Use of Solar Energy to Better Produce Ammonia
The production of ammonia (NH3) is one of the most important processes worldwide because of its use as fertilizer. Approximately 2-3% of all energy produced in the world is consumed each year in the Haber-Bosch process of making ammonia. Work toward a three-reaction solar thermochemical cycle using manganese as a transition metal reactant and a tube furnace is reported within. In reaction one, pure manganese powder is reacted with nitrogen gas and heated to 700-1000°C, producing manganese nitrides of varying stoichiometries. In reaction two, these nitrides are then heated with steam, producing ammonia and manganese oxide. The third and final reaction is the reduction of the manganese oxide back to pure manganese to close the cycle. All three reactions were tested using various reaction conditions and all were successful. Reaction 1 converted 80.9% of the manganese to manganese nitride, with 70% being in the form of Mn6N2.58, and the remaining 30% being Mn4N. Reaction 2 showed that approximately 20% of the lattice nitrogen in the manganese nitride converted to ammonia. Reaction 3 was carried out using multiple methods, but the most successful produced Mn6N2.58 as the only product besides unreacted starting material (MnO), showing that reactions 3 and 1 were performed in a single step, further simplifying the reaction cycle. The solar thermochemical cycle seems a promising alternative to the intense Haber-Bosch process for ammonia production. The heating for these experiments was carried out through the use of an electric tube furnace. Future work will focus on using a Fresnel-lens solar concentrator to provide heat using solar energy, increasing process viability as compared to the Haber-Bosch method by lowering both energy consumption and carbon dioxide emissions.
Ammonia is a major component in many fertilizers because it is one of the best ways of delivering nitrogen to plants, which they need to grow well. And as the world's population continues to exponentially increase, so must the food supply, and even more ammonia will be needed to fertilize the crops.
The current process for producing ammonia is the Haber-Bosch method, shown below. It combusts natural gas as a heat/energy source, uses very high pressures, and produces carbon dioxide as a byproduct. We want a cheaper, greener, and less intensive process. Our proposed synthesis, also shown below, uses solar energy as a heat/energy source, runs at ambient pressure, and produces no greenhouse gases.
Slack, A.V., Russell James, G., 1977. Ammonia, Part III. Marcel Dekker, Inc., New York.
Our proposed ammonia synthesis is a three-step cycle (displayed below). In the first step we take pure manganese and add nitrogen to it, producing a manganese nitride. We then flow steam over the nitride to remove the nitrogen and use it to produce ammonia, leaving us with manganese oxide, which we then reduce back to manganese and start the process over again. Reaction 2 was accomplished and shown to work before I arrived this summer, so I mostly worked on Reaction 3, and a little with Reaction 1.
Sievert's Law says that if you melt a metal oxide and flow an inert gas over it, some of the oxygen will be removed and leave in the gas stream. This method was used in Step 3 and is discussed more under the results tab.
All experiments were run in a tube furnace (shown below). A tube furnace allows a sample to be heated up to 1200°C while also allowing a gas to flow over it as the reaction occurs. The tube furnace is very helpful for allowing control of process variables such as temperatures and reaction times, but once reaction conditions are refined we hope to run the same experiments using a solar concentrator (shown below) instead. The solar concentrator allows for higher temperatures (around 1600°C) and uses only energy from the sun for reaction heating, making the process more energy efficient, which will be very critical during the transition to industry.
We used X-ray diffraction to characterize our solid samples before and after each reaction. This method works by sending a stream of X-rays at a powder sample and then measuring the angles at which the beams diffract. Every compound has its own unique set of angles that show up as a set of peaks when an XRD test is placed on a graph.
Step 2 of our cycle had already been completed before I arrived here this summer, and the production of ammonia was confirmed by the use of draeger tubes and ammonia probes. Therefore I worked a little bit with step 1, but mostly step 3.
In step 1, we took pure manganese and flowed nitrogen gas over it at 850°C for four hours to produce manganese nitride. Our results (below) show a mixture of products. We obtained manganese nitride in two different stoichiometries and some manganese oxide. The MnO was likely produced from a small amount of air in the furnace that didn't get purged out well enough.
In step 3, we began by attempting to use Sievert's Law to reduce MnO2 to just Mn. As our furnace heated up, we melted the oxide and some oxygen was removed, converting it to a stoichiometry with a decreasing oxygen to manganese ratio. However, this also came with a corresponding increase in melting temperature. Our tube furnace is only capable of reaching 1200°C, so the last stoichiometry we could melt was Mn2O3. And so we were left with mostly Mn3O4, and a small amount of MnO was produced, but we were unable to produce pure manganese. If our furnace was capable of reaching 2000°C, this method would likely succeed in completely removing oxygen, yielding pure manganese.
Since our furnace couldn't get hot enough for us to use Sievert's Law, we decided to try a different reduction method. A Gibbs energy analysis of various possible reducing agents showed that methane would work at temperatures above approximately 1100°C. We flowed 4% methane in nitrogen over MnO at 1200°C for 30 minutes and got very promising results. About 76% of the MnO stayed unreacted, likely because of our short run time, but the 24% that did react formed manganese nitride. This shows that not only did the methane complete step 3, but the excess nitrogen immediately took that product and performed step 1, shortening our cycle even more by accomplishing two reactions in one step.
All three reactions in our cycle have been shown to be successful, but they can be improved. Trying different reactants and reaction conditions will likely improve yields.
We hope to run all three reactions in the solar concentrator soon to show that it can be done using just solar energy as the heat source for the reaction.
Once consistent yields are determined and we have a better idea of the best reaction conditions, we can perform an economic feasibility analysis. This would allow us to see how the profitability of this method would compare to that of the Haber-Bosch method and then hopefully use our process in industry.
- University of Idaho
- Double Majoring in Chemical Engineering and Chemistry, Minoring in Math
- Senior, expected graduation: Spring 2016
- Hometown: Coeur d' Alene, ID
- Mentor: Michael G. Heidlage, IGERT in Biorefining, Department of Chemical Engineering
- Advisors: Dr. Mary E. Rezac and Dr. Peter H. Pfromm, Department of Chemical Engineering
This material is based upon work supported by National Science Foundation Grant: REU Site: Summer Academy in Sustainable Bioenergy; NSF Award No.: SMA-1062895, awarded to Kansas State University
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