A novel low-cost method for Hg0 removal from flue gas by visible-light-driven BiOX (X = Cl,Br, I) photocatalysts

BiOX (X = Cl, Br, I) photocatalysts were synthesized by a simple coprecipitation method and were characterized by SEM, TEM, HRTEM, XRD, TG, DRS, PL, and ESR techniques. The photocatalytic activity of Hg⁰ removal and the effects SO₂ and NO were investigated under fluorescent light. The Hg⁰ removal performance was in the sequence of BiOI > BiOBr > BiOCl. Compared with BiOBr, BiOI showed much excellent SO₂ resistance on Hg⁰ removal. In the BiOBr reaction system, h⁺ and O₂⁻ could play key roles in Hg⁰ removal, while for BiOI photocatalytic system, I₂ might be an important species for higher Hg⁰ removal.     

Photooxidative removal of Hg0 from simulated flue gas using UV/H2O2 advanced oxidation process: Influence of operational parameters

Element mercury (Hg⁰) from flue gas is difficult to remove because of its low solubility in water and high volatility. A new technology for photooxidative removal of Hg⁰ with an ultraviolet (UV)/H₂O₂ advanced oxidation process is studied in an efficient laboratory-scale bubble column reactor. Influence of several key operational parameters on Hg⁰ removal efficiency is investigated. The results show that an increase in the UV light power, H₂O₂ initial concentration or H₂O₂ solution volume will enhance Hg⁰ removal. The Hg⁰ removal is inhibited by an increase of the Hg⁰ initial concentration. The solution initial pH and pH conditioning agent have a remarkable synergistic effect. The highest Hg⁰ removal efficiencies are achieved at the UV light power of 36W, H₂O₂ initial concentration of 0.125 mol/L, Hg⁰ initial concentration of 25.3 μg/Nm³, solution initial pH of 5, H₂O₂ solution volume of 600 ml, respectively. In addition, the O₂ percentage has little effect on the Hg⁰ removal efficiency. This study is beneficial for the potential practical application of Hg⁰ removal from coal-fired flue gas with UV/H₂O₂ advanced oxidation process.     

TiO2-V2O5纳米纤维光催化氧化烟气中的Hg0

对静电纺丝法制备的TiO₂和TiO₂-V₂O₅纳米纤维进行光催化脱除模拟烟气中Hg⁰的研究。对纳米纤维进行了SEM、TEM、XRD、BET和UV-Vis检测。结果表明TiO₂-V₂O₅纳米纤维为锐钛矿,V₂O₅高度分散在TiO₂中。纤维直径在200 nm左右,由粒径为10 nm左右的微粒组成。掺杂V₂O₅后,纤维的吸光范围扩大,在可见光范围内的吸光度比纯TiO₂时有了很大提高。实验研究了不同光照条件、V₂O₅的掺杂量和循环次数对脱汞的影响,分析了TiO₂-V₂O₅催化脱汞的机理。当V₂O₅的质量含量为3%时,TiO₂-V₂O₅在可见光下的脱汞率可达到66%,比纯TiO₂时的7%有了显著提高;纤维的脱汞性能稳定,多次循环后紫外光和可见光下的脱汞率仍分别保持在80%和65%左右。电子的跃迁和电子、空穴的快速分离是TiO₂-V₂O₅在可见光下脱汞率提高的主要原因。     

Removal of elemental mercury from simulated coal-combustion flue gas using a SiO2–TiO2 nanocomposite

A novel silica–titania (SiO₂–TiO₂) nanocomposite has been developed to effectively capture elemental mercury (Hg⁰) under UV irradiation. Previous studies under room conditions showed over 99% Hg⁰ removal efficiency using this nanocomposite. In this work, the performance of the nanocomposite on Hg⁰ removal was tested in simulated coal-fired power plant flue gas, where water vapor concentration is much higher and various acid gases, such as HCl, SO₂, and NO_x, are present. Experiments were carried out in a fix-bed reactor operated at 135 °C with a baseline gas mixture containing 4% O₂, 12% CO₂, and 8% H₂O balanced with N₂. Results of Hg speciation data at the reactor outlet demonstrated that Hg⁰ was photocatalytically oxidized and captured on the nanocomposite. The removal efficiency of Hg⁰ was found to be significantly affected by the flue gas components. Increased water vapor concentration inhibited Hg⁰ capture, due to the competitive adsorption of water vapor. Both HCl and SO₂ promoted the oxidation of Hg⁰ to Hg(II), resulting in higher removal efficiencies. NO was found to have a dramatic inhibitory effect on Hg⁰ removal, very likely due to the scavenging of hydroxyl radicals by NO. The effect of NO₂ was found to be insignificant. Hg removal in flue gases simulating low rank coal combustion products was found to be less than that from high rank coals, possibly due to the higher H₂O concentration and lower HCl and SO₂ concentrations of the low rank coals. It is essential, however, to minimize the adverse effect of NO to improve the overall performance of the SiO₂–TiO₂ nanocomposite.     

Development of iron-based sorbents for Hg0 removal from coal derived fuel gas: Effect of hydrogen chloride

Laboratory studies were conducted to develop an elemental mercury (Hg⁰) removal process based on the reaction of H₂S and Hg⁰ using iron-based sorbents for coal derived fuel gas. It is well known that hydrogen chloride (HCl) is present in fuel gases derived from some types of coal, but the effect of HCl on the Hg⁰ removal performance of iron-based sorbents in coal derived fuel gas is not yet well understood. In this study, the effects of HCl on the removal of Hg⁰ from coal derived fuel gases over iron-based sorbents such as iron oxide (Fe₂O₃), supported iron oxides on TiO₂, iron oxide–Ca(OH)₂, and iron sulfides were investigated. The Hg⁰ removal experiments were carried out in a laboratory-scale fixed-bed reactor at 80 °C using simulated fuel gas. In the case of iron oxide (Fe₂O₃), the presence of HCl suppressed the Hg⁰ removal rate. In the case of Fe₂O₃ (2 or 5 wt%)/TiO₂, the presence of HCl did not suppress the Hg⁰ removal rate and the activity was stable. The Hg⁰ removal performance of reagent FeS₂ was higher than that of the iron oxide, and not affected by the presence of HCl. The Hg⁰ removal rate of iron oxide–Ca(OH)₂ was not effected by the presence of HCl, because HCl was captured by Ca(OH)₂. The reagent FeS₂ showed higher Hg⁰ removal activity than that of FeS₂ ore. However, the Hg⁰ removal performance of ground and kneaded FeS₂ ore was comparable to that of reagent FeS₂ probably due to the increase in porosity of the FeS₂ ore by grinding and kneading.     

Enhanced Hg2+ removal and Hg0 re-emission control from wet fuel gas desulfurization liquors with additives

Secondary atmospheric pollutions may result from wet flue gas desulfurization (FGD) systems caused by the reduction of Hg²⁺ to Hg⁰. The present study employed three agents: Na₂S, 2,4,6-trimercaptotiazine, trisodium salt nonahydrate (TMT) and sodium dithiocarbamate (DTCR) to precipitate aqueous Hg²⁺ in simulated desulfurization solutions. The effects of the precipitator’s dosing quantity, the initial pH value, the reaction temperature, the concentrations of Cl^? and other metal ions (e.g. Cu²⁺ and Pb²⁺) on Hg²⁺ removal were studied. A linear relationship was observed between Hg²⁺ removal efficiency and the increasing precipitator’s doses along with initial pH. The addition of chloride and metal ions impaired the Hg²⁺ removal from solutions due to the complexation of Cl^? and Hg²⁺ as well as the chelating competition between Hg²⁺ and other metal ions. Based on a comprehensive comparison of the treatment effects, DTCR was found to be the most effective precipitating agent. Moreover, all the precipitating agents were potent enough to inhibit Hg²⁺ reduction as well as Hg⁰ re-emission from FGD liquors. More than 90% Hg²⁺ was captured by precipitating agents while Hg²⁺ reduction efficiency decreased from 54% to just less than 3%. The additives could efficiently control the secondary Hg⁰ pollution from FGD liquors.     

Removal of Hg0 from containing‐SO2/NO flue gas by ultraviolet/H2O2 process in a novel photochemical reactor

A novel photochemical spray reactor is first developed and is used to remove Hg⁰ and simultaneously remove Hg⁰/SO₂/NO from flue gas by ultraviolet (UV)/H₂O₂ process. The effects of several parameters (UV wavelength, UV power, H₂O₂ concentration, Hg⁰ inlet concentration, solution temperature, liquid–gas ratio, solution pH, SO₂ concentration, NO concentration, and O₂ concentration) on removal of Hg⁰ by UV/H₂O₂ process were investigated. Removal mechanism of Hg⁰ is proposed and simultaneous removal of Hg⁰, NO, and SO₂ is also studied. The results show that the parameters, UV wavelength, UV power, H₂O₂ concentration, liquid–gas ratio, solution pH, and O₂ concentration, have significant impact on removal of Hg⁰. However, the parameters, Hg⁰ inlet concentration, solution temperature, SO₂ concentration, and NO concentration, only have small effect on removal of Hg⁰. Hg²⁺ is the final product of Hg⁰ removal, and Hg⁰ is mainly removed by oxidations of H₂O₂, ·OH, · O, O₃, and photoexcitation of UV. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2275–2285, 2014     

Removal of Elemental Mercury by Sodium Chlorite Solution

The important step for increasing gaseous elemental mercury (Hg⁰) removal in wet scrubber systems is altering the chemical form of the Hg⁰ to a water‐soluble oxidized species. This work focuses on the removal of elemental mercury from simulated flue gas by aqueous sodium chlorite in a bubble reactor. The effects of initial oxidizing solution concentration, reaction temperature, pH and mercury concentration in the inlet of the reactor on mercury oxidative absorption in sodium chlorite were investigated. The results indicate that higher concentrations of sodium chlorite favor Hg⁰ removal, with a greater efficiency observed in acidic than in alkaline solution. High temperature inhibits Hg⁰ absorption in aqueous sorbent when the reaction temperature is lower than ca. 40 °C, and the removal efficiency increases when the temperature is higher than that value. In conclusion, the major influencing factors on the levels of Hg⁰ removal are pH and chlorite concentration in solution.     

Experimental and theoretical analysis of element mercury adsorption on Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/Ag composites

A novel magnetic nano-sorbent Fe₃O₄/Ag was synthesized and applied to capture the elemental mercury from the simulated flue gas. The morphology, components and crystal phase of the sorbents were characterized by transmission electron microscope (TEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD), respectively. The mercury removal performance of the sorbents was investigated through the fixed-bed tests. The results indicated that silver was successfully loaded on the surface of Fe₃O₄ particles, which could significantly enhance the Hg⁰ removal performance of the sorbents. Flue gas components, including CO₂, SO₂, and NO, have little impact on the Hg⁰ removal performance of Fe₃O₄/Ag sorbents, while O₂ has a slightly positive effect. The Hg⁰ removal efficiency decreased with the increasing of temperature, Hg⁰ inlet concentration and gas hourly space velocity. Only one broad mercury desorption peak at approximately 210 °C could be observed during the temperature-programmed desorption (TPD) process, which indicated that mercury species existing on the surface of Fe₃O₄/Ag sorbents might be elemental mercury instead of oxidized mercury. Furthermore, the reusability tests showed that the Fe₃O₄/Ag sorbents could be efficiently regenerated and reused. Finally, the theoretical analysis based on the DFT method showed that a weak chemisorption of Hg⁰ on Fe₃O₄ sorbents changed to a strong chemisorption when silver was loaded. The results of theoretical analysis conformed to the experiments results well.     

Mercury emissions from six coal-fired power plants in China

Mercury emission field measurements based on the Ontario Hydro Method (OHM) were conducted for six coal-fired power plants in China. The mercury mass balances for the six power plants varied from 100.3% to 139.5% of the input coal mercury for the whole system. About 0.02%–1.2% of the mercury remained in the bottom ash. In the first five power plants equipped with pulverized coal boiler, most of the mercury was emitted from the stack to the atmosphere. The plants with Electrostatic Precipitator (ESP) system emitted more Hg⁰ than Hg²⁺, while the plants with the Fabric Filter (FF) emitted less Hg⁰ than Hg²⁺. Virtually all of the Hg^P enter the ESP or the FF was removed. The FF systems had better Hg⁰ and Hg²⁺ removal efficiencies than the ESP systems. The flue gas desulfurization (FGD) system removed up to 78.0% of Hg²⁺ and only 3.14% of Hg⁰ in the flue gas, while 8.94% of the original mercury in the coal was removed by the FGD system. The average mercury removal efficiencies of the ESP systems was 11.5%, that of the FF systems was 52.3% and that of the combined ESP + FGD system was 13.7%, much lower than the average removal efficiencies of pollution control device systems in US plants which have been used in previous studies of Chinese mercury emission inventory. Hg⁰, rather than Hg²⁺ as assumed in previous estimates, has been found to be the dominant species emitted in the atmosphere. The average emission factor was found to be 4.70 g/TJ (10.92 bl/Tbtu), which is much higher than for US plants burning bituminous coals due to the high mercury content in the Chinese coal and the low mercury removal efficiency of air pollution control devices of power plants.     

Catalytic performance of CuCl<Subscript>2</Subscript>-modified V<Subscript>2</Subscript>O<Subscript>5</Subscript>-WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> catalyst for Hg<Superscript>0</Superscript> oxidation in simulated flue gas

CuCl₂-SCR catalysts prepared by an improved impregnation method were studied to evaluate the catalytic performance for gaseous elemental mercury (Hg⁰) oxidation in simulated flue gas. Hg⁰ oxidation activity of commercial SCR catalyst was significantly improved by the introduction of CuCl₂. Nitrogen adsorption, XRD, XRF and XPS were used to characterize the catalysts. The results indicated that CuCl₂ was well loaded and highly dispersed on the catalyst surface, and that CuCl₂ played an important role for Hg⁰ catalytic oxidation. The effects of individual flue gas components on Hg⁰ oxidation were also investigated over CuCl₂-SCR catalyst at 350 oC. The co-presence of NO and NH₃ remarkably inhibited Hg⁰ oxidation, while this inhibiting effect was gradually scavenged with the decrease of GHSV. Further study revealed the possibility of simultaneous removal of Hg⁰ and NO over CuCl₂-SCR catalyst in simulated flue gas. The mechanism of Hg⁰ oxidation was also investigated.     

Development of iron oxide sorbents for Hg removal from coal derived fuel gas: Sulfidation characteristics of iron oxide sorbents and activity for COS formation during Hg removal

The aim of this study is to develop a process for the removal of Hg⁰ using H₂S over iron oxides sorbents, which will be located just before the wet desulfurization unit and catalytic COS converter of a coal gasification system. It is necessary to understand the reactions between the iron oxide sorbent and other components of the fuel gas such as H₂S, CO, H₂, H₂O, etc. In this study, the sulfidation behavior and activity for COS formation during Hg⁰ removal from coal derived fuel gas over iron oxides prepared by precipitation and supported iron oxide (1 wt% Fe₂O₃/TiO₂) prepared by conventional impregnation were investigated. The iron oxide samples were dried at 110 °C (designated as Fe₂O₃-110) and calcined at 300 and 550 °C (Fe₂O₃-300 and Fe₂O₃-550). The sulfidation behavior of iron oxide sorbents in coal derived fuel gas was investigated by thermo-gravimetric analysis (TGA). COS formation during Hg⁰ removal over iron oxide sorbents was also investigated using a laboratory-scale fixed-bed reactor. It was seen that the Hg⁰ removal activity of the sorbents increased with the decrease of calcinations temperature of iron oxide and extent of sulfidation of the sorbents also increased with the decrease of calcination temperature. The presence of CO suppressed the weight gain of iron oxide due to sulfidation. COS was formed during the Hg⁰ removal experiments over Fe₂O₃-110. However, in the cases of calcined iron oxides (Fe₂O₃-300, Fe₂O₃-550) and 1 wt% Fe₂O₃/TiO₂, formation of COS was not observed but the Hg⁰ removal activity of 1 wt% Fe₂O₃/TiO₂ was high. Both FeS and FeS₂ were active for Hg⁰ removal in coal derived fuel gas without forming any COS.     

Gaseous Elemental Mercury Removal Using Combined Metal Ions and Heat Activated Peroxymonosulfate/H2O2 Solutions

Several novel oxidation removal processes of elemental mercury (Hg⁰) from flue gas using combined Fe²⁺/Mn²⁺ and heat activated peroxymonosulfate (PMS)/H₂O₂ solutions in a bubbling reactor were proposed. The operating parameters (e.g., PMS/H₂O₂ concentration, Fe²⁺/Mn²⁺ concentration, solution pH, activation temperature, and Hg⁰/NO/SO₂/O₂/CO₂ concentration), mechanism and mass transfer-reaction kinetics of Hg⁰ removal were investigated. The results show that heat and Fe²⁺/Mn²⁺ have significant synergistic effect for activating PMS and PMS/H₂O₂ to produce free radicals to oxidize Hg⁰. Hg⁰ removal is strongly affected by PMS/H₂O₂ concentration, Fe²⁺/Mn²⁺ concentration, activation temperature, and solution pH. · and ·OH produced from combined heat and Fe²⁺/Mn²⁺ activated PMS/H₂O₂ play a leading role in Hg⁰ removal. Under optimized experimental conditions, Hg⁰ removal efficiencies reach 100, 94.9, 66.9, and 58.9% in heat/Fe²⁺/PMS/H₂O₂, heat/Mn²⁺/PMS/H₂O₂, heat/Fe²⁺/PMS, and heat/Mn²⁺/PMS systems, respectively. Hg⁰ removal processes in four systems belong to fast reaction and were controlled by mass transfer under optimized experimental conditions. © 2018 American Institute of Chemical Engineers AIChE J, 65: 161–174, 2019     

Removal of elemental mercury from simulated flue gas by cerium oxide modified attapulgite

A novel catalyst CeO₂/ATP was developed to remove Hg⁰ from coal fired gas. This is new way to use the facile, cheap and larger BET specific surface area catalyst attapulgite (ATP) as support to remove Hg⁰ from coal fired gas. The Hg⁰ removal and oxidation efficiency of CeO₂/ATP (1: 1) is up to 97.75% and 92.23% at 200 °C, respectively. We also found that ATP plays an important role in improving the catalyst activity of CeO₂/ATP, which can make CeO₂/ATP have more stable catalyst activity at broader temperature range and obtain lower optimum activity temperature. Other influencing factors, such as temperature and flue gas environment (SO₂, Cl₂, NO), are also investigated in order to get a clear understanding of the experiment. The formation mechanisms are also proposed.     

Catalytic oxidation and capture of elemental mercury from simulated flue gas using Mn-doped titanium dioxide

Titanium dioxide (TiO₂) and Mn-doped TiO₂ (Mn(x)-TiO₂) were synthesized in a sol-gel method and characterized by BET surface area analysis, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Gasphase elemental mercury (Hg⁰) oxidation and capture by the Mn-doped TiO₂ catalyst was studied in the simulated flue gas in a fixed-bed reactor. The investigation of the influence of Mn loading, flue gas components (SO₂, NO, O₂, and H₂O) showed that the Hg⁰ capture capability of Mn(x)-TiO₂ was much higher than that of pure TiO₂. The addition of Mn inhibits the grain growth of TiO₂ and improves the porous structure parameters of Mn(x)-TiO₂. Excellent Hg⁰ oxidation performance was observed with the catalyst with 10% of Mn loading ratio and 97% of Hg⁰ oxidation was achieved under the test condition (120 °C, N₂/6%O₂). The presence of O₂ and NO had positive effect on the Hg⁰ removal efficiency, while mercury capture capacity was reduced in the presence of SO₂ and H₂O. XPS spectra results reveal that the mercury is mainly present in its oxidized form (HgO) in the spent catalyst and Mn⁴⁺ doped on the surface of TiO₂ is partially converted into Mn³⁺ which indicates Mn and the lattice oxygen are involved in Hg⁰ oxidation reactions.     

Removal of Hg0 from flue gas using two homogeneous photo‐fenton‐like reactions

Removal of Hg⁰ using two homogeneous Photo‐Fenton‐Like reactions was first investigated in a photochemical reactor. Effects of process parameters on Hg⁰ removal were studied. Free radical and reaction products were analyzed. Removal pathways of Hg⁰ were discussed. Simultaneous removal of Hg⁰, NO, and SO₂ is also studied briefly. The results show that UV power, wavelength, H₂O₂ concentration, and solution pH have great effects on Hg⁰ removal. Hg⁰, and SO₂ concentrations, solution temperature, Fe³⁺, Cu²⁺, , and concentrations also have significant effects on Hg⁰ removal. However, concentrations of CO₂, NO, O₂, Cl^?, , , SiO₂, Al₂O₃, and Fe₂O₃ only have slight effects on Hg⁰ removal. Hg⁰/NO/SO₂ can be simultaneously removed by Photo‐Fenton‐Like reactions. ·OH was captured, and / /Hg²⁺ were also detected. Removals of Hg⁰ by photochemical oxidation and ·OH oxidation play a major role, and removal of Hg⁰ by H₂O₂ oxidation only plays a secondary role in removal of Hg⁰. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1322–1333, 2015     

Mercury oxidation and adsorption characteristics of chemically promoted activated carbon sorbents

Previous entrained-flow tests conducted under elemental mercury (Hg⁰)-laden air found that significant amounts of oxidized mercury (Hg²⁺) are not adsorbed onto cupric chloride-impregnated carbon (CuCl₂-AC) and brominated activated carbon (DARCO Hg-LH), but entrained to the gas phase. In this study, these sorbents were tested in a fixed-bed system and a filter-added entrained-flow system to further investigate Hg⁰ oxidation and adsorption characteristics of CuCl₂-AC and DARCO Hg-LH. These test results suggested that CuCl₂-AC has different sites available for Hg⁰ oxidation and Hg adsorption, and the resultant oxidized mercury generated from the reaction between Hg⁰ and CuCl₂ is re-adsorbed at the site of CuCl₂-AC available for adsorption. The resultant oxidized mercury was also found to be easily re-adsorbed onto CuCl₂-AC and DARCO Hg-LH in the filter connected to the entrained-flow reactor.     

Experimental study on ZnO-TiO<Subscript>2</Subscript> sorbents for the removal of elemental mercury

ZnO-TiO₂ sorbents synthesized by an impregnation method were characterized through XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy) and EDS (Energy dispersive spectrometer) analyses. An experiment concerning the adsorption of Hg⁰ by ZnO-TiO₂ under a simulated fuel gas atmosphere was then conducted in a bench-scale fixed-bed reactor. The effects of ZnO loading amounts and reaction temperatures on Hg⁰ removal performance were analyzed. The results showed that ZnO-TiO₂ sorbents exhibited excellent Hg⁰ removal capacity in the presence of H₂S at 150 °C and 200 °C; 95.2% and 91.2% of Hg⁰ was removed, respectively, under the experimental conditions. There are two possible causes for the H₂S reacting on the surface of ZnO-TiO₂: (1) H₂S directly reacted with ZnO to form ZnS, (2) H₂S was oxidized to elemental sulfur (S _ad ) by means of active oxygen on the sorbent surface, and then S _ad provided active absorption sites for Hg⁰ to form HgS. This study identifies three reasons why higher temperatures limit mercury removal. First, the reaction between Hg⁰ and H₂S is inhibited at high temperatures. Second, HgS, as the resulting product in the reaction of mercury removal, becomes unstable at high temperatures. Third, the desulfurization reaction strengthens at higher temperatures, and it is likely that H₂S directly reacts with ZnO, thus decreasing the S _ad on the sorbent surfaces.     

Analysis of mercury species present during coal combustion by thermal desorption

Mercury in coal and its emissions from coal-fired boilers is a topic of primary environmental concern in the United States and Europe. The predominant forms of mercury in coal-fired flue gas are elemental (Hg⁰) and oxidized (Hg²⁺, primarily as HgCl₂). Because Hg²⁺ is more condensable and far more water soluble than Hg⁰, the wide variability in mercury speciation in coal-fired flue gases undermines the total mercury removal efficiency of most mercury emission control technologies. It is important therefore to have an understanding of the behaviour of mercury during coal combustion and the mechanisms of mercury oxidation along the flue gas path. In this study, a temperature programmed decomposition technique was applied in order to acquire an understanding of the mode of decomposition of mercury species during coal combustion. A series of mercury model compounds were used for qualitative calibration. The temperature appearance range of the main mercury species can be arranged in increasing order as HgCl₂ < HgS < HgO < HgSO₄. Different fly ashes with certified and reference values for mercury concentration were used to evaluate the method. This study has shown that the thermal decomposition test is a newly developed efficient method for identifying and quantifying mercury species from coal combustion products.     

Effects of NOx, α-Fe2O3, γ-Fe2O3, and HCl on mercury transformations in a 7-kW coal combustion system

Bench-scale investigations indicate that NO, NO₂, hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃), and HCl promote the conversion of gaseous elemental mercury (Hg⁰) to gaseous oxidized mercury (Hg²⁺) and/or particle-associated mercury (Hg[p]) in simulated coal combustion flue gases. In this investigation, the effects of NO_x, α-Fe₂O₃, γ-Fe₂O₃, and HCl on Hg transformations were evaluated by injecting them into actual coal combustion flue gases produced from burning subbituminous Absaloka and lignitic Falkirk coals in a 7-kW down-fired cylindrical furnace. A bituminous Blacksville coal known to produce an Hg²⁺-rich combustion flue gas was also burned in the system. The American Society for Testing and Materials Method D6784-02 (Ontario Hydro method) or an online Hg analyzer equipped to measure Hg⁰ and total gaseous mercury (Hg[tot]) was used to monitor Hg speciation at the baghouse inlet (160–195 °C) and outlet (110–140 °C) locations of the system. As expected, the baseline Blacksville flue gas was composed predominantly of Hg²⁺ (Hg²⁺/Hg[tot]=0.77), whereas Absaloka and Falkirk flue gases contained primarily Hg⁰ (Hg⁰/Hg[tot]=0.84 and 0.78, respectively). Injections of NO₂ (80–190 ppmv) at 440–880 °C and α-Fe₂O₃ (15 and 6 wt.%) at 450 °C into Absaloka and Falkirk coal combustion flue gases did not significantly affect Hg speciation. The lack of Hg⁰ to Hg²⁺ conversion suggests that components of Absaloka and Falkirk combustion flue gases and/or fly ashes inhibit heterogeneous Hg⁰–NO_x–α-Fe₂O₃ reactions or that the flue gas quench rate in the 7-kW system is much different in relation to bench-scale flue gas simulators.An abundance of Hg²⁺, HCl, and γ-Fe₂O₃ in Blacksville flue gas and the inertness of injected α-Fe₂O₃ with respect to heterogeneous Hg⁰ oxidation in Absaloka and Falkirk flue gases suggested that γ-Fe₂O₃ catalyzes Hg²⁺ formation and that HCl is an important Hg⁰ reactant. The filtration of Absaloka and Falkirk combustion flue gases at 150 °C through fabric filters with ≈60 g/m² γ-Fe₂O₃ indicated that about 30% of the Hg⁰ in Absaloka and Falkirk flue gases was converted to Hg²⁺ and/or Hg(p). HCl injection (100 ppmv) into the Absaloka combustion flue gas converted most of the Hg⁰ to Hg²⁺, whereas HCl injection into the Falkirk flue gas converted most of the Hg⁰ and Hg²⁺ to Hg(p). Additions of γ-Fe₂O₃ and HCl did not have a synergistic effect on Hg⁰ oxidation. The filtration of Absaloka and Falkirk flue gases through much greater fabric filter loadings of 475 g/m² γ-Fe₂O₃ essentially doubled the baghouse Hg[tot] removal efficiency to about 50%. Results from this investigation demonstrate the importance of evaluating potential Hg⁰ reactants and oxidation catalysts in actual coal combustion flue gases.