Can inert gas systems be included in decarbonisation drives?

Oct 07 2021


Inert gas systems themselves use energy. The gas needs to be provided for inerting at the same time as the tanks are emptied, and inert gas cannot be stored onboard, with the volumes so large. Sometimes gas or fuel is burned specifically to create the gas. So are they a target for decarbonisation drives?

Being a reader of Tanker Operator magazine, you probably know that the purpose of inert gas is to prevent an explosion in the cargo tanks. An explosion can happen when there is oxygen together with a hydrocarbon. So inert gas must have oxygen below a certain level, about 11 per cent by volume.

 

This can be achieved by using exhaust gas from the engine or boiler, because the oxygen in the air has already combusted with the gas to form carbon dioxide, it cannot react again.

 

Some inert gas system manufacturers we spoke to for this article said that the market is highly competitive, with nearly all systems installed at the newbuilding stage, rather than replaced over the life of the vessel. Shipyards are in a strong negotiating position to push down prices, or drive for commodity products.

 

If shipowners wish to have a gas inerting system with special environmental features, they must pay the shipyard extra for this.

 

Inert gas on tankers can be sourced from the ship’s main auxiliary boiler, or an independent inert gas generator, burning gas just for this

 

The exhaust from a diesel generator is usually under 5 per cent oxygen. It is first passed through a scrubbing tower where it is cleaned and cooled with seawater.

 

Chemical tankers sometimes use pure nitrogen for inert gas, where there are concerns that standard hydrocarbon flue gases could contaminate the cargo. These systems are more expensive.

 

Inert gas drives different steels

In December 2020, DNV said that pitting corrosion on tanks is being caused by "aggressive chemicals contained in the inert gas”.

 

This is a bigger problem in double hull tankers, DNV said, because in single hull tankers, the seawater is closer to the cargo tanks. It can cool the tanks down and slow down bacterial growth. On double hull tankers, the cargo is able to get warmer, which is better condition for microorganisms to thrive and cause corrosion.

 

Three Japanese steel manufacturers, JFE Steel Corporation, Nippon Steel Corporation and Kobe Steel, Ltd, submitted a proposal to IMO to accept the use of more corrosion resistant steels in crude oil tanks. Consequently, IMO issued a new “Performance Standard for Alternative Means of Corrosion Protection for Cargo Oil Tanks of Crude Oil Tankers” in 2010.

 

DNV has since revised its relevant ship construction rules to incorporate corrosion-resistant steels for cargo tanks, and recently added the new classifier “CA” (for “corrosion protection by alternate means”) to its existing corrosion protection class notation. This new notation is labelled “COAT-PSPC (CA)” and was announced in July 2020.

 

Alfa Laval

Alfa Laval makes an Alfa Laval Automatic Fuel Efficiency Module (AFEM) which can be integrated into inert gas generators on product tankers, reducing fuel consumption by generating the exact amount of inert gas required, so no unnecessary fuel is used.

 

It can reduce fuel consumption by as much as 40 per ecnt, the company says.

 

The module is not so much a piece of equipment, more an adjustment to the combustion control in the inert gas generator.

 

A two year pilot project was run with Italian tanker operator Navigazione Montanari S.p.A, where it found it could reduce fuel by 30 per cent. The system was tested on 40,000 DWT product carrier Valle di Navarra, built in 2002, carrying gasooil and gasoline cargo, and discharging up to 3 times per week.

 

In one customer example, the customer was consuming 148 tonnes of fuel over 18 months, and anticipated saving 68 tonnes, or 46 per cent, from the AFEM.

 

It works together with Alfa Laval combustion units, which burn gas to make the inert gas.

 

The module includes a control panel, a fuel oil pump, an oxygen analyser, a burner actuator, and a burner oil supply line modification kit.

 

Alfa Laval says it is not aware of any corrosion problems being caused by flue gas from its inert gas generators, because they have systems to treat any harmful contamination.

 

Wärtsilä / Saacke

In February 2021, ship equipment / power systems company Wärtsilä signed a ‘strategic partnership’ with marine firing plans manufacturer Saacke, to combine their offerings for inert gas systems for the LNG carrier and LPG carrier markets.

 

Wärtsilä has over 50 years experience making inert gas systems, with 2,500 vessels on its reference list, including crude oil tankers, product tankers, chemical carriers and for LPG and LNG gas carries and FPSOs.

 

The companies will work together to provide a package for shipyards and shipowners.

 

They will look in particular at combining Saacke’s gas combustion units with Wartsila’s inert gas systems.

 

The agreement covers SAACKE’s Boilers, Exhaust Gas Economizers (which use heat in exhaust gas to make steam), and air-cooled Gas Combustion Units. On the Wartsila side, it covers Wärtsilä’s Inert Gas Systems and combined Inert Gas & Gas Combustion Units.

 

Wärtsilä will make its Flue Gas system available to SAACKE, making it possible for a single delivery of a combined Boiler and Flue Gas System. Putting this in a package should be a means of improving system performance and reducing cost.

 

Wartsila recently added a system for regulating the inert gas system based on the tank pressure. This means that the system optimises the gas production to only produce the necessary amount of inert gas to maintain the tank pressure.

 

The company claims that its burner / scrubber unit uses less deck space than any other known design. This reduced space requirement is achieved by having the combustion chamber located concentrically inside the scrubber unit, for cleaning the flue gas immediately after it is crated.

 

Scanjet

Scanjet of Sweden provides the Scanjet Feen Inert Gas Generating system, which combusts gas specifically to make inert gas. There are blowers to transport the inert gas through the system into the cargo tanks.

 

It also offers a flue gas system (F-IGS), a multi inert gas generator (M-IGG), an inert gas generator for LPG carriers (IGG-L).

 

It also offers nitrogen inert gas systems (usually used on chemical tankers). It has a system which separates nitrogen from air using membranes, and a system which separates nitrogen from air using  pressure swing adsorbation. This separates gases on the basis that different gases tend to be attracted to different solid surfaces more or less strongly.

 

All the main components are manufactured in house. Scanjet has been making inert gas generators and systems for 20 years.

 

The inert gas generating system can control the amount of inert gas which is needed, based on monitoring the cargo discharge rate. This minimises the amount of fuel which is used, and avoids excess inert gas being vented to the atmosphere.

 

In case of system failure, the system can be manually operated.

 



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