METHOD OF CONTROLLING COMBUSTION IN FLUIDIZED BED INCINERATOR

Abstract: A method of controlling combustion in a fluidized bed incinerator. In the fluidized bed incinerator (5), a waste feed rate is measured by a feed rate measuring device (11) using photoelectric elements (1) provided in a chute (2) for feeding the waste to be burnt (3) from a dust feeder (4) to the incinerator (5) and, when the feed rate of the waste exceeds a predetermined value: the feed rate of the waste to be burnt from the dust feeder (4) is decreased to suppress the combustion; an induction gas volume by an exhaust gas induction blower (9) of the incinerator is increased to suppress the rise of pressure in the incinerator; an air volume of a forced draft blower (7) for fluidizing air is decreased to suppress the combustion; or an air volume of a blower (8) for feeding air to a free board section is increased to prevent the occurrence of unburnt gas.

Description: The present invention relates to a method of controlling combustion of a fluidized-bed incinerator. The document Patent Abstracts of Japan, Vol. 10, No. 278 (P-499), September 20, 1986, discloses an apparatus for measuring the charging amount of waste for an incinerator. Photoelectric detection means are provided in an input chute, which detect the waste material falling into the incinerator. Processing means are provided to control incinerator operation on the basis of the detection signal. The document EP-A-0 358 760 discloses a combustion control method for a fluidized bed incinerator comprising the steps of detecting the combustion rate in the incinerator by a combustion rate detecting means. When the combustion rate is above a predetermined level, the flow rate of the combustion air on the lower side of the bed is reduced. At the same time, the flow rate of air into a space above the bed is increased, thereby maintaining the combustion rate in the incinerator at a predetermined level. As shown in Fig. 9, in a fluidized-bed incinerator 5, fluidized sand of a fluidized bed 6 constituted by the fluidized sand circularly supplied to the lower portion of the incinerator is preheated to a temperature of about 600°C, and fluidizing air 16 allows the sand to flow. In this state, combustion materials supplied from a charge chute 2 are combusted for a short time. Unburnt gas on the fluidized bed 6 is combusted by secondary air 15 supplied to a free board portion 13, and combustion exhaust gas 14 is exhausted from a top portion of the incinerator 5. Incombustibles contained in the combustion materials are exhausted from the bottom portion of the incinerator together with the fluidized sand. The fluidized sand is separated from the incombustibles and recycled in the incinerator. In order to control a combustion state of the fluidized-bed incinerator, as is disclosed in Published Unxamined Japanese Patent Application JP-A-53-148265, the following method is employed. That is, an oxygen concentration in exhaust gas is detected to control the supply rate of combustion materials such as garbage. A combustion state in an incinerator is determined by brightness or the like in the incinerator. For example, when the combustion becomes strong, a fluidizing air flow rate is decreased to cause the combustion to be weak. This method is disclosed in PCT WO88/8504. For example, when a large amount of waste is charged into an incinerator from a charge chute at one time, since a combusting operation is performed for a short time, e.g., several seconds, a large amount of unburnt gas may be generated for a lack of combustion air. The large amount of unburnt gas is exhausted out of the incinerator without being completely combusted even on a free board portion. The exhaust gas has a low oxygen concentration and contains harmful gases such as CO gas, thereby causing environmental pollution. In this case, even when the operation of the incinerator is controlled by measuring the oxygen concentration of the exhaust gas, the oxygen concentration meters available at present only measure an oxygen concentration with a time lag of several ten seconds. Therefore, the meter cannot respond to a change in combustion state within a short period of time. When a combustion state in an incinerator is determined by brightness in the incinerator, the time lag is smaller than that caused when the combustion state is controlled by measuring the oxygen concentration. However, since the combustion state is determined after combustion materials are combusted, the incineration cannot always be stably controlled. It is an object of the present invention to provide a method of controlling combustion of a fluidized-bed incinerator so as to solve the above problem. According to the present invention, a method of controlling combustion in a fluidized-bed incinerator is provided as defined in claim 1. In one embodiment, the flow rate of the fluidizing air is instantaneously decreased, and at the same time, air is supplied to the free board portion at a flow rate corresponding to the decrease, thereby suppressing a combusting operation and preventing production of unburnt gases. Since the waste supply rate is measured by the supply rate measuring unit arranged on the chute for supplying incineration waste to the incinerator, the waste supply rate of can be instantaneously measured before the waste is supplied to the incinerator. Therefore, an appropriate control operation can be performed in accordance with an incineration combustion state changed by a change in supply rate of the waste. Preferably, when a signal from the supply rate measuring unit exceeds a predetermined value, fluidizing air is instantaneously decreased, and at the same time, air is supplied to the free board portion at a flow rate corresponding to the decrease to rapidly suppress a combusting operation. On the other hand, unburnt matter produced with the decrease in fluidizing air is combusted in the free board portion to prevent generation of unburnt matter, thereby performing a stable combustion control operation of the fluidized-bed incinerator. Fig. 1 is a view for explaining an arrangement of a unit for carrying out the method according to the present invention; Figs. 2A, 2B, and 2C are graphs for explaining changes in supply rate, incinerator internal pressure, and oxygen concentration of exhaust gas as a function of time, respectively; Fig. 3 is a graph for explaining a change in incinerator internal pressure as a function of time when a combustion control operation is performed by a conventional method; Fig. 4 is a graph for explaining a change in incinerator internal pressure as a function of time when a combustion control operation is performed by the method according to an embodiment of the present invention; Fig. 5 is a graph for explaining a change in CO concentration as a function of time when a combustion control operation is performed by the conventional method; Fig. 6 is a graph for explaining a change in CO concentration as a function of time when a combustion control operation is performed by the method according to an embodiment of the present invention; Figs. 7A, 7B, and 7C are graphs for explaining changes in supply rate, control signal, and flow rate of fluidizing air as a function of time, respectively, according to the second embodiment of the present invention; Fig. 7D is a graph for explaining a change in flow rate of fluidizing air as a function of time in the conventional method; Fig. 8 is a graph showing a change in CO concentration as a function of time according to the second embodiment of the present invention; and Fig. 9 is a view for explaining a fluidized-bed incinerator. An application of the first embodiment of the present invention to a fluidized-bed municipal-waste incineration plant will be described below with reference to the accompanying drawings. A photoelectric element 1 for detecting a supply rate of combustion materials is mounted on a charge chute 2 for waste 3 between a dust feeder 4 and an incinerator 5. As the photoelectric element 1, e.g., a transmission photoelectric switch constituted by a light-emitting portion 1a and a light-receiving portion 1b is used. A detection signal from the photoelectric element 1 is arithmetically processed by a measuring unit 10 in accordance with a method disclosed in, e.g., Japanese Patent Application JP-A-2-77381, thereby instantaneously measuring a supply rate of the waste 3. In this case, the detection signal is output every millisecond, and the supply rate is measured in units of seconds, so that a combustion control operation is performed by a a combustion control unit 11. An incinerator internal pressure is input from a detector 18 to the combustion control unit 11. A control signal from the control unit 11 is output to the dust feeder 4, a blower 7 for supplying fluidizing air 16, a blower 8 for supplying secondary air 15 to a free board portion 13, and an induced blower 9 for exhaust gas 14. A tube 17 is branched from the fluidized air 16 to the free board portion 13, an ON/OFF valve 17a is arranged in the tube 17, and the control signal from the control unit 11 is output to the ON/OFF valve 17a. The dust feeder 4, the fluidizing air blower 7, the ON/OFF valve 17a, and the secondary air blower 8 or the induced blower 9 are selectively controlled in accordance with the nature of the waste 3. In a conventional method, as shown in Fig. 2A, when waste is supplied at a flow rate exceeding to a rated value, an overloaded combusting operation is performed in the incinerator, the incinerator pressure is increased as shown in Fig. 2B. Unburnt gas may be produced by a lack of combustion air, and as shown in Fig. 2C, an oxygen concentration of an exhaust gas is sharply decreased. In this case, the supply rate of waste is measured by the measuring unit 10, and the dust feeder 4, the secondary air blower 8, the induced blower 9, and the like are controlled by the control unit 11. For example, as shown in Fig. 2A, when a supply rate exceeds a predetermined level, the dust feeder 4 is stopped for a predetermined period, and the secondary air blower 8 is controlled to increase a secondary air flow rate for a predetermined period. At the same time, the induced blower 9 is controlled to increase an exhaust gas inducing rate. Therefore, strong combusting is prevented, and a stable supply rate, a stable incinerator pressure, and a stable oxygen concentration indicated by broken lines of Figs. 2A, 2B, and 2C, respectively, can be obtained. As a result, harmful gas components of the exhaust gas are prevented from exhaust out of the incinerator. When municipal waste (Hu = 2,000 Kcal/kg) is incinerated, operation results obtained in cases wherein the combustion control operation of the present embodiment is performed and not performed are shown in Figs. 3 to 6. A supply rate was measured by a measuring unit using a photoelectric element, and a waste supply rate was instantaneously measured on the basis of a detection signal from the measuring unit. A combustion control operation was performed so as to control the dust feeder 4, the fluidizing air blower 7, the secondary air blower 8, and the induced blower 9 (except for an operation of the ON/OFF valve 17a). The incinerator was operated to set an incinerator internal pressure at -50 mmAq. A combustion control operation was performed by, e.g., an oxygen concentration of an exhaust gas and the incinerator internal pressure without using a waste supply rate measuring unit. As a result, a change in incinerator internal pressure and a change in CO gas concentration shown in Figs. 3 and 5 were obtained. When a combustion control operation was performed using a waste supply rate measuring unit, as shown in Fig. 4, a change in incinerator internal pressure was considerably decreased. In addition, as shown in Fig. 6, a CO gas concentration was considerably reduced. As described above, since the waste supply rate was measured using the waste supply rate measuring unit to perform the combustion control operation, production of an unburnt gas can be substantially prevented, a change in incinerator internal pressure could be suppressed. Therefore, a possibility of the incinerator internal pressure being a positive pressure could be extremely reduced. The second embodiment wherein a combustion control operation is performed while controlling an ON/OFF valve 17a will be described below. The first, second, and third control steps are performed by a combustion control unit 11 in accordance with a supply rate of waste. The first, second, and third control steps are performed when the supply rates are set at predetermined values of, e.g., 120%, 150%, and 200%, respectively. In the first control step having the supply rate slightly higher than the predetermined value, the flow rate of secondary air 15 is increased, a set value of an incinerator internal pressure is decreased, and an exhaust gas induction rate is increased. Therefore, production of an unburnt gas is prevented, and an increase in incinerator internal pressure is prevented in advance. In the second control step having a supply rate considerably higher than the predetermined value, in addition to the operations of the first control step, the ON/OFF valve 17a arranged in a tube 17 connected to a free board portion 13 is opened. For this reason, fluidizing air 16 flows as secondary combustion air to the free board portion 13 having low air resistance, and the former fluidizing air flowing to the fluidized bed 6 is instantaneously decreased. Thus, the combusting operation on the fluidized bed 6 is suppressed, and the unburnt gas produced in this combustion operation is combusted in the free board portion 13. Finally, the third control step having a flow rate extremely higher than the predetermined value, the operation of the dust feeder 4 is stopped for a predetermined time. After the above operations of these steps are performed, when a predetermined time has elapsed, the incinerator internal pressure is stabilized. At this time, these controls parameters are set to their initial states. Operations in the second step will be described below with reference to Figs. 7A to 7C. (a) When a supply rate of combustion materials exceeds a predetermined level, (b) a signal for opening the ON/OFF valve 17a of the branch tube 17 is generated by the combustion control unit 11. Therefore, fluidizing air flows from the branch tube 17 to the free board portion, and (c) a fluidizing air flow rate is almost instantaneously (t1) reduced. When the supply rate is decreased to the predetermined level or less, a predetermined time has elapsed, and the incinerator internal pressure is at the preset level or a lower level (not shown), the control parameters are set to their initial states. In this case, when the fluidizing air flow rate is controlled by a damper arranged in a tube 16, since the operation of the damper takes time, as shown in Fig. 7D, it takes the time (t2) until the fluidizing air flow rate is decreased by a predetermined value. Since the time difference between t2 and t1 is generally 4 to 8 seconds, the control operation performed by the damper has a time lag, and a preferable control operation cannot be performed. The second embodiment will be described in detail below. Various elements set during a normal operation will be described as follows. An incinerator internal pressure is set at -70 mmAq; a flow rate of the fluidizing air 16 is 6,500 Nm³/h (in this case, a fluidization magnification is about 7); a flow rate of the secondary air 15 is 6,000 Nm³/h; and a flow rate of the branch tube 17 is zero. Various elements set during a combustion control operation will be described as follows. The incinerator internal pressure is set at -80 mmAq; the flow rate of the fluidizing air 16 is increased to 7,000 Nm3/h; the ON/OFF valve 17a is opened; and the fluidizing air 16 flows in the branch tube 17 at a flow rate of 2,000 Nm³/h. As a result, a flow rate of fluidizing air flowing from the bottom of the fluidized bed is to be 5,000 Nm³/h. In this case, a fluidization magnification is to be about 5. Therefore, a combustion time can be increased to 3 to 6 times a normal combustion time, and a combusting operation can be weakly performed. In addition, the flow rate of the secondary air 15 is increased to 6,600 Nm³/h. When municipal waste having a lower calorific value of about 2,000 kcal/kg is incinerated by the present method of controlling combustion, a change in CO concentration of the exhaust gas is shown in Fig. 8. The change in CO gas concentration is remarkably improved compared when a control operation of the ON/OFF valve 17a is not used as shown in Fig. 6. The CO concentration of the exhaust gas can be considerably reduced. In the above embodiment, although a transmission photoelectric switch is used as a photoelectric element, a reflection photoelectric element, a laser transmission/reception element, or the like can be used as the photoelectric element. Since a waste supply rate is measured by a supply rate measuring unit arranged on a chute for supplying incineration waste to an incinerator, before the waste is supplied to the incinerator, the waste supply rate can be instantaneously measured. Therefore, the fluidized-bed incinerator can be appropriately controlled in accordance with an incinerator combustion state changed by a change in supply rate of waste. As described above, production of an unburnt gas is prevented, and a change in incinerator internal pressure is suppressed to extremely reduce the possibility that the incinerator internal pressure becomes a positive pressure, thereby performing a stable combustion control operation.

Claims: A method of controlling combustion in a fluidized-bed incinerator comprising the steps of: measuring a waste fuel supply rate by a supply rate measuring unit (11) using a photoelectric element (1) arranged on a waste supply chute (2), said chute (2) supplying waste (3) from a dust feeder (4) to said incinerator, characterized by the steps of: under the condition that the waste supply rate is higher than a predetermined value, a) decreasing the waste supply rate from the dust feeder (4) to suppress the combustion operation, and b) increasing the exhaust gas flow rate of an exhaust gas induced blower (9) of said incinerator to limit an increase in the incinerator internal pressure. The method according to claim 1, further comprising the step of: c) decreasing the flow rate of the fluidizing air of a forced blower to suppress the combustion operation. The method according to claim 1 or 2, further comprising the steps of: d) increasing the flow rate of a blower (8) for supplying secondary air to a free board portion (13) of the incinerator to prevent production of unburnt gas. The method according to claim 1, 2 or 3, comprising the step of: e) instantaneously decreasing the flow rate of fluidizing air and at the same time, supplying secondary air to the free board portion (13) at a flow rate corresponding to the decrease in flow rate of the fluidizing air to suppress the combustion operation and to prevent production of unburnt gas.

Inventors: KAWAKAMI MASAAKI NKK CORPORATI, NOTO TAKASHI NKK CORPORATION, SUZUKI YASUO NKK CORPORATION, TSUBOI HARUHITO NKK CORPORATIO, YAMAGISHI MIKI NKK CORPORATION, YOKOYAMA TAKASHI NKK CORPORATI, KAWAKAMI, MASAAKI, NKK CORPORATION, NOTO, TAKASHI, NKK CORPORATION, SUZUKI, YASUO, NKK CORPORATION, TSUBOI, HARUHITO, NKK CORPORATION, YAMAGISHI, MIKI, NKK CORPORATION, YOKOYAMA, TAKASHI, NKK CORPORATION
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1991
Main classification: F23G5
IPC codes: F23G5, F23N5