Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS, OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D17/00—Arrangements for using waste heat; Arrangements for using, or disposing of, waste gases
  • F27D17/008—Arrangements for using waste heat; Arrangements for using, or disposing of, waste gases cleaning gases
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B7/00—Hydraulic cements
  • C04B7/36—Manufacture of hydraulic cements in general
  • C04B7/364—Avoiding environmental pollution during cement-manufacturing
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B7/00—Hydraulic cements
  • C04B7/36—Manufacture of hydraulic cements in general
  • C04B7/364—Avoiding environmental pollution during cement-manufacturing
  • C04B7/365—Avoiding environmental pollution during cement-manufacturing by extracting part of the material from the process flow and returning it into the process after a separate treatment, e.g. in a separate retention unit under specific conditions
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27B—FURNACES, KILNS, OVENS, OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
  • F27B7/00—Rotary-drum furnaces, i.e. horizontal or slightly inclined
  • F27B7/20—Details, accessories, or equipment peculiar to rotary-drum furnaces
  • F27B7/2016—Arrangements of preheating devices for the charge

Definitions

• the present invention refers to an integrated apparatus for the production of clinker starting from raw meal.
• the process of production of cement comprises the following steps:
• composition corrective substances such as gypsum, and additives (limestone, slag, pozzolana) .
• the process of production of clinker and then of cement provides on industrial scale a series of phases connected and subsequent to each other, and the phase of baking of raw materials is the one that mostly characterizes the whole production process.
• the raw mixture is fed to the kiln in form of dust.
• a dust is produced in a grinding plant, directly connected to the same kiln, which exploits the residual content of thermal energy of the gases coming from the kiln, to dry the wetness naturally associated with the raw materials to grind.
• the clinker is obtained by means of backing at high temperature of a mixture of raw materials mainly composed of limestone (calcium carbonate) and clay (silica, alumina, iron oxides, additional to crystallization water) .
• the raw materials are mixed in solid state in the desired proportions and then finely grinded up to obtain a homogeneous dust, the so-called "raw meal".
• the "raw meal” shall be therefore intended as the homogeneous dust so obtained, used as starting material for the production of clinker.
• the raw meal before being fed to the rotary kiln, is subjected to a pre-heating treatment and, possibly, to precalcination .
• One of the pre-heating methods currently most used is based on the employ of the so-called “suspension preheater” or “multistage cyclone preheater” (hereinafter referred to only as “preheater” or PRS) , composed of a tower of cyclones in which each step of pre-heating occurs in one or more cyclones.
• preheater or PRS
• the first cyclone is the one in which it takes place the first step of pre-heating and the first separation between pre-heated meal and combustion fumes
• the second cyclone is the one in which it takes place the second step of pre-heating and the second separation between pre-heated meal and combustion fumes and analogously are defined the subsequent cyclones of the multistage cyclone preheater.
• the first cyclone of the preheater, as well as the subsequent cyclones shall always be intended according to the above provided definition.
• FIG. 1A wherein it is illustrated a schematic representation of a plant of production of clinker according to the state of the art, comprising a rotary kiln provided with a 4-steps suspension preheater;
• FIG. IB wherein it is illustrated a schematic representation of a plant of production of clinker according to the state of the art, comprising a rotary kiln provided with a 5-steps suspension preheater and with a precalciner.
• the steps of pre-heating and pre-calcining are carried out, respectively, in the preheater 1 and in the precalciner 2 ( Figures 1A and IB) .
• the presence of these steps allows to feed to the rotary kiln 3 the meal partially calcined (30-40%) and pre-heated at a temperature of about 950°C, with a noticeable energy save in the subsequent reaction of clinkerization .
• step of pre-heating possibly associated with the step of pre-calcination, further allows to use rotary kilns smaller in size, reducing thus the heat losses which occur in such kilns and increasing the whole energy efficiency of the clinker production process.
• the task of the preheater is that of heating the raw materials from 40°C to 950°C and it is composed, as said, of a multistage cyclone tower (usually from 4 to 6 steps) , said stages being placed one on the other to form a tower with a variable height (up to 130-150 m) .
• a preheater can be defined as multistage cyclone preheater. In each step the thermal exchange between the material and the combustion gases is achieved in two steps:
• the thermal energy is supplied to the process from the main burner (placed on the rotary tube) and, in case of kilns provided with a precalciner, from the burners placed in the same precalciner.
• the gases go up the rotary tube and the tower of the PRS from the bottom to the top, suctioned by a ventilator arranged on the duct of exit of the first step.
• the thermal exchange between gases and raw materials determines a temperature lowering of the gases from 900-1000°C to 310-330°C.
• the fumes exiting from the PRS are used, partially or totally, for drying the same raw materials.
• the suspension preheater is composed of a series of cyclones specifically to allow an optimal contact time between the solid phase and the gaseous phase.
• the first step of pre-heating which occurs at the top of the tower, can be achieved in two parallel cyclones to ensure the best efficiency of separation of the meal from the gaseous stream before the exit of the latter from the preheater (as shown in Figures 1A, IB and 2) .
• the combustion fumes coming from the rotary kiln 3 and having a temperature of about 900-1000°C cross the cyclones from the bottom to the top (from IV to I) .
• the starting raw meal is mixed with the combustion fumes in the preheater 1, in which it is inserted through an entrance 4, arranged at the top of the preheater, between the first (I) cyclone and the second (II) cyclone.
• the raw meal crosses the preheater up to the exit in the lower portion, transported from one cyclone to the other, by the flow of combustion fumes.
• each cyclone about 80% of the solid phase (meal) is separated from the gaseous phase (combustion fumes) to be then newly introduced in the gaseous phase entering the cyclone arranged below.
• the gaseous phase containing the residual solid fraction (about 20% of the meal) flows, instead, to the next cyclone arranged above .
• a pre-heated meal is obtained having a temperature of about 950 °C. From the last pre-heating step in the multistage cyclone preheater, the meal is directly discharged in the rotary kiln 3 for the next reaction of clinkerization .
• the pre-heated meal is fed from the preheater 1 to a proper combustion chamber 5, provided with a burner 6, in said chamber being subjected to a partial process of calcination.
• the pre-calcined meal leaves the precalciner 2 and is fed, along with the combustion fumes of the precalciner 2, to the last step (V) of the preheater 1 to advance then towards the rotary kiln 3.
• the combustion fumes of the precalciner 2 flow together with those of the rotary kiln 3 and go up the preheater 1 unto the head exit 7, after the first cyclone.
• the gaseous stream exiting from exit 7 of the preheater comprising the combustion fumes of the rotary kiln 3 and, possibly, those of the precalciner 2, as well as the C0 2 produced by the dissociation of calcium carbonate, has a temperature of about 300- 330°C.
• this stream Prior to be released in the atmosphere, this stream is generally used in other phases of the process of production of cement (for example, for grinding and drying raw materials or as combustion air in the rotary kiln or in the precalciner) to recover its caloric content .
• the raw meal is turned into clinker by means of baking at a temperature of about 1450°C in a rotary kiln, mainly composed of a tilted rotary cylinder.
• the kiln (rotary tube) is mainly composed of a tilted rotary cylinder and its task is that of heating the raw materials from 950°C to 1450°C.
• the material calcined up to 95% when the plant is provided with a precalciner, is fed to the kiln from the discharge of the lower step of the PRS and undergoes, through progressive heating, a complete calcination and subsequently the reaction of formation of calcium silicates (mainly tricalcium and dicalcium silicate - reactions of clinkerization) which represent the main constituents of the clinker.
• the term clinkerization shall be intended as a series of chemical reactions among calcium, silica, alumina and iron oxides promoted by melting of a portion of the same raw materials (alumina oxides, iron and other minor elements) .
• the energy necessary to achieve the reactions of clinkerization (about 40% of the total) is basically related to the need to increase the temperature of the material up to 1450°C, temperature at which the clinkerization reactions are completed, being weakly exothermal.
• the required energy is supplied to the rotary tube through a burner arranged at the opposite end with respect to the loading zones of the raw materials, and it is transferred to the material by irradiation in the zone of the burner (the flame has a temperature of about 2000°C) and by convection and conduction through the combustion gases in the remaining part of the kiln.
• the fuels generally used are coal, pet-coke, fuel oil, methane, additional to alternative fuels such as, for example, animal meals.
• the clinker so obtained is discharged from the rotary kiln and it is quickly cooled in an air-cooler to make it stable.
• the cooler arranged in the discharge zone of the rotary kiln, has the task to cool the clinker from the temperature of 1300-1350°C to a temperature of about 100°C. It is mainly composed of a plurality of hollowed plates which allow the passage of cooling air blown-in by means of appropriate ventilators.
• the gas leaving the PRS must be cooled to reach a suitable temperature for a subsequent use.
• two systems are diffusely used:
• Heat exchangers subdivided into: o Gas/air exchangers, in which ambient air is used as cooling means; in this case the exchanged heat is then dissipated in the atmosphere ;
• Diathermal gas/oil exchangers in which diathermal oil is used as cooling means; in this case the exchanged heat is then recovered for other uses;
• the gaseous stream leaving the preheater is characterized by the presence of polluting substances, such as nitrogen oxides (NO x ) , sulphur oxides (in particular SO 2 ) and by a high concentration of dusts.
• polluting substances such as nitrogen oxides (NO x ) , sulphur oxides (in particular SO 2 ) and by a high concentration of dusts.
• NO x mainly derive from combustion processes which take place in the rotary kiln and, possibly, in the precalciner.
• the main methods used for reducing NO x in the gaseous stream leaving the preheater are two:
• Selective Catalytic Reduction - SCR - which provides the reaction of NO x with a reducing agent (for example, ammonia or urea) in the presence of a catalyst .
• a reducing agent for example, ammonia or urea
• the SNCR technique is effective if used on a gaseous stream having a temperature of about 800-900°C and allows to reduce up to 65% the NO x present.
• the application of the SCR technique of recent development in the field of production of cement, allows to lead to very high yields of reduction (higher than 90%) when used on a gaseous stream having temperature values comprised between about 300 and 400°C.
• the SCR system of removal is arranged in the plants of production of clinker in correspondence of the head exit of the preheater, after the first cyclone, wherein the gaseous stream leaving that exit, comprising the combustion fumes of the rotary kiln and, possibly, those of the precalciner, has a temperature of about 300-330°C.
• the system of catalytic removal of the nitrogen oxides is mainly composed of a series of layers/modulus of catalyst and of a series of nozzles for injection of ammonia.
• the fumes exiting from the preheater at a temperature of 320-350°C and with a concentration of NOx of 1200-1500 mg/Nm ⁇ are treated with an ammonia solution and conveyed towards the modulus of the catalyst, in which the reaction of reduction between NOx and ammonia occurs, with formation of elementary nitrogen and water.
• the removal of SO 2 is generally achieved through injection of compounds based on calcium oxides and/or hydroxides in the combustion fumes with subsequent formation of calcium sulphate, which can be advantageously recycled in the production process of clinker.
• the removal of SO 2 can be achieved through injection of sodium bicarbonate in the combustion fumes leaving the PRS .
• Sodium bicarbonate at 180-200°C turns into sodium carbonate and allows the removal of SO 2 with high efficiency. Also in this case the product of reaction is recycled inside the production process.
• the combustion fumes leaving the preheater after their depuration from NO x and SO x and after their possible recycling through other phases of the production process to recover the residual heat, must be finally freed from dust before their release in the atmosphere.
• the process of dust removal is normally achieved by filtration with electro-filters (also named as electrostatic precipitators) or with fabric filters, the latter being extensively used in the plants of production of clinker.
• Fabric filters are however capable to work at most at temperatures of 250°C and therefore the use of fabric filters implies the arrangement of appropriate systems of reduction of temperatures of the gases to be filtered (conditioning towers, heat exchangers, injection of dilution air).
• conditioning towers heat exchangers, injection of dilution air.
• the combustion fumes leaving the PRS contain a high quantity of sensitive heat that, if not re-used in the production process (for example, to dry the raw materials) , is generally dissipated by injection of water in the conditioning tower .
• the sensitive heat contained in the combustion fumes leaving the PRS is specifically used for drying the raw materials, but very often, according to the wet degree of the same raw materials, a remarkable amount of residual heat remains available and it is equally dissipated in the conditioning tower.
• FIG. 2 A conventional process according to the state of the art is represented in Figure 2, wherein it is illustrated a schematic representation of a production plant of clinker according to the known art, comprising a system of SCR removal 8 directly downstream of the PRS (preheater) , followed by a system of residual heat dissipation through a conditioning tower 13.
• a further problem in the existing processes and plants is just connected to the arrangement of a catalytic system of NOx removal and a subsequent system of recovery or dissipation of the residual heat contained in the combustion fumes.
• the Applicant has surprisingly identified a solution which allows to overcome the above reported drawbacks and makes possible the achievement of a system of thermal treatment of the combustion fumes leaving the PRS, integrated with a catalytic system of NOx removal (SCR) , in small spaces, ensuring optimal performances in the removal of pollutants.
• SCR catalytic system of NOx removal
• An objective of the present invention is therefore that of achieving an apparatus to overcome the drawbacks met by the state of the art.
• the object of the present invention is therefore an integrated process for the production of clinker starting from raw meal, comprising
• a multistage cyclone preheater 1 connected downstream of said rotary kiln 3 with respect to the direction of flow of the fumes 10 of a combustion, taking place in said kiln 3; possibly a precalciner 2 ; a catalytic system of NOx removal 8, connected downstream of said preheater 1 with respect to said direction of flow of the combustion fumes 10;
• said apparatus being characterized in that the catalytic system of NOx removal 8 and the system of heat treatment 9, 13 of the fumes leaving the catalytic system of NOx removal 8 are integrated in a single tower structure 12.
• the integrated apparatus according to the present invention can be moreover provided with a non-catalytic system of NOx removal (SNCR) .
• SNCR non-catalytic system of NOx removal
• the system of heat treatment of the integrated apparatus according to the present invention can be a system of heat recovery 9 or a system of heat dissipation 13, preferably it is a system of heat recovery 9.
• the system of heat treatment is a tube bundle heat recovery (WHR) system.
• WHR tube bundle heat recovery
• the fumes leaving the catalytic system of NOx removal (SCR) are cooled by a plurality of tube bundles crossed by a fluid (diathermal oil or water) , in its turn used in the production cycle (for example, for production of electric power, for production of vapour, for production of warm water, etc.) .
• a fluid diathermal oil or water
• the temperature of the fumes leaving the WHR is controlled by the flow of fluid crossing the tube bundles .
• the system of dissipation of heat is generally constituted by a conditioning tower (TC) .
• the integrated apparatus exploits, for the arrangement of the SCR system and of the heat exchanger or of the conditioning tower, the space conventionally occupied by the duct leaving the PRS which goes down.
• the main advantage of the integrated process according to the present invention is that it allows at the same time to effectively recover the residual heat in the fumes, reducing the dispersion surface, consisting of the tubes connecting the preheater, the SCR system and the tube bundle exchanger, and reducing for the same reason the load losses of the gaseous stream, thus eliminating the energy losses.
• the integration in a structure of the SCR system and of the energy recovery system allows to make the gas duct "active" which, from the top of the preheater goes down, where it is positioned the gas exhauster of the baking kiln (16 in figures 3 and 4), namely the ventilator suctioning the gases produced by the baking plant to be sent to the process phase of grinding of raw materials.
• the gas duct (downcomer) which from the top of the preheater comes down is quite fully substituted by the two elements integrated in a single structure, eliminating thus the related dissipations of thermal energy through its walls and of electric energy necessary to withstand the related load loss.
• the integrated apparatus according to the present invention allows to recover the residual heat contained in the combustion fumes, improving the whole efficiency of the production process and allowing also a noticeable reduction of water consumption for cooling the fumes.
• the apparatus according to the present invention allows also to provide, where necessary, a system of withdrawal of high temperature fumes leaving the catalytic system of NO x removal (SCR) and downstream of the heat treatment, where the fumes thus withdrawn are subjected to injection of sodium bicarbonate (NaHCC ⁇ ) for SO 2 removal.
• SCR catalytic system of NO x removal
• NaHCC ⁇ sodium bicarbonate
• Sodium bicarbonate must be indeed injected in a gaseous stream having a temperature of about 180-200°C, which can be obtained by dilution with ambient air of the withdrawn high temperature stream.
• Sodium bicarbonate once it is turned into sodium carbonate (Na2CC>3) , is transported to the entrance of the process filter where it can effect removal of SO2 with high efficiency.
• Embodiments of the integrated apparatus according to the present invention are schematically represented in the attached Figures 3 and 4.
• the combustion fumes 10 enter the preheater 1 from the bottom and go up the cyclones of the multistage cyclone preheater 1 unto the upper exit 7.
• the raw meal entering 4 is mixed with the combustion fumes 10 leaving the preheater 1, with pre-heating of the raw meal by contact with the combustion fumes 10, and formation of a gaseous stream containing a raw meal partially pre-heated in suspension moving towards the next step of the PRS .
• the raw meal subjected to pre-heating in a suspension preheater starts from a temperature of about 40 °C and reaches temperature values in the range of 270-360°C after having crossed at least the first two steps of preheating.
• the preheating of the raw meal up to the temperature of entrance to rotary kiln (about 950°C) is completed.
• the raw meal preheated up to a temperature of about 950 °C is discharged from the bottom of the preheater 1 in the rotary kiln 3 for the subsequent reaction of clinkerization .
• the combustion fumes leaving the preheater 1 through 7 are thus subjected to further treatments of removal from pollutants and/or heat treatments.
• the apparatus object of the present invention provides a system of removal of NO x 8.
• a system of removal of NO x 8 is a selective catalytic reduction system (SCR) where it is achieved a process of selective catalytic reduction by means of reducing agents (for example, ammonia) .
• SCR selective catalytic reduction system
• the reducing agent can be fed to the gaseous stream upstream of the device SCR 8.
• reducing agent it can be used also the ammonia possibly present in the same flow of combustion fumes subjected to the SCR treatment.
• This ammonia derives from the heat treatment of the raw materials fed to the preheater and it is conveyed by the combustion fumes up to the catalyst of the SCR system. If the amount of ammonia deriving from the raw materials is not sufficient, it is possible to feed in the gaseous stream subjected to SCR an additional amount of ammonia or another reducing agent.
• the apparatus according to the present invention can comprise, for example, a heat exchanger of the type air/air, air/diathermal oil, air/water-vapour (see heat exchanger 9 in Figure 3) or a water conditioning tower (conditioning tower 13 in Figure 4 ) .
• a further treatment to which it is possible to subject the combustion fumes leaving the preheater 1 is a process of removal of sulphur oxides (desulphurization), in particular of removal of SO 2 .
• this process provides the injection of compounds based on calcium oxides and/or hydroxides in the combustion fumes, by means of a proper system of injection.
• the above process of desulphurization can be achieved either before or after the process of removal of NO x , even though in the solution according to the present invention illustrated in Figure 5 it is achieved after the removal of NO x .
• the fumes leaving the system of removal SCR 8 are withdrawn through a system of extraction 14 and are subjected to injection of sodium bicarbonate ( aHC03) in 15 for removal of SO 2 .
• the combustion fumes so treated can be fed to other phases of the process of production of clinker and, more in general, to other phases of the process of production of cement (for example, for grinding and drying raw materials or as combustion air in the rotary kiln and/or in the precalciner) to recover the residual heat before the release in the atmosphere.
• the solution according to the present invention can be applied also to plants of production of clinker provided with a precalciner.
• the combustion fumes of the rotary kiln are fed to the precalciner and from it, along with the combustion fumes of the precalciner, to the suspension preheater 1.

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• Engineering & Computer Science (AREA)
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• 2012
  • 2012-02-23 IT IT000269A patent/ITMI20120269A1/it unknown
• 2013
  • 2013-02-21 WO PCT/EP2013/053484 patent/WO2013124372A1/en active Application Filing
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