Methane slip is the escape of methane gas from production, processing, transport, operation or combustion. In terms of internal combustion (IC) engines, “methane slip” refers to the unburned methane present in IC engine exhaust emissions. The amount of methane contained in the IC engine exhaust varies greatly between engine combustion types (Otto or Diesel), specific engine designs and engine loads.
Methane is of primary concern due to its increased
Global Warming Potential (GWP) over other greenhouse gases (GHGs). There are
various studies on the life-cycle GHG emissions, the results of which are
typically shown on a 100-year or 20-year GWP basis.
It is known that methane emissions in the atmosphere can trap solar radiation more than carbon dioxide (CO2). Methane emissions are estimated to be 84 times more severe than CO2 on a 20-year basis and 28 times more severe than CO2 over the 100-year basis by the IPCC AR5 report
According to the report, there are three primary
causes of methane slip:
- Scavenging leakage
- Incomplete
combustion
- Trapped methane in the combustion chamber
crevices.
Scavenging leakage takes place when the methane and
air mixture passes directly to the exhaust, for example when gas injection to
the cylinder occurs prior to closing the exhaust valve. Incomplete combustion
occurs in all IC engine types but is primarily an issue for lean burn Otto
process gas engines.
As ABS says, incomplete combustion can occur for many
reasons (including trapped methane, detailed below) but it is typically due to
flame quenching close to the cylinder walls and extinguishing of the combustion
flame at low pressure and temperature.
This is effectively fuel quenching at the coldest part
of the combustion chamber while the engine is running. This results in
increased methane emissions during transient operation and operation at low
engine loads.
To keep combustion stable and reduce methane slip,
lean burn Otto engines need to accurately control combustion between knock and
misfire conditions.
Dead volumes, or crevices, within an IC engine
cylinder and combustion chamber are also a source for incomplete combustion and
an opportunity for methane to leak directly to the exhaust. The amount of
methane slip emitted is highly dependent on the installed engine technology.
For example,
high-pressure gas injection engines using the diesel combustion process in gas
mode can reduce levels of methane slip to the engine exhaust more so than
low-pressure engines applying the Otto combustion process in gas mode. A
two-stroke engine, when compared to a four-stroke engine, is also typically
more effective at reducing methane slip due to the reduced quantities of
geometric gas traps.
Methane slip can be reduced by running engines at
higher power output. While this is not possible in all ship propulsion and
power generation arrangements, it can be used in power generation load sharing
to optimize power plant operation to reduce methane emissions.
The IMO’s Intersessional Working Group on Reduction of
GHG Emissions from Ships continues to consider approaches to control methane
slip, which is part of the 37 Candidate Measure Proposals submitted to IMO for
adoption. Options to address methane slip include direct methane emission
controls or indirect means through fuel carbon factors. The engine
manufacturers’ latest specifications and latest updates on the dual-fuel (DF)
engine concepts regarding possible primary reductions of methane slip, should
be referenced, the report concludes.
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