Page 105 - New Trends In Coal Conversion

68 New Trends in Coal Conversion

Group 3
Hg
Volatized and emitted fully
in the vapor-phase not
Br Cl F
enriched on the fly ash
Increasing volatility As Cd Ga Ge Pb Enriched in the fly ash and
BSe I
Group 2
depleted in the bottom ash
Sb Sn Te Ti Zn
Ba Be Bi Co Cr
Cs Cu Mo Ni Sr Ta
UV W Group 1
Equally distributed between
Eu Hf La Mn Rb
bottom ash and fly ash
Sc Sm Th Zr

Figure 3.4 Classification of trace elements by their behavior during coal combustion and
gasification
Reproduced with permission from Swanson, S.M., Engle, M.A., Ruppert, L.F., Affolte, R.H., Jo,
K.B., 2013. Partitioning of selected trace elements in coal combustion products from two coal-
burning power plants in the United States. Int. J. Coal Geol. 113, 116-126.© 2013. Elsevier.

Zevenhoven and Kilpinen (2001) found that temperature, not particle size, dictates
whether a certain TE or TE compound is volatile. For example, due to the lower
temperatures in pressurized fluidized bed combustion and integrated gasification
combined cycle, fewer TEs are released compared with pulverized coal combustion.
Díaz-Somoano and Martínez-Tarazona, (2003) found that other operational parame-
ters and gaseous components such as HCl or SO 2 are important in these systems.
Although the TE behavior from coal combustion and gasification processes is a rela-
tively new research topic, many studies have been developed in the last decades. These
studies have shown that TEs can be categorized into three main groups in regard to their
partitioning during coal combustion (Clarke and Sloss, 1992; Xu et al., 2003)(Fig. 3.4).

Group 1: elements that are approximately equally concentrated in the fly ash and bottom ash
or show little or no small particle enrichment. Examples of these elements are Be, Co, Mn, Th.
This group of elements is normally removed from the streams by particulate control systems.
Group 2: elements that are enriched in the fly ash relative to bottom ash or show increasing
enrichment on the fine-grained particles which may escape particulate control systems.
Examples include As, Cd, Cu, Pb, Sb, Se, Zn.
Group 3: elements which are readily volatilized during the combustion process and are mainly
concentrated in the vapor or gas phase. Examples are primarily Hg and in some cases Se.
However, some elements, such as Cr, Ni, U, and V, may show intermediate parti-
tioning behavior between Group 1 and 2, and volatile elements, such as Se, may
display a partitioning behavior intermediate between Group 2 and Group 3.