How do scrubbers on ships really work
Long before scrubbers or even before SOx was considered an issue for the shipping industry it was being regulated around the globe in connection with the use of large diesel engines in power production and other shore-based industries. As a consequence, exhaust gas cleaning or scrubbing technology is already a long established reality in shore-based situations cleaning up emissions from oil and coal-based power plants. Below is an explanation of how the technology works (you can also read a case study).
Scrubbers - Wet scrubbing technology
In a scrubber, the sulphur oxides in the exhaust are passed through a water stream reacting with it to form sulphuric acid and are removed from the exhaust gas which then passes out of the system. Sulphuric acid is highly corrosive but when diluted with sufficient alkaline seawater it is neutralised and the wash water can be discharged into the open sea after being treated in a separator to remove any sludge.
The alkalinity of seawater varies due to a number of reasons. In estuaries and close to land it may be brackish and closer to neutral and in some areas where underwater volcanic activity takes place the water may naturally be slightly acidic.
In the shipping sector, wet scrubbers are divided into two types; open loop and closed loop which were developed separately but which are now usually combined into a hybrid system that can employ the most appropriate technology depending upon prevailing circumstances. There is one type of wet scrubber that has been developed which combines wet scrubbing with membrane technology.
In an open loop scrubber seawater is used as the scrubbing and neutralising medium and no additional chemicals are required. The exhaust gas from the engine or boiler passes into the scrubber and is treated with seawater. The volume of seawater will depend upon engine size and power output but equates approximately to around 40m3 per MWh meaning a quite high pumping capability is required. The system is around 98% effective and even allowing for fuel oil with 3.5% sulphur should have no problem reaching the maximum 0.1% 2015 ECA level.
An open loop system can work perfectly satisfactorily only when the seawater used for scrubbing has sufficient alkalinity. Fresh water and brackish water are not effective and neither is seawater at high ambient temperature. For this reason, an open loop scrubber is not considered as suitable technology for areas such as the Baltic where salinity levels are not high. MARPOL regulations require the wash water to be monitored before discharge to ensure that the PH value is not too low.
A closed loop scrubber works on similar principals to an open loop system but instead of seawater it uses fresh water treated with a chemical (usually sodium hydroxide but some systems others) as the scrubbing media. This converts the SOx from the exhaust gas stream into harmless sodium sulphate. Unlike the flow through method of open loop scrubbers, the wash water from a closed loop scrubber passes into a process tank where it is cleaned before being recirculated. The fresh water can either be carried in tanks or else produced on board if a fresh water generator is installed on the ship.
In order to prevent build-up of sodium sulphate in the system, a small amount of wash water is moved at regular intervals either over side or to a holding tank and new freshwater added. The volume of wash water required in a closed loop system is around half that of the open loop version however, more tanks are required. These are a process or buffer tank in the circulation system, a holding tank where discharge to sea is prohibited and a storage tank able to have a controlled temperature between 20º and 50ºC for the sodium hydroxide which is usually used as a 50% aqueous solution. There must also be storage space for the dry sodium hydroxide.
The hybrid system is a combination of both wet types that will operate as an open loop system where water conditions and discharge regulations allow and as a closed loop system at other times. Hybrid systems are proving to be the most popular because they can cope with every situation.
The wet systems are not the most compact pieces of equipment and would take up considerable space if it were necessary to install them in under deck machinery spaces. Fortunately they can be installed in the funnel casing and can in some cases replace part of the conventional exhaust system.
The new membrane scrubber recently introduced is a wet scrubber but instead of the exhaust coming into direct contact with the scrubbing water in a spray or cascade system, nanoporous ceramic membrane separation tubes are used to extract SOx from the engine exhaust.
The Membrane Scrubber consists of an array of ceramic tube membranes, suspended in the exhaust stream. A manifold system circulates the absorbent solution through the membrane tubes. Exhaust gases pass over the membranes where the SOx is dissolved into the absorbent solution. The ceramic tubes have temperature limits exceeding 800°C and the use of stainless steel ensures the acidic nature of the sulphur oxides does not corrode the membrane modules. Ionada, the maker of system says one of the benefits of membrane scrubbing is the amount of effluent resulting from the system is significantly lower than typical closed loop scrubbers.
The absorbent solution discharge rate is much lower than existing closed loop scrubbers and allows the Membrane Scrubber to store the absorbed effluent onboard for discharge ashore. If sodium hydroxide is used as the absorbent fluid, the effluent can be regenerated for reuse with a sulphuric acid by-product. Other absorbents such as potassium carbonate are converted into potassium sulphate which has a commercial value equal to the base absorbent making the system cost neutral in terms of consumables.
It is claimed that due to the smaller volume of discharge water and the reduced amount of exhaust gas contaminants that are absorbed, the discharge water cleaning is much simpler. Removing the exhaust contaminants generates a small amount of sludge that must be stored onboard as part of the vessel’s oily water. The amount of sludge generated is less than 0.05tonnes/MW hr.
In the system seawater pumps provide cooling water to the heat exchanger to cool the circulating absorbent solution but no seawater is used for scrubbing of the exhaust gases.
The membranes require periodic cleaning to remove soot fouling on the membrane outer surfaces. The frequency of cleaning is dependent on operating conditions of the engines. The membranes are cleaned by circulating the absorbent solution under pressure to ‘back wash’ the membranes. The cleaning solution sludge is collected, and sent to the general sludge tanks. The amount of sludge produced is said to be typical of an economiser cleaning.
Scrubbers - Dry cleaning technology
No water or fluid of any sort is needed for the final scrubbing technology. A dry system employs pellets of hydrated lime to remove sulphur. An additional benefit is that the high temperature in the scrubber burns off any soot and oily residues. The lime pellets absorb sulphur and transform to gypsum.
Although spent pellets need to remain on board for discharge at ports, they are not considered as waste because they can be used for fertiliser and to produce plasterboard among other things. The dry system has a lower power consumption than wet systems as no pumps are required. However, the weight of the unit is much higher than wet systems. Only one dry system is currently marketed for marine use.
All scrubber systems require a treatment bypass for when the ship is operating without the need to use the scrubber. This prevents damage to the scrubber and reduces maintenance. Care needs to be taken to ensure that the scrubber is not causing backpressure to the engine as this could be damaging and will affect NOx reduction systems.
Proving scrubber performance
Flag states that decide to permit scrubbers on board ships will need to ensure that operators can prove compliance. Under ANNEX VI regulation 4 there are two schemes allowed for a system to be permitted that mirror the requirements for NOx compliance.
One demands that the performance of any scrubber is certified before use and, as with the NOx systems, providing it is always operated within approved parameters there is no need for continuous exhaust emission measurements on the ship. Parameters that must be continuously recorded include scrubbing water pressure and flow rate at the scrubber inlet, exhaust pressure before the scrubber and the pressure drop, fuel oil combustion equipment load, and exhaust gas temperature either side of the scrubber. A record of chemical consumption must also be maintained.
Under the second scheme, the exhaust gas must be continuously monitored when the equipment is in use and there is no need for the system’s performance to be certified. Under both schemes the condition of any washwater discharged to sea must be continuously monitored for acidity, turbidity and PAH (a measure of the harmful components of oil) and data logged against time and ship’s position. A test for nitrate content is also required at each renewal survey.
In May 2015 at MEPC 68 the meeting adopted amendments to the guidelines for exhaust gas cleaning systems which permits a calculation based methodology for verification of washwater discharge criteria. The revision allows for calculation or modelling to verify the discharge of wash water pH at a point of 4 m from the point of discharge. This will be reviewed after two-year’s time, if necessary changes are made to the wash water discharge controls however any changes will apply only to new installations.
Wet scrubbers are good at removing particulate matter and soot which although not currently regulated for specifically are likely to be so in future. Typically a scrubber will remove at least 500kg of particulate matter for every 100 tonnes of fuel oil burned and possibly more depending on how much wash water is used. These solids must be removed before the wash water is discharged overboard and to conserve space the system should have a separation phase included that removes as much of the water as possible before sending the sludge to be stored for later disposal ashore.
What are the costs of scrubbers?
Scrubbers are increasingly being fitted to newbuildings but the majority now in operation have been retrofits. The time for a retrofit is currently more than a typical scheduled drydocking meaning that extra lost earning days add to the capital outlay. The capital cost of scrubbers is currently high at between $500,000 to $5M depending upon maker and vessel size but that would conceivably reduce if volume sales materialise.
Payback time for a scrubber depends upon three variables;
- the capital and installation cost of the system,
- annual fuel consumption in ECAs and
- the price differential between distillate fuel and the normal fuel used on the vessel.
Take up rates for scrubbers may be improved if flag states and others offer state aid or attractive financial deals. So far aid has been limited to a small number of projects in Europe and some finance houses have begun offering schemes that assist with capital expenditure and which link repayments to savings made.
The number of ships fitted with scrubbers is growing and with it the number of scrubber manufacturers. At the time when the first commercial system was fitted in 2006, there were just a few organisations interested in the potential. Today the number of makers active in the field exceeds 20 and newcomers are appearing regularly.