Charge air humidification

Another way of introducing water into the combustion zone is by humidifying the scavenge air: warm water is injected and evaporated in the air intake, whose absolute humidity is thereby increased. An early drawback was that too much water in the air can be harmful to the cylinder condition but the introduction of the anti-polishing ring in the liner (see the chapter Medium Speed Engines— Introduction) has allowed a much higher humidity to be accepted.

In the Combustion Air Saturation System (CASS), which resulted from co-operation between Wartsila and the Finnish company Marioff Oy, special Hi-Fog nozzles introduce water directly into the charge air stream after the turbocharger in the form of very small droplets. Only a few microns in size, the droplets evaporate very quickly in an environment of more than 200oC and 75 m/s air velocity. Further heat for evaporation is provided by the air cooler (but acting now as a heater), resulting in combustion air with a humidity of around 60 g per kg of air. With such an amount of water, Wartsila reports, experiments demonstrated it was possible to secure NOx levels of less than 3 g/kWh (assuming the starting value is 10-15 g/kWh).

MAN B&W Diesel has also pursued NOx reduction by increasing the humidity of the charge air with water vapour. The process is based on the Humid Air Motor (HAM) system developed by the German company Munters Euroform (Figure 3.8). Compressed and heated air from the turbocharger is passed through a cell that humidifies and cools the air with evaporated water, the distillation process making it possible to use sea water rather than fresh water. The relative humidity of the inlet air can be kept constant at 99 per cent, yielding a reported maximum NOx reduction potential of around 70 per cent, exploiting all the waste heat of the engine.

Test results from an SEMT-Pielstick 3PC2.6 medium speed engine equipped with a prototype HAM system showed a NOx reduction from 13.5 g/kWh to 3.5 g/kWh. There was no significant influence

Figure 3.8 Schematic diagram of Humid Air Motor (HAM) system for NOx emission reduction

on specific fuel consumption, no significant increase in carbon monoxide and hydrocarbon emissions, and no smoke deterioration. These results—along with no trace of water in the lube oil, no corrosion and cleaner engine internals—were repeated in subsequent seagoing installations. In retrofit projects, the HAM system replaced the original intercoolers.

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Responses

  • jessica
    What is charge air humidifier?
    3 months ago

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