Preparation Of Nickel-basd Catalysts And Their Application In The Treatment Of Cyano-containing Exhaust Gases

Cyano-containing exhaust gases belonging to unconventional organic pollutants are extremely toxic and carcinogenic.Cyano-containing exhaust gases from carbon fiber production can be consed into tar after cool down.They are usually eliminated after combustion in factories.But problems are still remained such as NOx may be produced and HCN cannot be combusted completely.The catalysts for tar elimination mainly contain transition metal catalysts,natural minerals,zeolites and alkali metals.Nickel-based catalysts have gained more attention for its high activity and low price.Zeolites and natural minerals have been chosen as the support materials of nickel-based catalysts for the catalytic conversion of cyano-containg exhaust gases.The target is to develop one efficient technology for the treatment of cyano-containg exhaust gases.Different kinds of nickel-based catalysts were synthesized and tested in.the steam cracking of model compound benzonitrile.Theses nickel-based catalysts were further checked in conversion of cyano-containg exhaust gases in the lab.The physicochemical properties of catalysts have been characterized and analyzed simultaneously.Some new and valuable results have been got.The composition of cyano-containg exhaust gases from the carbon fiber carbonized under low temperature period was firstly analyzed.The pre-oxidized carbon fiber would discharge hot gases during the carbonization process.The permanent gases include H2,CO,CH4,CO2,HCN and NH3.In the toxic gases,NH3 has the highest content which accounts for 23.8%.HCN has the lowest content which accounts for 1.6%.But the toxicity of HCN is the highest.The composition of tar gases is very complex.The most abundant organic components are chain nitriles,cyano-containing or N-containing cyclic compounds and it also has small amount of alkanes and O-containing organics.In the following study,benzonitrile will be selected as the model compound to discuss for its difficult to be transformed from the point of cracking.The catalysts Ni supported on HZSM-5 were prepared by the volumetric immersion method,which were tested in steam cracking of benzonitrile.The amount of nickel active sites over the catalyst surface played an important role in the reaction activity.Compared with the support of USY,most of nickel was supported outside the holes of HZSM-5 which has smaller cages.It was beneficial to the primary cracking of benzonitrile.What's more,the diameter of channels for HZSM-5 was approaching to the kinetic diameter of chain molecules,which made the secondary cracking reactions fully occurred in the channels.With CeO2 modification,the oxygen mobility and nickel dispersion over the catalyst surface were improved resulting into the increase of efficiency for the benzonitrile cracking.Suitable reaction temperature,S/C ratio and space time are very important to the activity of catalysts and the composition of products.Nickel supported on CaO-MgO was prepared by the precipitation method.Ni and MgO can form the stable solid solution,while CaO promotes the catalyst reducibility.Ni/CaO-MgO shows high benzonitrile conversion rate.When the nickel loading in catalyst reached 15 wt%,Ni/CaO-MgO could catalyze the conversion of benzonitrile and reduce the yield of HCN or NH3 in high efficiency under 550-650 0C.CeO2 modification promoted the nickel dispersion over the catalyst and was especially beneficial to the increase of catalyst activity at low reaction temperature.With 10 wt%CeO2 addition,Ni/10CeO2/CaO-MgO shows good modification efficiency.Increasing the reaction temperature and space time was beneficial to the complete conversion of benzonitrile and reduction of HCN and NH3.When the S/C was greater than 1.2,the ratio of S/C has little effect on benzonitrile conversion,but can influence the composition of final products.Two optimized catalysts 10Ni/5CeO2/HZSM-5 and 15Ni/10CeO2/CaO-MgO were further studied with the probe reaction of steam cracking of benzonitrile.The reaction kinetics and catalyst stability were discussed,respectively.Under the conditions of excluding the internal and external diffusion,by fitting the kinetic data,it satisfied the assumption of first order kinetics well.The activation energies for 10Ni/5CeO2/HZSM-5 and 15Ni/10CeO2/CaO-MgO were 73.68 kJ/mol and 49.72 kJ/mol,respectively.After stability test at low S/C ratio,the catalysts with CeO2 modification show much better performance.The stability of 15NilOCeO2/CaO-MgO for benzonitrile conversion was superior to 10Ni/5CeO2/HZSM-5.CaO could strength the adsorption of O-containing species and the self-redox reaction of CeO2 could release lattice oxygen which improved the oxygen mobility over the catalyst surface.The synergy between CaO and Ce02 promoted the C-containing species over the catalyst surface reacted in time,which made 15Ni/10CeO2/CaO-MgO have excellent anti-coking performance.But,10Ni/5CeO2/HZSM-5 has better mechanical strength which was suitable for the reactor with circulating fluidized bed.15Ni/10CeO2/CaO-MgO has better stability and poor mechanical strength which was suitable for the fixed-bed reactor.10Ni/5CeO2/HZSM-5 and 15Ni/10CeO2/CaO-Mgo were tested with cyano-containing exhaust gases in the lab.The influence of reaction conditions was studied.The two catalysts with CeO2 modification have better catalytic performance for the organic compounds conversion in the cyano-containing exhaust gases.HCN and NH3 in the exhaust gases could be transformed into N2.The gas products mainly include the fuel gases H2,CO and CH4.The concentration of HCN and NH3 in the products could be kept in a low level.Reaction route for the cyano-containing exhaust gases from carbon fiber carbonization reacting over the nickel-based catalysts could be classified into two processes.The first process is that the organic compounds with large molecular weight in the exhaust gases went on primary craking reactions.Large amounts of gas products with small molecular weight including H2,CO,CH4,CO2,C2+,Ccoke,N2,HCN and NH3 would be produced.The second process is that gas products form the primary cracking went on secondary catalytic reactions.More amounts of H2,CO,CH4,CO2 and N2 would be produced.When steam introduced into the reaction system,the ratio of H2/CO in the final gas products could be increased.