Hot end of medium-sized modern cement kiln, showing tyres, rollers and drive gear Cement kiln From Wikipedia, the free encyclopedia Cement kilns are used for the pyroprocessing 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 tonnes of cement are made per year, and cement kilns are the heart of this production process: their capacity usually defines 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. Contents ◾ 1 The manufacture of cement clinker ◾ 2 Early history ◾ 3 The rotary kiln ◾ 4 The wet process and the dry process ◾ 5 Preheaters ◾ 5.1 Grate preheaters ◾ 5.2 Gas-suspension preheaters ◾ 6 Precalciners ◾ 7 Ancillary equipment ◾ 7.1 Coolers ◾ 7.2 Fuel mills ◾ 7.3 Fans ◾ 7.4 Gas cleaning Page 1 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln ◾ 8 Kiln fuels ◾ 8.1 Alternative fuels ◾ 9 Kiln control ◾ 10 Cement kiln emissions ◾ 10.1 Carbon dioxide ◾ 10.2 Dust ◾ 10.3 Nitrogen oxides (NOx) ◾ 10.4 Sulfur dioxide (SO2) ◾ 10.5 Carbon monoxide (CO) and total carbon ◾ 10.6 Dioxins and furans (PCDD/F) ◾ 10.7 Polychlorinated biphenyls (PCB) ◾ 10.8 Polycyclic aromatic hydrocarbons (PAH) ◾ 10.9 Benzene, toluene, ethylbenzene, xylene (BTEX) ◾ 10.10 Gaseous inorganic chlorine compounds (HCl) ◾ 10.11 Gaseous inorganic fluorine compounds (HF) ◾ 10.12 Trace elements ◾ 11 References The manufacture of cement clinker 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. Page 2 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln A preheater tower, rawmix silo and exhaust stack. Bottom left: rawmill. Bottom right: rotary kiln with tertiary air duct above. The U-shaped duct leading from the kiln inlet is an "alkali bleed". Typical clinker nodules ◾ 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 Page 3 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln 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 . Early history Portland cement clinker was first made (in 1825) in a modified form of the traditional static lime kiln.[1][2][3] 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. Around 1885, experiments began on design of continuous kilns. One design was the shaft kiln, similar in design to a blast furnace. Rawmix in the form of lumps and fuel were continuously added at the top, and clinker was continually withdrawn at the bottom. Air was blown through under pressure from the base to combust the fuel. The shaft kiln had a brief period of use before it was eclipsed by the rotary kiln, but it had a limited renaissance from 1970 onward in China and elsewhere, when it was used for small-scale, low- tech plants in rural areas away from transport routes. Several thousand such kilns were constructed in China. A typical shaft kiln produces 100-200 tonnes per day. From 1885, trials began on the development of the rotary kiln, which today accounts for more than 95% of world production. The rotary kiln Page 4 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln General layout of a rotary 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, based on a design by Frederick Ransome,[4] 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 Page 5 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln % of North American Capacity using Wet Process 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. The wet process and the 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%.[5] 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 Page 6 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln Mean Fuel Energy used in North American Kilns 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.[6] This led to two significant developments: Page 7 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln ◾ 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.[7] 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. Gas-suspension preheaters 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 Page 8 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln Cutaway view of cyclone showing air path 4-Stage preheater, showing path of feed takes place: the gas is efficiently cooled, hence producing less waste of heat to the atmosphere, and the raw mix 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. Page 9 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln % of North American Capacity using Precalciners 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. Precalciners In the 1970s the precalciner was pioneered in Japan, and has subsequently become the equipment of choice for new large installations worldwide.[8] 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 Page 10 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln Mean Daily Output (tonnes) of North American Kilns 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 Page 11 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln A pair of kilns with satellite coolers in Ashaka, Nigeria Sysy 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. Ancillary equipment Essential equipment in addition to the kiln tube and the preheater are: ◾ Cooler ◾ Fuel mills ◾ Fans ◾ Exhaust gas cleaning equipment. Coolers Early systems used rotary coolers, which were rotating cylinders similar to the kiln, into which the hot clinker dropped.[9] 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, Page 12 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln cement plant 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 . Fuel mills Fuel systems are divided into two categories:[10] ◾ Direct firing ◾ Indirect firing In direct firing, the fuel is fed at a controlled rate to the fuel mill, and the fine product is immediately blown into the kiln. The advantage of this system is that it is not necessary to store the hazardous ground fuel: it is used as soon as it is made. For this reason it was the system of choice for older kilns. A disadvantage is that the fuel mill has to run all the time: if it breaks down, the kiln has to stop if no backup system is available. In indirect firing, the fuel is ground by an intermittently run mill, and the fine product is stored in a silo of sufficient size to supply the kiln though fuel mill stoppage periods. The fine fuel is metered out of the silo at a controlled rate and blown into the kiln. This method is now favoured for precalciner systems, because both the kiln and the precalciner can be fed with fuel from the same system. Special techniques are required to store the fine fuel safely, and coals with high volatiles are normally milled in an inert atmosphere (e.g. CO2). Page 13 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln Used tires being fed mid- kiln to a pair of long kilns Fans A large volume of gases has to be moved through the kiln system.[11] Particularly in suspension preheater systems, a high degree of suction has to be developed at the exit of the system to drive this. Fans are also used to force air through the cooler bed, and to propel the fuel into the kiln. Fans account for most of the electric power consumed in the system, typically amounting to 10–15 kW·h per tonne of clinker. Gas cleaning The exhaust gases from a modern kiln typically amount to 2 tonnes (or 1500 cubic metres at STP) per tonne of clinker made.[12] The gases carry a large amount of dust—typically 30 grams per cubic metre. Environmental regulations specific to different countries require that this be reduced to (typically) 0.1 gram per cubic metre, so dust capture needs to be at least 99.7% efficient. Methods of capture include electrostatic precipitators and bag-filters. See also cement kiln emissions. 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.[13] 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. Page 14 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln Alternative fuels In addition to these primary fuels, various combustible waste materials have been fed to kilns. These alternative fuels (AF) include: ◾ Used motor-vechile tires ◾ Sewage sludge ◾ Agricultural waste ◾ Landfill gas ◾ Refuse-derived fuel (RDF) ◾ Chemical and other hazardous waste 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. A notable example is the use of scrapped motor-vechile tires, which are very difficult to dispose of by other means. 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, partially replacing iron that must otherwise be fed as raw material. A high level of monitoring of both the fuel and its combustion products is necessary to maintain safe operation.[14] Page 15 of 26 Cement kiln - Wikipedia 1/2/2017 https://en.wikipedia.org/wiki/Cement_kiln