The global 0.5% sulfur cap will enter into force in 2020, and more than 70,000 ships will be affected by the regulation. Stricter sulfur limits (SOx) emissions are already in place in Emission Control Areas (ECAs) in Europe and America, and new control areas are being established in ports and coastal areas in China. As a result, ship owners are weighing their options to ensure compliance. To ensure compliance, various methods have been adopted by ship owners and shipbuilders to achieve 0.5% sulfur content in open waters and 0.1% in Emissions Control Areas. For existing vessel technologies like scrubber tower installment, usage of marine diesel oil and very low sulfur fuel oil (VLSFO) are most commonly used. For the vessel under construction are looking for dual-fuel engines, LNG powered engine, use of variable geometry turbocharger are coming into the picture.


The International Maritime Organization (IMO) has set norms related to marine pollution and related to sulfur emissions. IMO has made it mandatory for shipping companies to regulate the emissions and reduce sulfur emissions to 0.5% from 3.5% from 1 January 2020. While emissions for Emission control areas (ECA) are subjected to countries, and they have set norms for 0.1% sulfur emissions. Today most of the countries are keen to regulate sulfur emissions in their waterways and ECAs are increasing; China is all set to declare all its international ports as ECAs. To achieve this milestone, certain technological and market advancements are required. To ensure compliance with the emissions norms, various technologies have been developed, but all of them are not practical, or the adopting procedure is very complicated. In this paper, technologies are discussed which are widely used and accepted globally. First of the most important technology is the usage of scrubber towers or exhaust gas cleaning systems followed by the use of very low sulfur fuel oil (VLSFO) & ultra-low sulfur fuel oil (ULSFO), Marine diesel oil, LNG as a fuel.


Scrubber technology was introduced in 2015, and this is the only technology that supports the use of High sulfur fuel oil (HSFO) while maintaining the emission standards of 0.5% m/m sulfur content. Scrubber washes the exhaust gas using an alkaline medium to neutralize the acidic exhaust gas and produces salts of sulfur that are not harmful and can be disposed of easily. Process water is treated in a treatment plant to filter sludge.


1. Open-loop scrubber system

Open Loop Scrubber
Figure 1, source: GLOBAL SULPHUR CAP 2020 (scrubber tower focused) by DNV-GL 2019 update. page 11.

In the open-loop scrubber system, seawater acts as a cleaning solvent due to the alkalinity of seawater. Amount of seawater required, dimensions of pipes & tower, the capacity of the pump depends upon the power of the main engine, auxiliary engines & boiler capacity. Exhaust gas from engines and boiler it is firstly cooled in a venturi using seawater and then passed through a fine mist of seawater in two stages where water droplets neutralize the exhaust gas and leftover water can be treated with wastewater and discharge to the sea.

Chemical reactions

SO2 (gas) + H2O + ½O2 → SO4 2- + 2H+ (Sulfate ion + Hydrogen ion)

HCO3 + H+ → CO2 + H2O (Carbon di-oxide + Water)

The disadvantage of open-loop systems

In an open-loop system, water is taken from the sea chest. There have been incidents when the sea chest becomes clogged and the supply of water reduces which may damage the pump impeller and inefficient exhaust gas cleaning. The main setback of this system is that the alkalinity of seawater is not uniform throughout like the Baltic Sea. If a vessel has to enter freshwater this system is of no use and also in ECAs discharge of scrubber water is not allowed in any proportions. So, the vessel has to take highly expensive VLSFO for operation in ECAs, freshwater, where alkalinity of seawater drops.

2. The closed-loop scrubber system

Closed Loop Scrubber
Figure 2, source: GLOBAL SULPHUR CAP 2020 (scrubber tower focused) by DNV-GL 2019 update. Closed-loop scrubber page 11

In a closed-loop system, process water is circulated in a closed-loop. The process water is cooled in a heat exchanger to reduce the evaporation of water. The ship’s technical water is added to the process water to compensate for evaporation and water loss, while alkali is added to maintain the alkalinity. Some process water is drained and treated using chemicals. Treated process water is transferred to a holding tank and discharged when permitted, and sludge is transferred to a sludge tank. The closed-loop system requires less water than an open loop. To meet the alkalinity of process water Sodium hydroxide in a 50% aqueous state is required in a separate tank. This system is independent of are of operation, having less maintenance cost & cooling of exhaust gas is not required as of in the open-loop system.

Chemical reactions-

2NaOH + SO2 → Na2SO3 + H2O (Sodium Sulphite);

Na2SO3 +SO2 +H2O → 2NaHSO3 (Sodium Hydrogen Sulphite);

SO2 (gas) + H2O + ½O2 → SO42- + 2H+ ;

NaOH + H2SO4 → NaHSO4 + H2O (Sodium Hydrogen Sulphate);

2NaOH + H2SO4 → Na2SO4 + 2H2O (Sodium Sulphate).

The disadvantage of closed-loop systems

This system requires additional tanks like process tanks, holding tanks, heat exchangers. Exhaust gas has to be treated with selective catalyst reduction (SCR) which makes the system quite complex.


Hybrid scrubber turns to b a much effective alternative from an open-loop, closed-loop, and dry scrubbers. As hybrid scrubber overcomes the shortcomings of all other kinds of scrubber systems like it can operate in the sea & freshwater also. Hybrid scrubber as defined by WÄRTSILÄ is a combination of open and closed-loop systems. The hybrid system increases the operating flexibility of scrubber and ensures the continuation of HSFO. Hybrid scrubber systems are getting popular with most of the cruise ships and are now getting a good response with the cargo ships, tankers, Ro-ro vessels. As this system is expensive and very complex, installation requires a change in engine room design to accommodate the scrubber system.

Choosing between many options of the scrubber system is an objective task as it depends upon vessel operations. If a vessel is only sailing in seawater on a fixed root there is no need for a hybrid scrubber system open-loop system can do the job. As fitting of a scrubber is a very complex task for older ships as it requires minimum 3 weeks of dry dock for a VLCC (very large crude carrier) to install it and installation cost for an older VLCC vessel is around $4.3 million and for a new build VLCC is around $2.7 million. As ship-owners are interested in the payback of the cost in which new-build ships can offer more because it has more sailing life and due to sulfur cap 2020 prices of HSFO are expected to drop. The main worries for ship owners are the price of LSFO if, prices of VLSFO decrease the installation of scrubber tower will not be cost-effective.

Payback Curve
Figure-3, Source: Drewry Maritime Research

Usage of very low sulfur fuel oil (VLSFO)

The main reason for the sulfur emissions is the burning of fuel with high sulfur content. The simplest way to reduce sulfur emissions is to completely remove sulfur from fuel and produce a non-sulfur fuel option. However, this is not as simple as it’s looked. Refineries are still in the process to adopt the method to make crude oil sulfur-free after distillation. HFO (heavy fuel oil) or fuel oil is a residual fuel remaining from distillation and cracking (breaking of the long-chain organic molecule or hydrocarbon) process in refineries. HFO is a residual fuel is having the highest sulfur content and before the global sulfur cap, it was used by blending it with IFO( intermediate fuel oil)180 & IFO 380. 

Combustion of HFO

CH4 + 2 O2 + N2 + H2S → 2 H2O + CO2 + CO + NO + NO2 + SO2 + Energy

As to produce low sulfur oil with sulfur content less than 0.5% m/m blending of HFO is done with distillate fuels in which the percentage of distillate fuel is higher than the residual fuel (40% RESIDUAL + 60% DISTILLATE ). By producing blend with the limits of emission norms need for scrubber tower is no longer needed. For ECAs dependency on ULSFO and Marine gas oil will be there.

Drawbacks of VLSFO-

As VLSFO is blended fuel and due to low sulfur content, the water content in fuel increases and if blending is done with biodiesel water content will increase. Microbial growth is very common for diesel fuel and heavy fuel oil. For microbes, gasoline is not a feeding ground for them because it contains lead which kills the bacteria. A microbial infestation will increase as the water content in the fuel increases. Microbial infestation decreases the quality of fuel and causes various acids to be formed in the sludge tank. Due to excessive laying of eggs, there occurs a foul smell in sludge which may indicate that fuel has become incompatible. to prevent the growth of microbe’s water content in the fuel needs to be controlled and the temperature at which fuel is stored needs to be regulated. Inspections must be taken for sludge analysis.

As fuel systems like fuel pumps are designed for certain viscosity of the fuel. As the viscosity difference of HSFO and VLSFO is significant so the fuel pump may not be able to work efficiently or they may fail during ongoing operation.As one of the most reported incidents and a real-life incident in all the ports that the sedimentation is way too higher in sludge tank and problems with the purifier which needs to be cleaned and service in a very short period. In a few cases, the fuel tank was found to be frozen due to the high pour point (pour point is the temperature below which liquid loses its flow characteristics).

Use of marine gas oil:-

Marine gas oils defines usually the distillates. distillates are those components are which are produced from the vapors of crude oil condensed in a distillation process. The composition of MGO has a very less sulfur content in it nearly less than 0.1% which is a benchmark standard for ECAs. As marine gas oil is a distillate it does not require any pre-heating before use. As it is a cleaner fuel it does not produces soot after combustion. It is also having fewer carbon deposits on valve seats and prevent it from choking. Due to fewer carbon emissions, the overhaul period of an engine increases, and less maintenance is required.

Issues related to MGO

As MGO is the most viable option for ECAs it comes with certain disadvantages of both market and technology. Bunker fuel prices for MGO are very much high with an average of 419.50 $/mt. As a vessel has to operate on HFO in global areas with scrubber towers and heated fuel tanks are available for fuel. To carry MGO as fuel a special service tank is required with a capacity of 8 hours normal operation as stated by the SOLAS Chapter II-1/26.11. Service tanks need to be well insulated to prevent any fire hazard. Entering ECAs fuel switch has to be done from HFO to MGO using a 3-way valve. The mixing of HFO and MGO is inevitable and will take place. Due to this mixing fuel change has to be done at a considerable distance from ECAs and sulfur emission to 0.1% will take time to be achieved. Due to the fuel mixing and presence of warm HFO when contacted with MGO it may gas up. the operating temperature of HFO & MGO is having a difference of 100 degrees Celcius thus, the changeover can cause a thermal shock to the equipment. There were many incidents reported for the increase in cylinder corrosion due to fuel switch over. MGO is a distillate fuel that yields very less acid after combustion and lubrication oils are chosen such that they neutralize the acidity caused by fuel. As lube oil is compatible with HFO so it is highly basic to neutralize the acid forms by HFO but in the case of MGO, it will not neutralize the reaction but make it more basic in nature which causes wear and corrosion of cylinder. To achieve maximum operation efficiency certain changes need to be made. By controlling and lowering the temperature of MGO the fuel systems can be saved from any damage and soot formations. For lube oil control feed of lube oil when working with MGO or changing the lube oil of a certain base number can be used for better results.

LNG as a fuel

LNG is the only marine fuel which is having shore and offshore applications. The natural gas is used from cooking gas to fuel for CNG powered automobile. All the countries require LNG to quench their energy needs and it is one of the best alternative fuels. Most of the LNG carriers are powered by a steam turbine. This steam is produced in the boiler by using boil-off LNG or oil. As natural gas is carried in a liquid state and it needs to maintain -161 degrees Celsius from getting evaporated. As the efficiency of all the insulation is never 100% so some amount of gas gets evaporated and the process is called boil off. On average around 0.1-0.17% of gas is evaporated. Consider a journey of 10 days for a gas carrier carrying 30000 m3 of cargo therefore, the total boil-off gas will be around 450000 liters of gas. This gas can’t be flushed into the atmosphere as it is harmful so the best way is to use it as fuel for producing steam in boilers and as fuel for dual-fuel engines. As production of natural gas is increasing manifolds and the USA becoming the largest producer of natural gas and abundance of natural gas in the north and south America region is giving backbone to the idea of adopting natural gas as a potential alternative. Today prices of LNG are lower than most conventional fuels.

The leading producers of marine engines and marine technologies like Finland’s WÄRTSILÄ and Germany’s MAN Diesel and turbo (MAN DT) have introduced dual-fuel engines. These dual-fuel engines are the most versatile engines not for only gas carriers but other types of vessels also.

Dual fuel engines can run on 3 different modes like Gas mode: where only gas is used, Gas and fuel mode: oil and gas are used where oil is used around 6-8%., Fuel mode: only fuel oil is used. The engine when working only with gas works on the Otto cycle and while working with fuel works on the dual cycle. In dual fuel when working on gas mode requires a little amount of oil as pilot fuel for combustion this mode maintains emissions in ECAs. While working with gas and fuel mixture a proportion of gas and oil are used for combustion. Working on fuel mode only consumes oil without any loss in power. In all 3 modes no loss in power, the engine speed is noted at 100% load conditions. The practical thing about these engines is that the vessel can carry natural gas from it is easily available and cheap and store it in tanks for future operations; this will reduce the cost of operations. When working on natural gas a good efficiency of 47% is achieved which is highest for an IC engine. Using gas as fuel saves fuel oil. working on gas and fuel mode does not require any exhaust gas cleaning systems like scrubbers. Exhaust gas recirculation can be used using turbocharger which will add up to the power and efficiency of the engine.

Disadvantages of LNG as fuel

The main issue for natural gas is space, volume-wise it needs more space than oil. For long voyages dependence only on LNG can’t be ensured as of now. To store more amount of LNG huge tanks need to be constructed which comes on the cost of reducing cargo space and ship-owners loses money from freights. Using LNG is not a viable option for older ships as they have to change the type of engines and fuel systems which is not practically possible. The issue of gas leakage is a potential danger as gas is odorless and colorless it makes detection of any leakage virtually impossible. The main and most hazardous risk is the ‘BLEVE’: Boiling liquid expanding vapor explosion, which is caused by a decrease in internal pressure due to cracks that evaporate the gas rapidly and valves can’t handle such pressures resulting in an explosion followed by fire. A fire caused by LNG can only be controlled by removing oxygen or removing methane. Production of LNG is increasing but the refineries are still struggling to quench the LNG supply. Not all the ports are having a well-established LNG bunkering infrastructure as it is in the developing process. it takes a very big investment to deliver a well planned and safe LNG bunkering infrastructure. The dual-fuel engines with gas mode & gas and fuel mode can’t be used for long voyages due to less volume of LNG onboard. Although, LNG is one of the most feasible options for the future. it is still in the development process for maximum usability and efficiency. With new ships with design favoring to storage and safety measures along with educating and training crew for the operation will yield the maximum output.


This paper has stressed the practical options to comply with sulfur cap 2020. In today’s scenario, no single technology is 100% efficient in the way towards emission norms. Economical and availability factor plays a great role in the non-acceptance of some options. In this paper compliance with emission norms was treated as paramount but the practicality is also well-considered. In the future, there will be a change from sulfur emissions to other emissions like noxious, PM & GHG. The proper education and training of crew will play a key role in maintaining the voyages without any accidents. The major risk onboard a vessel is a fire hazard. Shifting from conventional fuels and adopting distillate or natural gas as fuel needs to change the fire fighting systems as required by the desired fuel type. The best option is still a land to be discovered but globally for older ships out of ECAs the best-suited option is the use of scrubber tower systems and entering ECAs using MGO. If the market remains stable the scrubber tower systems can be used for newly constructed and planned vessels. As global trends are shifting towards the LNG and due to increased production it can be a viable solution for modern vessels but GHG needs to be equally stressed and acknowledged. Few other options are trending all over the world like electric propulsion, hydrogen as fuel, etc.


Authored By:- Cdt. Hardik Vats, TMI

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