Aftertreatment of methane slip from marine gas engines

Projekt:

Branschprogrammet hållbar sjöfart

Sammanfattning:
The emissions from marine gas engines are determined partly by the fuel and partly
by the combustion technology used. Natural gas, bunkered on ships in liquefied form
as Liquefied Natural Gas (LNG), is a clean fuel compared to fuel oils, causing low levels of emissions of sulphur dioxide, particles and soot. Also, CO2 emissions per energy unit is relatively low from LNG combustion due to more chemically bound energy per carbon content in natural gas than in fuel oil. Further, the Engines operating on natural gas are often of a “low-pressure” type. These engines have low NOX emissions compared to “high-pressure” diesel engines. The LNG engines are either spark ignited using the gas as the only fuel, or they use a dual fuel technology where a pilot fuel injection is used for ignition. The pilot fuel is responsible for a large part of emissions of SO2 and particles, although it only contributes 5% or less of the energy. Another type of dual fuel engine is the high-pressure engine using LNG as fuel in a diesel combustion cycle. Like in other diesel cycle engines, the NOX emissions from those engines are high, comparable to emissions from fuel oil combustion. The low-pressure dual fuel engines are by far the most used engine type on ships that are not LNG carriers.
A side effect from the combustion in the low-pressure engines is a slip of unburnt methane through the combustion process. For some engines using LNG as main fuel, the methane slip causes total emissions of CO2-equivalents to be higher than from comparable engines using only marine gasoil. The issue of methane slip is addressed by engine manufacturers aiming for improved designs and combustion technology.
Another way to approach the matter is to treat the exhaust gases. In this study we
have analysed different ways to oxidise methane in the exhaust pipe of marine engines. Methane engines used on land are often equipped with oxidation catalysts. There are however still no systems commercially available for marine applications. Factors that present a challenge to the use of catalysts on ships include a high sulphur content of the pilot fuel, low temperatures of the exhausts, and high contents of water vapor. Our analyses also include studies of a more innovative solution for methane oxidation based on a non-thermal plasma technology. Laboratory tests are positive and indicate a good potential, although tests at a larger scale are needed before installation on a ship is possible. Costs of methane catalysts on ships are expected to mainly depend on technical challenges at installation, and the needed catalytic metals. Since no regulations cover methane emissions from ships, installations are completely voluntary and require ship owners to accept the extra costs. In a case study, we study the costs associated with installation and operation of an oxidation catalyst on one of Furetank’s vessels. Yearly operating costs are estimated to be approximately 110 000 euro, and installation cost to be 450 000 euro. The uncertainties are however high since no real examples could be used for benchmark values.
A full-scale demonstration of methane oxidation catalysts on ships is needed and
should be technically possible. Before any demonstration, further guidance on operational practices should be developed and be accurately addressed for the specific
case. High costs may slow down the introduction of methane aftertreatment technology on ships on a commercially viable scale. It is therefore urgent to investigate potential
incentives and regulatory means to facilitate development and introduction of methane oxidation technology for the marine sector. Methane emissions can be expected to increase in the future, with an increasing number of ships driven by LNG, which makes these studies even more relevant.

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Författare: Hulda Winnes, Erik Fridell, Joanne Ellis, Björn Forsman, William Ramsay, Henrik Westermark
Utgivare: Lighthouse
Utgivningsdatum: 2020-09-30
Diarienummer: TRV 2019/27023
Antal sidor: 48
Språk: Engelska
Kontaktperson: Charlott Andersson, PLa1us