Oxyfuel combustion boiler system

Abstract

A combined heat exchanger including a flue gas cooler heat-transfer unit supplied with cooling fluid by a supply pump and an upstream GGH heat-transfer unit for circulation of circulating fluid with a downstream GGH by a circulation pump is arranged at an outlet of a gas air heater for heat exchange of flue gas from a boiler body for oxyfuel combustion with recirculation flue gases. A low-low temperature ESP is arranged at an outlet of the combined heat exchanger. A heat-exchange-duty adjustment device is arranged to regulate heats exchanged in the heat-transfer units in the heat exchanger such that at least flue gas temperature at the inlet of the low-low temperature ESP is kept to an inlet set temperature.

Claims

1. An oxyfuel combustion boiler system comprising: a boiler body for oxyfuel combustion, a gas air heater for heating recirculation flue gas with flue gas from said boiler body during oxyfuel combustion, a combined heat exchanger arranged at an outlet of said gas air heater, a low-low temperature electrostatic precipitator arranged at an outlet of said combined heat exchanger, a downstream gas-gas heat exchanger arranged downstream of said low-low temperature electrostatic precipitator, a compartment wall arranged in said combined heat exchanger to provide first and second chambers, a flue gas cooler heat-transfer unit arranged in said first chamber in said combined heat exchanger, an upstream gas-gas heat exchanger heat-transfer unit arranged in said second chamber in said combined heat exchanger, a supply pump which supplies cooling fluid to said flue gas cooler heat-transfer unit, a circulation pump which circulates circulating fluid between said downstream gas-gas heat exchanger and said upstream gas-gas heat exchanger heat-transfer unit, and a heat-exchange-duty adjustment device which adjusts heat exchange duties in said flue gas cooler heat-transfer unit and said upstream gas-gas heat exchanger heat-transfer unit to keep at least a temperature of flue gas at an inlet of the low-low temperature electrostatic precipitator at an inlet set temperature, wherein said heat-exchange-duty adjustment device comprises: first and second inner vanes which independently regulate flow rates of flue gas flowing to said flue gas cooler heat-transfer unit and said upstream gas-gas heat-exchanger heat-transfer unit, respectively, a cooling-fluid bypass valve that enables the cooling fluid to bypass said flue gas cooler heat-transfer unit, a circulating-fluid bypass valve that enables the circulating fluid to bypass said upstream gas-gas heat exchanger heat transfer unit and return to the downstream gas-gas heat exchanger, an inlet thermometer which detects a temperature of flue gas at an inlet of said low-low temperature electrostatic precipitator, an outlet thermometer which detects a temperature of flue gas at an outlet of said downstream gas-gas heat exchanger, a vane opening-degree controller which independently controls opening degrees of said first and second inner vanes, an inlet temperature controller which controls opening degrees of said cooling-fluid and circulating-fluid bypass valves to adjust a temperature of the flue gas at the inlet of said low-low temperature electrostatic precipitator detected by the inlet thermometer and make the temperature of the flue gas at the inlet of said low-low temperature electrostatic precipitator equal to the inlet set temperature, an outlet temperature controller which controls the opening degree of said second inner vane via said vane opening-degree controller to adjust a temperature of flue gas at the outlet of said downstream gas-gas heat exchanger detected by the outlet thermometer and make the temperature of the flue gas at the outlet of said downstream gas-gas heat exchanger equal to an outlet set temperature, and an operational controller which command-controls said vane opening-degree controller and said inlet and outlet temperature controllers depending on an operational condition.

2. The oxyfuel combustion boiler system as claimed in claim 1, wherein said vane opening-degree controller is adapted to: control the first inner vane to a fully closed position and control the second inner vane to a fully opened position during air combustion at startup of the oxyfuel combustion boiler system, control the first inner vane from the fully closed position, by gradually increasing its opening degree, to a fully opened position and control the second inner vane from the fully opened position, by gradually decreasing its opening degree, to a minimum opening degree during switching between air and oxyfuel combustions, and control the first inner vane to the fully opened position and control the second inner vane to a predetermined opening degree during oxyfuel combustion; wherein said inlet temperature controller is adapted to: control the opening degree of said circulating-fluid bypass valve such that the temperature at the inlet of said low-low temperature electrostatic precipitator detected by the inlet thermometer is made equal to the inlet set temperature during said air combustion, wherein an amount of the cooling fluid flowing to the flue gas cooler heat-transfer unit is regulated to a minimum flow rate using said cooling-fluid bypass valve and wherein an amount of the circulating fluid flowing to the upstream gas-gas heat exchanger heat-transfer unit is regulated to a predetermined flow rate using said circulating-fluid bypass valve, control the opening degree of said circulating-fluid bypass valve in a step before an intermediate point of a switch process of said first and second inner vanes such that the temperature of the flue gas at the inlet of said low-low temperature electrostatic precipitator detected by the inlet thermometer is made equal to the inlet set temperature during said switch process, wherein amounts of the cooling and circulating fluids flowing to the flue gas cooler heat-transfer unit and the upstream gas-gas heat exchanger heat-transfer unit, respectively, are preliminarily regulated to predetermined flow rates using the cooling-fluid and circulating-fluid bypass valves, respectively, and control the opening degree of said cooling-fluid bypass valve such that the temperature of the flue gas at the inlet of said low-low temperature electrostatic precipitator detected by the inlet thermometer is made equal to the inlet set temperature in a step after the intermediate point of the switch process during said switch process and during said oxyfuel combustion; and wherein said outlet temperature controller is adapted to control the opening degree of the second inner vane via said vane opening-degree controller such that the temperature of the flue gas detected by the outlet thermometer at the outlet of said downstream gas-gas heat exchanger is made equal to the outlet set temperature during said oxyfuel combustion.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a block diagram showing an embodiment of an oxyfuel combustion boiler system according to the invention;

(2) FIG. 2 is a block diagram showing an operation during air combustion in the oxyfuel combustion boiler system according to the invention;

(3) FIG. 3 is a block diagram showing an operation during switching between air and oxyfuel combustions in the oxyfuel combustion boiler system according to the invention; and

(4) FIG. 4 is a block diagram showing an operation during oxyfuel combustion in the oxyfuel combustion boiler system according to the invention.

DESCRIPTION OF EMBODIMENT

(5) Next, an embodiment of the invention will be described in conjunction with the drawings.

(6) FIG. 1 shows an embodiment of an oxyfuel combustion boiler system according to the invention in which reference numeral 1 denotes a boiler body for oxyfuel combustion. Arranged at an outlet of the boiler body 1 through a denitrator 3 for denitration of flue gas 2 is a gas air heater 4 which heat-exchanges the flue gas 2 with recirculation flue gas to be referred to hereinafter, thereby preheating the recirculation flue gas.

(7) Arranged at an outlet of the gas air heater 4 through a combined heat exchanger 5 is a dry electrostatic precipitator or low-low temperature ESP 6. Arranged at an outlet of the low-low temperature ESP 6 through an induced draft fan 7 is a desulfurizer 8. Arranged at an outlet of the desulfurizer 8 is a downstream GGH 9. Arranged at an outlet of the downstream GGH 9 through a booster fan 10 is a carbon dioxide capture unit 11.

(8) The combined heat exchanger 5 has therein a compartment wall 12 for compartmented flow of the flue gas 2 into left and right chambers 5a and 5b. Arranged in one 5a of the chambers compartmented by the compartment wall 12 is a flue gas cooler heat-transfer unit 16 supplied with cooling fluid 15 (low-pressure supply water) by a supply passage 14 having a supply pump 13. Arranged in the other chamber 5b compartmented by the compartment wall 12 is an upstream GGH heat-transfer unit 20 connected to a the downstream GGH 9 by a circulation passage 18 having a circulation pump 17 for circulated supply of circulating fluid 19 (circulating water).

(9) The combined heat exchanger 5 is provided with a heat-exchange-duty adjustment device 21 constructed as follows.

(10) First and second inner vanes 22 and 23 are arranged at outlets of the one and the other chambers 5a and 5b and are regulatable in opening degree by first and second drives 22a and 23a, respectively.

(11) Arranged between upstream and downstream sides of the supply passage 14 connected to the flue gas cooler heat-transfer unit 16 is a cooling-fluid bypass valve 24. Arranged between upstream and downstream sides of the circulation passage 18 connected to the upstream GGH heat-transfer unit 20 is a circulating-fluid bypass valve 25.

(12) The first and second inner vanes 22 and 23 are adapted to be independently regulated by signals transmitted to the drives 22a and 23a from a vane opening-degree controller 27 which in turn is operated by a command 26a from an operational controller 26 depending on an operational condition (air combustion, switch between air and oxyfuel combustions and oxyfuel combustion).

(13) Arranged at an inlet of the low-low temperature ESP 6 is an inlet thermometer 28, a temperature detected by the thermometer 28 being inputted to an inlet temperature controller 29. The inlet temperature controller 29 is operated by the command 26a from the operational controller 26 depending on the operational condition to regulate the opening degrees of the bypass valves 24 and 25 such that the detected temperature of the inlet thermometer 28 at the inlet of the low-low temperature ESP 6 is made equal to an inlet set temperature T.sub.1 (in FIG. 1, any temperature within a range of 85-90 C.)

(14) Arranged at an outlet of the downstream GGH 9 is an outlet thermometer 30, a temperature detected by the outlet thermometer 30 being inputted to an outlet temperature controller 31. The outlet temperature controller 31 is operated by the command 26a from the operational controller 26 depending on the operational condition to regulate the opening degree of the second inner vane 23 such that the detected temperature of the outlet thermometer 30 at the outlet of the downstream GGH 9 is made equal to an outlet set temperature T.sub.2 (in FIG. 1, any temperature within a range of 45-75 C.). When the signal from the outlet temperature controller 31 is inputted, the vane opening-degree controller 27 cuts off the command 26a from the operational controller 26 on the operational condition, and regulates the opening degree of the second inner vane 23 on the basis of the signal from the outlet temperature controller 31.

(15) In the embodiment shown in FIG. 1, for simplification, the description is made on a case where the supply and circulation pumps 13 and 17 are driven at a uniform rate of rotation. However, in addition to the operation of the heat-exchange-duty adjustment device 21, the flow rates of the cooling and circulating fluids 15 and 19 by the supply and circulation pumps 15 and 17, respectively, may be concurrently regulated.

(16) In the oxyfuel combustion boiler system shown in FIG. 1, part of the flue gas from which soot dust has been removed by the low-low temperature ESP 6 at the outlet thereof is extracted as secondary recirculation flue gas 34 by a secondary recirculation line 33 with a secondary booster fan 32. The secondary recirculation flue gas 34 is guided to and preheated by the gas air heater 4, and is mixed with oxygen (O.sub.2) 35 and supplied to the boiler body 1.

(17) Part of the flue gas at the outlet of the downstream GGH 9 is extracted as primary recirculation flue gas 38 by a primary recirculation line 37 with a primary booster fan 36. Then, the primary recirculation flue gas 38 is divaricated into two; one of them is supplied to the gas air heater 4 where it undergoes heat exchange into preheated flue gas 38a. The other of the primary recirculation flue gas 38 bypasses the gas air heater 4 and is mixed, as it remains low-temperature flue gas 38b, with the preheated flue gas 38a through the dampers 39a and 39b and the like into temperature-regulated primary recirculation gas. The temperature-regulated primary recirculation gas is guided to, for example, a pulverized coal mill (not shown) to be accompanied by pulverized coal 40 and then is supplied to the boiler body 1. Illustrated in FIG. 1 is a case where part of the flue gas at the outlet of the low-low temperature ESP 6 is extracted as secondary recirculation flue gas 34 and part of the flue gas at the outlet of the downstream GGH 9 is extracted as primary recirculation flue gas 38; however, secondary and primary recirculation flue gas 34 and 38 may be extracted anywhere downstream of the low-low temperature ESP 6.

(18) As mentioned in the above, the combined heat exchanger 5 is constituted to have therein the flue gas cooler and upstream GGH heat-transfer units 16 and 20, so that a construction between the gas air heater 4 and the low-low temperature ESP 6 can be simplified, leading to reduction of the installation space required.

(19) Further, the vane opening-degree controller 27 regulates the opening degrees of the first and second inner vanes 22 and 23 depending on the operation condition from the operational controller 26, and the inlet temperature controller 29 regulates the opening degrees of the bypass valves 24 and 25 such that the detected temperature of the inlet thermometer 28 at the inlet of the low-low temperature ESP 6 is made equal to the inlet set temperature T.sub.1. As a result, the flue gas regulated to 85-90 C. suitable for soot dust removal is supplied to the low-low temperature ESP 6, whereby high soot dust removal performance is exhibited.

(20) On the other hand, the flue gas supplied from the low-low temperature ESP 6 to the wet type desulfurizer 8 is cooled to, for example, anywhere from 40-50 C. The flue gas at the outlet of the desulfurizer 8 may contain sulfuric acid mist so that, if the flue gas is discharged downstream as it is, it may disadvantageously bring about sulfate corrosion of downstream pipings and equipment.

(21) Thus, depending on the operational condition from the operational controller 26, the outlet temperature controller 31 controls the opening degree of the second inner vane 23 via the vane opening-degree controller 27 such that the detected temperature of the outlet thermometer 30 at the outlet of the downstream GGH 9 is made equal to the outlet set temperature T.sub.2. As a result, the flue gas temperature at the outlet of the downstream GGH 9 is regulated to anywhere from 45-75 C., so that the downstream equipment is prevented from being corroded by sulfuric acid mist in the flue gas. Here, it has been known to be effective that the temperature of the flue gas at the outlet of the desulfurizer 8 is enhanced by the temperature of 5-25 C. by the downstream GGH 9. Since excessively enhanced flue gas temperature by the downstream GGH 9 would increase recovery load by way of cooling in the downstream carbon dioxide capture unit 11, it is preferable that to obtain the flue gas temperature in the rage of 45-75 C. by the temperature increase of 5-25 C.

(22) Next, with reference to FIGS. 2-4, the oxyfuel combustion boiler system will be described.

(23) FIG. 2 shows flue gas temperature control during the air combustion at the startup of the oxyfuel combustion boiler system. The boiler body, which is cold at the startup of the oxyfuel combustion boiler system, is heated by the air combustion of oil or gas.

(24) <Air Combustion>

(25) For the air combustion of the fuel in the boiler body 1 in FIG. 1, no recirculation of the secondary and primary recirculation flue gases 34 and 38 is conducted. The air is taken in by air-intakes upstream of the secondary and primary booster fans 32 and 36 and is boosted in pressure by secondary and primary booster fans 32 and 36, respectively, and is heated by the gas air heater 4 and supplied to the boiler body 1. For the air combustion, as shown in FIG. 2, the vane opening-degree controller 27 receives the command 26a on the operation condition (air combustion) from the operational controller 26 to control the first inner vane 22 for the flue gas cooler heat-transfer unit 16 to full close and the second inner vane 23 for the upstream GGH heat-transfer unit 20 to full open.

(26) The inlet temperature controller 29 receives the command 26a on operation condition (air combustion) from the operational controller 26 to controlswith an amount of cooling fluid 15 to the flue gas cooler heat-transfer unit 16 being regulated to a minimum flow rate using the cooling-fluid bypass valve 24 and with an amount of the circulating fluid 19 to the upstream GGH heat-transfer unit 20 being regulated to a rated flow rate using the circulating-fluid bypass valve 25the opening degree of the circulating-fluid bypass valve 25 such that the detected temperature of the inlet thermometer 28 at the inlet of the low-low temperature ESP 6 is made equal to the inlet set temperature T.sub.1. In this case, a temperature at the outlet of the downstream GGH 9 remains as it is and is not controlled; only effected is the above-mentioned control for keeping the inlet temperature of the low-low temperature ESP 6 to the inlet set temperature T.sub.1.

(27) <Switch Between Air and Oxyfuel Combustions>

(28) For the switch of the air and oxyfuel combustions shown in FIG. 3, the vane opening-degree controller 27 receives the command 26a on the operational condition (switch) from the operational controller 26 to control the first inner vane 22 for the flue gas cooler heat-transfer unit 16 from full close via gradually increased opening degree to full open, and the second inner vane 23 for the upstream GGH heat-transfer unit 20 from full open via gradually decreased opening degree to a minimum opening degree.

(29) The inlet temperature controller 29 receives the command 26a on the operational condition (switch) from the operational controller 26 to controlwith the amounts of the cooling and circulating fluids 15 and 19 to the heat-transfer units 16 and 20 being initially regulated to rated flow rates by the bypass valves 24 and 25, respectivelythe opening degree of the circulating-fluid bypass valve 25 in a step before an intermediate point during switch process of the first and second inner vanes 22 and 23 (for example, the opening degrees of the first and second inner vanes 22 and 23 being 50%, respectively) as switch point such that the detected temperature of the inlet thermometer 28 at the inlet of the low-low temperature ESP 6 is made equal to the inlet set temperature T.sub.1, and control the opening degree of the cooling-fluid bypass valve 24 at a step after the switch point during the switching such that the detected temperature of the inlet thermometer 28 at the inlet of the low-low temperature ESP 6 is made equal to the inlet set temperature T.sub.1. In this case, the temperature at the outlet of the downstream GGH 9 remains as it is and is not controlled; and only effected is the above-mentioned control for keeping the inlet temperature of the low-low temperature ESP 6 to the inlet set temperature T.sub.1.

(30) <Oxyfuel Combustion>

(31) For the oxyfuel combustion shown in FIG. 4, the vane opening-degree controller 27 receives the command 26a of the operational condition (oxyfuel combustion) from the operational controller 26 to control the first inner vane 22 for the flue gas cooler heat-transfer unit 16 to full open and the second inner vane 23 for the upstream GGH heat-transfer unit 20 to a controlled opening degree.

(32) The inlet temperature controller 29 receives the command 26a on the operational condition (oxyfuel combustion) from the operational controller 26 to control the opening degree of the cooling-fluid bypass valve 24 such that the detected temperature of the inlet thermometer 28 at the inlet of the low-low temperature ESP 6 is made equal to the inlet set temperature T.sub.1. Moreover, the outlet temperature controller 31 receives the command 26a on the operational condition (oxyfuel combustion) from the operational controller 26 to control the opening degree of the second inner vane 23 for the upstream GGH heat-transfer unit 20 via the vane opening-degree controller 27 such that the detected temperature of the outlet thermometer 30 at the outlet of the downstream GGH 9 is made equal to the outlet set temperature T.sub.2.

(33) Thus, during the oxyfuel combustion, the flue gas temperature at the inlet of the low-low temperature ESP 6 is kept to the inlet set temperature T.sub.1 (for example, 85-90 C.) to thereby ensure high soot dust removal performance by the low-low temperature ESP 6, and the flue gas temperature at the outlet of the downstream GGH 9 is kept to the outlet set temperature T.sub.2 (45-75 C.) to thereby prevent the downstream equipment from undergoing sulfate corrosion.

(34) It is to be understood that an oxyfuel combustion boiler system according to the invention is not limited to the above embodiment and that various changes and modifications may be made without departing from the scope of the invention.

INDUSTRIAL APPLICABILITY

(35) An oxyfuel combustion boiler system according to the invention may be extensively used for a boiler or other oxyfuel combustion installation.

REFERENCE SIGNS LIST

(36) 1 boiler body 2 flue gas 4 gas air heater 5 combined heat exchanger 6 low-low temperature ESP 9 downstream GGH 12 compartment wall 13 supply pump 15 cooling fluid 16 flue gas cooler heat-transfer unit 17 circulation pump 19 circulating fluid 20 upstream GGH heat-transfer unit 21 heat-exchange-duty adjustment device 22 first inner vane 23 second inner vane 24 cooling-fluid bypass valve 25 circulating-fluid bypass valve 26 operational controller 26a command 27 vane opening-degree controller 28 inlet thermometer 29 inlet temperature controller 30 outlet thermometer 31 outlet temperature controller 34 secondary recirculation flue gas (recirculation flue gas) 38 primary recirculation flue gas (recirculation flue gas) T.sub.1 inlet set temperature T.sub.2 outlet set temperature