Simultaneous removal of SO2 and NOx from sintering flue gas using ammonia-Fe(II)EDTA combined with electrolytic regeneration

Environment Protection Engineering Vol. 44 2018 No. 2 DOI: 10.5277/epe180202 YAN LIANG1, XI YAO1, LINBO QUIN2, WANGSHENG CHEN1, JUN HAN1, 2 SIMULTANEOUS REMOVAL OF SO2 AND NOx FROM SINTERING FLUE GAS USING AMMONIA-Fe(II)EDTA COMBINED WITH ELECTROLYTIC REGENERATION Sulfur dioxide and nitrogen oxide are health hazardous gases, which contribute to the formation of submicron acidic particulates. To reduce SO2 and NOx emission from the sintering flue gas, the combination of ammonia-Fe(II)EDTA solution scrubbing with Fe(III) electrolytic regeneration is proposed. The above method has the following advantages: direct conversion of NOx and SO2 to harmless N2 and SO24−, recovery of the by-product (NH4)2SO4), simultaneous removal of NOx and SO2 emission from flue gas in the reactor. The effect of the pH, initial Fe(II)EDTA concentration, and voltage on the desulfurization and denitration efficiencies was investigated using a bench-scale reactor. The maximal desulfurization and denitration efficiencies were 98% and 52%, respectively. The optimum parameters were pH ˃ 5.0, 2.1 V, and 0.05 mol·dm–3 Fe(II)EDTA concentration. SO2 and NOx removal from the sintering flue gas by ammonia-Fe(II)EDTA solution scrubbing combined with electrolytic regeneration was also demonstrated in a pilot-scale reactor. 1. INTRODUCTION Sulfur dioxide and nitrogen oxide are gases hazardous to human health and contribute to the formation of submicron acidic particulates that can penetrate into the human lungs and even be absorbed in the bloodstream [1,2]. The main sources of SO2 and NOx are the exhaust gas from fossil fuel combustion in industries such as coal combustion power plants, paper mills, iron and steel plants, and in waste incinerators [3]. In 2014, it was reported that the iron and steel industry in China emitted about 2.15×106 t of SO2 and 1.01×106 t of NOx [4]. Moreover, SO2 and NOx emitted from the sintering process _________________________ 1Hubei Key Laboratory for Efficient Utilization and Agglomeration of Metallurgic Mineral Resources, Wuhan University of Science and Technology, Wuhan, 430081, P.R. China, corresponding author J. Han, e-mail address: 2Industrial Safety Engineering Technology Research Center of Hubei Province, Wuhan University of Science and Technology, Wuhan, 430081, P.R. China. 20 Y. LIANG et al. accounted for 85% and 40% of the total emissions in the iron and steel industry [5]. In order to improve the air quality, the Ministry of Environmental Protection of China issued a new emission standard for decreasing air pollutants from the sintering and pelletizing process in the iron and steel industry (GB28662-2012). This standard stipulates that the SO2 and NOx concentration in the exhaust flue gas from sintering plants must be below 200 and 300 mg·Nm–3, respectively. However, the current NOx emission from most sintering plants is about 400 mg·Nm–3, which is above the emission limit [6]. At present, various desulfurization technologies such as dry desulfurization, semidry desulfurization, and wet desulfurization techniques have been utilized. The ammonia-based wet flue gas scrubber is regarded as one of the most reliable approaches and has been widely applied in China because of its low investment cost, high desulfurization efficiency, the lack of generation of secondary pollutants, and the production of useful byproducts [7, 8]. In the ammonia-based wet flue gas scrubber system, (NH4)2SO3 and small amounts of NH4HSO3 coexist in the scrubbing solution without free NH3. It was reported that (NH4)2SO3 has the capacity to absorb NOx (but is ineffective for desulfurization [9]), and 20−40% denitration efficiency could be achieved with the ammoniabased wet flue gas scrubber [10]. The reaction between NOx and (NH3)2SO3 is described below 2NO 2  H 2 O  SO32  2NO 2  SO 24  2H  (1) NO + SO32  NOSO32 (2) However, the NOx concentration in the sintering flue gas after passing through the ammonia-based wet flue gas scrubber is still above the emission limit. In order to further reduce NOx emission, technologies such as ammonia-based selective catalytic reduction (NH3-SCR) [11] and complex absorption [12, 13] have been developed. Although ammonia-based selective catalytic reduction (NH3-SCR) is widely used in coal-fired power plants, SCR is not suitable for sintering flue gas because the temperature (120−180 °C) of the sintering flue gas is lower than the optimal reaction temperature of the current commercial catalysts [2, 11]. Complex absorption is considered as one of the most promising approaches due to its rapid absorption rate and moderate operation cost [14, 15]. The addition of Fe(II) (EDTA) (EDTA – ethylenediaminetetraacetate) to the scrubbing solution (such as ammonia-based solutions and calcium-based solutions) could improve the solubility of NOx via formation of the Fe(II)(EDTA)(NO) complex [16, 17]. However, Fe(II)EDTA is easily oxidized to Fe(III)EDTA by oxygen in the flue gas (the sintering flue gas contains 15−18% of oxygen) and loses its capability to bind NO [18]. Hence, the challenge in the denitration process by Fe(II)EDTA solution is reducing Fe(III)EDTA. Chandrashekhar et al. [15] combined biomass from municipal sewage sludge with ethanol to reduce Fe(III)EDTA and Fe(III)NTA with the maximum reduc- Simultaneous removal of SO2 and NOx from sintering flue gas 21 ing rates 0.0021 and 0.0026, mmol·dm–3·d–1·mg–1 biomass. They proposed that the denitration reaction by Fe(II)EDTA solution and Fe(EDTA reduction proceeded according to the following stoichiometric equations: 2Fe(II)EDTA(NO ) 2  2H   N 2 O  H 2O  2Fe(III)EDTA  (3) N2O + 2Fe(II)EDTA2  2H  N2  H 2O  2Fe(III)EDTA (4) 12Fe(III)EDTA  C2 H5OH  5H 2O  2HCO3  12Fe(II)EDTA2  14H (5) Guo et al. [17, 19] demonstrated that the electrochemical method employing an activated carbon catalyst was effective for reducing Fe(III)EDTA to Fe(II)EDTA, and found that 99% denitration efficiency could be achieved over 10 h. However, activated carbon and a Nepem-117 proton exchange membrane were used to accelerate the reduction process in their experiments, which resulted in higher operation costs. Notably, there was a large amount of fly ash in the sintering flue gas, which would form a slurry in the scrubber and block the membrane in the case of real sintering flue gas. In order to simultaneously remove NOx and SO2, ammonia-Fe(II)EDTA scrubbing combined with electrochemical regeneration is proposed herein. Fe(II)EDTA is added to the ammonia-based solution to increase the solubility of NO by forming the Fe(II)(EDTA)(NO) complex. The Fe(II)(EDTA)(NO) complex is then reduced electrochemically, assisted by (NH4)2SO3 in the ammonia-based solution. The final products are N2 and (NH4)2SO4. When the amount of (NH4)2SO3 in the ammonia-based solution is insufficient, Fe(II)(EDTA)(NO) can also be reduced to N2 directly by the electrochemical process. This method has several advantages su (...truncated)