Overview of Scrubber Technologies

With the IMO’s 2020 global sulphur cap deadline getting closer, owners and operators are faced with having to make a significant operational and commercial decision – whether to continue operating on low sulphur fuel, post 2020, or whether to finance the retrofitting of a scrubber system to the vessel.

Depending on the type of scrubber selected, retrofitting the system to an existing vessel can be a complex and expensive process. Setting aside the significant capital cost of the technology itself, the engineering required to integrate the scrubber system, supporting structure, pipework and all required modifications to the vessel can be vast.

To assist owners and operators in understanding the different types of scrubber technology available, Cleanship Solutions has developed this guide on scrubber technologies, including the benefits and drawbacks of each system.

 

Overview

All scrubber systems are designed to be installed in the exhaust system of the vessel, immediately after the power plant and/or boilers. Whilst varying between different technologies, all scrubbers use one or more substances to remove most of the SOx from the exhaust gas, whilst simultaneously reducing the particulate matter (PM) too. Once through the scrubber system, the exhaust gas, which is now compliant with the legislative requirements, is simply emitted to atmosphere.

Whilst new technologies are being developed, existing scrubber systems generally use one of two basic concepts:

  • Dry scrubbers
  • Wet scrubbers

 

Dry Scrubbers

Intuitively, dry scrubbers do not use any sea water or other liquids to execute the scrubbing process. Instead, dry scrubbers use chemically treated granulates to instigate a chemical reaction that removes the SOx from the exhaust gas. Dry scrubbers are predominantly used in land based applications.

Below is a typical schematic of a dry scrubber system:

Dry Scrubber System Overview

Courtesy of ABS Exhaust Gas Scrubber Advisory

Benefits:

  • No requirement for large water pumps or wash water separation systems
  • Can be used before waste heat recovery systems – since no cooling of exhaust gas
  • Can be used in conjunction with selective catalytic reduction (SCR) systems for concurrent NOx reduction

Drawbacks:

  • Requires significant storage, conveying and bunkering system in place for handling granulates
  • Reactants can be expensive & must be closely controlled to ensure availability onboard
  • Requires discharge of solid waste from scrubber process

 

Wet Scrubbers

Wet scrubbers, again intuitively, use liquid to execute the scrubbing process – removing the SOx from the exhaust gas through chemical reaction with the wash water. Each manufacturer employs unique methods of instigating the chemical reaction – through a combination of liquid spraying or cascading systems within the scrubber unit – to maximise the exposure of the exhaust gas to the liquid.

Wet scrubbers are generally available in three formats:

  • Open Loop
  • Closed Loop
  • Hybrid (offering benefits of both open and closed loop systems)

 

Open Loop

Open loop wet scrubber systems generally use sea water as the scrubbing liquid, which is typically pumped directly from a dedicated sea chest (which is often a new sea chest due to the significant volumes of water required). Once the scrubbing process has been completed, the wash water is filtered and separated accordingly and discharged overboard. Sludge is collected separately for later discharge to shore. The wash water is generally closely monitored by an effluent monitor to ensure the discharging water meets the legislative requirements.

Open Loop Scrubber System Overview

Courtesy of ABS Exhaust Gas Scrubber Advisory

 

Open loop scrubber systems rely on the alkalinity of the water to establish the necessary chemical reactions with the exhaust gas, hence it is important that owners and operators work closely with the scrubber manufacturer to understand the trading areas, and water characteristics within.

Benefits:

  • No need for storage or handling of granulates or other chemicals
  • Generally requires only sea water
  • Comparatively simple to operate and install

Drawbacks:

  • Wet systems cool the exhaust gas and may hence cause issues with waste heat recovery systems of SCR systems
  • Requires regular maintenance & potentially de-fouling
  • May not be permitted for use in some ports

 

Closed Loop

Closed loop wet scrubber systems generally use chemically controlled sea or fresh water as the scrubbing liquid. Rather than discharge the wash water overboard, as open loop scrubber systems do, closed loop systems simply re-circulate the wash water.

Closed Loop Scrubber System Overview

Courtesy of ABS Exhaust Gas Scrubber Advisory

 

Alkalinity is closely controlled in a closed loop scrubber system to ensure the efficacy of the chemical reaction with the exhaust gas. The wash water is generally stored temporarily in a process tank, with settled residuals removed from the bottom whilst the remaining water is filtered and separated to remove the remaining residues from the scrubbing process. The wash water is then dosed with caustic soda to restore the alkalinity. The wash water is then re-used in the scrubbing process.

Benefits:

  • Requires significantly less wash water than an open loop system
  • Very little water is discharged overboard
  • Alkalinity of wash water is independent of vessel trading locations

Drawbacks:

  • Wet systems cool the exhaust gas and may hence cause issues with waste heat recovery systems of SCR systems
  • Complex wash water systems
  • Additional process tank(s) required
  • Additional chemical storage required
  • Requires significantly more space onboard than open loop system

 

Hybrid Systems

Given the inherent benefits of both open and closed loop scrubber systems, the hybrid system offers the benefits of both. Hybrid systems have the ability to be operated as either open loop or closed loop systems – enabling the efficient open loop system to be used whilst at sea, and the alkalinity independent closed loop system to be used whilst in fresh or brackish water, or in areas where open loop systems are not permitted.

Hybrid Scrubber System Overview

Courtesy of ABS Exhaust Gas Scrubber Advisory

 

Benefits:

  • Benefit from either open loop or closed loop operation
  • Flexible for vessel trading patterns

Drawbacks:

  • Wet systems cool the exhaust gas and may hence cause issues with waste heat recovery systems of SCR systems
  • Complex wash water systems
  • Additional process tank(s) required
  • Additional chemical storage required
  • Requires significantly more space onboard than either open or closed loop systems