DESCRIPTION COKE DRY QUENCHING METHOD AND COKE DRY QUENCHER [Technical Field] The present invention relates to coke dry quenching method and coke dry quencher. {Background Art] A, coke dry quencher (CDQ) is used for achieving energy conservation by recovering the sensible heat of red-hot coke that is cooled when discharged from coke ovens. A coke dry quencher comprises a quenching chamber where the sensible heat of red-hot coke is exchanged by using an inert gas and a prechamber disposed above the quenching chamber. Red-hot coke is changed into the prechamber from above. The prechamber is provided to absorb the variation in red-hot coke charging times and realize operation stabilization. After exchanging heat with inert gas and quenched close to 200 °C" in the quenching chamber, coke is discharged in given quantities. The inert gas heated to 900 °C after the heat exchange is discharged from the upper part of the quenching chamber to the ring duct and delivered through the primary dust catcher to the waste-heat boiler where heat is recovered and then back to the quenching chamber under pressure applied by the circulating blower. The coke charged in contains volatile substances and coke fines. Being highly combustible, volatile substances are likely, when contained in circulating gases in high percentages, to give rise to abnormal coiabustion. The volatile substances and coke fines remaining in coke lumps can be burnt if air is injected into the prechamber. The injected air sometimes burns part of the surface layer of red-hot coke. Mixing of the air and waste gas of combustion thus heated to high temperatures with the inert gas increases the heat release of the gas discharged from the quenching chamber. Because the temperature of the coke arriving at the quenching chamber via the prechamber is also raised/ the quantity of heat collected by the inert gas in the quenching chamber also increases. As a consequence, the quantity of steam collected by the waste-heat boiler increases. The air injection into the prechamber not only increases the quantity of heat recovered in the waste-heat boiler in steady-state operation of the dry quenching system, but also keeps the quantity of heat recovery in the waste-heat boiler invariant even when the coke temperature in the quenching chamber drops as a result of a decrease in the supply of red-hot coke or a drop in the temperature of red-hot coke. Japanese Unexamined Patent Publication (Kokai) No. 61-37893 discloses a method for injecting air into the prechamber. Japanese Unexamined Patent Publication (Kokai) No. 59-75981 discloses a method for supplying a moistened gas into the prechamber in the dry quenching system, producing a gas containing large quantities of carbon monoxide and hydrogen by the reaction of the gas with red-hot coke, and mixing the produced gas with the circulating gas in the quenching tower. It is stated that the carbon monoxide and hydrogen gas recovered as components of the circulating gas can be recovered as fuel gas after passing the boiler or as steam in the boiler after burning carbon monoxide and hydrogen by adding air,into the gas duct. Generally, the steam generated in the waste-heat boiler of the coke dry quenching system is often converted to electric energy the steam turbine generator. In order to stably run steam turbine generators at the most efficient point, it is important to maintain the steam generation by the waste-heat boiler at the required constant level. If generation of steam used for general purposes varies with respect to the requirement/ steam falls short of the requirement when generation decreases and exceeds the requirement and is wastefully dissipated when generation increases. In order to ensure effective use of generated steam, it is necessary to maintain steam generation at a constant level. The presence of the air injected into the prechamber/ the water and steam injected into the prechamber and the air injected into the high temperature gas recovered from the quenching tower increases the quantity of gas circulated between the quenching tower and waste-heat boiler. In order to maintain the quantity of the circulated gas at a constant level, it is necessary to let off part of the circulated gas to outside. If the circulating gas contains unburnt gases such as carbon monoxide and hydrogen/ it is difficult to effectively recover the energies of such unburnt gases. It is therefore preferable to ensure that the circulating gas contains no unburnt gases at least when the circulating gas has passed the waste-heat boiler by converting the unburnt gases contained in the circulating gas to heat energy by burning with injected air. The oxygen contained in the circulating gas is unfavorable because it burns coke in the quenching chamber and lowers the quenching ability thereof. When injecting oxygen into the circulating gas recovered from the quenching tower, it is therefore necessary to ensure that oxygen remains in the circulating gas by injecting an excessive amount of oxygen. The temperature in the prechamber rises when part of the residual volatile substances, coke fines and coke lumps are burnt by injecting air into the prechamber. When the temperature in the prechamber reaches approximately 1400 °C, the ash in coke is melted and gasified and the gasified ash is carried with the air and mixes with the inert gas ascending in the quenching chamber. Since the temperature of the inert gas at the exit end of the quenching chamber is approximately 900 °C, the gasified ash coagulates and adheres to sloping flue in the upper part of the quenching chamber. This accretion is called clinker, clogs the gas flue, increases the flow resistance of gases, and impairs the circulation of the hot coke quenching gas. Therefore, it is necessary to constantly control and maintain the temperature in the prechamber below a certain level even when air is injected into the prechamber. In order to meet the above requirement, the temperature of the gas supplied to the waste-heat boiler must be varied. The upper limit of the service temperature for the boiler tubes constituting the waste-hear coiler, however, is specified depending on the material and structure thereof. Since the use at a temperature above the specified upper limit leads to thermal fracture, it is necessary to keep the temperature of the gas supplied to the waste-heat boiler below the specified limit. If the temperature of the gas supplied to the waste-heat boiler drops, the heat exchange efficiency of the boiler drops, with a resulting decrease in steam production. Therefore, it is necessary to constantly control the temperature of the gas supplied to the waste-heat boiler within a certain range. The high-temperature waste gas discharged from the quenching tower is supplied to the waste-gas boiler via the sloping flue. If the quantity of the waste gas exceeds the upper limit, coke floats and bursts from the sloping flue. The bursting coke causes a sudden increase flow resistance of the circulating gas and wears away or damages the boiler tubes. Therefore, it is necessary to control the flow rate of the high-temperature waste gas below a certain level. In order to achieve energy saving by maximizing the recovery of sensible heat from the red-hot coke in the quenching chamber, it is important to increase the supply of inert gas to the quenching chamber as much as possible. Because the upper limit of the waste gas from the quenching tower is set as described earlier, it is necessary to constantly control the supply at the upper limit. [Summary of the Invention} A first object of the present invention is to provide, in the coke dry quenching method to recover the sensible heat of red-hot coke as steam by using the coke dry quencher, a quenching method to invariably maintain the recovery of steam at a required level. A second object of the present invention is to provide a quenching method to maintain, the quantities of combustible gas and oxygen in the circulating gas at a minimum level. A third object of the present invention is to provide a quenching method to prevent the accretion of foreign substances in the sloping flue by maintaining the temperature in the prechamfaer below a certain level. A fourth object of the present invention is to prevent heat fracture of boiler tubes and lowering of the heat recovery efficiency of the boiler by maintaining the temperature of the gas supplied to the waste-heat boiler within a certain range. A fifth object of the present invention is to prevent an increase in the flow resistance of the circulating gas and the wear and fracture of boiler tubes by the floating and bursting coke and maximize the recovery of sensible heat from red-hot coke in the quenching chamber by invariably maintaining the flow rate of waste gas from the quenching tower at a constant level. A sixth object of the present invention is to provide a coke dry quenching method and coke dry quencher that do not form an accretion of clinker even when air is injected into the precharaber and combustible gases and coke fines are burnt to enhance safety and increase waste heat recovery. (I) The gist of the present invention to achieve the first to fifth objects described above is as set out below: (1) A coke dry quenching method that uses a quenching tower 1 comprising a quenching chamber 2 and a prechamber 3 on top thereof, comprises the steps of charging red-hot coke 9 from above the prechamber, injecting air and/or water or steam into the prechamber 3, exchanging heat with the sensible heat of the red-hot coke by using an inert gas as a medium in the quenching chamber and recovering heat in the form of steam in a waste-heat boiler 7, the method is characterized by adjusting the quantity of air and/or water or steam to be injected into the prechamber 3 so that the quantity of heat input into the waste-heat boiler becomes equal to the target value. (2) The method described in (1) above, in which the ratio of the adjusted quantity of water or steam injected into the prechamber (hereinafter referred to as the "PC water-steam 26") to the adjusted quantity of air injected into the prechamber (hereinafter referred to as the "PC air 24") is set so that a constant temperature is maintained in the prechamber. (3) The method described in (1) or (2) above, in which air (hereinafter referred to as the "SF air 25") is supplied to the high-temperature gas 22 discharged from the quenching tower 1 before the gas reaches the waste- heat boiler, the quantities of the PC air 24 and/or PC water-steam 26 are adjusted so that the quantity of heat input to the waste-heat boiler is maintained constant, and the ratio of the adjusted quantity of the SF air 25 to the adjusted quantity of the PC air 24 and/or the PC water-steam 26 is set so that the concentration of the combustible gaa and oxygen in the circulating gas is maintained constant. (4) The method described in (I), (2) or (3) above, in which adjustment is done so that the quantity of the steam generated in the waste-heat boiler, instead of the quantity of heat input into the waste-heat boiler, becomes equal to the target value. (5) The method described in (I), (2) or (3) above, in which adjustment is done so that the gas temperature at the entry end of the waste-heat boiler, instead of the quantity of heat input into the waste-heat boiler, becomes equal to the target value, (6) The method described in (1), (2), (3.) or (5) above, in which the target value for the heat input to the waste-heat boiler or the gas temperature at the entry end of the waste-heat boiler is corrected so that the quantity of the steam generated in the waste-heat boiler becomes equal to the target value. (7) The method described in any of (1) to (6) above, in which adjustments of the PC air 24, PC water-steam 26 and SF air 25 are done so that the quantity of heat input to the waste-heat boiler is maintained constant by detecting variation in the quantity of the coke 10 discharged from the quenching tower 1 and compensating for variation in the quantity of sensible heat recovered from coke due to the detected variation in the quantity of coke discharge. {8; The method described in any of (1) to (7) above, in which adjustments of the PC air 24, PC water-steam 26 and SF air 25 are done so that the quantity of heat input to the waste-heat boiler is maintained constant by detecting variation in the quantity of the circulating gas 37 and compensating for variation in the quantity of sensible heat recovered from coke due to the detected variation in the quantity of the circulating gas. (9) The method described in any of (1), (2) and (4) to (8) above, in which air (SF air 25) is supplied to the high-temperature discharge gas 22 from the quenching tower before the waste gas reaches the waste-heat boiler 7 and adjustments of the PC air 24 and PC water-steam 26 are done so that the quantity of heat input to the waste-heat boiler becomes equal to the target value by detecting variation in the quantity of the SF air and compensating for the variation in the quantity of heat input to the boiler due to the detected variation in the quantity of the SF air. (10) The method described in any of (1) to (9) above, in which part of the gas discharged from the waste-heat boiler 7 and supplied to the quenching chamber 2 is bypassed, the bypassed gas (hereinafter referred to as the "bypassed gas 29") is merged with the gag supplied to the waste-heat boiler, and adjustments of the PC air 24, PC water*steam 26 and SF air 25 are done so that the quantity of heat input to the waste-heat boiler is maintained constant by detecting variation in the quantity of the injected gas 21 supplied to the quenching chamber and compensating for variation in the quantity of sensible heat recovered from coke due to the detected variation in the quantity of the injected gas 21 supplied to the quenching chamber. (11) The method described in any of (1) to (10) above, in which when the detected temperature of the gas supplied to the waste-heat boiler exceeds the predetermined upper or lower limit the gas temperature at the entry end of the boiler is brought back to between the upper and lower limits by increasing or decreasing the flow rate of the circulating gas. (12) A coke dry quenching method that uses a quenching tower 1 comprising a quenching chamber 2 and a prechamber 3 on top thereof, comprises the steps of charging red-hot coke 9 from above the prechamber, exchanging heat with the sensible heat of the red-hot coke by using an inert gas as a medium in the quenching chamber, supplying air (hereinafter referred to as the "SF air 25") to the high-temperature gas 22 discharged from the quenching tower 1 before the gas reaches the waste-heat boiler and recovering heat in the form of steam in the waste-heat boiler 1, the method is characterized by adjusting the quantity of the SF air so that the concentration of carbon monoxide or hydrogen in the gas circulating in the waste-heat boiler 7 or the heat release of the circulating gas due to the presence of the gases is maintained constant and the concentration of oxygen is kept below a certain level. (13) The method described in (12) above, in which the quantity of the SF air 25 is adjusted by setting a target value for the concentration of carbon monoxide in the gas circulating in the waste-heat boiler and an upper limit, a lower limit and a target value for the concentration of oxygen in the gas, adjusting the quantity of the SF air so that the carbon monoxide concentration becomes equal to the target value, adjustment of the quantity of the SF air based on the carbon monoxide concentration is suspended to make the oxygen concentration equal to the target value when the oxygen concentration exceeds the upper limit therefor, and adjustment of the quantity of the SF air based on the carbon monoxide concentration is resumed when the oxygen concentration falls below the target value or lower limit therefor or when the oxygen concentration falls below the target value or lower limit therefor and the carbon monoxide concentration exceeds the target value therefor. (14) The method described in (12) above, in which the quantity of the SF air 25 is adjusted by setting a target value for the concentration of hydrogen in the gas circulating in the waste-heat boiler and an upper limit, a lower limit and a target value for the concentration of oxygen in the gas, adjusting the quantity of the SF air so that the hydrogen concentration becomes equal to the target value, adjustment of the quantity of the SF air based on the hydrogen concentration is suspended to make the oxygen concentration equal to the target value when the oxygen concentration exceeds the upper limit therefor, and adjustment of the quantity of the SF air based on the hydrogen concentration is resumed when the oxygen concentration falls below the target value or lower limit therefor or when the oxygen concentration falls below the target value or lower limit therefor and the hydrogen concentration exceeds the target value therefor. (15) The method described in (12) above, in which the quantity of the SF air 25 is adjusted by determining the heat release of the circulating gas by adding the product of the concentration and heat release of hydrogen in the circulating gas in the waste-heat boiler to the product of the concentration and heat release of carbon monoxide in the circulating gas, setting a target value for the heat release of the circulating gas, setting an upper limit, a lower limit and a target value for the concentration of oxygen in the circulating gas, and adjusting the quantity of the SF oxygen so that the heat release of the circulating gas becomes equal to the target value therefor, adjustment of the quantity of the SF air based on the heat release of the circulating gas is suspended to make the oxygen concentration equal to the target value when the oxygen concentration exceeds the upper limit therefor, and adjustment of the quantity of the SF air based on the heat release of the circulating gas is resumed when the oxygen concentration falls below the target value or lower limit therefor or when the oxygen concentration falls below the target value or lower limit therefor and the heat release of the circulating gas exceeds the target value therefor. (16) The method described in any of (12) to (15) above, in which the quantity of the SF air 25 is adjusted by detecting variation in the quantity of the coke 10 discharged from the quenching tower 1 so that variation in the concentration of carbon monoxide or hydrogen in the circulating gas in the waste-heat boiler, or the heat release of the circulating gas and the concentration of oxygen due to the detected variation in the quantity of the coke discharged is prevented. (17) The method described in any of (12) to (16) above/ in which the quantity of the SF air 25 is adjusted by detecting variation in the quantity of the PC air 24 and/or the PC water-steam 26 so that variation in the concentration of carbon monoxide or hydrogen in the circulating gas in the waste-heat boiler, or the heat release of the circulating gas and the concentration of oxygen due to the detected variation in the quantity of the PC air 24 and/or the PC water-steam 26. (18) The method described in any of (12) to (16). above, in which air {the PC air 24) and/or water or steam (the PC water-steam 26) is injected into the prechamber 3, the quantity of the SF air 25 and the PC air 24 and/or the PC water-steam 26 is adjusted in the adjustment of the SF air 25 based on the concentration of carbon monoxide or hydrogen or the heat release of the circulating gas and the ratio of the decrement of the PC air 24 and/or the PC water-steam 26 to the increment of the SF air 25 is determined so that the heat input to the waste-heat boiler 7 is maintained constant. (19) The method described in (18) above, in which the ratio of the adjusted quantity of the PC water-steam 26 to the adjusted quantity of the PC air 24 is determined so that the temperature in the prechamber is maintained constant, (20) A coke dry quenching method that uses a quenching tower 1 comprising a quenching chamber 2 and a prechamber 3 on top thereof, comprises the steps of charging red-hot coke 9 from above the prechamber, injecting air and/or water or steam into the prechamber 3, exchanging heat with the sensible heat of the red-hot coke by using an inert gas as a medium in the quenching chamber and recovering the sensible heat of the high-temperature gas discharged from the quenching tower 1 in the form of steam in the waste-heat boiler 1, the method is characterized by bypassing part of the gas discharged from the waste-heat boiler 7 and supplied to the quenching chamber 2, merging the bypassed gas (hereinafter referred to as the "bypassed gas 29") with the gas supplied to the waste-heat boiler, and adjusting the quantity of the bypassed gas 29 so that the quantity of the high-temperature discharge gas 22 from the quenching tower 1 becomes equal to the target value therefor. (21) The method described in (20) above, in which the quantity of the bypassed gas is adjusted so that the pressure of the gas supplied to the waste-heat boiler measured between the exit end of the quenching tower and the entry end of the waste-heat boiler, instead of the quantity of the high-temperature discharge gas 22 from the quenching tower 1, becomes equal to the target value. (22) The method described in (2) or (19) above, in which .the temperature in the prechamber is measured and, when the measured temperature in the prechamber differs from the target value therefor, the ratio of the adjusted quantity of the PC water-steam 26 to the PC air 24 is corrected so that the temperature in the prechamber becomes equal to the target value therefor. (23) A coke dry quencher comprising a quenching tower 1 that comprises a quenching chamber 2 and a prechamber 3 disposed on top thereof from above which red-hot coke 9 is charged, an injector (14, 16) that injects air and/or water or steam into the prechamber, and a waste-heat boiler 7 that exchanges the sensible heat of the red-hot coke in the quenching chamber by using an inert gas as a heat-exchange medium and recovers the sensible heat of the high-temperature gas discharged from the quenching tower in the form of steam, the quencher is characterized by comprising an SF air injector 15 that supplies air (SF air 25) to the high-temperature gas 22 discharged from the quenching tower 2 and a bypass tube 19 that branches part of the gas discharged from the waste-heat boiler and supplied to the quenching chamber as an inert gas and merges the bypassed gas (the bypass gas 29} with the gas supplied to the waste-heat boiler, and disposing the merging point of the bypass tube 19 upstream (opposite to the boiler) of the SF air injector in the path of the high-temperature gas discharged from the quenching tower 1 and led to the waste-heat boiler 7. The inventions described in (I) to (6) and (11) above relate to the feedback control to maintain constant the quantity of steam recovered from the waste-heat boiler. * The inventions described in (7) to (10) above relate to the feedforward control to maintain constant the quantity of steam recovered from the waste-heat boiler in the presence of disturbances. The inventions described in (12) to (15), (18) and (19) above relate to the feedback control to minimize the contents of combustible components and oxygen in the circulating gas. The inventions described in (16) and (17) above relate to the feedforward control to minimize the contents of combustible components and oxygen in the circulating gas in the presence of disturbances. The inventions described in (20) and (21) above relate to the feedback control to maintain constant the high-temperature gas discharged from the quenching chamber. The invention described in (22) above relate to the feedback control to maintain constant the temperature in the prechamber. The invention described in (23) above relate to the coke dry quenching system to avoid damage due to abnormal local temperature increases of the sloping flue brick. (II) In the inventions to achieve the sixth object described earlierf water or steam is injected, together with air, into the prechamber. The water-gas reaction that occurs when red-hot coke comes in contact with steam is an endothermic reaction generating hydrogen gas and carbon monoxide. When water is injected/ an endotherraic reaction due to vaporization of water also occurs, in addition to the endothermic reaction due to water-gas reaction. Therefore, while air injection into the prechamber heats the inside of the prechamber, injection of water or steam irvto the prechamber gives rise to an endothermic reaction, with the result that the temperature in the prechamber is maintained at or below a certain level. Specifically, controlling the temperature in the prechamber at or below 1150 °C prevents the melting and gasification of ash in the prechamber and adhesion of clinker to the gas circulation system.. A first feature of the present invention is that it involves an advantageous method and apparatus to inject air and water into the prechamber. A second feature of the present invention is that it involves an advantageous method and apparatus to measure the temperature in the prechamber in controlling the temperature in the prechamber when controlling the temperature at or below a certain level by injecting air with water or steam into the prechamber. The gist of the present invention to achieve the sixth object thereof is as described below: (24) A coke dry quenching method that uses a quenching tower 1 comprising a quenching chamber 2 and a prechamber 3 on top thereof, comprises the steps of charging red-hot coke 9 from above the prechamber, injecting air 24 with water 26 into the prechamber from above thereof, exchanging heat with the sensible heat of the red-hot coke by using an inert gag as a medium in the quenching chamber and recovering heat in the form of steam, the method is characterized by atomizing the water 26 to be injected into the prechamber to a fine mist and injecting the mist into the prechamber together with the air 24. (25) The method described in (24) above, in which two or more injection ports 45 to inject the air and water into the prechamber are disposed along the circumferential direction of the prechamber, with the angle 6 between the adjoining injection ports disposed along the circumferential direction of the prechamber kept within the following range: 0.5 x (360/N) <. 9(°) £ 1.5 x (360/N) where 0 is the angle between adjoining injection ports disposed along the circumferential direction of the prechamber and N is the number of injection ports. (26) The method described in (24) or (25) above, in which the injection ports 45 to inject the air and water into the prechamber are disposed at a level above the preset upper limit of the coke charge in the prechamber. (27) The method described in (26) above, in which the injection of the air and water into the prechamber is interrupted or decreased when the top end of the coke charge in the prechamber exceeds the upper limit therefor and the injection of the air and water is resumed or increased when the top end of the coke charge falls below the upper limit or other preset level. (28) A coke dry quenching method that uses a quenching tower 1 comprising a quenching chamber 2 and a prechamber 3 on top thereof, comprises the steps of charging red-hot coke 9 from above the prechamber, injecting air 24 with water or steam into the prechamber from above thereof, exchanging heat with the sensible heat of the red-hot coke by using an inert gas as a medium in the quenching chamber and recovering heat in the form of steam, the method is characterized by measuring the surface temperature of the coke directly below the exit port of the prechamber with a non-contact optical thermometer 18 and managing or controlling operation by using the measured temperature as the temperature in the prechamber. (29) The method described in (28) above, in which air is injected into the prechamber from above thereof together with water or steam and either or both of the quantity of water or steam injected and the quantity of air injected into the prechamber are controlled so that the temperature in the prechamber becomes lower than the preset temperature. (30) A coke dry quencher comprising a quenching tower 1 that comprises a quenching chamber where the sensible heat of red-hot coke is exchanged with inert gas and a prechamber 3 disposed thereabove, a waste-heat boiler 7 to recover the heat of the inert gas in the form of steam, and an injector 46 disposed on top of the prechamber to inject air and water into the prechamber, the quencher is characterized by the injector 46 that atomizes water into a fine mist, mixes the fine mist with the air, and injects the mixture into the prechamber. (31) The quencher described in (30) above, in which the injector 46 comprises two water spray nozzles 42 disposed one on top of the other in an air injection tube 47 and the water spray nozzles spray water at wide angles horizontally and at narrow angles vertically. (32) The quencher described in (30) or (31) above, in which two or more injection ports 45 constituting the injector 46 are disposed along the circumferential direction of the prechamber, with the angle 8 between the adjoining injection ports disposed along the circumferential direction of the prechamber kept within the following range: 0.5 x (36Q/N) < e(0) S 1.5 x (360/N) where 0 is the angle between adjoining injection ports disposed along the circumferential direction of the prechamber and N is the number of injection ports. (33) The quencher described in (30), (31) or (32) above, in which the injection ports 45 constituting the injector 46 are disposed at a level above the preset upper limit of the coke charge in the prechamber. (34) A coke dry quencher comprising a quenching tower 1 that comprises a. quenching chamber where the sensible heat of red-hot coke is exchanged with inert gas and a prechamber 3 disposed thereabove, a waste-heat boiler 7 to recover the heat of the inert gas in the form of steam, and an injector 46 disposed on top of the prechamber to inject air and water into the prechamber, the quencher is characterized by comprising a non-contact optical thermometer 18 that is provided to measure the surface temperature of the coke directly below the exit port of the prechamber. [Brief Description of the Drawings] Fig. 1 is a schematic view of a coke dry quenching method of the present invention. Fig. 2 comprises block diagrams showing the outline of control according to the present invention. Fig. 2(a) is a block diagram for the invention described in claim 1, (b) is a block diagram for the invention described in claim 2, and (c) is a block diagram for the invention described in claim 3. Fig, 3 comprises other block diagrams showing the outline of control according to the present invention. Fig. 3(a) is a block diagram for the invention described in claim 6, and (b) is a block diagram for the invention described in claim 7. Fig. 4 comprises still other block diagrams showing the outline of control according to the present invention. Fig. 4(a) is a block diagram for the invention described in claim 12, (b) is a block diagram for the invention described in claim 16, and (c) is a block diagram for the invention described in claim 19. Fig. 5 is a schematic view of another coke dry quenching method of the present invention. Fig. 6 is a partial cross-sectional view of the air and water injector of the present invention. Fig. 7 is a perspective view showing the spraying condition of the water spray nozzle of the present invention. Fig. 8 shows the condition of the water sprayed according to the present invention. Fig. 8(a) shows the path of the sprayed water in the cross section of the cross section of the prechamber, and (b) shows the area in which water is sprayed at the top surface of red-hot coke. Fig. 9 shows the area in which water is sprayed at the top surface of red-hot coke by two or more injectors. Fig. 9(a) shows a case in which two injectors are provided, and (b) shows a case in which three injectors are provided. [The Most Preferred Embodiment] (I) The embodiment for practicing the present invention to achieve the first to fifth objects described earlier is described by reference to Fig. 1. The quenching tower 1 to quench red-hot coke has a vertically long profile and comprises a prechamber 3 and a quenching chamber 2 that are disposed one on top of the other. A sloping flue 4 formed along the inner wall of the prechamber 3 and quenching chamber 2 divides the gas flow therein. The red-hot coke 9 having a temperature of approximately 980 °C is charged from above the prechamber 3, gradually moves downward, and quenched in the quenching chamber 2 by the inert gas 27 blown in through the injection tube 11 disposed in the lower part thereof. The temperature of the coke 10 discharged from the lower part of the quenching chamber is approximately 200 °C. The inert gas 27 injected into the quenching chamber exchanges heat with the red-hot coke while ascending through the quenching chamber, becomes hotter, and flows out to a ring duct 5 through the sloping flue 4 in the upper part of the quenching chamber. Flowing further from the ring duct 5 to a waste-heat boiler 7 via a primary dust catcher 6, the inert gas is injected into the quenching chamber 2 again via a circulating blower 8 after the temperature thereof has fallen to approximately 180 °C as a result of heat exchange in the waste-heat boiler 7. In the present invention, air is injected into the prechamber as required. The air injected into the prechamber is hereinafter sometimes referred to as the "PC air 24". The oxygen in the injected air reacts with part of the residual volatile substances, coke fines and coke lumps. The reaction is mainly an endothermic reaction generating carbon monoxide. The injected air, product gas and coke descend in the prechamber while becoming hotter and becomes hottest in the lower part of the prechamber. The injected air and product gas mix with the inert gas ascending from below in the lower part of the prechamber and flow out to the ring duct 5 via the sloping flue 4. In the present invention, air is injected, as required, into the prechamber together with water or steam. The water or steam injected into the prechamber is hereinafter sometimes referred to as the "PC water'steam 26", The injected water absorbs heat when it evaporates to steam and the steam absorbs heat when it comes in contact with red-hot coke and generates hydrogen gas and carbon monoxide via the water-gas reaction. Therefore, injection of water or steam lowers the temperature of the gas and coke in the prechamber, and thus the temperature of the gas and coke in the prechamber can be controlled by controlling the quantity of the water or steam injected. The hydrogen gas and carbon monoxide produced by the water-gas reaction descend in the prechamber, mix with the ascending, inert gas in the lower part of the precharaber, and flow out to the ring duct 5 via the sloping flue 4. In the present invention, air 25 is injected, as required, into the ring duct 5 or a gas discharge tube 12 near the sloping flue 4 (SF). The injected gas is hereinafter sometimes referred to as the "SF air 25". The carbon monoxide produced by the reaction between the PC air 24 and red-hot coke 9 and the carbon monoxide and hydrogen produced by the reaction between the PC water-steam 26 and red-hot coke 9 burn when they come into contact with the SF air 25 after having flowed out through the ring duct 5 and release heat after changing into carbon dioxide and water. Steam energy recovery based on the injection of the PC air can be maximized by injecting as much of the SF air 25 as is appropriate for the quantity of the PC air 24 injected and necessary and sufficient for burning the carbon monoxide and other combustible gases produced by the injection of the PC air 24. Also, steam energy recovery based on the injection of the PC water-steam can be maximized by injecting as much of the SF air 25 as is appropriate for the quantity of the PC water-steam 26 injected and necessary and sufficient for burning the combustible gases produced by the water-gas reaction between the PC water-steam 26 and coke 9. If more oxygen than is required for burning the combustible gases contained in the discharge gas 22 is supplied as the SF air 25, excess oxygen remains in the circulating gas 37 and is contained in the injected gas 21 and blown into the quenching chamber 2, Therefore, it is preferable to supply only as much quantity of the SF air 25 as is required for burning the combustible gases contained in the discharge gas 22. In the present invention, part of the gas discharged from the waste-heat boiler 7 and supplied to the quenching chamber via an inert gas injection tube 11 is bypassed, as required, to a bypass tube 19. The bypassed gas (hereinafter referred to as the "bypass gas 29") is merged with the discharge gas 22 in a gas discharge tube 22 and becomes the gas 23 that is supplied to the waste- heat boiler 23. In order to maximize the recovery of sensible heat from the red-hot coke in the quenching chamber 2, it is preferable to inject as much gas 21 as possible into the quenching chamber 2. However, there is an upper limit for the flow rate of the gas 22 discharged from the quenching tower 1. Therefore, it is preferable to control the quantity of the gas 21 injected into the quenching chamber 2 so that the quantity of the gas 22 discharged from the quenching tower 1 is maintained in the vicinity of the upper limit therefor. It is sometimes desirable to increase the supply of the gas 23 to the waste-heat boiler to greater than the required quantity of the injected gas 21 in order to prevent an excessive increase of the temperature of the gas 23 supplied to the waste-heat boiler or for other purposes. On such occasions, the quantity of the gas 23 supplied to the waste-heat boiler can be increased while maintaining the quantity of the injected gas 21 in the vicinity of the upper limit for the quantity of the gas discharged from the quenching tower 1 by bypassing part of the circulating gas 37 to the bypass tube 19 and merging the bypassed gas with the discharged gas 22. The inventions described in (1) to (6) and (11) above relate to the feedback control to maintain constanf the quantity of steam recovered from the waste-heat boiler. The inventions described in (7) to (10) above relate to the feedforward control to maintain constant the quantity of steam recovered from the waste-heat boiler in the presence of disturbances. In the inventions described in (1) to (11) above, either the PC air alone or both of the PC air and PC water-steam are injected from the prechamber. If the quantity of the PC air 24 injected is increased, the rate of reaction with the red-hot coke in the prechamber increases which, in turn, increases the quantity of steam recovered. If, conversely, the quantity of the PC air 24 injected is decreased, the quantity of steam recovered decreases. The relationship between the variation in the quantity of the PC air and that in the quantity of the recovered steam depends primarily on the quantity of heat generated by carbon monoxide through the reaction between the oxygen in the PC air and coke. The heat generated by the combustion of the volatile substances contained in the red-hot coke is also involved. The relationship between the variation in the quantity of the PC air and that in the quantity of the recovered steam for each coke dry quenching system can be determined accurately, based on actual operation data. The relationship between the variation in the injected quantity of the PC water-steam and that in the quantity of the recovered steam can also be determined accurately, based on actual operation data. When the quantity of steam recovered from the waste-heat boiler ig maintained constant by feedback control, the heat input to the waste-heat boiler is usually chosen as the controlled variable. The invention described in (1) is based on the knowledge described above and maintains constant the quantity of steam generated by the waste-heat boiler by controlling the quantity of the PC air 24 when the PC water-steam is not injected or the quantity of the PC air 24 and that of the PC water-steam when the PC water-steam is injected so that the heat input to the waste-heat boiler becomes equal to the target value therefor. Fig. 2(a) shows a block diagram in which the PC air 24 alone is controlled. Concretely, the heat input to the waste-heat boiler is made equal to the target value therefor by adjusting, when the heat input deviates from the target value therefor, the heat release by varying the quantity of the PC air or that of the PC air and the PC water-steam in accordance with the degree of deviation. More concretely, good control can be achieved by optimizing the individual parameters for the PID control. If the temperature in the prechamber becomes too high, ashes contained in coke melt and gasify. The gasified ashes agglomerate on being cooled by the inert gas near the exit port of the quenching chamber and adhere to the sloping flue in the upper part of the quenching chamber. When the quantity of the PC air 24 is increased to keep constant the quantity of steam recovered in the invention described in (1), it is preferable to maintain constant the temperature in the prechamber. The temperature in the prechamber can be lowered by increasing the quantity of the PC water-steam injected. The invention described in (2) above is based on this knowledge, in the control to maintain constant the quantity of steam recovered, the quantity of the PC air and that of the PC water-steam are increased at a fixed ratio. Fig. 2{b) shows a block diagram. The fixed ratio is determined based on experimental or other data so that the temperature increase in the prechamber caused by the increase in the quantity of the PC air matches with the temperature decrease in the prechamber caused by the increase in the quantity of the PC water-steam. Thus, the feedback control to maintain constant the quantity of heat input to the waste-heat boiler can be achieved while the temperature in the prechamber is maintained constant. When the PC air 24 or the PC water-steam 26 is injected into the prechamber, the SF air 25 is usually injected to burn the combustible gases generated by the PC air 24 or the PC water-steam 26. If the quantities of the PC air and PC water-steam are adjusted in the inventions described in (!) and '{2) to control the quantity of heat input to the waste-heat boiler 7f the quantity of the combustible gases in the gas 22 discharged from the quenching tower also varies. If the combustible gases increase, sufficient energy cannot be recovered because the increased gases are sent unburnt to the waste-heat boiler. If the combustible gases decrease, the oxygen from the SF air 25 becomes excessive. Then, the gas containing oxygen is supplied to the waste-gas boiler, and eventually the gas 21 containing oxygen is supplied to the quenching chamber. In the invention described in (3) above, the ratio of the controlled quantity of the PC air and/or the PC water-steam to the controlled quantity of the SF air is determined so that the concentration of the combustible gases and oxygen in the gas supplied to the waste-heat boiler is maintained constant. Fig. 2(c) shows a block diagram for this case. The carbon monoxide in the waste gas increases as the quantity of the PC air 24 increases. The quantity of the SF air 25 required for changing the increased carbon monoxide to carbon dioxide by combustion is substantially •i equal to the quantity of the increased PC air 24. To be precise, the ratio of the controlled quantity of the SF air to the controlled quantity of the PC air can be determined based on experimental data so that the combustible gases and oxygen in the circulating gas neither increase nor decrease. The same can be done for the ratio of the controlled quantity of the SF air 25 to the controlled quantity of the PC water-steam 26. In the invention described in (3) above, the quantity of the SF air 25 increases or decreases with an increase or a decrease in the quantity of the PC air 24 and/or the PC water*steam 26. As the quantity of gas burnt in the gas discharge tube 12 also changes with an increase or a decrease in the quantity of the SF air 25, the quantity of heat input into the waste-heat boiler also increases or decreases. Therefore, in the feedback control implemented when the heat input to the waste-heat boiler diverges from the target value therefor, the amount of adjustment of the PC air 25 along with the PC air 24 or the FC air 24 and the PC water-steam 26 to make constant the heat input to the waste-heat boiler is smaller than that of the PC air, etc. in the inventions described in (1) and (2) above. If the quantity of the PC water-steam 26 alone is increased in the invention described in (1), the quantity of heat input into the waste-heat boiler decreases. If the quantity of the PC water-steam 26 is increased along with that of the SF air 25 in the invention described in (3), the quantity of heat input into the waste-heat boiler increases. Therefore, control parameters must be determined by considering the above factors. Accordingly, the parameters used in the PID control are naturally different from those used in the invention described in (1) above. Simultaneous implementation of the inventions described in (1), (2) and (3) above permits carrying out a feedback control to maintain constant the temperature in the prechamber and the heat input to the waste-heat boiler without increasing the quantities of combustible gases and oxygen contained in the gas supplied to the waste-heat boiler. In the feedback control to maintain constant the steam recovery from the waste-heat boiler 1, control to maintain constant the quantity of heat input thereto is usually practiced by choosing the quantity of heat input to the waste-heat boiler 7 as the controlled variable. However, even if control is done to make constant the heat input to the waste-heat boiler, the quantity of heat generated by the waste-heat boiler 7 sometimes varies, instead of becoming constant. The inventors learned through studies that the first reason for the variation in the quantity of s.team generation is that the unburnt combustible gases and oxygen remain in the gas 23 supplied to the boiler and the unburnt gases burn therein and generate steam having an energy greater than the quantity of heat input to the boiler. Oxygen will remain when the air introduced as the SF air 25 has not completed combustion before reaching the entry end of the boiler and when outside air comes into the boiler. The invention described in (4) above is based on the above knowledge and makes it possible to maintain constant steam generation by controlling fluctuation by adjusting so that the quantity of steam generation by the waste-heat boiler, instead of the quantity of heat input thereto, becomes equal to the target value therefor. The actual quantity of steam generation can be determined by providing an orifice flowmeter in the steam main. The actual quantity of steam can also be estimated from the quantity of pure water supplied to the waste-heat boiler. In the invention described in (5) above, adjustment is made so that the gas temperature at the entry end of the waste-heat boiler, instead of the quantity of heat input thereto, becomes equal to the target value therefor. Determining the quantity of heat input to the boiler requires the temperature, quantity and specific heat of the gas at the entry end of the boiler, involves complex measurement and lowers accuracy. By comparison, the gas temperature control at the entry end of the boiler according to the invention described in (5) above requires nothing more than the measurement of temperature. While the quantity and specific heat of the gas at the entry end of the boiler do not change greatly in short time periods, control to maintain constant temperature permits acquiring constant heat input in short periods of time. The feedback control based on actual steam generation, as described in (4) above, is difficult when the boiler has a large heat capacity because a large time lag occurs between the quantity of heat input to the boiler and the quantity of heat generated. Meanwhile, the divergence between the quantities of heat input and heat generated fluctuates in very long cycles. It is therefore preferable for the short cycle feedback control to maintain constant the quantity of steam generation to use the heat input to the waste-heat boiler as the controlled variable, determine the long-cycle relationship between the quantities of steam generation and boiler heat input, derive the heat input from the relationship that makes the quantity of steam generation equal to the target value therefor, and correct the target value for the heat input to the waste-heat boiler or the gas temperature at the entry end of the waste-heat boiler used in the short-cycle feedback control. The invention described in (6) above is based on this concept and has a feature in that the target value for the quantity of heat input to the waste-heat boiler or the gas temperature at the entry end of the waste-heat boiler is corrected so that the quantity of heat generated by the waste-heat boiler becomes equal to the target value therefor. Fig. 3(a) shows a block diagram for this case. The invention described in (7) above implements a feedforward control to maintain constant the quantity of steam generation by detecting fluctuations in the quantity of coke 10 discharged from the quenching tower 1 and picking up the detected fluctuations in the quantitv of coke discharge as a disturbance. When the quantity of coke discharged from the quenching tower 1 increases, the quantity of the sensible heat of coke recovered also increases. The increase in the quantity of the sensible heat of coke recovered accompanying the increase in the quantity of coke discharge can be approximately estimated by thermal calculation and accurately determined based on experiments. Meanwhile, one or more of the quantities of the PC air 24, PC water'Steam 26 and SF air 25 can be controlled to cancel the increase in the quantity of the sensible heat of coke recovered. The controlled variable can be accurately determined by thermal calculation and based on experiments so that the quantity of heat supplied to the boiler is maintained constant. Fig. 3(b) shows a block diagram for the case in which the PC air is chosen as the controlled variable. This figure also shows as block diagram for the feedback control. The invention described in (8) above implements a feedforward control to maintain constant the quantity of steam generation, as in the invention described in (7), by using the quantity of the circulating gas as a disturbance. When the circulating gas is not bypassed but supplied direct to the "quenching chamber, the balance left after deducting the quantity of the liberated gas from the quantity of the circulating gas and that of the liberated gas is used as the quantity of the gas injected into the quenching chamber. If the quantity of the gas injected to the quenching chamber fluctuates, the heat exchange efficiency in the quenching chamber changes, as a result of which, the quantity of the sensible heat the cooling gas recovers from the red-hot coke changes. One or more of the quantities of the PC air 24, PC water-steam 26 and SF air 25 is adjusted by predicting the fluctuation in the quantity of sensible heat recovered by thermal calculation and based on experiments, making up for the fluctuation and implementing a feedforward control so that the quantity of heat input into the waste-heat boiler is maintained constant. The quantity of the gas injected into the quenching chamber, instead of the quantity of the circulating gas, may be detected for use directly. The invention described in (9) above implements a. feedforward control to maintain constant the quantity of heat input to the boiler by using the fluctuation in the quantity of the SF air as a disturbance and adjusting the PC air and PC water-steam. The invention described in (10) above implements, where the bypass gas 29 is passed through the bypass tube 19, a feedforward control to maintain constant the quantity of steam generation, as with the invention described in (7), by using the fluctuation in the flow rate of the gas injected into the quenching chamber as a disturbance. When part of the circulating gas is bypassed to the waste-heat boiler, the quantity of the gas injected into the quenching chamber is equal to the balance left after deducting the quantities of the bypassed and liberated gases from the quantity of