Renewable powered calcium chemistry offers an alternative route for ammonia production

Professor Ruth Signorell and her colleagues at the University of Cambridge have reported a new approach to ammonia production that replaces fossil fuel derived hydrogen with a calcium based chemical cycle powered by renewable electricity. The work explores how ammonia could be synthesized using electricity and solid state reactions rather than relying on natural gas and high pressure hydrogen, addressing a major source of emissions in the global chemical industry.

Rodriguez, G., Watkins, N. B., Faraji, X., Lee, E., & Sepunaru, L. (2025). Quantification of redox thermodynamics shifts within coacervates. Proceedings of the National Academy of Sciences, 122(46). https://doi.org/10.1073/pnas.2521526122

Ammonia is produced on a scale of more than one hundred and eighty million tonnes per year, primarily for fertilizer manufacturing. Almost all of this production depends on the Haber Bosch process, which combines nitrogen and hydrogen at elevated temperatures and pressures. Although the process is highly efficient, it is closely tied to fossil fuel use, both as an energy source and as a feedstock for hydrogen. As a result, ammonia production accounts for a significant share of industrial carbon dioxide emissions.

Professor Ruth Signorell and her colleagues at the University of Cambridge stated,

“Inside these droplets, the chemistry is very different from normal water. So, you can make chemical and biochemical reactions that are otherwise impossible in water, which is very important for the origin of life.”

The Cambridge study builds on growing efforts to decouple ammonia synthesis from fossil fuels by redesigning the underlying chemistry rather than modifying existing plants.

The researchers propose a cyclic process based on calcium compounds that can store and release nitrogen in a controlled way. In this system, calcium nitride forms when calcium reacts with nitrogen under moderate conditions. When exposed to hydrogen containing species derived from water splitting, the nitride converts into ammonia while regenerating calcium oxide.

The calcium oxide is then recycled back to metallic calcium using electricity, closing the loop. Crucially, this regeneration step can be powered entirely by renewable electricity, allowing the system to operate without fossil fuels if paired with clean power sources.

By separating nitrogen activation from hydrogen generation, the process avoids several constraints of Haber Bosch chemistry. Nitrogen fixation occurs in the solid state, while hydrogen is introduced later in a lower pressure step. This separation offers more flexibility in how and when energy is supplied to the system.

A key element of the work is the use of electricity as the main energy carrier rather than heat generated from combustion. Calcium oxide reduction is energy intensive, but it is well suited to electrified processes such as electrochemical or plasma assisted reduction.

The researchers emphasize that the proposed cycle aligns more naturally with intermittent renewable energy sources. Unlike Haber Bosch plants, which require continuous operation to remain efficient, a calcium based system could potentially operate in stages, storing reactive intermediates when electricity is abundant and producing ammonia when demand requires it.

This characteristic may be particularly relevant for regions with high solar or wind capacity but limited access to natural gas infrastructure.

From an engineering perspective, the work highlights both opportunities and challenges. Calcium compounds are abundant and inexpensive, but they are also highly reactive and require careful handling. Reactor design must account for solid state reactions, heat management, and materials compatibility over repeated cycles.

The study reports laboratory scale demonstrations of the individual reaction steps, along with thermodynamic analysis showing that the overall cycle is feasible under realistic conditions. However, the researchers acknowledge that significant development is needed before the process could approach industrial deployment.

Scaling the system would require advances in solid handling, reactor integration, and high efficiency electrification. The durability of calcium based materials over many cycles will also be a critical factor in determining economic viability.

Interest in low carbon ammonia is growing beyond agriculture. Ammonia is increasingly viewed as a potential energy carrier for shipping, grid storage, and hydrogen transport. These applications amplify the need for production methods that do not shift emissions upstream.

The calcium mediated approach adds to a broader portfolio of alternative ammonia technologies, including electrochemical nitrogen reduction and plasma based synthesis. Rather than competing directly with Haber Bosch in the near term, such systems may initially serve niche roles where renewable electricity is plentiful and decarbonization is a priority.

Professor Signorell and her team frame the work as a platform for rethinking how ammonia chemistry can be integrated with future energy systems. By redesigning the reaction pathway itself, the study moves beyond incremental efficiency improvements and toward fundamentally different production models.

While commercial implementation remains a long term prospect, the research demonstrates that solid state chemistry combined with renewable electricity can open new directions for one of the world’s most important industrial chemicals.