Understanding the effects of sulfur on mercury capture from coal-fired utility flue gases

Abstract

Coal combustion continues to be a major source of energy throughout the world and is the leading contributor to anthropogenic mercury emissions. Effective control of these emissions requires a good understanding of how other flue gas constituents such as sulfur dioxide (SO 2 ) and sulfur trioxide (SO 3 ) may interfere in the removal process. Most of the current literature suggests that SO 2 hinders elemental mercury (Hg 0 ) oxidation by scavenging oxidizing species such as chlorine (Cl 2 ) and reduces the overall efficiency of mercury capture, while there is evidence to suggest that SO 2 with oxygen (O 2 ) enhances Hg 0 oxidation by promoting Cl 2 formation below 100 • C. However, studies in which SO 2 was shown to have a positive correlation with Hg 0 oxidation in full-scale utilities indicate that these interactions may be heavily dependent on operating conditions, particularly chlorine content of the coal and temperature. While bench-scale studies explicitly targeting SO 3 are scarce, the general consensus among full-scale coal-fired utilities is that its presence in flue gas has a strong negative correlation with mercury capture efficiency. The exact reason behind this observed correlation is not completely clear, however. While SO 3 is an inevitable product of SO 2 oxidation by O 2 , a reaction that hinders Hg 0 oxidation, it readily reacts with water vapor, forms sulfuric acid (H 2 SO 4 ) at the surface of carbon, and physically blocks active sites of carbon. On the other hand, H 2 SO 4 on carbon surfaces may increase mercury capacity either through the creation of oxidation sites on the carbon surface or through a direct reaction of mercury with the acid. However, neither of these beneficial impacts is expected to be of practical significance for an activated carbon injection system in a real coal-fired utility flue gas.

Influence of a CO2-enriched flue gas on mercury capture by activated carbons

“…O 2 improves mercury retention [20,23] whereas reactive gases such as SO x and NO x may modify the surface of the carbons and favor or inhibit mercury adsorption [24][25][26][27]. Other gases such as H 2 O may also play an important role in the mechanism of Hg adsorption on the surface of the activated carbon [20,[28][29][30], and it has also been suggested that CO 2 might compete with Hg for the same adsorption sites [22].…”

“…In general, the study of the mechanism of mercury capture on activated carbon is based not only on the interactions between mercury and carbon but also on the competitive reactions of these gases with the solid. Special attention has been paid to the role of SO x and the formation of sulfuric acid (H 2 SO 4 ) [22,[24][25][26][27] which may condensate on the carbon surface in different amounts preventing the adsorption of Hg 0 or favoring the reaction with mercury to form Hg 2 SO 4 [20]. Most of the knowledge acquired until now on Hg-activated carbon interactions has been obtained from studies carried out in atmospheres typical of coal combustion in air but there is a lack of similar knowledge on the interactions of Hg-activated carbons in oxy-combustion atmospheres.…”

Uncertainties in the Mercury Mass Balance in a Coal-Based IGCC Power Plant (Puertollano, Spain)

“…Previous data on this aspect were not available, but these values are particularly important if sulphur is to be sold as a by-product and not treated as a waste. The capacity of sulphur to retain gaseous Hg has to be related to the proportions of Hg, S and Cl in the raw materials [22]. Daily evolution trends were not significantly different for these solid waste and byproduct streams, except for sulphur, which showed a decreasing trend that mirrored the raw materials trend.…”

Activated carbons from biocollagenic wastes of the leather industry for mercury capture in oxy-combustion

“…According to previous results by the authors and other researchers [33][34] a higher proportion of oxidation of SO 2 to SO 3 occurs during oxycombustion than in air combustion. After several investigations into the role of SO 2 /SO 3 in mercury retention by activated carbons [35][36][37][38][39] it was concluded [12] that SO 3 in flue gases can cause the formation of H 2 SO 4 on the activated carbon surface, which would enhance Hg 0 adsorption. However, in the present study SO 3 and/or H 2 SO 4 might not be favoring Hg capture in the oxy-combustion atmosphere due to the slow kinetics of the reaction [12] where the activated carbon-gas contact time is lower than 1 second, similar to the contact time when activated carbon is injected into a coal-fired power plant.…”