Patent Publication Number: US-2023137348-A1

Title: A method and arrangement for capturing CO2

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The instant application is a U.S. National Stage application of and claims priority to PCT/NO2021/050095, filed on Apr. 7, 2021, which is a PCT application of and claims priority to NO Application No. 20200431, filed on Apr. 7, 2020, the subject matter of both aforementioned applications is hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method for the capture and subsequent storage of CO 2  from a CO 2  source ranging from sources with extremely low concentrations such as ambient air, via sources with higher concentrations such as air from vegetable storage spaces to sources with relatively high concentrations such as flue gas from combustion processes. The product contains up to more than 90% CO 2 , and no use of chemicals in any form ensures safety. Energy consumption is minimized by means disclosed here. More specifically, the present invention relates to the enhancement of a clean adsorption method and process for CO 2  capture, by maximization of the adsorbent CO 2  storage capacity combined with storage and re-use of heat required in the process, providing energy savings and thus extending the range of useful applications. 
     BACKGROUND ART 
     An earlier invention, WO2018/034570, presented a system for closed or semi-closed greenhouses. The closing of the greenhouse is accomplished by extracting air at a high rate from the closed greenhouse, dehumidifying it, adjusting the temperature, adding supplementary CO 2  captured from the outside air, and subsequently returning this CO 2  enriched air to the greenhouse. This stabilizes the greenhouse temperature and humidity and eliminates possibilities for CO 2  emissions. 
     Plants growing in closed or confined areas such as greenhouses will consume CO 2  and thus deplete the CO 2  in the air. Therefore, most greenhouses have an artificial supply of CO 2 , such as from a tank with liquid CO 2 . However, on warm days or with powerful artificial light in the greenhouse, the opening of hatches may be required to reduce the temperature. Air from the greenhouse then flows via the hatches to the outside atmosphere, and added CO 2 , which is mixed with the greenhouse air, escapes via the hatches along with the escaping air. Up to 75% of added CO 2  may be lost as result of this. Therefore, to minimize losses, the CO 2  concentration in the air tends to be much lower than the optimum concentration, limiting the CO 2  usage and emission to acceptable levels. Furthermore, air inside greenhouses tends to become very humid as most of the irrigation water is transpired and thus humidifies the air. This also forces the opening of the hatches in many cases. 
     Open greenhouses tend to get too high humidity. Plant transpiration in greenhouses increases the relative humidity in the local air. About 90% of the humidity taken up by plants is used for transpiration while 10% is used for growth. The transpiration cools the plant to 2° C. or more below the ambient temperature. The rate of transpiration is a function of, among other factors, the radiative heat input and the air relative humidity. High relative humidity, near water vapour saturation in the local air, reduces transpiration. If the temperature then drops, water may precipitate on plant leaves and elsewhere. This increases the risk of fungal diseases. Low relative humidity, such as below 50% in combination with high temperature, may result in excessive transpiration rates. The plant may then start to close the stomata openings, through which transpiration occurs, to reduce transpiration. However, CO 2  uptake also occurs through the stomata openings, so this may restrict plant growth. It is important to maintain the local air relative humidity at acceptable if not optimum levels. 
     Plant growth rate depends heavily on light including solar radiation or artificial lights. Solar radiation up to at least 600 W/m 2  benefits the plants. Artificial light provides about 250 to 300 W/m 2  and is used whenever the solar radiation is insufficient. However, both sources of light also provide heat and thus affect the greenhouse temperature. The optimum temperature depends on plant species and time of day. Day temperature of 20 to 25° C. is suitable for most plants. Optimum night temperatures may be in the range from 15 to 18° C. Typically, the heat input is too high during sunny days and as a result, greenhouse hatches are opened. This helps reduce the temperature and humidity. 
     The opening of the hatches also reduces the concentration of CO 2  and this may hamper plant growth rate. The depletion may happen very quickly, within minutes. Greenhouses with artificial CO 2  addition, such as from liquid CO 2  tanks, typically operate with CO 2  concentrations in the range from 600 to 800 ppm. With open hatches this drops to about 400 ppm. Up to three quarters of all CO 2  artificially injected into the greenhouse is emitted. This is costly and reduces plant growth rates. It limits the economic optimum CO 2  concentration in the greenhouse; without such emissions the optimum CO 2  concentration might have been much higher such as 1200 ppm. 
     The earlier invention, WO2018/034570, solves these issues. Further work with this technology has uncovered new areas where the technology disclosed in can provide great benefits. One example is vegetable storage facilities, where the air tends to become too CO 2  rich, such as for example 10 000 ppm, caused by slow vegetable degradation. The CO 2  capture technology, WO2018/034570, could solve this problem by closing the storage facility and capturing excess CO 2 . The question if the capture system could capture CO 2  from flue gas, typically containing 40 000 ppm CO 2 , has also been raised. 
     Among several such other possibilities, the direct capture of CO 2  from air, abbreviated DAC (Direct Air Capture) has recently gained significant interest as a means to limit climate change. Such facilities could provide negative emissions, and this could be assigned to positive emissions to annul these. 
     Based on the above, there is a need for a CO 2  capture system that can capture CO 2  from air, from intermediate concentration CO 2  sources, and from flue gas without major modifications. The energy consumption should be minimized, the CO 2  captured must be available at randomly fluctuating rates, and the CO 2  must be clean and suitable for closed spaces where people work. 
     CO 2  capture systems, from sources where the CO 2  is at least partly mixed with components from air and are not pressurized, are usually tailor made to the CO 2  source. As an example, CO 2  from flue gas is typically captured by adsorption using a reactive amine solution. The main issue with this is amine degradation in the presence of oxygen, forming carcinogenic compounds such as nitrosamines. 
     Currently there are no known CO 2  capture systems suitable for CO 2  sources with intermediate CO 2  concentrations such as 10 000 ppm. 
     The capture of CO 2  from air is under development by some players. One example is the contacting of aqueous potassium hydroxide with air. The hydroxide reacts with CO 2  to form potassium carbonate. A major challenge with this is the complexity and energy needed to regenerate the potassium hydroxide solution. Competing adsorption systems, under development to reduce energy requirements, typically use quite complex chemical solutions. Commercializing such systems typically takes a long time. Alternatives include adsorption systems 
     WO 2013075981 A3 describes a method for extracting CO 2  from air by adsorption on a solid adsorbent. The solid sorbent is functionalized using amine compounds that react with CO 2 . These chemicals enhance the adsorption capacity and reduce adsorbent sensitivity to humidity. However, during regeneration of the adsorbent, the amine compounds are exposed to hot air with high concentrations of oxygen, causing potential degradation to toxic and possibly carcinogenic products. CO 2  from such sources can therefore not be used in the enclosed space of a greenhouse. Performance during long term operation is uncertain, and functionalized solid sorbents may not be commercially available. Among non-functionalized adsorbents that are available commercially zeolites, as used in WO2018/034570 are among the most promising 
     Zeolites have some very significant advantages. CO 2  capture is extremely fast even from dilute sources such as air. Zeolites consist of pure aluminium and silicon oxides. These are inert and safe compounds, much as natural rocks. Zeolites are commercially available from numerous manufacturers and proven. They are suitable for DAC and also CO 2  sources with higher CO 2  concentrations than air. 
     One disadvantage with zeolites is the affinity for H 2 O. H 2 O is preferentially adsorbed. If the zeolite contains more than 2 to 4 weight % H 2 O, the capacity to adsorb CO 2  is reduced. However, if there is moisture in the air or gas containing CO 2 , this moisture will be adsorbed quickly at the zeolite bed inlet. Further into the zeolite bed CO 2  may still be adsorbed. The overall effect is a slightly reduced capacity to store CO 2 , while the ability to quickly adsorb CO 2  further into the zeolite bed is less affected. 
     From WO 2019/238488 and U.S. Pat. No. 4,536,197 there are known methods for capturing CO 2  in CO 2  adsorbent beds involving the use of heat recovery units for transferring heat to and from said beds. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention is to provide a method and an arrangement for efficient capture of CO 2 , temporarily store the captured CO 2  and release the CO 2  as nearly 100% CO 2  following the adsorption sequence. The invention shall not introduce any new contaminants in the produced CO 2  or in the exhaust air. An additional object is to reduce energy consumption, in particular high value energy such as electric or high temperature (above 80 to 100° C.) to an absolute minimum. Furthermore, the adsorbent shall be used as efficiently as possible, requiring the least amount for a pre-defined CO 2  capture capacity. Beyond this, the latest commercially available technologies, including air handling which is developing rapidly to reduce energy consumption, shall be utilized to the extent possible. The invention shall have the capability to work with varying CO 2  concentrations in the incoming gas, from as low as 50 ppm (which may be desirable in greenhouses during the night) via 400 ppm as in ambient air and 10 000 ppm as in vegetable storage facilities, to 40 000 ppm as in flue gas from combustion engines. 
     These objects are achieved in a method and arrangement as defined in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in detail in reference to the appended drawings, in which: 
         FIG.  1    is a schematic diagram showing one implementation of the invention, with air pre-treatment, CO 2  adsorption and energy conservation measures. 
         FIG.  2    is a plot of adsorbent performance when capturing CO 2  from air and desorbing at CO 2  partial pressure near atmospheric pressure. 
         FIG.  3    is a schematic diagram of adsorbent containment with flow direction and temperature profiles during CO 2  adsorption and during adsorbent heating. 
         FIG.  4    is a schematic diagram of CO 2  containment with flow direction and temperature profiles during CO 2  desorption and during adsorbent cooling. 
         FIG.  5    is a schematic diagram of compact CO 2  adsorbent containment systems with adsorbent stacked vertically 
         FIG.  6    is a schematic diagram of heat storage with air or CO 2  flow directions during storage of coldness and delivery of heat, and during storage of heat and delivery of coldness. 
         FIG.  7    is a schematic diagram showing one implementation of the invention, with air pre-treatment, dual CO 2  adsorption facility and energy conservation measures. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the present description and claims the terms “humidity” and “absolute humidity” are used as a measure of the true water vapour content of air (g/m3). The term “relative humidity” of an air-water vapour mixture is used as a measure of the ratio of the actual partial pressure of water vapour in the air to the partial pressure of water vapour in the air if the air had been saturated at the temperature in question. The term “CO 2  concentration” is a measure of the number of molecules of CO 2  in the air relative to the total number of gas molecules in the air. It is measured in ppm or parts per million. 
     The pressure is herein given in the unit “bara” is “bar absolute”. Accordingly, 1.013 bara is the normal atmospheric pressure at sea level. In SI units, 1 bar corresponds to 100 kPa. 
     The expression “ambient temperature” as used herein may differ with the climate for operation of a closed or semi-closed system served by a process according to the present invention. Normally, the ambient temperature is from about 0 to 40 C, but the ambient temperature may also be from sub-zero levels to somewhat higher than 40° C., such as 50 C. 
       FIG.  1    is a principle overall sketch of a system according to a preferred embodiment of the present invention. Part  1  is a process for air cooling and de-humidification and re-heating after CO 2  capture, comprising two desiccant wheels and a heat exchanger illustrated as a heat exchange wheel. The desiccant wheels are for air de-humidification. The heat exchange wheel is for air re-heating after CO 2  capture, preserving energy. Part  2  is the CO 2  capture system with zeolite bed, CO 2  desorption gas circulation system, heat storage and CO 2  storage. 
     Part  1 , air cooling and de-humidification with subsequent re-heating, comprises an air inlet conduit  1  where ambient air enters the process driven by a not shown fan. This air may be pre-cooled in a cooler  3  before proceeding to a first desiccant wheel  9 . The desiccant wheel comprises a rotating cylinder, typically 10 to 30 cm thick, where the air passes over a water adsorbent such as silica gel. The cylinder has two sections  19  and  8 , the first used for air dehydration and the second used for adsorbent regeneration. The sections are shown as 270° for dehydration and 90° for regeneration, but this is for illustration purposes and may vary depending on system design. As an example, 180° for dehydration and 180° for regeneration may also be used. In the section  19  the air gets in contact with or in close proximity to the adsorbent. The adsorbent physically adsorbs humidity according to known equilibria between amount of humidity adsorbed and partial pressure of humidity in the air. Typically, and depending on air temperature and relative humidity, about two thirds of the moisture in the air will be adsorbed in the desiccant wheel  9 , section  19 . The air pressure drop through the desiccant wheel is very low, in the order of 100 Pa. The slow rotation of the wheel, perhaps 10 revolutions per hour, continually moves the humid adsorbent exposed to air from the moisture adsorption section  19  to the regeneration section  8  and after regeneration back to the section  19 . 
     Downstream the desiccant wheel  9  the air flows in a conduit  20  via a cooler  16  and a conduit  52  to a second desiccant wheel  14 . Similar to the desiccant wheel  9 , this cylinder may be 10 to 30 cm thick and there is a 270° moisture adsorption section  22  and 90° regeneration or desorption section  13 . Silica gel is typically used as desiccant. In the section  22  most of the humidity in the air from the desiccant wheel  9  is removed. The partial pressure of H 2 O in the air exiting the wheel, conduit  17  leading to the process part  2 , shall be such that no water precipitation or ice forms in downstream low temperature processes. Typically, this means water dew point in the range −30 to −60° C., preferably about −50° C. 
     Dry air returning from the process part  2 , conduit  25 , is slightly colder than the air going to the process part  2 , the conduit  17 . This air flows via a small side draw, a conduit  21 , typically zero or a few percent of the air in the conduit  25 , and then in a conduit  24  to a second side draw, a conduit  5 . Air flow in the conduit  5  may be from about one third to two thirds or more of the air in the conduit  25 . This dry air flows to a heater  6  where it is heated to typically 50 to 100° C., with preferred temperature in the range 60 to 80° C. After heating, this air flows in a conduit  7  to the regeneration section  13  of desiccant wheel  14 . This desorbs water from the H 2 O adsorbent and thus shifts the equilibrium in the desiccant wheel, section  13 , towards lower amounts of H 2 O in the adsorbent and more humidity in the air. The remaining amount of humidity in the adsorbent is such that, as the adsorbent moves with the rotating wheel into the water adsorption section  22 , the adsorbent is capable of reducing the H 2 O dew point in the conduit  17  to desired values, about −30 to −60° C. Energy for H 2 O removal in the section  13 , essentially vaporization energy for the H 2 O removed from the adsorbent, is supplied as sensible heat in the air from the heater  6 . As H 2 O is removed from the adsorbent, the air temperature drops by about 25° C. and exits the desiccant wheel, a conduit  12 , at roughly 35 to 55° C. 
     Downstream the desiccant wheel  14  the regeneration air flows in the conduit  12  to a point of mixing with small amounts of extra air from the conduit  21 . This reduces the relative humidity of the resulting air mixture, which next flows in a conduit  11  to a heater  10 . In the heater  10  the air is trim heated, as required, to desired temperature which, similar to air in the conduit  12 , is in the range 35 to 55° C. or higher such as 60 to 100° C. 
     After the trim heating the air flows in a conduit  15  to the adsorbent regeneration section  8  of desiccant wheel  9 . In the regeneration section H 2 O is removed from the desiccant. This shifts the equilibrium in the desiccant wheel, section  13 , towards lower amounts of H 2 O in the adsorbent and more humidity in the air. The shift progresses to a level sufficient for required air dehydration as the desiccant moves with the revolving wheel into the adsorption section  19 . Similar to the desiccant wheel  14 , the regeneration energy in desiccant wheel  9  is supplied as sensible heat in regeneration air from the conduit  15 . The amount of energy required is determined by the vaporization energy of the H 2 O removed from the adsorbent. 
     Moist regeneration air, at a temperature slightly higher than the temperature of the ambient air, is returned to the atmosphere. Excess dry air from the process part  2 , a conduit  2 , is also returned to the atmosphere or may alternatively be utilized in a not shown vaporization chilling unit for the supply of low temperature coolant to for example coolers  3  or  16 . 
       FIG.  1    part  2  shows a process for the reception of dehydrated air from part  1 , cooling of this air, adsorption and desorption of CO 2 , re-heat of the dehydrated air and return of this air to the process part  1 . Air from the conduit  17  is cooled in a heat exchange wheel  18 , section  23 . The air flows through passages in the wheel and gets in close contact with cold substance, such as a metal, in these openings. This cools the air and at the same time heats the wheel heat storage substance. The wheel rotates slowly, thus moving heated heat storage substance from partition  23  to partition  26  of the wheel, where the wheel heat storage material is re-cooled by cold air from a conduit  33 . In this process, the air from the conduit  33  is heated. It exits the heat exchange wheel  18 , section  26 , in the conduit  25 . People skilled in the art will understand that the heat exchange wheel  18  can be replaced by an air-air heat exchanger. 
     Cooled air exits the heat exchange wheel  18  in a conduit  28 . The temperature is typically in the range −25 to −45° C. Next, the air is trim cooled in a heat exchanger  29  by heat exchange with a coolant provided by a not shown heat pump. After trim cooling, in a conduit  38 , the air is about 2 to 5° C. colder than in the conduit  28 . This air may bypass downstream equipment via a valve  30 , enabling the continued operation of the upstream air dehydration and cooling process whenever the downstream CO 2  capture process does not need air, such as during CO 2  desorption. 
     The process downstream of the conduit  38  has four operating modes. These are CO 2  adsorption at low temperature in an adsorbent bed  34  located in a container  35 , heating of the adsorbent bed, desorption of CO 2  from the adsorbent bed and re-cooling of the adsorbent bed before the cycle is repeated. 
     During CO 2  adsorption cold, dehydrated air flows via a conduit  39 , a valve  31  and a conduit  32  to the adsorbent bed  34 .  FIG.  2    shows typical range for low temperature and high adsorbent loading operation,  63 , of this operation. The adsorbent bed adsorbs virtually all CO 2  from the air. The duration of the adsorption process may be from less than one and up to several hours.  FIG.  3   , part  1 , graph  59  shows spatial variation in the air temperature within the adsorbent bed during CO 2  adsorption. The temperature is low, in the range from −10 to −50° C., and increases slightly, perhaps by roughly 1° C., from the bed inlet to the bed outlet. The air flow, which is large especially when adsorbing CO 2  from lean air as opposed to for example flue gas, flows downwards through the CO 2  adsorbent beads. This prevents fluidization of the beads. 
     CO 2  depleted air from the adsorbent bed  34  is directed via the exit conduit  36  and a valve  37  to the return conduit  33 . 
       FIG.  5   , parts  1  and  2  shows compact designs of the adsorbent beds and adsorbent container  35 . Air from the conduit  32  flows to two adsorbent beds stacked in a vertical direction, arranged for parallel air flow, such that equal fractions of the total air flows through each bed. An air exit conduit  36  may be on the opposite side of the air inlet flow  32 , as shown in  FIG.  5    part  1 , or, for very compact designs, on the same side as shown in part  2 .  FIG.  5    shows two adsorbent beds arranged vertically, but larger numbers of vertically stacked beds such as 3, 4, 5 or more is possible. 
     After completion of the first operation mode, CO 2  adsorption, the process switches to the second operating mode. The adsorbent bed is heated to the temperature required for CO 2  desorption.  FIG.  2    shows the range  64  of preferred new high temperature and low loading area. 
     The heating is accomplished by gas, a mixture of air and CO 2 , flowing from the warm side  58  of a combined high and low temperature heat storage system  54 ,  55 ,  56 ,  57  and  58 . This gas flows via a conduit  53 , a conduit  46  and trim heater  41 , a conduit  42 , the valve  37  and the conduit  36  to the adsorbent bed  34 , thus heating the adsorbent bed. The flow direction through the adsorbent bed is the opposite of the direction used during the first operation mode, adsorption. 
     When a gas such as air flows through a bed of CO 2  adsorbent, comprised of adsorbent beads with diameter typically in the range 4 to 6 mm, the heat transfer area between the beads and the air is very large. The heat is therefore transferred rapidly from the air to the beads. This transfer occurs and is completed in a temperature transition zone within the adsorbent bed.  FIG.  3    part  2  illustrates this. A transition zone  67  receives warm air from below. When the warm air encounters colder adsorbent beads, the beads are warmed and the air is cooled. Thus, at the inlet of the transition zone, the adsorbent beads reach nearly the same temperature as the warm incoming air. In the transition zone the air transfers heat to the beads and is cooled in the process. At the end of the transition zone the air has been cooled to nearly the same temperature as the original low bead temperature, and retains this low temperature as it flows through the remaining adsorbent bed.  FIG.  3   , part  2 , graph  61  shows this. Thus, as more heat is supplied with hot incoming air, the transition zone is moving from the adsorbent bed inlet towards the bed outlet, graph  61 ′. This transition zone movement through the bed may take from minutes up to an hour or more, and the volume of air that has flowed through the adsorbent bed will have been many times larger than the volume of the beads being warmed. 
     The key phenomenon is that the adsorbent beads are heated to nearly the temperature of the warm incoming air, thus preserving the valuable high temperature heat of the air. The air going out of the adsorbent bed will be at the low temperature originally in the bed, thus preserving the valuable low temperature heat originally in the adsorbent bed, until the transition zone reaches the end of the bed. It is thus possible to store this coldness for later use. As shown in  FIG.  6    part  1  this is done in the stacked heat storage beds  54  to  58 , located within a container  66 . The cold heat transfer gas flows via the conduit  32 , the valve  31  now adjusted for the second operating mode, via a fan  43 , a conduit  44 , through a valve  49  to a conduit  51 . 
     The stacked heat storage bed receives cold air from the conduit  51 . The beds contain metal or ceramic heat storage material with large surface area such as beads with diameters 3 to 10 mm. There are several beds,  5  shown in  FIGS.  1  and  6   , to minimize heat transport from cold end, near the conduit  51  to warm end, near the conduit  53 , when heat and coldness are both stored. 
     The cold air and CO 2  from the conduit  51  flows through bed  54 , then to the bed  55  which contains an upwards moving temperature transition zone  65 , with temperature profile as shown in graph  69 . This transition zone could also be in beds  54 ,  56 ,  57  or  58  but not at the top end of the bed  58 . This depends on the size of the heat storage and the system operation. People skilled in the art will also understand that the exact shape of the transition zone depends on air flow rate end temperature, heat transfer to and from the heat storage medium and the amount and heat capacity of heat storage medium. 
     Air from the transition zone is warm. As the air flows through beds  56 ,  57  and  58  it stays warm and thus supplies heat to the CO 2  adsorbent bed  34  via the conduits  53  and  46 , the trim heater  41 , the conduit  42 , the valve  37  and the conduit  36 . The overall effect of this process is to move valuable coldness from the adsorbent bed  34  to the heat storage  66 , while at the same time moving heat from the heat storage  66  to the adsorbent bed  34 . This continues until the CO 2  adsorbent bed is warm and ready for the third operating mode. 
     After completion of the second operating mode, CO 2  adsorbent heating, the third operating mode, CO 2  desorption, starts. Warm air from the adsorbent  34  flows via the conduit  32 , in opposite way of the arrow shown in  FIG.  1   , via the valve  31  to the conduit  40 . Similar to the second operating mode, valve  31  allows flow from conduit  32  to conduit  40 , but is closed to conduit  39 . The gas flows from the conduit  40  via the fan  43  and the conduit  44  to the valve  49 . This valve is adjusted such the gas can flow to a conduit  45  but not to the conduit  51 . Gas from the conduit  45  is directed via conduit  46  to the trim heater  41 . Small amounts of heat are provided in the heater  41  to compensate for CO 2  desorption heat requirement, typically around 30 kJ/mole CO 2 . In addition, any water remaining in air from the adsorbent wheel  14  that has been adsorbed in the adsorbent  34  is removed and requires some extra heat, about 4 kJ/g. The temperature profile within the adsorbent bed  34  during CO 2  desorption is indicated in  FIG.  4   , part  1 . The temperature is high and nearly constant especially towards the end of the desorption cycle, as shown in graph  60 . CO 2  desorption proceeds until the temperature is in the range 260 to 300° C. as indicated in  FIG.  2   , area  64 . 
     During operating mode three, when CO 2  is desorbed, large volumes of CO 2  are supplied from the CO 2  adsorbent to the gas phase. In order to keep the system pressure constant, and to preserve the CO 2  for later use, the produced CO 2  is directed via a conduit  48 , a valve  50  and a cooler  65  to CO 2  storage  47 . This storage may preferably be of the inflatable type. 
     People skilled in the art may notice, from  FIG.  2   , that some CO 2  may be released from the adsorbent bed  34  during the second operating mode as the adsorbent is heated. This CO 2  is also directed to the storage  47 . 
     After completion of the third operation mode, CO 2  desorption, the process switches to the fourth operating mode, adsorbent bed cooling. The adsorbent bed is cooled to the temperature required for CO 2  adsorption. This procedure is somewhat similar to the second mode of operation, adsorbent bed heating, but the gas now flows in the opposite direction. The cooling is accomplished by gas, a mixture of air and CO 2 , flowing from the cold side  54  of the combined high and low temperature heat storage system  54 ,  55 ,  56 ,  57  and  58 . This gas flows via the conduit  51 , the valve  49 , the conduit  44  and the fan  43 , via the conduit  40 , the valve  31  and the conduit  32  to the adsorbent bed  34 . The flow direction through the adsorbent bed is the same as used during the first operation mode, adsorption. Similar to operating mode  2 , instead of gradually cooling the whole adsorbent bed the bed will be cooled to nearly the temperature of the incoming cold gas in initially a heat transfer zone near the gas inlet into the bed. 
     This temperature transition effect is shown in  FIG.  4   , part  2 , graphs  62  and  62 ′. Graph  62  shows the temperature transition zone at some time into the heating process, graph  61 ′ at a later time. This transition zone occurs initially at the bed inlet. As cooling progresses, it moves gradually into the bed until it finally reaches the end at the bottom of the bed. As  FIG.  4    part  2 , graph  62  shows, gas from the CO 2  adsorbent transition zone is warm, very near the temperature of the adsorption bed before cooling started. This high temperature is preserved for later bed heating by directing the warm gas via the conduit  36 , through the valve  37  in a direction to the conduit  42  and the trim heater  41 , via conduits  46  and  53 , to the warm storage side of heat storage system  54  to  58 . 
     In a similar manner as in the adsorbent bed  34 , this incoming warm air pushes air through the heat storage system. As the air reaches a cold-hot transition zone, shown in  FIG.  6    part  2 , transition zone  65 ′, it is cooled to nearly the temperature required for the cooling of the adsorbent bed  34 .  FIG.  6    shows the transition zone in heat storage bed  57  but could typically be in the heat storage beds  56  or, towards the end of the adsorbent cooling operation, bed  55  or some distance into bed  54  but not at the end of this bed. Below the transition zone the air is cold. The cold air flows via the conduit  51 , the valve  49 , the conduit  44 , the fan  43 , the conduit  40 , the valve  31  and the conduit  32  back to the adsorbent bed  34 . This moves the transition zone here further into the bed. Completing of the adsorbent bed cooling is accomplished when the air has done many such circulations between the heat storage beds  54  to  58  and adsorbent bed  34  and thus has carried enough coldness from the beds  54  to  58  to the CO 2  adsorption bed  34 , moving the transition zone to the end of the bed. Simultaneously, all the high temperature heat from CO 2  adsorbent bed  34  is transported to the warm end of heat storage units  54  to  58 . 
     The warm-cold transition zone in the heat storage system has been pushed from the warm end bed  58  towards the cold end bed  54  but not through the cold end bed  54  all the way to the outlet of this unit, the conduit  51 . The direction of movement of the warm-cold transition zone is shown in  FIG.  6    part  2 , transition zone  65 ′ and graph  70 . 
     After completion of the fourth operating mode, the first operating mode can start, repeating the cycle. 
       FIG.  7    shows a second embodiment of the invention. A second CO 2  adsorption unit  35 ′ is connected in parallel to the first unit  35 . The operation of the second unit is similar to the operation of the unit  35 . Operation may be by alternating CO 2  adsorption between the two units. For example, one of the units, for example the unit  35 ′, is in the first operating mode, receiving cold and dry air through a conduit  39 ′, a valve  31 ′ and a conduit  32 ′. After adsorption, CO 2  and H 2 O depleted air is routed via a conduit  36 ′ and a valve  37 ′ to the conduit  33 . During this operation, the other unit,  35 , goes through operating modes two, three and four. In this way, the cold and dry air supply may operate continuously and may therefore be smaller in order to supply a specified amount of CO 2 . In addition, in CO 2  adsorbent beds the heat storage may be smaller than would be required according to  FIG.  1   . 
     Example 
     This example will follow the four operating modes. As before, these are CO 2  adsorption at low temperature in an adsorbent bed  34  located in a container  35 , heating of the adsorbent bed, desorption of CO 2  from the adsorbent bed and re-cooling of the adsorbent bed before the cycle is repeated. The initial state before the CO 2  adsorption starts is the same as the state after the completion of the fourth operating mode. The CO 2  adsorbent bed has been cooled and high temperature heat is stored in the heat storage  54  to  58 . Furthermore, the CO 2  adsorbent contains about 15 g residual CO 2  per kg CO 2  adsorbent from a previous run. The example refers mainly to the  FIG.  1   . 
     It is desirable to produce about 600 kg CO 2  corresponding to 80 kg per hour over 7.5 hours. In the first operating mode, CO 2  adsorption, 30 kg/s ambient air at 15° C., 95% relative humidity and containing 400 ppm CO 2 , corresponding to 80 kg CO 2  per hour, is forced through the conduit  1  by a not shown fan. There is no cooling in the cooler  3  and the total H 2 O flow in this stream is about 1334 kg/h. In the desiccant wheel  9  about 854 kg/h H 2 O is adsorbed and thus removed from the air flow. Air from the desiccant wheel, the conduit  20 , now contains about 480 kg/h H 2 O. This corresponds to a water dew point of about 0° C. The adsorption of the H 2 O in the desiccant wheel is exothermic and the temperature of the air in the conduit  20  is about 33° C. This air is cooled to 15° C. in the cooler  16  and then forwarded via the conduit  52  to the desiccant wheel  14 , section  22 . 
     In the desiccant wheel  14  about 477 kg/h H 2 O is removed from the air. The remaining 3 kg/h flows with the air in the conduit  17 . This corresponds to a water dew point of roughly −50° C. and the temperature is about 25° C. This air is cooled to −40° C. in the heat exchanger wheel  18  and further to −45° C. in the trim cooler  29 . This trim cooler is operated by a not shown heat pump. Next, the air flows via the valve  31  to the CO 2  adsorbent bed  34  where virtually all CO 2 , 80 kg/h, and virtually all H 2 O, 3 kg/h, are adsorbed. The CO 2  and H 2 O depleted air next flows via the valve  37  and the conduit  33  to the heat exchange wheel  18  where it is heated to about 20° C. 
     This air flows in conduits  25  and  24 , with no side draw in the conduit  21 . One third of the air, about 10 kg/h, flows via the conduit  5  to the heater  6  where it is heated to about 90° C. Subsequently it flows via the conduit  7  to the desiccant wheel  14 , regeneration section  13 . The air exits the regeneration section  13  in the conduit  12  at a temperature of about 60° C. The H 2 O flow with the air is about 477 kg/h. The air is then re-heated to 90° C. in the heater  10  and enters the desiccant wheel  9 , regeneration section  8  via the conduit  15 . Downstream the regeneration, the conduit  4 , the temperature has dropped to about 39° C. and the H 2 O flow with the air is about 1331 kg/h. 
     The table below shows a summary of the desiccant wheel operation. The stream numbers refer to  FIG.  1   , part  1 . Silica gel is used as H 2 O adsorbent. The objective is to show that in the example, air entering and exiting the desiccant wheel regeneration sections has a lower amount of water adsorbed in the silica gel than the air entering and exiting the adsorption sections. Therefore, the regeneration air will regenerate the desiccant wheel adsorbent by removing water. In this process, the temperature of the air drops from the air inlet to the air outlet. As the table shows, when comparing streams  7  and  17 ,  12  and  52 ,  15  and  20  and  4  and  1 , the equilibrium amount of H 2 O adsorbed in the desiccant wheel silica gel is lower into and out of regeneration sections than out of and into, respectively, H 2 O adsorption sections. People skilled in the art will however notice that the operation of the desiccant wheels is not optimized, especially with respect to the use of heat at the lowest possible temperature for adsorbent regeneration. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                   
                   
                 Adsorbed in 
               
               
                   
                   
                   
                   
                 Pressure 
                 silica gel at 
               
               
                   
                 Air flow 
                 H 2 O flow 
                 Temperature 
                 H2O 
                 equilibrium 
               
               
                 Stream 
                 kg/s 
                 kg/h 
                 ° C. 
                 Pa 
                 moles/kg 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 7 
                 10 
                 &lt;&lt;3 
                 90 
                 &lt;&lt;14 
                 &lt;&lt;0.06 
               
               
                 17 
                 30 
                 3 
                 25 
                 5 
                 0.28 
               
               
                 12 
                 10 
                 477 
                 61 
                 2160 
                 4.24 
               
               
                 52 
                 30 
                 480 
                 15 
                 720 
                 12.5 
               
               
                 15 
                 10 
                 477 
                 90 
                 2160 
                 1.45 
               
               
                 20 
                 30 
                 480 
                 33 
                 720 
                 6.22 
               
               
                 4 
                 10 
                 1331 
                 39 
                 5700 
                 17.5 
               
               
                 1 
                 30 
                 1334 
                 15 
                 2020 
                 23.6 
               
               
                   
               
            
           
         
       
     
     Following operating mode one, operating mode  2  is initiated by adjusting the valve  31  such that there is free flow between the conduits  32  and  40 , but no flow into the valve from the conduit  39 . Instead, cold air may be bypassed via the valve  30  or the complete cold air supply may be stopped. 
     As indicated in  FIG.  2   , the CO 2  adsorbent will contain about 80 g CO 2  per kg adsorbent. After desorption of CO 2 , with a CO 2  partial pressure of 1.0 bar and desorption temperature about 280° C., there will be a residual amount of about 15 g CO 2  per kg adsorbent. In this example, the net amount of CO 2  adsorbed will be 65 g per kg CO 2 . People skilled in the art will understand that with lower partial pressure CO 2 , such as if diluted by air, or higher desorption temperature such as up to 300° C., the residual amount of CO 2  in the adsorbent can be much lower. 
     Based on 65 g CO 2  per kg adsorbent, the adsorption and storage of 600 kg CO 2 , about 9 metric tons of adsorbent is required. 10 metric tons will be assumed in this example, corresponding to about 12.5 m 3  adsorbent beads. Furthermore, the volume of the 30 kg/s air flow at −45° C. is about 23.5 m 3 /s. With superficial air velocity of 0.6 m/s the total area of adsorbent becomes roughly 40 m 2 ′ Combined with the adsorbent volume of 12.5 m3, the thickness of the adsorbent bed is about 0.31 m. 
     The heating of 10 metric tons of adsorbent, heat will be supplied from the heat storage  54  to  58 , stored in an earlier run, by flowing 30 kg/s gas from the heat storage  58  via the trim heater  41 , through the adsorbent bed  34  where the gas gives off heat to the adsorbent and is cooled to the adsorbent temperature of near −45° C. such as shown in  FIG.  3    part  2  graphs  61  and  61 ′. The cooled gas flows via the fan  43  to the cold end of the heat storage  54 - 58 , cooling the cold end and thus preserving the low temperature and pushing more hot gas from the heat storage  58  to the adsorbent bed  34 . 
     After completion of the CO 2  adsorbent heating, the third operating mode CO 2  desorption starts. CO 2  and air flow out of the CO 2  adsorbent  34  via the conduit  32 , the valve  31 , the conduit  40 , the fan  43  which enforces the gas flow and then via the conduits  45  and  46  to the trim heater  41  where heat is supplied for the CO 2  and any H 2 O desorption. The warmed gas then flows via the conduit  42 , the valve  37  and the conduit  36  to the adsorbent  34 . This continues until the required amount of CO 2 , 600 kg, is desorbed. The duration of this operation may in the order of one hour depending on the heat input in the heater  41  and the gas circulation rate. Desorbed CO 2 , about 600 kg, flows via the conduit  48 , the valve  50  and the cooler  65  to the CO 2  storage  47 . 
     After completion of the operating mode three, operating mode  4 , CO 2  adsorbent cooling, starts. This is similar to the operating mode  2 , but the air flowing between the heat storage  66  and the CO 2  adsorbent unit  35  now flows in the opposite direction, with cold gas flowing from the heat storage via the conduit  51  and downstream equipment to the CO 2  adsorbent  34 . Here, the gas is heated by remaining heat from the CO 2  desorption. This warm gas flows via the conduit  36  to the heat storage  66 , heating heat storage beds near the top by pushing a heat transfer zone towards the cold end of the heat storage. Simultaneously, cold gas from the cold end of the heat storage is pushed to the CO 2  adsorbent bed  34 , further cooling this bed. When the adsorbent bed is cold throughout, the operating mode four is completed and the system is again ready to start with operating mode  1 . 
     People skilled in the art will notice that the detail design of the air dehydration and cooling before CO 2  adsorption is dependent on ambient conditions. A cold environment and therefore low H 2 O content in the air may result in a much simpler system than shown in  FIG.  1    part  1 , for example one of the desiccant wheels may be omitted. 
     People skilled in the art will also notice that much energy can be saved by reducing the mean temperature difference in the heat exchanger wheel  18 , and that a normal heat exchanger may be used instead. This reduces the amount of cooling needed in the trim heater  29 . 
     In addition to this, people skilled in the art will understand that instead of zeolite adsorption beds for CO 2  and H 2 O, potentially more efficient and less H 2 O sensitive CO 2  adsorption systems such as amine functionalized alumina may be used, but this may introduce the disadvantage of potential degradation of chemicals and production of toxic substances. 
     Furthermore, it will be understood that while the dehydration and cooling of the air may seem equipment and energy intensive, the current emphasis on low emission has forced and is forcing the development of extremely efficient air dehydration and cooling systems. The desiccant wheel is an example. Such systems have been and will continue to be adapted in order to simplify the air pre-treatment.