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.

While there is great intuitive appeal to this statement, Ferrari’s claim reveals a profound misunderstanding of the term “carbon neutrality.” The term does not refer to a mere equivalence in the quantity of carbon atoms in two quantities of gas, rather, as commonly understood, carbon neutrality refers to a net zero change in carbon dioxide, or equivalent, emissions — where carbon dioxide 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 the produced methane is irrelevant.

When methane is burned under ideal circumstances, combustion products are limited to carbon dioxide and water. (i.e. CH4 + 2O2 → 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 one 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 will impact air quality more greatly than does carbon dioxide. Thus, unless the 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 the synthesized methane were ideal and complete, if any of the methane leaks prior to an even ideal combustion at the point of use, that leaked methane will have a dramatically higher impact on global warming than either the methane which is burned or the original source of 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 prior to combustion, 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 natural gas distribution system will result in anything but carbon-neutrality.

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 gas that would otherwise have been extracted from the earth, the result can be a substantial reduction in the relative emissions of global warming gases in the atmosphere. 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 power 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 power production is high but demand is low. Rather than simply curtailing their output when demand is insufficient, renewable generators would redirect their power to power-to-gas facilities whose output would be stored for later use. Then, when low winds or cloud cover reduce power generation below that which is required, the stored gas would be burned in turbines or fuel cells to increase the quantity of power injected to 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 be. 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. (e.g. 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 effective 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 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 does not produce a carbon neutral fuel. However, when used as part of an energy storage system in concert with renewable electricity generation it allows for a more optimal dispatch of renewable power within the 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.