Patent Publication Number: US-2013230442-A1

Title: Method and apparatus for collecting carbon dioxide from flue gas

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of International Patent Application No. PCT/CN2011/078394 with an international filing date of Aug. 15, 2011, designating the United States, now pending, and further claims priority benefits to Chinese Patent Application No. 201010510906.X filed Oct. 18, 2010. The contents of all of the aforementioned applications, including any intervening amendments thereto, are incorporated herein by reference. Inquiries from the public to applicants or assignees concerning this document or the related applications should be directed to: Matthias Scholl P.C., Attn.: Dr. Matthias Scholl Esq., 14781 Memorial Drive, Suite 1319, Houston, Tex. 77079. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an emission reduction and resource utilization technology of carbon dioxide from flue gas of a power plant boiler, and more particularly to a method and an apparatus for collecting carbon dioxide from flue gas by using active sodium carbonate. 
     2. Description of the Related Art 
     Several methods for capturing CO 2  have been developed. A chemical absorption method is widely applied in industries, and principle of the chemical absorption method is as follows: CO 2  in the flue gas is prone to react with and be absorbed by a chemical solvent. A rich solution of the chemical solvent is acquired after absorbing CO 2  to an equilibrium state; then the rich solution is introduced into a regeneration tower, heated and decomposed for releasing CO 2  gas and being transformed into a barren solution. After that, the barren solution is recycled to absorb CO 2  from the flue gas. Thus, by circulating an absorbent solution between an absorption tower and the regeneration tower, CO 2  in the flue gas is captured, separated, and purified. Currently, the chemical absorption method using an amino alcohol solution to absorb CO 2  is the most widely applied method, which specifically includes: an MEA method, an MDEA, and a mixed organic amines method. In productive practice, it has been proved that, although the chemical absorption method using the amino alcohol solution which has been applied for about twenty years in chemical field has the characteristics of fast absorption speed, strong absorption ability, it still has a common defect when it is utilized in treating flue gas from power plant that the oxidative degradation of the amino alcohol affects a long term and stable operation of the apparatus, thereby resulting a serious corrosion on the apparatus, and high energy consumption in regeneration. This is mainly because the flue gas of coal-fired power plant has the following characteristics, compared with that of the common chemical gas resource: 1) a large amount of the flue gas having a relatively low concentration of carbon oxide (10-15%); 2) the flue gas contains a relative high content of oxygen (5-10%), and dust including metal ion Fe and others, which accelerates the oxidative degradation of the organic amines, and results in a large consumption of the expensive amino alcohol absorbent. All these reasons account for the high cost of the method for collecting carbon dioxide by using amino alcohol. 
     Sodium carbonate is first used in industrialized manufacture of CO 2  absorbents, which absorbs CO 2  and produce NaHCO 3 . A temperature for complete decomposition of NaHCO 3  into Na 2 CO 3  and CO 2  is 20-30° C. lower than a temperature of the regeneration of amino alcohol. Thus, for energy consumption of regeneration, the method by using sodium carbonate as the absorbent has an obvious advantage that it has 20-30% energy consumption lower than the method using amino alcohol as the absorbent. However, the alkalinity of sodium carbonate is weaker than that of amino alcohol, and has a low absorption speed, poor effect of absorption when sodium carbonate is used alone. Furthermore, a comprehensive energy consumption and cost of the method using sodium carbonate is not superior to the method using the organic amines, and the method using sodium carbonate has almost been abandoned. 
     SUMMARY OF THE INVENTION 
     In view of the above-described problems, it is one objective of the invention to provide a method and an apparatus for collecting carbon dioxide from flue gas by using active sodium carbonate that have a simple processing, simple structure of apparatus, low investment, and low production cost. The method and the apparatus are in accordance with the characteristics of the flue gas of the power plant boiler, solve problems of oxidative degradation of the organic amines, serious corrosion on the apparatus, and high energy consumption. 
     To achieve the above objective, in accordance with one embodiment of the invention, there is provided a method for collecting carbon dioxide from flue gas by using active sodium carbonate. The method is a reprocessing of the flue gas of power plant boilers after common dust removal and desulfurization treatment, and comprises the following steps: 
     1) mixing an aqueous solution of sodium carbonate with an amino alcohol activator to yield a CO 2  absorbent; evenly spraying the CO 2  absorbent into the flue gas after the common dust removal and desulfurization treatment for fully contacting the flue gas flowing upwardly with the downwardly sprayed CO 2  absorbent and for allowing CO 2  in the flue gas to react with the amino alcohol activator and the aqueous solution of sodium carbonate: the amino alcohol activator first contacting with CO 2  to form a zwitterionic intermediate and being free again in a subsequent hydration reaction of the zwitterionic intermediate, H +  produced from the hydration reaction being neutralized by alkali ion CO 3   2−  in the aqueous solution of sodium carbonate, and HCO 3   −  produced from the hydration reaction contacting with metal ion Na +  in the aqueous solution of sodium carbonate and precipitating to produce a sodium bicarbonate slurry. Hereinbelow a reaction principle between the amino alcohol activator which is represented by capital letter A and the aqueous solution of sodium carbonate (Na 2 CO 3 ) is explained: 
     First, the amino alcohol activator A contacts with CO 2  to form the zwitterionic intermediate A.CO 2 , which is summarized by the following chemical equation: 
       CO 2 +A→A.CO 2   (1-1)
 
     Second, the hydration reaction of the zwitterionic intermediate A.CO 2 , the amino alcohol activator A is free again; HCO 3   −  and CO 3   2−  are also produced, which is summarized by the following chemical equation: 
       A.CO 2 +H 2 O→HCO 3   − +H + +A  (1-2)
 
     Third, H +  produced from the hydration reaction is neutralized by alkali ion CO 3   2−  in the aqueous solution of sodium carbonate, which is summarized by the following chemical equation: 
       CO 3   2− +H + →HCO 3   −   (1-3)
 
     Fourth, HCO 3   −  contacts with metal ion Na +  in the aqueous solution of sodium carbonate to precipitate gradually, which is summarized by the following chemical equation: 
       Na + +HCO 3   − →NaHCO 3 ↓  (1-4)
 
     Because the amino alcohol activator A is prone to combine with CO 2 , the zwitterionic intermediate A.CO 2  is immediately produced in a reaction zone of the reaction (1-1). As the hydration reaction of the zwitterionic intermediate A.CO 2  is much faster than that of CO 2 , the production speed of HCO 3   −  and H +  is very fast. Therefore, the amino alcohol activator A is recycled in the reaction zone between a combined state and a free state, which assures the neutralization reaction (1-3) continuously occurs, and a whole CO 2  absorption speed of the method is much faster than the CO 2  absorption speed by using Na 2 CO 3  alone. Furthermore, because NaHCO 3  has a relative small solubility in the CO 2  absorbent solution, NaHCO 3  crystallizes and precipitates with the increase of a production thereof, which decreases HCO 3   −  in the absorbent solution, and further propels the whole reaction to a direction of CO 2  absorption. Thus, the whole CO 2  absorption effect of the method is relatively equal to a whole CO 2  absorption effect by using the amino alcohol alone, but the production cost of the method is largely decreased. 
     2) thermally decomposing the sodium bicarbonate slurry obtained in step 1) to produce a highly concentrated CO 2  gas and an aqueous solution of sodium carbonate, which is summarized by the following chemical equation: 
       2NaHCO 3 →Na 2 CO 3 +CO 2 ↑+H 2 O
 
     3) returning the aqueous solution of sodium carbonate obtained in step 2) to step 1) to form the CO 2  absorbent for recycling; 
     4) cooling the highly concentrated CO 2  gas separated from step 2) for condensing hot water vapor therein; 
     5) carrying out gas-liquid separation on the highly concentrated CO 2  gas after cooling treatment of step 4), removing condensed water to yield highly purified CO 2  gas having a purity exceeding 99%; and 
     6) drying, compressing, and condensing the highly purified CO 2  gas to transform the highly purified CO 2  gas into a liquid state, and obtaining a highly concentrated liquid CO 2 . 
     In a class of this embodiment, a concentration of the aqueous solution of sodium carbonate is 10-30 wt. %. The amino alcohol activator is monoethanolamine (MEA) or diethanolamine (DEA). A weight of monoethanolamine or diethanolamine being added is 0.5-6% of a weight of sodium carbonate being added. A circulating liquid-gas ratio between the CO 2  absorbent and the flue gas is 5-25 L/m 3 . Thus, appropriate proportion of the amino alcohol activator and concentration of the aqueous water solution assure a fast reaction with CO 2 , decrease a dosage of the expensive absorbent to the utmost, prevent the corrosion of the apparatus caused by oxidative degradation of the amino alcohol, and largely decrease the investment on the apparatus and the operation cost. 
     In a class of this embodiment, a temperature of the reaction between CO 2  in the flue gas and the CO 2  absorbent in step 1) is controlled at 40-55° C.; and a pressure of the reaction is controlled at 3-300 kPa. Thus, the absorbent solution is capable of completely reacting with CO 2  in the flue gas at a suitable temperature and pressure. 
     In a class of this embodiment, a temperature of the thermal decomposition of the sodium bicarbonate slurry in step 2) is controlled at 80-130° C. Within such a temperature range, sodium bicarbonate is quickly decomposed for releasing a large amount of CO 2  and acquiring the highly concentrated CO 2  gas. 
     In a class of this embodiment, the highly concentrated CO 2  gas is cooled to a temperature of 20-35° C. Thus, a large amount of water vapor is condensed, thereby improving the purity of the CO 2  gas. 
     An apparatus for collecting carbon dioxide from flue gas by using active sodium carbonate to carry out the above method, comprises: an absorption tower, a regeneration tower, a slanting board sedimentation pool, a cooler, a gas-liquid separator, a desiccator, a compressor, and a condenser. The absorption tower comprises a flue gas inlet at a lower part, a flue gas outlet at a top, and a slurry outlet at a bottom. The regeneration tower comprises a feed inlet and a decomposed gas outlet at an upper part, and a feed outlet at a lower part. The slanting board sedimentation pool comprises a slurry inlet and an absorbent inlet at an upper part, a supernatant outlet, and an underflow outlet. A plurality of absorbent spray layers and at least one demister device are arranged one after another from bottom to top between the flue gas inlet and the flue gas outlet of the absorption tower. The slurry outlet of the absorption tower communicates with the slurry inlet of the slanting board sedimentation pool. The absorbent inlet of the slanting board sedimentation pool communicates with an absorbent container. The supernatant outlet of the slanting board sedimentation pool is connected to the absorbent spray layers of the absorption tower via a circulating pump. The underflow outlet of the slanting board sedimentation pool is connected to the feed inlet of the regeneration tower via a sodium bicarbonate pump. The feed outlet of the regeneration tower is connected to the absorbent inlet of the slanting board sedimentation pool via a sodium carbonate pump. The decomposed gas outlet of the regeneration tower is connected to an inlet of the gas-liquid separator via the cooler. A gas outlet of the gas-liquid separator is in series connected with the desiccator, the compressor, and the condenser. Thus, during the absorption of CO 2 , treatments of CO 2  gas comprising regeneration, dehydration, desiccation, compression, and condensation are continuously carried out until a highly purified liquid carbon dioxide is acquired. 
     In a class of this embodiment, the underflow outlet of the slanting board sedimentation pool is connected to the feed inlet of the regeneration tower via the sodium bicarbonate pump and a heat exchanger. The feed outlet of the regeneration tower is connected to the absorbent inlet of the slanting board sedimentation pool via the sodium carbonate pump and the heat exchanger. Thus, an exhaust heat of a barren solution of sodium carbonate in the regeneration tower is fully utilized, that is, preheating a rich solution of sodium bicarbonate introduced into the regeneration tower, and meanwhile cooling down the barren solution of sodium carbonate; thereby realizing a benign recycling of the heat exchange and saving the heat energy resource. 
     In a class of this embodiment, a liquid outlet of the gas-liquid separator is connected to the absorbent inlet of the slanting board sedimentation pool. Therefore, the condensed water separated from the gas-liquid separator is returned to the slanting board sedimentation pool for water recycling, thereby reducing the water consumption in the whole process and lowering the production cost. 
     In a class of this embodiment, three absorbent spray layers are employed. A filler layer is arranged beneath an upmost absorbent spray layer. A uniform flow sieve plate is arranged beneath each of the other two absorbent spray layers. Furthermore, a ratio between an aperture area and a plate area of the uniform flow sieve plate is 30-40%. On one hand, through the uniform flow sieve plate, the upward gas flow becomes more uniform, which effectively eliminates a dead angle of the flue gas and is conducive to full contact between the flue gas and the absorbent; on the other hand, under the spraying action of absorbent through a plurality of absorbent spray layers, a spraying coverage of the absorbent on a cross section of the absorption tower is 300% above, thereby assuring a full contact between CO 2  in the flue gas and the absorbent and a complete chemical reaction for absorbing CO 2 . 
     Compared with a conventional processing that employs an amino alcohol to remove carbon dioxide, advantages of the invention are summarized as follows:
         First, the CO 2  absorbent is formed by adding the amino alcohol activator into the aqueous solution of sodium carbonate. The whole CO 2  absorption effect of the CO 2  absorbent is relatively equal to a whole CO 2  absorption effect by using the amino alcohol alone; however, difficulties of CO 2  absorption by using the amino alcohol alone, such as large consumption of the amino alcohol absorbent, difficulty in later treatment after degradation, and high operation cost, and so on, are solved. Besides, sodium carbonate is a widely used chemical product, which is very easy to purchase with a price being 1/10 of that of the amino alcohol; thereby largely decreasing the cost for capturing CO 2  gas.   Second, after CO 2  is absorbed by the aqueous solution of sodium carbonate, the produced sodium bicarbonate has a decomposing temperature that is 20-30° C. lower than a regeneration temperature of the amino alcohol. Not only the energy consumption of the regeneration process is low, but also low exhaust heat of the flue gas and other heating means are effectively utilized, thereby being conducive to energy conservation. Thus, the invention is particularly applicable to treat flue gas from coal-fired power plant boiler that has a large flow of flue gas and a low concentration of carbon dioxide.   Third, the amino alcohol activator is only a small part in the aqueous solution of sodium carbonate, so that the problem of oxidative degradation of the amino alcohol activator is prevented. Besides, the corrosion phenomenon on the apparatus is far few than that when using the amino alcohol activator alone. Thus, the method of the invention can be stably operated; availability of the apparatus is much higher than that of the amino alcohol absorption method, and the apparatus investment and operation cost are much lower than that of the amino alcohol absorption method.   Finally, the method fully utilizes the flue gas from the power plant boiler; effectively decrease the emission of carbon dioxide while acquiring the liquid state carbon dioxide having the purity of 99% above, which meets the carbon dioxide standard of international industrial grade. The method is not only beneficial to the comprehensive management of atmospheric pollution, but also propels a benign development of recycling economy. Furthermore, the method realizes a harmless and resource utilization of the flue gas from the power plant boiler, which is very suitable for coal-fired power plants.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described hereinbelow with reference to accompanying drawings, in which the sole FIGURE is a connection structure diagram of an apparatus for collecting carbon dioxide from flue gas by using active sodium carbonate. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     For further illustrating the invention, experiments detailing an apparatus and a method for collecting carbon dioxide from flue gas by using active sodium carbonate are described below combined with the drawing. 
     As shown in the FIGURE, an apparatus for collecting carbon dioxide from flue gas by using active sodium carbonate comprises: an absorption tower  1 , a regeneration tower  10 , a cooler  17 , a gas-liquid separator  16 , a desiccator  15 , a compressor  14 , and a condenser  13 . Three absorbent spray layers  20  and one demister device  21  are arranged one after another from bottom to top between a flue gas inlet  5  and a flue gas outlet  22  of the absorption tower  1 . The flue gas inlet  5  is disposed at the lower part of the absorption tower  1  and the flue gas outlet  22  is disposed at the top of the absorption tower  1 . A filler layer  3  is arranged beneath an upmost absorbent spray layer  20 . A uniform flow sieve plate  4  is arranged beneath each of the other two absorbent spray layers  20 . A ratio between an aperture area and a plate area of the uniform flow sieve plate  4  is 38%. Such a combination of spraying structure assures a spraying coverage of the absorbent on a cross section of the absorption tower is 350% above. The demister device  21  comprises: an upper demisting filer screen, a lower demisting filer screen, and a cleaning spray component arranged between the two demisting filer screens for removing absorbent droplets in the flue gas. 
     A slurry outlet at the bottom of the absorption tower  1  communicates with a slurry inlet  6   a  at the upper part of a slanting board sedimentation pool  6 , and the sodium bicarbonate slurry is capable of flowing into the slanting board sedimentation pool  6  under the gravity thereof. An absorbent inlet  6   b  at the upper part of the slanting board sedimentation pool  6  communicates with an absorbent container  19  for replenishing sodium carbonate, amino alcohol activator, and a process water. Sodium carbonate is a predominate ingredient in a supernatant of the slanting board sedimentation pool  6 , and a sodium bicarbonate slurry is the predominate ingredient in an underflow. A supernatant outlet  6   c  of the slanting board sedimentation pool  6  is connected to the three absorbent spray layers  20  of the absorption tower  1  via a circulating pump  8 . An underflow outlet  6   d  of the slanting board sedimentation pool  6  is connected to a feed inlet at the upper part of the regeneration tower  10  via a sodium bicarbonate pump  7  and a heat exchanger  18 . A feed outlet at the lower part of the regeneration tower  10  is connected to the absorbent inlet  6   b  of the slanting board sedimentation pool  6  via a sodium carbonate pump  9  and a heat exchanger  18 . A supporting boiling unit  11  of the regeneration tower  10  is arranged outside a bottom of the regeneration tower  10 . 
     A decomposed gas outlet at the upper part of the regeneration tower  10  is connected to an inlet of the gas-liquid separator  16  via the cooler  17 . A liquid outlet of the gas-liquid separator  16  is connected to the absorbent inlet  6   b  of the slanting board sedimentation pool  6 . A gas outlet of the gas-liquid separator  16  is in series connected with the desiccator  15 , the compressor  14 , and the condenser  13 . An outlet of the condenser  13  is connected to a storage tank of liquid carbon dioxide  12 . Each of the above devices are commonly used devices in the chemical industry, thus, structures thereof are not described herein. 
     In operation test of the above apparatus, parameter of mix proportion of the CO 2  absorbent is selected from the following in accordance with different content of CO 2  in the flue gas:
         1) if a concentration of an aqueous solution of sodium carbonate is 10 wt. %, a weight ratio between monoethanolamine (MEA) or diethanolamine (DEA) and sodium carbonate is 1.5-6%;   2) if a concentration of an aqueous solution of sodium carbonate is 15 wt. %, a weight ratio between monoethanolamine (MEA) or diethanolamine (DEA) and sodium carbonate is 1-5%;   3) if a concentration of an aqueous solution of sodium carbonate is 20-25 wt. %, a weight ratio between monoethanolamine (MEA) or diethanolamine (DEA) and sodium carbonate is 0.8-4%; and   4) if a concentration of an aqueous solution of sodium carbonate is 30 wt. %, a weight ratio between monoethanolamine (MEA) or diethanolamine (DEA) and sodium carbonate is 0.5-3%.       

     Specific process flow of the invention is as follows: flue gas discharged from a coal-fired power plant is input into the absorption tower  1  via the flue gas inlet  5  after common dust removal and desulfurization treatment. The flue gas passes through the uniform flow sieve plate  4  and the filler layer  3  and flows upwards. Meanwhile, the aqueous solution of sodium carbonate added with the amino alcohol activator is sprayed downwards via the absorbent spray layers  20 . A circulating liquid-gas ratio between the CO 2  absorbent and the flue gas is controlled within 5-25 L/m 3 , particularly within 12-22 L/m 3 . A temperature of a reaction between CO 2  in the flue gas and the CO 2  absorbent is controlled at 40-55° C.; a pressure of the reaction is controlled at 3-300 kPa, particularly at 5−200 kPa. Thus, the upwardly flowing flue gas fully contacts with the downwardly sprayed CO 2  absorbent at the filler layer  3  and the uniform flow sieve plates  4  for allowing CO 2  in the flue gas reacting with and being absorbed by the amino alcohol activator and the aqueous solution of sodium carbonate. 
     The flue gas after being removed from a large amount of CO 2  continues flowing upward, passes through the demister device  21  for further removing absorbent droplets from the flue gas, and finally a cleaning flue gas is discharged directly into the atmosphere. The sodium bicarbonate slurry produced by absorbing CO 2  falls down to a bottom of the absorption tower  1 , and is introduced into the slanting board sedimentation pool  6  for stratifying after passing through the slurry outlet of the absorption tower  1 . Sodium carbonate is the predominate ingredient in the supernatant of the slanting board sedimentation pool  6 , and the sodium bicarbonate slurry is the predominate ingredient in the underflow. 
     The sodium bicarbonate slurry is transported to an endothermic tube of the heat exchanger  18  via the sodium bicarbonate pump  7  and input into the regeneration tower  10  from the feed inlet after heat absorption. The sodium bicarbonate slurry is prayed into each sieve plate of the regeneration tower, heated and decomposed by an upward flowing water vapor; CO 2  is released. Incompletely decomposed sodium bicarbonate slurry falls down to the bottom of the regeneration tower  10 , and is heated by the supporting boiling unit  11  of the regeneration tower  10  to a temperature of 80-130° C. and further decomposed for releasing high concentrated CO 2  gas while an aqueous solution of sodium carbonate is acquired. 
     The aqueous solution of sodium carbonate in the regeneration tower  10  is raised up by the sodium carbonate pump  9  and input into an exothermic tube of the heat exchanger  18  for heat release. Thereafter, the aqueous solution of sodium carbonate is input into the slanting board sedimentation pool  6  from the absorbent inlet  6   b , and further transported into the absorbent spray layers  2  of the absorption tower  1  via the circulating pump  8  for recycling. 
     The highly concentrated CO 2  gas released from the regeneration tower along with a large amount of water vapor flow out of the regeneration tower  10  through the decomposed gas outlet of the regeneration tower  10 , and into the cooler  17  in which CO 2  gas is cooled to a temperature of 25-35° C. and most of the water vapor is condensed. 
     A highly concentrated CO 2  gas is acquired after being treated by the cooler  17 , and transported into the gas liquid separator  16 , in which the condensed water is completely separated from CO 2  gas under a centrifugal force, and a highly purified CO 2  gas having a purity exceeding 99% is obtained. The separated condensed water is transported through the water outlet of the gas liquid separator  16  and the absorbent inlet  6   b  of the slanting board sedimentation pool  6 , and finally into the slanting board sedimentation pool  6  for recycling. The separated highly purified CO 2  gas is transported to the desiccators  15  for drying treatment, and then into the compressor for compression. The compressed CO 2  gas is transported into the condenser  13  for being condensed into the liquid state and obtaining a highly concentrated industrialized liquid CO 2  product, which is finally input into the storage tank of liquid carbon dioxide  12  for storing. 
     While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.