By Brentan Alexander, PhD; Chief Science Officer & Chief Commercial Officer

 

In the fight to supplant fossil fuels and build a climate friendly global economy, renewable natural gas (RNG) has been proposed as a prudent and cost-effective method of decarbonizing fossil-based natural gas. As a carbon-negative fuel source that works interchangeably in the hydrocarbon infrastructure we already have, RNG promises a rapid offsetting of the anthropogenic carbon dioxide (CO₂) emissions from fossil gas without the need to reconfigure large portions of our energy delivery system. Mix some carbon-negative RNG in with fossil natural gas and voilà: The overall mix is carbon neutral. New research out of Georgia Tech, however, paints a different picture: At any meaningful scale, RNG is likely to be more carbon intensive than flaring is.

The promise of RNG is rooted in the comparatively high global warming impact of methane, the primary ingredient in natural gas. Although CO₂ gets all the attention, methane is a significantly more potent greenhouse gas, having a global warming potential approximately 30 times greater than CO₂. Methane is generated as a natural byproduct of many modern processes, from landfills to waste-water treatment to dairy farming. In these systems, organic materials are broken down by bacteria, which generate methane that generally escapes to the atmosphere, contributing to global warming. If that waste methane is instead captured and utilized in an engine, water heater, cooking stove, or other device normally fueled by natural gas, the global warming impact of the methane emissions is avoided. As a result, RNG under this scenario is carbon negative.

This math works if the captured methane is truly a waste methane that was otherwise going to be released to the atmosphere. Unfortunately, the amount of waste methane that can be made into useful RNG is a small fraction of the overall need (most estimates cap out at 10%). The authors of the new research argue that any structure that rewards the production of RNG is likely to create an incentive for producers to make more “waste” methane for capture from other sources (e.g. wood or other biomass). Methane purposefully created to be captured and used as RNG isn’t really a waste: If it weren’t for the desire to make RNG, methane from these sources never would have been created, and therefore was never at risk for release to the atmosphere. As a result, any action to scale RNG production will make it increasingly difficult to determine what portion of produced RNG is resultant from methane that was truly a waste product (i.e. that it would not have been otherwise captured and utilized). That means a lot of RNG would simply be, at best, carbon neutral.

“Carbon neutral” still sounds pretty good compared to fossil natural gas, but as the authors point out, the very gas infrastructure RNG hopes to utilize partially undermines that promise. The problem is that our gas infrastructure is leaky, losing between 1% to 3% of gas, and if most of the RNG produced is merely carbon neutral, the release of even a small fraction through these leaks degrades its climate-friendly bonafides since methane has such a high global warming impact. The result is RNG turned upside down: A system designed to prevent the release of waste methane instead becomes a system that leaks manufactured methane.

The utilization of RNG at scale to supplant meaningful quantities of natural gas would still be less carbon-intensive than fossil natural gas, but the authors argue this may not be the best strategy if greenhouse gas reduction is your primary goal. If decarbonization is the sole consideration, waste methane is best utilized in an on-site flare (or other on-site usage); the fossil gas grid is better replaced with electrification, green hydrogen, or other solutions. Compared to this outcome, RNG usage in the gas grid is necessarily more carbon-intensive.

This doesn’t mean that RNG is of no value in the fight to decarbonize: RNG is demonstrably better than fossil gas, is still carbon negative (even with manufactured RNG mixed in) when compared against uncontrolled methane release, and it has the ability for rapid deployment through existing infrastructure. Despite the arguments of the authors, economic costs and time-to-market have to be considered in addition to decarbonization potential. And currently, the potential of RNG hasn’t approached the available waste methane resource, leaving lots of low-hanging fruit. But the production of RNG at scales that rival our natural gas demand will also be more carbon-intensive than alternative solutions to decarbonize the gas grid, as the authors demonstrate, and incentivizing its use now will lock in its production for decades. Policymakers aiming for a carbon-neutral future should take note: Blanket support for any RNG will miss the benefits and drawbacks when compared with other solutions. The best policies will account for the full life-cycle impact of RNG, like California’s LCFS program, to incentivize the use of RNG derived from true waste methane. Even then, the math says carbon-negative RNG can only supplant a small fraction of our natural gas demand. Anything else is best seen as an important but incremental step to bridge the gap while the slow change away from gas usage takes hold.

 

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