1 mm of Soot Reduces 10% Heat Transfer Efficiency – Keep the Exhaust Stacks Clean – WSS



Exhaust gas boilers (Photo: WSS)

Wilhelmsen Ships Service, a provider of products and services to the shipping industry, is advising vessel operators to wage war on soot.  According to WSS, soot accumulation in a ship’s exhaust gas boiler (EGB) is a serious issue, leading to reduced EGB efficiency, significant cleaning costs, corrosion, and the risk of soot fires.  However, a simple post-combustion fuel treatment can solve all these problems, at negligible cost.

Refined heavy marine fuels are full of contaminants, such as Vanadium, Sodium, Aluminium, Silica, and Potassium, which leave deposits when burnt.  These particles, soot, pass through vessel exhaust valves and turbochargers, continuing with the flue gases until they reach the EGB.  Here they stick to the boiler’s heat transfer surfaces and impair its efficiency, as Jonas Östlund, WSS Product Marketing Manager, Marine Products, Oil, explains:

“Just like any heat exchanger, an obstructive deposit will reduce the transfer of heat and therefore reduce the efficiency of the equipment,” he notes.  “Soot deposits are particularly effective at reducing heat transfer as they insulate extremely well.  A 1mm layer of soot can reduce EGB efficiency by 10%.  Left to build up to a 3mm layer, EGB efficiency can be reduced by up to 50%.”

“Obviously this calls for cleaning, with typical EGBs requiring around five hours of cleaning during port calls, usually every two to three months.  This means labour, equipment, and the disposal of wash water that contains acidic soot – the latter being something currently up for regulatory discussion, with an expectation that it will have to be disposed of onshore, at extra cost.”

“So, the cleaning task is more than a minor hassle – it’s a major inefficiency, cost and labour burden for the engine room.”

Cleaning and EGB inefficiency are the most obvious problems, but it doesn’t stop there. Östlund states that un-burnt fuel and lubricants can also be deposited in the EGB.  These lower the ignition temperature of soot, and increase the risk of soot fires.

“These can permanently damage the ECB and, although uncommon, pose a very real threat,” he says.

“In addition, cold corrosion is also a factor.  Sulphur in the fuel reacts with oxygen during combustion to form Sulphur Dioxide and Sulphur Trioxide.  When the temperature drops below 135°C, which occurs in EGBs operating at low velocities, the Sulphur Trioxide reacts with the moisture in the air and forms sulphuric acid.  This is very corrosive to tube surfaces, and affects metal in a similar way to rust.”

The problems are myriad, but the solution is simple, according to Östlund.  He stresses that post-combustion fuel treatments – such as WSS’ Unitor’s Fuel Power Soot Remover Liquid Plus – provide easy relief, adding “fuel treatment shouldn’t just end in the fuel tanks.”

Source: Wilhelmsen Ship Service

There are many additives available in the market.  WSS has unitor soot cleaning products which can help remove the soot accumulation.

While the above news is from WSS, we refer to proactive measures how the soot itself shall be prevented from building up.  As clearly explained by WSS, it is always recommended to keep the stacks clean to improve efficiency and enhance safety by preventing fires.

The best solution would be to:

  1. Run the engines on a higher loads or reduce the time the engine running on low-loads.
  2. Use distillate fuels at low loads where if it demonstrates being economical.
  3. Especially diesel generators perform better when the load is kept as high as possible.
  4. Regular cleaning of EGB stacks.
  5. Use of additives to improve combustion quality.
  6. Use good quality fuel as far as possible.


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