Buckling of Chimney - A Diagnostic Investigation

by

Y P Sethi, S K Chaturvedi, S N M Khan and M Vasudeva

National Council for Cement and Building Materials, Ballabgarh (India)

1.0 INTRODUCTION

A cement plant, located in one of the Southern States of India, was facing the problem of

corrosion of chimney shell of precalciner string of kiln system. The rotary kiln is equipped with

two strings of preheater; kiln string having four-stage preheater cyclones and precalciner string

having six-stage preheater cyclones. The chimneys in both the preheater and precalciner strings

of kiln circuit were made of mild steel and installed in 1983 and 1998 respectively. The service

life of kiln string chimney was around 20 years. When chimney shell thickness was measured in

July 2007, a high wear rate of the chimney shell of precalciner string was observed that

subsequently led to the state of buckling. The chimney shell was gradually chipped off in the

form of flakes leaving the residual shell thickness as low as 2 mm. National Council for Cement

and Building Materials (NCB) undertook the diagnostic investigations to identify the causes of

buckling of chimney due to corrosion and suggested remedial measures.

2.0 CAUSES OF BUCKLING OF CHIMNEY

The buckling of metallic shell is caused by the progressive deterioration and wear rate of

shell material mainly due to corrosion. Corrosion of metallic shell is influenced by a number of

factors such as composition of the metallic shell, nature of the gases in contact with the shell and

their temperature etc. Steel, when exposed to an industrial atmosphere, reacts to form the

corrosion product (Fe2O3.H2O), which wears out the material gradually.

Corrosion also occur because of condensation of flue gas containing H2O, SO2 / SO3,

NOx and HCl. Massive and rapid corrosion of metal is usually caused by the action of sulphuric

acid formed by the oxidation of sulphur. This condensation of gas occurs on surfaces that are

below its dew point. The dew point of water, nitric acid and hydrochloric acid is much lower

than that of sulphuric acid. Hence, the limiting dew point is that of sulphuric acid. Typically, the

maximum corrosion rate occurs at 15-20oC below the dew point as shown n Figure 1.

FIG 1 CORROSION RATE AS A FUNCTION OF WALL TEMPERATURE

110 120 130100

WALL TEMPERATURE OC

Dew Point

CO

RR

OS

ION

RA

TE

Peak Corrosion

Fig 1 : Corrosion Rate as a Function of Wall Temperature

Page 1 of 4 Article downloaded from 11th NCB International Seminar

3.0 MECHANISM OF CORROSION OF METALLIC CHIMNEY

The sulphur originates mainly from the fuel being used for the burning of the raw

materials in the cement rotary kiln where as the raw materials used for cement making is a major

source of sulphide sulphur. The possible reactions for the oxidation of sulphur leading to SO42-

formation from pyritic sulphur are shown below:

2 FeS2 + 7 O2 + 2 H2O = 2 Fe2+

+ 4 SO42-

+ 4 H+

2 FeS2 + 7 O2 + 2 H2O = 2 FeSO4 + 2 H2SO4…………………...(1)

4 Fe2+

+ O2 + 4 H+ = 4 Fe

3+ + 2 H2O

4 FeSO4 + O2 + 2 H2SO4 = 2 Fe2(SO4)3 + 2 H2O……………….(2)

The Fe3+

ion (ferric ion) can also oxidize pyrite that facilitates the formation of compound

sulphuric acid under the influence of excess oxygen and water as per the reactions shown below.

FeS2 + Fe2(SO4)3 = 3 FeSO4 + 2 S…………………………….(3)

2 S +3 O2 + 2 H2O = 2 H2SO4…………………………………(4)

4.0 DIAGNOSTIC INVESTIGATIONS IN A CEMENT PLANT

4.1 Observations

The measurements of operating parameters were carried out at preheater outlet of both

preheater strings and at various points along the height of the chimneys as shown in Figure 2.

The observations and their impact particularly on corrosion of mild steel chimney are as under:

1C

PC

2

COAL

E.S.P.

PA FAN

3

KILN

GRATE COOLER

SEC. AIR

4

COOLER FANS

1A

1B

3

1

6

5

4

2

G

C

T

E.S.P.

T

C

G

STACK

STACK

ESPFAN

FANPH

COAL

T A D

FEED

FEED

PCFAN

FIG. 2 PROCESS MEASUREMENTS IN KILN CIRCUIT

Fig 2 : Process Measurements in Kiln Circuit

i) O2 and CO levels at the preheater outlet of calciner string were measured and

found to be 2.3-2.4 % and 270 ppm respectively, which indicates satisfactory

combustion in the precalciner. However, O2 level at preheater outlet of kiln string

was found to be 6.3%.

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ii) The temperature of gases at the outlet of calciner and kiln strings was found to be

3420C and 364

0C respectively. The temperature at this location for calciner string

with 6-stage preheater cyclones should be around 3000C.

iii) The temperature of preheater gases in the calciner string is brought down to a

temperature of 2100C after GCT. It indicates that high quantity of water is sprayed

inside GCT for the purpose of conditioning of hot preheater gases.

iv) The temperature was also measured at four different floors of the chimney at

different time intervals, which was found to vary between 94 and 970C.

v) At the prevailing conditions of the kiln, the dew point of sulphuric acid was

calculated to be between 93 and 950C. In order to combat the corrosion of

chimney, the gases in contact with its surface should always be above 950C. The

data provided by the plant indicated that the temperature of gases at the chimney

of calciner string was low at around 800C. The condition of low temperature at

ESP chimney is conducive to initiate and intensify corrosion of its shell.

vi) The temperature of gases after the GCT in kiln string is relatively high (170-

1800C). Low quantity of water is thus needed for spraying in GCT to bring down

the gas temperature from 3600C at preheater outlet to170-180

0C, which possibly

reduces the severity of chimney corrosion. The temperature of gases at chimney

of kiln string was found to be 158 0C, much above the dew point of sulphuric acid

and a safe condition to avoid the danger of acid corrosion attack.

4.2 Characterisation of Samples and Interpretation

The analysis of limestone indicated the presence of 1.11-1.24% total sulphur. The soluble

sulphur as SO3 in limestone was found to be 0.14-0.15% whereas the chloride content was found

to be 0.004-0.009%. The analysis of additives such as bauxite, iron ore and laterite indicated that

sulphur was present only in laterite to the extent of 0.51% as SO3. However, the chloride content

was found to be 0.005%, 0.005% and 0.009% respectively in these additives.

The characterisation of samples of ESP dust, collected from kiln and precalciner strings,

indicated the concentration of SO3 as 0.62% and 0.39% respectively. The ESP dust sample of the

second day was also evaluated for SO3 and the same was found to be 0.45% and 0.20%

respectively. The samples of GCT dust, collected from kiln and precalciner strings, indicated the

concentration of SO3 as 0.42% and 0.49% respectively. The analysis of GCT dust sample of the

second day also indicated the presence of SO3.

The analysis of corroded chimney flake samples, collected near the stack area, indicated

that the concentration of SO3 was 5.37% and 4.71% respectively for first day and second day.

Since these samples were collected from outside near the bottom of the chimney, it was

presumed that these are the material coatings on the surface of the corroded steel of chimney.

The samples of limestone and corroded chimney flakes were also subjected to XRD

investigations to find out the minerals present. The results indicated the presence of sulfur

containing minerals such as FeS2 (pyrite).

4.3 Recommendations

i) Preheater exit gas temperature particularly of precalciner string should be

controlled close to 300 0C.

ii) The temperature of gases after the ESP of precalciner string should always be

maintained above 95 0C. If the need arises, gases after the GCT should be allowed

at a relatively higher temperature.

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iii) Sulphur bearing additives namely laterite, should be discouraged and substitute

additive may be explored.

iv) The inner surface of the chimney stack should be painted with a polysiloxane

based air drying aluminium containing suspension, which protects the shell from

corrosion due to acid attack. Alternately, corrosion resistant coating materials

such as water glass bonded materials may be considered for the coating purpose.

v) Stainless steels containing high percentage of alloying elements (18% Cr, 8% Ni

and 35 Mo) are non corrodible. As this type of steel is highly expensive, low alloy

steel containing 2-3% Cr, which is also resistant to acid corrosion may be

considered, as a last resort, as a material of construction for the chimney.

5.0 CONCLUSION

The following measures could help in overcoming the extent of severity of corrosion of

mild steel that is responsible for high wear rate leading to buckling of chimney in kiln system:

i) Preheater exit gas temperature should be maintained as low as possible through

improved process efficiency. There could be various reasons for high gas

temperature that includes high gas velocity profile in the preheater resulting in

poor heat transfer between gas and the material, delayed combustion of fuel, by

passing of gases and irregular distribution of gas and material in two strings of

preheater etc. Low preheater gas temperature will not only improve the energy

efficiency of the plant but also reduce the demand of excess water requirement for

cooling the hot gases. The reduced water addition in the GCT will bring down the

extent of severity of corrosion.

ii) The temperature of gases after the ESP should be maintained above the dew point

of gases thereby alleviating the possibility of acid attack on mild steel chimney.

iii) The inner surface of chimney stack should be painted with polysiloxane based air

drying aluminium containing suspension. Alternately, corrosion resistant material

such as water glass bonded materials may be considered for coating purpose.

iv) The interaction of a metal or alloy with gases below the dew point is clearly of

vital importance in the performance of materials of construction. Low alloy steel

containing 2-3% Cr, which is resistant to acid corrosion may be considered as a

material of construction for the chimney.

ACKNOWLEDGEMENT

The authors have freely drawn upon completed R & D work / status reports of NCB and some of

the unpublished work in NCB as well as the published literature. This paper is being published with the

permission of Director General, NCB.

REFERENCES

1) Latest advances in the understanding of acid dewpoint corrosion : Corrosion and stress

corrosion cracking in combustion gas condensates - W.M.M. Huijbregts, R. Leferink;

Anti-Corrosion Methods and Materials, Vol. 51, 3 (2004), pg 173-188

2) Chemical Engineers' Handbook, 5th edition - Perry R H and Chilton C H ed., McGraw-

Hill, New York, 1973

3) Predicting Dew Points of Flue Gases - Verhoff F H. and Banchero J T Chem. Eng. Prog.,

August, 1974

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