A New Approach for NOx Removal from Flue Gas Using a Biofilm

Apr 28, 2014 - The contribution of H2 and glucose in reduction of Fe(III)EDTA was .... increase of the O2 concentration from 0 to 10% in the simulated...
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A New Approach for NOx Removal from Flue Gas Using a Biofilm Electrode Reactor Coupled with Chemical Absorption Yinfeng Xia,†,‡ Yun Shi,†,‡ Ya Zhou,§ Nan Liu,† Wei Li,*,†,‡ and Sujing Li*,† †

Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Institute of Industrial Ecology and Environment, Department of Chemical and Biological Engineering, Zhejiang University (Yuquan Campus), Hangzhou 310027, People’s Republic of China ‡ Institute of Environmental Engineering, Zhejiang University (Zijingang Campus), Hangzhou 310058, People’s Republic of China § Taihu Basin Authority of Ministry of Water Resources, Shanghai 200434, People’s Republic of China ABSTRACT: A chemical absorption−biofilm electrode reactor (CABER) integrated system was used for removal of nitrogen monoxide (NO) from flue gas. Effects of the electric current on NO removal efficiency, concentration of Fe(II)EDTA, and consumption rate of glucose in the stabilization phase were investigated. Results indicate that the optimum impressed current was 0.04 A [i.e., 66.7 A m−3 net cathodic compartment (NCC) of the current density]. Under this condition, the consumption rate of glucose was 0.462 g h−1. Performance evaluation of this new approach was investigated under optimum conditions as well. It is noted that minimum residence time was only 20 s, maximum oxygen tolerability was 10%, and maximum elimination capacity of NO was 104.2 g of NO m−3 h−1. The contribution of H2 and glucose in reduction of Fe(III)EDTA was also studied. The results indicated that increasing the H2 supply appropriately could reduce the consumption of glucose. This new approach showed a better performance on NO removal and a larger processing load than those of the chemical absorption−biological reduction (CABR) integrated system.

1. INTRODUCTION Nitrogen oxides (NOx) are one of the main air pollutants because of their harmful impacts on human health and the environment. Gaseous NOx have several deleterious effects on human health.1 Currently, conventional post-combustion controls include selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), absorption, and adsorption methods.2 However, these conventional methods suffer from high cost and the risk of second pollution.3 To overcome these drawbacks, researchers proposed several new technologies, such as chemical absorption4 and bioreactors.5,6 One promising technology for NOx removal from flue gas is a chemical absorption−biological reduction (CABR) integrated system.7−9 As the name implies, absorption and biological reduction are two main parts in this approach. The main reactions involved in these two parts are as follows:10−13 Fe(II)EDTA + NO(aq) ⇔ Fe(II)EDTA−NO

12Fe(II)EDTA−NO2 − + C6H12O6 microorganism

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 12Fe(II)EDTA2 − + 6N2 + 6H 2O + 6CO2 (5)

2Fe(II)EDTA−NO2 − + 2Fe(II)EDTA2 − + 4H+ microorganism

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 4Fe(III)EDTA− + N2 + 2H 2O

(6)

Although this approach improves the limitation of the absorption rate and regeneration of the absorbent, it was far from industrial application because of its long gas residence time, low removal capacity of NO, and poor tolerability of oxygen.8 Hence, a biofilm electrode reactor (BER) was introduced to enhance the process of bioreduction.14 It is expected that H2 generated from the cathode could be used by microorganisms as an electron donor and CO2 yielded from the anode could serve as a carbon source of the autotrophic microorganism as well as a pH buffer. The electrode reactions were listed as follows:15

(1)

anodic reaction 4Fe(II)EDTA2 − + O2 (aq) + 4H+ −

→ 4Fe(III)EDTA + 2H 2O

C + 2H 2O → CO2 + 4H+ + 4e− (2)

cathodic reactions

24Fe(III)EDTA− + C6H12O6 + 24OH− microorganism

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 24Fe(II)EDTA2 − + 18H 2O + 6CO2

(3)

2Fe(III)EDTA− + H 2 + 2OH− microorganism

2−

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2Fe(II)EDTA + 2H 2O © 2014 American Chemical Society

(7)

2H 2O + 2e− → H 2 + 2OH−

(8)

1 O2 + 2e− + H 2O → 2OH− 2

(9)

Received: March 18, 2014 Revised: April 26, 2014 Published: April 28, 2014

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Figure 1. Configuration of the chemical absorption−biofilm electrode reactor (CABER) integrated system: (1) mass flow controller, (2) mixing chamber, (3) NOx analyzer, (4) sieve-plate tower, (5) temperature controller, (6) biofilm electrode reactor, (7) thermostatic water bath, (8) submersible pump, (9) rotameter, and (10) magnet pump. Escherichia coli strain FR-2,10 were mix-cultivated to form biofilm. The basal medium consisted of the following compositions (per liter): glucose (1000 mg), KH2PO4 (300 mg), Na2SO3 (70 mg), MgCl2 (100 mg), CaCl2 (20 mg), NaHCO3 (5400 mg), and trace elements (2 mL). The trace elements for the bacteria growth include the following components (per liter): CoCl2 (240 mg), MnCl2·4H2O (990 mg), CuSO4·5H2O (250 mg), Na2MoO4·2H2O (220 mg), NiCl2·6H2O (190 mg), H3BO4 (14 mg), and ZnCl2 (100 mg). 2.3. Reactor Configuration. A schematic diagram of the continuous apparatus is shown in Figure 1. A sieve-plate column with an inner diameter of 0.04 m and an effective volume of 0.57 L was used to absorb the simulated flue gas. A BER with an inner diameter of 0.12 m and an effective volume of 1.2 L was used to bioreduce Fe(II)EDTA−NO and Fe(III)EDTA. One graphite rod was installed along the central axis of the BER as the anode, and four others that surrounded it were used as cathodes. The remaining space in the cathode area was filled with graphite granules (6 mm in diameter and 8 mm in length), which serves not only as a biofilm carrier but also as the third electrode. A direct current (DC) power with a precision of 1 mA (QJ2002A, Shanghai Yice Electronic Technology Co., China) was used to provide a constant current. A peristaltic pump (BT00-100M, Baoding Gelan Constant Flow Pump Co., China) was used to inject glucose for microorganism growth. The simulated flue gas consisted of N2, O2, CO2, and NO, which were premixed in a mixing chamber after metering using mass flow meters (Beijing Sevenstar Qualiflow Electronic Equipment Manufacturing Co., Ltd., model D07-12A). Gas sampling points were located at the inlet and outlet of the sieve-plate column, and the liquid sampling points were located in the holding tank (2 L). The gas flow rate was controlled at 1000 mL min−1. A magnet pump was used to continuously introduce the solution into the reactor at a flow rate of 10 L h−1. The holding tank was placed in the thermostatic water bath with a temperature of 50 °C. In addition, water was pumped into the heating jacket of BER by a submersible pump to simulate a typical flue gas temperature (45−55 °C) after the flue gas desulfurization (FGD) process. The pH of the scrubber liquor was kept at the range of 6.2− 6.8 with the help of pH buffer (NaHCO3). 2.4. BER Startup. To start up the BER, a solution containing 5 mM Fe(III)EDTA, basal medium, trace nutrients, and mixed culture was fed into the reactor with a pH of 6.8. A total of 300 mL of fresh Fe(III)EDTA medium was added daily to the reactor with the

Previous experimental results showed that the bioreduction process was enhanced using this approach.14,16 Nevertheless, the removal efficiency of conventional BER is not ideal because of the limited cathode surface area. A threedimensional biofilm electrode reactor (3D-BER) was applied to increase the cathode surface area.13 Graphite granules were used to fill the cathodic compartment, which was regarded as the third electrode. The expanded cathodic compartment provides more cathode surface area for electrolysis and biofilm formation. Experimental results demonstrated that use of the 3D-BER significantly promotes the bioreduction rate of Fe(II)EDTA−NO and Fe(III)EDTA simultaneously because of the increase of biomass and the presence of H2 as a new electron donor. On the basis of the pilot studies, this work was to set up a bench-scale integrated system, optimize the key parameters, evaluate the performance of this new approach, and compare the performance between this new approach and the CABR integrated system. The consumption rate of glucose was innovatively investigated. The contribution of different electron donors in reduction of Fe(III)EDTA was also studied for the first time. The aim of this study was to provide theoretical foundation and basic data for the industrial application of this new approach.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Na2EDTA (99.95%), FeSO4(NH4)2SO4·6H2O (99.5%), FeCl3·6H2O (99.5%), and D-glucose (99.5%, cell culture tested) were obtained from Shanghai Chemical Reagent Co., China. NO (5% in N2, v/v) and N2 (99.999%) were purchased from Zhejiang Jingong Gas Co., China. Graphite electrode (spectral grade) was obtained from Shanghai New Graphite Material Co., China. All other chemicals were analytical-grade, commercially available, and used without further purification. Fe(II)EDTA−NO and Fe(III)EDTA complexes were prepared according to the procedure in the previous study.10 2.2. Microorganism Cultivation and Media Composition. Two kinds of microorganisms, Pseudomonas sp. strain DN-211 and 3333

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Figure 2. NO removal efficiency and concentration of Fe(II) during the startup of the integrated system: [Fe(III)EDTA]0, 5 mmol L−1; I, 0.02 A; feed rate of glucose, 1 g h−1; NO, 500 ppm; CO2, 15% (v/v); O2, 0−3% (v/v); and inlet gas flow rate, 1 L min−1. simultaneous withdrawal. To stimulate the NO removal and regeneration of Fe(II)EDTA, the simulated flue gas that was oxygen-free contained NO (500 ppm), CO2 (15%, v/v), and N2 in the first 18 days. After a high removal efficiency of NO was achieved, O2 (1−3%, v/v) was added to the simulated gas stream to improve oxygen tolerability of the microorganisms. The glucose fed into the reactor continuously at a rate of 1000 mg h−1, and thus, glucose was present in excess of the microorganism growth requirement. 2.5. Experimental Procedure. After the startup period, all of the absorption liquid was replaced by the fresh Fe(II)EDTA (10 mM) medium. Five batch experiments were carried out under different impressed current (0−0.08 A) to optimize the impressed current. Also, a continuous experiment was carried out to determine the consumption rate of glucose via monitoring the glucose concentration in the liquid phase. The minimum gas residence time was determined by a gradual increase of the inlet gas flow rate from 0.5 to 2.0 L/min. The maximum oxygen tolerability was determined by a gradual increase of the O2 concentration from 0 to 10% in the simulated flue gas. Similarly, the maximum elimination capacity was determined by a gradual increase of the NO concentration from 180 to 900 ppm in the simulated flue gas. To study the reduction mechanism, seven batch experiments were carried out to detect the accumulation of H2 and glucose in the biofilm electrode reactor. 2.6. Analytical Techniques. The inlet and outlet NO concentrations were measured using a chemiluminescent NOx analyzer (Thermo, model 42i-HL). A cold trap was used to remove moisture before the gas entered the analyzer. The Fe(II)EDTA and total iron concentrations were determined by a modified 1,10-phenanthroline colorimetric method at 510 nm.10 The concentrations of glucose were determined by a 3,5-dinitrosalicylic acid colorimetric method (DNS method). The sample solution containing microorganisms should be filtrated using a microporous membrane with 0.22 μm pore size before being measured. The biofilm on the graphite granules was observed by a scanning electron microscope (TM-1000, Hitachi, Japan). The concentrations of H2 were measured with gas chromatography (GC7890, Agilent, Santa Clara, CA). All of the data shown in this study were the mean values of the duplicate or triplicate experiments. The confidence level used in this paper was 95%, while the probability of different results was determined by t distribution.

3. RESULTS AND DISCUSSION 3.1. Performance during the Startup Period. The removal efficiency of NO and concentration of Fe(II)EDTA in the absorption liquid are two key indices, which were used to evaluate the formation of biofilm. According to Figure 2, the concentration of Fe(II)EDTA in the absorption liquid gradually increased to 5 mM at the initial stage, while the removal efficiency of NO first increased to 95% and then decreased to 90%. This is because iron-reducing bacteria were more active than the denitrifier at the initial stage, resulting in the increase of Fe(II)EDTA and Fe(II)EDTA−NO in absorption liquid. Despite the fact that the concentration of Fe(II)EDTA increased, the accumulation of Fe(II)EDTA−NO leads to a decline of absorption efficiency of NO. At the second stage, 1% O2 was added in the simulated flue gas to further activate the iron-reducing bacteria. Because of the presence of O2, the concentration of Fe(II)EDTA in absorption liquid slightly fluctuated around 3 mM. Nevertheless, the removal efficiency of NO was kept at about 90% because of the accumulation of biomass and enhancement of microbial activity. At the third stage, the removal efficiency of NO dropped to 70%, while the concentration of Fe(II)EDTA remained stable, with 3% O2. It indicated that the biofilm was gradually formed and the concentration of total iron (5 mM) became the limitation factor. This is because oxygen and nitric oxide would compete of limited Fe(II)EDTA. Biofilm could be observed on the surface of graphite granules after 60 days of cultivation. To confirm the formation of biofilm, samples of graphite granules were taken out from the middle section of the cathode area and were observed using scanning electron microscopy (SEM). According to the SEM images (Figure 3) before and after the startup period, it was clearly seen that biofilm was successfully formed on the graphite granules. After the startup period, continuous experiments were carried out to evaluate the NO removal efficiency of BER in the presence of 3% oxygen. As shown in Figure 4, the increase of total iron resulted in a high NO removal efficiency and a stable operating system, indicating the startup of the system. Also, another experiment was carried 3334

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theoretical generation of H2 and its residual amount. According to Figure 6, the consumption of H2 in the first 4 h was close to the theoretical generation of H2. It seems that H2 is the primary electron donor in reduction of Fe(III)EDTA. The contribution of H2 was in the range of 42−61% in the reduction process, while the contribution of glucose was in the range of 39−58%. Moreover, nearly 74% of glucose served as the carbon source. It indicated that increasing the H2 supply appropriately could reduce the consumption of glucose, simplify the process, and save the operating cost. 3.3. Optimization of Operating Parameters. An impressed current is introduced to activate the bacteria and offer electrons for bioreductions.16 Glucose is used as the organic carbon source for the bacteria growth and electron donor for the bioreductions.10,11 These are two key operating parameters. Hence, it is crucial to optimize the current intensity and feed rate of glucose that provides sufficient electrons and carbon source without high energy consumption. A series of experiments were designed to evaluate the effect of different impressed currents on the regeneration rate of Fe(II)EDTA and the consumption rate of glucose. The results are listed in Table 1. It is noted that the optimum condition on the regeneration rate of Fe(II)EDTA (0.183 mmol L−1 h−1) and the glucose consumption rate (0.462 g h−1) was achieved when the impressed current was set as 0.04 A [66.7 A m−3 net cathodic compartment (NCC)]. Considering the shock resistance of the system, the actual feed rate of glucose should be a little higher than its consumption rate. Therefore, the optimum impressed current was set as 0.04 A (66.7 A m−3 NCC), and the optimal feed rate of glucose was set as 0.5 g h−1 (0.125 g L−1 h−1). 3.4. Performance Evaluation of the CABER Integrated System, Comparison between CABER and CABR. 3.4.1. Minimum Gas Residence Time. Inlet gas flow rate affects the flue gas residence time in the absorption tower. Withstanding a higher inlet gas flow rate is essential for industrial application of the CABER integrated system. As shown in Figure 7, the gas residence time was 60 s and the removal efficiency of NO was 98% when the inlet gas flow rate was 0.5 L min−1. The gas residence time decreased to 20 s, and the removal efficiency of NO declined to 90% when the inlet gas flow rate increased to 1.5 L min−1. Furthermore, the removal efficiency of NO was 78% when the gas residence time was 15 s. Hence, the optimal inlet gas flow rate was 1.5 L min−1 to maintain a short gas residence time and a sufficient removal efficiency of NO. 3.4.2. Maximum Elimination Capacity of NO. The inlet concentration of NO has a direct influence on the operating performance of this system. The higher the concentration of NO in the flue gas, the larger the required processing load of the system. As shown in Figure 8, the removal efficiency of NO was within the range of 92.5−97.2% when the inlet NO concentration was between 180 and 900 ppm. Meanwhile, the concentration of Fe(II)EDTA fluctuated between 6.19 and 4.72 mM along with the increase of the inlet NO concentration. It indicated that the biofilm electrode reactor could provide sufficient elimination capacity of NO under experimental conditions. In general, the NO concentration in a typical flue gas is under 1000 ppmv.17 Hence, this system provides a high NO removal efficiency for a typical flue gas. 3.4.3. Maximum Oxygen Tolerability. The presence of oxygen in the flue gas can react with Fe(II)EDTA to form Fe(III)EDTA that is incapable of absorbing NO.18 On the

Figure 3. SEM images of the electrode surface: (a) before the startup period and (b) after the startup period.

out to evaluate the changes of the glucose concentration in the circulating fluid. As shown in Figure 5, the amount of glucose in the absorption liquid gradually increased with time, indicating an excessive addition of glucose. From an economic standpoint, the feed rate of glucose should be optimized. 3.2. Contribution of H2 and Glucose in Reduction of Fe(III)EDTA. According to the previous study, both H2 and glucose can be used by microorganisms as electron donors to reduce Fe(III)EDTA.13 In this study, a series of experiments were designed to study the contribution of these two electron donors. As shown in Figure 6, the consumption of glucose by microorganisms increased with time. However, the consumption of H2 was synchronized with the reduction of Fe(III)EDTA. It indicated that H2 was the electron donor for reduction, while glucose served as both the electron donor and carbon source. Because Fe(II)EDTA is the dominant electron donor in reduction of Fe(II)EDTA−NO,13 the contribution of H2 and glucose in reduction of Fe(III)EDTA could be calculated. The theoretical generation of H2 was calculated through the following equation: Q = 1/2(Ct − C0)nVF, where Ct and C0 are the final and initial concentrations of the reaction product (M), V is the volume of the reactor (L), n is the stoichiometric coefficient, and F is Faraday’s constant (C mol−1). The consumption of H2 is the difference between the 3335

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Figure 4. NO removal efficiency and concentration of Fe(II) after the startup of the integrated system: [Fe(III)EDTA]0, 5 mmol L−1; I, 0.02 A; feed rate of glucose, 1 g h−1; NO, 500 ppm; CO2, 15% (v/v); O2, 3% (v/v); and inlet gas flow rate, 1 L min−1.

Figure 6. Consumption of electron donors and reduction of chelate complexes during different times: T, 323 K; I, 0.02 A; [Fe(III)EDTA]0, 8 mmol L−1; [Fe(II)EDTA−NO]0, 4 mmol L−1; pH, 6.8; and glucose, 2000 mg L−1.

Figure 5. Concentration of glucose in the liquid after the startup of the integrated system: I, 0.02 A; feed rate of glucose, 1 g/h; NO, 500 ppm; CO2, 15% (v/v); O2, 3% (v/v); and inlet gas flow rate, 1 L/min−1.

in the previous studies.8 However, the biological reduction rate is always a limiting factor.19 The CABER integrated system was proposed to overcome this limitation. Comparisons of these two systems were made in this study. As listed in Table 2, the minimum residence time of the CABER integrated system was only 20 s, while that of the CABR integrated system was 45 s. A shorter residence time indicates a greater elimination capacity. It is a step further to make it commercialized. Another improvement is the higher oxygen tolerability of 10% in the CABER integrated system. Also, the maximum elimination capacity of the CABER integrated system was 104.2 g of NO m−3 h−1, while that of the CABR integrated system was only 18.78 g of NO m−3 h−1. In conclusion, the CABER integrated system is more advantageous than the CABR integrated system

other hand, the presence of oxygen would inhibit the activity of the bacteria. As shown in Figure 9, the removal efficiency of NO was kept above 90% as the oxygen concentration increased from 1 to 10%. Meanwhile, the concentration of Fe(II)EDTA reduced from 7.60 to 3.85 mM with the increase of the oxygen concentration. It is noted that a higher oxygen concentration results in less Fe(II)EDTA. However, separation of the absorption tower from the bioreactor in this system alleviated the negative effect of oxygen on microbial activity. Moreover, the introduction of electrochemical action helped the system maintain a relatively high concentration of Fe(II)EDTA, even when the oxygen concentration was 10%. 3.4.4. Performance Comparison between CABER and CABR. The CABR integrated system has been fully reported 3336

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Table 1. Regeneration Rate of Fe(II)EDTA and Consumption Rate of Glucose under Different Current Densities

a

impressed current (A)

current density (A m−3 NCC)a

apparent regeneration rate (initialization phase) (mmol L−1 h−1)

apparent regeneration rate (stabilization phase) (mmol L−1 h−1)

consumption rate of glucose (g/h)

0 0.02 0.04 0.06 0.08

0 33.3 66.7 100 133.3

0.136 0.293 0.284 0.339 0.513

N/A 0.195 0.183 0.141 0.170

0.56 0.627 0.462 0.693 0.587

NCC = net cathodic compartment.

Figure 9. Effects of the O2 concentration on the performance of the integrated system: [Fe]0, 10 mmol L−1; CO2 concentration, 15% (v/ v); NO concentration, 720 ppm; inlet gas flow rate, 1 L min−1; I, 0.04 A; feed rate of glucose, 0.5 g h−1; and circulating flow rate, 10 L h−1.

Figure 7. Effects of the inlet gas flow rate on the performance of the integrated system: [Fe]0, 10 mmol L−1; CO2 concentration, 15% (v/ v); O2 concentration, 3% (v/v); NO concentration, 720 ppm; I, 0.04 A; feed rate of glucose, 0.5 g h−1; and circulating flow rate, 10 L h−1.

Table 2. Performance Comparison between the CABR Integrated System and the CABER Integrated System parameters

CABR integrated system

CABER integrated system

4520 320 18.788

20 10 104.2

minimum residence timea (s) maximum oxygen tolerancea (%) maximum elimination capacity (g of NO m−3 h−1) a

The comparisons were based on the removal efficiency of 90%.

increasing the H2 supply appropriately could reduce the consumption of glucose. The performance of this new approach was also evaluated under a stable state. In comparison to the CABR integrated system, the new approach has a shorter gas residence time, larger elimination capacity of NO, and higher oxygen tolerability.

■

Figure 8. Effects of the NO concentration on the performance of the integrated system: [Fe]0, 10 mmol L−1; CO2 concentration, 15% (v/ v); O2 concentration, 3% (v/v); inlet gas flow rate, 1 L min−1; I, 0.04 A; feed rate of glucose, 0.5 g h−1; and circulating flow rate, 10 L h−1.

AUTHOR INFORMATION

Corresponding Authors

*Fax: 86-571-87952513. E-mail: [email protected]. *Fax: 86-571-87952513. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

in terms of NO removal, processing load and higher oxygen tolerability in the flue gas.

■

ACKNOWLEDGMENTS This work was sponsored by the National Natural Science Foundation of China (21276233 and 21306166), the China Postdoctoral Science Foundation (2013M541783), and the Postdoctoral Science Preferential Funding of Zhejiang Province, China (BSH1301019).

4. CONCLUSION A bench-scale integrated system was successfully built up to continuously, stably, and efficiently remove NOx in simulated flue gas. The system was optimized via selecting the optimum impressed current and feed rate of glucose. It is found that 3337

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