Gas-fired power plants are the largest source of heat-trapping carbon pollution from the US power sector. They also bring with them a host of other problems for people and communities. A flurry of new gas power plant proposals threatens to exacerbate these problems.
Some gas plant developers and their backers are talking up the prospect of burning hydrogen in the plants as a way to address carbon pollution and keep the plants from becoming irrelevant as we make the necessary transition to a low-carbon economy.
But how much would adding hydrogen actually address the problems of gas plants—carbon pollution and more? And how would that approach compare to using renewable energy? While the answers can get a little numbers-heavy, they can be really illuminating. Like the fact that—spoiler alert—using solar panels or wind turbines directly is three times more efficient than an approach involving hydrogen production and gas plants.
The Union of Concerned Scientists (UCS) interrogated hydrogen cofiring as one possible approach to cutting gas plant carbon pollution in our recent report Beyond the Smokestack, and in the related free, publicly available Gas Plant Alternatives Tool (GPAT), which can help you do interrogation of your own.
Before you approach hydrogen cofiring as an alternative, here are seven important questions to ask, and understand the answers to—numbers and all.
1. Does burning hydrogen in a gas plant cut carbon?
In its basic molecular form, hydrogen (H2) consists of two hydrogen atoms, and nothing else. It’s flammable, and it doesn’t emit carbon when it combusts. This means that burning it in place of some of the fossil methane typically used in a gas plant, a process sometimes referred to as “cofiring,” can reduce the carbon pollution coming out of the plant’s smokestack.
But there’s a lot more to the picture, and those complications must be part of any conversation around using hydrogen in a gas plant.
One issue is the reduction in smokestack carbon pollution that hydrogen cofiring can actually offer. Hydrogen packs a lot less energy—about two-thirds less—than the same volume of methane gas. That means that, for a plant to generate the same amount of electricity, it needs to burn more of a gas-hydrogen blend. In turn, this means a given percentage of hydrogen (by volume) blended in doesn’t lead to the same percentage reduction in the amount of gas—or to the same reduction of carbon emissions. A blend with 50% hydrogen by volume, for example, gets you only 23% less smokestack carbon pollution. The graph below from Beyond the Smokestack underscores the non-linearity of that relationship.
Cofiring a certain amount of hydrogen gets you a lot less carbon reduction. (Source:Beyond the Smokestack)
An even bigger issue arises when it comes to sourcing the hydrogen itself—it has to come from somewhere, after all. Hydrogen atoms are almost always found tied up with other atoms, and there are real implications from the energy-intensive process of separating out the H2 for subsequent use. For a carbon-free molecule, hydrogen can sure come with a lot of carbon.
2. Does hydrogen production cause carbon pollution?
While hydrogen burns without emitting carbon, producing hydrogen can be carbon intensive, sometimes very carbon intensive. Almost all hydrogen production in the United States right now is actually from gas, produced via a process known as steam methane reforming (SMR). SMR involves using steam to separate the hydrogen from the carbon in methane (CH4).
In addition to producing hydrogen, this process results in emissions of carbon dioxide (CO2) and other heat-trapping gases. In fact, a typical SMR operation results in 12 kilograms of carbon dioxide equivalent (CO2e) emissions for every kilogram of hydrogen produced, while each kilogram of hydrogen displacing gas in a gas plant would cut CO2 by only 6 kilograms. Given that math, the carbon pollution from producing hydrogen would be twice the amount reduced via burning hydrogen.
Some fossil fuel industry proponents have pitched coupling carbon capture and storage (CCS) with the methane reforming process to limit the resulting carbon pollution. However, as our Beyond the Smokestack report details, CCS can have significant implications of its own.
Another way to produce hydrogen is through the electrolysis of water. The carbon implications of electrolysis vary widely depending on the specifics, as discussed in the next section.
3. Does electrolysis make carbon-free hydrogen?
Electrolysis involves using electricity to separate water into hydrogen and oxygen. Water itself (H2O) is carbon-free. So far, so good. But where that electricity comes from—how it’s generated—matters. A lot. Because electrolysis requires large amounts of electricity.
Hydrogen can be made with zero-carbon electricity, including renewable energy like solar or wind (the result is sometimes referred to as green hydrogen). But an electrolyzer might instead be powered by sources that emit a lot more carbon. The resulting hydrogen, then, carries with it a lot of carbon baggage.
An electrolyzer powered by electricity with the average carbon intensity for the US grid (roughly equal to that from a gas plant) could produce hydrogen with a carbon intensity that is 70% more than that of SMR-produced hydrogen—and more than 45 times what low-carbon options would produce.
Even “green hydrogen,” however, can have carbon implications, as discussed below.
4. Is green hydrogen carbon-free?
Producing hydrogen via solar- or wind-powered electrolysis produces hydrogen, water, and no carbon. But there can still be carbon implications from using renewable energy, even if the energy itself is zero-carbon.
This possibility has to do in part with the implications of diverting existing renewable energy for the electrolysis—electricity from solar or wind farms that had already been in place before the electrolyzer came into being. If that electricity powers the electrolyzers, then it isn’t available to meet the other electricity needs it had previously served. That means the other electricity demand has to be supplied from other power sources, as this great UCS animation explains. Those other sources are typically gas or coal plants, meaning that “renewables diversion” comes with a sizeable carbon pollution cost.
The graph below from UCS’s report uses results from GPAT’s assessment of a 250 megawatt plant operating 60% of the time and targeting 90% reductions in smokestack carbon emissions with hydrogen cofiring, which would require cofiring with 97% hydrogen. It shows the potential carbon implications of different hydrogen production scenarios, including that renewables diversion effect.
Carbon emissions involved with producing hydrogen can easily outweigh carbon pollution reductions at the plant. (Source:Beyond the Smokestack)
There are ways to ensure that green hydrogen can live up to its carbon-reduction potential, including by addressing that diversion effect. UCS and others have been pushing strongly to make government support for hydrogen production, like tax credits, contingent on it being powered by new/incremental low-carbon sources, in the same general region, and with renewable electricity produced at the same time as the particular electrolyzer needs it. My colleague Julie McNamara explains more about those “three pillars” for clean hydrogen here.
5. How many solar panels does green hydrogen need?
Electrolysis is power hungry, and GPAT shows the renewable energy implications of that hunger. Take, for example, our hypothetical 250 megawatt gas plant, running 60% of the time with 30% hydrogen and 70% gas. Powering electrolysis to produce that much hydrogen could take 500,000 solar panels (500 watts each), or more than 50 wind turbines (3 megawatts each).
Renewable energy goes a lot further if it isn’t routed through an electrolyzer and burned in a gas plant, given the inefficiencies inherent in both the electrolyzer and the power generation stages in the renewables-to-hydrogen-to-cofiring pathway. Using solar or wind energy to power an electrolyzer with a typical efficiency of 75%, and then using the resulting hydrogen in a gas plant with an efficiency of 45%, for example, would mean losing two-thirds of the original electricity in the process. Using that same solar or wind energy directly could meet three times as much electricity need as the hydrogen-to-gas-plant approach.
6. What are other costs from burning hydrogen in gas plants?
While carbon reductions are the main driver of discussions of burning hydrogen in gas plants, and overall heat-trapping emissions should be a major consideration, the hydrogen use brings other potential concerns. Those include:
More NO x – The higher fuel flow rate (faster use of fuel, given hydrogen’s lower energy content) and the higher temperatures produced by gas-hydrogen blends produce more nitrogen oxides (NOx), which can cause and exacerbate asthma and other respiratory diseases. A plant cofiring with 30% hydrogen, for example, could generate around 17% more NOx than the plant being fueled with only gas. Keeping the NOx from adding to the pollution burden of the surrounding community would require changes in operation or pollution controls.
More water use – Hydrogen production consumes water. With a 30% blend using electrolytic hydrogen, the water consumed in producing that hydrogen could be almost 1.3 times as much as the power plant itself consumes in its operations. Hydrogen production via SMR is even more water intensive, consuming potentially almost 1.7 times as much for that same case.
More costs – At low levels of hydrogen cofiring, the added NOx would need to be addressed, potentially with investments in new controls. Higher blending levels would require upgrades to other pieces of the plant. Any level of hydrogen use would require investment in pipelines or storage for the fuel. And, given that hydrogen costs several times more than gas, hydrogen use would likely involve higher fuel costs for the foreseeable future. At current costs, a 30% blend could almost double a plant’s fuel costs.
More electricity use – This is one of the more surprising facts about the gas-hydrogen approach to power generation**: it may actually consume more electricity than it offers.** Producing the hydrogen for 80% cofiring via electrolysis would use up almost twice as much energy as the gas-hydrogen plant itself would generate with that hydrogen. Even making the hydrogen for 30% blending would require the equivalent of two-fifths of the resulting electricity from the plant.
Perpetuation of gas use – This one tops all the others. Investing in hydrogen burning for a gas plant presupposes that the full, ongoing use of that plant—perpetuating a range of negative aspects while introducing new ones, and potentially failing to address the carbon pollution—is the right way to go. None of those other issues would be in play if we chose a route that is less polluting, more efficient, and lower cost.
That leads to one more really important topic: what better options do we have? One big answer, it turns out, lies in those same wind turbines and solar panels.
7. Are there better options than hydrogen cofiring for cutting gas plant pollution?
Renewable energy can directly displace gas generation. Using the renewable electricity to serve homes and businesses directly, rather than channeling it into producing hydrogen for burning in a gas plant, can lead to much greater reductions in the gas plant’s carbon pollution. And it can do it without all the problems that come with the hydrogen, with renewable energy’s own impacts paling in comparison with those of hydrogen burning.
That should be the key takeaway from this type of Q&A. When you compare cutting gas plant pollution by cofiring with hydrogen, on the one hand, to cutting the plant pollution by simply obviating the need for that generation by offering a better option, on the other, the math is clear. And so is the solution: use gas plants less by using renewables more.