Multi-air-pollutant removal by using an integrated system: Key parameters assessment and reaction mechanism.

How to cost-efficiently and cooperatively remove SO2, NO and Hg0 in flue gas is a hot topic in the field of air pollution control. This work developed an integrated system that consists of a dual-absorption system and a vapor oxidation system, in which Na2CO3 and H2O2/Na2S2O8 were used as the absorbent and oxidant. The results indicated that the efficiencies of SO2 removal and NO conversion reached 99.5% and 93% respectively. Rising the vaporization temperature and decreasing the pH of H2O2/Na2S2O8 could facilitate the NO conversion. The spent Na2CO3 after desulfurization was demonstrated to be a good absorbent for NO2 removal. The best conditions of pH and temperatures for the dual-absorber were determined as 10/8 and 60/60 °C, respectively. The presence of 1000 mg/m3 SO2 and 300 mg/m3 NO favored the Hg0 removal. TMT-15, an organic sulfur compound, was demonstrated to be useful in retaining Hg2+, with an efficiency of 92%. According to the analyses of electron spin resonance (ESR), ion chromatography (IC), atom fluorescence spectrometry (AFS) and X-ray photoelectron spectroscopy (XPS), SO4- and HO were proved to be the key radicals, and the existing forms of N- and Hg- species in the product were identified as NaNO2/NaNO3 and HgCl2.

Radical-induced oxidation removal of multi-air-pollutant: A critical review.

Sulfur dioxide (SO2), nitric oxide (NO) and elemental mercury (Hg0) are three common air pollutants in flue gas. SO2 and NO are the main precursors for chemical smog and Hg0 is a bio-toxicant for human. Cooperative removal of multi-air-pollutant in flue gas using radical-induced oxidation reaction is considered as one of the most promising methods due to the high removal efficiency, low cost and less secondary environmental impact. The common radicals used in air pollution control can be classified into four types: (1) hydroxyl radical (OH), (2) sulfate radical (SO4-), (3) chlorine-containing radicals (Cl, ClO2, ClO, HOCl-, etc.) and (4) ozone. This review summarizes the generation methods and mechanism of the four kinds of radicals, as well as their applications in the removal of multi-air-pollutant in flue gas. The reactivity, selectivity and reaction mechanism of the four kinds of radicals in multi-air-pollutant removal were comprehensively described. Finally, some future research suggestions on the development of new technique for cooperative removal of multi-air-pollutant in flue gas were provided.

Simultaneous Removal of SO2 and NO Using a Novel Method of Ultraviolet Irradiating Chlorite-Ammonia Complex.

A novel advanced oxidation process (AOP) using ultraviolet/sodium chlorite (UV/NaClO2) is developed for simultaneous removal of SO2 and NO. NH4OH, as an additive, was used to inhibit the generation of ClO2 and NO2. The removal efficiencies of SO2 and NO reached 98.7 and 99.1%. NO removal was enhanced by greater UV light intensity and shorter wavelengths but was insensitive to changes in pH and temperature. SO2 at 500-1000 mg/m3 improved NO removal, especially in the absence of UV. The coexistence of SO2 and O2 facilitated the removal of NO by ClO2-. HCO3-, Cl-, and Br- enhanced NO removal, but their roles were negligible when UV was added. The generation of ClO2 and ClO•/HO• was verified by an UV-vis spectrometer, electron spin resonance (ESR), and radical-quenching tests. The mechanisms responsible for the removal of SO2 and NO were attributed to the synergism between acid-base neutralization and radical-induced oxidation. The ClO2- evolution and product composition were demonstrated by UV-vis and X-ray photoelectron spectroscopy (XPS). Kinetics analyses showed that the Hatta numbers were 329-798 and 747-1000 without and with UV. Thus, the gas-film resistance mainly controlled the mass-transfer process.

Elemental mercury removal by a novel advanced oxidation process of ultraviolet/chlorite-ammonia: Mechanism and kinetics.

A novel advanced oxidation process (AOP) of ultraviolet/chlorite-ammonia (UV/NaClO2-NH4OH) was developed to remove Hg0 from flue gas. The distribution of mercury concentration in three solutions of NaClO2-NH4OH, KCl, and H2SO4-KMnO4 was determined by cold atom fluorescence spectrometry (AFS). The role of NH4OH was to help NaClO2 preserving and/or stabilizing Hg2+ meanwhile inhibiting the photo-production of ClO2. In the absence of UV, decreasing pH promoted the release of Hg2+ from NaClO2-NH4OH; introducing NO, SO2, O2, Br-, Cl-, and HCO3- suppressed Hg0 oxidation. In the presence of UV, rising temperature accelerated the release of Hg2+ from NaClO2-NH4OH; while SO2, Br- and HCO3- facilitated Hg0 oxidation. In the absence and presence of UV, Hg0 oxidation was controlled by ClO2- and by ClO/Cl2O2/HO/ClO2, respectively. The formations of ClO/HO/ClO2 were confirmed by electron spin resonance (ESR). X-ray photoelectron spectroscopy (XPS) revealed that the products of Hg0 and ClO2- were HgCl2, and ClO2, Cl-, ClO3-, Cl2, and ClO4-, respectively. Analysis of kinetics showed that the Hatta numbers were 23-133 and 69-305 without and with UV, respectively, thus, the gas-film mass transfer was the rate-determining step. This paper gives a new insight in radical behavior in Hg0 oxidation.

A novel method of ultraviolet/NaClO2-NH4OH for NO removal: Mechanism and kinetics.

The key step for nitric oxide (NO) removal using oxidation method is to efficiently oxidize NO. This study developed a novel advanced oxidation process (AOP) of ultraviolet light (UV) catalysis of chlorite (NaClO2) to oxidize NO. The production of nitric dioxide (NO2) and photo-production of chlorine dioxide (ClO2) were suppressed by adding ammonium hydroxide (NH4OH). The NO conversion efficiency was 98.1% using UV/NaClO2-NH4OH. Electron spin resonance (ESR) tests confirmed the roles of hydroxyl radical (HO) and oxychloride radical (ClO/Cl2O2) in the oxidation of NO. Kinetics analyses showed that NO flux was significantly enhanced by radical-induced (HO/ClO) oxidation of NO. In the presence of UV, the overall reaction rates (kov1*) were 3-8 times higher than those without UV. The Hatta number, namely the enhanced factor, was calculated in the range of 229-403 and 730-780 corresponding to without and with UV light, suggesting that NO oxidation belonged to fast and/or instantaneous reaction. Thus, the gas-film mass transfer resistance was the rate-determining step. N-containing product was determined as NH4+ and NO3- according to X-ray photoelectron spectroscopy (XPS).

Combustion behavior, emission characteristics of SO2, SO3 and NO, and in situ control of SO2 and NO during the co-combustion of anthracite and dried sawdust sludge.

The combustion behaviors of anthracite and dried sawmill sludge (DSS) were studied using thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG). DSS was found to be a promoter for anthracite combustion, the addition of DSS in anthracite decreased the burnout temperature and time. But DSS caused the rapid releases of SO2 and NO in the initial combustion stage. In overall, the increasing of DSS significantly decreased the emission factor of SO2 from 13.42 ±â€¯1.80 to 0.31 ±â€¯0.08 g/kg; while the emission factor of NO was not obviously changed and stable at 0.7-0.8 g/kg in all cases. The oxygen-rich atmosphere was helpful for the rapid and sufficient combustion of blend; the oxygen-lean atmosphere delayed the combustion process and slowed down the releases of SO2 and NO. The increasing combustion temperature improved the anthracite combustion, and the emission factors of SO2 and NO were all increased with the temperature increasing. 900 °C was found to be the best combustion temperature for NO generation. SO3 was detected in the combustion of anthracite under 21% and 30% of O2. Two promising ways for control of SO2 and NO were provided: 1) urea-fuel mixture combustion combined with the post-combustion wet absorption by Na2CO3; 2) post-combustion wet absorption by NaClO/Na2CO3. The removal efficiencies of SO2 and NO could reach 100% and over 95% respectively. The removal products were determined as sulfate, sulfite and nitrate by IC, with no toxic byproducts being produced.