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Wet exhaust from a marine leisure craft diesel engine: environmental aspects
2001 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

In most leisure marine craft a wet exhaust system is used to avoid hot exhaust lines and to provide silencing. In a wet exhaust system the exhaust from the engine is mixed with seawater before it is discharged under or near the water surface. The use of a wet exhaust system is suspected to result in an increased possibility for exhaust constituents to be transferred into the aquatic environment. The objective of the study was to review current knowledge on wet exhaust and related subjects to form an understanding of how the exhaust is effected, to what extent constituents are transferred into the aquatic environment and to make a general assessment of the potential environmental effects from exhaust released through a wet exhaust system. A theoretical study system with a Volvo Penta KAD44PEDC engine installation was defined and the exhaust from that system was characterized. The main constituents of diesel engine exhaust are carbon dioxide, carbon monoxide, nitrogen oxides, sulfur dioxide and various organic compounds, mainly hydrocarbons. Emission factors for these constituents and speciated organics were obtained from KAD44PEDC certification testing data and from applicable data for other engines. The exhaust is partitioned in a gas phase and a particle phase. The fine size fraction of the particulate matter was recognized to be of particular concern due to suspected health risks. Transfer of gaseous constituents into the water phase was first approximated using an idealized system, assuming steady state equilibrium conditions. After considering the limitations of this model it was concluded that it does not reflect the real situation very well. Instead the transfer is believed to be characterized by kinetics with boundary layer diffusion being rate determining. Thus, the transfer is governed largely by the flow situation and the contacting time between the exhaust gas and the water phase. The dominant particle phase transfer mechanisms are inertial and diffusional collection. These findings imply that a larger part of the transfer occurs in the propeller wash, since the conditions there are very turbulent and the detention time is believed to be longer than in the exhaust system. It was concluded that some transfer of the inorganic constituents might occur. For the organics the situation is more complicated but a significant potential for transfer was found to exist. A significant potential for transfer of particulate matter is also believed to exist and effects on the size distribution are plausible. Three studies of transfer of exhaust from gasoline outboard engines were reviewed as well as four studies on the effects of water scrubbing on exhaust from diesel powered underground mining equipment. From these studies it was found that a significant transfer of hydrocarbons, averaging around 40%, could be achieved. Reactions involving aldehydes may also take place in the water phase. It was noted that turbulence and exhaust gas to water ratio could have a significant influence on the transfer processes. Water scrubbing was found to be a viable way of removing particulate matter from diesel engine exhaust and a reduction of about 40% was observed for a diesel engine with a wet exhaust system. Some results indicated that increased turbulence could improve the removal of smaller particles. Further transport and transformations of the exhaust constituents after discharge need to be considered in order to assess their potential environmental impact. No well established or otherwise obvious way to assess the potential for adverse environmental impact was found. Overall, the inorganics potential for adverse effects in the aquatic environment and on human health is relatively clear while the effects of the organic constituents and the particulate matter are somewhat uncertain. The exhaust contains a number of constituents that are toxic and bioaccumulative and some compounds that are known human carcinogens. It is considered clear that there is a potential for adverse effects on the physical environment in the form of contributions to the greenhouse effect, acidification, eutrophication, formation of groundlevel ozone and visibility impairment. It is also considered that there is a potential for adverse effects on aquatic organisms due to transfer of toxic compounds into the aquatic environment. A human health risk due to direct exposure to the exhaust or indirect exposure to exhaust constituents is believed to be present. It was concluded that: The established knowledge on wet exhaust is very limited and that due to the use of a wet exhaust system a transfer of exhaust constituents to the water phase is very likely, The transfer probably occurs largely in the propeller wash. The transfer will probably affect the composition of the exhaust released to the atmosphere. A significant transfer of organics and particulate matter is likely and the transfer of organics into the water phase is likely to have an adverse impact on aquatic biota. Potential health risks from inhalation of the exhaust could possibly be reduced. It was also concluded that more data is needed to verify these hypotheses and to enable a reliable quantification of the processes. A proposal for further studies, including exhaust measurements is made.

Place, publisher, year, edition, pages
Keyword [en]
Technology, marine wet exhaust, diesel engines, exhaust constituents, aquatic environments, environmental effects, diesel engine, emissions, emission factors, hydrocarbons, organic particles, nanoparticles, particulate matter, scrubbing, environmental, impact
Keyword [sv]
URN: urn:nbn:se:ltu:diva-50268ISRN: LTU-EX--01/084--SELocal ID: 7875a83c-1031-40f3-97b2-0a34373191d3OAI: diva2:1023625
Subject / course
Student thesis, at least 30 credits
Educational program
Environmental Engineering, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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