Abstract:
A method for generation of electrical power mainly from a coal based fuel, where the combustion gas is separated into a CO 2  rich stream and a CO 2  poor stream in a CO 2  capturing unit, the CO 2  poor stream is released into the surroundings, and the CO 2  rich stream is prepared for deposition or export, is described. A plant for executing the method and a preferred injector for the plant, is also described

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates a method for generation of electrical power mainly from a coal based fuel, where the combustion gas is separated into a CO 2  rich stream which is exported e.g. for safe deposition, and a CO 2  poor stream that is released into the surroundings. The invention additionally relates to a plant for performing the method and a part of the plant. 
       BACKGROUND 
       [0002]    The concentration of CO 2  in the atmosphere has increased by nearly 30% in the last 150 years, mainly due to combustion of fossil fuel, such as coal and hydrocarbons. The concentration of methane has doubled and the concentration of nitrogen oxides has increased by about 15%. This has increased the atmospheric greenhouse effect, something which has resulted in:
       The mean temperature near the earth&#39;s surface has increased by about 0.5° C. over the last one hundred years, with an accelerating trend in the last ten years.   Over the same period rainfall has increased by about 1%   The sea level has increased by 15 to 20 cm due to melting of glaciers and because water expands when heated up.       
 
         [0006]    Increasing discharges of greenhouse gases is expected to give continued changes in the climate. The temperature can increase by as much as 0.6 to 2.5° C. over the coming 50 years. Within the scientific community, it is generally agreed that increasing use of fossil fuels, with exponentially increasing discharges of CO 2 , has altered the natural CO 2  balance and is therefore the direct reason for this development. 
         [0007]    It is important that action is taken immediately to stabilise the CO 2  content of the atmosphere. This can be achieved if CO 2  generated in a thermal power plant is collected and deposited safely. It is assumed that the collection represents three quarters of the total costs for the control of CO 2  discharges to the atmosphere. 
         [0008]    Discharge gas from thermal power plants typically contains 4 to 10% by volume of CO 2 , where the lowest values are typical for gas turbines, while the highest values are only reached in combustion chambers with cooling, for example, in production of steam. 
         [0009]    Capturing of CO 2  from CO 2  containing gas by means of absorption is well known, see e.g. EP 0 551 876. The CO 2  containing gas is here brought into contact with an absorbent, usually an amine solution which absorbs CO 2  from the gas. The amine solution is thereafter regenerated by heating the amine solution. The absorption is, however, dependent on the partial pressure of CO 2 . If the partial pressure is too low, only a relatively small part of the total CO 2  is absorbed. Normally the partial pressure of CO 2  in combustion gas is relative low, for gas turbines a value of 0.04 bar is typical. The energy consumption in such a plant is about 3 times higher per weight unit CO 2  than if the partial pressure of CO 2  in the feed gas is 1.5 bar. The cleaning plant becomes expensive and the degree of cleaning and size of the power plant are limiting factors. 
         [0010]    Therefore, the development work is concentrated on increasing the partial pressure of CO 2 . According to WO 00/48709, the combustion gas that has been expanded over a gas turbine and cooled, is re-pressurized. The pressurized gas is then brought in contact with an absorbent. In this way, the partial pressure of CO 2  is raised, for example to 0.5 bar, and the cleaning becomes more efficient. An essential disadvantage is that the partial pressure of oxygen in the gas also becomes high, for example 1.5 bar, while amines typically degrade quickly at oxygen partial pressures above about 0.2 bar. In addition, costly extra equipment is required. 
         [0011]    Another possibility to raise the partial pressure of CO 2  is air separation. By separating the air that goes into the combustion installation into oxygen and nitrogen, circulating CO 2  can be used as a propellant (for gas turbines) or as a cooling gas (for coal fired boilers) in gas turbine combined cycle or coal fired power plants, respectively. Without nitrogen to dilute the CO 2  formed, the CO 2  in the exhaust gas will have a relatively high partial pressure, approximately up to 1 bar. Excess CO 2  from the combustion can then be separated out relatively simply so that the installation for collection of CO 2  can be simplified. However the total costs for such a system becomes relatively high, as one must have a substantial plant for production of oxygen in addition to the power plant. Production and combustion of pure oxygen represent considerable safety challenges, in addition to great demands on the material. This will also most likely require development of new turbines. 
         [0012]    From WO 2004/001301 it is known to let the combustion take place under elevated pressure, cool down the combustion gas by generation of steam, split the combustion gas in a CO 2  rich stream for deposition, and a CO 2  poor stream, and expanding the CO 2  poor stream over a turbine before it is released into the atmosphere. The plant in question is however a gas powered plant, and there is no mentioning of the use of coal as a fuel. 
         [0013]    WO 2004/026445 relates to a method for separation of combustion gas from a thermal gas fired power plant into a CO 2  rich stream and a CO 2  poor stream. The combustion gas from the power plant is here used as an oxygen containing gas in a secondary combined power plant and separation plant. 
         [0014]    The methods described above mostly relates to natural gas fired power plants. Today, however, coal is a more widely used fuel for thermal power plants than natural gas. Coal fired thermal power plants do, additionally, produce more CO 2  per unit of electrical power than plants based on natural gas. Additionally, coal is an easy available and compared with natural gas, less expensive fuel. 
         [0015]    Introduction of a coal based fuel, such as pulverized coal, into a pressurized combustion chamber is connected with technical challenges. Using air as a propellant for the coal dust will give an explosive mixture that will cause the combustion to start before entering the combustion chamber, an may even result in an explosion in the means for mixing air and coal or in connecting lines or in the combustion chamber. Using an inert gas as nitrogen would be another possibility but purification of nitrogen would add unacceptable cost to the plant. Additionally, addition of nitrogen would increase the total gas flow and result in a reduced partial pressure of CO 2  in the combustion gas, which is disadvantageous for the separation of CO 2 . 
         [0016]    According to the so-called PFBC process, pulverized coal is mixed with water to give a paste-like mixture that is squeezed into the combustion chamber. The water-coal paste mixture is required in order to pump the fluid and thereby overcome the boiler combustion pressure. The water in the paste will vaporize with resulting loss of efficiency. In order to fire the water-coal paste, a fluid bed combustor is required. This is large and expensive equipment. In addition, the fluid bed gives a significant pressure drop, in the order of 2 bar. This reduces the downstream turbine power. 
         [0017]    Accordingly, there is a need for a cost effective method for generation of electrical power from a coal based fuel where the combustion gas is split into a CO 2  rich stream for deposition and a CO 2  poor stream that may be released into the atmosphere. 
         [0018]    According to a first aspect, the present invention relates to a method for generation of electrical power mainly from a coal based fuel, where the coal based fuel and an oxygen containing gas is introduced into a combustion chamber and combusted at an elevated pressure, the combustion gases are cooled down in the combustion chamber by generation of steam for production of electricity, the combustion gas is further cooled down and separated into a CO 2  rich stream and a CO 2  poor stream in a CO 2  capturing unit, the CO 2  poor stream is reheated and expanded over a turbine to produce electrical power, before the CO 2  poor stream is released into the surroundings, wherein the CO 2  rich stream is split into a stream for deposition or export, and a stream that is recycled to the combustion chamber. In the combustion chamber, the recycled CO 2  is used to bring the pulverized coal into the combustion zone. If the pulverized coal is fed into the boiler by air instead of CO 2 , there is severe explosion hazard. By use of CO 2  instead of air, the explosion hazard is removed. Additionally, the pressure drop mentioned for fluidized bed reactors, is eliminated. 
         [0019]    According to a preferred embodiment, at least a portion of the CO 2  rich stream that is recycled to the combustion chamber is mixed with the coal based fuel before introduction to the combustion chamber and is injected into the combustion chamber together with the coal based fuel. The CO 2  rich stream that is recycled to the combustion chamber may be used to fluidize the fuel in the tanks in the intermediary storage means, to avoid that settled coal fuel may hinder the injection into the combustion chamber. Additionally, the CO 2  rich stream may be used as a propellant for the fuel to force the fuel from the tank into the combustion chamber. 
         [0020]    The CO 2  poor stream is preferably heated by heat exchanging against combustion gas from a secondary combustion chamber fired by gas, before the CO 2  poor stream is expanded over a turbine. This is done to optimize the energy output from the plant and increase the part of the electricity that is produced by expansion of this stream before it is released into the surroundings. 
         [0021]    The pressure in the combustion chambers may be from 5 to 35 bar, preferably 10 to 20 bar, more preferably from about 12 to about 16 bar. The absorption of CO 2  in the CO 2  capturing device is more effective at an elevated pressure than at a lower pressure. Combustion at an elevated pressure delivers combustion gas at an elevated pressure to the capturing device without energy consuming compressors. By keeping the combustion chamber nearly fully fired, the mass flow of flue gas to be purified is minimized, and the concentration and hence the partial pressure of CO 2  are thus maximized. 
         [0022]    It is preferred that the temperature in the combustion gas leaving the combustion chamber, is reduced to below about 350° C. by production of steam. By keeping the temperature in the combustion gas leaving the combustion chamber below 350° C., normal quality steel may be used in the equipment for further handling of the gas. Additionally, a high energy output is taken out as steam that is used for production of electric energy. 
         [0023]    According to an embodiment, natural gas in introduced into the combustion chamber to support the combustion. The combustion becomes more effective when supported by addition of natural gas. 
         [0024]    According to a second aspect, the invention relates to a thermal power plant mainly fired with a coal based fuel, the thermal power plant comprising a combustion chamber, means for introducing the coal based fuel and an oxygen containing gas into the combustion chamber, cooling means for cooling the combustion gas in the combustion chamber and means for separation of the combustion gas into a CO 2  rich stream and a CO 2  poor stream, wherein the power plant additionally comprises a line for recirculation of a part of the CO 2  to the combustion chamber and a CO 2  line for delivering the remaining CO 2  rich stream for deposition or export. 
         [0025]    The cooling means are preferably cooling coils inside the combustion chamber, where the cooling coils are cooling the combustion gas by generation of steam. Cooling coils inside the combustion chamber are effective in cooling the combustion gases at the same time as steam for generation of electric power is produced. 
         [0026]    Preferably, the thermal power plant further comprises a steam turbine connected to a generator for the production of electrical power. 
         [0027]    According to a preferred embodiment, a secondary combustion chamber fired by gas, for generation of heat for heating the CO 2  poor stream, and turbine for expanding of the heated CO 2  poor stream before it is released into the surroundings, are employed. Heating of the stream before it is released into the surroundings, adds energy to the gas. As a result the production of electrical power from the expansion of the CO 2  poor stream over a turbine becomes more efficient and improves the total efficiency of the plant. 
         [0028]    It is preferred that the turbine for expansion of the CO 2  poor stream is connected to a generator for production of electrical power. 
         [0029]    According to a third aspect the invention relates to an injector for a coal based fuel and an oxygen-containing gas into a pressurized combustion chamber, comprising a central pipe for injection of a mixture of pulverized coal based fuel and CO 2  gas, surrounded by a plurality of injectors for oxygen containing gas. The construction of the injector having a central tube for injection of the coal and CO 2  surrounded by injectors for oxygen containing gas ensures rapid and intimate mixing of the coal based fuel and the oxygen containing gas. This rapid and intimate mixing of the fuel and oxygen containing gas ensures optimal combustion in the combustion chamber. 
         [0030]    According to a preferred embodiment, the injector additionally comprises one or more gas injectors for injection of natural gas. Addition of additional fuel in the form of natural gas may be used both in starting up the combustion and for maintenance of the combustion. Combustion of natural gas in the combustion chamber results in a better and more optimal combustion of the coal as the additional heat ensures that lighter components in the coal evaporates and are more effectively combusted. 
         [0031]    Helically ribs may additionally be provided inside the central pipe. The helical ribs will cause the mixture of coal based fuel and CO 2  have a vortex motion out of the central tube. This motion ensures even better mixing of the coal based fuel, the oxygen containing gas and any added natural gas. 
         [0032]    According to one embodiment, the gas injectors are orientated so the gas rotates the opposite way relative to the coal powder. Rotating of the gas and coal powder opposite relative to each other ensures optimal mixing of gas and coal powder. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  is a schematic diagram of a preferred embodiment of the invention; 
           [0034]      FIG. 2   a ) illustrates a longitudinal section through an injector according to the invention; 
           [0035]      FIG. 2B  illustrates the section A-A in  FIG. 2   a;    
           [0036]      FIG. 3  illustrates an exemplary grinding and intermediate fuel storage device for the plant according to the invention; 
           [0037]      FIG. 4  is a longitudinal section through a combined heat exchanger and secondary combustion chamber for plant according to the invention; 
           [0038]      FIG. 5  is a schematic diagram of an intermediate fuel storage device and means for taking care of CO 2 ; and 
           [0039]      FIG. 6  is a schematic diagram of an exemplary CO 2  capturing unit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    An exemplary embodiment of a thermal power plant fired by natural gas and coal is illustrated in  FIG. 1 . Coal and optionally limestone, are introduced into a coal mill  12  through a coal line  10  and a lime stone line  11 , respectively. The coal and the optional limestone, are milled to a ground mixture in the coal mill  12  to a particle size suitable for feeding into a combustion chamber. 
         [0041]    The ground coal and optional limestone are carried on a conveying means  13  to intermediate storage means  14 . The intermediate storage  14  in the illustrated embodiment comprises two or more storage units, each unit operated in a batch wise manner. Two or more units are necessary to give a continuous operation of a combustion chamber. 
         [0042]    Each intermediate storage unit comprises an inlet valve  15 , a storage tank  16  and an outlet valve  17 . Additionally, each unit comprises one or more inlets for CO 2  coming in from a CO 2 -line  18 . The ground mixture from the coal mill is conveyed to the intermediate storage device and filled into one storage tank at a time. The inlet valve  15  for the tank  16  to be filled is opened and the outlet valve  17  is closed. During or after filling of a tank  16 , air is preferably purged from the tank by means of CO 2  from the CO 2 -line  18  to avoid creation of dangerous mixtures of air and coal dust. 
         [0043]    The CO 2  is controlled by means of a CO 2  valve  19 . After filling the tank and purging air from the tank, the inlet valve  15  is closed. Before the mixture in the tank is to be introduced into a combustion chamber  25 , CO 2  is filled into the tank to give a pressure in the tank that is higher than the pressure in the combustion, for example 0.5 to 1 bar, such as 0.7 bar, higher. 
         [0044]    According to one embodiment, the CO 2  inlets in the tank are placed so that the mixture in the tank is at least partly fluidized by the incoming stream of CO 2 . The outlet valve  17  is thereafter opened and the mixture is led to an injector  21  through a line  20 . The mixture is introduced into the combustion chamber  25  by the injector  21  together with CO 2 , compressed oxygen containing gas from an air line  23  and optionally natural gas from a gas line  22 . The injector  21  is described in more detail below with reference to  FIG. 2 . Gas from the gas line  22  is used to promote the combustion in the combustion chamber and to adjust internal combustion therein. 
         [0045]    The oxygen containing gas may be air, oxygen enriched air or oxygen. The terms air and oxygen containing gas in the description and claims, used as synonyms to describe these possibilities. 
         [0046]    The combustion in the combustion chamber  25  occurs at an elevated pressure, for example from 5 to 25 bar, more preferred from about 10 to about 20 bar, and most preferably about 15 bar. 
         [0047]    Solid matter in the combustion chamber, such as non-combustible residues from the coal and calcium sulphate produced in binding of sulphur compounds in the combustion gases, is collected in the bottom of the combustion chamber and removed through a solids removal line  24 . 
         [0048]    The above described combustion chamber  25  is a presently preferred combustion chamber. The skilled man in the art will, however, understand that other constructions and principles of operation are possible. The described combustion chamber may, e.g. be substituted by a fluidized bed combustion chamber. 
         [0049]    A substantial amount of the heat produced from the combustion is removed from the combustion chamber by producing steam in cooling coils  9  inside the combustion chamber. Most of the heat is removed from the top of the combustion chamber to reduce the temperature of the combustion gas leaving the combustion chamber  25  through a combustion gas line  35 . 
         [0050]    The steam produced in the cooling coil  9  is removed from the combustion chamber through a steam line  26  and is expanded over a turbine  28  to produce electricity in a generator  27 . The expanded steam is led in a line  29  to a condenser  30 , where the expanded gas is cooled and condensed. The condensed water is pumped by a pump  31  and pre-heated by heat exchanging in a pre-heater  32  before the water again is introduced through a line  33  into the cooling coil  9  in the combustion chamber  25 . It must be noted that this circuit may be far more complex. The cooling coil  9  may be divided into two or more cooling coils each taking out a part of the heat to one or more steam turbines. 
         [0051]    The combustion gas leaving the combustion chamber  25  through the combustion gas line  35  has preferably a temperature of about 350° C., or lower. A temperature of less than 350° C. in the combustion gas leaving the combustion chamber makes it possible to use relatively inexpensive steel in the construction of lines and processing equipment, and reduces the building cost. 
         [0052]    The combustion gas in line  35  contains dust from the combustion chamber. This dust may be harmful for the further processing of the combustion gas. Accordingly, the dust has to be removed in a dust removal unit  36  comprising a plurality of cyclones and/or filters  38 . 
         [0053]    The illustrated dust removal unit  36  comprises two lines in parallel each comprising a number of cyclones and or filters in series. The unit may, however, comprise of more than two lines in parallel. To allow continuous operation of the dust removal unit, one or more of the parallel lines may be shut down for cleaning and service as long as at least one of the parallel lines are open and in operation at all times. 
         [0054]    The inlet side of one of the parallel lines may be closed by means of an upstream valve  37 , whereas the other side of the parallel lines, may be closed by a downstream valve  40 . Dust, separated in the cyclones and/or filters, is removed through dust removal lines  39 . 
         [0055]    From the dust removal unit, the dust free combustion gas is led via a line  41  to a selective catalytic reduction unit (SCR unit) for substantial reduction of NOx produced in the combustion chamber. In the SCR unit  42 , NOx can be removed with NH 3 , according to the reaction 3NO+2NH 3 =2.5N 2 +3H 2 O. This cleaning has up to 90% efficiency at atmospheric pressure, but is assumed to be much better at the working pressure which is typically above 10 bara. It will therefore be possible to clean NOx down to a residual content of 5 ppm or better. By adapting the heat exchangers, the gas can be given a temperature that is optimal for this process. Other known methods for NOx removal without using NH 3  may also be used. The NH 3  method has the disadvantage that it gives some NH 3  “slip”. 
         [0056]    The cleaned gas, is leaving the SCR unit in a line  43  and is cooled in a heat exchanger  44 . From the heat exchanger  44 , the gas is led into a condenser  47  in a line  46 . In the condenser, the gas cooled further down and condensed water is removed from the gas. The gas leaving the condenser is led to a CO 2  capturing unit  49  in a line  48 . 
         [0057]    Alternatively, a gas scrubber may be provided upstreams of the condenser. In the optional gas scrubber the gas is saturated with water vapor, and the gas is cooled by countercurrent contact with water at suitable temperatures. The scrubber may employ chemicals to oxidize and/or absorb multiple flue gas stream residuals including NOx, SOx, other acids or gases, and particulates. Such chemical may be the NH 3  “slip” from the SCR system which provides an alkaline solution, or a special chemical with alkaline and/or oxidizing properties. In the latter case, the scrubber may replace the SCR unit  42  completely. 
         [0058]    The purification of the flue gas is essential to minimize the formation of heat stable salts in the CO 2  capturing absorbent, and to minimize the degradation of CO 2  capturing performance with time. 
         [0059]    The CO 2  capturing unit typically comprises an absorber where the flue gas flows countercurrent to an absorbent such as an amine, hot carbonate or a physical absorbent. The amount of CO 2  in the flue gas is typically reduced by 90 to 99% in the absorber before the flue gas leaves the absorber as a CO 2  poor stream. The absorbent with absorbed CO 2  (rich absorbent) is heated in a solvent/solvent heat exchanger and regenerated in a stripper column. The regenerated solvent is cooled in the solvent/solvent exchanger, cooled in a trim cooler and returned to the CO2 absorption tower, whereas the CO 2  is removed from the stripper column as a CO 2  rich stream.  FIG. 6  illustrates an exemplary CO 2  capturing unit. The detailed design the unit will, however, depend on the type of solvent used. 
         [0060]    The CO 2  capturing unit  49  may be any kind of unit capable of splitting the partly cleaned combustion gas in a CO 2 -rich stream leaving the unit through a CO 2 -line  51 , and a CO 2 -poor stream leaving the unit through a line  50 . The CO 2 -rich stream in line  51  is compressed to a pressure of about 100 bar in a compressor  52  powered by a motor  53 . A part of compressed CO 2 -rich stream is leaving the compressor in line  54  and is recycled as a source of CO 2  for the intermediate storage means  14 . The remaining CO 2  is compressed further and is removed from the plant in a CO 2 -line  55 . 
         [0061]    The CO 2 -poor stream leaving the CO 2  capturing unit  49  through line  50  is introduced into a re-humidifier, where the gas is heated and saturated with water before it is led through a line  57  to the heat exchanger  44  where the CO 2 -depleted gas is heated against the hot gas in line  43 . Preferably, air or another suitable gas is introduced into line  57  (or alternatively line  50 ) through an air line  73  to make up for the mass of the CO 2  that has been removed from the combustion gas so that the heat capacity of the CO 2 -poor stream is approximately the same as the heat capacity of the combustion gas in line  43 . The air is taken into the system through an air intake  70  and is compressed by means of a compressor  71  powered by a motor  72 . As an alternative, some air from the compressor  78  may be by-passed the combustor  25  and downstream equipment, and introduced in line  50  or line  57 . (This is not shown in  FIG. 1 ). 
         [0062]    The heated CO 2 -poor stream leaves the heat exchanger  44  through a line  58  and is introduced into a heat exchanger  59  where the CO 2 -poor stream is heated against combustion air entering the heat exchanger in a line  82  from a secondary combustion chamber  81 . The secondary combustion chamber  81  is fired by natural gas from a gas inlet line  80 . Oxygen for the combustion in the secondary combustion chamber  81  is introduced into the secondary combustion chamber through a line  87 . 
         [0063]    The cooled down gas from the heat exchanger  59  leaves the heat exchanger in a line  86  that is introduced into the line  41  for CO 2 -removal. A part of the gas in line  86  may be taken out in a line  83  and recycled into line  82  by means of a fan  84  and a line  85 . The recirculation through line  83  is used to increase the mass flow of heated gas through the heat exchanger  59  from line  82 . If the heat exchanger is built of material that stand high temperature, such as up to 2000° C., the recirculation is superfluous. 
         [0064]    The heated CO 2 -poor stream leaving the heat exchanger  59  in a line  60 , is expanded over a turbine  61 . The expanded CO 2 -poor stream leaving the turbine  61  through a line  62  is cooled further in heat exchangers  63  before the gas stream is released into the atmosphere through a line  64 . The heat exchanger(s)  63  may be identical to the preheater  32 , preheating the water entering the cooling coils in the combustion chamber so that energy in the expanded CO 2 -poor stream is used to heat the water in the preheater  32 . 
         [0065]    Air for both the combustion chamber  25  and the secondary combustion chamber  81  is in the illustrated embodiment introduced to the system through an air intake  75 . The air in air intake  75  is compressed, preferably in a two step compressor, having two compressors  76  and  78  and an intercooler  77 . The compressed gas leaving the compressor  78  in a line  79 , is split into two streams into the air line  23  leading to the injector  21 , and into the second air line  87  leading into the secondary combustion chamber  81 . A leakage in the compressors  76 ,  78  and/or the turbine  61  is illustrated by a leakage line  88 . The compressors at the illustrated embodiment is placed on a shaft  66  that is common to both the compressors  76 ,  78 , the turbine  61  and a generator  65  for generation of electric power. As an alternative, there may be a two stage compressor  76 ,  78  (as shown) and a two stage turbine  61  (low pressure stage and high pressure stage)—not shown—such that the low pressure turbine drives the low pressure compressor  76 , and the high pressure turbine drives the high pressure compressor  78  plus the generator  65 . 
         [0066]      FIG. 2   a  represents a length section through the combustion chamber and a preferred embodiment of an injector  21 . The injector  21  is supported by a collar  101  welded to the wall of the combustion chamber. The injector is inserted into the collar  101  and fastened to the collar by means of a holding plate  100 . The injector comprises a central tube  102  for injection of coal, air injectors  103  and gas injectors  104  surrounding the central tube. The collar  101  is preferably cooled down by means of air from air inlet  109  circulating in a cooling jacket  106  surrounding the collar. Preferably the air heated by cooling the collar in the cooling jacket is led in a line  107  and is introduced into the air injectors  103  and injected into the combustion chamber. 
         [0067]    The mixture of coal, CO 2  and optionally lime stone entering the injector  21  through line  20 , is introduced into a central pipe  102 . The mixture is blown through the tube by means of pressurized CO 2  and injected into the combustion chamber. By using nozzles, as indicated in the figure, to inject the air into the combustion chamber, the venturi effect caused by the nozzles will cause an additional drag of material from the central pipe into the combustion chamber. 
         [0068]    The hot and burning gas/coal mixture leaving the injector  21  may be harmful to the wall of the combustion chamber and steam heating coils  9 . To avoid damage to the wall of the combustion chamber and steam heating coils  9 , a reflector plate  111  is arranged opposite the injector  21  for reduction of velocity of remaining unburned particles and avoid or reduce damages to the inner wall of the combustion chamber. Preferably, the reflector is cooled by means of CO 2  delivered through a gas line  110  being circulated trough cooling channels  112  at the rear side of the reflector plate. Normally, one reflector plate is arranged per injector if more than one injector is arranged in the wall of the combustion chamber. Alternatively, the reflector may be frustoconical having openings for the injectors. 
         [0069]      FIG. 2   b  illustrates the cross section A-A in  FIG. 2   a . The central pipe  102  is surrounded by a plurality of air injectors  103 . The gas injectors, for injection of natural gas introduced into the injector in gas line  22 , are in the illustrated injector, situated inside one or more of the air injectors. A plurality of helically shaped ribs  105  at the inner wall of the central pipe, causes the coal mixture to rotate and accordingly create turbulence in the combustion chamber. The creation of turbulence is important to assure proper mixing of the injected coal, gas and air to promote optimal conditions for combustion. 
         [0070]      FIG. 3  illustrates a combined mill and intermediate storage device  14 . Coal and lime stone are transported on conveying means  10 ,  11 ,  13  into a funnel  150  leading to a mill  12 . The funnel  150  has a plurality of internal flaps  151  for reduction of the coal/limestone feeding velocity into the mill  12 . The reduced feeding velocity will allow for optimum abatement of air. The mill  12  preferably comprises more than one mill, where the incoming coal and limestone firstly are introduced into a mill and thereafter into a fine mill to give the preferred particle size. 
         [0071]    The mill and lower part of the funnel is preferably purged by CO 2  entering from a purge line  152  to reduce the amount of oxygen or air that is carried with the coal and limestone, as a mixture of coal dust and oxygen may be explosive. The stream of CO 2  in the purge line is controlled by a valve  153 . 
         [0072]    From the mill, the coal and limestone dust is vertically fed by an Archimedes screw  13  to the tank  16 . A valve  15  inserted between the conveyor  13  and the tank  16  is used to close the inlet of the tank when the tank is full of coal and limestone dust. When the tank  16  is to be emptied into the combustion chamber, the valve  15  is closed, CO 2  is introduced into the tank at the top of the tank through a CO 2  line  154  controlled by a valve  155 , and/or through a CO 2  line  157  controlled by a valve  158 . The introduction of CO 2  either through the line  154  or line  157  will boost the pressure in the tank. The pressure in the tank is increased to a pressure that is higher than the pressure in the combustion chamber. Preferably, the pressure in the tank is from 0.5 to 1 bar higher than in the combustion chamber. Introduction of CO 2  through line  157 , close to the bottom of the tank, will at least partly fluidize the content of the tank. The valve  17  in line  20  is then opened, and the mixture of CO 2 , coal dust and limestone is forced through the line  20 , through the injector  21  and into the combustion chamber as described above. After the tank  16  is emptied, the valve  17  is again closed, valve  15  is opened, and the tank again filled with dust as described above. 
         [0073]      FIG. 4  illustrates a combined secondary combustion chamber and heat exchanger  200  to substitute for the secondary combustion chamber  81 , heat exchanger  59  and lines connecting them. This combination is more heat efficient and avoids or reduces the use of connection lines. 
         [0074]    Air and natural gas are introduced through an air line  203  and a gas line  202 , respectively, into a combustion chamber  201 . CO 2  is introduced from a CO 2  line  204  through a cooling jacket  205  to cool down the upper part of the combustion chamber, and is released into the combustion chamber to adjust the gas composition in the combustion chamber. The burning gas in the combustion is forced downwards in the combustion chamber and through openings  206  near the bottom of the combustion chamber. The warm flue gas from the combustion chamber is circulated in a flue gas chamber surrounding the combustion chamber. The hot flue gas in the flue gas chamber is cooled by heat exchange against the CO 2 -poor stream from line  58  entering the device through an inlet  212 . The CO 2  poor stream circulates in the circulation space defined between the outer wall of the flue gas chamber  207  and a heat exchanger shell  210 . 
         [0075]    The flue gas from the secondary combustion chamber  201  leaves the device through a flue gas outlet  208  and is introduced into line  86 . The heated CO 2  poor stream leaves the device through a heat exchanger outlet  213  into line  60 . The air to be introduced into air line  203  is preferably preheated by heat exchanging against the CO 2  poor stream, as the air is introduced into an air inlet to a jacket  216  surrounding at least a part of the heat exchange shell  210 . The heated air is removed through an air outlet  217  and is introduced into air line  203 . 
         [0076]    This combined combustion chamber and heat exchanger gives a more compact construction of the combined device. A high temperature difference over the wall separating the combustion chamber and the heat exchange part of the device, results in the need of a relatively small heat exchange area. 
         [0077]      FIG. 5  illustrates an embodiment of the intermediate storage means  14 , including storage means  250  for CO 2 . The CO 2  storage means  250  comprises a CO 2  storage tank  255 , a compressor  259  run by a motor  263 , a dust filter  252  and connecting lines  257  and  261 , and several valves  253 ,  254 ,  258 ,  260  and  262 , controlling the flow in the system. The CO 2  storage means  250  may be closed of from the intermediate storage means  14  by means of an optional valve  251 . 
         [0078]    When CO 2  under pressure in the tank  255  is to be filled into one of the tanks  16 ,  16 ′ or  16 ″, the valve connected to the tank  16 , i.e.  248 ,  248 ′ or  248 ″ is opened. The valves  256  and  262  are then opened to allow the gas in tank  255  flow through the lines  256 ,  261  and  249 ,  249 ′ or  249 ″. When the flow from tank  255  into tank  16 ,  16 ′ or  16 ″ declines due to lower pressure difference, valve  256  is closed, valves  254 ,  260  and  258  are opened and the CO 2  from the tank  255  is compressed by the compressor  259  until the pressure in the tank  255  is about atmospheric pressure. All valves  253 ,  254 ,  256 ,  258 ,  260 ,  262  and  248  are subsequently closed. 
         [0079]    To fill excess CO 2  from a tank  16 ,  16 ′ or  16 ″, into the tank  255 , the corresponding valve  248 ,  248 ′ or  248 ″ is opened. The CO 2  is then allowed to flow through the filter  252  from the tank  16 ,  16 ′ or  16 ″ into the tank  255  by opening valves  253  and  254 . As soon as the flow decreases due to reduced difference in pressure between the tanks, valve  254  is closed, the valves  260 ,  258  and  256  are opened and the gas from the tank  16 ,  16 ′ or  16 ″ is compressed and led to tank  255  for temporary storage. When the pressure in the tank  16 ,  16 ′ or  16 ″ is about atmospheric pressure, all the valves  248 ,  248 ′,  248 ″,  253 ,  254 ,  256 ,  258 ,  260  and  262  are closed. 
         [0080]    It is obvious for the skilled man that CO 2  may be introduced or removed from the tank  16  through any CO 2  lines into the tank, such as line  154 ,  157  or  18  and that line  249  is illustrative and may cover any of the mentioned lines alone or in combination. 
         [0081]      FIG. 6  illustrates an exemplary and somewhat simplified CO 2  capturing unit  49 . The cooled down combustion gas enters the unit  49  through line  48  and is introduced into an absorber  300  near the bottom. The cleaned combustion gas leaves the absorber  300  in line  50  close to the top of the absorber. An absorbent, such as an amine or hot carbonate solution, is introduced into the absorber through a line  301  close to the top of the absorber, and leaves the absorber as a rich absorbent (rich in CO 2 ) through a line  302  close to the bottom of the absorber. The countercurrent flow of gas to be cleaned and absorber through the absorber ensures optimal conditions for absorption of CO 2 . 
         [0082]    The rich absorbent in line  302  is heated in a heat exchanger  303  against regenerated (lean) absorbent before the rich absorbent is introduced into a stripping column  305  close to the top thereof. The temperature in the stripping column is higher and the pressure is lower than in the absorber  300 , causing CO 2  to be released from the absorbent. CO 2  released from the absorbent is removed from the stripping column through a CO 2  line  306 . The CO 2  in line  306  is cooled in a reflux condenser  307  to remove humidity in the CO 2  rich stream leaving the CO 2  capturing unit through line  51 . Humidity that is condensed in the reflux condenser  307  is returned to the stripping column in a reflux line  308 . 
         [0083]    The stripped or lean absorbent is taken out close to the bottom from the stripping column  305  in line  301 . The lean absorbent in line  301  is cooled in heat exchanger  303  and cooler  311  before it is reentered into the absorber  300 . A part of the lean adsorbent may be taken out in a heating circuit  309  where it is heated in a reboiler  310  before the heated lean absorbent is reintroduced into the stripping column  305 . 
         [0084]    In an exemplary plant according to  FIG. 1 , key figures for temperature, pressure and mass flow may be as follows: 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Pressure, temperature, mass flow and effect for different 
               
               
                 units/at different locations in a 400 MW plant 
               
             
          
           
               
                   
                   
                 Temperature 
                 Mass flow 
                   
               
               
                 Ref. No. 
                 Pressure (bara) 
                 (° C.) 
                 (kg/s) 
                 Effect (MW) 
               
               
                   
               
             
          
           
               
                 13 
                 1,013 
                 30 
                 21 (coal) 
                   
               
               
                 22 
                 20 
                 15 
                 2.3 
               
               
                 23 
                 16 
                 300 
                 300 
               
               
                 26 
                 300 
                 600 
                 272 
               
               
                 27 
                   
                   
                   
                 428 
               
               
                 35 
                 16 
                 350 
                 323 
               
               
                 46 
                   
                 120-130 
               
               
                 48 
                   
                 40-90 
               
               
                 55 
                 100 
                 30 
                 78 
               
               
                 58 
                 15 
                 330 
                 385 
               
               
                 60 
                 15 
                 850 
                 385 
               
               
                 65 
                   
                   
                   
                 80 
               
               
                 73 
                 16 
                 145 
                 50 
               
               
                 75 
                 1,013 
                 15 
                 400 
               
               
                 80 
                 20 
                 15 
                 5 
               
               
                 82 
                   
                 870 
               
               
                 86 
                 15 
                 330 
                 90 
               
               
                 87 
                 16 
                 300 
                 85 
               
               
                 88 
                 16 
                 300 
                 15 
               
               
                   
               
             
          
         
       
     
         [0085]    The skilled man in the art will understand the mentioned heat exchangers, turbines, compressors and the like may represent two or more parallel and/or serially connected devices. Additionally, where two or more parallels are mentioned, the number of parallels may be different from the exemplified embodiment.