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Modeling and Control of the Sintering Process
7b-Ol 6
Copyright © 1996 IFAC 13th Triennial World Congress, San Francisco, USA
MODELING AND CONTROL OF THE SINTERING PROCESS Illlrieh Kostial, Jail Terpak, tubollllr Dorcak
Department of Management and Control Engineering, BERG Faculty, Technical University of J( o,~ice Bozeny Nemcovej 3, 042 00 J(o,~ice, Slovakia phone (+4295) 6332729, e-mail: dorcak(qiccsun.tuke.sk
Abstract: This paper deals with the application of a mathematical model for analysis, optimization and control of the sintering process, The model includes processes in the ignition furnace (fuel combustion, heat transfer) and in the sinter bed (heat transfer, coke combustion, oxidation-reduction reactions of iron oxides, dissociation of carbonates, vaporization), The simulations results were used for instructive representation of system Lwhaviour, its interpretation, optimization and control system design. Keywords: control
mathematical model, simulation, dynamic behaviour, optimization,
1. INTRODUCTION The quality and production rate of the pig iron from a blast furnace is considerably influenc.ed by the sintering process, The main aim of the sintering plant is to effectively produce sinter of desired quality. The sinter strand converts raw material to sinter by physical and chemical transformations. The strand consists of a conveyor with a grate comparising uniform raw-mix layer, through which the air can pass. The sintering bed eonsists of crushed ore, coke and watf~r. The bed is ignited by means of an ignition furnace IIsing the combustion gas. Then hot reaction zone passes through the bed under the influence of a downward draught produced by an exhaust fan. When reaction zone reaches the grate the sintering process is complete, The ideal output of the sintering process is a material with desired properties independent on variations in the
property of the input materials. The behaviour of the reaction zone is vitally important in determining the sinter quality. If the temperature bec.omes too high or the reaction zone progresses too slowly the sinter becomes rich on the glass phase which prevents the reaction in the blast furnaee. If the temperature in the reacting zone is too low the sintering is not processed suffieiently and the sinter has not the desired mechanical, physir.al and metallurgical properties. Between these two extreme conditions various grades of sinter can be produeed and for given input materials by proper choice of sintering wnditions good quality sinter ean be gained. The inhomogeneity of the sinter quality through the sintering bed is the most important challenge for its improvement. The most deficient from this aspect is its surface layer. The solution of the ignition furnace with conveniently divided heat input to the sintering bed with subsequent heat t.reatment section belongs to the usual
6221
6222
and convection from the ignition furnace, Qa is the heat released by c.oke combustion, Q.QO is the heat lost by waste gases, Qm is the heat of endothermic reactions and water evaporation and Q a., is the accumulated heat in the sintering material. For the calculation of radiation heat transfer the zone method was used. The total amount of heat transferred to the i - tll zone is calculated by the approximate formula (Kostial and Dorcak, 1988)
Fe304
= 3PeO + ~02
(13)
1 = Fe + -0 2 2
(14)
FeO
Modeling of water vaporization or condensation is based on the functional dependence of the partial pressure of the saturated steam on the temperature. The volume of the vaporized steam or condensed steam is determined from the difference of the apparent partial steam pressure and the actual partial steam pressure
Ne
L:: QEj
Qi
=
j=1 Ne
HmH 2 0 = V Ai - QEi,
[W]
L:: Aj
where QEj = (J"A/r/ [W] is the radiation flux of the j-th zone, (J" is the Stefan-Boltzmann c.onstant, Ai [m 2] is equal to c:S for the surface zone with area Sand emisivity c: and 4 V(\', for volume zones with volume V and absorptivity c.oeffic.ient (\', and 7j [K] is the temperature of the j-th zone. Ne is total number of zones.
= A + B T = -R T
In(pco,). [J /mo/]
(8)
(9)
By the substitution of the numerical values for A, Band R Peo 2 can be calculated for the temperature T, and, therefore, the amount of CO 2 and the amount of the dissociated carbonate can be determined. The equation (9) can describe ferrous oxide dissociation as well. The difference is only in the expression for the balance constant independent on the oxygen partial pressure. Below 570°C, the dissociation is governed by equations :JFe 2 0 a
=
Fe:J04
1 2Fe304 + 202
= aF, + 2()2
100)'
[kg/s]
(15)
The carbon combustion is t~xpressed by the following equations (16) C + O2 = n0 2 - HI
n + ~02 = CO -
Under the assumption that the activities of carbonates and oxides are equal to one, the balance constant of equation (8) is equal to the partial pressure Peo 2 • The c.onstants A and B ofthe function 6.G = A+BT, which represent the change of the standard free enthalpy in carbonate dissociation, where taken from the physical chemistry tables. For the dissociation reaction (8) holds 6.G
H2 0
where V[m 3 / s] is the total volume of the gases, f! [kg/m 3 ] is the density of the steam depending on the temperature, Pp [Pal is the partial pressure of saturated steam, p [Pal is the total pressure of the gas mixture, and H 2 0 [%] is the percentage of steam in the mixture.
The carbonate dissociation follows the equation
= MeO + (,'(h.
Pp (-p -
(7)
j=l
MeC0 3
f!
(17)
H2
(18) H'l.O+C= H 2 +CO+H3 The relative proportion of carbon in CO 2 is 0.712, in CO 0.178, and in the reaction with H 2 0 is 0.11 kg/kg (Drabina, 1987). Then the carbon combustion heat is
Q = HI 0.712 + H~ 0.178 - H3 0.11
(19)
Carbon burning takes place under certain conditions. However, if the temperature is above the carbon ignition temperature (750°C), then the rate of carbon combustion is a function of oxygen c.ontent in combustion gas according to the relationship V02
C
0.712
0.178
= (1- 0.11)0.89(Ofo, + Of o ),
[kg/s]
(20)
where V02 [m3 / s] is the oxygen volume in combustion gas, Of02 and Ofo [m3 / s] is the amount of oxygen needed for the reaction C + O 2 = CO 2 and C + ~02
=
CO. The simulation algorithm consists of the following steps: 1. Data about gas, air, and sintered material.
(10)
2. Combustion gas volume
(11)
3. Heat generation and heat balance calculation acc.ording to the equations (1) and (7).
and above 570°C
4. Heat transfer in the ignition furnace, equation (2). (12)
5. Heat balance in the sinter bed, equation (6).
6223
6224
6225
6226
Copyright © 1996 IFAC 13th Triennial World Congress, San Francisco, USA
MODELING AND CONTROL OF THE SINTERING PROCESS Illlrieh Kostial, Jail Terpak, tubollllr Dorcak
Department of Management and Control Engineering, BERG Faculty, Technical University of J( o,~ice Bozeny Nemcovej 3, 042 00 J(o,~ice, Slovakia phone (+4295) 6332729, e-mail: dorcak(qiccsun.tuke.sk
Abstract: This paper deals with the application of a mathematical model for analysis, optimization and control of the sintering process, The model includes processes in the ignition furnace (fuel combustion, heat transfer) and in the sinter bed (heat transfer, coke combustion, oxidation-reduction reactions of iron oxides, dissociation of carbonates, vaporization), The simulations results were used for instructive representation of system Lwhaviour, its interpretation, optimization and control system design. Keywords: control
mathematical model, simulation, dynamic behaviour, optimization,
1. INTRODUCTION The quality and production rate of the pig iron from a blast furnace is considerably influenc.ed by the sintering process, The main aim of the sintering plant is to effectively produce sinter of desired quality. The sinter strand converts raw material to sinter by physical and chemical transformations. The strand consists of a conveyor with a grate comparising uniform raw-mix layer, through which the air can pass. The sintering bed eonsists of crushed ore, coke and watf~r. The bed is ignited by means of an ignition furnace IIsing the combustion gas. Then hot reaction zone passes through the bed under the influence of a downward draught produced by an exhaust fan. When reaction zone reaches the grate the sintering process is complete, The ideal output of the sintering process is a material with desired properties independent on variations in the
property of the input materials. The behaviour of the reaction zone is vitally important in determining the sinter quality. If the temperature bec.omes too high or the reaction zone progresses too slowly the sinter becomes rich on the glass phase which prevents the reaction in the blast furnaee. If the temperature in the reacting zone is too low the sintering is not processed suffieiently and the sinter has not the desired mechanical, physir.al and metallurgical properties. Between these two extreme conditions various grades of sinter can be produeed and for given input materials by proper choice of sintering wnditions good quality sinter ean be gained. The inhomogeneity of the sinter quality through the sintering bed is the most important challenge for its improvement. The most deficient from this aspect is its surface layer. The solution of the ignition furnace with conveniently divided heat input to the sintering bed with subsequent heat t.reatment section belongs to the usual
6221
6222
and convection from the ignition furnace, Qa is the heat released by c.oke combustion, Q.QO is the heat lost by waste gases, Qm is the heat of endothermic reactions and water evaporation and Q a., is the accumulated heat in the sintering material. For the calculation of radiation heat transfer the zone method was used. The total amount of heat transferred to the i - tll zone is calculated by the approximate formula (Kostial and Dorcak, 1988)
Fe304
= 3PeO + ~02
(13)
1 = Fe + -0 2 2
(14)
FeO
Modeling of water vaporization or condensation is based on the functional dependence of the partial pressure of the saturated steam on the temperature. The volume of the vaporized steam or condensed steam is determined from the difference of the apparent partial steam pressure and the actual partial steam pressure
Ne
L:: QEj
Qi
=
j=1 Ne
HmH 2 0 = V Ai - QEi,
[W]
L:: Aj
where QEj = (J"A/r/ [W] is the radiation flux of the j-th zone, (J" is the Stefan-Boltzmann c.onstant, Ai [m 2] is equal to c:S for the surface zone with area Sand emisivity c: and 4 V(\', for volume zones with volume V and absorptivity c.oeffic.ient (\', and 7j [K] is the temperature of the j-th zone. Ne is total number of zones.
= A + B T = -R T
In(pco,). [J /mo/]
(8)
(9)
By the substitution of the numerical values for A, Band R Peo 2 can be calculated for the temperature T, and, therefore, the amount of CO 2 and the amount of the dissociated carbonate can be determined. The equation (9) can describe ferrous oxide dissociation as well. The difference is only in the expression for the balance constant independent on the oxygen partial pressure. Below 570°C, the dissociation is governed by equations :JFe 2 0 a
=
Fe:J04
1 2Fe304 + 202
= aF, + 2()2
100)'
[kg/s]
(15)
The carbon combustion is t~xpressed by the following equations (16) C + O2 = n0 2 - HI
n + ~02 = CO -
Under the assumption that the activities of carbonates and oxides are equal to one, the balance constant of equation (8) is equal to the partial pressure Peo 2 • The c.onstants A and B ofthe function 6.G = A+BT, which represent the change of the standard free enthalpy in carbonate dissociation, where taken from the physical chemistry tables. For the dissociation reaction (8) holds 6.G
H2 0
where V[m 3 / s] is the total volume of the gases, f! [kg/m 3 ] is the density of the steam depending on the temperature, Pp [Pal is the partial pressure of saturated steam, p [Pal is the total pressure of the gas mixture, and H 2 0 [%] is the percentage of steam in the mixture.
The carbonate dissociation follows the equation
= MeO + (,'(h.
Pp (-p -
(7)
j=l
MeC0 3
f!
(17)
H2
(18) H'l.O+C= H 2 +CO+H3 The relative proportion of carbon in CO 2 is 0.712, in CO 0.178, and in the reaction with H 2 0 is 0.11 kg/kg (Drabina, 1987). Then the carbon combustion heat is
Q = HI 0.712 + H~ 0.178 - H3 0.11
(19)
Carbon burning takes place under certain conditions. However, if the temperature is above the carbon ignition temperature (750°C), then the rate of carbon combustion is a function of oxygen c.ontent in combustion gas according to the relationship V02
C
0.712
0.178
= (1- 0.11)0.89(Ofo, + Of o ),
[kg/s]
(20)
where V02 [m3 / s] is the oxygen volume in combustion gas, Of02 and Ofo [m3 / s] is the amount of oxygen needed for the reaction C + O 2 = CO 2 and C + ~02
=
CO. The simulation algorithm consists of the following steps: 1. Data about gas, air, and sintered material.
(10)
2. Combustion gas volume
(11)
3. Heat generation and heat balance calculation acc.ording to the equations (1) and (7).
and above 570°C
4. Heat transfer in the ignition furnace, equation (2). (12)
5. Heat balance in the sinter bed, equation (6).
6223
6224
6225
6226