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SOx Scrubbing of Marine Exhaust Gases
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Legislation governing SOx emissions of ships affect ship design and operation. If the vessel operates in areas where SOx emissions are controlled, compliance can be achieved by using low sulphur fuel, or by cleaning exhaust gases using SOx scrubbers, or by a combination of both.
There are two different technologies which are developed for marine application, namely sea water (“open loop”) scrubbing, and fresh water (“closed loop”) scrubbing with an added chemical (typically caustic soda).
Alkalinity is needed in the scrubbing water to neutralize acids and therefore achieve SO2-reductions. Such alkalinity to some extent available in sea water, but it can also be injected artificially in the form of an alkaline chemical. As ships have sea water available in unlimited quantities, sea water seems to be a good choice. However, there are some limitations in this concept. The main advantage of a seawater scrubber is simplicity; it requires neither additional chemicals nor fresh water for operation. In order to operate properly, a seawater scrubber needs a high flow of seawater with an adequate level of alkalinity. Water in different seas has different alkalinity. Ocean alkalinity is usually constant and high, whereas the alkalinity in the Baltic Sea is lower than normally in other seas. At low alkalinity levels the sea water scrubber can still operate, but it leads to lower cleaning efficiency.
Fresh water scrubber is a good alternative if high efficiency cleaning is needed, or as a means of avoiding sea water alkalinity problems. In such scrubbers, a caustic soda (NaOH) solution is used for neutralizing the sulphur. Fresh water scrubber cleaning efficiency is typically higher than 90 per cent. A figure as high as 97 per cent can be specified for generator engines to reach an equivalent of 0.1 per cent fuel sulphur, as will be required in, for example, European Union ports and California. Thus, engines can run on conventional HFO.
The power demand for pumps is between 0.5 and 1 per cent of the engine in question.
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Working Process of the Scrubber
The washing solution is pumped from the process tank through a system cooler to the scrubber. From the scrubber the washing solution returns to the process tank by gravity.
NaOH is fed to the system via a small feed pump. Topping-up of fresh water is needed to the extent that the evaporated or discharged water exceeds the humidity in the exhaust gases (from engine combustion).
A small portion (the “bleed-off”) of the scrubbing water flow is conducted to the treatment unit. The treated effluent is discharged overboard or alternatively to a clean bilge water tank or other suitable holding tanks. This feature is very important, as the system can periodically be operated in a “Zero Discharge Mode” without discharging any wash water overboard.
The captured contaminants (sludge) are transferred to the vessel’s sludge tank.
The process tank can be large enough to temporarily hold some bleed-off for periods when the scrubber is running but the treatment plant is not, or vice versa.
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Task 1. Study the Scrubber System Layout below and comment on:
1) the construction of the scrubber (main parts);
2) the type of scrubbing used
3) the operation of the system (processes taking part during the operation).
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Waste Incinerators – Use on Board Ships
How to handle waste?
Due to Marpol annexes getting stricter day by day, the problem related to disposing of the ship’s waste at sea is always vital. So what can be done about this waste? Accumulating it till the next port of call is the only option but what if the voyages are long? Also collecting the waste for a longer time is unhygienic and it also generates an unbearable stench. If it is chemicals or oily toxic substances then it is better to dispose of it as soon as possible as it might emit poisonous gases or foul odour. We had already discussed how bilge water is handled but these wastes cannot be pumped overboard.
Incinerators are used for this purpose. Incinerators burn the food, sewage and oil waste at high temperatures, reducing the waste to disposable ash. These wastes are present in almost every type of ship and in this text we will see how the incinerator works and what is its importance.
Construction
Incinerator is in the shape of a vertical cylindrical chamber with an inverted funnel shaped chimney at the top. The cylindrical chamber consists of a burning chamber which is lined with refractory materials inside. An oil fired burner is provided to initiate the ignition process. It is extremely important that the temperature inside the cylinder is controlled and for this reason thermostats are used. To provide an uninterrupted flow of air for the combustion, forced draft fans are provided. The air supplied is directed upwards in swirls with the help of strategically designed ports.
A rotating shaft with blades is attached at the center, which helps for a faster combustion process and also prevents incomplete combustion. The ash and the residue thus generated due to the combustion are forced to the periphery by this rotating shaft. The ash is pushed into an ash hopper and it gets collected there. A door is provided to dump the waste inside the incinerator. This door is pneumatically operated and when opened shuts down the fan and the burner automatically.
Not all the ash gets collected in the ash hopper. Some of the ash due to the forced air goes up to the chimney with the smoke. To remove this ash from the smoke a char eliminator is used. A char eliminator is similar to a filter paper. A sight glass is provided at the side of the incinerator to monitor the burning process. All the processes are controlled with the help of a control panel that is fitted on or near the incinerator.
Working and disposing
Solid waste is put inside the incinerator through the waste door, in properly arranged stacks so that the chances of incomplete combustion are minimized. The oily sludge or waste oil is not directly put into the combustion chamber neither it is put through the waste door. A separate tank is made which has its outlet into the combustion chamber. The oily sludge is first heated up to a temperature which can facilitate the difficult process of burning oily waste. Once optimum temperature is reached, the oil is passed into the combustion chamber.
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The burner then burns the mixture at a thermostatically controlled temperature so as to ensure a complete combustion. The forced draught fan provides continuous supply of air and the rotating shaft creates the required swirling of air. The ash that is created is collected in the ash slide. This ash can be disposed of to the sea or stored to dispose of it at the next port.
In many ships these incinerators are not used properly and it should be made a practice so that all staff right from the junior engineer, deck cadet to the chief engineer and master are aware of its use. This would go a long way in improving the quality of sea water as well as reduce stress on their minds in that they are doing everything as per the maritime law.
Incinerator installed in ships after 1 January 2000 are to have a Type Approval Certificate as well as Operating Manual on board and should not be subject to any modification which may affect their Certificate status.
Operators must be trained as to how to correctly operate incinerators. While use of plastic incineration is discouraged, a list of other items which are prohibited from incineration, as specified in the operation manual, must be posted at site. Incinerators must never be used in ports.
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Sewage Treatment
Today there are three main sewage treatment processes: biological, physical-chemical and electrocatalytic oxidation, and which process to choose depends on a number of factors.
Biological technology requires a steady and relatively constant intake of solidified sewage so that aerobic bacteria can feed on it in order to keep the discharge within regulatory limits. This type has some disadvantages: it must operate day in – day out, needs to be de-sludged on a regular basis although a plant using this technology cannot operate without some accumulation of sludge.
Physical-chemical treatment of sewage is relatively simple in concept, design and operation but there are some essential shortcomings limiting its application in marine conditions. It requires much space, its holding capacity is enough only for a 14-day accumulation of solids and therefore needs to dispose of accumulated solids regularly. So the system is not always suitable for all ships, especially those staying away from port for more than two weeks.
Electrocatalytic oxidation is a good key to a successful treatment of sewage on a ship. This type of treatment plant can be up to 50% smaller than a biological plant and doesn’t need to carry a ready quantity of calcium hypochlorite which is required to produce sodium hypochlorite. The latter is produced of seawater using electrolysis. Another advantage of this system is that it can be switched on and off as needed.
Electrolytic treatment process
Since the system (See “The process diagram” below) operates on demand, the surge tank (V-1) acts as a reservoir to store incoming black and gray water. As soon as enough wastewater has been received to reach the high level start switch, the treatment process begins. When the system starts, the macerator pump (comminuting pump) moves wastewater out of the surge tank and through the electrolytic cell.
As sewage is removed from the surge tank, the waste level drops to the low level float switch, which stops the process. A third level alarm switch in the surge tank activates a high level alarm.
The macerator grinds and reduces solids in the sewage to particles with a maximum size of 1.5 mm before sending it through the electrolytic cell. A restriction orifice forces a measured amount of macerated sewage through the cell while the remaining sewage returns to the surge tank.
The electrolytic cell is the heart of the treatment process. Here the sodium chloride in seawater is used to produce sodium hypochlorite, which disinfects the wastewater. Before entering the cell, a regulated flow of seawater is added to the macerated sewage stream. The sewage and seawater then pass between the charges cell plates where a number of reactions occur simultaneously. The result is rapid and almost complete elimination of organic compounds and a total bacteria kill.
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After that the stream enters the effluent tank (V-2) where it will remain for at least 30 minutes. The effluent tank remains full at all times. The treated liquid is discharged overboard. The material which was not completely treated during the first passage through the cell, is returned to the surge tank and treated again.
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