LAUSR

Abstract Primary NOx control methods used on coal fired utility boilers utilize ammonia- and urea-based processes. These processes result in some ammonia exiting the treatment zone, which can react with SO3/H2SO4 in the coal fired flue gas to form ammonium bisulfate (ABS). This ABS formation can lead to potential balance of plant operational issues: • deposition on the SCR NOx control ammonia injection components in cases of insufficiently heated dilution air for ammonia transport • deposition on catalyst surfaces during low load operation, reducing catalyst activity • deposition in the cold end baskets of the air preheater Mitigating these impacts requires an accurate understanding of the ABS formation temperature as a function of flue gas composition. Five past studies of ABS formation temperature exhibit a very large variation. This paper provides a critical review of these studies, along with new experimental data, in order to show that ABS formation is best described by recent laboratory work, with the ABS formation temperature effectively described by the equation: (1) P NH 3 ( atm ) ∗ P SO 3 ( atm ) = 2.97 ∗ 10 13 ∗ e ( - 54 , 950 / RT ) where R is the universal gas constant (1.987 cal/K-mol) and T is the flue gas temperature in degrees Kelvin (K). With the increase in renewable fuel sources and the increasing use of natural gas for electricity generation, coal fired boilers are called upon to operate over a broader load range. Using the above expression, this work shows that the generally accepted value for ABS formation temperature is nominally 5% (14 K or 25°F) too high. While this difference may seem small, from a practical perspective it can represent substantial operating cost savings, as a large coal fired plant can operate at a lower minimum load than is currently required by the catalyst vendor‘s prescribed SCR minimum operating temperature based on ABS formation temperature from prior work.