Evaluation of the Variables that Influence Mercury Capture in Solid Sorbents

Abstract

A B S T R A C TNewly obtained results on mercury retention in fly ashes and activated carbons during coal combustion are compared with previous findings. The influence of different variables on mercury capture, including the composition and nature of the sorbents, the composition of the flue gas and the mercury species in gas phase is investigated.Coal-fired utility boilers are currently the largest single-known source of mercury emissions. According to the USEPA's Information Collection Request (ICR), on average, only 40% of the mercury entering a coal-fired power plant is captured, while 60% is emitted into the atmosphere. Several solid materials, including activated carbons, are considered to be effective sorbents for Hg control in flue gases from coal combustion. However, more economical alternatives to retain mercury such as the optimization of conditions for improving mercury capture in fly ashes in the installation also need to be studied.In this work, the mercury retention capacity of different fly ashes was compared with retention by commercial activated carbons in different experimental conditions. As might be expected, the results obtained indicate that the quantity of mercury captured depends on the characteristics of the fly ashes and on the mode of occurrence of mercury in gas phase. The retention of elemental mercury in the fly ashes, unlike activate carbons, was greatly influenced by the gas composition. Although the efficiency of the mercury retention was higher in activated carbons than in fly ashes, some activated carbon showed a lower retention capacity for elemental mercury than some fly ashes in the simulated flue gas.

Speciation of mercury in fly ashes by temperature programmed decomposition

“…Therefore, in addition to HgCl 2 , the presence of Hg 0 cannot be ruled out. (Table 1) are what one would expect in a gasification atmosphere containing H 2 S. In previous studies [19,34], it was suggested that the Hg species formed in the N 2 and gasification atmospheres were the same because the retention capacities in both atmospheres were similar. However, the results obtained from thermal desorption in this study show that, while Hg is present mainly as HgCl 2 in the N 2 atmosphere (Figure 3), in the gasification atmosphere it is in the form of HgS ( Figure 5).…”

“…The amount of Hg 0 captured in the gasification atmosphere and in N 2 was similar, the highest capture being achieved in the combustion atmosphere [19]. However, when the source of mercury was HgCl 2 , the retention was similar in all three atmospheres [34].…”

“…In order to confirm the results of the previous studies carried out by Lopez-Anton et al [19,34] in which fly ashes were used as sorbents for Hg retention in combustion and gasification atmospheres, the temperature programmed decomposition method was employed to identify the Hg species formed in the different atmospheres. Figures 3-6 show the thermal decomposition profiles of the fly ashes obtained post-retention of Hg 0 and HgCl 2 in inert, combustion and gasification atmospheres.…”

“…The third fly ash (CTP) was taken from a fluidised bed combustion plant (FBC) that burns coal mixtures and coal wastes with a high mineral matter content, and uses limestone in the bed. These fly ashes had been previously employed as sorbents of Hg 0 (g) and HgCl 2 (g) [19,34]. The Hg species present in the gas atmosphere were obtained by evaporating Hg 0 and HgCl 2, respectively.…”

The role of unburned carbon concentrates from fly ashes in the oxidation and retention of mercury

“…Several full-, pilot-, and bench-scale studies have revealed that fly ashes can both adsorb and oxidize Hg in coal combustion flue gases [6][7][8][9][10][11][12][13][14]. A number of investigations have attempted to correlate Hg retention and oxidation with the content and characteristics of unburned carbon in raw fly ashes [15][16][17] and have found that, in general, Hg capture increases in fly ashes with a high carbon content [18].…”