SOx Scrubber

What defines this type

A SOx scrubber is an exhaust gas cleaning system (EGCS) fitted downstream of the main and auxiliary engines to strip sulphur oxides from the exhaust before it reaches the funnel, so the vessel can keep burning residual fuel with a sulphur content above the global cap instead of switching to low-sulphur distillate. The exhaust is routed through a tower where it is contacted with a spray of alkaline water; the SO2 reacts with the water to form sulphurous and sulphuric acid, which is then neutralised. The three configurations differ only in where the wash water comes from and what happens to it afterward: open-loop uses raw seawater's natural alkalinity and discharges it overboard, closed-loop doses caustic soda (NaOH) into a fresh water circuit and recirculates it through a holding tank, and hybrid units can switch between the two depending on trading area.

Main components

• Scrubber tower / absorber — the vessel where exhaust meets spray water, fitted with demister pads to strip carried-over droplets before the gas exits to the funnel.
• Seawater lift pumps — supply the spray water in open-loop and hybrid operation, sized for a large flow at modest pressure.
• Washwater treatment unit (WWTU) — separates oil residue and solids from the effluent before discharge or storage, typically using hydrocyclones and coalescers.
• Residue and NaOH tanks — hold caustic soda for closed-loop dosing and collected scrubber sludge for shore disposal.
• Exhaust bypass damper — lets exhaust skip the scrubber during maintenance or failure, usually at the cost of running on compliant fuel while bypassed.
• Monitoring skid — continuous SO2/CO2 ratio analyser, PAH, turbidity, pH and nitrate sensors on the washwater discharge line, feeding the compliance record.

Selection / sizing

Sizing starts from total installed exhaust flow at MCR across all connected engines, not just the main engine, since many installations scrub auxiliary engine and boiler exhaust through the same or a parallel unit. The decisive commercial choice is loop type: open-loop needs no consumables and no holding tank but is banned from discharge in a growing list of ports, estuaries and coastal waters; closed-loop carries NaOH storage and periodic bunkering of caustic soda but can operate almost anywhere; hybrid gives flexibility at the cost of extra tankage and complexity. Available seawater flow, space for the WWTU and holding tanks, and the backpressure budget the engine maker allows are the other limits that decide whether a scrubber fits at all.

Regulations / Class

The applicable limit is MARPOL Annex VI Regulation 14: 0.50% m/m sulphur globally, 0.10% m/m inside Emission Control Areas, with an EGCS accepted as an equivalent means of compliance under Regulation 4. Design, testing and washwater discharge criteria — pH, PAH, turbidity, nitrate — follow the IMO 2015 EGCS Guidelines (MEPC.259(68)). Continuous monitoring records must be kept onboard and are checked at port state control; several administrations and port authorities restrict or prohibit open-loop discharge within their waters independently of IMO, so the trading pattern has to be checked against the current list before a loop type is chosen.

Typical faults

• Scaling and fouling of spray nozzles and packing from seawater minerals — reduces SO2 removal efficiency and raises exhaust backpressure toward the engine's allowed limit.
• Washwater pump or mechanical seal failure — loss of spray flow, immediate non-compliance on the SO2/CO2 ratio, forcing a switch to bypass and compliant fuel.
• SO2 analyser drift or contamination — false compliant readings that surface as a discrepancy at PSC inspection, or false alarms that force an unnecessary bypass.
• Bypass damper seizure — leaves no route around the scrubber during an internal failure, forcing an engine load reduction.
• WWTU coalescer fouling — oil content in discharge water creeping toward the discharge limit, risking a violation.

What to look for in a supplier

• Documented reference installations at a comparable total exhaust flow and engine combination, not just the main engine.
• Independent verification of washwater treatment efficiency claims, not only the manufacturer's own test data.
• Availability of wear parts — nozzles, demister pads, analyser cells — through a network that reaches the vessel's actual trading routes.
• Backpressure figures guaranteed in writing and checked against the engine maker's allowable limit at full load.
• Support for both loop types if the trading pattern is not yet fixed, rather than a single-loop unit that locks in a discharge strategy.

Check the discharge ban list for the actual trading area before ordering — a closed-loop or hybrid unit costs more up front but avoids being forced onto expensive compliant fuel every time the ship enters a port that has banned open-loop discharge.