Experimental Study On Urea Catalytic Pyrolysis To Prepare Ammonia And Pyrolysis Furnace Flow Field Simulation

With the maturation of urea ammonia preparation technology,urea is gradually used as a reducing agent in SCR systems.The catalyst can promote the hydrolysis of the urea pyrolysis intermediate HNCO,increase the ammonia production rate of urea,and reduce the reaction energy consumption.In order to further explore the effect of metal oxide catalysts on the pyrolysis of urea to ammonia,this paper starts with studying the mechanism of urea-catalyzed pyrolysis and ammonia formation.The specific parameters of the factors affecting the yield of urea pyrolysis and ammonia production are studied in this paper.Taking the urea-catalyzed pyrolysis furnace with a reasonable flow field and temperature field as the starting point,the optimized design of the designed urea pyrolysis furnace under different parameters was performed using CFD software.The main results are as follows:The study of urea solid pyrolysis shows that urea crystals mainly produce NH₃ and a small amount of CO₂ in the temperature range of room temperature to 300°C,and urea is completely converted to other products at 300°C,and HNCO is mainly generated in the temperature range of 300-400°C.The residue of pyrolysis is mainly nitrogen-containing heterocyclic organic compounds mainly composed of cyanuric acid.At 450°C,the pyrolysis residue is completely reacted.The catalyst TiO₂ can simultaneously shift the pyrolysis of urea and its pyrolisis intermediate product-cyanuric acid to the low temperature side and shorten the reaction process.HNCO and water vapor are prone to react on the surface of the catalyst TiO₂.The kinetics of the first stage of urea pyrolysis shows that the activation energy E of the first stage of pure urea pyrolysis is 113.25 kJ·mol^-1,and the activation energy E is 77.42 kJ·mol^-1,indicating that the catalyst TiO₂ can effectively reduce the activation energy of urea pyrolysis reaction and accelerate the reaction rate of the first step of urea pyrolysis.On a small experimental device,the experimental conditions of catalyzed pyrolysis of ammonia to urea solution were optimized.Finally,the optimum experimental conditions for pyrolysis temperature of 350°C,carrier gas flow rate of1.0L,50%urea solution initial concentration and airspeed of 18000h^-1 were determined.condition.In the activity of single-component catalysts,TiO₂ and ZrO₂ have better catalytic effect,and under the experimental conditions of 300°C or more,the catalytic effects of the two are basically consistent.The catalytic effect of Al₂O₃ is only inferior to that of TiO₂,and the molecular sieves with two different silica-alumina ratios are inferior to Al₂O₃,and SiO₂ has almost no catalytic effect.For the composite catalysts with anatase TiO₂ as the carrier,the activity of the catalysts at the temperature conditions of 200°C and 250°C is:5Cu-TiO₂>5Fe-TiO₂>TiO₂>1V5W-TiO₂>3V5W-TiO₂;Under the temperature conditions above 300°C,the order of the catalyst activity is:TiO₂>5Cu-TiO₂>5Fe-TiO₂>1V5W-TiO₂>3V5W-TiO₂.The mechanical properties of the shaped catalyst are mainly affected by the addition of the molding agent.In the experiment,it was found that the catalyst prepared from the glass fiber as the forming agent has better mechanical properties.While the mechanical properties are improved,the catalyst activity is reduced.The numerical simulation results of the urea pyrolysis furnace show that the six swirl vanes plus the center solid small cylinder and the outer cylinder surface have the best swirl strength and relatively small relative velocity deviation.Reducing the size of the spray gun into the furnace can effectively shorten the height of the evaporation section of the droplets and increase the average temperature of the main section of the urea pyrolysis furnace,which is favorable to the catalytic reaction of the HNCO gas and water vapor.The arrangement of the flow plates has a certain rectifying effect on the pyrolysis furnace,so that the flow field under the flow plates is relatively uniform,and the droplets form a good swirling effect in the furnace.