Cement is made by heating limestone (calcium carbonate) with small quantities of other materials (such as clay) to 1450 °C in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which is then blended with the other materials that have been included in the mix. The resulting hard substance, called ‘clinker’, is then ground with a small amount of gypsum into a powder to make ‘Ordinary Portland Cement’, the most commonly used type of cement (often referred to as OPC).
Portland cement is a basic ingredient of concrete, mortar and most non-specialty grout. The most common use for Portland cement is in the production of concrete. Concrete is a composite material consisting of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element. Portland cement may be grey or white. Crusher: Crusher is used to crush the big limestone pieces into small and required range. Stacker: A stacker is a large machine used in bulk material handling.
Its function is to pile bulk material such as limestone, ores and cereals on to a stockpile. A reclaimer can be used to recover the material. Stackers are nominally rated for capacity in tonnes per hour (tph). They normally travel on a rail between stockpiles in the stockyard. A stacker can usually move in at least two directions: horizontally along the rail and vertically by luffing (raising and lowering) its boom. Luffing of the boom ehavior dust by reducing the distance that material such as coal needs to fall to the top of the stockpile.
The boom is luffed upwards as the height of the stockpile increases. Some stackers can rotate the boom. This allows a single stacker to form two stockpiles, one on either side of the conveyor. Stackers are used to stack in different patterns, such as cone stacking and chevron stacking. Stacking in a single cone tends to cause size segregation, with coarser material moving out towards the base. In raw cone ply stacking, additional cones are added next to the first cone. In chevron stacking, the stacker travels along the length of the stockpile adding layer upon layer of material.
Other than stacking, a stacker has three basic movements: •Luffing: This is vertical movement. Old stackers used a winch mechanism with metal wire, but newer ones are powered by hydraulic cylinders, generally two. •Travelling: The stacker moves on a rail track, which may be broad or narrow gauge, enabling it to move around the stockyard as required. For this purpose, traction motors powered by direct current (DC) are connected by bevel gears to between 12 and 22 wheels. For manual control, all the controls are in a controller’s cabin above the boom conveyor or boom.
Modern stackers can be controlled remotely. •Slewing: This is rotation of the stacker around its central axis to align or place the stockpile where required. This works mostly by a slew pinion that rotates around a slew base. This type of gear assembly is called a sun and planet gear. The axles may be multiple and are driven by DC-powered axle motors which transmit the torque via bevel or helical gears. The conveyor belts used in stackers may be made of fabric or metal wire, depending upon the material to be handled. They are driven by pulleys, which in turn are driven by DC motors.
The motors and gear are coupled by fluid coupling. Most stackers are electrically powered by way of a trailing cable. There are basically two types of cable trailing: power cord rotating drum (PCRD) and control cable rotating drum (CCRD). Pendulum adjustments are made to ensure the proper alignment of these cables while the stacker is travelling. Reclaimer: A reclaimer is a large machine used in bulk material handling applications. A reclaimer’s function is to recover bulk material such as ores and cereals from a stockpile. A stacker is used to stack the material.
Reclaimers are volumetric machines and are rated in m3/h (cubic meters per hour) for capacity, which is often converted to t/h (tonnes per hour) based on the average bulk density of the material being reclaimed. Reclaimers normally travel on a rail between stockpiles in the stockyard. A bucket wheel reclaimer can typically move in three directions: horizontally along the rail; vertically by “luffing” its boom and rotationally by slewing its boom. Reclaimers are generally electrically powered by means of a trailing cable. Rawmill:
Raw milling involves mixing the extracted raw material to obtain the correct chemical configuration and grinding them to achieve the proper particle size to ensure optimal fuel efficiency in the cement kiln and strength in the final concrete product. Three types of process may be used the dry process, the wet process or the demidry process. If the dry process is used the raw material are dried using impact driers, drum dryers, paddle –equipped rapid dryers, air seperators, or autogenious mills before grinding or in the grinding process itself.
In wet process, water is added during grinding. In the semidry process the material are formed into pellets with addition of water in a pelletizing device. Kiln: Cement kilns are used for the pyro processing stage of manufacture of Portland and other types of hydraulic cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates. Over a billion tones of cement are made per year, and cement kilns are the heart of this production process: their capacity usually define the capacity of the cement plant.
As the main energy-consuming and greenhouse-gas–emitting stage of cement manufacture, improvement of kiln efficiency has been the central concern of cement manufacturing technology. A typical process of manufacture consists of three stages: •grinding a mixture of limestone and clay or shale to make a fine “rawmix” (see Rawmill); •heating the rawmix to sintering temperature (up to 1450 °C) in a cement kiln; •grinding the resulting clinker to make cement (see Cement mill). In the second stage, the rawmix is fed into the kiln and gradually heated by contact with the hot gases from combustion of the kiln fuel.
Successive chemical reactions take place as the temperature of the rawmix rises: •70 to 110 °C – Free water is evaporated. •400 to 600 °C – clay-like minerals are decomposed into their constituent oxides; principally SiO2 and Al2O3. Dolomite (CaMg(CO3)2) decomposes to calcium carbonate, MgO and CO2. •650 to 900 °C – calcium carbonate reacts with SiO2 to form belite (Ca2SiO4). •900 to 1050 °C – the remaining calcium carbonate decomposes to calcium oxide and CO2. •1300 to 1450 °C – partial (20–30%) melting takes place, and belite reacts with calcium oxide to form alite (Ca3O•SiO4).
Alite is the characteristic constituent of Portland cement. Typically, a peak temperature of 1400–1450 °C is required to complete the reaction. The partial melting causes the material to aggregate into lumps or nodules, typically of diameter 1–10 mm. This is called clinker. The hot clinker next falls into a cooler which recovers most of its heat, and cools the clinker to around 100 °C, at which temperature it can be conveniently conveyed to storage. The cement kiln system is designed to accomplish these processes
Portland cement clinker was first made (in 1824) in a modified form of the traditional static lime kiln. The basic, egg-cup shaped lime kiln was provided with a conical or beehive shaped extension to increase draught and thus obtain the higher temperature needed to make cement clinker. For nearly half a century, this design, and minor modifications, remained the only method of manufacture. The kiln was restricted in size by the strength of the chunks of rawmix: if the charge in the kiln collapsed under its own weight, the kiln would be extinguished.
For this reason, beehive kilns never made more than 30 tonnes of clinker per batch. A batch took one week to turn around: a day to fill the kiln, three days to burn off, two days to cool, and a day to unload. Thus, a kiln would produce about 1500 tonnes per year. A kiln is basically an industrial oven, and although the term is generic, several quite distinctive designs have been used over the years. Although perhaps more normally associated with pottery making, both ‘Bottle’ and their very close relatives ‘Beehive’ kilns, were also the central feature of any cement works.
Early designs tended to be updraft kilns, which were often built as a straight sided cone into which the flame was introduced at, or below, floor level. Reaching heights of up to 70 ft, the dome or bottle shape of the kiln, known as the ‘hovel’, would be quite a prominent landmark. As well as protecting the inner kiln or ‘crown’, the opening at the top of the hovel also acted as a flue, to remove the smoke and exhaust gases that were produced during the production process. There was a three to four foot gap between the outer wall of the hovel and inner shell of the crown.
Due to the fact that the 1-foot-thick (0. 30 m) crown wall would expand and contract during firing, it was strengthened with a number of iron bands, known as ‘bonts’. These were set twelve inches apart and ran right around the circular oven. The development of downdraft kilns in the early 20th Century proved to be much more fuel efficient and were designed to force the heated air to circulate more around the kiln. The design incorporated a gentle curve at the ‘shoulders’ of the kiln, which served to reflect the rising heat from the fire at the bottom of the kiln, back down again over the material.
The smoke and exhaust was then sucked out through holes at the bottom of the kiln via a flue, which was connected to a nearby chimney. The chimney would also serve a number of neighbouring kilns as well. The kiln would be fired for several days to achieve the high temperatures required to produce cement clinker, and although the above methods were successful, the problem with any batch kiln was that it was intermittent and once the product had been produced, the fire had to be extinguished and the contents allowed to cool. This not only wasted a lot of the heat, but also added to the expense of the finished product.
In order to save money on fuel, a kiln was required that could run almost continuously, whilst the raw material was somehow fed through it. It was this scenario that lead to the development of the ‘Chamber’ kiln in the late 1850s. This particular kiln comprised a number of individual chambers, which were arranged so that the hot flue gases from one chamber, were drawn off and used to pre-heat the material in the following chambers, before they were drawn up the chimney. Once the first chamber had been filled with raw material, coal was added through the roof holes of the chamber and was then set alight.
At the same time, the second chamber was being filled with raw material. The airflow from the first chamber was then adjusted, using a number of dampers, to funnel the hot air through to the second chamber to pre-heat the material. More coal was then poured into the second chamber and ignited, as the third chamber was being filled and so on. This process continued along the length of the kiln, so that by the time the last chamber had been fired, the first chamber had already been cleared and re-filled with more raw material so that the process could start again.
Although such chamber kilns were still being installed as late as 1900, the development of the rotary kiln was already starting to have a major impact. The rotary kiln was a major advancement for the industry as it provided the continuous production of a much more uniform product in larger quantities. Rotatory kiln: The rotary kiln consists of a tube made from steel plate, and lined with firebrick. The tube slopes slightly (1–4°) and slowly rotates on its axis at between 30 and 250 revolutions per hour. Rawmix is fed in at the upper end, and the rotation of the kiln causes it gradually to move downhill to the other end of the kiln.
At the other end fuel, in the form of gas, oil, or pulverized solid fuel, is blown in through the “burner pipe”, producing a large concentric flame in the lower part of the kiln tube. As material moves under the flame, it reaches its peak temperature, before dropping out of the kiln tube into the cooler. Air is drawn first through the cooler and then through the kiln for combustion of the fuel. In the cooler the air is heated by the cooling clinker, so that it may be 400 to 800 °C before it enters the kiln, thus causing intense and rapid combustion of the fuel.
The earliest successful rotary kilns were developed in Pennsylvania around 1890, and were about 1. 5 m in diameter and 15 m in length. Such a kiln made about 20 tonnes of clinker per day. The fuel, initially, was oil, which was readily available in Pennsylvania at the time. It was particularly easy to get a good flame with this fuel. Within the next 10 years, the technique of firing by blowing in pulverized coal was developed, allowing the use of the cheapest available fuel. By 1905, the largest kilns were 2. 7 x 60 m in size, and made 190 tonnes per day.
At that date, after only 15 years of development, rotary kilns accounted for half of world production. Since then, the capacity of kilns has increased steadily, and the largest kilns today produce around 10,000 tonnes per day. In contrast to static kilns, the material passes through quickly: it takes from 3 hours (in some old wet process kilns) to as little as 10 minutes (in short precalciner kilns). Rotary kilns run 24 hours a day, and are typically stopped only for a few days once or twice a year for essential maintenance.
One of the main maintenance works on rotary kilns is tyre and roller surface machining and grinding works which can be done while the kiln works in full operation at speeds up to 3,5 rpm. This is an important discipline, because heating up and cooling down are long, wasteful and damaging processes. Uninterrupted runs as long as 18 months have been achieved. Wet and dry process: From the earliest times, two different methods of rawmix preparation were used: the mineral components were either dry-ground to form a flour-like powder, or were wet-ground with added water to produce a fine slurry with
the consistency of paint, and with a typical water content of 40–45%.  The wet process suffered the obvious disadvantage that, when the slurry was introduced into the kiln, a large amount of extra fuel was used in evaporating the water. Furthermore, a larger kiln was needed for a given clinker output, because much of the kiln’s length was used up for the drying process. On the other hand, the wet process had a number of advantages. Wet grinding of hard minerals is usually much more efficient than dry grinding. When slurry is dried in the kiln, it forms a granular crumble that is ideal for subsequent heating in the kiln.
In the dry process, it is very difficult to keep the fine powder rawmix in the kiln, because the fast-flowing combustion gases tend to blow it back out again. It became a practice to spray water into dry kilns in order to “damp down” the dry mix, and thus, for many years there was little difference in efficiency between the two processes, and the overwhelming majority of kilns used the wet process. By 1950, a typical large, wet process kiln, fitted with drying-zone heat exchangers, was 3. 3 x 120 m in size, made 680 tonnes per day, and used about 0. 25–0. 30 tonnes of coal fuel for every tonne of clinker produced.
Before the energy crisis of the 1970s put an end to new wet-process installations, kilns as large as 5. 8 x 225 m in size were making 3000 tonnes per day. An interesting footnote on the wet process history is that some manufacturers have in fact made very old wet process facilities profitable through the use of waste fuels. Plants that burn waste fuels enjoy a negative fuel cost (they are paid by industries needing to dispose of materials that have energy content and can be safely disposed of in the cement kiln thanks to its high temperatures and longer retention times).
As a result the inefficiency of the wet process is an advantage—to the manufacturer. By locating waste burning operations at older wet process locations, higher fuel consumption actually equates to higher profits for the manufacturer, although it produces correspondingly greater emission of CO2. Manufacturers who think such emissions should be reduced are abandoning the use of wet process. Preheaters: In the 1930s, significantly, in Germany, the first attempts were made to redesign the kiln system to minimize waste of fuel.  This led to two significant developments: •the grate preheater •the gas-suspension preheater.
Grate preheaters The grate preheater consists of a chamber containing a chain-like high-temperature steel moving grate, attached to the cold end of the rotary kiln.  A dry-powder rawmix is turned into a hard pellets of 10–20 mm diameter in a nodulizing pan, with the addition of 10-15% water. The pellets are loaded onto the moving grate, and the hot combustion gases from the rear of the kiln are passed through the bed of pellets from beneath. This dries and partially calcines the rawmix very efficiently. The pellets then drop into the kiln. Very little powdery material is blown out of the kiln.
Because the rawmix is damped in order to make pellets, this is referred to as a “semi-dry” process. The grate preheater is also applicable to the “semi-wet” process, in which the rawmix is made as a slurry, which is first de-watered with a high-pressure filter, and the resulting “filter-cake” is extruded into pellets, which are fed to the grate. In this case, the water content of the pellets is 17-20%. Grate preheaters were most popular in the 1950s and 60s, when a typical system would have a grate 28 m long and 4 m wide, and a rotary kiln of 3. 9 x 60 m, making 1050 tonnes per day, using about 0.
11-0. 13 tonnes of coal fuel for every tonne of clinker produced. Systems up to 3000 tonnes per day were installed. The key component of the gas-suspension preheater is the cyclone. A cyclone is a conical vessel into which a dust-bearing gas-stream is passed tangentially. This produces a vortex within the vessel. The gas leaves the vessel through a co-axial “vortex-finder”. The solids are thrown to the outside edge of the vessel by centrifugal action, and leave through a valve in the vertex of the cone. Cyclones were originally used to clean up the dust-laden gases leaving simple dry process kilns.
If, instead, the entire feed of rawmix is encouraged to pass through the cyclone, it is found that a very efficient heat exchange takes place: the gas is efficiently cooled, hence producing less waste of heat to the atmosphere, and the rawmix is efficiently heated. This efficiency is further increased if a number of cyclones are connected in series. The number of cyclones stages used in practice varies from 1 to 6. Energy, in the form of fan-power, is required to draw the gases through the string of cyclones, and at a string of 6 cyclones, the cost of the added fan-power needed for an extra cyclone exceeds the efficiency advantage
gained. It is normal to use the warm exhaust gas to dry the raw materials in the rawmill, and if the raw materials are wet, hot gas from a less efficient preheater is desirable. For this reason, the most commonly encountered suspension preheaters have 4 cyclones. The hot feed that leaves the base of the preheater string is typically 20% calcined, so the kiln has less subsequent processing to do, and can therefore achieve a higher specific output. Typical large systems installed in the early 1970s had cyclones 6 m in diameter, a rotary kiln of 5 x 75 m, making 2500 tonnes per day, using about 0.
11-0. 12 tonnes of coal fuel for every tonne of clinker produced. A penalty paid for the efficiency of suspension preheaters is their tendency to block up. Salts, such as the sulfate and chloride of sodium and potassium, tend to evaporate in the burning zone of the kiln. They are carried back in vapor form, and re-condense when a sufficiently low temperature is encountered. Because these salts re-circulate back into the rawmix and re-enter the burning zone, a recirculation cycle establishes itself. A kiln with 0. 1% chloride in the rawmix and clinker may have 5% chloride in the mid-kiln material.
Condensation usually occurs in the preheater, and a sticky deposit of liquid salts glues dusty rawmix into a hard deposit, typically on surfaces against which the gas-flow is impacting. This can choke the preheater to the point that air-flow can no longer be maintained in the kiln. It then becomes necessary to manually break the build-up away. Modern installations often have automatic devices installed at vulnerable points to knock out build-up regularly. An alternative approach is to “bleed off” some of the kiln exhaust at the kiln inlet where the salts are still in the vapor phase, and remove and discard the solids in this.
This is usually termed an “alkali bleed” and it breaks the recirculation cycle. It can also be of advantage for cement quality reasons, since it reduces the alkali content of the clinker. However, hot gas is run to waste so the process is inefficient and increases kiln fuel consumption. Precalcinery: In the 1970s the precalciner was pioneered in Japan, and has subsequently become the equipment of choice for new large installations worldwide.  The precalciner is a development of the suspension preheater. The philosophy is this: the amount of fuel that can be burned in the kiln is directly related
to the size of the kiln. If part of the fuel necessary to burn the rawmix is burned outside the kiln, the output of the system can be increased for a given kiln size. Users of suspension preheaters found that output could be increased by injecting extra fuel into the base of the preheater. The logical development was to install a specially designed combustion chamber at the base of the preheater, into which pulverized coal is injected. This is referred to as an “air-through” precalciner, because the combustion air for both the kiln fuel and the calciner fuel all passes through the kiln.
This kind of precalciner can burn up to 30% (typically 20%) of its fuel in the calciner. If more fuel were injected in the calciner, the extra amount of air drawn through the kiln would cool the kiln flame excessively. The feed is 40-60% calcined before it enters the rotary kiln. The ultimate development is the “air-separate” precalciner, in which the hot combustion air for the calciner arrives in a duct directly from the cooler, bypassing the kiln. Typically, 60-75% of the fuel is burned in the precalciner.
In these systems, the feed entering the rotary kiln is 100% calcined. The kiln has only to raise the feed to sintering temperature. In theory the maximum efficiency would be achieved if all the fuel were burned in the preheater, but the sintering operation involves partial melting and nodulization to make clinker, and the rolling action of the rotary kiln remains the most efficient way of doing this. Large modern installations typically have two parallel strings of 4 or 5 cyclones, with one attached to the kiln and the other attached to the precalciner chamber.
A rotary kiln of 6 x 100 m makes 8,000–10,000 tonnes per day, using about 0. 10-0. 11 tonnes of coal fuel for every tonne of clinker produced. The kiln is dwarfed by the massive preheater tower and cooler in these installations. Such a kiln produces 3 million tonnes of clinker per year, and consumes 300,000 tonnes of coal. A diameter of 6 m appears to be the limit of size of rotary kilns, because the flexibility of the steel shell becomes unmanageable at or above this size, and the firebrick lining tends to fail when the kiln flexes.
A particular advantage of the air-separate precalciner is that a large proportion, or even 100%, of the alkali-laden kiln exhaust gas can be taken off as alkali bleed (see above). Because this accounts for only 40% of the system heat input, it can be done with lower heat wastage than in a simple suspension preheater bleed. Because of this, air-separate precalciners are now always prescribed when only high-alkali raw materials are available at a cement plant. The accompanying figures show the movement towards the use of the more efficient processes in North America (for which data is readily available).
But the average output per kiln in, for example, Thailand is twice that in North America. Coolers: Early systems used rotary coolers, which were rotating cylinders similar to the kiln, into which the hot clinker dropped.  The combustion air was drawn up through the cooler as the clinker moved down, cascading through the air stream. In the 1920s, satellite coolers became common and remained in use until recently. These consist of a set (typically 7–9) of tubes attached to the kiln tube. They have the advantage that they are sealed to the kiln, and require no separate drive.
From about 1930, the grate cooler was developed. This consists of a perforated grate through which cold air is blown, enclosed in a rectangular chamber. A bed of clinker up to 0. 5 m deep moves along the grate. These coolers have two main advantages: they cool the clinker rapidly, which is desirable from a quality point of view (to avoid that alite, thermodynamically unstable below 1250 °C, revert to belite and free CaO on slow cooling), and, because they do not rotate, hot air can be ducted out of them for use in fuel drying, or for use as precalciner combustion air.
The latter advantage means that they have become the only type used in modern systems. Kiln fuels: Fuels that have been used for primary firing include coal, petroleum coke, heavy fuel oil, natural gas, landfill off-gas and oil refinery flare gas.  High carbon fuels such as coal are preferred for kiln firing, because they yield a luminous flame. The clinker is brought to its peak temperature mainly by radiant heat transfer, and a bright (i. e. high emissivity) and hot flame is essential for this.
In favorable circumstances, high-rank bituminous coal can produce a flame at 2050 °C. Natural gas can only produce a flame of, at best 1950 °C, and this is also less luminous, so it tends to result in lower kiln output. In addition to these primary fuels, various combustible waste materials have been fed to kilns, notably used tires, which are very difficult to dispose of by other means. In theory, cement kilns are an attractive way of disposing of hazardous materials, because of: •the temperatures in the kiln, which are much higher than in other combustion systems (e. g.
incinerators), •the alkaline conditions in the kiln, afforded by the high-calcium rawmix, which can absorb acidic combustion products, •the ability of the clinker to absorb heavy metals into its structure. Whole tires are commonly introduced in the kiln, by rolling them into the upper end of a preheater kiln, or by dropping them through a slot midway along a long wet kiln. In either case, the high gas temperatures (1000–1200 °C) cause almost instantaneous, complete and smokeless combustion of the tire. Alternatively, tires are chopped into 5–10 mm chips, in which form they can be injected into a precalciner combustion chamber.
The steel and zinc in the tires become chemically incorporated into the clinker. Other wastes have included solvents and clinical wastes. A very high level of monitoring of both the fuel and its combustion products is necessary to maintain safe operation. For maximum kiln efficiency, high quality conventional fuels are the best choice. When using waste materials, in order to avoid prohibited emissions (e. g. of dioxins) it is necessary to control the kiln system in a manner that is non-optimal for efficiency and output, and coarse combustibles such as tires can cause major product quality problems.
Cement kiln emissions: Emissions from cement works are determined both by continuous and discontinuous measuring methods, which are described in corresponding national guidelines and standards. Continuous measurement is primarily used for dust, NOx and SO2, while the remaining parameters relevant pursuant to ambient pollution legislation are usually determined discontinuously by individual measurements. The following descriptions of emissions refer to modern kiln plants based on dry process technology. Carbon dioxide During the clinker burning process CO2 is emitted. CO2 accounts for the main share of these gases.
CO2 emissions are both raw material-related and energy-related. Raw material-related emissions are produced during limestone decarbonation (CaCO3) and account for about 60% of total CO2 emissions. Dust To manufacture 1 t of Portland cement, about 1. 5 to 1. 7 t raw materials, 0. 1 t coal and 1 t clinker (besides other cement constituents and sulfate agents) must be ground to dust fineness during production. In this process, the steps of raw material processing, fuel preparation, clinker burning and cement grinding constitute major emission sources for particulate components.
While particulate emissions of up to 3,000 mg/m3 were measured leaving the stack of cement rotary kiln plants as recently as in the 1950s, legal limits are typically 30 mg/m3 today, and much lower levels are achievable. Nitrogen oxides (NOx) The clinker burning process is a high-temperature process resulting in the formation of nitrogen oxides (NOx). The amount formed is directly related to the main flame temperature (typically 1850–2000 °C). Nitrogen monoxide (NO) accounts for about 95%, and nitrogen dioxide (NO2) for about 5% of this compound present in the exhaust gas of rotary kiln plants.
As most of the NO is converted to NO2 in the atmosphere, emissions are given as NO2 per cubic metre exhaust gas. Without reduction measures, process-related NOx contents in the exhaust gas of rotary kiln plants would in most cases considerably exceed the specifications of e. g. European legislation for waste burning plants (0. 50 g/m3 for new plants and 0. 80 g/m3 for existing plants). Reduction measures are aimed at smoothing and ehavior plant operation. Technically, staged combustion and Selective Non-Catalytic NO Reduction (SNCR) are applied to cope with the emission limit values.
High process temperatures are required to convert the raw material mix to Portland cement clinker. Kiln charge temperatures in the sintering zone of rotary kilns range at around 1450 °C. To reach these, flame temperatures of about 2000 °C are necessary. For reasons of clinker quality the burning process takes place under ehavior conditions, under which the partial oxidation of the molecular nitrogen in the combustion air resulting in the formation of nitrogen monoxide (NO) dominates. This reaction is also called thermal NO formation.
At the lower temperatures prevailing in a precalciner, however, thermal NO formation is negligible: here, the nitrogen bound in the fuel can result in the formation of what is known as fuel-related NO. Staged combustion is used to reduce NO: calciner fuel is added with insufficient combustion air. This causes CO to form. The CO then reduces the NO into molecular nitrogen: 2 CO + 2 NO > 2 CO2 + N2. Hot tertiary air is then added to oxidize the remaining CO. Sulfur dioxide (SO2) Sulfur is input into the clinker burning process via raw materials and fuels.
Depending on their origin, the raw materials may contain sulfur bound as sulfide or sulfate. Higher SO2 emissions by rotary kiln systems in the cement industry are often attributable to the sulfides contained in the raw material, which become ehavior to form SO2 at the temperatures between 370 °C and 420 °C prevailing in the kiln preheater. Most of the sulfides are pyrite or marcasite contained in the raw materials. Given the sulfide concentrations found e. g. in German raw material deposits, SO2 emission concentrations can total up to 1. 2 g/m3 depending on the site location.
In some cases, injected calcium hydroxide is used to lower SO2 emissions. The sulfur input with the fuels is completely converted to SO2 during combustion in the rotary kiln. In the preheater and the kiln, this SO2 reacts to form alkali sulfates, which are bound in the clinker, provided that oxidizing conditions are maintained in the kiln. Carbon monoxide (CO) and total carbon The exhaust gas concentrations of CO and organically bound carbon are a yardstick for the burn-out rate of the fuels ehavior in energy conversion plants, such as power stations.
By contrast, the clinker burning process is a material conversion process that must always be operated with excess air for reasons of clinker quality. In concert with long residence times in the high-temperature range, this leads to complete fuel burn-up. The emissions of CO and organically bound carbon during the clinker burning process are caused by the small quantities of organic constituents input via the natural raw materials (remnants of organisms and plants incorporated in the rock in the course of geological history).
These are converted during kiln feed preheating and become oxidized to form CO and CO2. In this process, small portions of organic trace gases (total organic carbon) are formed as well. In case of the clinker burning process, the content of CO and organic trace gases in the clean gas therefore may not be directly related to combustion conditions. Dioxins and furans (PCDD/F) Rotary kilns of the cement industry and classic incineration plants mainly differ in terms of the combustion conditions prevailing during clinker burning.
Kiln feed and rotary kiln exhaust gases are conveyed in counter-flow and mixed thoroughly. Thus, temperature distribution and residence time in rotary kilns afford particularly favourable conditions for organic compounds, introduced either via fuels or derived from them, to be completely destroyed. For that reason, only very low concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (colloquially “dioxins and furans”) can be found in the exhaust gas from cement rotary kilns. Polychlorinated biphenyls (PCB) The emission ehavior of PCB is comparable to that of dioxins and furans.
PCB may be introduced into the process via alternative raw materials and fuels. The rotary kiln systems of the cement industry destroy these trace components virtually completely.  Polycyclic aromatic hydrocarbons (PAH) PAHs (according to EPA 610) in the exhaust gas of rotary kilns usually appear at a distribution dominated by naphthalene, which accounts for a share of more than 90% by mass. The rotary kiln systems of the cement industry destroy virtually completely the PAHs input via fuels. Emissions are generated from organic constituents in the raw material.
Benzene, toluene, ethylbenzene, xylene (BTEX) As a rule benzene, toluene, ethylbenzene and xylene are present in the exhaust gas of rotary kilns in a characteristic ratio. BTEX is formed during the thermal decomposition of organic raw material constituents in the preheater. Gaseous inorganic chlorine compounds (HCl) Chlorides are minor additional constituents contained in the raw materials and fuels of the clinker burning process. They are released when the fuels are burnt or the kiln feed is heated, and primarily react with the alkalis from the kiln feed to form alkali chlorides.
These compounds, which are initially vaporous, condense on the kiln feed or the kiln dust, at temperatures between 700 °C and 900 °C, subsequently re-enter the rotary kiln system and evaporate again. This cycle in the area between the rotary kiln and the preheater can result in coating formation. A bypass at the kiln inlet allows effective reduction of alkali chloride cycles and to diminish coating build-up problems. During the clinker burning process, gaseous inorganic chlorine compounds are either not emitted at all or in very small quantities only. Gaseous inorganic fluorine compounds (HF)
Of the fluorine present in rotary kilns, 90 to 95% is bound in the clinker, and the remainder is bound with dust in the form of calcium fluoride stable under the conditions of the burning process. Ultra-fine dust fractions that pass through the measuring gas filter may give the impression of low contents of gaseous fluorine compounds in rotary kiln systems of the cement industry. Trace elements The emission ehavior of the individual elements in the clinker burning process is determined by the input scenario, the ehavior in the plant and the precipitation efficiency of the dust collection device.
The trace elements introduced into the burning process via the raw materials and fuels may evaporate completely or partially in the hot zones of the preheater and/or rotary kiln depending on their volatility, react with the constituents present in the gas phase, and condense on the kiln feed in the cooler sections of the kiln system. Depending on the volatility and the operating conditions, this may result in the formation of cycles that are either restricted to the kiln and the preheater or include the combined drying and grinding plant as well.
Trace elements from the fuels initially enter the combustion gases, but are emitted to an extremely small extent only owing to the retention capacity of the kiln and the preheater. Under the conditions prevailing in the clinker burning process, non-volatile elements (e. g. arsenic, vanadium, nickel) are completely bound in the clinker. Elements such as lead and cadmium preferentially react with the excess chlorides and sulfates in the section between the rotary kiln and the preheater, forming volatile compounds.
Owing to the large surface area available, these compounds condense on the kiln feed particles at temperatures between 700 °C and 900 °C. In this way, the volatile elements accumulated in the kiln-preheater system are precipitated again in the cyclone preheater, remaining almost completely in the clinker. Thallium (as the chloride) condenses in the upper zone of the cyclone preheater at temperatures between 450 °C and 500 °C. As a consequence, a cycle can be formed between preheater, raw material drying and exhaust gas purification. Mercury and its compounds are not precipitated in the kiln and the preheater.
They condense on the exhaust gas route due to the cooling of the gas and are partially adsorbed by the raw material particles. This portion is precipitated in the kiln exhaust gas filter. Owing to trace element ehavior during the clinker burning process and the high precipitation efficiency of the dust collection devices trace element emission concentrations are on a low overall level. Reverse air bag house: In reverse-air baghouses, the bags are fastened onto a cell plate at the bottom of the baghouse and suspended from an adjustable hanger frame at the top.
Dirty gas flow normally enters the baghouse and passes through the bag from the inside, and the dust collects on the inside of the bags. Reverse-air baghouses are compartmentalized to allow continuous operation. Before a cleaning cycle begins, filtration is stopped in the compartment to be cleaned. Bags are cleaned by injecting clean air into the dust collector in a reverse direction, which pressurizes the compartment. The pressure makes the bags collapse partially, causing the dust cake to crack and fall into the hopper below.
At the end of the cleaning cycle, reverse airflow is discontinued, and the compartment is returned to the main stream. The flow of the dirty gas helps maintain the shape of the bag. However, to prevent total collapse and fabric chafing during the cleaning cycle, rigid rings are sewn into the bags at intervals. Space requirements for a reverse-air baghouse are comparable to those of a shaker baghouse; however, maintenance needs are somewhat greater. Cement mill: A ball mill is a horizontal cylinder partly filled with steel balls (or occasionally other shapes) that rotates on its axis, imparting a tumbling and cascading action to the balls.
Material fed through the mill is crushed by impact and ground by attrition between the balls. The grinding media are usually made of high-chromium steel. The smaller grades are occasionally cylindrical (“pebs”) rather than spherical. There exists a speed of rotation (the "critical speed") at which the contents of the mill would simply ride over the roof of the mill due to centrifugal action. The critical speed (rpm) is given by: nC = 42. 29/vd, where d is the internal diameter in metres. Ball mills are normally operated at around 75% of critical speed, so a mill with diameter 5 metres will turn at around 14 rpm.
The mill is usually divided into at least two chambers,(Depends upon feed input size presently mill installed with Roller Press are mostly single chambered), allowing the use of different sizes of grinding media. Large balls are used at the inlet, to crush clinker nodules (which can be over 25 mm in diameter). Ball diameter here is in the range 60–80 mm. In a two-chamber mill, the media in the second chamber are typically in the range 15–40 mm, although media down to 5 mm are sometimes encountered.
As a general rule, the size of media has to match the size of material being ground: large media can't produce the ultra-fine particles required in the finished cement, but small media can't break large clinker particles. Mills with as many as four chambers, allowing a tight segregation of media sizes, were once used, but this is now becoming rare. Alternatives to multi-chamber mills are: A current of air is passed through the mill. This helps keep the mill cool, and sweeps out evaporated moisture which would otherwise cause hydration and disrupt material flow. The dusty exhaust air is cleaned, usually with bag filters.