WO2002004098A1

WO2002004098A1 - Process for separating carbon dioxide, co2, from combustion gas

Info

Publication number: WO2002004098A1
Application number: PCT/FI2001/000629
Authority: WO
Inventors: Matti Nurmia
Original assignee: Nurmia, Wendie
Priority date: 2000-07-11
Filing date: 2001-07-02
Publication date: 2002-01-17
Also published as: AU2001282155A1

Abstract

The invention relates to a process for separating carbon dioxide, CO2, from combustion gas, in which process: the combustion gases entering the process are brought to a high pressure, and they are washed (10.1) with a solvent, to transfer the CO2 from the combustion gases to the solvent, and the combustion gases are removed from the process after washing, and the solvent is led from the washing process to a lower pressure evaporation process (12, 13), in which the CO2 is separated from the solvent and led out of the process, and the solvent is pumped back into the dissolution process, and a solvent, such as methanol, which has a strong CO2 solubility pressure gradient, deviating from Henry's law, over a suitable temperature range, is selected as the washing liquid. The dissolution and evaporation are carried out nearly isothermally in the said temperature range near the condensation point of CO2 at the partial pressure of CO2 used in the process, and thermodynamically in nearly reversible conditions.

Description

PROCESS FOR SEPARATING CARBON DIOXIDE, C0₂, FROM COMBUSTION GAS

The present invention relates to a process for separating carbon dioxide, C0₂, from combustion gas, in which process: - the combustion gases entering the process are brought to a high pressure, and

- they are washed with a solvent to transfer the C0₂ to the solvent, and

- the combustion gases are removed from the process after washing, and

- the solvent is led from the washing process to a lower pressure evaporation process, in which the C0₂ is separated from the solvent and led out of the process, and

- the solvent is pumped back into the dissolving process, and - the solvent selected as the washing liquid is a solvent, such as methanol, which has a solubility pressure gradient deviating from Henry's law over a suitable temperature range.

It is wished to limit carbon dioxide emissions globally, due to their effect on climate. Large amounts of carbon dioxide are released during the combustion of fossil fuels. The recovery of carbon dioxide from combustion gas has been technically difficult and financially unprofitable, due to its large energy consumption.

The liquefaction of carbon dioxide is one way to separate it. However, the triple point of carbon dioxide l^es at quite a high pressure (5,1 bar) so that the partial pressure of carbon dioxide in the exiting gas is always at least equal to this. Therefore the degree of separation remains modest.

US publication 2,863,527, among others, discloses the so-called Rectisol process. In the example of the publication (Fig. 4 of the publication) , carbon dioxide is separated from process gas, in which its content is 30 %, at a pressure of 20 bar. Dissolution stops at a temperature of -30°C, which determines the C0₂ content in the wash solution. Flash evaporation takes place at a temperature of -60°C, at which only about 40 % of the C0₂ can be separated. The low separation percentage means that much of the C0₂ solution obtained from the flash evaporation must be distilled in a rectification column, which consumes a great deal of heat, and from which the heat of evaporation of the C0₂ cannot be recovered to cool the dissolution column, but instead a corresponding amount of cooling energy must be produced by means of separate machinery. All in all, the process is complex and thermodynamically quite disadvantageous.

The energy consumption of generally known C0₂ separation processes is in the range 600 - 900 kJ/kg C0₂. In an oxygen combustion process, under ideal conditions when burning pure carbon, even the production of oxygen requires 850 kJ/kg C0₂.

The invention is intended to create a separation method for C0₂ that is both more economical of energy and simpler than previously. The characteristic features of the separation process according to the invention are stated in Claim 1. By carrying out the dissolution of the C0₂ from the combustion gas and its evaporation from the solvent in an almost reversible manner, this method achieves nearly the thermodynamic minimum value for the separation work, which is about 160 kJ/kg C0₂, if there is 10 - 14 % C0₂ in the combustion gas and the separated C0₂ is obtained at normal pressure. This demands the exploitation of the pressure energy of the pressurized combustion gas, for example according to Claim 10.

The invention exploits a solvent of a type such as methanol, in which the dissolution of C0₂ deviates from Henry's law, in such a way that its solubility increases steeply as the condensation point of C0₂ is approached. This is shown in Figure 1, which shows the solubility of C0₂ in methanol at different partial pressures of C0₂, as a function of temperature. For example, at a partial pressure of 8 bar, the condensation point of C0₂ is -46°C, while its solubility approaches infinity as the temperature drops towards the condensation point.

The figure shows that, if the partial pressure of the C0₂ in the combustion gas is 8 bar, and dissolution is carried out at - 45°C, then in a solution with 85 % saturation there will be 2000 kg of C0₂/tonne of methanol. By carrying out evaporation at -50°C and a pressure of 4 bar, 485 kg of C0₂/tonne of methanol will remain in the solution, in other words, 75 % of the C0₂ will evaporate.

It is possible to get even closer to a reversible process in the examples examined below. In these, C0₂ is also more thoroughly removed from the combustion gases by using two-stage flash evaporation and/or air scavenging of the evaporation chamber .

The invention offers the following advantages :

- as the evaporation takes place at a temperature slightly below the dissolving temperature and close to the partial pressure of the C0₂ prevailing in the solution, the dissolution- evaporation cycle can be made nearly reversible and its energy consumption made smaller than before,

- the large amount of thermal energy (in the order of 250 kJ/kg) released in the dissolution can be absorbed in the evaporation by binding the dissolution and evaporation chambers to each other thermally,

- these large thermal fluxes are transferred from one liquid flow to another, without large dry' heat exchangers of low efficiency,

- the cooling effect necessary to cover losses can also be produced in the evaporation, - the possibility of creating an ^integrated' process, in which the same liquid mixture acts as both a solvent of the C0₂ and a heat transfer liquid, by means of which the combustion gas is cooled to the process temperature and heated after the process.

Figure 1 shows that the solubility of C0₂ in methanol deviates strongly from Henry's law already when its partial pressure is 4 bar. As the C0₂ content of the combustion gas from air combustion is in the order of 8 - 16 % in nearly stoichiometric combustion, a partial pressure of 4 bar will require the combustion gas to be boosted to a pressure of 25 - 50 bar.

If oxygen enrichment, in which there is 50 % oxygen, is used in the combustion, a C0₂ content in the range 20 - 40 % will be obtained in the combustion gas while a partial pressure of 4 bar will be achieved by boosting the combustion gas to a pressure of 10 - 20 bar. The pressure energy of the boosted combustion gas can be exploited, for example, in a gas turbine cycle adapted to the C0₂ separation process.

In the following, the invention is examined with the aid of examples, which are shown in the accompanying figures. In the example of Figure 2, the partial pressure of the C0₂ in the combustion gas is assumed to be 8 bar, i.e. if there is 16 % C0₂ in the combustion gas, dissolution will take place at a pressure of 50 bar.

Figure 1 shows the dissolution of carbon dioxide in methanol at different pressures.

Figure 2 shows a separation column using two-stage flash evaporation for removing C0₂.

Figure 3 shows a column arrangement equipped with s separate heat-transfer circulation. Figure 4 shows a system equipped with air scavenging.

Figure 5 shows a wet' heat exchanger for cooling/heating the combustion gas before/after the separation of the C0₂.

Figure 6 shows an integrated system, including a wet heat exchanger and a C0₂ separation component in the same column.

In the example of Figure 2, the dissolution and evaporation chambers 10.1, 12, 13 are built into the same column 10. They form a heat exchanger, so that the combustion gas flows upwards in one or several pipes, in which the solvent flows downwards, the jacket surrounding the pipes acting as an evaporation chamber. A construction of the opposite type, in which evapora- tion takes place in the pipes and dissolution in the jacket surrounding them, is also possible.

The combustion gas connections are marked with the reference numbers 11.1 and 11.2. A high-pressure pump 15 pumps C0₂-lean methanol into a dissolution chamber 10.1, in which it enriches as it flows downwards against the flow of combustion gas. A fluid motor 16 can exploit part of the energy obtained from the pressure reduction of the C0₂ solution, if the solvent flow is led through a line 17 to the first evaporation chamber 12 (4 bar) , in which it is sprayed into the solvent in the evaporation chamber. Most of the carbon dioxide separates at this pressure, and this portion is sucked into a separate intake of a compressor 20. Next, the solvent flow is led through a pipe line 18 to a second evaporation chamber 13 (2 bar) , in which additional C0₂, which is sucked into the compressor 20, separates . The C0₂ flows coming from the evaporation chambers can also be combined by means of an ejector located before a regenerator R, which is used as a heat exchanger, in which case the separate intake of the compressor will not be needed. Finally the depleted methanol flow is pumped back to a pressure of 50 bar and a new cycle begins. In this example, the evaporation chambers are nearly full of solvent, which enters a strong circulation, due to the C0₂ bubbles created in the solvent by the heat being transferred to the circulation in the dissolution. This increases the effi- ciency of the heat transfer to the solvent and improves the isothermal nature of the process.

As in the other examples, packings, bases, or other structures for controlling the flow of gas and liquid and for making the heat transfer and mass transport more efficient, can be placed in the dissolution and evaporation chambers. In a carefully designed dissolution chamber, the dissolution can take place practically reversibly, because when the combustion gas rises in the dissolution chamber, the partial pressure of its C0₂ drops, at the same time as the C0₂ content of the solvent flowing in contact with the combustion gas diminishes.

In this case, the evaporation process is two-stage flash evaporation. If the process is carried out in conditions in which the solubility of the C0₂ diminishes rapidly as the pressure drops, a nearly reversible process is achieved in the evaporation. This ^'can be seen in the fact that, if C0₂ is bubbled into the chambers 12 and 13, the process would transfer the C0₂ into the clean combustion gas, in which its partial pressure would rise to nearly the partial pressure of the fed C0₂ (4 bar) . Reversibility has been sacrificed in the example of Figure 2, in that most of the C0₂ is obtained at 4 bar, whereas its partial pressure in the combustion gas was 8 bar. The Rectisol process is considerably less reversible.

The flows of the combustion gas entering and leaving the process travel through the regenerator R or some other heat exchanger. If external cooling power is brought into the process, the flow of exiting C0₂ can be boosted when cold, without having to lead it through the regenerator. This reduces the boosting power required. In the solution of Figure 3, the dissolution and evaporation take place in separate columns 10' and 10". The same reference numbers as above are used for functionally similar components . The heat flow is transferred from the solution to the evapora- tion using a separate liquid circulation (pump 21) . The thermal capacity flow of this circulation can be selected in such a way, that the desired temperature difference, in the range 5...10°C, prevails at the upper and lower ends of both columns. In the final evaporation at a high temperature, the C0₂ is separated more completely from the solution, so that less C0₂ remains in the combustion gas.

Figure 4 shows an example of a process using air scavenging, in which the partial pressure of the C0₂ is assumed to be 4 bar, i . e . in combustion gas with 25 % C0₂, dissolution will take place at a pressure of 16 bar.

The combustion gas is led to the dissolution part 10.1 of column 10, in which it flows upwards as its C0₂ flows downwards into the solvent. The C0₂ solution then flows into the enrichment part of the column and from there through the heat exchanger 23, the turbine or liquid motor 16, and line 17 to the evaporation pipe 26 in the dissolution column, in which a pressure somewhat higher than the original partial pressure prevails, and in which the heat released in the dissolution transfers the C0₂ to the mixture of C0₂ and air that returns to the pipeline. The evaporation pipes 26 continue through the enrichment column into the chamber at its bottom end, in which scavenging air is bubbled (air connection 27) through the solution and from which depleted solvent is pumped to the upper end of the dissolution column through the line 19, by means of a pump 15.

The gas flow coming from the evaporation pipe is boosted to a suitable pressure and the boosted gas is cooled, until most of its C0₂ has liquefied. The gaseous mixture of C0₂ and air remaining from the liquefaction is led cold into the enrichment part of the column (connection 28), in which it increases the C0₂ content of the solution coming from the dissolution part, and from which the remaining gaseous mixture combines with the combustion gas flow going to the dissolution part. Thus, the C0₂ content of the solution going to evaporation and the pressure of the C0₂ evaporated from it are increased more than would be possible on the basis of only the partial pressure of the C0₂ in the combustion gas .

By means of the air scavenging, the evaporation process becomes inverse to the dissolution, i.e. the partial pressure of the C0₂ of the gas increases as it moves upwards , at the same time as its content in the downward flowing solution decreases. In this way, it is possible to avoid the dilution of the C0₂ solution associated with evaporation carried out in constant-pressure C0₂, while the process comes closer to reversibility.

The combustion gas can be cooled to the process temperature and the C0₂ can be heated after separation, by means of the Vet' heat exchanger of Figure 5, in which liquid, e.g., a 1:2 mixture of propylene glycol and water, with a low vapour pressure and congealing point, is circulated between the two columns 30 and 31. These are connected to each other by means of pipe lines 32 and 34, in which there are transfer pumps 33 and 33'. The combustion gas is fed into the connection 30.1 and cleaned and heated combustion gas exits from the connection 31.1. The connections 30.2 and 30.1 are joined to the connec- tions 11.1 and 11.2 in the previous Figures. Separation of S compounds, e.g. using the Wellman-Lord process, can also be combined with the heat exchange. As in the other columns examined here, the columns of the wet heat exchanger can also be equipped with packing, bases, or other structures to guide the gas and liquid flow and to increase the efficiency of the heat transfer and mass transport. The wet heat exchanger reduces the loss into the combustion gas of the solvent used for the separation of the C0₂, as it washes the solvent out of the exiting combustion gas and transfers it to the incoming combustion gas, which thus arrives in the C0₂ 5 separation saturated with solvent.

The combination of a wet heat exchanger with the C0₂ separation creates the integrated' process of Figure 6, in which all sub- processes are carried out in the column 10. The same reference ιo numbers as above are used for functionally similar components.

The combustion gas is cooled to the process temperature in the cooling part 10.2, by means of the cold solvent coming from the evaporation chamber 13. It then rises into the dissolution part 15 10.1, in which the C0₂ dissolves into the solvent flowing against it. The solvent is collected in an intermediate base 24, from which it is led to the two-stage evaporation (chambers 12 and 13), or to some other evaporation process described above .

20

If the solvent also acts as a heat exchanging liquid, the liquid selected for this purpose must have, besides the solvent properties described above, also a sufficiently low vapour pressure and low congealing point. Such are, for example, 25 aqueous solutions of propylene carbonate and polyatomic alcohols .

The efficiency of the heat exchange process is maximized when the thermal capacity flow of the solvent/heat-exchange liquid 30 more or less corresponds to that of the combustion gas.

In order to improve the economy of the separation process, the pressurized combustion gas exited from it is used to produce mechanical or electrical energy by heating it and expanding it 35 in a turbine or other engine in such a way that there can be one or several heating and expansion stages.

Claims

1. A process for separating carbon dioxide, C0₂, from combustion gas, in which process: - the combustion gases entering the process are brought to a high pressure, and

- they are washed (10.1) with a solvent, to transfer the C0₂ from the combustion gases to the solvent, and

- the combustion gases are removed from the process after washing, and

- the solvent is led from the washing process to a lower pressure evaporation process (12, 13) , in which the C0₂ is separated from the solvent and led out of the process, and

- the solvent is pumped backed into the dissolution process, and

- a solvent, such as methanol, which has a strong C0₂ solubility pressure gradient, deviating from Henry's law, over a suitable temperature range, is selected as the washing liquid, characterized in that - the dissolution and evaporation are carried out nearly isothermally in the said temperature range near the condensation point of C0₂ at the partial pressure of the C0₂ used in the process, and

- the dissolution and evaporation are carried out thermodynami- cally in nearly reversible conditions.

2. A process according to Claim 1, characterized in that the dissolution and evaporation are carried out in the same column

(10) in chambers (10.1, 12, 13) separated by intermediate walls, in order to transfer the heat from the dissolution to the evaporation.

3. A process according to Claim 1, characterized in that the dissolution and evaporation are carried out in two or several columns (10', 10"), which are connected to each other by means of a separate heat exchange circuit (21) .

4. A process according to any of the above Claims 1 - 3, characterized in that the difference between the dissolution and the evaporation temperatures is in the range 4 - 8°C, to transfer the dissolution heat to the evaporation.

5. A process according to any of the above Claims 1 - 4, characterized in that the evaporation (12, 13) is carried out in two or several stages and the C0₂ is removed by means of a separate intake compressor (20) or an ejector at a common exhaust pressure.

6. A process according to any of the above Claims 1 - 5, characterized in that the solvent is led from the higher pressure dissolution through a liquid motor or turbine (16) to the lower pressure evaporation, in order to exploit the pressure energy of the solution.

7. A process according to any of the above Claims 1 - 6, characterized in that the incoming flow of combustion gas is cooled by the exiting flow of combustion gas, by using a separate liquid circulation equipped with a wet heat exchanger (30, 31) , which at the same time binds the residual solvent from the exiting flow of combustion gas and transfers it to the incoming flow of combustion gas.

8. A process according to any of the above Claims 1 - 6, characterized in that the incoming flow of combustion gas is cooled by the exiting flow of combustion gas, by using a wet heat exchanger (10.2, 10.3, 32) integrated with the column (10) used for the dissolution of the C0₂, in which the cold solvent obtained from the evaporator cools (10.2) the incoming combustion gas, after which the heated solvent is led to the upper part (10.3) of the column (10), in which it heats the exiting combustion gas and at the same time cools itself before descending into the dissolution part.

9. A process according to any of the above Claims 1 - 8, characterized in that

- at least part of the evaporation process of the C0₂ solution obtained in the dissolution process is carried out in a chamber

5 into which air is fed (27), which is removed from the C0₂-air mixture obtained by pressurizing it to a suitable pressure and cooling it, until most of the C0₂ liquefies,

- the liquefied C0₂ is removed from the separation process either as a liquid or as a gas,

10 - the gaseous C0₂-air mixture remaining from the liquefaction is returned (28) to the dissolution process, in which it enriches the C0₂ content of the solvent used to wash the combustion gas .

15 10. A process according to any of the above Claims 1 - 9, characterized in that the pressurized combustion gas exiting from the separation process is used to produce mechanical or electrical energy by heating it and expanding it in a turbine or other engine in such a way that there can be one or several

20 heating and expansion stages.