A Study On Nitrogen Removal Of Various Compounds During Supercritical Water Oxidation

Supercritical water oxidation?SCWO?has been considered as a greatly promising process,due to fast reaction rate,short time consumption,good airtightness,high removal rate and little secondary pollution.Up to the present,data is still sparse on the distribution of N-containing products,transformation and denitrification mechanism during SCWO process.In this work,lots of N-containing compounds were selected to better systematically investigate N conversion mechanism in SCWO process.Density functional theory?DFT?method was used to calculate the quantum properties of the compounds in order to further explore the relationship between TN removal and molecular structural characteristics from the aspect of quantization parameter.The main contents and conclusions of this work are as follows:?1?Denitrification mechanism of nitrogen containing inorganics?NO₃^-/NH₄⁺?during SCWO.The result showed that NH₄⁺could react with·OH and formed·NH₂,·NH and HNO:.Among these intermediates,·NH and HNO:could further react each other and generate N₂.However,due to structural stability of NH₄⁺,few NH₄⁺could be oxidized into·NH₂ by·OH,which resulted in unsuccessful denitrification during SCWO process?removal efficiency<15%?.In terms of NO₃^-,it could be converted into NO₂^-and O₂ via pyrolysis.NO₂^-could be reduced into NO and N₂ by active H formed during hydrothermal reaction.However,the presence of·OH would lead to little formation NO₂^-of from NO₃^-?removal efficiency<10%?.As for NO₃^-/NH₄⁺,they could generate·NO₂,NO₂^-,·NH₂ and·NH during SCWO.·NO₂ and·NH₂ could easily react and form N₂.The experimental result showed that NO₃^-/NH₄⁺,with a ratio of 1:1,achieved 21.2%of TN removal.Isopropyl alcohol,as a co-fuel,could also play a positive role on TN removal,because the existence of heating value as well as C and H atoms could facilitate the reduction of NO_x^-into N₂.?2?Denitrification mechanism of different nitrogen containing organics during SCWO.After 350-550 ^oC and 0.5-6 min,TN removal efficiencies were 43.7⁹3.3%.The average TN removal rates showed an order of diazo group compounds?84.4%?>amino ones?61.9%?>nitro ones?60.6%?>N-heterocyclic ones?57.8%?.The result of N-containing products showed that org-N could be rapidly decomposed to inorg-N during SCWO.As for amino and N-heterocyclic group compounds,they produced much NH₄⁺and few NO₃^-.While for nitro-group compounds,NO₃^-was the predominant and NH₄⁺was little.The amounts of NH₄⁺and NO₃^-changed little in general,the both could partly convert into N₂,which resulted in unsuccessful denitrification.Therefore,the decline of org-N made a main contribution on TN removal during SCWO process.?3?Denitrification mechanism of 2-,3-and 4-nitroaniline?2-,3-and 4-NA?during SCWO.TN removal presented a general trend that 2-NA>3-NA>4-NA,which indicated that the TN removal behavior between amino and nitro groups would prefer to happen in the molecule.Furthermore,the smaller interval between the amino and nitro group was the more easily to interreact.Density functional theory?DFT?method was used to calculate the molecular structures of 2-,3-and 4-NA to further explore their conceivable reaction pathways.It showed that.OH could attack the amino and nitro groups,and formed the active amino and nitro sites,respectively,which resulted that the both could react with each other,and generated N₂.Then,.OH would attack the benzene ring,which resulted in ring-open.At last,all the intermediates would be mineralized into H₂O,N₂,CO₂ and few salts.According to the result of aniline/nitrobenzene?1:1,by mol?,the mixture obtained more TN removal than the average of the both initial TN removal.as a result,it might exist synchronous denitrification behavior between the nitro and amino/heterocyclic compounds,which lead to a higher TN removal after mixing.?4?Synchronous denitrification mechanism between nitro and amino/heterocyclic compounds?1:1,by mol?during SCWO.The results showed that TN removal of the nitro and amino/heterocyclic compounds?66.7%?was higher than that of the average between the both initial TN removal?58.1%?.It indicated that the synchronous denitrification behavior did exist between the nitro and amino/heterocyclic compounds.According to the result that the org-N could be rapidly decomposed to inorg-N,as well as unsuccessful denitrification between NO₃^-/NH₄⁺during SCWO,the overall reaction process was proposed,including the formation of active amino and nitro groups at the early stage of SCWO,as well as the limiting step of N₂ in the late stage of SCWO.Based on the equilibrium hypothesis,the TN removal rate of mixture(k_MIX-TN)was related to that of nitro and amino/heterocyclic compounds?kNO2-TN or kNH2-TN?.It presented that kMIX-TN=4.2608kNO2-TNkNH2-TN,with 0.7910 of R²,as well as kMIX-TOC=2.6745kNO2-TOCkNH2-TOC,with 0.8274 of R².It implied that the equilibrium hypothesis was credible.The result of calculation based on quantum descriptor showed that BO_x,E_HOMO,E_LUMOUMO and E_gapap could greatly affect the synchronous denitrification effect,which meant the mixture,containing similar values of BOx,EHOMO,ELUMO and Egap,would result in a higher kMIX-TN value.?5?Quantitative-Structure-Activity-Relationship?QSAR?models for the degradation rate and temperature of nitrogenous organic compounds in SCWO.The QSAR models have been built based on the degradation rate of TN(k _TN)and the temperature for TN removal efficiency of 50%(T_N50),as well as 19 kinds of quantum parameters.Model 4 and Model 8,lgk_TN=-1.279-3.371f?-?_n–1.608E_gap-0.097q?N?+0.471BO_x and T_N50=483+647f?-?_n+318E_gap+28q?N?–68BO_x,respectively,presented were stable,robust and accurate.A three factor theory was proposed based on the optimal models,which was the selectivity of active site?f?-?_n?,the transfer of electrons(E_gap),and the breaking of chemical bond?BO_x?.More values of f?-?_n and E_gap,as well as less value of BO_x could lead to the target compound was easily to be attacked during SCWO,which resulted in a alump of TOC.When the amount of TOC tended to none,lack of heating value,it was hard to facilitate the conversion of N₂from NH₄⁺and NO₃^-.As a result,It would lead to a lower TN degradation rate(k_TN)and a higher temperature of 50%TN removal(T_N50).Based on the aboved results,we found that the denitrification behaviour of nitro and amino group prefered to happen in the molecule rather than occur between two molecules,and the smaller interval between the amino and nitro group was the more easily to interreact.Synergetic denitrification effect was found in SCWO of nitro and amino containing compounds.BO_x,E_LUMO,E_HOMO and E_gap played significant role in denitrification during SCWO process.