Air Pollution Control Innovations

Seawater Scrubber Removes SO2 from Marine Diesel Engine Exhaust

Posted by Andy Olds on Thu, Apr 08, 2010 @ 12:32 PM

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On March 18th, 2010 I participated on a panel discussion for the Cruise Lines International Association's Inc. (CLIA) Exhaust Gas Scrubber (EGS) Workshop in Miami, Fl. The workshop was professionally managed by BMT Designers & Planners, a navycruise ship architecture and marine engineering firm.   The panel was comprised of potential marine exhaust gas scrubber vendors.  The intent of the workshop was to provide information to cruise line participants to assess the maturity of the industry and the likelihood that exhaust gas cleaning systems will be a feasible response to the challenges of changes in regulations.

The industry is evaluating alternatives for meeting upcoming SOx emission limits under Annex VI of Marpol 73/78.  The SOx emission limits will require ships to achieve at least a SOx reduction equivalent to 0.1% sulfur fuel by 2015.  This requirement can be met by using more expensive, low sulfur fuel, or by scrubbing the exhaust gas stream.  The rules essentially require > 97% SOx removal assuming 3.5% sulfur fuel.   The International Maritime Organization (IMO) has issued Guidelines for Exhaust Gas Cleaning Systems, Annex 4, Resolution MEPC.170(57), adopted April 4th, 2008 to specify the requirements for testing, survey certification, and verification of exhaust gas cleaning (EGS) systems to ensure compliance with Annex VI.   

Envitech first started evaluating the marine scrubber application in early 2008 at the request of one of the major cruise lines.  The cruise line was interested in working with a company that could apply industrial air pollution control equipment experience to marine diesel exhaust streams on board a ship.   Envitech has deployed many particulate and acid gas scrubbers on a wide range of combustion processes including a seawater scrubber for an industrial waste incinerator at a pharmaceutical plant.  Many of these systems are similar process requirements for a diesel engine exhaust.   As a result of our evaluation Envitech developed, and recently filed a patent application for, the Hysea Marine Scrubber which is a hybrid seawater scrubber system.  We introduced this technology to the industry during the CLIA EGS workshop.

The Hysea Marine Scrubber uses available seawater alkalinity to scrub SOx.  The system is chemically assisted with caustic solution (NaOH) to achieve high SOx removal and reduced water flow rates.  The chemical consumption is minimal and estimated to be less than 7% of the usage of a closed loop, recirculation system.  The system is designed to provide flexibility to operate in two modes:

  • Open Loop/Caustic Reduced Mode - Continuous, once-though liquid discharge.
  • Closed loop/bunkering Mode - Re-circulated seawater with a small discharge stream that can be temporarily bunkered on board the ship.

The discharge liquid in both operating modes is treated to meet regulatory requirements.  Because chemical assistance with caustic substantially reduces the water flow rate, the water treatment system becomes more manageable on board a ship.  The water treatment system also re-oxygenates the water to meet chemical oxygen demand (COD) standards.

The table below shows a comparison of three different marine scrubber configurations, including:

  • Open Loop - Using once through seawater
  • Closed Loop - Using re-circulated water
  • HySea Marine Scrubber - Using chemically assisted Seawater

 

 

ITEM OPEN LOOP CLOSED LOOP HYSEA
Water
   Open Loop Mode
   Closed Loop Mode
Sea Water
Once-Through
-
Fresh Water
Re-circulated
-
Sea Water
Once-through
Re-circulated
Blowdown, gpm
    Open Loop Mode
    Closed Loop Mode

10,000
-

-
5

2,300
5
Energy Consumption, kW
    Open Loop Mode
    Closed Loop Mode

290
-

45
-

90
45
Caustic Consumption
  (est. average), gph
    Open Loop Mode
    Closed Loop Mode


0
-


-
60


4
60

A comparison of the operating parameters highlights the reduced water and power consumption of the hybrid system compared to an open loop system.  It also shows the substantial caustic reduction compared to a closed loop system.  The main advantages of the Hysea Marine scrubber include:

  • Reduced seawater flow rates - 75% - 80% Reduction
    • Reduced power consumption - 70% - 75% Reduction
    • Smaller piping - Simplified installation
    • Smaller water treatment system - Simplified installation
  • High removal efficiency -  0.1% sulfur fuel equivalent
    • Including low alkalinity seawater conditions
  • Operating flexibility to bunker a low flow discharge stream
  • Reliance on reliable and proven process technology
  • Water discharge that exceeds  discharge requirements
  • Water treated for chemical oxygen demand (COD)

Although the Hysea scrubber was designed for ship board use for a diesel engine exhaust, the same design principals also apply to acid gas scrubbing for land based industrial processes that have access to seawater.

A lot of interest in Marine exhaust gas cleaning systems was expressed during the EGS workshop. However, the cruise line industry is still evaluating the full range of options for complying with Annex VI of Marpol 73/78.  The general consensus of the panel participants is that exhaust gas cleaning is not only technically feasible, but provides a compelling financial case as a means for meeting the new regulations.

Please read our case study on reducing SO2 emissions for ships docked at ports by clicking the link below.

Download  Case Study

Photo Credit: Saint Seminole

Topics: Scrubbers, cleaning systems, Acid Gas