Abstract:
The present invention relates to a cement manufacturing plant includes a system for capturing carbon dioxide (CO 2 ) from an exhaust gas stream generated in the kiln  110  of the cement manufacturing plant by using regenerative calcium cycle (RCC) into the cement manufacturing process. Also, the invention relates to a method of capturing carbon dioxide CO 2  from a carbon dioxide CO 2  rich exhaust gas stream generated in a cement kiln, wherein the method includes recirculation of the carbon dioxide CO 2  rich exhaust gas stream to a unit for carbonization.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT/IB2012/001586 filed Aug. 14, 2012, which in turn claims priority to European application 11006647.9 filed Aug. 15, 2011, both of which are hereby incorporated in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates cement manufacturing plant wherein a system for capturing of carbon dioxide (CO 2 ) in a carbon dioxide (CO 2 ) rich exhaust gas stream has been integrated. The invention relates also to a method for capturing carbon dioxide. 
       BACKGROUND 
       [0003]    In the cement manufacturing process carbon dioxide CO 2  is released into the atmosphere directly when the calcium carbonate is heated, producing lime and carbon dioxide. This is the process taking place in the kiln and the calciner, thus the heart in the cement production. Carbon dioxide CO 2  is also, indirectly, emitted through the use of energy if its production involves the emission of CO 2 . 
         [0004]    The cement industry is the second largest CO 2  emitting industry behind power generation. The cement industry produces about 5% of global man-made CO 2  emissions, of which 60% is from the chemical process, and 40% from burning fuel. The amount of CO 2  emitted by the cement industry is nearly 900 kg of CO 2  for every 1000 kg of cement produced. 
         [0005]    Use of the Carbone Capture and Storage (CCS) is becoming or more interest as the production of the carbon dioxide cannot be avoided when producing the cement, as it is involved in the chemical process as well as generated in the energy consumption. The carbon dioxide is generated during the calcination, thus during the extraction of lime from the limestone. 
         [0006]    The calcination is an endothermic process and it is required that energy is supplied to the reaction taking place in the calciner. The calciner is operating at temperature of 900° C. In the conventional cement plants this energy is generated and supplied from the kiln exhaust gas. Also a firing system dedicated to supply energy to the calciner may be required to generate the required energy. 
         [0007]    There is a need for processes that improve and optimize the utilization of energy in such systems. The reduction or removal of the carbon dioxide is most often a process of heat generation. Therefore there is a demand to find a process where the generation of heat is limited. 
       SUMMARY 
       [0008]    The present invention relates to a cement manufacturing plant wherein the carbon dioxide CO 2  is captured to an almost pure CO 2  stream by including a closed loop of exhaust gas from the cement kiln together with a unit for carbonation. 
         [0009]    In the unit for carbonation, hereafter denoted the ‘carbonator’, the lime (CaO) reacts with carbon dioxide CO 2  for form limestone (CaCO 3 ), thus a carbonation reaction. 
         [0010]    During the calcination process included in the process for manufacturing cement, and taking place in the unit for calcinations, hereafter denoted the ‘calciner’, the limestone CaCO 3  is converted, or decomposed, to lime and carbon dioxide CO 2  when treated with heat. 
         [0011]    The CO 2  capture process according to the above described scheme, using lime to capture CO 2  from flue gases and routing the limestone to the calciner for CO 2  release, is called the regenerative calcium cycle (RCC). 
         [0012]    An object of the present invention is to provide an integrated system for capturing CO 2  in a cement manufacturing plant. 
         [0013]    An object of the present invention is to improve the utilization of energy generated in a cement manufacturing plant. 
         [0014]    According to an aspect illustrated herein, there is provided a cement manufacturing plant wherein the plant comprises a system for capturing carbon dioxide (CO 2 ) from an exhaust gas stream generated in the cement kiln of the cement manufacturing plant and incorporating the regenerative calcium cycle (RCC) into the cement manufacturing process. 
         [0015]    An embodiment of the invention is a cement manufacturing plant wherein the system comprises a carbonator for capturing the carbon dioxide CO 2  present in the exhaust gas present in the plant. 
         [0016]    Another embodiment of the invention is the cement manufacturing plant wherein the system comprising means for recirculation of the exhaust gas from the kiln to the carbonator downstream the kiln. 
         [0017]    Another embodiment of the invention is the cement manufacturing plant wherein the carbonator is operating at temperature in the range between 550 and 750° C., preferably at 650° C. 
         [0018]    Another embodiment of the invention is the cement manufacturing plant wherein the solids comprising calcium carbonate from the carbonator; is transferred to the unit for treatment of the cement raw material; and the separated solids comprising calcium carbonate CaCO 3  are transferred to the calciner. 
         [0019]    Another embodiment of the invention is the cement manufacturing plant wherein the solids comprising calcium oxide, CaO, from the calciner is partly transferred to the carbonator. 
         [0020]    The system may comprise one or more solid/solid heat exchangers wherein the calcium carbonate CaCO 3  from the carbonator is pre-heated to a temperature of, for example, 750° C. by a counter-current flow of the calcium oxide rich stream fed from the calciner. 
         [0021]    Another embodiment of the invention is the cement manufacturing plant wherein the calciner is supplied with energy by recycling of carbon dioxide CO 2  rich exhacust gas stream being heated by heat exchange with the hot exhaust gas stream generated in the cement kiln. 
         [0022]    This overall arrangement separates the exhaust gas stream generated in the cement kiln, and the optional firing systems, from the calciner. The arrangement generates a gas stream downstream the unit for treatment of the cement raw material being almost pure carbon dioxide CO 2  gas. Further, advantageously, no arrangement for separation of air, for example an air separation unit (ASU), is needed in the system of the invention. 
         [0023]    Another embodiment of the invention is the cement manufacturing plant wherein the exhaust gas generated in the cement kiln is subject for preheating air in a heat exchanger. 
         [0024]    Another embodiment of the invention is the cement manufacturing plant wherein the exhaust gas generated in the cement kiln is to be used for generating steam in a steam generator. 
         [0025]    Another embodiment of the invention is the cement manufacturing plant wherein said system comprises means for dust removal. 
         [0026]    Exhaust gas directly from the kiln outlet may be at high temperature, typically in the range of 800-1200° C. The exhaust gas after use in pre-heating of the cement raw materials and after being subjected to dust removal may typically be at a temperature in the range of 100-300° C. 
         [0027]    Another embodiment of the invention is the cement manufacturing plant wherein said system comprises means for adjusting pressure of the exhaust gas generated in the kiln. 
         [0028]    According to other aspects illustrated herein, there is provided a method for capturing carbon dioxide wherein the method of capturing carbon dioxide CO 2  from a carbon dioxide CO 2  rich exhaust gas stream generated in a cement kiln, the method comprising recirculation of the carbon dioxide CO 2  rich exhaust gas stream to a unit for carbonation, the carbonator. The carbon dioxide CO 2  is absorbed by the lime, more specifically; it participates in the reaction with lime, CaO, to form limestone, CaCO3. 
         [0029]    Another embodiment of the invention is the method for capturing carbon dioxide CO 2  from a carbon dioxide CO 2  rich exhaust gas stream generated in a cement kiln, wherein the carbonization is taking place at temperature in the range between 550 and 750° C., preferably at 650° C. 
         [0030]    Another embodiment of the invention is the method for capturing CO 2 which comprises transferring solids comprising CaCO 3  generated in a carbonation reaction, to be combined with cement raw material before the calcination reaction of the said solid material and the cement raw material to form calcium oxide, CaO. 
         [0031]    Also, another embodiment of the invention is the method for capturing CO 2  wherein at least a portion of the solid materials comprising calcium oxide, CaO, produced by the calcination reactions is recirculated to the carbonation reaction. 
         [0032]    Another embodiment of the invention is the method wherein the solid material comprising CaCO 3  transferred from the carbonation reaction is subjected for heating by the solid material comprising CaO being recirculated to the carbonation reaction. 
         [0033]    The above described and other features are exemplified by the following figures and detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    Referring now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike: 
           [0035]      FIG. 1  schematically depicts a cement manufacturing plant comprising a carbonator and recirculated of the cement kiln exhaust gas. 
           [0036]      FIG. 2  schematically depicts a cement manufacturing plant (prior art). 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    The cement manufacturing plant, wherein the system for capturing CO 2  as described above is included will be further described. 
         [0038]    Cement raw materials, generally lime (calcareous), silica (siliceous), alumina (argillaceous), and iron (ferriferous) are obtained by mining or quarrying. In the clinker production process, the raw materials are crushed and homogenised, e.g. in a raw mill, into a mixture which is fed into a rotary kiln. This process, generally referred to as raw milling, may be characterized as a dry, wet, semi-dry or semi-wet process depending on the state of the raw material. In the dry process, the raw materials are ground dry and fed to the kiln as a powder. In the wet process, the raw materials are ground wet and fed to the kiln as a slurry. In the semi-dry process, the raw materials are ground dry and then moistened to form nodules which can then be fed to the kiln. In the semi-wet process, the raw materials are ground wet in a slurry, which is dewatered before it is fed to the kiln. 
         [0039]    The kiln may be a large rotating pipe, generally about 50 to 100 m long and up to 6 m in diameter. The kiln is heated by a flame at a temperature of about 2000° C. inside of it. The kiln is slightly inclined to allow for the materials to slowly reach the other end, where it is quickly cooled to 100-200° C. 
         [0040]    Generally, four basic oxides in the correct proportions are used to make cement clinker, calcium oxide (about 65%), silicon oxide (about 20%), alumina oxide (about 10%) and iron oxide (about 5%). These elements mixed homogeneously will combine when heated by the flame to a temperature of approximately 1450° C. During this process, generally referred to as pyroprocessing, new compounds are formed, such as silicates, aluminates and ferrites of calcium. Hydraulic hardening of cement is caused by hydration of these compounds. The product obtained is called clinker, and is generally present in the form of 1-10 mm nodules formed due to partial melting of the material in the kiln. 
         [0041]    During the pyroprocessing process, CO 2  is emitted. CO 2  emissions are both raw material-related and energy-related. Raw material-related emissions, i.e. CO 2  emissions produced during limestone decarbonation, account for about 60% and combustion of fuels account for about 40% of total CO 2  emissions from the cement process. 
         [0042]    The exhaust gases from a modern cement kiln may typically amount to about 2 tonnes per tonne of clinker made. The kiln outlet gas may for example have a temperature of about 1000° C. when it leaves the kiln. The hot kiln exhaust gas is generally used for pre-heating the raw materials prior to introduction of the materials into the kiln. The pre-heating may for example be performed in one or more grate pre-heaters and/or gas-suspension pre-heaters. 
         [0043]    The exhaust gases from the cement kiln carry a large amount of dust, typically about 30 grams per cubic metre. Environmental regulations generally require that this amount be reduced to about 0.1 gram per cubic metre, so dust removal needs to be at least 99.7% efficient. Methods of dust removal include electrostatic precipitators and bag-filters. The temperature of the kiln outlet gases after use in pre-heating the raw materials and after dust removal may typically be about 150° C. 
         [0044]    In order to reduce the amount of CO 2  which is released to the atmosphere, the cement plant is provided with a gas capturing system for capturing CO 2  from the cement kiln exhaust gas (kiln gas). 
         [0045]    An embodiment of the cement manufacturing plant of the invention is further illustrated in  FIG. 1 . 
         [0046]    Herein, the cement manufacturing plant  1  comprises the cement kiln  110  wherein lime CaO is treated into clinker/CaO, which is then transported to further treatment via duct  113 . 
         [0047]    The hot exhaust gas rich in carbon dioxide CO 2  generated during the process in the cement kiln is forwarded to the carbonator  130 , thus the unit for carbonation. First the dust in the kiln exhaust gas is removed in a cyclone type dust removal system  114 . The dust is transferred to pipe  125  and reintroduced to the cement kiln. The heat of the hot exhaust gas from the cement kiln may be exchanged in a heat exchanger (gas/gas heater)  112 . The gas from unit for treatment of the cement raw material  170  is split and one part transferred via a recycle fan  175  to the heat exchanger  112  where it is heated and forwarded to the calciner  120  via duct  115 . The hot exhaust gas from the cement kiln may also be used to heat air in the heat exchanger  140  which air is then introduced to the firing system  119  via the ducts  141  and  142 . 
         [0048]    The heat of hot exhaust gas from the cement kiln may also be heat exchanged for steam generation, forwarded via duct  131  to steam generator  130 , and reconnected to duct  111  via duct  132 . 
         [0049]    Optionally, the flue gas may be conditioned in a unit for conditioning  150  before its introduction into the unit for carbonation via duct  151 . Contaminants are most often introduced in the carbonator. The conditioning in unit  150  may, for example, be treatment of the exhaust gas like removal of NOx gases (DeNOx), or dust removal for example by electrostatic precipitation (ESP) or any other suitable dust removal device. 
         [0050]    The exhaust gas is then pressurized by the fan unit  154  and heated by the heat exchanger  152  before introduction to the carbonator  160 . 
         [0051]    The exhaust gas is pressurized to 50 mbar to 400 mbar, preferably pressurized 100 mbar, and heated to a temperature between 300° C. and 600° C. for an optimized sorption of the carbon dioxide. The exhaust comprising the carbon dioxide is forwarded to the carbonator  160 , the CO 2  is added to the reaction taking place in the carbonator, thus the reaction between lime, CaO, and carbon dioxide CO 2  to form limestone, CaCO 3 . 
         [0052]    The carbonator is typically operating at a temperature of between 550 and 750° C., for example between 600 and 700° C., preferably at about 650° C. 
         [0053]    The exhaust gases with lean content of carbon dioxide CO 2  forwarded from the carbonator  160  via duct  161  may be cooled in multiple steps, the heat may be recovered in unit  162 , may be included in the heat exchanger  152 , and may also be used as drying agent for the raw material introduced in the plant for the cement manufacturing. 
         [0054]    The solid material and the exhaust gas is then introduced into the unit for treatment of cement raw material  170 . The solid material forwarded from the carbonator comprises mainly calcium carbonate (limestone) but may also comprise some parts CaO (lime). The solid material from the carbonator is included and mixed with the raw material fed to the unit for pretreatment  170 . The unit for pretreatment  170  comprises an arrangement of multiple steps pretreatment of the raw material. 
         [0055]    In one arrangement the solid material comprises, for example, limestone CaCO 3  which is transported via pipe  121  from the carbonator to the unit for treatment of cement raw material  170 . A heat exchange (solid/solid heat exchange) may be performed in a heat exchanger  128  where the heat of stream of solid material (comprising mainly lime) from the calciner forwarded via pipe  126  is transferred to the solid material (comprising mainly limestone) forwarded from the carbonator to the calciner via pipe  121  and  122 . The heat exchanger may for example be a cross heat exchanger transferring heat from the solid material of the calciner with the solid material of the carbonator (solid/solid heat exchanger). 
         [0056]    In another arrangement, the solid material may forwarded via pipe  121  to the unit for treatment of cement raw material  170 , and may be introduced and mixed with the raw material at a higher level of the unit for treatment of raw material, The solid material (mainly CaO, lime) forwarded from the calciner via pipe  125  and pipe  126  is preferably passing heat exchanger  129  for heat recovery. 
         [0057]    The carbon dioxide rich exhaust gas generated in the calciner is separated e.g. in cyclones or by other methods separating solid material from a gas stream. 
         [0058]    The separated carbon dioxide CO 2  rich exhaust gas is then introduced in the unit for pretreatment of cement raw material, and is transferred with counter-current flow through the unit for treatment of the cement raw material and forwarded via duct  171 . The said limestone forwarded to the unit for treatment of the cement raw material is then mixed with the cement raw material during its pretreatement and before it is introduced to the calciner  120  via pipe  124 . The calciner  120 , the limestone CaCO 3  is split to lime CaO and carbon dioxide CO 2  by heating. 
         [0059]    The lime resulting by the reaction in the calciner may be divided into different portions, where one first portion is forwarded back to the carbonator via pipe  126 . The second part is forwarded to the cement kiln  110  for producing clinker and lime (CaO) and forwarded for further processing via pipe  113 . The clinker and lime may be cooled by heat exchanger  116  wherein the air introduced to the plant may be heated. 
         [0060]    The exhaust gas generated in the calciner and the unit for pretreatment of cement raw material may typically comprise contaminants in the form of, for example, dust particles, hydrochloride acid, HCl, nitrous oxides, NOx, sulphur oxides, SOx, and heavy metals including mercury, Hg. If the concentration of gases like oxygen (O 2 ), nitrogen (N 2 ) etc. in the CO 2  rich exhaust gas is from  170  is high, a separation step like the gas processing unit (GPU)  182  is needed. Otherwise, the following treatment may be performed in compression- and/or drying systems. The dust present in this carbon dioxide CO 2  rich exhaust gas which has passed the unit for pretreatment of cement raw material may be removed before it is further treated in a gas processing unit (GPU). The dust generated during the calcination process may be extracted and removed from the recycle loop for further processing in a unit for conditioning  180 , like in a Electrostatic Precipitator (ESP) process. Further, the heat of the gas may be recovered by a heat exchanger  181  before transferred for further processing. 
         [0061]    Optionally, the carbon dioxide CO 2  enriched gas may be forwarded to a gas processing unit (GPU)  182 . 
         [0062]    Optionally, the gas may also be reintroduced into the cement manufacturing plant and the unit for carbonation via the duct  151  connected via duct  185 . 
         [0063]    Another option is to introduce the residual inert gas fraction, after carbon dioxide purification in the GPU, back to the duct feeding the air heater, via duct  186 , in case the oxygen concentration is high. Otherwise the gas can be routed via  185  to the main kiln exhaust gas. 
         [0064]    The heating in the cement plant may be performed in one or more indirect heat exchangers. By the term “indirect heat exchanger” as used herein, is meant a heat exchanger configured for heat exchange between two fluid streams without mixing of the two fluid streams occurring. The heat exchanger may also be of type solid/solid heat exchanger. The heat exchanger may for example be of the shell and tube type. Suitable types and dimensions of heat exchangers are readily identified by the person skilled in the art. 
         [0065]      FIG. 2  shows a cement plant and its kiln  210  and calciner  220  according to the prior art. Energy is supplied to the calciner  220  via the kiln  211  exhaust gas and from an energy source  218  added to system. Also air  219  may be added to the system. The kiln exhaust any of preceding gas may be further heated by a dedicated firing system  219 . Also, the exhaust gas  211  may have to be dedusted, for example by treatment in cyclone (not shown) prior its mixing with the flue gases from the firing system(s). This is performed to reduce or minimize cement losses and depositions in the system. The cement raw material is pre-treated in the raw material unit  270  before it entering the calciner  220 . 
         [0066]    The cooled exhaust gas  271  from the raw material unit  270  is removed from the system. A draft fan  254  may be included, by which the pressure may be adjusted, for example to compensate the pressure losses. For protecting this fan from the components in the exhaust gas, a flue gas conditioning system  250  may be required. This conditioning system may comprise or include an electrostatic precipitator (ESP) or any other suitable dust removal device. 
         [0067]    Advantages obtained by the present invention are: 
         [0068]    Low impact of the efficiency of the cement manufacturing plant; 
         [0069]    A system being independent on other energy sources; 
         [0070]    High purity of carbon dioxide CO 2  is obtained due to the closed system; 
         [0071]    A system is provided wherein no air separation unit (ASU) is needed. 
         [0072]    While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.