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Preparing steam LNGCs for low sulphur gas oil

Hamworthy Combustion has completed its first LNGC burner conversions to meet the EU Fuel Directive's requirements for reduced sulphur emissions

LNG Bayelsa ­ the first LNGC to have its Hamworthy Combustion burner equipment converted to use low sulphur MGO

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or many years steam boilers used in land-based applications have been subject to tight emission controls. These controls mainly focus on limiting

However, EU Directive 2005/33/EC, known as the EU Fuels Directive, mandates the use of fuel containing not more than 0.1 per cent sulphur by weight on all merchant ships alongside a berth or at anchor in European waters. The date for entry into force is 1 January 2010 although some leeway is being allowed to provide existing ships with a reasonable amount of time to implement the measures necessary to achieve this goal. The implication of the EU's 0.1 per cent sulphur limit is that it will only be satisfied with distillate diesel fuels. As a result, the use of HFO for steam generation when the vessel is in European waters will be phased out over the coming years. Similar restrictions are now also being introduced for ships in Californian coastal waters and it is expected that they will be extended to the whole North American coastline in the next few years. As a result of the EU Fuels Directive, all existing steam generation burner equipment supplied for firing HFO will need to be modified to enable the burning of low sulphur distillate fuel when the ship is in European waters. The fuel that will enable the maximum sulphur content of 0.1 per cent requirement to be met is referred to as low sulphur DMA or, to give it its commercial name, low sulphur marine gas oil (LSMGO). "For the last 18 months we have been developing

emissions of nitrogen oxides (NOx), sulphur dioxide (SO2), carbon monoxide (CO), carbon dioxide (CO2)

and particulates. All are pollutants and widely regarded as being the main causes of the global greenhouse effect. Until recently, shipping has been largely exempt from these controls, even when in port. In addition to burning cargo boil-off gas, when available and permitted, main and auxiliary steam boilers on LNG carriers have traditionally used heavy fuel oil (HFO), which has a relatively high sulphur content. In recent years IMO has been working to tighten its Marine Pollution (Marpol) Convention Annex VI global regulatory regime governing ship atmospheric pollution. In addition, both the European Union (EU) and the California Air Resources Board (Carb) have drawn up their own regional measures aimed at reducing atmospheric emissions from shipping. As a result of the latter initiatives, tighter sulphur emission controls are being brought into force in both European and American waters. Until now, ships have been able to fire boilers with HFO with a sulphur content in the 1.5-3.5 per cent range.

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and test firing, at our headquarters in Poole on the south coast of England, a conversion package that will enable existing burner equipment to be operated safely on LSMGO," states Ian Neill, general sales manager, industrial and marine packaged burners for Hamworthy Combustion Engineering. "By providing this conversion package Hamworthy Combustion has sought to meet the changing needs of the shipowners and existing customers who currently operate HFO burner equipment." Hamworthy Combustion is an established supplier of combustion equipment for the marine sector, including LNG carriers. Over 60 per cent of the main steam boilers on LNG carriers are fired with either Hamworthy HXG or Airoil 5000 dual-fuel burners, and the first LNG vessel with the company's burner equipment entered into service in 1972. Hamworthy Combustion also installs many of its divided flow register (DF) and rotary cup burners on the auxiliary boilers used on board tankers, bulk carriers, passenger vessels, etc and is making guidance available which highlights the modifications that need to be made to enable these other ship types to switch to the combustion of LSMGO safely and efficiently. The company has marked the finalisation of its conversion package development work with the completion of its first successful conversion of the main boilers on an LNG carrier. The vessel in question is LNG Bayelsa, a 137,500m3, spherical tank ship built in 2003 which is managed by Shell and owned by Nigeria LNG Ltd. The conversion kit enables LNG Bayelsa's main boilers to be operated on either LSMGO or HFO. This is achieved by using steam as the atomising medium for each fuel oil and either fuel can be used in combination with boil-off gas. The same burner sprayer and atomiser are used for both fuel oils and there is no need for any manual intervention in the operation of the burners. "The Hamworthy Combustion package also enables the burner to complete an online changeover from HFO to LSMGO, and vice versa," continues Mr Neill. "Such a capability offers a number of advantages which can be itemised as follows: (a) as the same burner sprayer and atomiser can be used for both types of oil fuel, there is no risk of human error arising out of having to change burner sprayers or atomisers; (b) the risk of human error is further reduced because the changeover between HFO and LSMGO is completed automatically using the burner control system; (c) there is no need to control the shutdown of individual burners; (d) the changeover between HFO and LSMGO, and back again, is seamless and, as it is completed under load conditions, the fuel oil flame is always present. Therefore, the loss of the ship steam supply is never a factor; and, (e) the system is approved by the relevant classification society." A critical factor for Hamworthy Combustion when developing the LNG carrier conversion package was the need to ensure that the firing of LSMGO in the ship's main high-pressure boiler is carried out safely. That consideration ensured the inclusion of the following key components in the company's burner conversion package: (a) atomiser tips suitable for HFO and LSMGO; (b) an oil sprayer gun assembly suitable for HFO and MGO with atomising steam; (c) high-energy igniter/exciter unit, complete with spark diagnostic feedback to ensure the ignition source is proved before opening the main fuel valves;

A Hamworthy Combustion HXG burner for LNG carriers The Hamworthy Combustion conversion package enables the burner to switch online between HFO and LSMGO

(d) Smartscan 121 flame scanner units with ultraviolet

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The self-checking Smartscan 121 flame scanner can be set to reduce the 'flame off' time to 1 second

flame viewing and a self-checking function which provides a flame failure signal in less than 1 second; (e) relevant spares in accordance with class society regulations; and (f) relevant classification society approvals for this full conversion package. Historically, there has been some reluctance on the part of shipowners and maritime authorities to fire distillate fuel oils in high-pressure main steam boilers. This is due to safety concerns over the high furnace temperatures and the tendency for distillate oils to `flash off' into highly explosive gases if the combustion is incomplete. There have been several incidents in the past where boiler explosions have resulted in fatalities on vessels on which the combustion light-up sequence has been faulty and unburnt fuel has vaporised in the furnace. Manufacturers of the combustion equipment for these boilers have therefore had to focus on ensuring safe and reliable combustion of the low sulphur marine gas oils which will now be required to be used on main and auxiliary steam boilers. One of the major implications of firing LSMGO is the effect it has on the fuel oil supply to the boilers and on the burners themselves. Because LSMGO has poor lubricating properties, it is not suitable for most

existing fuel oil supply pumps. As a result, it is necessary to install new supply pumps as well as a separate fuel supply system. Such an arrangement is also necessary because LSMGO should be delivered to the burner at ambient temperatures. If it is heated, it will vaporise in the pipework, causing combustion issues. Using separate systems for HFO and MGO also allows for online fuel oil changeover, a significant benefit for an LNG carrier. For these reasons modifications to the burner equipment are vital to safe combustion of the LSMGO. Furthermore, the heating value and viscosity of LSMGO require fuel nozzle sizes and controls issues to be addressed to ensure that the boilers can be changed over to the new fuel safely whilst ensuring that combustion efficiency is maintained, irrespective of the fuel in use. "We have found that all different burner types are suitable for conversion to the new fuel," explains Mr Neill. "However, extreme care must be taken to minimise the risk of accidents. The most vulnerable time for unburnt fuel to enter the furnace space would be during the boiler's light-up sequence. The ignition equipment and flame scanners are considered to be the two most crucial components of the burner when it comes to ensuring the safe and efficient combustion of any fuel. "Older types of ignition systems such as carbon rod

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are reliable as a source of ignition for HFO. However, there is no automatic check on their performance before the fuel valves open to allow fuel to enter the furnace. There is thus a possibility that the ignition will fail, allowing unburnt fuel to enter the furnace space to create an explosive mixture. "Hamworthy Combustion, through our US group company Chentronics, has developed a high-energy igniter with spark diagnostics," continues Mr Neill. "The diagnostics feature of the system measures both the spark rate and the spark intensity and provides a signal to the boiler control system which prevents the fuel valves to the burner opening in the event of poor performance of the ignition system. This system thus ensures that the ignition source is proven to be in full operating condition before the fuel valves allow fuel to enter the furnace space." As regards the flame scanner equipment, the most critical aspect of its operation is the "flame off" safety time, i.e. the time between flame failure and the shutoff of the fuel valve which stops the flow of fuel to the furnace. Many of the older types of scanner equipment have flame off safety times of around 4 seconds. While this is acceptable for firing heavy fuel oils, 4 seconds worth of distillate fuel oil being discharged into a hot furnace creates an extremely hazardous situation which could result in vaporisation and the production of explosive gases from the unburnt fuel oil. Hamworthy Combustion's self-checking Smartscan 121 flame scanner

can be set to reduce the flame off time to 1 second, thus minimising the time that potentially unburnt fuel oil can enter the furnace. In the past atomising steam has rarely been used when firing distillate fuel oil due to the perceived dangers of this approach. However, both the Royal Navy in the UK and the Italian Navy have used Hamworthy steam atomised burners on distillate oils on their vessels. Hamworthy Combustion has years of experience with twin fluid atomisers, both steam and air, and this expertise has enabled the company to consider all aspects of firing distillate oils using steam atomisation. One of the main problems is that caused by the differences in thermal expansion and heat transfer from the steam to the oil due to the high atomising steam temperature compared with the oil temperature. Current HFO burners use concentric tube atomisers in which the steam is supplied to the atomiser by means of a tube either through the centre of the burner gun or outside it, depending on the design. The oil is supplied either around the outside of the tube or inside it. This causes the higher temperature of the steam, which can be up to 340°C, to transfer to the oil, which is supplied at ambient temperature, or around 40°C. "To overcome this problem and eliminate any heat transfer issues, the Hamworthy Combustion solution utilises a parallel tube sprayer to supply the steam and oil down completely separate tubes," explains Mr Neill. "The difference in thermal expansion between the steam and the oil is solved by the inclusion of expansion pieces in the steam supply tube. "Hamworthy Combustion has been supporting and advising shipowners and operators on the requirements associated with the conversion of both main and auxiliary boilers. These conversions are focused on the aforementioned safety requirements for the combustion of distillate oils and, as such, the conversion kit has been produced and offered for the conversion of the company's HXG, DF and rotary cup burners. "It is vital that boiler conversion work is carried out and supervised by fully qualified engineers," he concludes. "With engineers based in Europe, Dubai, Singapore, Korea, China and Japan, Hamworthy Combustion offers a full service package which encompasses the timely installation, supervision and commissioning of burner conversions on a worldwide basis."

The high-energy igniter/exciter unit's spark diagnostic feedback capability enables the ignition source to be proved before the main fuel valves are opened

Since the completion of the burner conversion on LNG Bayelsa, Hamworthy Combustion has carried out similar projects on a further five LNG carriers. LNG

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