A scrubber sprays alkaline water into the vessel’s exhaust, which removes SOx from the ship’s engine and boiler exhaust gases. In a seawater system, the sea’s natural alkalinity largely neutralizes the results of SO2 removal before discharge back to the sea. In a freshwater system, the wash-water used for scrubbing and neutralization is treated with an alkaline chemical such as sodium hydroxide. In both cases, the sulfates resulting from the SO2 removal will be discharged with the wash-water to the sea.
Sulfur in the form of sulfates (SO4) is the end product of the scrubbing process and is also a naturally occurring constituent of seawater – and therefore not harmful to the sea. The oceans are the Earth’s natural reservoir of sulfur and play a key role in the sulfur cycle. Sulfur is the 4th most common element in the sea and is essential to marine life, both biologically necessary and critical to many metabolic processes.
To provide perspective, consider the following: If all the sulfur deposited in the oceans from all of time were accumulated at the bottom of the ocean the layer would be five feet thick; adding all the sulfur from all the oil and gas reserves in the world would add only the thickness of a sheet of paper. Compared with the quantity of sulfate existing in the oceans, the small amounts of sulfate contributed by exhaust gas scrubbing are not only insignificant and benign, but as SO4 are also beneficial to marine life.
Removing sulfur almost entirely (over 98%, leaving less than in liquid compliant fuels) from the exhaust via the scrubbing process has the important primary intended environmental benefit of reducing the negative impact of emissions to air, as air emissions containing SO2 can add to the formation of particulate matter, and also the possibility of acid rain.
During the desulfurization process within the scrubber, SO2 gas is absorbed by the seawater spray. Through a series of reactions within the wash-water, the SO2 is converted to an end product of sulfate (SO4), water (H2O) and carbon dioxide (CO2).
During the scrubbing process any decrease in the pH of the wash-water is largely neutralized by the natural alkalinity found in seawater. This ensures that the pH of the discharged water is in compliance with guidelines established by the International Maritime Organization (IMO), which requires a minimum pH of 6.5 is reached within four meters of discharge.
Direct measurements on 40 ships monitored by maritime classification societies while the ships were in port have shown that pH levels of scrubber discharge water revert to neutral pH (7.0) almost immediately, and reach ambient seawater pH levels of 8.2 within two to four meters of the discharge point – well exceeding the IMO requirement. There is no residual acidity, no threat to marine life.
For comparison, consider that rainwater is typically in the range of 5.0 – 6.0pH.
As shown by a 2012 Danish Environmental Agency study, there is negligible acidification effect from scrubbers, even in semi-enclosed ocean areas with high traffic levels of scrubber-fitted ships.
The following scrubber discharge water parameters were developed by the IMO after years of scientific evaluation, and in consideration of major national and international water quality standards, including the EU’s Water Framework Directive.
In accordance with IMO Resolution MEPC 259(68) 2015m, Guidelines for Exhaust Gas Cleaning Systems, during scrubber operations these parameters are continuously monitored through specialized water analysis instruments, with the results securely recorded against time, the ship’s position and other operating parameters:
(a) pH (with temperature compensation),
(b) polycyclic aromatic hydrocarbons (PAH), and
(c) turbidity.
All recorded data must be retained on board for a period of not less than 18 months from the date of recording and is subject to auditing and verification by port states and by classification societies, on behalf of flag states.
Yes, they do. In addition to the findings of several studies, IMO Guidelines for EGCS require a certain minimum consistent emissions performance that all ships operating scrubbers must comply with, and which ensures that scrubbers on ships using HFO remove more than 98 percent of the sulfur oxides from the exhaust. This results in emissions consistently lower in sulphur oxides than those of marine gasoil (MGO), which is considered the benchmark for compliant operations in the IMO’s Sulphur Emissions Control Areas (ECA’s). As a result, scrubbers are an approved method of compliance with the sulphur regulation by the IMO, European Union, US Environmental Protection Agency and all other countries that ratified MARPOL VI.
Scrubber systems contain an emissions gas analyzer, routinely calibrated by certified technicians and which continuously monitors and records the key exhaust variables including SO2/CO2 ratio to ensure ongoing compliance with IMO Guidelines. Any excursion of the readings from the compliant ratio will cause an alarm which the crew must respond to and resolve.
(Source: Fridell, Erik & Salo, Kent. (2014). Measurements of Abatement of Particles and Exhaust Gases in a Marine Gas Scrubber. Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment. 230.10.1177/147509021454371)
No, there is no link between scrubbers and vessel speed. All practical methods which reduce CO2 emissions are welcome, and the option of slow steaming is one way of reducing fuel usage and CO2 emissions that is always available. Scrubbers do not interfere with this in any way.
Scrubbers allow vessels to continue using heavy fuel oil (HFO), a residual fuel with high energy content that is difficult to refine, providing a widely available and, when paired with exhaust cleaning systems (EGCS), an environmentally sound alternative marine fuel which is equivalent in sulphur reduction to the sulphur compliant fuel marine gasoil (MGO).
The combination HFO + EGS generally removes more sulphur from air emissions than the 0.1% sulphur MGO, and also significantly more than the 0.5% very low sulphur fuel oil (VLSFO), which at 60% of the marine fuel market is by far the most common marine fuel in use today. The HFO + EGCS combination is roughly the same as MGO in air emission of metals, PAHs and particulates, but much less than the same components of VLSFO.
And, although the use of scrubbers does create a slight (1%) increase in the amount of fuel burned and therefore CO2 release to atmosphere, this is far outweighed by the life cycle energy penalty from the refining process for MGO, which has a CO2 footprint up to 25% higher.
Inert gas systems installed on tanker vessels have been using exhaust gas scrubbers for decades. Scrubbers specifically for SOx removal have been in global operation in the maritime sector for over 20 years. Currently, about 10% of the vessels in the global maritime trading fleet are successfully using scrubbing technology to comply with the 2015 sulfur regulation in existing Sulfur Emission Control Areas (ECAs) and the IMO’s 2020 Global Sulphur Cap under Guidelines from the IMO/Marine Environment Protection Committee (MEPC), and also under the EU’s 2018 Sulfur Directive, the US EPA Vessel General Permit and related US Coast Guard Directives.
Scrubbers for SOx removal have also been in successful use on land in power plants, refineries and industry for many years all over the world and continue today, including for oil-fired power generation plants.
Both scrubbers and compliant fuel are equally acceptable and environmentally sound methods of compliance with the 2020 sulfur cap, and scrubber use is an equivalency under Marpol VI for global operations. CSA believes that both are essential tools for successfully reducing the sulphur emissions environmental impact of the maritime sector.
EGCS (“scrubbers”) used with HFO or VLSFO provide a practical, transitional solution to alternative fuels, providing cleaned air emissions with sulfur and particulate levels as low or lower than the other major marine fuels in general use today – with the exception of LNG – and with no adverse impact on the marine environment.
On a well-to-wake basis, EGCS-fitted vessels have a lower CO2 footprint than the most widely used alternative fuel options of VLSFO (by far the highest volume fuel used globally) and MGO, and therefore supports the IMO ambitions for lowering CO2 emissions by 2030. In addition, the combination of EGCS + HFO provides a significant local air emissions benefit, including lower SOx and PAH’s than when using VLSFO or MGO.
EGCS also play a vital role in the development and operation of marine carbon capture systems (CCS), which require a cleaned exhaust stream by (EGCS) before the CO2 separation process. EGCS are part of every marine CCS development, and their general availability will encourage the wider uptake of CCS, which has the potential to make a significant technical contribution to meeting maritime climate goals.