Abstract:
A method in which CO 2  is placed on an adsorber and an adsorption reaction with ammonia, that is used as a chemical absorption agent, occurs, is provided. The CO 2  extracted from the waste gas is joined to the ammonia on the catalytic surface using a heterogeneous, catalytic reaction. At least two reactors are provided in the associated device. The reactors, which operate alternately, are switched between the adsorption of CO 2  and the regeneration of the absorption agent.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2008/058240, filed Jun. 27, 2008 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2007 030 069.9 DE filed Jun. 29, 2007, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a method for separating carbon dioxide (CO 2 ) from flue gases by using an adsorption method, wherein the CO 2  is accumulated on an adsorber. In addition, the invention relates to an associated device for executing the method. 
       BACKGROUND OF INVENTION 
       [0003]    The reduction in the emission of the greenhouse gas carbon dioxide (CO 2 ) from power plants and industrial plants can be achieved through the use of low-carbon fuels. 
         [0004]    The latter is however not a solution for existing plants which are designed to use high-carbon fuels, such as lignite-fired power plants in particular. Separation processes are required here which remove the CO 2  for example from the flue gas, or the waste gas. 
         [0005]    All the gases arising during the combustion process are referred to as flue gases, whereby the expression waste gas is used throughout in the following. 
         [0006]    The removal of CO 2  from waste gas can take place by means of physical or/or chemical binding in the bulk (“absorption”) or by means of accumulation on active surfaces (“adsorption”). 
         [0007]    With regard to the physical or chemical absorption, in both cases these are multi-step processes in which the waste gas containing CO 2  is brought into contact with a physical or chemical absorber until the latter is completely charged with CO 2 . Thereafter the absorber must be discharged, whereby the CO 2  is released in the presence of a scrubbing gas and is finally separated from the absorber. 
         [0008]    Potential problems in this situation are the slippage of the substance required for the binding, in other words the absorber, the separation of the CO 2  from the scrubbing gas in a form which permits the further use of the CO 2 , and where applicable the high energy requirement for the regeneration, particularly in the case of chemical binding. 
         [0009]    The binding of CO 2  by means of ammonia has recently been proposed, a method that has long been known from the synthesis of ammonia (see Parrish, Roger Warren: “Process for manufacture of ammonia”, EP 0247 220 B1 and the publication cited therein Uhde, Georg Friedrich: “Method of separating ammonia from gases and mixtures of gases containing ammonia”, U.S. Pat. No. 1,745,730 A), whereby ammonia slip can occur in the case of the separation of CO 2  from waste gas, and moreover the separation of CO 2  and NH 3 , which is present in bound form as ammonium carbonate or ammonium hydrogen carbonate, presents problems. 
         [0010]    Alternatively, it is possible to work with adsorbers, on which the CO 2  is accumulated in a first process step for example at low temperature or high pressure and is desorbed in a second process step at high temperature or low pressure (so-called “pressure swing adsorption” or “temperature swing adsorption”). A problem regarding the level of efficiency exists here because the adsorption capacity is considerably less than the capacity of absorbers, whereby a high energy requirement results in order to be able to handle temperature and pressure cycles. 
         [0011]    A method is described in DE 1 911 670 A for cleaning gases which contain acidic components such as CO 2 , whereby here as well as in the following three publications the further use of the cleaned gas is the primary objective and not the further use of the gas bound on an adsorber. The separation of CO 2  from process gases for the semiconductor industry by means of adsorption on zeolites charged with ammonia is known from JP 04-022415, whereby the CO 2  remains chemically bound at ambient temperature through reaction with the ammonia. The separation of CO 2  from waste gases, for example from thermal power plants, through carbonation at temperatures between 600° C. and 800° C., is described in JP 10-272336 A. Finally, it is demonstrated in a paper published from “Applied Surface Science”, Vol. 225, No. 1-4, pp. 235-242 (2004) that a preparation of activated carbon using ammonia leads to improved binding of CO 2 . In none of the cases is a method disclosed which enables the CO 2  removed from the gas undergoing cleaning to be prepared for further use or disposal with a justifiable expenditure of energy. 
         [0012]    In addition, membrane methods for separating CO 2  are possible which hitherto however have been unsuitable for applications in large plants for reasons of cost and efficiency level, in other words the low selectivity of the separation process between CO 2  and for example N 2 . 
       SUMMARY OF INVENTION 
       [0013]    Starting from this basis, the object of the invention is to propose an improved method for the large-scale reduction in emissions of carbon dioxide (CO 2 ) and to create an associated device. In this situation, the CO 2  should be separated from the waste gases such that the subsequent use for disposal of the CO 2  is made possible. 
         [0014]    This object is achieved in respect of the method by the measures described in the claims, whereby the invention emerges as a sequence of individual process steps. An associated device is the subject matter of the claims. Developments of the method and the associated device are set down in the respective dependent claims. 
         [0015]    According to the invention it is proposed that the binding of CO 2  from waste gas be performed in an adsorption reactor by means of a heterogeneous catalytic reaction with ammonia as the chemical absorber which is bound to the catalytic surface. In this situation, the process is conducted at a low temperature T with the result that the reaction products containing carbon, such as isocyanic acid (HNCO) and urea ((NH 2 ) 2 CO) for example, are also bound to the catalytic surface in accordance with the following reaction equations, whereby the molecules bound to the catalytic surface are identified by an “s”: 
         [0000]      NH 3 (s)+CO 2 ⇄HNCO(s)+H 2 O   (1) 
         [0000]      HNCO(s)+NH 3 (s)⇄(NH 2 ) 2 CO(s)   (2) 
         [0016]    A suitable temperature window is dependent on the catalyst used, in particular at temperatures below T=200° C., and in the case of the invention is advantageously: 
         [0000]      70° C.&lt;T&lt;140° C.   (1a) 
         [0017]    At low temperatures and high surface concentrations of NH 3  the equilibrium of the reaction (2) lies on the right-hand side of the reaction equation, at high temperatures or low surface concentrations of NH 3  however on the left-hand side. 
         [0018]    The catalyst is subsequently regenerated, whilst excluding the waste gas, at a higher temperature in a gas mixture consisting of water vapor and CO 2 , whereby CO 2  is selectively released and is thereby definitively separated, while the absorption agent is returned to its original state and remains bound to the surface in this situation: 
         [0000]      HNCO(s)+H 2 O→NH 3 (s)+CO 2    (3) 
         [0019]    Reaction (3) represents the converse of the reaction (1), which is forced to take place as a result of the fact that water vapor is made available in excess and the temperature is raised such that it lies above the window specified in (1a). As a result the equilibrium of the reaction (2) is shifted to the left-hand side because isocyanic acid (HNCO) is constantly eliminated through the hydrolysis reaction (3). 
         [0020]    The subsequent separation of water vapor and CO 2  can be achieved through condensation by means of suitable pressure and temperature control. 
         [0021]    Alternative reaction mechanisms, which for example lead to the formation of ammonium carbamate NH 2 CO 2   − NH 4   + , can likewise be represented when suitable reaction parameters (low temperature) and catalysts are chosen: 
         [0000]      2NH 3 (s)+CO 2 →NH 2 CO 2   − NH 4   + (s)   (4) 
         [0022]    Ammonium carbamate (NH 2 CO 2 NH 4   + ) can be converted to ammonium carbonate by hydrolysis in an aqueous solution or on a suitable catalytic surface even at low temperatures: 
         [0000]      NH 2 CO 2   − NH 4   + (s)+H 2 O→(NH 4 ) 2 CO 3    (5) 
         [0023]    The ammonium carbonate decomposes thermally on an increase in temperature into NH 3  and CO 2  and water is split off: 
         [0000]      (NH 4 ) 2 CO 3 →2NH 3 (s)+CO 2 +H 2 O   (6) 
         [0024]    By using suitable catalysts it is possible to ensure that NH 3  remains bound on the surface. The subsequent separation of water vapor and CO 2  can again be achieved through condensation by means of suitable pressure and temperature control. 
         [0025]    With regard to the device according to the invention, at least two reactors are present. In this situation, for the purposes of execution of the inventive method described above in terms of flow implementation it is possible to implement the following as different arrangements:
       Two parallel reactors are fed alternately with waste gas for the adsorption of the CO 2  and the respective working reactor is taken out of the waste gas stream when the adsorber has been largely converted and fed with the regeneration gas which has been brought to the required temperature.   One adsorption reactor and one regeneration reactor are located parallel to one another such that the catalyst required for the reaction process can be guided through a gas lock from the adsorption reactor into the regeneration reactor and back.       
 
         [0028]    One option for the latter is to design the catalyst for example as a rotatable stack of disks which is arranged such that the catalytic surfaces alternately pass through the adsorption reactor and the regeneration reactor. As an alternative to this, the passage of catalyst particles in counterflow is possible. 
         [0029]    The advantages of the invention described above compared with the previous methods, which function with liquids such as ammonia in aqueous solution, lie essentially in the fact that by choosing suitable catalysts with binding sites for NH 3  the ammonia slip can be greatly reduced. In addition, the reaction kinetics can as a result be configured considerably more selectively, such that the formation of undesired byproducts is suppressed, which consume the absorber or result in a binding of the CO 2  which is energetically strong and can be released only with a high expenditure of energy. 
         [0030]    It is furthermore advantageous that a considerable portion of the CO 2  from waste gases is separated in such a form which enables the subsequent use of the CO 2  with a low expenditure of energy in order to arrive at a sustainable reduction in CO 2  emissions. 
         [0031]    By preference, oxides and mixtures of oxides such as TiO 2  and V 2 O 5  for example come into consideration as catalytic materials, whereby titanium dioxide for example is a suitable hydrolysis catalyst, while V 2 O 5  is favorable for binding ammonia on the surface. Alternatively, ion exchanged zeolites can be employed as catalysts, which are likewise capable of binding ammonia very selectively. 
         [0032]    Use of the method according to the invention is of particular interest with regard to CO 2  separation by way of ammonia solutions at process temperatures &gt;10° C. since in this case a proportion of NH 3 , dependent on temperature, in the region of several percent by volume, is still present in the separated CO 2 , which cannot be eliminated economically by conventional methods. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    Further details and advantages of the invention will emerge from the following description of figures for exemplary embodiments with reference to the drawing in conjunction with the claims. 
           [0034]    In schematic representation in the drawings: 
           [0035]      FIG. 1  shows an arrangement for separating CO 2  from waste gas with the aid of a solid catalyst and 
           [0036]      FIG. 2  shows an alternative arrangement to  FIG. 1  with a rotatably arranged plate-type catalyst, 
           [0037]      FIG. 3  shows a further arrangement with catalyst particles and 
           [0038]      FIGS. 4 and 5  show the sensor system in the case of an alternating function of the reactors in accordance with the  FIGS. 1 to 3  as an adsorption reactor on the one hand or as a desorption reactor on the other hand. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0039]    In the following, the two figures are described individually in each case. In this situation the essential elements, such as the reactors and valves including the lines which comprise the same function have the same reference characters. 
         [0040]    With regard to the  FIGS. 1 to 3 , two identical reactors  10  and  10 ′,  20  and  20 ′, and  30  and  30 ′ respectively are present in each case, which are run in alternating operation. In other words, for example for 
         [0041]      FIG. 1 , while the one of the two reactors  10 ,  10 ′ is used for the adsorption of the CO 2  in the waste gas containing CO 2 , the other of the two reactors  10 ,  10 ′ is discharged, which is described in detail further below. For such a type of alternating operation, fluid lines with a series of valves are required as well as additionally a storage container for an absorption agent for CO 2  and a unit for separating the CO 2  from the regenerate. 
         [0042]    The two reactors  10  and  10 ′ in  FIG. 1  each have a respective catalyst bed  11  and  11 ′. Waste gas containing CO 2  is delivered by way of a waste gas line  1  and directed by way of the branch  2  either into the first reactor  10  or by way of a parallel line la with branch line  2   a  into the second reactor  10 ′ connected in parallel. Valves V 1 , V 2 , V 7  and V 8  are connected into the lines  1 ,  1   a  for this purpose. The associated control unit is not shown. 
         [0043]    At any one time one of the reactors  10 ,  10 ′ is therefore in adsorption operation while the other reactor is in regeneration operation. To this end, a reaction gas mixture, which is also referred to as regenerate, is fed from the other side of the reactors by way of the line  6  with the parallel line  6   a  and the respective branch lines  7  and  7   a  to the two reactors in alternating operation. For this purpose, valves V 3 , V 4 , V 5  and V 6 , whose function emerges from the description of the alternating operation, are connected in the lines  6 ,  6   a.  CO 2 -reduced waste gas is taken away by way of the line  8  and regenerate containing CO 2  is delivered to the unit  5  by way of the line  3 . The separation of CO 2  from the regenerate takes place in the unit  5 , with the result that pure recovered CO 2  is taken away here. The container for the absorption agent is designated by  4  and is operatively connected by way of a valve V 9  to the fluid circuit. 
         [0044]    The two reactors  10 ,  10 ′ in  FIG. 1  have—as already mentioned—catalyst beds  11  and  11 ′ which contain a solid catalyst, which is designed for example as a plate-type catalyst. Alternatively, such a catalyst bed can also be designed as a so-called particle bed (“packed bed”). 
         [0045]      FIG. 2  shows a simplified illustration of an alternative arrangement to  FIG. 1 , having reactors  20  and  20 ′. The individual valves are not shown here, apart from valve V 9 . In this situation, the two reactors  20 ,  20 ′ are connected to each other by way of a gas-tight lock. Instead of the catalyst beds from.  FIG. 1 , plate-type catalysts  15  are present here, arranged rotatably around a vertical axis. Both reactors are linked by way of a gas-tight lock  30 , whereby through rotation of the catalyst plate arrangement in each case a fully charged catalyst half can be brought into the second reactor for generation purposes and the discharged catalyst plate half is available for recharging. Otherwise, the alternation principle with delivery of a regeneration gas mixture (“regenerate”) on the one hand and separation (CO 2 ) on the other hand from the waste gas is identical. It is important in both cases that absorption agent can be added in regulated fashion to the regenerator after the CO 2  separation in order to compensate for an unavoidable loss of regeneration agent occurring during practical operation. 
         [0046]    Other embodiments according to the invention include plate reactors coated with catalysts, in particular those having movable plates or other structures having a large specific surface, in which the plates are transported in a rotary fashion from the charging area (flue gas, CO 2  gas stream) by way of a lock system into the discharging area for CO 2  separation purposes and back into the charging area. 
         [0047]    In deviation to the  FIGS. 1 and 2 , the invention defined on the basis of the application for protection also includes arrangements in which the gas stream to be cleaned is passed through a fluidized bed of small catalyst particles (“fluidized bed reactor”), whereby in particular small particles and those having a high specific surface, for example porous particles, are advantageous. The charged particles are continuously removed from the charging area, delivered to a desorption area and then fed back again into the adsorption reactor. 
         [0048]    In the simplified illustration according to  FIG. 3 , in a further arrangement the gas stream to be cleaned is delivered in counterflow through a “shower” of catalyst coated particles having a high specific surface (“trickle-bed reactor”), whereby the charged particles are likewise continuously removed, regenerated and delivered back again to the trickle-bed reactor. 
         [0049]    To this end, two reactors  30  and  30 ′ are shown in  FIG. 3 , which operate on the counterflow principle. In this situation, both reactors  30 ,  30 ′ have catalyst plates  31  and  31 ′ respectively, which are implemented in each case as a packed bed of catalytic particles. Both reactors  30  and  30 ′ are connected at their ends in each case by means of a gas-tight lock  32  and  32 ′ respectively. 
         [0050]    Otherwise, the device according to  FIG. 3  operates in corresponding fashion to  FIG. 2 . It is however important here that the waste gas containing CO 2  is brought into the reactor  30  by way of the line  1  and flows there in counterflow to the catalyst particles. Corresponding conversely, the regenerate is brought into the reactor  30 ′, whereby the catalyst particles here flow again in counterflow according to the arrow. In practice, a through-flow pump is used for this purpose, which is not shown individually in  FIG. 3 . 
         [0051]    The sensor system on the one hand and also the signal processing are not contained in the examples illustrated in  FIGS. 1 to 3 . Essentially it is the same for all the three examples according to  FIGS. 1 to 3  and is explained in detail with reference to  FIGS. 4 and 5 . 
         [0052]    In  FIG. 4 , an adsorption reactor is designated all-inclusively by  40 . A valve V 10  is provided for the flue gas inlet by way of a line  41  and a valve V 11  is provided for the gas take-off at the outlet from the reactor by way of the line  49 . A temperature sensor  42  and also a gas sensor  43  for the CO 2  concentration are situated on the input side in the adsorption reactor  40 . A further gas sensor  44  for the CO 2  concentration is present on the outgoing side. It is therefore important that the concentrations c(CO 2 ) at the input on the one hand and at the output on the other hand can be measured and are correlated with the temperature T in accordance with a thermally activated process. The adsorption capacity of the adsorber can be determined from the decrease in the CO 2  concentration at a particular temperature T. When the adsorption capacity decreases below a particular limit value a regeneration is initiated. 
         [0053]    In  FIG. 5 , a desorption reactor  50  is illustrated which has input lines  51 ,  51   a  and an output line  59 . Valves V 12  and V 13  are again provided at the input and at the output, and a valve V 14  is additionally provided in the feed line  51   a  for delivering an absorption agent. 
         [0054]    In the desorption reactor  50 , a sensor  52  is provided at the input for the temperature T and a sensor  53  is provided at the output for the concentration c(Abs) of the absorption agent. The signals for the concentrations on the one hand and the temperatures on the other hand are processed in a control device which is not described individually, a known microprocessor control unit for example. An important criterion concerning the control in this situation is the fact that the adsorption capacity of the catalytic material for CO 2 , which is determined from current CO 2  measurement values at the adsorption reactor, is maintained in an adequate manner through the storage of absorption agent on the catalytic surface. For this purpose, a valve V 12  is closed in order to stop the delivery of desorption gas mixture. Valve V 14  is then opened in order to deliver absorption agent (ammonia for example). Shortly thereafter valve V 13  is closed in order to avoid any slip of the absorption agent. As soon as the sensor  53  in the output area of the desorption reactor  50  identifies absorption agent concentrations above a first limit value, valve V 14  is closed. 
         [0055]    During operation of the device as intended, temperature T and absorption agent concentration c(Abs) are monitored by sensors: As the temperature T drops, with an intact catalyst the storage capacity for the absorption agent rises such that the concentration c(Abs) of the absorption agent still contained in the gas phase falls below a second limit value classed as uncritical after a short waiting time and the reactor, or the catalyst charged with absorption agent, can be taken into operation again. Deviations from this behavior give indications of damage to the catalyst as a result of either mechanical, thermal or also chemical influences, whereby maintenance of the system can be undertaken where necessary. 
         [0056]    The devices having two reactors described with reference to the figures can advantageously be used for separating CO 2  from waste gases containing CO 2 . In this situation, the following process steps in particular take place:
       The waste gas containing CO 2  is passed over a catalyst, at the active centers of which NH 3  is accumulated.   The CO 2  is transformed at a first process temperature by chemical reactions with the NH 3  into a stabile compound which is likewise bound to the catalyst surface. This temperature is designated T 1 .   At a second process temperature which is higher than the first process temperature, the catalyst thus charged with CO 2  is subjected to a scrubbing gas stream consisting of CO 2  and water vapor (H 2 O). This temperature is designated T 2  (T 2 &gt;T 1 ). At this temperature the compound comprising CO 2  and NH 3  decomposes, the CO 2  is given off into the scrubbing gas stream while the NH 3  remains accumulated on the surface of the catalyst.   The scrubbing gas stream enriched with CO 2  is conveyed into the further reactor and cooled down there to a temperature which is lower than the first process temperature. This temperature is designated T 3  (T 3 &lt;T 1 ). At this temperature the water is condensed and discharged.       
 
         [0061]    As a result of this temperature control the pure dry CO 2  above the water surface can then be pumped away and delivered for a further use. 
         [0062]    Overall, it can be noted that by using the method described above and also the devices created to implement the method the separation of CO 2  in particular from flue gases and in principle from CO 2 -containing waste gases of all types takes place in a single manner. A reduction in the CO 2  emissions of climate damaging greenhouse gases is thereby made possible. It is important in this situation that the CO 2  is present at the end of the stated process in an almost pure form in order that it can be compressed for example for storage in natural gas fields or oil fields whilst simultaneously increasing the production volume (so-called “enhanced oil recovery”, “enhanced gas recovery”). Of secondary significance however is the completeness of the separation of the CO 2  from the flue gas: A residual quantity of 10% of the original CO 2  content can remain in the flue gas without further ado if as a result for example the energy requirement for the separation can be minimized compared with the energy turnover of the plant which is emitting the flue gas. 
         [0063]    An oxidic catalyst is advantageously used as the adsorber for the plant. In this situation the catalyst consists for example of titanium oxide (TiO 2 ) or a mixture of titanium oxide (TiO 2 ) and a further metal oxide, in particular dosed with vanadium oxide (V 2 O 5 ). It can also consist of an ion exchanged zeolite. 
         [0064]    The desired objective of being able to produce carbon dioxide (CO 2 ) in pure form at the end of the process according to the invention for the purpose of further use or disposal can henceforth be achieved in an efficient manner.