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A New Debate Emerges: LNG or CNG for the Long Haul

Amidst the constant discussion of plentiful domestic natural gas and its use as a transportation fuel, an unusual technological and philosophical debate has emerged. Those familiar with the industry know that until recently, fleet managers considering the conversion from gasoline or diesel to natural gas had basically two options: compressed natural gas (CNG) was the choice for any return-to-base, short mileage vehicles, and liquefied natural gas (LNG) was the option for long-haul on-highway Class 8 trucks, also known as tractor trailers or semis. The reasoning behind this was relatively straightforward, and more or less a product of a few issues inherent to gaseous rather than liquid fuel (energy density, tank storage capacity, re-fueling time). However, due to a variety of innovations, a paradigm shift may be under way.

In comparing alternative fuels to gasoline or diesel, a major consideration is the relative energy density and associated cost, weight and size of on-board fuel storage. For natural gas, when compressed, its energy density is only about a quarter that of diesel, and when liquefied just 60% of the energy density of diesel. Therefore, either option requires greater fuel storage capacity to achieve a comparable range, which means more and/or larger tanks.

Compared to CNG, LNG contains 2.4 times more energy per diesel gallon equivalent (DGE). Moreover, since LNG, like diesel and gasoline, is a liquid, one could achieve comparable refueling speed, whereas the level of compression required to “fast-fill” with CNG is very high (~3,600 psi). As a result, for the long-haul trucking sector, the energy density and associated cost(s), weight and on-board storage capacity of LNG have long been viewed as the more attractive, viable option.

Relatively recent advances in tank storage capacity and “fast fill” refueling technology have allowed room for debate as to whether LNG really is the only natural gas option for the long-haul trucking industry. To best highlight the philosophical nature of this emerging debate, it may be best to look at two of the leading natural gas refueling infrastructure providers, Clean Energy Fuels (CLNE) and Trillium CNG (TEG subsidiary), each of which has taken an opposing view on this topic.

Clean Energy was the first mover in the industry and is now by far the largest provider of natural gas refueling infrastructure in the US. They are betting big on the fact that CNG is the choice for local urban fleets (refuse vehicles, delivery trucks, etc.) but that LNG is the option for long-haul tractor-trailers. Alternatively, Trillium CNG, along with their partners at AMP Americas, a Chicago-based investment firm, strongly believe that CNG should be the choice for all heavy-duty fleets, regardless of distance traveled or route. Without commenting on which approach is better, the following will help to explain each company’s thought process.

CNG and LNG are both proven forms of natural gas storage, with distinct advantages over diesel and gasoline when used as a transportation fuel. To produce CNG, natural gas is taken directly out of the United States’ expansive network of natural gas pipelines, whereas LNG must be cryogenically liquefied to -260 degrees F (to become a liquid) and often must travel via ground transportation (tanker truck) to stations across the US. With pipeline access, LNG can alternately be produced from the gas grid through MMLS (movable modular liquefaction system) units. On-site, CNG is compressed immediately and enters a truck in a process that is almost identical to traditional fueling practices, from the driver’s perspective. On the other hand, LNG requires drivers to wear a mask and gloves to protect themselves against cryogenic burns.

For the Class 8 truck sector, Trillium/AMP have made the decision to build CNG stations because, in their words, “it is a cheap, simple and safe way to transport and store natural gas.”  They have also found that the additional simplicity of CNG over LNG makes it an easier product to maintain, as well as a less expensive product to produce.  For example, according to their general pricing model and marketing materials, on average, “end users of CNG gain a $.48 advantage over LNG, for a product that works equally well, has less associated hazards and a greater built-in infrastructure across the US.”

Alternatively, Clean Energy has invested heavily in LNG infrastructure, including two liquefaction facilities, to supply their network of 150 existing refueling stations and more in the works that they refer to as America’s Natural Gas Highway. While the production and transport of LNG require greater technical expertise and significantly more capital than for CNG, LNG cost savings are realized on refueling infrastructure/operation. This is primarily a result of the high electricity demand/cost required to achieve the compression necessary for a “fast-fill” CNG station. There is also greater flexibility in where a station can be located (no need for natural gas pipeline access) and in future expansion of existing stations. For more on Clean Energy’s take on this debate, you can watch their CNG vs. LNG educational video here.
In either case, the technology is fully commercial, if not mature, so the debate is rooted primarily in economics. Regardless, the pace of innovation across the natural gas industry has been astounding over the past decade, from extraction processes, to refueling infrastructure, fuel storage and engine technology. So although the rapid rate of natural gas vehicle adoption may intensify this fuel debate, both options are viable and economically/environmentally superior to diesel for the Class 8 sector and time will tell which option is truly best suited for the long haul. While the question at the pump may well be, “liquid or gas,” the one thing that is clear is that NGVs of all sizes are here to stay!  

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John Miller's picture
John Miller on Jul 17, 2013 12:01 am GMT

CNG and LNG both have their pros and cons as far as operational flexibility and infrastructure costs.  When reviewing the different physical properties of each form of natural gas motor fuels the economics appear to favor LNG for longer range HDV transport and possibly CNG for shorter range vehicles with more centralized fueling infrastructures.

The advantages in both cases are directionally lower fuel costs and cleaner tailpipe emissions, including lower carbon.  This is a very viable solution to possibly improving MDV/HDV transport performance in many areas.  The question that needs to also be addressed is how efficient long-haul or cross country NG HD 18-wheelers or semi’s compare to the alternative of Rail freight transport?  Refer to my recent post on this subject.  

By the way, your first website link “Clean Energy Fuels (CLNE)” has an error:  You need to remove the ending back-slash in your post link.

Gal Sitty's picture
Gal Sitty on Jul 17, 2013 5:31 pm GMT

Why not also consider methanol? It can be made just about easily and cheaply from natural gas, and as an alcohol fuel, would require significantly less investment to use in our current vehicle fleet and infrastructure. It also has significant environmental benefits over gasoline and diesel.

Phil Hughes's picture
Phil Hughes on Jul 20, 2013 12:10 am GMT

On a per energy basis, CNG and especially LNG are far too dangerous to give over to. LNG has exactly the same low-clinging aspect as Butane and Propane except that it is far worse in that whereas LP gas always clings to the ground, in case of a rupture LNG will have a very large expanse of optimally explosive mixture. That means that explosions occuring with semi-tank ruptures, at neighborhood service stations, and in home fueling systems will have absolutely devastating consequences. Also, Chesapeake and others are tailoring CNG as a solution for low income individuals who typically  cannot afford OEM CNG vehicles but instead would be looking at Conversion.


This is the last thing we need in terms of raising the probability that any one car out of potentially tens of millions will explode in traffic. History is replete with situations which have been allowed to take hold with dangerous substances which had a similar seemingly low probability of death per unit but when coupled with human greed and then human error have ended disastrously. With this mode on the scale that is desired there will be at least one explosion per city per month. Can you imagine the terror of say a mother driving her child down the road wondering if a CNG car is going to explode? Can you imagine being a firefighter going to the scene of a semi-truck accident or a house fire wondering whether a rupture in the storage system, a car’s relief valve may have started to vent, or a malfunction in a residential refueling system has occurred? After just a few accidents nationwide which have the potential to level entire neighborhoods and force rebuilding of houses no emergency personnel in their right minds will go close, leave alone put out a fire. It would be like the first-responders during 9/11.

There have been numerous accidents oversees, again per amount of energy used, of systems and vehicles blowing up. We have essentially used SE Asian nations as guinea pigs on a problem that has so many modes of failure that there is no solution. Just one example is that they have found that CNG tanks can rupture when they are heated in random patterns and rates. Does any one of the proponents which to risk their life due to a failure mode which has not been predicted and when multipled by massively increased usage? I would think not.

The solution is Gas to Liquids conversion so it is not like CNG and LNG are required at all. Also, we must have quality train systems in the US because a large segment of the population here cannot afford cars at all, and again certainly not OEM equpped CNG models.

I believe that all who would proceed without a study of lage scale use considering also human greed and errors are guilty of criminal negligence and if they persist should be prosecuted as such.

Vailhem Vailhem's picture
Vailhem Vailhem on Aug 7, 2013 3:10 am GMT

Why no mention of ANG? It’s by far the most efficent route to go as far as transportation is concerned. Amazes me that ANG isn’t deeper into the natural gas, or just gas storage industry’s mindset in general

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