Abstract:
The present disclosure relates to a method of operating a boiler system that includes an oxyfuel boiler in which an oxygen stream and a fuel stream are combusted to generate a stream of flue gas, an oxygen gas source producing the stream of oxygen for the boiler, and a gas processing unit for cleaning and compressing at least a portion of the stream of flue gas generated in the boiler for producing a stream of pressurized fluid comprising carbon dioxide. The method includes operating the boiler system, at least for a period of time, in an evaporation mode, in which a stream of oxygen from the oxygen gas source in liquid form is evaporated prior to being introduced in the boiler by transferring heat energy to the stream of liquid oxygen from a first stream of carbon dioxide of the gas processing unit. The present disclosure further relates to a boiler system for an oxy-fuel process as well as to a power plant comprising such a system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European Application 12182733.1 filed Sep. 3, 2012, the contents of which are hereby incorporated in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an oxy-fuel boiler system and a method of operating the system. The present disclosure also relates to an oxyfuel combustion power plant comprising such a system. 
       BACKGROUND 
       [0003]    In the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant, a hot process gas is generated, such process gas containing, among other components, carbon dioxide CO 2 . With increasing environmental demands various processes for removing carbon dioxide from the process gas have been developed. One such process is the so called oxy-fuel process. In an oxy-fuel process a fuel, such as one of the fuels mentioned above, is combusted in the presence of a nitrogen-lean gas. Oxygen gas, which is provided by an oxygen gas source, is supplied to a boiler in which the oxygen gas oxidizes the fuel. In the oxy-fuel combustion process a carbon dioxide rich flue gas is produced, which can be treated using various CO 2  capture technologies in order to reduce the emission of carbon dioxide into the atmosphere. 
         [0004]    CO 2  capture often comprises cooling, or compression and cooling, of the flue gas to separate CO 2  in liquid or solid form from non-condensable flue gas components, such as N 2  and O 2 . 
         [0005]    After purification and separation of carbon dioxide, a carbon dioxide rich stream is obtained and need to be handled, such as by storing and transportation in tanks (stationary or on a truck or ship), transporting via pipelines and/or pumping into the ground for prolonged (definitive) storage and mineralisation. 
         [0006]    Different components used in an oxy-fuel process may not always be used to their full capacity. Components downstream of the boiler are designed in view of the output from the boiler. Also, components upstream of the boiler are designed in view of the input needed in the boiler. Some of the apparatuses used in an oxy-fuel process are thus oversized since the oxy-fuel process not always is operated at full capacity all the time. 
         [0007]    There is always a need to improve the flexibility of an oxy-fuel process. It would be desirable to find new ways to lower overall energy consumption, scale down the size/capacity of the components and better utilize the components present in an oxy-fuel process. 
       SUMMARY 
       [0008]    By using heat energy from parts of a process and forward it to other parts where it is needed the overall energy consumption may be lowered. Also by using different heat exchanging systems depending on the load of a process increases the flexibility in the running of such a process. By, during periods of low load, accumulate oxygen, an important ingoing component for the combustion of fuel, the process may not need as high capacity oxygen gas supply as otherwise would be needed if it was to produce all oxygen needed during high load on the system. Thus, this presents an opportunity to decrease the capacity of the oxygen gas supply. Also, using heat energy available in the process the required energy input may be decreased and/or adjusted to periods of the day when the energy price is lower in order to save costs. 
         [0009]    A combination of load dependent heat transfer systems results in a decreased overall energy consumption, decreased operational costs for the process and decreased scale of some apparatuses needed. 
         [0010]    An object of the present invention is to provide a method of operating a boiler system comprising an oxyfuel boiler in which an oxygen and a fuel stream are combusted to generate a stream of flue gas, an oxygen gas source producing the stream of oxygen for the boiler, and a gas processing unit for cleaning and compressing at least a portion of the stream of flue gas generated in the boiler for producing a stream of pressurized fluid comprising carbon dioxide, the method comprising: 
         [0000]    operating the boiler system, at least for a period of time, in an evaporation mode, in which a stream of oxygen from the oxygen gas source in liquid form is evaporated prior to being introduced in the boiler by transferring heat energy to the stream of liquid oxygen from a first stream of carbon dioxide of the gas processing unit. 
         [0011]    According to one embodiment, the method further comprises operating the boiler system, at least for a period of time, in a condensing mode, in which a stream of gaseous oxygen from the oxygen gas source is condensed prior to being forwarded to a liquid oxygen storage unit by transferring heat energy from the stream of gaseous oxygen to a second stream of carbon dioxide of the gas processing unit. 
         [0012]    According to one embodiment, the method further comprises transferring, in the condensing mode, heat energy from the gaseous oxygen to the second stream of carbon dioxide which has the form of a stream ( 34 ) of pressurized carbon dioxide from the gas processing unit ( 45 ), wherein the pressurized carbon dioxide is made to expand before the transfer of heat energy. 
         [0013]    According to one embodiment, the method further comprises: 
         [0000]    establishing whether the boiler system operates at a first load or at a second load,
 
wherein the second load is a lower load than the first load,
 
controlling the boiler system to operate in the evaporation mode when the boiler system operates at the first load, and controlling the boiler system to stop operation in the evaporation mode when the boiler system operates at the second load.
 
         [0014]    According to one embodiment, the method further comprises: 
         [0000]    establishing whether the boiler system operates at a third load or at a fourth load,
 
wherein the fourth load is a lower load than the third load,
 
controlling the boiler system to operate in the condensing mode when the boiler system operates at the fourth load, and
 
controlling the boiler system to stop operation in the condensing mode when the boiler system operates at the third load,
 
wherein a stock of liquid oxygen is built up when the boiler system operates at the fourth load.
 
         [0015]    According to one embodiment, the fourth load is a lower load than the first load. 
         [0016]    According to one embodiment, the method further comprises controlling the boiler system to avoid, at least for a period of time, simultaneously operating in the evaporation mode and in the condensing mode, 
         [0000]    wherein a stock of liquid oxygen is built up in the condensing mode when operating at the fourth load, and that said stock of liquid oxygen is evaporated in the evaporation mode for use in the boiler when operating at the first load, wherein the fourth load is a lower load than the first load. 
         [0017]    According to one embodiment, the method comprises an intermediary heat transfer fluid which is utilized in the evaporation mode for transferring heat to the stream of liquid oxygen from the stream of carbon dioxide. 
         [0018]    According to one embodiment, the method further comprises transferring, in the evaporation mode, heat directly to the liquid oxygen from the stream of carbon dioxide. 
         [0019]    Another object of the present invention is to provide a boiler system comprising an oxyfuel boiler in which a stream of oxygen and a fuel are combusted to generate a stream of flue gas, an oxygen gas source producing the stream of oxygen for the boiler, a gas cleaning system for cleaning at least a portion of flue gas generated in the boiler, a flue gas condenser for condensing the cleaned flue gas, a gas processing unit for further cleaning and compression of the flue gas for producing a stream of pressurized fluid comprising carbon dioxide, said boiler system further comprising: 
         [0000]    a first heat exchanging system, operative at least for a period of time in an evaporation mode, in which a stream of oxygen from the oxygen gas source in liquid form is evaporated prior to being introduced in the boiler by transferring heat energy to the stream of liquid oxygen from a first stream of carbon dioxide of the gas processing unit. 
         [0020]    According to one embodiment, the boiler system further comprises a second heat exchanging system operative at least for a period of time in a condensing mode, in which a stream of gaseous oxygen from the oxygen gas source is condensed prior to being forwarded to a liquid oxygen storage unit by transferring heat energy from the stream of gaseous oxygen to a second stream of carbon dioxide of the gas processing unit. 
         [0021]    According to one embodiment, the boiler system further comprises a controlling device which controls the boiler system to operate in the evaporation mode or condensation mode based on a measured load on the boiler system. 
         [0022]    According to one embodiment, the boiler system further comprises an oxygen storage unit for storing oxygen from the oxygen storage unit in liquid form. 
         [0023]    According to one embodiment, the second heat exchanging system comprises a heat exchanger in which a compressed liquid carbon dioxide stream is evaporated by heat energy transferred from condensation of a gaseous oxygen stream from the oxygen gas source. 
         [0024]    Another object of the present invention is to provide an oxy-fuel combustion power plant comprising said boiler system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Referring now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike: 
           [0026]      FIG. 1  is a schematic view of a boiler system  1  in an oxy-fuel process, disclosing one embodiment a heat transfer system  46 . 
           [0027]      FIG. 2  is a schematic view of a boiler system  1  in an oxy-fuel process, disclosing another embodiment a heat transfer system  46 . 
           [0028]      FIG. 3  is a schematic view of a boiler system  1  in an oxy-fuel process, disclosing yet another embodiment a heat transfer system  46 . 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0029]    The present method establishes the load at which the boiler system operates. By providing limit values on the load for when the evaporation mode and the condensing mode, respectively, are to be active the method is switching between the modes based on a load value measured in the process during operation. The evaporation mode is to be active during a high load on the boiler system. The load on the boiler system is considered high when the load is at least 75% of maximum capacity of the boiler, i.e. 75-100%, preferably 95-100%. The condensation mode is to be active during a low load on the boiler system. The load on the boiler system is considered low when the load is below 75% of maximum capacity of the boiler, e.g. 0-74%, preferably 25-60%. 
         [0030]    The load on the boiler system could be measured and controlled using the oxygen demand or flowrate to the boiler. Additionally, the operation may be influenced by a demand of emptying the O 2  buffering vessel of the present system, which vessel contains oxygen to be fed to the boiler. However, the operation is not necessarily measured and controlled only by the oxygen demand but also the operating point of the CO 2  compressor(s) in the gas processing unit. This operating point is indicating whether condensation of O 2  can be done. The load of big centrifugal compressors is measured typically by flow control. Below 75% load such machines are operated using gas recirculation to prevent damages to the equipment from surge conditions. 
         [0031]    As an example, a low load on the boiler system involves a low O 2  demand from the boiler, but with the present method the CO 2  compressor may operate at &gt;75% capacity and O 2  for storage is made available. 
         [0032]    The limit values of the boiler system to operate in the evaporation mode are set as a first and a second load. The limit value for a first load is set to at least 75% of maximum capacity of the boiler, at which the boiler system activates and operates in the evaporation mode. The limit value for a second load is set to below 75% of maximum capacity of the boiler. At which the boiler system deactivates the evaporation mode. 
         [0033]    The limit values of the boiler system to operate in the condensation mode are set as a third and a forth load. The limit value for a third load is set to at least 75% of maximum capacity of the boiler, at which the boiler system deactivates the condensation mode. The limit value for a forth load is set to below 75% of maximum capacity of the boiler, at which the boiler system activates and operates in the condensation mode. 
         [0034]    The limit values for the first and third load may be different or the same. The limit values for the second and forth load may be different or the same. 
         [0035]    The mode of operation of the boiler system is controlled by a controlling device, such as a computer, microprocessor or controller, which compares the value of a measured current load with the set limit values and then regulate the process accordingly. 
         [0036]    In order to exemplify the use of the different operation modes, evaporation and condensation mode, an example of an embodiment of the present method of operation is given below. If the oxygen gas source  26  have a maximum production capacity of 80 kg/h of O 2  and the boiler  4  have a capacity during full operation (high load) to use 100 kg/h of O 2  but during low load only uses 50 kg/h. The boiler  4  is lacking 20 kg/h of O 2  during high load but during low load 30 kg/h of O 2  which is available is not needed. Thus by providing an oxygen storage unit, excess oxygen during periods of low load may be stored to be used when the load increases. 
         [0037]    When the evaporation mode is in operation in the boiler system the heat transfer used in this mode may be done in different ways. Liquid oxygen may be provided from an oxygen gas source, which may be an air separation unit, via a heat exchanging device condensing an oxygen gas stream supplied within or outside the air separation unit, or from a liquid oxygen storage unit. The liquid oxygen may be evaporated using heat energy transferred from a stream of carbon dioxide of the gas processing unit. The heat energy may be transferred by using a heat exchanging medium which transfers the heat energy from e.g. a carbon dioxide stream in a carbon dioxide condenser by cooling and optionally condensing said carbon dioxide stream and forwarding the heat energy to the liquid oxygen and evaporating it into gaseous oxygen. The heat exchanging medium may be any medium useful for transferring heat energy. The heat exchanging medium may not need to be condensed and evaporated itself during the heat transfer. Another way to transfer the heat energy between the oxygen stream and the carbon dioxide stream could be to forward the liquid oxygen into a carbon dioxide condenser directly, generating evaporation of the liquid oxygen stream and cooling and optionally condensing the carbon dioxide stream in the carbon dioxide condenser. Yet another way could be to forward a part of a carbon dioxide stream from a carbon dioxide condenser in the gas processing unit to a heat exchanging unit in which heat energy is transferred from condensation of said carbon dioxide stream to evaporation of the liquid oxygen entering the heat exchanging unit. The gaseous oxygen then obtained is forwarded to the boiler for use in the combustion of fuel. The oxygen streams of the boiler system are controlled, e.g. in terms of temperature or flow. The liquid and/or gaseous oxygen streams in the boiler system is be forwarded by controlling the flow of oxygen in a per se known manner. 
         [0038]    By the term “carbon dioxide rich” used throughout the application text is meant that the gas stream referred to contains at least 40% by volume of carbon dioxide (CO 2 ). 
         [0039]    The heat transfer process and the system involved will now be disclosed more in detail with reference to  FIGS. 1-3 . It is to be noted that not all streams or controlling means needed to operate an oxy-fuel process are disclosed in the figures. The  FIGS. 1-3  are focusing on the main flow of the fuel becoming a CO 2  stream, which then is purified, cooled, separated and compressed but also on the provision of an additional flow of oxygen to the fuel combustion, which flow is dependent of fluctuations in process load, in order make the oxy-fuel process more flexible in terms of energy resource allocation, apparatus scaling and capacity. 
         [0040]      FIG. 1  is a schematic representation of a boiler system  1 , as seen from the side thereof. The boiler system  1  comprises, as main components, a boiler  4 , being in this embodiment an oxy-fuel boiler, and a gas cleaning system  6 . The gas cleaning system  6  comprises a particulate removal device, which may, for example, be a fabric filter or an electrostatic precipitator, and a sulphur dioxide removal system, which may be a wet scrubber. 
         [0041]    A fuel, such as coal, oil, or peat, is contained in a fuel storage  2 , and can be supplied to the boiler  4  via a supply stream  3 . An oxygen gas source  26  is operative for providing oxygen gas in a manner which is known per se. The oxygen gas source  26  may be an air separation unit operative for separating oxygen gas from air, an oxygen separating membrane or any other source for providing oxygen gas to the boiler system  1 . Oxygen in the form of a gas stream  50  from the oxygen gas source  26  is continuously, during operation of the boiler  4 , directly fed into the boiler. In order to make the boiler system  1  more flexible an additional flow of oxygen is provided between the oxygen gas source  26  and the boiler  4 . An oxygen storage unit  30 , e.g. a tank, for storage of liquid oxygen is placed between the oxygen gas source  26  and the boiler  4 . The produced oxygen gas to be feed to the boiler  4 , comprises typically 90-99.9 vol. % oxygen, O 2 . A re-circulation of flue gas (not shown), which contains carbon dioxide, to the boiler  4  is provided in the boiler system  1 . The re-circulation of flue gas may be taken from stream  9 . The re-circulation of flue gas and the oxygen gas may become mixed with each other to form a gas mixture containing typically about 20-50% by volume of oxygen gas, the balance being mainly carbon dioxide and water vapour, upstream of the boiler  4 . The boiler  4  is operative for combusting the fuel, which is supplied via the supply stream  3 , in the presence of the oxygen gas. Oxygen gas is continuously during operation fed directly from oxygen gas source  26  to boiler  4  as a stream  50  but may also depending on the load on the boiler system  1  be supplied by streams  27 ,  29 ,  31 ,  33 . The flow of oxygen of streams  50 ,  27 ,  29 ,  31  and  33  may be controlled by a controlling system which may e.g. comprise computer, micro processor, controller, valves, actuators and/or pumps, which system is not shown in the figures for the purpose of maintaining clarity of the illustration. Controlling the flow of oxygen is done in a per se known manner. A part of the oxygen produced in the oxygen gas source  26  may not directly fed into the boiler  4 . When the boiler  4  is not operating at full capacity i.e. during a low load on the boiler system, the oxygen gas source  26  may still be operating at full capacity and forwarding the oxygen not needed in the boiler  4  via a stream  27 ,  29  to an oxygen storage unit  30 . Thus, the excess part of oxygen produced in the oxygen gas source  26  is stored in the oxygen storage unit  30 . When the load on the boiler system  1  then increases and the boiler  4  operates at full capacity the oxygen stored in the oxygen storage may be used and fed into the boiler  4  via streams  31 ,  33 . The excess of oxygen gas during low load on the boiler system  1  is forwarded via stream  27  to a heat exchanging unit  28 , wherein the oxygen gas is condensed and forwarded as stream  29  into the oxygen storage unit  30 . During high load on the boiler system  1  the stored liquid oxygen is forwarded via stream  31  through a heat exchanging unit which vaporizes the oxygen and the oxygen gas is forwarded in stream  33  into the boiler  4 . 
         [0042]    A stream  5  is operative for forwarding carbon dioxide rich flue gas generated in the boiler  4  to the gas cleaning system  6 . By “carbon dioxide rich flue gas” is meant that the flue gas leaving the boiler  4  via the stream  5  will contain at least 40% by volume of carbon dioxide, CO 2 . Often more than 50% by volume of the flue gas leaving the boiler  4  will be carbon dioxide. Typically, the flue gas leaving boiler  4  will contain 50-80% by volume of carbon dioxide. The balance of the “carbon dioxide rich flue gas” will be about 15-40% by volume of water vapour (H 2 O), 2-7% by volume of oxygen (O 2 ), since a slight oxygen excess is often preferred in the boiler  4 , and totally about 0-10% by volume of other gases, including mainly nitrogen (N 2 ) and argon (Ar), since some leakage of air can seldom be completely avoided. 
         [0043]    The carbon dioxide rich flue gas generated in the boiler  4  may typically comprise contaminants in the form of, for example, dust particles, hydrochloric acid, HCl, sulphur oxides, SO X , and heavy metals, including mercury, Hg, that should be removed, at least partly, from the carbon dioxide rich flue gas prior to disposing of the carbon dioxide. 
         [0044]    The gas cleaning system  6  removes in different steps most of the dust particles from the carbon dioxide rich flue gas and also sulphur dioxide, SO 2 , and other acid gases from the carbon dioxide rich flue gas. 
         [0045]    An at least partly cleaned carbon dioxide rich flue gas is forwarded from cleaning system  6  via a stream  7  to a flue gas condenser  8 . From the flue gas condenser  8  via a stream  9  the flue gas is forwarded to a gas processing unit (GPU)  45  in the form of a gas compression and purification unit of the boiler system  1 . In the GPU  45  the cleaned carbon dioxide rich flue gas is further cleaned and is compressed for disposal. Compressed carbon dioxide hence leaves the GPU  45  via a stream  24  and is transported away for disposal, which is sometimes referred to as “CO 2  sequestration”. 
         [0046]    The cleaned carbon dioxide rich flue gas enters the GPU  45  via the stream  9  and is introduced into the flue gas compression unit  10 , optionally comprising intercooling and separation steps. A stream  11  forwards the compressed gas from the flue gas compression unit  10  to a trace substance removal unit  12 , which removes any trace components still present in the stream, e.g. by adsorption or absorption to remove mercury and other substances. 
         [0047]    The GPU  45  further comprises a drier unit  14 , e.g. an adsorption drier, operative for removing at least a portion of the content of water vapour of the flue gas. The drier unit  14  may comprise more than one drier. 
         [0048]    The drier unit  14  is arranged downstream of trace substance removal unit  12  connected by stream  13 , but upstream of a CO 2  condenser unit  16  connected by stream  15 . 
         [0049]    The adsorption drier contains an adsorbent or desiccant capable of adsorbing water molecules from a gas stream. The desiccant may, for example, be silica gel, calcium sulfate, calcium chloride, montmorillonite clay, molecular sieves, or another material that is, as such, known for its use as a desiccant. The molecular sieves have a pore size suitable for adsorption of water, e.g. molecular sieves having a pore size in the range of 3 to 5 Å. 
         [0050]    The GPU  45  may optionally comprise a flue gas economizer (not shown) arranged between the trace substance removal unit  12  and the drier unit  14  and configured to recover heat from the flue gas stream leaving the trace substance removal unit using, e.g. boiler feed water. 
         [0051]    The cleaned carbon dioxide rich flue gas is forwarded via a stream  15  from drier unit  14  to a CO 2  condenser unit  16 , in which the gas is cooled in a heat-exchanger, often called a cold-box, to cause liquefaction of the carbon dioxide such that the carbon dioxide can be separated from gases, such as nitrogen, that are not liquefied at the same temperature as carbon dioxide. The liquefied carbon dioxide is forwarded to a CO 2  separation unit  18  by a stream  17 . For example, a heat exchanger, also called a cold box, of the CO 2  separation unit  16  may often be made from aluminium. Residual water may cause formation of ice in the cold box, eventually resulting in problems with reduced cooling capacity and clogging of the heat exchanger. By providing a drier unit  14  upstream of the CO 2  separation unit  18 , such problems are avoided, or at least minimized. 
         [0052]    Furthermore, the GPU  45  may comprise a high pressure compression unit  23  arranged downstream, as seen with respect to the transport direction of the carbon dioxide, of the CO 2  separation unit  18 , and comprising one or more compression stages for compressing the carbon dioxide to a suitable pressure for a following sequestration. After compression of the gas in the high pressure compression unit  23 , the compressed carbon dioxide, which may be in a supercritical or subcritical liquid state, may be forwarded, via stream  24 , for further use. The compressor unit  23  comprises at least one compressor having at least one, and typically two to ten compression stages for compressing the liquefied carbon dioxide. Each compression stage could be arranged as a separate unit. As an alternative, several compression stages could be operated by a common drive shaft. The high pressure compression unit  23  may be run under subcritical conditions or supercritical conditions. 
         [0053]    A heat transfer system  46  comprises two heat exchanging systems  47 ,  48  which operate at different modes. The first heat exchanging system  48  is active during high load in the boiler system  1 . 
         [0054]    In the first heat exchanging system  48 , a stream  31  comprising liquid oxygen is to be evaporated before entry as an evaporated stream  33  into the boiler  4 . Stream  31  is evaporated in a heat exchanger  32 , in which heat energy is transferred and evaporates the oxygen, resulting in the evaporated stream  33  which is forwarded into the boiler  4 . The heat energy transferred in the heat exchanger  32  is recovered from a heat exchanging media circulated between heat exchanger  32  and the CO 2  condenser unit  16 . The heat exchanging media is retrieves heat energy in the CO 2  condenser unit  16  by aiding the condensation of the CO 2  in stream  15 , which than is transferred to the heat exchanger  32  and there aids the evaporation of the liquid oxygen stream  31 . The heat exchanging media is forwarded using a pumping device  38 . The pumping device  38  and a regulating device  39 , e.g. a valve, are used to control the flow of the second heat exchanging system  48  using a controlling device  41 . 
         [0055]    The second heat exchanging system  47  is active during low load in the boiler system  1 . In the second heat exchanging system  47  a stream  34  is taken from outgoing stream  24  comprising compressed CO 2 . The stream  34  is forwarded to a heat exchanger  28 . Into the heat exchanger is forwarded stream  34  and stream  27 . Heat energy from stream  34  comprising compressed CO 2  is used do condense the stream  27 , resulting in an evaporated CO 2  stream  36  which is forwarded back to stream  21 . The condensed stream  29  is then forwarded into an oxygen storage unit  30 . The flow of the stream  34  is controlled by a controlling device  41  using a valve  35 . 
         [0056]    The second heat exchanging system  47  is active during low load in the boiler system  1 . 
         [0057]      FIG. 2  is a schematic representation of a boiler system  1 , as seen from the side thereof. The boiler system  1  comprises the same features as in previously mentioned  FIG. 1  apart from the construction of the heat transfer system  46 . 
         [0058]    The heat transfer system  46  comprises two heat exchanging systems  47 ,  48  which operate at different modes. The second heat exchanging system  47  is the same as disclosed for  FIG. 1  above. 
         [0059]    In the first heat exchanging system  48 , a stream  31  comprising liquid oxygen is to be evaporated before entry as an evaporated stream  33  into the boiler  4 . Stream  31  is forwarded to the CO 2  condenser unit  16 , in which heat energy is transferred and evaporates the oxygen, resulting in the evaporated stream  33  which is forwarded into the boiler  4 . The heat energy transferred to the oxygen stream  31  is recovered from the condensation reaction taking place in CO 2  condenser unit  16 . 
         [0060]      FIG. 3  is a schematic representation of a boiler system  1 , as seen from the side thereof. The boiler system  1  comprises the same features as in previously mentioned  FIGS. 1 and 2  apart from the construction of the heat transfer system  46 . 
         [0061]    The heat transfer system  46  comprises two heat exchanging systems  47 ,  48  which operate at different modes. The second heat exchanging system  47  is the same as disclosed for  FIG. 1  above. 
         [0062]    In the first heat exchanging system  48 , a stream  31  comprising liquid oxygen is to be evaporated before entry as an evaporated stream  33  into the boiler  4 . Stream  31  is evaporated in a heat exchanger  32 , in which heat energy is transferred and evaporates the oxygen, resulting in the evaporated stream  33  which is forwarded into the boiler  4 . The heat energy transferred in the heat exchanger  32  is recovered from condensation of a CO 2  containing stream  42  taken from stream  21  leaving the CO 2  condenser unit. The condensed CO 2  stream  43  leaving the heat exchanger  32  is forwarded to and added to stream  24  leaving the compressor unit  23 , forming stream  25  to be forwarded for further use. The condensed CO 2  stream  43  is forwarded by use of a pumping device  44  which is controlled by a controlling device  41 . 
         [0063]    The load on the boiler system  1  is not always constant. Thus, during periods of low load, e.g. during night time, the boiler system  1  is using a heat exchanging system  47 . During periods of low load, e.g. below 75%, the compression unit  23  work partly on recirculation. Therefore a surplus of cold energy may be extracted out of the stream  24  coming from the compression unit  23  or any recirculation streams from it. The surplus of cold energy may then be used to liquefy oxygen gas produced in an oxygen gas source  26 . Also, during periods of low load part of the formed liquefied oxygen is stored in an oxygen storage unit  30 . The stored oxygen may then be used when the load increases on the boiler system  1 . During periods of high load, e.g. during day time (normal working hours), the boiler system  1  is using a heat transfer system  48 . Heat energy is transferred to evaporate liquid oxygen stream  31  before entering the boiler  4  as stream  33  is performed, using streams connected with either the CO 2  condenser unit  16  or the CO 2  compression unit  23 . 
         [0064]    The difference in load on the boiler system  1  triggers the use of either the first the heat exchanger system  48  or the second heat exchanger system  47 . A measuring unit could be connected to the boiler  4 , measuring the load on the boiler  4 . When the load on the boiler  4  reaches a specified value the heat transfer system  46  changes mode from the first heat exchanger system  48  to the second heat exchanger system  47 , or the other way around. When the load is high, e.g. at least a load of at least 75% on the boiler  4 , the first heat exchanger system  48  is activated to facilitate more oxygen being fed into the boiler  4 . When the load is lower, e.g. below a load of 75% on the boiler  4 , the second heat exchanger system  47  is activated to facilitate oxygen being stored in oxygen storage unit  30 , which oxygen will be fed into the boiler  4  when the load increases and the first the heat exchanger system  48  is activated. 
         [0065]    This combination of heat transfer systems results in a decreased overall energy consumption. The use of the surplus of cold energy for the liquefaction of oxygen also influences the design of the oxygen gas source  26 , since it may be scaled down. The design of CO 2  condenser unit  16  may also be influenced due to the evaporation energy transferred from the oxygen stream, since heat energy needed for the condensing of CO 2  may be taken from other parts of the process. Thus, by using heat energy from other parts of the process the overall energy consumption is decreased. Also by adapting the boiler system process to different load cycles the process may be optimized in view of energy consumption. 
         [0066]    It will be appreciated that numerous variants of the embodiments described above are possible within the scope of the appended claims. 
         [0067]    While the invention has been described with reference to a number of preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.