CO2 recovery unit and CO2 recovery method

Abstract

A CO.sub.2 recovery unit and a CO.sub.2 recovery method capable of having an excellent CO.sub.2 absorption rate and saving energy are provided. A CO.sub.2 recovery unit of the invention includes: a CO.sub.2 absorber which includes an upper CO.sub.2 absorption unit obtaining a CO.sub.2 absorbent by causing a flue gas containing CO.sub.2 to contact a CO.sub.2 absorbent and a lower CO.sub.2 absorption unit obtaining a CO.sub.2 absorbent by causing the CO.sub.2 absorbent to contact a flue gas containing CO.sub.2; a CO.sub.2 absorbent regenerator which obtains the CO.sub.2 absorbent by heating the CO.sub.2 absorbent a thermometer which measures a temperature of the CO.sub.2 absorbent supplied from the CO.sub.2 absorber to the CO.sub.2 absorbent regenerator; and a control device which controls a temperature of the CO.sub.2 absorbent supplied to the lower CO.sub.2 absorption unit based on the temperature of the CO.sub.2 absorbent measured by the thermometer.

Claims

1. A CO.sub.2 recovery method comprising: obtaining a first CO.sub.2 absorbent by causing a CO.sub.2 containing gas to be treated to contact a CO.sub.2 absorbent in a first CO.sub.2 absorption unit of a CO.sub.2 absorber so that CO.sub.2 contained in the gas to be treated is absorbed to the CO.sub.2 absorbent and obtaining a second CO.sub.2 absorbent by causing the first CO.sub.2 absorbent to contact the CO.sub.2 containing gas to be treated in a second CO.sub.2 absorption unit of the CO.sub.2 absorber so that CO.sub.2 contained in the gas to be treated is absorbed to the first CO.sub.2 absorbent; regenerating the CO.sub.2 absorbent by heating the second CO.sub.2 absorbent in a CO.sub.2 absorbent regenerator so that CO.sub.2 is discharged from the CO.sub.2 absorbent; and measuring a temperature of the second CO.sub.2 absorbent supplied from the CO.sub.2 absorber to the CO.sub.2 absorbent regenerator and controlling a temperature of the first CO.sub.2 absorbent supplied to the second CO.sub.2 absorption unit based on the measured temperature of the second CO.sub.2 absorbent, wherein controlling the temperature of the first CO.sub.2 absorbent supplied to the second CO.sub.2 absorption unit such that the measured temperature of the second CO.sub.2 absorbent is in a range of 62 C. to 67 C., wherein a CO.sub.2 partial pressure of the CO.sub.2 containing gas to be treated is in a range of 50 kPa to 200 kPa.

2. The CO.sub.2 recovery method according to claim 1, wherein the temperature of the first CO.sub.2 absorbent supplied to the second CO.sub.2 absorption unit is controlled so that the temperature is in the range of 50 C. to 60 C.

3. The CO.sub.2 recovery method according to claim 1, wherein a ratio (the first CO.sub.2 absorption unit:the second CO.sub.2 absorption unit) between a filling material charging height in the first CO.sub.2 absorption unit and a filling material charging height in the second CO.sub.2 absorption unit is in the range of 1:3 to 3:1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic diagram illustrating a CO.sub.2 recovery unit according to an embodiment of the invention.

(2) FIG. 2 is a diagram illustrating a relation between a temperature of a CO.sub.2 absorbent (a semi-rich solution) supplied to a lower CO.sub.2 absorption unit and a ratio of a CO.sub.2 absorption rate of a rich solution.

(3) FIG. 3 is a diagram illustrating a relation between a temperature of a CO.sub.2 absorbent (a semi-rich solution) supplied to a lower CO.sub.2 absorption unit and a ratio of a heat amount necessary to regenerate the CO.sub.2 absorbent.

(4) FIG. 4 is a diagram illustrating a relation between a temperature of a CO.sub.2 absorbent (a semi-rich solution) supplied to a lower CO.sub.2 absorption unit and a temperature of a CO.sub.2 absorbent (a rich solution) supplied to a CO.sub.2 absorbent regenerator.

(5) FIG. 5 is a diagram illustrating a relation of filling material charging height ratios of a lower CO.sub.2 absorption unit and an upper CO.sub.2 absorption unit of a CO.sub.2 absorber with respect to a ratio of a CO.sub.2 absorption rate of a rich solution.

DESCRIPTION OF EMBODIMENTS

(6) The present inventors have paid attention to a conventional CO.sub.2 recovery unit which recovers CO.sub.2 from a gas having a relatively low CO.sub.2 partial pressure (for example, 10 kPa to 15 kPa) such as a combustion flue gas discharged from a boiler of a thermal power station and does not recover CO.sub.2 from a synthetic gas having a relatively high CO.sub.2 partial pressure (for example, 50 kPa to 200 kPa) and discharged from a direct reducing furnace. Then, the present inventors have found that a CO.sub.2 recovery unit and a CO.sub.2 recovery method capable of both having an excellent CO.sub.2 absorption rate and saving energy are obtained by a configuration in which a CO.sub.2 absorber is provided with a plurality of CO.sub.2 absorption units and a temperature of a CO.sub.2 absorbent supplied to the plurality of CO.sub.2 absorption units is controlled based on a temperature of a CO.sub.2 absorbent supplied to a CO.sub.2 absorbent regenerator when CO.sub.2 is recovered from a synthetic gas having a relatively high CO.sub.2 partial pressure, whereby the invention is obtained.

(7) Hereinafter, an embodiment of the invention will be described in detail with reference to the accompanying drawings. The invention is not limited to the embodiments below and can be appropriately modified. Further, the components of the CO.sub.2 recovery unit according to the embodiments below can be appropriately combined with one another.

(8) FIG. 1 is a schematic diagram illustrating a CO.sub.2 recovery unit according to an embodiment of the invention. As illustrated in FIG. 1, a CO.sub.2 recovery unit 1 is an apparatus which recovers CO.sub.2 in a reducing furnace flue gas (a gas to be treated) 11A, corresponding to a synthetic gas discharged from a direct reducing furnace, in the form of a high-concentration CO.sub.2 gas. The CO.sub.2 recovery unit 1 includes a cooling tower 12 which cools a flue gas 11A containing CO.sub.2 discharged from a direct reducing furnace, a CO.sub.2 absorber 14 which is provided at a rear stage of the cooling tower 12 and causes the cooled flue gas 11A to contact a CO.sub.2 absorbent 13 so that CO.sub.2 in the flue gas 11A is absorbed to the CO.sub.2 absorbent 13 to be removed from the flue gas, and a CO.sub.2 absorbent regenerator 15 which is provided at a rear stage of the CO.sub.2 absorber 14 and discharges CO.sub.2 from a CO.sub.2 absorbent 13C having CO.sub.2 absorbed thereto to regenerate the CO.sub.2 absorbent 13.

(9) In the CO.sub.2 recovery unit 1, the CO.sub.2 absorbent 13 is circulated between the CO.sub.2 absorber 14 and the CO.sub.2 absorbent regenerator 15. The CO.sub.2 absorbent 13 (a lean solution) is supplied as the CO.sub.2 absorbent 13C having CO.sub.2 absorbed thereto (a rich solution) in the CO.sub.2 absorber 14 to the CO.sub.2 absorbent regenerator 15. Further, CO.sub.2 is removed from the CO.sub.2 absorbent 13C (the rich solution) by the CO.sub.2 absorbent regenerator 15 and a resultant gas is supplied as the regenerated CO.sub.2 absorbent 13 (the lean solution) to the CO.sub.2 absorber 14.

(10) The cooling tower 12 includes a cooling unit 121 which cools the flue gas 11A. Further, a circulation line L.sub.1 is provided between a bottom portion of the cooling tower 12 and a top portion of the cooling unit 121. The circulation line L.sub.1 is provided with a heat exchanger 122 which cools cooling water W.sub.1, a circulation pump 123 which circulates the cooling water W.sub.1 in the direction line L.sub.1, and an adjustment valve 124 which adjusts the amount of a waste liquid separated as a liquid from the circulation line L.sub.1 and discharged therefrom.

(11) In the cooling unit 121, the flue gas 11A is cooled by a counterflow contact between the flue gas 11A and the cooling water W.sub.1 and thus a cooled flue gas 11B is obtained. The heat exchanger 122 cools the cooling water W.sub.1 which is heated by exchanging heat with the flue gas 11A. The circulation pump 123 supplies the cooling water W.sub.1 flowing down to the bottom portion of the cooling tower 12 through the heat exchanger 122 to a top portion of the cooling unit 121. In the cooling tower 12, when the amount of moisture in the flue gas 11A is small, a liquid level of the cooling tower 12 decreases and thus water is supplied from a tower top portion. Further, when the amount of the moisture in the flue gas 11A is large, the liquid level of the cooling tower 12 increases and thus a part of the cooling water W.sub.1 circulated in the circulation line L.sub.1 is separated as waste liquid.

(12) The CO.sub.2 absorber 14 includes a CO.sub.2 absorption unit 141 which is provided in a lower portion of the CO.sub.2 absorber 14 and to which the CO.sub.2 absorbent 13 and the flue gas 11B cooled by the cooling tower 12 are supplied, and a water washing unit 142 which is provided in an upper portion of the CO.sub.2 absorber 14.

(13) The CO.sub.2 absorption unit 141 includes a lower CO.sub.2 absorption unit (a second CO.sub.2 absorption unit) 141A which is provided in a lower portion of the CO.sub.2 absorption unit 141 and an upper CO.sub.2 absorption unit 141B (a first CO.sub.2 absorption unit) which is provided in an upper portion of the CO.sub.2 absorption unit 141. A filling material is charged into the lower CO.sub.2 absorption unit 141A at a charging height H1. A filling material is charged into the upper CO.sub.2 absorption unit 141B at a charging height H2. The CO.sub.2 absorbent 13 which is regenerated by the CO.sub.2 absorbent regenerator 15 is supplied to the upper CO.sub.2 absorption unit 141B. A CO.sub.2 absorbent 13B which absorbs CO.sub.2 in a flue gas 11C by the upper CO.sub.2 absorption unit 141B is supplied to the lower CO.sub.2 absorption unit (the second CO.sub.2 absorption unit) 141A.

(14) A liquid storage unit 143A, which stores a CO.sub.2 absorbent (a first CO.sub.2 absorbent) 13A flowing down from the upper CO.sub.2 absorption unit 141B and staying at a lower portion of the upper CO.sub.2 absorption unit 141B, and a chimney tray 143B are provided between the lower CO.sub.2 absorption unit 141A and the upper CO.sub.2 absorption unit 141B. The liquid storage unit 143A is provided with an extraction line L.sub.11 which extracts the CO.sub.2 absorbent 13A stored in the liquid storage unit 143A from the CO.sub.2 absorber 14 and supplies the liquid to the lower CO.sub.2 absorption unit 141A.

(15) The extraction line L.sub.11 is provided with a heat exchanger 24 which cools the CO.sub.2 absorbent 13A to obtain the cooled CO.sub.2 absorbent 13B and a pump 25 which supplies the CO.sub.2 absorbent 13A as the CO.sub.2 absorbent 13B to the lower CO.sub.2 absorption unit 141A. The heat exchanger 24 is configured to adjust a refrigerant supply amount by a control device 101. Further, the pump 25 is configured to adjust the amount of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A by the control device 101. The control device 101 can be realized by, for example, a general or dedicated computer such as a CPU (Central Processing Unit), a ROM (Read Only Memory)/and a RAM (Random Access Memory) and a program operated on this computer.

(16) A bottom portion of the water washing unit 142 is provided with a liquid storage unit 144A which stores washing water W.sub.2 for washing a flue gas 11D obtained by removing CO.sub.2 therefrom in the flue gas 11C. A circulation line L.sub.2 which supplies the washing water W.sub.2 containing the CO.sub.2 absorbent 13 recovered by the liquid storage unit 144A from a top portion of the water washing unit 142 so that the washing water is circulated is provided between the liquid storage unit 144A and the water washing unit 142. The circulation line L.sub.2 is provided with a heat exchanger 21 which cools the washing water W.sub.2 and a circulation pump 22 which circulates the washing water W.sub.2 containing the CO.sub.2 absorbent 13 recovered by the liquid storage unit 144A through the heat exchanger 21 so that the washing water is circulated in the circulation line L.sub.2. Further, the circulation line L.sub.2 is provided with an extraction line L.sub.3 which extracts a part (washing water W.sub.3) of the washing water W.sub.2 and supplies the water to the CO.sub.2 absorbent 13 (the lean solution). The extraction line L.sub.3 is provided with an adjustment valve 23 which adjusts the amount of the washing water W.sub.3 supplied to the CO.sub.2 absorbent 13.

(17) In the CO.sub.2 absorption unit 141, a counterflow contact between the flue gas 11A containing CO.sub.2 by the upper CO.sub.2 absorption unit 141B and the CO.sub.2 absorbent 13 containing alkanolamine occurs. Accordingly, CO.sub.2 in the flue gas 11C is absorbed to the CO.sub.2 absorbent 13 by a chemical reaction expressed in the following formula. As a result, CO.sub.2 in the flue gas 11C is removed so that the flue gas 11C becomes the flue gas 11D obtained by removing CO.sub.2 therefrom and the CO.sub.2 absorbent 13 becomes the CO.sub.2 absorbent 13A. Then, in the lower CO.sub.2 absorption unit 141A, a counterflow contact between the flue gas 11B containing CO.sub.2 and the CO.sub.2 absorbent 13B having CO.sub.2 absorbed thereto occurs. Accordingly, CO.sub.2 in the flue gas 11B is absorbed to the CO.sub.2 absorbent 13B by the chemical reaction expressed in the following formula. As a result, CO.sub.2 in the flue gas 11B is removed so that the flue gas 11B becomes the flue gas 11C of which a CO.sub.2 concentration is decreased and the CO.sub.2 absorbent 13B becomes the CO.sub.2 absorbent 13C. In this way, when the flue gas 11B containing CO.sub.2 passes through the CO.sub.2 absorption unit 141, the flue gas 11D obtained by removing CO.sub.2 therefrom is obtained. Further, the CO.sub.2 absorbent 13 absorbs CO.sub.2 to become the CO.sub.2 absorbent 13B (the semi-rich solution) in the upper CO.sub.2 absorption unit 141B and the CO.sub.2 absorbent 13B further absorbs CO.sub.2 to become the CO.sub.2 absorbent 13C (the rich solution) in the lower CO.sub.2 absorption unit 141A.
RNH.sub.2+H.sub.2O+CO.sub.2.fwdarw.RNH.sub.3HCO.sub.3

(18) In the water washing unit 142, the flue gas 11D obtained by removing CO.sub.2 therefrom after passing through the upper CO.sub.2 absorption unit 141B rises through a chimney tray 144B. Then, a gas-liquid contact occurs between the flue gas 11D and the washing water W.sub.2 supplied from the top portion of the water washing unit 142 so that a flue gas 11B is obtained by recovering the CO.sub.2 absorbent 13 accompanied by the flue gas 11D through circulating and washing processes. After mist in the flue gas 11E is trapped by a mist eliminator 145, the flue gas is discharged to the outside from a tower top portion 14a of the CO.sub.2 absorber 14.

(19) A rich solution supply pipe 50 which supplies the CO.sub.2 absorbent 13C having CO.sub.2 absorbed thereto (the rich solution) in the CO.sub.2 absorber 14 to an upper portion of the CO.sub.2 absorbent regenerator 15 is provided between a tower bottom portion 14b of the CO.sub.2 absorber 14 and the upper portion of the CO.sub.2 absorbent regenerator 15. The rich solution supply pipe 50 is provided with a thermometer (a temperature measurement device) 102 which measures a temperature of the CO.sub.2 absorbent 13C, a rich solution pump 51 which supplies the CO.sub.2 absorbent 13C having CO.sub.2 absorbed thereto in the CO.sub.2 absorber 14 to the CO.sub.2 absorbent regenerator 15, and a rich-lean solution heat exchanger 52 which heats the CO.sub.2 absorbent 13C by the CO.sub.2 absorbent 13 (the lean solution) heated by the CO.sub.2 absorbent regenerator 15 to remove CO.sub.2 therefrom. The control device 101 adjusts the amount of a refrigerant supplied to the heat exchanger 24 based on a temperature of the CO.sub.2 absorbent 13C measured by the thermometer 102 and controls the amount of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A by the pump 25. Additionally, the thermometer 102 may be provided at a position where the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A can be controlled by the control device 101. For example, the thermometer may be provided at a rear stage of the heat exchanger 24 of the extraction line L.sub.11.

(20) A CO.sub.2 absorbent supply unit 151 to which the CO.sub.2 absorbent 13C having CO.sub.2 absorbed thereto is supplied is provided at a enter portion of the CO.sub.2 absorbent regenerator 15. A tower bottom portion 15b of the CO.sub.2 absorbent regenerator 15 is provided with a circulation line L.sub.4 which circulates the CO.sub.2 absorbent 13C flowing down to the tower bottom portion. The circulation line L.sub.4 is provided with a regenerating heater 31 which heats the CO.sub.2 absorbent 13 by saturated steam S, an adjustment valve 32 which supplies the saturated steam S to the regenerating heater 31, and a circulation pump 33 which supplies the CO.sub.2 absorbent 13 of a tower bottom portion of the CO.sub.2 absorbent regenerator 15 to a lower portion of the CO.sub.2 absorbent supply unit 151 of the CO.sub.2 absorbent regenerator 15 through the regenerating heater 31.

(21) A tower top portion 15a of the CO.sub.2 absorbent regenerator 15 is provided with a gas discharge line L.sub.5 which discharges a CO.sub.2 gas 41 accompanying steam. The gas discharge line L.sub.5 is provided with a condenser 42 which condenses moisture in the CO.sub.2 gas 41 and a separation drum 43 which separates water W.sub.5 condensed by the CO.sub.2 gas 41. A CO.sub.2 gas 44 from which the condensed water W.sub.5 is separated is discharged to the outside from the upper portion of the separation drum 43. A condensed water line L.sub.6 which supplies the condensed water W.sub.5 separated in the separation drum 43 to an upper portion of the CO.sub.2 absorbent regenerator 15 is provided between the bottom portion of the separation drum 43 and the upper portion of the CO.sub.2 absorbent regenerator 15. The condensed water line L.sub.6 is provided with a condensed water circulation pump 45 which supplies the condensed water W.sub.5 separated in the separation drum 43 to the upper portion of the CO.sub.2 absorbent regenerator 15. An adjustment valve 46 which controls the amount of the condensed water W.sub.5 supplied to the CO.sub.2 absorbent regenerator 15 is provided between the condensed water circulation pump 45 and the CO.sub.2 absorbent regenerator 15. Further, a re-circulation line L.sub.12 which divides a part of the condensed water W.sub.5 supplied to the CO.sub.2 absorbent regenerator 15 and re-circulates the condensed water W.sub.5 supplied to the water washing unit 142 of the CO.sub.2 absorber 14 is provided between the condensed water circulation pump 45 and the circulation line L.sub.2. The re-circulation line L.sub.12 is provided with an adjustment valve 47 which adjusts the amount of the condensed water W.sub.5 supplied to the water washing unit 142.

(22) Further, the tower bottom portion of the CO.sub.2 absorbent regenerator 15 and the upper portion of the CO.sub.2 absorption unit 141 of the CO.sub.2 absorber 14 are provided with a lean solution supply pipe 53 which supplies the CO.sub.2 absorbent 13 (the lean solution) of the tower bottom portion of the CO.sub.2 absorbent regenerator 15 to the upper portion of the CO.sub.2 absorption unit 141. The lean solution supply pipe 53 is provided with the rich-lean solution heat exchanger 52 which heats the CO.sub.2 absorbent 13C having CO.sub.2 absorbed thereto (the rich solution) by the CO.sub.2 absorbent 13 (the lean solution) heated by the steam in the CO.sub.2 absorbent regenerator 15 so that CO.sub.2 is removed therefrom, a lean solution pump 54 which supplies the CO.sub.2 absorbent 13 of the tower bottom portion of the CO.sub.2 absorbent regenerator 15 to the upper portion of the CO.sub.2 absorption unit 141, and a cooling unit 55 which cools the CO.sub.2 absorbent 13 (the lean solution) to a predetermined temperature.

(23) Next, a relation between a CO.sub.2 absorption rate and a temperature of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A of the CO.sub.2 absorber 14 in the CO.sub.2 recovery unit 1 according to the embodiment will be described with reference to FIG. 2. FIG. 2 is a diagram illustrating a relation between the temperature of the CO.sub.2 absorbent 13B (the semi-rich solution) supplied to the lower CO.sub.2 absorption unit and a ratio of the CO.sub.2 absorption rate of the rich solution. Additionally, in FIG. 2, a horizontal axis indicates the temperature of the CO.sub.2 absorbent 13B and a vertical axis indicates the ratio of the CO.sub.2 absorption rate of the rich solution. Further, in FIG. 2, a case where a flue gas having a low CO.sub.2 partial pressure (for example, about 10 kPa) such as a combustion flue gas discharged from a boiler is used is indicated by a dotted line and a case where a flue gas having a high CO.sub.2 partial pressure (for example, about 60 kPa) such as a synthetic gas discharged from a direct reducing furnace is used is indicated by a solid line. Additionally, a plot indicates an analysis value. Further, in FIG. 2, the flue gas having a low CO.sub.2 partial pressure and the flue gas having a high CO.sub.2 partial pressure are displayed on the same axes at a ratio in which a maximal value of the absorption rate is 1 in a temperature range equal to or higher than 40 C. and equal to or lower than 70 C., but have different maximal values of the absorption rates.

(24) As indicated by the dotted line of FIG. 2, when the flue gas having a low CO.sub.2 partial pressure is used, the CO.sub.2 absorption rate of the CO.sub.2 absorbent 13B increases in accordance with a decrease in temperature. For this reason, it is desirable to decrease a temperature of the CO.sub.2 absorbent 13B in order to efficiently recover CO.sub.2 in the flue gas having a low CO.sub.2 partial pressure.

(25) Meanwhile, when the flue gas having a high CO.sub.2 partial pressure is used as indicated by the solid line of FIG. 2, the CO.sub.2 absorption rate has a different tendency from the case where the flue gas having a low CO.sub.2 partial pressure is used. When the flue gas having a high CO.sub.2 partial pressure is used, the CO.sub.2 absorption rate becomes maximal at about 55 C., and the CO.sub.2 absorption rate decreases as the temperature falls from 55 C. However, in the embodiment, compared with the case where the flue gas having a low CO.sub.2 partial pressure is used, it is possible to improve the CO.sub.2 absorption rate in the lower CO.sub.2 absorption unit 141A by controlling the temperature of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A in an operation state and to decrease the heat amount of the CO.sub.2 absorbent 13C in the CO.sub.2 absorbent regenerator 15 by decreasing the circulation amount of the CO.sub.2 absorbent. Accordingly, energy saving is realized.

(26) As illustrated in FIG. 2, in the CO.sub.2 recovery unit 1 according to the embodiment, it is desirable to control the temperature of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A in a range equal to or higher than 50 C. and equal to or lower than 60 C. by the control device 101. Accordingly, the CO.sub.2 recovery unit 1 can set the temperature of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A within an appropriate range. For this reason, it is possible to further improve the CO.sub.2 absorption rate of the flue gas 11A by the CO.sub.2 absorbent 13B in the lower CO.sub.2 absorption unit 141A and to decrease the circulation amount of the CO.sub.2 absorbent 13B in accordance with the improved CO.sub.2 absorption rate. Thus, it is possible to decrease the amount of the saturated steam S consumed to regenerate the CO.sub.2 absorbent 13C. Further, the CO.sub.2 recovery unit 1 can set the temperature of the CO.sub.2 absorbent 13C supplied to the CO.sub.2 absorbent regenerator 15 to an appropriately high temperature and thus an effect of decreasing a steam consumption amount is obtained.

(27) Additionally, in the embodiment, the CO.sub.2 partial pressure of the flue gas 11B is desirably equal to or higher than 50 kPa and equal to or lower than 200 kPa. When the CO.sub.2 partial pressure is equal to or higher than 50 kPa, the CO.sub.2 absorption rate of the lower CO.sub.2 absorption unit 141A has a different tendency when the CO.sub.2 partial pressure is low (for example, about 10 kPa) as indicated by the solid line of FIG. 2. Further, when the CO.sub.2 partial pressure is equal to or lower than 200 kPa, the amount of CO.sub.2 in the flue gas 11B can be sufficiently decreased by the CO.sub.2 absorber 14. From the viewpoint of improving the above-described operations and effects, the CO.sub.2 partial pressure of the flue gas 11B is more desirably 55 kPa or more, further desirably 60 kPa or more, more desirably 150 kPa or less, and further desirably 100 kPa or less. When the above-described fact is taken into consideration, the CO.sub.2 partial pressure of the flue gas 11B is more desirably equal to or higher than 55 kPa and equal to or lower than 150 kPa and further desirably equal to or higher than 60 kPa and equal to or lower than 100 kPa.

(28) Next, a relation between the temperature of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A in the CO.sub.2 recovery unit 1 according to the embodiment and the heat amount necessary to regenerate the CO.sub.2 absorbent 13 in the CO.sub.2 absorbent regenerator 15 will be described with reference to FIG. 3. FIG. 3 is a diagram illustrating a relation between the temperature of the CO.sub.2 absorbent 13B (the semi-rich solution) supplied to the lower CO.sub.2 absorption unit 141A and a ratio of the heat amount necessary to regenerate the CO.sub.2 absorbent 13. Further, in FIG. 3, a horizontal axis indicates the temperature of the CO.sub.2 absorbent 13B and a vertical axis indicates the ratio of the heat amount necessary to regenerate CO.sub.2. Further, in FIG. 3, the flue gas having a high CO.sub.2 partial pressure is displayed as a ratio in which a minimal value of the heat amount necessary to regenerate the CO.sub.2 absorbent 13B is 1 in a temperature range equal to or higher than 40 C. and equal to or lower than 70 C. Additionally, a plot indicates an analysis value.

(29) As illustrated in FIG. 3, when the flue gas having a high CO.sub.2 partial pressure is used, the heat amount necessary to regenerate the CO.sub.2 absorbent 13B in the CO.sub.2 absorbent regenerator 15 becomes minimal at about 55 C. and the heat amount necessary to regenerate the CO.sub.2 absorbent 13C increases as the temperature falls from 55 C. Thus, in the embodiment, as illustrated in FIG. 2, when the temperature of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A is set to a range in which the CO.sub.2 absorption rate of the flue gas 11A using the CO.sub.2 absorbent 13B of the lower CO.sub.2 absorption unit 141A is high, it is possible to decrease the amount of the saturated steam S consumed to regenerate the CO.sub.2 absorbent 13C. This is because the circulation amount of the CO.sub.2 absorbent 13 can be decreased in accordance with improvement in absorption rate.

(30) FIG. 4 is a diagram illustrating a relation between the temperature of the CO.sub.2 absorbent 13B (the semi-rich solution) supplied to the lower CO.sub.2 absorption unit 141A and the temperature of the CO.sub.2 absorbent 13C (the rich solution) supplied to the CO.sub.2 absorbent regenerator 15. Further, in FIG. 4, a vertical axis indicates the temperature of the CO.sub.2 absorbent 13C supplied to the CO.sub.2 absorbent regenerator 15 and a horizontal axis indicates the temperature of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A.

(31) As illustrated in FIG. 4, in the embodiment, there is a direct proportional relation between the temperature of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A and the temperature of the CO.sub.2 absorbent 13C supplied to the CO.sub.2 absorbent regenerator 15. Thus, when the control device 101 controls the amount of the refrigerant supplied to the heat exchanger 24 and the amount of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A by the pump 25 so that the temperature of the CO.sub.2 absorbent 13C supplied to the CO.sub.2 absorbent regenerator 15 is measured by the thermometer 102 and the measured temperature falls within a predetermined range (for example, a range equal to or higher than 62 C. and equal to or lower than 67 C.), it is possible to control the CO.sub.2 absorbent 13B of the lower CO.sub.2 absorption unit 141A at a desired temperature. Accordingly, it is possible to obtain a high CO.sub.2 absorption rate and to decrease a heat amount necessary to heat the CO.sub.2 absorbent 13C in the CO.sub.2 absorbent regenerator 15.

(32) FIG. 5 is a diagram illustrating a relation of filling material charging height ratios H1 and H2 of the lower CO.sub.2 absorption unit and the upper CO.sub.2 absorption unit of the CO.sub.2 absorber with respect to a ratio of the CO.sub.2 absorption rate of the rich solution. Additionally, in FIG. 5, a case where the filling material charging height ratios (the upper CO.sub.2 absorption unit 141B:the lower CO.sub.2 absorption unit 141A) H1 and H2 of the lower CO.sub.2 absorption unit 141A and the upper CO.sub.2 absorption unit 141B are changed in a range of 1:3 to 3:1 is displayed at a ratio in which a maximal value of the CO.sub.2 absorption rate is 1.

(33) As illustrated in FIG. 5, in the embodiment, when the filling material charging height ratios of the lower CO.sub.2 absorption unit 141A and the upper CO.sub.2 absorption unit 141B are changed, the CO.sub.2 absorption rate changes. For this reason, in the embodiment, it is desirable that the charging height ratio (the upper CO.sub.2 absorption unit 141B:the lower CO.sub.2 absorption unit 141A) between the charging height of the filling material the filling material H2 in the upper CO.sub.2 absorption unit 141B and the charging height of the filling material H1 in the lower CO.sub.2 absorption unit 141A be equal to or larger than 1:3 and equal to or smaller than 3:1. Accordingly, since the absorption efficiency for CO.sub.2 in the flue gas 11A of the upper CO.sub.2 absorption unit 141B and the absorption efficiency for CO.sub.2 in the flue gas 11A of the lower CO.sub.2 absorption unit 141A are respectively improved, it is possible to further improve a CO.sub.2 absorption rate and to save energy. As the charging height ratio, 1:1 is more desirable from the viewpoint of further improving the above-described operations and effects.

(34) Next, an overall operation of the CO.sub.2 recovery unit 1 according to the embodiment will be described. The flue gas 11A such as a synthetic gas containing CO.sub.2 discharged from the direct reducing furnace is introduced into the cooling tower 12 and is cooled by a counterflow contact with respect to the cooling water W.sub.1 to become the flue gas 11B. The cooled flue gas 11B is introduced into the CO.sub.2 absorber 14 through a flue gas duct 16 and a flow rate of the flue gas 11B introduced into the CO.sub.2 absorber 14 is measured. A counterflow contact occurs between the flue gas 11B introduced into the CO.sub.2 absorber 14 and the CO.sub.2 absorbent 13 containing alkanolamine in the lower CO.sub.2 absorption unit 141A and the upper CO.sub.2 absorption unit 141B of the CO.sub.2 absorption unit 141 so that CO.sub.2 in the flue gas 11B is absorbed to the CO.sub.2 absorbent 13 and the flue gas 11D is obtained by removing CO.sub.2 therefrom.

(35) The flue gas 11D obtained by removing CO.sub.2 therefrom rises through the chimney tray 144B and causes a gas-liquid contact with respect to the washing water W.sub.2 supplied from the top portion of the water washing unit 142 so that the flue gas 11E is obtained by recovering the CO.sub.2 absorbent 13 accompanied by the flue gas 11D through a circulating and washing process. Mist is the flue gas 11E is trapped by the mist eliminator 145 and the flue gas is discharged to the outside from the tower top portion 14a of the CO.sub.2 absorber 14.

(36) The CO.sub.2 absorbent 13C having CO.sub.2 absorbed thereto in the CO.sub.2 absorber 14 exchanges heat with the CO.sub.2 absorbent 13 (the lean solution) in the rich-lean solution heat exchanger 52 through the rich solution supply pipe 50 and is supplied to the upper portion of the CO.sub.2 absorbent regenerator 15 by the rich solution pump 51. Here, in the embodiment, the CO.sub.2 absorbent 13C flowing in the rich solution supply pipe 50 is measured at all times by the thermometer 102 and the measured temperature of the CO.sub.2 absorbent 13C is transmitted to the control device 101. The control device 101 adjusts the amount of the refrigerant supplied to the heat exchanger 24 and the amount of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A by the pump 25 so that the temperature of the CO.sub.2 absorbent 13C measured by the thermometer 102 falls within a predetermined range.

(37) CO.sub.2 is removed from the CO.sub.2 absorbent 13C supplied to the CO.sub.2 absorbent regenerator 15 while the CO.sub.2 absorbent 13C flows down to the tower bottom portion through the CO.sub.2 absorbent supply unit 151 and thus a semi-lean solution is obtained. This semi-lean solution is circulated in the circulation line L.sub.4 by the circulation pump 33 and is heated by the saturated steam S in the regenerating heater 31 so that the CO.sub.2 absorbent 13 (the lean solution) is obtained. The heated saturated steam S becomes the steam condensed water W.sub.4. The CO.sub.2 gas 41 removed from the CO.sub.2 absorbent 13 passes through the condenser 42 so that moisture is removed therefrom and is discharged as the CO.sub.2 gas 44, from which the condensed water W.sub.5 is separated, to the outside from the upper portion of the separation drum 43, The separated condensed water W.sub.5 is supplied to the CO.sub.2 absorbent regenerator 15 and a part of the water is divided so that the water is supplied to the water washing unit 142 of the CO.sub.2 absorber 14 through the re-circulation line L.sub.12.

(38) The CO.sub.2 absorbent 13 (the lean solution) of the tower bottom portion 15b of the CO.sub.2 absorbent regenerator 15 exchanges heat with the CO.sub.2 absorbent 13C (the rich solution) by the rich-lean solution heat exchanger 52 through the lean solution supply pipe 53 and is supplied to the upper portion of the CO.sub.2 absorption unit 141 of the CO.sub.2 absorber 14 by the lean solution pump 54. The CO.sub.2 absorbent 13 supplied to the CO.sub.2 absorption unit 141 absorbs CO.sub.2 of the flue gas 11A in the upper CO.sub.2 absorption unit 141B to become the CO.sub.2 absorbent (the semi-rich solution) 13A and is extracted from the lower portion of the upper CO.sub.2 absorption unit 141B to the extraction line L.sub.11. The extracted CO.sub.2 absorbent 13A is cooled to a predetermined temperature range by the heat exchanger 24 to become the CO.sub.2 absorbent (the semi-rich solution) 13B and is supplied to the lower CO.sub.2 absorption unit 141A by the pump 25 to absorb CO.sub.2 in the flue gas 11B by the lower CO.sub.2 absorption unit 141A so that the CO.sub.2 absorbent (the rich solution) 13C is obtained. The CO.sub.2 absorbent (the rich solution) 13C is extracted from the tower bottom portion 14b of the CO.sub.2 absorber 14 and is supplied to the CO.sub.2 absorbent regenerator 15.

(39) As described above, according to the embodiment, since the temperature of the CO.sub.2 absorbent 13B supplied to the lower CO.sub.2 absorption unit 141A is controlled based on the temperature of the CO.sub.2 absorbent 13C supplied to the CO.sub.2 absorbent regenerator 15, the CO.sub.2 absorption rate of the flue gas 11B of the lower CO.sub.2 absorption unit 141A can be increased. Accordingly, since the CO.sub.2 recovery unit 1 has an excellent CO.sub.2 absorption rate even when the synthetic gas having a high CO.sub.2 partial pressure in the flue gas 11B is treated, energy can be saved.

(40) Additionally, in the above-described embodiment, an example of treating the flue gas 11A such as a synthetic gas containing CO.sub.2 discharged from a direct reducing furnace has been described, but the invention can be applied to various gases including a natural gas (a methane gas) containing CO.sub.2.

REFERENCE SIGNS LIST

(41) 1, 2 CO.sub.2 Recovery Unit 11A, 11B, 11C, 11D, 11E Flue Gas 12 Cooling Tower 121 Cooling Unit 122 Heat Exchanger 123 Circulation Pump 124 Adjustment Value 13 CO.sub.2 Absorbent (Lean Solution) 13A CO.sub.2 Absorbent 13B CO.sub.2 Absorbent (Semi-Rich Solution) 13C CO.sub.2 Absorbent (Rich Solution) 14 CO.sub.2 Absorber 14a Tower Top Portion 14b Tower Bottom Portion 141 CO.sub.2 Absorption Unit 142 Water Washing Unit 143A Liquid Storage Unit 143B Chimney Tray 144A Liquid Storage Unit 144B Chimney Tray 145 Mist Eliminator 15 CO.sub.2 Absorbent Regenerator 15a Tower Top Portion 151 CO.sub.2 Absorbent Supply Unit 16 Flue Gas Duct 21 Heat Exchanger 22 Circulation Pump 23 Adjustment Valve 24 Heat Exchanger 31 Regenerating Heater 32 Adjustment Valve 33 Circulation Pump 41, 44 CO.sub.2 Gas 42 Condenser 43 Separation Drum 45 Condensed Water Circulation Pump 46, 47 Adjustment Valve 50 Rich Solution Supply Pipe 51 Rich Solution Pump 52 Rich-Lean Solution Heat Exchanger 53 Lean Solution Supply Pipe 54 Lean Solution Pump 55 Cooling Unit 101 Control Device 102 Thermometer (Temperature Measurement Device) L.sub.1, L.sub.2, L.sub.4 Circulation Line L.sub.3, L.sub.11 Extraction Line L.sub.5 Gas Discharge Line L.sub.6 Condensed Water Line L.sub.12 Re-Circulation Line S Saturation Steam W.sub.1 Cooling Water W.sub.2, W.sub.3 Washing Water W.sub.4 Steam Condensed Water W.sub.5 Condensed Water