Nitrous oxide packs a dangerous climate punch, but much goes unabated

When it comes to greenhouse gases, most global focus is on carbon dioxide and methane. Nitrous oxide, the third-most-emitted greenhouse gas, seems to go unnoticed, despite its gut punch to the climate.

Known as laughing gas, nitrous oxide is almost 300 times as potent a greenhouse gas as CO₂; it also depletes the stratospheric ozone layer. Nitrous oxide emissions from human activities have gone up by 30% since 1980, faster than scientists had previously assumed, according to a recent study in Nature.

Yet, N₂O has received little attention in international climate or ozone treaties. That’s mainly because agriculture is responsible for over two-thirds of N₂O emissions. Both nitrogen-based fertilizer and manure emit the gas as they break down. Policy makers and farm lobbies make the case that these agricultural emissions are difficult to monitor.

But emissions by the chemical industry, the next-biggest source, can be abated at what advocates say are reasonable costs. N₂O is released as a by-product in the manufacture of nitric acid, a key fertilizer raw material, and of the nylon raw materials adipic acid and caprolactam. Firms in the nylon business have taken or are starting to take action. In contrast, the nitric acid sector has mostly failed to heed calls to reduce N₂O.

The chemical industry’s curtailment of N₂O started about 30 years ago, when major adipic acid producers—at the time, firms including DuPont, Bayer, and Rhône-Poulenc—installed technologies to abate nearly all their emissions. Later, the European Union Emissions Trading System spurred adoption of technologies to eliminate N₂O in European nitric acid plants.

Climate urgency and investor pressure are now pushing others to catch up. The caprolactam producer Fibrant just announced that it has completed installation of an N₂O abatement system at its facility in Geleen, the Netherlands. The firm points to “the current tightening of greenhouse gas (GHG) emission ambitions in Europe.”

In February, Lanxess inaugurated a $10 million N₂O reduction system at its caprolactam plant in Antwerp, Belgium. The firm plans to open a second, bigger abatement facility in 2023. And in June 2020, Ascend Performance Materials installed technology at its adipic acid plant near Pensacola, Florida, that cuts N₂O emissions in half.

Meanwhile, the Nitric Acid Climate Action Group (NACAG), an initiative launched by the German government at the 2015 Paris climate conference, is incentivizing countries around the world to eliminate N₂O emissions from their nitric acid plants.

Mitigating N₂O emissions from chemical production is cheap compared with costs in agriculture or transportation, says NACAG director Enrico Rubertus. The process costs $1–3 per metric ton (t) of carbon dioxide equivalent, he says, as opposed to a few hundred dollars per metric ton to switch a public bus system to electric. “It’s low-hanging fruit, something quite easy to manage, and should be done immediately,” he adds.

Many effective technologies to abate N₂O emissions are available. One process, selective catalytic reduction (SCR), uses catalysts—usually oxides of metals such as cobalt, zirconium, and titanium—to break down N₂O into nitrogen and oxygen.

Installing SCR is relatively straightforward and affordable, because it’s “effectively putting the catalyst in an existing reactor,” says Emilio Martin, a chemical engineer at NACAG. SCRs can reduce up to 85–90% of N₂O emissions. The downside of being inside the reactor is that “when you start and shut down plants you have huge temperature differences, and this causes problems in catalyst functioning,” Martin says.

The other main abatement technology uses thermal reduction units (TRUs), dedicated reactors placed at the end of a plant’s flue gas pipes, where they destroy N₂O by incinerating it. If SCRs are Fords, TRUs are Porsches, Martin says, costing over five times as much, at upward of $2 million apiece, but they can mitigate 95–99% of emissions.

At its Pensacola plant, Ascend uses SCR and TRUs to destroy nitrogen oxides (NO_x), volatile compounds that contribute to ground-level ozone, smog, and acid rain, because the US Environmental Protection Agency regulates NO_x emissions from chemical factories. The system also destroys some N₂O as a side effect.

Ascend is now making an effort to cut its N₂O emissions directly, says Vikram Gopal, its senior vice president of technology.

In doing so, the company is catching up with other adipic acid makers that installed abatement equipment decades ago. Industrial N₂O emissions in the US dropped dramatically in 1996, when three of the four adipic acid plants operating then installed emissions control technology. Nylon 6,6 is made by reacting adipic acid and hexamethylenediamine. Making adipic acid involves oxidizing a cyclohexanone-cyclohexanol mixture with nitric acid, a reaction that generates one N₂O molecule for each adipic acid molecule.

Ascend’s engineers have been developing a customized TRU for the past 5 years to cut those emissions, Gopal says. While conventional TRUs reduce N₂O into nitrogen and oxygen, Ascend’s process uses a specially designed reactor and catalysts to reform N₂O into nitric acid. “The advantage is that you’re making desirable nitric acid rather than breaking up into nitrogen and oxygen,” Gopal says.

The system halved the plant’s emissions after it was installed last June, he says. A second-generation technology could reduce emissions by over 95%. Ascend has tested a 1/10th scale version and is assessing whether to go to full scale. “It’s a brand-new technology, so there’s a scale challenge that comes with any new process,” Gopal says. “We’ve been at it for a little over 5 years. You have to evaluate how new technology sustains itself.”

If employed, Ascend’s technology could have an outsize climate impact. The Pensacola plant was the largest industrial source of N₂O emissions in the US in 2019, according to the EPA, emitting almost 18,000 t. That’s the same as about 5.5 million metric tons of CO₂.

The only other adipic acid plant in the US is Invista’s facility in Victoria, Texas. The former DuPont plant uses an SCR system to cut its N₂O emissions, which were under 3,000 t in 2019.

But nearly half of the world’s adipic acid supply comes from 11 plants in China, which generate hundreds of thousands of tons of N₂O every year—as much as the greenhouse gas emissions of all the cars in California, Shanghai, and Beijing, according to the publication Inside Climate News.

Last September, the Beijing-based industrial gas firm Linggas started capturing over 6,000 t of N₂O at an adipic acid facility in China. The company purifies the gas to sell for use in flat-panel display manufacturing and other applications. It expects to capture an additional 12,000 t of N₂O at the plant this year.

Another N₂O culprit is the production of caprolactam, a raw material for nylon 6. Globally, caprolactam facilities emit about 35,000 t of N₂O per year, equal to about 10 million t of carbon dioxide. Producing caprolactam involves oxidizing ammonia at high temperatures to make ammonium nitrite, which is then hydrogenated with sulfuric acid to give hydroxylammonium sulfate. Both the oxidation and hydrogenation steps result in N₂O as a side product, says Kris Devoldere, a chemical engineer at Lanxess.

The N₂O reduction unit that Lanxess inaugurated at its caprolactam plant in Antwerp early this year will destroy 500 t per year of N₂O made in the hydrogenation step. The company plans to cut an additional 1,000 t emitted during the ammonia oxidation step starting in 2023. “This would eliminate the vast majority of laughing gas produced at our facilities,” Devoldere says.

Lanxess uses a two-step mitigation process to reduce both N₂O and NO_x emissions. In the first stage, N₂O is broken down into nitrogen and oxygen using a 1,000 °C regenerative thermal oxidation process typically used to eliminate volatile organic compounds. The second step involves reacting NO_x with ammonia in an SCR unit operating at 250–450 °C to produce nitrogen and water.

Since 2009, the company has been eliminating 5,000 t per year of N₂O at its adipic acid plant near Krefeld, Germany, its only other significant source of those emissions.

Even if all the world’s adipic acid and caprolactam makers abated their N₂O, though, emissions from nitric acid plants would negate much of the effect. Such factories are the biggest industrial emitters of N₂O. Just as with caprolactam, making nitric acid involves high-temperature oxidation of ammonia, which generates N₂O. But because nitric acid is a key ingredient of fertilizer, the world produces a lot more of it than it does caprolactam.

Of the 580 nitric acid plants worldwide, just over 100 abate their emissions. The rest emit about 350,000 t of N₂O per year, 10 times as much as the world’s caprolactam plants and equal to all the greenhouse gas emissions of the Netherlands, according to NACAG’s Rubertus.

In the absence of regulations, few businesses want to pay for abatement. “Mitigation from a business point of view doesn’t make a lot of sense,” Rubertus says. “Nobody is doing it voluntarily unless they have a good climate heart.”

Enter Europe’s climate progressiveness. The EU’s mandatory emissions trading scheme imposes limits on N₂O emissions from adipic acid and nitric acid plants. Australia enacted a similar policy recently to reduce its emissions from chemical plants and other sources.

The group’s mission is to coax nitric acid plants elsewhere to cut N₂O emissions by at least 90% in the next 4 years. NACAG provides technical and financial support to any developing country that pledges to mandate N₂O cuts but that lacks the resources to support the effort. The group is technology neutral but prefers the end-of-pipeline TRU systems because of their higher reductions and because they also cut NO_x emissions.

NACAG is in discussions with 30 countries—most of them in Latin America, Africa, and Asia. Seven have already signed formal political commitments in exchange for grants that can go up to $5 million. The regulations that accompany the money should ensure long-term mitigation even after support from the German government ends.

This approach is intended to avoid the shortcomings of an earlier incentive program, the United Nations’ Clean Development Mechanism, which also incentivized cuts in industrial N₂O emissions. In the mid-2000s, spurred by this program, 18 nitric acid plants and 2 adipic acid plants in China destroyed tens of thousands of tons of N₂O. They sold those reductions as carbon credits worth billions of dollars to developed countries. But, according to an Inside Climate Newsinvestigation, the companies stopped reporting their emissions to the UN after the funding ended in 2012—and likely stopped abatement.

In 2016, the Chinese government said it would cut N₂O emissions from the country’s adipic acid and nitric acid plants by 2020. It is unclear that this has happened, but if it has, it would eliminate 350,000 t of emissions per year, according to a 2019 report by the World Resources Institute.

Today, nitric acid factories in the US, China, and India are the world’s biggest N₂O emitters, Rubertus says. The US remains one of a few developed countries to skimp on reducing N₂O emissions. Of the country’s 32 nitric acid plants, half use abatement technology. The rest emitted 31,000 t of N₂O in 2019, according to EPA data.

US interest in N₂O mitigation was minimal during the Trump administration, Rubertus says, but things are changing with the Biden administration’s climate focus. And at the Ammonium Nitrate-Nitric Acid conference, last held in Vienna in 2019, industry representatives seemed more willing to talk about their N₂O problem. “We can see a shift in the last 4 years from not talking about emissions to really looking at it,” Rubertus says.

Another round of European regulations could drive further action in the chemical industry. In March, the European Parliament approved a legislative proposal called the Carbon Border Adjustment Mechanism that would, starting in 2023, tax industrial commodities imported into the EU that are made in a carbon-intensive manner.

Just like the European emissions trading scheme, the carbon tax would cover both carbon dioxide and nitrous oxide emissions, Rubertus says. “So from a business perspective it must be clear to operators that they won’t be able to sell products in the future if they don’t produce in a more climate-friendly way.”