J. Cent. South Univ. (2014) 21: 4109−4114 DOI: 10.1007/s11771-014-2405-6

Fundamental study on iron ore sintering new process of flue gas recirculation together with using biochar as fuel

GAN Min(甘敏), FAN Xiao-hui(范晓慧), JIANG Tao(姜涛), CHEN Xu-ling(陈许玲),

YU Zhi-yuan(余志元), JI Zhi-yun(季志云)

School of Minerals Process and Bioengineering, Central South University, Changsha 410083, China

© Central South University Press and Springer-Verlag Berlin Heidelberg 2014

Abstract: It is of great significance for cleaner production to substitute bio-energy for fossil fuels in iron ore sintering. However, with the replacement ratio increasing, the consistency of heat front and flame front is broken, and the thermal utilizing efficiency of fuel is reduced, which results in the decrease of yield and tumble index of sinter. Circulating flue gas to sintering bed as biochar replacing 40% coke, CO in flue gas can be reused so as to increase the thermal utilizing efficiency of fuels, and the consistency of two fronts is recovered for the circulating flue gas containing certain CO2, H2O and lower O2, which contributes to increasing the maximum temperature, extending the high temperature duration time of sintering bed, and results in improving the output and quality of sinter. In the condition of circulating 40% flue gas, the sintering with biomass fuels is strengthened, and the sintering indexes with biomass fuel replacing 40% coke breeze are comparative to those of using coke breeze completely. Key words: iron ore sintering; biomass fuel; flue gas recirculation

1 Introduction

Sinter is the main iron-bearing burden accounting for a ratio of more than 75% in blast furnace for ironmaking. However, sintering process produces a large amount of flue gas that contains diverse pollutants, such as COx, SOx, NOx and dioxins, etc [1−4]. It is reported that the combustion of fossil fuels is the main source of CO2, SOx, NOx in sintering process [5−6]. Therefore, it is of great importance to find clean and renewable energy sources to replace fossil fuels, which makes sinter production clean. Biomass is a type of potential clean energy that can replace fossil fuels. The research results of CSIRO [7] and Corus [8] showed that, biomass fuels are low in nitrogen and sulfur contents, and CO2 liberated during combustion can be sequestered back into growing biomass [9−15], so that they were effective to reduce the emissions of COx, SOx and NOx in sintering process. However, the yield and tumble index of sinter were decreased by substituting biomass for fossil fuels due to the great difference in burning characteristics of two fuels. In order to ensure the output and production quality of sinter, the new process of flue gas recirculation together with using biochar as fuel was studied in this work.

2 Materials and methods 2.1 Properties of raw materials

Iron ores blend, fuels and fluxes were mixed to produce sinter with TFe 57.5% (mass fraction), SiO2 4.82%, basicity (w(CaO)/w(SiO2)) 2.0 and MgO 2.0%. The chemical compositions of raw materials and their contents are given in Table 1. Table 1 Chemical composition (%) of raw materials and their

contents (%) in mixture Type of raw

material TFe FeO SiO2 CaO MgO Al2O3 LOI

Totalpercent

Iron ores blend

63.02 6.50 4.58 0.35 0.28 1.42 3.10 60.73

Dolomite 0.21 0.13 0.71 32.64 19.83 0.56 46.47 5.58

Limestone 0.14 0.10 1.49 50.66 2.28 0.43 40.72 2.16

Quicklime 0.4 0.23 2.86 76.69 1.18 1.20 12.36 4.62

Return fines 56.81 6.25 5.11 9.02 1.86 2.00 0.00 23.08Note: The ratio is calculated in the conditions of coke being 3.85% (mass fraction).

Two types of solid fuels were applied, one of which

is coke breeze, and the other is biochar that comes from carbonized biomass. Their ultimate and proximate analyses are illustrated in Table 2. It can be seen that biochar is low in N, S contents and ash content.

Foundation item: Projects(51174253, 51304245) supported by National Natural Science Foundation of China Received date: 2013−05−31; Accepted date: 2013−10−10 Corresponding author: FAN Xiao-hui, Professor, PhD; Tel: +86−731−88830542; E-mail: csufanxiaohui@126.com

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Table 2 Ultimate and proximate analyses of fuels

Fuel type

Ultimate analyses/%

Proximate analyses

(dry base)/% Calorific

value/(MJ·kg−1)Ctotal H S N

Fixed carbon

Ash Volatile

Coke breeze

78.89 2.46 0.500 0.72 74.68 19.54 5.88 26.84

Biochar 94.64 2.77 0.037 0.19 87.34 5.10 7.55 30.77

2.2 Experimental methods

Sintering process was simulated in a sinter pot of d 100 mm×500 mm, as shown in Fig. 1. The procedure involved ore proportioning, blending, granulation, ignition, sintering and cooling. As using flue gas recirculation, a sealed gas hood was covered on the top of sintering pot to circulate flue gas into sintering bed. Circulating gas was simulated by pure gas (N2, O2, CO, CO2) and SO2(mass fraction, 1%), NO (mass fraction, 0.6%), H2O(g) from the steam generator and hot air from a heating furnace. An infra-red analyzer was used to detect the components and temperature of exhaust gas, and combustion efficiency of w(CO2)/w(CO+CO2) was calculated to assess the burning degree of fuels. And the temperature of sinter bed was tested by thermocouple. As sintering ended, indexes such as vertical sintering speed, yield, productivity and tumble index were tested.

Fig. 1 Schematic of main equipments for flue gas recirculation

sintering: 1—Pure gas; 2,17—Valve; 3,13,16—Flowmeter;

4—Mixing gas room; 5—Flue gas analyzer; 6—Dryer;

7—Vacuum chamber; 8—Temperature display; 9—Gas hood;

10—Thermocouple; 11—Sinter pot; 12—Heating furnace;

14 — Electric furnace; 15 — Steam generating furnace;

18—Funnel; 19—blower; 20—temperature controller

A sinter pot with the same dimension was used to

research heat front and flame front of sintering. The iron balls used to simulate size distribution of sinter mixture were loaded into the sinter pot, and five thermocouples are placed into bed evenly to test the change of temperature. The heat front test starts with a thermal impulse of 1250 °C for 1 min. Heat front achieves when temperature of sinter bed increases uniformly, which generally takes 100 °C isothermal as a benchmark.

Unlikely, flame front test needs to add solid fuel with a ratio of 2.5% into mixture, and igniting under 1150 °C for 1 min. While the flame front achieves when temperature sinter bed increases rapidly, which is subjected to the isothermal of 600 °C. When the bottom of the bed gets to 100 °C (600 °C), the heat(flame) transfer has finished, and the speed of heat(flame) front is the height of bed divided by the transfer time. 3 Characteristics of sintering with biochar

The influence of replacing coke breeze with biochar on sintering was studied on the heat equivalent basis. The results are shown in Fig. 2. With the increase of replacing proportion, the flame front speed is accelerated due to the excellent combustion property of biomass. When the proportion of biochar replacing coke breeze increases from 0% to 40% and 100%, respectively, the flame front speed rises from 34.11 mm/min to 41.67 mm/min and 46.90 mm/min, but the heat front speed maintains about 35 mm/min, which indicates that the consistency of heat front speed and flame front speed is ruined remarkably when the replacing proportion reaches 40%. Meanwhile, the combustion efficiency of fuels decreases from 87.83% to 86.92%, 85.15%, which indicates that the thermal utilizing efficiency of fuel is lowered as biomass replacing coke breeze. As a result of the inconsistency between heat transfer and flame spread and the decrease of thermal utilizing efficiency of fuel, the maximum temperature of sintering layer and high temperature (≥1200 °C) duration decrease as biomass replacing coke breeze, which is disadvantageous to the mineralization of sintering mixture.

Fig. 2 Effect of biomass replacing coke breeze on process of

sintering

All the above behaviors result in the change of

sintering characteristics when coke breeze is replaced by biomass fuels, and the results are demonstrated in Fig. 3. With the replacing proportions increasing, sintering vertical speed is accelerated because of the increase of

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fuel burning speed. While yield and tumble index decrease, especially when the ratio exceeds 40%, product quality indexes of sinter are deteriorated remarkably because the heat front falls behind the flame front, the combustion efficiency reduces, and the maximum temperature and high temperature duration time of sintering bed decrease. So, the proper proportion of biochar replacing coke breeze is 40%, but its indexes are not good as that of using coke breeze as fuel.

Fig. 3 Effect of proportion of biochar replacing coke breeze on

sinter indexes

4 Effect of flue gas recirculation on biomass

fuelled sintering 4.1 Effects of circulating flue gas on combustion

efficiency of fuel When biochar substituting coke in sintering, the

ratio of w(CO2)/w(CO+CO2) in flue gas decreases. If flue gas circulates, CO will flow through the sintering bed again. Circulating flue gas flows through sinter zone first, and the behavior of CO in sinter zone is shown in Fig. 4. CO would not react below 500 °C, but most of CO is

Fig. 4 CO combustion behavior in sinter zone

burning when the temperature increases to 700 °C, and CO is completely burned as the temperature increases to 900 °C. Therefore, CO can be reused completely in the sinter zone because its latent heat will be released and transferred to the sintering bed, which increases the utilizing efficiency of biochar.

It makes great sense to the temperature of sintering bed when CO is combusted in the sinter zone, and the temperature curve is shown in Fig. 5. Compared with the case that no recirculation, the temperature increases from 1262 °C to 1304 °C, and the high temperature duration (≥1200 °C) increases from 2 min to 3.17 min with the recirculation of 1% (mass fraction) CO, which benefits the mineralization of sintering materials.

Fig. 5 Effect of recycling CO on temperature curve of sintering

bed

4.2 Effects of circulating flue gas on combustion and heat transfer

Compared with conventional sintering of using air, O2 content in circulating gas is lower, CO2 and H2O(g) contents are higher, and gas temperature is higher. In the condition of biochar replacing 40% coke, the effects of compositions and temperature of circulating gas on flame front and heat front are shown in Fig. 6.

The change of O2 content in circulating gas influences mainly the flame front. The flame front speed slows down with the reduction of O2 content in circulating gas. When O2 (mass fraction) is reduced to 15%, the speed of flame front and heat front can regain consistency. While CO2 increases the heat front speed obviously, the flame front speed increases slightly, which makes the gap of two fronts speed narrowed. With the H2O(g) content of circulating gas increases, both front speeds improve with different extents, and the gap of two front speeds is also reduced. Along with the increase of flue gas temperature, both the flame front speed and heat front speed slow to a certain degree and the gap of two front speeds has a little change.

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Fig. 6 Effect of compositions and temperature of circulating gas on heat front speed and flame front speed: (a) O2 mass fraction; (b)

CO2 mass fraction; (a) H2O mass fraction; (d) Temperature of Circulating gas

Consequently, the gas components of circulating

flue gas, such as CO2 and H2O(g), have a higher heat capacity, which is helpful to improve the heat front speed. The flame front speed can properly be decelerated because of the lower O2 content in circulating gas compared with air, which indicates that it is possible to regain consistency of two front speeds through circulating flue gas together with biochar as fuel. 5 New process of flue gas recirculation

together with using biochar as fuel

It is significant to make sure the appropriate circulating mode which is beneficial for the consistency of two front speeds. Figure 7 shows the sketch map of flue gas recirculation. The circulating gas obtains from the tail of the sintering machine. The flue gas recirculation scheme is worked out according to the recirculation ratio and the concentration of each composition in flue gas, which is given in Table 3.

Table 4 shows the effect of flue gas recirculation ratio on sintering of biochar replacing 40% coke breeze. With the increase of the flue gas recirculation ratio, the

Fig. 7 Sketch map of flue gas recirculation: 1—Chimney;

2—Wind machine; 3—Duster; 4—Wind box; 5—Bedding

material bin; 6—Mixture bin; 7—Igniter; 8—Sintering layer;

9—Gas hood; 10—Star wheel; 11—Sinter; 12—Single roller

crusher; 13—Flue; 14—Circulating flue

Table 3 Scheme of flue gas recirculation

Recirculation ratio/%

Recirculation gas composition/% Recirculation gas

temperature/°CO2 CO2 CO H2O

30 15.29 5.52 0.74 6.12 220

35 14.79 5.99 0.80 6.80 200

40 14.24 6.38 0.84 7.28 175

50 10.20 11.01 0.87 13.81 150

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sintering speed decreases, especially when increasing to 50%. The sintering productivity increases firstly and then decreases, which gets maximum when recirculation ratio is 30%. Both the yield and tumble index increase firstly and then decrease, of which maximum recirculation is 40%. Synthesizing the effect of recirculation ratio on sintering speed, specific productivity, finished product, tumble strength, it can be concluded that the appropriate recirculation ratio is 40%.

As biochar replacing 40% coke breeze, compared with no recirculation, the yield increases from 66.18% to 72.03%, the productivity increases from 1.50 t/(m2·h）to 1.55 t/(m2·h), and the tumble index increases from 58.68% to 61.45% as circulating 40% flue gas. It means that the sinter output and quality indexes can be improved obviously through circulating the flue gas, and can reach the level that of using 100% coke as fuel. Table 4 Effect of flue gas recirculation on biomass fuelled

sintering

Ratio of biomass

fuel replacing coke/%

Flue gas recirculation

ratio/%

Sintering speed/

(mm·min−1)

Yield/%

Productivity/(t·m−2·h−1)

Tumble index/%

0 0 22.50 72.68 1.55 60.98

40 0 24.15 66.18 1.50 58.68

40 30 23.56 70.28 1.62 60.63

40 35 23.42 71.17 1.59 61.26

40 40 23.15 72.03 1.55 61.45

40 50 18.23 64.58 1.18 55.32

The heat transfer and combustion wave curves of

sintering as circulating 40% flue gas were tested in the condition of biochar replacing 40% coke. Compared with no recirculation, the result shows that the heat front speed improves from 34.11 mm/min to 35.71 mm/min, and flame front speed reduces from 41.66 mm/min to 35.88 mm/min. So, the two speeds can regain consistency after flue gas recirculation. And the maximum temperature of sintering bed increases from 1262 °C to 1292 °C, the time of sustaining high temperature increases from 2 min to 2.5 min after circulating. All of these can improve the mineralization of sintering mixture. The microstructures of sinter are shown in Fig. 8. When flue gas recirculation combines using biochar replacing 40% coke (Fig. 8(b)), there is a large amount of acicular calcium ferrite in sinter, and the interleaving structure with excellent strength is formed by calcium ferrite and magnetite, which the microstructure is similar to that of using 100% coke (Fig. 8(a)).

Fig. 8 Microstructures of sinter (CF — Calcium ferrite;

M—Magnetite; P—Pores): (a) No recirculation and using

100% coke; (b) Circulating flue gas together with biochar

replacing 40% coke

6 Conclusions

1) With the increase of biomass fuel replacing coke ratio, the flame front speed increases, which makes the flame front speed surpass the heat front speed and result in breaking their consistency, and the combustion efficiency of fuel decreases which reduces the fuel heat efficiency. All that lead to the reduction of the maximum temperature of sintering bed and the high temperature duration time, and finally decrease of the yield and tumble index of sinter.

2) Circulating the flue gas to sintering bed, CO can be reused through secondary reaction in the sinter zone, which improves the utilizing efficiency of biomass fuel. Low O2 content in recirculation gas can properly reduce the flame front speed, and high CO2 and H2O content can improve the heat transfer front speed, which can make the two speeds of biomass fuelled sintering regain consistency.

3) In the condition of circulating 40% flue gas, the sinter output and quality indexes of substituting biomass fuel for 40% coke is improved, and the indexes are comparative to that of using 100% coke.

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(Edited by DENG Lü-xiang)