Article pubs.acs.org/EF
Evaluation of NOx Removal from Flue Gas by a Chemical Absorption−Biological Reduction Integrated System: Glucose Consumption and Utilization Pathways Nan Liu,† Yan Jiang,† Lei Zhang,†,‡ Yinfeng Xia,†,‡ Bihong Lu,† Bailong Xu,† Wei Li,*,†,‡ and Sujing Li† †
Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Institute of Industrial Ecology and Environment, College of Chemical and Biological Engineering, Zhejiang University Yuquan Campus, Hangzhou, Zhejiang 310027, People’s Republic of China ‡ Institute of Environmental Engineering, Zhejiang University Zijingang Campus, Hangzhou, Zhejiang 310058, People’s Republic of China S Supporting Information *
ABSTRACT: Biological reduction of nitric oxide (NO) with ferrous chelate is the main step for the chemical absorption− biological reduction (CABR) integrated method to remove nitrogen oxide (NOx) from flue gas. Heterotrophic bacteria play a dominant role in the CABR process, and their reactivity is seriously affected by carbon source and electron donor. Therefore, the consumption and utilization pathways of glucose were investigated. The study on glucose metabolites shows that the accumulation of acetate should be alleviated, which make it possible to keep running the bioreactor normally, although the volatile fatty acids (VFAs) may be beneficial as an electron donor for the reactions in CABR. The reduction of complex NO mainly depends upon the concentration of Fe(II) and acetate. The main utilization pathway of glucose can be expressed as glucose → pentanoic acid → butyric acid → propionic acid → acetic acid → CO2. Under experimental conditions of 670 mg m−3 NO inlet concentration, 0−10% O2 concentration, and 8 h of hydraulic retention time (HRT), more than half of inlet elemental carbon (glucose) was released in the form of gas after 240 h of operation. VFAs, especially acetic acid, mainly existed in the liquid phase, and CO2 was mainly observed in the gas phase. flue gas.10−13 The reaction mechanism is shown as follows, when glucose is used as an electron donor:14,15
1. INTRODUCTION Nitrogen oxides (NOx) in the flue gas of fossil fuel power plants are major atmospheric pollutants. They contribute to the formation of acid rain, photochemical pollution, and smog. Moreover, they are a main cause for the destruction of the ozone layer and photochemical smog.1−3 Therefore, the Chinese Environmental Protection Agency released a new NOx emission control policy in 2011. The amount of NOx emissions needs to be reduced to 20.462 million tons in 2015, a decrease of 10% compared to that in 2010 according to China’s 12th Five-Year Plan for National Economic and Social Development. The main component of NOx is nitric oxide (NO).4 It is very difficult to remove NO with conventional wet flue gas desulfurization (FGD) because of its low solubility in aqueous solution.5 Thus, several technologies, including selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), and other combined chemical and biological approaches, have been proposed for NO removal.5−8 Among these approaches, SCR with NH3 as a reductant has been widely used for NOx removal from stationary sources.6,9 However, all of these conventional processes still suffer from some problems, such as high cost and risk of secondary pollution, especially the disposal of catalysts in the SCR system, which makes them less attractive for industrial applications.8 To tackle these challenges, studies have been conducted on using a highly effectively integrated chemical absorption− biological reduction (CABR) approach to remove NO from © XXXX American Chemical Society
Fe(II)L2 − + NO(aq) ↔ Fe(II)L−NO2 −
(1)
4Fe(II)L2 − + O2 (aq) + 2H 2O → 4Fe(III)L− + 4OH− (2)
12Fe(II)L−NO2 − + C6H12O6 microorganism
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 12Fe(II)L2 − + 6H 2O + 6CO2 (g) + 6N2(g) (3)
24Fe(III)L− + C6H12O6 + 24OH− microorganism
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 24Fe(II)L2 − + 18H 2O + 6CO2 (g)
(4)
Fe(II)L [L = chelate agent, such as ethylenediaminetetraacetic acid (EDTA)] is employed to form stable complexes with NO [Fe(II)L−NO] and enhance the absorption of NO with scrubber liquid.10,16−18 Denitrifying bacteria and iron-reducing bacteria are introduced in this system to convert the nitrosyl complex into N2 and Fe(II)L at around 50−55 °C, which is the adiabatic temperature for the scrubber liquid. Recently, the research on the complexed NO conversion pathway in the CABR system has proven that about 64% of nitrogen was Received: July 2, 2014 Revised: November 13, 2014
A
dx.doi.org/10.1021/ef5014852 | Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. Configuration of the lab-scale bioreactor: (1) gas cylinders, (2) mass flow controllers, (3) gas mixing chamber, (4) bioreactor, (5) thermostatic water bath, (6) holding tank, (7 and 8) pump, (9) liquid flow meter, (10) cooler, and (11) NO−NO2−NOx analyzer.
reduced as N2 under steady-state conditions.19 Typical flue gas usually contains 3−10% O2. Fe(II)L will be oxidized into Fe(III)L in the presence of oxygen, instead of binding NO in this process.20,21 The regeneration rate of Fe(II)EDTA is critical to facilitate the NO reduction process.12,22 Previous studies have investigated the limiting factors of the biological regeneration process that converts Fe(III)EDTA into Fe(II)EDTA. Li et al.12 reported that Fe(II)EDTA−NO existing in scrubber solution can inhibit both the cell growth and Fe(III)EDTA reduction. Furthermore, it is confirmed that some terminal electron acceptors, such as dissolved oxygen, NO2−, and NO3−, can also inhibit Fe(III) reduction.23−25 The carbon source is one of the major components needed in the NO removal process.26 Studies have showed that both organic and inorganic compounds with reducing property, such as Fe(II), sulfides, etc., can participate in the anti-nitrification process in the form of an electron donor.27 Typically, a supplement of organic compounds is used to serve as the electron donor and carbon source for denitrification in activated sludge systems of biological wastewater treatment.28 Thus far, several kinds of carbon sources have been used in NO removal and Fe(III)EDTA reduction because of their low cost,22 including methanol, ethanol, and acetate.22 From our previous studies, glucose is used as the sole carbon source and electron donor for the bacteria in the CABR. Glucose is proven to be a suitable carbon source for both Fe(III)EDTA reductase bacteria Escherichia coli and dissimilatory (NO removal) bacteria Pseudomonas sp.10,12 However, the level of glucose concentrations and its metabolite products could significantly affect the NO removal efficiency and the reaction balances.13 At neutral pH, the redox potential of NO reduction (NO/N2O = +1180 mV, and N2O/N2 = +1350 mV) is much larger than that of iron reduction [Fe(III)EDTA/ Fe(II)EDTA = +96 mV].29 van der Maas22 and Zhang14
reported that Fe(II)EDTA−NO reduction was dependent upon the availability of Fe(II)EDTA rather than that of an external electron donor, i.e., ethanol. In contrast, Kumaraswamy et al.27 found that Fe(II)EDTA would not be used as an electron donor, if methanol was added in the Fe(II)EDTA− NO reduction process. Therefore, there is a need to conduct a pilot study on the glucose consumption in the Fe(III)EDTA reduction process and the competition of glucose consumption between the NO removal and Fe(III)EDTA reduction processes. Moreover, glucose might be decomposed into different organic compounds under anoxic conditions. The intermediate product of glucose degradation was usually regarded as toxic to bacteria because of intercellular acidification. Therefore, it is crucial to investigate the effect of glucose on the Fe(III)EDTA and Fe(II)EDTA−NO reduction processes and its utilization pathways. The aim of this project was to study if the two major reactions in this system compete for the utilization of glucose and whether the NO and Fe(III)EDTA reduction was inhibited by glucose metabolite accumulation. In addition, the distribution of the glucose degradation product needs to be quantified. The experiments were conducted in a bioreactor. The advantage of the bioreactor is high efficiency of NO removal at a low temperature compared to other methods. This paper will give a better understanding on the whole NO removal system, including the configuration and operation mechanism of the bioreactor.
2. EXPERIMENTAL SECTION 2.1. Chemicals. NO (5% in N2, v/v), O2 (99.999%), and N2 (99.999%) were obtained from Zhejiang Jin-gong Gas Co. (Hangzhou, China). FeSO4·7H2O (99.5%), disodium ethylenediaminetetraacetate (Na2EDTA, 99.95%), and D-glucose (99.5%, cell culture tested) were from Shanghai Chemical Reagent Co., China. All of the other B
dx.doi.org/10.1021/ef5014852 | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
chemicals were analytical-grade reagents, commercially available, and used without further purification. 2.2. Microorganism Cultivation. The microorganisms used in this work were isolated and identified by 16S rDNA amplification and sequencing as Pseudomonas sp. (GenBank: DQ811956) and E. coli (GenBank: DQ411026) in our previous study.10,12 After the microorganisms were cultivated for microbial adaption, they were inoculated into the bioreactor. The nutrient media used for cultivation contains glucose (1000 mg L−1), KH2PO4 (300 mg L−1), MgCl2 (100 mg L−1), Na2SO3 (70 mg L−1), CaCl2 (20 mg L−1), and trace components (2 mL L−1). The trace components for the bacteria growth include MnCl2·4H2O (990 mg L−1), CuSO4·5H2O (250 mg L−1), CoCl2 (240 mg L−1), NiCl2·6H2O (190 mg L−1), ZnCl2 (100 mg L−1), and H3BO4 (14 mg L−1).14 2.3. Bioreactor Configuration. A schematic illustration of the laboratory-scale bioreactor is shown in Figure 1. The bioreactor column is made of a poly(methyl methacrylate) (PMMA) cylinder with an inner diameter of 8 cm and an effective volume of 3 L. The bioreactor has a packed bed volume of 1.5 L. The packing in the bioreactor is made of polyethylene and a specific surface area of 1200 m2 m−3. The temperature of the bioreactor was maintained at 50 ± 0.5 °C with a water jacket to simulate the typical flue gas temperature (45−55 °C) after the FGD process. The liquid sampling points were located in a holding tank (6 L). The bioreactor was operated in a countercurrent mode. The scrubber liquor from the holding tank was continuously pumped into the top of the bioreactor. The gas flow went through the bioreactor from the bottom to the top. 2.4. Experimental Procedure. The bioreactor was filled with 4 L of Fe(II)EDTA (10 mmol L−1) medium inoculated with 200 mg [dry cell weight (DCW)] L−1 Fe(III) and Fe(II)L−NO reducing organisms. The simulated flue gas consisting of N2, O2, CO2, and NO was mixed in a mixing chamber after metering by the mass flow meters (Beijing Sevenstar Qualiflow Electronic Equipment Manufacturing Co., Ltd.). The gas flow rate (QG) was set at 2000 mL min−1. A pump was used to circulate the solution in the reactor at a flow rate (QL) of 40 L h−1. The experiments were first carried out to keep a stable operation under different O2 concentrations in the bioreactor. At this stage, the inlet oxygen loading rate was increased gradually by increasing the O2 concentration from 0 to 10% (v/v), while the concentration of NO was maintained around 670 mg m−3. Glucose was added to the bioreactor as the sole organic carbon source. The glucose added in the recycled liquid was kept at the concentration of no less than 0.8 g h−1 to maintain a sufficient glucose loading for the microorganisms in the bioreactor. The effect of glucose consumption on the Fe(III)EDTA and Fe(II)EDTA−NO reduction was mainly determined under a glucose loading of 0−2000 mg L−1 added in the recycled liquid. Moreover, degradation products of glucose by microorganisms were also investigated. The experiments were conducted under a pH value between 6.5 and 7.5. The liquid samples were obtained by the sampling needle, while the gas samples were obtained through the reactor outlet by the sampling bags. 2.5. Analytical Methods. The concentrations of Fe(II)EDTA and total iron were determined by a modified 1,10-phenanthroline colorimetric method at 510 nm.10 All data shown in this paper were the mean values of duplicate or triplicate experiments. Glucose was determined by high-performance liquid chromatography (HPLC, Agilent, 1200), using 5 mmol L−1 H2SO4 as the mobile phase at a flow rate of 0.8 mL min−1.30 Ethanol, volatile fatty acid (VFA) in the liquid phase [Agilent 7890A, chromatographic column HP-fatty, the detector of flame ionization detection (FID) at 280 °C, and detector limit of
