Exhaust Emissions From Ship Engines - Significance, Regulations, Control Technologies

• Author: Laurence Goldsworthy
• Position: Australian Maritime College, University of Tasmania
• Pages: 21-30

21

EXHAUST EMISSIONS FROM SHIP ENGINES - SIGNIFICANCE,

REGULATIONS, CONTROL TECHNOLOGIES

La urie Go ld swo rthy∗

1 Overview

Shipping is the most fuel efficient means of moving freight, carrying more than 70% of the global freight task.

Around 70% of ship emissions occur within 400km of land.1 The impact of ship engine exhaust emissions on

terrestrial air quality is under focus. Terrestrial air emission controls are outpacing controls on ship emissions.

Ships generally use low quality fuel to reduce costs. This low quality fuel tends to have a high sulphur content.

Emissions of oxides of sulphur (SOx) from shipping represent about 60% of global transport SOx emissions.

Emissions of oxides of nitrogen (NOx) from shipping represent about 15% of global anthropogenic NOx

emissions and around 40% of global NOx emissions from transport of freight. Shipping represents around 15%

of global freight transport CO2 emissions (2 to 3% of total global CO2 emissions).2

Engine exhaust emissions can be divided into two broad categories – those which directly affect air quality and

those which directly affect global warming.

2 Air Quality Emissio ns

2.1 Formation

In diesel engines, fuel is injected at high pressures into air which has been compressed by the moving pistons.

This compression raises the temperature of the air sufficiently to cause the fuel to ignite. Combustion proceeds

around the periphery of the fuel spray at temperatures around 2000°C.

Oxides of nitrogen are formed during the combustion process due to combination of nitrogen and oxygen from

the air at these high temperatures. The diesel combustion process inherently produces relatively high levels of

NOx, and the fuel properties only have a minor influence on the amount produced.

Oxides of sulphur form during the combustion process, by combination of the sulphur in the fuel with oxygen.

The prime constituent of SOx is SO2. The amount of SOx formed in an engine depends primarily on the

concentration of sulphur in the fuel. SOx emissions from ship engines are relatively high because they burn high

sulphur content fuels.

Particulate matter (PM) emissions are primarily formed by two separate mechanisms:

• Nuclei mode particles consist mainly of condensed hydrocarbons and sulphates. The gaseous

precursors condense as temperature decreases in the exhaust system and after mixing with cold air in

the atmosphere. The sulphates arise fr om combination of SOx and water in the exha ust.

• Accumulation mode particulates are formed during combustion by agglomeration of primary

carbonaceous particles (99% C by mass) and other solid materials. The majority of the accumulation

mode particulates form in the core of the burning fuel spray. They are known as ‘black carbon’ or

‘soot’. Further, gases and condensed hydrocarbon vapours are absorbed into the surface of the particles.

The formation of accumulation mode soot is inherent in the diesel combustion process and is only

partially dependent on fuel quality.

The condensed hydrocarbons in the nuclei mode particles and on the surface of the accumulation mode particles

contain toxic and carcinogenic hydrocarbons. The high sulphur content of marine fuels leads to relatively high

levels of sulphate particulates.

∗ Australian Maritime College, University of Tasmania.

1 Jana Moldanováa et al, 'Characterisation of Particulat e Matter and Gaseous Emissions from a Larg e Ship Diesel Engine' (2009) 43

Atmospheric Environment, 2632.

2 S.B. Dalsøren et al, 'Update on Emissions and Environmental Impacts from the International Fleet of Ships. The Contribution from Major

Ship Types and Ports' (2009) 9 Atmospheric Chemistry and Physics 2171; James J. Corbett et al 'Mortality from Ship Emissions: A

Global Assessment' (2007) 41(24) Environmental Science and Technology 8512; Øyvind Buhaug et al, Second IMO GHG Study 2009

International Maritime Organization (IMO) London UK April 2009; V Eyring et al, 'Emissions from International Shipping: 1. The Last

50 years' (2005) Journal of Geophysical Research 110.