Of the many transport fuel choices available today, none plays a more strategic role than natural gas in having an immediate and major positive economic and environmental impact. But of the two most widely available – CNG and LNG – which is most appropriate?
There are various considerations that affect fuel selection. Vehicle range, type of cargo, type of vehicle, driving conditions, turning radius, length and/or weight limitations all affect fuel choice. In the case of natural gas, the availability of infrastructure or supply chain and/or the cost of establishing these is key to the choice between LNG and CNG.
LNG applications – transportation and shipping
The market for LNG use in transportation and shipping is growing.
- A large vessel carrying LNG may use LNG to propel it. Since evaporated cargo provides a source of clean fuel, most LNG tankers have a steam-turbine propulsion system, which can efficiently use the existing terminal infrastructure and the ‘boil over’ – the part of LNG that turns into gas due to heat absorption in tanks. The economic and environmental benefits will more than pay for the small loss of cargo.
- For container ships, there are challenges to overcome for smooth transcontinental routes, but this is a significant area of potential for LNG.
- In areas where LNG terminals are available, ferries have begun to use natural gas as a transport fuel. LNG is increasingly being used in response to new rules that require vessels in the Baltic Seal in Northern Europe and off the coast of North America to significantly reduce emissions of SO2 and other pollutants.
- Offshore drill vessels are another good candidate for LNG, as well as for natural gas that would normally be flared off and burnt from a well.
LNG applications – inland
LNG is well-positioned for growth in inland applications as well. It is deployed to run rail engines, mining trucks, gas compression stations, and on-shore well equipment. Typically, a hub-and-spoke relationship drives growth, facilitating the adoption of the fuel by on-road long-distance trucks with access to LNG at their terminal points. According to NGV Global, which represents the natural gas vehicle industry, the success of LNG for heavy-duty vehicle and off-road applications, along with the advent of LNG-fueled marine transportation, will serve to push natural gas into new territory as it delivers significant economic and environmental benefit.
Cryogenic pump key to LNG use
Central to the fluid system design for LNG is a submersible cryogenic pump installed in the storage tank in fueling stations, as well as in some storage systems on vehicles or vessels. The pump moves LNG into the vaporizer, which gasifies the liquid. Aerospace pumping technologies, which have very low losses, are currently being tested for this application to limit heat loss and enhance the ability to use the whole tank. Other systems use the pressure created in the tanks to move the gas out of storage. However, for larger engines with heavy demand or at fueling stations, a submerged cryogenic pump is preferable to move large volumes of gas.
In recent years, CNG has grown in acceptance and use around the world due to strong endorsements from China and India, where it has been widely adopted as a public transport fuel. Spurred on by an increased availability of technology from Cummins Westport, Daimler, Volvo, Ford, and GM, as well as key systems providers, such as Parker Hannifin, AC Delco and Bosch, CNG has gathered strong momentum.
- In the U.S., fixed-route trucks such as beverage delivery vans, waste trucks, and cement mixers have increasingly adopted CNG as a fuel of choice, driven by larger-sized engines and long distances and performance demands.
- As of 2013, there were over 120,000 natural gas reciprocating engines produced in North America, reflecting 10% year-over-year growth. In China, growth is running at 25% year-over-year.
- CNG use and acceptance has been further advanced by the development of lighter weight, fully composite tanks, allowing on-road trucking to adopt CNG in a much bigger way
Markets are driving innovation in natural gas engines, storage, and fuel systems, with the aim of achieving near-zero emissions. Further innovations in fuel systems are focused on robust systems with improved stable flow performance, accurate flow and pressure control, and near-zero fugitive emissions to further reduce the carbon footprint of natural gas. Many forward-thinking organizations engaged in exploration and delivery are funding such efforts. The long-term goal is to reduce NOx levels of transport systems/engines to the same level as power plants (0.02 g/bhp-hr). This will potentially make natural gas vehicles and transportation cleaner than electric options.
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Article contributed by Veriflo Division of Parker Hannifin