For achieving green production of iron ore sintering, it is significant to substitute biochar, which is a clean and renewable energy, for fossil fuels. In this paper, the gasification reaction between CO2 and biochar was investigated. The results showed the initial temperature and the final temperature of the gasification reaction between biochar and CO2 were lower, while the maximum weight loss rate and the biggest heat absorption value were much higher than those of coke breeze, which indicated gasification reaction between the biochar and CO2 occurred rapidly at lower temperature. The gasification activation energy of biochar was 131.10 kJ/mol, which was lower than that of the coke breeze by 56.26 kJ/mol. Therefore, biochar had a higher reactivity and easily reacted with CO2 to generate CO. As a result, when biochar replaced coke powder at equal heat condition in sintering process, the combustion efficiency of fuel decreased and was disadvantage to the mineralization of iron ores at high temperature. With the increase of substitute proportion, the sinter yield, tumble strength, and productivity were decreased. The proportion of biochar replacing coke breeze should not be higher than 40%. By reducing the heat replacement ratio of biochar, the yield and quality of sinter got improved.
The energy consumption of the iron ore sintering process generally accounts for about 10% of iron and steel enterprises, 75%~80% of which are consumed in the form of solid fossil fuels like coke breeze or anthracite [
CSIRO in Australia conducted a study on the application of charcoal in iron ore sintering process. The results showed that the ash content of charcoal produced by red eucalyptus was low and the residual impurities were few after burning. When applied to sintering process, the biochar could replace part of the coke breeze but reduced the strength of the sinter product. In particular, the tumble strength was reduced significantly when biochar dosage was high [
There were lots of differences between biochar and conventional fuels in terms of chemical composition, physical properties, proximate analysis, and so on. Biochar replacing coke breeze would bring a series of changes to the behavior of the fuel in sintering process. Therefore, in this paper the differences between biochar and coke breeze in reactivity were studied, as well as the kinetic characteristics of biochar reacting with CO2. In addition, the mechanism how biochar replacing coke breeze affected the yield and quality of sinter was revealed by researching the influences of biochar on combustion efficiency.
Iron ore blend, solid fuels, fluxes (dolomite, limestone, and quicklime), and return fines were utilized to produce sinter. The chemical compositions of raw materials are shown in Table
Chemical compositions of raw materials and their proportions in mixture.
Types of raw materials | Chemical composition/wt-% |
|
||||||
---|---|---|---|---|---|---|---|---|
TFe | FeO | SiO2 | CaO | MgO | Al2O3 | LOI | ||
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 | 23.08 |
Two types of solid fuels were applied in the experiments. One was coke breeze, which came from an industrial sintering plant, and the other was biochar carbonized from acutissima at 700°C for 30 min in nitrogen gas. Ultimate and proximate analyses of fuels are illustrated in Table
Ultimate and proximate analyses of solid fuels.
Fuel types | Ultimate analyses/wt-% | Proximate analyses (dry base)/wt-% | Calorific value/MJ·kg−1 | ||||
---|---|---|---|---|---|---|---|
|
S | N | Fixed carbon | Ash | volatile | ||
Coke breeze | 78.89 | 0.500 | 0.72 | 74.68 | 19.54 | 5.88 | 26.84 |
Biochar | 94.64 | 0.037 | 0.19 | 87.34 | 5.10 | 7.55 | 30.77 |
Chemical compositions of ash in fuels.
With the help of optical microscope, microstructures of coke breeze and biochar were obtained, as shown in Figure
Microstructure of solid fuels: (a) coke breeze; (b) biochar; C: carbon; P: pore.
The reactivity of biochar under the nonisothermal condition was studied using the synchronous heat analyzer (NETZSCH STA 449C, German). 5.0 mg sample was put in the Al2O3 crucible of the thermobalance stent and heated by controlled computer process. The gas flow velocity was controlled, 0.5 m/s, and the speed of temperature increasing was 15°C/min. The TG-DTG curve and DSC curve of gasification reaction between the biochar and CO2 were analyzed to obtain the characteristic parameters of gasification reaction, including reactions starting temperature (
The reactivity of the biochar under isothermal conditions was studied in a vertical furnace. Using a fused silica tube with Φ38 × 550 mm as reaction tank, there was a cup for placing sample, which charged by 25 g dried fuel with a size fraction of 3 mm. The weight was measured and recorded by electronic balance and computer, respectively, and the system read the data every 20 s. Before starting the test, nitrogen as protective gas with flow rate of 5 L/min was passed into the tube until the temperature reached the preset temperature again. Then, the reaction tank was weighed and the nitrogen gas was cut off. Then inlet CO2 with the flow rate of 10 L/min until the weight loss reached a constant value. Thus the gasification reaction conversion rate (
The rate of gasification reaction was evaluated using the instantaneous rate
Sintering process was simulated in a sinter pot of 180 mm diameter × 700 mm deep. The procedure involved ore proportioning, blending, granulation, ignition, sintering, and cooling. Raw materials were granulated in a drum of 600 mm diameter × 1400 mm deep for 4 min and then charged into the sinter pot. A hearth layer approximately 20 mm thick was used to protect the grate from thermal erosion. After charging, the fuel in the surface layer was ignited at 1150 ± 50°C for 1 min by an ignition hood. The combustion front moved downwards with the support of a downdraught system with a negative pressure of 10 kpa. In the sintering process, a infrared analyzer was used to detect the CO and CO2 contents in exhaust gas, and combustion efficiency of CO2/(CO + CO2) was calculated to assess the burning degree of fuels. After sinter cake discharging, dropping test (2 m ×3 times), screening, and tumble strength were carried out to evaluate the physical strength of sinter. Yield was the proportion of product sinter which deducted the hearth layer material and the fines −5 mm. Productivity was defined as the weight of product sinter produced per area per time. The test of tumble strength was conducted in a drum of
During the sinter pot tests, the mass of biochar was calculated on the basis of replacement percentage biochar replacing coke and the heat replacement ratio by
The TG-DSC curves of nonisothermal gasification of fuels are shown in Figure
TG-DSC curves of nonisothermal gasification of fuels.
Coke breeze
Biochar
The TG-DTG and DSC curves of fuels’ gasification were analyzed to obtain the characteristic parameters. It showed that biochar started to gasify at low temperature, and the reaction starting temperature (
The reaction rates of biochar and coke breeze with CO2 were studied by isothermal thermogravimetric analysis and the conversion rates are shown in Figure
Effect of temperature on gasification of fuels.
Coke breeze
Biochar
In this paper, a typical shrinking core reaction model was used to study the gasification reaction kinetics of solid fuels. The reaction can be divided into three zones: chemical reaction kinetics zone, inner diffusion zone, and outer diffusion zone. In the chemical reaction kinetics zone, the controlling factor of the reaction rate is the chemical reaction of coke and CO2. In the outer diffusion zone, the controlling factor is the effect of CO2 diffusing to coke surface, while the internal diffusion zone is affected by both chemical reaction and diffusion. Tseng and Edgar [
The internal diffusion rate constant of coke gasification in the internal diffusion zone is described in the following formula:
In the outer diffusion zone, the external diffusion rate constant of coke gasification is represented by the following formula:
In the formula,
The reaction rate constant
Combining the above equations, the following formula (
The activation energy
Activation energy of fuel gasification and the transition temperature of every zone.
Fuel | Activation energy/kJ·mol−1 | Transition temperature/°C | |||
---|---|---|---|---|---|
Chemical reaction | Internal diffusion | External diffusion | Chemical |
Internal |
|
Coke breeze | 187.36 | 77.16 | 33.51 | 950 | 1100 |
Biochar | 131.10 | 71.82 | 31.88 | 900 | 1000 |
Relationship between
It was shown in Table
The effect of proportion of biochar replacing coke at equal heat substitution on sintering process was studied. The effect of biochar replacing coke breeze on the emission of CO2 and CO during sintering was shown in Figure
Effect of proportion of biochar replacing coke on COx concentrate of flue gas.
Combustion efficiency refers to the ratio of complete combustion to the whole combustion. The ratio of CO2/(CO + CO2) can reflect the combustion efficiency. When C is combust at high temperature, the reaction on the surface of C is the gasification reaction between C and CO2, and the produced CO diffuses outward and reacts with O2 diffusing inward to generate CO2. Therefore, combustion efficiency was affected by the rate of CO2 + C = 2CO reaction on the surface of carbon particle. The influences of biochar replacing coke breeze on combustion efficiency were illustrated in Figure
Influence of biochar replacing coke breeze on combustion efficiency.
From the above, it is known that biochar was characterized by higher reactivity than coke breeze and could react with CO2 rapidly. Therefore, more CO was generated on the surface of biochar. Furthermore, as biochar burned quickly, more O2 was consumed per unit time and the concentration of O2 in flue gas was relatively low, which limited the secondary combustion reaction of CO and finally made combustion efficiency drop, which finally decreased the production and quality index of sintering.
The effect of biochar replacing coke breeze at equal heat substitution on the yield and quality of sinter was shown in Table
Effect of biochar replacing coke breeze on sintering indexes.
Biochar replacing coke ratio/% | Suitable moisture/% | Sintering speed/mm·min−1 | Yield/% | Tumble strength/% | Productivity/t·m−2·h−1 |
---|---|---|---|---|---|
0 | 7.25 | 21.94 | 72.66 | 65.00 | 1.48 |
20 | 7.25 | 24.58 | 68.69 | 64.40 | 1.52 |
40 | 7.50 | 24.73 | 65.30 | 63.27 | 1.43 |
60 | 7.50 | 27.20 | 55.35 | 54.67 | 1.32 |
100 | 7.75 | 27.17 | 41.11 | 23.87 | 0.93 |
The main reason why sinter yield and tumble strength decreased was that the biochar burnt too fast, causing the deterioration of combustion efficiency and the drop of bed layer temperature. Consequently, reducing the heat replacement ratio of biochar for raising the temperature of bed layer could improve the yield and tumble strength of sinter. The effect of heat replacement ratio on sintering indexes was shown in Table
Effect of heat replacement ratio on sintering indexes.
Heat replacement ratio | Sintering speed/mm·min−1 | Yield/% | Tumble strength/% | Productivity/t·m−2·h−1 |
---|---|---|---|---|
1.00 | 24.73 | 65.30 | 63.27 | 1.43 |
0.85 | 24.29 | 67.27 | 64.47 | 1.41 |
0.75 | 23.88 | 69.63 | 64.18 | 1.46 |
The initial temperature and the final temperature of the gasification reaction between biochar and CO2 were low, the speed was fast, and the maximum weight loss rate and heat absorption were both higher than those of the coke breeze. Dynamic parameters showed that the gasification activation energy of biochar was 56.26 kJ/mol, lower than coke breeze, which indicated that the biochar had better reactive activity. Due to biochar’s high reactivity, the degree of incomplete combustion in the sintering process increased and the thermal efficiency reduced, which was not conducive to the high-temperature mineralization process. As a result, the sinter yield, tumble strength, and productivity decreased with the increase of biochar’s substitute proportion. Therefore, the proportion of biochar replacement of coke breeze at equal heat substitution should be controlled no more than 40%. Reducing the heat replacement ratio of biochar could improve the temperature of sinter bed, improving the yield and tumble strength of sinter.
The authors declare that they have no conflicts of interest.
The research was financially supported by the State Key Program of National Natural Science Foundation of China (no. U1660206), Natural Science Foundation of Hunan Province in China (no. 2015JJ3164), Hunan Provincial Co-Innovation Center for Clean and Efficient Utilization of Strategic Metal Mineral Resources and Innovation Driven Plan of Central South University (no. 2015CX005), Hunan Provincial Innovation Foundation for Postgraduate (CX2016B054), and Open-End Fund for the Valuable and Precision Instruments of Central South University (CSUZC201703).