System and method for efficient district heating and flue gas cleaning

Abstract

The invention relates to an oxyfuel combustion system and method for efficient district heating. Particularly, the invention relates to a system comprising a burning facility, a washing system comprising a plurality of wet scrubbers, and a heat exchanger for district heating. The corresponding method relates to combusting fuel using oxygen as an oxidant in the burning facility, capturing combustion-originating contaminants from flue gas by the washing system, and circulating scrubbing liquid from the washing system to the heat exchanger for district heating.

Description

[0001] System and method for efficient district heating and flue gas cleaning

[0002] Technical field

[0003] The invention relates to an oxyfuel combustion system and method for efficient district heating and flue gas cleaning. Particularly, the invention relates to a system comprising a burning facility, a washing system comprising a plurality of wet scrubbers, and a heat exchanger for district heating. The corresponding method relates to combusting fuel using oxygen as an oxidant in the burning facility, capturing combustion-originating contaminants and water vapor from flue gas by the washing system, and circulating scrubbing liquid from the washing system to the heat exchanger for district heating.

[0004] Background

[0005] Conventionally, washing systems such as wet scrubbers are used in combined heat and power plants (CHP plants) for cleaning flue gas, mainly removing combustion-originating contaminants such as ash, and water vapor present in the flue gas originating from the fuel. CHP plants are typically connected to district heating systems, where the heat produced in the plant may be used to warm residential and commercial housing. Especially the combustion of biofuels may lead to considerable ash concentrations in the flue gas.

[0006] Wet scrubbers are typically also utilized in recovering heat from the hot flue gas by heating water. The heat recovery capacity of conventional wet scrubbers is limited by the difference of a dew point of water in the flue gas and temperature of scrubbing liquid returning from the heat exchanger 112 for district heating. The higher the dew point, the more heat can be transferred from the flue gas to the scrubbing liquid, thus heating the scrubbing liquid. The difference between the dew point and the temperature of the returning scrubbing liquid needs to be at least 10°C (the dew point temperature being higher) to facilitate heat transfer from the flue gas to the scrubbing liquid without an external heat pump, i.e., to heat the scrubbing liquid that can then be circulated to a heat exchanger for district heating. For example, in a conventional CHP plant, where the fuel combusted in the plant has a moisture content of 45% (by mass), the dew point is simulated to be ca. 62°C. A typical temperature of the returning scrubbing liquid is 50°C. Thus, the heat transfer capacity is limited by the specific heat capacity of the flue gas. For a 100 MW CHP plant, the specific heat capacity of the flue gases is about 8,5 MW while the enthalpy of condensation for water vapor originating mainly from the water content and the hydrogen content of the biofuel is about 19 MW. This means that with present design, only one third of the theoretical energy content is recovered.

[0007] By using oxygen as an oxidant in the combustion process instead of air, the dew point of water in the flue gas may be increased to more than 80 °C, drastically improving the possible heat recovery capacity. However, if the heat recovered from enthalpy of condensation is to be maximized, the wet scrubber should be operated close to its minimum operating temperature, i.e., close to 60°C, and the temperature difference between the flue gas and the returning scrubbing liquid is again too small for efficient heat transfer into the district heating system. Operating the wet scrubber at higher temperatures in turn reduces the amount of heat recovered from the flue gas.

[0008] Heat exchangers, such as countercurrent heat exchangers, would be more efficient in heat recovery. However, as wet scrubbers are also used for cleaning the flue gas, wet scrubbers cannot be replaced by heat exchangers.

[0009] Thus, improved systems for heat recovery from flue gases in wet scrubbers are needed.

[0010] Summary

[0011] An object of the invention is to overcome the drawbacks in the state of the art.

[0012] More precisely, an object of the invention is to improve heat recovery from flue gases in combustion systems.

[0013] Another object of the invention is to improve energy efficiency of district heating systems. A yet another object of the invention is to produce high purity raw gas for synthetic fuel or polymer production.

[0014] The invention is characterized by the independent claims. Embodiments of the invention are described in the dependent claims.

[0015] The features recited in the dependent claims and the embodiments in the description are mutually freely combinable unless otherwise explicitly stated. The exemplary embodiments presented in this text and their advantages relate by applicable parts to all aspects of the invention, both the system and the method, even though this is not always separately mentioned.

[0016] An oxyfuel combustion system is provided. The system comprises:

[0017] - a burning facility configured to combust fuel using oxygen as an oxidant, generating wet flue gas;

[0018] - a washing system connected to an exhaust line of the burning facility configured to receive the wet flue gas from the exhaust line and to capture combustion-originating contaminants, e.g., ash, and water vapor and to recover heat energy from the wet flue gas, thus generating dry flue gas, wherein the washing system comprises a plurality of wet scrubbers connected in series, wherein each wet scrubber of the plurality of wet scrubbers is configured to bring the flue gas into contact with a scrubbing liquid, e.g., water; and

[0019] - a heat exchanger for district heating.

[0020] The system is configured to circulate the scrubbing liquid, e.g., water, from the washing system to the heat exchanger for district heating.

[0021] The system comprises a burning facility configured to combust fuel using oxygen as an oxidant. This is generally referred to as an oxyfuel combustion or oxyfuel process.

[0022] A method for recovering heat from flue gas is also provided. The method comprises:

[0023] - combusting fuel in a burning facility using oxygen as an oxidant, and thus generating wet flue gas; - receiving, by a washing system connected to an exhaust line of the burning facility, the wet flue gas from the exhaust line;

[0024] - capturing, by the washing system, combustion-originating contaminants, e.g., ash, and water vapor from the wet flue gas, thus generating dry flue gas, while simultaneously

[0025] - recovering, by the washing system (206), heat energy from the wet flue gas, wherein the washing system comprises a plurality of wet scrubbers connected in series, wherein each wet scrubber of the plurality of wet scrubbers is configured to bring the flue gas into contact with a scrubbing liquid, e.g., water; and

[0026] - circulating the scrubbing liquid, e.g., water, from the washing system to a heat exchanger for district heating.

[0027] The inventors have discovered that by using a washing system comprising a plurality of wet scrubbers, the amount of heat energy recovered from the flue gas may be drastically increased. In certain examples, the amount of heat recovered from the flue gas may be increased by more than 100% compared to conventional systems.

[0028] The increased heat recovery from the flue gas will correspondingly increase the efficiency of district heating systems coupled with CHP plants. The heat originally contained in the flue gas is transferred to the scrubbing liquid to be circulated from the washing system to the heat exchanger for district heating. In other words, the scrubbing liquid entering the heat exchanger for district heating can be heated to a higher temperature with the presented method compared to conventional district heating methods. Thus, water for the district heating system requires less external heating, e.g., by a primary steam from a heat plant or a CHP plant, to reach a desired temperature upon entering the district heating system. The need of primary steam may be reduced by ca. 10- 15 MW in a 100 MW CHP plant coupled with a district heating system by using the presented system and method.

[0029] Another advantage of the presented system and method is increased purity of the obtained dry flue gas. The washing system comprising a plurality of wet scrubbers can remove a larger amount of combustion-originating contaminants and water vapor from the flue gas compared to conventional systems, thus leading to lower-originating contaminant and water vapor concentrations and water vapor content in the dry flue gas. This is particularly beneficial for systems where the dry flue gas is utilized, e.g., as a raw gas in a chemical reaction, such as fuel or polymer synthesis.

[0030] Brief description of the drawings

[0031] Figure 1 schematically presents a system according to the invention;

[0032] Figure 2 schematically presents a system according to an embodiment of the invention;

[0033] Figures 3a, 3b schematically present washing systems according to embodiments of the invention;

[0034] Figures 4a, 4b schematically presents washing systems according to embodiments of the invention;

[0035] Figure 5 schematically presents a system according to an embodiment of the invention; and

[0036] Figure 6 schematically presents a method according to the invention.

[0037] Detailed description

[0038] In this application, the following reference numerals will be used:

[0039] 100 system

[0040] 101 burning facility

[0041] 102 synthetic fuel or polymer production facility

[0042] 103 hydrogen production facility

[0043] 104 oxygen dilution equipment

[0044] 105 carbon dioxide refining equipment

[0045] 108 exhaust line

[0046] 110 gas mixer

[0047] 112 heat exchanger

[0048] 114 oxygen production facility

[0049] 201 primary wet scrubber

[0050] 202 secondary wet scrubber

[0051] 203 intermediate wet scrubber

[0052] 205 scrubbing liquid generator 206 washing system

[0053] Referring to Figure 1 , the presented system 100 comprises a burning facility 101 and a washing system 206 connected to an exhaust line 108 of the burning facility 101. The system 100 is configured to circulate a scrubbing liquid, e.g., water or an aqueous solution of water, from the washing system 206 to a heat exchanger 112 for district heating. The scrubbing liquid, denoted in Figure 1 as H2O, is at a discharge temperature Th when entering the heat exchanger 112 for district heating. The scrubbing liquid is returned back from the heat exchanger 112 for district heating to the washing system 206. The scrubbing liquid is at a return temperature Ti when returning to the washing system 206. The return temperature Ti is lower than the discharge temperature Th. Typically, Th is in the range of 80-95°C and Ti in the range of 40-70°C.

[0054] The burning facility 101 is configured to combust fuel using oxygen (O2) as an oxidant. As a result of the combustion process, the burning facility generates wet flue gas. The wet flue gas comprises gaseous combustion products, such as carbon dioxide (CO2) and water vapor (H2O). The wet flue gas typically also comprises combustion-originating contaminants, i.e., components that are desirably removed from the flue gas in the washing system 206. Typical examples of combustion-originating contaminants comprise ash, dust, particulates, sulphur oxides, nitrogen oxides originating from the fuel, as well as chlorine and / or fluorine components. Water vapor originating from the combustion of the fuel is also present in the flue gas, and it can be removed from the flue gas in the washing system 206. The system 100 can be utilized in various different industrial plants or power plants. Suitable burning facilities may be power plant furnaces or boilers, as well as industrial plant furnaces. In certain embodiments, the burning facility 101 may be a heat plant boiler, a power plant boiler, a combined heat and power plant (CHP) boiler, a fluidized bed boiler, a recovery boiler, a rotary kiln, a cement kiln or a lime kiln.

[0055] The fuel combusted in the burning facility 101 may be a fossil fuel, such as a crude oil distillate, coal or lignite, natural gas or shale gas. Preferably, the fuel is a renewable fuel, more preferably a biofuel, even more preferably a solid fuel or biomass fuel, such as sugar-producing crops, starch-producing crops, oil-producing crops, or wood-based fuel. Suitable solid fuels or biomass fuels may originate from, e.g., grass, bagasse, sugarcane, corn, rapeseed, palm, straw, hardwood, softwood, bark, or any combination thereof. In an embodiment, the fuel is a solid wood-based biomass fuel, such as bark. In other embodiments, the fuel is a waste-based fuel, preferably solid or gaseous industrial or municipal waste, such as gas from animal waste, landfill gas, gas from coal mines, sewage gas, or combustible industrial waste gas. The fuel may comprise any of said fossil fuels, renewable fuels, waste-based fuels, or any combination thereof.

[0056] The washing system 206 is connected to the exhaust line 108 of the burning facility 101 . The washing system 206 is configured to receive the wet flue gas from the exhaust line 108 and to capture combustion-originating contaminants, e.g., ash, and water vapor from the wet flue gas, thus generating dry flue gas. The flue gas may comprise combustion-originating contaminants originating from the fuel in particle form, gaseous form, or both. Typical examples of- originating contaminants include ash, dust, particulates, sulphur oxides, nitrogen oxides originating from the fuel, as well as chlorine and / or fluorine components. The chorine and / or fluorine components refer to elemental or ionic chlorine and fluorine, as well as chlorine and fluorine compounds, such as acids of chlorine and fluorine. The washing system 206 is further configured to recover heat energy from the flue gas.

[0057] The washing system 206 comprises a plurality of wet scrubbers connected in series. Each wet scrubber of the plurality of wet scrubbers is configured to bring the flue gas into contact with a scrubbing liquid, by spraying it with the scrubbing liquid, by forcing it through a pool of scrubbing liquid, or by some other contact method with the scrubbing liquid, so as to remove the combustion-originating contaminants and the water vapor. Water vapor constitutes a majority of components to be removed from the flue gas in terms of volume of the components to be removed, i.e., the sum of the combustionoriginating contaminants and water vapor. Therefore, the flue gas that is received from combustion may be referred to as a wet flue gas, and the flue gas at the output of the washing system 206, after removal of the combustionoriginating contaminants and water vapor, may be referred to as a dry flue gas. At least a part of the wet flue gas is fed to the washing system 206. Each wet scrubber of the plurality of wet scrubbers has a mist eliminator for separating droplets of scrubbing liquid from the output flue gas. Each wet scrubber of the plurality of wet scrubbers in the washing system 206 is assembled countercurrent towards the flue gas flow. That is, the flow direction of the scrubbing liquid in each wet scrubber of the plurality of wet scrubbers in the washing system 206 is opposite to the flow direction of the flue gas.

[0058] Typical scrubbing liquids may be selected from water, aqueous solutions of sodium hydroxide, calcium hydroxide, sodium carbonate, or any combination thereof.

[0059] The washing system 206 may be integrated into the burning facility 101 , or it can be a stand-alone equipment. In an example, the washing system 206 is integrated into the burning facility 101 such that the exhaust line 108 is fixed to the wet scrubber for conducting at least a part of the flue gas through the washing system 206. On the other hand, the washing system 206 may be a stand-alone equipment, such that the washing system 206 can be detached from the exhaust line 108 without a service break of the burning facility 101 .

[0060] The system 100 may further comprise a cleaning system for the scrubbing liquid. As the combustion-originating pollutants are transferred from the flue gas to the scrubbing liquid, the scrubbing liquid needs to be continuously cleaned. The cleaning system is omitted from the figures for clarity. To reduce the need of scrubber water purification, an electrostatic precipitator can be added prior to the washing system 206. The electrostatic precipitator is configured to separate out most of the combustion-originating pollutants, e.g., ash, dust and particulates, from the flue gas in dry form.

[0061] The system 100 is configured to circulate the scrubbing liquid from the washing system 206 to the heat exchanger 112 for district heating. District heating is a system for distributing heat generated in a centralized location, i.e., a district heating plant, through a system of insulated pipes for residential and commercial heating requirements such as space heating and water heating. The heat is typically transformed through hot water as the heat carrier. The hot water is transferred into a heat distribution center of a building. The heat energy is delivered to the heating network of the building directly from the hot water, or preferably via a heat exchanger. The heat may be used to heat indoor spaces and domestic water in the building. The heat may also be utilized in ventilation. The heat carrier water is recirculated back to the district heating plant for reheating, and may be termed returning water. Temperature of the water entering the system of insulated pipes is typically higher than 50°C, preferably higher than 90°C or higher than 100°C. In certain examples, the water entering the system of insulated pipes is pressurized, and the temperature of the water is 110-130°C. Temperature of the returning water is typically 40-60°C lower than temperature of the water entering the system of insulated pipes. Typical temperature of the returning water may be in the range of 15-50°C, preferably 25-45°C.

[0062] The heat exchanger 112 for district heating is typically a countercurrent heat exchanger. That is, flow direction of the scrubbing liquid in the heat exchanger 112 is opposite to flow direction of the heat carrier water. Heat contained in the scrubbing liquid is transferred into the heat carrier water, whereby the scrubbing liquid cools down while the heat carrier water heats up. The flows of scrubbing liquid and heat carrier water are not mixed in the heat exchanger 112, but they form separate cycles.

[0063] Referring to Figure 2, the heat exchanger 112 for district heating is typically operatively connected with the washing system 206, with the hydrogen production facility 103 for receiving cooling water used in the hydrogen production facility 103 (not shown), or both.

[0064] Especially water electrolysis systems for hydrogen production require cooling, most conveniently with water. While cooling the electrolysis process, the cooling water receives heat energy and thus heats up. In a typical water electrolysis system, the cooling water can achieve temperatures of ca. 40- 60°C. It is beneficial to utilize the heat bound in the electrolysis cooling water in the district heating and thereby to reduce the so-called thermal pollution caused by the cooling water to the environment. This can be achieved by directing the cooling water to the heat exchanger 112 for district heating, whereby heat contained in the cooling water may be transferred to the district heating network.

[0065] The washing system 206 condenses steam, or water vapour, contained in the flue gas into the liquid phase. The water condenses when the temperature decreases below the dew point, or, in other words, when the relative humidity reaches 100%. Condensation occurs typically at a range of temperatures, the higher end of which is determined by the dew point. Lower end of the condensing range is typically dictated by capacity of a heat reservoir, e.g., the heat exchanger 112. Typically, in systems where the wet scrubber is operatively connected to the heat exchanger 112 for district heating, the lower end of the condensing range is in the range of 40-50°C. The condensing temperature increases with an increasing partial pressure of water vapour in the flue gas. After the wet scrubbing treatment, the dry flue gas comprises a minor water vapour content according to the dew point at the actual gas temperature.

[0066] Partial pressure of water vapour in the flue gas in the system 100 is considerably higher than compared to a flue gas from conventional air combustion, leading to higher condensing temperatures. In this context, the term “condensing temperature” should be understood as the higher end of the condensing range, i.e., the dew point of water in a wet scrubber of the plurality of wet scrubbers. In conventional systems, the condensing temperature is typically in the range of 60-65°C. With the present invention, however, the condensing temperature may be increased to be in the range of 70-100°C. With an increased condensing temperature compared to conventional systems, more heat energy may be transferred to the district heating via the heat exchanger 112. Thus, the heat carrier water for the district heating system requires less external heating, e.g., by a primary steam from a heat plant or a combined heat and power (CHP) plant, to reach a desired temperature upon entering the district heating system.

[0067] Using the presented system and method, condensing temperature of the water at a wet scrubber of the plurality of wet scrubbers may be increased to 70- 90°C, preferably to 85-90°C, i.e., 5-30°C higher, such as 10-15°C higher, preferably 25-30°C higher than when using a conventional air combustion. Thus, the temperature difference of the heat carrier water compared to the entrance temperature of the district heating system is also increased by ca. 5- 30°C, such as 10-15°C, or even 25-30°C. Heating the heat carrier water consumes a considerably smaller amount of energy with the higher condensing temperature, compared to a conventional system. The decrease in energy consumption may be in the range of 10-50%, such as 10%, 20%, 30%, 40% or even 50% compared to a conventional system. Thus, less external heating, e.g. by a primary steam from a heat plant or a combined heat and power (CHP) plant, is needed to reach the same entrance temperature for the heat carrier water entering the district heating system.

[0068] Operating temperatures of each wet scrubber of the plurality of wet scrubbers in the washing system 206 may be selected according to a desired balance in heat capacities of the flows of the flue gas and the scrubbing liquid, as well as efficiency of each wet scrubber of the plurality of wet scrubbers. Operation parameters such as nozzle type and atomization pressure of the scrubbing liquid in each wet scrubber may be selected and optimized as desired.

[0069] The washing system 206 functions as a carbon capture equipment in the system. Thus, the need of an external carbon capture equipment is eliminated. The wet flue gas received from the exhaust line 108 of the burning facility 101 is lead through the washing system 206 to obtain dry flue gas. The dry flue gas comprises at least 70% by volume, preferably at least 90% by volume, volume carbon dioxide (CO2), of the total volume of the dry flue gas. The dry flue gas may comprise 70-100% by volume, preferably 80-99% by volume, more preferably 90-99% by volume, such as 95-98% by volume carbon dioxide (CO2), of the total volume of the dry flue gas. The dry flue gas may also comprise less than 10% by volume, preferably less than 5% by volume, such as 1-10% or 2-4% by volume oxygen, of the total volume of the dry flue gas, due to the oxygen excess at the combustion. The dry flue gas comprises a minor water vapour content according to the dew point at the actual gas temperature. The dry flue gas may also comprise trace amounts of other elements or compounds originating from the fuel, such as nitrogen, sulphur and / or their oxides, as well as chlorine and / or fluorine components.

[0070] Referring to Figure 2, the system 100 may further comprise at least one selected from a hydrogen production facility 103, an oxygen production facility 114, and a synthetic fuel or polymer production facility 102.

[0071] The hydrogen production 103 facility can be any facility, equipment or reaction vessel capable of producing hydrogen as a product of a chemical reaction using suitable reactants. The hydrogen may be produced e.g. by steam reforming, methane pyrolysis, partial oxidation of heavy hydrocarbons, plasma reforming, coal gasification, electrolysis, radiolysis, thermochemical methods, photocatalytic water splitting, or biocatalysed electrolysis. Preferably, the hydrogen production 103 facility is a water electrolysis equipment. In an embodiment, the hydrogen production facility is configured to feed the produced hydrogen to the synthetic fuel or polymer production facility 102.

[0072] The oxygen production facility 114 can be any facility, equipment or reaction vessel capable of producing oxygen as a product of a chemical reaction using suitable reactants. The oxygen may be produced e.g. by air separation, such as cryogenic distillation, pressure swing adsorption, membrane separation; or oxygen evolution, such as electrolysis or chemical oxygen generation. Preferably, the oxygen production facility 114 is a water electrolysis equipment.

[0073] The oxygen production facility 114 is preferably a part of the hydrogen production facility 103, and the oxygen production facility 114 is configured to produce oxygen as a by-product of hydrogen production. The oxygen production facility is configured to feed the produced oxygen to the burning facility 101. In an example, the oxygen production facility is a part of the hydrogen production facility 103, when the oxygen production facility is integrated into the hydrogen production facility 103. The oxygen production facility is integrated into the hydrogen production facility 103 for example, when the produced oxygen is obtained from the same process input raw material, e.g. water, where the hydrogen is obtained from. On the other hand, the oxygen production facility is integrated into the hydrogen production facility 103, when the oxygen production and the hydrogen production are performed in parallel and the production of oxygen cannot take place without the production of hydrogen. In an example, the oxygen production facility is configured to feed the produced oxygen to the burning facility 101 via an oxygen line.

[0074] The hydrogen production facility 103 and the oxygen production facility 114 may be parts of an electrolysis equipment, wherein hydrogen is produced through electrolysis of water. The produced hydrogen is fed to the synthetic fuel or polymer production facility 102 as a raw gas for fuel or polymer synthesis. The electrolysis equipment produces oxygen as a by-product. The produced oxygen is fed to the burning facility 101 for combustion of fuel using at least a part of the produced oxygen. Conventional water electrolysis techniques may be used. The electrolysis equipment may be, for example, a polymer electrolyte membrane (PEM) cell, a solid oxide electrolysis cell, or an amine electrolysis cell. The electrolysis reaction produces hydrogen gas at the cathode and oxygen gas at the anode. The electrolysis equipment may produce, e.g., 1700 kg / h hydrogen and 13600 kg / h oxygen. In an embodiment, the water electrolysis is powered by renewable electricity, preferably wind power.

[0075] The synthetic fuel or polymer production facility 102 may be configured to capture CO2 generated at the combustion in a fuel synthesis. The synthetic fuel production facility utilizes the hydrogen produced at the hydrogen production facility 103 and the CO2 originating from combustion at the burning facility 101 to produce fuels for, e.g., traffic, transportation, or shipping purposes. The synthetic fuel may be selected from low-molecular weight aliphatic hydrocarbons or alcohols, such as methane, methanol, ethane, ethene, ethanol, propane, propene, propanol, butane, butene, butanol, and biodiesel.

[0076] The synthetic fuel produced may be methanol (CH3OH), synthesized in a direct CO2 hydrogenation process according to the following reactions:

[0077] CO2 + H2-> CO + H2O

[0078] CO + 2 H2-> CH3OH

[0079] Compared to hydrogen, methanol is easier and safer to transport, to handle and to store. The need for pressurized containers is eliminated. The synthesis process may utilize suitable catalysts, such as heterogeneous catalysts, homogeneous catalysts, photocatalysts, enzymatic catalysts, and / or electrocatalysts.

[0080] The production facility 102 may alternatively or additionally be configured to capture CO2 generated at the combustion in a polymer synthesis. The polymer synthesis typically uses an intermediate product, such as at least one selected from low-molecular weight aliphatic hydrocarbons or alcohols, such as methane, methanol, ethane, ethene, ethanol, propane, propene, propanol, butane, butene, butanol, and any combination thereof as a starting material for the polymer synthesis.

[0081] The synthetic fuel or polymer production facility 102 may require a high purity in the raw gases hydrogen and CO2 for an efficient reaction. Typically, water electrolysis systems may produce hydrogen with a very high purity, and no additional purification is necessary. For the CO2 originating from the combustion, however, purification by the wet scrubbing process in the washing system 206 is needed to obtain high-quality raw gas for the fuel or polymer synthesis. With the washing system of the present invention, it is possible to produce CO2 with a higher purity, e.g., with lower water and ash content, compared to conventional wet scrubbing systems.

[0082] The washing system 206 may be operatively connected to the synthetic fuel or polymer production facility 102. The washing system 206 may be configured to feed at least a part of the dry flue gas from the washing system 206 to the synthetic fuel or polymer production facility.

[0083] The system 100 may further comprise a carbon dioxide refining equipment 105 configured to remove traces of nitrogen, sulphur and / or their oxides, chlorine and / or fluorine components, and / or oxygen from the dry flue gas. The carbon dioxide refining equipment is typically located downstream of the washing system 206. When the dry flue gas is treated with the refining equipment 105 to remove traces of nitrogen, sulphur and / or their oxides, chlorine and / or fluorine components, and / or oxygen, essentially pure carbon dioxide is obtained. After refining, the dry flue gas comprises at least 99 % by volume, such as 99-100 % by volume CO2, of the total volume of the dry flue gas. The carbon dioxide refining equipment may be selected from, e.g., chemical treatment systems and cryogenic cleaning systems. Preferably, a cryogenic cleaning system is used as the carbon dioxide refining equipment. Cryogenic cleaning comprises compression and cooling of the dry flue gas, inducing phase changes of carbon dioxide in the flue gas. Since each component of the dry flue gas has different phase transfer properties, carbon dioxide may be extracted from the dry flue gas very efficiently. The present system enables the use of cryogenic cleaning, as the flue gas only contains a minor amount of nitrogen oxides (NOX) that originate from the fuel, because combustion is carried out with diluted oxygen instead of air. Cryogenic cleaning is more cost- effective compared to conventional chemical methods. In addition, the use of harmful chemicals may be reduced.

[0084] Referring to Figure 2, the system 100 may further comprise a gas mixer 110. The gas mixer 110 is configured to form an output gas based on a mixture of the wet flue gas originating from combustion received from the exhaust line 108 prior to the washing system 206 and the dry flue gas obtained from the washing system 206 The gas mixer 110 is operatively connected to the washing system 206 for receiving the dry flue gas and to the exhaust line 108 for receiving the wet flue gas. With the use of the gas mixer 110, output gas comprising CO2, water, and possibly trace amounts of nitrogen, sulphur and / or their oxides, and oxygen, with a highly controllable water content may be generated. In case the system also comprises the carbon dioxide refining equipment 105, the gas mixer 110 may be placed either upstream or downstream (in terms of flow direction of the flue gas) of the carbon dioxide refining equipment 105. If the gas mixer 110 is placed upstream of the carbon dioxide refining equipment 105, trace amounts of nitrogen, sulphur and / or their oxides, chlorine and / or fluorine components, and oxygen remain in the output gas generated by the gas mixer 110. For purposes of recirculating flue gas as an oxygen diluent, these trace amounts are essentially inert and may be utilized in the oxygen dilution without further purification. If the gas mixer 110 is placed downstream of the carbon dioxide refining equipment 105, trace amounts of nitrogen, sulphur and / or their oxides, chlorine and / or fluorine components, and oxygen are removed from the gas stream before entering the gas mixer 110. In this case, the gas mixer 110 may generate output gas consisting essentially of CO2 and water, with a highly controllable water content. Due to the high cost of the carbon dioxide refining, it is preferable to place the gas mixer 110 upstream of the carbon dioxide refining equipment 105.

[0085] The desired volume ratio of the wet flue gas to the dry flue gas may be defined at the gas mixer 110. The volume ratio of wet flue gas to dry flue gas at the gas mixer may be determined based on a need of heat energy at the heat exchanger 112 for district heating.

[0086] In cold circumstances, e.g. during winter, the need for heat energy at the district heating may be high, and as much of the heat contained in the flue gas is directed to district heating as possible. The ratio of the wet flue gas to the dry flue gas at the gas mixer may therefore be 50:50, 40:60, 30:70, 20:80, 10:90, or even 0:100 [vol-%:vol-%]. In certain situations, the wet flue gas is fed to the washing system 206 in its entirety, and the ratio of the wet flue gas to the dry flue gas at the gas mixer 110 may be 0:100 [vol-%:vol-%]. When all flue gas is directed to the washing system 206, the amount of heat energy withdrawn from the flue gas with the use of the heat exchanger 112 is at its maximum.

[0087] Conversely, in warm circumstances, e.g. during summer, the need for heat energy at the district heating is low. In this situation, it is beneficial to recirculate as much of the heat energy back to the combustion process as possible. Therefore, the ratio of the wet flue gas to the dry flue gas at the gas mixer 110 may be 50:50, 60:40, 70:30, 80:20, 90:10, or even 100:0 [vol-%:vol-%]. In other words, the minority or even none of the flue gas is circulated to the washing system 206. During summertime, the combustion process produces a surplus of waste heat. Together with the low energy need at the district heating, the quality of recovered heat and not the amount is of importance. Circulating a majority of the hot, wet flue gas back to the combustion process increases the partial pressure of water vapor in the flue gas leading to a higher condensing temperature in the scrubber, and thus a higher temperature of the scrubbing liquid. This reduces the need for the expensive external primary steam to increase the temperature to desired district heating temperature.

[0088] Referring to Figure 2, the system may further comprise an oxygen dilution equipment 104. The oxygen dilution equipment 104 is configured to feed diluted oxygen to the burning facility 101. Especially in burning facilities designed for air combustion, dilution of the oxygen is of essential importance. Too high oxygen content in the burning facility 101 may increase the temperature inside the burning facility to such an extent that may destroy the burning facility. In a fluidized bed boiler, for example, feeding pure oxygen would probably melt the bed. Controlling the oxygen dilution enables a precise regulation of the combustion process at the burning facility 101. In an optimal situation, the combustion profile of the burning facility may be maintained identical to conventional combustion with air. Thus, no technical modifications to the burning facility 101 itself are needed.

[0089] The gas mixer 110 may be connected to the oxygen dilution equipment 104. Thus, the output gas from the gas mixer 110 may be fed to the oxygen dilution equipment 104. The oxygen dilution equipment 104 then dilutes oxygen using the output gas generated at the gas mixer as the diluent. The advantage of the connection between the gas mixer 110 and oxygen dilution equipment 104 lies in the circulation of the flue gas back to the burning facility 101 as the oxygen diluent. The output gas, originating from the flue gas of the burning facility 101 , consists entirely of combustion products. Therefore, the output gas is an inert oxygen diluent that does not react at the combustion. Compared to conventional air oxidant, the oxygen diluted with the output gas does not produce any nitrogen oxides at the combustion. The oxygen content in a combustion chamber of the burning facility 101 can be precisely determined by determining the ratio of the output gas to the oxygen at the oxygen dilution equipment 104. The highly controlled oxygen content in the combustion results in the high dew point of water vapor in the flue gas, beneficial for the district heating as discussed above.

[0090] In a preferred system, the oxygen diluted at the oxygen dilution equipment 104 is received from the oxygen production facility 114.

[0091] Referring to Figure 2, the system may comprise a first control device CNTL1 operatively connected to the oxygen dilution equipment 104 and the burning facility 101. The first control device CNTL1 is configured to measure one or more operational characteristics of the burning facility 101 and / or the exhaust line 108. The one or more operational characteristics, such as pressure, temperature, flow rate of combustion gas, carbon monoxide concentration, oxygen concentration, or any combination thereof, may be measured at one or more points within burning facility 101 and / or at the exhaust line 108. Preferably, the one or more operational characteristics are measured at multiple points within the burning facility 101 and / or at the exhaust line 108 to create a combustion profile for the burning facility 101 .

[0092] The first control device CNTL1 controls dilution of oxygen received by the oxygen dilution equipment 104 based on the measured one or more operational characteristics of the burning facility 101 and / or the exhaust line 108. Controlling the oxygen dilution enables a precise regulation of the combustion process at the burning facility 101. In an optimal situation, the combustion profile of the burning facility may be maintained identical to conventional combustion with air. Thus, no technical modifications to the burning facility 101 itself are needed. Referring to Figure 2, the system may comprise a second control device CNTL2 operatively connected to the gas mixer 110. The second control device CNTL2 controls a volume ratio of the wet flue gas to the dry flue gas at the gas mixer for forming the output gas. The volume ratio of the wet flue gas to the dry flue gas may be varied according to different needs. The volume ratio of the wet flue gas to the dry flue gas may vary from 100:0 to 0:100 [vol-%:vol- %], such as 100:0, 90:10, 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, 20:80, 10:90, or 0:100 [vol-%:vol-%].

[0093] In certain embodiments, the second control device CNTL2 is configured to control the volume ratio of the wet flue gas and the dry flue gas at the gas mixer for forming the output gas based on determining a need to control a combustion temperature of the burning facility 101 . The wet scrubbing process decreases the temperature of the flue gas. Thus, in case the temperature of the burning facility 101 needs to be decreased, a larger amount of dry flue gas may be used to form the output gas, and the volume ratio of the wet flue gas to the dry flue gas may be 50:50, 40:60, 30:70, 20:80, 10:90 or 0:100 [vol-%:vol-%]. On the other hand, leading the hot, wet flue gas back to the burning facility 101 keeps the temperature decrease at the burning facility 101 to a minimum. Thus, if needed, the volume ratio of the wet flue gas to the dry flue gas may be 100:0, 90:10, 80:20, 70:30, 60:40, or 50:50 [vol-%:vol-%].

[0094] In certain embodiments, the second control device CNTL2 is configured to control the volume ratio of wet flue gas and dry flue gas at the gas mixer for forming the output gas based on determining a need to feed dry flue gas to the synthetic fuel or polymer production facility 102. The synthetic fuel or polymer production facility 102 may require dry CO2 for an efficient reaction. Thus, it may be beneficial to keep the volume ratio of wet flue gas to dry flue gas towards a majority of dry flue gas, e.g., 50:50, 40:60, 30:70, 20:80, 10:90, or even 0:100 [vol-%:vol-%]. Referring to Figures 3a and 3b, the washing system 206 may comprise two wet scrubbers connected in series. A primary wet scrubber 201 is configured to receive the wet flue gas from the exhaust line 108. The primary wet scrubber 201 is also configured to partially remove the combustion-originating contaminants and water vapor from the wet flue gas to generate an intermediate flue gas. The primary wet scrubber (201 ) is configured to simultaneously recover heat energy from the flue gas and to transfer the heat energy to the scrubbing liquid, thereby heating the scrubbing liquid. Further, the primary wet scrubber 201 is configured to feed the intermediate flue gas to a secondary wet scrubber 202. The secondary wet scrubber 202 is in turn configured to receive the intermediate flue gas from the primary wet scrubber 201 . The secondary wet scrubber 202 is configured to further remove combustion-originating contaminants and water vapor from the intermediate flue gas. The secondary wet scrubber (202) is configured to simultaneously further recover heat energy from the intermediate flue gas and to transfer the heat energy to the scrubbing liquid, thereby further heating the scrubbing liquid. Further, the secondary wet scrubber 202 is configured to generate the dry flue gas.

[0095] It is possible to direct the flue gas out of the washing system 206 for further use, e.g., to direct the flue gas to the synthetic fuel or polymer production facility 102, either after the primary wet scrubber 201 or after the secondary wet scrubber 202. In other words, the system 100 may be configured to direct the intermediate flue gas or the dry flue gas to the synthetic fuel or polymer production facility 102. It is also possible to direct a part of the intermediate flue gas out of the washing system 206 and another part of the intermediate flue gas to the secondary wet scrubber 202. This may have an advantage of cost-efficiency in planning plant layout in the system. For example, it may be possible to arrange shorter flue gas lines, thus saving both material and cost.

[0096] As presented in Figure 3a and 3b, the primary wet scrubber 201 is configured to operate at a temperature Ti, and the secondary wet scrubber is configured to operate at a temperature T2. The operating temperatures T1 and T2 are selected such that T1 > T2. Therefore, the flue gas undergoes a two-stage wet scrubbing process. The primary wet scrubber 201 may be operated at a temperature close to the maximum dew point of water vapor in the flue gas, i.e., at 70-90°C, preferably in the range of 75-85°C, such as at ca. 80°C. Therefore, the maximum heat transfer capacity can be utilized and heat may be transferred to the scrubbing liquid at a high efficiency. The secondary wet scrubber 202 may be operated at a temperature close to the lower end of the condensing range, i.e., in the range of 50-70°C, preferably in the range of 55- 65°C, such as at ca. 60°C for maximizing the heat transfer capacity originating from enthalpy of condensation of water. The system may further comprise one or more intermediate wet scrubbers connected in series with the primary wet scrubber 201 and secondary wet scrubber 202. Figures 4a and 4b present a washing system 206 comprising one intermediate wet scrubber 203. The one or more intermediate wet scrubbers 203 are connected between said primary wet scrubber 201 and secondary wet scrubber 202, such that the one or more intermediate wet scrubbers 203 are configured to receive the intermediate flue gas from the primary wet scrubber 201. The one or more intermediate wet scrubbers 203 are configured to further remove combustion-originating contaminants and water vapor from the intermediate flue gas to generate a second intermediate flue gas. The one or more intermediate wet scrubbers 203 are configured to simultaneously further recover heat energy from the flue gas and to further transfer the heat energy to the scrubbing liquid, thereby further heating the scrubbing liquid. The one or more intermediate wet scrubbers 203 are further configured to feed the second intermediate flue gas to the secondary wet scrubber 202. The secondary wet scrubber 202 is in this embodiment configured to receive the second intermediate flue gas from the one or more intermediate wet scrubbers 203, to further remove combustion-originating contaminants and water vapor from the intermediate flue gas and further to generate the dry flue gas, while simultaneously further recovering heat energy from the flue gas and to further transfer the heat energy to the scrubbing liquid, thereby further heating the scrubbing liquid. The one or more intermediate wet scrubbers 203 are operated at temperatures T3 in the range of T1 > T3 > T2, such that heat energy can be efficiently transferred from the flue gas to the scrubbing liquid.

[0097] It is possible to direct the flue gas out of the washing system 206 for further use, e.g. direct the flue gas to the synthetic fuel or polymer production facility 102, either after the primary wet scrubber 201 , after the one or more intermediate wet scrubber 203, or after the secondary wet scrubber 202. In other words, the system 100 may be configured to direct the first or the second intermediate flue gas or the dry flue gas to the synthetic fuel or polymer production facility 102. It is also possible to direct part of the first intermediate flue gas out of the washing system 206 and another part of the intermediate flue gas to the one or more intermediate wet scrubber 203. It is likewise possible direct a part of the second intermediate flue gas out of the washing system 206 and another part of the second intermediate flue gas to the secondary wet scrubber 202. This may have an advantage of cost-efficiency in planning plant layout in the system. For example, it may be possible to arrange shorter flue gas lines, thus saving both material and cost.

[0098] The multiple-stage wet scrubbing process obtained by using the one or more intermediate wet scrubbers 203 further enhances the efficiency of heat transfer from the flue gas to the scrubbing liquid. Heat contained in the flue gas may be more efficiently transferred into the scrubbing liquid in the multi-phase wet scrubbing process compared to conventional one-stage wet scrubbing. Both the specific heat capacity of the flue gas and the enthalpy of condensation can be utilized in the heat transfer process, thus obtaining enhanced energy transfer. The operating temperature Ti at the primary wet scrubber 201 is typically chosen as close to the dew point of water vapor in the flue gas. Thus, T 1 may be in the range of 80-100°C, preferably in the range of 90-100°C. The operating temperature of the secondary wet scrubber T2, on the other hand, is advantageously chosen near the lower end of the condensing range, i.e., near the temperature of the scrubbing liquid returning from the heat exchanger 112 for district heating, while still maintaining the possibility to transfer heat to the scrubbing liquid without an additional heat pump. Thus, T2 may be in the range of 50-70°C. The one or more intermediate wet scrubbers 203 are operated at temperatures T3 falling between T1 and T2 for maximizing the heat transfer capacity of the washing system 206. T3 may be in the range of 60-80°C, with the proviso that T1 > T3 > T2. If the washing system 206 comprises more than one intermediate wet scrubbers, the operating temperatures should be selected such that the operating temperature decreases gradually, such that an intermediate wet scrubber placed adjacent to the primary wet scrubber 201 is operated at a temperature closest to T1 and another intermediate wet scrubber placed adjacent to the secondary wet scrubber 202 is operated at a temperature closest to T2.

[0099] The multiple-stage wet scrubbing process further enhances the efficiency of the district heating system. It is estimated that the need of primary steam to heat the heat carrier water entering the district heating system may be reduced up to 20-25 MW in a 100 MW CHP plant with the use of the multiple-stage wet scrubbing process, compared to a conventional process. The present system has the advantage of providing dry flue gas with a higher purity compared to conventional systems. When using the washing system 206, a larger amount of the combustion-originating contaminants and water vapor can be removed compared to conventional systems. As the combustionoriginating contaminants and water vapor are transferred from the flue gas to the scrubbing liquid, the concentration of the combustion-originating contaminants in the scrubbing liquid gradually increases with the cleaning process. When entering the washing system 206, the scrubbing liquid is clean of the combustion-originating contaminants. Thus, at the secondary wet scrubber 202, the last stage of scrubbing in the flow direction of the flue gas, is carried out with fresh and contaminant-free scrubbing liquid, resulting in enhanced removal of the combustion-originating contaminants and water vapor. This applies for two-stage and multi-stage processes alike.

[0100] In the multiple-stage wet scrubbing process, the contaminant removal efficiency is even further enhanced compared to the two-stage approach. With the one or more intermediate wet scrubbers 203 in the system, a majority of the combustion-originating contaminants and water vapor are already removed from the flue gas in the primary wet scrubber 201 and in the one or more intermediate wet scrubbers 203. Thus, the clean scrubbing liquid entering the secondary wet scrubber 202 can efficiently remove the remaining combustion-originating contaminants and water vapor from the flue gas, resulting in a high-purity CO2 for the fuel or polymer synthesis process.

[0101] The washing system 206 may comprise a plurality of individual wet scrubber units connected in series, as illustrated in Figures 3a and 4a. The wet scrubbers of the plurality of wet scrubbers may also be arranged as a stack of wet scrubbers overlying on each other as a multi-stage wet scrubber tower, as illustrated in Figures 3b and 4b. The advantage of this multi-stage wet scrubber tower is reduced need of space at the industrial site, e.g., at the CHP plant. The operation principle of the washing system 206 remains the same in both wet scrubber arrangements.

[0102] The system 206 may comprise a wet scrubber configured to dedicatedly purify the dry flue gas according to gas purity requirements for the synthetic fuel or polymer production. The wet scrubber may be operated in a separate scrubbing liquid cycle, not connected with the liquid cycle of the remaining washing system. The scrubbing liquid may in this case comprise additives configured to remove any contaminants that would jeopardize function of the catalyst in the fuel or polymer synthesis process. The secondary wet scrubber 202 may be utilized in this purpose, or an additional wet scrubber may be installed into the system.

[0103] Referring to Figure 5, the system 100 may further comprise a scrubbing liquid generator 205. The scrubbing liquid generator 205 is configured to combine scrubbing liquid returning from the heat exchanger 112 for district heating with cooling water from the hydrogen production facility 103 and / or fresh water from a water supply. Especially water electrolysis systems for hydrogen production require cooling, most conveniently with water. While cooling the electrolysis process, the cooling water receives heat energy and thus heats up. In a typical water electrolysis system, the cooling water can achieve temperatures of ca. 40-60°C. It is beneficial to utilize the heat bound in the electrolysis cooling water in the district heating and thereby to reduce the so-called thermal pollution caused by the cooling water to the environment. This can be achieved by combining the cooling water with the scrubbing liquid returning from the heat exchanger 112 for district heating at the scrubbing liquid generator 205. The cooling water from the hydrogen production facility 103, the scrubbing liquid returning from the heat exchanger 112 for district heating, fresh water from a water supply, or any combination thereof may be directed to the scrubbing liquid generator 205 to generate the scrubbing liquid.

[0104] At the scrubbing liquid generator 205, scrubbing agents may possibly be added to the combination of the cooling water and the returning scrubbing liquid, thus generating anew the scrubbing liquid. Typical scrubbing agents may be selected from the group comprising sodium hydroxide, calcium hydroxide, sodium carbonate, or any combination thereof.

[0105] The cleaning system for cleaning the scrubbing liquid is most conveniently placed such that the scrubbing liquid can be cleaned before being directed to the scrubbing liquid generator. The cleaning system may be placed, for example, between the washing system 206 and the heat exchanger 112, either at the discharge side or at the return side. The cleaning system may also be placed between the heat exchanger 112 and the scrubbing liquid generator 205 to remove the combustion-originating contaminants from the scrubbing liquid before the next cycle of scrubbing liquid being directed to the washing system 206.

[0106] The system 100 may comprise one or more flow rate meters configured to measure a flow rate for one or more gas flows in the system. The one or more gas flow may be selected from the group comprising hydrogen generated at the hydrogen production facility 103, oxygen generated at the oxygen production facility 114, oxygen diluted at the oxygen dilution equipment 104, wet flue gas at the exhaust line 108, first and second intermediate flue gas at the washing system 206, dry flue gas exiting the washing system 206, dry flue gas entering the synthetic fuel or polymer production facility 102, output gas generated at the gas mixer 110, and any combination thereof.

[0107] In an example in accordance with at least some embodiments, a control system comprising at least one control device may be operatively connected to one or more equipment of the system 100, for example one or more of the oxygen dilution equipment 104, the burning facility 101 , the gas mixer 110, the synthetic fuel or polymer production facility 102, the oxygen production facility 114, the carbon dioxide refining equipment 105, the washing system 206, the one or more flow rate meters and other device(s) for receiving and sending information for example messages comprising measurements and / or control commands. Accordingly, the control system may send control commands to one or more of the oxygen dilution equipment 104, the burning facility 101 , the gas mixer 110, the synthetic fuel or polymer production facility 102, the oxygen production facility 114, the carbon dioxide refining equipment 105, the washing system 206, the one or more flow rate meters and the other device(s). On the other hand, the control device may receive information such as measurements from one or more of the oxygen dilution equipment 104, the burning facility 101 , the gas mixer 110, the synthetic fuel or polymer production facility 102, the oxygen production facility 114, the carbon dioxide refining equipment 105, the washing system 206, the one or more flow rate meters and the other device(s). Examples of the measurements comprise temperature measurements, pressure measurements, gas flow rate measurements and content of flue gas. Content of the flue gas may be measured for example regarding content of carbon monoxide, content of oxygen and / or content of CO2, whereby combustion at the burning facility may be monitored. Examples of other device(s) of the system comprise may be sensors for example one or more of temperature sensors, pressure sensors, flow rate sensors, oxygen sensors, carbon monoxide sensors and CO2 sensors. The other device(s) may be deployed to the system for measuring operation of the oxygen dilution equipment 104, the burning facility 101 , the gas mixer 110, the synthetic fuel or polymer production facility 102, the oxygen production facility 114, the carbon dioxide refining equipment 105 and / or the washing system 206. It should be noted that instead of having a single control device connected to the one or more of the oxygen dilution equipment 104, the burning facility 101 , the gas mixer 110, the synthetic fuel or polymer production facility 102, the oxygen production facility 114, the carbon dioxide refining equipment 105 and the washing system 206, the one or more flow rate meters, one or more further control devices may be provided. For example, one control device may be connected to the gas mixer 110 and optionally to other device(s) such as a sensor configured to measure operation of the gas mixer. Another control device may be connected to the oxygen dilution equipment 104 and the burning facility and optionally to other device(s) such as a sensor configured to measure operation of the oxygen dilution equipment 104 and / or the burning facility 101 . In an example, communications between a control device and the oxygen dilution equipment 104, the burning facility 101 , the gas mixer 110, the synthetic fuel or polymer production facility 102, the oxygen production facility 114, the carbon dioxide refining equipment 105, the washing system 206, the one or more flow rate meters, and / or the other device(s) may be digital communications for example over a wired or wireless connection. Examples of the connections comprise field bus technologies such as Profibus, Scanbus, Internet Protocol and Ethernet connections. In an example, any control device in the control system may comprise memory that stores instructions that when executed by the control device cause one or more functionalities described with an example and / or embodiment described herein.

[0108] In an embodiment an apparatus, or a control device, comprises at least one processor and a communications unit, for example a transceiver. The processor is operatively connected to the communications unit for controlling the communications unit. The apparatus may comprise a memory. The memory may be operatively connected to the processor. It should be appreciated that the memory may be a separate memory or included to the processor and / or the transceiver. The memory may store instructions that, when executed by the at least one processor causes execution of one or more functionalities in accordance with a method described herein. In an example, the transceiver is configured to perform digital communications for example over a wired or wireless connection. Examples of the connections comprise field bus technologies such as Profibus, Scanbus, Internet Protocol and Ethernet connections.

[0109] Embodiments may be implemented in software, hardware, application logic or a combination of software, hardware and application logic. The software, application logic and / or hardware may reside on memory, or any computer media. In an example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer- readable media. In the context of this document, a “memory” or “computer- readable medium” may be any media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer.

[0110] Reference to, where relevant, “computer-readable storage medium”, “computer program product”, “tangibly embodied computer program” etc., or a “processor” or “processing circuitry” etc. should be understood to encompass not only computers having differing architectures such as single / multi- processor architectures and sequencers / parallel architectures, but also specialized circuits such as field programmable gate arrays FPGA, application specify circuits ASIC, signal processing devices and other devices. References to computer readable program code means, computer program, computer instructions, program instructions, instructions, computer code etc. should be understood to express software for a programmable processor firmware such as the programmable content of a hardware device as instructions for a processor or configured or configuration settings for a fixed function device, gate array, programmable logic device, etc.

Claims

Claims:1 . An oxyfuel combustion system (100) comprising:- a burning facility (101 ) configured to combust fuel using oxygen as an oxidant, generating wet flue gas;- a washing system (206) connected to an exhaust line (108) of the burning facility (101 ) configured to receive the wet flue gas from the exhaust line (108) and to capture combustion-originating contaminants, e.g., ash, and water vapor and to recover heat energy from the wet flue gas, thus generating dry flue gas, wherein the washing system (206) comprises a plurality of wet scrubbers connected in series, wherein each wet scrubber of the plurality of wet scrubbers is configured to bring the flue gas into contact with a scrubbing liquid; and- a heat exchanger (112) for district heating, wherein the system (100) is configured to circulate the scrubbing liquid from the washing system (206) to the heat exchanger (112) for district heating.

2. The system (100) according to claim 1 , wherein the washing system (206) comprises two wet scrubbers connected in series such that- a primary wet scrubber (201 ) is configured to receive the wet flue gas from the exhaust line (108), to partially remove the combustionoriginating contaminants e.g., ash, and water vapor and to recover heat energy from the wet flue gas to generate an intermediate flue gas, and to feed the intermediate flue gas to a secondary wet scrubber (202);- the secondary wet scrubber (202) is configured to receive the intermediate flue gas from the primary wet scrubber (201 ), to further remove combustion-originating contaminants e.g., ash, and water vapor and to recover heat energy from the intermediate flue gas to generate the dry flue gas; and- the primary wet scrubber is configured to operate at a temperature Ti, the secondary wet scrubber is configured to operate at a temperature T2; and T1 > T2.

3. The system (100) according to claim 2, wherein T1 is in the range of 70- 90°C, preferably in the range of 75-85°C, such as at ca. 80°C, and / orwherein T2 is in the range of 50-70°C, preferably in the range of 55-65°C, such as at ca. 60°C.

4. The system (100) according to claim 2 or 3, further comprising one or more intermediate wet scrubbers (203) connected in series with the primary wet scrubber (201 ) and secondary wet scrubber (202), between said primary wet scrubber (201 ) and secondary wet scrubber (202) such that- the one or more intermediate wet scrubbers (203) are configured to receive the intermediate flue gas from the primary wet scrubber (201 ), to further remove combustion-originating contaminants e.g., ash, and water vapor and to recover heat energy from the intermediate flue gas to generate a second intermediate flue gas, and to feed the second intermediate flue gas to the secondary wet scrubber (202);- the secondary wet scrubber (202) is configured to receive the second intermediate flue gas from the one or more intermediate wet scrubbers (203), to further remove combustion-originating contaminants e.g., ash, and water vapor and to recover heat energy from the intermediate flue gas to generate the dry flue gas; and- the one or more intermediate wet scrubbers (203) are operated at temperatures T3 in the range of T1 > T3 > T2.

5. The system (100) according to claim 4, wherein T1 is in the range of 80- 100°C, preferably in the range of 90-100°C, and / or wherein T2 is in the range of 50-70°C, and / or wherein T3 is in the range of 60-80°C.

6. The system (100) according to any of the preceding claims, comprising a wet scrubber configured to dedicatedly purify the dry flue gas according to gas purity requirements for the synthetic fuel or polymer production, preferably wherein the wet scrubber is operated in a separate scrubbing liquid cycle, optionally wherein the wet scrubber configured to dedicatedly purify the dry flue gas is the secondary wet scrubber 202, or an additional wet scrubber assembled in the system (100).

7. The system (100) according to any of the preceding claims, further comprising a scrubbing liquid generator (205) configured to combine returning scrubbing liquid from the heat exchanger (112) for district heating with cooling water of a hydrogen production facility (103) and / or fresh waterfrom a water supply, and possibly configured to add scrubbing agents, thus generating the scrubbing liquid.

8. The system (100) according to any of the preceding claims, further comprising a gas mixer (110) configured to form an output gas based on a mixture of the dry flue gas and the wet flue gas, whereby the gas mixer (110) is connected to the washing system (206) for receiving the dry flue gas and to the exhaust line (108) for receiving the wet flue gas.

9. The system (100) according to any of the preceding claims, wherein the washing system (206) is operatively connected to a synthetic fuel or polymer production facility (102) and configured to feed dry flue gas from the wet scrubber to the synthetic fuel or polymer production facility (102).

10. The system (100) according to claim 8 or 9, further comprising an oxygen dilution equipment (104) configured to feed diluted oxygen to the burning facility (101 ), such that the gas mixer (110) is connected to the oxygen dilution equipment (104) for feeding the output gas to the oxygen dilution equipment (104) for diluting oxygen received by the oxygen dilution equipment (104); and / or wherein the oxygen is received from an oxygen production facility (114).11 . A method for recovering heat from flue gas, comprising- combusting fuel in a burning facility (101 ) using oxygen as an oxidant, and thus generating wet flue gas;- receiving, by a washing system (206) connected to an exhaust line (108) of the burning facility (101 ), the wet flue gas from the exhaust line (108);- capturing, by the washing system (206), combustion-originating contaminants, e.g., ash, and water vapor from the wet flue gas, thus generating dry flue gas, while simultaneously- recovering, by the washing system (206), heat energy from the wet flue gas, wherein the washing system (206) comprises a plurality of wet scrubbers connected in series, wherein each wet scrubber of the plurality of wet scrubbers is configured to bring the flue gas into contact with a scrubbing liquid; andcirculating the scrubbing liquid from the washing system (206) to a heat exchanger (112) for district heating.

12. The method according to claim 11 , wherein the washing system (206) comprises two wet scrubbers connected in series such that the method comprises- receiving, by a primary wet scrubber (201 ), the wet flue gas from the exhaust line (108);- partially removing, by the primary wet scrubber (201 ), the combustionoriginating contaminants e.g., ash, and water vapor from the wet flue gas to generate an intermediate flue gas, while simultaneously- recovering, by the primary wet scrubber (201 ), heat energy from the wet flue gas;- feeding, by the primary wet scrubber (201 ), the intermediate flue gas to a secondary wet scrubber (202);- receiving, by the secondary wet scrubber (202), the intermediate flue gas from the primary wet scrubber (201 );- further removing, by the secondary wet scrubber (202), combustionoriginating contaminants e.g., ash, and water vapor from the intermediate flue gas to generate the dry flue gas, while simultaneously- recovering, by the secondary wet scrubber (202), heat energy from the intermediate flue gas; such that the primary wet scrubber operates at a temperature Ti, the secondary wet scrubber operates at a temperature T2; and T1 > T2.

13. The method according to claim 12, wherein T1 is in the range of 70- 90°C, preferably in the range of 75-85°C, such as at ca. 80°C, and / or wherein T2 is in the range of 50-70°C, preferably in the range of 55-65°C, such as at ca. 60°C.

14. The method according to claim 12 or 13, wherein the washing system (206) further comprises one or more intermediate wet scrubbers (203) connected in series with the primary wet scrubber (201 ) and secondary wet scrubber (202), between said primary wet scrubber (201 ) and secondary wet scrubber (202), the method comprising- receiving, by the one or more intermediate wet scrubbers (203), the intermediate flue gas from the primary wet scrubber (201 );- further removing, by the one or more intermediate wet scrubbers (203), combustion-originating contaminants e.g., ash, and water vapor from the intermediate flue gas to generate a second intermediate flue gas, while simultaneously- recovering, by the one or more intermediate wet scrubbers (203), heat energy from the first intermediate flue gas;- feeding, by the one or more intermediate wet scrubbers (203), the second intermediate flue gas to the secondary wet scrubber (202);- receiving, by the secondary wet scrubber (202), the second intermediate flue gas from the one or more intermediate wet scrubbers (203),- further removing, by the secondary wet scrubber (202), combustionoriginating contaminants, e.g., ash, and water vapor from the intermediate flue gas to generate the dry flue gas, while simultaneously- recovering, by the secondary wet scrubber (202), heat energy from the second intermediate flue gas; such that the one or more intermediate wet scrubbers (203) are operated at temperatures T3 in the range of T1 > T3 > T2.

15. The method according to claim 14, wherein T1 is in the range of 80- 100°C, preferably in the range of 90-100°C, and / or wherein T2 is in the range of 50-70°C, and / or wherein T3 is in the range of 60-80°C.

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