Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 27

4
Mercury Speciation Transformation During Coal
Combustion
4.1 Introduction
During the coal combustion process and the cooling process of flue gas, mercury
experiences a complex physical and chemical transformation. Various factors will
affect the speciation distribution of mercury in flue gas. Such factors include the
type of coal being used, temperature, reaction conditions, flue gas composition, fly
ash composition, and so on.
Mercury is a volatile element, and mercury compounds in coal are thermally
unstable. There is a special characteristic that when the temperature is higher than
700 °C, mercury compounds will be decomposed into Hg 0 . In the coal combustion
chamber where the temperature is usually more than 1,000 °C, almost all mercury
compounds in coal, such as pyrite and cinnabar containing mercury, are converted
to gaseous Hg 0 .
When the temperature gradually decreases as the flue gas flows through the
various heat transfer equipment, the Hg speciation continuously changes. Some
gaseous Hg 0 are adsorbed by coal cinders or fly ash through several methods of
physical adsorption, chemical absorption, and chemical reaction to form Hg p , which
may exist as a speciation of HgCl 2 , HgO, HgSO 4 , HgS, etc., on the surface of a
particle. Some gaseous Hg 0 react with other flue gas components to form Hg 2+
compounds when the flue gas temperature falls. Many researchers believe that
chlorine containing substances play a key role in the process of Hg oxidation. The
reaction between Hg 0 and Cl is usually considered as one of the main mechanisms
for mercury transformation in the cooling flue gas. Gaseous Hg 2+ in the flue gas is
mostly considered to be HgCl 2 (g) [1] . Some Hg 2+ compounds are emitted as the
gaseous form with the other flue gases. There is still some gaseous Hg 0 existing in
the flue gas without changing speciation.
In this chapter we will describe in depth the mercury speciation transformation
during coal combustion using different research methods. We mainly focus on the
mercury emissions from coal of different combustion types, such as grate-firing,