Tsubame BHB, a Japanese cleantech startup founded in 2017, has developed an innovative ammonia synthesis technology. Their process employs electride catalysts to produce ammonia at low temperatures and pressures, resulting in a more energy-efficient and environmentally friendly method compared to the traditional Haber-Bosch process. This novel approach enables small-scale, distributed ammonia production, which can be customized for on-site and on-demand requirements.
Challenges: green ammonia
Ammonia is a vital chemical commodity with widespread applications. It serves as the foundation for fertilizers that underpin modern agriculture, contributing to about half of global food production. Beyond agriculture, ammonia shows promise as a renewable energy storage and transport medium. Its potential is being explored in various sectors, including Samsung in shipping, Toyota in automotive, and Boeing and NASA in aircraft, owing to its superior energy density compared to lithium-ion batteries and compressed hydrogen.
However, conventional ammonia production is a significant contributor to global carbon emissions. The process, which involves steam methane reforming, water-gas shift reaction, and the Haber-Bosch process, consumes approximately 1.8% of global energy output annually. This results in the emission of about 500 million tons of carbon dioxide, accounting for roughly 1.8% of global carbon dioxide (COâ‚‚) emissions.
Given the environmental impact of current production methods, there is an urgent need to develop more sustainable approaches to ammonia synthesis.
Tsubame BHB Technology
The Haber-Bosch process, utilizing iron-based catalysts, is the current industrial method for ammonia synthesis:
N₂ + H₂ ⇆ NH₃
Owing to strong N≡N triple bond of nitrogen gas (N₂), Haber-Bosh process for ammonia synthesis must be conducted at high reaction temperatures of 400−500 ºC to break the strong N≡N bond. Furthermore, high pressure (100−300 bar) is required due to thermodynamic limitations. Therefore, the Haber-Bosh process for ammonia synthesis requires large scale and robust plants.
A more sustainable approach to ammonia synthesis at small scale, operating at lower pressures and temperatures, has been in recent demand for onsite ammonia production. Electrochemical ammonia synthesis, which can occur at room temperature and ambient pressure, is one such promising alternative. This method requires stable electrocatalysts capable of producing ammonia with high yields and faradaic efficiency. Several startups, including NitroFix, are working to commercialize this technology.
Masashi Hatton and colleagues have developed a novel heterogeneous catalyst for ammonia synthesis that operates at just 1 bar and 50 ºC (also ref. JP2022070143A). This catalyst consists of ruthenium (Ru) nanoparticles deposited on a cubic CaFH solid solution. The catalyst is created by introducing fluoride anions into calcium hydride, resulting in a stable electron-donating material. However, the fabrication of this complex system remains challenging.
Tsubame BHB has developed a unique electride-supported catalyst that facilitates efficient ammonia synthesis from clean hydrogen and nitrogen at low temperature (300-400 ºC) and pressure (30-50 bar).
Tsubame BHB ammonia production system
Tsubame uses a reaction gas containing a small amount of ammonia gas to produce ammonia at low temperature and pressure with at least two series-connected ammonia synthesis reactors. The diagram below depicts the basic principle of Tsubame’s ammonia synthesis process.
The ammonia synthesis process involves two reactors operating in series. Reactor 1 receives a mixture of N₂ and H₂ with 4% volume NH₃ at 300 ºC (Point A). The pressure is raised to 30 bar to promote ammonia production. Due to the exothermic nature of the reaction, the gas temperature increases to 400 ºC, resulting in an outlet gas with 9% volume NH₃ (Point B).
The gas from reactor 1 is then cooled to 300 ºC while maintaining the 9% NH₃ concentration (Point C). This cooled gas feeds into reactor 2, where the pressure is again increased to 30 bar. The ongoing exothermic reaction raises the temperature to 350 ºC, producing an outlet gas with 13% volume NH₃ (Point D).
Through this two-stage process, the initial reaction gas containing 4% volume NH₃ is converted to a product gas with 13% volume NH₃.
As depicted in the diagram below, Tsubame's ammonia synthesis system consists of at least two ammonia synthesis reactors connected in series, along with a heat exchanger, a cooler, a gas-liquid separator, and gas compressors.