Ammonia Synthesis Using Atmospheric DBD Plasma Coupled With Al₂O₃ Supported Catalyst

Ammonia（NH₃）synthesis technology is the pillar industry of chemical industry,which is directly related to the development of agriculture and national economy.Ammonia is the main source of nitrogen fertilizer,and hence plays a very important role in agricultural development.In addition,ammonia has a great potential as a new energy carrier.Dielectric barrier discharge（DBD）plasma coupled with catalyst can be used to prepare ammonia under atmospheric pressure,reducing the energy consumption and providing a new thought of distributed mode for ammonia synthesis.DBD resonant characteristics reflect the matching degree between the DBD reactor and DBD power,which directly affect the ammonia yield.However,few studies on the correlation between ammonia synthesis and DBD resonance were reported.Moreover,γ-Al₂O₃ carrier can adsorb ammonia because of its acidity,and thus the surface acidity needs to be reduced to improve the catalyst activity.In addition,circulation mode can be used to further improve the ammonia production since it can eliminate the external diffusion.In this study,packed bed DBD plasma coupled with catalyst was used for ammonia synthesis under atmospheric pressure.The research mainly focused on three aspects:resonant characteristics,catalyst modification and circulation operation.The main research contents and achievements are as follows:（1）The effects of operating parameters and structure parameters on ammonia yield and resonant characteristics of DBD system were investigated,respectively.The correlation between ammonia synthesis using DBD plasma coupled with catalyst and resonant characteristics was elaborated.Results showed that highest ammonia yield was obtained at the resonance frequency of DBD system.The resonance frequency of DBD system decreased with the increase in the input voltage and discharge zone width.The decrease in the resonance frequency was advantageous to the ammonia production.Thetotal gas flow rate,N₂/H₂ volume ratio,catalyst filling,catalyst particle size and discharge position had little effect on the DBD resonant characteristics.However,the ammonia yield was increased with the increase in the total gas flow rate,the increase in N₂/H₂ volume ratio,catalyst filling,decrease in catalyst particle size and discharge position around middle.（2）Alumina was modified with CaO and MgO（CaO-Al₂O₃,MgO-Al₂O₃）,respectively,followed by further impregnation of Ru to obtain Ru-based catalysts（Ru/CaO-Al₂O₃,Ru/MgO-Al₂O₃）.Modified catalysts were characterized using BET,TEM and NH₃-TPD techniques,and tested in the DBD plasma reactor for ammonia synthesis.Results showed that the surface area was increased and surface acidity was decreased with the increase in the contents of CaO and MgO in the catalyst,resulting in the increase in the catalyst activity.However,the surface area started to decline at a certain amount of CaO or MgO,but no obvious change was found in catalyst activity.Moreover,the catalyst activity was further improved with the impregnation of Ru.Ru/5%CaO-Al₂O₃ showed the best catalytic effect,and the maximum ammonia yield and energy yield were obtained at around 240 ℃.（3）The improvement of reactor included the adsorption and circulation systems.The process parameters（N₂/H₂ volume ratio,discharge power and gas circulation flow rate）were optimized using the improved DBD reactor.Results showed that the optimal N₂/H₂ volume ratio was 2:1 for both circulation and non-circulation processes.The one-way conversion rate of H₂ was increased while that of N₂ was decreased with the increase in N₂/H₂ volume ratio.The ammonia yield and energy yield were increased with the increase in input voltage.In addition,the increase in gas circulation flow rate improved the diffusion of synthesized ammonia from the catalyst surface to the gas flow,enhancing the ammonia synthesis.