Study Of Optimal Control On Environment Pollution Brought By Cokery

1. IntroductionCokery gas collector pressure is the main factor of coke techniques. It is also one of the parameters that is hard to be controlled in coke production. It can affect the quality of coke, longevity of cokery and environment pollution. Generally, coke factories adopt the technique that two cokeries share one blast cooling system. The distance between cokery and blast cooling system is fairly far and disposition of gas pipelines is not symmetrical. Gas collector pressure can not be stabilized in the reference value. The problem of smoking and sparkling is highly severe. Instability of gas collector pressure will bring serious environment pollution.This paper firstly analyses characteristics of the technique that two cokeries share one blast cooling system and then present a control scheme that is based on fuzzy sensor technique and double closed-loop two layer feedbacks. Figure 1 shows the structure of the control system.Figure 1. Structure of double closed-loop two layer feedbacksTwo gas collector pressure and main pipe suction composed three inner loops. According to the difference between actually measured pressure and reference value, every inner loop adopts PID algorithm to control respective object valve and finishs the first layer feedback control. In the second layer,fuzzy control technique is adopted. Fuzzy sensors output linguistic value of the two collector pressures' current state according to valve position. The linguistic value is used to modify main pipe reference value dynamically.The system use main pipe suction as supervisory level control, to avoid coupling disturb of two gas collector. Actualize valve position of the two controllers adapted to main pipe suction and achieve the outer-loop feedback control. Without the outer-loop control, gas collector pressures can never trace gas producing state and can only just adjust the object valve of their own passively to adapt to the change of gas flux. When gas is sufficient, opening degree of the valve is big. Whereas, when gas is insufficient, opening degree of the valve is small. That will make valve deviating from its best adjusting position. Using main pipe suction as supervisory level control has some innovative significance on gas collector pressure control.2. Principle of gas collector pressure controlAccording to principle of coke techniques, gas collector pressure is required to be maintained in the range of (90Pa±20Pa) and then it can be ensured those problems of negative pressure in the bottom of cokery , cokery smoking and environment pollution will not appear.According to experience of manual manipulation, some control rules can be summarized below:(D Gas collector pressure reflects gas turnout of each cokery directly and opening degree of the valve directly affects gas output. If pressure is above the normal level, opening degree should be increased. Whereas, opening degree should be reduced.(2) Adjusting one valve, the other collector pressure will change too. Because pipelines are connected and gas is flowing, manipulation on one valve will affect gas flux in other pipelines.(3) When the blast fan is working, a biggish suction (about -4000Pa) will be kept in front of the fan. Generally, the best opening degrees of adjusting valve which is before the fan and two collector valves are 50%.(D If the two collector pressure are all more than reference value, opening degree of adjusting valve which is before the fan should be increased.(5) If the two collector pressures are all less than reference value, opening degree of adjusting valve which is before the fan should be reduced.3. Research on control strategy of two layer feedbacksOptimal control of the fan's suction is a main guarantee to stabilize gascollector pressure. When adjusting gas collector pressure, different fan's suctions can all guarantee gas collector pressure stabilize in the range that technique criterion requires. This is because each gas collector has different valve position. When gas flux is same, if opening degree of valve is big, the suction required is small. If opening degree of valve is small, the suction required is big. Therefore, appropriate suction should be chosen to satisfy current gas producing situation. In order to associate gas collector pressure and main pipe suction, we design the second layer feedback control which uses two gas collector pressures and valve position state to modify main pipe reference value dynamically and then actualizes outer-loop control.Outer-loop control adopts fuzzy control technique. Using the measuring knowledge of valves position state, fuzzy sensors output linguistic value of 1# and 2# gas collector pressure. The two linguistic values will be used as input parameters to control main pipe reference value and to change suction to make current gas producing state adapted to the two collector valves' station.Significance of the second layer feedback control is to actualize a double closed loop of main pipe suction and gas collector control and then to use suction control as supervisory level control. That means to make collector valve position adapted to main pipe suction through adjusting reference value of main pipe suction and then to make them working in a balanceable state. Control logic can refer to Figure 1.3.1 Inner-loop PID controlThree inner-loops contain 1 #, 2# gas collector and main pipe suction. The structures are basically same, as Figure 2 shows:PressurePIU-Controlu(k)D/A ConversionObject ValveA/D ConversionPressure SensorFigure 2. The first layer PID closed-loop feedback controlPg(k) is a reference value. P(k) is gas collector pressure value converted by pressure sensor. E(k)=P(k)-Pg(k) is system error. The first layer feedback control adopts PID control algorithm and uses error signal E(k) to control opening degree of each valves, to change gas flux in pipeline and to achieve the aim of adjusting gas collector pressure and main pipe suction.3.2 Gas collector pressure state fuzzy sensorL. Foully bring forward the concept of fuzzy sensor firstly in 1988. It is an intelligent sensor that can convert measured numerical value to linguistic symbol description. Along with development of modern measure technology, people do not need to know the precise measured result in some situation and fuzzy linguistic description can be better to illuminate problems. For example, when people are measuring blood pressure, the result being 140mmHg or 139mmHg is not important. The important is to draw a conclusion that for elder the result is a normal blood pressure and for youngling the result is a bit of higher.In gas collector pressure control system, techniques require pressure in the range of 90Pa~110Pa. For different fan suction, gas collector pressure can be adjusted to be in the range because collector can adopt different valve opening degree. When gas is sufficient, a big valve opening degree is needed to satisfy producing situation. Different linguistic description should be given for pressure corresponding to different valve opening degree. Supposing such a situation that measured pressure of 95Pa and that is in the range techniques permit. But now valve opening degree is 80% and that means a very big opening degree is needed to satisfy gas flux. Although collector pressure is normal, a linguistic description of "pressure being above the normal level" should be given actually. Therefore, actual gas collector pressure state should be modified suitably on the background of considering measure.We treat fuzzy set of gas collector pressure P={ OPa < p < 250Pa } as basic set of value and select three linguistic values { pressure being below the normal level, pressure being moderate, pressure being above the normal level }, corresponding to three fuzzy subsets{B, M, A} respectively. Triangle membership functions are adopted, as Figure 3 shows:no130 150PaFigure 3. Membership functions of gas collector pressure state B denotes "pressure being below the normal level", M denotes "pressure being moderate" and A denotes "pressure being above the normal level".After pressure sensor outputs a pressure value, the membership degree of pressure value P tq the three fuzzy subsets will be calculated firstly. According to the principle of max membership degree, find out which fuzzy set the gas collector pressure state belongs to. And then find out the corresponding language output value from simulative human language output set that has been defined before. But this is not the final output. It is just a linguistic description of measured value. It needs to learn and to be modified according to measuring knowledge (valve position).3.3 Adaptability of gas collector pressure state to the environmentValve position plays a very important role in gas collector pressure state fuzzy sensors. We regard valve position as measuring background knowledge and adopt adaptability handling methods based on expert qualitative learning. Through comparing the difference of measuring description between experts and fuzzy sensors, we ascertain description of fuzzy concept.Considering valve opening degree is the measuring background knowledge and it has three conditions: "opening degree being below the normal level", "opening degree being moderate" and "opening degree being above the normal level". Measured pressure results should be modified in different conditions respectively. According to integration of actual manipulation experiences and experts' knowledge, we describe qualitative difference as two conditions below:? "Opening degree being below the normal level"According to experience, valve opening degree being below the normal level is because gas is insufficient and fan suction is excessively strong. And that will make the gas collector valve opening degree below the normal level. Though the pressure's numerical value is normal, the pressure state is below the normal level and it should be modified using background knowledge. The method is to shift membership functions of pressure state 40Pa rightward, as Figure 4 shows.Mrno130150170 190PaFigure 4. Membership function revision when valve opening degree is below? "Opening degree being above the normal level"Valve opening degree being above the normal level is because gas is excessively sufficient and fan suction is weak. And that will make the gas collector valve opening degree above the normal level. Though the pressure's numerical value is normal, the pressure state is above normal level and it should be modified using background knowledge. The method is to shift membership functions of pressure state 40Pa leftward, as Figure 5 shows:Mr90UOPaFigure 5. Membership function revision when valve opening degree is above 3.4 Supervisory level controlValve opening degree of 1# and 2# gas collector plays a very important role in control. It reflects current balancing state of suction and gas collector. Supervisory level control is to control main pipe reference value and actualize the second layer feedback control using the output value pi and Pi which are outputted by gas collector pressure state fuzzy sensor.Supervisory level control adopts fuzzy control technique. Output variable G is main pipe reference value and its basic set of value is {-400Pa, +400Pa}. Quantification set of G is selected as {-2, -1, 0, +1, +2}. Proportion factor ha = 400/2 = 200 .We select three linguistic values {NB,ZE,PB} for 5 and they denote modification degree of main pipe reference value { modification negative , modification zero, modification positive } respectively.According to expertise and manual manipulation experiences, we summarize 9 control rules:K\. If Pi pressure is below the normal level and Pi pressure is below the normal level, then modify G to negative.R2: If P\ pressure is moderate and Pi pressure is below the normal level, then modify G to negative.R9: If Pi pressure is above the normal level and Pi pressure is above the normal level, then modify g to positive.Every fuzzy condition clause decides a fuzzy relation. Though the AND operation over the 9 fuzzy relations: Rt(i = 1,2,...., 9), we can get the general fuzzy relation R:According to characters of fuzzy ratiocinative composing method (maximum-minimum method), when input value $\ and P2 are known, we can get output of fuzzy set 5 through ratiocinating3.5 Pressure State Measurement AnalyzingSupposing output value of the two gas collector pressure state fuzzy sensors are : Fi ="A-pressure being above the normal level", Ti ="A- pressure being above the normal level", then:'0 N 0 0 0.5A(0,0,0,0.5,l)Oi? =/0 0 0 0 00 0 0 0 00 0 0 0 00 0 0 0.5 0.50 0 0 0.5 1= [0,0.5,0.5,0.5,1]Assume that we use center-average defuzzification so that the formula describing how to compute the output is0.5x(-l) + 0.5x0 + 0.51 + 0.5 + 0.5 + 0.5= 0,5Through measurement transform, the value can be transformed to modification value of the reference value. Main suction reference valve is:Grefirence = -(4000 + 0.8 x kg) = -(4000 + 0.8 x 200) = -4160PaThe signification is increasing reference value of main pipe suction from -4000Pa to -4160Pa to reduce gas collector pressures when both of them are above the normal level. So it is obvious that, two layers feedback control can correct the problem of imbalance in gas producing state and gas collector pressure state. Otherwise, only relying on single loop adjustment, gas collector will never adapt to the current working state. Through supervisory level control of main pipe suction, double closed loop feedback control can be achieved.4. ConclusionThrough investigating and researching for several years, we have accumulated abundant experiences in cokery optimal control. We got sustentation fund from science and technology office of Jilin province in 2002 and set up the researching project of "Study of optimal control on environment pollution brought by cokery" (NO.20020421). The project had gotten through experts' appraisal by science and technology office of Jilin province in December 2004. The two layer feedback control system based on fuzzy sensors which presented in this paper has been running on actual gas collector pressure control for 2 years. The facts prove that the control system is reliable, stable and practical. The system can reduce pollution brought by cokery effectively, prolong working longevity of cokery, improve quality of coke, reduce working intensity of workers and improve management level of enterprises. It has potential social benefits and economic benefits. Particular research productions list below:(D Adopts fuzzy sensor technique on measuring background of valve position state. Using linguistic descriptions of the two collector pressure state which is outputted by fuzzy sensor actualize dynamic modification of main pipe reference value and resolve the problem that it is hard to control when gas is insufficient or excessively sufficient.(2) Analyze the problem of collector pressure adjustment in double cokeries system. Design a two loops feedback control system based on characteristics of the valves position dynamic changes.(3) Adopts adjusting suction control as supervisory level control and avoid coupling disturb of two gas collector.