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THE ANNALS OF DUNREA DE JOS UNIVERSITY OF GALAIFASCICLE V, TECHNOLOGIES IN MACHINE BUILDING,

ISSN 1221- 4566, 2012

23

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 Engineeringion.ion@ugal.ro

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.