Abstract:
A method for removing CO 2  in a exhaust gas from combustion of carbonaceous material, in which method the exhaust gas is brought in countercurrent flow with a liquid CO 2  absorbent in a contact section of an absorption column, to produce a CO 2  depleted exhaust gas that is released into the atmosphere and a CO 2  rich absorbent that is withdrawn from the absorption column, wherein the exhaust gas is introduced into the absorption column at a temperature above about 90 and the exhaust gas is cooled in the absorption column by countercurrent flow of the liquid absorbent in the absorption column, is described.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to field of CO 2  capture from a gas mixture. More specifically the present invention relates to CO 2  capture from an exhaust gas from the combustion of carbonaceous material. Most specifically the present invention relates to an improved method and plant for cooling and absorbing CO 2  from a combustion gas. 
       BACKGROUND 
       [0002]    The continually increasing combustion of fossil fuel, such as coal, natural gas and oil, during the last centuries has resulted in an increase in the concentration of CO 2  in the atmosphere. The increasing concentration of CO 2  has caused concern due to the greenhouse effect caused by CO 2 . The greenhouse effect is suspected already to have caused at least some of the changes in the climate that have been seen during the last decades, and is according to simulation models suspected to cause even more and potentially dramatic changes in the climate of planet earth. 
         [0003]    This has caused a call for action from scientists, environmentalists and politicians throughout the world, to stabilize or even reduce the discharge of CO 2  from combustion of fossil fuel to the atmosphere. This may be achieved by capturing and safe depositing of CO 2  from the exhaust gas from thermal power plants and other plants where fossil fuel is combusted. 
         [0004]    The captured CO 2  may be injected in sub terrain formations such as aquifers, oil wells for enhanced oil recovery or in depleted oil and gas wells for deposition. Tests indicate that CO 2  remains in the sub terrain formation for thousands of years and is not released into the atmosphere. 
         [0005]    Capturing of CO 2  from a gas by means of absorption is well known and has been used for decades, e.g. for removal of CO 2  (and other acid gases) from produced natural gas at gas fields. The absorbents used or suggested in the prior art have been different aqueous alkaline solutions, such as potassium carbonate, see e.g. U.S. Pat. No. 5,528,811, and different amines, see e.g. U.S. Pat. No. 4,112,051, U.S. Pat. No. 4,397,660 and U.S. Pat. No. 5,061,465. Separation of CO 2  from exhaust gas from thermal power plants by means of an amine solution, is known e.g. from U.S. Pat. No. 4,942,734. 
         [0006]    Common for these CO 2  capturing solution is that the gas mixture to be separated is introduced countercurrent to the aqueous adsorbent in an absorber column. The gas leaving the absorber column is CO 2  depleted (or acid gas depleted), whereas the CO 2  (or other acid gas) leaves the absorber column together with the absorbent. The absorbent is regenerated in the regenerator column and returned to the absorber column. Amine is regenerated by stripping the amine solution with steam in the regeneration column. The steam is generated in the reboiler at the base of the column. 
         [0007]    As illustrated above CO 2  as such is well known in the art. However, there is a need for several improvements in the CO 2  capturing process to make CO 2  free or low CO 2  emission thermal power plants economically profitable. 
         [0008]    The plants for capturing of CO 2  are relative large, complex and expensive constructions. It is therefore desired to reduce the size, complexity and cost of the Plants. 
         [0009]    Capturing of CO 2  is carried out at the expense of the efficiency of a thermoelectric power plant utilizing fossil fuel, so that the output of electrical power and/or medium temperature heat from a thermoelectric power plant is reduced. The reduced efficiency compared with a traditional plant makes these facilities less profitable. Improvements in the efficiency, i.e. reducing the energy cost in the CO 2  capturing process, are therefore sought. 
         [0010]    The currently preferred absorbents are aqueous solutions of different amines. The commonly used amines are alkanol amines, such as e.g., diethanol amine, mono methyl ethanolamine, aminoethyl ethanolamine, 2-(Methylamino)etanol, MDEA as well as other amines known by skilled man in the art. The absorption of CO 2  to the amine absorbents is a reversible, exothermic reaction. Accordingly, heat has to be supplied to the regenerator column to reverse the absorption and release the CO 2 . 
         [0011]    The amine absorbents are exposed for degradation during the absorption/desorption cycle described above. The exhaust gas from the combustion of carbonaceous material comprises oxygen which will react with the amines dependent on the type of amine, the oxygen concentration, the total contact time between oxygen and amine, temperature etc. High temperature, long contact time and high oxygen content in the gas to be treated, are factors that increase the degradation of the amine. The degradation of the amine is also a factor adding cost to the capture of CO 2 . The degraded amines have to be replaced and deposited resulting in increased operational costs of the plant. 
         [0012]    The exhaust outlet temperature from a traditional thermal power plant is normally 80 to 120° C. as the gas at lower temperatures has no value in production of electrical power, and no or limited value as a heat source in other processes. 
         [0013]    A cooler is therefore introduced between the outlet of a thermal power plant and the absorber column. The most common cooler is a tower in which the exhaust gas is cooled by a countercurrent flow of water spray both to cool the gas and saturate the gas with water vapor. The cooled and saturated gas leaving the cooler has normally a temperature of about 50° C. and is directly introduced into the absorption column. 
         [0014]    The cooler and absorber for a CO 2  capturing plant, constitutes a substantial part of the total investments for the plant. A decrease of the size and complexity and thus the total cost of investments for the cooler and absorber is therefore sought. Additionally, there are significant operating costs connected to the cooler. There is a pressure loss across the cooler resulting in increased power consumption. Additionally, the cooler requires large power consuming pumps. 
         [0015]    An objective of the present invention is thus to provide solutions that makes it possible reduce to cost of investments for the cooler and absorber in a CO 2  capturing plant, without an unacceptable increase in operational costs and/or decrease in efficiency of the plant. 
       SHORT DESCRIPTION OF THE INVENTION 
       [0016]    The present invention provides a method for removing CO 2  in a exhaust gas from combustion of carbonaceous material, in which method the exhaust gas is brought in countercurrent flow with a liquid CO 2  absorbent in a contact section of an absorption column, to produce a CO 2  depleted exhaust gas that is released into the atmosphere and a CO 2  rich absorbent that is withdrawn from the absorption column, wherein the exhaust gas is introduced into the absorption column at a temperature above about 90° C. and the exhaust gas is cooled in the absorption column by countercurrent flow of the liquid absorbent in the absorption column. It has surprisingly been found that the inlet cooler for the absorption column may be omitted, and thus both cost of investment and cost of operation, may be reduced without a corresponding loss in absorption capacity of the absorption column. The loss of absorption capacity due to the cooling in the absorption column may be compensated by an increase of about 10% of the contact section. The omission of the inlet cooler has, however a significant positive effect on the cost of investment and the area requirement for a CO 2  capturing plant, as the inlet cooler is an expensive component requiring valuable area in the plant. The savings for the omission of the inlet cooler is far greater than the added cost for increasing either the size of or circulation rate through the contact section. 
         [0017]    According to one embodiment, the absorbent is withdrawn from the absorption column immediately after leaving the contact section. By removing the absorbent from the absorption column immediately after leaving the contact section, the time in which the absorbent is in contact with the hot exhaust gas is reduced, thus reducing the degradation of the absorbent. 
         [0018]    According to a second embodiment, the present invention provides an absorption column for removing CO 2  from an exhaust gas from combustion of carbonaceous material comprising an inlet section for the exhaust gas, one or more packed contact sections(s) for effecting countercurrent flow of the exhaust gas and a liquid absorbent, and means for withdrawing CO 2  depleted exhaust gas from column, and means for withdrawing CO 2  rich absorbent from the column, wherein the means for withdrawing the CO 2  rich absorbent comprises a collection tray arranged between the inlet section ( 10 ) and the contact section(s). 
     
    
     
       SHORT DESCRIPTION OF THE FIGURES 
         [0019]      FIG. 1  illustrates an absorption column and an inlet cooler according to the prior art, 
           [0020]      FIG. 2  illustrates an absorption column according to a first embodiment of the present invention, 
           [0021]      FIG. 3  illustrates an absorption column according to a second embodiment or the present invention, 
           [0022]      FIG. 4  illustrates an absorption column according to a third embodiment of the present invention, 
           [0023]      FIG. 5  is a graphical presentation of temperature as a function of height in a contact section for an inlet temperature of 50° C. and a saturation temperature of 47° C., 
           [0024]      FIG. 6  is a graphical presentation of temperature as a function of height in a contact section for an inlet temperature of 90° C., and saturation temperature of 50° C., and 
           [0025]      FIG. 7  is a graphical presentation of the absorption capacity of an absorbent at an inlet temperature of 50° C. compared to 90° C. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIG. 1  illustrates an inlet cooler  1  and an absorption column  2  according to the prior art. Combustion gas, typically having a temperature of 90-140° C. is introduced through an exhaust line  3  into an inlet cooler chamber  4 . The combustion gas is cooled in a contact zone  5  in the inlet cooler  1  by a countercurrent flow of cooling water. The cooling water is introduced through a cooling water line  6  and is sprayed by means of liquid distribution means  7  over the contact zone  5 . Due to the direct countercurrent contact with the cooling water, the combustion gas is both cooled and saturated with water vapor. The cooled combustion gas is withdrawn from the top of the inlet cooler through a line  8  and introduced into the absorption column, whereas cooling water is collected in the bottom of the cooler and withdrawn through a withdrawal line  9 . 
         [0027]    The cooled and saturated combustion gas in line  8  is introduced in an inlet chamber  10  at the bottom of the absorption column  2 . From the inlet chamber  10 , the gas flows upwards in the absorption column and countercurrent to liquid absorbent in one or more contact section(s)  12 ,  16 . The contact section (s) is(are) preferably comprises a structural packing to increase the surface area, or the contact area between the liquid absorbent and the gas. The illustrated absorption column is provided with two serially connected contact sections  12 ,  16 . After leaving the contact sections  12 ,  16 , the gas is washed by a countercurrent flow of water in a washing section  21  to remove any amine in the gas flow. Washing water is introduced through a washing water line  23  and sprayed at the top of the washing section  21 , is collected at a water collection plate  19  below the washing section and is withdraw through a washing water removal line  20 . Water droplets in the washed gas flow is then preferably removed in a demister section  24  before the lean exhaust gas, from which 80 to 99% of the CO 2  has been removed by adsorption, is withdrawn through a lean exhaust gas withdrawal line  25 . 
         [0028]    Lean, or CO 2  pure, absorbent is introduced into the absorption column through a lean absorbent line  18  and is sprayed onto the top of the upper contact section  16  by means of liquid distribution means  17 . The absorbent flows through the upper contact section  16  and is collected at an absorbent collection plate  15  below the contact section  16 . The absorbent collection plate  15  may be a bubble cap collection plate or any other type of collection plate known by the skilled man in the art. The semi lean absorbent having absorbed CO 2  from the exhaust gas, is withdrawn in a bypass line  14  and sprayed at the top of the lower contact section  12 . After flowing through the lower contact section  12 , the rich absorbent having absorbed more CO 2  from the exhaust gas, is collected at the bottom of the stripper column and is withdrawn to a rich absorbent withdrawal line  11 . 
         [0029]    The rich absorbent may be regenerated in a regenerator, e.g. as described in the applicants own Norwegian patent applications NO2006 5411 NO2006 5412 or NO2006 5413, the description of which are incorporated in the present description as reference. Regenerated absorbent is then introduced as lean absorbent in line  18 . 
         [0030]    The liquid distribution means  7 ,  17 , respectively, may be any means suitable to distribute a liquid over the contact sections. Suitable liquid distribution means  7 ,  17  may be nozzles, troughs or piping with small holes to allow the liquid to be distributed over the contact sections. 
         [0031]    The embodiment illustrated in  FIG. 2 , corresponds to the above described solution according to prior art, with the difference that the cooler is omitted. By introducing exhaust gas not saturated with water and at a temperature typically from about 90-130° C., the exhaust gas is cooled by evaporation of water from the absorbent to saturate the exhaust gas and results only a slight increase of temperature of the absorbent in the lowermost part of the contact section. This slight increase of temperature and the corresponding slight decrease in absorbing capacity of the absorbent, may be compensated by an increase in the height of the contact sections(s) and/or an increased flow of absorbent. The extra costs for the increased height of the packing or increased flow of absorbent to absorb the same amount of CO 2  are, however, relatively small compared with the cost of a cooler according to the prior art. 
         [0032]      FIGS. 3 and 4  illustrates two different variations of an embodiment where the rich absorbent leaving the lower contact section is collected in a collection tray  37  beneath the structured packing of the lower contact section  12 , to collect the rich absorbent flowing out of the contact section. The collected rich absorbent is withdrawn from the collection tray  37  and is transferred to a rich absorbent storage tank  31 ,  35  through lines  30 ,  38 , respectively. By removing the rich absorbent from the absorption column immediately after leaving contact section the exposure time of the absorbent for oxygen and high temperature is minimized. This results in a reduced degradation of the absorbent compared with the solutions according to the prior art. 
         [0033]    To reduce the degradation of the absorbent even more, an inert gas, such as e.g. nitrogen, may be introduced into the tank  31 ,  35  through an inert gas line  30 ,  32 , respectively, as a blanketing system to further reduce the contact between absorbent and oxygen. As illustrated in  FIG. 4 , the tank  35  is optionally vented into the inlet chamber of the absorbing column through a venting line  39  to avoid the need for cleaning the gas removed from the tank  35  to adjust the pressure therein. 
         [0034]    The rich absorbent in the tank  31 ,  35  is carried to a regeneration plant through a rich amine return line  32 ,  40 , respectively. 
         [0035]    To reduce degradation of the absorbent in the tank  31 ,  35 , respectively, by oxygen present in the absorbent, a not shown stripper column in which a stripper gas flows countercurrent to the rich absorbent, may be introduced in line  30 . The stripper gas may be CO 2  from the above mentioned absorbent regenerator. The CO 2  removed from the stripper column may be dried and compressed and exported from the plant as the oxygen level even after stripping of the absorbent is sufficiently low for most applications of the CO 2 . 
       EXAMPLE 
       [0036]    The simulation programs ProTreat and CO2Sim were used for simulation of the temperature profile in a contact section in an absorber for CO 2 . 
         [0037]    The simulated values from the simulations were compared with measured values from a test contact section having a diameter of 300 mm using MEA as absorbent amine. 
         [0038]    Simulated values and measured values from the test absorber for gas inlet temperature of 50° C. and 90° C. are presented in  FIGS. 5 and 6 , respectively. The gas entering at 50° C. corresponds to a typical cooled and humidified exhaust gas, having a dew point of about 47° C. and thus being almost saturated with water, whereas the gas entering at 90° C. corresponds to a typical exhaust gas that is released from a natural gas fired thermal power plant, having a dew point of about 44° C. 
         [0039]    As illustrated in the  FIG. 5  the temperature in the contact section increases substantially linear as a function of the height, due to the exothermal absorption reaction between CO 2  and the amine.  FIG. 6  illustrates the fact that the gas entering the contact section at a temperature of 90° C. is effectively cooled by evaporation of water from the absorbent solution. From a height of about 0.5 m the temperature again increases due to the exothermal absorption reaction as mentioned above. From the height of 0.5 m the temperature difference between the two cases is about 2° C. 
         [0040]      FIG. 7  illustrates the CO 2  loading of the absorbent (mole CO 2 /mole MEA) as a function of the height of the contact section for the two test and simulation cases having inlet temperatures of 50 and 90° C., respectively. Due to the higher temperature in the lower part of the contact section, the loading is slightly lower for the higher inlet temperature than the lower temperature exhaust gas. 
       DISCUSSION 
       [0041]    The test results and simulations confirm a close agreement between the simulations and the experimental tests. The simulation model may therefore be used to predict the performance of the absorber under varying temperatures and humidity of exhaust gas. 
         [0042]    The results confirms that the inlet cooler upstream for the absorption column in a plant for capturing of CO 2  from the exhaust gas from a thermal power plant fired by natural gas, may be omitted according to the present invention. The cost for omitting the inlet cooler is somewhat lower absorption efficiency. This lower absorption efficiency may be compensated for by increasing the circulation rate of the absorbent by about 6%, or by increasing the height of the contact section(s) by less than 10%, or about 0.5 m. 
         [0043]    The experimental results and the simulations also confirms that the hot (90° C.) and dry exhaust gas is cooled to a temperature that is slightly higher than the temperature of a typical cooled and humidified exhaust gas, in the lower 0.5 m or less of the contact section. This cooling effect is mostly contributed to evaporation of water from the absorbent. This evaporation will concentrate the amine solution, which may be favorable in the lower part of the absorber. 
         [0044]    Accordingly, both the simulated values and the results from an actual test of an absorption tower confirm the present inventor&#39;s theory that the inlet cooler between a natural gas powered thermal power plant may be omitted and thus make substantial savings possible. This finding is in contradiction to the accepted view in the field that an inlet cooler is necessary for cooling the exhaust gas from a thermal power plant before introduction into an absorption column for absorption of CO 2 .