Ronald Michalsky - Chromium as reactant for solar thermochemical synthesis of ammonia from steam, nitrogen, and biomass at atmospheric pressure

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  Ronald   Michalsky, Peter H. Pfromm: Chromium   as reactant for solar thermochemical synthesis of ammonia from   steam, nitrogen, and biomass at atmospheric   pressure (in press, Solar   Energy).


  Ammonia for fertilization plays a crucial role in agriculture. It   is an important commodity chemical, and it can serve as a fuel   for combustion engines or as a carrier molecule for hydrogen.   Global NH3 production of over 100 million metric tons   per year relies almost entirely on natural gas for energy and   hydrogen. About 2% of the world’s energy budget is spent to   produce NH3. Experiments towards a solar   thermochemical cycle for NH3 synthesis at near   atmospheric pressure using a transition metal reactant and a   Fresnel-lens solar furnace are reported here: reacting Cr metal   powder with gaseous N2 to Cr nitride, hydrolyzing Cr   nitride powder with steam to NH3 and   Cr2O3, and finally reducing   Cr2O3 powder back to Cr with mixtures of   H2, CO, and N2. At about 1000 °C it was   found that Cr readily fixes N2 from the gas phase as   Cr nitride (4.13 × 10−2 mol N2/mol Cr/min,   85 ± 4 mol% of hexagonal Cr2N after 5.6 min).   Cr2N converts over time to a cubic CrN phase.   Corrosion of Cr nitride with steam at 1000 °C and about 1 bar   forms Cr2O3 and CrO while liberating 53 ±   11 mol% of the nitrogen contained in the solid Cr nitride in 60   min. Of the N liberated, 0.28 ± 0.07 mol% forms the desired   NH3. This results in a yield of 0.15 ± 0.02 mol%   NH3 relative to the N in the nitride (1.07 ×   10−4 mol NH3/mol Cr/min). Addition of   CaO/Ca(OH)2 powder or quartz wool to provide more   reactive sites and promote protonation of N increased the yield   of NH3 only slightly (0.24 ± 0.01 or 0.39 ± 0.03 mol%   NH3 relative to the N in the nitride respectively).   The thermochemical cycle is closed by heating   Cr2O3 to 1200–1600 °C with a reduction   yield near the surface of the particles of approximately 82.85   mol% (40 min at 1600 °C) in a gas stream of H2 and CO   (2.7 × 10−3 mol Cr/mol   Cr2O3/min). An unreacted core model was   applied to estimate the activation energy of   Cr2O3 reduction with 128 ± 4 kJ/mol. Cr   appears promising to promote nitridation and oxide reduction as a   basis for a future custom-designed reactant with high specific   surface area enabling sustainable and more scalable   NH3 production from N2 and H2O   at ambient pressure without natural gas consumption.

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