Modeling and Control of Industrial Tunnel-type Furnaces for Brick and Tile Production

Transcription

1 Modeln and Control of Industral Tunnel-type Furnaces for Brck and Tle Producton Panaots Mchael Unversty of Patras Electrcal & Computer Enneern Dept. Patras 26500, Greece Abstract The dynamc behavor of an ndustral tunneltype furnace s of the fundamental mportance for the qualty of the processed products. The knowlede of the furnace dynamcs can make the control of the system easer as well as t can elmnate certan operatonal problems. The varous subsystems exstent n a tunnel-type furnace are usually controlled by conventonal technques and ndependent controllers offern smply an acceptable operaton of the whole system. Ths paper approaches the overall furnace system by consdern both mathematcal modeln and fuzzy loc based technques. It proposes a control scheme for model-based coordnaton of the ndvdual subsystems whch are controlled locally by fuzzy controllers combned wth conventonal ones. The proposed control structure can be easly appled to the process envronment and the mplementaton may be realzed drectly on a prorammable loc controller. Index Terms Industral Furnaces, System Modeln, Control, Smulaton, Fuzzy loc I. INTRODUCTION System modeln and control s an mportant ssue n ndustral enneern applcatons and partcularly n complcated thermal processes. Conventonal approaches to system modeln need many assumed condtons and rely heavly on mathematcal tools, such as dfferental equatons, transfer functon and so on, whch emphasze the precson and exact descrpton of each quantty nvolved. The use of these mathematcal tools s sutable and well-appled only n smple or well-defned systems. However, when the system s complcated, these conventonal approaches become less effectve [1]. Fuzzy loc based technques have become a sutable alternatve and have been successfully employed n varous areas where conventonal methods fal to provde satsfactory results. Althouh they can be used n system modeln as well as n control, t seems that ther use n control s more feasble, attractve and wdely accepted. Tunnel-type contnuous furnaces are used n brck and tle producton plants to fre the products at hh temperatures of more than 1000 derees C. A man characterstc of these furnaces are ther lare dmensons wth tunnel lenth usually reater than 100 m and tunnel cross secton about 15 m 2, dvded n a number of thermal processn zones. The products travel throuh the tunnel, Stamats Maness Unversty of Patras Electrcal & Computer Enneern Dept. Patras 26500, Greece stam.maness@ee.upatras.r typcally wth a pecewse constant flow, and et subjected to ths successve thermal processn. A bref revew of the fundamental prncples and applcatons of thermal systems control s ven n [2], where the heat exchann procedure s manly treated. Hh relablty of furnace systems s a crucal factor n achevn hh product yeld. A ood mathematcal model of the system s requred n order to mplement a better control scheme. The model wll have to nclude contnuous and dscrete dynamcs, but may also be nfnte dmensonal, dependn on whether the temperature and aerodynamc states are modeled as dstrbuted or lumped parameter systems. The frst step n the desn of a modelbased controller s the development of a thermal model whch accurately captures the actual physcal behavor of the system to be controlled. Ths hh-fdelty thermal model s based on the applcaton of the dynamc heat transfer equatons to the system. The model may contan physcal varables whose values are not known n advance (e.. heat transfer coeffcents) and are dentfed from expermental data. A comparson of the model response wth the actual system output provdes a measure of model accuracy. The modeln and control of dstrbuted thermal systems s nvestated n [3] where model-based control desn technques are appled. A dynamc model of reheatn furnace based on fuzzy system and enetc alorthm s proposed n [1]. A basc oal n all research efforts concernn the control of tunnel-type furnace systems s the mathematcal modeln of them as has been done n other classcal types of furnaces [4, 5, 6]. The obtaned models can be used effcently for the analyss as well as for the synthess of the control stratey of these furnace thermal systems. Alon a tunnel-type furnace there are several subsystems n operatonal nterrelaton whch are responsble for the domnant thermal and aerodynamc states nsde the furnace. Accordn to a standard practce, n brck and tle manufacturn we use a seres of snle loop controllers n order to control these dfferent subsystems. The control task s today performed wth conventonal PID controllers that provde an acceptable operaton of the plant [7]. The PID controllers however cannot ensure that there s not devaton from the desrable operatn ponts. Ths means that there are stll snfcant

2 open control problems; one such problem s the dfference n temperature between the top and the bottom of the tunnel, despte the acton of the ar-recycln and sdeburners subsystems. Furthermore, current operaton s far from an optmal one from the enery consumpton pont of vew. There are snfcant thermal losses and the fuel consumpton can be substantally reduced. Mult-loop controllers have also been used n other types of ndustral furnaces as s the mult-loop temperature control scheme for a televson lass furnace descrbed n [8]. It s dffcult, tme-consumn and expensve to coordnate a reat number of snle-loop controllers requred for many subsystems whle n parallel achevn precse control s not always possble. A control tool has been proposed by ControlSoft Inc., the MANTRA 47 [9], whch can be used nstead of snle loop controllers n lass manufacturn furnaces. One control loop refers to the combuston procedure performed n ndustral burners that use ether fuel ol or natural as. A passve and actve control scheme of NOx n ndustral burners s proposed n [10]. The oal of ths control loop s to acheve ood mxn of fuel and ar n order to obtan the desred stochometrc concentratons mposed by envronmental condtons. In ths paper, the proposed approach for modeln and control of ndustral tunnel-type furnaces combnes conventonal mathematcal modeln wth fuzzy loc based technques. Ths combnaton of both technoloes ves n some cases better results [1]. Partcularly, ths paper ntroduces a fuzzy supervsory control scheme n order to coordnate local controllers and to evaluate whether they satsfy prespecfed performance crtera. A two-level herarchcal structure has been appled concernn the prmary system varables and the process behavor respectvely. Furthermore, two supervsory confuratons have been tested one for parameters adjustment and another for outputs addton n order to estmate the mpact of the hh level control stratey Preheatn Zone Heatn Zone Furnace lenth=105 m II. THE TUNNEL FURNACE FOR BRICK AND TILE PRODUCTION A tunnel-type ndustral furnace conssts of several subsystems n operatonal nterrelaton and s dvded usually n three zones, the pre-heatn zone, the heatn zone and the cooln zone as shown n F. 1. Two basc subsystems are responsble for the thermal and aerodynamc states created nsde the furnace. The system s heart s the heatn zone consstn of a matrx-set of burners. A typcal number s ehty burner flames or more located on the roof of the furnace. In order to acheve sothermal dstrbuton of heat from top to bottom, there are also sde burners at each sde of the furnace for rapd actons and temperature correctons. In some cases, the fuel used n these two burner-roups s dfferent and the correspondn combuston control presents addtonal dffcultes. In some producton processes, the products have to be dehydrated before entern the furnace. In such cases a parallel passve kln operates wth hot ar from furnace. Hence, another subsystem s that of hot ar extracton from the furnace and specfcally from the cooln zone. The hot ar flow s reulated n varous ponts of the furnace ether by on/off tampers or by analo poston tampers performn mxn wth envronment ar n order to keep hot ar flow and temperature constant. Immedately after the heatn zone, there s a small subsystem performn a rapd reducton of the temperature. Another subsystem wth multple ar fans feeds cool ar from the envronment at the end of the tunnel. To keep the tunnel temperature constant from top to bottom, n addton to the sde burners, there s also a subsystem for recycln hot ar before the heatn zone. Fnally, a chmney at the bennn of the tunnel forces exhaust emssons to the envronment affectn so the overall aerodynamc state nsde the furnace Cooln Zone Chmney Ar-lock Chamber Product flow Hot Ar Recycln Sde Burners Ar flow Roof Burners Rapd Cooln System Null Pressure Lne Hot Ar Extracton for Kln Operaton Cold Ar Feedn System F. 1. Schematc overvew of a tunnel-type furnace for brck and tle producton.

3 III. PHYSICAL MODEL OF THERMAL PROCESS The tunnel-type ndustral furnace s a very complex object for modeln and control, because multple exchane of enery occurs n t due to the nature of process that leads to complex mathematcal models descrbn ts dynamcs. In the past, tunnel-type ndustral furnaces were often bult wth a central heatn zone and thermal nsulaton that mnmzed heat loss throuh furnace walls. The result of such a desn was to create an almost statc system wthn the furnace from the aerodynamc pont of vew. A typcal brck producton process would nvolve placn a car of brcks nsde the furnace, rasn the temperature slowly to process temperature, holdn for a specfed tme and then quckly cooln the furnace. Whle such processes are stll common today, ncreasn demands for better temperature unformty and reater yeld are drvn furnace makers to address complcatons related to the dynamcs of the heatn and cooln processes [3]. Today, furnace makers ntroduce forced mult-nput multoutput ar flow n order to acheve the desred temperature dstrbuton across the furnace and to explot as much as possble the heat losses n secondary zones. The thermal effcency of a fuel-fred furnace s usually expressed by the rato of heat transfer to the load to the enery nput n the fuel. There are three basc and nterdependent mechansms of heat transfer, the thermal radaton, convecton and conducton. The steady-state enery balance s ven by Q& G + Q& pa = Q& + Q& L + Q& l (1) where Q & G s the enery nput suppled by the fuel, Q & pa s the rate of enery supply n preheated combuston ar, Q & s the heat content of the flue products, Q & L s the rate of heat transfer to the load and Q & l expresses any knd of enery losses. The enery nput suppled by the fuel s ven by Q & G = V & GC (2) V where V & G s the volumetrc flow rate of fuel and C V s the calorfc value. The rate of enery supply n preheated combuston ar s ven by Q & pa = V & GRs(1 + X) ρae (3) th, a where R s s the stochometrc ar/fuel volume rato, X s the percentae excess ar level, ρ a s the densty of ar at the reference temperature and pressure and E th, a s the specfc enthalpy of the preheated ar. Assumn complete combuston, the heat content of the flue products at temperature T s ven by Q & = V & P + R X ρ E T (4) ( ), ( ) s s th where P s s the combuston product/fuel volume rato, s the densty of the combuston products and E th, s the specfc enthalpy of the combuston products as a functon of temperature. Fnally, assumn a process load ρ throuhput of b & L, the rate of heat transfer to the load s ven by Q & L = b& L[ Eth( To) Eth( T) ] (5) where Eth ( T o ) and Eth( T ) are the specfc enthalpes of the load at the outlet and nlet temperature respectvely. The domnant mode of heat transfer from the flame and combuston products nsde furnace s the nonlumnous aseous radaton. In fossl-fuel fred combuston processes, carbon doxde and water vapor are the most mportant emtters of aseous radaton. Carbon monoxde and methane also absorb and emt radaton, but they are usually absent or exst at very low concentratons. Thermal radaton transfer can occur from surfaces and ases wthn a tunnel-type furnace. All surfaces wthn ndustral furnaces, emt, reflect and absorb radaton from ther surroundns and thereby partcpate n the overall exchane of radant enery to a load. For modeln a tunnel-type ndustral furnace, we consder that all furnace surfaces are rey Lambert surfaces where emssvty s assumed to be ndependent of both wavelenth and drecton of radaton. For a rey Lambert surface the emssvty (ε) s equal to absorptvty (α) and ε=1-ρ, where ρ s the reflectvty coeffcent. The enery transfer between two rey Lambert surfaces A 1 and A 2 of emssvtes ε 1 and ε 2 respectvely s ven by Q& 1 2 = ( Eb,1 Eb,2) / + 1 (6) A1ε1 A2 ε2 where E b s the emssve enery of surface. The aseous atmosphere n any fuel-fred furnace partcpates n the overall nterchane of radaton. A parallel beam of radaton passn throuh an absorbn rey as s attenuated n proporton to ts ntensty and the dstance traversed throuh the as. The radant emsson from a volume V of as at temperature T and wth 4 attenuaton coeffcent K s ven by Q& = σ KVT where 4 σ s the Boltzman constant. When a beam of radaton s ncdent upon a partcle, scattern occurs by dffracton, refracton and reflecton. However, because the dameter of soot partcles n aseous and lqud fuel flames s usually qute small, scattern of radaton by soot may be consdered nelble compared wth emsson and radaton. On the other hand, methods of calculatn scatter and for predctn the scattern coeffcent are dependent on the partcle dstrbuton/concentraton and ths nformaton s dffcult to obtan. IV. SYNTHESIS OF THE SUPERVISORY CONTROL STRATEGY Most ndustral furnaces are controlled usn classcal control alorthms such as ON-OFF or PID controllers. The popularty of PID control can be attrbuted to both ts ood performance over a wde rane of operatn condtons and to ts functonal smplcty. A common problem wth PID controllers used for control of hhly nonlnear processes s that the set of controller parameters

4 produces satsfactory performance only when the process s wthn a small operatonal wndow. Outsde ths wndow, other parameters or set ponts are necessary, and these adjustments may be done automatcally by a hh level stratey. The new control systems requre lookn for new and better control alorthms. Neural networks and fuzzy systems are ben used more often now due to development of mcroprocessors. A supervsory system s a system that evaluates whether local controllers satsfy prespecfed performance crtera, danoses causes for devaton from the performance crtera, plans actons, and executes the planned actons. For hh level control and supervsory control several smple controllers can be combned n a prorty herarchy. The appled control system wth supervsory fuzzy controller conssts of two herarchcal levels as shown n F. 2. The process control stratey System and Envronment Measurements T d Sensors Decsons from Model-based Smulaton Results PID Parameters SYSTEM F. 2. Herarchcal fuzzy supervsory structure. 1 st Level Process Behavor Quantzaton 2nd Level Output s expressed lnustcally as a set of mprecse condtonal statements whch form a set of decson rules. A fuzzy expert controller typcally takes the form of a set of IF- AND-THEN rules whose antecedents and consequences are membershp functons. The membershp functons of the fuzzy sets used to formulate the lnustc terms can be defned n many ways and ther choce depends on each practcal stuaton. In ths approach, the classcal tranular membershp functons, from neatve b to postve b, have been used. In the frst level, all the heurstc rules derved from model-based smulaton tests, are ncluded and refer to the prmary system varables ncludn the ar flow rate to kln, the temperature n varous ponts alon the furnace and the correspondn set ponts. The evaluaton of these (frst set) decson rules leads to fuzzy control actons whch must farther determned. Ths s done n the second level where the rules contan IF condtons whch refer to the process behavor. The overshoot, dampn and perod varables characterze the system transent response and are calculated before the adjustment of the controller parameters. Most heatn plant models are based on ether the zone method for radaton analyss or the computatonal flud dynamcs models referred to n [11]. Because of the lare lenth (105 m) of the ndustral furnace for brck and tle producton, t s more convenent to consder the multdmensonal zones model shown n F.3. In multdmensonal zone models, both lontudnal and crosssectonal or radal varaton n temperature and heat flux are consdered. Accordn to zone method, the furnace radatn enclosure s dvded nto sothermal volume and surface zones. A total enery balance s wrtten over each zone n terms of the radaton arrvn at t from all the zones n enclosure. Thus the radant enery balance ncludn radaton arrvn at surface s n uuuuv l uuuuuv Q& = S S E + G S E Aε E (7) where j bj j j b j= 1 j= 1 Q & s the enery flow to zone, SS uuv uuuv and GS are the exchane factors known as surface-surface and assurface drected flux areas respectvely, E s the black body emssve power of a as, n s the number of surface zones and l s the number of volume zones. The eometry and nput data for each volume and surface zone can be vared to match the specfed desn and operatn condtons of the furnace. For a lon-furnace model wth fuel and Zone 1 Load flow & m + 1 m& m & + Zone m + 1 F. 3. A lon-furnace model wth recrculaton between zones. ar nput to the specfed zones shown n F.3, the flow of combuston products between zones s resolved nto forward and reverse flow components m & + and m& respectvely. The enery balance s then ven by Q& G, + Q& pa, + Q& th, Q& co, Q& ra, = 0 (8) where Q & G, s ven by (2), Q & pa, s the heat nput n the combuston ar ven by (3). Q & co, and Q & ra, are the convectve and radatve heat transfer from the combuston products to the surroundn surfaces n zone. Q & th, s the flow of enthalpy to zone n the combuston products and s ven by Q& + th, = m& 1 Eth, ( T, 1) + m& + 1 Eth, ( T, + 1) (9) + m & + m& + m& 0,1 Eth, ( T, ) The above model s capable of smulatn the effects of recrculaton between zones (descrbed n secton II) provded the forward and reverse flow components are specfed. The soluton of each volume zone enery balance equaton s dependent on the combuston product temperatures n the nehborn zones. The equatons for all volume zones must therefore be solved smultaneously n the 1 st level of the supervsory structure n order to & Zone n Heat nput Flue mass flow m& 0,

5 derve the temperature T,. Fuzzy controllers may be nterated wth other controllers n varous confuratons. Two basc (a) T d (b) T d FUZZY PID FUZZY PID F. 4. Fuzzy controller confuratons. confuratons of the furnace supervsory control, shown n F. 4, were tested. Fuzzy n F. 4 refers to the hh level furnace control stratey whle PID stands for a conventonal control scheme whch n furnace case conssts of varous ndependent or coupled PID loops. In confuraton (a), the supervsory stratey s used for adjustments of the parameters of the PID control loops. Normally, conventonal PID controllers are capable of controlln the process when the operaton s steady and close to normal condtons. However, f sudden chanes occur or f the process enters abnormal stuatons, then the confuraton (b) s useful to brn the process back to normal operaton as fast as possble. SYSTEM SYSTEM forced nlet of ar at the end of the furnace (whch results n hh pressure at one end of the tunnel) whle there s a forced outlet at the bennn of the furnace (whch results n low pressure at the other end). Ths dfference n pressure results n a net flow of ar from one end to the other. In addton to controlln the temperature of the furnace, one has also to reulate and control the stablty of ths aerodynamc state. Between hh and low pressure there s an manable lne whch presents null pressure. Ths lne must le n a constant locaton between the heatn and cooln zone. A possble shftn of nullpressure lne n the heatn zone means that cool ar enters the heatn zone whch s forbdden. On the other hand, a possble shftn n the cooln zone mples that exhaust emssons wll enter the kln, whch s also undesrable. To keep the null-pressure lne constant, there must be a contnuous control of the cool ar nlet flow. The chmney flow s treated as a dsturbance, snce t vares ndependently to keep exhausts temperature at a low level for savn enery. The desrable temperature pattern succeeded manly by the matrx-set of roof burners, shown n F. 6, s stronly affected by the convenent or no operaton of the overall ar flow system. From the above descrpton t s obvous V. FURNACE PERFORMANCE RESULTS AND DISCUSSION Typcal oals of a supervsory controller are safe operaton, hhest product qualty, and most economc operaton. All three oals are usually mpossble to acheve smultaneously, so they must be prortzed. The temperature alon the whole lenth of the tunnel must follow a predefned curve dependn on the knd of the process. For example, n furnaces for conventonal Temperature o C Furnace lenth (m) F. 5. Furnace temperature pattern. ceramc products, the temperature pattern must follow the one depcted n F. 5. As mentoned above there s a F. 6. Overvew of the roof burners. that the supervsory control system has to perform contnuous control of varous physcal varables such as temperature, ar flow and stochometrc combuston, and dscrete control (ON/OFF) of varous tampers, burners, fans and doors. The most basc of the PID control loops s the one concernn the combuston of the roof burners shown n F. 6. The controller reulates the operaton of burners based on the measurement of ar flow nlet and temperature, and on the stochometrc analyss data. A prelmnary parametrc study was made to nvestate the mpact of flame-vortex nteractons and fuel/ar mxture nhomoenety on combuston nstablty. The dependency of combuston nstablty on fuel mean velocty s stron and suests that the domnant mechansm causn combuston nstablty s hydrodynamcs and ts nteracton wth flame. The proposed fuzzy supervsory control scheme has been tested wth expermental data obtaned from a 250 ton/24 hour brck furnace n Greece. The fuzzy loc part of the

6 controller and the correspondn rule base were mplemented n a S7-300 PLC usn the FuzzyControl++ S7 software packae by Semens. Fure 7 shows typcal Preheatn Zone Temperature ( o C) Bottom temperature Top temperature Furnace lenth (m) CONCLUSIONS Control problems n the furnace-based process ndustry are domnated by non-lnear and tme-varyn behavor, many nner loops and much nteracton between the control loops. The ntroducton of a snle multple-nput multpleoutput controller s not useful because of the rather hh desn effort and the low transparency of ts complex structure. A more sutable herarchcal fuzzy-loc-based supervsory control scheme has been descrbed n ths paper. In the upper level of the herarchy the supervsory controller classfes the actual system stuaton based on the mathematcal model of the process and n the lower level performs the specfc control mode selecton. F. 7. Temperature varaton alon the tunnel furnace at the top and the bottom wthout supervsory control. Temperature ( o C) Preheatn Zone Bottom temperature Top temperature Furnace lenth (m) F. 8. Temperature dfference of F. 6 after the acton of the fuzzy supervsory controller. temperature varatons at the top and the bottom of the tunnel furnace wthout the acton of the supervsory controller. The temperature devaton must be null n the optmum case and f possble wthout the ad of the ar recycln and sde-burners subsystems. Snce somethn lke ths t seems mpossble today, new and better control alorthms are requred. Fure 8 shows the reducton of the temperature dfference between the top and the bottom of the tunnel obtaned after the acton of the fuzzy supervsory controller. As mentoned above, typcal oals for a supervsory controller are safe operaton, hhest product qualty and most economc operaton. All three oals are usually mpossble to acheve smultaneously. Hence, they must be prortzed and presumably safety ets the hhest prorty. The proposed herarchcal supervsory control scheme s the most sutable to assn prortes to the varous control objectves. REFERENCES [1] B. Zhan, L. Xu, J. Wan and H. Shao, Dynamc Model of Reheatn Furnace Based on Fuzzy System and Genetc Alorthm, Proceedns of the Amercan Control Conference 2002, vol.5, pp , May [2] M. Sen, A Revew of the Prncples and Applcatons of Thermal Control, Journal of the Mexcan Socety of Mechancal Enneern, vol.1, No.4, pp , [3] A.E. Naen, J.L. Ebert, D. Roover, R. L. Kosut, M. Dettor, L. Porter and S. Ghosal, Modeln and Control of Dstrbuted Thermal Systems, IEEE Transactons on Control Systems Technoloy, vol.11, No.5, September [4] S. Pal and A.K. Lahr, Mathematcal Model of COREX Melter Gasfer: Part II, Dynamc Model, Metallurcal and Materals Transactons B, Vol.34B, pp , September [5] D.Lj. Debelkovc, I. Buzurovc, M.V. Rancc and S.D. Sarbon, Mathematcal Model of the Pulversn Plant as a Mass Accumulator wth Assocated Dynamcs of Both Impeller and Coal Dryn Process, Scentfc Journal Facta Unverstats Mechancal enneern, vol.1, No.6, pp , [6] Q. He, S.J. Qn and A.J. Torpac, Computatonally Effcent Modeln of Wafer Temperatures n an LPCVD Furnace, Journal on Advanced Process Control and Automaton, vol.5044, pp , [7] S. Martneau, K.J. Burnham, O.C.L. Haas, G. Andrews and A. Heeley, Four-term blnear PID controller appled to an ndustral furnace, Control Enneern Practce, 12, pp , [8] U.C. Moon and K.Y. Lee, Mult-Loop Control of Temperature for TV Glass Furnace, Proceedns of the 39 th IEEE Conference on Decson and Control, Sydney, December [9] Temperature Mantenance and Fuel Flow n Glass Furnace usn MANTRA 47, ControlSoft Publcatons No. AN303, rev , [10] O. Delabroy, E. Hale, F. Lacas, S. Candel, A. Pollard, A. Sobesak and H.A. Becker, Passve and actve control of NOx n ndustral burners, The Expermental Thermal and Flud Scence Journal, 16, pp.64-75, [11] J.M. Rhne and R.J. Tucker, Modeln and Control of Gas-Fred Furnaces and Bolers, McGraw-Hll, New York 1991.