Civil Engineering Reference
In-Depth Information
advanced heat integration technology, where the energy not used to generate
electricity, is re-used by the plant, or by adjacent industries or municipal
district housing schemes.
For more detailed discussions of waste incineration, readers are referred
to the topics by Brunner (1991), Wentz (1989), Williams (2005) and Dean
(1988) which in combination, give a comprehensive view of the processes
involved. In addition, The World Bank Technical Guidance report (1998)
on 'Municipal Solid Waste Incineration' provides an excellent overview of
the issues influencing these activities.
12.3 Combustion products
Combustion of MSWI waste produces two principal combustion products:
those which fall from the incinerator and those which rise from it during
combustion. Both Sabbas et al. (2003) and Wiles (1996) describe in some
detail the combustion products and distinguish between the following streams
(note that mass fraction estimates are from Sabbas et al. (2003)):
∑
Bottom ash
: Solids discharged from the furnace bottom, principally
by falling through the grate, noting that the older term 'clinker' is in
common usage in europe and that 'grate siftings' or 'riddlings' (materials
discharged during movement of the grate) are usually incorporated with
the bottom ash. Together they represent 20-30% by mass of the original
waste on a wet basis.
∑
Heat recovery ash; 'economizer' ash
: Modern incinerators incorporate
heat recovery devices and particulate ashes collected from this stage
may be combined with either the bottom ash or air pollution control
residues depending on the reactor design. They represent around 10%
of the original wet mass of waste.
∑
Fly ash
: Particles recovered from the flue gasses, prior to their chemical
cleaning by electrostatic precipitation and/or injection of reactive solvents.
Such ashes are enriched in volatile components (Pb, Zn, Cd, Sn, Sb, Na,
K, Cl, S etc.) making their bulk composition distinct from the bottom
ashes. In total, fly ashes account for 1-3% of the wet waste mass.
∑
Air pollution control
(APC) residues: The products formed after flue
gas cleaning, by reaction with injected solvents and sorbents. These
contain precipitates from reactions with solutes in the spray in addition
to fine ash particles and represent 2-5% of the original waste mass on
a wet basis. These APC residues may be in solid, liquid or sludge form
(Sabbas et al. 2003) depending on the scrubbing technology adopted
and may be mixed with the fly ash fraction.
The different composition of these materials governs their potential re-use
in construction and only certain fractions lend themselves readily to concrete
