Nine fly ash samples were collected from the particulate collection devices (baghouse or electrostatic precipitator) of four full-scale pulverized coal (PC) utility boilers burning eastern bituminous coals (EB−PC ashes) and three cyclone utility boilers burning either Powder River Basin (PRB) coals or PRB blends (PRB−CYC ashes). As-received fly ash samples were mechanically sieved to obtain six size fractions. Unburned carbon (UBC) content, mercury content, and Brunauer−Emmett−Teller (BET)−N2 surface areas of as-received fly ashes and their size fractions were measured. In addition, UBC particles were examined by scanning electron microscopy, high-resolution transmission microscopy, and thermogravimetry to obtain information on their surface morphology, structure, and oxidation reactivity. It was found that the UBC particles contained amorphous carbon, ribbon-shaped graphitic carbon, and highly ordered graphite structures. The mercury contents of the UBCs (Hg/UBC, in ppm) in raw ash samples were comparable to those of the UBC-enriched samples, indicating that mercury was mainly adsorbed on the UBC in fly ash. The UBC content decreased with a decreasing particle size range for all nine ashes. There was no correlation between the mercury and UBC contents of different size fractions of as-received ashes. The mercury content of the UBCs in each size fraction, however, generally increased with a decreasing particle size for the nine ashes. The mercury contents and surface areas of the UBCs in the PRB−CYC ashes were about 8 and 3 times higher than UBCs in the EB−PC ashes, respectively. It appeared that both the particle size and surface area of UBC could contribute to mercury capture. The particle size of the UBC in PRB−CYC ash and thus the external mass transfer was found to be the major factor impacting the mercury adsorption. Both the particle size and surface reactivity of the UBC in EB−PC ash, which generally had a lower carbon oxidation reactivity than the PRB−PC ashes, appeared to be important for the mercury adsorption.
This work explores surface changes and the Hg capture performance of brominated activated carbon (AC) pellets, sulfur-treated AC pellets, and sulfur-treated AC fibers upon exposure to simulated Powder River Basin-fired flue gas. Hg breakthrough curves yielded specific Hg capture amounts by means of the breakthrough shapes and times for the three samples. The brominated AC pellets showed a sharp breakthrough after 170−180 h and a capacity of 585 μg of Hg/g, the sulfur-treated AC pellets exhibited a gradual breakthrough after 80−90 h and a capacity of 661 μg of Hg/g, and the sulfur-treated AC fibers showed no breakthrough even after 1400 h, exhibiting a capacity of >9700 μg of Hg/g. X-ray photoelectron spectroscopy was used to analyze sorbent surfaces before and after testing to show important changes in quantification and oxidation states of surface Br, N, and S after exposure to the simulated flue gas. For the brominated and sulfur-treated AC pellet samples, the amount of surface-bound Br and reduced sulfur groups decreased upon Hg capture testing, while the level of weaker Hg-binding surface S(VI) and N species (perhaps as NH 4 + ) increased significantly. A high initial concentration of strong Hg-binding reduced sulfur groups on the surface of the sulfur-treated AC fiber is likely responsible for this sorbent's minimal accumulation of S(VI) species during exposure to the simulated flue gas and is linked to its superior Hg capture performance compared to that of the brominated and sulfur-treated AC pellet samples.
This document presents and discusses results from Cooperative Agreement DE-FC26-06NT42778, "Full-scale Testing of a Mercury Oxidation Catalyst Upstream of a Wet FGD System," which was conducted over the time-period July 24, 2006 through June 30, 2010. The objective of the project was to demonstrate at full scale the use of solid honeycomb catalysts to promote the oxidation of elemental mercury in pulverized-coal-fired flue gas. Oxidized mercury is removed downstream in wet flue gas desulfurization (FGD) absorbers and collected with the byproducts from the FGD system. The project was co-funded by EPRI, the Lower Colorado River Authority (LCRA), who also provided the host site, Great River Energy, Johnson Matthey, Southern Company, Salt River Project (SRP), the Tennessee Valley Authority (TVA), NRG Energy, Ontario Power and Westar. URS Group was the prime contractor and also provided cofunding.The scope of this project included installing and testing a gold-based catalyst upstream of one full-scale wet FGD absorber module (about 200-MW scale) at LCRA's Fayette Power Project (FPP) Unit 3, which fires Powder River Basin coal. Installation of the catalyst involved modifying the ductwork upstream of one of three wet FGD absorbers on Unit 3, Absorber C. The FGD system uses limestone reagent, operates with forced sulfite oxidation, and normally runs with two FGD modules in service and one spare. The full-scale catalyst test was planned for 24 months to provide catalyst life data. Over the test period, data were collected on catalyst pressure drop, elemental mercury oxidation across the catalyst module, and mercury capture by the downstream wet FGD absorber.The demonstration period began on May 6, 2008 with plans for the catalyst to remain in service until May 5, 2010. However, because of continual increases in pressure drop across the catalyst and concerns that further increases would adversely affect Unit 3 operations, LCRA decided to end the demonstration early, during a planned unit outage. On October 2, 2009, Unit 3 was taken out of service for a fall outage and the catalyst upstream of Absorber C was removed. This ended the demonstration after approximately 17 months of the planned 24 months of operation. This report discusses reasons for the pressure drop increase and potential measures to mitigate such problems in any future application of this technology.Mercury oxidation and capture measurements were made on Unit 3 four times during the 17-month demonstration. Measurements were performed across the catalyst and Absorber C and "baseline" measurements were performed across Absorber A or B, which did not have a catalyst upstream. Results are presented in the report from all four sets of measurements during the demonstration period. These results include elemental mercury oxidation across the catalyst, mercury capture across Absorber C downstream of the catalyst, baseline mercury capture across Absorber A or B, and mercury re-emissions across both absorbers in service. Also presented in the report are estim...
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