Abstract:
To produce cement clinker by baking of raw meal in a kiln, use is conventionally made of a raw meal preheater in which the heat of the flue gas emerging from the kiln is transferred to the raw meal. In order to remove impurities which accumulate in circulation between the kiln and the raw meal preheater, a part of the flue gas is extracted from the kiln, bypassing the raw meal preheater. The heat generated during the baking of cement clinker can be used particularly efficiently if the flue gases extracted and diverted past the raw meal pre-heater are used in a boiler to generate hot steam which can subsequently be expanded in a turbine.

Description:
PRIORITY CLAIM 
       [0001]    This application is a continuation of pending International Application No. PCT/EP2012/058942 filed on May 14, 2012, which designates the United States and claims priority from German Patent Applications No. 10 2011 050 694 filed on May 27, 2011 and No. 10 2011 052561 filed on August 10, 2011, all of which are incorporated by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to an apparatus and a method for manufacturing cement clinker, also referred to as clinker. The apparatus comprises a kiln for transforming the raw meal into clinker and at least a first heat exchanger for heating a fluid, in order to subsequently expand it in at least one turbine. 
         [0004]    2. Description of Relevant Art 
         [0005]    Clinker is produced by baking the raw meal in a kiln. This requires a temperature of about 1450° C. fuels such as coal, natural gas, petroleum, petroleum products (including plastic residues), paper or wood or other substitute fuels are burned in the kiln to produce this temperature. 
         [0006]    Patent CH 689 830 A5 describes the production of clinker with simultaneous power generation. The preheated raw meal is baked in a rotary kiln to produce clinker as usual. The flue gases generated in the kiln are extracted from the kiln and the heat stored therein is used initially to preheat the raw meal. The flue gases cooled down in this process to about 600° C.-1200° C. are subsequently fed to a heat exchanger to produce superheated steam, which is then expanded in a turbine. The turbine drives a generator, such that the mechanical energy released during expansion is at least partially made available for use as electrical energy. 
         [0007]    The published patent application DE 25 58 722 also describes a possibility for converting the heat energy contained in flue gases from the clinker process into electrical energy. This option uses the flue gases to preheat raw meal and produce steam. As usual, the raw meal is preheated in a heat exchanger tower with cyclone separators. A steam boiler for generating steam is arranged in the flue gas flow between the cyclone separators. 
         [0008]    The raw meal usually consists of a mixture of limestone and clay or marl, often with iron ore and/or sand. In this way, chlorides, alkali metal compounds and sulphur compounds are also borne into the kiln with the raw meal. These impurities evaporate in the oven, leave it with the flue gas and then condense in the raw meal preheater and on the raw meal. A portion of the impurities are introduced back into the kiln with the raw meal, where they re-evaporate and leave the kiln with the flue gas to subsequently condense again. In this way, the impurities accumulate in the kiln and in the region of the raw meal preheater. This leads to a narrowing of the cross-section of the flue gas flow in the area where the contaminants condense. These impurities are therefore usually removed from the flue gas by means of a bypass system. These bypass systems remove a portion of the dust-laden kiln exhaust gases prior to entry into the raw meal preheater. However, there is a problem with subsequent processing of the dust accumulated from dedusting the bypass gases. The coarse dust can first be removed for instance from the bypass gas in a cyclone separator, before cooling the gas and then removing the fine dust in a fibrous filter. The coarse dust contains only a little chloride and can be fed into the kiln again. The fine dust has a high chloride content and can to a limited extent be added to the clinker, for instance during the cement milling process, without compromising the quality of the cement (cf. “A new chloride-bypass system with stable kiln operation and recycling of waste” Sutou et al., ZKG International, Vol. 54, No. 3, 2001, pp. 121-128). The problem of this solution is that the dust particles are needed as condensation nuclei and after coarse dust separation these condensation nuclei are no longer available in sufficient quantity. 
       SUMMARY OF THE INVENTION 
       [0009]    The object of the invention is to provide an apparatus and method to enable better use of heat generated by baking clinker from raw meal. 
         [0010]    This aim is achieved by an apparatus and a method according to the independent claims. Advantageous embodiments of the invention are specified in the dependent claims. 
         [0011]    The apparatus for the production of clinker has at least a minimum of one kiln for baking the raw meal to convert it to clinker. The kiln has at least one outlet for flue gases, which is connected with at least one raw meal preheater in such a way that the heat stored in the flue gases coming from the outlet in the raw meal preheater is delivered to the raw meal. The preheated raw meal can then be fed to the kiln and burned there to form clinker. The device also has at least one branch for flue gases to divert a portion of the flue gases in order to remove impurities, i.e. a so-called bypass outlet. The bypass outlet can for example be arranged on the kiln or between the kiln and the raw meal preheater. To convert the heat stored in the flue gas into electric energy the apparatus has at least a first heat exchanger, in which the heat stored in the flue gas is transferred to the fluid so that it can then be expanded in a turbine. The turbine then drives, for example, a generator. In particular, the fluid can therefore be water or water vapour. According to the invention, the first heat exchanger is preferably connected to the branch, i.e. the bypass outlet, in such a manner that the heat from the diverted portion of the flue gas is supplied to the fluid. Thus, the amount of heat available for steam generation is increased per unit of time. 
         [0012]    The term ‘heat’ denotes the thermal energy Q=c(T, p, V) *m*T, stored at a given temperature in an amount of a substance, wherein c (T, p, v) describes the specific heat capacity, m the mass and T the temperature. As is customary, V and p stand for volume and pressure respectively. Heat can be partially transferred, for example in a heat exchanger, to a different material having lower temperature. Heat can be transferred from one substance to another substance and conveyed by transporting substances, for example with the stream of a flowing fluid. In such processes the term heat refers to the transferred and/or transported thermal energy within a time interval. 
         [0013]    Preferably, the branch is connected to at least one mixing chamber in order to mix the diverted part of the exhaust gases with fresh air. Although the temperature of the flue gas is reduced as a result, for example in the order of magnitude of 450° C. (especially expedient 300°-500° C.), at these temperatures the chloride condenses on the dust particles and can be separated from the flue gas, for example by means of electric or ceramic filters. Nevertheless, this temperature is more than sufficient to efficiently heat the fluid mixed with the diverted and dedusted flue gas mixed with fresh air in the first heat exchanger. 
         [0014]    Therefore, the mixing chamber preferably has an outlet that is connected to a hot-gas dust separator, for example a ceramic filter, to remove dust from the flue gases that have been blended with fresh air and therefore cooled. 
         [0015]    As already described, the hot gas dust separator preferably has an outlet connected to an inlet of the first heat exchanger in order to heat the fluid in the first heat exchanger with the dust-free flue gas, i.e. heat is removed from the dedusted flue gas to generate the steam. The first heat exchanger can thus also be referred to as a chloride bypass boiler. The prior removal of dust means that the function of the chloride bypass boiler is not affected by dust, which could otherwise settle on heat exchange surfaces and shorten the life of the components of the chloride bypass boiler due to their abrasive properties. 
         [0016]    The flue gas exiting the chloride bypass boiler is preferably recycled to the kiln. As a result, the residual heat contained in the flue gases and not transmitted to the fluid can on the one hand be utilised and on the other, fed into the conventional exhaust gas treatment, e.g. denitrification in an SCR plant. Accordingly, the chloride bypass boiler is preferably connected to the kiln in such a manner that flue gas exiting the chloride bypass boiler is returned to the kiln system. 
         [0017]    More preferably, the flue gas leaving the chloride bypass boiler is used as a coolant for cooling clinker, heating it more. Therefore, the chloride bypass boiler is particularly preferred in connection with a clinker cooler whereby flue gases expelled from the first heat exchanger are blown via the clinker cooler into the kiln as secondary air. As a result, the residual heat still present in the flue gas after the chloride bypass boiler can be fed back into the oven. 
         [0018]    Highly preferable is that at the least a second heat exchanger, e.g. a steam boiler, is connected to the first heat exchanger in such a manner that the fluid is heated sequentially in the two heat exchangers. The amount of heat supplied to the fluid and thus the energy (per unit of time) released during expansion of the fluid can thus be increased further. This solution involving a serial-type coupling of both heat exchangers has the advantage that only one turbine is required, compared with two fluids conveyed parallel. In addition, the achievable energy density of the fluid in the series circuit is greater than with a parallel coupling of heat exchangers, with the turbine having to expand a smaller flow volume. 
         [0019]    For example, the fluid, e.g. water or steam (no further distinction to be made here), can be heated in one of the two heat exchangers, first to a first temperature (T 1 ), e.g. in the order of 250° C. (200° C.-300° C.) at a first pressure (p 1 ). Then, further heat is supplied to the fluid (300° C.-500° C.) to heat it, e.g. to a second temperature (T 2 ) e.g. 400° C. (300° C.-500° C.) at a second pressure (p 2 ). Preferably, the first pressure is greater than the second pressure, i.e. p 1 &gt;p 2 , simplifying the fluid feed to the downstream heat exchanger. The second heat exchanger can for instance be a steam boiler heated by flue gases exiting the raw meal preheater. 
         [0020]    Preferably, the fluid supplied to the first and second heat exchangers is preheated in at least one third heat exchanger. In this way the heat stored in heat media, which have a lower temperature than the flue gases exiting the kiln and the raw meal preheater can be used effectively, i.e. the energy stored in the fluid (per time unit) can be increased further. For example, cooling air heated in a clinker cooler, so-called exhaust air, can be fed into the third heat exchanger as a heat source. Similarly, denitrified flue gases can be supplied to the third heat exchanger. Highly preferable is for the third heat exchanger to have at least two stages, with heat stored in denitrified flue gas supplied to the fluid in one of the stages and heat stored in exhaust air of the clinker cooler supplied to the fluid in another stage. For example, in a first stage the fluid can be heated from about 50° C. (30° C.-80° C.) with the heat of denitrified flue gases to about 115° C. (80° C.-150° C.). In a second stage, which may be spatially separated from the first stage, the fluid can then be heated to about 200° C. (150° C.-250° C.). During preheating, the fluid is preferably under a pressure in the order of about 20 to 30 bar. At this pressure and temperature, water, a suitable fluid, is still liquid. This is how supply fluid preheating can be easily differentiated from steam generation. 
         [0021]    Heating of the fluid in the third heat exchanger occurs at a somewhat constant pressure of at least p 3 , which is preferably greater than the pressure specified above for p 1  and/or p 2 , simplifying feeding of the pre-heated fluid in the first and second heat exchanger. 
         [0022]    The main process steps according to the invention can be summarised as follows:
       1. Burn raw meal to produce clinker in a kiln.   2. Heat the raw meal in a raw meal preheater using flue gases from the kiln.   3. Divert a portion of the flue gases from the kiln in order to conduct them past the raw meal preheater.   4. Generate steam with the heat produced during the combustion process in the kiln, with the heat for producing the steam being extracted from the diverted flue gases.   5. Expand the vapour by means of at least one steam turbine.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    In the following, the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment and with reference to the drawings. 
           [0029]      FIG. 1  shows an example of a flow diagram of an apparatus for baking clinker from raw meal, and 
           [0030]      FIG. 2  shows details of the flow chart. 
       
    
    
       [0031]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Core of the apparatus according to  FIG. 1  is a rotary kiln  10  between a clinker cooler  20  and a heat exchanger tower  30 . The rotary kiln  10  has one burner (not shown) projecting into the rotary kiln  10  from the side of the clinker cooler to generate the heat required for producing clinker by means of combustion in the rotary kiln  10 . A main flow of flue gases generated during combustion leaves the rotary kiln  10  from an outlet  31  on the heat exchanger side. The outlet is connected to the heat exchange tower  30  as an example of a raw meal preheater. Reciprocally, the raw meal is introduced into the rotary kiln  10  from this side. As an example, the heat exchange tower  30  has here four cascaded inter-connected cyclone separators  32  connected in series for preheating the raw meal using the heat of the flue gas and for removing the coarse particles from the flue gas. Any other number of cyclones could be cascaded as well. 
         [0033]    In addition to the outlet  31 , the kiln has a branch  91  for flue gases to divert a portion of the flue gases for separating impurities. The branch is therefore the beginning of a bypass line, a chloride bypass in the illustrated example. The diverted flue gas flow is de-noted as a partial flue gas flow or bypass flow solely to distinguish it better from the main flue gas flow in particular. 
         [0034]    The main flue gas flow coming from the heat exchanger tower  30  typically has a temperature of about 250-550° C., usually from 300 to 500° C. Before the flue gas is fed to a flue gas filter  50  for further dust removal, it is cooled to less than 150° C. On the one hand this makes it possible to drastically reduce the volume to be dedusted (per time unit) and the cheaper fabric filter technology can be used. In addition, heavy metals contained on the dust in the flue gases, such as mercury or thallium, condense during the cooling of the flue gases to less than 150° C. and the dust can be separated with this during dedusting. Consequently, the flue gas filter  50  can also be referred to as a cold trap. Provision is made for three options to cool the main flue gas flow:
       (i) guiding the main flue gas flow to a boiler  100  in order to generate steam that is expanded in a turbine assembly  120  to drive for example a generator G.   (ii) guiding the main flue gas flow to a raw meal mill  34  for drying and preheating the material to be ground and   (iii) guiding the main flue gas flow to an evaporative cooler  36 .       
 
         [0038]    Provision is made in the respective lines for valves  38  to divide the flue gas flow into the three means of cooling. During normal operation, no or as little flue gas as possible should be cooled by the evaporative cooler  36 , because the heat removed from the flue gas in the evaporative cooler  36  is no longer available as process heat. The evaporative cooler therefore preferably only has the function of an emergency cooler should it not be possible to use the boiler  100 . 
         [0039]    The heat contained in the diverted partial flue gas flow is also used to generate steam: For this purpose, the branch  91  is connected to a mixing chamber  90  in which the partial flue gas flow is mixed with fresh air. Chloride condenses on dust particles contained in the flue gas during this process. The mixing temperature is set in the order of 400° C. (about 350° C.-450° C.) and allows dust to be removed from the partial flue gas flow in a hot gas filter  94 . The outlet of the mixing chamber is thus connected to the inlet of the hot gas filter  94 ; this is indicated by a line  92 . 
         [0040]    The dedusted partial flue gas flow is then fed to a heat exchanger  110 , which is also referred to below as a chloride bypass boiler  110  (indicated by a compressor symbol and connecting line  93 ). Heat is transferred from the partial flue gas flow to water in the chloride bypass boiler  110  to generate steam, with the partial flue-gas flow cooled to about 230° C. The chloride bypass boiler  110  is connected to the clinker cooler  20 , in particular with the area of the clinker cooler  20  where the clinker falling out of the kiln  10  are stored in order to cool the clinker with the partial flue gas flow, heating the partial flue gas flow. The partial flue gas flow is fed back into the kiln  10  via the clinker cooler  20  as secondary air. This allows one to dispense with separate flue gas purification of the partial flue gas flow, e.g. removal of nitrogen. Moreover, this reintroduction makes energetic sense as the heat stored in the partial flue gas flow after exiting the chloride bypass boiler  110  is fed back into the kiln. 
         [0041]    The main flue gas flow is dedusted and denitrified. For this purpose, the main flue gas flow in the steam boiler  100  is cooled to about 170° C., preferably to less than 150° C. At this temperature, heavy metals contained in the flue gas condense on the dust and can be deposited with the dust in the downstream flue gas filter  50 . The flue gas filter  50  thus has the function of a cold trap for heavy metals. The dedusted flue gas is supplied to an SCR plant  60  for catalytic denitrification of the flue gases. To do so it must be heated to at least 230° C. Therefore, coming from the flue gas filter  50  it is fed firstly into a recuperator  62 , which is also fed a counterflow of flue gas that has been previously denitrified in the SCR system  50  so that heat is transferred from the denitrified gas to the flue gas to be denitrified. The flue gas leaving the recuperator that is to be denitrified is fed into another heat exchanger  64  in order to heat it further. The requisite heat for heating the flue gas is supplied to the next heat exchanger  64  via a so-called thermal oil acting as a heat transfer fluid. The flue gas heated in this way in two stages (first stage: recuperator  62 ; second stage, “second heat exchanger  64 ”) is supplied to the SCR plant  60  and denitrified there 
         [0042]    The denitrified flue gas heats the flue gases to be denitrified in the recuperator  62  as previously described and is cooled accordingly. The flue gas is then cooled in a further heat exchanger  102 , preferably to about 110° C. and can be discharged as indicated via a flue. The heat extracted from the flue gas in the heat exchanger  102  is used to preheat the feedwater for the boiler  100  and/or the chloride bypass boiler  110 . 
         [0043]    In addition, heat is removed from the rotary kiln  10  with the preferred continuous removal of clinker from the rotary kiln  10 . This hot clinker, initially about 1450° C., is cooled in the clinker cooler  20 . Air serves as the preferred coolant and in the simplest case, ambient air. The clinker cooler  20  is thus a heat exchanger. A portion of the air heated in the clinker cooler  20  is discharged from the clinker cooler via a so-called central air outlet  24 . The thermal oil in its capacity as a heat transfer fluid is heated in a heat exchanger  80  with heat stored in the discharged air, hereinafter denoted as exhaust air, after coarse dedusting by a cyclone separator  77 . The heat transferred to the heat transfer fluid can be transported over long distances with only minimal heat loss, especially to heat the flue gas to be denitrified to the temperature required for denitrification in the second heat exchanger  64 . 
         [0044]    The heat exchanger  80  has an inlet  81  for the exhaust air, which is first conveyed to the heat exchanger  80  via a first conduit  83  to heat the heat transfer fluid flowing through the first conduit  83 . A second conduit  84 , through which the exhaust air is conducted, is arranged subordinate to the first conduit  83 . Another heat transfer fluid flows in the second conduit  84  and is heated by the exhaust air. In the example shown, the additional heat transfer fluid is water, which is pre-heated as feedwater for the boiler  100  and/or a boiler  110 . The exhaust air exits the heat exchanger  80  through an outlet  82 . The exhaust air is conveyed in a flow channel in the heat exchanger  80 . The flow channel is for example U-shaped, i.e. has two free arms  85 ,  86  which are interconnected by an underlying transverse arm  87 . One of the two lines  83 ,  84  is in each of the two free arms  85 ,  86 . Deflecting the air in the region of the transverse arm  87  causes the clinker dust borne by the exhaust air to collect at the bottom of the transverse arm, where it can be separated. 
         [0045]    The outlet  82  is connected to a further heat exchanger to control the temperature in the downstream flue gas filter  75 . The filtered exhaust air is discharged through an implied fireplace. 
         [0046]    Steam generation takes place in several stages. The feedwater is obtained mainly by condensation of steam expanded previously in the steam turbine assembly  120 . For this purpose, provision is made for various condensors  130 ,  140 ,  150 . Losses are preferably compensated with demineralized water. The feedwater at the outlet of the condenser  150  is about 55° C. and is removed from there and conveyed to the other heat exchanger  102  by means of a pump. There it is heated to about 135° C. (100° C.-150° C.) in a first stage using the heat of the main denitrified flue gas flow. During this process, some of the feedwater is recycled after it has left the heat exchanger, resulting in a constant temperature of about 110° C. (&gt;100° C. to 150° C.) at the feedwater inlet of the heat exchanger  102 . In this way, the flue gas emerging from the heat exchanger  102  has a temperature of at least about 110° C. (100° C. to 150° C.), preventing condensation from the water contained in the main flue gas flow. It is preferable for this temperature to be selected as low as possible, but high enough to prevent or reduce to a minimum the formation of condensation in the flue adjoining the heat exchanger  102 . 
         [0047]    A portion of the feedwater preheated in the heat exchanger  102  is fed directly to the boiler  100 . The remaining part of the preheated feedwater in the heat exchanger  102  is heated further in the second conduit  84  of the heat exchanger  80  by the exhaust air from the clinker cooler further, for example to about 200° C. (150° C.-250° C.). Some of this portion of the feedwater is also fed to the boiler  100  and the remainder into the chloride bypass boiler  110 . 
         [0048]    The steam generated from the feedwater in the boiler  100  and the chloride bypass heater  110  is then fed to the turbine assembly  120 . For this purpose, the boiler  100  has two conduit systems  101 ,  102 . The first  101  of the two conduit systems is used to generate steam under relatively low pressure and relatively low temperature, e.g. about 200° C. (150°-250° C.) at about 4 bar (2-6 bar). This first conduit system  101  is arranged downstream from the second conduit system  102  in the main flue gas flow and is fed with feedwater pre-heated by the heat exchanger  102 . Therefore, the second conduit system is used to generate steam at a much higher temperature, e.g. about 400° C. (300° C. or more), e.g. at about 15 bar (10-30 bar), preferably superheated steam. It is fed with feedwater coming from the heat exchanger  80  as follows: A first part of the feedwater coming from the heat exchanger  80  is heated in a first section  103  of the second conduit  102  of the steam boiler  100 . The other part of the feedwater coming from heat exchanger  80  is heated to approx. 250° C. (200° C. to 300° C.) in the chloride bypass boiler  110  at about 10-30 bar. The two parts of the feedwater coming from the heat exchanger  80  heated in this manner are then heated to the final temperature of about 400° C. at about 10-30 bar in a second section  104  of the conduit  102  of the boiler  100  immediately downstream from the first section  103 . Consequently, two vapour flows stream to the turbine assembly from the boiler  100 : a first, which has been heated by the main flue gas flow in the first conduit  101  and a second, which has been heated in the second conduit  102  in conjunction with the chloride bybass boiler  110 . The first vapour flow has a lower temperature and lower pressure in comparison with the second vapour flow. These two vapour flows are then expanded in the corresponding two-stage turbine assembly  120 . The second vapour flow is initially expanded in a first turbine stage  121  to approximately the pressure of the first vapour stream. The two steam flows are then expanded together in a second turbine stage  122 . The expanded steam is then condensed in a plurality of condensors  130 ,  140 ,  150 . The water obtained in this manner can be resupplied to the feedwater preheating system from the heat exchangers  102  and  80 . 
         [0049]    It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention is believed to provide a method and an apparatus for producing cement clinker. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           10  Kiln, here as a rotary kiln 
           20  Clinker cooler 
           24  Centre air outlet 
           30  Heat exchanger tower 
           31  Outlet for primary flue gas stream 
           32  Cyclone separator 
           34  Raw meal mill 
           36  Evaporative cooler 
           38  Valve 
           50  Flue gas filter for particulate removal 
           60  SCR system 
           62  Recuperator/heat exchanger 
           64  (Second) heat exchanger 
           70  Cold trap/Cooler 
           75  Flue gas filter for particulate removal 
           77  Cyclone separator for coarse particulate matter 
           80  Heat exchanger 
           81  Exhaust air inlet 
           82  Exhaust air outlet 
           83  First heat transfer fluid line 
           84  Second heat transfer fluid line 
           85  Free arm 
           86  Free arm 
           87  Transverse arm 
           90  Mixing chamber 
           91  Branch for partial flue gas flow 
           92  Connection 
           93  Connection 
           94  Vapour particulate removal 
           100  Heat recovery boiler/Steam boiler 
           101  First conduit 
           102  Second conduit 
           103  First section of the first conduit 
           104  Second section of the second conduit 
           102  Heat exchanger for feedwater preheating 
           110  Vapour extraction boiler 
           120  Turbine assembly 
           121  First turbine stage 
           122  Second turbine stage 
           130  Condensor I 
           140  Condensor II 
           150  Condensor III