Seawater is the main absorbing medium used in the exhaust gas cleaning process. Seawater is alkaline and thus offers a buffer and neutralization capacity. Seawater also contain large amounts of dissolved sulphates. The absorbed SO2 will only marginally increase the seawater content of sulphates. Commercial available NaOH solution (50%) is injected in both cleaning stages to boost the seawater alkalinity and thus increase the neutralization capacity and the absorption efficiency in the EGCS. This boost will also compensate for the fluctuating alkalinity content in the ambient seawater, and will reduce the amount of seawater used and raise the pH of the washwater.
Through efficient interaction between the highly alkaline seawater and the exhaust gas stream, SO2 and also particulate matter (PM) is dissolved in, or captured by, the liquid.
When dissolved in the liquid, SO2 originating from the burnt fuel oil will make the liquid acidic before the alkalinity in the water convert the sulphur to harmless sulphites/sulphates. Such sulphites/sulphates may be discharged to sea without further treatment.
SO2 is dissolved in seawater (reaction a), it reacts with water and forms an acid, H+ ions are produced (i.e pH decreases). Bicarbonate and carbonate in the seawater reacts with the released H+ , thus neutralizing part of the acidic effect from the absorbed SO2. The sulphite will react with the oxygen in the seawater and form sulphate (reaction c). Adding caustic soda (NaOH) to the seawater will increase the neutralizing capacity, last two reactions.
The washwater is monitored by measuring pH, PAH and turbidity. If the washwater exceeds given limits for turbidity and PAH, then a part of the washwater ("1st stage injection drain") may be cleaned through the Water treatment filter before being discharged to sea. Residue from the filter will be contained on board before being discharged to shore.
The system may operate in the following modes:
• Open loop mode
• Closed loop mode
• Open loop reduced load mode
• Closed loop reduced load mode
OPEN LOOP
Seawater is used as cleaning liquid and pass through the system once before being
monitored and then discharged to sea.
The seawater alkalinity is boosted with NaOH to increase the SOx absorption
A part of the washwater ("1st stage injection drain") is collected in the gas inlet section of
the scrubber, and this stream may be cleaned in the Water treatment filter before
discharge.
CLOSED LOOP (Alternative system)
Washwater is used as cleaning liquid and is recirculated through the system via the System
tank and Recirculation pump.
The washwater alkalinity is boosted with NaOH to increase the SOx absorption.
To avoid saturation, a bleed stream is drained from the Decanter tank to the Holding tank and a balancing amount of makeup water (seawater) is added to the System tank.
The recirculated washwater is cooled before being re-injected to the EGCU.
A part of the washwater ("1st stage injection drain") is collected in the gas inlet section of the scrubber, and this stream is cleaned in the Water treatment filter before recirculation.
Provision of two basic Schemes for compliance to be used for EGCS approval, Scheme A or Scheme B, as the choice of the equipment manufacturer.
The two EGCS schemes apply the following concepts:
• Scheme A - based on initial emission performance unit certification together with a continuous check of operating parameters and daily exhaust emission monitoring
• Scheme B - based on continuous exhaust emission monitoring together with a daily check of operating Parameters
REQUIRED EGC SYSTEM DOCUMENTATION
On board Documentation
SECP Sox emission compliance Plan
ETM-B EGCS technical Manual Scheme – B (ETM – B)
OMM Onboard Monitoring Manual
EGC Record Book or Electronic Logging system
The system consists of 2 water monitors:
No. 1 is installed close to the scrubbing water pumps measuring the (inlet) scrubbing water parameters pH and Turbidity, the pH value is used for monitoring purpose and to give alarm is it is too low, the Turbidity value is used to deduct it from the outlet value as the outlet limitations are above the inlet.
No. 2 is installed on the outlet of the system close to the over boards, measuring the
Wash water parameters pH, Turbidity and PAH (Poly Cyclic Aromatic Hydrocarbons)
-pH limits are different for each system and is specified in the ETM-B for the specific vessel.
-Turbidity limits are 25NTU with an exceedance allowance of 20% for every 12-hour period.
-The PAH limits are also different for each system and is stated in the ETM-B for the specific vessel. This parameter has also has an exceedance allowance of 100% of the limit for every 12-hour period.
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