(220f) Evaluation of Mixed-Absorbent in Chemical Absorption-Biological Reduction Integrated Approach for NOx Removal From Flue Gas

Authors: 
Liu, N., Insititute of Industrial Ecology and Environment
Li, W., Zhejiang University
Zhang, L., Zhejiang University


Evaluation of mixed-absorbent in chemical absorption-biological reduction integrated approach for NOx removal from flue gas

Nan Liu, Lei Zhang and Wei Li*

Institute of Industrial Ecology & Environment, Zhejiang University,

Hangzhou, 310027, China

Abstract:

Nitrogen oxides (NOx) are usually released from the combustion of fossil fuels, which has contributed to various environmental problems, such as global warming [1]. So far, several technologies have been developed for NOxremoval. These include selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) [2, 3]. However, these conventional control systems suffer from high cost and the risk of secondary pollution [4].

Presently, a new method, called chemical absorption-biological reduction integrated method for NOx removal from flue gas, is a promising approach [5]. It involves NO enhanced absorption by ferrous complex (Fe(II)L, L: EDTA or Citrate et. al.) and reduction of ferric complex (Fe(III)L, caused by oxygen in flue gas), Fe(II)L-NO by bacteria. Previous studies showed that the Fe(II)EDTA system had obvious superiority, but it is still restricted by some factors, such as high market price and poor environmental-friendly characteristics.

A review of research indicated that Citrate was originally used in the desulfurization of flue gas, such as SO2 absorption by sodium citrate [6] and H2S removal by Ferric Citrate Complex [7]. Our preliminary researches [8] also confirmed that Fe(II)Cit (Cit: Citrate) was cheaper to absorb NO, and could also be used as carbon source to lessen cost, but the capability of NO removal is much lower than Fe(II)EDTA.

In present studies, an innovation mixed-absorbent of Fe(II)Cit/Fe(II)EDTA (Fe(II)L) system was introduced to make up for each other's deficiencies. After studying the mixed- absorbent system through long-term operation, some influencial factors, such as the concentration of NO, O2, gas flow rate, were also investigated in the biofilter.

The study was conducted under anaerobic conditions with a pH value of 6.3-7.0. The bioreactor [9] was supplied with NO (250ppm), O2 (3%, v/v), CO2 (15%, v/v), N2 (simulated actual flue gas). The molar ratio of Fe(II)Cit to Fe(II)EDTA in the mixed absorbent was 3:1 [10]. The initial concentration of Fe(II)L was 10mmol·L-1. Bacterial strains used in this bioreactor were Pseudomonas sp. DN-2 (GenBank accession No.: DQ811956) [11] and Escherichia coli FR-2 (GenBank accession No.: EU693909)[12] with terminal electron acceptors of Fe(II)L-NO and Fe(III)L respectively, which were isolated and enriched in our previous study [11,12].

After a long time operation period, all the recycled liquid solution was replaced by the fresh Fe(II)L (10mmol·L-1) medium. Then the effects of key factors on the biofilter performance were investigated. The biofilter was loaded with simulated flue gas. The influence of NO, O2, gas flue rate was determined by a gradual increase of the concentration. REwas introduced to stand for NO removal efficiency.

The Fe(II)L concentrations were determined by modified 1, 10-phenanthroline colorimetric method at 510nm [12]. The inlet and outlet NO concentrations were measured with a chemiluminescent NOx analyzer (Thermo, Model 42i-HL). Microscopic analyses were made by an environment scanning electron microscope (Philips, model XL30E, SEM). All data shown in this paper were the mean values of duplicate or triplicate experiments.

The result shows that RE maintained at about 90% during 170h operation, while Fe(II)L concentration kept at about 6mmol·L-1. To investigate the growth of biofilm on the polyethylene fillers, samples of polyhedral sphere were taken out from the upper and bottom sections of the medium bed for analysis by SEM at the end of long-term operation stages. The results show that the microorganisms can grow well on filter medium in comparison with the contrast one. The formation of the biofilm was enhanced with the biofilter operation. Subsequently, high level of REcould be attributed to the abundance of microorganisms in the biofilm.

The kinetic behavior accords with the Michaelis-Menten model. It can be concluded that Fe(II)L existed as absorbent definitely improved the NO diffusing process from gas phase to liquid phase.

The effect of NO concentration on RE was determined in the range of 250-800ppm during 28h operation. It was observed that REmaintained over 90%, which had no significant changes in current experimental conditions. The experimental result indicates that the system has  potential to remove NO at high loading conditions.

Flue gas from power plants usually contains amount of oxygen depending on combustion conditions. Oxygen has undesirable effects on NO absorption with the iron chelate. The concentration of NO was 250ppm and O2 was increased from 1% to 5% (v/v) in 34h periodic steps. It can be observed that the system was highly effective (most up to 90%) for RE. The RE can return to steady level within 6h during each step in O2 inlet concentration. The Fe(II)L concentration maintained over 5mmol·L-1. As the O2 concentration increasing from 1% to 5% (v/v), Fe(II)L reduced by microorganisms substantially kept a balance with Fe (II) oxidation with O2. So it appeared that the oxidation of Fe(II)L is not strongly affected by the fluctuation of oxygen concentration. The system shows a well adaptability to the shock load of the oxygen.

The experiment was conducted to evaluate the interferences of gas flow rate in the process. RE and Fe(II)L concentrations remained steady, even if the inlet gas flow rate was increased (1-2.5L·min-1).

  In conclusion, a mixed-absorbent constituted by Fe(II)Cit and Fe(II)EDTA was utilized to achieve the CABR process. The system could maintain at 90% of NOx removal efficiency. In addition, the biofilter presented a high degree of flexibility in the changes of operating conditions (inlet concentration fluctuations of NO, O2, and gas flow rate). The mixed-absorbent provides the possibility for continuous removal of NO from flue gas in industrial application prospect.

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