POST-MINAMATA ACTIVITIES IN SOUTHEAST ASIA AIMED AT REDUCING MERCURY EMISSIONS
Several countries in Southeast Asia with sizeable generation capacity installed in existing coal-fired power plants are preparing to implement the Minamata Convention on Mercury. In the process, countries are considering application of best available techniques (BAT) and best environmental practices (BEP) to reduce mercury emissions, as described in United Nations Environment Programmes (UN Environments) draft BAT/BEP Guidance Document, called for in Article 8 of the Convention. This paper presents the various capacity building activities related to BAT/BEP application and carried out by UN Environment in Vietnam and Thailand. The paper also describes results of mercury measurements carried out at coal-fired power plants in the region under a U.S. project in cooperation with UN Environment.
NOVEL LDH-DERIVED NIALMO (M=ZN, BA, MN) MIXED OXIDES FOR HG0 ADSORPTION AND OXIDATION IN THE PRESENCE OF NH3
Simultaneous removal of Hg0 by NH3-SCR catalysts has a profound developing potential. NH3 is the most widely used reductant in low temperature SCR. But it is well known that NH3 exhibited negative effects on Hg0 removal at low temperature because ad-NH3 species prevent Hg0 adsorption. For instance, traditional VWTi catalyst is easily poinsoned by NH3 in Hg0 removal. It is worth mentioning that mixed oxides obtained from hydrotalcite-like compounds exhibit high ecient NH3-SCR activities due to their acid-base character and high surface area, such as MgCuAlO, MgCuFeO, MgAlFeO and MgAlFeCeO.
The hydrotalcite-like NiAlM (M=Zn, Ba, La, Ce, Cr) compounds were synthesized by urea method and calcinated at 773 K. The samples after calcination were denoted as NiAlMO. Before calcination, the XRD pattern of NiAlM precursor shows a hydrotalcite-like structure. After calcination, the hydrotalcite layered structure completely collapsed and only NiO crystalline phases could be observed on all of these samples. It indicated that doping elements might be highly dispersed in the catalyst. The BET surface areas of NiAlBaONiAlZnONiAlMnONiAlLaO and NiAlCrO were 106.01, 110.63, 121.26, 126.44 and 151.48 m2/g, respectively. The result indicated that hydrotalcite-based NiAlM synthesized by urea method indeed had higher specific surface area.
In Hg0-TPD experiments, the amount of Hg0 desorption showed in sequence as: NiAlZnO(125.12mg/g)>NiAlLaO(44.52mg/g)≈NiAlBaO(43.48mg/g)≈NiAlMnO(40.07mg/g)>NiAlCrO(32.81mg/g)>VWTi(11.65mg/g). After introduction of 100ppm NH3, the Hg0 adsorption capacity decreased on all of these samples. But the order of Hg0 adsorption capacity changed as: NiAlZnO(43.89mg/g)≈NiAlBaO(39.44mg/g)> NiAlMnO(31.55mg/g)>NiAlLaO(14.22mg/g)≈NiAlCrO(13.15mg/g)>VWTi(3.32mg/g). Apparently, the performance of the five NiAlMO catalysts was superior to that of VWTi, whether NH3 existed or not. In addition, the desorption peaks of all NiAlMO catalysts were in the range of 100-300℃. When the desorption temperature was higher than 300 ℃, the effect of NH3 was negligible. It indicated that NH3 mainly affected Hg in the weakly adsorbed state. The large BET surface area may be one of the reasons for the better adsorption performance of Hg0 on NiAlZnO and NiAlMnO. However, the larger surface area of NiAlCrO didn’t result in higher Hg0 adsorption capacity. It could be speculated that specific surface area was not the crucial factor for Hg0 absorption.
In summary, the calcined hydrotalcite-based NiAlM, particularly NiAlZnO, NiAlBaO and NiAlMnO have good performance in Hg0 capture even when NH3 exists. It’s promising to be used for simultaneous removal of Hg0 and NO at low temperature.
Keywords: hydrotalcite, Hg-TPD, ammonia, urea, manganese
IMMOBILIZATION OF ELEMENTAL MERCURY FROM COAL COMBUSTION FLUE GAS BY NOVEL MINERAL SULFIDE SORBENT
A novel zinc sulfide sorbent with large surface area was successfully synthesized by a liquid-phase precipitation method to immobilize elemental mercury from coal combustion flue gas. Surface area was found to play important role in Hg0 adsorption by zinc sulfide. At relatively high temperatures (140 to 260 °C), the Nano-size ZnS (Nano-ZnS) with the largest surface area of 196.1 m2×g-1 exhibited far greater Hg0 adsorption capacity than the conventional bulk ZnS sorbent due to the abundance of surface sulfur sites, which have a high binding affinity for Hg0. The Nano-ZnS Hg0 was first physically adsorbed on the sorbent surface, then reacted with the adjacent surface sulfur to form the most stable mercury compound, HgS, which was confirmed by X-ray photoelectron spectroscopy analysis and a temperature-programmed decomposition test. The optimized temperature for Hg0 removal using Nano-ZnS was 180 °C, at which a maximum Hg0 adsorption capacity of 497.84 µg×g-1 was achieved when adsorption bed was 50 % penetrated (inlet Hg0 concentration of 65.0 μg×m-3). Negligible effects of H2O, SO2, HCl on Hg0 adsorption was observed, while NO, especially with the aid of O2, inhibited Hg0 removal by the Nano-ZnS. Compared with several commercial activated carbons used exclusively for gas-phase mercury removal, the Nano-ZnS was superior in both Hg0 adsorption capacity and adsorption rate. With this excellent Hg0 removal performance, non-carbon Nano-ZnS may prove to be an advantageous alternative to activated carbon for Hg0 removal in coal-fired power plants equipped with particulate matter control devices and selective catalytic reduction devices.
THE INVESTIGATION OF MOS2 NANOSHEETS FOR HG0 CAPTURE BY MATERIALS GENOME APPROACH
Compared with graphene, the graphene-like 2D TMDs materials with layered structure exhibit abundant surface chemistry that is importance for adsorption/catalysis processes. There is a great potential for such layered materials decorated with abundant sulphur for mercury capture to address the urgent global challenges. However, the types of TMDs nanosheet and their effects on Hg0 capture remain unexplored. In this research, the Materials Genome approach that integrated computational (stage I), experimental (stage II), and mechanisms (i.e., informatics and data analysis, stage III) study has developed for accelerating discovery of MoS2 nanosheets features for Hg0 capture. Based on computational chemistry simulation results, it is found that MoS2 nanosheets performed the strongest adsorption capability for Hg0 capture. The MoS2 containing adsorbent is then prepared, characterized and evaluated by using various techniques such as XPS, Raman, XRD, HRTEM, NH3-TPD and, in-situ DRIFTS, dynamic transient and steady-state Hg0 capture evaluation in the stage II experimental study. The charge density difference analysis, PDOS analysis and adsorption pathways and energy profiles predictions are further adopted as data informatics tools to reveal the mechanisms of Hg0 captured on the MoS2 surface. The demonstrated materials genome approach in atomic-level shows a great potential for the discovery of nano-materials with defective features as advanced functional environmental remediation materials.
STUDY ON MERCURY EMISSION AND MERCURY ADSORPTION CHARACTERISTICS OF FLY ASH AND BIOMASS CHAR IN UTILITY BOILERS
The mercury emission and adsorption characteristics of fly ash and biomass char were studied in this paper. The mercury emission was obtained by measuring the mercury content in solid samples including coal, limestone, slag, fly ash and gypsum in PC and CFB utility boilers. The relationships were obtained between of the mercury emission and adsorption with physical and chemical characteristics of fly ash and preparation conditions of biomass char. The parameters include unburned carbon content in fly ash, particle size and pore structure, biomass char preparation temperature, heating rate and air. And the mercury adsorption kinetic characteristics of fly ash and biomass char were investigated. The results show that the majority of mercury releases to the atmosphere with the flue gas in PC boiler, while the one is enriched in fly ash and captured by the precipitator in CFB boiler. The coal factor was proposed to characterize the impact of coal property on mercury emissions in this paper. The larger the coal factor is, the lower the mercury emission to the atmosphere is. The results also show that the capacity of adsorbing mercury is directly related to the particle size of fly ash. The mercury content first increases and then decreases with the increase of the particle size in the range of 24.5 to 362.5µm. It is found that the mesoporous content4-6nmis the biggest among 77.5-106µm in the fly ash, which is beneficial to adsorbing the mercury. The pore structure of biomass char influences greatly in physical adsorption. The more cumulative pore volume of biomass char and the more mercury adsorbed. The pores with smaller pore size had better adsorption effect on mercury than those with larger pore size at the early stage of adsorption, while the latter could enhance the total adsorption amount of mercury. The adsorption process was affected by both chemical adsorption and physical absorption, and the former is an important rate-limiting process.
DEVELOPMENT OF SELECTIVE SORBENT FOR OXIDIZED MERCURY AND ITS APPLICATION ON SPECIATION MERCURY MEASUREMENT IN COAL FIRED FLUE GAS
The emission of mercury derived from coal-fired power plant has been brought into global focus for its heavy impact on both environment and human health. Mercury species are difficult to measure accurately in coal-fired flue gas because of their low concentration and distinct physicochemical property. Alkali metal oxides, salts and chlorides efficient selectivity for gaseous oxidized mercury (Hg2+) and are preferred materials for mercury speciation in flue gas. Several novel calcium-based sorbents were manufactured in this study with mesoporous silica support and their features for oxidized mercury selective adsorption were analyzed. By bench-scale fixed bed experiments, calcium-based sorbents mingled with different active ingredients like CaO, KCl, NaOH were compared on their selective efficiency for Hg2+ and anti-interference ability for other flue gas components. Meanwhile, the crystalline structure and surface characteristics are obtained though X-Ray Powder Diffraction (XRD) study, Brunauer-Emmett-Teller (BET) analysis and SEM (Scanning Electron Microscope) analysis. Compared with the original CaO sorbent (C-S), which selectively adsorbed 64.61% of HgCl2 under the simulated flue gas, the composite sorbents (CK-S) mingling CaO with KCl were able to absorb more HgCl2 (85.62%). The surface imperfection formed on CK-S surface during the CaO and KCl eutectic melting process can be responsible for the performance improvement. The increase of oxygen vacancy on CK-S surface effectively enhanced HgCl2 selection and the diffusion of products layer (CaSO4). Furthermore, another calcium-based sorbent labeled as NCK-S was prepared by doping NaOH to CK-S. The selective adsorption efficiency of NCK-S for HgCl2 was up to 95.36%. Actually, NCK-S obtained more basic sites on sorbent surface with the addition of NaOH. Moreover, NaOH can limit CaO dissolutionthrough the common ion effect when the calcium-based sorbents are exposed to moist flue gas. Therefore, NCK-S possessed better sulfur resistance and HgCl2 adsorption capacity than the other two calcium-based sorbents. Finally, the NCK-S along with activated carbons was applied to speciation mercury field sampling on a bench-scale coal-fired unit by EPA Method 30B speciation traps. The gaseous mercury species distribution results by speciation traps coincided to those by Ontario Hydra Method (OHM).
STUDY ON THE MERCURY EMISSION AND TRANSFORMATION IN A COAL-FIRED POWER PLANT WITH ULTRA-LOW EMISSION CONTROL DEVICE
The field test of the mercury emission from the Chinese ultra-low emission coal-fired power plant was investigated by the Ontario Hydro method. Temperature-programmed decomposition desorption, scanning electron microscope and X-ray power diffraction spectrometry were used in this work. Results show that the emitted Hg0 in the stack occupies the largest proportion of the total amount (49.55%) with 16 times of the emitted Hg2+. Mercury in bottom ash occupies the smallest ratio with 0.13%. Hg0 oxidation across SCR (53.26%) plays an important role for the Hg0 removal. HgT removal rate across the APCDs obeys the order of ESP > WFGD > WESP. WESP can further reduce the mercury emission. The emission factor is 1.39 g/1012 J, less than the mean value of Chinese plants. Mercury content in gypsum is higher than the limit while bottom and ESP ash have no effects on the soil. Leaching is not likely but thermal treatment should be paid more attention to the gypsum disposal. Mercury concentration in the WFGD and WESP waste water is higher than the limit, and enough importance should be given on the waste water from WFGD. Mercury transformation mechanism across the power plant is discussed systematically.
OPTIMIZING MERCURY CONTROLS IN POWER AND CEMENT PRODUCING PLANTS
Over the course of its 15 years of industry experience, Ohio Lumex has developed a variety of measurement techniques, products, and services which provide coal-fired utilities and cement kilns with effective means for reducing mercury emissions. The data provided by these measurements has been critical in helping the affected industries minimize operating costs required to meet regulatory limits, via optimization of control technologies and injected materials.
Mercury sorbent traps and portable sorbent trap analyzers allow for quick and reliable on-site and accurate determination of total mercury concentration as well as mercury oxidation ratio, in any sampling environment from the SCR inlet to the stack. Sorbent traps have become the industry standard, and are well-renowned for their self-validation criteria, ease of use, and reliability. In addition to mercury sorbent traps, Ohio Lumex has designed sorbent traps to measure a variety of common analytes of interest, such as NH3, SO3, HCl, HBr, Se, and As.
Portable mercury speciating monitors yield real-time total and oxidized mercury data, and are designed to function for extended durations in nearly any sampling environment. Permanent mercury monitoring systems are generally installed in stacks or at the inlet to FGDs, and are equipped to send live mercury data directly to the plants data integration system.
Hundreds of engineering studies have utilized these technologies and have demonstrably reduced the cost of mercury abatement via tuning of plant control technologies as well as optimized injection rates of activated carbon, dry sorbents, calcium bromide, sulfides and other materials.