Methane Slip | Definition, Causes and How to Reduce
Introduction
The maritime industry has been moving towards greener shipping to reduce its carbon footprint for some time now. Along the way, we have seen many initiatives that can help us reduce our emissions by using cleaner fuel more efficiently.
Some of these include shifting to slow steaming, reducing redundant loads, efficient ballast water management, use of renewable energy and shifting to LNG as a fuel.
Methane slip is an issue affecting the use of LNG as a marine fuel. In this article, we take a deeper look at what it is, how it occurs and what we can do to reduce it.
But let’s first understand why LNG engines are so irresistible considering our climate objectives.
Shift to LNG engines
The shift to LNG-powered ships has the potential to drastically reduce CO2 emissions. It can be the bridging fuel of choice to work on cutting out carbon dioxide, SOx, NOx and other problematic pollutants.
Currently, LNG or natural gas-powered engines are used in dual-fuel engines powering LNG carriers. These carriers are capable of working on traditional fuels along with LNG.
The LNG powering the engines is sourced from the boiloff of LNG cargo in the ship’s tanks.
Using natural gas reduces carbon dioxide emission by 40% when compared to coal combustion and 25% compared to conventional marine fuels while providing the same propulsive power.
LNG-fuelled ships also do not emit soot, particulates or dust. Their sulphur dioxide, mercury and other harmful agent emissions are also insignificant making these LNG fuelled ships the cleanest and greenest of all.
But the problem of methane slip has the power to undo all of our progress. Let us understand how.
What is methane slip?
Methane slip is the escape of unused methane from an engine’s combustion chamber and other parts of the ship where it is stored and transferred. The fuel is not used by the engine. As a result, methane escapes in its pure form to the environment.
Now, why is that a problem?
Methane happens to be a greenhouse gas. Greenhouse gases restrict the escape of heat from the earth’s environment. Over time, the accumulated heat increases the average temperature of the planet and results in consequential effects on the planet’s ecosystem. These effects are irreversible.
While CO2 is among the most discussed greenhouse gases, methane is 80 times worse than CO2, at least in the short term.
Methane also happens to be an ozone destroyer. In the severe sunlight of the upper atmosphere, methane reacts with water vapour to form chemicals that destroy the ozone layer. Ozone layer depletion is directly responsible for the development of non-melanoma skin cancer in humans due to more UVB rays from the sun reaching the earth’s surface.
These compelling reasons make it very important to reduce methane emissions. And methane forms 85 to 95 per cent of liquefied natural gas (LNG).
But the use of methane in current engines will inevitably lead to increased methane emissions due to methane slip.
The positive effects of using a cleaner fuel like LNG will be outperformed by the negative effects of methane slip were these engines to be put into wider use.
Methane slip can undermine the advantages of using LNG-powered ships. Without effective methods to eliminate or at least reduce it, LNG engines run the risk of never being adopted widely.
Causes of methane slip
Methane slip from engines is a concern among engine manufacturers and marine organizations alike. In order to combat it, we need to first find what causes it.
Engine manufacturers and other experts list the causes of methane slip as follows.
- Supply chain leakage
- Combustion chamber leakage
Supply chain leakage
LNG is vulnerable to higher leakage than fossil fuels in the supply chain. It has higher well-to-wake emissions when compared to fossil fuels. LNG escapes during all the stages such as storage, transportation and usage.
Combustion chamber leakage
Leakage from the combustion chamber is estimated to be much more than through the supply chain. Factors such as engine type, load, fuel, duty cycle, and speed affect the rate of leakage.
Among these, three important factors that are common to all engine types are the amount of dead volume, piston ring leakage, and incomplete combustion.
The above factors can lead to increased methane slip either through the engine exhaust system or the crankcase ventilation.
We risk leaking small amounts of unburned methane with every rotation of the engine. The cumulative effect of such incessant leakage can throw a wrench into our climate goals.
How can we reduce methane slip?
Eliminating or at least minimizing methane emissions to acceptable levels is an essential step before we can start endorsing LNG engines on all types of vessels. Several methods have been discussed to reduce methane slip. Three of the most promising solutions are as follows:
- Reduction of crevice volume
- Direct gas injection
- Aftertreatment solutions like oxidation catalysts
Reduction of crevice volume
The crevice volume refers to the sum of all the small volumes in the combustion chamber where the flame is not able to propagate and unburned methane remains.
The reduction of crevice volume through design changes can minimize methane slip by up to 70 per cent.
Direct gas injection
LNG engines use a mixture of air and LNG supplied through the air inlet valve. The necessary ignition for the LNG fuel is supplied by a small amount of diesel fuel used as the pilot fuel.
Engine manufacturers are now exploring the use of direct gas injection that allows the introduction of LNG directly into the combustion chamber instead of mixing it with the air first.
The LNG enters the combustion chamber directly late into the compression stroke through its separate injector. One more injector for diesel supplies a small amount of diesel fuel for ignition.
High-pressure (1500+ bar) LNG injection provides a better flow rate per unit area. It also allows us to shape a favourable heat release rate curve further improving fuel efficiency.
Up to 90 per cent reduction in methane slip is possible by improving the direct gas injection technology.
After-treatment solutions like oxidation catalysts
Oxidation catalysts are also in use to treat the methane that slips out of the engine and prevent it from escaping into the atmosphere.
They are in use in many industries that release hydrocarbons. The use of such catalysts can reduce methane slip by about 70% if employed correctly.
Currently, engine manufacturers are looking into after-treatment solutions as a more viable means of methane slip control compared to direct gas injection.
Conclusion
Liquefied natural gas is an abundant resource. It is more affordable and practical than alternatives such as electric and fuel cell propulsion.
The advantages such as high thermal efficiency, lower emissions and mature regulation framework provide further incentives for its wider use.
But methane slip can be a deal breaker. Even small amounts are of environmental concern due to methane’s potency as a greenhouse gas.
Methane slip thus remains a very important issue for the maritime industry. But looking at the present research, we can confidently infer that methane slip will be minimized in a few years’ time.
