CA Silicate & Other Insulation - Surface Dew Point

7/26/2019 CA Silicate & Other Insulation - Surface Dew Point

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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


2/5

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

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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


4/5

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)

CA Silicate & Other Insulation - Surface Dew Point

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Transcript of CA Silicate & Other Insulation - Surface Dew Point