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.

The myth of “carbon-neutral” Power-To-Gas

In a recent Utility Dive article, it is wrongly claimed that power-to-gas technology can be used to produce methane (CH4) which is:

a “renewable, carbon-neutral fuel” since its production is powered by renewables, its ingredients are air and water, and any carbon released in the process was originally taken from the air.

Joseph Ferrari, Wärtsilä North America General Manager of Utility Market Development, quoted in “Power-to-gas could be key to California’s long-duration storage needs, stakeholders say,” by Kavya Balaraman, Utility Dive, May 6, 2020.

There is great intuitive appeal to this statement. If the quantity of carbon is not increased, then a process “must be” carbon-neutral. But, no matter how intuitively appealing the claim may be, it reveals a profound misunderstanding of the term “carbon neutrality.” The term does not refer to a mere equivalence in the number of carbon atoms in two quantities of gas, rather, as commonly understood, carbon neutrality refers to a net zero change in either carbon dioxide or equivalent emissions — where emission equivalents are measured in terms of Global Warming Potential (GWP). Given that, over a 20-year period, methane has a GWP eighty-six times greater than does carbon dioxide, the quantity of carbon contained in methane is irrelevant.

When methane is burned under ideal circumstances, combustion products are limited to carbon dioxide and water. (i.e. CH4 + 3O2 → CO2 + 2H2O) Only under such ideal conditions would the quantity of carbon dioxide emitted by burning methane be exactly equal to the quantity consumed in synthesizing that methane, and thus, only in this ideal case might one argue for carbon-neutrality. However, in the real world, methane combustion is never “ideal.” In fact, incomplete combustion, due to insufficient oxygen supply, poorly maintained equipment, and many other factors, will produce unwanted emissions such as methyl alcohol, formaldehyde, formic acid, carbon monoxide, and other combustion products, as well as some amount of unburned methane. Some of these combustion products have a GWP greater than that of carbon dioxide and others are pollutants that impact air quality more than does carbon dioxide. Thus, unless “green” synthesized methane is burned ideally and completely, (but, it won’t be) the resulting combustion products will have a greater impact on global warming and on air quality than would have resulted from simply leaving the carbon dioxide in the air and avoiding the power-to-gas process entirely.

Even if the combustion of synthesized methane were ideal and complete, if any of the methane leaks prior to combustion, that leaked methane will have a dramatically higher impact on global warming than either the methane which is burned or the original source carbon dioxide. Of course, in the real world, there will always be leakage and the quantity of leakage is likely to increase as the distance between the power-to-gas system and the point of combustion increases. Leak-Prone-Pipes are a significant problem in the gas utility business.

Given that methane has a GWP-20 which is eighty-six times higher than carbon dioxide’s, a leak of as little as 1.16% (i.e. 1/86) of the methane will have a GWP-20 equivalent to that of the quantity of carbon dioxide that would be produced if 100% of the methane were burned under ideal conditions. Thus, if only 1.16% of the produced methane were leaked, the global warming impact of producing, transporting, and consuming that gas would be almost twice as great as that of the carbon dioxide input to the power-to-gas process. Unfortunately, leakage in today’s natural gas transmission and distribution networks is often found to exceed 3% to 4%. It should be clear that pumping the output of power-to-gas systems through the leaky natural gas distribution system will not be carbon-neutral.

Because of incomplete combustion and leakage, power-to-gas schemes simply cannot be described as “carbon neutral.” Power-to-gas will always be somewhat carbon-positive. Nonetheless, if the methane produced by “clean” power-to-gas, such as that described by Ferrari, offsets or substitutes for methane that otherwise would have been extracted from the earth, the result can be a substantial reduction in the relative emissions of global warming gases. In general, reuse or recycling of carbon dioxide already in the environment should be preferred to releasing additional methane from underground sequestration.

In order to limit the potential for leakage and to ensure that the synthesized methane is more likely to undergo complete combustion, we should prefer to consume methane as close as possible to its point of synthesis and we should seek to use it only in professionally managed equipment, rather than in the often poorly maintained furnaces, water heaters, or cooking appliances of natural gas utility consumers. We will enjoy the greatest environmental benefit from power-to-gas if the methane produced is used to generate electricity using co-located generators.

Given that “clean” power-to-gas must rely on clean, renewable electricity for power, it would make the most sense to co-locate power-to-gas facilities with renewable electricity producers. Doing this, combined with local storage of gas, would isolate renewable generators, such as wind or solar farms, from the unfortunately too-frequent requirement that they curtail their output during times when electricity production is high but demand is low. Rather than simply curtailing generation when demand is insufficient, renewable generators might redirect their excess electricity to power-to-gas facilities whose output would be stored for later use. Then, when low winds or cloud cover reduce generation below that which is required, the stored gas would be burned in turbines or fuel cells to increase the quantity of electricity injected into the grid. By exploiting this combination of power-to-gas, local storage, and gas-fueled generators, as a kind of “battery” system, both the usefulness and the revenues of renewable generators would be increased. We would all benefit from a reduction in the number of facilities that must be built in order to address our peak power requirements.

Some may argue that, in at least some parts of the country, there is such a quantity of “excess” renewable power generation that it would be more effective to simply pump the synthesized gas into the local natural gas transmission and distribution network. They will say: “If we’re producing methane, why not use it to offset fracked gas?” In the short-term, such claims might sound compelling, however, in the longer-term, we can be confident that they won’t remain compelling. First, a more useful solution to the problem of excess local electricity production would be to enhance the regional or national electric transmission systems to allow a greater ability to move excess local production to remote areas where it is more needed. (NREL has been studying such transmission network enhancements in their Interconnections Seam Study (SEAMS).) Second, it is important to recognize that as more and more energy consumption is satisfied by Beneficial Electrification, rather than by burning fossil fuels at the point-of-use, our aggregate demand for electricity will increase dramatically. (Brattle Group estimates that by 2050, electricity demand may more than double) Thus, what may be considered excess production today will not remain excess for much longer. The development of costly interconnects with the natural gas distribution system, if only to relieve a temporary problem, cannot be justified. Increasing our ability to deliver electricity to where it is needed would be a better long-term solution.

Using power-to-gas to augment supply to the existing gas system would also probably delay and complicate efforts to convince consumers to abandon gas and other fossil fuels, in favor of Beneficial Electrification (i.e. heat pumps, induction cooking, electric vehicles, etc.). The Second Great Electrification of our nation may be delayed, with potentially serious consequences.

Using power-to-gas to replace some of our nation’s natural gas would certainly have the short-term effect of somewhat reducing the aggregate emissions now due to gas combustion. However, many studies have shown definitively that even our maximum technical capacity to synthesize methane is far below that which would be needed to replace more than a small portion of gas we use today. (A National Grid study found, for instance, that the maximum technical capacity for all Renewable Natural Gas (RNG), including power-to-gas, in New York State would only provide for 17% of our State’s current annual gas demand.) Thus, power-to-gas, while it might temporarily help reduce emissions, is simply not able to provide much help in achieving necessary emission reductions, such as the 85% reduction, by 2050, that is required by New York’s Climate Leadership and Community Protection Act (CLCPA) or the similar commitments made by other states.

Even if we decide that it is worth the expense to use power-to-gas to provide a temporary, partial reduction of gas utility emissions, it is likely that doing so will result in hindering our long-term development of more effective, sustainable solutions. This is, in part, due to the fact that the introduction of “clean, renewable” gas into the gas supply would probably confuse consumers, as well as clean energy advocates, and present a partial, inadequate solution that results in a reduction in the effort put into advocating for the managed decapitalization of gas assets and the gas use reductions which are needed. The potential of power-to-gas to weaken efforts to replace gas assets is well known to gas industry insiders who recognize that:

“”technologies to decarbonize the pipeline can serve as a conduit to environmental organizations, thereby seeking to mitigate the opposition’s fervor against [gas] infrastructure expansion.”

“[Power-to-gas is the] prime opportunity for utilities to continue using existing pipeline infrastructure, and expanding infrastructure.”

American Gas Association (AGA), Sustainable Growth Committee Meeting, March 15-16,2018, Meeting Summary.

In summary: Power-to-gas schemes do not, can not, and will not produce a carbon neutral fuel for distribution by gas utilities. However, when used as part of an energy storage system in concert with renewable electricity generation power-to-gas allows for a more optimal dispatch of renewable power within a transmission-constrained electric grid. The use of power-to-gas to displace utility distributed natural gas should be discouraged in order to avoid reduction of both the “fervor against infrastructure expansion” and the efforts which are needed to reduce gas use.

See also: The Futility of RNG