why coating formed in kiln inlet

kiln inlet coating - page 1 of 1

1) whats are the reasons of kiln inlet coating. 2) why plysious kiln have differnt dia. 3)polysius kiln have no grith gear and having electromechanic roller drive system is it more usefull than grith gear

Hi Armankhan, I can answer your first question;- Kiln Inlet buildups are normally due to an increase in recirculation of one or more of the volatile components within the kiln/preheater system. ie alkalis, sulphur or chloride. The usual culprit is sulphur, whose volatility is strongly influenced by the O2 content at the kiln inlet. If insufficient O2 is present in the kiln gases at this point, the following reaction cannot completely take place;- 2K2O(g) + 2 SO2(g) + O2(g) --> 2K2SO4(s). Alkali sulphates are far less volatile than alkali oxides and chlorides. So, if this reaction isimpeded, SO3 recirculation will increase suddenly and dramatically, causing buildups and possibly evenrings at the kiln inlet. Lack of sufficient oxygen at the kiln inlet could be caused by either insufficent secondary air flow or poor combustion in the main burner flame. Of course an increase in alkalis, chlorideor sulphur inputs to the kiln (ie in raw materials or AFRs) can also cause buildups and rings in the kiln inlet area, particularly if the SO3/(K2O + Na2O) molar ratio is not in balance (~1.0). Also hard burning can increase the recirculation of alkalis, sulphur and chloride as well, by increasing the evaporation rate of these components in the burning zone.The burning zone temperature and kiln inlet temperature should be carefully controlled to minimise this. I will leave your other two questions re Polysius kilns to one of theprocess engineering experts... Regards, Ted.

The usual culprit is sulphur, whose volatility is strongly influenced by the O2 content at the kiln inlet. If insufficient O2 is present in the kiln gases at this point, the following reaction cannot completely take place;-

Alkali sulphates are far less volatile than alkali oxides and chlorides. So, if this reaction isimpeded, SO3 recirculation will increase suddenly and dramatically, causing buildups and possibly evenrings at the kiln inlet. Lack of sufficient oxygen at the kiln inlet could be caused by either insufficent secondary air flow or poor combustion in the main burner flame.

Of course an increase in alkalis, chlorideor sulphur inputs to the kiln (ie in raw materials or AFRs) can also cause buildups and rings in the kiln inlet area, particularly if the SO3/(K2O + Na2O) molar ratio is not in balance (~1.0).

Also hard burning can increase the recirculation of alkalis, sulphur and chloride as well, by increasing the evaporation rate of these components in the burning zone.The burning zone temperature and kiln inlet temperature should be carefully controlled to minimise this.

re re: kiln inlet coating - page 3 of 5

Hello GKS, Due to the use of high sulphur petcoke and insufficient alkalis in the raw materials, your alkali/SO3 ratio in both hotmeal and clinker is far too low (~0.5 in both cases). To balance the sulphur you will need to increase your alkali content in the raw mix by a factor of two or so, either by selective mining at the quarry or adding a high alkali corrective such as potassium/sodium feldspar. Regards, Ted.

To balance the sulphur you will need to increase your alkali content in the raw mix by a factor of two or so, either by selective mining at the quarry or adding a high alkali corrective such as potassium/sodium feldspar.

Thank you sir, But adding K and Na element not balance hot meal to react to form stable componet to come out with clinker. But some time KCL like salt formed which caused heavy corrosion effect.So3 will increased upto 3.0 % in hot meal and k2o will reach upto 2.0 % . We have used feldspar ( Caf2 ) for reduced combinibilty temp . for incresed C3s in clinker with pet coke ( 100%) but Hot meal not control and kiln not running normal, Best regards GKS

But adding K and Na element not balance hot meal to react to form stable componet to come out with clinker. But some time KCL like salt formed which caused heavy corrosion effect.So3 will increased upto 3.0 % in hot meal and k2o will reach upto 2.0 % .

Hello GKS, I think you mean fluorospar (CaF2). Feldspars are silicates of potassium, sodium or calcium and are commercially used to balance sulphur in kilns burning a high proportion of petcoke as fuel. However, it's not enough to simply balance the sulphur with alkalis. You must also ensure that there is enough oxygen at the kiln inlet so that the sulphur can react with the alkalis to form alkali sulphate compounds which are less volatile, otherwise the alkalis and sulphur cannot escape and will continue to cause blockages. With petcoke, you need to have significantly higher O2 content in the gas stream at the kiln inlet, due to the higher sulphur loading... say 4-6% O2 instead of 2-3%, or sometimes even higher. Also the petcoke needs to be ground fine enough to ensure rapid combustion, otherwise an oxygen deficiency (CO)in the kiln inlet may result, increasing sulphur volatility and again causing blockages. Regards, Ted.

However, it's not enough to simply balance the sulphur with alkalis. You must also ensure that there is enough oxygen at the kiln inlet so that the sulphur can react with the alkalis to form alkali sulphate compounds which are less volatile, otherwise the alkalis and sulphur cannot escape and will continue to cause blockages.

Sir, We have sufficient O2 level ( 5.0 %) with high flame momentum ( 8.0 N /MW) and fine coal residue on 90 micron 2.0 % . But input s from pet coke 4.5 to 6.5 % . To control VF we have reduced KF feed upto 10 to 20 tph for last to result become normal and avoid co formation at kiln inlet and riser duct & timely cleaned coating formation zone by water jet and air blaster. I want to know any other process in kiln operation for help to avoid blockage. With best regards GKS

To control VF we have reduced KF feed upto 10 to 20 tph for last to result become normal and avoid co formation at kiln inlet and riser duct & timely cleaned coating formation zone by water jet and air blaster.

reasons for drop in kiln inlet temperature - page 1 of 2

Please any body suggest what would be the possible reason for drop in kiln inlet temp, we have rotary kiln of 3.0 x 42.5m length, with suspension preheater with secondary firing system, all temp profiles including preheater are normal only the kiln inlet temp we are getting around 820 to 840 Deg C, which is almost equal to Cyclone No.5 gas temp, this is happening since from one month, earlier we were getting around 950 to 1000 deg C at kiln inlet, we have checked all Cy dip tubes, dispersion plates etc, we are not finding any problem, calcination also drop down, please suggest the possible reasons for the same. Thank You, Ramesh Aital

Please any body suggest what would be the possible reason for drop in kiln inlet temp, we have rotary kiln of 3.0 x 42.5m length, with suspension preheater with secondary firing system, all temp profiles including preheater are normal only the kiln inlet temp we are getting around 820 to 840 Deg C, which is almost equal to Cyclone No.5 gas temp, this is happening since from one month, earlier we were getting around 950 to 1000 deg C at kiln inlet, we have checked all Cy dip tubes, dispersion plates etc, we are not finding any problem, calcination also drop down, please suggest the possible reasons for the same.

Dear Kiln inlet temp drop due to following reason 1. Heavy coating at kiln inlet - check temp. profile in kiln calcination zone ( kiln inlet to 10. mtr ) 2. Can having co formation at kiln inlet .- To avoid co formation for it. PL send your sreen shot of kiln and calciner on same time on forum. Thank for sharing your problem GKS

Dear Sir, We have checked inlet oxygen and Co%, oxygen is around 3.0% and Co is 0.03%, the calcination zone area is clean and there is no coating, we have checked shell temp profile, it has been observed that on an average there is 100 Deg drop in shell temp, our burning zone length became short, this problem happened just 4 days after burner pipe replacement, burner is conventional burner and we don't have precalciner in the system, we are not clear about what coarse of action to be taken, kindly help in this regards.

We have checked inlet oxygen and Co%, oxygen is around 3.0% and Co is 0.03%, the calcination zone area is clean and there is no coating, we have checked shell temp profile, it has been observed that on an average there is 100 Deg drop in shell temp, our burning zone length became short, this problem happened just 4 days after burner pipe replacement, burner is conventional burner and we don't have precalciner in the system, we are not clear about what coarse of action to be taken, kindly help in this regards.

Hello R. Aital Have you change in flame formation pattern by changing primary air flow ( which is indicate by your pressure in radial and axial flow). and any change in hood draft control. If preheater fan flow is less and cooler vent flow is high than say burning zone become short and all zone shifting toward kiln hood . Which indicat by cooler vent temp. , high clinker temp and flame become short and wide and kiln inlet temp less. How you say that burning zone short With regards Gks

If preheater fan flow is less and cooler vent flow is high than say burning zone become short and all zone shifting toward kiln hood . Which indicat by cooler vent temp. , high clinker temp and flame become short and wide and kiln inlet temp less.

kiln inlet coating - page 1 of 5

Hello Dogra,Usually coatings or buildups in this area are due to an elevated sulphur cycle causedby either poor combustion in the main flame, or use of high SO3 fuels such as petcoke. The key is to ensure enough O2 at the kiln inlet to control excessive SO3 recirculation. High sulphur levels in the raw materials can also be a cause.Also an excessive alkali/chloride cyclecan be the cause. (ie excessive burning zone temperatureand/or high alkali/chloride inputs in the raw materials, alternative fuels or waste streams)The correct alkali/sulphur balance is another important indicator. If the molar ratio ofalkalis to SO3 in the totalraw material and fuel inputs to the kilnis not close to 1, then eitherincreased alkali or sulphur recirculation can be problematic.Sometimes if the main burner flame is too long, the temperature at the kiln inlet can increase sufficiently to cause part of the alkali/sulphur rich hot meal entering the kiln to melt and become 'sticky' or even partially clinkerize, resulting in rings and buildups.For adiscussion on the same subject, see this thread;-http://www.cemnet.com/cs/forums/thread/2208.aspxRegards,Ted.

Usually coatings or buildups in this area are due to an elevated sulphur cycle causedby either poor combustion in the main flame, or use of high SO3 fuels such as petcoke. The key is to ensure enough O2 at the kiln inlet to control excessive SO3 recirculation. High sulphur levels in the raw materials can also be a cause.

The correct alkali/sulphur balance is another important indicator. If the molar ratio ofalkalis to SO3 in the totalraw material and fuel inputs to the kilnis not close to 1, then eitherincreased alkali or sulphur recirculation can be problematic.

Sometimes if the main burner flame is too long, the temperature at the kiln inlet can increase sufficiently to cause part of the alkali/sulphur rich hot meal entering the kiln to melt and become 'sticky' or even partially clinkerize, resulting in rings and buildups.

Hello Ted, In our case, SO3 in the hot meal has increased from 1.0 to 1.9, but SO3 in clinker is normal. I too also expecting high Sulphur recirculation, as it is evident from heavy coatings in smoke chamber. Our Kiln is operating at 2.5% to 3.0% O2 level. However,NOX level has increased in the kiln inlet, which is evident that the temperature indise the kiln is high. In the discussion above, you have said that, the coatings may also because of alkali and chloride. I have seen the coatings too, physicaly, its color is light yellow, and i infer that these coatings may be mainly due to Sulphur. What do you suggest in the condition above mentioned, and how can we deffrientiate the nature of coatings, (i.e. its is due to alkali, or sulphur or chloride)

In our case, SO3 in the hot meal has increased from 1.0 to 1.9, but SO3 in clinker is normal. I too also expecting high Sulphur recirculation, as it is evident from heavy coatings in smoke chamber. Our Kiln is operating at 2.5% to 3.0% O2 level. However,NOX level has increased in the kiln inlet, which is evident that the temperature indise the kiln is high.

In the discussion above, you have said that, the coatings may also because of alkali and chloride. I have seen the coatings too, physicaly, its color is light yellow, and i infer that these coatings may be mainly due to Sulphur.

Hello Manish, The only real way is to tell is to somehow obtain a sample, once the kiln has stopped. This is difficult as many buildups fall out due to thermal shock once the kiln inlet temperature drops. Sulphur buildups are generally hard and solid while alkali/chloride buildups are usually much softer. The hotmeal analysis is a better guide, if you have a long enough history of results. If the hotmeal SO3 has risen but the alkalis and chloides have not, the buildups in the smokebox are likely to be related to oxygen deficiency or an increase in sulphur input . If the alkalis/chlorides have risen but not the sulphur, then the buildups are likely to be temperature or alkali input related. If sulphur, alkalis and chloride have all increased in the hotmeal the buildups are likely to be due to increased burning zone temperature or an increase in the sulphur, alkali and chloride inputs. Regards, Ted.

numerical investigation of the impact of coating layers on rdf combustion and clinker properties in rotary cement kilns - sciencedirect

CFD-simulations of rotary cement kilns considering coating layers are conducted.Numerical models for RDF, coating and clinker bed are presented and combined.The impact of coating regions on RDF combustion and clinker properties is investigated.Light and evenly distributed coating profiles are found to be beneficial for the process.Heavy and locally concentrated coating can result in a high free lime content of 2wt%.

The formation of regions of solid coating, where agglomerated clinker material adheres to the refractory lining of the kiln wall, is very common during cement clinker production. While a thin coating layer protects the refractory lining, strong deposit formation can impair the material flow through the kiln. In this study, the impact of these coating layers on the clinker production process within a rotary kiln is investigated with CFD simulations. The fuel injected at the main burner is a mixture of pulverized coal and refuse derived fuel (RDF). Advanced models were developed to accurately describe the trajectories and thermal conversion of non-spherical RDF particles in the gas phase. These models are based on a detailed fuel analysis of major RDF fractions. A blocked-off region approach is used to consider different coating profiles within the simulation domain. The thermochemical processes in the clinker bed of the kiln are approximated with a one-dimensional model that calculates heat and mass exchange with the gas phase, the incorporation of fuel ashes into the bed and the chemical-mineralogical reactions of the clinker. The blocked-off region approach is also employed to account for the clinker bed geometry in the kiln, which greatly depends on the considered coating profile. Two cases, one with a thin and evenly distributed coating profile and one with a thick and locally concentrated coating, are simulated. The resulting impact on RDF conversion, gas phase properties and clinker phase formation are assessed and compared to a reference case without any coating. Results show that the insulation effect of a thin coating profile increases the gas phase temperature in the kiln and helps to reduce the free lime content of the final clinker product. In the case of heavy coating, a temperature shift towards the solid material inlet of the kiln occurs, which outweighs the beneficial insulation effect of the coating in the sintering zone and leads to lower local gas phase temperatures. In combination with reduced clinker residence times, this results in a slight increase of the free lime content in the clinker.

coating formation in kiln inlet - page 1 of 3

Sir, In our plant we are getting more coating problems at Kiln inlet and Riser duct. Please tell me what is reason for the coating problem and give me some suggetion to minimize this problem. Fuel using :Crude OilRegardsRegards

HiCoating at kiln inlet ia mainly due to the volatiles like Chlorides, alkalies ( Na2O and K2O) and SO3. To minimise the coating formation, the alkali sulfate ratio should be around 1.1 so that these volatiles will come out as alkali sulfate along with the clinker so that the recirculation inside the kiln circuit will be less.Chlorides should be controlled from the inputs like limestone, clay etc. If sea water is used in rawmill , or GCT or anywhere , then it will also increase the chloride contentWhen we use high sulfur coal or petcoke, the SO3 will increase. If Sulfur is deficient in the circuit and alkalies are more, then we can add gypsum along with the rawmaterial to reduce the alkali sulfate ratio.Even after doing all the above, there will be some residual coating will be formed in the kiln inlet. It can be cleaned with out stopping the plant, by!. Kiln inlet area may be lined with silicon carbide based bricks or castable, which will form a glassy phaseat high tempand thus avoids the coating formation2. You can also use high pressure water pumps( 250 to 300bar)for the removal of the coating3. Any CO at kiln inlet will aggrevate the coating formtion.So reduced atmosphere should always be avoided at kiln inlet4. Air blasters can be installed at the kiln inlet and can be operated using automatic timers5. we can also use CARDOX system, which will blast the coating usiong CO2 blasting.RegardsVinayak

Coating at kiln inlet ia mainly due to the volatiles like Chlorides, alkalies ( Na2O and K2O) and SO3. To minimise the coating formation, the alkali sulfate ratio should be around 1.1 so that these volatiles will come out as alkali sulfate along with the clinker so that the recirculation inside the kiln circuit will be less.

contribution to kiln tyre contact stress analysis | open access journals

In this paper contact stress analysis of kiln tyre is presented. Contact stress plays key roll in pitting of tyre. In some cases, contact stress cause cracks in tyre. Highest value of contact stress is not on the surface but slightly below. Tyre is supported by rollers, contact pattern arrangement of tyre and roller is similar to two cylinders in contact and axes are parallel. Load on tyre is calculated considering kiln is simply supported. Contact stress analysis is performed using analytical and finite element simulation.

impact of coating layers in rotary cement kilns: numerical investigation with a blocked-off region approach for radiation and momentum - sciencedirect

CFD-simulations of rotary cement kilns considering coating layers.Iterative determination of coating thickness based on kiln shell temperature profiles.Blocked-off region approach successfully used to describe coating layers in the kiln.Coating layers found to have a significant impact on process conditions.

In this study CFD simulations of an industrial scale rotary kiln for cement clinker production are conducted. A solid layer of agglomerated clinker material, which adheres to the kiln wall and forms a stable coating of variable thickness during kiln operation, is considered in the simulations of the furnace. During operation of the kiln the thickness of the coating layer is unknown, but the routinely measured temperature profile along the kiln shell is an indicator for the local layer thickness. Therefore, a first estimate of the initially unknown thickness of the coating layer is calculated by a one-dimensional heat transfer model, based on the temperature profile along the kiln shell, and introduced into the CFD simulations. As the process conditions within the furnace and, thus, the heat transfer to the kiln wall and the resulting coating thickness change over time, an adaptive modelling approach is required to describe the solid coating region in the CFD simulations. A method to represent the geometry of the coating layer as a blocked-off region for momentum is presented and extended for radiation. The results are compared to the conventional approach where the coating layer is represented by a highly resolved CFD mesh following the wall contour. Two generic temperature profiles of the kiln shell are assumed and used to simulate the corresponding coating profile in the furnace. The effect on the process conditions within the kiln are assessed and compared to a reference case without coating regions. Results show that the blocked-off approach employed is suitable to model the additional solid boundary profile in the furnace. The coating regions are found to have a significant impact on the process conditions within the kiln, especially if the coating layers are located towards the burner end of the kiln.

clinkerization - cement plant optimization

The process of clinkerization signifies conversion of raw meal into clinker minerals mainly consisting of C4AF(Aluminoferite), C3A(Aluminite), C2S(Belite) and C3S (Alite) phases along with small percentage of free lime CaO, MgO, Alkalies, Sulphates etc. The conversion taking place in kiln system as raw meal is heated gradually to clinkerization temperature (1450 0C) as shown below in table 1.

Kiln system has seen a sea of development since 1950s to till date, from vertical shaft kilns to modern pre-calciner kiln. Capacity has increased from as low as 50 tpd to as high as 12000 tpd from kiln. Heat consumption reduced from 1400 kcal/kg to 670 kcal/kg of clinker. Specific heat consumption of various kiln systems is tabulated (Table 2) below to assess the progress in the development of clinkerization technology.

The overall process of conversion from raw meal to clinker being endothermic demands a theoretical heat of about 380-420 kcal/kg-clinker. However, the rest of the specific heat consumption as tabulated above constitutes heat losses from preheater exhaust gases, clinker, cooler exhaust gases, preheater dust and radiation losses. Heat loss distribution across different elements can be established through heat balance and process audit of pyro section. Fuels used commonly to provide heat for the conversion processes are coal, fuel oil, and natural gas. Alternative fuels like petcoke, rubber tyres, wood chips, etc. have been introduced to economize cement making process.

Lime Saturation Factor (LSF) is the ratio of the actual amount of lime in raw meal/clinker to the theoretical lime required by the major oxides (SiO2, Al2O3 and Fe2O3) in the raw mix or clinker. It is practically impossible to complete the reaction to 100%, in a reactor like rotary kiln, therefore there will always be some unreacted lime (CaOf) known as free lime. The amount of free lime in clinker indicates incomplete burning and needs to be monitored. When coal is used as fuel, the ash content and its composition should be considered in raw mix design. LSF of clinker lies in the range of 92-98. Higher LSF at controlled free lime content translates to better quality of clinker (high C3S), difficult clinkerization, high heat consumption.

Silica Modulus (SM) is the ratio of content of oxides of silica to the oxides of alumina and iron. SM signifies the ratio of solid content to the melt content. Therefore, when SM is too high, nodulization becomes weak and clinkerization reaction (C3S formation) rate slows down, kiln becomes dusty and difficult to operate. While as when SM is too low, more melt is formed in kiln, issues like thick kiln coating, kiln melting, snowman formation in cooler are more prone. Normal range of SM is 2.3-2.7.

Alumina Modulus (AM) is the ratio of content alumina oxide to iron oxide. AM signifies the temperature at which liquid formation starts, the nature of liquid formed and the color of clinker formed. The lowest temperature is obtained at AM equal to 1.6, which is the optimum for clinker formation and nodulization. Higher the AM, lighter the color of clinker (cement). Normal range of SM is 1-2.5.

MgO is commonly present in raw meal. Some of the MgO (2%) is accommodated into the clinker mineral structure, while as extra MgO forms a crystal called periclase and causes mortar expansion. MgO up to 4 % is found common in clinker. Rapid cooling of clinker can mitigate the expansion problems, however higher MgO causes ball formation in kiln, increases melt phase etc. and therefore, can disturb kiln operation.

Alkalies A part of alkalies Na2O and K2O combines chemically with clinker minerals, while as the major part remains as water soluble and affects adversely cement strength (28 Day Strength). If alkalies are not balanced by sulphates, volatile recirculation phenomenon starts disturbing kiln operation due to kiln inlet, bottom cyclone coating. Alkali content is generally expressed in terms of sodium equivalent as under:

SO3 in clinker comes from raw materials and fuel. Sulphate can form a stable compound with potassium and comparatively lesser stable compound with sodium as potassium sulphate (K2SO4) and sodium sulphate (Na2SO4) respectively. Sulphur in raw meal increases SOx emission and causes build-up in preheater. Sulphates needs to balance alkalies in kiln system. Excess sulphates can be calculated as:

Cl chlorides can come from raw materials and fuel. Chlorides form stable compounds with alkalies and are highly volatile. 1% of chlorides is generally considered the maximum in hot meal sample. Excess chlorides needs to be bypassed at kiln inlet through bypass duct. Clinker can contain about 0.012 to 0.023 % cl.

Liquid Phase (%) mainly consists of the aluminium, iron and magnesium oxides. However, alkalies and sulphates also contribute to liquid phase. The liquid phase plays an important role in coating formation and nodulisation. The liquid percentage at 1450 0C can be estimated using the formula

Burnability is a reference value for raw meal indicating how difficult it is to burn. Hard burning is indicated from incomplete burning in terms of free lime content. Although the burning atmosphere in kiln is different from a laboratory oven, nevertheless it is believed that under similar conditions of temperature and residence time, free lime content will depend only on the physical and chemical characteristics of raw meal. Different cements groups have been using different ways to estimate burnability. In FLSmidth the following procedure is followed.

Raw meal samples are placed in laboratory oven at 1400 0C, 1450 0C and 1500 0C respectively for 30 minutes and free lime is measured for each. The FLS burnability is indexed with 100 at following free lime values 3.6% for the sample at 1400 0C, 2.6 for the sample at 1450 0C and 1.6 for the sample at 1500 0C.

Degree of Calcination: is determined by loss on ignition (LOI) of hot meal sample. To reduce the influence of alkalies, sulphur etc. the loss should be measured at 950 0C. Formulas used to approximate degree of calcination are as under.

Clinker free lime (CaOf) should be as high as possible to avoid hard burning of clinker, but safely below value, inviting mortar expansion; normally, between 0.5% and 1.5%. Free lime indicates incomplete clinker burning, therefore should be monitored regularly and maintained closely in the acceptable range. Kiln feed rate fluctuations and composition inconsistency makes difficult to control free lime in clinker

Clinker litre weight (grams/litre): A convenient supplement for free lime measurement is the more rapid determination of litre weight of clinker sample from the cooler discharge to approximately +6/-12 mm and weighing a standard 1 litre volume. Normal range of litre-weight is 1100-1300 g/L. Low litre weight means high free lime and dusty clinker in general (for higher AM free lime can be higher instead of high litre weights).

Kiln Speed should be such that volumetric loading is within the range 10-15% and heat transfer is maximized. Pre-calciner kilns generally rotate at 3.5-4.5 rpm. Under normal conditions, kiln should be run with as high rpm as possible. Higher kiln rpm improves clinker mineralogy and grindability. Speed control is used to take care of usual kiln disturbances like coating fall down with the other controlling parameters like, fuel rate, preheater fan rpm and kiln feed rate.

Fuel Rate is frequently used as a controlling parameter in kiln operation. Fuel is regulated in kiln and precalciner to maintain required temperature. O2 and CO must be considered first before increasing fuel rate.

Feed Rate is generally maintained in a stable kiln operation. When the control actions, like kiln speed, fuel rate and air control fails or is expected to be insufficient to control kiln disturbance, feed rate is changed as required.

Preheater Fan Speed is varied to fulfill air requirement in kiln system and maintain oxidizing conditions in kiln. ID fan speed is not changed frequently in normal kiln operation, unless feed or fuel changed significantly.

Kiln Inlet Analyser gas composition reveals the process (kiln) stability and combustion efficiency. With a good flame in kiln O2 at kiln inlet will be about 1-2% and CO less than 200 ppm, while as it has been observed that an unstable flame may yield in excess of 500 ppm CO with even 3% O2. NOx measurements at kiln inlet gives an early indications of changing burning zone temperatures conditions, before it is reflected in kiln torque trend. It is important to mention that kiln inlet gas analyser probe position should be inside the kiln to avoid leakage air through inlet seal to be sucked with sample gas.

PC-Gas Analyser is generally installed in the outlet duct from the bottom cyclone to avoid frequent jamming of gas filter due to high dust load in PC outlet duct. Oxygen level should as between 0.5-1.5 at CO less than 100 ppm.

Preheater outlet Analyser: In preheater down-comer analyser serves both purposes, to measure leakage across the tower and the overall combustion conditions in kiln system. Moreover, it serves as a safety equipment for all critical equipments in upstream gas circuit like, ESP, Bag house etc. Oxygen content of 1.5 -2.5% is considered good at preheater outlet. Prompt action is recommended if CO increases more than 0.5%.

Lower Cyclone Temperature is considered most important and stable temperature in preheater to control pre-calciner fuel rate. It is generally maintained manually or by PID loop in the range of 10 0C, in the range 850-900 0C to ensure calcination between 90% to 95 %.

Burning Zone Temperature is monitored by radiation pyrometer. Maintaining constant burning zone temperature means, clinker of constant quality and grindability from a consistent kiln feed. Radiation pyrometer gives a relative value of temperature on the basis of visibility (color) in burning zone and can be used as a decisive parameter in stable kiln operation.

Secondary air Temperature should be as high as possible, It reflects the stability of clinker bed in cooler and the heat recuperation from hot clinker. The higher the best, in the range of 800-1050 0C.

Cyclone Cone Drafts. In operation of kiln it is a life line to monitor all preheater drafts, particularly cyclone cone drafts. Cone drafts in preheater cyclone gives an important indication of cyclone jamming along with the other parameters like temperature.

Kiln Back End Temperature indicates the overall stability of kiln operation. It is generally maintained very closely. Variation in kiln back-end temperature indicates either change in burning zone or a change in calciner, hence is of pivotal importance to infer both areas of interest. Back-end temperature is normally maintained at 1050 0C.

Flame Geometry will determine the flame length and therefore, burning zone length. Flame should be as shot as possible, But, care should be taken to avoid thermal abuse of refractory due to shorter and one sided flames.

Kiln Hood Draft should be slightly negative and must be maintained closely between 0 to -2 mm H2O preferably by PID control loop with Cooler vent fan speed. More negative will increase cold air leakages into the kiln through outlet seal and hood, while as positive pressures are unsafe.

Cooler bed height, Undergrate Pressures. Maintaining constant clinker bed height is a key to stable cooler operation. Undergrate pressure reflects bed resistance and changes with clinker size. To maintain constant Undergrate pressure cooler speed is varied manually or in auto-mode by PID control loop. Constant bed height ensures stable secondary and tertiary air temperatures.

Cooling Air Quantity is maintained to ensure cooling of clinker and heat recuperation from hot clinker from kiln. Specific air usage is generally considered as key performance indicator of cooler. New generation coolers can cool clinker to the temperatures to as low as 65 0C over ambient with a specific air consumption of 1.7 kg/kg-clinker. Generally cooling fans are designed at 2.7 kg-air/kg clinker.

Preheater: Preheaters as name implies serves the purpose of heating raw meal to a temperature where calcination or dissociation of CO2 begins in calciner. Preheater consists of 4-6 low pressure cyclones one over the other. Number of cyclones depends on the natural humidity (moisture) in raw materials, in other words the drying capacity required to dry out raw materials in raw mill. Five stage-cyclones are commonly existing in cement plants. In order to increase heat utilization in kiln system, six stage cyclones are as well installed in many cement plants. However, increasing cyclone stages beyond six does not look economic any more, as the quantum of heat saving is not significant to justify it, moreover the increased pressure drop across preheater outbalances the improvements due to additional cyclone.

Raw meal enters (at 50 0C) in the riser duct of second cyclone (from top) and is picked up with the hot gases to first (top) cyclone, where raw meal is separated from gas stream and passed down to second cyclone. Heat transfer takes place in suspension phase between hot gases and raw meal. In this way raw meal passes from top cyclone to lower cyclone (bottom cyclone but one) and enters calciner at about 800 0C. As a whole the heat transfer process is counter current as feed moves from top to bottom and hot gases from bottom to top, however in actual the whole heat transfer takes place in co-current heat transfer mode. Rate of drying, dehydration and calcination are governed by heat transfer rate. Efficient heat transfer can be generally assessed from the difference of the gas and material temperature of cyclones.

With the evolution of low pressure cyclone, it became feasible to go from 4-stage cyclone preheater to 6-stage preheater and harvest more heat economy in kiln section. For reference are tabulated the pressure drop values across preheater of 4,5 and 6-stage preheater.

Brick Lining of Cyclones: All preheater components need to be lined from inside with appropriate refractory to save shell/components from heat and to hold heat inside for process use. Refractory castable, Bricks, Insulation Bricks are used in preheater.

Calciner.Calciner serves the purpose of decomposition of carbonates into reactive oxide calcium oxide. Calcination is an endothermic process and needs heat energy of about 420 kcal. Raw meal is taken in calciner from the last but one stage of preheater. Heat for calcination is supplied through secondary firing in calciner and combustion air is taken from cooler through tertiary air duct. Various configurations of calciner are existing in modern kiln systems. Fundamentally calciner can be either in line with kiln or can be a off-line, thus, taking only air from cooler in different configurations in itself. With coal as a fuel the recommended retention time in calciner should be at least of 3.3 seconds to ensure fuel combustion in calciner. With the development of cement technology, 60% of the fuel required is fired in calciner and 90 to 95 % of calcination duty is done outside the kiln.

Rotary Kilns.Rotary kiln is a rotating cylinder, installed at an inclination of 3.5 to 4 % to facilitate material movement. Length and diameter of kiln is decided for the required capacity throughput. Main factors dictating size of kiln are the retention time (25-30 minutes) of material in kiln, degree of filling (10-17%) and thermal loading of burning zone (2.8-4.8 x 106 kcal/h/m2). Pre-calciner kilns are shortest in length, as 90-95 % calcination is completed outside the kiln. L/D of three tyre kiln is between 14-17 and for new kiln like Rotax kiln it is only 12-13. Kilns are commonly supported on three supporting stations. Each supporting station has 2 rollers and 4 bearings. All rollers are mounted on one fabricated bed plate. Tyre rests on rollers which have an angle of about 30 degrees at the center of the kiln. Kiln is lined with refractories bricks of 150-250 mm thickness, depending on the diameter of kiln. Basic bricks are preferred in burning zone, however, 75 % alumina bricks are still used for cost consideration. Rest of the kiln is lined with ~ 45 % alumina bricks.

Clinker cooler serves two main objective of cooling clinker from temperature of about 1350 0C to the temperature (65-150 0C) where it can be handled by conveyors like pan conveyors, chain, Elevators etc. and heat recovery from hot clinker coming out of kiln. A huge development has happened in clinker coolers designs and types as well. Grate cooler with a take-off for pre-calciner is generally required for pre-calciner kilns. Cross bar coolers are used in new plants to achieve cooling efficiencies (>70%) and less maintenance burden. New coolers are designed for the capacity to be handled with the loading of 40-55 tpd of clinker cooled/m2 of grate area. Cooling air requirement is generally designed at 2.2-2.5 nm3/kg-clinker. Either hammer crusher or roller crusher is used to break lumps of clinker before coming out from cooler. Water spray or Air to Air heat exchanger is used to cool down cooler vent air before de-dusting in ESP or bag filter.