17 September 2023 |

Tackling a triumvirate of greenhouse gasses


Nitric acid is likely not the first ‘product’ that comes to mind when you think about the uphill battle our society faces to decarbonize. Odds are you think of a car or a plane or a cow. 

That said, nitric acid is emblematic of the chemical production that makes heavy industry one of the largest emitters of greenhouse gasses. There are many ways to slice global greenhouse gas emission calculations, but by some measures, industry accounts for 34% of them. Industrial emissions are also among the sectors where emissions are growing most rapidly. 

Winnowing down the industrial emissions funnel, within industry, cement, and steel are often the main emissions-producing culprits people single out and focus on. However, it’s worth noting global chemical production accounts for more emissions annually than steel and cement do.

Chart via Generation’s Sustainability Trends Report 2023 

A triumvirate of emissions

Nitric acid (HNO3) is a strong, corrosive mineral acid primarily used in service of other industrial applications. The main industrial use for nitric acid is as a raw material for fertilizer production. It’s also used to manufacture dyes, plastics, pharmaceuticals, and metal processing, where it’s used to dissolve metals like copper and zinc.

Much like we discussed last week concerning steel production, one of the challenges with nitric acid is that the most common production technique often releases greenhouse gasses into the atmosphere as a direct byproduct of reactions. Unlike steel or cement production however, which produce CO2 as a direct byproduct, nitric acid production can release nitrous oxide (N2O), a greenhouse gas that depletes ozone and is a 250-300x stronger warming agent in the atmosphere than CO2. 

Nitric acid is typically produced through the Ostwald process, which involves: 

  • Oxidation of ammonia (NH3) to nitric oxide (NO) using air.
  • Oxidation of nitric oxide to nitrogen dioxide (NO2).
  • The nitrogen dioxide is then absorbed in water to form nitric acid. 

During these reactions, nitrous oxide can also be unintentionally formed during ammonia oxidation. While this occurs in side reactions that can be mitigated relatively economically, producers don’t necessarily have an economic incentive to take on that work (even if it’s cheap).

Industrial nitric acid production units at a fertilizer plant (via Shutterstock)

Beyond nitrous oxide, nitric acid production also uses natural gas as a feedstock, and the entire process is typically fossil fuel-powered. Nitric acid thus represents a triumvirate of emissions: You’re dealing with N2O produced in side reactions, CH4 (methane) by virtue of using natural gas as a feedstock, and CO2 emissions from energy use.

Making nitric acid out of thin air

In probably the smallest deal covered in today’s email, Arctura, a Rhode Island-based climate tech startup, received $1.15M in grant funding from the DOE to create greener nitric acid. Their approach concentrates on using renewable wind energy to power a novel atmospheric plasma process. 

The company isn’t best known for trying to decarbonize nitric acid production. It’s commercializing a suite of different products related to wind energy, including coatings to protect turbines from lightning and software to help maximize power generation from turbines.

That makes it a bit surprising that they’re also trying to build green nitric acid plants. Specifically, Arctura wants to use Low Temperature Plasma processes that would require only air, water, and electricity to generate nitrogen dioxide. 

For one, it’s important to note nitrogen is the most abundant element in our atmosphere. Fixing it from air is possible, though challenging. Arctura’s process would use air and electricity to generate nitrogen dioxide (NO2) from plasma:

  • Electricity itself can create a plasma, within which specific energy and temperature conditions can lead to the dissociation and recombination of nitrogen and oxygen molecules from air.
  • These reactions can drive nitrogen oxide formation, including nitrogen dioxide (NO2).
  • Water can then be added to make nitric acid, as in the same third step from the Ostwald process.

Notably, this approach could eliminate all three greenhouse gasses from the production of nitric acid: 

  • It omits natural gas as a feedstock, solving the methane component.
  • If powered by renewable, zero-emissions electricity, it cuts CO2 emissions from fossil fuel use for energy.
  • Its proposed nitric acid production process also skips ammonia oxidation, solving the problem of nitrous oxide production in side reactions. 

This brings us back to why it makes sense to a wind energy company. Renewable energy is often curtailed when electricity demand is low. In a sense, green nitric acid production could become an alternative to batteries, which are neither cheap nor easy to interconnect. When renewable energy developers have excess energy, they could use it flexibly to produce nitric acid.

This could work especially well if energy developers co-locate green nitric acid production with renewable energy in areas where there’s demand for nitric acid, reducing transportation costs. 

The net-net

Like many of the things we cover here, Arctura’s green nitric acid ambitions represent an as-of-yet commercially unproven approach. I wouldn’t have written about it if the DOE hadn’t provided the grant funding, as they’re a lot better at doing robust technical due diligence than I am. 

Still, discussing it offered us a window into yet another under-discussed industrial emissions source and decarbonization challenge. A report commissioned by The German Federal Ministry for Economic Affairs and Climate Action estimated that cleaning up nitric acid production could mitigate up to gigatonne of CO2e emissions over the next decade. Sometimes, gigatonne level reductions are hiding in the darndest places! 

It’s my job to give stuff like this a bit of air time instead of writing article after article about massive EV battery plant investments. Otherwise, we all risk losing sight of the breadth of where climate tech innovation is required.