Effect of air pollution control devices on mercury isotopic fractionation in coal-fired power plants

Coal combustion is an important source of anthropogenic mercury (Hg) emissions. Air pollution control devices (APCDs) are widely used in coal-fired power plants (CFPPs) to reduce pollutant emissions including Hg. To gain insight into the Hg isotopic fractionation produced by APCDs, the Hg isotopic compositions of feed coal (FC) samples and combustion byproducts from 7 CFPP utility boilers in Hebei Province of China were studied. Significant mass-dependent fractionation (MDF) was observed in combustion byproducts when comparing with FC samples, while mass-independent fractionation (MIF) was not identified. Compared to FC samples, no significant difference in bottom ash (BA, δ202HgBA-FC = 0.08 ± 0.16‰, 1SD, n = 3) samples while light isotopes were enriched in fly ash (FA, δ202HgFA-FC = −0.38 ± 0.75‰, 1SD, n = 7) samples. Boilers lacking of a selective catalytic reducer (SCR) unit produced gypsum (GY) samples with negative MDF signatures compared to FC samples (δ202HgGY-FC, no SCR = −0.22 ± 0.10‰, 1SD, n = 3). In contrast, boilers equipped with a SCR unit produced GY samples with higher MDF signatures (δ202HgGY-FC, SCR = 0.54 ± 0.40‰, 1SD, n = 3), probably as a result of the oxidation of Hg0 in the SCR unit. Calculated isotopic signatures of gaseous Hg in the stack emission (SE, δ202HgSE-FC = 0.03 ± 0.20‰, 1SD, n = 7) had no significant difference from that of FC samples. The results suggested distinct isotopic compositions should be fully considered during tracing Hg pollution via Hg isotopic signatures, especially when solid produced by CFPPs were involved.