Study On The Removal Of NO_x And SO₂ From Flue Gas By A Novel BTF-ABR Integrated System

Coal-fired flue gas typically contains 3-8% O₂ and 100-1100 mg/m³ of NO.After wet scrubbing and other treatments,the temperature of flue gas is typically around 50°C and often contains a certain amount of SO₂.Due to the low solubility of NO in water,the removal of NO from flue gas is extremely challenging.Fe（Ⅱ）EDTA complexation denitrification technology has been emerged as an important method for controlling NOx emissions.However,Fe（Ⅱ）EDTA can be easily oxidized to Fe（Ⅲ）EDTA,which can reduce the ability of the system to remove NO and increase operating costs.This study aims to develop a novel biotrickling filter–anaerobic baffled reactor（BTF–ABR）integrated system to achieve efficient removal of NOx with the simultaneous regeneration of Fe（Ⅱ）EDTA.Firstly,the combined process of BTF and ABR was employed to achieve the enhancement of the removal efficiency of NOx and the regeneration of Fe（Ⅱ）EDTA.The 16S rRNA gene amplicon sequencing and real-time polymerase chain reactions（real-time PCR）were used to elucidate the mechanism of the BTF-ABR integrated system from the molecular biology perspective.Secondly,the NOx removal performance of the system at different chemical oxygen demand（COD）/NO ratios were investigated and the NOx removal mechanism of the system under various COD/NO ratios was further studied as well.Finally,the simultaneous removal performances of NOx and SO₂ in the presence of SO₂ were investigated,and 16S rRNA amplicon gene sequencing combined with metagenome sequencing technologies were conducted to reveal the mechanism of simultaneous denitrification and desulfurization of the system at different COD/NO ratios.The results are summarized as follows:Firstly,a novel BTF-ABR integrated process was developed and the feasibility of the process for simultaneous removal of NOx and regeneration of Fe（Ⅱ）EDTA was demonstrated.The NOx removal effeciency could be maintained at over 90%,with a maximum removal rate of 98.4%,under micro-aerobic conditions at 50±0.5℃.Real-time PCR analysis showed that coordinated expression of denitrification functional genes was the primary reason for no secondary pollution.The 16S rRNA gene amplicon sequencing analysis demonstrated that the cooperation of denitrifying bacteria（Klebsiella,Petrimonas and Rhodococcus）and iron-reducing bacteria（Klebsiella,Geobacter and Petrimonas）as well as the stability of community structure and function in the system was the key to the stable and efficient removal of NOx and the regeneration of Fe（Ⅱ）EDTA simultaneously.Secondly,we investigated the NOx removal performance and stability of the BTF-ABR integrated system at different COD/NO ratios under 3.5%O₂ and 50±0.5℃conditions.The results showed that the minimum operating cost in terms of glucose was 4.79 g of glucose/g of NO.However,a COD/NO ratio of 12.18 led to carbon source waste,while a ratio of 4.63resulted in the emission of approximately 20 mg/m³ of N₂O at the end of the study.Highly bacteria diversity and positive co-occurrence networks at the COD/NO ratio of 6.71 were the main reasons for no secondary pollution.In addition,although the decrease in COD/NO ratio significantly impacted the microbial community structure,the NOx removal effeciency was stabilized at over 90%because the micro-aerobic environment produced by ABR combined highly diverse microbes and functions in BTF,as well as the coordinated expression of denitrifying genes.Thirdly,we investigated the simultaneous removal performance of NOx and SO₂ in the BTF-ABR integrated system at different COD/NO ratios under 500 mg/m³,5%O₂ and 50±0.5℃conditions.The results showed that as the COD/NO ratio decreased,the average removal efficiency of NOx and SO₂ decreased reduced from 99.1%and 100%to 89.8%and 96.6%,respectively,with N₂ and S⁰ as the main products.When the COD/NO ratio was greater than30.87,the release of H₂S and N₂O was detected,and when it was less than 4.63,there was a release of N₂O.The 16S rRNA gene amplicon sequencing and metagenomic sequencing analysis results demonstrated that most of the microorganisms in the system had multiple nitrogen and sulfur metabolic functions.Therefore,the stable microbial structure and diverse nitrogen and sulfur metabolic function bacteria in the BTF-ABR integrated system were the fundamental reasons for maintaining its stable and efficient simultaneous denitrification and desulfurization ability at different COD/NO ratios.In summary,this study demonstrated that efficient,stable,and cost-effective simultaneous removal of NOx and SO₂ could be achieved in the BTF-ABR integrated system.These findings provide a theoretical basis for the industrial application of the BTF-ABR integrated process.