Modeling and Simulation of a Power Plant Drum Boiler Combustion Control System

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MODELING AND SIMULATION OF A POWER PLANT DRUM BOILER COMBUSTION CONTROL SYSTEM R. T. Pena, L. A. Aguirre and E. M. A. M. Mendes /) 1'/1111" F lIg:l'ldllll;a FiI'IItJII;m. /,"/," [ '1-".\1(; . H" /:"' /);1";111 Sal/Ill. 35. 311/ 611. lidll H II I; :II IIII'. ,\1 (;. li m:iI

Abstract. This paper describes a power plant combustion control system modeling and simulating work. The system is part of an integrated steel making plant , which belongs to Acominas, a company located in Minas Gerais state - Brazil . Three boilers, working in parallel, feed main steam, at 60 kgf/cm2 - 485 C to a single steam collector drum . The combustion control system was designed in a boiler following strategy to burn, simultaneously, four different fuels . The boiler furnace combustion control system is, as usual, made out of two main control loops, fuel and air, that are interlocked . The combustion control system process was modeled using input-output type models , assuming a linear behavior for the plants in each control loop. The developed model was simulated in an IBM-PC XT compatible microcomputer. The simulation results show that the fuel control loops, under certain restrictions, can work all in automatic. Also, a new air fuel interlock strategy was modeled and showed good results in the simulation . The conclusions of this study are being successfuly implemented in the actual plant. KeyWords. Power Plant Modeling; Process Contr o l; Computer Applications ; Steam Plants .

INTRODUCTION The modeling and simulating work described in this paper was used to study an actual power plant combustion control system . The steam power plant , which belongs to an integrated steel making plant located in Brazil, has three 140 ton / h drum boilers working in parallel. The modeling work was done on boiler number 2 . The boilers can burn , simultaneously, blast furnace gas (BFG) , coke oven gas (COG), basic oxygen furnace gas (BOFG) and E-type oil, in this order of priority. The former three fuels are by-products of the steel making process, while the fourth one is to be used in specific situations only (Pena and others, 1986) . The fuel gases, by-products of the steel making process, are stored and dispatched by the plant gas dispatch center . There are many gas users in the whole steel making plant . After serving all other users, the excess gas is dispatched to be burned in the power plant boilers furnaces . Therefore each boilp.r must work ill d large load range, 50 ton/h to 140 ton/h. The plant was designed to work with all four fuel control loops in full automatic . However, because of the la c k uf knowledge about the fuel control system dYlldW1CS, the plant technical staff decided to wurk with just one fuel loop in automatic . The other three are set to the point

Control Engineering

determined by the gas dispatch center and are always put to work on manual . The originally designed fuel-air interlock system was not working properly in the load-increase situation. A new interlock strategy was then designed . A simple input-output type model was developed for the complete combustion control system. The objective was to study the fuel control loops dynamics and to check the proposed air-fuel interlock strategy. In this paper, after a brief description of the studied power plant, a d i scussion of the designed and new air fuel interlock strategy is presented . The model development and the combustion control system simUlation results are also discussed . DESCRIPTION OF THE POWER PLANT The power plant has three 140 ton/h drum boilers , two 15 MW turbo generators and two 24.7 MW 4700 Nm3/min turbo blowers. The steam produced by all three boilers is sent to a single 60 kgf/cm2 steam collector and from there it goes to the turbines of both turbo generators and both turbo blowers . The 60 kgf/cm2 steam also goes, through reducing valves, to the 25 and 12 kgf/cm2 steam collectors . This low pressure steam is supplied to the steel making plant process . The process steam

R. T. Pe na. L. :'I.. :\ g u ir re a nd E. ;'1. .-\ . ;'1. ;' kn d es ~etu~n

doesn't

p~ocess va~iable fo~ the Qlobal cont~olle~ (FIC 202) ~ep~esents the~mal powe~ due to all the fuels bu~ned at th~t moment. This siQnal

The

to the cycle.

F~om

the condense~s to the boile~ feedpumps, the feedwate~ path is the same fo~ the th~ee boile~s .

output of the The

The powe~ plant ope~ates in a boile~ followinQ cont~ol mode. In this ope~at i nQ mode, the main steam p~essu~e is cont~olled by ~eQulatinQ the fuels fi~inQ ~ate, while load is cont~olled by ~eQulatinQ the Qove~no~ valve . The load chanQe affects the m~in steam p~essu~e. The boile~ input demand siQnal (BID) is the main steam p~essu~e cont~olle~ output. It is used as the fuel cont~ol system Qlobal cont~olle~ set point . Thus the ove~all combustion ~ate follows the load new load level is ente~ed the actuates manually on the Qove~no~ valve se~vomoto~ and keeps obse~vinQ the Qene~ated powe~ wattmete~ until the Qene~ation e~~o~ is null. a

THE COMBUSTION CONTROL SYSTEM

The

The combustion cont~ol system includes the fuel and the ai~ cont~ol systems . The set point fo~ both Ryst~ms is the BID siQnal which is the main steam p~essu~e cont~olle~ output.

Ai~

Cont~ol

Systea

The ai~ cont~ol system has to assu~e safe combustion conditions in steady state and du~inQ load chanQes . The~efo~e , when load is to be inc~eased the ai~ flow has to inc~ease befo~e the fuels flows do . When load is to be dec~eased the ai~ flow has to dec~ease afte~ the fuels flows do. Thus the ai~ flow cont~ol system ensu~es that the fu~nace will neve~ o pe~ate in a ~educinQ condition (neQative excess oxygen) .

Systea

Cont~ol

202 output is sent to the fuels acco~dinQ to a definite

Each fuel flow siQnal (FBFG , FCOG, FBOFG and FOL) is sent to th~ee diffe~ent blocks : the ~espective FIC st~uctu~e, as p~ocess va~iable ; the FZ 202 st~uctu~e , as desc~ibed befo~e; the a~ithmetic blocks FZ 204-2, FZ 205-2 and FZ 206-2, as shown in FiQ . 1 . Each a~ithmetic block outputs a setpoint , 10, to the ~espective cont~olle~, f~om which the therm~l powe~ cont~ibution of the fuels of hiQhe~ p~io~ity a~e deduced . Thus , as it can be seen f~om the fiQu~e , BFG has the hiQhest p~io~ity and the oil the lowest one. The blocks at the output of the individual cont~olle~s , as well as the FZ 203-2 block, a~e max/min limit i nQ st~uctu~es.

ope~ato~

The Fuel

st~uctu~e

cont~olle~s st~ateQY.

~equi~ement.

When

FIC

a~ithmetic

1 shows the fuel cont~ol system . blocks labeled as plants include the cu~~ent-pneumatic conve~te~s (liP), the diaph~aQm valves and the pipinQ fo~ each fuel . Each plant output is the measu~ed diffe~ential pr es sure obtained in the flowmete~ . This signal Qoes th~oUQh a squa~e ~oot calculato~ to oive the actual fuel flow. Fiou~e

The

The

flow is cont~olled by th~ee as shown if FiQ. 2. Dampe~ 01 is located just afte~ the fo~ced draft fan, in the main ai~ duct. Downst~eam, the ai~ flow is divided into two flows . These flows a~e cont~oled by two othe~ dampe~s (02 and 03) . ai~

dampe~s,

lID

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flow the beinQ is the FZ 202.

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The fuel flow

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systea_

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rz 101-1V'"

t OPOIL

I OIL

Po,,'er Plallt Dnllll Boiler Combustioll COlltrol S\'st l' 1lt

In the oriQinally de~igned air flow control system (not shown in the fiQure). the damper 01 controller received as set point the master siQnal . The set points for dampers 02 and 03 controllers were siQnals obtained by the sum of the oil and COG flows and the BFG and BOFG flows. respectively . This strat~QY was conceived to assure the air-fuel flow interlock scheme .

combustion and damper 03 is kept controllinQ air flow for COG and oil combustion. Therefore. 01 controls pressure while 02 and 03 control air flow.

However. this oriQinal control structure doesn't work properly when the load increases . In this case. when the air is suposed to lead the fuels flows. damper 01. controlled by the PlO siQnal . would open before the increase in fuels flows. However. dampers 02 and 03 would have to wait the respective fuel flow sum siQnal increase because this siQnal was their controller set point. Therefore the total air flow would not be able to increase before the fuels flows .

The derivative equation:

In FiQ . 2. equation:

all

summers

(8)

solve

the

(OR)

solve

the

if dE2/dt if dE2/dt

< 0

Eo - K1.E1 + K2 . E2 relays

Eo ~ El + K.dE2/dt. Eo - El.

The PlO flow controller FIC 209 set point is a function of the state of consumption of BFG and BOFG. This controller controls damper 02. The BFG and BOFG flow measurements are combined in the summer 82 . The output of this summer is the El input for the derivative relay OR1 . The other input for this relay . E2. is the output of 81 . This summer receives as inputs the BFG and BOFG

The proposed air control strateQY is shown in FiQ. 2. The damper 01 has been converted to control the air duct pressure. Damper 02 is maintained controllinQ air flow for BFG and BOFG

'F.

~ALVI

r---------------------~~--------~ aOFG YAlV(

••••

I,

S'

r-----------------~--------_e.~

I, 10

SI

eo.

I,

VALVI

••

1I 10

0

Thus. the derivative of the input affects the output of the derivative relay only when it is positive .

The system would work well when the plant load decreases because the air flow always lags the fuels flows due to the system delays.

10

~

S)

I, ~------~~------__..co.

I, I, 10

L-____________________---_______ .O'L

"1."" 'UIltNACI

COl + OIL

.. F" 101

FiO. 2. The air flow control systea.

R. T . Pl' lI a. 1.. .\ ..-\ g llirtl' a lld L \1. .-\ . \1. \klld l's flow controllers OFIC 205).

outputs (OFIC

203

and

This control structure, due to the anticipatory effect of the derivative relays, assures that the FIC 209 controller set point signal leads the BFG and COG flows when the load increases. In the load reduction case, the flow controllers outputs reduce . Their derivatives are negative and, therefore, don ' t actuate . Between the derivative relay and the FIC 209 controller set point Input there is a ratio-bias structure for gain adjust (ratio , t) and for assuring a minimum value (bias, B) for the an flow set point . The control for damper 03 (COG + Oil) is similar and, therefore, isn't described here.

structures parameters used in the were those set in the actual plant.

model

As it can be seen from Fig. 1 and Fig. 2, the outputs of the fuel con t rol system (fuels flows) are inputs for the air flow control system. Therefore the air control model and the fuel control model are assembled together to obtain the entire combustion control system model. Actual plant data were used to determine the air plants gains . Gain polynomials were determined in the same way as for the fuel system . The time constants for the air plants (COG + Oil damper and BFG + BOFG damper) were assumed to be 3.0 sec . , based upon actual power plant observations. No dynamic tests could be done on the actual dampers. The controllers and arithmetic structures parameters were tuned in the model . SIMULATION RESULTS

THE COMBUSTION CONTROL SYSTEM MODEL The plants transfer functions, shown in Fig. I, were assumed to be first order lags . static and dynamic tests were done to identify the process parameters. Pulse tests (Hougen, 1979) and their resulting data were used to estimate the process time constants (Pena and others , 1986) . It was observed that the fuel plants gains change with the respective fuel flow. Least mean square methods were used to determine polynomials that give the first order lag gains as functions of the respective fuel flow . Due to plant operational conditions the pulse tests were only done on the COG and Oil plants. The determined time constants for the COG and Oil plants were 5.5 and 1 . 1 sec. respectively. Time constants for BFG and BOFG plants were considered equal to that of COG. All fuel I ike PI. and Ti)

controllers are PlO operating The controllers parameters (Rp as well as the arithmetic

The 13th order , dynamic, nonlinear, PASCAL coded, combustion control system model runs in an IBM-PC-XT compatible microcomput.er, with 80~7 coprocessor, approximately in real time . Figure 3 shows plots of the four fuels and air flows during changes In the boiler input demand signal. Figure 4 shows the first 50 seconds of Fig . 3. These figures show that both air flows , for COG+Oil and for BFG+BOFG, lead the respective fuel flows when the load increases and lag them when load decreases . Therefore the new air-fuel interlock strategy works well. This strategy is being succesfully implemented in the actual plant. Figure 3 also shows that the BFG flow always tries to get the whole load . That is, when the system asks for more steam from the boiler (increase in BID signal) the BFG control valve acts in order to take over the whole increase in load. To be able to burn the other three fuels a limitation on the BFG is needed . The other three fuels don't dispute among themselves or with the BFG.

p . u. 0 . 6~------------------------------------------------~

~ 0. 3

"'~, Bfg +Bofg

O.O~~---L---r---------r______~-'~~~__~r-______~

o

40

80

120

160

200 s

FiO . 3. Coabustion control systea siaulation during a load change .

Powcr Planl Drum Boile r Combusl ion Col1lrol SYSle m

p.u. 0 . 55

Dem Ai r Bfg +Bofg

/~~ ---------~ --------

0.275

/'

Bf g

Ai r Co g +0 i 1 ____________- - -

~

__

.~-~C.:. Og::.-.---;;;-~= Oil

0.0

o

10

20

30

40

50 s

Fig. 4. Detail of Fig. 3. The results of the simulation yield a fuel control system operating mode in full automatic . To operate in this mode. the system limiting structures need to be set to their respective bands . These bands (max/min consumption for each fuel) have to be opflned by the gas dispatch center. To implement a change in a particular band. the operator will have to change the plant load accordingly and simultaneously. As the power plant operates in a boiler following mode. all changes in the BID signal have to be iniciated in the turbo generator steam governor valve. An all automatic plant load control system linked to the gas dispatch cent er computer is being designed. based upon these simulation results. CONCLUSIONS A power plant combustion control system was simulated using the model which is described in this paper . The system is part of an actual integrated steel making plant located in Brdzil.

A new proposed air fuel interlock strategy . also described in the paper. was successfully tested by simulation. A full automatic fuel control system operating mode was devised using the simulation results . The conclusions of this work have been aproved by the plant technical staff and are gOing tu be implemented in the actual plant. In fact. the air-fuel interlock strategy has been already successfully installed in the prototype boiler. at the power plant.

REFERENCES Hougen. J . o. (1979). Measurements and Control Aplications. Instrument Society of America. Research Triangle Park - NC. USA. Pena. R . 1' . • LUlld. H. P. L .• and Seara. C. M. (1986). Acominas Power Plant Fuel control System Study (in Portuguese). - Technical Report - CPGEE - UFMG. Belo Horizonte-MG. Brazil .