Study On The Performance Of Adjusting NO_x Oxidation Ratio Cooperated With Chemical Absorption-biological Reduction Method To Remove NO_x From Flue Gas

At present,fossil fuels account for the majority of the world's energy supply.In China,nearly three-quarters of total energy demand is supplied from coal resources.Flue gas from coal combustion usually contains large concentrations of nitrogen oxides(NO_x).Excessive emission of NO_x will cause serious air pollution phenomena,such as photochemical smog,acid rain,and ozone layer destruction,and will seriously threaten the health of the human respiratory system.Therefore,controlling NO_x emissions from coal-fired flue gas is an effective way to reduce air pollution.In recent years,the biological reduction method has attracted much attention in the field of NO_x treatment due to its advantages of low investment and operation cost and no secondary pollution.Its core is to reduce NO_x to non-toxic and harmless N₂ under the action of microorganisms.However,its industrial application is limited by two critical issues:anaerobic conditions are difficult to achieve and NO mass transfer efficiency is low.To solve both problems,this paper proposed a new technology of adjusting the NO_x oxidation ratio and cooperating with the chemical absorption–biological reduction method(CABR)for NO_xremoval from flue gas.The CABR system was established based on selecting excellent aerobic denitrifying bacteria.The effects of different NO_x oxidation ratios,absorbent types,and concentrations on the denitration efficiency and products of the CABR system were investigated,and the optimal NO_x oxidation ratio and chemical absorption system was determined.Moreover,the NO_x removal characteristics of the CABR system under the optimal NO_x oxidation ratio were studied emphatically:The effects of different operating conditions on NO_x removal performance were surveyed to identify the critical parameters for ensuring stable operation of the integrated system;High-throughput sequencing technology was used to analyze the variation of microbial community structure in the biotrickling filter under different operating parameters;The existing forms of NO_x in the gas phase,liquid phase and biological phase,and the aerobic denitrifying pathways of microbe were investigated to clarify the NO_x removal mechanism.The research results will provide an environmentally friendly reference method for NO_x treatment,and also provide an important theoretical reference for realizing the industrial application of the technology.The main research contents and conclusions of this article are as follows:(1)In this study,a microbial consortium with efficient aerobic denitrification ability was selected from wastewater of sewage treatment plant and named HN-04.The aerobic denitrification characteristics of HN-04 were studied using^-–N and^-–N as single nitrogen sources.The results showed that the degradation efficiency of^-–N and^-–N reached 100%after 24 h of incubation,and the maximum removal rates were 14.5 mg/L/h and9.33 mg/L/h,respectively.The microbial community structure of HN-04 was analyzed by high-throughput technology,and it was found that the proportion of Pseudomonas and Azomonas was higher,which reached 34.41%and 8.06%,respectively,except for the unclassified bacteria genus.These two bacteria were the common types of aerobic denitrifying bacteria.(2)In the shaking flask experiment,the denitrification performance of the HN-04 was optimized by a single factor experiment.The results showed that the optimal^-–N removal conditions of the HN-04 were sodium citrate as carbon source,C/N ratio of 12,p H of 7,and temperature of 30?.And HN-04 could tolerate a high concentration of^-–N.The microbial consortium HN-04 was inoculated into the biotrickling filter for biofilm formation under the optimal nitrogen removal conditions,and then simulated flue gas was introduced to investigate the effects of different NO_x oxidation ratios and chemical absorbent on the denitration efficiency of the system.The results showed that when the NO_x oxidation ratio in the flue gas was adjusted to 50%and the 0.02 mol/L Na HCO₃ was selected as the absorbent,the system can maintain a high NO_x removal efficiency,and a stable operation CABR system was established on this basis.(3)In NO_x oxidation ratio was 50%,the absorbent was 0.02 mol/L Na HCO₃ conditions,the effects of different operation parameters,such as temperature,oxygen concentration,NO_xconcentration,gas flow rate,SO₂ concentration,the concentration of^-–N and^-–N in absorption solution,absorption liquid replacement rate,absorption liquid flow rate and the amount of carbon source added in the fresh absorption liquid,on NO_x removal of CABR system were investigated.The results showed that the CABR system can effectively remove more than 98%of NO_x at a high temperature of 50?.The denitration efficiency of the system exceeds 94%in the range of oxygen concentration of 0-10 vol%with the _xconcentration was 500 ppm.When the inlet NO_x concentration rose to 800 ppm,the NO_xremoval efficiency of the system was higher than 98%.The gas flow rate higher than 1.5L/min hurt the denitration efficiency of the system.And the concentration of SO₂ in the flue gas was up to 800 ppm,the denitration efficiency of the system will not be affected.The addition of 10–200 mg/L^-–N and^-–N into the absorption solution will not affect the NO_x removal effect of the system and the denitrification performance of microorganisms.Replacing 50%of the absorption liquid every day can meet both the requirements for efficient removal of NO_x and the number of electron donors needed for microbial growth.The removal efficiency of NO_x can be maintained above 99%in the absorption liquid flow rate range of0-50 L/h.When the amount of carbon source added in the daily replacement absorption solution is 3.0 g/L,it can not only maintain the efficient removal of NO_x in the gas phase and liquid phase but also avoid the accumulation of chemical oxygen demand(COD)in the liquid phase.(4)The microbial community structure in the biofilm of the reactor under different operating parameters was analyzed by high-throughput sequencing.The results showed that the richness and diversity of the microbial community in the reactor at 50? were significantly increased compared with that in the 40? shake flask experiment,and the dominant bacteria included Chelatococcus?Pannonibacter?Pseudomonas and Alishewanella.At different oxygen concentrations,Proteobacteria was the dominant bacteria phylum in all samples,and the dominant genus was Chelatococcus.Changing the inlet NO_x concentration in the flue gas,the dominant bacteria phylum in the reactor remained Proteobacteria,while the dominant genus changed to Pseudomonas.The dominant strains were all aerobic denitrifying bacteria under different operating parameters.(5)The conversion path of NO_x in the CABR system was analyzed according to the enzyme activity test,the amount of intermediate product N₂O,and the existing form of N element in the final gas phase,liquid phase and biological phase.And under the condition of stable operation of the system,the total nitrogen in the gas phase,liquid phase,and biological phase in the process of NO_x purification by CABR system within 24 h was statistically calculated.It was found that about 59%of the nitrogen in the inlet NO_x was discharged in the form of N₂ through microbial denitrification,37%of the nitrogen was converted into biomass nitrogen for microbial growth,and only 1.1%of the nitrogen was accumulated in the liquid phase.The results indicate that the process of NO_x removal in the CABR system mainly relies on microbial denitrification and assimilation.