Renewable Natural Gas

What is Natural Gas?

Natural Gas is a fossil energy that contain many different compounds, the majority of which is methane, a hydrocarbon containing one atom of carbon and four atoms of hydrogen (CH4). Other compounds found in natural gas are hydrocarbon gas liquids, water vapor, and carbon dioxide. Natural gas can be found in large cracks and spaces between layers of overlying rocks. This type of natural gas is called conventional natural gas. Other source of natural gas are shale, sandstone, and other types of sedimentary rocks. Natural gas extracted from these sources is called unconventional natural gas while those extracted from crude oil deposits are called associated natural gas.¹

Natural Gas as a transportation fuel

Because of its lower greenhouse gas emissions and its inherently lower price than gasoline and diesel, natural gas has been considered an alternative fuel.² Vehicles can be upfitted to run on both conventional fuel and natural gas without undergoing costly conversion. Natural gas is used in transportation in the form of compressed natural gas (CNG) or liquefied natural gas (LNG). Recent developments in fuel storage technology have introduced the use of adsorbed natural gas (ANG) in transportation in a commercial scale.^3,4,5 ANG technology operates at considerably lower pressure than that of CNG and allows refilling at 900 psi instead of 3600 psi.^6,7

A well-to-wheel (WTW) analysis of natural gas conducted by the Energy Systems Division of Argonne National Laboratory showed that it yields 14% less CO2 equivalent (CO2e) per Megajoule than gasoline. In the same study, the waste-to-well analysis of renewable natural gas sourced from anaerobic digestion of animal waste showed 82 to 91% reduction GHG emission reduction versus gasoline and 86 to 94% reduction versus diesel.⁸ 

What is Renewable Natural Gas (RNG)?

Renewable Natural Gas or RNG is an upgraded or refined biogas produced from a variety of renewable sources including solid waste landfills, water treatment facilities, livestock farms, waste from food and beverage production, and organic waste management operations.^9,10 Compared to conventional natural gas (derived from fossil), RNG has zero to very low amounts of other hydrocarbons such as ethane, propane, butane, and pentane.¹¹

RNG reduces methane emissions significantly in that it captures methane that would otherwise be released to the atmosphere from landfills and other industrial and agricultural wastes. Methane has a significantly higher global warming impact than CO2, about 25 times more potent.¹² Based on waste-to-well studies conducted by Argonne National Laboratory, RNG sourced from landfills produces 80 to 82% lower CO2e emissions per MJ versus gasoline, and 77% versus diesel.¹³ The following table summarizes the WTW GHG emission of RNG from different sources based on studies conducted by Argonne National Laboratory.

Table source: CSB author based on references cited

Learn more >> Renewable Natural Gas Webinar

This virtual event was co-hosted by the Carnegie Mellon University Wilton E. Scott Institute for Energy Innovation. Industry leaders and academia experts came together for a moderated energy panel on renewable natural gas. Panelists included Marc Fetten, CEO of GreenGasUSA, Sharon Frank, VP of Environmental, Health & Safety, Montauk Renewables, Inc., Sam Lehr, Policy Manager at The Coalition for Renewable Natural Gas, Götz Veser, Professor at the University of Pittsburgh Swanson School of Engineering, and Larry Frederick, PTAC SC. Guest Speakers and Moderators included Katelyn Haas-Conrad, Assistant Director at the CMU Scott Institute for Energy Innovation, and Chris Gassman, Associate Director at the University of Pittsburgh Center for Sustainable Business. Watch the full recording here.

With fugitive methane emissions, is RNG climate-positive or climate-negative?

While natural gas is considered a viable accelerator of the transition to clean energy, reservations on scaling up the use of this fuel arise because of fugitive emissions coming from leakages in its production, storage, and delivery, and its end use in transportation. Murphy and Holstein (2021) of the Environmental Defense Fund cited research conducted by Weller et al. (2020) where they estimated around 630,000 leaks in the natural gas distribution system in the U.S. resulting to 690,000 metric tons of methane emissions per year.¹⁷ This figure is five times higher than the estimates published in US EPA greenhouse gas inventory.¹⁸

Grubert (2020) evaluated the GHG intensity of RNG as the amount of methane leakage, on the assumption that combustion GHG emission of RNG is climate neutral. She classified the GHG intensity of RNG for three production pathways: (1) RNG produced from waste methane that would have otherwise been released to the atmosphere [as methane]; (2) RNG produced from waste methane that would have otherwise been flared off [i.e. burned in a flaring facility and released as carbon dioxide]; and (3) intentionally produced methane. In the assumption that burning RNG is carbon neutral, i.e., the carbon that was previously captured by the RNG source(s) is returned to the atmosphere as carbon dioxide, Pathway 1 is considered highly GHG negative [or climate positive]. Pathway 2 will only be GHG negative if the total leakage of the whole RNG system, that is to say leakage from production, storage, and transportation of RNG, is lower than the leakage from the flare. For comparison, leakage from flare is estimated at 1% while leakage in the downstream of production alone is already estimated at 0.8%. Thus, Grubert, E. assessed Pathway 2 as highly unlikely to be GHG negative [or climate-positive]. For Pathway 3, since methane is intentionally produced as opposed to no methane produced, any system leakage makes it GHG-positive [or climate-negative].¹⁹ To curb methane emissions from the pipeline, the U.S. senate has passed the Protecting our Infrastructure of Pipelines and Enhancing Safety or PIPES Act of 2020 in August of 2020. The Act focuses on setting standards to reduce methane emissions from all three pipeline categories: gathering lines, long distance transmission lines, and local distribution lines.²⁰ It also details the appropriations for pipeline safety programs and requires adoption of advanced leak detection technologies and practices.²¹

The end use of methane in transportation is also seen as a significant contributor to methane emissions, specifically from the tailpipes, engine crankcase,²² and filling of compressed natural gas.²³ While there is not an abundance of resource on the pump-to-wheels (PTW) methane emissions of natural gas systems, researchers from West Virginia University have developed a novel measurement system to quantify methane leaks and conducted a pilot study on heavy duty transportation sector. Based on this study, engine crankcase methane emissions contributed 39% while engine tailpipes, 30% of the total PTW emissions. Vehicle and station tanks combined accounted for 8% of the total PTW emissions.²⁴ Opportunely, EPA has recently certified the use of engines with closed crankcases which addresses the emission problems from engine crankcase.²⁵ The following chart from Clark et al. (2016) shows the absolute (in g methane loss per kg of fuel used) and relative contribution of different sources to the total PTW emissions.

Does investment in renewable natural gas deter the development of electric alternatives?

In the country's journey towards zero emission in 2050, an optimal combination of different pathways and technologies will be necessary instead of a single “all in” approach. In the process of transitioning to zero emission technologies, RNG can serve as a displacement fuel to fossil fuels and for some geographical and sectoral areas that are difficult to decarbonize or electrify, RNG can play a significant role as well.²⁶ In a study done for the California Air Resources Board, Mahone et al. (2018)²⁷ concluded that biomethane, alongside hydrogen or synthetic fuels, is a pillar upon which cross-sector decarbonization could be achieved. For instance, RNG can displace additional gas demand for buildings, while in the transport sector, compressed biomethane sees more utility in medium and heavy-duty vehicles as the LDV segment tilts more towards electrification.

Cyrs, T. et al (2020) from World Resources Institute (WRI) summarized multiple studies on the role of RNG in Deep Decarbonization in the following table. From these studies, it can be deduced that RNG will play a significant role in achieving net zero and is complementary to electrification. Investing in the development and deployment of RNG as well as the assoiciated infrastucture, therefore, does not and should not hamper electrification of geographical and sectoral areas where electrification proves to be the best alternative.