Exhaust gas cleaning system

Alternative emission abatement equipment includes exhaust emissions – a scrubber. It  can be open or closed loop or hybrid (open and closed loop). Scrubbers ensure that emissions from low-sulfur fuels are reduced. Table 1 shows documents, compulsory to be carried on board:

Table 1. Documents, compulsory on board a vessel, related to the exhaust emissions

NOTE: Exhaust gas cleaning (EGC) equipment suitable for use on board ships is recognized not only as having a continuous emission monitoring system (Scheme B) but also without it (Scheme A). When using scheme A EGS equipment, it is necessary to check its operating parameters and emissions periodically.

Exhaust gas cleaning system

The scrubber system shall be equipped with bypass devices designed to protect the engine(s) in the event of a scrubber malfunction. The scrubber bypass mode is also selected when no flue gas cleaning is required. The exhaust flue has a bypass valve, and the scrubber inlet pipe has an intake valve. The bypass and intake valves shall be operated in such a way that the intake valve cannot close until the bypass valve is fully opened and vice versa.
In order to maximize the contact between the exhaust gas and the water in the scrubber, it is necessary to mix the water thoroughly with the exhaust gas. For this purpose, when the bypass valve is closed, the exhaust gases are directed to the narrowing jet compartment of the scrubber. As the cross-sectional area decreases, the gas flow rate increases (Bernoulli equation). At the same time, washing water is fed from the nozzles on the side along the narrow channel to the narrowing compartment. Intense turbulence occurs in the gas flow as the exhaust gas travels in the narrow channel at high speed. This turbulence breaks down the flow of the supplied liquid into very fine droplets. Alkaline water droplets, meanwhile, neutralize the sulfur compounds in the gas and the solids deposited on the surface of the droplets.
The exhaust gas, mixed with a cloud of fine liquid droplets, moves into the expanding section of the scrubber absorber. Here the wet gas velocity decreases and the flow turbulence decreases. The droplets of liquid begin to bind, swell, and separate from the exhaust gas, at the same time taking away the absorbed sulfur compounds and particulates.
Exhaust gases passing through the jet and absorber sections are monitored by a continuous scrubber flue gas analysis system and inform the operator about the quality of the cleaning process.
The exhaust gas cleaning system can operate in two modes:

• Sea water (SW) mode (also called “open cycle”);
• Fresh water (FW) mode (also called “closed loop”).