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February 23, 2001

Wärtsilä cooperates with water mist company on emissions reduction
Wärtsilä Diesel has found what, at first, seems an unlikely partner in its efforts to reduce emissions. That partner is Marioff, developer of the Hi-Fog system of fire suppression and smoke reduction using water mist.

Marioff is thus a specialist in creating humidity. And Wärtsilä needs humidity to reduce NOx in diesel and perhaps gas engines.

It is well known that introduction of water into the combustion space of a diesel engine in one way or another reduces NOx formation.

There are two main mechanisms for this phenomenon, one being the addition of mass in the combustion space, which reduces the temperature increase created by the combustion. The other effect may be evaporation of the water in the combustion space during combustion, which means certain areas of the combustion space can be cooled down.

Both mechanisms are utilized in the Direct Water Injection system, which is one of the technologies promoted by Wärtsilä. However, these technologies cannot be used for very low NOx, because the evaporation of the water in the combustion space will steal some of the combustion energy. For the lowest NOx values it is therefore desirable to go another way, and that is to evaporate the water before it enters the combustion space.

Wärtsilä created such a system in the 1980s called Combustion Air Humidification. At that time it was not possible to add very much water to the combustion air due to sensitive cylinder liner piston ring technology. Since the introduction of the antipolishing ring, however, diesel engines have been virtually insensitive to humidity in the combustion air, and therefore Wärtsilä decided to again take up the development of this system.

Here the Marioff technology of creating very small water droplets, HI-FOG, offers a neat fit. When water in this form is introduced directly after the turbocharger in an environment of more than 200 degrees C and more than 75 m/s air velocity, the small water droplets will evaporate very fast. The enclosed figure shows the system. Evaporation of water needs a lot of energy, and for that reason additional energy is introduced via the charge air cooler, which in this case is working as an air heater. The result is combustion air saturated with water. Experiments have shown that it is possible to reach a NOx level of less than 3 g/kWh with this system, assuming that the starting value is 10-15 g/kWh.

According to the agreement Wärtsilä will take care of the engine optimization, while Marioff will provide all water-related technologies, i.e. nozzles and water systems.

The benefits of the Combustion Air Saturation System are:

  • -Reduction of NOx without increase of fuel consumption.
  • The system does not change the engine architecture.
  • The system can be switched on and off without affecting the engine function. The charge air cooler simply becomes a cooler instead of a heater. This feature may be valuable in certain areas, because increased humidity in the exhaust air may make the exhaust more visible, although it is a white mist containing mainly water. However, in some cases it may be more sensible to go with no visible exhaust rather having to explain that it is just water mist.

A possible disadvantage of the system is that the water consumption is pretty high, i.e. around two times the fuel consumption. In a normal cargo ship this is not a problem because at least in the case of four-stroke engines there is enough heat to produce the necessary water by sea water evaporation.

In cruise ships, the concept is to use purified waste water.

The agreement between Marioff and Wärtsilä also covers another development idea, a scrubber based on Hi-Fog technology.

Up to now scrubbers have not been very successful for removing particulates from the exhaust gas because the exposed surface between the gas and the liquid is too small.

The total surface of a liquid increases in inverse proportion to the size of droplets, i.e. if the droplet size changes from one millimeter to one micron, the total surface of the droplets is one thousand times larger. This fact may make a scrubber based on HI-FOG technology able to remove particulates from the exhaust gas.

It is too early to say that this technology will be successful in all respects, and there are problems to solve concerning cooling of the exhaust gas, etc. However, there is also a great demand for a technology with the ability to reduce particulates in the exhaust of heavy fuel burning engines.

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