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Distributed Microprocessor-Based Control Systems for Flue Gas Cleaning
© IFA C \[icroCOll1pllter Application in Process Control. Istanbul Turke\.19R6 Copyri~ht
DISTRIBUTED MICROPROCESSOR-BASED CONTROL SYSTEMS FOR FLUE GAS CLEANING W. Latzel* and H. Kahle** "'L'IIi;'l'I'sitiit. CH Paderborll, Federal Republic of Gen!WN\' **81'011'11, Bm'fI'i (3 Cie AG, M(JIIII/zeim, FRG
Ab,stI.a,c,t~"
Various microprocessor-based control systems are known today . They are either centralized, dedicated-centralized or decentralized in their functional structure of data acquisition and computation of control algorithms. This paper presents a microprocessor-based, busoriented control system with freely sele ctable centralized, dedicated-centralized and decentralized functional structures for both data acquisition and computation of control algorithms within one overall system. Demand in flexibility of functional structures is demonstrated in flue gas cleaning plants for coal-fired power plants. An analysis of the flue gas cleaning process is presented. The analysis shows that, with the exception of flue gas treatment itself, the following process stages of cleaning, product processing and end product treatment are separated from each other by intermediate storage containers. Resulting variable availability requirements for distinct process stages are demonstrated. These requirements are met by variable degrees of centralization within the functional structure of the control system. A control system meeting the above requirements is presented . Some special system characteristics like closed-loop control via the data highway, modular system structure, event-driven communication are shown. flue gas cleaning; distributed control system; availability requirements; centralized functional structure; dedicated-centralized functional structure; decentralized functional structure.
Key.h'.QXQ,s",!"
1. ANALYSIS OF THE FLUE GAS CLEANING PROCESS Flue gas c l e aning pr o c e sses for p o wer plants belong t o the state o f the art. Several meth o ds o f pr ocess engineering have rea c hed the stat u s o f technical a pplicati o n o r hav e even gained sub stantial operational experience . In nearly all used methods, appropriate chemical reagents a re added to the flue gas after having left the boiler and the electrostatic filter. In this way the pollutants 502 and NOx are absorbed chemically and can be washed out of the flue gas stream / 1/ .
gyp Sum slurry proceSSing
Out of appro ximately 20 methods available o n the market, th o se metho ds using abs o rption liquid c o ntaining limestone or milk of lime have gained a market share o f ab o ut 90%. The final end product resulting from these pr o cess engineering methods is commercially valuable gypsum. Because of the dominating market share, only these methods with gypsum as an end product will be considered furthermore.
FIg .'
end produc1 treatment
Outline of flue gas clean ing process
2. AVAILABILITY REQUIREMENTS FOR THE FLUE GAS CLEANING PROCESS Among other criteria, the individual process stages differ in required availabili ty level.
The flue gas cleaning process as depicted 1 can be subdivided into five in Fig. stages: - absorption - flue gas reheating - reagent preparation - gypsum slurry processing - sewage and end product treatment.
2 .1. Availability requirements for distinct process stages
Process stages ass o ciate d with the flue gas stream itself t o gether with abs o rption and flue gas reheating functions must provide the same high availability as the power generation process itself. 123
124
W. LaLZei and H. Kahle
According to applicable statutory regula tions for air pollution control it is necessary to shut down or to drastically reduce production rate of the power plant in case these process stages go out of operation. The stages o f reagent preparation and gypsum slurry processing have to meet medium availability requirements, as they are equipped with mechanical storage containers.In order to bridge short disturbances of associated components, the volume of containers as well as the normally existing overdosing of absorbent within the storage co ntainers can be utilized . Moreover, several components within the stages of reagent preparation, gypsum slurry processing as well as oxydation units are often installed in parallel /2/. In case of a component failure, these pr ocess stages can remain in operati on by using parallel equipment.
functional structure of FGC process
levets of control
PCL process control level (open/ closed loop)
GCL group control level (open/ closed loop)
ICL individual control level (open /closed loop)
Fig.2.1
Sewage and end product treatment stages call for the lowest availability require ments . Their mec hani ca l storage containers provide such large buffer effects, that disturbances in these stages may last some hours , bef o re the production rate of power ge neration must be reduced . 2.2. Availability requirements for the control system To get a technically and commer c iall y opti mal solution, the different availability requirements for individual process stages must be i mposed upon the control system as well. In case a uniform cont rol system for the whole plant is desired, this system shou ld p ossess inherent graded fa c ilit ies to realize the full scale from high to low availab ility requir e ment s. In sta lling such a control system req ui r es a division of the ove rall p r ocess into s ubfuncti ons of process contro l. Only with the resulti ng functional st ru cture, the gr ad e o f centralization f or the installed control system and hence the grade of required availability level can be exact ly determined. 2 . 2.1. Hierarchically structured control functions As an example, Fig. 2.1 shows a section of the flue gas clea ning process with hierar chica lly structured contro l functi ons within pro cess, group and individual contr o l l eve l. Clear delimitations, dependencies as well as red undancies of con trol fun ctions are re vealed. The r equi r ed availability level can now be determined by combining a mo re o r less la rge number of control functions i n one hardwa re module. As a result , differently centralized structures f or installed control system comp one nts are obt aine d. 2.2.2. Various centralization in the control system for flue gas cleaning subprocesses Decentralized functional structure For components associated with the flue gas stream itself, high availability is
Levels of control for flue gas cleaning (FGC) process
availability requirem ents
0) high availabili ty
b)
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GCL
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Availability requirement s and corresponding fun ctional structure of control for flue gas cleaning (FGC) process
required. This ca ll s for a st ri ctly decentralized and hierarchical structure of t he control system ( Fig. 2.2.al. This functional structure has successfully been used in power plant applications for more than 20 years . It comprises onl y one cont r ol function per ha rdwar e module in the individual control level as well as in the group cont r ol level. In case of overall automation , several group control functions are coord inated by a supe rim posed process control level. Failure o f modules used for p r ocess as well as group level control can be bridged by manual cont r ol actions in the next lower level o f hierarchy until repair is accomplished. Only t he degree of automation and comfort of handling is lowered during d own time. Fai lure of a mo dul e used in the individual control level does not cause t o tal shut - down of a plant , since those devices, which are decisive for the overall proces s perf o rmance are installed redundantly anyhow. For high availability requirements, mechanical redundancy i s repli cated i n the i nsta lled control equipment in a consequent manner. Foll ow ing this principle, fail u re o f any control module does n ot affect ove rall plant availability .
12:i
COlltrol Systems for Flue Gas ClclIling
Dedicated-centralized functional structure A medium level of availability is required for components o f reagent preparation and gypsum slurry processing. Modules with allocated multiple control functions are used in group and individual control levels. Concentration of several functi ons in one control module is only permissable within each te chno logical coherent control function (Fig. 2.2.b). The hierarchical structure is maintained.
process control
group control
Centralized functional structure Low availability is required for sewage and end product treatment. In this application. control functions in individual as well as in group control level are perf ormed cent rally in one common module (Fig . 2.2.c). Concent ration of control functi ons in one control module is only limited by its maximum processing power. All three functional structures offer an optimum for the ratio of availability to cost. The above mentioned cOllsiderations on differently centralized structures may be applied to all control functions like measurement. o pen and closed loop control . 3. PRESENTATION OF A CONTROL SYSTEM WITH FLEXIBLE FUNCTIONAL STRUCTURE FOR FLUE GAS CLEANING PLANTS The control system PROCONTROL P, produced by Brown, Boveri & Cie AG. offers a selectable degree of centralization for realizati on of control functions. Before introducing those equipment components allowing this concept, a brief ove rview of the system configuration and types of control modules shall be given for better understanding.
3.1. PROCONTROL P system configuration A generalized configuration of the system is shown in Fig. 3.1. The system can comprise up to 250 stations. which may be installed either locally decentralized through out the whole plant site or locally centralized within the elect r onics room. According to the assigned control functions. electronic modules for measurement. open and closed loop control. indication and annunciation may be installed in any mix and any number in every station. Modules within one station communicate with each other via the station bus. All stations are connected with each other via the redundant remote bus. Both channels A and B of the rem ote buses are working non-reactively. Up to e ight redundant remote buses of 1 .5 km length each may be coupled and laid out within the plant site. All process signals transmitted via a station bus are forwarded to the remote bus and are received by all connected stations in the same instant. Thus, all process signals within the overall system are available for further processing in all connected stations.
Fig. 3.1
PROCONTROl P System configuration
Use of a programming. diagnosing and indicating device (PDAG). which may be plugged in at any station, allows monitoring and substitution of all process signals on the remote bus. Equally, application-specific control algorithms may be read or written from or to any control module via the remote bus during full system ope rati on. Process monitoring and control may be performed either with CRT ope rat o r control stations o r with conventional control instrumentati o n or a combinati o n o f both . Depending on the type of process, a c ontrol system for a flue gas cleaning plant will consist of typically six to twelve stations. connected via the remote bus. 3.2. Structure of a PROCONTROL P station A PROCONTROL P station is a cabinet for electronic equipment consisting of a maximum of four nineteen~inch racks. Each rack provides twenty-one slots for inser tion of electronic mo dules. Figure 3.2 depicts typical control modules. Nearly all modules are micropro cessor - controlled. There are signal input and output modules as well as modules for device control (open and closed loop) with appropriate hardware for signal I/O to the process and (optionally) to a hardwired manual control station in the control room. Programmable processing modules contain application-specific control algorithms for use in pr ocess and group control level. In case of centralized or dedicated-centralized functional structures, processing modules perform individual control level functions as well. Connection to the CRT operator control station is achieved by the operator station coupler. The remote bus coupler connects the station bus to the remote bus and thus to all other stat ions and their modules throughout the system.
W. Latzel and H. Kahle
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Fig.3.3
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PROCONTROl P Binary control. centralized functional structure
3.3. Realization of graded centralization For the example of binary open loop control, the equipment facilities of PROCONTROL P to set up differently centralized structures are presented. In the following figures, station bus and remote bus are combined under the name of data bus.
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3.3.1. Centralized functional structure Several group and as many as possible device control functions together with associated interlocking logic are installed in the programmable processing module (Fig.
3.3.).
Coupling of devices is perf ormed by simple device I/O and basic control modules. These modules have no microprocessor. The number of coupled devies is limited only by the maximal possible number of wires connectable to the module. Control commands by CRT operator control station within group and individual control level are directed to the programmable control module only. With additional cost, optional manual control stations for group or individual control level may be installed. Their communication with the programmable processing module is conducted via the data bus, because the large required number of contacts for parallel wiring cannot be installed on the processing module.
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The centralized functional structure has the disadvantage to set out of operation several group and device control functi ons in case of a failed programmable processing module. Only device protection functions remain in operation. This structure has the advantage of lowest installation cost compared to the following variants.
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3.3.2. Dedicated-centralized functional structure Only one group control function and the hierarchically corresponding device control functions together with their inter-
dev ice
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PROCONTROl P Binary control. decentralized funct ional structure
o perator conlfol station
127
Control Systems for Flue Gas Cleaning locking logic are installed in the programmable processing module (Fig. 3.4). Coupling of devices is performed by simple device I/O and basic control modules, comparable to those used in centralized functional structures. These modules are not equipped with microprocessors, however an additional hard-wired manual control station is installed in the control room. Manual interference via CRT operator control station within group and individual control level is directed to the programmable processing module only, just like in the centralized functional structure . As an option with additional cost, a manual control station for group control can be installed . This function works in parallel to the CRT operator control station via the data bus . The dedi cat ed- cent ralized functional structure will limit consequences of a disturbance to a clearly predefinable subfunction within the hierarchy of ove rall control. Failure of the pr ogrammable pr ocess ing module will affect only one group control fun c tion and its associated individual control functions together with interl oc king logic. However, each individual device c an still be controlled manually by use of the manual control station, ove rriding interlocking logic status. 3 . 3.3 . Decentralized functional structure For high availability requirements, a decentra lized functional structure shown in Fig . 3.5 is chosen. Only one cont r o l function each within group and individual control level is installed o n each hardware module. Ever y module i s connected to a hard-wired manua l cont r o l station. Manual interference via CRT operator control stati on can be directed to the programm able processing module as well as to the device cont r o l module. Consequences o f a failed programmable pro cess i ng module are lim ited to the group control function only. Manual interfe re nce within the indi v i dual cont r o l level can be pe rf o r med either via CRT operator contr ol station or via manual control stat i on. Failure of a devic e contro l module i s not decisive for ove rall plant ava il abi lity , since important devices are installed redundantly anyhow. Compared to the previ ous structures, the decentralized, hie r archical functi onal st ru ctu re provides a hi gh ove rall plant availability. This high availability requires the comparatively highest installation cost. 3.3.4. Physical location of control equipment installations The majority o f FGC plants is installed in already existing power plants . Changes in physi cal const ructi on are only possible within narr ow limits . Thus , on ly limited space i s available f or installati on of additional control equipment. Fig . 3.6 shows two variants in allocation o f contro l equipment. It is assumed in both cases, that control of the FGC plant has to be perf o rmed by the o perat o r staff in the p ower plant control room.
power plant control room
power plant electronics room
distance " 400 m
FGC plant building
FGC process area allocation A
Fig.3.6
allocation B
Various allocations for control equipment of flue gas cleaning process
Variant A assumes that there is still sufficient space for installation of the control equipment cabinets within the power plant electronics room . Signals for measurement and control are connected with multi-conductor cables to the FGC process area. In variant B, electroni c equipment cabinets are installed in the FGC plant building. In this case, the distance between the power plant electronics ro om and the FGC plant building is bridged by th e remote b us . Since all process data are available on the rem ote bus , only stations with modules for signal I /O for wired push buttons, indi cato rs and meters are se t up in the power p l ant electronics r oom. There , the CRT operator control station is hooked to the control system in the same fashi on. The fle xi ble s tru ctu re of the pre sented control system allows a locally central ized o r decentralized installati o n of the control e quipment cab inet s. 4 . SPECIAL CHARACTERISTICS OF PROCONTROL P CONTROL SYSTEM In this section, an ove r v i ew of those system characteristics is given, whi ch a llow the flexibility of the sys tem structure as des c ribed in section 3. 4.1. Source addressing and broadcasting with common acknowledge Within the PROCONTROL P system, all process signals are transmitted in a digitized format to every stati on bus and re mote bus . This task is autonomously perf o rmed by the system and need not be pro grammed . Signals are transmitted with messages. Messages contain mainly the binary code d proce ss signal and the correspo nding address. Systemwide, address and process signal represent a nonseparable entity according to the principle: "Systemwide, one signal is named identi cally" or "Systemwide, one message is addressed identically", respectively.
w. Latzel and H. Ka hle
128
Every station can indicate a disturbed receipt of one or several messages by means of a convenient common acknowledge fun c tion, working in synchronism with message transmission . Missing common acknowledge provokes immediate retransmission :
[me~a_ge~.::..c.........c::=.--'==':""--=:""::"'..L:.:...:.--~ J station 2
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"A message is retransmitted to all receivers if at least one receiver does not acknowledge proper receipt." Use of the above mentioned principles al l ows the separati on and distributi on o f one overall control function t o nearly any number of fun c ti ona lly and locally distributed mo dules . 4 .2. Data flow within the system Fig.4.1
Principle of source addressing
The address is defined by that location, where the process signal is fed into the system. This location is th e source for the process signal. The co rresponding address is thus called source address. Figure 4.1 depicts in a simplified manner the principl e of source addressing. In thi s example, an ana l og s ignal is fed into un it 12 of analog input module 11 in station 12 in system 11. The process signal with the address 1 , 2, 1, 2 is tran smitted over the stati on bus to the remote bus and furtheron to the bu se s of all other stations within the system. Trans mission is thus perf ormed co rresponding to the broadcast principle: "During transmission from one source, all destinations are receiving". Every module pr ovided with a receiving function compa r es the address of every newly r eceived message with a list of application-specific add r esses that is sto r ed within it s memory. This check de cides on further processing of the re ceived message in this mod ule. In the above example. the address comparison matches on unit 11 on analog output module 1 3 i n station 19 in system 11. The value of the tr ansmitted process signal i s stored in the output r egiste r and the signal is forwarded to the hard-wired panel meter. The connect i on between ana l og input and analog output module is established simply by storing the signal's source address in the analog out put module . With an equal pr og ramming procedure, any desired number o f receiving modu les may store and process the same message simultaneously . Connection between data source and one o r several rec eiv ing modules is established as f o ll ows: "OnlY the acceptance messages."
receiving module decides and further processing
on of
Princ ipa l advantages of th is transmission scheme especially for real -t ime application within industrial control systems are shown in /3/ and / 4/.
All functionally and locally distributed modules within the control system have to exchange process data in real-time . Guar anteing the necessary data flow within the system is of extreme importance for overall system perf o rman ce. To ensure this, the presented contr o l system uses a mix of cyclic and event-driven communication modes. Accord ing to momentar y pro cess signal dynami c performance, the system autonomously selects eithe r cyclic or eve nt -driven communication mode. Positive operational experience of this scheme in appl icati ons f o r ove ra ll cont r ol of coa l fired power plants is shown in /5/, /6/. 4 .2.1. Event-driven transmission
An event-driven t ran s mission is stimulated by a change of value of any process signal within the control system. In this way, an excellent dynamic performance of process signal transmission is ensured throughout the whole system. Cri teria for stimulation of a transmission are different for binary and a nal o g process signals: a) for binary process signa ls : An event-driven transmission is stimulated whenever a binar y process signal c hanges state. b) for analog process signals: An event-driven transmission is stimul ated when eve r the signal level of an anal og process s ig nal exceeds a presel ectab le increment compared to last transmission B.Il. d after a preselectable inhibit-time has expired since last transmission. Typi ca l values for increments are 0 . 4% of full scale and 50. 200 or 1000ms for inhibit-time. Resp onse times of typically 10ms for transmission in ove rall systems with more than 3 . 000 analog and 10.000 binary s ig nals are obtained. /5/, /6/. Event -d riven transmi ss i on ensu res the ne cessary response ti me for process signa ls. 4 . 2.2 . Cyclic transmission
The system PROCONTROL P performs event -driven tr ans missi ons with fi r st p r io r ty. Ho wever, a minimum portion of available bandwidth is reserved for cyc lic transmission . During this mode o f operati on, the system itself activates all data
Control Systems for Flue Gas Cleaning sources to transmit t"heir present value of proc ess signal. This procedure is repeated continuously. In this fashion, all rec eiv ing modu les within the system are updated with the latest value of process signal. Depending o n ove rall system size, cyclic update of a given process signal is repeated in a scan of 1 to 6 seconds. Control mod ules, distributed within the system, are continuously synchronized in this way. In addit i on, this mode o f operation allows to cyc li cl y monitor the perf ormance of all system components with the help of appropriate diagnostic provisions. The cyclic mode provides the robustness for transmission.
necessary
4.3. Closed loop control within the locally and functionally distributed control system
5. CONCLUSION An analysis of the flue gas cleaning process shows va riable de grees of availability requirements for certain process stages, while ensuring a high availability for the main power plan t . Variing availability r equ irements can be met by variable degrees o f centralization in the setup o f the presented control system . A technical and economi cal optimum will result . Any mix of cent ra lized, dedicated-centralized and decentralized functi onal structures may be chosen. The system PROCONTROL P has been successfully installed f o r co ntrol of industrial processes and power plants as well as in flue ga s cleaning plants. 6. REFERENCES
/ 1/
Zi~nerma nn , H. (1986). Kon zept und Einsatzerfahrungen bei de l' Leit technik von Rauchgasentschwe f e 1 ungs an 1 age n . y.DJ:. :12~X.tQJ::Lt_<;,._:;. l;t~ , VDI- Ve rl ag , DUsseldorf . pp. 87-99.
/2/
Atzger , J. (1978). Entschwefelungs verfahren mit Endprodukt Gips - ein wirksamer und kostengUnstiger Beitrag zur Reinhaltung del' Luft . T.QIl.inQIJ.:;tx.i.eil.ei:t1J))g, .7.. 4 1 3 - 4 1 8 . Sprechsaal Verlag. Cobu r g.
/3/
Gueth, R.and T.La l ive , ( 19 83). The distributed data fl ow aspect of industrial compute r systems. In PX.Q.c..,._.l1..th._l .E.A.CW.or.k.sbop . . o.I\ .D.i.:l.tr..i.h1Lt.e.Q..._C.Q!!lP.l.lt.Sl.r.....CoIlt.ro_l SY.:Lte.!llil.. Pergamon Press. pp. 1-8.
/ 4/
Gueth, R ., J. Kriz and S. Zueger. (1985). Broadcasting sourceaddressed messages. In J2xOQ ... .. 5th C.oui.ere.ns;;"e .. .Q.I\ .. lU.s:triRl.lte.d .. ColDP'\.l.:t. iM Qxste.ms. IEEE Compute r Society Press. pp. 108- 115 .
/5/
Latzel. W. and H. Kahle ( 1984 ). Dynamik verte ilt e r Pr ozeSl eit systeme, insbeso ndere fUr Regelungen. In GME:.f .
/6/
Herrma nn, R. and W. K6rner (1986). Einsatz del' Bus-Leittechnik in einem 750-MW -Kohlekr a ftw e rk . In
The principles shown in sect i ons 4.1 and 4 .2 allow the setup of a locally distribu ted c l osed l oop cont rol function with data transmission via station and remote bus (Fig . 4.2 ).
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Closed loop control \l'la station and remote bu s In PROCONTROl P distributed control system
In / 5/, the ve rifica t i on of stab ility for closed l oop contro l functions is demon strated in power plant applications. Thi s feature enables to set up all func tions of control and instrumentati on such as measurement, o p en and closed loop control, indicati on and annunciation with one homogeneous set of control equipment.
129
YD I.:M.t::i.sili.t&_l>_6_9.• VDI-Verlag , DUsseldorf. pp. 35-57.
DISTRIBUTED MICROPROCESSOR-BASED CONTROL SYSTEMS FOR FLUE GAS CLEANING W. Latzel* and H. Kahle** "'L'IIi;'l'I'sitiit. CH Paderborll, Federal Republic of Gen!WN\' **81'011'11, Bm'fI'i (3 Cie AG, M(JIIII/zeim, FRG
Ab,stI.a,c,t~"
Various microprocessor-based control systems are known today . They are either centralized, dedicated-centralized or decentralized in their functional structure of data acquisition and computation of control algorithms. This paper presents a microprocessor-based, busoriented control system with freely sele ctable centralized, dedicated-centralized and decentralized functional structures for both data acquisition and computation of control algorithms within one overall system. Demand in flexibility of functional structures is demonstrated in flue gas cleaning plants for coal-fired power plants. An analysis of the flue gas cleaning process is presented. The analysis shows that, with the exception of flue gas treatment itself, the following process stages of cleaning, product processing and end product treatment are separated from each other by intermediate storage containers. Resulting variable availability requirements for distinct process stages are demonstrated. These requirements are met by variable degrees of centralization within the functional structure of the control system. A control system meeting the above requirements is presented . Some special system characteristics like closed-loop control via the data highway, modular system structure, event-driven communication are shown. flue gas cleaning; distributed control system; availability requirements; centralized functional structure; dedicated-centralized functional structure; decentralized functional structure.
Key.h'.QXQ,s",!"
1. ANALYSIS OF THE FLUE GAS CLEANING PROCESS Flue gas c l e aning pr o c e sses for p o wer plants belong t o the state o f the art. Several meth o ds o f pr ocess engineering have rea c hed the stat u s o f technical a pplicati o n o r hav e even gained sub stantial operational experience . In nearly all used methods, appropriate chemical reagents a re added to the flue gas after having left the boiler and the electrostatic filter. In this way the pollutants 502 and NOx are absorbed chemically and can be washed out of the flue gas stream / 1/ .
gyp Sum slurry proceSSing
Out of appro ximately 20 methods available o n the market, th o se metho ds using abs o rption liquid c o ntaining limestone or milk of lime have gained a market share o f ab o ut 90%. The final end product resulting from these pr o cess engineering methods is commercially valuable gypsum. Because of the dominating market share, only these methods with gypsum as an end product will be considered furthermore.
FIg .'
end produc1 treatment
Outline of flue gas clean ing process
2. AVAILABILITY REQUIREMENTS FOR THE FLUE GAS CLEANING PROCESS Among other criteria, the individual process stages differ in required availabili ty level.
The flue gas cleaning process as depicted 1 can be subdivided into five in Fig. stages: - absorption - flue gas reheating - reagent preparation - gypsum slurry processing - sewage and end product treatment.
2 .1. Availability requirements for distinct process stages
Process stages ass o ciate d with the flue gas stream itself t o gether with abs o rption and flue gas reheating functions must provide the same high availability as the power generation process itself. 123
124
W. LaLZei and H. Kahle
According to applicable statutory regula tions for air pollution control it is necessary to shut down or to drastically reduce production rate of the power plant in case these process stages go out of operation. The stages o f reagent preparation and gypsum slurry processing have to meet medium availability requirements, as they are equipped with mechanical storage containers.In order to bridge short disturbances of associated components, the volume of containers as well as the normally existing overdosing of absorbent within the storage co ntainers can be utilized . Moreover, several components within the stages of reagent preparation, gypsum slurry processing as well as oxydation units are often installed in parallel /2/. In case of a component failure, these pr ocess stages can remain in operati on by using parallel equipment.
functional structure of FGC process
levets of control
PCL process control level (open/ closed loop)
GCL group control level (open/ closed loop)
ICL individual control level (open /closed loop)
Fig.2.1
Sewage and end product treatment stages call for the lowest availability require ments . Their mec hani ca l storage containers provide such large buffer effects, that disturbances in these stages may last some hours , bef o re the production rate of power ge neration must be reduced . 2.2. Availability requirements for the control system To get a technically and commer c iall y opti mal solution, the different availability requirements for individual process stages must be i mposed upon the control system as well. In case a uniform cont rol system for the whole plant is desired, this system shou ld p ossess inherent graded fa c ilit ies to realize the full scale from high to low availab ility requir e ment s. In sta lling such a control system req ui r es a division of the ove rall p r ocess into s ubfuncti ons of process contro l. Only with the resulti ng functional st ru cture, the gr ad e o f centralization f or the installed control system and hence the grade of required availability level can be exact ly determined. 2 . 2.1. Hierarchically structured control functions As an example, Fig. 2.1 shows a section of the flue gas clea ning process with hierar chica lly structured contro l functi ons within pro cess, group and individual contr o l l eve l. Clear delimitations, dependencies as well as red undancies of con trol fun ctions are re vealed. The r equi r ed availability level can now be determined by combining a mo re o r less la rge number of control functions i n one hardwa re module. As a result , differently centralized structures f or installed control system comp one nts are obt aine d. 2.2.2. Various centralization in the control system for flue gas cleaning subprocesses Decentralized functional structure For components associated with the flue gas stream itself, high availability is
Levels of control for flue gas cleaning (FGC) process
availability requirem ents
0) high availabili ty
b)
medium availabi lity
functional structure 01 control
~~
GCL
~
GCl
dedicatedcenltall zed
IC l
st ruc ture
c)
low availability
@
typical appllcalton eKample
decentralized structure
flue gas lan
reagent transport
ICl devices
devices
GCL ICl
centralized st ructur e
gypsum transport
devices
GCL "" group control level
ICl
Rg.2 .2
"" mdlvldual con trol level
Availability requirement s and corresponding fun ctional structure of control for flue gas cleaning (FGC) process
required. This ca ll s for a st ri ctly decentralized and hierarchical structure of t he control system ( Fig. 2.2.al. This functional structure has successfully been used in power plant applications for more than 20 years . It comprises onl y one cont r ol function per ha rdwar e module in the individual control level as well as in the group cont r ol level. In case of overall automation , several group control functions are coord inated by a supe rim posed process control level. Failure o f modules used for p r ocess as well as group level control can be bridged by manual cont r ol actions in the next lower level o f hierarchy until repair is accomplished. Only t he degree of automation and comfort of handling is lowered during d own time. Fai lure of a mo dul e used in the individual control level does not cause t o tal shut - down of a plant , since those devices, which are decisive for the overall proces s perf o rmance are installed redundantly anyhow. For high availability requirements, mechanical redundancy i s repli cated i n the i nsta lled control equipment in a consequent manner. Foll ow ing this principle, fail u re o f any control module does n ot affect ove rall plant availability .
12:i
COlltrol Systems for Flue Gas ClclIling
Dedicated-centralized functional structure A medium level of availability is required for components o f reagent preparation and gypsum slurry processing. Modules with allocated multiple control functions are used in group and individual control levels. Concentration of several functi ons in one control module is only permissable within each te chno logical coherent control function (Fig. 2.2.b). The hierarchical structure is maintained.
process control
group control
Centralized functional structure Low availability is required for sewage and end product treatment. In this application. control functions in individual as well as in group control level are perf ormed cent rally in one common module (Fig . 2.2.c). Concent ration of control functi ons in one control module is only limited by its maximum processing power. All three functional structures offer an optimum for the ratio of availability to cost. The above mentioned cOllsiderations on differently centralized structures may be applied to all control functions like measurement. o pen and closed loop control . 3. PRESENTATION OF A CONTROL SYSTEM WITH FLEXIBLE FUNCTIONAL STRUCTURE FOR FLUE GAS CLEANING PLANTS The control system PROCONTROL P, produced by Brown, Boveri & Cie AG. offers a selectable degree of centralization for realizati on of control functions. Before introducing those equipment components allowing this concept, a brief ove rview of the system configuration and types of control modules shall be given for better understanding.
3.1. PROCONTROL P system configuration A generalized configuration of the system is shown in Fig. 3.1. The system can comprise up to 250 stations. which may be installed either locally decentralized through out the whole plant site or locally centralized within the elect r onics room. According to the assigned control functions. electronic modules for measurement. open and closed loop control. indication and annunciation may be installed in any mix and any number in every station. Modules within one station communicate with each other via the station bus. All stations are connected with each other via the redundant remote bus. Both channels A and B of the rem ote buses are working non-reactively. Up to e ight redundant remote buses of 1 .5 km length each may be coupled and laid out within the plant site. All process signals transmitted via a station bus are forwarded to the remote bus and are received by all connected stations in the same instant. Thus, all process signals within the overall system are available for further processing in all connected stations.
Fig. 3.1
PROCONTROl P System configuration
Use of a programming. diagnosing and indicating device (PDAG). which may be plugged in at any station, allows monitoring and substitution of all process signals on the remote bus. Equally, application-specific control algorithms may be read or written from or to any control module via the remote bus during full system ope rati on. Process monitoring and control may be performed either with CRT ope rat o r control stations o r with conventional control instrumentati o n or a combinati o n o f both . Depending on the type of process, a c ontrol system for a flue gas cleaning plant will consist of typically six to twelve stations. connected via the remote bus. 3.2. Structure of a PROCONTROL P station A PROCONTROL P station is a cabinet for electronic equipment consisting of a maximum of four nineteen~inch racks. Each rack provides twenty-one slots for inser tion of electronic mo dules. Figure 3.2 depicts typical control modules. Nearly all modules are micropro cessor - controlled. There are signal input and output modules as well as modules for device control (open and closed loop) with appropriate hardware for signal I/O to the process and (optionally) to a hardwired manual control station in the control room. Programmable processing modules contain application-specific control algorithms for use in pr ocess and group control level. In case of centralized or dedicated-centralized functional structures, processing modules perform individual control level functions as well. Connection to the CRT operator control station is achieved by the operator station coupler. The remote bus coupler connects the station bus to the remote bus and thus to all other stat ions and their modules throughout the system.
W. Latzel and H. Kahle
126 remote bus A
r------------...,
programmable module
remote bus B
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I optional manual control:
I
I
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I 1
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I devices) 1 1
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status
~ manual control slallon
PROCONTROl P
Fig.3.3
Station bus with types of plug-in modules
devICe
PROCONTROl P Binary control. centralized functional structure
3.3. Realization of graded centralization For the example of binary open loop control, the equipment facilities of PROCONTROL P to set up differently centralized structures are presented. In the following figures, station bus and remote bus are combined under the name of data bus.
programmable
manual control
processing module
1- - o-plional:group control (1 group)
: lnter!OCk I device !control
+
:
.~
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,
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!(~32
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3.3.1. Centralized functional structure Several group and as many as possible device control functions together with associated interlocking logic are installed in the programmable processing module (Fig.
3.3.).
Coupling of devices is perf ormed by simple device I/O and basic control modules. These modules have no microprocessor. The number of coupled devies is limited only by the maximal possible number of wires connectable to the module. Control commands by CRT operator control station within group and individual control level are directed to the programmable control module only. With additional cost, optional manual control stations for group or individual control level may be installed. Their communication with the programmable processing module is conducted via the data bus, because the large required number of contacts for parallel wiring cannot be installed on the processing module.
contact status
Fog.3.4
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programmaNe processing module
--,
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r
The centralized functional structure has the disadvantage to set out of operation several group and device control functi ons in case of a failed programmable processing module. Only device protection functions remain in operation. This structure has the advantage of lowest installation cost compared to the following variants.
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+
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i
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3.3.2. Dedicated-centralized functional structure Only one group control function and the hierarchically corresponding device control functions together with their inter-
dev ice
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Fig.3.S
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PROCONTROl P Binary control. decentralized funct ional structure
o perator conlfol station
127
Control Systems for Flue Gas Cleaning locking logic are installed in the programmable processing module (Fig. 3.4). Coupling of devices is performed by simple device I/O and basic control modules, comparable to those used in centralized functional structures. These modules are not equipped with microprocessors, however an additional hard-wired manual control station is installed in the control room. Manual interference via CRT operator control station within group and individual control level is directed to the programmable processing module only, just like in the centralized functional structure . As an option with additional cost, a manual control station for group control can be installed . This function works in parallel to the CRT operator control station via the data bus . The dedi cat ed- cent ralized functional structure will limit consequences of a disturbance to a clearly predefinable subfunction within the hierarchy of ove rall control. Failure of the pr ogrammable pr ocess ing module will affect only one group control fun c tion and its associated individual control functions together with interl oc king logic. However, each individual device c an still be controlled manually by use of the manual control station, ove rriding interlocking logic status. 3 . 3.3 . Decentralized functional structure For high availability requirements, a decentra lized functional structure shown in Fig . 3.5 is chosen. Only one cont r o l function each within group and individual control level is installed o n each hardware module. Ever y module i s connected to a hard-wired manua l cont r o l station. Manual interference via CRT operator control stati on can be directed to the programm able processing module as well as to the device cont r o l module. Consequences o f a failed programmable pro cess i ng module are lim ited to the group control function only. Manual interfe re nce within the indi v i dual cont r o l level can be pe rf o r med either via CRT operator contr ol station or via manual control stat i on. Failure of a devic e contro l module i s not decisive for ove rall plant ava il abi lity , since important devices are installed redundantly anyhow. Compared to the previ ous structures, the decentralized, hie r archical functi onal st ru ctu re provides a hi gh ove rall plant availability. This high availability requires the comparatively highest installation cost. 3.3.4. Physical location of control equipment installations The majority o f FGC plants is installed in already existing power plants . Changes in physi cal const ructi on are only possible within narr ow limits . Thus , on ly limited space i s available f or installati on of additional control equipment. Fig . 3.6 shows two variants in allocation o f contro l equipment. It is assumed in both cases, that control of the FGC plant has to be perf o rmed by the o perat o r staff in the p ower plant control room.
power plant control room
power plant electronics room
distance " 400 m
FGC plant building
FGC process area allocation A
Fig.3.6
allocation B
Various allocations for control equipment of flue gas cleaning process
Variant A assumes that there is still sufficient space for installation of the control equipment cabinets within the power plant electronics room . Signals for measurement and control are connected with multi-conductor cables to the FGC process area. In variant B, electroni c equipment cabinets are installed in the FGC plant building. In this case, the distance between the power plant electronics ro om and the FGC plant building is bridged by th e remote b us . Since all process data are available on the rem ote bus , only stations with modules for signal I /O for wired push buttons, indi cato rs and meters are se t up in the power p l ant electronics r oom. There , the CRT operator control station is hooked to the control system in the same fashi on. The fle xi ble s tru ctu re of the pre sented control system allows a locally central ized o r decentralized installati o n of the control e quipment cab inet s. 4 . SPECIAL CHARACTERISTICS OF PROCONTROL P CONTROL SYSTEM In this section, an ove r v i ew of those system characteristics is given, whi ch a llow the flexibility of the sys tem structure as des c ribed in section 3. 4.1. Source addressing and broadcasting with common acknowledge Within the PROCONTROL P system, all process signals are transmitted in a digitized format to every stati on bus and re mote bus . This task is autonomously perf o rmed by the system and need not be pro grammed . Signals are transmitted with messages. Messages contain mainly the binary code d proce ss signal and the correspo nding address. Systemwide, address and process signal represent a nonseparable entity according to the principle: "Systemwide, one signal is named identi cally" or "Systemwide, one message is addressed identically", respectively.
w. Latzel and H. Ka hle
128
Every station can indicate a disturbed receipt of one or several messages by means of a convenient common acknowledge fun c tion, working in synchronism with message transmission . Missing common acknowledge provokes immediate retransmission :
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"A message is retransmitted to all receivers if at least one receiver does not acknowledge proper receipt." Use of the above mentioned principles al l ows the separati on and distributi on o f one overall control function t o nearly any number of fun c ti ona lly and locally distributed mo dules . 4 .2. Data flow within the system Fig.4.1
Principle of source addressing
The address is defined by that location, where the process signal is fed into the system. This location is th e source for the process signal. The co rresponding address is thus called source address. Figure 4.1 depicts in a simplified manner the principl e of source addressing. In thi s example, an ana l og s ignal is fed into un it 12 of analog input module 11 in station 12 in system 11. The process signal with the address 1 , 2, 1, 2 is tran smitted over the stati on bus to the remote bus and furtheron to the bu se s of all other stations within the system. Trans mission is thus perf ormed co rresponding to the broadcast principle: "During transmission from one source, all destinations are receiving". Every module pr ovided with a receiving function compa r es the address of every newly r eceived message with a list of application-specific add r esses that is sto r ed within it s memory. This check de cides on further processing of the re ceived message in this mod ule. In the above example. the address comparison matches on unit 11 on analog output module 1 3 i n station 19 in system 11. The value of the tr ansmitted process signal i s stored in the output r egiste r and the signal is forwarded to the hard-wired panel meter. The connect i on between ana l og input and analog output module is established simply by storing the signal's source address in the analog out put module . With an equal pr og ramming procedure, any desired number o f receiving modu les may store and process the same message simultaneously . Connection between data source and one o r several rec eiv ing modules is established as f o ll ows: "OnlY the acceptance messages."
receiving module decides and further processing
on of
Princ ipa l advantages of th is transmission scheme especially for real -t ime application within industrial control systems are shown in /3/ and / 4/.
All functionally and locally distributed modules within the control system have to exchange process data in real-time . Guar anteing the necessary data flow within the system is of extreme importance for overall system perf o rman ce. To ensure this, the presented contr o l system uses a mix of cyclic and event-driven communication modes. Accord ing to momentar y pro cess signal dynami c performance, the system autonomously selects eithe r cyclic or eve nt -driven communication mode. Positive operational experience of this scheme in appl icati ons f o r ove ra ll cont r ol of coa l fired power plants is shown in /5/, /6/. 4 .2.1. Event-driven transmission
An event-driven t ran s mission is stimulated by a change of value of any process signal within the control system. In this way, an excellent dynamic performance of process signal transmission is ensured throughout the whole system. Cri teria for stimulation of a transmission are different for binary and a nal o g process signals: a) for binary process signa ls : An event-driven transmission is stimulated whenever a binar y process signal c hanges state. b) for analog process signals: An event-driven transmission is stimul ated when eve r the signal level of an anal og process s ig nal exceeds a presel ectab le increment compared to last transmission B.Il. d after a preselectable inhibit-time has expired since last transmission. Typi ca l values for increments are 0 . 4% of full scale and 50. 200 or 1000ms for inhibit-time. Resp onse times of typically 10ms for transmission in ove rall systems with more than 3 . 000 analog and 10.000 binary s ig nals are obtained. /5/, /6/. Event -d riven transmi ss i on ensu res the ne cessary response ti me for process signa ls. 4 . 2.2 . Cyclic transmission
The system PROCONTROL P performs event -driven tr ans missi ons with fi r st p r io r ty. Ho wever, a minimum portion of available bandwidth is reserved for cyc lic transmission . During this mode o f operati on, the system itself activates all data
Control Systems for Flue Gas Cleaning sources to transmit t"heir present value of proc ess signal. This procedure is repeated continuously. In this fashion, all rec eiv ing modu les within the system are updated with the latest value of process signal. Depending o n ove rall system size, cyclic update of a given process signal is repeated in a scan of 1 to 6 seconds. Control mod ules, distributed within the system, are continuously synchronized in this way. In addit i on, this mode o f operation allows to cyc li cl y monitor the perf ormance of all system components with the help of appropriate diagnostic provisions. The cyclic mode provides the robustness for transmission.
necessary
4.3. Closed loop control within the locally and functionally distributed control system
5. CONCLUSION An analysis of the flue gas cleaning process shows va riable de grees of availability requirements for certain process stages, while ensuring a high availability for the main power plan t . Variing availability r equ irements can be met by variable degrees o f centralization in the setup o f the presented control system . A technical and economi cal optimum will result . Any mix of cent ra lized, dedicated-centralized and decentralized functi onal structures may be chosen. The system PROCONTROL P has been successfully installed f o r co ntrol of industrial processes and power plants as well as in flue ga s cleaning plants. 6. REFERENCES
/ 1/
Zi~nerma nn , H. (1986). Kon zept und Einsatzerfahrungen bei de l' Leit technik von Rauchgasentschwe f e 1 ungs an 1 age n . y.DJ:. :12~X.tQJ::Lt_<;,._:;. l;t~ , VDI- Ve rl ag , DUsseldorf . pp. 87-99.
/2/
Atzger , J. (1978). Entschwefelungs verfahren mit Endprodukt Gips - ein wirksamer und kostengUnstiger Beitrag zur Reinhaltung del' Luft . T.QIl.inQIJ.:;tx.i.eil.ei:t1J))g, .7.. 4 1 3 - 4 1 8 . Sprechsaal Verlag. Cobu r g.
/3/
Gueth, R.and T.La l ive , ( 19 83). The distributed data fl ow aspect of industrial compute r systems. In PX.Q.c..,._.l1..th._l .E.A.CW.or.k.sbop . . o.I\ .D.i.:l.tr..i.h1Lt.e.Q..._C.Q!!lP.l.lt.Sl.r.....CoIlt.ro_l SY.:Lte.!llil.. Pergamon Press. pp. 1-8.
/ 4/
Gueth, R ., J. Kriz and S. Zueger. (1985). Broadcasting sourceaddressed messages. In J2xOQ ... .. 5th C.oui.ere.ns;;"e .. .Q.I\ .. lU.s:triRl.lte.d .. ColDP'\.l.:t. iM Qxste.ms. IEEE Compute r Society Press. pp. 108- 115 .
/5/
Latzel. W. and H. Kahle ( 1984 ). Dynamik verte ilt e r Pr ozeSl eit systeme, insbeso ndere fUr Regelungen. In GME:.f .
/6/
Herrma nn, R. and W. K6rner (1986). Einsatz del' Bus-Leittechnik in einem 750-MW -Kohlekr a ftw e rk . In
The principles shown in sect i ons 4.1 and 4 .2 allow the setup of a locally distribu ted c l osed l oop cont rol function with data transmission via station and remote bus (Fig . 4.2 ).
go
~
ope,al a r conlfol station
~
I
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I
-
- - anal og s'9nal
<:::=_ dlgrt al ~tgnal Via slatlon bus and remote bus Fig 4 .2
Closed loop control \l'la station and remote bu s In PROCONTROl P distributed control system
In / 5/, the ve rifica t i on of stab ility for closed l oop contro l functions is demon strated in power plant applications. Thi s feature enables to set up all func tions of control and instrumentati on such as measurement, o p en and closed loop control, indicati on and annunciation with one homogeneous set of control equipment.
129
YD I.:M.t::i.sili.t&_l>_6_9.• VDI-Verlag , DUsseldorf. pp. 35-57.