NO Removal From Flue Gas By Cobalt(Ⅱ)-Histidine Complex Chemical Absorption-Biological Reduction Integrated Approach

Increasing fuel demand has led to a continuous increase in NO_x emissions from flue gas with the development of economy,and NO_x pollution is becoming increasingly serious.Flue gas denitrification is an important way to control NO_x emission.The chemical absorption-biological reduction（CABR）flue gas denitrification technology,combining the advantages of high absorption rate from complexation absorption approach,and economical and secondary-pollution-free from biological approach,has gathered wide attention.However,the O₂resistance of the ferrous absorbent of CABR technology is low,leading to declining NO removal efficiency under high O₂ content of flue gas.In order to improve the O₂ resistance of CABR technology,a cobalt（Ⅱ）complex chemical absorption-biological reduction（CABR-Co（Ⅱ）L）integrated approach for NO_x removal from flue gas is proposed.In CABR-Co（Ⅱ）L approach,the ferrous absorbent in the chemical absorption process of CABR technology is replaced by Co（Ⅱ）L,which has the ability to absorb NO and is more resistant to O₂.In this paper,the suitable NO absorbent cobalt（Ⅱ）-histidine（Co（Ⅱ）His）complex was screened out first,and then the Co（Ⅱ）His absorbent was studied from two aspects as chemical absorption and biological reduction of Co（Ⅱ）His chemical absorption-biological reduction（CABR-Co（Ⅱ）His）integrated system.Finally,the performance of NO removal from simulated flue gas by CABR-Co（Ⅱ）His system was explored preliminarily.The following main research results have been obtained:（1）Screening out suitable NO absorbent is the first task for establishing the CABR-Co（Ⅱ）L integrated system for NO removal from flue gas.In this study,the absorbent Co（Ⅱ）His with strong stability and NO absorption capacity was screened out by examining the stability and NO absorption capacity of the Co（Ⅱ）L containing mainly amino acid cobalt（Ⅱ）at a solution p H of about 7.It was found that the O₂ resistance of Co（Ⅱ）His was significantly stronger than that of Fe（Ⅱ）EDTA by investigating the oxidation process of Co（Ⅱ）His and Fe（Ⅱ）EDTA.The effect of Co（Ⅱ）His absorbent on the subsequent aerobic denitrification process of the denitrification products NO₃^-and NO₂^-was investigated then.Results showed that histidine（His）contained in Co（Ⅱ）His absorbent could promote the growth and aerobic denitrification process of aerobic denitrification bacteria Paracoccus versutus LYM.Dihistidine cobalt（Ⅱ）ion(Co（His）₂²⁺)could significantly promote the growth of the strain LYM while caused the lag of NO₃^-reduction process.Hexahydrate cobalt（Ⅱ）ion(Co（H₂O）₆²⁺)could inhibit the growth and aerobic denitrification of the strain LYM significantly.The concentration of free Co（H₂O）₆²⁺in Co（Ⅱ）His absorbent could be significantly reduced by increasing the initial molar ratio of His to Co（Ⅱ）（His/Co（Ⅱ））from 2 to 4-6,which could eliminate the inhibitory effect of Co（Ⅱ）His absorbent on the strain LYM.The strain LYM could tolerate 20 m M Co（Ⅱ）His absorbent when His/Co（Ⅱ）was 4.In addition,strain LYM had the ability to reduce Co（Ⅲ）His under aerobic condition.（2）NO absorption into Co（Ⅱ）His absorbent is an important link of CABR-Co（Ⅱ）His integrated system.Firstly,the reaction characteristics of Co（Ⅱ）His with NO were investigated.Results showed that the reaction process of Co（Ⅱ）His with NO could be promoted by decreasing the temperature of Co（Ⅱ）His solution from 30-35 to 5-10 ^oC,increasing the concentration from3 to 10 m M,or increasing the p H from 7.2 to 9.4.Then,the process characteristics of NO removal from simulated flue gas by Co（Ⅱ）His chemical absorption approach were investigated.Results showed that Co（Ⅱ）His could promote NO conversion to NO₃^-under aerobic condition.The NO removal efficiency of packed column in the presence of O₂ or bubble packed column in the absence of O₂ was higher than that of packed column in the absence of O₂ or bubble packed column in the presence of O₂ when Co（Ⅱ）His concentration was 5-20 m M.NO removal efficiency of Co（Ⅱ）His chemical absorption approach in the packed column reached 77.4-77.9%with 10 m M Co（Ⅱ）His concentration,2 L min^-1 simulated flue gas flow rate,208 mg m^-3 inlet NO concentration,10%O₂ content.（3）The removal of denitrification products NO₃^-and NO₂^-and the reduction of Co（Ⅲ）His need to be achieved by biological reduction process under aerobic condition in CABR-Co（Ⅱ）His integrated system to prevent secondary pollution and to regenerate Co（Ⅱ）His absorbent.Paracoccus versutus LYM with the capacities of aerobic denitrification and Co（Ⅲ）His reduction was utilized.For the one aspect,the process characteristics of the simultaneous reduction of 10 m M NO₃^-and 10 m M Co（Ⅲ）His under aerobic condition were investigated.Results showed that NO₃^-and the accumulated denitrification intermediate product NO₂^-could be reduced rapidly by the strain LYM.The concentration of Co（Ⅱ）in the solution was determined by the chemical oxidation process of Co（Ⅱ）His and the biological reduction process of Co（Ⅲ）His,which were both affected by O₂ supply.When O₂ was supplied with high-to-low time order,the maximum apparent Co（Ⅱ）formation efficiency and formation rate were 65.4%and 0.40 m M h^-1 respectively.Co（Ⅲ）His reduction process could be promoted by increasing the initial biomass or the nutrient concentration,and adding nutrients regularly.For the other aspect,the effect of NO₃^-on Co（Ⅲ）His reduction process under aerobic condition was investigated.Results showed that the Co（Ⅲ）His reduction process of the strain LYM was promoted by 5-20 m M NO₃^-.The Co（Ⅲ）His reduction performance of the strain LYM co-acclimated by NO₃^-and Co（Ⅲ）His was better than that of the strain LYM acclimated only by Co（Ⅲ）His under the conditions of no NO₃^-and 10 m M NO₃^-.The Co（Ⅲ）His reduction process of the strain LYM acclimated only by Co（Ⅲ）His was promoted by 10 m M NO₃^-.The Co（Ⅲ）His reduction process of the strain LYM co-acclimated by NO₃^-and Co（Ⅲ）His was firstly inhibited and then promoted by 10 m M NO₃^-.（4）The NO removal capacity from simulated flue gas by CABR-Co（Ⅱ）His integrated system was investigated in a biotrickling filter which was inoculated with Paracoccus versutus LYM.Results showed that NO removal efficiency was 22.9-45.5%when simulated flue gas flow rate was 2 L min^-1,O₂ content was 10%,Co（Ⅱ）His concentration was 10 m M.During NO removal process,Co（Ⅱ）concentration increased and there was no accumulation of NO₃^-or NO₂^-in the absorption liquid.In order to improve the absorption performance of Co（Ⅱ）His absorbent regarding NO,a packed column was added before the biotrickling filter,and NO removal experiment was carried out by using the series device of packed column and biotrickling filter.Results showed that under the conditions of 1 L min^-1 simulated flue gas flow rate,3%O₂content,and 10 m M Co（Ⅱ）His concentration,NO removal efficiency of CABR-Co（Ⅱ）His integrated system maintained at 75.2-86.2%during 24 h continuous operation.However,NO removal efficiency of biological reduction approach without Co（Ⅱ）His decreased from 76.3%to 54.2%after 7.5 h.