NO332809B1 - Absorption medium and method for removing carbon dioxide from a gas stream, as well as using the absorption medium. - Google Patents

Abstract

The present invention relates to a process for capturing carbon dioxide, CO 2 from exhaust gas using CaO dissolved in a mixture of NAF and CaF2 as the absorption medium.

Description

The present invention relates to the capture of carbon dioxide, CO2 from exhaust gas using calcium oxide dissolved in a salt melt as an absorption medium.

In a conventional thermal power plant, coal is oxidized with air with a ratio of »1.1 in such a conventionally fired boiler, there is 10-15% CO2 in the exhaust gas and the temperature is about 800°C at the outlet from the boiler. The hot exhaust gas is heat exchanged with water and generates superheated steam at high pressure which is used to drive a turbine which in turn drives a generator for electricity. The electrical efficiency is relatively low, approx. 40-60%. The hot steam is condensed before it is again exchanged for heat with the hot exhaust gases from the combustion process. In the condensation process, a lot of heat is released which can be used for district heating purposes in so-called CHP (combined heat and power) plants. This increases the total efficiency in a plant to around 70%.

The most current technology for cleaning CO2 from thermal power plants is currently based on the absorption of CO2i amines. After decompression and cooling in the turbine, the exhaust gases are passed through a large reactor where CO2 is absorbed in an amine-based liquid at 30-40°C. The other exhaust gases are released into the atmosphere, while the C02-rich amine liquid is fed into another chamber where the temperature is raised to 120-130X and CO2 is selectively released. The released gas can then be compressed into a liquid and deposited in a suitable place. The amine absorbent is cooled to 30-40X and fed into the absorption chamber where the process starts again. The temperature change of large amounts of absorbent is energy-intensive and lowers the electrical efficiency of the plant by about 10%.

A general thermal energy conversion process can be represented by the sketch in Figure 2. Heat (Qh) flows from a reservoir of high temperature through a machine to a reservoir of low temperature. On the road, work W is performed while heat Ql is supplied to the low-temperature reservoir. The efficiency of the process is given by equation (1).

The theoretical efficiency (carnot efficiency) for a thermal energy conversion process in general is given by equation (2) where TH and TL are respectively high and low temperature in the energy conversion process.

This represents a fundamental limit for the efficiency of thermal processes. In general, it is beneficial to have as large temperature differences as possible for increased efficiency.

When recovering energy from the temperature change between 130°C and 40°C in an amine purification process, the theoretical efficiency according to equation (2) is 66.9%. In practice, it is far lower, while the energy from the recovery process is available as relatively low-quality heat energy that can primarily only be used for heating purposes. A gas power plant with amine purification is outlined in Figure 3.

RU2229335 C1 relates to a CO2 absorbent which is a mixture of calcium oxide with a eutectic mixture of alkali metal carbonates produced in the form of grains.

JP11028331 A deals with electrochemical separation of CO2, where CO2 is converted to CO at the cathode through an electrochemical reaction.

JP 10085553 deals with the separation of CO2, by bringing the exhaust gas through a membrane where the fibers consist of a compound oxide that forms CO2 through a chemical reaction with C02 and an oxide.

US2005036932 deals with a method for absorbing and removing C02 from an exhaust gas. The exhaust gas is blown through an agglomerate of solid particles containing CaO and or Ca(OH)2 so that the CO2 in the exhaust gas is converted to CaC03.

Terasaka et al. (Chem. Eng. Technol. 2006, 29 No 9, pages 1118-1121) has described a process in which CO2 is absorbed by solid, particulate lithium silicate (LiSiO4) in a slurry with molten salt as working medium. U2CO3 and Li2Si03 are formed, which are present as solid particles in the slurry.

Known technology describes many different methods for capturing CO2. The disadvantages are mainly the size of the treatment plants and low energy efficiency for the energy conversion process.

It is an aim of the invention to come up with a method for cleaning C02 from exhaust gases from combustion plants which will provide an increased degree of efficiency and energy quality for the energy recovery from the cleaning process. In addition, it would be desirable to increase the efficiency of cleaning CO2 by using faster chemical reactions so that a treatment plant can be made physically smaller.

The present invention provides a method for removing carbon dioxide from a gas stream, where the gas stream in a first step in a reaction chamber is brought into contact with an absorption medium in a molten state comprising 99-50% by weight of a mixture of NaF and CaF2 and 1-50% by weight of dissolved CaO which reacts with the carbon dioxide to form a calcium carbonate. The method is carried out at a pressure close to atmospheric pressure above the absorption medium.

Furthermore, the invention relates to the use of an absorption medium comprising 99-50% by weight of a salt melt which is a mixture of NaF and CaF2 and 1-50% by weight of dissolved CaO for the removal of carbon dioxide from a gas stream.

The invention also provides an absorption medium for removing carbon dioxide from a gas stream comprising 99-50% by weight of a salt melt which is a mixture of NaF and CaF2 and 1-50% by weight of dissolved CaO.

Figures:

Fig. 1: Schematic representation of a conventional thermal power plant (B. Sørensen)

Fig. 2: Schematic visualization of a general energy conversion process.

Fig. 3: Schematic representation of gas power plant with amine purification plant, from SINTEF Fig. 4: Gibbs free energy for reaction (5). Positive values indicate that the reaction goes to the left. For negative values, the reaction goes to the right. Fig. 5: Principle sketch for high-temperature salt melt absorption of C02 from gas power plants.

Fig. 6: Phase diagram CaF2-NaF.

Fig. 7: Sketch of experimental apparatus (cell).

Fig. 8: Gas composition from the cell during absorption experiments.

Fig. 9: Gas composition during desorption experiments.

Salt smelters are used in the chemical process industry in a number of contexts. Specifically, the following can be mentioned: Electrolyte in electrolysis processes, as a catalytic medium in pyrolysis processes and as electrolyte in batteries and photoelectrochemical solar cells. In general, salt melts consist of metal-anion compounds of different composition. These are very thermodynamically stable in themselves at the same time as they are effective solvents for other substances and elements. Some salt melts show a certain solubility of carbon in the form of C02. This particularly applies to chlorides such as M-CIX, fluorides M-Fx, nitrates M-(N03)y where M is a metal with valence x or y/2.

In general, oxides will be soluble in salt melts with opposite Lewis acid-base characteristics so that acidic melts dissolve basic oxides and vice versa. A good example is CaCb, which in itself shows slightly acidic characteristics. This means that basic (CaO, MgO) and partially amphoteric oxides dissolve easily, while acidic oxides (SiO2, Ti02) will dissolve only to a small extent. Basic oxides have an affinity for CO2 during the formation of carbonates according to equation (3)

Calcium carbonate, like calcium oxide, has basic properties and is generally dissolved in acidic melts. However, this is not exclusively valid since both CaO and CaCO3 also dissolve in fluorides such as CaF2 which exhibit basic properties. Calcium carbonate is very stable at room temperature, but decomposes to CaO and C02 according to equation (4) at temperatures above 850-900°C. Table 1 shows AG for reaction (3) as a function of temperature. This is the opposite reaction of (4) so that the same numbers apply to (4) with the opposite sign.

Table 1: Gibbs free energy for reaction (3) as a function of temperature.

By using CaO's affinity for CO2, reactions (3) and (4) can be used to capture such gas from a diluted gas mixture, for example from a coal-fired power station, by passing the gas mixture through a salt melt with dissolved CaO, which will thus attract separate the carbon dioxide from the gas mixture and form CaC03. The temperature of salt melts is normally in the range of 600-1500°C, and the high temperature combined with the catalytic properties of the salt melts means that such capture can be very effective.

The Gibbs free energy for reaction (3) and (4) for the other alkaline earth metal oxides has been calculated using the HSC Thermodynamic software tool, Outotech 2008.

In Figure 4 it appears that the free energy according to (4) for the different alkaline earth metal oxides changes sign at higher temperature as we move down the periodic table. This can be exploited in the way that the heavier oxides are used for absorption in combustion processes where the exhaust gas temperature is high. It is desirable to have a large driving force (large, negative AG) in order for the processes to go quickly. At the same time, process speed normally increases with increasing temperature. This constitutes conflicting effects in the absorption of CO2 in salt melts and an optimal operating point must be found experimentally in each individual case. When oxide and corresponding carbonate are present dissolved in a salt melt, this will constitute a different medium and environment than when they are present in free form at unit activity as assumed in (4), but qualitatively the behavior will be the same for the different cations. Corresponding calculations for the alkali metal oxides show that these are more stable so that we do not achieve a change in sign for reaction (4) at relevant operating temperatures. This applies to the conditions at which the calculations were made (STP, unit activity, solid phase) and is not necessarily representative when the reactants are dissolved in a salt melt. In this state, the stability of the compounds will be lower so that alkali metal oxides/carbonates also function as active compounds. The gaseous CC-2 solubility in salt melts is a function of temperature in that the solubility decreases with increasing T. This is partially described in E. Saido, et al., J Chem Eng, Data, 25, (1), 1980, pages 45-47. The solubility of gaseous CO2 is in the order of 0.1-1% and can lead to a loss in process efficiency through the formation and decomposition of CaC03.

The present invention does not include any solid-phase reactions since the reactants are present dissolved as complexions in the described salt melt. This means that the kinetics are much faster than previous techniques and you don't have to deal with a slurry, but keep everything in liquid form. Instead of absorbing CO2 from the combustion gases in the low-temperature zone after heat exchange with steam, this can be done before the combustion gases cool down. Immediately after combustion, the gases maintain temperatures in the range of 800-1400°C. For a coal-fired power plant, the gases are present at approximately atmospheric pressure and energy is extracted in a conventional steam boiler where water is heated to approximately the same temperature as the exhaust gases for operation of a steam turbine which in turn drives an electric generator (Figure 1). There are no fundamental reasons why C02 should not be able to be absorbed from the exhaust gases in a hot and/or pressurized state. Salt smelters as described above typically have a melting point in the range 600-1412°C, precisely in the temperature range for the combustion process. Such an absorption process can be thought of as carried out in principle as an amine plant, but at a higher process temperature. The absorption temperature will be in the range 600 - 1600°C. The higher temperature is believed to make the kinetics of the process much faster so that such a plant can be made physically much smaller than an amine absorption plant which is physically very large. Such a plant is outlined in Figure 5. For a gas power plant, the combustion gases before the turbine will also be under high pressure, which will further increase the efficiency of the process if it is placed here since the partial pressure of CO2 in the gas mixture will be far higher than at atmospheric pressure (10- 20 atm).

According to the invention, hot exhaust gases can be passed through salt melts consisting of NaF and CaF2 at approx. 850°C where CO2 is absorbed by dissolved CaO in the salt melt in a chamber with the formation of CaC03. The melt with a high content of CaC03 is then fed to a desorption chamber and heated to approx. 950°C for release of gas.

Theoretically, this is explained by equation (5) which shifts to the left at T>850°C when AG>0 (see Table 1).

The cleaned exhaust gases and CO2 heat are then exchanged in separate circuits with water to generate high temperature steam which drives a steam turbine to operate an electric generator. The salt melt with dissolved, regenerated CaO is cooled to 800°C before it is fed back into the absorption chamber. In this process, heat is exchanged with steam, which in turn can drive a turbine to generate electricity - preferably the same one that is driven by steam generated by the hot exhaust gases. The benefit of cleaning the gas while it is at high temperature will be a higher electrical efficiency from the overall energy conversion process. The theoretical Carnot efficiency (see equation 2) in such a process for the recovery of process heat from cooling the salt melting medium from 950°C to 800°C in the absorption plant will, - if we assume TL about 40°C after energy withdrawal from the cooling medium (and condensation in the case of steam) to electrical energy as well as TH approx. 800°C, be 95%. Moreover, this will be able to generate high-quality electrical energy instead of low-quality water-borne heat.

Alternatively, changing pressure between the absorption and desorption chambers can in principle function in the same way as changing temperature. For a gas power plant where a large proportion of the energy in the fuel is linked to the hydrogen content in the gas, there will also be a third way to remove the CO2 emission to the atmosphere, namely by precipitating the carbon electrochemically from dissolved carbonate form in the molten salt in the desorption chamber. In this case, elemental carbon will be generated which can be taken out of the process path and deposited. This will be an alternative to pre-combustion reformation of natural gas into carbon (carbon black) and hydrogen before burning the hydrogen.

An immediate challenge with the concept described above is that water in the combustion gas to be cleaned can lead to the hydrolysis of certain salt melts - primarily chlorides, into oxyhydrochlorides. This can be avoided by using melts that are not exposed to this, preferably basic fluorides, or you can run a continuous regeneration process where some melt is taken out for treatment at any time.

The salt melt used in the method according to the present invention comprises NaF and CaF2, where the content of NaF is preferably in the range from 45-95 mol%, preferably from 55-85 mol%, more preferably from 60-75 mol%.

The amount of CaO dissolved in the salt mixture is preferably in the range from 1-50% by weight, preferably from 5-35% by weight, more preferably from 10-25% by weight.

The melting point of the absorption medium (the mixture of salt + oxide/carbonate) must lie below the temperature at which metal oxide and metal carbonate change to be the stable phases. This can be seen from Table 1 and Figure 4. It is important to have control over the melting point of the mixture. Addition of an oxide should normally mean a lowering of the melting point. In the present method, the temperature in the reaction chamber will preferably be in the range from 700-1200°C. The absolute pressure in the reaction chamber will lie in the range 0.5-200 bar.

Experimental and results

A salt mixture consisting of 65 mol% NaF and 35 mol% CaF2 was prepared by fusing chemical grade substances (Merck) at 900°C in a Ni crucible. This is close to the eutectic minimum in the system NaF-CaF2, which is outlined in Figure 6. After solidification, the mixture was crushed into smaller pieces for preparation into electrolyte. 850 g of salt mixture was then added to 150 g of CaO and filled into a Ni crucible. The mixture was melted down at 850°C in a gas-tight cell outlined in Figure 7. The height of the melting phase was about 20 cm. A simulated flue gas consisting of 22 wt% C02 + 78 wt% N2 (200 ml/min) was first introduced into the area above the melt phase through a centrally placed nickel tube and then out through a stainless steel tube. The gas composition was monitored using an FTIR analyzer (Thermo Nicolet 6700) equipped for gas analysis for highly accurate measurements of CO 2 . The cell had an analysis volume of approximately 200 ml, which, together with the volume above the molten salt, meant that stable analyzes were only achieved after a certain time when the entire volume had been filled with the gas from the cell. The pressure in the cell will thus be approximately atmospheric pressure above the melt mixture, while a certain overpressure will exist at the bottom of the cell where gas is bubbled through the melt. The overpressure can be estimated at about 0.03 atm and is due to the weight of a 20 cm molten column with a density of about 2.2 g/cm<3>. When stable values corresponding to the specifications of the gas mixture were achieved (22% by weight C02), the central gas supply pipe was lowered into the melt to 1 cm above the bottom while gas was continuously supplied. The composition of the gas stream was monitored and after a short time the C02 content in the gas stream was observed to decrease. After about 20 minutes, the C02 content in the gas stream had dropped to about 500 ppm. The Ni tube was then raised above the melt again and the gas composition was again found to rise and then stabilize at 22 wt%. The results from the gas analyzes are outlined in Figure 8. The cell was then left in a warm state overnight without gas flow.

The following day, the gas supply was changed to argon (Ar 5.0) and 250 ml/min was passed through the Ni tube which was again immersed in the salt melt mixture. A certain initial content of C02 was measured - which was probably due to residues from yesterday that were still present in the pipelines and analysis volume. However, this decreased quickly and when the C02 content was measured below 100 ppm, the temperature of the furnace controller was increased to 950°C. Temperature in the area above the melt mixture was monitored and when this rose above about 900°C, C02 was again observed in the gas stream, which now consisted of Ar. As the temperature increased to 950°C, the C02 content rose and peaked at about 0.3% before falling. This was interpreted as that C02 was now released by reaction [9] now going to the left due to changed thermodynamics according to Table 1 and Figure 4. The course is outlined in Figure 9.

The experimental parameters are summarized below.

Melt: 35% CaF2/65% NaF added 15% CaO.

Absorption temperature: 850°C, Desorption temperature: 950°C.

Pressure: Approximate atmospheric pressure, estimated excess pressure in inlet 0.03 bar.

Gas composition in: Absorption - 22% C02residual N2,

Desorption 100% Ar (5.0). Degree of purity >99.995%.

Claims (8)

1. Method for removing carbon dioxide from a gas stream, characterized in that the gas stream is brought in a first step in a reaction chamber into contact with an absorption medium in a molten state comprising 99 - 50% by weight of a mixture of NaF and CaF2 and 1 - 50% by weight of dissolved CaO which reacts with the carbon dioxide and forms calcium carbonate, the process is carried out at a pressure close to atmospheric pressure above the absorption medium. 2. Method according to claim 1, where the mixture of NaF and CaF2 contains 45 - 95 mol% NaF. 3. Method according to claims 1 or 2, where the absolute pressure in the reaction chamber is in the range 0.5 - 200 bar. 4. Method according to any one of claims 1 to 3, where the temperature in the reaction chamber is in the range 700 - 1200 °C. 5. Method according to claims 1 to 4, where in a next step the absorption medium is fed to a desorption chamber, heated above the melting point and releases CaO and carbon dioxide. 6. Use of an absorption medium comprising 99-50% by weight of a salt melt which is a mixture of NaF and CaF2 and 1-50% by weight of dissolved CaO for the removal of carbon dioxide from a gas stream. 7. Absorption medium for removing carbon dioxide from a gas stream, characterized in that it consists of 99-50% by weight of a salt melt which is a mixture of NaF and CaF2 and 1-50% by weight of dissolved CaO. 8. Absorption medium according to claim 7, where the mixture of NaF and CaF2 contains 45 - 95 mol% NaF.