1) The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081 China. 2) Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan, 430081 China. 3) School of Metallurgy, Northeastern University, Shenyang, 110004 China. 4) State Key Laboratory of Advanced Steel Process and Products of Central Iron & Steel Research Insti-tute, Beijing, 100081 China.
(Received on February 14, 2016; accepted on April 13, 2016; J-STAGE Advance published date: June 7, 2016)
An unsteady model of the top gas recycling oxygen blast furnace (TGR-OBF) process was established according to material and thermal balance principles. The restrictive kinetic step of the reaction in the bosh of oxygen blast furnace was considered, as well as the key nodes’ gas recycling processes and combus-tion equilibrium of the shaft injection gas. Typical unsteady analyses of the TGR-OBF processes are demonstrated by the unsteady model. It is concluded that the whole TGR-OBF processes could reach equilibrium in case of reasonable operation. Based on the default conditions in this paper, it takes about 6 times for the main gas elements travelling from tuyere zone to the furnace top, until the unsteady pro-cess turns to be a stable one. The fuel rate decreases from 496 kg/t to 426 kg/t and 403 kg/t when the blast oxygen content is increased from 21.5% to 50% and 98%, and instantaneously, the top gas volume drops from 1 582 Nm3 to 1 462 Nm3 and 1 031 Nm3. Because most carbonaceous gases are deprived in the VPSA (Vacuum Pressure Swing Absorption) segment and also recycled inside of the blast furnace, the CO2 emissions in the TGR-OBF cases are 206.06 Nm3 and 99.56 Nm3/t compared to the CO2 emission of 664.58 Nm3/t in the conventional blast furnace.
KEY WORDS: oxygen blast furnace; thermal balance; mass balance; unsteady model; CO2 emission.
1. Introduction
The oxygen blast furnace ironmaking process uses high oxygen enriched blast instead of air blast compared to the traditional blast furnace, and in some special cases, it may refer to full oxygen blast furnace or nitrogen free blast fur-nace. It is often united with top gas recycling technology and called top gas recycling oxygen blast furnace (TGR-OBF). Taking advantages of high productivity, high PCR (pulverized coal rate), low fuel rate, low CO2 emission, etc., the TGR-OBF process is considered to be one of the promising ironmaking processes in future.1–10)
The exact process was presented by Wenzel in an American patent,1) whereafter, fundamental studies such as static bal-ance models and mathematical models were established by different researchers.2–10) Meanwhile, industrial trails have been implemented in different countries to reduce carbon consumption and CO2 emissions.11–15)
However, after adopting high oxygen enriched blast, high pulverized coal injection (PCI) and top gas recycling sys-tem, the technical and economic indexes usually deteriorate in practice, due to the complexity of the whole systems
increasing the difficulty in operation technique, and also due to lack of unsteady analyses of the TGR-OBF systems. Fur-thermore, the optimization and overall arrangement of the hot blast stove and TGR-OBF systems are not determined, so the industrial operation is usually aimless and uncertain. This article aims to solve the unsteady model of the TGR-OBF process, which is significant for the industrial trial but has not been reported in other papers.2–15)
2. Unsteady Model
The unsteady mathematical models for the blast furnace operation based on the equations of mass, momentum and heat balances and reaction kinetics have been successfully developed.20) However, it is not applied in TGR-OBF sys-tems because of the complicated change of top recycled gas.
2.1. DescriptionThe investigated TGR-OBF process is shown in Fig. 1.
The hot stove and VPSA system (Vacuum Pressure Swing Absorption, a system for capture of CO2) are both consid-ered in the whole process, as well as the different oxygen enrichment ratios. At first, the blast furnace is operated as a conventional one. It is instantly injected with high oxygen enriched blast and high coal rate in the raceway zone, then
gasified raceway gas rises up and increases in volume after passing through direct reduction zone. The new generated gas mixture is called bosh gas, which continues go up to take part in the indirect reduction of solid iron oxide charge. When the indirect reduction finishes, the outcome gas goes up, meets the shaft injected gas, and at last makes up the top gas.
Firstly, the volume of the shaft injected gas is zero. One path for the top gas is going to the tuyere after H2O, dust and CO2 are removed. The other path for the top gas is recycled without deprivation of CO2 (de-CO2), mixed with industrial O2 and combust in a burner and then injected into the shaft. If CO2 is removed, the top gas can be utilized in four routes. First, it is recycled into the shaft and hearth tuyere; second, it is used as fuel gas to heat the hearth recycled gas; third, it is recycled as fuel gas to heat the hot stove; fourth, it is supplied as fuel gas to other users in the plant.
Hearth recycled gas, oxygen enriched blast and pulver-ized coals are mixed and then react in the raceway zone. The new generated gas goes up, mixing with direct reduc-tion product gas, and forms new bosh gas. The bosh gas reacts with solid charges as an indirect reduction reactant and the new generated product gas mix with shaft injected gas, forming new top gas, which continues recycling as the context described.
In conclusion, there are six key nodes being in unsteady status during the unsteady operation of the TGR-OBF pro-cess. First, the raceway gas contents are unsteady when the hearth recycled gas changes in composition and volume. Second, the bosh gas contents are unsteady when the race-way gas and direct reduction degree are changing. Third, the direct reduction degree is unsteady when the indirect reduction degree is changing. Fourth, the indirect reduction degree is unsteady when the bosh gas is changing. Fifth, the top gas content is unsteady when the product gas of the indi-rect reduction and the shaft injected gas are changing. Sixth, the contents of the shaft injected gas and the hearth injected gas are unsteady when the top gas is changing. These six nodes interact as both cause and effect. It is hard to say that the unsteady process must come to a steady one at last without theoretical analyses, so the relevant approaches are urgent and significant.
2.2. AssumptionsThe blast furnace is one of the most complicated chemical
reactors in the world. Simultaneously, it contains a lot of physicochemical reactions, accompanying with heat, mass and momentum transportation. It is hard to establish an accurate unsteady model without some adequate assump-tions. In order to analyze the presented complicated object, some assumptions are made as follows:
(1) Methane accounts for a very small fraction in the top gas composition, so it is neglected in the blast furnace process;
(2) Phosphor in the charges all gathers into the hot metal, which has a ferrous loss ratio of 0.25%. 5% of S gasified in the gas, 50% of Mn and 80% of V dissolve in the hot metal.
(3) 50% of CaCO3 decomposes in the high temperature zone;
(4) The utilization ratio of H2 and CO in the oxygen blast furnace satisfy the Bogdandy formula, η_H2 = 0.88 × η_CO + 0.1;16,17)
(5) The indirect reduction degree is proportional to the reducing gas ratio of the bosh gas, and is independent on the volume of bosh gas.18)
(6) After CO2 is removed from the top gas by VPSA, the de-CO2 ratio is 98%;
(7) The main chemical reactions happening in the oxygen blast furnace are similar to that in traditional blast furnace, and they are shown in Table 1.
2.3. CalculationIn the calculations, coke rate is fixed first, and heat loss
is a default value. Thermodynamic data are cited from JANAF,19) and Heat capacities are calculated by:
C Tx H x H x H x H
Tp i
To
To
To
To
,, , , ,( ) = × + × + × + × +
−( )1 1 2 2 3 3 4 4
298
.......................................... (2)
2.3.1. Calculation of the Direct Reduction Degree(1) Carbon in the coke combusted in the racewayFirstly, the direct reduction degree of the oxygen blast
furnace is given as Rd_0, and the carbon balance equation is:
m m m m m mcoal coal C_cok C_b C_dFe C_dA C_HM(C)⋅ + = + + +ω e ... (3)
So the carbon in the coke combusted in the raceway is:
Fig. 1. The investigated object in the TGR-OBF system.
Table 1. Main chemical reaction in oxygen blast furnace.
m m m m mC_bK C_coke C_dFe C_dA C_HM= + + + ......... (4)
Where,
m RC_dFe d_= ×[ ]× ×1012
560Fe ................... (5)
mC_HM C= ×[ ]10 ............................ (6)
m KC_coke coke(C)= ×ω ........................ (7)
mC_dA
Si Mn P V
102
Ti
=
×[ ]
× +[ ]
× +[ ]
× +[ ]
× +
[ ]10
2824
5512
6260 24
48824×
.......................................... (8)
(2) Parameters in the racewayThe pulverized coal rate could be calculated by the C–O
balance:
m m
V x
C_bK coal coal
blast
+ ⋅ + ( )
=× × +
(C) Ccoal_vol
O H O_bl2 2
122 ϕ aast CO
coal2
2
(H O) (O)
( ) + ⋅
+ × +
V
m
hearth _hearth
coal coal
.
ϕ
ω ω22 4
18 116
.... (9)
Then the theoretical combustion temperature in raceway is:
tQ Q Q H H H H
_
_
tct
carbon blast hearth blast hearth H O decomp c2
=+ + + + − − ooal_decomp
p race racewayC V_ ×
........................................ (10)
where,
Q C m tpcarbon _= × × −( )Carbon C_bK bK 25 .......... (11)
Q C V tpblast blast blast blast= × × −( )_ 25 ............. (12)
Q C V tphearth hearth hearth hearth= × × −( )_ 25 .......... (13)
H
V x m
blast
blast O H O_blast coal
2 coa2 2
H O
= ×
× × +( )+ × ×( )
4934
2 22 4φω
. ll coal_volO+ ( )
ω
16
........................................ (14)
H Vhearth hearth CH _hearth CO _hearth4 2= × × − ×( )1 595 7 697φ φ ... (15)
H
Vm
H O decomp
blast H O_blastcoal coal
2
2
H O
_ .= ×
× +⋅ ( )
×
10 795 3
182
2φ
ω22 4.
........................................ (16)
H mcoal_decomp coal= ×900 ..................... (17)
V V x Vraceway blast O H O_blast hearth CO _hearth2 2 2= × + +( ) + × +( )1 1φ φ ++
×⋅ ( )
+( )
+( )
+( )
mcoalcoal coal coal coal
H O H O N2
18 2 16 28
2ω ω ω ω
++( )
×
ω Ccoal_vol
1222 4.
........................................ (18)
Gaseous components in the raceway are calculated as,
V V x VCO race blast O H O_blast hearth
CO _hearth
2 2
2
_ = × ⋅ +( ) +× ⋅ +
2
2
φ
φ φCCO_hearth CH _hearth
coalcoal coal
4
H O O
+( )
+ ×( )
+( )
φ
ω ωm
2
18 16
×22 4.
...... (19)
V V VH _race blast H O_blast hearth CH _hearth H _heart2 2 4 2= × + × ⋅ +φ φ φ2 hh
coalcoal coal
H O H
( )
+ ×( )
+( )
×m
ω ω2
18 222 4.
........................................ (20)
V V V mN _race blast N _blast hearth N _blast coal
coal
2 2 2
N
= × + × +
×( )
φ φ
ω
22822 4× .
... (21)
(3) Gaseous components in the bosh gasThe bosh gas volume is calculated according to param-
eters known,
V V V Vbosh CO_bosh H bosh N _bosh2 2= + +_ ............. (22)
where the gaseous components are:
V V
m m K K
CO_bosh CO_race
C_dA C_dFe coke_volC
= + ×
++
⋅ ( )+
⋅
22 4
12 12
.
ω ω OOcoke( )
16
.... (23)
V VK
H _bosh H _racecoke
2 2
H= + ×
⋅ ( )22 4
2.
ω ......... (24)
V VK
N bosh N _racecoke
2 2
N_ .= + ×
⋅ ( )22 4
28
ω ......... (25)
So the volume fraction of the reducing gas in bosh gas could be calculated, and the iterated direct reduction degree is,
R Rd i_1 1= − .............................. (26)
where Ri is substituted by (1). The new got direct reduction degree Rd_1 should be input to Rd_0, again and again, until the following equation is satisfied.
So the chemical components of slag could be calculated by,
m m jj i ii
_ slag = × ( )∑ ω ...................... (33)
where,
i = ore, coke, coal, flux, -dust ............... (34)
j = ⋅ ⋅⋅ ⋅
CaO,SiO ,MgO,Al O FeO,
V O , TiO ,P O , S2 2 3 Fe V
2 5 Ti 2 2 5 S
,λ λλ λ
...... (35)
2.3.3. Calculation of the Top Gas(1) Utilization ratio of the reducing gasMole value of the oxygen in the furnace charges which
is related to iron oxides and could be reduced by reducing gas is:
n mii i
i
t_O2 3Fe O FeO
= ⋅× ( )
+( )
∑3
160 72
ω ω ...... (36)
where i is the same as (34).Mole value of the oxygen in wustite directly reduced by
C is:
nm
dC_OC_dFe=12
............................ (37)
Mole value of the oxygen in high valent ferric oxides reduced by CO is:
nV
CO_OCO_bosh _CO=
×η22 4.
...................... (38)
Mole value of oxygen in wustite and high valent iron oxides reduced by H2 is:
nV
H _OH bosh _CO
2
2=× × +( )_ . .
.
0 88 0 1
22 4
η ........... (39)
Oxygen balance in the iron oxides is:
n n n nt_O dC_O CO_O H _O2= + + .................. (40)
At last, the utilization ratio of CO and H2 could be calcu-lated by Eqs. (36)–(40).
(2) Calculation of the top gas compositionComponents in the top gas could be calculated by the
utilization ratio of CO and H2:
V V
m m
CO_top CO_bosh _CO
flux flux oreIg I
= ⋅ −( ) +
× × ( ) +
122 4
44
0 5
η
ω ω
.
. ggore CO_shaft( ) +V
... (41)
V V
m m
CO _top CO_bosh _CO
flux flux ore
2
Ig Ig
= ⋅ + ×
× ( ) + ( )
η
ω ω
22 4
440 5
..
oore CO shaft2 +V _
...... (42)
V V VN top N _bosh N _shaft2 2 2_ = + ................... (43)
V V VH _top H bosh _H H _shaft2 2 2 2= ⋅ −( ) +_ 1 η ........... (44)
V V m Vi ii
H O_top H _bosh _H 2 H O_shaft2 2 2 2H O= ⋅ + × ⋅ ( ) +∑η ω22 4
18
.
........................................ (45)
where i = ore, coke, flux, and the total volume of the top gas is:
V V V V V Vtop CO_top CO top H _top N _top H O_top2 2 2 2= + + + +_ ... (46)
At the start of the unsteady process, the volume of shaft injection is zero in (41)–(46).
2.3.4. Calculation of the Shaft Injected GasAfter water and dust are removed, the top gas is recycled
to the burner and mixed with some industrial O2, and then the mixture is combusted to a pre-settled temperature t_shaft. The product gas is injected into the shaft tuyere to heat the furnace shaft.
According to the water - gas equilibrium,
∆G RTV V
V Vwater-gaso H CO
H O CO
2 2
2
+⋅⋅
=ln 0 .............. (47)
Furthermore,
V V V VCO CO CO_0 CO _02 2+ = + =α1 ............... (48)
V V VH H O H _02 2 2+ = =α2 ..................... (49)
C V t V V H V Hp_0 25⋅ ⋅ −( )+ −( )× + ×top_shaft top CO CO _0 CO-CO H O H -H2 2 2 2 2 22
2 2 2
O
top_shaft
CO CO _0 H O
= × +× −( )+ ×
×
C VV V V
p_1
12
12
982 ×× −( )t_shaft 25
........................................ (50)
On combination of Eqs. (47)–(50),
VH2 =− −( ) + +( ) + × −( )
× −( )
γ γ γ γ γ ββ
λ
ββ
λ
0 1 0 12
22
10
2
10
4 1
2 1
... (51)
V VH O H2 2= −α2 ............................ (52)
S C V t V H0 0 025= × × −( ) − × −p_ top_shaft _top CO _0 CO-CO2 2 ξ ... (57)
β1
25
98=
× −( )−
C tH
p_1CO-CO
_shaft2
............ (58)
β21 25
98=
× −( )−
C tH
p_H -H O
_shaft2 2
............ (59)
γα ββ
00 2 2
1
=− ×S .......................... (60)
γ λ αλ α β λ
β1 0 1
0 2 2 0 0
1
2= × +
− S ................ (61)
γ λ α αλ α α β
β2 0 1 2
0 2 0 2 2
1
= × −−( )S
............. (62)
In above equations, Cp_1 is an unknown value. It is firstly given as 1.14 kJ/m3, and it would result in a group of com-positions of the combusted gas. Then the new capacity could be calculated by the product gas compositions. Iterations continue until the following formula is satisfied,
2.3.5. Distribution of the Top Gas after Removing of CO2
The volume of CO2 removed from the top gas is:
V V Ratio xdeCO top_dust CO _top_dust2 2_shaft= × × −( )×0 98 1. ... (65)
The volume of the top gas after removing CO2:
V V Ratio Vtop_deCO top_dust deCO2 2_shaft= × −( ) −1 .... (66)
The volume of reducing gas recycled to hearth tuyere is:
V V Ratiotop_hearth top_deCO2 _hearth= × ............ (67)
The heat exchange ratio of the recycled gas is settled at 85%, and the heat required is:
QC V tp
0_hearth_hearth hearth hearth
=× × −( )_
.
25
0 85 ..... (68)
The calorific value of the top gas after removing CO2:
q x xtop_deCO CO_top_deCO H _top_deCO2 2 2 2= × + ×12 632 3 10 795 3. .
........................................ (69)
The volume of fuel gas required to heat the hearth recy-cled reducing gas is:
VQ
qfuel_hearth
0_hearth
top_deCO2
= ....................... (70)
The heat exchange efficiency of the hot stove is settled as 85% according to the practical data in Baosteel, so the volume of the fuel gas required to heat the hot stove is:
VC V t
q
pfuel_blast
_blast blast
top_deCO
_blast
2
=× × −( )
×
25
0 85. ........ (71)
The volume of the de-CO2 top gas supplied to the other users in the plant is:
V V V V V Vtop_output top_deCO hearth deCO fuel_hearth fuel_b2 2= − − − − llast
........................................ (72)
2.3.6. Mass and Thermal BalanceMass input of the oxygen blast furnace is:
m K m m m m m m minput ore flux coal blast hearth shaft dust= + + + + + + − ........................................ (73)
Mass output of the oxygen blast furnace is:
m m m m moutput topgas slag HM loss= + + + ............ (74)
The general formula of mass of the mixed gases is:
m xV M
ii
i
gasgas= ××∑
22 4. ..................... (75)
where, i = CO, CO2, H2, N2, H2O, O2.Additionally, the volume of the industrial O2 required in
the blast is:
V
V x V xO _blast
blast O blast O H O_blast
2
2
=× − × × − −( )
−
2 2
2179
1
0 98 0
φ
. .0022179
×
........................................ (76)
When every item above is calculated and the mass bal-ance could be established, thermal balance of the oxygen blast furnace could be settled by three methods.17)
In the first method, it is calculated according to the input and output energy based on Hess’ law, without taking into account of real chemical reactions in the blast furnace. The input energy carriers include coke, coal, blast, shaft recycled gas, hearth recycled gas and furnace charges. The output energies include heat of iron oxides decomposition, heat of sulfur deprivation, heat of flux decomposition, heat of coal decomposition, heat of water decomposition, slag enthalpy, hot metal enthalpy, top gas enthalpy, calorific value of top gas, calorific value of un-combusted coal and the heat loss.
In the second method, the real chemical reactions are con-sidered in comparison of the first method. The input calorific value of material is abnegated. Instead, the input energies include but not limited to carbon gasification enthalpies in raceway, carbon gasification in direct reduction, carbon combustion in indirect reduction and hydrogen combustion in indirect reduction. The heat of coal decomposition, calo-rific value of top gas and un-combusted coal are abnegated in the output items. Besides, the other input and output items are the same as those in the first method.
The items in the third method are similar to the second one. The input energies include enthalpies of carbon gasifi-
done by a carefully programmed application, and the pro-gram flow chart is designed as Fig. 2.
3. Results and Discussion
When the application is finished, it should be applied to not only the traditional blast furnace but also the TGR-OBF process. A typical chemical composition of raw materials and fuels used is shown in Tables 2 and 3. The designed hot metal components are shown in Table 4. The usage ratios of the sinter, pellet and lump are 80%, 15% and 5% respec-tively. Heat loss of the third method is settled as 8.2–8.5%.
As shown in Table 5, oxygen content in the blast is 21.5% and the blast temperature is 1 200°C in the traditional blast furnace process, while blast temperatures are lowered and oxygen contents are increased in the other cases. The condition of raw materials and fuels are same in every case. In recent research, Helle and Saxén21) has also focused on different operation conditions, but the unsteady model of the TGR-OBF process is not reported.
3.1. Calculation ResultsMass flow of the traditional blast furnace process is
shown in Fig. 3 (case 0) according to the typical raw mate-rial and operation conditions. The coke rate is settled as 350 kg/t, the coal rate is 146 kg/t, the volume of top gas is about 1 582 Nm3, the direct reduction of Fe is 0.38, and the theoretical combustion temperature is 2 079°C.
The unsteady analyses of TGR-OBF processes are char-
Fig. 2. Flow chart of the unsteady analysis of the TGR-OBF pro-cess.
Table 2. Chemical composition of raw materials (mass%).
Material TFe FeO CaO SiO2 MgO Al2O3 MnO S P2O5 H2O Ig TiO2 V2O5
Table 4. Chemical composition of hot metal (mass%).
Fe C Si Mn P S V Ti
95.14 4.12 0.42 0.20 0.03 0.05 0 0.04
Table 5. Operation conditions of the calculated blast furnace pro-cesses.
Case K/kg x_O2/ %
t_blast/ °C
Ratio_ shaft/%
Ratio_ hearth/%
t_shaft /°C
t_hearth /°C
0 350 21.5 1 200 0 0 – –
1 300 50.0 1 000 0 60 – 1 200
2 200 98.0 500 30 60 900 1 200
cation in raceway, heat of hot blast, heat of shaft recycled gas and hearth recycled gas. The corresponding output energies are the same as items in the second method except decrease in enthalpy of iron oxides decomposition and increase in enthalpy of direct reduction.
2.4. ProgrammingThe complicated calculation process described above is
acterized by the number of circled times, referring to the tracks of elements from raceway to furnace top.
In case 1: the coke rate is 300 kg/t; the blast oxygen content is 50%; the blast temperature is 1 000°C; the shaft recycled ratio is 0%; the hearth recycled ratio is 60% and the temperature of hearth recycled gas is 1 200°C. The unsteady evolution of key note gas compositions, input and output materials are shown in Tables 6–8 during the transition of normal BF to TGR-OBF (case 1). At first, a conventional BF is operated, and then it is suddenly injected with high oxygen blast and high pulverized coal. Consequently, the coal rate decreases from 220 to 170 kg/t because of the recycled reducing gas injecting from hearth. Simultane-ously, the required blast decreases from 758 to 564 kg/t. Because the volume of top gas increased from 1 047 to 1 461 Nm3 and the hearth recycled ratio is 60% constantly, the fuel gas required for heating hearth recycled gas is increased from 128 to 234 kg/t. It takes about 6 times for the unsteady process to turn stable in case 1 (Note: the “1
unique unit” of traveling time of gas from tuyere zone to the furnace top, means a real periodic time from unsteady process to a stable one).
When it becomes stable, mass flow of the whole process is established in Fig. 4, and the three types of thermal balance are calculated in Table 9. The energy of hearth
Fig. 3. An example of mass flow of the traditional blast furnace process (case 0).
Table 6. The evolution of composition of the key node gases (Case 1).
*Note: Energy of the shaft recycled gas includes both calorific value and enthalpy of the shaft recycled gas, and similarly for the energy of the hearth recycled gas.
Table 10. The evolution of composition of the key node gases (Case 2).
Table 11. The evolution of input materials during the transition of normal BF to TGR-OBF (kg, Case 2).
Times m_ore K m_coal m_blast m_flux m_shaft_O2 m_total
0 1 593 200 212 336 33 26 2 401
1 1 593 200 223 343 35 24 2 418
2 1 594 200 209 329 33 25 2 389
3 1 594 200 205 325 32 25 2 380
4 1 594 200 203 323 32 25 2 377
5 1 594 200 203 323 32 25 2 377
6 1 594 200 203 323 32 25 2 377
Table 12. The evolution of output materials during the transition of normal BF to TGR-OBF (Case 2).
TimesItems/kg
error /%
Vtopgas /m3
m_hm m_slagm_
deCO2
m_H2O_
topm_dust
m_output
_top
mfuel_
hearthm_total
0 1 000 292 780 105 15 54 19 2 265 5.65 568
1 1 000 294 818 116 15 86 29 2 358 2.49 971
2 1 000 291 825 114 15 95 32 2 372 0.72 1 013
3 1 000 290 828 113 15 97 33 2 376 0.20 1 025
4 1 000 290 828 112 15 98 33 2 376 0.04 1 028
5 1 000 290 828 112 15 98 33 2 377 0.02 1 029
6 1 000 290 828 112 15 98 33 2 377 0.01 1 030
recycled gas accounts for 28.54%, 11.40% and 29.62% in the three types of thermal balance respectively. The fuel rate of the TGR-OBF process in case 1 decreases by 70 kg/t compared to the conventional BF in case 0.
In case 2: the coke rate is 200 kg/t; the blast oxygen content is 98%; the blast temperature is 500°C; the shaft recycled ratio is 30% and the temperature of shaft recycled gas is 900°C; the hearth recycled ratio is 60% and the tem-perature of hearth recycled gas is 1 200°C. The unsteady evolution of key note gas compositions, input and output materials are shown in Tables 10–12 during the transition of normal BF to TGR-OBF (case 2).
When it becomes stable in case 2, mass flow of the whole process is established in Fig. 5, and the three types of thermal balance are calculated in Table 13. The energy of both shaft and hearth recycled gas accounts for 24.59%, 8.60% and 26.21% in the three types of thermal balance respectively. The fuel rate of the TGR-OBF process in case
*Note: Energy of the shaft recycled gas includes both calorific value and enthalpy of the shaft recycled gas, and similarly for the energy of the hearth recycled gas.
Fig. 5. Mass flow of a TGR-OBF process (case 2).
2 decreases by 93 kg/t compared to the conventional BF in case 0.
The role of coke and pulverized coal as reducer and thermal supplier is substituted by shaft and hearth recycled gas, so the final fuel savings of the TGR-OBF processes are 14.1% (case 1) and 18.8% (case 2) compared to the case 0.
3.2. Discussion(1) Restrictive kinetic conditionIn assumption (5), the indirect reduction degree is pro-
portional to the reducing gas ratio of the bosh gas, and is independent on the bosh gas volume. As shown in Eq. (77), the reducing gas ratio of the bosh gas of conventional blast furnace is settled as 49%, and the direct reduction degree of full oxygen blast furnace is set as 0.08.
Ri = +−−
× −[ ]0 620 92 0 62
100 49.
. .%(Reducing gas) 49 .... (77)
However, the Eqs. (1) and (77) are reasonable but empiri-cal, so a further analysis such as one dimensional kinetic blast furnace model should be implanted to clarify the mechanism of the complicated unsteady TGR-OBF process.
(2) Top gas temperatureAs a designed TGR-OBF blast furnace, the top gas tem-
perature of each process is set as 200°C, which is question-able when the shaft recycled ratio exceeds a limit. But as a practical TGR-OBF blast furnace, the top gas temperature could be input by the VB interface of the program. Of course, a more proper method is required to describe the result of gas-solid heat exchange in the complex process in further.
(3) Carbon consumptionWith depriving CO2 from the top gas and recycling it
to the furnace shaft and hearth, the carbon consumption is reduced by 14% (case 1) and 19% (case 2) compared to the case 0. Nevertheless, the deprivation of CO2 is also an energy consumed process. Helle and his partners9) have considered the economic input of the CO2 stripping, but they only analyzed the full oxygen blast furnace process. The energy consumption of de-CO2 segment in different TGR-OBF processes should be considered in the whole process optimization. It would be discussed by exergy analyses in another paper soon.
(4) Reduction of CO2 emissionThere are usually two methods to calculate the CO2 emis-
sion: method one is only calculating the practical CO2 gas emission, the other is not only calculating practical emission but also considering the carbon savings which would lead to reduction of CO2 gas emission. Danloy and his partners8,14) use method two and presented that the ULCOS oxygen blast furnace would reduce 50–80% CO2 gas emission. However, the energy consumption of de-CO2 segment is not mentioned in their articles, and the carbon savings are on the precondition of disregarding economic input of the CO2 stripping.
In this paper, the method one is used to calculate the CO2 emission for the three cases. Because the carbonaceous gases are all used as fuels, the CO2 emission of conventional BF is calculated as Eq. (78).
V VBF_CO emission Topgas 2 Topgas2 CO% CO %= × +( ) / 100 ... (78)
Similarly, the CO2 emission of different TGR-OBF pro-cesses is calculated by Eq. (79). As a result, the CO2 emis-sions of the three cases are 664.58, 206.06, 99.56 Nm3/t, respectively.
V V V VTGR-OBF_CO emission output fuel_blast fuel_hearth2
CO
= + +( )× %% CO %2 hearth_injected
+( ) / 100 .... (79)
4. Conclusions
(1) An unsteady model of the TGR-OBF blast furnace is developed, and the unsteady analyses are characterized by the number of circled times, which indicate tracks of ele-ments from raceway to furnace top. It takes about 6 times for the unsteady process to turn to a stable one in case 1 and case 2 in this paper.
(2) The role of coke and pulverized coal as reducer and thermal supplier is substituted by shaft and hearth recycled gas, so the final fuel savings of the TGR-OBF processes are 14.1% (case 1) and 18.8% (case 2) compared to the conven-tional blast furnace in case 0.
(3) Because the most carbonaceous gases are deprived in the VPSA segment and circled inside of the blast furnace, the TGR-OBF processes calculated in this paper is reduced by 69% and 85% CO2 emissions compared to the conven-tional BF in case 0.
AcknowledgementsThis work is financially supported by the National
Key Technologies R & D Program of China (Grant No. 2011BAE04B02) and the Postdoctoral Science Foundation of the State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology.
Nomenclature Cp: Heat capacity at constant pressure (kJ/mol, kJ/
kg or kJ/mol·K −1) m: Weight of material (kg, kg/t or kg/thm) V: Volume of gas (Nm3, Nm3/t, or Nm3/thm) K: Coke rate (kg/t or kg/thm) Q: Heat of material (kJ, kJ/kg or kJ/m3) H: Enthalpy of material (kJ, kJ/kg or kJ/m3) Ri: Indirect reduction degree (–) Rd: Direct reduction degree (–) n: Kilo mole numbers of material (kmol) Ig: Burning loss (–) G: Gibbs free energy (kJ/mol) t: Temperature of degree Celsius (°C) S0: Medial parameter in calculation (kJ) Ratio: Distribution ratio of recycled gas (–) q: Calorific value of material (kJ/kg, kJ/m3) x: Volume fraction of gas (–) M: Chemical formula weight (kg/mol)Greek ω: Weight fraction of solid material
φ: Volume fraction of gaseous material ϕ: Volume fraction of gaseous material η: Mass fraction distributed in hot metal Utilization of CO or H2 in blast furnace λ: Mass fraction distributed in slag α: Medial parameter in calculation (m3) β: Medial parameter in calculation (kJ) γ: Medial parameter in calculation (–) ξ0: Medial parameter in calculation (kJ)Subscripts C_coke: Carbon in the coke C_b: Carbon combusted in raceway C_dFe: Carbon consumed in the direct reduction of Fe C_HM: Carbon in hot metal C_dA: Carbon consumed in the direct reduction of
the other elements top_dust: Dry top gas after removing dust top_deCO2: Status of top gas flows after de-CO2
Fuel_hearth: Top gas flows after de-CO2 and partly used as fuel to heat hearth recycled gas
Output_top: Summary of top gas output and top gas used as fuel in the hot stove
Note: the other subscripts not mentioned are simply under-stood just as their names imply.
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