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burning factor means in cement kiln

the cement kiln

the cement kiln

Most Portland cement is made in a rotary kiln. Basically, this is a long cylinder rotating about its axis once every minute or two. The axis is inclined at a slight angle, the end with the burner being lower.The rotation causes the raw meal to gradually pass along from where it enters at the cool end, to the hot end where it eventually drops out and cools. They were introduced in the 1890s and became widespread in the early part of the 20th century and were a great improvement on the earlier shaft kilns, giving continuous production and a more uniform product in larger quantities.For information on reactions in the kiln see the clinker pages.

The original rotary cement kilns were called 'wet process' kilns. In their basic form they were relatively simple compared with modern developments. The raw meal was supplied at ambient temperature in the form of a slurry.A wet process kiln may be up to 200m long and 6m in diameter. It has to be long because a lot of water has to be evaporated and the process of heat transfer is not very efficient.The slurry may contain about 40% water. This takes a lot of energy to evaporate and various developments of the wet process were aimed at reducing the water content of the raw meal. An example of this is the 'filter press' (imagine a musical accordion 10-20 metres long and several metres across) - such adaptions were described as 'semi-wet' processes.The wet process has survived for over a century because many raw materials are suited to blending as a slurry. Also, for many years, it was technically difficult to get dry powders to blend adequately.Quite a few wet process kilns are still in operation, usually now with higher-tech bits bolted on. However, new cement kilns are of the 'dry process' type.

In a modern works, the blended raw material enters the kiln via the pre-heater tower. Here, hot gases from the kiln, and probably the cooled clinker at the far end of the kiln, are used to heat the raw meal. As a result, the raw meal is already hot before it enters the kiln.

Secondly, and less obviously, the process of transferring heat is much more efficient in a dry process kiln. An integral part of the process is a heat exchanger called a 'suspension preheater'. This is a tower with a series of cyclones in which fast-moving hot gases keep the meal powder suspended in air. All the time, the meal gets hotter and the gas gets cooler until the meal is at almost the same temperature as the gas. The basic dry process system consists of the kiln and a suspension preheater. The raw materials, limestone and shale for example, are ground finely and blended to produce the raw meal. The raw meal is fed in at the top of the preheater tower and passes through the series of cyclones in the tower. Hot gas from the kiln and, often, hot air from the clinker cooler are blown through the cyclones. Heat is transferred efficiently from the hot gases to the raw meal. The heating process is efficient because the meal particles have a very high surface area in relation to their size and because of the large difference in temperature between the hot gas and the cooler meal. Typically, 30%-40% of the meal is decarbonated before entering the kiln. A development of this process is the 'precalciner' kiln. Most new cement plant is of this type. The principle is similar to that of the dry process preheater system but with the major addition of another burner, or precalciner. With the additional heat, about 85%-95% of the meal is decarbonated before it enters the kiln

An integral part of the process is a heat exchanger called a 'suspension preheater'. This is a tower with a series of cyclones in which fast-moving hot gases keep the meal powder suspended in air. All the time, the meal gets hotter and the gas gets cooler until the meal is at almost the same temperature as the gas.

The basic dry process system consists of the kiln and a suspension preheater. The raw materials, limestone and shale for example, are ground finely and blended to produce the raw meal. The raw meal is fed in at the top of the preheater tower and passes through the series of cyclones in the tower. Hot gas from the kiln and, often, hot air from the clinker cooler are blown through the cyclones. Heat is transferred efficiently from the hot gases to the raw meal.

The heating process is efficient because the meal particles have a very high surface area in relation to their size and because of the large difference in temperature between the hot gas and the cooler meal. Typically, 30%-40% of the meal is decarbonated before entering the kiln.

A development of this process is the 'precalciner' kiln. Most new cement plant is of this type. The principle is similar to that of the dry process preheater system but with the major addition of another burner, or precalciner. With the additional heat, about 85%-95% of the meal is decarbonated before it enters the kiln

Since meal enters the kiln at about 900 C, (compared with about 20 C in the wet process), the kiln can be shorter and of smaller diameter for the same output. This reduces the capital costs of a new cement plant. A dry process kiln might be only 70m long and 6m wide but produce a similar quantity of clinker (usually measured in tonnes per day) as a wet process kiln of the same diameter but 200m in length. For the same output, a dry process kiln without a precalciner would be shorter than a wet process kiln but longer than a dry process kiln with a precalciner.

In the diagram above of a precalciner kiln, raw meal passes down the preheater tower while hot gases rise up, heating the raw meal. At 'A,' the raw meal largely decarbonates; at 'B,' the temperature is 1000 C - 1200 C and intermediate compounds are forming and at 'C,' the burning zone, clinker nodules and the final clinker minerals form. A preheater tower is likely to have 4-6 stages, not the three shown here. Many designs are more complex but this diagram illustrates the principle. See the 'Clinker' pages for more information on reactions in the kiln.

The kiln is made of a steel casing lined with refractory bricks. There are many different types of refractory brick and they have to withstand not only the high temperatures in the kiln but reactions with the meal and gases in the kiln, abrasion and mechanical stresses induced by deformation of the kiln shell as it rotates.

Bricks in the burning zone are in a more aggressive environment compared with those at the cooler end of the kiln (the 'back end'), so different parts of the kiln are lined with different types of brick.

Periodically, the brick lining, or part of it, has to be replaced. Refractory life is reduced by severe changes in temperature, such as occur if the kiln has to be stopped. As the cost of refractories is a major expense in operating a cement plant, kiln stoppages are avoided as far as possible.

As the meal passes through the burning zone, it reaches clinkering temperatures of about 1400 C - 1500 C. Nodules form as the burning zone is approached. When the clinker has passed the burning zone, it starts to cool, slowly at first, then much more quickly as it passes over the 'nose ring' at the end of the kiln and drops out into the cooler.

The cooled clinker is then conveyed either to the clinker store or directly to the clinker mill. The clinker store is usually capable of holding several weeks' supply of clinker, so that deliveries to customers can be maintained when the kiln is not operating.

Articles like this one can provide a lot of useful material. However, reading an article or two is perhaps not the best way to get a clear picture of a complex process like cement production. To get a more complete and integrated understanding of how cement is made, do have a look at the Understanding Cement book or ebook. This easy-to-read and concise book also contains much more detail on concrete chemistry and deleterious processes in concrete compared with the website.

Almost everyone interested in cement is also concerned to at least some degree with concrete strength. This ebook describes ten cement-related characteristics of concrete that can potentially cause strengths to be lower than expected. Get the ebook FREE when you sign up to CEMBYTES, our Understanding Cement Newsletter - just click on the ebook image above.

clinkerization - cement plant optimization

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.

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