What is renewable natural gas and how can it be used to fuel vehicles?
Renewable natural gas (RNG) is pipeline-quality natural gas made by collecting and purifying biogas, the methane produced from decomposing organic matter. Biogas can be collected from sources such as landfills, livestock operations, wastewater treatment plants, food manufacturing and wholesalers, supermarkets, restaurants, and hospitals. Once purified to remove contaminants and increase its heat content, the gas is called RNG and is a “drop-in” fuel that can be transported with conventional natural gas in pipelines, dispensed at the same fueling stations, stored in the same storage tanks, and used in natural gas vehicles without any engine modifications.
Despite its advantages, there are only 60 operational RNG production facilities in the United States. Many more use the biogas to generate electricity. This is due to federal and state programs, such as the federal Investment Tax Credit and state renewable portfolio standards, which incentivize the use of biogas for power generation rather than for vehicle fuel.
The purification process for biogas is called conditioning or upgrading, and it involves removing water, carbon dioxide, hydrogen sulfide, and various contaminants and trace elements. From there, RNG can be compressed to make renewable compressed natural gas (R-CNG) or super-cooled to make renewable liquefied natural gas (R-LNG).
RNG is produced from feedstocks that come from a wide range of industrial sectors, many of which already collect and process biomass as part of their daily operations:
Landfills: Landfill gas (LFG) is collected from decomposing waste in landfills. According to the U.S. Environmental Protection Agency (EPA), landfills are the third largest source of human-related methane emissions in the United States. Landfills account for 70% of the operational RNG projects in the United States. One of the largest LFG-to-vehicle fuel projects is Waste Management’s Altamont Landfill near Livermore, California. This project produces up to 13,000 gallons of R-LNG each day to fuel 300 refuse trucks.
Livestock Operations: Animal manure can be collected and taken to an anaerobic digester for RNG production. A few farms across the country have started to use biogas to produce RNG vehicle fuel, including Hilarides Dairy in California and Fair Oaks Dairy in Indiana.
Wastewater Treatment Plants: Approximately 9% of the more than 16,000 wastewater treatment plants in the United States use anaerobic digestion to produce biogas. The Janesville Wastewater Treatment Plant in Wisconsin is an example of a plant that uses biogas to produce RNG for use in vehicles.
Other Biomass Sources: RNG can also be produced from lignocellulosic material, such as crop residues and dedicated energy crops, through thermochemical conversion, co-digestion, and dry fermentation. These technologies are being used in Europe, but have limited applications in the United States. RNG also can be produced from food waste, either alone or in conjunction with biosolids from livestock operations or wastewater treatment plants. CleanWorld Partners’ Sacramento BioDigester and quasar’s Central Ohio BioEnergy project convert food waste to RNG for vehicle fueling.
RNG qualifies as a cellulosic biofuel under the EPA’s Renewable Fuel Standard (RFS2) program. In fact, RNG accounted for more than 50 million renewable identification numbers (RINs) in 2014 – 98% of all cellulosic biofuel RINs. According to organizations that track biofuels market data, cellulosic biofuel RINs were valued at $0.70– 0.85 per diesel gallon equivalent in 2014;this value is expected to increase in the future.
Like conventional natural gas, RNG can be produced domestically and can displace the petroleum currently being imported for transportation use. However, RNG offers some additional benefits. RNG has practically a net zero carbon impact. On a lifecycle basis, RNG accounts for fewer greenhouse gas (GHG) emissions than most currently available motor fuels. RNG can reduce GHG emissions by 95% compared to conventional gasoline and diesel fuel. This is partially because capturing biogas from landfills and livestock operations can reduce GHG emissions by preventing methane releases that were occurring into the atmosphere. Additionally, RNG produced through anaerobic digestion eliminates odors and results in nutrient-rich liquid fertilizer as a by-product. Also, biogas feedstocks are plentiful, so RNG could make use of the 450 million pounds of municipal solid waste dumped in landfills, 160 billion pounds of food waste generated, or the 500 million tons of animal waste produced each year.
Like conventional natural gas, the main barriers to RNG are lack of vehicle availability and fueling infrastructure, though efforts are underway to address both of these obstacles. However, RNG production costs exceed those for conventional natural gas, especially for small-scale operations. Small-scale RNG production can cost around $5.50–$9per million British thermal units compared to $4.50 for conventional natural gas. Additional financing and incentive opportunities, as well as state renewable portfolio standards that encourage the investment in biogas for vehicle fuel production, may spur additional production.
For more information on RNG, please see the following additional resources:
- Alternative Fuels Data Center’s RNG Production page
- Clean Cities’ presentation: RNG and RINs
- American Biogas Council
- Clean Cities Technical Response Service Team