How renewable natural gas decarbonizes natural gas engines
What is renewable natural gas?
Renewable natural gas, or RNG, is sometimes known as biomethane or upgraded biogas. Anaerobic digestion, a process in which bacteria break down organic matter, produces biogas. Biogas can generate heat and electricity with only a minor clean up. Additional refining removes contaminants such as CO2 and nitrogen. At that point, biogas becomes renewable natural gas—nearly pure methane. For many applications, RNG is functionally identical from standard natural gas. Most natural gas distribution networks allow for blending of renewable biogas, and natural gas engines use it.
Here are some of the main sources of organic matter used to feed biogas-producing bacteria:
Organic matter dumped at landfills tends to ferment spontaneously. The resulting biogas emissions account for nearly a fifth of the human-caused methane emissions in the United States according to the Environmental Protection Agency. Methane actually produces 25 times more greenhouse gas emissions compared to CO2. Not only is it a great source of fuel, but landfill methane capture also prevents the emissions of powerful greenhouse gases.
Cattle farms and chicken farms tend to produce large amounts a manure—a great snack for the bacteria responsible for anaerobic digestion. Industrial biogas production comes from manure using, for example, large airtight fermentation tanks known as digesters.
Wastewater treatment plants produce a lot of sludge. It’s basically what remains from wastewater after most of the clean water has been removed. Sewage sludge is normally trucked to a landfill, or sometimes used as fertilizer; but, due to its high content in organic matter, it can also serve as a feedstock to produce biogas. Lots of wastewater plants do this and use the biogas themselves, for example to heat fermentation ponds.
What are the benefits of renewable natural gas on natural gas engines?
When used in an engine, natural gas has similar performance characteristics compared to diesel, but is quieter and much cleaner. Its simplified aftertreatment systems result in near zero NOx levels. Natural gas, however, remains a fossil fuel and its use always results in CO2 emissions. This is where the additional benefits of RNG shine through.
The carbon content of RNG, in contrast, is non-fossil. Burning RNG is thus carbon-neutral since it doesn’t add carbon to the atmosphere. When accounting for the total well to wheels carbon emissions, the use of RNG remains extremely low-carbon. In some cases, such as the use of landfill gas, it may even be carbon-negative, as previously mentioned.
Whether used to produce heat and electricity or to power your fleet, RNG helps reduce net carbon emissions. RNG is classified as an Advanced Biofuel under the renewable fuel standard in the United States, contributing to the role natural gas engines play in our renewable future.
There are also other benefits to the production of biogas. After the bacteria are done and the water is removed, the solids left in digesters can be used as fertilizer, mulch, or animal bedding. Researchers are even assessing the use of these solids to produce ethanol—a way to squeeze even more energy out of the original feedstock.
Rural areas can now diversify their economies beyond agriculture alone by producing biogas and digestate. Many farmers have invested in digesters and are thus able to produce and sell biogas and renewable natural gas. In rural areas not reached by natural gas distribution networks, this can make RNG available for transportation and other purposes. At Fair Oaks Farm, a large-scale dairy operation in Indiana, RNG is produced on-site. The RNG is then used to fuel the trucks used by the farm to deliver the milk it produces to its customers. The milk trucks feature Cummins’ 9-liter ISL G natural gas engines. RNG is also used in other applications such as vocational trucks, transit and school buses, and medium-duty trucks.
How does RNG compare to other fuels?
Natural gas vehicles are cleaner, quieter and require less maintenance than diesel vehicles among other benefits. RNG vehicles are low-carbon and even sometimes carbon-negative. In commercial applications, RNG may be the most widely used alternative fuel. NGVAmerica, a trade association promoting the use of natural gas in vehicles, reports that RNG accounted to 64% of on-highway natural gas use in 2021. It’s safe, effective and relatively affordable, but fleet managers who want to switch to natural gas engines need to make additional considerations.
One consideration with RNG is whether we have enough RNG to fulfill the needs of commercial mobility. While RNG can’t fulfil all the energy needs of humankind, it has the capacity to play a play a role in decarbonizing select commercial mobility applications. The last few years have seen growth in RNG production, a trend expected to continue at one of the quickest rates of fuel growth in the segment. According to the International Energy Agency, RNG made up 1% of biofuel production in 2020 and is expected to increase to 20% by 2050.
Read more about how RNG compares other alternative fuels.
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