Hybrid Scrubbers – An Overview

With the IMO’s 2020 global sulphur cap deadline getting closer, owners and operators are faced with having to make a significant operational and commercial decision – whether to continue operating on low sulphur fuel, post 2020, or whether to finance the retrofitting of a scrubber system to the vessel.

Depending on the type of scrubber selected, retrofitting the system to an existing vessel can be a complex and expensive process. Setting aside the significant capital cost of the technology itself, the engineering required to integrate the scrubber system, supporting structure, pipework and all required modifications to the vessel can be vast.

However, perhaps the most critical decision to be made by owners and operators is the type of scrubber system itself. With three different types of scrubbers available, namely open loop, closed loop and hybrid systems, each with their own inherent benefits and drawbacks, the selection of the most appropriate system for the vessel’s trading patterns is highly important.

Sticking to our commitment to being owners and operators trusted advocate in achieving environmental compliance, Cleanship Solutions has developed a three part deep-dive series on each of these scrubber types, highlighting critical considerations of each option.

Part three is the hybrid scrubber.

Please note that this article does not discuss the physical retrofit practicalities of the hybrid scrubber option – as this will be covered in detail in future scrubber related articles. Rather, this article has been developed to identify critical commercial considerations of the hybrid scrubber option, and their potential impact on the vessel’s performance.


Hybrid scrubber systems are generally a combination of both the open loop and closed loop systems – offering owners and operators the flexibility of operating on one of the two modes, dependent on the alkalinity of the local water.

As we discovered in previous articles, closed loop scrubber systems generally use chemically controlled sea or fresh water as the scrubbing liquid. Unlike open loop scrubber systems, which pump water directly from the sea chest, through the scrubber, and directly overboard, closed loop systems re-circulate the scrubbing liquid, with only a small bleed off discharged overboard. Once passed through the exhaust gas, the wash water is typically held in a process tank, treated, and then re-circulated.


Primary Benefits of Hybrid Scrubbers

Hybrid scrubbers offer the best of both worlds in terms of scrubber technology. With the open loop mode offering the simplest and most cost effective process, the hybrid system enables owners and operators to operate on open loop mode where local water alkalinity permits. Similarly, where local water alkalinity levels do not permit the open loop mode, the closed loop mode can be selected, and by virtue of their chemical dosing process, can be used to artificially control the alkalinity of the scrubbing liquid and ensure scrubbing efficacy.

The ability to switch between both modes can also have significant appeal when faced with other water quality problems. Issues such as the intake of sand or sedimentation, which, in earlier articles we discovered can cause fouling and wear of scrubber nozzles, can be particularly expensive. The ability to switch out of open loop mode, into closed loop mode, and avoid these problems, is inherently attractive.

For areas of strict legislation on discharge water, the hybrid also offers the ability to continue using the scrubber in closed loop mode, rather than switch to LSFO, which would be required in an open loop only scrubber system.

Critical Considerations

Like both the open and closed loop scrubber systems, hybrid scrubber systems do have a number of critical considerations that owners and operators are well advised to examine when determining their suitability for specific vessels. As outlined in the first two parts of this three part series, Cleanship Solutions take great care to analyse these aspects when conducting scrubber feasibility studies on behalf of our ship owner and operators clients, and feel it important to share these considerations with the wider community.

Total Retrofit Cost

Let’s face it, scrubbers are expensive to retrofit, period. The not so subtle combination of significant structural changes to the vessel’s funnel, stability issues, shoe-horning in vast pieces of new equipment and pipework and then, of course, the capital investment cost of the scrubber system itself, puts scrubber retrofit bills well into the millions of dollars. However, when comparing all three options, the hybrid scrubber option is considerably more expensive than either the open or closed loop systems individually. Whilst offering the best of both worlds operationally, the hybrid scrubber ultimately also requires the best of both options technically – resulting in a significantly complex and costly installation.

System Complexity

Compared to both the open loop and closed loop systems, the hybrid system is intuitively the most complex of the three, and requires a substantial number of components installed onboard. To facilitate the open loop mode, the system requires the significant wash water supply system that is prevalent on standalone open loop models. Similarly, to ensure the system can also operate on closed loop mode, the entire wash water treatment system (which requires a number of storage and holding tanks, as well as additional water treatment equipment) is also a necessity.

Fundamentally, the hybrid scrubber system is complex and space hungry, and requires even more careful planning and attention paid at the survey and feasibility stage.

Is the System Truly Hybrid?

In marine scrubber lingo, the term “hybrid” is often loosely used, and can cause some confusion. A true hybrid system is one which offers both the open and closed loop options, with accompanying equipment to ensure so. However, some manufacturers offer open loop systems with optional alkaline dosing, terming them “hybrid.” Whilst offering some ability to chemically control the scrubbing liquid alkalinity, these systems do not offer all the bells and whistles of both open and closed loop functions, and hence are not truly hybrid. It is important that owners and operators investigate the technical detail of any hybrid offering to ensure it is truly hybrid.

Chemical Handling & Storage

The closed loop mode of the scrubber will require artificial adjustment of scrubbing liquid alkalinity levels – which is typically achieved using caustic soda (NaOH) dosing. Regardless of the chemical or compound used for alkalinity adjustment, additional storage tanks will be required onboard, to safely store the chemicals for use, and may, in many cases, require careful temperature and humidity control to ensure chemical stability.

System Wear and Fouling

The open loop mode of hybrid scrubbers can have a detrimental effect on the scrubber materials – primarily in the form of system wear and long term fouling. Even with smaller filter meshes installed in sea-strainers, it is likely that the scrubbing liquid will enter the scrubber unit inclusive of small particles such as sand, as well as small organic species such as jellyfish and algae. The former will, over time, cause wear on the scrubber components, particularly the injection nozzles, and the latter will likely build up in way of the nozzles of the scrubber, and cause unwanted fouling – decreasing performance and requiring frequent maintenance.

Owners and operators should remain conscious of these limitations and potentially examine the use of self-backflushing filters as an additional step to minimise these impacts.

Fresh Water Consumption

With the closed loop scrubber mode requiring bleed off of the wash water, to maintain chemical composition and efficacy of the wash water, there is a very real, and comparatively significant, need for fresh water top up. In some cases, this fresh water top up can be as high as 2.5m3 per day, per MW. Even for modestly powered vessels, this additional fresh water consumption can add up.

In more extreme cases, owners and operators may be faced with upgrading the existing fresh water production plants onboard to meet these requirements, adding not only to the upfront investment costs in the scrubber retrofit, but also ongoing operational costs of the vessel.

Power Availability & Operating Costs

With the open loop scrubber mode requiring such vast quantities of water being pumped continuously from the sea-chests up to near the highest points in the funnel, the additional power consumption for these pumps alone is likely to be significant. Indeed, owners and operators may be faced with upwards of 150kW of new sea water pumps, due to not only significant volume demand from the scrubber, but also the discharge head necessary to reach the top of the funnel.

Setting aside the obvious impact on fuel consumption onboard, which is not insignificant considering the constant requirement to run these pumps, such significant power requirements may also cause issues with some existing vessels – particularly those that are already underpowered. Whilst this is most likely to manifest itself during power hungry operational modes onboard – such as cargo operations of thruster operations, it is, nonetheless, something owners and operators should pay close attention to during the determination of the feasibility of an open loop scrubbers system.

Impact on Waste Heat Recovery and SCR Systems

One of the most detrimental effects of scrubber systems is their impact on the temperature of the exhaust gas. Scrubber systems, due to using low temperature wash water, can significantly lower the temperature of the exhaust gas to the extent that it has a negative impact on any existing exhaust based systems – such as waste heat recovery systems or selective catalytic reduction systems (SCR). Whilst one may intuitively relocate such existing technologies upstream of the scrubber, this too can have negative impacts on the temperature of the exhaust gas, and cause detrimental performance issues on the scrubber. Therefore this process is somewhat iterative and requires close discussions between the scrubber manufacturer and the manufacturer of the waste heat recovery or SCR systems, to ensure both are operating optimally, or, at the very least, within an acceptable compromise.


The hybrid scrubber, operationally, is the gold standard of marine scrubbers. The flexibility it offers in terms of efficiency (running complex chemical dosing systems only when required), as well as preparedness (to switch modes depending on local water quality and/or legislation) is unparalleled, making it, on paper, a very attractive option to owners and operators.

However, whilst the hybrid scrubber combines all of the attractive aspects of both the open loop and closed loop options, it also, intuitively, combines all of the inherent negatives too. All of the technical and commercial complications of the open and closed loop systems are prevalent in the hybrid option, and in many instances, the sum of these complications is much more detrimental than the constituent parts.

Owners and operators are well advised to pay close attention to the critical considerations highlighted in this article, and to study the technical documentation from each manufactuter carefully. An investment of time and energy at the initial concept and feasibility stages can not only reap dividends further down the retrofit line, but in many instances avoid some significant headaches too!