Environmental Engineering Reference
In-Depth Information
paper industry, a fluidized-bed incinerator (Latva-Somppi et al., 1997). The incinerator yields
products of steam and byproducts of ash and flue gas. Ash can be disposed directly into the
nearest landfill or it can be used as the raw materials of light-weight aggregate and
constructional brick in the building industry (Liaw et al., 1998; Monte et al., 2008). The flue
gas requires treatment through air pollution control before discharge through the stack.
However, specifically designed fluidized-bed combustors produce fewer pollutants through
the flue gas (Kumar, 2000). For power generation the steam is directed through steam
turbines, which work to produce power through the electric generator. Alternatively steam
can be used locally for process steam reducing the mill's dependence on costly fossil fuels for
steam production (Monte et al., 2008). Typical fluidized bed operating temperature is
between 700-900
o
C (Latva-Somppi et al., 1997). While depending on specific waste
regulations in Europe 850
o
C must be achieved for at least 2 seconds, and if hazardous waste
with a content of more than 1% of halogenated organic substances, the temperature is raised
to 1100
o
C for 2 seconds in order to reduce the formation of the toxic compounds such as
polychlorinated dibenzodioxins (PCDDs) (Monte et al., 2008). Efficient heat and mass
transfer in the fluidized bed enables the endothermic fuel drying and devolatilization to occur
simultaneously with exothermic combustion of char and volatiles (Latva-Somppi et al., 1997).
Incineration of residues (both rejects and sludge), combined with power and steam
generation, is one of the most commonly applied disposal methods in Europe (Monte et al.,
2008). Sludge incineration is widely practiced on a full-scale basis in many highly populated
urban areas such as in Japan and Germany (Stark et al., 2006). A recent review by Monte et
al. (2008) cited eleven European pulp mills which burn some portion of pulp sludge in
combination with other biomass. Oral et al. (2005a/b) described a three-stage retrofit of an
incinerator in the Czech Republic and concluded that the thermal treatment through
incineration of sludge for energy recovery is favorable both economically and
environmentally.
Figure 3. General flow diagram of incineration (modified from Kumar, 2000)
