At Riviera Maritime Media’s ‘Chemicals for emissions abatement’ webinar, a panel of experts weighed the merits of the two primary alkalis used with scrubbers
Shipowners opting for the closed-loop variety of exhaust gas cleaning systems (EGCS) to comply with IMO sulphur emissions rules must use an alkaline cleaning agent. The most popular choices are sodium hydroxide (caustic soda) and magnesium hydroxide. There are others available, but, as the panel noted, their uses are “rather limited”.
When choosing an alkali, Europiren managing director Henk Don said shipowners must consider several factors, including the ready availability of the product globally, local stock levels, potential for disruption to supply of the product and any available alternatives. Europiren, the webinar’s premier partner, is a subsidiary of the Russian Mining Chemical Company which mines brucite – the mineral form of magnesium hydroxide that is used for a variety of magnesia-based products.
A comparison of the two alkali cleaning agents reveals trade-offs, among them the volatility of each substance, their relative efficiency and efficacy of use and the potential for price instability in global markets. For instance, Mr Don said, sodium hydroxide is far more easily accessible than magnesium hydroxide, but it is also more hazardous to use and prone to price fluctuations.
“Whether we like it or not, often the first question we get is about the price. It’s impossible to say one is cheaper than the other on a cost per tonne basis because of the difference in efficiency – magnesium [hydroxide] is up to 40% more efficient because of the chemistry – but also because the price of caustic soda is very volatile, and the picture can change completely within a couple of months. The pricing of magnesium hydroxide is much more stable,” Mr Don said.
“The biggest advantage of caustic soda is its abundant availability. It is available all around the globe, basically on every corner of the street,” he said. “The biggest disadvantage of caustic soda is that it is a hazardous product because of its high pH and therefore you need to take special precautions for the handling and storage of it. Magnesium hydroxide is completely safe to handle.”
Another difference between the two substances is in their physical properties, Mr Don noted, with caustic soda being liquid and magnesium hydroxide being a suspension of solids in water. Magnesium hydroxide requires agitation or circulation to stay in suspension or it will settle – a consideration not required with use of caustic soda. However, while magnesium hydroxide’s freezing point is the same as water, caustic soda crystalises at temperatures below 15°C, meaning storage tanks and pipes need to be [heat] traced to ensure the substance remains in its liquid form.
Synthetic versions of the products are available as substitutes, but can pose another set of cost and storage challenges, Mr Don said.
Whereas magnesium hydroxide is mined as a solid material and then milled, synthetic magnesium hydroxide can be obtained as a by-product from the production of magnesium oxide. The synthetic form, while offering the benefit of not requiring mining and milling operations to acquire, has a lower solid content than mineralised magnesium hydroxide, making it less effective and more costly by volume – and requiring more hold space to store.
“The synthetic form of magnesium hydroxide is, in almost all cases, an intermediate form of the product in the production of magnesium oxide. And the solids content, the concentration, is given in the process. The natural magnesium hydroxide is mined as a solid material and then milled to fine powder and then suspended in water,” Mr Don said, noting the concentration of magnesium hydroxide solids in the substance’s milled form could be 100%.
“The higher the solid content, the lower the cost per tonne of the product. And it means less storage space on board. Storage capacity [that would otherwise be used to store greater volumes of less concentrated magnesium hydroxide] can be used for cargo instead,” he explained.
Finally, in calculating which alkali option to use, Mr Don said shipowners should consider a supplier’s capacity to cope with changes at short notice without heavy cost implications. As an example, he said that Europiren recently received an order for loading in Antwerp. A few days later, the location was changed to Italy, then Barcelona, before Europiren eventually met the customer in Valencia.
TIMAB Magnesium development director Francois Mazaré also made a case for magnesium hydroxide.
Mr Mazaré said the chemical is capable of neutralising sulphuric acid, and the lack of hazardous properties gives the substance a “natural advantage” during bunkering operations, as compared to caustic soda.
Mr Mazaré said magnesium hydroxide’s chemical composition adds to its efficiency. A tonne of pure magnesium hydroxide can neutralise the same solution as effectively as 1.38 tonnes of pure caustic soda, he said.
Mr Mazaré also discussed how his company addresses magnesium hydroxide’s primary operational shortcoming of settling out of suspension.
He said TIMAB, a webinar sponsor, uses a combined, two-step approach, keeping particle sizes small to slow down sedimentation and weaking the sediment structure if settlement happens, so that the substance can be pushed back into suspension with only mild agitation.
“We also use additives that will enhance those two [remedial measures] to slow the condition and to smooth over dispersion” he said.
To lower the risk of clogging, TIMAB filters material that has nothing to do with the scrubber performance (such as calcium or silicates) and produces a separate treatment product with high reactivity to encourage smooth and consistent pumping within EGCSs.
“That’s the reason why we promote [use of] high purity [magnesium hydroxide],” he added.
Speaking from a scrubber manufacturer’s perspective, and countering the proponents of magnesium hydroxide, TECO 2030 chief executive Stian Aakre said he considers sodium hydroxide to be easier to work with, controlling pH levels and offering a more immediate effect.
For bunkering procurement, Mr Aakre agreed that sodium hydroxide’s fluctuating price works against the alkali’s advantageous ready availability but, he said, sodium hydroxide is the easier of the two substances to bunker.
While rare in practice, in theory, magnesium oxide could be bunkered – with the necessary suspension generated on board using a slurry generation plant. However, the substance faces port-side restrictions that sodium hydroxide does not.
Dedicated, closed-loop scrubbers account for a mere 3% of total sales, Mr Aakre said.
Despite this, the director of EGCSA – the association representing scrubber manufacturers – Don Gregory noted that closed-loop scrubbers remain essential in some circumstances pointing to the low alkaline waters of the Great Lakes in Canada, where vessels exclusively use closed loop scrubbers.
Mr Aakre said assuming hybrid scrubber systems (20% of total sales) run on closed loop for even 10% of the operations, that only brings total closed-loop operations to 5%, with 95% of scrubbing operations using open-loop configurations.
“If we couple these closed-loop operations with the non-existant environmental impact of the chemical, that leads me to a conclusion that it’s very important to not lose sight of what the scrubber is actually doing. It does reduce SOx by 98%, CO2 by approximately 10-20% compared to the use of compliant fuel and it gives you a significant PM reduction” said Mr Aakre.
Some projections show scrubber installations could pick up again as increasing crude oil prices prop up marine fuel price spreads that make their installation economically feasible for shipowners.
Mr Gregory is bullish on the short-term prospect for scrubbers. “Let’s not forget all ships will be fitted with scrubbers by the end of this decade. You can take me up on that in nine years’ time if I’m wrong, but I think I will be right,” he said, adding that scrubber manufacturers are likely to soon turn to removal of black carbon, particulate matter and other toxic compounds as the IMO expands the scope of its regulations. A strong majority of webinar attendees, 83%, felt that the use of scrubbers is the most economical way of complying with IMO 2020.
From Europiren’s perspective, Mr Don said he too believes scrubbers are here to stay and called on manufacturers to collaborate on R&D to make alternative alkalis available.
Premier Chemicals Group sales director Stephen Lyons said selecting chemicals for use with scrubbers is an individual choice but pointed out the regulatory hurdles faced by the primary chemicals currently in use.
The transportation of sodium hydroxide by road tankers (the preferred method of delivery) is prohibited by some terminals which call for the use of barges. In addition, stored supplies, in many ports, are designated as exports for customs purposes, and Mr Lyons cautioned shipowners to check their suppliers are aware of the added financial hurdle and have plans to keep costs manageable.
Discussing operational safety, Mr Lyons said owners need a comprehensive safety plan, particularly in light of the hazards of caustic soda. And, ultimately, Mr Lyons advised owners to work with a reputable supplier who can serve the fleet globally.
Nearly a third (30%) of attendees felt the most important criteria for choosing the alkali for use with a scrubber system is the availability of a distribution network for the product. Almost a fifth (19%) felt the safety hazards posed by the alkali should be the most important criteria when making the choice of product, while 27% said cost was paramount.
Ultimately, when choosing suppliers, Mr Lyons said owners must consider whether or not suppliers demonstrate an understanding of port and vessel deliveries, customs and tax compliance, and can ensure quality of a product while delivering the product safely and at an effective cost.
On the question of substituting one alkali with another, over half (54%) of webinar attendees feel the process is prohibitively complicated. Mr Mazare, however, said this need not be true in every instance.
“We’ve experienced that kind of switch from caustic soda to magnesium hydroxide, and there are several aspects that need to be addressed,” he said. “The circulation line of course, the bumps need to be adapted to the alkali you are going to use, and typically managing your pH setpoints will be [needed] to make the best out of the product.”
“The reverse is true as well. To switch back to magnesium hydroxide from caustic soda,” he added, noting that in either case, the most important point is “not to mix those two products in the same tank”.
Mr Don said operators using magnesium hydroxide will need to flush the lines to avoid blockages of the pumps when idle. Because of its properties, with slow reaction times, magnesium hydroxide builds much larger crystals during treatment, resulting a more dense sludge than with caustic soda.
Mr Lyon said the choice of combinations of scrubber technology and treatment chemicals at the design phase is crucial, as switching once a system becomes operational is more difficult.
“Both products have merits,” he said, adding that the practicality and availability of a product will be crucial to shipowner operations. The opinion was echoed by 59% of attendees. Others (17% each) feel cost and environmental impact, respectively, are the key driver in the choice of an alkali for exhaust gas treatment.
Webinar Panellists (click the image above to view the webinar on demand):
Europiren managing director Henk Don
TIMAB Magnesium Industrial Applications development director Francois Mazare
Premier Chemicals Limited sales director Stephen Lyons
TECO2030 chief executive Stian Aakre
EGCSA director Don Gregory
Riviera Maritime Media will provide free technical and operational webinars in 2021. Sign up to attend on our events page