Panel Discussion "Exhaust Gas Emissions from Ships"
Reduction and Control of Emissions
Stephen G. DEXTER *
Large Engines and Compressors
Exhaust emissions from marine engines can be reduced by many methods, each with its own penalty in cost or durability. These methods are reviewed. A secondary issue is the question of the control required to maintain the emissions at the level of the engine when new. It is predicted that on-board diagnostics will replace the currently proposed Technical File approach.
Key Words: Emissions, Marine Engines, Control, On-board Diagnostics
The technology required to reduce exhaust gas emissions from diesel engines has been available for many years and has been applied to automotive engines. The more difficult problem is how marine engines can be both environmentally friendly and efficient. In many ways it is easier to reduce the emissions from the marine engine than from an automotive engine because of the less frequent changes in load and speed and the fewer limitations of space, weight and cost. However, the problem is made more difficult by the type of fuel burnt by marine engines and by the greater demand for low fuel consumption and good durability. From the year 2000 NOx reduction methods are being applied to marine engines and it is interesting to consider these and how they may be monitored.
2. EMISSIONS REDUCTION
The primary concern over marine emissions is NOx. The formation of NOx is caused by high temperatures during combustion and by the length of time held at that temperature. Most in-cylinder NOx reduction methods therefore rely upon reducing the local temperature during combustion and/or reducing the time at that temperature. The alternative method is to reduce the NOx by after-treatment in the exhaust gas stream after it has left the engine.
The first reduction method is to retard the injection and then to raise the compression ratio to recover some of the loss in fuel consumption. At the same time the injection period may be reduced, again to improve the fuel consumption, and improved fuel atomisation and distribution may be used to reduce local temperatures. In addition a control of the rate of injection can reduce the large NOx production caused during the initial uncontrolled combustion of the fuel injected in the delay period as in the AVL Fuel Injection Rate Shaping Technology (FIRST) system .
Reduced air intake temperature is another method of reducing NOx and it also reduces fuel consumption. A method of reducing local combustion temperatures is exhaust gas recirculation and this has been very successful in smaller engines. It is more difficult to apply in marine engines because the boost pressure is higher than the exhaust pressure. Also the particles contained in exhaust from heavy fuel burning engines can cause component wear.
Secondary methods such as water injection and selective catalytic reduction (SCR) are currently used and are very effective but a careful balance is required between effectiveness, cost and inconvenience. These reduction methods and their effect on engine design are discussed in . The conclusion is that many techniques will be combined and continued effort is required to make sure that emissions reductions can be achieved without large cost penalties.
3. CONTROL OF EMISSIONS
The present method of controlling exhaust emissions proposed by IMO is a compromise between what is ideal and what is practical. It relies upon an engine, or an engine group, being certified for a particular emissions level when the engine is new. The ship owner or operator is then required to maintain a Technical File which may be inspected at intervals to make sure that the emissions level of the engine has not been allowed to increase. This has already raised, and will continue to raise, many questions about which spare parts may be used during maintenance and what adjustments may be made to the engine during its life. If the requirements are rigidly interpreted it will not be possible for engine operators to use spare parts which have not been purchased from suppliers authorised by the engine manufacturer unless the part suppliers go to the considerable cost of testing and proving that their parts do not change the emissions levels of the engine. A less rigid interpretation will inevitably introduce some leniency into this situation, as a result of commercial pressures from the ship owners and operators, and by the inevitable situation when a ship is held up because authorised spare parts are not available and unauthorised parts are available locally . How these disputes and negotiations will be settled remains to be seen but the situation is likely to cause much anguish for all concerned unless another approach can be found.
Another aspect of control is whether the engine operator will make deliberate adjustments to the engine to reduce cost or fuel consumption with the result of an increased emissions level. This adjustment, if returned to its original setting before the ship comes in for its next inspection, may then remain undetected. The current system seems to rely upon the integrity of owners and operators and will only be controlled by an inspection of parts and of the Technical File.
* AVL List GmbH, Hans-List-Platz 1, 8020 Graz, Austria
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