The Futility of RNG

Elon Musk recently gained quite a bit of attention with his offer of a $100 million prize for development of the “best” technology to capture carbon dioxide emissions. This increased attention to Carbon Capture and Utilization (CCU) must have quite pleased our nation’s gas utilities who have recently been arguing that they can maintain the value of their massive infrastructure, while achieving climate goals, by replacing natural gas (methane) with Renewable Natural Gas. RNG is methane that might be either synthesized from captured carbon or harvested from existing methane sources such as landfills. But, while Musk’s prize might motivate development of practical technologies that will facilitate the capture and sequestration of atmospheric Carbon, it won’t help the gas utilities produce a climate-friendly fuel that they can sell in lieu of natural gas.

The unfortunate reality is that as long as there is leakage in the gas transmission and distribution systems (and there always will be) even the best technologies for producing RNG will result in significant climate impacts and social costs. The many claims that RNG is either “carbon negative” or “carbon neutral” are, quite simply, false. Gas utility regulators should carefully review the science and then prohibit gas utilities from wasting ratepayers’ money on futile RNG schemes and soon to be stranded assets. Gas utilities should focus their efforts on creating processes for the managed decapitalization and decommissioning of their existing gas assets and they should stop making the problem worse by marketing gas and investing in gas expansion.

The false claim that burning methane (CH4) produced from captured carbon dioxide (CO2) has no net negative impacts is intuitively appealing since burning methane “should” emit the same amount of CO2 which was used to produce it. As Stephen Colbert would say, the claim has truthiness. But truthiness isn’t truth. In fact, as long as there is any leakage in the gas system, any amount of methane produced from captured carbon dioxide will not only increase global warming but also impose a variety of other social costs.

The New York Department of Environmental Conservation (DEC) recently published Value of Carbon Guidance which establishes, for New York, the monetary values or “social costs” to be used when estimating the impact of emissions of carbon, methane, or nitrous oxide. The DEC estimates that, in 2025, the global social cost of one metric ton of carbon dioxide will be $134 (assuming a 2% discount rate). Thus, injecting a ton of carbon dioxide into the atmosphere will have a social cost of $134. Similarly, removing one ton of carbon dioxide will result in a $134 savings, or social cost reduction. In theory, any process that both removes a ton and then injects a ton should have a net impact of $0 (i.e. $134 – $134 = $0). Often, it is wrongly claimed that RNG affords such a net-zero process. But, due to leakage, it does not.

Our experience in New York indicates that it is reasonable to assume that about 3.14% of the methane which is injected into the gas utilities’ transmission and distribution systems is leaked unburned. So, if we were to convert some quantity of captured carbon dioxide to RNG (methane) and then distribute it for burning at customer sites, only about 96.86% of the RNG would actually be burned and thus converted back into carbon dioxide. Unfortunately, the 3.14% which was leaked, as methane, would have a social cost of $3,113 per metric ton which is a bit more than 23 times as high as the social cost of the carbon dioxide from which it was synthesized. The net social cost of burning methane produced from one ton of captured carbon dioxide would be $31.33[1] or, about 19% of the avoided $165.33[2] social cost of burning fossil methane gas. While a reduction to 19% is significant, 19% is still much higher than the 0% impact that is often claimed. Of course, the social cost of burning RNG can be reduced by reducing the leakage rate. However, it will prove impossible to eliminate all leakage. Thus, RNG from captured carbon will always have a net social cost greater than zero.

Of course, the social cost of emissions includes costs of impacts other than those related to global warming. These costs include those of ocean acidification and corral reef death. The social cost also includes some offsetting benefits such as increased crop yields. So, given that a primary focus of decarbonization efforts is to address global warming impacts, we should estimate just the global warming impacts, in isolation. I do so below.

By definition, carbon dioxide has a GWP (Global Warming Potential) of 1. Methane has a GWP-20 (GWP integrated over 20 years) of about 86. (New York law requires the use of GWP-20, not the more common GWP-100.) Thus, the net GWP-20 for RNG produced from one metric ton of captured carbon dioxide will be 2.669, which is about 73% of the 3.669 GWP-20 of burning an equivalent amount of fossil gas.[3] Once again, we see that RNG produced from captured carbon is “better” than fossil methane, but not much better.

Some might argue that as long as RNG has both a lower social cost and a lower GWP-20 than does fossil gas, it is clear that we should adopt its use. However, putting aside the fact that RNG is much more expensive than fossil gas, it is necessary to recognize that New York’s law (The CLCPA) requires that by 2050 global warming emissions must be reduced by 85% from their 1990 levels. It should be clear that a mere 27% reduction in GWP-20 is insufficient to be very useful in achieving this requirement — especially when we consider that gas consumption today is actually much greater than it was in 1990. If we were to invest today in the equipment needed to synthesize RNG, we would find that doing so wouldn’t help much in achieving the necessary GWP-20 reductions. Any RNG equipment installed today, or in the future, is likely to become a stranded asset long before the end of its expected useful life because state policy will eventually require a reduction in gas sales as the only way that to achieve the required 85% reduction. The real impact of employing RNG today will be to give the false impression that emissions are being reduced sufficiently to meet the 2050 goal. This false impression is likely to distract policy makers from what should be our primary focus: defining a process for the managed decapitalization and decommissioning of the existing gas distribution network.

It would be wonderful if RNG produced from captured atmospheric carbon dioxide was actually useful. Unfortunately, it is not. At least, it isn’t useful as a means to provide a “carbon-neutral” substitute for the fossil gas currently delivered by our gas utilities. As long as there is any leakage in the gas system, and there always will be, RNG will always have both a social cost and a GWP-20 greater than 0. It is futile to try to get around this problem. RNG is not the solution to any problem currently faced by our gas utilities. Investing in RNG wastes ratepayers’ money.

See Also: The myth of “carbon-neutral” Power-To-Gas

[1] The ratio of the molecular weight of CO2 (~44g/mol) to that of CH4 (~16g/mol) is about 16/44 or 0.3636. Because one molecule of CH4 is produced from each molecule of CO2, the ratio of weight between any identical number of CO2 and CH4 molecules will be equal to the ratio of the atoms’ molecular weights. If we produce 0.3636 tons of methane from one ton of captured carbon dioxide and then burn it in a system that has 3.14% leakage, the burning of methane will produce carbon dioxide having a value of ($134/ton * 0.9686 tons) = $129.79. The leaked methane will have a value of ($3,113 * 0.3636 * 0.0314) = $35.54. Thus, the net social cost of one metric ton of carbon dioxide which is converted to methane that is partially burning with some leakage, will be: ($134 – $129.79 – $35.54) = -$31.33.
[2] The social cost of burning a quantity of fossil gas equivalent to that which can be produced from one metric ton of captured carbon dioxide would be ($129.79 + $35.54) = $165.33. The social cost of RNG is thus ($31.33/165.33) = 19% of the social cost of fossil gas.
[3] The GWP-20 for methane consumed, with 3.14% leakage, will be the sum of the GWP-20 of the methane burned and the GWP-20 of the methane leaked. (i.e. 0.9686 * 1 + 0.0314 * 86 = 3.669) The net GWP-20 will be the GWP-20 of the consumed methane minus the GWP-20 of the captured carbon or 3.669 – 1 = 2.669.