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Natural gas for ship propulsion in Denmark

4 Emissions to air

• 4.1 Reduction of emissions
• 4.2 The potential in Denmark
• 4.3 Comparison of reductions achieved in Scenarios 1 and 4
• 4.4 Emissions of greenhouse gasses
• 4.5 Summary and conclusion on emissions

4.1 Reduction of emissions

A primary driver for the conversion of the existing fuel consumption is the sulphur caps being rolled out globally over the coming decade and in particular the stricter requirements in the Baltic Sea and the North Sea, both ECAs. Here the sulphur content of fuel must not exceed 1.0% as of 1 July 2010 and is further reduced to 0.1% on January 1 2015. If natural gas is not considered this leaves only MGO or exhaust gas cleaning as alternatives. Installation and operating scrubbers adds capital needs and running cost and there is an expectation in the market that the future cost of low sulphur fuel may soar with the increased demand.

This section compares the emissions related to the consumption of fuel under the 1% sulphur cap and those emissions a conversion to natural gas achieves. Almost all data regarding emissions from natural gas in ships are from LNG and will also be used to represent the emissions using CNG^[22]. There are also emission control measures underway for NO_x introducing a worldwide 20% reduction for new engines in 2011 (IMO Tier II level) and an 80% reduction (IMO Tier III level) for new engines from 2016 operating in ECAs.

Table 4-1 compares the emissions to air from LNG and liquid petroleum fuels for ships. The table is from an EU financed study MAGALOG carried out during 2007-2008 on LNG as a clean fuel for ships in the Baltic and North Seas, and it compares the emissions from HFO (Residual oil) with 3.5% sulphur, Marine diesel oil with 0.5% sulphur, Gasoil with 0.1% sulphur and finally natural gas (LNG).

Table 4‑1 Estimated emissions to air from LNG and liquid petroleum fuel for ships. Emissions are related to the engine output in kWh and for typical medium speed engines built after year 2000 without exhaust cleaning. Emission may vary with fuel quality and engine type (Marintek in MAGALOG 2008)

LNG and natural gas in general is a cleaner fuel for internal combustion engines than other liquid petroleum fuels, which as mentioned in Chapter 1, is a considerable attraction regarding the reduction of sulphur. The SO_x and PM emissions from the LNG itself compared to residual fuel are close to zero, but in practise there will still be a contribution from the lube oil^[23]. In two-stroke engines the PM reduction operating on LNG compared to HFO in 60-70%, which is similar to the effect achieved by scrubbers. The NO_x from LNG compared to residual fuel is reduced by 80-90%. As the overwhelming bonus and providing the cash injection to finance at least part of the conversion costs there is a reduction in fuel consumption and hence a CO₂ emission reduction. In Table 4-1 the CO₂ is shown to be reduced up to 25% based on theoretical considerations, but due to volatile organic compounds (VOC) in the exhaust gas this is not achievable in practice (see below).

The technical advantages^[24] of using LNG or natural gasses as fuel for internal combustion engines are in the MAGALOG study and listed below:

• High methane number, allowing a high power ratio within the knocking margin of the engine.
• Easily mixed with air to obtain a homogenous charge, which burns with high flame velocity even at high air access. This avoids high peak temperatures and pressures during combustion, resulting in reduced emissions of NO_x of as much as 90% in comparison with residual oil or marine diesel oil. It also allows for high efficiency.
• Contains no sulphur, therefore emits no SO_x, and this also result in very low particle emissions.

A major disadvantage when using LNG or natural gas as fuel for ships is more un-combusted hydrocarbons, mainly methane, in the engine exhaust. The cause is the relatively low combustion temperature when burning a lean gas/air mixture, compared to HFO or diesel, but this is also the reason for the lower NO_x emission. Depending on the design and operation with respect to the VOC-exhaust level the overall climate benefit of using LNG/natural gas as a substitute for liquid petroleum fuel oil is estimated at up to 15%, but it is emphasised that the potential emissions of VOCs are mainly methane, which is a powerful greenhouse gas^[25].

4.2 The potential in Denmark

In the previous chapter the future for natural gas vessels in Denmark is expected to be in the ferry traffic and short sea traffic, because this type of shipping operation at the first glance meets a basic requirement for introduction of new technology: the immediate future is foreseeable as the ships usually operate on fixed voyages in time and geography, which enables a projectable investment window. However, as concluded in Chapter 3, at least the short sea cargo traffic is in reality not all that fixed since routes are frequently altered to suit customer needs. Regarding, ferries the operation is carried out under time limited concessions thus potentially reducing the pay back window, but the distances and ports of call remain fixed.

This section aims at evaluating the reduction of air emissions achieved by the different scenarios outlined in Chapter 3. The potential of emission reduction from the Danish ships will be dependent on the present types of fuel in use. Due to the MARPOL VI restrictions already in place or imminent at the time of this study (summer of 2010) in the ECAs, many shipowners have already changed to marine fuels with less than 1.0% sulphur.

The study has interviewed shipowners’ representatives concerning the fuel type of the different ships^[26] and this information is used as a base for estimating the reduction potential. It was found that that the smaller island ferries and fast ferries operate either on MDO or MGO, whereas larger ferries, which operate on longer distance, mostly use HFO with 1.0% sulphur or MDO. Ships operating on HFO switch over to MDO while berthed in ports^[27], so in addition to the HFO consumption, there is a minor MDO consumption.

It is assumed that all short sea shipping mainly operates on fuel with a sulphur content of 1.0% except when in port. For the purpose of calculating the future air emission reduction the minor reduction already achieved from using MDO when operating in port is neglected and the estimated reductions are based on a level of 1.0% sulphur fuel.

The Table 4-2 below are from MAGALOG report and indicates the potential of switching from one fuel type to another. When changing from oil with 1.5% or 1.0% sulphur contents the percentages change proportionally for SO_x and PM. It is assumed that 1% added pilot fuel and lubrication oil still leads to emission of SO_x and PM also when operating on LNG.

* This PM reduction may be 85-92%, but a rounded estimate is given here.

To calculate the fuel consumption and emissions from the ships from Scenario 1, 2, 3 and 4, these data are used together with the vessels technical information from the respective shipping companies and engine load function provided by DTU^[28]. When no data could be withdrawn from the shipping companies the following assumptions concerning fuel types have been made: Ferries operate on MDO and ferries with long voyages on HFO; short sea shipping operates on 1% sulphur fuel. In the main body of the report only the range defining scenarios 1 and 4 are presented, with further details on the scenarios provided in appendix 6.

4.3 Comparison of reductions achieved in Scenarios 1 and 4

Table 4-3 shows the estimated current fuel consumption and the estimated comparable LNG consumption in Scenarios 1 and 4. The expected reduction potential realised, if all ships were converted from the existing fuel type to LNG or CNG is presented in Table 4‑4. The presented result for ferries comprise fast ferries, smaller ferries within the Danish boarders and RoPax vessels on routes within the Danish boarders and to our neighbour countries. The short sea traffic comprises cargo ships, including RoRo cargo ships, with at least one Danish port call on their routes. All cargo ships are assumed to operate on fuel with 1.0% sulphur.

The reduction potential linked to Scenario 1, includes 65 ferries in 41 ports and 78 short sea cargo ships in 14 ports, and in Scenario 4 includes 27 ferries in nine ports and 20 cargo ships in four ports.

The absence of sulphur and almost non-existing PM contents in natural gas leads to minimal emissions of SO_x and PM only caused by pilot diesel and lube oil, when a ship is operated on LNG or CNG. It should be noted that the sulphur and PM emissions are indicative as LNG or CNG exhaust gas from vessels with dual fuel engines may contain a fraction of sulphur and PM.

The reduction potential for NO_x is more than 80%, if all the selected ferries and short sea cargo ships are converted to LNG or CNG operated engines.

The ports in Scenario 4 are Sjællands Odde, Rønne, Rødby, CMP/Copenhagen, Gedser, Hirtshals, Helsingør, Esbjerg and Aarhus port for ferries and the four short sea line traffic ports are Aarhus, Esbjerg, CMP/Copenhagen and ADP/Fredericia.

4.4 Emissions of greenhouse gasses

Using natural gas enables the ship to reduce CO₂ emissions. The reduction of CO₂ differs slightly between LNG and CNG mainly caused by less energy requirement for compression compared to liquefaction. Presently, the combustion technology for a typical medium speed engine without exhaust cleaning allows a 10-15% saving, with a theoretical reduction potential for CO₂ of up to 25% when converting to natural gas^[29]. As a crude estimate the following table presents reductions in CO₂ emissions in the four scenarios based on a conservative 10% realised reduction.

The MAGALOG study estimated the reduction of greenhouse gases by converting the ferries in the Baltic Sea and the North Sea. The study concluded that 1 million tons CO₂ equivalents would be saved if the efficiency was set at a conservative 10% less emission level for LNG.

However, one important issue when assessing the impact on global warming by choosing LNG and CNG is the potential for emission of methane due to its potency as a GHG. In a recent study from Chalmers comparing LNG with other fuels in shipping in a life cycle perspective it was in fact stated that “The crude oil base fuel alternatives have lower global warming potential if about 2.5% of the LNG used for transport leaks”.^[30] Thus, control of the emissions of VOCs and in particular methane is of paramount importance to the positive effects on global warming of converting to LNG.

4.5 Summary and conclusion on emissions

The scenarios have revealed that relative to the number of ships and ports involved a large potential for reduction of emissions to air by conversion to LNG or CNG is achievable in a limited number of ferries operating from nine Danish ports and by targeting short sea line traffic in four ports. The reduction potential for short sea traffic is mainly from RoRo cargo vessels.

Click here to see: Figure 4‑1 Reduction in emissions for conversion to natural gas in Scenarios 1-4

The reduction potential from Scenario 1, which includes 65 ferries in 41 ports and 78 vessels in short sea line traffic in 14 ports, to Scenario 4, which includes 27 ferries in nine ports and 20 vessels in short sea line traffic in four ports is still achieving 70-80% of the maximum scenario. It appears to be beneficial to target the installations of the LNG or CNG storage and refilling plant to the most consuming routes/ports and yet reap a large emission reduction potential. It is also clear the focusing on the ferry trade will give the most immediate and large reductions in fuel, SO_x, PM and NO_x,

[22] There are further minor differences such as the use of pilot fuels.

[23] The lube oil contribution, when using gas in combustion engines is for a four-stroke engine up to 0.1 g PM/kWh.

[24] Technical features from MAGALOG report (p. 16)

[25] MAGALOG report (MARINTEK) (p. 17)

[26] Interviews with shipowners and Hans Otto Holmegaard Kristensen, DTU.

[27] To fulfil requirements in Directive 2005/33/EC on sulphur content of marine fuels

[28] Hans Otto Holmegaard Kristensen, DTU

[29] See table 4-1

[30] Selma Bengtsson, Chalmers University of Technology, Lighthouse Eco Ship Theme Day Program, May 27th 2010.

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Version 1.0 November 2010, © Danish Environmental Protection Agency