Carbon dioxide (CO2) is understandably seen as the primary cause of global warming due to the greenhouse effect: when fossil fuels are burned, they produce CO2 which absorbs heat energy from sunlight much more vigorously than the nitrogen and oxygen in the air, trapping that heat in the atmosphere. The subsequent rise in the atmosphere’s temperature is likely to lead to potentially disastrous climatic effects.
Some fuels produce far less CO2 than others, and some produce none at all: hydrogen burns to give off only water (H2O), and in general fossil fuels with a higher hydrogen content and a lower carbon content are seen as desirable. This is why there is such an emphasis on gases such as methane (CH4), propane (C3H8) and butane (C4H10).
Methane has a calorific value (CV or specific energy content) comparable to fuel oil, but its carbon content is significantly less, at 75% by weight rather than ~85%. This means that the life-cycle CO2 emissions of natural gas (which is typically 85-95% methane) are around 27% lower than those of fuel oil. This is why many heating and power operations have moved from coal or oil to LNG (liquefied natural gas) and LPG (a mixture of propane and butane).
Shipping operations are also moving from heavy fuel oil (HFO) towards gaseous fuels to reduce their particulate (PM) and sulphur dioxide (SO2) emissions — the latter in particular is heavily regulated in emissions control areas (ECAs) such as the Baltic and North Sea and the North American coasts.
MAN Energy Solutions, a large manufacturer of maritime and stationary engines, calls this change the ‘Maritime Energy Transition’. The firm says large marine engines need to change “from oil fuel to flexible dual-fuel capability, so that engines are able to run on a wide range of low-carbon or carbon-neutral fuels. The shift to LNG is a logical and important first step in preparing engines for the broader use of synthetic fuels.”
But CO2 is far from the only culprit in global warming; for instance, nitrous oxide (N2O) is a common byproduct of agriculture, industry and transport, and another greenhouse gas (GHG). Its global warming potential (GWP — see sidebar) is around 298 times that of the same weight of CO2 over 100 years.
And methane itself is another substantial driver of global warming: each kilogramme of CH4 is equivalent in GWP to around 30kg of CO2.
As MAN puts it: “to maximise the environmental benefits of LNG, the complete natural gas supply chain must be escape-proof on a ‘well-to-wake’ basis. This means from the moment natural gas emerges from the ground to the moment exhaust gases emerge from the gas-burning engine”.
Some natural gas is released to the atmosphere during its extraction, processing and distribution. A 2019 well-to-tank (WtT) analysis on behalf of SEA/LNG and the Society for Gas as a Marine Fuel concluded that these stages release the equivalent of 67g/kWh of CO2 — almost half of which occurred during the purification and liquefaction of the gas. By comparison, heavy fuel oil (HFO) itself emits the equivalent of about 49g/kWh of CO2 during the well-to-tank phase.
A SLIPPERY CONCEPT
‘Methane slip’ is used to describe any loss of methane as it is used to fuel an engine. The gas can be lost during the intake phase, as it ‘escapes’ from a poorly-sealed fuel system, intake manifold or crankcase ventilation, particularly in the case of a two-stroke engine. Or, as a result of incomplete combustion, unburnt methane and other hydrocarbons can be emitted from the exhaust.
In Otto-cycle engines, the fuel is mixed with air before ignition — whether that means spark ignition or using a ‘pilot’ injection of liquid fuel to initiate combustion in a dual-fuel engine. This allows methane to escape during cylinder scavenging, when the exhaust gases are replaced by a fresh charge of fuel and air.
Conversely, MAN’s large ME-GI two-stroke dual-fuel (DF) engines operate on a full diesel cycle, directly injecting the gaseous fuel into the combustion chamber once the air has been compressed and the liquid fuel ‘pilot’ has already ignited. No methane can escape during scavenging, and the efficient combustion process minimises unburnt fuel in the exhaust. These engines (which operate at only around 100rpm) have guaranteed methane slip levels of just 0.2-0.3g/kWh across the entire load range — with the lowest figure as the engine approaches 100% load. The pilot injection of liquid fuel can be as little as 0.5% of the total fuel used, and overall, the latest ME-GI engines have shown a 23% reduction in GHG emissions.
MAN also produces four-stroke spark-ignition gas and dual-fuel engines operating on the Otto cycle (where fuel is pre-mixed with air before ignition). Here, it is looking at several ways to reduce methane slip. Improvements to internal engine design include minimising valve overlap (when inlet and exhaust valves are both open) and removing ‘crevice volumes’ in the combustion chamber where gas can remain unburnt. For the methane that does get through, exhaust aftertreatment involves oxidation catalysts. MAN is also investigating direct gas injection (as used on the two-stroke engines), which it says will give the potential to reduce methane slip by over 90%.
MAN’s marine engines are designed to be retrofitted with new technology. In the roadmap for development is the ability to burn liquefied ethane gas (LEG), LPG, synthetic fuels and even ammonia (NH3) — the firm has announced that its first ammonia-fuelled engines will be available in 2024, and available as a retrofit just a year later.
BOX: GLOBAL WARMING POTENTIAL (GWP)
Global Warming Potential (GWP) allows comparisons of the global warming impact of different gases, and lets analysts add up emissions estimates of different gases. GWP indicates how much energy one tonne of a gas (and its subsequent emissions) will absorb over a given period of time, relative to one tonne of CO2. The time period usually used for GWPs is 100 years. CO2 remains in the atmosphere for thousands of years, whereas methane only usually stays in the atmosphere for around a decade; however, it absorbs much more sunlight energy than CO2, so it has a GWP of 28–36 over 100 years.
BOX: METHANE SLIP IN ROAD ENGINES
Methane slip can be a problem in road vehicle engines too. A number of dual-fuel (DERV + LNG) engines have been built (both diesel and spark-ignition) some using a low-pressure injection system which was prone to methane loss. Unburnt methane has also been known to contaminate exhaust catalysts, compromising the emissions rating of the engines. High-pressure fuel injection setups and (in the case of SI engines) sophisticated ignition systems can mitigate these problems, which are less common in dedicated LNG-only engines.