United States Patent (19) Smith

Transcription

1 United State Patent (19) Smith (54 CONTROL FOR A POWER PLANT COAL MILL PULVERIZER HAVING FEEDFORWARD DAMPER POSITONING 75 Inventor: 73) Aignee: William J. Smith, Verona, Pa. Wetinghoue Electric Corp., Pittburgh, Pa. 21 Appl. No.: 878, Filed: Feb. 16, 1978 (51) Int. Cl... B02C 25/00 52 U.S. Cl /33; 24.1/34 58) Field of Search /33, 34 (56) Reference Cited U.S. PATENT DOCUMENTS 2,003,985 6/1935 Wallene /34 X 2,060,375 11/1936 Ihimura A34 2,152,367 3/1939 Smith /34 2,226,923 12/1940 Cro /34 X 2,419,436 4/1947 Boho /34 11) 4,177,950 45) Dec. 11, ,894,696 7/1959 Miller /34 X Primary Examiner-Robert L. Spicer, Jr. Attorney, Agent, or Firm-E. F. Poeky 57 ABSTRACT Power plant boiler fuel demand i tranmitted a a coal feeder peed demand to a coal pulverizer control. A peed controller operate the feeder in accordance with the peed demand, and a poition controller for a hot coal tranport air damper poition the hot air damper to hold the mill outlet temperature to a etpoint value and to increae or decreae damper poition in accor dance with a feedforward ignal repreenting the feed ing peed demand. A poition controller for a cold air damper regulate the total primary air flow to a value needed for afe and mooth tranport of the pulverized coal to the boiler burner, and it accordingly act a a proce trim on the feedforward control applied by the hot damper controller. 4. Claim, 5 Drawing Figure 42 EMPERATURE CONTROLER OTDAMPER COLD DAMPER CONTROLER SENSOR

2 U.S. Patent Dec. 11, 1979 Sheet 1 of 4 4,177, Pl TEMPERATURE 6 FUEL CONTROL 32 e 30 R al f-2- PULVERIZER 34 COAL FEEDER ER AIR FOW SENSOR HOT DAMPER COLD DAMPER FIG.I.

3 U.S. Patent Dec. 11, 1979 Sheet 2 of 4 4,177, TFE AIR P HOT DAMPER MILL OUTLET SUMMING TE: AMP - O HOT DAMPER FUEL SES HER 37 CONTROL UFEEDERSPEED AIR DEMAND COLD, COAL SENSOR DAMPER 38 FEEDER 53 SET PT ER AMPER COLD DAMPER P OLD AIR 26 DEMAND COLD SENSOR FIG.2. AR HOT 43 TEMPERATURE 42 SET PT P Hal or t 28 TEMPERATURE Ex HOAAPER MLL OUTLET Head TEMPERATURE SENSOR 36 y FUEL COAL 25 FEEDER a 37 CONTROL ER SET PT 38 Pyr Pra m C OLD AIR DAMPER 26 LE DEMAND SENSOR ONTROLLER FIG.3. SUMMING AMP

4 U.S. Patent Dec. 11, 1979 Sheet 3 of 4 4,177,950 to 14 MILL OUTLE PRESSURE 3. L 12 7 a MILL BOWL DIFF, to > IO 6 a O -l 75 i 5 ge 70 PR, +O w/ m, l M TEMP. SE O? 70, -IO K 60 FEEDER SPEED 50 WITH FEEDER. SPEED FEED FORWARD S 40. TO HOT AIR DAMPER 4 ES a 12 MILL OUTLET 7 in PRESSURE O 6 a 75 MELL BOWL DIFF 5 & tr. To O 1. g an A TEMR O H 8O N-FEEDER SPEED 70 WITHOUT FEEDER: SPEED FEED S. FORWARD TO HOT AIR DAMPER O 6O O ELAPSED TIME (MINUTES) FIG4. -IO K

5 U.S. Patent Dec. 11, 1979 Sheet 4 of 4 4,177, i Cl a Sg 2 O 75 MILL OUTLET PRESSURE MILL BOWLDIFF -l AR W 7O 7O 6O A TEMP H : 5O FEEDER SPEED WTH FEEDER SPEED FEED FLO O 40 FORWARD TO HOT AIR DAMPER 4. MILL OUTLET PRESSURE h l up 5 2 MILL BOWL DIFF C a C- g 0. SS Ll O 75 7O PRI AR FLOW C - A TEMP 8O WITHOUT FEEDER SPEED FEED FORWARD TO HOT AR DAMPER d ; 70 FEEDER SPEED i 60 O 2 ELAPSED TIME (MINUTES) FIG5.

6 1. CONTROL FOR A POWER PLANT COAL MLL PULVERIZER HAVING FEED FORWARD DAMPER POSITONING BACKGROUND OF THE INVENTON The preent invention relate to electric power plant and more particularly to control ytem employed with coal pulverizer in uch plant. In the operation of a coal fired electric power plant, coal i fed to a pulverizer mill where it i finely ground for mixture with the primary airflow and tranported to the burner. In many cae, the inlet hot and cold air flow mix i varied by damper poitioning to hold a deired mill exit temperature for the primary air and coal mixture. Further, the total primary air flow i typi cally regulated to a etpoint value by damper poition ing. In one particular prior art arrangement, the hot and cold air damper are moved in the ame direction for flow correction and they are moved in oppoite direc tion for outlet temperature correction. When a feeder peed change occur in repone to a change in fuel demand from the boiler control, the rate at which coal i fed to the pulverizer change. Then, a the amount of pulverized coal produced for tranport by the primary air begin to change, the preure drop acro the mill change cauing the air flow and the exit temperature to change. For example, with a feeder peed increae the amount of pulverized coal begin to increae cauing greater preure drop and reduced primary air flow and reduced outlet temperature. The pulverizer control operate the damper to make needed flow correction during the proce tranient condition. In the known prior art, pulverizer control configura tion have not been a effective a deirable becaue the repone time needed for achieving primary air flow correction ha been exceive. Thu, the time for flow correction i o great in many exiting intallation that undeirable diturbance occur in the boiler outlet team preure when fuel change are required. The primary reaon for thi i that pulverizer control typically em ploy feedback control loop which provide correction only after proce error begin to occur. Thu, the time required for a change in feeder peed to be effected through the pulverizer proce ignificantly affect the repone time of prior art pulverizer control. While feedforward control may generally avoid proce error and improve proce repone, there i no known prior art which dicloe how feedforward control can be uccefully employed in achieving improved pulver izer control repone time without diturbing the mooth and continuou flow of coal to the burner, i.e., without cauing ettlement of fine and without cauing overize coal to be held in upenion above the pulver izing mechanim. SUMMARY OF THE INVENTION A control i provided for an electric power plant pulverizer mill which ha a variable peed feeder and hot and cold air duct upplying primary air flow through the pulverizer under the control of repective hot and cold air damper. The control comprie mean for ening the temperature of the mill outlet flow, mean for ening the primary airflow, mean for gener ating a feeder peed demand and mean for operating the feeder in accordance with the feeder peed demand. The hot damper i poitioned in accordance with a hot damper poition demand and the cold damper i poi 4,177, tioned in accordance with a cold damper poition de mand. The damper poition demand are generated in repone to the flow and temperature ening mean to control total primary air flow in accordance with a predetermined function and to control the mill outlet temperature in accordance with another predetermined function. The feeder peed demand i applied to the damper poition demand generating mean to produce anticipatory movement of at leat one of the damper in repone to feeder peed demand change. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 how a chematic view of a coal pulverizer mill and the manner in which it i controlled in accor dance with the principle of the invention; FIG. 2 how a block diagram of the pulverizer con trol of FIG. 1 in greater detail; FIG. 3 how a block diagram of a modified pulver izer control in accordance with another embodiment of the invention; FIG. 4 how mill repone to a tep feeder increae both with the employment of feedforward control in accordance with the invention and without uch feed forward control in accordance with the typical prior art; and FIG. 5 i like FIG. 4 but it how repone to a tep feeder peed decreae. DESCRIPTION OF THE PREFERRED EMBODIMENT More particularly, in FIG. 1 there i hown a conven tional coal mill pulverizer 10 having a belt or other conventional feeder 12 which receive coal from bun ker (not hown). Generally, the coal i pulverized by roller (not hown) o that nominally, at leat 80% of the ground coal would pa through a 200 meh creen. A claifier ection 14 return the overize coal particle back to the pulverizing ection for recycling. A minimum primary air flow i required to prevent unafe ettlement of the fine coal in the tranport line into eparate fuel lug which could caue exploive activity at the burner. To provide for mooth and afe boiler operation the primary air flow i pecified in thi embodiment to be a contant value which i ufficient to tranport the pulverized coal and which i well above the minimum afe value. However, the primary airflow etpoint value i not o great a to have that force which would caue overize coal particle to be carried up ward from the roller and thu inhibiting grinding and creating a recycling load within the pulverizer. Al though, the primary air flow i controlled to a etpoint value in thi embodiment, uch flow could be variable a the pulverizer i controlled to atify boiler demand in other application of the invention. Hot air i obtained from a preheater (not hown) through a duct 16 under the control of a variably poi tioned damper 18. Cold air i upplied through a duct 20 under the control of a variably poitioned damper 22. The mixed inlet air pae through a primary air duct 24 where a flow enor 25 i located. In the pulverizer 10, the primary air pae through the grinding ection and carrie pulverized coal through the claifier 14 to the mill exit where a temperature enor 28 i located. The pulverized coal i then tran ported to the burner through duct 30 and 32. A pulverizer control 25 include a conventional coal feeder poitioner 34 which control the feeder peed in

7 3 repone to peed demand from a uitable fuel control ler 36. Similarly, a flow controller 38 operate a poi tioning control 40 for the cold air damper 22. A control 44 for the hot air damper operate in repone to the output of a pulverizer temperature controller 42 and in repone to a feedforward application of the feeder peed demand a indicated by the reference character 46. The tranient repone of the pulverizer proce to feeder peed change i ignificantly and afely hort ened by operation of the decribed control ytem. Thu, the control in effect anticipate the way in which the proce would normally deviate when a feeder peed change occur and action i taken immediately to reduce the proce deviation. Such improvement i realized in the pulverizer 10 a well a other known pulverizer having different tructural deign from that hown for the pulverizer 10. Portion of the pulverizer control 25 are eparately hown in greater detail in FIG. 2. The temperature controller 42 i preferably a conventional proportional plu integral (PI) controller which repond to a tem perature etpoint 43 and the temperature enor 28 to drive the hot air damper poition control to zero mill outlet temperature error. A ummer 45 combine the temperature controller ignal and the feedforward feeder peed demand ignal from the fuel control 36 to apply a ingle poition demand ignal to the hot air damper poition control 44. A PI controller i alo preferably ued for the flow controller 38. A hown, the flow controller 38 re pond to a primary air flow etpoint 37 and the flow enor 26in developing a control ignal which i applied a a cold air damper poition demand to the cold air damper poition control 40. Poition enor 51 and 53 are ued by the repective damper poition control 44 and 40 in meeting damper poition demand. In operation, an increae in feeder peed demand needed to avoid a drop in boiler outlet team preure imultaneouly caue (1) the feeder to increae it fuel feed rate and (2) the hot air damper poition control to open the damper to a poition needed to maintain the primary air flow and mill outlet temperature in anticipa tion of the the effect of the increae in the fuel feed rate. If the hot air damper poition i till omewhat in error a the proce change i tranported to the mill outlet, the temperature controller 42 and the flow con troller 38 jointly operate a a trim on the feedforward control action. Any tranported outlet temperature error reult in ome minor repoitioning of the hot air damper through the control 42 and 44. Any tran ported primary air flow error reult in ome minor cutback poitioning of the cold air damper through the control 38 and 40. Normally, feedback trim control action i not ignifi cant and the overall control repone time i ignifi cantly reduced while providing mooth and table oper ation of the pulverizer 10 by operation of the invention. Decreae in feeder peed demand reult in proce control action which i oppoite to that decribed for feeder peed increae. A hown in FIGS. 4 and 5, actual mill tet demon trate the improvement realized in time repone. With a feeder peed increae at time=0 in FIG. 4, the prior art mill repond with a diturbance for nearly three minute in primary air flow, a drop of nearly 10 F. in the mill outlet temperature over a three-minute time period (with correction occurring later), and a period of nearly three minute before mill outlet preure and mill 4,177, bowl differential ettle out at higher value. Thu, the tranient condition clearly perit for nearly three min ute in the pulverizer. In the pulverizer mill 10, the primary air flow ditur bance occur for about 45 econd, an outlet tempera ture drop i limited to about 2 F. over a three-minute time period, and mill outlet preure and mill bowl differential ettle out in le than one minute. Ue of the invention thu clearly provide an improvement in y tem repone. In FIG. 3, there i hown an alternative embodiment of the invention. In thi cae the feedforward feeder peed demand i applied to provide oppoite move ment of the hot and cold damper on feeder peed change. For example, the hot damper i further opened and the cold damper i further cloed on feeder peed increae. Thi embodiment provide a lower proce repone to feeder peed change than doe the pre ferred embodiment, but the peed of repone i till fater than that of the prior art. What i claimed i: 1. A control for an electric power plant pulverizer mill having a variable peed feeder and hot and cold air duct upplying primary air flow through aid pulver izer under the control of repective hot and cold air damper, aid control compriing mean for ening the temperature of the mill outlet flow, mean for ening the primary air flow, mean for generating a feeder peed demand, mean for operating the feeder in accor dance with the feeder peed demand, mean for poi tioning the hot damper in accordance with a hot damper poition demand, mean for poitioning the cold damper in accordance with a cold damper poition demand, mean for generating the damper poition de mand in repone to aid flow and temperature ening mean to control total primary air flow in accordance with a predetermined function and to control the mill outlet temperature in accordance with another prede termined function, and mean for applying the feeder peed demand to aid damper poition demand generat ing mean to produce anticipatory movement of at leat one of the damper in repone to feeder peed demand change. 2. A control a et forth in claim 1 wherein the total primary air flow and the mill outlet temperature are controlled to repective etpoint value and wherein aid damper poition demand generating mean applie a poition demand to aid hot damper poitioning mean on the bai offeeder peed demand and further applie a poition demand to aid cold damper poitioning mean on the bai of the flow etpoint and an output from aid flow enor. 3. A control a et forth in claim 2 wherein mean are provided for generating a mill outlet temperature et point, controller mean are provided for generating a temperature correction baed on the actual and etpoint temperature, and aid damper poition demand gener ating mean generate the hot damper poition demand a a function of the feeder peed demand and the tem perature correction. 4. A control a et forth in claim 1 wherein the total primary air flow and the mill outlet temperature are controlled to repective etpoint value and wherein aid damper poition demand generating mean applie a poition demand baed on feeder peed demand to aid hot damper poitioning mean and aid cold damper poitioning mean to move the damper in oppoite direction on change in feed peed demand. k.....