FI111607B - A process for producing liquid carbon dioxide from flue gas under normal pressure - Google Patents

Description

111607

PROCESS FOR THE PRODUCTION OF LIQUID CO 2 CO2 FROM NORMAL PRESSURE FLUE GASES

The invention relates to a process for the production of liquid carbon dioxide (CO2) 5 from a normal pressurized flue gas, wherein the process gas: to the top, - CO 2 is desorbed from the top of the desorption column into a CO 2 gaseous concentrate, which is removed from the top by passing said absorbent solution to the bottom of the column, from where it is recycled to the top of said absorption column, is liquefied.

There is a global desire to limit CO2 emissions due to their climatic effects. Large amounts of carbon dioxide are released, for example, when fossil fuels are burned. Carbon capture from flue gases has been: technically difficult and economically unviable due to its high energy consumption.

One known CO 2 separation technique from flue gases is represented by the 30 MEA method described, inter alia, in DE-549556 and DE-606132. In these techniques, CO2 is absorbed. from flue gas to aqueous solution of monoethylarnine (MEA) or other amines. After absorption, the solution is heated, for example, to 120 ° C, and the dissolved CO 2 is desorbed at a pressure of about 2 bar-35.

• · 2 111607 In this process, the desorption heat of CO 2, which is 1.6 MJ / kg CO 2 using MEA, must be introduced into the process at 120 ° C. One of the major drawbacks of this # process is the high amount of energy required in principle to transfer CO 2 from its original 5 partial pressure of about 0.15 bar in the flue gas to about 2 bar of desorption.

Further, various methods of separation are known from, inter alia, US-4,797,141, WO-0048709 and JP-59073415. However, it is strongly characteristic of all of these that desorption occurs by heating the solution with external energy, which makes the processes extremely energetically disadvantageous. US-6,228,145 discloses a membrane-based separation method and JP-1230416 a method in which, as a batch process, CO 2 is first bound by adsorption to a solid and then released in vacuo.

PCT / FI01 / 00629 discloses a thermodynamically advantageous method for separating CO2 from flue gas. This process utilizes a sharply abnormal Henry's solubility of CO 2, for example, in methanol near its dew point. The method works best with a partial pressure of CO 2 of several bars and is therefore particularly suitable for gas turbine cycles and other processes in which flue gas pressurization can be used to generate energy.

DE-843 545 discloses a process in which blast furnace gas, pressurized to 2.5 bar and containing 24% CO 2, is washed with ethanol at a temperature of from -74 ° C to -67 ° C. The resulting CO 2 - 30 solution is then evaporated in two steps at 0.2 and 0.04 bar. in the same temperature range. The CO2 obtained at 0.2 bar is fed to the supercharger via a regenerator, but the fraction obtained at 0.04 Bar is supercharged as it is too low for the normal regenerator process. The resulting cooling-35 power shortage and other cooling power needs are covered by expanding the gas fraction leaving the scrubbing process in the cooling turbine.

111607 3 In the process disclosed in this publication, the separation process is performed approximately reversibly without the heating of the CO2 solution and the resulting energy loss. However, the extremely low process temperature of 5 temperatures, the low pressure of the 0.02 bar of CO 2 fraction, the volatility of the ethanol as solvent and its high viscosity under the process conditions make the whole process uneconomical. Furthermore, when it requires a very high CO 2 rich process gas, such a process has not become common use in flue gas cleaning.

The object of the present invention is to provide a process in which CO 2 can be liquefied from normal pressure flue gas and which avoids the above disadvantages. Characteristic features of the process of the invention are set forth in claim 1.

The deviation from the reversibility of the absorption / desorption process can advantageously be reduced by the use of air purifier 20 in the desorption step, whereby the air accumulated in the CO 2 concentrate can be used to liquefy the CO 2 utilizing the compressed energy of residual waste gas.

The absorption / desorption process of the invention does not require refrigeration because the processes are chemically performed at a temperature range which is advantageous to the process, rather than leaching and evaporation at cryogenic temperatures. As a result, the viscosity of the absorption solution is less than one tenth of the viscosity of the 30 solutions used in the process of the above-mentioned DE. The low viscosity of the absorption solution enhances heat and mass transfer and requires less energy to recycle.

The process of the invention can avoid supercharging very low pressure CO 2 fractions using air flushing or other methods described later. Unlike the prior art, these processes allow the economic separation and liquefaction of CO 2 from, for example, low CO 2 flue gas from natural gas combustion.

Expensive, volatile or oxidative chemicals are not required in the process of the invention. Other advantages of the process according to the invention are listed in the specification.

The process will now be described with reference to the accompanying drawings, which illustrate some embodiments of the process of the invention.

Figure 1 is a schematic diagram of a process according to the invention and

Figure 2 schematically illustrates another embodiment of the process.

Figure 1 shows a basic design of the process according to the invention, which preferably uses air rinsing in the desorption step. The carbon dioxide concentrate separation unit 10 shown in the left half of this figure is formed by an absorption column 11 and a desorption column 12 disposed in the same structure, thermally coupled to provide heat transfer.

The absorption and desorption processes are performed in a chemically preferred temperature range of 35 to 75 ° C, which corresponds to the temperature of the flue gas introduced into the process. The temperature of the desorption column 12 is 2-5 ° C lower than that of the absorption column 11 to transfer the heat released in the absorption ** 'of desorption column 11, with both columns 11, 12 operating in near-isothermal conditions most preferably without the introduction of external heat. The thermal energy required for desorption using, for example, potassium carbonate is about 35,790 kJ / kg CO2.

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At the bottom of the absorption column 11, a normal pressure flue gas is introduced which flows upwardly as the CO 2 therein is absorbed against the flue gas, into an absorption solution flowing along the column structures (not shown) which is introduced into the column 11 from its upper part. For example, a 2-N aqueous solution of potassium or sodium carbonate is suitable as the absorption solution. These compounds are evaporative and non-oxidizing. The said solutions have a low viscosity (1.0 - 0.55 cP) in the temperature range shown, whereby heat and material transfer is efficient and the recycling of 10 solutions requires little energy.

A CO 2-rich absorption solution is conducted from the lower part of the absorption column 11 to the lower part of the desorption column 12, provided with a heat exchanger 16 underneath the surface of the absorption thiol solution where the absorption solution introduced from the lower 11 column transfers heat to the bottom solution 12.

From the heat exchanger 16, the absorption solution is led to the top of the desorption column 12, from where it flows downwardly through the structures 20 arranged in the column 12 (not shown). A pressure of about 0.2 bar is applied to the desorption column 12 and, in accordance with a preferred embodiment of the invention, flushing air is passed to its lower end which flows through the absorption solution at the bottom of the column 12 and rises against the downstream absorption solution 25. The air flow maintains a constant pressure in the column 12 with a partial pressure of CO 2 in the column 12 at each height approximately equilibrium with the composition of the absorption solution at the same height. The CO2 released from the solution evaporates into the rinsing air. The normal-30 CO 2 concentrate leaving the top of the column 12 under a pressure of 50 to 55% below the pressure is passed through a water vapor condenser 13 to be pressurized to the sets of the supercharger intercooler 14 and further to the liquefaction process 15.

The CO2-poor absorption solution accumulated at the bottom of column 12 is transferred by pump 17 back to column 11 to capture the CO2.

6 111607

In the liquefaction process, a preferred embodiment of which is illustrated in the right half of Figure 1, the CO 2 concentrate pressurized, for example, to 100 bar, is introduced into the lower part of the liquefaction column 18 in the intercooler sets 14. In the liquefaction column 18, the CO 2 concentrate flows upward as the CO 2 liquefies on the surface of the series 18 heat exchangers 19.1, 19.2, 19.3, 19.4. Liquefied CO2 is removed from the bottom of the column 18 for its remote transfer or other use.

The non-liquefied waste gas is led from the top of the column 18 through the heat exchanger cooling tube or conduit group 19.1 to the first cooling turbine 20.1 where it loses some of its pressure energy. From the cooling turbine 20.1, the cycle continues in a similar manner to the cooling pipes 19.2. The cooling-15 expansion cycle 24 is repeated in the turbines 20.2, 20.3 and the heat exchangers 19.3, 19.4 until the pressure of the waste gas has dropped to 1 bar, whereupon it is removed from the cooling-expansion circuit 24 at the bottom of the last heat exchanger 19.4. The heat exchangers 19.1, 19.2, 19.3, 19.4 are arranged on the liquefaction column 18 20 so that their temperature levels are substantially aligned and their cooling power is weighted on the lower part of the column 18. The cooling turbines 20.1, 20.2, 20.3 are coupled to a generator 21 or other device to utilize the mechanical energy produced by them.

*. 25

For example, if the liquefaction of CO 2 is carried out at a pressure of 100 bar at a temperature between + 15 ° C and -40 ° C, the so-called "Press effect" (Z. phys. Chemie 110, 768- (1924)) will be left in the non-liquefied waste gas. barium CO2 and 30 liquefaction binds cooling power of about 250 kJ per kg of liquefied * CO2. When the heat flux released in the liquefaction is concentrated at the warm lower end of column 18, it is preferable to use a lower pressure ratio in some of the cooling turbines and concentrate the cooling power they produce to the lower end of column 18.

35 1 · f 7 111607

About 88% of the CO 2 concentrate in this basic solution of the invention can be liquefied. Under these conditions, the expansion and cooling process 24 produces about 360 kJ cooling power per Nm3 of expanded gas, or 230 to 260 kJ per 5 kg of CO2. Further, expanding the purge air of the enrichment stage in a separate turbine (not shown) before feeding it to the desorption column 12 can further provide cooling power of about 45 kJ / kg CO2, or a total of 280-310 kJ / kg co2.

10

Both the absorption-desorption cycle 10 and the liquefaction 15 may use a double column having the structure disclosed in patent application FI-20011969, "Process under normal pressure for producing oxygen or oxygen enriched air" in claim 11 and Figures 4a and 4b.

Because the flue gas to be treated is often warm (60-90 ° C), its heat can be utilized by performing absorption and desorption at temperatures so high that the pressure of the water vapor in the flue gas has a significant effect on the process. If, in this second preferred embodiment of the process according to the invention, the temperature of the desorption column 12 is, for example, 62.5 ° C, then the water evaporating from the absorption solution maintains a partial pressure of water vapor of 0.22 bar.

··. 25

If the CO 2 concentrate leaving the desorption column 12 contains 0.1 bar CO 2 and an equal amount of flushing air, then the total pressure of the CO 2 concentrate, taking into account the partial vapor pressure of water at 62.5 ° C, is 0.42 bar. If the CO 2 concentrate contains, after the condenser 13, 0.02 bar 30 of water vapor, it will still have a total pressure of 0.42 bar, of which · '0.2 bar will be CO 2 and also 0.2 bar air. In other words, the partial pressures of CO2 and air are doubled, thereby reducing the size of the superchargers required for CO2 pressurization and their energy requirements. Energy for this is obtained from the "sliding" condensation of water vapor in the desorption column 12 35 from the absorption solution or introduced at a pressure below normal pressure in the condenser 13. The condensate water is removed from the condenser 13 (not shown) before the CO 2 concentrate is pressurized.

In many cases, especially when the dew point of the flue gas to be treated is higher than the absorption temperature, the thermal energy required to evaporate the water in the desorption column 12 may be taken from the flue gas in the absorption column 11. For example, 0.5 bar tap steam.

10

In the third embodiment of the invention shown in Figure 2, two separate columns 11 'and 12' are used instead of the double column shown in Figure 1. In this case, the absorption heat is stored in the column 11 'in the absorption solution and exited there by desorption in the column 12'. If one kilo mole (= 44 kg) of CO 2 is absorbed per tonne of solution, the temperature of the solution increases by about 9 ° C and the CO 2 concentration achieved at a given CO 2 partial pressure is reduced by about 21% with potassium carbonate. To limit this effect, a dilute absorption solution and a high solution flow can be used.

In this embodiment, the lower part of the liquefaction column 18 is provided with a water-cooled · *, 25 pre-condenser 22 separate from the rest of the cooling circuit 24, which is close to the inlet of CO2 concentrate 27. In cold climates, cold water (6 ° C) also from other water bodies. If the pre-condenser 22 is maintained at 10 ° C and the CO 2 partial pressure in the CO 2 concentrate 30 is 60 bar (for example, the concentrate pressure is 110 bar and its 9 · t 'CO 2 content is 55%), about 25% of the CO 2 liquefies in the pre-condenser 22 when the cooling power requirement is reduced accordingly.

In the embodiment shown in Figure 2, an additional absorption coil <9 111607 column 23 is placed under suitable pressure in the liquefaction cooling circuit 24, whereby the CO 2 rich solution from the lower part of the first absorption column 11 'absorbs more CO 2 from the liquefaction cooling circuit 24. This increases the CO 2 fractional pressure of the desorption concentrate while returning a significant amount of the CO 2 from the waste gas to the liquefaction process 15. The impoverished waste gas is returned from the top of column 23 to the cooling circuit 24 of the liquefaction process 15.

A portion of the liquefaction waste gas may also be introduced at suitable pressure into the ejector (dotted lines) adjacent the evaporation column 12 'to increase the pressure of the evaporated CO 2 concentrate and reduce the size of the required superchargers 14.

In a fourth embodiment of the invention, very little or no air rinsing is used, whereby the concentrate generated in the desorption column 12 is almost pure CO 2 and most of it can be liquefied at 100 bar using water cooling.

If the CO 2 concentration of the absorption solution corresponds to a partial pressure of 0.15 bar and a CO 2 partial pressure of 0.05 bar on the evaporation column, then the separation efficiency without air purging would then be 67%. At such a low pressure, it is advantageous to utilize the pressure of the water vapor as described in the second embodiment. Already at evaporation temperature +47. At 25 ° C, the water has a vapor pressure of 0.11 bar and thus the pressure of the CO 2 concentrate entering the superchargers 14 is 0.16 bar instead of 0.05 bar.

CO 2 pressurized with pressures 14 to 100 bar is led to a liquefaction column 18 where it liquefies in a water-cooled liquefaction 30 (not shown) that fills the entire column 18. If other gases are present as impurities in the concentrate, at a temperature of 15 ° C in the liquefier (not shown), the gas exiting the top of the column 18 will contain 50% CO 2. If this loss of CO 2 is harmful, CO 2 is recovered by the liquefaction column 18 described in the embodiment shown in Figure 1, which has the required cooling power of φutt · 111607 10 by expanding the waste gas leaving column 18 in cooling turbines 20.1, 20.2, 20.3, 20.4.

Process conditions, such as CO 2 pressurization and CO 2 concentration, can preferably be selected such that the need for cooling power required by the liquefaction process can be accurately masked by the pressure energy of the non-liquefied waste gas in the process and other process cooling power sources. Hereby, the process energy consumption per kilo of liquefied CO2 is brought close to its minimum value.

One preferred embodiment of the liquefaction process of the invention is its use in connection with oxygen combustion. This embodiment lacks the absorption and desorption cycles described, since in oxygen or oxygen concentrate combustion, CO2 can be liquefied from the flue gas without enrichment. An example is the liquefaction of CO2 from pressurized flue gas from a Värtan-type PFBC power plant.

In this case, the main part of the flue gas of the pressure burner boiler flows through the gas turbine, the waste heat boiler and the water vapor condenser back to the supercharger where it is mixed with oxygen concentrate and returns the gas mixture to the boiler. The lower temperature side stream is cooled and charged to a liquefaction pressure of. After 25, CO 2 is liquefied as described in the previous examples.

If the CO 2 content of the flue gas is <50%, it is preferable to extend the liquefaction near the triple point of CO 2 (-56.5 °, 5.1 bar) to minimize its loss in the waste gas. If 30% CO 2 is present and * the liquefaction pressure is 100 bar, about 81% liquefies between -5 and -52.5 ° C. In this case, only a portion of the pressurized energy of the waste gas from the liquefaction is needed to produce cooling power, and cooling circuit 24 only has 1 to 35 cooling turbines. Unless additional cooling capacity is required, for example, in an oxygen enrichment unit adjacent to the power plant, the waste gas stream leaving the cooling circuit at excess pressure is heated, for example, in process heat exchangers before being fed to a separate turbine to generate energy.

In the process of the present invention, nitrogen oxides (NOx) in the flue gas are absorbed into the absorption solution and converted into nitrate ions. The nitrate ions can easily be separated from the absorption solution as nitrates, while the cations leaving it are replaced by the corresponding hydroxides. This 10 provides a degree of freedom in the combustion of the power plant without the need to minimize the NOx content of the flue gas. Because nitrates are significantly more valuable than hydroxides, it may even be economically advantageous to aim for high concentrations of NOx in the flue gas.

In the process of the invention, large heat fluxes are either transferred from one stream to another in the embodiments based on Figure 1 without large size and low efficiency "dry" heat exchangers, or stored in the absorption solution, which releases it in desorption, as occurs in the Figure 20 embodiments.

The process and apparatus of the invention achieve a much more energy efficient carbon dioxide separation method compared to the prior art. Application of the above. It is clear from the exemplary embodiments that there are numerous other hardware concepts that implement the process of the invention. Thus, the various implementation variations of the process are extremely versatile and are therefore not limited to the application examples described above.

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Claims (12)

12 111607 A process for producing liquid carbon dioxide (CO 2) from a normal pressurized flue gas, comprising: (12) to the upper part, - CO 2 is desorbed from the absorption solution introduced into the top of the desorption column (12) to a gaseous CO 2 concentrate which is removed from the top by passing said absorbent solution to the lower part of the column (12) is subjected to high pressure and - most of the concentrate CO 2 is liquefied, characterized in that the CO 2 is transferred from normal pressure flue gas by near-reversible absorption and desorption processes to a lower pressure CO 2 concentrate at the temperature of the flue gas being treated a, more preferably in the temperature range of 35-75 ° C such that the absorption and desorption processes take place in columns (11, 12) at approximately the same temperature, most preferably without the introduction of external heat into said processes. Process according to Claim 1, characterized in that, in order to improve the reversibility of the desorption process, a desorption column. An air flow is introduced into the lower portion of the 30 n (12) to maintain a constant pressure in the column (12) with a partial pressure of CO 2 in the column (12) at each height approximately equilibrium with the composition of the absorption solution. Process according to Claim 1 or 2, characterized in that the liquefaction of the CO 2 concentrate desorbed in the desorption column (12) is cooled (13), subjected to high pressure (14) and introduced into the lower part of the liquefaction column (18). from where it flows upward while part of the CO 2 concentrate liquefies on the heat exchange surfaces (19) provided on the column (18) and the non-liquefied waste gas is removed from the top of the column (18) and the liquefied CO 2 from the bottom. A process according to claim 3, characterized in that said heat pump columns (18) are provided in series with a plurality of heat exchangers (19.1, 19.2, 19.3, 19.4) between which the expansion of the non-liquefied waste gas is arranged to take place by cooling turbines (20.3, 20.1, 20.1, 20). ) such that the non-liquefied waste gas discharged through the top 15 of the liquefaction column (18) is led to a cooling-expansion circuit (24) in which at least part of the cooling power required to liquefy the CO 2 is produced by the pressurized energy of the waste gas. the waste gas is supplied to the top of the heat exchanger (19.1, 19.2, 19.3, 19.4), - the waste gas is discharged from the bottom of the heat exchanger (19.1, 19.2, 19.3, 19.4) and supplied to the cooling turbine (20.1, 20.2, 20.3) for expansion and * '· 25 , 20.3) the expanded waste gas is recycled to the upper part (19.2, 19.3, 19.4) of the next heat exchanger, and said cooling-expansion cycle (24) is continued until the waste gas pressure is substantially 1 bar, whereupon it is discharged 30 revolutions (24) at the lower part of the last heat exchanger (19.4). ta. A process according to claim 3 or 4, characterized in that said heat exchange means (19.1, 19.2, 19.3, 35 19.4) are arranged on the liquefaction column (18) so that they are substantially aligned with their temperature levels and their cooling power is weighted on the column ( 18) at the bottom. A process according to any one of claims 3 to 5, characterized in that a cold water-cooled pre-condenser (22) substantially separate from the cooling-expansion circuit (24) is provided in the lower part of the liquefaction column (18) near the inlet of the CO 2 concentrate. a portion of the CO 2 entering the column (18) is liquefied before flowing to the surfaces of the heat exchangers (19). Process according to one of Claims 1 to 6, characterized in that below the surface of the absorption solution are heat exchange means (16) arranged at the lower part of the desorption column (12), by means of which the absorption solution introduced from the lower part of the column (11) from the solution before transferring it to the absorption column (11), and said absorption solution from the heat exchange means (16) is led to the top of the desorption column (12). 20 Process according to one of Claims 1 to 7, characterized in that the absorption heat is stored in an absorption solution which releases it during desorption. Process according to one of Claims 1 to 8, characterized in that an additional absorption unit (23) is provided between the absorption and desorption columns (11, 12) and the CO 2 content of the absorption solution introduced from the lower part of the absorption column (11). to increase the CO 2 partial pressure of the concentrate, wherein - the lower part of the auxiliary absorption unit (23) feeds the non-liquefied waste gas from the cooling-expansion cycle (24) of the liquefaction column (18) and outwardly from the upper 111607 for the next step of the cooling-expansion cycle (24). Process according to one of Claims 1 to 9, characterized in that an aqueous solution of potassium and / or sodium carbonates is used as the absorption solution. Process according to one of Claims 1 to 10, characterized in that the vaporization / condensation cycle and / or the ejector of water vapor evaporated in the concentrate or introduced into the column (12) is utilized to increase the pressure level of the CO 2 concentrate. Process according to one of Claims 1 to 11, characterized in that the energy consumption of the CO2 liquefaction process is minimized by adjusting the concentrate composition, pressure and other conditions of the liquefaction process so that the cooling power produced by the process exactly covers the cooling capacity of the liquefaction process. 1 • · 16 111607