wet electrostatic precipitator (wet ESP or WESP) is an excellent solution as a retrofit to an existing EGCS, or as part of a new EGCS installation. It can also be installed as a standalone unit. A WESP works by electrically charging a tube bundle to attract the PM in the exhaust gas, which is then caught by the running water on the tubes. This is a robust technology that works well for shipping and can be used broadly across ship types and operational patterns.

Diesel particulate filters (DPF) are another well-known technology. These filters collect PM larger than their mesh size and must periodically be cleaned of collected soot, which means ash is a by-product. The technology is simple and easy to service, but its downside is high backpressure. The filters must therefore be installed on the pressure side of the turbocharger. Although the main application so far has been two-stroke engines and deep-sea shipping, there are no technical limitations preventing DPF deployment on four-stroke engines as well.  

A third technology that is a good solution for shipping is catalytic DPF (or CDPF). These filters have a catalyst that “burns” off the PM, and the very fine ash that remains is collected. This means that by-products are more or less eliminated, and backpressure issues are eliminated with them. We’ve experienced more than 98% PM removal with such filters, based on mass. Since the operational temperature of the catalyst is around 350°C, the main application is four-stroke engines operating in NOx ECAs, making CPDF a perfect technology for cruise ships, ferries, etc.

With these technologies in ongoing development, Alfa Laval welcomes the outcome of PPR 8. We’re working actively with the marine solutions to achieve a cleaner world, whether the focus is on BC or PM as a whole.

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Fact box

Particulate matter (PM) is a collective name for fine solid or liquid particles added to the atmosphere by processes at the Earth's surface. Particulate matter includes dust, smoke, soot, pollen and soil particles [European Environment Agency, April 27, 2021,]. In relation to exhaust gas emissions, PM therefore comprises soot, dust and smoke.

The amount of PM is normally expressed in terms of mass. However, it can also be expressed in combination with specific particulate sizes. An example is PM10, which is the mass of particulates larger than 10 micrometres. 

Particulate number (PN) is just another means of quantifying PM. Within a given volume, the number of particles, regardless of size, can be counted.

Black carbon (BC) is the black share of the particulate matter in the exhaust gas. IMO has defined BC* as:

[…] a distinct type of carbonaceous material, formed only in flames during combustion of carbon-based fuels. It is distinguishable from other forms of carbon and carbon compounds contained in atmospheric aerosol because it has a unique combination of the following physical properties:

1.    it strongly absorbs visible light with a mass absorption cross section of at least 5 m2g-1 at a wavelength of 550 nm;

2.    it is refractory; that is, it retains its basic form at very high temperatures, with vaporization temperature near 4000 K;

3.    it is insoluble in water, in organic solvents including methanol and acetone, and in other components of atmospheric aerosol; and

4.    it exists as an aggregate of small carbon spherules.


*Based on: Bond, T.C. et al. (2013). Bounding the role of Black Carbon in the climate system: A scientific Assessment, Journal of Geophysical Research: Atmosphere, 118, 5380-5552, doi:10.1002/JGRD.50171