A study of nitric oxide and hydrogen production based on chemical looping
2017-03-03T01:22:14Z (GMT) by
Hydrogen is considered as a fuel of the future with several potential benefits whereas nitric oxide has varied medical and pharmaceutical applications in addition to being the major intermediate in the production of nitric acid which has itself extended applications. The conventional processes employ highly exothermic catalytic oxidation of ammonia to produce nitric oxide, and energy intensive steam methane reforming of natural gas to produce hydrogen. These fossil fuel based commercial processes are effective, but face future challenges due to the depletion of fossil resources and atmospheric emissions. There have been several efforts in the direction of improvement of these processes energetically and economically, however, there have been no significant changes in the main process reactions. As hydrogen is the starting precursor for ammonia and hence nitric oxide production, the cost-benefit analysis is performed for different hydrogen producing technologies where the benefits are evaluated using multi-criteria decision making approach such as Analytic Hierarchy Process. Water splitting by a chemical looping approach proved to be the best renewable technology for the chosen perspective of selecting a cost effective environmentally benign technology. In the present work, a chemical looping based process is proposed for production of nitric oxide and hydrogen with a major focus on the reaction of ammonia with different metal oxides. Nitric oxide and hydrogen are produced by reduction of metal oxide by ammonia and the metal oxide is regenerated by oxidation thus completing the cycle. Based on the method of regeneration, the two chemical looping based processes are possible, namely chemical looping using air oxidation (CLAO) and chemical looping using hydrolysis (CLHYD). The objective of the thesis is to carry out both thermodynamic and experimental studies of the proposed chemical looping based processes to produce nitric oxide and hydrogen. The thermodynamic feasibility analysis and the experimental constraints resulted in the selection of cupric oxide (CuO), ferric oxide (Fe₂O₃) and cobalt oxide (Co₃O₄) for reaction with ammonia at 825 °C, 830 °C and 530 °C, respectively. The experimental results confirm the feasibility of the ammonia-metal oxide reaction for each of these metal oxides with conversions as high as 90 % in semi-batch mode reactor. The effect of varying the different parameters such as temperature, ammonia concentration, and particle size on the yield of nitric oxide is reported for the case of CuO and a reaction mechanism is proposed to explain these results. This simultaneous production of nitric oxide and hydrogen in endothermic manner can prove to be a better alternative for conventional exothermic oxidation of ammonia in presence of platinum catalyst at 900 °C. The proposed chemical looping based processes are simulated in Aspen Plus and are compared with the steam methane reforming (SMR) approach for nitric oxide production on the basis of energy and exergy analysis. The exergy efficiency for three processes, namely, SMR, CLHYD and CLAO including steam generation potential is calculated to be 36.9 %, 62.9 % and 66.5 % respectively. The chemical looping approach is found to be exergetically more efficient and environmentally more benign than the reforming based process. The chemical looping based processes offer the advantages such as operation at lower pressures, avoidable expensive catalysts, independent of fossil fuels as feed, negligible nitrous oxide (N₂O) emission, and the hydrogen product that can be totally or partially used for production of ammonia. The proposed process scheme for nitric oxide and hydrogen production holds the potential for significant reductions in cost, energy and environmental emissions. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of the Indian Institute of Technology Bombay, India and Monash University, Australia.