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Chemical Absorption-microbial Reduction for the Removal of NOx from Flue Gas

Author: JiangJinLin
Tutor: LiWei
School: Zhejiang University
Course: Environmental Engineering
Keywords: flue gas denitrification chemical absorption microbial reduction ferrous chelates Fe(II)Cit NO_x
CLC: X701
Type: Master's thesis
Year: 2008
Downloads: 181
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Abstract

The combustion of fossil fuels generates NO_x pollutants which cause air pollution and acid rain. It is reported that the NO_x pollutants in China is seriously. The development of the technologies for NO_x removal from flue gas is a problem we must to resolve. So it stares us in the face to develop a new technology for NO_x removal with the characteristics of low cost, completely reduction of NO and high removal efficiency.The newly proposed and adopted method in our work for NO_x removal from flue gas, i.e. chemical absorption-biological reduction integrated process, is deemed as a promising method. As a part of this work, in this paper, the influence factors of NO absorption capacity by ferrous citrate （Fe（Ⅱ）Cit） solution was studied in an absorber with a sieve plate, and a comparison was made between NO absorption by Fe（Ⅱ）Cit and Fe（Ⅱ）EDTA, Fe（Ⅱ）（CyS）₂. Selected and bred a strain Enterococcus sp. FR-3 which had a high active Fe（HI）Cit reduction ability under the flask conditions and 40℃,.and a model of the microbial growth as function of the consumption ofsubstrate was established. The character and the Fe（Ⅱ）Cit-NO reduction ability of strain Pseudomonas sp. DN-2 was studied. The aim of this work was to provide a new method and some fundamental data for NO_x removal from flue gas. The main experimental results were as follows:The NO absorption capacity by ferrous chelate ligants solutions was studied in an absorber with a sieve plate. The effects of the kinds of ferrous chelates, the molar rate of Fe（Ⅱ）/Cit, the pH value, the temperature, the concentration of O₂ in the simulated flue gas, and the concentration of SO₃^2- were examined. The results showed that the NO absorption capacity of Fe（Ⅱ）Cit was much better than that of ferrous oxalate, ferrous lactate and ferrous gluconate, but poorer than that of Fe（Ⅱ）EDTA and Fe（Ⅱ）（Cys）₂. Citrate had a powerful pH buffer capacity. Higher absorption amounts of NO could be achieved when Fe（Ⅱ）/Cit was kept at 1:2, pH was between 7～7.5 orlower temperature. 5% O₂ in the simulated flue gas reduced the absorption amount of NO by 82%. With the increase of SO₃^2- in the solution, the absorption capacity would be improved. When SO₃^2-/Fe（Ⅱ）=5, the absorption amounts increased by 2.3 times than that without SO₃^2-.Strain Enterococcus sp. FR-3 could reduce Fe（Ⅲ）Cit in absorption solution efficiently with adding of carbon sources and nitrogen sources. Among the carbon sources and nitrogen sources investigated, glucose and NH₄Cl were suitable for the strain FR-3 growth reduction of Fe（Ⅲ）Cit, the optimal adding amounts was 1000 mg·L^-1 and 100 mg·L^-1, respectively. The bio-reduction could be achieved efficiently with 150 mg·L^-1 cells inoculation and reduction rate did not increase with adding more amount of cells inoculation. The optimal reduction temperature and pH range were 35～40℃and 6.6～7, respectively; for strain FR-3 growth, which were 30～35℃and 6～7, respectively. There was no obvious inhibiton on strain FR-3 when Fe（Ⅲ）Cit concentration was as high as 40 mmol·L^-1, but the inhibiton phenomenon existed when there was a certain amount of NO₂^- or SO₃^2- in the absorption solution, especially there esisted SO₃^2-. When the concentration of NO₂^- or SO₃^2- was small than 4 mmol·L^-1 or 2 mmol·L^-1, respectively, the inhibition was not obvious. SO₄^2- had no inhibiton on strain FR-3 in this experiment.The models of cells concentration （X） as function of Fe（Ⅲ）Cit （S） and cells concentration （X） as function of time （t） based on Logistic equation could describle the relations between strain FR-3 growth and reduction of Fe（Ⅲ）Cit very well, and which also helpful to the study of that between strain FR-3 growth and time. The production of Fe（Ⅱ）Cit was controlled by mixed mechanism because the non-growth coefficient Strain Pseudomonas sp DN-2 could reduce Fe（Ⅱ）Cit-NO in absorption solution efficiently, yet the microbial growth amounts were very small. Glucose was suitable for bio-reduction of Fe（Ⅱ）Cit-NO, the optimal glucose adding amounts or cells inoculation were 500 mg·L^-1 or 150 mg·L^-1, respectively, and reduction rate did not increase with adding more amounts of glucose or cells inoculation. The optimal reduction temperature and pH range were 40～45℃and 6.75～7.1, respectively. Using of FeSO₄ or FeCl₂ agents would bring SO₄^2- or Cl^- into solution, each ion had no obvious influence on the growth of strain DN-2 and the bio-reduction of Fe（Ⅱ）Cit-NO. Comparatively, the sample using FeSO₄ had higher reduction rate than the one using FeCl₂ under the same experimental conditions. Strain DN-2 could reduce Fe(ⅢCit, but the reduction rate was only 20.5%. NO₂^- would inhibit the bio-reduction of Fe（Ⅲ）Cit because it could act as competitive electron acceptor of Fe（Ⅲ）Cit.

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