No matter where you go, people are discussing natural gas. While the publicity often far exceeds the current infrastructure, it’s beneficial to understand the basics, as this fuel source is poised to play a more significant role in the future.
Natural gas is an odorless, nontoxic, gaseous mixture of hydrocarbons — predominantly methane — and it possesses the lowest carbon content of any hydrocarbon fuel. Although it does produce greenhouse gases, it is much cleaner than petroleum-based fuels. It is lighter than air and disperses quickly into the atmosphere when spilled. Plus, it has a narrow flammability range, which makes it a relatively safe fuel.
Consequently, natural gas is quickly gaining favor as an affordable domestic alternative to gasoline or diesel. Any vehicle traditionally powered by gasoline or diesel can be converted to natural gas.
Capacity a Challenge
When people refer to compressed natural gas (CNG) or liquefied natural gas (LNG), they are really describing the state in which the natural gas is stored. A natural gas engine is not specific to a particular gas type. It doesn’t care whether fuel is being supplied by a CNG or LNG tank.
There are benefits and drawbacks to both storage solutions, particularly when discussing mobile equipment. CNG is stored as a gas at 2,900 to 3,600 psi, while LNG is stored as a liquid at a very low cryogenic temperature. With CNG, the compressed gas travels through a series of highly pressurized stainless steel lines until it reaches the regulator, where the pressure is reduced to around 115 psi to match the amount needed by the fuel injection system.
According to Paul Blomerus, senior director, High Horsepower Corporate Development, Westport Innovations Inc., the amount of natural gas an engine receives is regulated to the temperature and pressure required by the engine, typically 110 to 150 psi. The customer will pick the most appropriate tank solution.
Whether you use CNG or LNG, the natural gas storage tanks and more complex fuel delivery systems will add cost to the initial purchase price vs. a traditional petroleum-fueled vehicle. And fuel storage will require a larger footprint on a mobile vehicle than with an equivalent diesel fuel tank solution.
The storage volume required per unit of energy, is larger for natural gas compared to diesel. “LNG requires a volume of about 1.7 times that of diesel to store an equivalent amount of energy,” says Scott Fiveland, GAS new technology manager, Caterpillar. “CNG systems, depending upon the degree of pressurization (usually 3,600 psi), require upwards of four times relative to diesel and 2.5 times relative to LNG. In key industries, LNG fuel systems will be designed to continue to meet key customer requirements while not sacrificing payload, etc.”
CNG has been more readily used in the past. “Up until now, it has been more CNG than LNG, but that is changing very rapidly,” says Blomerus. Many recent announcements tout additional capacity being added for LNG. “CNG is very popular on return to base vehicles.” This is because trickle compression filling is a very efficient way to fill a CNG tank. You plug in the compressor and fill the tank overnight when electricity is cheaper. “On an over-the-road haul truck, the LNG is going to be a better choice for fuel — better range, faster filling.”
Caterpillar works with customers to match fuel system storage requirements with customer needs, and executes the appropriate packaging on an application basis. “CNG does have some energy density storage challenges. But in some cases where CNG is directly available at a cheap cost, it can be utilized,” says Fiveland. “In this case, the appropriate storage technology and cycle times, derived via customer needs, will be designed into the products to maximize the customer value.”