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THERMAL INSULATION
L
owering energy consumption and heat
loss in cement plants has gained in
importance due to economic, legislative
and sustainability factors. Reducing
a plant's carbon
footprint
and the
implementation
of
energy management
systems, together with mitigating
greenhouse gas emissions have been
milestones in this development process.
Thermal insulation reduces heat loss of
refractories with low thermal conductivity
while, at
the same time, allowing the
cement manufacturing process
to
continue
smoothly without affecting the properties
or quality of the cement being produced .
Although the production of cement
demands a theoretical energy of 1650-
1800kJ/kg clinker,l.l the real values are
higher. They range from 3000kJ/kg clinker
over an average of -3500kJ/kg clinker
up
to
8000kJ/kg/clinker for plants with
low thermal efficiency. Reducing energy
input
is
based on the optimisation of plant
technology and increasing the thermal
insulation capability
of
the refractories.
Insulation is possible
in
the kiln and
in static units, mainly the pre heater.
In addition to the essential insulating
properties, mater ials have
to
enable an
uninterrupted cement production process.
Demands on therm l insulation
As air
is
the best thermal insu lator (aside
from a vacuum), insulating refractory
products are characterised by high
porosity. However, increased porosity
means
that
the strength, refractoriness
and corrosion resistance of refractory
linings decreases.
The correct selection of materials can
counter these challenges to some extent.
The most commonly-used materials for
insulating refractories include calcium
silicate (thermal application limit: 1100
C),
expanded clay 1200 C) and fireclay
1300
C).
4
Additionally, vermiculite,
Figure 1: wet corrosion of steel shell by temperatures below dewpoint for differen t
insulation concepts
160
regular insulation thickness
140
120
.a
100
0
E
Q;
{;3
Q;
so
60
optimized area
low heat
losses
low
risk df corrosion
Q
:;;
.
40
area of wet
corrosion
below dew
point
20
10
20 30 40
5
60 7
BO
90 100
insulation thickness
[mm]
refractory
lining
example
wear lining: 114 mm fireclay brick
insulation
: e
lightweight expanded clay
fireclay
gunnable gunnable
T[inside] = 5 C
T[ambient]
= 2
C
no wind
which ha s
an
application limit of
950
C,
is
gaining more attention due to its thermal
stabi lity and chemical resistance. This
is of
particular importance given the increasing
use of alternat ive fuels.
Typical requirements for insulating
materials are:
low thermal conductivity
thermal resistance
chemical resistance
mechanical strength
volume stability.
orrosion mechanisms
in preheaters
Corrosion reactions t low and high
temper tures
In
the upper cyclone stages
with
a gas
temperature of 180 -500
C,
corrosion of
the steel shell
is
a wet process (rusting), as
0
vermiculite
gunnable
calcium
silicate
plates
its temperature
is
below the dewpoint of
water of about 60 C see Figure
1).
In
the presence of chlorides, electrolytic
reaction reduces the strength
of
the
metallic anchors
of
a monolithic lining
and of the steel shell and thus the stabi lity
of the construction. If the dewpoint of
sulphuric acid (-70-140 C) is reached,
wet corrosion is accelerated. Although the
reaction mechanism is different, the result
is similar
but
is reached much earlier, so
the deiA point must be avoided.
In the lower cyclone stages,
hot
gas
corrosion may take place, especially in the
presence
of
hydrochloric acid. Reaction
with
sulphatic compounds, which in the
presence
of
oxygen and chlorides will
cause corrosion
of
the metallic shell, are
possible see Figure 2).
Therefore, in all preheater stages
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THERMAL INSULATION
igure 2: hot gas corrosion y increased chloride concentration
~ b o v e
9
OJ insulating
concretesore superior to
calcium silicate plates
because they maintain
their
dimensional
stability
deposits
oxide
metal
Source German Associa tion
of aterial
Science
not only must the cement production
parameters be observed but also the
process parameters for secondary
compounds, temperatures and
atmospheres
to
prevent the formation
of
deleterious substances.
In addition,
hot
gas corrosion of
chromium-containing anchors has to be
prevented as the formation
of
hexavalent
chromates is possible. This must
be
avoided for structural stability and
environmental reasons.
Sigma phase embrittlement o
metallic anchors
Sigma phase is
an
intermetallic chrome
iron phase
that
occurs between 700-
900
C n heat-resistant steel alloys
with chromium contents of>17 per
cent. Continuously exposed
to
these
temperatures, the steel embrittles
because of the sigma phase formation
see Figure 3). A reduction in resistance to
corrosion mechanisms (like the depletion
of
chromium) , plus a loss
of
refractory
properties and oxida tion resistance is
therefore possible. Anchors exhibiti ng the
sigma phase are very brittle and break
easily under mechanical loads. A dropping
of anchored concrete fields may occur.
ining concepts for insulation
Calcium silicate boards
A worldwide accepted l ining design is the
installation of calcium silicate boards.
These should
be
installed carefully
so
that hot
gases cannot reach the kiln shell
through gaps and joints see Figure 4).
Apart from the environmental benefits,
proper ins tallat ion reduces energy loss and
increases the l ifetime of the lining.
A drawback of this concept
is
thermally
induced shrinkage when calcium silicate
Figure 3: sigma phase
embrittlement
of metallic anchors with
chromium contents> 17
ndependent welding in the
oil &
gas industry has
become reality
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plates are exposed
to
higher temperatures
see
Figure
5).
This
will
generate thermal
bridges in the lining from the
hot
calciner
atmosphere
to
the shell. Kiln gases then
diffuse more easily through the gaps
between the calcium silicate and the
working lining. Consequently, acids and
alkalis can condense on the anchors or
on the steel shell, causing subsequent
corrosion and damage
to
the lining.
If calcium silicate materials are used
as insulation in statically-stressed plant
units, shrinkage can endanger the entire
system. In the statically-defined system a
closed ring can lose its locking force
so
th t
it opens in the keying section, which may
lead to the collapse of the entire lining.
nsulating castable lining
As
an alternative
to
calcium silicate plates,
various lightweigh t refractory concretes
are available see Table 1). These can
be
installed by rodding, trowelling, casting
or
gunning. The mos t advantageous of these
three processes is gunning as
it
offers the
quickest installation time and high lining
quali ty. Gunning mixes also have the
following benefits:
selection in terms of heat transfer and
insulation thickness
improved homogeneity of the
insulation layer
good enclosure
of
the anchors for a
homogeneous lining
castables with improved
refractoriness. The emergency running
properties are sufficient
to
continue
production temporarily without fatal
failure.
For example, in the uppermost
((
nstallation
of
nsulating
concretes
on the
basis
of
l r e c l a ~ expanded
clay or vermiculite
give
considerable
advantages
regarding thermal
stability and
emergency
properties
over the time
-
consuming
installation
of
calcium
silicate plates .
Fo
-
x
a w rle
Y ~ r ' r ~ r
W ~ o . Y
cyclone stage, a one-layer lini ng
with
hig
strength and optimised insulation using
a lightweight fireclay concrete has been
installed.
Figure 5: thermal stability of calcium silicates and an expanded clay insulation castable
calcium
silicate
plates
700
oc
1000
oc 1200 oc
expanded
clay gunnable
Figure 6: relationship between porosity and conductivit y
temperature
I
C
100
800
900
I I
10 00
11 00
00
1
7Q0
I
12 00 13 00
90
0.14 0.18 W/mK 95 apparent porosity
80
0.17 0.20 0.23 W/m K
*
70
85
%
apparent
porosity
i "
;:;;
60
0
0
c:
50
.25 0.30 W/mK
65 apparent porosity
a
40
-
.40 0.50
WmK
30
40
%apparent porosity
+ calcium silicate plates vermiculite + expanded clay + lightweight clay
gunnable gunnable gunnable
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As Figure 6 shows, a vermiculite -
based concrete comes very close to the
calcium silicate plates in terms of thermal
conductivity. Above 900 C insulating
concretes are superior to calcium silicate
plates because they maintain their
dimensional stability. A homogeneous
concrete lining can be applied in
thicknesses starting from abou t 35mm
and meets demands in terms
of
calculated
and expected shell temperatures.
An
optimum balance between heat loss and a
shell temperature that provides corrosion
protect ion can be ensured.
Figure 7 shows vessels that can be lined
with insulating vermicul ite concrete. The
areas down to
the lowest cyclone stage
should still be lined
with
materials based
on expanded clay and lightweight fireclay,
given
their
higher refractoriness.
Following the loss of the wear lining, an
insulating concrete
that
remains
in
place
is
the better option. Production usually
has to stop in this case, as calcium silicate
plates are less resistant to kiln gases and
kiln feed dusts. Therefore, it
is
possible
to
obtain some additional service life after
loss
of
the wear lining, enabling repair at a
later stage.
ractical application
o
an
insulating concrete
A direct comparison of the different
systems used in a plant is demonstrated
using a satellite cut
see
Figure
8).
This
involves the lining of a double dedusting
THERM L INSUL TION
cyclone that was installed on the central
air outlet
of
a cement cooler.
shell temperatures
but
gave more security
regarding emergency properties and also
kept the lining away from the dewpoint of
acidic compounds.
Comparison ofthe lining with calcium
silicate plates in one cyclone and a
vermiculite
concrete lining
in the other
is
possible,
covering the
installation and
the resulting
outside
temperatures .
The vermiculite
lining was
gunned for
insulation of the
areas shaded in
grey, while all
other areas were
insulated with
calcium silicate
plates.
The concrete
lining was
installed at
twice the rate
of the calcium
silicate plate.
According to
measurements,
the gunnable
vermiculite
lining concept
showed only
slightly higher
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THERMAL INSULATION
he importance
o
good
insulation and corrosion
prevention
Industry. Third edition. Wiesbaden / Berlin,
Germany: Bauverlag.
3
THOMAS SAND BEIMDIEK K (2012) REFRALITE
20 G the
innovative
supplement to
Refratechnik s
insulating
concrete concept.
Proceedings Refratechnik Symposium
Berlin
2012, p199-208.
4
ROUTSCHKA, G AND WUTHNOW, H (2011)
Praxishand buch Feuerfeste Werkstoffe 5
Auf/age. Essen, Germany : Vulkan Verlag.
To reduce heat loss in a cement kiln good
insulation and corrosion prevention
are essential. This includes considering
the metallic corrosion
of
vessel shells
and anchors
so
that
the possibility
of
acidic reaction as well as sigma-phase
embrittlement
is
reduced or even
prevented.
Figure 7: fields of application of a vermiculite concrete
s
two layer lining
Installation of insulating concretes
on the basis
of
fireclay, expanded clay
or vermiculite provide considerable
advantages in terms
of
thermal stability
and emergency properties over the time
consuming installation of calcium silicate
plates. This ensures a long-term and
satisfactory service life of the refractory
lining and a prolonged lifetime
of
the steel
structures, thereby improving performance
of the entire cement plant.
REFEREN ES
1
CEMENT SUSTAINABILITY INITIATIVE CSI)
(2009)
Developmentof he State of he Art-
Techniques in Cement Manufacturi ng.
CSI
/
ECR Technology Papers. Geneva,
Switzerland/
Dusseldorf, Germany.
2
DUDA WH (1985)
Cement Data Book. Volume 1:
nternational
Process Engineering in the Cement
inside
temperature
:5 850 o
peak temperature
1150 o
Figure 8: installation of a
gunn ble
vermiculite concrete
comp red to
calcium silicate plat es
outside
temperature:
5
C I
wind
velocity: approx. 2.5 mls
temperature
of
central air before entering
double
cyclone:
measuring plane :
469
C I measuring plane 2:
447
C
D vermiculite gunnable D
calcium
silicate
plates
measuring
points: vermiculite gunnable
right
side)
calcium silicate plates
left
side)