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THE ANNALS OF DUNREA DE JOS UNIVERSITY OF GALAIFASCICLE V, TECHNOLOGIES IN MACHINE BUILDING,
ISSN 1221- 4566, 2012
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DYNAMIC MODEL OF A STEAM BOILER FURNACE
I on V. I ON, Fl or i n POPESCU
Dunrea de Jos University of Galai, Thermal Systems and Environmental [email protected]
ABSTRACT
The pressure pulsation phenomena inside a furnace of a steam boiler affect thesteam boilers efficiency and safety. A simple mathematical model describing the
thermodynamic processes, which take place inside the furnace, is presented. Thesimulation model is applied to a coal fired boiler furnace and a gas fired boilerfurnace. The results of the simulation provide useful information for the controlsystem of the both air blowers and exhauster of the steam boiler.
KEYWORDS: boiler furnace, mathematical model, air-gas circuit control
system.
1. INTRODUCTION
A power utility boiler has an unsteady state
operation due to the change in power demand.During the unsteady operation of the boiler steam apulsatory flow of the flue gas is created. Thispulsatory flow allows the occurrence of certain
pressure rhythmic variations inside the furnaceleading to negative effects upon the air-flue gascircuit safety and efficiency in operation.
Pressure rhythmic variations cause unsteadiness
of combustion, diminishing its efficiency andincreasing air pollution, and alternative mechanicalstress upon the furnace walls (in case it has beenequipped with membrane walls) which may reduce intime the outfits durability and even destroy it.
The causes of the pressure rhythmic variationsinside the furnace may fall into two distinctcategories:
-external causes:
pulsatory inlet flow of the fuel into thefurnace when burning solid fuel, caused byvariations of the fuel heating value and
malfunction of the coal mills;
pulsatory inlet flow of the air into furnace,caused by fluctuations of the fuel flow;
lack of the synchronization between the airblowers and exhausters when fluctuations of theflow rate air supply occurs.
- internal causes:
flame turbulence and/or difference betweenthe combustion speed and the admission speedof the air-fuel mixture.In practice, the pressure rhythmic variations
inside the furnace may be a result of both internal
and external causes, the interference phenomenamaking difficult the identification of the each type ofcauses.
Knowing the dynamic characteristics of thefurnace is useful to develop its control system. The
most practical way to know the dynamic propertiesof the furnace is simulation. Performing test runs is
almost impossible because of the safety andeconomical reasons [13]. The simulation model isbased on the mass, heat and momentum conservationprinciples.
2. MODEL OF THE FURNACE
DYNAMICS
To develop the mathematical model so as tosimulate the dynamic behaviour of the furnace, we
take into account the working chart in figure 1. Theinput quantity, which varies in time, is the fuel mass
flow rate,f
m and the output value is the pressure
inside the furnace, pf. The model includes the mass,energy, and momentum balances, the heat transferfrom hot flue gases to water and steam model and theflue gas flow through the boiler model:- combustion heat balance:
iifbaa
ggf Qmim
d
idV
lfgg QQim [kW] (1)
- combustion mass balance:
gafbg
f mmmd
dV
[kg/s] (2)
- flue gas flow through the boiler:
ffg pkm [kg/s] (3)
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THE ANNALS OF DUNREA DE JOS UNIVERSITY OF GALAI FASCICLE V
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- furnace gas pressure:
gggg TRp [N/m2] (4)
- combustion dynamics:
fTemm ffb
1 (5)
Fig. 1.Physical model of the boiler furnace.
where:VF[m
3] - furnace volume;
g[kg/m3] flue gas density;
fm [kg/s] - fuel mass flow rate;
fbm [kg/s] burning fuel mass flow rate;
am [kg/s]air mass flow;
gm [kg/s]exhaust gas mass flow;
LHV [kJ/kg]-lower heating value of the fuel;
fQ [kW] heat transferred by radiation to risers:
444 gRrtgRf TScTTScQ
lQ [kW] lost heat through wall of the furnace;
Tg[k] flue gas temperature;hg[kJ/kg] flue gas enthalpy:
refrefgpgg hTTch Tref[K] reference exhaust gas temperature;cpg [kJ/kg K] flue gas specific heat at constantpressure;ha[kJ/kg] - air specific enthalpy;cpa[kJ/kg K]-air specific heat at constant pressure;
Ta[K]air temperature;
refh [kJ/kg]- flue gas reference enthalpy;
Rg[kJ/kg K] - gas constant for flue gas;cR [kW/m
2K
4] the coefficient of mutual heat
transfer between flame and wall;S[m2]radiation heat transfer surface of riser;Trt[K] riser metal tube temperature;kf[ms] a friction coefficient.
3. SIMULATION OF THE FURNACE
DYNAMICS
To be quickly implemented within a simulationtool environment the developed mathematical model
is presented in a block diagram form. Figure 2depicts the furnace block diagram representation in
Simulink (Matlab). The dynamic behaviour of thefurnace has been found out introducing certainvariation laws of the control input which stands for
the fuel mass flow rate fm and specifying the time
variation of the gas pressure inside the furnace (themaximum risk and maximum stress conditions forcontrol system are those which have a suddenlyvariation of the fuel flow rate, such as the blockage
of one mill or in an extremely case the total blockageof the fuel supply). The furnaces of two power utilitysteam boilers have been analyzed: pulverized coalfired boiler (type C4) and natural gas fired boiler(type TGM89). The steady state operating conditionsare presented in table 1. The simulation results areshown in figures 3 and 4.
Table 1
Parameters Measureunit Steam boilerC4 Steam boilerTGM 89
fm kg/s 12.928 6.193
am kg/s 138.55 115.5
ha J/kg 260000 287000
VF m3 2813 1770
LHV J/kg, J/m3N 24704000 35523000
Tref K 1273 1473
href J/kg 1662538 1923738
kf ms 0.001514 0.001217
cR W/m2K4 4.110-8 3.6210-8
lQ W 28361000 614000
Tf s 4 0S m2 1200 1350
g kg/m3 0.3927 0.2265
xF J/m3 652878.6 435726,6
pg N/m2 105 105
4. CONCLUSIONS
From the figures above it can be concluded that:
the pressure variation inside the furnace, causedby variations of the fuel flow is higher inside thenatural gas fired boiler, because, as compared to coal,natural gas has a higher combustion rate;
due to the low gas dynamic resistance of theboiler, the phenomenon occurs very fast.
The control system of the air-gas circuit may bedeveloped on the basis of these characteristics. Thissystem requires that the flue gas pressure at the endof furnace should be measured and in consequencethe opening angle of the gas blowers guide blades
must be ordered in accordance with the obtainedresults.
For a good working of the regulating system itis required that the measurement of the pressure atthe end of furnace, should be done in all four sides ofthe furnace and all four values should be calculated
together to find out their mean value, which will beused further on by the regulating system.
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FASCICLE V THE ANNALS OF DUNREA DE JOS UNIVERSITY OF GALAI
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a)25% step in fuel flow
b).100% step in fuel flow (the total blockage of fuelsupply).
Fig. 3.Pulverized coal fired boiler - furnacepressure.
Fig. 4.Natural gas fired boiler-furnace pressurevariation at the total blockage of fuel supply.
REFERENCES
[1] Adam E.J., Marchetti L.,Dynamic simulation of large
boilers with natural recirculation, Computers and Chemical
Engineering, 1999; 23, pp. 10311040.
[2] Borman G.L., Ragland K.W., Combustion engineering,Singapore, McGraw-Hill Inc., 1998.
[3] Chien K.L., Ergin E.I., Ling C., Lee A.,Dynamic analysis
of a boiler, Transactions of ASME, 1958; 80, pp. 18091819.[4] de Mello F.P., Boiler models for system dynamic
performance studies, IEEE Transactions on Power Systems,
Vol 6, No. 1, 1991, pp. 753761.[5] Elmegaard B., Simulation of boiler dynamics -
Development, Evaluation and Application of a General
Energy System Simulation Tool, Ph.D. Thesis, TechnicalUniversity of Denmark, 1999.
[6] Falcetta M.F., Sciubba E., A Computational, Modular
Approach to the Simulation of Powerplants, Heat Recovery
Systems and Combined Heat and Power Production, 1995,Vol.15, pp.131145.
[7] Gaba R., Gaba A.,Mathematical model and computation
program of the chamber furnace of boilers for air pollutionreduction, Environmental Engineering and Management
JournalMarch 2012, Vol.11, No. 3, pp. 557-565.
[8] Maffezzoni C.,Dinamica dei generatori di vapore, Masson,Milano, 1989.
[9] Maffezzoni C.,Issues in modeling and simulation of power
plants, Proceedings of IFAC symposium on control of power
plants and power systems, Vol. 1, 1992, pp. 1927.[10]Neaga C., et al., Modelarea funcionrii cu pulsaii de
presiune a focarului cazanului de 420 t/h Nr.1 de la C.E.T.
Braov (The modelling of the furnace working with pressurevariations for 420 t/h steam boiler No. 1 from CET Braov),
Lucrrile Conferinei Naionale de Termotehnic, 23-24 Mai,1997, Braov, pp. 243-250.
[11] Ordys A.W., Pike A.W., Johnson M.A., Katebi R.M.,Grimble M.J., Modelling and Simulation of Power
Generation Plants, London, Springer-Verlag, 1994.[12] Saastamoinen J.J.,Modelling of dynamics of combustion of
biomass in fluidized beds, Thermal Science: Vol. 8 (2004),
No. 2, pp. 107-126.
[13] Tuomas Kataja, Yrj Majanne, Dynamic Model of a
Bubbling Fluidized Bed Boiler, 14th Nordic Process ControlWorkshop, Sirkka-Liisa Jms-Jounela (ed.), August 2007.
pp. 58-61.
0 100 200 300 400 500 600 700 800 900 10009.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
10
10.1
x 104
Time [s]
Furnace
pressure
pf
[Pa]
0 200 400 600 800 1000 1200 1400 1600 1800 20003
4
6
7
8
x 104
Time [s]
Furnace
pressure
pf
[Pa]
10
0 200 400 600 800 1000 1200 1400 1600 1800 20005
6
7
8
9
10
11x 104
Time [s]
Furnace
pressure
pf
[Pa]
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Mux
1/s
Integr._XF
1/s
Integr._Rog
ma_in5
VF_in6
href_in7
Fip_in15
ha_in14
Qii_in13
XF_in1
Rog_in3
hga_in8
Step_mc
cpg_in9
Tg_in10
mg_in11
FiR_in13
Tref_in2
Clock
time
To Workspace1
f(u)
F1
f(u)
F2
f(u)
F3
f(u)
F4
f(u)
F5
Scope5
Scope6
Scope2
tg_out
To Workspace3
Scope1
pg_out
To Workspace2
Rog_out
To Workspace4
f(u)
F6
f(u)
F7
f(u)
F8
f(u)
F9
f(u)
F10
u
To Workspace
Scope3Rog1[kg/m3]
ddtXF
iga
Tg
ddtRog
pg[N/m2]
mg[kg/s]
cpg[J/kg.K]
tg[C]
FiR[W]
Scope4
g
Fg
xh
Fig. 2. SIMULINK-furnace model block diagram.