Patent Application
20110104624
The present invention relates to a method and an apparatus of controlling combustion in an oxyfuel combustion boiler.
An increased carbon dioxide (CO2) density in the atmosphere has proved to be one of major factors of global warming which has recently come into attention as a global-scale environmental problem. A thermal power plant appears close-up as a fixed source of discharging these substances. Fuel for thermal power generation may be oil, natural gas and coal, among which coal is especially anticipated to have a large future demand due to its greater potential reserves.
Coal contains a higher percentage of carbon as compared with natural gas and oil, together with other components such as hydrogen, nitrogen and sulfur, and ash as an inorganic component. Therefore, when coal is burned in the air, most of the composition of the combustion exhaust gas is occupied by nitrogen (about 70%), with the remainder occupied by carbon dioxide CO2, sulfur oxide SOX, nitrogen oxide NOX, dust comprising ash and unburned coal particles, and oxygen (about 4%). The combustion exhaust gas is thus subjected to exhaust gas treatments such as denitration, desulfurization and dedusting so that NOX, SOX and particulates fall under their respective environmental emission standard values before the emission to the atmosphere through a stack.
NOX occurring in the combustion exhaust gas is divided into a thermal NOX generated from oxidization of nitrogen in the air by oxygen and a fuel NOX generated as a result of oxidization of nitrogen in the fuel. Up until now, a combustion method of lowering the flame temperature has been employed for reduction of the thermal NOX whereas another combustion method of forming a fuel-excess region for deoxidizing NOX within a burner has been employed for reduction of the fuel NOX.
In case of using a fuel containing sulfur such as coal, a wet or dry desulfurizing device has been provided to remove SOX occurring in the combustion exhaust gas as a result of the combustion.
It is desired on the other hand that a large amount of carbon dioxide generated in the combustion exhaust gas be also separated and removed with high efficiency. A possible method of capturing carbon dioxide in the combustion exhaust gas has hitherto been reviewed which includes a method of causing an amine or other absorbing liquid to absorb it, an adsorption method of causing a solid adsorbent to adsorb it or a membrane separation method, all of which have a low conversion efficiency, thus not yet reaching a practical use level of the CO2 capture from a coal burning boiler.
Accordingly, a combustion technology of a fuel with oxygen instead of air has been proposed as an effective manner to address at one time both the problem of separation of carbon dioxide in the combustion exhaust gas and the problem of suppression of the thermal NOX (see, for example, Patent Literature 1).
When coal is burned with oxygen, generation of the thermal NOX is not seen and most of the combustion exhaust gas is occupied by carbon dioxide with the remainder occupied by other gases containing the fuel NOX and SOX, consequently achieving a relatively easy liquefaction and separation of the carbon dioxide through cooling of the combustion exhaust gas.
In a conventional air-combustion coal burning boiler, nitrogen is a balance gas for oxygen which is a major component gas other than oxygen in the air used for combustion of pulverized coal whereas in an oxyfuel combustion boiler, carbon dioxide and steam become balance gases for oxygen since they are main component gases other than oxygen in the recirculating exhaust gas.
Thermal properties, however, differ between nitrogen and carbon dioxide and steam. Hence, there occurs a problem that when the oxygen density (boiler-brought-in oxygen density) for a total amount of gases introduced into the oxyfuel combustion boiler is set to approx. 21% which is an oxygen density in the air, the flame temperature lowers as compared with the air combustion, resulting in an insufficient furnace heat absorption.
The invention was made in view of the above and has its object to provide a method and an apparatus of controlling combustion in an oxyfuel combustion boiler, ensuring a sufficient furnace heat absorption through prevention of lowering of flame temperature to thereby achieve stabilized oxyfuel combustion operations.
The invention is directed to a method of controlling combustion in an oxyfuel combustion boiler where while oxygen fed from an air separation unit is introduced into a coal burning boiler, an exhaust gas in recirculation is introduced as primary and secondary recirculating exhaust gases into a mill and the coal burning boiler, respectively, the pulverized coal pulverized by the mill being transferred by said primary recirculating exhaust gas to a burner for oxyfuel combustion with said oxygen and said secondary recirculating exhaust gas, the method comprising
In the method of controlling combustion in the oxyfuel combustion boiler, preferably, the boiler-brought-in oxygen density falls within a range from 25 to 30%.
The invention is also directed to an apparatus of controlling combustion in an oxyfuel combustion boiler where while oxygen fed from an air separation unit is introduced into a coal burning boiler, an exhaust gas in recirculation is introduced as primary and secondary recirculating exhaust gases into a mill and the coal burning boiler, respectively, the pulverized coal pulverized by the mill being transferred by said primary recirculating exhaust gas to a burner for oxyfuel combustion with said oxygen and said secondary recirculating exhaust gas, the apparatus comprising
The invention is further directed to an apparatus of controlling combustion in an oxyfuel combustion boiler where while oxygen fed from an air separation unit is introduced into a coal burning boiler, an exhaust gas in recirculation is introduced as primary and secondary recirculating exhaust gases into a mill and the coal burning boiler, respectively, the pulverized coal pulverized by the mill being transferred by said primary recirculating exhaust gas to a burner for oxyfuel combustion with said oxygen and said secondary recirculating exhaust gas, the apparatus comprising
In the apparatus for controlling combustion in the oxyfuel combustion boiler, preferably, the boiler-brought-in oxygen density falls within a range from 25 to 30%.
According to a method and an apparatus of controlling combustion in an oxyfuel combustion boiler of the invention, there can be obtained an excellent effect of ensuring a sufficient furnace heat absorption through prevention of lowering of flame temperature to thereby achieve stabilized oxyfuel combustion operations.
Embodiments of the invention will be described with reference to the accompanying drawings.
Referring to
Incorporated in the oxygen feed line 18 for the wind box are an O2 density monitor 22 and a flowmeter 23 for measuring, respectively, an O2 density 22a and a flow rate 23a of oxygen to be directly fed to the wind box 5 of the coal burning boiler 4.
Incorporated in the primary recirculating exhaust gas line 12 at an inlet of the mill 3 are an O2 density monitor 24 and a flowmeter 25 for sensing, respectively, an O2 density 24a and a flow rate 25a of the primary recirculating exhaust gas to be introduced into the mill 3.
Incorporated in the secondary recirculating exhaust gas line 16 are an O2 density monitor 26 and a flowmeter 27 for sensing, respectively, an O2 density 26a and a flow rate 27a of the secondary recirculating exhaust gas fed with oxygen from the oxygen feed line 17 for secondary recirculating exhaust gas.
Incorporated in the a recirculating exhaust gas line 28 on an outlet side of the forced draft fan 11 and upstream of branch points to the primary and secondary recirculating exhaust gas lines 12 and 16 is a flow rate regulating damper 29 acting as a flow rate regulator for regulating a flow rate of total recirculating exhaust gases to be introduced into the mill 3 and the coal burning boiler 4.
Further, a controller 30 is disposed for calculating a boiler-brought-in oxygen density which is an oxygen density for a total amount of gases introduced into the coal burning boiler 4 on the basis of the O2 densities 22a, 24a and 26a measured by the O2 density monitors 22, 24 and 26, respectively, and the flow rates 23a, 25a and 27a measured by the flowmeters 23, 25 and 27, respectively, the controller 30 outputting an opening degree control signal 29a as flow rate control signal to the flow rate regulating damper 29 such that the boiler-brought-in oxygen density falls within a predetermined range. In place of the flow rate regulating damper 29, any other flow rate regulator such as a damper may be used to which the flow rate control signal is outputted from the controller 30.
As shown in
Operations in the above illustrated embodiment will be described.
In the normal operation of the coal burning boiler 4 as set forth hereinabove, the coal stored in the coal bunker 1 is fed by the coal feeder 2 to the mill 3 where coal is pulverized into pulverized coal. A part of the exhaust gas forcedly sent by the forced draft fan 11 from the recirculating exhaust gas line 28 is led as primary recirculating exhaust gas through the primary recirculating exhaust gas line 12 into the mill 3 via the air preheater 8 for preheating; the primary recirculating exhaust gas dries the coal fed to the mill 3 and transfers the pulverized coal produced by the mill 3 to the burner 6. Another part of the exhaust gas forcedly sent by the forced draft fan 11 from the recirculating exhaust gas line 28 is led as secondary recirculating exhaust gas through the secondary recirculating exhaust gas line 16, via the air preheater 8 for preheating, into the wind box 5 of the coal burning boiler 4 to which oxygen produced by the air separation unit 10 is directly fed through the oxygen feed line 18 for wind box, so that the pulverized coal is subjected to oxyfuel combustion within the coal burning boiler 4.
At the start-up of the coal burning boiler 4, air (not shown) in lieu of the primary recirculating exhaust gas is introduced into the mill 3 so that the air dries coal fed to the mill 3 and transfers pulverized coal obtained therein to the burner 6. Air (not shown) instead of the secondary recirculating exhaust gas and oxygen is fed to the wind box 5 of the coal burning boiler 4 so that the pulverized coal undergoes air combustion within the coal burning boiler 4. When the heat absorption of the coal burning boiler 4 reaches a predetermined value, the air is switched to the primary recirculating exhaust gas, the secondary recirculating exhaust gas and oxygen for shifting to oxyfuel combustion.
An exhaust gas from the coal burning boiler 4 is introduced through the exhaust gas line 7 into the air preheater 8 where the primary and secondary recirculating exhaust gases are heated and subjected to heat recovery. The exhaust gas passing through the air preheater 8 goes to the exhaust gas treating devices 9 such as a desulfurizer and a dust collector for desulfurization and dust collection, with the result that the exhaust gas purified by the exhaust gas treating devices 9 is inducedly sucked by the induced draft fan 20 before the emission through the stack 21 to the atmosphere. The exhaust gas passing through the exhaust gas treating devices 9 is partly recirculated by the forced draft fan 11 and partly introduced into the capture device 19 for the capture of CO2, etc. from the exhaust gas.
In the normal operation of the coal burning boiler 4 of the illustrated embodiment, the O2 density 22a of oxygen to be directly fed to the wind box 5 of the coal burning boiler 4 is sensed by the O2 density monitor 22; the flow rate 23a of oxygen to be directly fed to the wind box 5 of the coal burning boiler 4 is sensed by the flowmeter 23; the O2 density 24a of the primary recirculating exhaust gas to be introduced into the mill 3 is sensed by the O2 density monitor 24; the flow rate 25a of the primary recirculating exhaust gas to be introduced into the mill 3 is sensed by the flowmeter 25; the O2 density 26a of the secondary recirculating exhaust gas fed with oxygen from the oxygen feed line 17 for secondary recirculating exhaust gas is sensed by the O2 density monitor 26; the flow rate 27a of the secondary recirculating exhaust gas fed with oxygen from the oxygen feed line 17 for secondary recirculating exhaust gas is sensed by the flowmeter 27; and the boiler-brought-in oxygen density which is an oxygen density to the total amount of gases introduced into the coal burning boiler 4 is calculated by the controller 30 on the basis of the O2 densities 22a, 24a and 26a sensed by the O2 density monitors 22, 24 and 26, respectively, and the flow rates 23a, 25a and 27a sensed by the flowmeters 23, 25 and 27, respectively (see step S1 of
It is then determined whether the boiler-brought-in oxygen density is blow 25% (see step S2 of
If negative, i.e., if the boiler-brought-in oxygen density is not below 25%, then it is determined whether the boiler-brought-in oxygen density is over 30% (see step S4 of
The sufficient furnace heat absorption is thus obtained through prevention of lowering of the flame temperature, enabling the oxyfuel combustion operations to be performed in a stable manner.
In the normal operation of the coal burning boiler 4 of this illustrated embodiment, the O2 density 31a and the flow rate 32a of oxygen introduced into the coal burning boiler 4 are sensed by the O2 density monitor 31 and the flowmeter 32, respectively; the O2 density 33a and the flow rate 34a of the total recirculating exhaust gases to be introduced into the mill 3 and the coal burning boiler 4 are sensed by the O2 density monitor 33 and the flowmeter 34, respectively. The boiler-brought-in oxygen density of oxygen introduced into the coal burning boiler 4 is calculated in the controller 30 on the basis of the O2 densities 31a and 33a sensed by the O2 density monitors 31 and 33, respectively, and the flow rates 32a and 34a sensed by the flowmeters 32 and 34, respectively (see step S1 of
Thus, similar to the
It is to be understood that a method and an apparatus of the invention for controlling combustion in an oxyfuel combustion boiler are not limited to the above embodiments and that various changes and modifications may be made without departing from the scope of the invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/000473 | 3/6/2008 | WO | 00 | 12/3/2010 |
