Method of combustion

Abstract: A structure of a boiler of the type that pulverized fuel is burnt within a square barrel-shaped furnace (01) having a vertical axis, is improved so that ignitability is improved and a produced amount of NOx is decreased. Burners (21,22) for injecting pulverized fuel-air mixtures (09) are disposed at the central portions of the vertical furnace sidewalls and are inclined downwards with respect to a horizontal plane. Also, nozzles (27) for feeding air (29) are disposed under the respective burners.

Description: Field of the Invention: The present invention relates to improvements in operating boilers for electric utility or industrial use, furnaces for chemical industry, and the like which make use of pulverized solid fuel. Description of the Prior Art: At first, one example of a boiler furnace in the prior art which makes use of pulverized coal as fuel, will be described with reference to Fig. 6 showing a vertical cross-section view and Fig. 7 showing a horizontal cross-section view taken along line VII-VII in Fig. 6. In these figures, reference numeral 01 designates a furnace main body, numeral 02 designates burner main bodies, numeral 03 designates fuel nozzles, numeral 04 designates air nozzles for a main burner, numeral 05 designates pulverized coal transport pipes, numeral 06 designates combustion air lines, numeral 07 designates a coal pulverizer, numeral 08 designates a blower, numeral 09 designates pulverized coal-air mixture, numeral 10 designates combustion air, numeral 11 designates coal, numeral 12 designates conveying air, numeral 13 designates a furnace inner space, numeral 14 designates pulverized coal flames, numeral 15 designates main burner air lines, numeral 16 designates additional air lines, numeral 17 designates air for main burners, numeral 18 designates additional air, and numeral 19 designates additional air nozzles. The above-described furnace main body 04 is formed in a square barrel-shape having a vertical axis, and as shown in Fig. 7, it is provided with burner main bodies 02 at corner portions in a horizontal cross-section of a furnace wall. Each burner main body 02 is provided with a plurality of (three in the illustrated example) assemblies each consisting of a fuel nozzle 03 and air nozzles 04 assembled above and below the fuel nozzle 03, as aligned vertically, and these fuel nozzles 03 and air nozzles 04 are all directed horizontally towards the inner space of the furnace. Coal 11 fed to a coal pulverizer 07 is finely pulverized and mixed with conveying air (hot air) 12 which is fed simultaneously, to form pulverized coal-air mixture 09, and then the mixture sent to the burner main body 02 through pulverized coal transport pipes 05. The pulverized coal-air mixture sent to the burner main body 02 is injected to the furnace inner space 13 via the fuel nozzles 03. On the other hand, combustion air 10 is fed through combustion air lines 06 by a blower 08, then it is branched into main burner air 17 and additional air 18, and they are respectively injected to the furnace inner space 13 through air nozzles 04 provided in the burner main bodies 02 and through additional air nozzles 19 provided above the burner main bodies 02. The pulverized coal-air mixture 09 injected to the furnace inner space 13 is ignited by an ignition source not shown, and burns while forming pulverized coal flames 13. In the pulverized coal flames 14, the pulverized coal burns, in the proximity of an ignition point, as reacting with oxygen supplied by the conveying air 12 forming the pulverized coal-air mixture 09 together with the pulverized coal as well as a part (in the proximity of the ignition point) of the main burner air 17, and thereafter in a main combustion zone, combustion is continued by oxygen in the remainder of the main burner air 17. In a heretofore known boiler, since a total amount of the conveying air 12 and the main burner air 17 is made less than an amount corresponding to a stoichiometric ratio with respect to the pulverized coal injected through the fuel nozzles 03 for the purpose of suppressing production of nitrogen oxides (hereinafter abbreviated as NO x ), the furnace inner space 13 from the portion of the burner main bodies 02 up to the additional air nozzles 19 is held under a reducing atmosphere condition. Accordingly, the combustion gas produced by combustion of the pulverized coal-air mixture 09 would rise through the furnace inner space 13 initially in an incomplete combustion state, and the combustion is completed by the additional air 18 injected through the additional air nozzles 19. Also, in the heretofore known boiler, a mixing ratio of conveying air to pulverized coal in the pulverized coal-air mixture 09 was mostly chosen in the range of 2:1 to 4:1 in weight proportion generally in view of practical use of the coal pulverizer 07. That is, the pulverized coal-air mixture 09 was subjected to combustion at a mixing ratio of [conveying air]/[pulverized coal] (hereinafter abbreviated as A/C) = 2 - 4. Now let us consider the problems involved in the heretofore known boilers. [1] Generally, ignitability of the pulverized coal flames 14 is improved when the following condition is fulfilled. 1) A volatile constituent in pulverized coal is much, and a fuel ratio (fixed carbon/volatile constituent) is low; 2) A heat flow flux reaching a burner opening is large; 3) An A/C ratio of the pulverized coal-air mixture 09 is close to 1; and 4) An injection speed of the pulverized coal-air mixture 09 is small. Accordingly, boilers satisfying the above-mentioned conditions as much as possible are considered to be faborably. Fig. 8 is a diagram showing one example of results of practical measurement for distribution of a heat flow flux coming from a furnace inner space 13 and reaching a furnace wall with respect to a real boiler, and Fig. 9 is a diagram showing one example of results of experiments conducted in connection the relations between a flame propagation speed of pulverized coal and an A/C ratio of pulverized coal-air mixture. According to these diagrams, a heat flow flux coming from a furnace inner space 13 and reaching the furnace wall becomes maximum at the central portion of the furnace wall, and a flame propagation speed of pulverized coal becomes maximum at A/C ≒ 1 of the pulverized coal-air mixture. Since coal having a low volatile constituent and a high fuel ratio does not fulfil the condition 1) above, it is desirable to fulfil the other conditions 2), 3) and 4). However, in the heretofore known boiler, since the burner main bodies 02 were provided at the respective corner portions of the furnace main body 01 as shown in Fig. 7, a heat flow flux reaching the burner portion was small as shown in Fig. 8. On the other hand, in the case of employing coal having poor ignitability due to a low volatile content, it is necessary to improve the ignitability by making the A/C ratio of the pulverized coal-air mixture 09 fed to the burner main body 02 close to 1 (See Fig. 9), but in the heretofore known boiler the A/C ratio was generally 2 to 4 due to restriction in practical use of the coal pulverizer 07, and it could not be made close to 1. In addition, although the pulverized coal-air mixture 09 becomes ready to be ignited as its injection speed is slowed down in view of the relation to a flame propagation speed, as it is injected horizontally in the case of the boiler in the prior art, if the injection speed is too slow, pulverized coal in the pulverized coal-air mixture 09 would hang down and would accumulate at the fuel nozzle 03, and therefore, the injection speed cannot be made lower than a predetermined speed. As described above, the boilers in the prior art had a shortcoming that coal having a low volatile content or a high fuel ratio was difficult to be ignited. [2] With regard to combustion in a boiler, it is a well-known fact that an amount of production of NO x is in an inversely proportional relation to an amount of charging of additional air 18. However, in the heretofore known boiler system, since there is a problem in an ignitability in the case of coal having a low volatile content or a high fuel ratio, the amount of charging of the additional air 18 cannot be made much, and so, there was a bar against reduction of NO x . From US-A- 3 387 574 there is known a boiler furnace comprising the features of the preamble of claim 1. SUMMARY OF THE INVENTION: It is therefore the object of the present invention to provide a method of operating a boiler making use of pulverized solid fuel, in which ignitability is improved, even with fuel having a low volatile constituent and a high fuel ratio can be burnt, and a produced amount of NO x is decreased. According to the present invention, there is provided a method of combusting pulverized fuel in a boiler of the type that pulverized fuel is burnt within a square barrel-shaped furnace having a vertical axis, which comprises burners disposed at the central portions of respective sides in a horizontal cross-section of a furnace wall wherein pulverized fuel-air mixtures are injected in downwardly inclined directions with respect to a horizontal plane and air is fed into said furnace below the same burners. Since the burners are disposed at the central portions of respective sides in a horizontal cross-section of a furnace wall, an amount of heat received by a burner opening is extremely increased. In addition, as the burners are directed in downwardly inclined directions with respect to a horizontal plane, an injection speed of a pulverized fuel-air mixture can be set slow, and a stay time of combustion gas in a reducing atmosphere zone is prolonged. Furthermore, since air is fed below the burners, combustion at the furnace bottom portion becomes good. The above-mentioned and other objects, features and advantages of the present invention will become more apparent by reference to the following description of examples for carrying out the invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS: In the accompanying drawings: Fig. 1 is a vertical cross-section view showing a first example for carrying out the present invention; Figs. 2 and 3 are horizontal cross-section views taken respectively along line II-II and along line III-III, as viewed in the direction of arrows; Fig. 4 is a vertical cross-section view showing a second example for carrying out the present invention; Fig. 5 is a horizontal cross-section view taken along line V-V in Fig. 4 as viewed in the direction of arrows; Fig. 6 is a vertical cross-section view showing one example of a boiler furnace in the prior art which makes use of pulverized coal as fuel; Fig. 7 is a horizontal cross-section view taken along line VII-VII in Fig. 6 as viewed in the direction of arrows; Fig. 8 is a diagram showing one example of results of measurements for distribution of heat flow flux reaching from a furnace inner space to a wall surface, conducted in a real boiler; Fig. 9 is a diagram showing one example of results of experiments conducted with respect to a relation between a flame propagation speed of pulverized coal and an air-to-coal mixing ratio of a pulverized coal-air mixture; and Fig. 10 is a diagram showing one example of results of practical measurements for a relation between a stay time of combustion gas in the range from the center of the burner main body to an additional air nozzle portion and an NO x concentration at the outlet of the furnace. DESCRIPTION OF THE INVENTION: Now, the present invention will be described in greater detail in connection to the first example for carrying out the invention illustrated in Figs. 1, 2 and 3. For the purpose of avoiding redundant description, in these figures component parts similar to those of the heretofore known boiler described previously with reference to Figs. 6 and 7, are given like reference numerals and further explanation thereof will be omitted here. As new reference numerals in Figs. 1 to 3, reference numeral 20 designates pulverized coal separators, numeral 21 designates thick pulverized coal-air mixture nozzles, numeral 22 designates thin pulverized coal-air mixture nozzles, numeral 23 designates thick pulverized coal transport pipes, numeral 24 designates thin pulverized coal transport pipes, numeral 25 designates a thick pulverized coal-air mixture, numeral 26 designates thin pulverized coal-air mixture, numeral 27 designates under air nozzles, numeral 28 designates under air lines, and numeral 29 designates under air. Coal 11 fed to a coal pulverizer 07 is finely pulverized and mixed with simultaneously fed conveying air (hot air) 12 to form a pulverized coal-air mixture 09 (A/C = 2 - 4), and the mixture 09 is sent to the pulverized coal separators 20 through the pulverized coal transport pipes 05. Then it is separated into a thick pulverized coal-air mixture 25 (A/C ≒ 0.5 - 1.5) and a thin pulverized coal-air mixture 26 (A/C ≒ 5 - 20), and they are respectively sent to thick and thin pulverized coal-air mixture nozzles 21 and 22 assembled in the burner main bodies 02 through the thick pulverized coal transport pipes 23 and the thin pulverized coal transport pipes 24, respectively. As shown in Fig. 2, the above-mentioned burner main bodies 02 are disposed at the central portions of the respective ones of the four sides in a horizontal cross-section of the furnace wall of the square barrel-shaped furnace main body 01. This burner main body 02 is divided into a plurality of compartments, and each compartment is composed of both the thick and thin pulverized coal-air mixture nozzles 21 and 22 and a main burner air nozzle 04. Both the thick and thin pulverized coal-air mixture nozzles 21 and 22 are arrayed, in principle, in the sequence of thin-thick → thick-thin → thin-thick → thick-thin from the bottom or on the contrary in the sequence of thick-thin → thin-thick → thick-thin → thin-thick from the bottom, but in some cases, they may be assembled in the sequence of thick-thin → thick-thin → thick-thin (or in the opposite sequence to this). These plurality of thick and thin pulverized coal-air mixture nozzles 21 and 22 are all mounted as inclined downwards by 5 degrees to 45 degrees with respect to a horizontal plane, and the inject both the thick and thin pulverized coal-air mixtures 25 and 26 sent thereto into the furnace inner space 13. On the other hand, combustion air 10 is fed by a blower 08 through combustion air lines 06, and it is branched into main burner air 17, additional air 18 and under air 29. The main burner air 17 is injected into the furnace inner space 13 through the main burner air nozzles 04 assembled in the respective burner main body 02 and through the peripheral space of the both thick and thin pulverized coal-air mixture nozzles 21 and 22. The under air 29 is fed through the under air lines 28 and is blown into the furnace inner space 13 through the under air nozzles 27 provided separately below the burner main bodies 02. As shown in Fig. 3, the under air nozzles 27 are disposed at the central portions of the respective ones of four sides in a horizontal cross-section of the furnace wall so that each of their axes may be included in the same vertical plane as the axes of the corresponding burner main body 02. The total amount of the combustion air, the main burner air 17 and the under air 29 is made less than the amount corresponding to a stoichiometric ratio with respect to the amount of pulverized coal injected through the both thick and thin pulverized coal-air mixture nozzles 21 and 22 assembled in the burner main bodies 02, and the remainder of the air necessitated for completion of combustion is charged into the furnace inner space 13 through the additional air nozzles 19 as additional air 18. The thick pulverized coal-air mixture 25 injected into the furnace inner space 13 is ignited by a ignition source not shown and forms pulverized coal flames 14. Since the thick pulverized coal-air mixture 25 has a mixing ratio A/C ≒ 0.5 - 1.5 as described above, ignition is good and stable flames can be formed. While the thin pulverized coal-air mixture 26 simultaneously injected to the furnace inner space 13 is hard to keep flames and by itself cannot form flames because it has a mixing ratio A/C » 1 and a pulverized coal concentration is thin, it can continue combustion by the flames of the thick pulverized coal-air mixture 25 formed contiguously thereto. In addition, in the illustrated example, since the burner main bodies 02 are disposed at the central portions of the respective ones of four sides of the furnace wall where heat flow fluxes coming from the furnace inner space 13 become maximum on the same horizontal cross-section of the furnace wall, a heat receiving amount at the burner opening upon combustion is extremely increased as compared to the boiler in the prior art, and thus ignitability is improved. In general, in view of the relation to a flame propagation speed, ignitability becomes better as the injection speed of the thick pulverized coal-air mixture 25 is lowered, and in this example for carrying out the invention, owing to the fact that the thick pulverized coal-air mixture nozzles 21 are arranged as inclined downwards, hanging as well as accumulation on the pulverized coal-air mixture nozzles 21 of pulverized coal can be prevented, thus the injection speed can be set slower than that in the case of the boiler in the prior art, and accordingly, ignitability can be further improved. Fig. 10 is a diagram illustrating results of practical measurements conducted in a real system with respect to the relations between a combustion gas stay time in the range from the center of the burner main body 02 to the portion of the additional air nozzle 19 and an NO x concentration at the outlet of the furnace. In this diagram, as an NO x concentration value when the stay time is zero, an NO x concentration value when the additional air is not supplied is plotted. It is seen from this figure that an NO x concentration is greatly reduced by slightly extending the stay time. Since the total amount of air charged through the burner main bodies 02 and the under air nozzles 27 is less than the amount corresponding to a stoichiometric ratio with respect to the amount of pulverized coal fed through the burner main bodies 02, the furnace inner space 13 lower than the portion of the additional air nozzles 19 is a reducing atmosphere, where NO x produced by combustion of pulverized coal is reduced, and intermediate products such as NH₃, HCN and the like are produced. The amount of NO x at the outlet of the furnace is determined by an extent of this reducing reaction. If the stay time is long, then a reducing reaction time is also prolonged, and accordingly NO x is decreased. In this example for carrying out the invention, since the pulverized coal-air mixtures 25 and 26 are injected as inclined downwards, not only ignitability is improved as described above, but also a stay time in the furnace inner space 13 of the combustion gas becomes long, and there is an effect of decreasing NO x . However, if the pulverized coal-air mixtures 25 and 26 are injected as inclined downwards into the furnace inner space 13 held under a reducing atmosphere, there occur the following problems: 1 ○ Although the pulverized coal-air mixtures 25 and 26 injected from the thick and thin pulverized coal-air mixture nozzles 21 and 22, respectively, at the lowest level would form pulverized coal flames 14, since at the bottom of the furnace, a reducing atmosphere is present and a thermal load is low, the combustion product would fall under the state of charcoal (mainly a fixed carbon constituent) to the bottom of the furnace while the combustion is not proceeding sufficiently, then falls through ash discharge holes not shown into water within a clinker further below which is also not shown, and contaminates the clinker water into black. 2 ○ Under a reducing atmosphere, since a melting point of ash is lowered as compared to the case of an oxidizing atmosphere (a well-known fact), slagging becomes remarkable, and there is a fear that the ash discharge holes at the bottom of the furnace may clogged. 3 ○ At the bottom portion of the furnace, reducing corrosion is liable to occur. As a counter-measure against the above-mentioned problems, in this example, under air nozzles 27 are disposed under the burner main bodies 02 separately from the burner main bodies 02 in the same vertical planes as the axes of the burner main bodies 02. As the combustion of the pulverized coal-air mixtures 25 and 26 injected from the both thick and thin pulverized coal-air mixture nozzles 21 and 22 at the lowest level is promoted by the under air 29 fed through these under air nozzles 27 and the furnace inner space 13 under the burner main bodies 02 is held at an oxidizing atmosphere, contamination of the clinker water, clogging of the ash discharge holes at the bottom of the furnace, reducing corrosion of the bottom portion of the furnace, and the like can be prevented. Accordingly, the angle of downward inclination of the both thick and thin pulverized coal-air mixture nozzles 21 and 22 can be chosen large, hence a stay time within the furnace inner space 13 of the combustion gas in the range from the burner main bodies 02 to the portion of the additional air nozzles 19 is elongated by the corresponding amount, and the effect of decreasing NO x is enhanced. It is to be noted that the furnace inner space 13 lower than the portion of the additional air nozzles 19 is, as a whole, held at a reducing atmosphere. Next, a second example for carrying out the present invention will be described with reference to Figs. 4 and 5, which show a vertical cross-section view and a horizontal cross-section view taken along line V-V in Fig. 4. In these figures also, component parts similar to those of the first preferred embodiment described above, are given like reference numerals, and further explanation thereof will be omitted here. In this second example, pulverized coal separators 20 are not present in pulverized coal transport pipes 05 at an inlet portion of burner main bodies 20 as provided in the above-described first example. Accordingly, the distinction of the thick pulverized coal transport pipes 23 from the thin pulverized coal transport pipes 24 as well as the distinction of the thick pulverized coal-air mixture nozzles 21 from the thin pulverized coal-air mixture nozzles 22 are not present, and each pulverized coal-air transport pipe 05 is directly connected to one kind of pulverized coal-air mixture nozzle 03 disposed in the burner main body 02. The other structure is quite similar to that in the above-described first example. In this example also, the burner main bodies 02 are disposed at the respective central portions of four sides in a horizontal cross-section of the furnace wall where a heat flow flux reaching from the furnace inner space 13 becomes maximum similarly to the case of the first example, and thus provision is made such that a receiving amount of heat at a burner opening upon combustion may be remarkably increased as compared to the burner in the prior art. In this example, since the pulverized coal separator is not provided, the A/C ratio of the pulverized coal-air mixture 09 injected to the furnace inner space 13 is normally 2 - 4, and this is high as compared to the A/C ratio of the thick pulverized coal-air mixture in the first example. Accordingly, there is fear about ignitability in the case of coal having a low volatile constituent and a high fuel ratio, but owing to the fact that an injection speed of the pulverized coal-air mixture 09 can be made low and an amount of receiving heat at the burner opening because of the downwardly inclined (5° - 45°) pulverized coal-air mixture nozzles 03, the boiler furnace has extremely excellent ignitability as compared to that in the prior art. With regard to the other operation characteristics, this modified example is similar to the above-described first example, and there are almost equivalent advantages to the first example. As will be seen from the detailed description of the examples above, according to the present invention, the following advantages are obtained: 1) Owing to the fact that the burners are disposed at the central portions of the respective sides in a horizontal cross-section, of the furnace wall where a heat flow flux reaching from the furnace inner space becomes maximum, a receiving amount of heat at the burner opening is extremely increased, and thereby ignitability is improved. 2) As a result of the fact that fuel nozzles (fuel-air mixture nozzles) are disposed as inclined downwards, an injection speed of a pulverized coal-air mixture can be set slow as compared to that in the prior art, and hence even fuel having a low volatile constituent and a high fuel ratio which was hardly ignited in the prior art, can be properly burnt. 3) As a result of the downward inclination of the fuel nozzles, the time when combustion gas stays in a reducing atmosphere zone within the furnace becomes long, and so, the furnace is effective for decreasing NO x . 4) Thanks to feed of under air, combustion at the bottom portion of the furnace becomes good and an oxidizing atmosphere is formed there, so that contamination of clinker water does not occur, and sludging is also mitigated. Accordingly, the fear of clogging of the bottom of the furnace is eliminated, and also reducing corrosion of the bottom of the furnace can be mitigated.

Claims: A method of combusting pulverised fuel in a square barrel-shaped furnace having a vertical axis with main burners disposed at the central portions of respective sides in a horizontal cross-section of a furnace wall wherein pulverized fuel-air mixtures are injected from said main burners through nozzles in a down wardly inclined direction with respect to a horizontal plane and air is fed into said furnace below said burners characterized in that, the total amount of the main burner air and the under air is made less than the amount corresponding to a stoichiometric ratio with respect to the amount of the pulverized coal injected through the pulverized coal-air mixture nozzles and the remainder of the air necessitated for completion of combustion is charged to the furnace inner space through additional air nozzles provided above the main burners. A method as claimed in claim 1 wherein said pulverized fuel-air mixtures are separated into a coal-rich pulverized coal-air mixture and a coal-lean pulverized coal-air mixture which are then sent to respective pulverized coal-air mixture nozzles included in the burner main bodies. A method as claimed in claim 2, wherein the coal rich pulverized coal-air mixture has a mixing ratio of conveying-air/pulverized-coal of 0.5-1.5 and the coal-lean pulverized coal-air mixture has a mixing ratio of conveying-air/pulverized-coal of 5-20. A method as claimed in claim 2 wherein main burner air is injected into the furnace inner space through main burner air nozzles which are provided in a plurality of compartments together with the coal-rich and coal-lean pulverized coal-air mixture nozzles. A method as claimed in claim 1, wherein said under air is injected through under air nozzles from the respective central portions of four sides in a horizontal cross-section of a furnace wall in directions that may be included in the same vertical planes as the axes of the corresponding burners. A method as claimed in claim 1, wherein the pulverized coal-air mixture is injected through the pulverized coal-air mixture nozzles in a downwardly inclined direction of between 5 and 45 degrees with respect to the horizontal plane.

Inventors: NAKASHIMA FUMIYA MITSUBISHI JU, OGURI MASAHARU NAGASAKI TECHNI, TOKUDA KIMISHIRO NAGASAKI TECH, NAKASHIMA, FUMIYA, MITSUBISHI JUKOGYO K.K., OGURI, MASAHARU, NAGASAKI TECHNICAL INSTITUTE OF, TOKUDA, KIMISHIRO, NAGASAKI TECHNICAL INSTITUTE OF
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: F23C5
IPC codes: F23C5, F23C7, F23C99