Civil Engineering Reference
In-Depth Information
100
80
60
Average MSWI (a)
Average MSWI (b)
Highest value
40
20
Lowest value
0
0
2
4
6
8
10
12
14
16
18
20
Mean particle size (mm)
12.6
Particle size distribution in bottom ashes from Cataluña, Spain
After Chimenos et al. (1999).
of varying composition, which represents a percent or so of the total mass.
Carbon-rich, these particles have the potential to burn more fully, yet the
residence time in the incinerator was insufficient to allow them to do so.
Increasing the burn-time is an unattractive option as it reduces the overall
energy efficiency of the plant, so the compromise is to allow a small (~1% of
the original wet waste mass) to remain incompletely burnt. Bone fragments
may also be present in the bottom ash, converted wholly or partially to
apatite (Ca
5
(PO
4
)
3
(OH,F,Cl)).
12.3.2 Fly ashes
The fine material (see Fig. 12.7) rising up the combustion stack comprises
incombustible, inorganic solids such as molten glass fragments, and crystalline
products from combustion. This fraction is collected by electrostatic
precipitators and filters and represents 2-3% of the original wet mass of
waste. Incorporated with these unburned fragments are condensates and
reaction products from the gas phase and these distinguish the fly ash from
the bottom ash.
Youcai et al. (2002) groups flue gas impurities into four groups:
∑ acid gases such as HCL, HF, SO
2
, NO
x
;
∑ products of incomplete combustion, such as hydrocarbons, CO, dioxins
and furans;
∑ dust with heavy metals;
∑
volatile heavy metals.
