Environmental Engineering Reference
In-Depth Information
In a sludge incineration process, water in the sludge is completely evaporated and the
organic matter in the sludge is effectively oxidized at high temperatures to CO
2
and H
2
O, as
shown in the following two reactions:
Evaporation:
H
2
O (
l
) → H
2
O (
g
)
Oxidation and combustion:
sludge solids/organics + O
2
→ CO
2
+ H
2
O + ash + heat
Since the water evaporation reaction is highly endothermic, in order to sustain the
combustion for sludge with a low TS content, dewatering of the original sludge must be
conducted or additional fuels (bark, wood waste and oil, etc.) shall be added (Monte et al.,
2008). Monte et al. (2008) illustrated that self-supporting incineration (combustion) can be
attained for feedstock containing approximately > 25% combustible/organic content and an
ash content of 0-60%, where additional fuels are not necessary. Evaporation, oxidation and
combustion occur simultaneously. The energy required for evaporation can be estimated
using an equation from Kim and Parker (2008):
Q
drying
=
M
ws
×
W
× [(
C
p,water
× Δ
T
) + Δ
H
vap
] + [
M
ws
× (1-
W
)] ×
C
p,sludge
× Δ
T
(2)
Where
M
ws
is unit mass of wet sludge using a basis of 1kg,
W
is the water fraction in
sludge, Δ
H
vap
is latent heat for vaporization of water (2090 kJ/kg),
C
p,water
is heat capacity of
water (4.186 kJ/kg/
o
C),
C
p,sludge
is heat capacity of solids in sludge (1.95 kJ/kg/
o
C) and Δ
T
is
temperature difference between initial temperature of 25
o
C and the temperature of drying of
105
o
C. This results in an approximate energy input for drying of 2198 kJ/kg. Further energy
input is required to raise the temperature of the sludge from drying temperature of 105
o
C to
an average reactor temperature of 800
o
C. The further energy input can be estimated at 2753
kJ per kg of the sludge with 10% total solids (TS). An estimated energy consumption of the
evaporation energy input plus the energy required for heating the feedstock to the reactor
temperature is thus approximately 5000 kJ/kg. Approximately 30% of the solids remain in the
ash (Fytili and Zabaniotou, 2008), therefore 70% is combusted. Energy output of the
incinerator may be estimated given the steam generation data for the incinerator.
2.2. Pyrolysis
Pyrolysis is a thermal decomposition process in the absent of oxygen to convert biomass
or waste materials into solid charcoal, water, water-soluble organics (pyroligneous acids,
including methanol and acetic acid); water-insoluble organics grouped under the term of “tar”
or “bio-oil”, and non-condensable gases (H
2
,
CH
4
, CO, CO
2
).
The chemistry of pyrolysis mainly associates with degradation of carbohydrate polymers
(cellulose and hemi-cellulose) and conversion of carbohydrate components, which may be
illustrated in Figure 4.
When heated at a temperature higher than 300°C, the carbohydrate polymers de-
polymerize into short chains of sugars, accompanied by slow dehydration and subsequent
reactions to form unsaturated polymer intermediates that may be eventually condensed to
form char (Lomax et al., 1991). When heated at a higher heating rate to a higher temperature,
the de-polymerization reactions will liberate volatile products as oil or tar. Cleavage of C-C
