Environmental benefits of CNG

By George Kalet

Compressed natural gas (CNG) is often called the cleanest burning natural source of fossil fuel propulsion; much more environmentally friendly than conventional gasoline and diesel fuels. The combustion of natural gas produces negligible amounts of sulfur, mercury and particulates. This mostly has to do with its chemical composition.
The process of cleaner burning fuels involves the elimination of carbon molecules that are the root cause of contamination to the environment. The evolution of combustible fuels has moved from coal, which is essentially 100-percent carbon, to liquid petroleum that combines carbon and hydrogen, and then methane, which is comprised of one carbon molecule and four hydrogen molecules, and considered a greenhouse because of its lone carbon molecule. Burning natural gas does produce nitrogen oxides (NOx), which are precursors to smog, but at lower levels than gasoline and diesel used for motor vehicles.
The one downside: methane burns at approximately twice the combustion temperatures of liquid petroleum fuels; close to 1,100 F compared to 500-600 F. In terms of efficiency, the degree of power generated from the same amount of fuel is an energy to power ratio. Look at the two side by side, for the same amount of energy produced, and you’ll find that methane burns off more oxides of nitrogen at these temperatures, but not for as long a time.
Natural gas has often, and perhaps unjustly, received a bad reputation, but only because of its being burned in an engine designed for gasoline or diesel combustion. CNG burns at a lower compression ratio, and responds much more efficiently in engines engineered specifically to burn CNG. Compared with conventional diesel and gasoline powered vehicles, natural gas produces lower emission levels; and because CNG fuel systems are completely sealed, they do not produce any evaporative emissions. Only about one-tenth of 1 percent of processed and used CNG goes toward automotive transportation fuel.
In the event of a leak or emergency venting, because it is lighter than air, CNG dissipates much more rapidly into the atmosphere. Methane burns within a very narrow fuel-to-air ratio, between 5 and 15 percent in air, meaning it takes in less air to realize the same degree of combustion.
Released and mixed into the air, CNG becomes flammable only when the mixture is within 5 to 15 percent natural gas. At less than 5 percent natural gas, the mixture doesn’t burn; and at more than 15 percent natural gas, there is not enough oxygen to allow the mixture to burn. Methane is a cleaner, environmentally friendlier fuel than propane, which is a heavier-than-air gas that runs to the lowest point and typically pools on the ground.
In terms of efforts to meet U.S. Environmental Protection Agency (EPA) emissions standards, environmental protective appliances fitted on CNG-powered vehicles are less expensive than the emission systems and filters on fossil fuel engines. As the EPA continues to drill down on the emissions performances of various fuels, the requirements for efficient emissions control on gasoline and diesel engines have driven up the costs — but are not required for today’s natural gas engines, which already surpass the 2020 EPA emissions standards.
See the August /September issue of BUSRide for the next chapter in this series, detailing CNG conversions – installation, maintenance and training best practices!

George Kalet is CNG applications and key account manager for Atlas Copco, Gas and Process Group. Atlas Copco develops innovative sustainable solutions that create value for its customers in more than 180 countries. The company’s expertise is in compressors, vacuum solutions and air treatment systems, construction and mining equipment, power tools and assembly systems. Visit www.atlascopcogroup.com for more information.