To Make Green Ammonia, US Researchers Design a Novel Solar Process

Step 1 – reduction of the metal oxide; Step 2 – separation of nitrogen from the air and reoxidation of the metal oxide; Step 3: ammonia synthesis by reaction of H2 gas with a metal nitride *; and step 4 – re-nitriding the now nitrogen-poor nitride with the generated nitrogen.
* Note that the order of the steps, from top to bottom, is: 1, 2, 4 and 3 IMAGE @ Alberto de la Calle

As a vital component of fertilizers, ammonia (NH3) is the second most widely produced industrial chemical in the world. The annual production is around 180 million tons and almost 90% of this production covers the global demand for agricultural production.

Ammonia is also increasingly viewed as a potential green fuel for power generation and some difficult-to-decarbonize sectors like shipping, with many advantages over green hydrogen, its other competitor. But the way in which ammonia is currently produced is incompatible with a safe climate: the Haber-Bosch process.

In the Haber-Bosch (HB) process, ammonia is produced from hydrogen (H2) and nitrogen (N2) through an exothermic catalytic high pressure reaction (150-300 bar) at 350-500 ° C. In the industrial HB process, the main input materials for this NH3 production process are obtained through the consumption of hydrocarbons: H2 is generally obtained from methane (CH4) through steam reforming, and N2 is obtained from the air after removing oxygen (O2) through combustion won by CH4. Additional hydrocarbon fuels are burned to generate the heat and mechanical energy required for the process, which further increases carbon dioxide (CO2) emissions. Worldwide, an average of just under three tons of CO2 are generated per ton of ammonia produced.

But what if ammonia could be produced from renewable energy sources in an environmentally friendly way? And how could a new approach use solar heat?

In order to design such fundamentally new processes, much research on sustainable energy has been funded internationally by governments. To date, however, most researchers have studied how to switch the HB process to either green (not a fossil fuel) or blue (fossil fuel, with carbon capture and storage). These processes still have to do with very high pressures to produce the ammonia. Among the research institutions working on CO2-neutral ammonia production, the US DOE has funded a multi-agency project involving Sandia National Laboratories, Georgia Institute of Technology and Arizona State University to use concentrated solar energy as the sole source of energy.

This is how the novel ammonia process would work

A multi-institutional team led by Dr. Andrea Ambrosini from Sandia National Laboratories is working on a different route to climate-neutral ammonia that does not use the HB process at all. This team is examining the feasibility of a unique solar thermochemical ammonia production process without CO2 emissions.

Dr. Alberto de la Calle, Assistant Research Scientist on the ASU team who is co-author of the solar powered nitrogen separation process from air based on a two-step thermochemical cycle: thermodynamic analysis and a low pressure reactor design for solar thermal ammonia production, explained the rationale for the new approach in a recent one Call from Arizona.

“We propose a sustainable way of ammonia production that uses concentrated solar radiation for process heat, to separate nitrogen from the air instead of fossil fuels and to reduce the pressure required to synthesize NH3. To achieve this, we are developing advanced solar thermochemical looping technology to produce (and store) N2 from air for subsequent production of NH3 via an advanced two-step process, ”he said.

Your proposed process is divided into two phases, with two steps in each phase.

In the first phase, the team will separate nitrogen from the air using a two-stage thermochemical metal oxide cycle. The first step is to thermally reduce the metal oxide, which releases some oxygen from its structure. Since this reduction reaction is endothermic (it consumes heat), concentrated solar radiation supplies the energy in the form of heat. In the second step, this reduced metal oxide oxidizes in the air, whereby the O2 is consumed, which leads to a highly pure N2 flow. Once the reaction is complete, the metal oxide returns to the first step and closes the loop.

In the proposed second stage, they would produce the ammonia using a two-stage metal nitride thermochemical cycle. The first step of this second cycle is the ammonia synthesis reaction. In this step, a metal nitride is reduced by H2 (nitrogen removed), which directly produces NH3. In the second step, the nitrogen-poor metal nitride is renitrided by the purified N2 from the first stage, whereby the nitride is regenerated. As soon as the reaction is complete, the regenerated nitride can be reduced again, thereby closing the cycle. Investigation of an effectively working nitride for this very novel cycle is currently underway.

Make nitrogen emission-free

“The nitrogen separation from the air in the traditional HB process generates a lot of CO2,” explains de la Calle. “The traditional process extracts N2 from the air by removing the O2 by burning CH4. This heat is used to produce more H2 through steam reforming, but CO2 emissions increase. We propose using the sun to reduce a metal oxide, which then consumes the oxygen in the air without producing CO2. This technology is able to generate high-purity nitrogen without the need for post-processing (separation of CO2) as is the case with the HB process. “

Another advantage of this novel process is that it can store the reduced metal oxide particles – solids are much easier to store than gases – and thus produce nitrogen as required. By storing the reduced particles instead of a gas, no expensive pressure storage and compression work are required in order to introduce the gas into the interior of the pressure vessel.

At the online conference SolarPACES 2020, the team presented an ammonia reactor with an analysis of the conditions for the nitrogen separation process from air based on a solar thermal cycle.

“We think we may be able to drive the solar reduction of the metal oxide at 800 ° C by purging the O2 released from the metal oxide with air from the reactor and performing the nitrogen separation step at 500 ° C,” he commented.

As with other solar thermal reactors, this heat would be supplied by a solar array of heliostats that focuses thousands of “suns” of highly concentrated sunlight onto a receiver or reactor on a tower. The production would look a bit like a CSP plant just because this green ammonia process isn’t is needed to generate electricity, there would be no power plant block with generator or steam cycle, but only the solar field and the receiver / reactor, where the first step (where heat is required) of the thermochemical process is carried out.

IMAGE @ US DOE A test site in the Sandia National Laboratory is used by the team to test its process

Haber-Bosch is taking a new approach. the end

The second stage of the process is ammonia production through a thermochemical cycle at significantly lower pressure requirements than HB. Haber-Bosch needs 150-300 bar to drive the reaction, but de la Calle believes the new process could work at or below 30 bar pressure.

“HB’s high pressures make all components of the process more expensive: reactor, heat exchanger, piping and compression stages. In addition, the cost of the energy required for compression is significant, which accounts for around 20% of total energy consumption. If we can make ammonia at much lower pressures, it will save a lot of costs and carbon emissions, ”he noted.

On the other hand, these reactions would require higher temperatures than the Haber-Bosch process, which only requires between 350 and 500 ° C, and he notes that they are still working to screen materials for this thermochemical cycle.

“We aim to achieve close to 500 ° C and a maximum pressure of 30 bar for both the ammonia synthesis and the re-nitriding reactions. I think that with a well-designed heat recovery system we can meet all of our heat needs by using heat recovery in the nitrogen production sub-process, ”he said.

The reactions proposed in this project (metal oxide reduction, nitrogen production, ammonia synthesis and renitration) are at an early stage of technical maturity, and the ASU team has now started system modeling and detailed thermodynamic and techno-economic analysis in order to find the optimal ones Operating conditions or size. The three-year award would culminate in early 2022.

Relevant papers:

• “Modeling a concentrating solar reduction reactor for the separation of oxygen from the air” Matthew Kury, H. Evan Bush, Kevin Albrecht and Andrea Ambrosini.
• “Experimental screening of singly and doubly substituted strontium ferrites for solar thermal air separation” Tyler Farr, Nhu “Ty” Nguyen, H. Evan Bush, Andrea Ambrosini and Peter G. Loutzenhiser.
• “Solar-powered nitrogen separation process from air based on a two-stage thermochemical cycle: thermodynamic analysis” Alberto de la Calle, H. Evan Bush, Ivan Ermanoski, Xiang Gao, Andrea Ambrosini and Ellen B. Stechel.
• “Substituted Strontium Ferrite Thermodynamics for Solar Air Separation” H. Evan Bush, N. Ty Nguyen, Tyler P. Farr, Ellen Stechel, Peter G. Loutzenhiser and Andrea Ambrosini.
• “A low pressure reactor design for solar thermal ammonia production” Xiang Gao, Ivan Ermanoski, Andrea Ambrosini, Alberto de la Calle and Ellen B. Stechel.

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