How to control kiln shell corrosion

How to Control Kiln Shell CorrosionShree Cement Ltd, Beawar(Raj.)21/06/2012

6/21/2012 1How to Control Kiln Shell Corrosion

By:-Ankit karwa4th Year UG,MSEIIT Kanpur

Summer In Plant Training at Shree Cement Ltd,Beawar(Raj.)

6/21/2012 How to Control Kiln Shell Corrosion 2

Introduction

Use of Alternate raw Material and fuel in cement manufacturing process proved to be beneficial in terms of financial aspects.

the presence of deleterious volatile compounds in these fuels causes many problems such as kiln shell corrosion, build ups and attacking the refractory lining and reducing their campaign lives.

Among all these problems kiln shell corrosion is most serious problem as it acts silently and reduces the shell thickness to below critical structural and mechanical limits of stability of kiln shell.

Corrosion can be defined as the destruction or deterioration of material due to the reaction with its environment.

Best way to prevent corrosion is to prevent the volatiles and alkalis to reach upto kiln shell.

selection of proper refractory quality, adequate installation measures and highly oxidizing conditions in hottest zone of the kiln proved as the key to minimize the extent of kiln shell corrosion


Corrosion of cement Kiln

Corrosion of cement kiln shell is influenced by a number of factors composition of the metallic shell and its environment, temperature of the shell, cleanliness or roughness of the shell surface, its contact with other materials and severe process conditions.

Active Scaling agents of steel are: Carbon dioxide(CO2) and Sulphur dioxide(SO2)

sulphur dioxide increases rate of scaling and sometimes result in deep intergranular penetration of steel through formation of eutectic of liquid iron oxide – iron sulphide.This deleterious effect of SO2 can be controlled by providing excess Oxygen.

Rate of scaling also depends on presence of different element in steel for example chromium,aluminium and silicon through formation of protective oxide layer.Whereas sulphur in steel increases rate of attack. but influence of carbon is small.



Main reason of corrosion can be:alternate oxidation at high temperature acidic reaction at low temperatures when the kiln is stopped for repairs.

This corrosion phenomena mainly takes place because of oxides, chlorides and sulphide at high temperature.

Rate of corrosion depends uponShell material and surface conditionCorrosion mediumTime available and temperature



How corrosion takes place during kiln shutdownsdeposit of salts containing potassium chloride in particular on the shell becomes active because being hygroscopic it absorbs atmospheric moisture. the chloride can reach the kiln shell in the form of gases .

But alkalis can only penetrate refractory lining as a part of liquid potassium and Sodium salt melts.So if product of corrosion contain substantial amount of potassium and sodium ,the form of corrosion is termed as Hot Corrosion indicating that liquid phase take part in reaction.


Mechanism of Shell Kiln Corrosion

One of the most important reactions in the lining is the oxygen consumption where SO2 consumes oxygen and condenses as SO3 :

2 SO2 (g) + O2 = 2 SO3 (↓)This SO3 formed condenses as calcium or magnesium salts. The result can be that an oxidizing environment inside the kiln turns into a reducing environment at the kiln shell .

OxidationFormation of oxide scale by reaction of iron from steel to oxygenThese oxide layers will deposit in form of different layersThe oxide with highest oxygen content Fe2O3 ,will found at scale-brick interface The oxide with highest iron content FeO, will found at the metal-scale interface

Sulphidization SO2 acts as oxygen donor

4 Fe + 2 SO2 (g) = Fe3O4 + FeS2So a sulphidization reaction can be identified if corrosion product contain pyrite (FeS2) or

pyrrhotite (FeS).


Mechanism of Shell Kiln Corrosion

Sulphidization enhanced by presence of chloride mainly due tothey affect the morphology of the corrosion scale by hindering the formation of protective oxide layer

high temperature hydrolysis 2 KCl (g) + H2O (g) + SO2 (g) + ½ O2 (g) = K2SO4 + 2 HCl (g) (T > 900°C)

2 HCl (g) + ½ O2 (g) = Cl2 (g) + H2O (T < 400°C)

Once Cl2 (g) is formed, it can reach the kiln shell through the refractory bricks or through the gaps/joints within and between rings and will react with either the oxide-sulphide layers or, most likely, directly with the kiln shell

FeS + Fe3O4 + 4 Cl2 = 4 FeCl2 + SO2 + O2Cl2 + Fe = FeCl2

This how corrosion of kiln shell takes place.


Role of Refractories in tackling shell corrosion

protect the steel shell from the direct attack of deleterious gases and clinker melt to reduce the shell temperature so that steel of the shell does not loose its properties.

The entire system including, preheater, rotary kiln and cooler is lined with suitable size and quality of refractories to achieve the above mentioned advantages.

Passage of Volatiles through Bricks

As these volatiles pass through bricks they will form different feldsphatic compounds like Al6Si2O13, KAl.Si2O6, Ca.Al2Si2O8 and formation of most detrimental KCl will takes place.

Formation of these compounds in the brick matrix led to volume expansion and breaking of ceramic bonds ultimately leading to breaking or loosening of bricks.



Absorption of volatiles through the open pores in the bricks

Bricks have 3 types of pores:1. Through pores(open pores)2. Closed pores within brick body3. One side open and one side closed pores

Through pores are very dangerous from the point of view of damaging the brick structure as well as causing kiln shell corrosion .

These provide passage to volatiles to reach upto kiln shell. The best approach would be to check the permeability of these brick before finalizing them for kiln.



Passage of Volatiles through joints/gaps Volatile rich kiln gases and clinker reaches to kiln shell through joints and gaps between and within rings.

These gaps may occur due to various reasons such as: Individual brick behavior at working temperature Inadequate installation due to poor workmanshipActual service conditions prevailing inside the kiln

the passage of clinker liquids rich in volatiles and hot volatile gases is much more dangerous as their concentration at the place of corrosion becomes high and severe reaction takes place between kiln shell and clinker rich in volatile gases leading to shell corrosion.


Role of process parameter on shell corrosion

Process parameters also affect the shell corrosion

For ex. The kiln shell temperature is of major importance to the speed of shell corrosion .Carbon steel is poor in oxidation but as temperature increases rate of oxidation also increases.Rate of oxidation at 400°C is 14 times at 200°C and it increases manifolds with increase in temperature.

If temperature of surface in kiln is 300-400°C then inside temperature is much higher and rate of oxidation is very high which is not desirable.

Another process parameter example is concentration of O2 and CO at kiln inlet should be 1.5% minimum and .1 maximum respectively.


Role of process parameter on shell corrosion

Why high rate of corrosion in the burning/pre-burning zone is observed?sulphate bearing compounds gets recycled along with feed material and travel up to burning zone where they get partially dissociated and vaporized once again Due to this, the concentration of SO3 increases leading to the increase in partial pressure of these volatile gases over a period of time As a result, they get diffused through the open pores of refractory lining reach upto the kiln shell and initiate the corrosion process.

Even high levels of carbon monoxide due to incomplete combustion of fuel also increases the rate of corrosion.


Recommendations

The possible causes for the occurrence of shell corrosion in cement kiln could therefore be :Penetration of refractories and attack of alkalis on kiln shell Poorly designed Brick shape and size, Poor Brick quality in terms of porosity and permeability Inadequate Installation quality Improper combustion of fuel leading to reducing conditions inside kiln

The possible preventive measures coming out from the above studies to minimize the tendency and rate of shell corrosion phenomenon are as discussed below: A) Use of different gas tight coating or paints on the inner kiln shell to lessen the chloride attack

Shell should be coated with available coating or paint immediately after de-lining. This will also help in preventing the absorption of atmospheric moisture by hygroscopic compounds

such as KCl. Example of one such coating is :-KilnGard-600SCW manufactured by 3L&T This coating has already been used in some plants of Mexico, Colombia and Brazil.

These plants first used Sacrificial Stainless steel plate to combat corrosion but results was not good.


Recommendations

This figure shows after using SS plate how the thickness of shell varies.They observed a loss of more than 1.3 mm in shell thickness just in 11 months.Which is obviously not a good solution to corrosion problem.


Recommendations

This figure shows thickness of Shell after using KilnGard-600 SCW.

Thickness of the shell is practically same after 12 months of application.

This coating is applied by air spray on a sand blasted steel surface,The recommended thickness is .5mm.After drying at room temp. for about 12 hrs bricks can be installed.Coating will attain its maximum physical and chemical propertiesAfter heating the kiln during startup process.

Recommendations

B)Ultra-Z-Coat is zirconia based refractory coating.which is manufactured by wearresist technologies pvt. Ltd.

Ultra-Z-coat is characterized by a very high content of zirconia and other special ceramics imparts extremely high resistance to the aggressive environments typically encountered at temperatures up to 1900°C.A 3mm protective layer of Ultra Z coat can typically increase the working life of any underlying refractory by a factor of two to four times.

Coverage Ratio: 2mm thickness gained by application of 4 Kgs over a 1 Sq. meter area.

Benefits

1. Produced a gas tight surface and minimized energy losses. 2. Reduced the effects of thermal shock. 3. Reduced Spalling and eliminated cracking 4. Reduced Slag adhesion and increased resistance to corrosion 5. Increased the working life of kiln and reduced the costs of maintenance shutdowns.



Recommendations

C) Alkali by-pass systemis a duct between the feed end of the kiln and the preheater tower through which a portion of the kiln exit gas stream is withdrawnquickly cooled by air or water to avoid excess buildup of alkali, chloride and sulfur on the raw feed. This may also be referred to as the “kiln exhaust gas bypass”.

KHD manufactures alkali by pass system:

for using secondary raw materials and secondary fuels of significant chlorine and sulfur contents the Bypass System enables a flexible kiln operation at varying fuel and raw-material mixtures.

Recommendations

A portion of the volatile components will be removed from in the area of the inlet chamber by means of a bypass and thus be withdrawn from the circuit. This will lower the level of the volatile circuit to such an extent that coatings remain controllable and operational reliability.


Recommendations

D)The kiln shell temperature from outside should not exceed 300°C.

Shell Temperature profile of kiln 2.here maximum temperature Of shell is around 390°C which is close to 400.

Additional cooling fans should be provided for effective external cooling. the selection of bricks in this area should be reviewed with respect to their thermal conductivity and wear rate pattern. If no coating on bricks in that area, high wear rate leads to reduced residual lining thickness leading to higher shell temperature.


Recommendations

E) intrusions of volatiles also takes place through joints and gaps in between rings and individual rings.

So during selecting refractory lining 2 main characteristics should be given due attention namely

1.permeability :permeability plays a role in allowing gases/liquids to reach up to kiln shell.

2.reversal thermal expansion of bricks: The reversal thermal expansion gives an idea of extent of shrinkage of a particular brick undergo when the kiln temperature is reduced from its normal operating temperature to any lower temperature.


Recommendations

F) Application of spreader jack should be done in each ring at the time of closing the individual rings and due care should be taken so that bricks do not crush or develop internal cracks due to pressure applied by spreader jack.

Hydraulic Spreader jack By using Spreader jack we can fix the brick ring with very less gaps in between refractories. So chances of alkali attack through gap in between brick will be very less. In no case chiseling or hammering of bricks should be done. The ring should be closed using key bricks instead of cutting and fixing with the help of hammer.


Recommendations

G)In context of corrosion of kiln shell and heat loss use of Basic refractory is highly recommended. Although basic refractories are costlier than neutral refractories but they provide more service time and more resistance to corrosion and alkali attack.

H)The shell corrosion is attributed to a large extent to poor oxidizing conditions prevailing in the kiln. The best measure against chloride promoted sulphidization is to keep a steady supply of fresh air to the surface of the kiln shell. The CO/O2 analyzer installed at the kiln inlet should be made working properly to avoid incomplete combustion of coal which otherwise could lead to uncontrolled formation of CO and low oxygen level at kiln inlet. The oxygen at kiln inlet should be maintained at 1.5 % minimum to ensure proper oxidizing conditions with CO not exceeding 0.1 percent at this point.

Here at SCL the levels of O2 and CO are 5-6% and 0.01-0.15% respectively. by looking at above composition we can say that here oxidizing conditions are not poor. These compositions are not aid in corrosion of kiln shell. So in future also company should maintain these conditions.



Technology

How to control kiln shell corrosion