SULFURIZED BLACK CARBON SORBENTS FOR MERCURY VAPOR CAPTURE IN LOW-RESOURCE SETTINGS
Mercury (Hg) is a hazardous atmospheric pollutant released during fossil fuel combustion and small-scale gold mining. While impregnated activated carbon sorbents are well studied for Hg vapor capture in developed countries and large industrialized settings, there exist no suitable low cost alternatives for Hg capture from artisanal and small-scale gold mining (ASGM) in developing countries. This research seeks to develop an easy-to-manufacture carbon sorbent using elemental sulfur and activated carbon or hardwood-based biochar for potential use during ASGM Hg-amalgam heating. Consumer-grade sulfur powder was melted on granular activated carbon or hardwood biochar in a process feasible for a cook stove setting. Activated carbon and biochar were successfully sulfurized to more than 5% sulfur by weight. The products were tested for sorption of gaseous Hg(0) (500 μg Hg per cubic meter) in an air gas stream at 23°C. The sulfurized activated carbon achieved higher Hg(0) adsorption capacity (7800 mg/kg) relative to unsulfurized activated carbon (680 mg/kg) and sulfurized biochar (2400 mg/kg). Sorption isotherms were determined to discern Hg(0) sorption mechanisms, and indicated the process could be modeled as a pseudo-first order process. Analysis of surface speciation of sulfur and Hg by X-ray spectroscopy techniques revealed that these elements were bound in a mercury-sulfide-like structure in a single plane but were not in the form of highly crystalline structures. This research demonstrates a novel and effective Hg(0) sorbent using consumer-grade materials and synthesis equipment and provides an option for improved Hg vapor reduction for individuals located in resource-limited settings such as ASGM sites.
DEVELOPMENT OF SCR CATALYST REGENERATION PROCESS FOR ENHANCED MERCURY OXIDATION
The removal of mercury from flue gas in wet scrubbers is greatly increased if the flue gas mercury (Hg) is present as a water-soluble oxidized species (e.g. Hg2+). Increased mercury oxidation upstream of wet scrubbers improve overall mercury removal with minimum additional costs. The selective catalytic reduction (SCR) catalyst in a fossil fuel power plant plays a key role as a co-benefit for oxidizing elemental mercury (Hg0). However, there are influencing factors like the ammonia ratio applied for NOx-removal, which reduces the performance of regular SCR catalysts related to mercury oxidation.
Currently the SCR catalyst Original Equipment Manufacturers (OEMs) have provided commercial enhanced mercury oxidation catalysts to the coal-fired power plants for reducing stack Hg emission. To the SCR catalyst regeneration industry, developing enhanced Hg oxidation regenerated catalyst is important to help power plants further cost savings on NOx and Hg removal. The different proprietary methods have been studied, optimized and applied into the catalyst regeneration process for further improving Hg oxidation. A systematic study in a laboratory micro reactor has been performed to evaluate SCR catalyst mercury oxidation. The test results demonstrated that STEAG SCR-Tech’s Hg oxidation regeneration methods performed as well as commercial new enhanced Hg oxidation catalyst. Furthermore, regular deactivated SCR catalyst was converted to enhanced Hg oxidation catalyst by means of the SCR catalyst regeneration process.
SIMULTANEOUS REMOVAL OF NO AND HG0 FROM COAL-COMBUSTION FLUE GASES USING RICE HUSK SIO2 MODIFIED BY COPPER RECYCLED FROM INDUSTRIAL WASTE.
Mercury (Hg) and NOx discharged from coal-fired power plants (CFPPs) have both received special concern owing to the high toxicity and long retention time in the environment of Hg and the formation of acid rain, photocatalytic smog and secondary PM2.5 from NOx. In this research, rice husk derived SiO2 impregnated by copper recycled from industrial waste was tested under a simulated flus gas condition for simultaneous removal of NO and Hg. CuOx of 10, 25, and 50 wt% was impregnated onto the rice husk derived SiO2. The BET result showed that the presence of CuOx increased the surface area as compared to the raw SiO2. 50 wt% CuOx/SiO2 having the highest BET surface area may lead to its best Hg and NO removal efficiency. Surface-treated catalysts were analyzed with XRD; however, there is no significant peak at high angle of 35.8° and 38.2° among all the samples, indicating that CuOx was highly dispersed on the surface, which can enhance the contact with the pollutants and lead to a greater conversion in catalytic oxidation of Hg0 and reduction of NO.
CuOx/SiO2 showed great NO removal efficiency between 200 and 400 °C under the tested condition. 50% CuOx/SiO2 can achieve more than 60% NO removal efficiency under a broad operation temperature (250‒400 °C). The CuOx-modified SiO2 showed excellent Hg removal ability under 150 °C. These results indicate that using rice husk derived SiO2 impregnated with copper recycled from industrial waste can be a feasible way for multipollutant control of Hg and NOx.
PILOT-SCALE CAPTURE OF MERCURY, ARSENIC, AND SELENIUM FROM WARM SYNGAS AT ELEVATED PRESSURES BY PALLADIUM SORBENTS
Warm gas cleanup of fuel gas from integrated gasification combined cycle power plants is important in order to preserve both their higher thermal efficiencies and to eliminate dirty water circulation and treatment systems. Ten pilot-scale tests of palladium on alumina sorbents for the removal of trace contaminants from several types of coal-derived syngas at elevated temperatures and pressures were conducted at the Southern Company National Carbon Capture Center. Between 96 – 100% removal of mercury, arsenic, and selenium from all syngas types, sour and sweet, was observed at 500 °F and elevated pressures of 150 – 200 psig. The results indicate that the Pd sorbents exhibit large capacities for the capture of Hg, Se, and As under varying conditions and over extended test periods. Current preliminary work also shows that the sorbent is not only regenerable, but that the sorbent is just as effective at capturing these contaminants after regeneration. Future work and tests will focus on use of lower loadings of Pd, higher syngas flow rates, and further regeneration cycles in the removal of the trace contaminants, as well as the possible removal of other contaminants.
CONTROL OF MERCURY EMISSIONS – ALTERNATIVE METHODS
With implementation of the EPA Mercury Air Toxics Standard (MATS) in the United States, electric generating units (EGUs) are required to achieve high levels of mercury reduction (excess 90% in most cases). While activated carbon is widely utilized to reduce mercury emissions to meet the MATS regulation, it often results in high operational costs and significant maintenance expense. With mercury emission limits approaching in the European Union, it is important to recognize alternative methods that utilize existing capital equipment to minimize mercury control impacts. Nalcos MerControl Technologies reduce mercury emissions via liquid based reagents, reducing operational costs and greatly increasing ease of application. Nalcos presentation will provide full-scale results achieved with novel MerControl technologies; including MerControl 8034 Plus (designed for wFGDs) and MerControl SD-Hg (designed for semi-dry scrubbers).
MERCURY REMOVAL FROM WASTE INCINERATION FLUE GAS: HETEROGENEOUS OXIDATION AND CAPTURE BY WASTE-DERIVED FLY ASHES
Among all heavy metals, mercury (Hg) is a pollutant of special concern due to its major impact on human health and the environment. Incineration is one of the main waste management strategies used for the treatment of municipal solid waste (MSW), some which, consist of Hg-containing wastes, such as batteries, paint residues, thermometers, thermostats, light switches and others products which are discarded as household waste in MSW. Removal of Hg from MSW incineration flue gas is essential from the stand point of environmental pollution control. After incineration, all mercury content in waste is released as Hg0 which passes into the flue gas and is gradually oxidized (mainly into HgCl2 form), by both homogeneous and heterogeneous reactions. Some degree of Hg removal can be achieved by existing conventional air pollution control devices (APCDs), normally used to control NOx, SO2, and particulate matter. Hg bound to particles (Hgp) is usually the easiest species to be removed from flue gas as a co-benefit in existing emission control devices, such as fabric filters (FFs) or electrostatic precipitators (ESPs). Some Hg interaction/retention mechanisms have already been proposed in fly ashes from coal-fired power plants, however, fewer studies are still available concerned about fly ashes from MSW incineration. It must be taken into account that conditions of both processes are different, and therefore, the composition and characteristics of the resulting fly ashes cannot be always comparable. In this study, several samples of fly ashes (characterized by composition, surface area and carbon content) derived from MSW incineration were assessed for mercury removal under MSW incineration conditions at laboratory scale, using a fixed-bed quartz reactor packed with fly ash. The results obtained showed that unburned content, composition of flue gas (e.g. HCl and SO2 content) and operating temperature are important variables controlling capture of mercury. Fly ash enriched in unburned carbon can both, oxidize Hg0 and capture it effectively in presence of chloride. Surface area together with carbon content of fly ash and content of HCl in flue gas were correlated with the oxidation and adsorption of elemental mercury. The results obtained in this study may help to propose the interaction mechanism and to understand the fate/behavior of mercury in a baghouse, and provide a deeper knowledge of the impacts on fly ash properties in waste incineration.
COST EFFECTIVE REDUCTION OF MERCURY USING POWDER ACTIVATED CARBON INJECTION
With more than a decade of user experience at a variety of utility scale coal-fired power plants, powder activated carbon has been thoroughly demonstrated as a powerful tool for the reduction of mercury emissions. Nevertheless, in the past sorbent usage costs required to sufficiently reduce mercury emissions could be prohibitively high due to a variety of factors negatively impacting the efficiency of the activated carbon sorbent. Such factors include Air Quality Control System (AQCS) configuration, fuel type, and the presence of Flue Gas Conditioning (FGC) with SO3. An additional factor which can significantly increase the required sorbent usage rate is the application of Dry Sorbent Injection (DSI) for acid gas (i.e. HCl, SO2) control. With previous generations of sorbent products a combination of these factors could often drive up sorbent usage costs required to meet emissions compliance to the point of being economically unfeasible. In response, new generations of powder activated carbon sorbentsdemonstrating significantly increased mercury removal efficiency and tolerance to DSI have been recently developed. This increased mercury removal efficiency and DSI tolerance has not only allowed a substantial reduction in sorbent usage costs required to meet mercury emissions compliance under both the Mercury and Air Toxics Standards (MATS) and Canadian provincial regulations, but can also reduce compliance costs globally. As demonstrated in both full-scale field trials and long-term usage at standard operating conditions, this represents a significant cost savings to the electric utility.
A ZIRCONIUM-BASED METAL ORGANIC FRAMEWORK-CARBON HYBRID SORBENT FOR MERCURY AND OXYANIONIC HEAVY METAL REMOVAL
Adsorption using highly porous and highly functionalized sorbents is a straightforward removal technology currently being employed in wastewater treatment. Metal-organic frameworks are materials that have been gaining popularity in remediation technology due to their high surface areas, high specificity towards certain pollutants, as well as high structural integrity. Zirconium-based MOF, UiO66, has been shown to remove oxyanionic metal pollutants in water such as selenite, however, this study found out that it has low affinity for mercury. A novel sorbent was synthesized from activated carbon and UiO66 MOF via solvothermal method to remove both mercury and oxyanionic metals from aqueous solutions. The composite was characterized using FSEM-EDS, FT-IR, XRD, and TGA, and showed successful integration of the UiO66 and activated carbon components. The sorbent has a SBET of 1051 m2 g-1. Batch adsorption tests using CV-AFS and ion-chromatography reveal that the Hg 2+ and SeO3 2- uptake of the hybrid follows the pseudo-second order kinetics with observed Qmax values and rate constants of 249.9 mg g-1, k = 5.6E-05 and 177.2 mg g-1, k = 3.16E-05, respectively. The presence of equal concentrations of As, Cr, and Se does not significantly affect the adsorption performance of the hybrid for mercury. There was no definite effect of pH on Hg 2+ adsorption but a decrease in SeO3 2- uptake was observed at pH values higher than 7. The hybrid is a viable sorbent for both anionic and cationic heavy metal contaminants.
REMOVAL OF HG0 FROM SIMULATED COAL-FIRED FLUE GAS BY MN SUPPORTED MONTMORILLONTE
As one of the most important anthropogenic sources of mercury emission, coal combustion brings a large amount of Hg0 into the atmosphere. However, Hg0 removal is particularly difficult due to its low water solubility and high vapor pressure. And the widely application of activated carbon injection technology is limited for its high operation cost and poor capacity. Thus, it is of great significance to develop novel materials that are highly-efficient, low-cost and environmental-friendly.
In this study, natural montmorillonite (MK10) and Mn were used as support and active component, respectively. One the one hand, its Hg0 removal performance was compared with catalysts such as Mn/TiO2, Mn/Al2O3 and Mn/SiO2. Various characterizations (BET, XRD, TEM, XPS, H2-TPR and Hg-TPD) were conducted to analyze the interaction mechanism between active component and different supports. On the other hand, the effects of preparation methods, loading values, temperatures and gas components for Hg0 removal on Mn/MK10 were investigated. The Hg0 removal mechanism was discussed, on the basis of characterization results.
Hg0 removal efficiencies of four kinds of catalyst were evaluated and compared (Mn/MK10 > Mn/SiO2 > Mn/TiO2 > Mn/Al2O3), and different characterization were conducted to explain the proposed mechanism. Physical adsorption dominated Hg0 removal for Mn/Al2O3 and Mn/SiO2 at low temperature (150C), while chemical adsorption was more important at high temperature (350C). Chemical adsorption and oxidation played the main role for Mn/TiO2 and Mn/MK10, respectively. Besides, the effect of MnOx morphologies for Hg0 removal was investigated. Amorphous MnO2 benefited Hg0 oxidation most, amorphous Mn2O3 partly oxidized Hg0 at high temperature, and crystalline MnO2 basically promoted the chemical adsorption.
The Hg0 oxidation mechanisms of Mn/MK10 in different atmosphere were studied. NO had a promoting effect due to the formation of NO2 and Hg(NO3)2. The addition of only 5 ppm HCl led to excellent Hg0 removal performance as HCl enhanced Hg0 conversion to HgClx. The inhibition effects of SO2 and H2O(g) were ascribed to the formation of MnSO4 and the competitive adsorption, respectively. Moreover, Hg0 removal performance was maintained at 80%99% (NO/SO2=0.261.71) in simulated flue gas without HCl, which appeared to be promising in industrial application.
EXPERIMENTAL STUDY ON HG2+ REDUCTION AND RE-EMISSION IN THE WET FLUE GAS DESULFURIZATION SYSTEM
Wet flue gas desulfurization (WFGD) system has benefit Hg2+ pollution control performance in coal fired power plant. However, Hg2+ reduction and emission take place in the WFGD system, resulting in Hg0 emission to the atmosphere. In this work, the effects of operating temperatures, pH values and O2 concentration on the reduction of Hg2+ were investigated in the presence of SO32- and SO42-. The effect of SO42-/SO32- ratios were also investigated, since SO42- is considered as an important factor that affect the Hg2+ reduction reaction. The results indicated that Hg0 release was enhanced with the operating temperature increases. When the pH value of the liquid decreased, a remarkable emission process was obtained through the experiments. It is also found that the Hg2+ reduction will be inhibited with high availability of excess SO32- in the occurrence form of Hg(SO3)22-, but lower concentration of SO32- (<2mM) would lead to enhanced mercury re-emission by forming more redox unstable HgSO3. The content of sulfate ion, as well as SO42-/SO32- ratio nearly have no impact on Hg0 emission. In addition, O2 generally inhibited Hg0 emission with complex way. Situations where O2 exists induce a second emission by damage the stable redox complexes between Hg2+ and SO32-. However, as a result of oxidation of oxygen, the emission inhibited subsequently by forming complex HgSO3SO42- , which in turn had a negative effect on Hg0emissioin.
THE STUDY ON REMOVING HG0 FROM THE SIMULATED FLUE GAS BY ABANDONED SEMI-COKE DESULFURIZER
In this paper, the Hg0 removal from the simulated flue gas was investigated by the abandoned semi-coke desulfurizer (DUISC). As contrast samples, the semi-coke sorbents USC and UISC were also prepared by high-pressure impregnation under ultrasound-assistance. The mercury removal performances of the sorbents DUISCUISC and USC were tested in a fixed-bed reactor. The results show that the order of Hg0 removal capacity is DUISC> UISC> USC, and DUISC has the highest mercury capacity of 1.81 µg/g and the longest breakthrough time of 12.5 h. Adsorbing Hg0 is a kind of typical gas-solid reaction, thus the surface of sorbent is the location of the demercuration reaction. From BET results, the specific surface of DUISC is smaller than that of USC. Apparently, the mercury removal performance is also related to the components of sorbent. Compared with USC, the diffraction peak of ZnO can be seen from UISC, and the mercury activity of UISC increases to 1.24 µg/g and it is 2.2 times as high as USC. It is obvious that the existence of ZnO is beneficial to removing Hg0. However for DUISC, the diffraction peak of ZnS is also appeared besides ZnO. In accordance with DUISC’s higher mercury capacity, ZnS plays a main role in removing Hg0. The reason is that ZnS can be oxidized to S0, then S0 and Hg0 may react to HgS. So elemental sulfur (S0) formation leads to the best mercury removal performance of DUISC. In a word, the removing mercury of DUISC includes physisorption and chemisorption of Hg0, and the latter is the most important. ZnO and ZnS are active components of DUISC. Meanwhile, -OHC=O and COOH are the main functional groups by FTIR which adsorb and oxidize Hg0.