Technology: dual fuel LNG truck engines [Gas in Transition]
Long-distance road transport faces significant challenges when it comes to batteries. The key issue is the size and weight of the batteries required to provide sufficient run time. The large size leaves insufficient space and capacity for freight transport to be profitable, while the charging infrastructure remains limited.
In contrast, the low cryogenic temperature at which LNG is stored means that it has sufficient energy density to provide long runs – up to 1,600 km.
Compressed natural gas (CNG) appears to be the best option for gas-powered passenger cars, and is generally preferred for urban distribution trucks, garbage collection, and for city and suburban buses (where electrification is also a concern). option), but when it comes to international, LNG is a large-scale deployable alternative fuel today.
Europe saw a 60% increase in the number of LNG refueling stations in 2020, from 250 to 400. This reflects the growing number of vehicles running on natural gas, especially trucks running on CNG and LNG; 6,802 new registrations of gasoline trucks were registered last year, according to industry association NGVA Europe.
LNG engines – inherently cleaner
LNG engines are internal combustion engines (ICE) based on existing designs for diesel and gasoline engines.
ICE diesel engines produce particulate matter (PM), a major contributor to local air pollution. This is the result of diffusion combustion, where oxidizer and fuel are separated before being burned, which tends to lead to incomplete combustion and the production of soot.
ICE diesel engines also produce NOx because they are lean burn – that is, diesel is burned with an excessive amount of air and, since nitrogen is the main component of air , the production of NOx tends to be higher than for a rich combustion engine.
Both NOx and PM are treated after combustion in the exhaust system, but because this aftertreatment is imperfect, there is usually a trade-off between the level of NOx and PM emitted to the environment.
LNG, even injected in liquid form, vaporizes quickly and mixes with air to ensure much more complete combustion. PM emissions are negligible. This has a strong ripple effect because, since there is no trade-off between PM and NOx, combustion and aftertreatment processes can be optimized to reduce NOx emissions.
Carbon emissions are also lower for the simple reason that LNG contains less carbon than diesel. CH4 (methane / natural gas) has a carbon / hydrogen ratio of 1: 4, while diesel has a carbon / hydrogen ratio of around 1: 1.75, although this varies seasonally and in different markets and between Suppliers.
LNG is difficult to use on its own in a compression ignition engine because it has a low cetane number. The cetane value represents the ignition delay, or the time between the start of injection and the increase in pressure in the engine cylinder.
This is why a small amount of pilot fuel – diesel – is used to create the first phase of combustion in a compression-ignition LNG engine, which provides a rapid build-up in pressure, sufficient to allow the ignition of the second phase. of the LNG / air mixture.
While LNG has a low cetane number, it has a high octane number, which reduces pre-ignition or “knocking”. Pre-ignition usually results in incomplete combustion and therefore particulate emissions. As a result, the only PM emissions from dual fuel compression ignition LNG engines result from the low amount of pilot fuel used.
Compression-ignition diesel engines have gone through several stages of innovation and development to improve efficiency and reduce emissions, the two most important developments being turbocharging and the shift from single injection points to fuel points. multiple injection. Although the use of LNG presents new challenges, both innovations can be applied to dual-fuel LNG compression-ignition engines, promising increased efficiency and lower emissions in the future.
Reduce methane slip
According to LNG truck manufacturers, such as Sweden’s Scania, CO2 emissions reductions from LNG engines can reach up to 20%, compared to diesel engines, operating under optimal conditions. But studies, for example the Sustainable Gas Institute Can natural gas reduce transportation emissions? show that actual GHG reductions are lower, depending on engine type, mode of operation and methane slip level.
Methane slip is an area that has improved significantly since the first LNG engines arrived on the market and some older GHG emissions studies for road transport do not reflect the performance of modern engine designs. Methane slip, like particulate emissions from diesel, is the product of incomplete combustion, which results in methane leakage from the exhaust system.
Methane is a considerably more potent GHG than CO2, especially if its effect is measured over a 20-year period as opposed to a 100-year period.
Marine engine builder and developer WÃ¤rtsilÃ¤ notes that it has reduced methane slip from its dual fuel engines by 85% since 1993. He says there are a range of ways in which methane slip can be further reduced, primarily by focusing on engine designs that result in faster and more complete combustion. WÃ¤rtsilÃ¤ believes his next combustion concept will reduce methane slip by over 50% to around 1 gram per kWh. The first LNG engines had a methane slip of about 16 g / kWh.
However, the GHG reductions from LNG engines undergo a dramatic change when combined with liquefied biomethane to form bio-LNG, also known as liquid biomethane. This can be used in the same engines and with the same infrastructure without further modifications.
Using 100% bio-LNG can generate CO2 emissions reductions of up to 90% compared to diesel, according to Scania, but the share of bio-LNG in road transport fuel is more likely to increase gradually, in order to allow sufficient time for an expansion of the production of biomethane.
For dual-fuel engines there are also options to replace pilot diesel fuel, for example with hydrogenated vegetable oil or biodiesel (Fatty Acid Methyl Ester).
NGVA Europe claims that more than 25% of gas refueling stations in Europe already provide biomethane and that its use accounts for 17% of all gas used as transport fuel in the region. As the number of CNG and LNG vehicles increases, it can be difficult to maintain or increase this percentage even as the absolute volume of biomethane increases. Nonetheless, NGVA Europe predicts that by the end of the decade there will be around 10,000 CNG stations and 2,000 LNG stations in Europe, and that the proportion of biomethane will increase to 40% on average.