After some project experiences, Sox scrubber system from STI has been found to be effective in terms of installation space, weight, and power consumption. Our experience is to have one scrubber unit per each exhaust gas pipe; so called as “In-Line system with Single inlet”. This has been found to be useful when the installation takes place on existing ships as scrubber units can replace silencer units so that the amount of installation work and funnel casing modification work can be minimum.
The situation becomes a little different when comes to new building projects. Having a ‘large’ scrubber unit to accommodate exhaust gas flows from various diesel combustion units have some advantage over installing as many scrubber units as the number of diesel combustion units. Of course, there are some different considerations are to be made, like by-pass lines, isolation dampers, etc. This type of scrubbers are called as “In-Line system with Multi inlet”.
STI has modified its own scrubber design from “In-Line system with Single inlet” to “In-Line system with Multi inlet”. As a result, this is made as a new line of scrubber design together with the existing designs. As the scrubber units from STI are of “In-Line type” design, we call the new design as “Multi inlet system”.
Multi Inlet scrubber units are found to have some advantages of reducing the ‘overall volume’ of scrubber units installed onboard as the design is made by taking the operation modes into account. It, as a result, reduces overall installation weight as well as cost.
System sizing for multi inlet scrubber units are not very straight forward as there are many issues to be checked in advance. For example,
Number of diesel combustion units to be connected to the scrubber unit.
Operation modes (ex. Sea going, Port, Maneuvering, Cargo cooling, etc.) for the scrubber sizing and utility sizing
Considerations for the bypass lines by regulatory requirements? and/or maintenance point of view. (For the case of the scrubber system design from STI, bypass pipes are not necessary as it can be switches between “dry operation” and “(wet) operation” without making any changes.)
STI is pleased to include some design cases to help customers to have some ideas about scrubber unit size, weight, and utilities as below.
This is a 3D rendering image of an In-line Multi Inlet Scrubber unit for Case 1.
In this technical note, we have shown the corrosion rate of SUS316L material in the presence of sulfuric acid as per the weight concentration in %. Expected annual corrosion of the scrubber body, when SUS316L is used, is ~0.03mpy, corresponding to?0.000762mm/year of corrosion speed.
The corrosion speed depends on the weight concentration of sulfuric acid during scrubber operation. STI has shown that it is in the range of ~10-4% of weight concentration of sulfuric acid, which is well below the concentration ranges where other types of stainless steel material like duplex, super duplex, etc.
As a summary, SUS316L is good enough to make marine SOx scrubber units considering the weight concentration of sulfuric acid (being very low) and the operation temperature of <100degC.?
After more than 10 years of discussions, International Maritime Organization (IMO) finally set the global sulfur cap of 0.5% to be in place from 2020, during its?Marine Environment Protection Committee (MEPC), meeting for its 70th session in London during 27~28 Oct. 2016. (See the press release from IMO here.)
“Regulations for the Prevention of Air Pollution from Ships”, MARPOL Annex VI?adopted on 19 May 2005, which has been updated at later meetings. Regulation 14 of this guideline is about controlling SOx emission from marine diesel engines from 1)?All ships of 400 gross tonnage, 2)?Fixed or floating platforms (drilling rigs) and 3) Floating craft and submersibles.
The regulation 14 suggests the fuel is to be of ‘clean fuel’ with?low sulfur contents. As an alternative to using ‘clean fuel’, MEPC allows exhaust gas cleaning systems like ‘scrubbers’ to clean the emissions before the exhaust gas is release to atmosphere.
SOx scrubber systems from STI is a result of more than 10 years of development, onboard tests, and actual installations onboard ships. STI’s SOx scrubber system has?following features,
in-line system that can replace existing silencers,
dry/wet run without any provisions to switch the modes,
waste water treatment to comply with discharge water requirements,
compact design with low water consumption,
continuous emission monitoring system (Scheme B) as standard but Scheme A configuration is available
all-in-one control and monitoring system with independent emergency shutdown system
various configurations of open/close/hybrid operation as per requirements.
SOx scrubber system from STI is custom made for each project. STI is ready to provide technical and commercial proposals.
Wet scrubber systems using seawater or freshwater uses big amount of water to dissolve SOx included in the exhaust gas from marine diesel engines. Most of the sprayed water goes down to the drain, but some of the carry-over condensates by the exhaust gas flow and condensates created after the outlet of scrubber units may generate water drops falling on the ship.
Various efforts have been made to remove the water drops falling on the ship: large heater, induction fan, and mist eliminator. Large heaters heat-up the flow to evaporate condensates but requires electric power. Induction fans would be a good choice but the entire scrubber system depends on the fan, which may add some large back pressure to the exhaust gas pipe system.?Mist eliminator uses static filter so called as ‘chevron mist ‘, which collects condensates and drains. Problem with the mist eliminators are filter blockage and large size.
Manufacturers of ‘Chevron’ type mist eliminators recommend that the flow speed when passing through the filter units would be limited to be 2.5m/s (as slow as possible, of course). This is a tough requirements for applications like scrubber systems.
Exhaust gas flow speed is around 25~30m3/s within exhaust gas pipes.
To reduce the speed, cross section area should be increased by almost 10 times of exhaust gas pipes.
When the filter unit is installed horizontally, draining of collected condensates becomes an issue unless the cross section diameter is increased more than 3 times of existing exhaust gas pipes…this is why most of the scrubber manufacturers in the market should have a huge diameter in the demister section.
Through the past project experience, STI has gains enough experience to deal with the condensates. STI provides small-scale and blockage-tolerant demister system together with its SOx scrubber system.