4d-2: Technologies and approaches for mitigating 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.

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

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.

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.

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.