In 2008, Aalborg Industries of Denmark combined its experience from supplying scrubbers as an integrated part of inert gas systems onboard ships, with their experience of delivering exhaust boilers for large marine diesel engines. The result? They designed and erected an exhaust gas boiler and scrubber test rig to cool and clean the exhaust from MAN Diesel’s test engine in Denmark.
The actual test engine was a four stroke diesel engine running on heavy fuel oil with up to 2.7% sulfur content. Several physical designs like water injection principles, scrubber diameters, water flow and temperatures, freshwater, seawater and other parameters were tested, evaluated and compared during a wide variety of operating conditions. In total, 128 different tests were carried out and the reduction of sulfur dioxide (SO2) and particulate matter (PM) by the scrubber were checked.
The engine was a MAN 5L21/31 four-stroke diesel engine with a power output at maximum continuous rating (MCR) of 1000 kW. The main components making up the scrubber system were an exhaust boiler and a combined demister and absorber. Exhaust gas from the engine is first cooled to 180° to 200°C in the boiler before the first pre-cleaning stage, in which freshwater or seawater is injected together with sodium hydroxide (NaOH). Next, the exhaust gas enters the absorption tower, in which it slowly flows upward in counter current with water flowing downward. Water droplets are caught in a demister. The cleaned exhaust gas is finally reheated to avoid water condensation and visible vapor. The obtained SO2 reduction efficiency is a function of the mole relation between added NaOH and sulfur in the fuel.
The system is able to use seawater or freshwater and to switch between the media. Two different pre-cleaning methods were tested a simple jet nozzle and a more advanced adjustable venturi section. In the jet, the water is atomized by a nozzle in a straight downward flow with almost no pressure drop on the gas side. In this, 45 to 55% PM removal was obtained using this method.
In the venturi section, the water is atomized by increasing the gas velocity through an adjustable throat section, which results in a gas pressure drop of 100 to 400 mmWC (water column). By allowing a 400 mmWC pressure drop up to 78% PM was trapped. The selected pressure drop, however, will be a compromise between PM removal and engine efficiency, hence between PM and CO2 emission. “With the experience and knowledge from the testing with MAN Diesel in Copenhagen [Denmark], we designed the entire scrubber system to clean the exhaust gas from the ro-ro vessel owned by DFDS Tor Line, Tor Ficaria,” said Jens Peter Hansen, project manager, exhaust gas scrubber development, Aalborg Industries. “The vessel went into dock this summer, and during this docking the scrubber system was installed. When installed after the 21 MW two-stroke MAN diesel engine, it will be the world’s largest scrubber installation onboard a ship.”
The detailed design work has been carried out in close cooperation with DFDS to ensure that the installation will fit both existing ship installations and will lower the cost for retrofitting the installation at the yard. MAN Diesel has participated intensively to make sure that the design of the system will allow for a smooth and safe operation of the engine and the turbo-generator. This is important at all engine loads with the additional exhaust gas backpressure created by the scrubber.
The exhaust gas scrubber is designed to clean the 195 300 kg/h of exhaust gas from the 21 MW 9L60MC-C8 engine at 100% MCR. At open sea, up to 1000 m3/h of seawater is pumped directly to the adjustable venturi and the absorber unit. This means that significant quantities of NaOH are saved during the long journey with high engine load compared to operating with freshwater. In harbors and during trips through sensitive areas with strict rules regarding discharge water the system can switch to scrubbing with freshwater.
Seawater is then used only to cool the circulating freshwater in a plate heat exchanger. Then more than 99% of the water is re-circulated back to the absorber and the remaining amount, about 1%, has to be cleaned and returned to the sea. The low engine load, in cases like this, means that only a limited amount of NaOH is required. “When looking solely on the ship, it is true, that running on lower sulfur heavy fuel oil or distillate oil will result in lower CO2 emission,” said Hansen. “However, when taking into account the excessive energy consumption associated with desulphurization of residual oil, then significantly more greenhouse gases will be emitted to the atmosphere than with a scrubber solution.”
Significant cost savings for the ship owners can be obtained by combining high sulfur fuel with an abatement technology compared to using low sulfur fuel. Case studies show vessels trading primarily within emission control areas (ECAs) may see a payback profile for its scrubber investment in as little as one year.