Shipping Emissions - The Environmental Case for Sea Water Scrubbing


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The romantic image of a luxury liner cruising blue seas amid blue skies and trailing a plume of smoke is too close to the truth for comfort. The emissions from shipping make an increasingly conspicuous contribution to air quality, health and environmental problems of Europe, and indeed, the world. Yet an effective and safe technology to dramatically reduce these emissions is being discouraged, even blocked, by invalid perceptions of possible harm. This paper aims to explode the myths surrounding sea-water scrubbing for shipping, presenting the environmental case for rapid and full support to the wide introduction of this technology. Indeed, this is a win-win technology minimising environmental emissions at minimum cost to the industry.

Today's mercantile fleet, unlike the sailing fleets of just 150 years ago, uses fuel to propel itself. For the last 70 odd years, this fuel has been predominantly oil, and its burning, in engines or boilers, creates four general kinds of emissions:

  • Sulphur Oxides, or SOx, which come from burning the sulphur present in all oils. There are various geological cycles of sulphur, each with some impact on the environment. We explore what happens when mankind's activities modify them, and how scrubbing can minimises this disturbance.
  • Nitrogen Oxides, or NOx, which is made when engines heat up the Nitrogen and Oxygen in air. Scrubbing does little to modify these emissions, although it does not prevent use of NOx reduction technologies. Despite their environmental importance, they are not discussed further in this paper.
  • Soot, or Particulate Matter, which we will call PM in the rest of this paper. This consists of particles of many different materials, such as unburned fuel or incombustible elements in the fuel. Some if it is salts formed from SOx and NOx, which condense away from the plume. It also comes in many different sizes, and we are beginning to believe that it is the very small, or ultrafine, particles that do us the most harm. Scrubbing dramatically reduces PM emissions of all sizes.
  • Carbon Dioxide, CO2, which is an inevitable product when we get energy from burning the carbon in fuel, and which is the main culprit implicated in global warming and climate change. The best way of reducing CO2 is to use less energy, so, in our other discussions, we explore the energy efficiency of how we can reduce other emissions.

Natural Sulphur Cycles

Sulphur is a element we need to live and some 0.2% of our bodies is sulphur. We get the sulphur we need from what we eat, and plants get it from the soils they grow in. Every year huge quantities of this sulphur are washed from the land to the sea. Some of it is replenished from the rocks of the earth, or from volcanoes or other sulphurous sources, (when it is called brimstone). Most is replenished by rain, which carries sulphur from the sea. In its travels from sea to rain to land, and back again, sulphur also plays a crucial climate control role, as follows:

  • Micro organisms in the sea methylate the sulphur (that is they add methane groups (CH3) to it) and use this for their own biological processes. Other creatures may then eat them and capture this sulphur for their own uses. When they die, some of the sulphur turns to dimethyl sulphide (DMS), which is volatile, and some of it evaporates from the sea surface.
  • (Some of the sulphur also stays with the decaying organism, so some goes to make up the sulphur in the oil we now burn).
  • The DMS in air provides a nucleation site for moisture to condense into clouds. Tiny particles of DMS help damp air turn into tiny droplets. The amount of DMS influences how quickly clouds are formed, and thus influences how much sunlight reaches the sea, and so how much rain falls. We now recognise this as a critical climate control mechanism.
  • Much of the land upon which sulphur falls has plenty of sulphur around already, so the ecology is not constrained by lack of sulphur. If you add more sulphur, nothing happens, except that more of it runs off back to the sea. A few ecologies are constrained by lack of sulphur, and have evolved to live with only that which falls in the rain. If more sulphur lands on these ecological communities, then some of the organisms in it will be able to grow bigger or faster. For a few agricultural lands, this extra sulphur acts as a fertiliser, although the effect is rarely large, and other chemical fertilisers are also needed. For most sulphur constrained ecologies extra sulphur changes the balance of the ecology, often for the worse.

The SOx Cycle

Mankind, in its use of sulphur, influences these natural cycles, principally by the SOx cycle.

Most of the Sulphur Oxides (SOx) we release arise when we burn materials containing sulphur, such as oil and coal. We also create smaller quantities of SOx, with their sulphurous smell, when we smelt sulphate ores in the extraction of metals, such as copper, zinc and lead, and it was perhaps this smelting that caused it to be one of the first air pollutants ever to be regulated.

Once released into air, SOx will combine with water droplets to form sulphuric acid, or with other particles in air to form sulphates. Although emitted as a gas, the gas can condense to form tiny crystals of SOx, which are (like natural DMS) nucleation sites for moisture (which then form Sulphuric Acid). SOx can travel a long way from where they are emitted, and a 'rule of thumb' is that half of them will land within 720 kilometres of the source.

SOx in air cause a haze, which reduces visibility and increases the amount of sunlight reflected back. As well as damaging 'clear air', it thus also influences the climate. When breathed it is irritating to nasal passages and the lungs, and is harmful to our health (as well as that of animals).

When emitted SOx come to land, it is, unlike DMS, largely as sulphuric acid, giving rise to the term Acid Rain. In many ecosystems, there is enough 'buffering' in the water and the soil to neutralise the acid, and it behaves much as sulphur from natural sources. On many lands, this buffering capacity arises from the limestone or chalk geology of the lands, [and older, alluvial soils tend to have greater buffering capacity]. After providing sulphur to the ecosystem (perhaps with a fertiliser effect) it runs off to the sea. Many rivers in Europe and other industrialised countries carry some eight times more sulphur than natural sources would cause, apparently without harm from this particular extra load.

Some ecosystems, particularly those associated with granite geology or thinner soils, do not have enough buffering capacity to neutralise the load of acid that lands. When this happens, the soil and freshwater environment turns more acid. As well as being corrosive, it is harmful to all life, with the harm growing as the concentration increases. Fish cannot survive in the water, and plants cannot survive in the soil. If the acid is concentrated by sudden releases, such as when snow melts, the effect can be lethal and quick.
Eventually, the SOx are washed down the streams with the acid gradually neutralised by other materials carried in the rivers to the sea.

If not neutralised before reaching the sea, the massive buffering capacity of sea water rapidly neutralises any remaining acidity.

In short, SOx in air are always harmful, on land they are often harmful. The sea is where they live.

SOx and Sea Water Scrubbing

When SOx come into contact with seawater there is a fast and efficient reaction between the SOx and Calcium Carbonate (CaCO3) in the seawater, to form Calcium Sulphate (gypsum) and CO2. The reaction neutralises the acidity of SOx, and consumes some of the buffering capacity of the seawater. The reaction is complete in a very short time, so the equipment to bring the exhaust gas with SOx and the seawater into contact can be compact.

The key environmental benefit of scrubbing is that it short circuits the whole SOx cycle, and returns the sulphur to the sea in the safest, quickest and least harmful way(see Note 1). This is the core environmental justification for scrubbing.

But further questions about scrubbing need answering:

  • Does the SOx harm the sea water or the creatures in it?
  • Can the sulphur be burned in other ways?
  • Are there other better ways of returning the sulphur to the sea?

We then discuss the further benefits arising from scrubbing, and safely disposing of, the PMs in the emissions.

Does the SOx harm the sea water or the creatures in it?

Adding scrubbed sulphur to the sea does change its acidity. However, the evidence available suggests that, in seawater, this change is virtually undetectable within a meter or two of any discharge. Certainly, any increase in the concentration of sulphur is miniscule (a few parts per million (see Note2)) compared to the sulphur already there. No known environmental harm is caused by such small changes, and, as mentioned above, many rivers already have much larger increases in sulphur concentration.

We do not yet know how far seawater can be diluted before it loses it scrubbing effectiveness, or the changes in acidity of sulphur content become large enough to cause harm. [The evidence so far suggests that there are few places where shipping penetrates where there is a risk of harm. Except during the most prolific floods, the tidal reaches of rivers and estuaries receive enough new seawater to fully neutralise the largest feasible discharges. The Great Lakes appear to be the only large body of water carrying shipping where scrubbing is not appropriate. Even fully enclosed harbours, with lock access, probably turn over enough seawater to handle likely discharges.

Can the sulphur be burned in other ways?

Shipping, like other large industrial plant, is able to handle and use the difficult heavy fuels that are unsuitable for other purposes, such as domestic heating, car or lorry transport, or air transport. To do so they use complex fuel handling and processing machinery (quite unlike the filters in, for example, cars), but the extra cost is justified by the lower costs of the fuels they can then use.

These heavy fuels often concentrate the sulphur from the original crude oils. Although some heavy fuels can be made from the low sulphur crude oils, the average sulphur levels in marine fuels are 2.9% sulphur. (This is 29,000 parts per million, whereas low sulphur car fuels are around 50 ppm).

So if shipping were to use low sulphur fuels, what would happen to all the high sulphur fuel oils? There appear to be three possible answers:

  • This fuel is shipped to other parts of the world, where regulation is less strict. However, in most possible markets, SOx emissions are becoming a political issue, and the 'Asian Brown Cloud' in part created from SOx emissions, is threatening to impact the whole Northern hemisphere. There may be parts of the Pacific, the Atlantic and the Southern Ocean where SOx can be emitted without significant harm.
  • The fuel is burned on shore, and uses other Flue Gas Desulphurisation (FGD) technologies to remove the SOx from the emissions. Currently, the primary technology for FGD is to use limestone or chalk, which, when mixed with the SOx creates gypsum. This requires the limestone or chalk to be mined, transported and processed, and the gypsum to be recovered, processed and transported. In some processes, the gypsum can be used for building materials, but this is an energy intensive process, and most is disposed of in landfill. Environmentally, this compares unfavourably with shipping desulphurisation, where the ships are literally floating in the neutralising material .
    · The sulphur is removed from the fuel, which we discuss next.

Are there other better ways of returning the sulphur to the sea?

Fuel oil can be desulphurised. The process generally involves forming hydrogen (H2) and reacting this with the oil. The sulphur in the fuel reacts to form Hydrogen Sulphide, which is then further processed. The further processing may create Sulphuric Acid, a useful industrial chemical, or elemental sulphur, which is also of some value in chemical processing. It may also be processed into gypsum for use in building materials.

The potential environmental attraction of this is that the sulphur recovered could replace sulphur that is otherwise mined as a feedstock to the chemical industry, and the gypsum could replace the gypsum that is mined.

There are disadvantages:

  • Forming Hydrogen is an energy intensive process. Indeed, hydrogen is very attractive, clean fuel. [This energy is lost in the desulphurisation process.]
  • Hydrogen Sulphide is a dangerous gas. It is lethal at concentrations of 50 ppm, and at levels far below this will anaesthetise our sense of smell, which otherwise finds it pungent and unpleasant. At higher concentrations it is explosive.
  • Sulphuric Acid is a dangerous chemical to transport, and the environmental risks and energy cost of doing so may outweigh any benefit from reduced mining.
  • The energy costs of transporting gypsum to where it is useful may outweigh any environmental gains from reduced mining.
  • For many fuel uses, where no other means of removing SOx are available, the environmental benefits of desulphurisation can clearly outweigh these disadvantages, and continued investment and regulation to achieve lower sulphur levels is well directed. When directed at desulphurising shipping fuels, the overall environmental benefit is less clear.

Once removed from fuels, the sulphur, mostly as sulphuric acid, is used in a rich variety of industrial and chemical processes. Indeed, sulphuric acid production is a key indicator of industrial activity. Some is also used in to manufacture fertiliser. However, almost all uses are dissipative. That is, in use the acid is neutralised to form various sulphates. If soluble, these are eventually washed to the sea. If insoluble, they may be disposed of in landfill as well as carried to the sea as silt.

New processes may be found that give other ways of using the sulphur, and if we reduced our dependence on fossil fuels, less would be released. But, at present, sea water scrubbing looks like a benign way to return the sulphur that comes from extraction of fossil oils to the ocean from which it once came.

What about Soot?

All combustion processes, including ships engines, produce PMs, many of them very small and therefore carried a long way. Much depends upon the design and operation of the engine.

If the fuel used is low in sulphur, its PM emissions will generally be less. However, scrubbing reduces the emitted PMs much more than is currently possible using clean fuels and operating with well tuned and well designed engines, and does so across the spectrum of particle sizes.

It is only in the last decade or so that the scale of the health harm of PMs has become apparent, and the numbers are disturbingly large. In Denmark, which suffer particularly from PMs as well as from shipping emissions (it is nearly surrounded by shipping lanes), there are estimated to be some 5000 excess deaths per year, as well as many other morbidity effects. Other countries suffer similar effects. While the shipping contribution is not easily quantified, it is significant and the benefit of reductions in airborne PMs from shipping is substantial.

Such reductions are achieved as an additional (and arguable zero cost) environmental benefit of sea-water scrubbing by shipping.

A key perceived disadvantage of scrubbing the PMs from exhaust is that the PMs (as well as the SOx) end up in the scrubbing water. If this water is not further treated, then its discharge would put this waste into the sea in a more concentrated form than it would be if it was first dispersed through air emissions. Note that this does not add to the volumes of pollutants being put to the sea, just shortens the route by which it reaches the sea.

It is this discharge which is the focus of the main environmental objections to sea water scrubbing, and is quoted as a reason not to encourage (or even permit) sea water scrubbing.

Even without further treatment, the balance of environmental and health benefits will, in most ecosystems, outweigh any harm from these discharges. However, there are many ways of removing oily and solid wastes from water. Current shipping regulations have encouraged the installation and use of water cleaning technologies, and they are well understood by the industry. Sea water that has been used for scrubbing can readily be treated before discharge, and the technical standards for such equipment will undoubtedly require that this is done.

The Environmental Case

Sea-water scrubbing of shipping exhausts is an environmentally benign way of reducing sulphurous and particulate emissions from ships, and provides a safe way of reducing the overall sulphur burden our use of fossil fuels creates. There are substantial health and environmental benefits from the technology.

It is for other papers to show that this is also minimises the economic burden on shipping, its passengers and its users.


1. It is also currently by far the cheapest way, but the economic arguments are presented elsewhere.

2. Note that this is a measure of the increase in concentration, not a measure of new substances being added. The local effect increases the proportion of sulphur in the sea from 0.0928% to perhaps 0.0929%. By way of comparison, we have increased the concentration of CO2 in our atmosphere from ~275ppm to ~367ppm, an increase of over 30%.

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