Condensation Recovery Characteristics Of Low Partial Pressure NO₂ In Desorption Gas From Sintering Flue Gas Denitrification

Adsorption separation is one of the important technologies for flue gas denitrification,which can purify and enrich NO_x.NO₂ can be obtained from desorbed gas by condensation to achieve the dual purpose of flue gas purification and pollutant recovery.The condensation process of NO₂ in desorbed gas is affected by the non-condensable gas（NCG）and the dimerization reaction of NO₂.This paper focuses on the condensation and recovery process of NO₂ and the influence of NCG and NO₂ dimerization reaction on condensation heat and mass transfer.Experimental apparatuses for NO₂ condensation recovery and heat transfer were established to study the effects of gas source concentration,pressure and condensation temperature on the condensation recovery effect,as well as the effects of subcooling degree,NO₂ mole fraction and gas velocity on the heat transfer.The results showed that the NO₂ recovery rate increased with the increase of gas source concentration and pressure and the decrease of temperature.The upper pressure and lower temperature limits for NO₂ desorption gas condensation of 1～2%concentration were determined to be 1.5 MPa and-30℃,respectively.The variation laws of increasing condensation heat transfer coefficient（HTC）with the decrease of subcooling degree,the increase of mole fraction and flow rate were obtained.NO₂ condensation first undergoes exothermic gas-phase NO₂ dimerization transition,followed by exothermic condensation phase change.NO₂ condensation visualization experiments were carried out to obtain the evolution of NO₂ condensation modes at different temperatures,mole fractions and flow conditions with and without NCG.The results of quantitative analysis showed that the duration of each period of the condensation process decreased with the increase of mole fraction and flow rate,and the decrease of temperature.The occurrence frequency of streamlet at stabilization increased with the increase of mole fraction,and there were maximum values at-15℃ and 1000 ml·min-1.On this basis,the mechanisms of NO₂ dimerization and NCG effects on three typical condensation modes of fog,droplet-streamlet-filmlet coexistence（DSF）and freezing are elucidated:the dimerization reaction triggers the rapid formation of N2O4 fog with increased intermolecular forces,leads to the inhomogeneity of heat mass transfer in the DSF mode with complex thermal resistance network and condensation state,and induces the melting of NO₂ ice layer during the freezing process-thickening cycle with the release of dimerization heat.The presence of NCG leads to irregular shape of the fog,greater condensate diversity in the DSF mode,inhomogeneous initial ice layer in the freezing mode and thickening process.Further numerical simulations for high-pressure condensation of low partial NO₂ in a circular tube were carried out to obtain the results of flow,temperature and concentration fields under typical working conditions.The exothermic NO₂ dimerization reaction increased the gas-phase temperature in the inlet region,and the reverse radial velocity away from the wall appeared.The condensates on the wall were mainly stream-like and film-like,and the low concentration of NO₂ in the upper part of the gas phase region was condensed in a fog mode.The results showed that the condenser can withstand a wide range of flow rates and NO₂ mole fractions.The first 1.0 m of the condensing section played the main condensation role and the main condensation position moved up when the flow rate and NO₂ mole fraction increased.Based on the experimental law of recovery and heat transfer and the NO₂ condensation gas-liquid phase transition law,a skid-module for NO₂ recovery from desorption gas of～50,000 m3/h sinter flue gas denitrification was built in Handan Iron and Steel Group.According to the compression,cooling and total energy consumption per unit mass of NO₂ product,the operating temperature and pressure of the skid-mounted with optimal energy efficiency were determined to be 1.1 MPa and-25℃.The results showed that the condensation recovery of liquid NO₂ products with purity greater than 99%from NO₂ desorption gas of sintered flue gas was successfully achieved.The overall heat transfer coefficient of the condenser was 70-170 W/m2·K,and the local condensation heat transfer coefficient ranged from 200-1000 W/m2·K.The energy consumption analysis of the process operation was carried out.The energy consumption per ton of liquid NO₂ was obtained as 3293.28 kW·h,of which the combined energy consumption ratio of raw gas compressor energy consumption to refrigeration equipment energy consumption was about 8.5:1.The process economic analysis showed that the recovery of NO₂ from flue gas denitrification desorption gas has good economic benefits.The research results of the paper can provide a technical reference for the purification and recovery of pollutants in NO_x-containing flue gas,and is expected to provide a new way for flue gas pollutant treatment and resource recovery,which has important scientific and application significance.