JV Task 119 - Effects of Aging on Treated Activated Carbons

Abstract

For both the United States and Canada, testing has been under way for electric utilities to find viable and economical mercury control strategies to meet pending future mercury emission limits. The technology that holds the most promise for mercury control in low-chlorine lignite to meet the needs of the Clean Air Act in the United States and the Canada-Wide Standards in Canada is injection of treated activated carbon (AC) into the flue gas stream. Most of the treated carbons are reported to be halogenated, often with bromine. Under a previous multiyear project headed by the Energy & Environmental Research Center (EERC), testing was performed on a slipstream unit using actual lignite-derived flue gas to evaluate various sorbent technologies for their effectiveness, performance, and cost. Testing under this project showed that halogenated ACs performed very well, with mercury capture rates often {ge} 90%. However, differences were noted between treated ACs with respect to reactivity and capacity, possibly as a result of storage conditions. Under certain conditions (primarily storage in ambient air), notable performance degradation had occurred in mercury capture efficiency. Therefore, a small exploratory task within this project evaluated possible differences resulting from storage conditions and subsequent effects of aging thatmore » might somehow alter their chemical or physical properties. In order to further investigate this potential degradation of treated (halogenated) ACs, the EERC, together with DOE's National Energy Technology Laboratory, the North Dakota Industrial Commission (NDIC), the Electric Power Research Institute (EPRI), SaskPower, and Otter Tail Power Company, assessed the aging effects of brominated ACs for the effect that different storage durations, temperatures, and humidity conditions have on the mercury sorption capacity of treated ACs. No aging effects on initial capture activity were observed for any carbons or conditions in the investigation. As measured by the 50% breakthrough time, no changes in capacity were observed for Norit LH samples stored frozen and likely none for Norit LH samples stored under high humidity conditions. The major aging effects on capacity for the EERC brominated sample were seen as a decrease in capacity during the first week of storage under high humidity conditions. Storage of the Norit LH and the EERC 5% brominated samples under low humidity conditions resulted in slightly improved capacities. Storage of the 15% brominated sample under high humidity conditions also improved the capacity. Surface analysis using x-ray photoelectron spectroscopy (XPS) showed some migration of bromine to and from the surface occurred during storage, although no net loss of bromine was observed. The migration to the surface in the case of the 15% brominated carbon correlates with the increased capacity observed for this sample. The XPS elemental speciation data for the EERC brominated carbons showed a trend toward more bromide being formed at the expense of covalent bromine during storage. This was especially great for the 15% and the 5% stored in low humidity. The Norit LH samples did not show this increase in bromide concentration. This is consistent with the lack of change in capacity for the Norit LH during storage. The carbon speciation data showed generally more oxidization of carbon surfaces with storage, especially formation of carboxylate groups. The least increase in carboxylate groups was seen for the Norit LH stored under low humidity, which correlates with its increased capacity. In conclusion, the observed changes in surface chemistry can be related to the minimal changes in capacity observed, but the factors operate in different directions, so the relationships are complex. High-quality x-ray absorption fine structure spectra were obtained for most of the samples. Significant changes were observed in the x-ray absorption near edge structure and extended x-ray absorption fine structure spectra of the stored carbons, but proved difficult to interpret and correlate with structural changes.« less