Can Natural Gas Be Part of a Low-Carbon Future?

Mar 26, 2021 12:00 PM

Michael E. Webber, Scientific American | 2021 Issue

In the mid-2010s it became common to say that natural gas would be a bridge fuel to a zero-carbon future, in which solar, wind and other renewable technologies provide all of our energy without any carbon dioxide emissions to worsen climate change. But if natural gas is really a bridge, then it’s not part of the long-term plan. And if we actually build the bridge, we’re likely to stay on it.

Natural gas consumption in the U.S. has risen by a third in the past 15 years. Gas accounts for 32 percent of total energy consumption and is now the biggest source of electricity nationwide, largely displacing coal-fired power plants. Natural gas—primarily methane—burns much cleaner than coal does, and it provides ready backup to variable wind and solar farms. That sounds promising, except burning natural gas still creates CO2. Methane in wells and pipelines can leak into the atmosphere, amplifying global warming. And once the last coal plant closes, natural gas plants become the dirtiest electricity sources. [Disclosure: The author is a professor of energy resources at the University of Texas at Austin. He is also chief science and technology officer at ENGIE, a global energy and infrastructure firm in Paris that operates the world’s largest independent electricity company as well as large natural gas networks.]

To reduce CO2 emissions, society has to decarbonize its energy systems as quickly as possible. Building more wind and solar farms is relatively inexpensive and fast, and it accelerates the shutdown of coal plants. But exploiting the best locations—the windswept plains and sunbaked deserts—requires a greatly expanded transmission grid to bring the electrons to major cities and manufacturing complexes. Those wires and poles introduce risks from windstorms, floods and fires—all rising because of climate change—and township after township routinely fights expansion plans: “Not in my backyard.”

The natural gas infrastructure, almost all belowground, is far less prone to interruption. The U.S. has about three million miles of natural gas pipelines running underneath nearly every major city in the contiguous 48 states. After adding all the compressors, tanks and storage caverns, the infrastructure is worth several trillion dollars. The power plants themselves add hundreds of billions of dollars more. The nearly 70 million households served by natural gas have furnaces, water heaters and cooktops worth at least another $100 billion. Multiply all that sunk investment by about five for the entire world. Gas is also more intertwined than any other energy source with other sectors of society—transportation, buildings (for heating and cooking) and industry (for heat and as a feedstock for chemicals)—making it harder to replace.

Swapping out that infrastructure before its natural lifetime ends would also entail financial losses for the current owners, who will push back. The replacement technology could cost taxpayers, ratepayers and homeowners, who will push back, too. And more electricity does not readily solve the need for liquid fuels burned in trucks, ships and planes or for intense heat in industrial foundries, distilleries and refineries that make volumes of metals, cement, glass, jet fuel and chemicals. The energy density of liquid fuels is difficult to match.

If we can clean emissions out of the natural gas system, it could be part of a carbon-neutral future instead of a bridge. The technology exists to extract the carbon or to transform the gas so that carbon coming out and carbon going in balance to zero or near zero.

The first step in a comprehensive plan for decarbonizing the nation’s energy infrastructure would be improving energy efficiency and conservation to reduce consumption. The second would be to electrify as many cars, space heaters, water heaters and cooktops as is practical, using renewable sources. At the same time, tighten up the leaky gas infrastructure. And replace as much natural gas as possible with low-carbon alternatives such as biogas, hydrogen and synthesized methane or use a process called pyrolysis at the end of the natural gas pipes to get the carbon out.

Clean energy supporters rightly worry that any investment in gas infrastructure creates a lock-in effect. Each new power plant, pipeline or gas storage unit has an expected lifetime of 25 to 80 years, so each element could either become a trap for more emissions or a stranded asset. But we can solve the lock-in problem with drop-in alternatives to natural gas: low-carbon gases that can flow through existing pipes, tanks and power plants, taking advantage of those trillions of dollars of assets.

Zero-Carbon Gas

The drop-in substitute most ready for natural gas is biomethane—methane gas produced from biological sources. Microbes inside large drums called anaerobic digesters chew up organic matter such as crop waste, manure, sewage, and food waste and other garbage in landfills, producing methane. Biodigesters, already a mature technology, transform waste streams at landfills and the waste lagoons adjacent to concentrated animal feeding operations from environmental liabilities into valuable commodities, generating revenues for municipalities and farmers.

Biomethane is working in Austin, Tex. Waste Management, which operates one of the city’s landfills, collects biomethane from 128 wells on its site and burns it to generate enough electricity for 4,000 to 6,000 homes. And one of the city’s wastewater-treatment plants has eight biodigesters, each with two million gallons of capacity; microbes convert sewage into biogas that fuels on-site electricity generators. The process creates a solid by-product called Dillo Dirt, which feels and smells like a clumpy compost. A city contractor sells it by the bag in area stores to enrich soil.

About a quarter of the more than 2,000 U.S. landfills now harvest their gas or process their waste into biogas using biodigesters. That only offsets less than 1 percent of the country’s total natural gas use, however. Biogas can serve as a direct substitute for natural gas, but the relative volume, globally, is low. If a farm, landfill or sewage plant cannot readily use the gas to make electricity or is not next to a gas grid, the biomethane might need to be liquefied and trucked to another location, reducing the carbon payoff. Still, biomethane is a commercially ready technology that can begin to decarbonize part of the gas system.

Hydrogen Instead of Methane

Natural gas can be replaced altogether, with hydrogen. Turbines can burn hydrogen to generate electricity for the grid, and internal-combustion engines can burn it in heavy-duty vehicles. Hydrogen in fuel cells can produce electricity for cars, homes or offices. And hydrogen is a ready building block for many basic chemicals. Burning it, or reacting it in fuel cells, does not produce CO2. Leaked hydrogen has a warming effect that is just a fraction of that of methane.

Natural hydrogen seeps out of the ground from basins in many cratons in the earth—large blocks of ancient rock that form the central parts of continents. Scientists have stumbled across these seeps for more than a century. Oil and gas companies, however, have considered hydrogen a nuisance when they find it alongside underground reservoirs because it can catch fire and can degrade metal piping. But today corporate and university researchers are drilling hydrogen test wells and launching multiyear programs to search for hydrogen underground. Anticipation feels similar to what arose during the very early days of fracking shale: a huge resource is out there, if engineers can figure out how to harness it cheaply and safely.

We can also manufacture hydrogen. Right now most hydrogen for industry is produced from steam re-forming of methane—adding heat and hot water to methane to create hydrogen and CO2. Electrolysis—using electricity to split water into hydrogen and oxygen—can also create hydrogen gas. Both processes require significant amounts of energy, however.

Moving and storing gaseous hydrogen is also a challenge. Because of hydrogen’s low density, it takes a lot of energy to move it through a pipe compared with denser gases such as methane or liquids such as petroleum. After several hundred kilometers the inefficiency makes moving hydrogen more expensive than the value of the energy it carries. And hydrogen can embrittle steel pipelines unless that is mitigated by altering operating conditions or incorporating expensive alloys.

One way to integrate hydrogen is to mix it with methane in an existing natural gas pipeline. This blending decarbonizes some of the system by displacing a portion of the natural gas with hydrogen. Experiments in the U.K. and France show that a mixture of 80 percent methane and 20 percent hydrogen can be efficiently moved in a natural gas pipeline. As part of a study from mid-2018 to March 2020, Dunkirk, France, used an 80–20 blend to fuel 100 homes and a hospital boiler without any new equipment along the pipeline or in the buildings.

Fittings inside furnaces and stoves, such as burner tips, might need to be altered or replaced for blends with more than 20 percent hydrogen because, like pure hydrogen, blended gas burns at different temperatures and rates. Another consideration is that because of hydrogen’s low energy density, a 20 percent blend by volume provides 14 percent less energy per cubic foot than natural gas.

One way around certain cost and safety challenges is to pipe hydrogen as part of another chemical form we know how to handle, such as ammonia, which has one nitrogen atom and three hydrogen atoms. Molecules that include hydrogen atoms are known as hydrogen carriers. Hydrogen is converted, where it is found or produced, into the carrier, which is dropped into existing pipelines, and is either used in that form or converted again into hydrogen at the destination.

Common carriers such as ammonia, formic acid and methanol are liquid at near-ambient conditions, making them easier to transport than gaseous hydrogen. Although ammonia is caustic, it is already moved worldwide as a fertilizer ingredient, and it can be burned without producing any CO2. Methane could be the most efficient option because it carries four hydrogen atoms for every carbon atom and is already compatible with existing pipes, compressors, tanks, turbines and appliances.

Demonstration projects are growing quickly in number. Finnish industrial builder Wärtsilä is constructing a new ship for 2023 named Viking Energy that will run on ammonia with fuel cells, avoiding greenhouse gas emissions and other pollutants that plague the maritime sector. Air France and the Charles De Gaulle airport in Paris are very interested in hydrogen as a way to decarbonize aviation. Hydrogen carriers are still in the early stages of research, however, so it is difficult to say how successful they could be.

Power plants that burn hydrogen are on drawing boards, too. In Delta, Utah, the Intermountain Power Plant—one of the largest U.S. coal-fired plants—sends electricity hundreds of miles to Los Angeles. To meet the city’s long-term requirement for renewable and low-carbon energy, in 2025 plant owners will replace the coal boilers with turbines that can burn hydrogen. They will start with a blend of 30 percent hydrogen in natural gas and will shift to 100 percent hydrogen later. The hydrogen will be generated right there using electrolysis powered by wind and solar and will be stored in more than 100 existing, underground salt caverns, each about the size of the Empire State Building.

End of the Pipe

Instead of decarbonizing natural gas before it goes into the pipeline, we could remove the carbon at the end of the pipe, where customers consume the gas. Methane, for example, can be split at the user’s location into hydrogen and solid carbon, which looks like a fine, black dust. The process, called methane pyrolysis, is efficient and eliminates CO2 emissions. Every kilogram of hydrogen produced from pyrolyzed methane generates three kilograms of solid carbon instead of nine kilograms of CO2 gas that would be emitted if the methane was burned.

The pile of carbon dust that accumulates inside a collector in a furnace or stove would be carted away each month or so. We already pay garbage haulers and municipal wastewater-processing plants to clean up our solid and liquid wastes; we should pay to clean up the waste from our gas use, too. The carbon piles actually have value, though, because they can be sold as a basic ingredient for making graphite, rubber, coatings, batteries and chemicals, as well as a soil amendment for agriculture.

Although engineers have studied methane pyrolysis for decades, they have deployed it only in small demonstration projects. Some equipment at the end of the pipe has to be changed to separate the carbon, but no expensive hydrogen pipelines would have to be built, simplifying matters greatly. Pyrolysis of conventional natural gas can bring the entire system to nearly zero carbon. Adding methane from biodigesters or made from CO2 in the atmosphere using renewable electricity could make the system carbon-negative.

Imagining any of these decarbonized futures might conjure up visions of large new industrial complexes or millions of small equipment changes for consumers. But so do other proposals to curb emissions. Electrifying every heater, stove and vehicle would require widespread technology replacement. Plans to directly pull CO2 from the air would require millions of big machines to capture the gas and sequester it—sprawling enterprises that would also demand lots of new land and new electricity.

Decarbonized gas would let us take advantage of trillions of dollars of existing pipelines, equipment and appliances, saving huge sums of money and years of time in creating a zero-carbon energy system. We would, of course, have to fix the leaky infrastructure. Leaks can be minimized by replacing pneumatic equipment with electric devices at well sites, improving the automation of pipe and tank inspections with sensors on drones and robots, and writing regulations that no longer turn a blind eye to leaking, as well as deliberately venting or burning unwanted gas. This work would create jobs for workers in the oil and gas industries and would clean up the energy infrastructure, which in turn could lessen pollution in communities near energy facilities.

Reining in climate change requires many solutions. Declaring who cannot be part of those, such as natural gas companies, only raises resistance to progress. Because decarbonized gas can complement renewable electricity and because it might be a faster, cheaper and more effective path for parts of society that are difficult to electrify, we should not discard gas as an option. We have a massive gas infrastructure, and we have to figure out what to do with it. Scrapping it would be slow, expensive and incredibly difficult, but we could instead put it to work to help create a low-carbon future.