DOI 10.1007/s11015-017-0544-3 Metallurgist, Vol. 61, Nos. 7–8, November, 2017 (Russian Original Nos. 7–8, July–August, 2017)
STUDY OF FLUE-GAS RECIRCULATION SINTERING Yu. A. Frolov,1 Aggarwal Nokesh,2 and L. I. Polotskii3
UDC 669.162.16
The sintering process in a sinter machine with a sintering area of 224 m2 involving recirculation of flue gas to reduce the emission of harmful substances into the atmosphere and its flow through a bag filter to be installed after the existing electrostatic precipitator is analyzed. Alternatives of supplying a portion of the flue gas alone or mixed with atmospheric air or hot air from the sinter cooler to the bed to increase the oxygen content of the gas are considered. It is shown that forcing atmospheric air into the bed is inappropriate. It is recommended to recirculate a mixture of 70% flue gas and 30% air. Implementing the recommended alternative will reduce the gas flow through the bag filter by 30–35%, the CO emissions into the atmosphere by 8136 tons/yr, and the solid fuel consumption by 5600 tons/yr. Keywords: wind box, air, emissions, gas, oxygen, dust, mathematical model, flue gas, suction, leakages, consumption, bed, recirculation, temperature, fuel, mix, exhauster, sinter machine.
It is planned to install bag filters after the existing electrostatic precipitators in the exhaust duct of the sinter machines delivered to India from China (210 m2) and by the Outotec company from Finland (224 m2) and to arrange the recirculation of flue gas and forced supply of atmospheric air, as shown in Fig. 1. Currently, the dust concentration after the electrostatic precipitators is on the order of 100 mg/nm3. A bag filter will decrease it to 10–15 mg/nm3, and the recirculation of 30–35% of flue gas will greatly reduce the capital and current expenditures for smaller bag filters, the CO emissions into atmosphere, and the consumption of solid fuel. A preliminary analysis of the schematic in Fig. 1 shows that there is no need to force atmospheric air into the recirculation line. The air-flow rate in the mixture will be maintained automatically depending on the amount of flue (process) gas supplied to the sinter machine. This will allow reducing the expenditures for the introduction and maintenance of the recirculation system and simplifying the control of the system, including for safety reasons. An induced-draft fan is required only if hot air is supplied from the sinter cooler. We used a three-dimensional mathematical (dynamic) model of the sintering process [1]. The goal was to establish conditions that maximize the flow rate of recirculating gas, maintaining, if possible, the capacity of the sinter machine and decreasing the fuel consumption. Since the sintering area and design characteristics of sinter machines to be retrofitted and the charge conditions at sintering plants are similar, the analysis was performed for an Outotec sinter machine. The specifications and operating conditions of the sinter machine are given below: Width of traveling grate, m . . . . . . . . . . . . . . . . . . . . . . . . . . Length of sintering zone, m . . . . . . . . . . . . . . . . . . . . . . . . . . Number of wind boxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 68 17
1
Uralelektra Scientific and Production Enterprise, Ekateriburg, Russia; e-mail:
[email protected]. Boldrocchi India P Limited, Chennai, India; e-mail:
[email protected]. 3 Integrator Company, Berezovsky, Sverdlovsk Oblast, Russia; e-mail:
[email protected]. 2
Translated from Metallurg, No. 8, pp. 25–32, August, 2017. Original article submitted November 30, 2016.
0026-0894/17/0708-0629 ©2017 Springer Science+Business Media New York
629
Fig. 1. Schematic of the recirculation and cleaning of flue gas and forced supply of atmospheric air.
Height of charge bed, mm . . . . . . . . . . . . . . . . . . . . . . . . . . . 600–680 Vacuum in collector, kPa . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13–16 Dimensions of ignition hearth (L × B), m·m . . . . . . . . . . . . . 3.50 × 3.00 Flow rate of gas for charge ignition, m3/h . . . . . . . . . . . . . . . 400–600 Specific consumption of gas, m3/ton . . . . . . . . . . . . . . . . . . . 3.0 Length of wind box under hearth, m . . . . . . . . . . . . . . . . . . . 3.8 2 Specific output, tons/(m ·h) . . . . . . . . . . . . . . . . . . . . . . . . . . 1.38 Flow rate of flue gas, nm3/h . . . . . . . . . . . . . . . . . . . . . . . . . . 612 000 Flow rate of flue gas, m3/sec . . . . . . . . . . . . . . . . . . . . . . . . . 324 Dust concentration in flue gas, mg/nm3 . . . . . . . . . . . . . . . . . 100 Gas temperature at exhauster inlet, °C . . . . . . . . . . . . . . . . . . 160 Vacuum at exhauster inlet, kPa. . . . . . . . . . . . . . . . . . . . . . . . 17 Composition of gas after ESP, %: N2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68.0 O2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.0 CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.0 CO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.0 H2O. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.0 The input data are summarized in Tables 1 and 2. Charge constituents (kg/ton sinter): ore fines 790, returns 230, breeze 180, limestone 55, dolomite 75, precalcined lime 20, hard coal 60, total 1410. Mathematical model of sintering process. The three-dimensional sintering model [1] was supplemented by conditions of supply of atmospheric or hot air from the sinter cooler simultaneously with flue-gas recirculation. During computation, it was possible to change the height and moisture content of the charge bed, the gas flow rate needed to ignite it, the consumption of solid fuel, the vacuum level in wind boxes, and the velocity of traveling grate. The values of fields (5 mm along the bed height and 0.75 m along the bed length) of the following parameters can be stored as Excel text files: temperature of the material and gas; FeO and Fe2O3 content of the material; positions of the fronts of CaСO3 and combustible carbon in the bed; rate of filtration of gas through the bed; vacuum level and gas flow in the bed and exhaust duct; the number of leaks in the exhaust duct; content of O2, CO, СO2, H2O in gas; amount of melt; temperature of pallets. Also, the total and partial 630
TABLE 1. Composition of Charge Constituents and Sinter, % Constituents
Ore fines
Returns
Limestone
Dolomite
Lime
Hard coal
Sinter
CaO
0.12
8.70
51.51
38.10
85.29
0.6
8.9
MgO
0.07
1.87
3.16
15.10
3.89
0.1
1.9
SiO2
3.30
3.74
0.88
0.80
3.02
6.3
3.7
Al2O3
2.55
2.80
0.22
0.23
0.86
3.6
2.7
Fe
63.57
58.43
1.67
0.55
0.64
0.5
58.4
FeO
–
–
–
–
–
–
10.4
S
0.01
–
–
–
–
–
–
P
0.05
–
–
–
–
–
–
Na2O
0.04
–
–
–
–
–
–
K2O
0.02
–
–
–
–
–
–
MnO
0.17
–
–
–
–
–
–
Ignition losses
2.67
–
41.79
40
5
–
–
Moisture
6.50
–
0.6
0.6
0
–
–
TABLE 2. Proximate Analysis and Particle-Size Distribution of Fuel (hard coal) Proximate analysis
Particle-size distribution (%), mm
Ash content, %
Volatiles, %
Combustible carbon, %
Bulk density, tons/m3
Moisture content, %
+5
3–5
0.5–3.0
–0.5
11.50
2.77
84.18
0.92
12.0
2.95
9.13
45.49
42.21
material and heat balances are stored in the file. Another text file contains the integral parameters of the sintering process for the wind boxes or their groups. In adapting the mathematical model, the results of the reference case of computation should correspond to the input data: height of the charge bed, output of the sintering machine, FeO content of the sinter, sintering completion point, average sinter temperature at the exit from the traveling grate, vacuum level, gas temperature, and exhauster capacity. In addition to the input data, it is necessary to specify the distribution of bulk density and fuel concentration along the height of the charge bed and the amount of melt needed to form a sinter. To this end, literature data are used. The aerodynamic drag coefficients of the seals of the traveling grate and the leaks of the gas exhaust duct were taken from [2], which made it possible to calculate the leak area of the latter. Therefore, the air inleakage was determined automatically in the model. The aerodynamic characteristics of the exhausters were chosen from the input data and described by a polynomial equation of the second degree. The results of adaptation of the mathematical model are presented in Figs. 2–4 and Tables 3–6. In the reference case of computation, the height of the combustion zone increases from 35 mm at the initial stage of the process to 45 mm at its final stage, and the solid fuel stops burning in the sintering zone (63.6 m) of the sintering machine. The height of the limestone dissociation zone increases from 40 mm at the initial stage of the process to 80 mm at its final stage. Almost all of the limestone decomposes: undecomposed limestone particles 0.15–0.50 mm in diameter are only found at the boundary of the bed, in a sinter element 10 mm in height. The parameter values found are in good agreement with the input data and the modern physical interpretation of the sintering process, and the error of computing the material and heat balances is minimum. This allows using the adapted mathematical model to analyze the effect of the recirculation of flue gas on the sintering process and the performance of the sintering machine. 631
Fig. 2. Distribution, along the length of the sintering machine, of the filtration rate of air at the entry (win) to and exit (wout) from the charge bed and gas temperature (Tg) at the exit from the bed.
Fig. 3. Distribution, along the length of the sintering machine, of the composition of gas at the exit from the charge bed.
Gas recirculation analysis. The following seven alternatives of supplying flue gas and air to the sinter machine were analyzed using the model:* 1) a portion of flue gas after the ignition hearth over the total sintering area without addition of air; 2) same, plus filtration of air through the bed over two wind boxes after the hearth before the supply of flue gas to the remaining area of the sintering machine; 3) same as 1, plus filtration of hot (300°C) air from the sinter cooler through the bed over two wind boxes after the hearth before the supply of flue gas to the remaining area of the sintering machine; 4) same as 1, plus filtration of air through the bed over two wind boxes after the hearth before the recirculation of a mixture of 70% gas and 30% air; 5) same as 1, plus filtration of air through the bed over two wind boxes after the hearth before the recirculation of a mixture of 70% gas and 30% hot (300°C) air; 6) same as 4, but for a mixture of 50% gas and 50% air; 7) same as 5, but for a mixture of 50% gas and 50% hot (300°C) air. Preliminary model computations showed that supplying flue gas into the bed right after the ignition hearth considerably inhibits the combustion of the solid fuel, which is due to the short time of ignition of the charge (insufficient for the *
In all the alternatives, one wind box has a place to replace pallets, and gas is not recirculated into the bed over the last wind box for effective operation of the dust collector of the unloading unit.
632
TABLE 3. Total Charge Balance Input Components kg/h
Output 3
nm /h
kg/h
nm3/h
%
Neutral components
36668.9
36668.9
СаСО3
33 476.4
370
MgСО3
9016.2
66.3
FeO
14063.5
40751.4
CaO
18032.4
36571.9
MgO
2976.6
7238.5
Fe2O3
317163.7
287509.8
Solid fuel
17630
2197.5
Water
21159.6
0
N2
273653.5
218922.8
273653.5
218922.8
75.3
O2
81570.4
57099.3
46575.6
32602.9
11.2
CO2
838.7
427
67891
34562.7
11.9
CO
0
0
5590.9
4472.7
1.50
H2Ovapor
1086.4
1352
22242.9
27680
Total
827336.1
827328.3
821742.
–10.95
7.87
Accumulated error
Fig. 4. Distribution along the bed height (from top to bottom) of: a) final sinter temperature (tf) and the amount of melt (gm); b) maximum (FeOmax) and final (FeOf) content of FeO.
combustion zone to form in full) and the low oxygen content of the flue gas. Therefore, all the alternatives assume natural filtration of air through the bed over two wind boxes after the hearth or supply of hot air to this zone of the bed. The basic results on the effect of flue-gas recirculation on the sintering process are presented in Figs. 5–7 and Tables 7–11. For normal course of the sintering process, the oxygen content of gas should be no less than 15–16%. To derive a generalized relationship between the relative velocity of the maximum-temperature front in the bed (it is equal to the ratio of this velocity at the current О2 content of the gas to the velocity at the initial О2 content of air (21%)) and 633
TABLE 4. Partial Charge Balances, kg/h Components
Balance of O2
Balance of carbonates and CO2 in hearth
Input
Output
Input
Output
СаСО3
16068.7
177.6
4017.2
44.4
MgCO3
5152.1
37.9
1186
9.5
FeO
3125.2
9055.9
CaO
5152.1
10449.1
MgO
1190.7
2895.4
Fe2O3
95149.1
86252.9
H2O (water)
18808.5
0.0
O2
81570.4
46575.6
CO2
609.9
49375.3
CO
0
3194.8
В H2O
965.7
19771.5
Balance of iron Input
10938.3
222014.6
Output
Balance of CaO Input
Output
13390.6
148
12880.3
26122.8
Total
227792.4
227786
Accumulated error
–12.0
6.41
Input
Output
31695.6
201256.9 1876
228.7
18515.7 2396.1 2351.1
В H2O (vapor) Hard coal
Balance of hydrogen
17630
2197.5
23164
23163
232952.9
232952.4
3.5·10–3
0.437
120.7
2471.4
26270.8
2471.8
2471.4
–4.86·10–6 –9.8·10–3
0.353
0.347
26270.8
Fig. 5. Oxygen content of recirculated gas at the entry to (a) and at the exit from (b) the bed: 1) reference; 2) 30% atmospheric air; 3) 30% hot air; 4) 50% atmospheric air; 5) 100% recirculated gas.
the oxygen content of the gas over the bed, experimental data (102 data points) reported in the literature were statistically processed in [3]. The weak effect of enriching gas with oxygen over 15–16% was explained in [4], where similar results were obtained: “... if the fuel concentration in the charge during sintering is low, the reaction surface of fuel particles in the combustion zone is insufficient for full utilization of blast oxygen, i.e., enrichment of blast with oxygen naturally decreases the oxygen utilization.” In alternative 1, the average oxygen content of gas is 10.6% at the entry to the bed and 3.17% at the exit from it, the solid fuel at the end of the sintering zone having no time to fully burn down at a height of 55 mm. The FeO content of the sinter at the exit from the traveling grate is 16.9%, and the unsolidified melt occupies 175 mm of the bed height. 634
Fig. 6. Distribution of FeO content of the sinter at the end of the process: 1) reference; 2) 100% recirculated gas; 3) 30% atmospheric air; 4) 50% atmospheric air.
Fig. 7. Variation in the amount of melt with the height of the bed depending on the recirculation of flue gas: 1) reference; 2) 100% recirculated gas; 3) 30% atmospheric air.
Therefore, this alternative, as well as alternatives 2 and 3, cannot be used, and the flue gas should be diluted with air to increase the O2 conten.** An analysis of the calculated results shows that alternatives 4 and 5 are the best. In these cases, the process completes within the sintering zone, the gas flow rate through the bag filter decreases by 35%, the CO emission into atmosphere reduces by 8136 tons/yr, and the consumption of solid fuel reduces by 5600 tons/yr. It is expected that the FeO content of the sinter will inevitably increase from 9.9 to 10.8% in these cases, which can increase the coke rate in blast-furnace smelting. Conclusions. The recirculation of flue gas is, undoubtedly, environmentally friendly, yet should be subject to thorough preliminary analysis in each specific case. The recirculation of gas is inevitably accompanied by an increase in the FeO content of the sinter due to the reduction of iron oxides by carbon monooxide, i.e., the sinter becomes less reducible. As the consumption of limestone (but not lime) and, hence, the СО2 content of the recirculated gas increase (the О2 content decreases), the basicity of the sinter increases and the vertical sintering rate decreases. In cooling the sinter on the sintering machine, the recirculation system has to be switched off in summertime because of higher final temperature of the sinter. However, the higher the specific fuel consumption, the more the saving of the fuel and the lower the CO emission into the atmosphere. If combined with the installation of an electrostatic precipitator or a bag filter, the efficiency of the process increases due to a decrease in the load and capital gas-cleaning expenditures. **
The fraction of air in the mixture with flue gas at the entry to the bed of ChelMK sintering machines is minimum because the oxygen content of flue gas is ~16% (and higher at some sintering plants of CIS countries). This is due to the design and condition of the seals of sintering machines and exhaust duct, including multicyclones. At modern foreign sintering plants, the О2 content of gas before the exhauster is 14–15%.
635
TABLE 5. Heat Balance of Sintering Process Input
Balance items
Output
kJ/ton
%
kJ/ton
%
Coke gas
21352.86
4.40
0.0
0.0
Solid fuel
379563.7
77.90
0.0
0.0
Charge (40°C)
34431.79
7.10 53379.3
45.9
19141.7
16.4
Sinter Air
15424.49
3.20
Flue gas Melting of charge
0
0
14136.6
12.1
Dissociation of СаСО3
0
0
14075.9
12.1
Dissociation of MgСО3
0
0
3143.5
2.7
Evaporation of water
0
0
12588
10.8
Oxidation FeO
35973.87
7.4
0.00
0.00
Total
486747.1
100
116458.1
100
Accumulated error
–9.73
–206.1
TABLE 6. Parameters of Flue Gas before the Exhauster Vacuum, kPa
Temperature, °C
Flue gas flow, nm3/h (m3/h)
Total air inleakage, nm3/h (%)
N2
O2
CO2
CO
H 2O
1490
136.4
467335 (818901)
97071 (20.0)
72.1
13.67
7.40
0.96
6.10
Composition of flue gas, %
FeO content, %
136
467335 818901 13.67
7.40
1.21
9.16
97071/20.00
562
9.91
Vacuum before exhauster, kPa
Gas temperature before exhauster, °C
270711 14.90
Air flow rate, m3/h
Final temperature of sinter, °C
0
Air inleakage in exhaust duct, m3/h/%
Reference 17.630
Recirculated gas flow rate, m3/h
Alternatives
Consumption of solid fuel, tons/h
TABLE 7. Calculated Results for Alternatives of Flue-Gas Recirculation Exhauster output
nm3/h
m3/h
Composition of flue gas, %
О2
СО2
СО
Н2О
No. 4
16.930
166139 104897 15.25
127
466674 808520 10.10 11.24
0.79
12.18
98298/20.10
532
10.79
No. 5
16.590
163700
15.47
118
511874 863362 11.49 10.04
0.82
8.00
129564/25.03
601
11.58
No. 6
16.820
118343 151938 15.07
132
466717 813192 11.49
9.78
1.027
7.94
97663/20.10
522
10.27
No. 7
16.740
116072 148090 15.45
118
509017 857750 12.48 8.701
0.973
7.08
120075/22.90
668
9.55
90175
TABLE 8. Fuel Combustion Completion Point in the Bed Parameters
Alternatives Reference
Nos. 4 and 5
Nos. 6 and 7
Process completion point, m
63.6
66.8
66.0
Change of process completion point, m
0
+ 3.2
+ 2.4
Sintering rate, %
100
95.2
96.4
636
TABLE 9. Recirculation Flow Rate of Flue Gas and Air Recirculation flow rate of, nm3/h (m3/h) air
flue gas
Total circulation flow rate of flue gas and air, m3/h
71202 (81635)
166139 (243792)
237341 (325427)
Fraction of flue gas in mixture with air, %
Exhauster output, nm3/h (m3/h)
70.0 (74.9)
469674 (808520)
TABLE 10. Results of Calculation of CO Emissions into Atmosphere Alternatives
Exhauster output, nm3/h
Reference No. 4
Flow rate of, nm3/h
CO content of gas in stack, %
CO emission, m3/h (kg/h)
CO emission, tons/h
Reduction in CO emission, tons/h
467335
0.96
4486.4 (5608)
44864.2
–
303535
1.21
3672.8 (4591)
36727.7
–8136.4
recirculated flue gas
flue gas through stack
467335
0
469674
166139
TABLE 11. Results of Calculating the Saving of Solid Fuel Alternatives
Fuel consumption, tons/h
Fuel consumption, tons/yr
Fuel saving, tons/yr
Reference
17.63
141040
–
No. 4
16.93
135440
5600 (4%)
All this necessitates the substantiation of the optimal alternative of flue-gas recirculation. REFERENCES 1. 2.
3.
4.
Yu. A. Frolov and L. I. Polotskii, “Three-dimensional mathematical (dynamic) model of the sintering process. Part I,” Metallurg, No. 12, 42–47 (2014). L. K. Gerasimov, Yu. A. Frolov, V. I. Korotich, et al., “Determining the aerodynamic characteristics of sintering machines,” in: Scientific Foundations of Automated Process Control System for Agglomeration, Naukova Dumka, Kiev (1980), pp. 70–83. L. K. Gerasimov, Yu. A. Frolov, M. F. Vitushchenko, and G. G. Dobryakov, “Oxygen enrichment of hearth gas during external heating of the sintering mixture,” in: Operating Conditions and Design Parameters of Thermal Metallurgical Units, Metallurgiya, Moscow (1986), pp. 5–8. N. M. Babushkin and V. N. Timofeev, “Experimental study of the combustion of carbon in a bed of sintering mixture,” in: Collection of Research Papers of VNIIMT, Metallurgizdat, Moscow (1962), Iss. 7, pp. 17–46.
637
