Air Compliance Testings' comprehensive Mercury testing services provide our clients with accurate, low detection level, quality-assured data using EPA validated test methods. This data enables our clients to make informed decisions regarding Mercury emissions levels and control in their boiler systems at a reasonable cost.

Mercury in Coal-Fired Boiler Flue Gas

Minimizing mercury emissions in coal-fired boiler flue gas, by virtue of the new mercury reduction rules, constitutes a major concern for many industrial and utility facilities. Several factors serve to complicate the task, including the extremely low concentration of Mercury in flue gas (possibly in the parts per trillion range), as well as the complexity of Mercury chemistry throughout the boiler system. Among the numerous interrelated factors that affect the amount of Mercury ultimately emitted at the stack are: coal type and composition, ash composition, flue gas temperature, acid gases, retention time in air pollution control devices, and boiler system design.

Mercury typically exists as a trace element in coal on the average order of 0.1 ppmw, although this concentration varies considerably among different coal types. As coal is burned in large boilers (at around 1700°K), the Mercury in the coal volatilizes to form thermodynamically favored gaseous elemental Mercury. In the subsequent cooling of the combustion gases, interaction with other combustion products results in a portion of the elemental Mercury being converted into gaseous oxidized forms of Mercury. Oxidation is beneficial because oxidized Mercury compounds are generally water-soluble and are therefore, more effectively captured by wet scrubbers and other pollution control devices. Conversely, elemental Mercury  is difficult to control and is likely to enter the global atmospheric cycle because of its high vapor pressure and low water solubility. A small portion of elemental Mercury and oxidized Mercury adsorbs onto flue gas particulates, forming particle-bound Mercury. Thus, Mercury exists in three different forms in coal combustion gas - elemental, oxidized, and particle-bound - with the gaseous (vapor stage) forms being the most prevalent in stack emissions.

The task of obtaining relevant information regarding the specific behavior of Mercury in any given boiler system entails taking a variety of Mercury concentration measurements (speciated, particle-bound and/or total Mercury) along the flue gas stream. This data, along with information regarding the trace Mercury composition of the coal and material by-products (fly ash, bottom ash, scrubber liquor, etc.), can then be evaluated with respect to desired Mercury emission limits (or compliance with a facility's regulatory emission limit). This data can also be used to determine whether existing pollution control devices and/or operating parameters have the potential to oxidize or absorb the elemental mercury to affect the amount of mercury retained in the control device.