Patent GB-1245276-A (GB)
Title: Improvements in method and apparatus for cement kiln control
IPC: F27B7/42
Filed: 1968-10-23
Abstract: 1,245,276. Automatic control of rotary cement kilns. GENERAL ELECTRIC CO. Oct. 23, 1968 [Oct.30, 1967], No.50249/68. Heading G3R. The burning zone of a rotary cement kiln is automatically maintained at a desired length by controlling the rate of heat input into the kiln in accordance with variations of the torque supplied by the motor rotating the kiln, so that the torque is maintained constant at a value determined by the particular operating conditions of the kiln, e.g. type of material, rate of feed and nature of desired product. In addition to the torque control system, a further control system is provided to automatically maintain constant the temperature of the gases at the feed end of the kiln and thereby provide a relatively constant temperature profile over the length of the kiln. Also, an over-ride control system ensures that the oxygen content of the gases does not fall below a predetermined minimum value, by over-riding the temperature control system or, if this is not or no longer possible, by over-riding the torque control system. As shown, a control system 51, which may comprise a digital computer with suitable A/D, D/A converters and logic circuits, is supplied by sensers 50, 53, 55, 59 and 60 respectively responsive to rate of raw feed, gas temperature, gas (oxygen) content, rate of heating fuel flow and torque of an electric driving motor 20. The fuel may be natural gas, pulverized coal, oil or combinations thereof, the senser 53 may be a thermocouple and the motor 20 may be A. C., in which case the senser provides a signal representative of the watt power input to the motor, or may be D. C. as in the described embodiment wherein the senser 60 provides a signal representative of the armature current. A controller 68 adjusts the fuel flow in accordance with torque variations and a controller 69 adjusts a fan 31 to vary the rate of flow of the gases leaving the kiln in accordance with temperature variations. In the latter system, control of dampers 38 may replace control of the fan 31. Due to the long time delay between a control action to adjust the fuel flow and the resulting change in the burning zone condition (and hence in the motor torque), the torque control system includes, in addition to a comparator 82 for the measured and set point current signals, a further feedback loop comprising a process model 83 and a summing amplifier 84, which loop ensures that the system responds only to genuine kiln disturbances, that is, disturbances other than those directly caused by a previous control action. The model 83 comprises calculating and memory apparatus, Fig. 3 (not shown), which calculates, stores and selects feedback signals whose number depends on the interval between control value calculations and the time delay characteristic of the kiln. Apart from the members 83, 84, the torque and temperature control systems are generally similar and include filter and logic circuits 80, 81 and 88, 89, which compare each filtered signal with the previous one and use the latter if the latest signal falls outside a predetermined range, thereby masking momentary disturbances in the kiln. Also the torque controller 85 receives a signal representative of the rate of fuel flow to the mixing chamber 27, which signal varies up to the time when automatic control of the kiln is initiated, but remains constant during the period of automatic control. The controllers 85, 91 are of the type which calculate the output signal in accordance with the input signal(s) received and with a predetermined equation involving the signal(s). The controller 85 uses an equation relating the signal from amplifier 84, the constant signal of rate of fuel flow mentioned previously, and a constant which is a function of the characteristics of the kiln being controlled. The controller 91 uses an equation relating the signal from amplifier 90, the previous rate of gas flow, the previous signal from amplifier 90 and constants dependent on the controller, which includes both proportional and integral modes. The outputs of controllers 85, 91 are also fed to the oxygen over-ride logic circuit 95, which receives input signals, representative of rate of raw feed and the oxygen content of the exit gases and calculates what the new oxygen content will be after the contemplated control actions are taken. If the predicted oxygen content is less than the prescribed minimum safe value, a logic switch 92 is operated to allow a new exit gas flow signal calculated by the circuit 95 to be passed to the controller 69, the new signal ensuring that the oxygen content is held at the safe level. However, if adjustment of the gas rate of flow cannot achieve the desired safe level, a logic switch 86 is operated to permit a fuel rate of flow signal calculated by the circuit 95 to be applied to the controller 68. Operation of the oxygen override logic circuit 95 is described, including the equations used in the various calculations. All the circuit members shown in Fig. 2 may be incorporated in the digital computer. The setpoint signal applied to the amplifier 82 may be controlled by the kiln operator or by a value stored in the digital computer and will be based on a periodic chemical analysis of the kiln product. The initial setpoint will depend on the type and rate of feed and on a particular product derived from past experience. A flow chart of the whole control system is discussed, Fig. 4 (not shown) and characteristics of typical operations of the torque control system are described, Fig. 5 (not shown).