Furnace pressure control method

Abstract: A method of controlling the pressure in an incinerator [1] for incinerating municipal refuse, industrial waste, etc. Exhaust gas from the incinerator [1] is passed through an exhaust gas cooler [2] and an exhaust gas treating device [3] and then discharged into the atmosphere by the action of an induced draft fan [5]. A gas, e.g., part of the flow of exhaust gas induced by the induced draft fan [5] or air taken in from the atmosphere, is added to a gas flow path upstream the inlet of the induced draft fan [5] through an addition gas line provided with an addition gas control damper [4]. The addition gas control damper [4] is controlled on the basis of an output from a furnace pressure controller [12] such that when the furnace pressure is relatively high, the flow rate of the addition gas is reduced, whereas, when the furnace pressure is relatively low, the flow rate of the addition gas is increased.

Description: The present invention relates to a method of controlling the pressure in an incinerator used in equipment for incinerating municipal refuse, industrial waste, etc. by controlling the flow rate of exhaust gas. In an incinerator for municipal refuse or the like, the pressure in the furnace must be constantly kept at a predetermined negative pressure from the viewpoint of safety. If the negative pressure is excessively high, the amount of leakage air from the furnace, an exhaust gas cooler, a gas treating device, an exhaust gas duct, etc. increases, so that the amount of exhaust gas increases, resulting in an increase in the electric power consumed by an induced draft fan. Accordingly, it is necessary to control the pressure in the furnace at an appropriate negative pressure. In general, the control of the pressure in an incinerator has heretofore been effected by using a simple control system such as that shown in Fig. 2, in which reference numeral 1 denotes an incinerator, 2 a gas cooler, 3 a gas treating device, 4 a remote-control exhaust gas damper for controlling the flow rate of exhaust gas, 5 an induced draft fan for suction of exhaust gas, and 6 a stack. The incinerator 1 is fed with fuel 7 and combustion air 8. Reference numeral 13 denotes leakage. The pressure in the incinerator 1 that is detected by a detecting element 9 is transmitted by a pressure transmitter 10 to a controller 12 including a PID controller 11, where it is compared with a furnace pressure set value to obtain a manipulated variable signal, and the remote-control exhaust gas damper 4, serving as a final control element, is controlled on the basis of the manipulated variable signal to thereby control the flow rate of exhaust gas. Such a conventional control system is satisfactorily fit for practical use in the case of general combustion furnaces but not for incinerators designed for municipal refuse, which varies greatly in both quality and quantity because such incinerators have drastic, oscillatory and irregular variations in the furnace pressure in comparison with relatively stable furnaces such as heavy oil incinerators. Accordingly, it is difficult for a simple control system such as that described above to effect stable control of the pressure in the incinerators for municipal refuse. There is a prior art designed to cope with this problem, e.g., Japanese Patent Public Disclosure (KOKAI) No. 61-49929 (1986) entitled "Furnace Pressure Control System", filed by the present applicant. Fig. 3 shows the arrangement of this furnace pressure control system. Referring to the figure, a controller 12 includes a first-order lag filter 15, a subtracter 16, a non-linear operator 17, a differentiator 18, a non-linear operator 19 and an adder 20. The differentiator 18 and the non-linear operator 19 constitute in combination a differential output circuit 21. In the furnace pressure control system having the above-described arrangement, when the differential output circuit 21 is not employed, the pressure in the incinerator 1 is transmitted as an output PV 0 to the first-order lag filter 15 by the pressure transmitter 10. The filter 15 absorbs ripples to produce an output PV 1 . The subtracter 16 obtains a difference between the output PV 1 and a set value SV in the PID controller 11 and delivers an output PV 2 , which is input to the non-linear operator 17. The operator 17 delivers an output PV 3 with a gain selected in accordance with conditions, that is, whether SV<PV 1 or SV>PV 1 . More specifically, the gain that is selected when SV<PV 1 is larger than that in the case of SV>PV 1 . The output PV 3 is subjected to PID operation in the PID controller 11 to deliver an output MV 1 , which is input to the adder 20 to deliver an output MV 0 . In this case, there is no input to be added to MV 1 . Hence, MV 0 =MV 1 . With the output MV 0 , the remote-control exhaust gas damper 4 is controlled. However, since the gain is changed as described above, the value of the output MV 0 is larger in the case of SV<PV 1 than in the case of SV>PV 1 . Accordingly, the operating speed of the remote-control exhaust gas damper 4, which is a final control element, is higher in the case of SV<PV 1 than in the case of SV>PV 1 , thereby promptly suppressing the rise in the furnace pressure, and thus preventing it from becoming a positive pressure. When the differential output circuit 21 is employed in the furnace pressure control system shown in Fig. 3, the non-linear operator 17 may not necessarily need to change the gain on the basis of the size comparison between SV and PV 1 . The output PV 0 is differentiated in the differentiator 18 to deliver an output y 1 , which is input to the non-linear operator 19. The operator 19 delivers an output y 2 only when the differential value is positive. The output y 2 is added to the output MV 1 delivered from the PID controller 11 as a fundamental manipulated variable in the adder 20 to generate a corrected manipulated variable signal MV 0 , which is used to control the remote-control exhaust gas damper 4 as a final control element. An upward tendency of the furnace pressure is judged by the fact that the differential value is positive, and in such a case a larger manipulated variable is given to the final control element to increase the operating speed of the exhaust gas damper 4, thereby promptly suppressing the rise in the furnace pressure, and thus preventing it from becoming a positive pressure. Recently, exhaust gas treatment has been improved. That is, it has heretofore been common practice to employ an electrostatic precipitator for exhaust gas treatment, whereas it has recently become common practice to employ a bag filter or wet-type treatment or to pass exhaust gas through a chemical-packed bed. Thus, the pressure loss is increasing in the exhaust gas treatment. When the exhaust gas treatment is accompanied by a large pressure loss, it is likely with the method disclosed in Japanese Patent Public Disclosure (KOKAI) NO. 61-49929 (1986) that the induced draft fan 5 will transiently become deficient in capacity due to the delay in operation of the remote-control exhaust gas damper 4, resulting in an abnormally positive furnace pressure. If the furnace pressure becomes positive, the combustion gas leaks out of the system, which is unfavorable for the working environment. In view of the above-described circumstances, it is an object of the present invention to provide a furnace pressure control method which is capable of promptly following up a change in the flow rate of exhaust gas to stabilize the furnace pressure. To attain the above-described object, the present invention provides a method of controlling the pressure in an incinerator for incinerating municipal refuse, industrial waste, etc., by means of a gas control damper controlling the flow rate of a gas which is added to a gas flow upstream the inlet of an induced draft fan, comprising: passing exhaust gas from the incinerator through an exhaust gas cooler, and an exhaust gas treating device, then discharging it into the atmosphere by the action of the induced draft fan; feeding either part of the flow of exhaust gas induced by the induced draft fan or air taken in from the atmosphere to the inlet of the fan or the inlet of the exhaust gas treating device or the inlet of the exhaust gas cooler or the inside of the incinerator through an addition gas line provided with said gas control damper; and controlling the said gas control damper on the basis of an output from a furnace pressure controller such that when the furnace pressure is relatively high, the flow rate of the addition gas is reduced, whereas, when the furnace pressure is relatively low, the flow rate of the addition gas is increased. According to the present invention, the controller gain that is used when the furnace pressure is on the plus side of a set value for the furnace pressure controller is larger than the gain that is used when the furnace pressure is on the minus side of the set value so that when the furnace pressure is on the plus side, the gas control damper is operated at a relatively high speed. In addition, the gas control damper is operated even more rapidly when a sudden change of the furnace pressure toward the plus side of the set value is detected, In addition, the present invention is characterized by combining the control operation in which the gain is changed according to whether the furnace pressure is on the plus or minus side of the set value for the furnace pressure controller and the control operation in which the gas control damper is operated even more rapidly when a sudden change of the furnace pressure toward the plus side of the set value is detected. By virtue of the above-described arrangement, the gas control damper that is installed in the addition gas line is used as a final control element for the furnace pressure control, and the gas control damper is opened and closed so as to compensate for a change in the flow rate of combustion gas through the addition gas line. Therefore, the induced draft fan is allowed to operate with its maximum capacity at all times. Accordingly, when the flow rate of exhaust gas increases rapidly (i.e., when the furnace pressure rises), the gas control damper is closed, so that the exhaust gas is sucked with the maximum capacity of the induced draft fan, thus enabling the furnace pressure to be stabilized promptly. In addition, the controller gain that is used when the surface pressure is on the plus side of the set value for the furnace pressure controller is relatively large so that when the furnace pressure is on the plus side of the set value, the gas control damper is operated at a relatively high speed, thereby promptly suppressing the rise in the furnace pressure toward the plus side. When a sudden change of the furnace pressure toward the plus side of the set value of the furnace pressure controller is detected, the gas control damper is operated even more rapidly, thereby promptly suppressing the rise in the surface pressure toward the plus side. By virtue of the fact that the output of said furnace pressure controller that controls said gas control damper is the sum of an output of a PID controller forming a fundamental manipulated variable and a signal which is inversely proportional to the change of the furnace pressure, the fluctuation of the furnace pressure is reduced speedily and a stability thereof is recovered quickly. Since the gas control damper is fully closed when the furnace pressure is higher than a first set pressure and the damper is fully opened when the furnace pressure is lower than a second set pressure, the stability of the furnace pressure is speedily recovered and further, the exhaust gas is prevented from being released into the atmosphere without being treated through the addition gas line. The above and other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings, in which like reference numerals denote like elements, and of which: Fig. 1 shows the arrangement of a furnace pressure control system which may be employed to carry out the furnace pressure control method of the present invention; Figs. 2 and 3 show the respective arrangements of conventional furnace pressure control systems; Figs. 4, 5, 6, 7, 8, 9, 10 and 11 show other furnace pressure control systems, respectively, which may be employed to carry out the furnace pressure control method of the present invention; and Figs. 12, 13 and 14 are graphs respectively showing the output of an inversely proportional operator employed in the system shown in Fig. 8. Embodiments of the present invention will be described below with reference to the accompanying drawings. It should be noted that the present invention is not necessarily limited to these embodiments. Fig. 1 shows the arrangement of a furnace pressure control system which may be employed to carry out the furnace pressure control method of the present invention. In the figure, the same reference numerals as those in Figs. 2 and 3 denote the same or equivalent portions or elements (the same is the case with other drawings). As shown in Fig. 1, the furnace pressure control system is provided with an addition gas line L for feeding back part of the flow of exhaust gas induced by the action of an induced draft fan 5 to the inlet of a remote-control exhaust gas damper 4. The addition gas line L is provided with a gas control damper 22. In the furnace pressure control system having the above-described arrangement, the pressure PV 0 in the incinerator 1 is detected by a detecting element 9 and then transmitted to a controller 12 for furnace pressure control by a pressure transmitter 10. In the controller 12, the detected pressure PV 0 is compared with a set value SV in a PID controller 11 and subjected to PID operation to control the gas control damper 22 installed in the addition gas line L, thereby stabilizing the pressure in the incinerator 1. The remote-control exhaust gas damper 4 is a manual damper which is fully closed when the induced draft fan 5 is started. When the induced draft fan 5 is in operation, the damper 4 is opened to a predetermined degree (substantially full open). In this arrangement, the maximum capacity of the induced draft fan 5 is defined by the upper limit of the gas control damper 22. Since municipal refuse or the like cast in the incinerator 1 varies in both quality and quantity, the flow rate of combustion gas generated therefrom also varies. As a result, the furnace pressure also varies. Hitherto, the remote-control exhaust gas damper 4 has been controlled as a final control element to stabilize the furnace pressure, as shown in Figs. 2 and 3. Therefore, the operating point of the induced draft fan 5 has heretofore been set at a level where it operates with a reduced capacity. In this embodiment, the gas control damper 22 that is installed in the addition gas line L is used as a final control element for the pressure control, and the gas control damper 22 is opened and closed so as to compensate for a change in the flow rate of combustion gas through the addition gas line L. Therefore, the induced draft fan 5 is allowed to operate with its maximum capacity. Accordingly, when the flow rate of exhaust gas increases rapidly (i.e., when the furnace pressure rises), the gas control damper 22 is closed, so that the exhaust gas is sucked with the maximum capacity of the induced draft fan 5, thus enabling the furnace pressure to be stabilized promptly. Fig. 4 shows the arrangement of another furnace pressure control system which may be employed to carry out the furnace pressure control method of the present invention. This furnace pressure control system employs a controller 12 having the same arrangement as that of the controller 12 shown in Fig. 3 and controls the gas control damper 22 provided in the addition gas line L on the basis of the output of the controller 12. In the furnace pressure control system shown in Fig. 4, when the differential output circuit 21 is not employed, the pressure in the incinerator 1 is transmitted as an output PV 0 to the first-order lag filter 15 by the produce transmitter 10. The filter 15 absorbs ripples to provide an output PV 1 . The subtracter 16 obtains a difference between the output PV 1 and a set value SV in the PID controller 11 and delivers an output PV 2 , which is input to the non-linear operator 17. The operator 17 delivers an output PV 3 with a gain selected in accordance with conditions, that is, whether SV<PV 1 or SV>PV 1 . More specifically, the gain that is selected when SV<PV 1 is larger than that in the case of SV>PV 1 . The output PV 3 is subjected to PID operation in the PID controller 11 to deliver an output MV 1 , which is input to the adder 20 to deliver an output MV 0 . In this case, there is no input to be added to MV 1 . Hence, MV 0 =MV 1 . With the output MV 0 , the gas control damper 22 installed in the addition gas line L is controlled. However, since the gain is changed as described above, the value of the output MV 0 is larger in the case of SV<SV 1 than in the case of SV>PV 1 . Accordingly, the speed at which the gas control damper 22, which is a final control element, is closed is higher in the case of SV<PV 1 than in the case of SV>PV 1 , thereby increasing the flow rate of exhaust gas released in the atmosphere through the stack 6, and thus making it possible to promptly suppress the rise in the furnace pressure and prevent it from becoming a positive pressure. When the differential output circuit 21 is employed, the non-linear operator 17 may not necessarily need to change the gain on the basis of the size comparison between SV and PV 1 . The output PV 0 is differentiated in the differentiator 18 to deliver an output y 1 , which is input to the non-linear operator 19. The operator 19 delivers an output y 2 only when the differential value is positive. The output y 2 is added to the output MV 1 delivered from the PID controller 11 as a fundamental manipulated variable in the adder 20 to generate a corrected manipulated variable signal MV 0 , which is used to control the gas control damper 22 as a final control element. An upward tendency of the furnace pressure is judged by the fact that the differential value is positive, and in such a case a larger manipulated variable is given to the gas control damper 22 as a final control element to increase the closing operation speed of the gas control damper 22, thereby promptly suppressing the rise in the furnace pressure, and thus preventing it from becoming a positive pressure. Although in the furnace pressure control systems shown in Figs. 1 and 4 the adding end of the addition gas line L is connected to the inlet of the remote-control exhaust gas damper 4, it should be noted that a portion to which the adding end of the addition gas line L is connected is not necessarily limitative thereto. The adding end of the addition gas line L may be connected to the inlet of the gas treating device 3 as shown in Fig. 5, or to the inlet of the gas cooler 2 as shown in Fig. 6, or to the inside of the incinerator 1 as shown in Fig. 7. Figs. 8, 9, 10 and 11 show other furnace pressure control systems, respectively, which may be employed to carry out the furnace pressure control method of the present invention. Fig. 8 has an arrangement in which the differential output circuit 21 in the furnace pressure control system shown in Fig. 4 is replaced with a non-linear operator 23 and the addition gas line L is connected to the inlet of the gas treating device 3. The output of the non-linear operator 23 is inversely proportional to the change in the furnace pressure, as shown in Fig. 12, and is dominant over the output MV 1 from the PID controller 11. In the example shown in Fig. 12, when the furnace pressure is -50 mmAq or higher, the output of the non-linear operator 23 is always zero, so that the gas control damper 22 is fully closed and no exhaust gas is released through the addition gas line L. When the furnace pressure is -150 mmAq or lower, the output of the non-linear operator 23 is a high constant value, so that the gas control damper 22 is fully opened. Thus, the lower the furnace pressure, the higher that flow rate of exhaust gas returned to the upstream side of the exhaust gas duct 4. In the furnace pressure control system shown in Fig. 9, the exhaust gas treating device comprises an electrostatic precipitator 42 and a wet-type gas treating machine 43, and a protective damper 44 is disposed at the inlet of the wet-type gas treating machine 43. Part of the flow of exhaust gas induced by the induced draft fan 5 passes through the gas control damper 22 and the addition gas line L to flow back to the gas flow path between the electrostatic precipitator 42 and the protective damper 44. In the furnace pressure control system shown in Fig. 9, the protective damper 44 is closed when needed to prevent the wet-type gas treating machine 43 from being damaged by heat. When the protective damper 44 is closed, the operation of the furnace is stopped and the gas control damper 22 is opened, so that exhaust gas discharged from the electrostatic precipitator 42 is led to the stack 6 through the addition gas line L and the gas control damper 22. The furnace pressure control system shown in Fig. 10 has an arrangement in which the remote-control exhaust gas damper 4 and the induced draft fan 5 in the system shown in Fig. 6 are replaced with an inverter-driven induced draft fan 5. With this arrangement, the flow loss of exhaust gas is smaller than in the case where the damper 4 is provided, so that the exhaust gas can be induced to flow even more speedily when the furnace pressure rises. The furnace pressure control system shown in Fig. 11 has the same arrangement as that of the furnace pressure control system shown in Fig. 5 except that in the system shown in Fig. 11 the gas control damper 22 is communicated with the atmosphere through a line L, whereas in the system shown in Fig. 5 the discharge port of the induced draft fan 5 and the gas control damper 22 are communicated with each other. In the furnace pressure control system shown in Fig. 11, when the furnace pressure is relatively low, the air is supplied to the gas flow path between the gas cooler 2 and the gas treating device 3 under the control of the gas control damper 22. Fig. 12 shows a first type output of the non-linear operator 23 used in the controller shown in Fig. 8, which is inversely proportional to the change in the furnace pressure and is dominant in the output of said furnace pressure controller. When the furnace pressure is not lower than -50 mmAq, the first type output is always zero which make the gas control damper 22 to be fully closed. When the furnace pressure is not higher than -150 mmAq, the first type output is a high constant value which make the gas control damper 22 to be fully opened. Fig. 13 shows a second type output of the non-linear operator 23 which is inversely proportional to the change in the furnace pressure over major part of the furnace pressure and is dominant in the output of the furnace pressure controller. And when the furnace pressure is not lower than -50 mmAq, the second type output is also always zero which make the gas control damper 22 to be fully closed. When the furnace pressure is not higher than -150 mmAq, the second type output of the non-linear operator 23 is also a high constant value which make the gas control damper 22 to be fully opened. However, the second type output includes a constant linear value when the furnace pressure is in the vicinity of the furnace pressure set value SV of said PID controller. The constant linear set value of the second type output comprises a horizontal folded liner shape as shown in Fig. 13. When the second type output takes the constant linear value, the gas control damper 22 is retained in a position without being moved by the controller, thus it becomes possible to prevent the gas control damper 22 from excessively responding to the furnace pressure when the furnace pressure is in the vicinity of the set value of the PID controller, and to generate the advantageous effects of preventing the furnace pressure from being vibrated. Fig. 14 shows a third type output of the non-linear operator 23 which is similar to that shown in Fig. 13, except that the center of the constant linear value is changed in connection with the set value SV of the PID controller, whereby it becomes possible to change automatically the output profile of the non-linear operator 23 when the set value SV of the PID controller is changed, whereby which generates the advantageous effects of preventing the furnace pressure from being vibrated due to the in conformity of the set value SV of the PID controller and the center of the constant linear value. Thus, according to the present invention, an gas control damper is installed in an addition gas line L for feeding either part of the flow of exhaust gas induced by the induced draft fan or air taken in from the atmosphere to the inlet of the remote-control exhaust gas damper or the inlet of the exhaust gas treating device or the inlet of the exhaust gas cooler or the inside of the incinerator, and the gas control damper is controlled on the basis of the furnace pressure controller, thereby controlling the furnace pressure. Accordingly, it is possible to operate the induced draft fan with its maximum capacity at all times and hence possible to stabilize the furnace pressure promptly. Although the present invention has been described through specific terms, it should be noted here that the described embodiments are not necessarily exclusive and that various changes and modifications may be imparted thereto without departing from the scope of the invention which is limited solely by the appended claims.

Claims: A method of controlling the pressure in an incinerator comprising: passing exhaust gas from said incinerator (1) through an exhaust gas cooler (2) and an exhaust gas treating device (3) and then discharging it into the atmosphere by the action of an induced draft fan (5); characterized by adding a gas to a gas flow path upstream the inlet of said induced draft fan (5) through an addition gas line (L) provided with a gas control damper (22); and controlling said gas control damper (22) on the basis of an output from a furnace pressure controller (12) such that when the furnace pressure is relatively high, the flow rate of the addition gas is reduced, whereas, when the furnace pressure is relatively low, the flow rate of the addition gas is increased. The control method of Claim 1, wherein the gain of the controller (12) that is used when the furnace pressure is on the plus side of a set value (SV) for said furnace pressure controller (12) is larger than the gain that is used when the furnace pressure is on the minus side of said set value, so that when the furnace pressure is on the plus side, said gas control damper (22) is operated at a relatively high speed. The control method of Claim 1, wherein said control damper (22) is operated even more rapidly when a sudden change of the furnace pressure toward the plus side of a set value for said furnace pressure controller (12) is detected. The control method of Claim 1, wherein the output of said furnace pressure controller (12) is the sum of an output of a PID controller (11) forming a fundamental manipulated variable and an output of a non-linear operator (23) forming a signal which is inversely proportional to the change of the furnace pressure. The control method of Claim 4, wherein when the furnace pressure is higher than a first set pressure, said gas control damper (22) is fully closed, whereas, when the furnace pressure is lower than a second set pressure, said control damper is fully opened. The control method of Claim 5, wherein the output of said non-linear operator (23) includes a linear constant value irrespective of the furnace pressure in the vicinity of a furnace pressure set value SV of the PID controller. The control method of Claim 6, wherein the center of said linear constant value responds to the furnace pressure set value SV of the PID controller. The control method of Claim 1, wherein at least two of the following control operations are combined, viz. (i) a control operation in which the gain of the controller (12) that is used when the furnace pressure is on the plus side of a set value for said furnace pressure controller is larger than the gain that is used when the furnace pressure is on the minus side of said set value, (ii) a control operation in which said gas control damper (22) is operated even more rapidly when a sudden change of the furnace pressure toward the plus side of said set value is detected, and (iii) a control operation in which the output of said furnace pressure controller that controls said gas control damper (22) is the sum of an output of a PID controller forming a fundamental manipulated variable and an output of a non-linear operator forming a signal which is inversely proportional to the change of the furnace pressure. The control method of any one of Claims 1 to 8, wherein said addition gas is a flow of exhaust gas induced by said induced draft fan. The control method of any one of Claims 1 to 8, wherein said addition gas is air taken in from the atmosphere.

Inventors: FURUKAWA MASAAKI, FURUKAWA, MASAAKI
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1992
Main classification: F23N3
IPC codes: F23N3