19-07-2014, 12:18 PM
Maritime Institute Of William Barentsz e Of William Barentsz
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Abstract
The worldwide focus on fuels is generally increasing because of the focus on exhaust gas emissions.
During more than 10-15 years, the authorities have been focused on establishing rules for the exhaust gas emissions from marine engines, and today it seems that IMO Annex VI will be ratified in the course of 2004/2005. Hence, the marine industry will be under international regulation.
Compliance with IMO Annex VI requires the engines to be within the given NOx level limits documented in a technical file following the engine in operation. In order to give operators a unified technical file (MAN B&W Diesel A/S, 2004b) to be followed by MAN B&W licensees, a procedure has been developed by MAN B&W and accepted by the flag states’ representatives, the Classification Societies.
Until now, local rules have been introduced for areas such as Sweden, Norway, and the harbour of Hamburg, where for example a harbour-fee reduction is used as an incentive for the use of low-sulphur fuel, but with limited impact on the environment, especially with regard to emissions from ships in international operation.
A general worldwide emissions limitation seems to be the only way that all countries can benefit from a reduction in emissions. Emission limits must follow state-of-the-art technology and the ability of the market to adapt to such limits.
It is correct that emission limits can force the technology to be developed, but then the solution chosen will not necessarily be the optimum one. And the system chosen on ships built will, on average, stay there for the ship’s lifetime, which can be more than 25 years.
Exhaust Gas Recirculation (EGR) and Humid Air Motor (HAM)
Modification of combustion air properties. For both the Exhaust Gas Recirculation
(EGR) and the so called Humid Air Motor (HAM) systems, the NOx reduction
effect is achieved by reducing the local maximum combustion temperatures in
the combustion chamber, and reducing the concentration of oxygen by the addition of inert media with high specific heat: exhaust gas or water vapour. The NOx production only takes place at very high temperatures (2,200°K and above), and it increases exponentially with the temperature. The EGR method is based on a reduction of the oxygen content in the cylinder charge, and the HAM method is partly based on reducing the oxygen content of the cylinder charge and partly on increasing the heat capacity of the cylinder charge by the addition of water vapour.
As mentioned above, these methods (EGR and HAM) have, by calculations and tests, proved their capability for NOx reduction, but they have never before been developed to a commercial application level for large two-stroke engines, and they have not been fully optimized with regard to cross-over effects on fuel oil consumption, heat load conditions and other emission parameters.
Results from engine testing with EGR systems
Very promising operating conditions have been obtained during the tests, as outlined in the below summary of the main results. Emission results. The relative changes in measured emission parameters as a function of the recirculation amount at 75% engine load are illustrated in Fig 1 & 2.
As can be seen, at increased recirculation amounts, the HC and PM emissions are reduced corresponding to the reduction of the exhaust gas flow from the engine. This indicates that each engine cycle has the same production of HC and PM independent of the recirculation amount, and that the HC and PM in the recirculation gas is eliminated during the normal combustion process.
The increase in CO emissions with increased recirculation amount indicates, as expected, that the lower cylinder excess air ratios at increased recirculation amount result in larger local regions in cleaning the exhaust gas with scrubber.
As mentioned in the description of the EGR system, the EcoSilencer has been introduced in the EGR system to clean the exhaust gas and, if possible, also to reduce some of the emission components.
MAN B&W Diesel has accordingly measured the emission components at inlet and outlet of the scrubber at different engine loads. The results from these
Results from engine testing with HAM systems
As for the EGR system also for the HAM system very promising operating conditions have been obtained, as outlined in the following.
Emission results. The measured emission parameters, as a function of the HAM level at 100% engine load, are illustrated in Fig. 3. As can be noted, the HC and
PM emissions are nearly unaffected by the HAM level. The CO emissions increase significantly with increased HAM level, most likely due to the lower cylinder excess air ratios at increased HAM levels, which result in larger local regions in the combustion chamber with lack of oxygen. Furthermore, the expected significant reduction of the NOx level has been confirmed thatcombustion chamber with lack of oxygen. Furthermore, the expected significant reduction of the NOx level has been confirmed