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Recently deployment of renewable energy such as sunlight and wind power or deployment of process technologies for carbon dioxide capture and storage (CCS) is indispensable to reduce the CO2-emission. However, there are some issues to be solved in order to accelerate the mass deployment of renewable energy. Since amount of renewable energy quite changes unstably with time, which depends on weather and place, development of process systems technology is an issue for stable and effective utilization of electric power that is generated by fluctuating renewable energy.
Those in national institute of National Institute of Advanced Industrial Science and Technology (AIST) agree that process technologies on energy conversion of renewable electricity into hydrogen energy carrier are very useful to deploy long-term storage and long-distance transport of much more renewable inside and outside Japan. Ammonia is a potential hydrogen carrier that contains 17.6 wt% of hydrogen. Moreover, as an energy carrier, ammonia is thought to be a clean fuel as only water and nitrogen are produced on direct combustion.
Many researchers and engineers consider that ammonia plants using hydrogen produced by solar electricity or wind electricity will be much smaller than those currently used. This implies process development in a different direction than that practices in the traditional chemical industry, according to Minnesota research group that has developed a small-scale Haber process . For example, there is an issue of low pressure condition for feed of raw material gas, since hydrogen and nitrogen are produced by water electrolysis and pressure swing adsorption (or cryogenic air separation) respectively. Ammonia synthesis under the conventional pressure conditions needs increase in power for compression of the feed gas. Moreover, low temperature operation has advantage in getting higher ammonia concentration of equilibrium limitation. In order to promote the reaction under the lower pressure conditions, lower reaction temperature condition is desirable due to a limitation of equilibrium.
In Japan, the Japan Science and Technology Agency (JST) supports the research and development of catalyst with high catalytic activities in the low-temperature region, and our research group plans to demonstrate performance of developed catalysts by small-scale ammonia plants that will be built in Fukushima Renewable Energy Institute, AIST (FREA). Ruthenium (Ru) catalysts have received particular attention owing higher activity for ammonia synthesis, since the pioneering study of Aika et al. [2-3] Great efforts have been paid to develop supported Ru catalysts using Al2O3, MgO, Cs-MgO and so on. However, there is a small number of studies for influence of time-variation of flow rate of hydrogen to kinetics of ammonia synthesis by using the supported Ru catalysts from the viewpoint of design of the small-scale ammonia production systems by using renewable energy. In the present study, we investigate influence of flow ratio of hydrogen (H2) to nitrogen (N2) to dynamic behavior of ammonia production by using the developed Ru catalyst under low pressure conditions (< 2.5 MPa). Then effects of addition of alkaline earth metal to Ru catalyst will be discussed.
Laboratory kinetic experiments for the developed catalyst, which was called “Ru-catalyst A”, was carried out by using a quartz tubular reactor (Φ14 mm x 370 mm). The catalyst of about 2 milliliters was packed in the middle. As the preliminary step to activate the catalyst, hydrogen and nitrogen was flowed in a stoichiometry ratio for 30 minutes under condition of 600 °C and ambient pressure. In the present study, ammonia synthesis was carried out by changing temperature (T) in a range of 325 – 425 °C and gage pressure (P) in a range of 0.0 – 2.5 MPa. The produced ammonia was analyzed by the decrease of electron conductivity in the sulfuric acid solution.
First, data maps for space time yield (STY) for ammonia production by using Ru-catalyst A were created by changing the flow ratio of hydrogen to nitrogen (γ) in a range of 0.25 – 4.0. When a data map for T = 425 °C and P = 2.5 MPa was compared with one for T = 350 °C and P = 2.5 MPa, high values of STY were seen in a wide region of higher g than 0.67 that indicated maximum of STY for the case for 350 °C. Then a tendency of higher STY for γ = 1 and T = 400 °C than a case for stoichiometry ratio (γ = 3) was remarkably observed by increasing pressure. We analyzed kinetics of ammonia production under the ambient pressure condition, by using the power law expression (r = k PN2l PH2m PNH3n). In this kinetic analysis, the exponent of partial pressure for hydrogen (m) was positive. Then it was seen that order of m for T = 400 °C was higher than one for T = 325 °C in the analytical region of γ > 3. It was considered that the developed Ru-catalyst A could perform stable production of ammonia with high STY in an operation range of γ < 3 by increasing reaction temperature.
Next, effects of addition of alkaline earth metal to metal catalyst carrier were investigated by using three different preparation methods; co-precipitation and physical mixing. For physical mixing, the alkaline earth metal and the metal catalyst carrier were mixed in a ball mill (Retsch MM400). Data maps for STY for ammonia production by using Ru catalyst containing alkali metal promoter, which was called “Ru-catalyst B”, were created by changing experimental condition as mentioned previously. When a data map for application of physical mixing was compared with one for co-deposition for cases of T = 375 °C and P = 2.5 MPa, maximum of STY were seen at 1.5 of γ higher than 1.0 that indicated maximum of STY for the case for application of co-deposition. For these three catalysts prepared with different methods, it was seen that the trend for effect of temperature was quite comparable. At lower temperature, it was considered that the highest catalytic activity was achieved under operating condition of lower γ. Whereas, with increase in temperature, it was inferred that the catalytic activity increased with relative increase in hydrogen concentration in reactant stream.
Moreover, it was seen that maximum of STY for Ru-catalyst B that was prepared by that physical mixing method was very close to maximum STY for the above-mentioned Ru-catalyst A in a case of T = 375 °C, although the supported amount of ruthenium for Ru-catalyst B was less than one for Ru-catalyst A. Then, in the same previously mentioned kinetic analysis for ammonia production under the ambient pressure condition, lower value of the exponent of partial pressure for hydrogen (m) was estimated by comparing the analytical result for Ru-catalyst A. It is necessary for characterization of existence form of magnesium in reaction conditions to make clear influences of preparation method to retardation of ammonia decomposition and enhancement of the overall reaction efficiency in future work.
This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Energy carriers” (funding agency: Japan Science and Technology Agency, JST).
 M. Malmali et al., Ind. Eng. Chem. Res., 55(2016) 8922-8932.
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