TEXTE Environmental Impacts of Exhaust Gas Cleaning Systems for Reduction of SOx on Ships — Analysis of status quo 
Report compiled within the framework of the project ImpEx 
2.1 Open loop EGCS 
An OL system, also commonly referred to as seawater system, requires high amounts of 
seawater (45 m?/MWh) and relies on its natural alkalinity for the scrubbing process (Lloyd’s 
Register, 2012). The water is directly discharged back to the sea, in some cases with prior 
treatment for solids removal or dilution with seawater to increase the pH. The energy 
consumption of OL systems is 1-3% of the engine power output (US EPA, 2011). 
Although in most of the literature 45 m*/MWh is given as the typical flowrate for OL systems, 
the required flowrate varies significantly as a function of the physical-chemical properties of the 
water (temperature, alkalinity and salinity), the desired SOx removal efficiency (Hassellöv and 
Turner, 2007) and the effectiveness of the water-gas contact (system design) (EGCSA, 2012). 
Hassellöv and Turner (2007) mentioned that the initial factor determining the SOx uptake 
capacity is the alkalinity or buffering capacity of the water. Once it is consumed, the factor 
enabling further uptake is the solubility of sulphur dioxide, which decreases with higher 
temperature and salinity (ionic strength). They calculated the water volume required to reach a 
determined level of reduced emission as a function of temperature for six different waterbodies 
(see Figure 2). For the calculations, combustion of fuel with 3% sulphur content, engine power 
of 12 MW and specific fuel consumption of 185 kg/MWh were assumed. For instance, to achieve 
emissions equivalent to combustion of fuel with 0.1% sulphur content and water temperature of 
15 °C, a water flowrate ofaround 56 m*/MWh would be required in open sea and 80 m?/MWh in 
the Bothnian Sea. Buhaug et al. (2006) indicated a water consumption in the range of 40-100 
m*/MWh. Based on an extensive literature review, Teuchies et al. (2020) and Hassellöv et al. 
2020) reported an average flowrate of 87 + 50 m*/MWh and 90 + 14 m?/MWh, respectively. In 
the sampling campaign of the previous UBA/BSH study (Schmolke et al., 2020) water flowrates 
under stable conditions from around 60 to 140 m*/MWh were recorded. 
2.2 Closed loop EGCS 
A CL system, also commonly referred to as freshwater system, employs typically freshwater 
treated with an alkaline substance (e.g. caustic soda) to adjust the pH level. After the washing 
process in the spray tower, the water is passed into a process (or recirculation) tank. There, a 
small portion of the water is taken from the tank bottom, where the scrubbing products are 
settled, pumped out and discharged after being treated for solids removal. The water treatment 
units are typically hydrocyclones, centrifugal separators or dissolved air flotation, sometimes in 
combination with flocculants. The amount of water being discharged (bleed-off) is significantly 
lower (0.1 - 0.3 m?/MWh) than the water volume discharged from OL systems. Teuchies et al. 
(2020) reported an average discharge rate of 0.47 + 0.25 m?*/MWh. Alternatively, bleed-off can 
be stored in a holding tank and properly discharged. That temporarily zero discharge mode is 
very convenient in regions with existing restrictions for EGCS water discharge. Residuals 
removed from the water treatment are called sludge and must be properly disposed ashore. The 
amount of sludge generated may range from 0.1 to 0.9 kg/MWh (EGCSA, 2012; Lloyd’s Register, 
2012; US EPA, 2011; Den Boer and Hoen, 2015). Stena Teknik (Asplind, 2018) reported an 
amount of sludge disposal equivalent to around 1% of the burnt fuel. From the process tank, 
most of the water is recirculated (-20 m?/MWh) and, after addition of an alkaline solution for 
pH control and cooling by a seawater heat exchanger to prevent losses by evaporation, is 
9umped back to the absorption tower. Depending on the amount of water losses, freshwater 
(make-up water) is added to the system. The energy consumption of CL systems is reported to 
be about 0.5-1% of the engine power output. The rate of consumption of caustic soda ranges 
between 6-18 L/MWh and is directly proportional to SO; in flue gas; typical ratio is 1.25 kg 
caustic soda per 1 kg SO; or 6 L/MWh-%S (EGCSA, 2012; Lloyd’s Register, 2012; US EPA, 2011; 
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