Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering

Applied Energy xxx (2017) xxx–xxx

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering Zhilong Cheng, Jingyu Wang, Shangshang Wei, Zhigang Guo, Jian Yang, Qiuwang Wang ⇑ Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, PR China

h i g h l i g h t s
The heat pattern can be controlled by adjusting the gaseous fuel concentration.
Gaseous fuel segregation was firstly proposed for energy efficiency optimization.
The new method has great potential for green and efficient production of sinters.

a r t i c l e

i n f o

Article history: Received 13 January 2017 Received in revised form 1 June 2017 Accepted 12 June 2017 Available online xxxx Keywords: Iron ore sintering Imbalance of heat distribution Gaseous fuel segregation Injecting concentration Red-hot region Energy efficiency

a b s t r a c t It has been widely reported that the sinter strength and heat pattern would be weakened when adopting the low grade solid fuels, such as biomass, semi-coke and anthracite. Moreover, the imbalance of heat distribution in the sintering bed is considered to be problematic on the energy efficiency. To solve the above problems simultaneously, the gaseous fuel segregation method was firstly proposed in this paper. The gaseous fuel was injected to the melting zone from the top and auto-ignited near the solid fuel combustion zone. Firstly, methane concentrations of 0.0% and 0.5% vol. were tested, keeping the total calorific heat input unchanged. The heat pattern in melting zone was recorded by both contact thermocouples and non-contact thermal infrared imager. The results indicated that the methane injection could significantly extend the melting zone from the upstream and raise the sinter strength higher than that of coke sintering, without increasing the energy consumption. Then, the energy saving potential of the novel method was evaluated by reducing the calorific heat input 4, 6 and 8%. Furthermore, in the segregation case, the gaseous fuel injecting concentration was increased in the upper bed to enhance the weak heat pattern, and decreased in the lower bed to avoid the energy waste. It was observed that the melting zone became much more uniform in the infrared images. Finally, the optimum segregation degree of 1.0%/mm was recommended, where the sinter strength grew 2.31%. The present study provides an effective way for optimizing the energy efficiency in the sintering process. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Iron and steel industry is the highest (15% [1]) energy consumption sectors in China. Traditionally, coke breeze supplies heat for the iron ore sintering. Coking and sintering contribute the second highest (26% [1]) energy consumption in the iron and steel production. Iron ore sintering is a pre-treatment process to prepare porous sinters with suitable strength which are the burden materials for iron-making in the blast furnace. The detailed description of iron ore sintering process can be found in the previous published paper [2]. There are many similar applications with iron ore sintering in natural (e.g., smoldering [3,4]), technological (e.g., ⇑ Corresponding author. E-mail address: [email protected] (Q. Wang).

self-propagating high-temperature synthesis [5,6]) and industrial (e.g., biomass incineration [7] and power plant [8]) fields. The optimum energy efficiency with lowest cost and environmental impact has been the ultimate goal in actual sintering operations. However, as reported in the open works, there are two factors leading to the low energy efficiency in the sustainable production of sinters, which are the unreasonable combustion organization and cooling condition, and the imbalance of heat distribution in the sintering bed. 1.1. Unreasonable combustion organization and cooling condition in the sintering bed With the aim at reducing the operation cost and controlling the CO2 emission, researchers carried out investigations on seeking for

http://dx.doi.org/10.1016/j.apenergy.2017.06.024 0306-2619/Ó 2017 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

2

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx

On the contrary, in the lower bed, the heat pattern was enhanced due to the auto-accumulation of heat [21] in the sintering bed. Auto-accumulation of heat means the auto-recovering of ‘‘waste” energy in the combustion products and the hot sintered ores to the combustion zone. The excessive heat caused the energy waste in the lower bed. The uneven heat pattern and sinter strength were observed in the previous paper [22]. The imbalance of heat distribution in the sintering bed adversely affected the average sintering performance and energy efficiency which was reported not only in the theoretical study [23], but also in the experimental work [24]. Nath and Mitra [25] carried out the mathematical modelling of two layers with different fuel contents in the sintering process. The optimum fuel efficiency and sintering performance were obtained using the genetic algorithm. Zhao et al. [23] carried out the theoretical investigation for minimizing the imbalance of heat distribution in the iron ore sintering through the solid fuel segregation approach. Machida et al. [26] achieved the solid fuel segregation using a new charging device: magnetic braking feeder. Fundamental investigation was performed to examine the effect of solid fuel segregation on optimizing the heat distribution. However, the segregation of about 4.0% solid fuel in the sintering bed had problems that the controlling accuracy could not be guaranteed, the versatility was not high enough, as well as the adjusting range.

the alternative fuels. The properties of the potential alternatives, such as reactivity and microstructure, have significant influences on the flame front characteristics [9]. Wang et al. [10] numerically evaluated the effect of less expensive anthracite on the iron ore sintering performance. The reactivity of anthracite is much lower than that of coke, contributing to the much lower heat release rate in the combustion zone. The results show that the peak temperature, melting zone thickness and sintering speed decreased significantly in the anthracite case, which adversely affected the sinter quality and productivity. Luo et al. [11] adopted the semi-coke as by-product derived from coal-based direct reduction process in iron ore sintering. Ooi et al. [12] experimentally examined the influence of charcoal for partially replacing the coke breeze in the sintering process. Both semi-coke and charcoal have much higher reactivity than coke. Nunes et al. [13] reviewed the combustion characteristics of biomass pellets. Cheng et al. [14] analysed the reasons for weaker sinters at high charcoal proportion. Actually, the excessively high flame front speed caused by the much higher reactivity and specific surface area of charcoal decreased the red-hot zone thickness, as well as the residence time of melting temperature which is the holding time of temperature above 1100 °C (the initiating temperature of melting process) in the temperature-time profile. Moreover, under the limited oxygen supply, the combustion efficiency was deteriorated in a narrow combustion zone. Consequently, the sintering performance, such as sinter strength and productivity, cannot satisfy the actual need at the same energy consumption level. The heat released by fuel combustion rapidly raised the ores temperature above 1100 °C [15] where the melting phase (bonding phase) began to form. Generally, the melting zone was quite narrow. The ideal situation was that the temperature above 1100 °C in a certain position can be maintained long enough. However, when the local fuel burned out and combustion zone moved downwards, one can imagine that the melting zone was cooled by the fresh air very quickly. The high cooling rate brings negative influences on the melting phase generation and energy efficiency. A mass of work has been carried out with the purpose of improving the unreasonable combustion organization and cooling condition in the sintering process. The authors believe that changing the inlet air conditions related to heat and combustion is the most promising method in the long term due to its high precision, high flexibility and low environmental impact. The hot air (recycled from the sinter cooling machine) injection method was employed to add the extra heat to the weak melting zone. However, the temperature of recycled hot air was only 150–450 °C, which was not helpful to expand the red-hot zone effectively. Researchers [16] also adopted the flue gas recirculation technology to control the combustion zone by utilizing the sensible heat of recirculated gas and calorific heat of carbon monoxide in it. However, the low oxygen concentration and high humidity in the recirculated gas were the drawbacks of the flue gas recirculation technology [17]. Castro [18] numerically predicted the heat pattern in the sintering bed with combined usage of biomass and gaseous fuel. However, there is still no experimental data to support the feasibility of the new technology.

Raw materials, as listed in Table 1, contain iron ore fines, solid fuel, return fine and hydrated lime (95% purity). The basic analysis of the solid fuels for sintering tests is shown in Table 2. The available gaseous fuels in iron and steel production process are usually natural gas, coke oven gas and blast furnace gas. The main composition of these gases is carbon monoxide or methane. As an elementary research, methane (99.9% purity) is employed as the injecting gaseous fuel considering the operation safety and experimental cost.

1.2. Imbalance of heat distribution in the sintering bed

2.2. Experimental setup

In the actual operation, the solid fuel was distributed almost evenly in the sintering bed. Actually, the sintering bed permeability would not vary significantly until the flame front reached near the bottom of sintering bed according to Loo and Leaney’s [19] and Zhou et al.’s [20] work. Thus, bed permeability is not the main contributor to the imbalance of heat distribution. In the upper bed, the heat pattern and sinter strength were weak because that the fresh air directly cooled the high temperature zone without preheating.

The experimental system is schematically illustrated in Fig. 1. It mainly consists of infrared imager, video camera, gases supply system, sintering reactor, gaseous fuel injecting device, liquefied petroleum gas (LPG) ignitor, control cabinet and data acquisition system. There are two kinds of sinter pots adopted in the present study: stainless steel reactor and quartz glass reactor. Stainless steel reactor with 65 mm heat insulating layer is employed in the thermocouple data and sinter strength acquisition. Eight

1.3. The objective of present study Based on the above literature review, the motivation of the present study is to optimize the gaseous fuel injecting concentration for improving the unreasonable combustion organization and cooling condition, and minimizing the imbalance of heat distribution in the sintering bed simultaneously. Firstly, the gaseous fuel injection method was adopted to generate a secondary combustion zone for expanding the burning area and reducing the cooling rate in the co-firing of biomass and coke breeze case. Then, based on the influencing mechanism of gaseous fuel concentration on the heat pattern in the sintering bed, the gaseous fuel segregation method was designed to balance the unevenly distributed heat at higher accuracy and versatility. These were achieved by controlling the injecting concentration of gaseous fuel as the sintering process progressed. 2. Experimental method and materials 2.1. Experimental materials

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

3

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx Table 1 Blending ratio of raw materials in the reference case. Raw materials

VMS ore

WPF ore

Return fine

Hydrated lime

Coke

Charcoal

Mass fraction (%)

33.30

33.30

20.68

8.32

1.76

2.62%

Table 2 Proximate analysis and heat value of coke breeze and charcoal. Parameters

Coke breeze

Charcoal

Proximate analysis (% ad) Fixed carbon Volatiles Ash Moisture Calorific value (MJ/kg)

85.15 1.27 13.02 0.56 27.64

81.18 4.24 4.18 10.40 27.81

S-type thermocouples are positioned in the axis of reactor at an equal interval of 40 mm from y = 40 mm (TC 1) to y = 320 mm (TC 8) to record the temperature profiles. The temperature measurement accuracy was evaluated in the previous published paper [2]. Quartz glass reactor is used for recording the infrared and video images. It’s necessary to point out that the infrared imager

(FLIR, SC2500) and video camera capture the information in same field. The temperature calibration (800–1200 °C, ±1 °C) of infrared imager was adopted. Two measures are taken to achieve fully mixing of methane and air: double methane nozzles are arranged on the main pipe and the metal foam gas mixture (pore size: 0.8 mm, thickness: 10 mm) is installed in the pipe to enhance the mixing of methane and air, as shown in Fig. 1. A K-type thermocouple is inserted in the wind box to monitor the exhaust gas temperature. Before the formal tests, the parameters of infrared imager were corrected according to four thermocouples data at the same locations in the sintering bed, as shown in Fig. 2(a). Fig. 2(b) presents the thermal profiles recorded by the thermocouples and infrared imager. It was noted that, there existed temperature difference in the descending stage. It was due to the dirty film forming at high temperature. The film adhered to the inner surface of quartz glass, preventing the infrared wave from completely passing through the

Fig. 1. Schematic diagram of experimental system.

1200

P3

Unit: mm

P1

TC 1 (y=80)

P2

TC 2 (y=160)

P3 P4

TC 3 (y=240) TC 4 (y=320)

T ( o C)

P1

Thermocouple data Infrared data

P2

1000

y=0

P4

800 600 400 200 0 0

200

400

600

800

1000

1200

Time (s)

(a) Arrangement of thermocouples and infrared imager

(b) Comparison of thermocouple and infrared data

Fig. 2. Parameter correction of infrared imager.

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx

quartz glass, which was inevitable in the sintering process. Moreover, the uncertainty of raw mixture emissivity at different physical properties and temperature was another contributor to the temperature deviation. Actually, there were great challenges for quantitatively measuring the temperature field in the sintering process using an infrared imager. Fortunately, the temperature profiles recorded by the infrared imager and thermocouple agreed well in the rising stage, as shown in Fig. 2(b). And the basic shape and trend of the profiles were very similar in the descending stage, except the exact temperature. Thus, for qualitatively displaying the evolution of red-hot zone in the sintering bed, the difference in Fig. 2(b) was acceptable. What needs to be emphasized is that, the quantitative temperature data was collected from thermocouple reading, and the infrared images were used for visually displaying the evolutions of red-hot zones as a supplementary means in the present research.

Designed methane concentration (%)

4

Case 4 Case 7 Case 8 Case 9

0.8 0.6 0.4 0.2 0.0 0

2

4

6

8

10

12

14

Time (min)

(a) Designed gaseous fuel concentrations in segregation cases

2.3. Experimental procedures

Methane flowrate (ml/s)

75 Case 4 Case 7 Case 8 Case 9

60

45

30

15 120

240

360

480

600

720

Time (s)

(b) Methane flowrate in segregation cases Actual methane concentration (%)

Firstly, the raw materials are weighed and wetly mixed. Then, they are granulated in the cylinder granulation for 5 min where the moisture content is adjusted to the target value. The preparing procedures have been described in the previous published paper [22]. After all the preparations, the raw materials are ignited at the temperature of near 1100 °C by liquefied petroleum gas (LPG) burner for 60 s. Data acquisition system is initiated at the same time. After the ignition, the burner is lifted off from the reactor. And the gaseous fuel injection system is installed quickly. The suction pressure applied across the reactor is 4 kPa during the ignition period and 6 kPa during the sintering period. Then, the infrared imager and video camera are turned on. In our preliminary tests, time consumed in the coke sintering was 970 s. However, in the 60% charcoal sintering, the sintering time was only 840 s (14 min). Except for the ignition time and installation time of methane injecting system, there was 12 min left for methane injection. Moreover, the sucked airflow rate increased rapidly when the flame front arrived near the bottom bed, leading to significant variation of the actual methane concentration. Therefore, methane was injected for 10 min from 60 s after ignition, avoiding the sharp variation of methane concentration in the final sintering period. In the operation, we need to control the single variable, keeping the total methane input (calorific heat input) and injection time unchanged. Thus, it is necessary to design concentration profiles at the ideal conditions before the tests, as shown in Fig. 3(a). The actual gaseous fuel flowrates at different injecting periods in Fig. 3(b) are calculated according to the designed methane concentration and the average airflow rate in the preliminary tests. In the whole injection period, the total methane input in Fig. 3(b) is kept unchanged at different cases. Actually, the single variable is methane flowrate in this paper, which is able to control accurately. The methane flowrate is controlled by the mass flow controller (Bronkhorst, ±0.5% RD plus ±0.1% FS) by coding a simple script. The sucked airflow rate varied with the heat pattern in the sintering bed, resulting in the differences between the designed concentration and actual concentration. In fact, the sucked airflow rate was 125–1000 times higher than methane flowrate. Thus, controlling the gaseous fuel injecting concentration on-line was extremely difficult and expensive. At the current stage, the sucked airflow rate was not controlled actively. We recorded the actual methane concentration profiles in several typical cases, as shown in Fig. 3 (c). Usually, the sucked airflow rate would keep almost unchanged in the initial and middle periods, and gradually increase in the final period due to the solid fuel consumption and moisture evaporation. Therefore, the actual methane concentration is higher in the initial period and lower in the final period than the designed concentration which is based on the average airflow rate. Generally,

1.0

Case 4 Case 7 Case 8 Case 9

1.0

0.8

0.6

0.4

0.2 120

240

360

480

600

720

Time (s)

(c) Actual methane concentrations in segregation cases Fig. 3. Arrangement of methane injecting condition in segregation cases.

the actual methane concentration profiles could satisfy the need of fuel segregation at different bed heights. Finally, the maximum exhaust gas temperature on the monitor screen suggests the ending point of sintering process. 2.4. Parameters definition Major parameters appeared in this paper are defined as follows: (1) Melt quantity index (MQI) is defined as the enclosed area above 1100 °C (initiating temperature of melting process) in a temperature-time profile recorded by the thermocouple [2]. MQI can be obtained by Eq. (1):

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

5

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx

ðT  1100Þ  ds

ð1Þ

where s1 and s2 are the beginning and ending times of melting process. (2) Shatter strength is the resistance to degradation or breaking caused by the drop of iron ore sinters, which is expressed by the ratio, in percentage, of the sample mass of +10 mm in particle size after 4 droppings from a height of 2 mm to the sample before the test (Japanese Industrial Standard JIM-M8711). (3) The gaseous fuel segregation degree (a) is expressed by Eq. (2) for evaluating the optimum segregation condition. Considering the possibility of applying the result to different sintering machines, the bed height (H) is divided by the difference value of the designed initial concentration and the designed final concentration, as expressed in Eq. (2):

a ¼ ðC initial  C final Þ=H

Solid fuel rate*

No.

Case Case Case Case Case Case Case Case Case *

1 2 3 4 5 6 7 8 9

Methane concentration

Coke

Charcoal

100.0% 40.00% 40.00% 40.00% 40.00% 40.00% 40.00% 40.00% 40.00%

0.00% 60.00% 52.48% 48.48% 46.48% 44.48% 48.48% 48.48% 48.48%

0.00% 0.00% 0.50–0.50% 0.50–0.50% 0.50–0.50% 0.50–0.50% 0.60–0.40% 0.70–0.30% 0.80–0.20%

(a = 0.0%/mm) (a = 0.0%/mm) (a = 0.0%/mm) (a = 0.0%/mm) (a = 0.5%/mm) (a = 1.0%/mm) (a = 1.5%/mm)

solid fuel rate ¼mfuel =mcoke;case1 .

Solid: 0% charcoal and 100% coke Dash: 60% charcoal and 40% coke Case 1 Case 2

1400

ð2Þ

where C initial is the initial gaseous fuel injecting concentration (%), C final is the final gaseous fuel injecting concentration (%), H is the bed height (mm). (4) In order to present the benefit of gaseous fuel segregation on balancing the heat distribution in the sintering bed, a dimensionless variable gMQI is introduced, as expressed in Eq. (3):

gMQI ¼ MQIlocal =MQIaverage

Table 3 Arrangement of fuel supply in all experimental conditions.

ð3Þ

where the MQIlocal is the local MQI, MQIaverage is the average value of eight local MQI in a certain sintering case. 3. Results and discussion In this paper, two reference cases (Cases 1–2) were arranged, as reported in Table 3. Gaseous fuel injection method is employed in Case 3 for the improvement of weak energy efficiency and sinter strength in Case 2. Then, the energy saving potential of the novel method was evaluated in Cases 3–6. The above studies were included in Section 3.1. Furthermore, the optimization of unevenly distributed heat adopting the gaseous fuel segregation method was carried out in Section 3.2 where four segregation degrees were examined in Cases 6–9. 3.1. Development of gaseous fuel injection for improving the energy efficiency 3.1.1. Application of gaseous fuel injection for enhancing the weakened heat pattern Case 1 was arranged as the reference case where the coke breeze provided the heat for melting phase generation. Using biomass for partial replacement of coke is an attractive technique for controlling CO2 emission in the sintering process. Thus, the Case 2 was arranged as another reference case where 40% calorific heat was from coke and 60% calorific heat was from charcoal. Fig. 4(a) shows the comparison of typical thermal profiles in Cases 1–2. Obviously, the peak temperature in Case 2 became much lower, and the residence time of melting temperature was shorter. The main reasons were explained in the previous published paper [14]. Except for that, the burning zone of solid fuels in the sintering bed was usually quite narrow. The melting zone was exposed to the cold air once the solid fuel burned out. As shown in Fig. 4(a), the melting zone was cooled very quickly by the strong air convection. Consequently, the shatter strength decreased from 69.50% in Case 1 to 48.86% in Case 2. According to the analysis in Introduction section, reducing the cooling rate in melting zone and expanding the melting zone without increasing fuel consumption is the most immediate and prac-

TC 4

1200

TC 8

1000 800 600 400 200 0 0

200

400

600

800

1000

1200

Time (s)

(a) Typical thermal profiles in Cases 1-2 enhanced case

case 2

injecting gaseous fuel 1100 C

Temperature

s1

Temperature (oC)

MQI ¼

Z s2

MQI

gaseous fuel sintered zone

sintered zone

combustion zone

combustion zone

secondary combustion

Time

(b) Expansion of thermal profiles Fig. 4. Heat pattern analyses in reference cases.

tical way to improve the weak heat pattern and sinter strength in Case 2. As indicated in Fig. 4(b), gaseous fuel was injected at the inlet of sintering reactor to generate a secondary combustion zone, which was promising to reduce the cooling rate and increase the MQI simultaneously. Actually, the injecting methane with 0.5% vol. concentration equivalently replaced partial solid fuel input in Case 3, keeping the same total calorific heat input with Case 2. Fig. 5 presents the infrared images in Cases 1–3. The bright and colorful zone in the infrared image implies the red-hot region, while the black zone represents the low temperature region. In

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

6

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx

160 s

256 s

352 s

448 s

544 s

640 s

736 s

832 s

Case 1

red-hot region

Case 2

red-hot region

Case 3

red-hot region

temperature range: 800-1300 oC Fig. 5. Infrared images in Cases 1–3.

Table 4 Comparison of sintering performance in Cases 1–3. Case

Case 1

Case 2

Case 3

Average MQI Shatter strength

11856 °C s 69.50%

4887 °C s 48.86%

13345 °C s 72.13%

comparison with Case 1, the red-hot region was sharply narrowed at high charcoal proportion in Case 2. By contrast, the red-hot region was expanded at each moment with the help of methane injection method in Case 3 without increasing the calorific heat input. Moreover, the infrared images suggested that the heat pattern of Case 3 had already reached the same level with that in Case 1. The thermocouple data (TC1-TC8) indicated that average MQI increased from 4887 °C s in Case 2 to 13345 °C s in Case 3, as shown in Table 4. The shatter strength also increased significantly from 48.86% in Case 2 to 72.13% in Case 3. It was noted that, both average MQI and shatter strength in Case 3 had even exceeded those in Case 1 at the same calorific heat consumption.

3.1.2. Application of gaseous fuel injection for the further energy saving According to the results in Section 3.1.1, the sintering performance in Case 3 had already been higher than that in Case 1. Therefore, for further reducing the energy consumption in the sintering operations, Cases 4–6 where the calorific heat input reduced 4, 6 and 8% were arranged in this section, as shown in Table 3. Fig. 6 shows the average MQI and shatter strength at different reduction rates of calorific heat input. It can be seen that the variation of average MQI and shatter strength was not obvious when the calorific heat input reduced 4% in Case 4. They were still higher than those in Case 1, as shown in Fig. 6(a) and (b). However, further reducing the calorific heat input, both the average MQI and shatter strength dropped below the levels in Case 1. Generally, compared with Case 1, the solid fuel rate decreased 11.52%, while the shatter strength increased 1.44% in Case 4, with the help of gaseous fuel injection method, as shown in Fig. 7. These have significant contributions to the clean and economical sinter production. The reduction of solid fuels consists of 4% cut of calorific heat and 7.52% replacement by ultra-lean gaseous fuel.

76

Shatter strength (%)

Average MQI ( o s)

20000

16000

12000 Level of Case 1

8000

4000

72

68

Level of Case 1

64

60 0

2

4

6

Calorific heat reduction (%)

(a) Average MQI

8

0

2

4

6

8

Calorific heat reduction (%)

(b) Shatter strength

Fig. 6. Influence of reducing calorific heat on average MQI and shatter strength.

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

7

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx

Coke

Methane (0.5%)

Charcoal

11.52%

4%

90

6%

8%

Case 5

Case 6

80

88.48%

Share of calorific supply (%)

100

70 40

20

0 Case 1

Case 2

Case 3

Case 4

Operating condition Fig. 7. Share of calorific supply in Cases 1–6 (reduction of solid fuel rate and total calorific heat).

In fact, the increased energy efficiency was benefited from more reasonable heat supply approach in Case 4. Partial solid fuel was initially removed from the raw materials due to its low combustion efficiency. As mentioned in the previous paragraphs, the heat condition above the red-hot zone was the worst when the solid fuel burned out at a certain bed height, especially in Case 2. In Case 4, the gaseous fuel was injected to the subjected area to generate a self-sustained secondary combustion region adhered above the original solid fuel combustion zone, expanding the red-hot region. Besides, the method was able to provide a buffering zone, protecting the red-hot zone from being cooled directly by the strong air convection. According to the combustion theory, the burning of ultra-lean methane (only 0.5%) cannot be self-sustained at the ambient environment (methane lean flammable limit: 5.3%). However, the ultra-lean methane was preheated to its ignition point by the hot sinters as approaching to the solid fuel burning area where the bed temperature was more than 1000 °C. Moreover, the solid fuel combustion zone was able to provide sustained supports for the self-sustaining of the secondary combustion zone. Thus, the secondary combustion zone was always adhered above the original solid fuel combustion zone. More importantly, the effective utilization of ultra-lean gaseous fuel would not bring any challenge to the operational safety.

no-injection

low concentration

middle concentration

high concentration

MQI increasing

Temperature

1100 C

MQI

upper edge of original combust. zon

minimum sustaining temperature

stable interval of secondary combust. zone

shift of secondary combust. zone

Distance from the top

(a) Influencing mechanism of gaseous fuel concentration on heat pattern in sintering bed secondary combust. zone dash line original thermal profile solid line gaseous fuel injection thermal profile

secondary combust. zone solid fuel combust. zone

temperature

1100 ºC

high concentration

middle concentration

high concentration

middle concentration

low concentration

low concentration

time

(b) Control strategy of gaseous fuel concentration in segregation case Fig. 8. Research foundation of gaseous fuel segregation.

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

8

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx

avoiding the energy waste in the lower bed, the gaseous fuel injecting concentration should be raised in the upper bed and reduced in the lower bed in the segregation case, as shown in Fig. 8(b). Four cases were arranged to examine the effect of gaseous fuel segregation on the sintering performance, as shown in Table 3. Considering the high cost of continuous control of methane concentration, ten constant concentrations at different injecting periods were designed, as shown in Fig. 3(a). Fig. 9(a) presents the typical thermal profiles of the nonsegregation (Case 4) and segregation (Case 9) cases. In the gaseous fuel segregation case, the peak temperature was raised, and the residence time of melting temperature was extended significantly in the upper bed, such as TC 1. By contrast, the thermal profiles in the lower bed, such as TC 7, became lower and thinner in the segregation case. Thus, the residence time of melting temperature was shortened. Actually, these variations were easy to accept because of the increasing methane concentration (calorific heat input) in the upper bed and the decreasing methane concentration in the lower bed. Except for that, the burning of higher concentration methane with lower ignition energy can be sustained in relatively low temperature environment, shifting the secondary combustion zone upward in the upper bed. The shift of secondary combustion

3.2. Gaseous fuel segregation for improving the imbalance of heat distribution In the conventional method, the fuel was almost evenly distributed in the sintering bed. However, the imbalance of heat distribution in the vertical direction [22] was considered to be problematic on the energy efficiency and sinter strength in the upper bed. In this section, gaseous fuel segregation was proposed based on the method in Section 3.1 for improving the heat distribution at different bed heights. The minimum ignition energy of premixed methane/air was highly relied on the methane concentration. In the range of low methane concentration, the minimum ignition energy dropped with the increase of methane concentration. Consequently, as the methane concentration increased, the premixed methane/air started burning at a relatively lower temperature, which meant that the secondary combustion zone was shifted to the upper direction, as shown in Fig. 8(a). Then, the temperature profile at higher methane concentration was further expanded, resulting in the larger MQI. These provided the possibility of improving the heat pattern at different bed heights by controlling the gaseous fuel injecting concentration. Thus, with the aim at enhancing the weak heat pattern in the upper bed and

Solid: Case 9

1400

y = 0 mm

Dash: Case 4

Flange TC 1 TC 3 TC 5 TC 7

1200

400 mm

Temperature ( oC)

1000 800 600 400

y=40

TC 1

y=120

TC 3

y=200

TC 5

y=280

TC 7

200 0 0

200

400

600

800

1000

1200

Flange

Time (s)

(a) Comparison of thermal profiles in Case 4 and Case 9

Fig. 9. Effect of gaseous fuel segregation on heat pattern.

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

9

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx

zone contributed to the further expansion of melting zone and the improvement of sinter strength in the upper bed.Fig. 9(b) shows the distribution ofgMQI in the non-segregation (Case 4) and segregation (Cases 8–9) cases. In the non-segregation case, the gMQI was much lower than 1.0 in the upper bed, and higher than 1.0 in the lower bed. By employing the gaseous fuel segregation method, the gMQI in Case 9 approached close to 1.0, especially in the upper bed, as shown in Fig. 9(b). For reflecting the overall performance of heat distribution in the sintering bed, the standard deviation (SD) of gMQI was calculated by Eq. (4).

SD ¼

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X8 ðgMQI;i  1Þ2 =8 i¼1

ð4Þ

The SD of gMQI was 0.42 for Case 4, 0.35 for Case 8 and 0.26 for Case 9. These results indicated that the gaseous fuel segregation method was helpful to balance the heat distribution in the sintering bed. Actually, the added heat in the upper bed enhanced the weak heat pattern. Meanwhile, the heat pattern in the lower bed would not drop so much, which was close to the average level.

160 s

256 s

352 s

448 s

The authors must admit that the thermocouple data in Fig. 9(b) was not so perfect in the middle of sintering bed with some slight fluctuations. As the content of solid fuels in the raw mixture was quite low (less than 10% vol.), it was not surprising to observe the temperature fluctuations in different tests due to the randomly distributed fuel particles in the sintering bed. For example, the fuel particles closer to thermocouple tip would record a relatively higher MQI, while the fuel particles farther to the tip corresponded to a relatively lower MQI. Fortunately, most data and the overall trend were reasonable. For further proving the effectiveness of gaseous fuel segregation method, the red-hot zone was observed with a video camera, while its temperature distribution was acquired by an infrared thermography simultaneously. The infrared images and actual pictures in the non-segregation and segregation cases are shown in Fig. 10. It can be seen that, in the non-segregation case, the red-hot zone was very thin in the upper bed, and was excessively thick in the lower bed which was mainly resulted from the sufficient preheating of fresh air by the hot sintered ores. However, in the segregation case (case 9), the higher methane injecting concentration expanded the red-hot zone in the upper bed, and the lower methane injecting concentration

544 s

640 s

736 s

832 s red-hot region

(a)

red-hot region

temperature range: 800-1300 oC (a) Infrared and actual pictures in non-segregation case

160 s

256 s

352 s

448 s

544 s

640 s

736 s

(b)

832 s red-hot region

red-hot region

temperature range: 800-1300 oC

(b) Infrared and actual pictures in segregation case Fig. 10. Comparison of infrared and actual pictures.

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

10

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx

narrowed the excessively thick red-hot zone in the lower bed. This trend agreed well with the analysis results based on the thermocouple data. Generally, the red-hot zone in the segregation case became much more balanced in the whole process, which was very helpful for achieving higher energy efficiency and average sintering performance. For example, the shatter strength increased from 70.94% in Case 4 to 72.06% in Case 9. It was also noted that, the red-hot zone in the segregation case arrived the bed bottom earlier than that in the non-segregation case, suggesting the higher propagation speed of red-hot zone. With the acceptable sinter strength, higher red-hot zone propagation speed was preferred. The higher propagation speed was attributed from the reduced resistance which was caused by the narrowed red-hot zone in the lower bed. According to the above investigation, the gaseous fuel segregation method had positive impacts on the heat distribution and sintering performance. Actually, in the segregation cases, the heat distribution and sintering performance are highly depended on how the gaseous fuel was distributed during the whole injecting process. Therefore, the effects of segregation degree on the heat pattern and sinter strength were also discussed in this section. Four gaseous fuel segregation cases were arranged, as shown in Table 3. Fig. 9(b) indicates that the high gaseous fuel segregation degree was beneficial to balance the heat distribution in the sintering bed based on the standard deviation analysis. Fig. 11(a) shows that the MQI in the upper bed (average of TC1, TC2 and TC3) increased with the rise of segregation degree until it reached 1.0%/mm. The MQI in the lower bed (average of TC6, TC7 and TC8) dropped with the rise of segregation degree. Moreover, the shatter strength at different segregation degrees was plotted in Fig. 11(b). With the rise of segregation degree, the shatter strength increased until the segregation degree reached 1.0%/mm (Case 8). As the segregation degree further increased to 1.5%/mm (Case 9), the sinter strength stopped increasing, even started to decline slightly. High segregation degree in Case 9 contributed to the best performance on balancing the heat distribution in the studied range. But its sinter strength started to decline slightly, compared with

the sinter strength in Case 8. To explain the reason, the average MQI at different segregation degrees were examined in Fig. 11(a). It was interesting to find that the average MQI decreased with the rise of segregation degree. Actually, in the Cases 4 and 7–9 with the same total calorific heat input, the only difference was the fuel supply manner in the vertical direction of the sintering bed. At lower segregation degree, gaseous fuel segregation method was able to remove the excessive heat from the lower bed, and added to the upper bed where the heat pattern was very weak. It was noted that, energy efficiency in the upper bed was lower than that in the lower bed due to the low combustion efficiency and weak auto heat accumulation, which was widely accepted in the published papers [10,24]. Usually, the solid fuel combustion zone in the sintering bed was quite narrow. In the upstream of the combustion zone, the sintered ores were cooled by the strong air convection, where the temperature dropped to the ambient temperature very quickly. As a result, the secondary combustion zone stopped further moving to the upper direction at high methane concentration, because the combustion of ultra-lean methane/air could not be initiated nor self-sustained at too low temperature. Thus, at too high segregation degree (Case 9), when the improvement of sinter strength in the upper bed could not fill the sinter strength decline in the lower bed, the average sinter strength began to decrease. Therefore, the optimum segregation degree of 1.0%/mm was recommended in the present study. The key sintering parameters in the typical cases are summarized in Table 5, including the flame front speed, sinter yield, productivity and shatter strength. As indicated in Table 5, the sinter yield and shatter strength in Case 8 were the optimal in the tested cases. And the productivity in Case 8 was almost same with the reference case (Case 1). Other than the benefit of energy efficiency improvement, the influence of gaseous fuel injection on the pollutant emission should also be considered. Fig. 12 presents the NOx concentration profiles in the exhaust gas in several typical cases. The NOx concentration in all coke sintering (Case 1) was much higher than that in any other cases. When the 60% coke was replaced by charcoal

76

MQI ( o s)

Upper Average Lower

15000

10000

Shatter strength (%)

20000

74

72

70 Level of Case 1

68

66

5000 0.0

0.5

1.0

1.5

0.0

Segregation degree (%/mm)

0.5

1.5

1.0

Segregation degree (%/mm)

(a) MQI

(b) Shatter strength

Fig. 11. Influence of segregation degree on MQI and shatter strength.

Table 5 Comparison of key parameters in the typical cases. Case No.

Case 1

Case 2

Case 4

Case 7

Case 8

Case 9

Flame front speed (mm/min) Yield (%) Productivity (t/(m2 h)) Shatter strength (%)

25.26 78.41 1.39 69.50

28.57 64.98 1.29 48.86

25.00 78.38 1.37 70.94

24.79 78.42 1.36 72.72

24.95 78.94 1.38 73.25

25.08 78.23 1.38 72.06

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

11

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx

NOx concentration (ppm)

250 Case 1 Case 2 Case 3 Case 4 Case 8

200

150

100

50

0 0

200

400

600

800

1000

1200

Time (s) Fig. 12. NOx emission in exhaust gas in typical cases.

(Case 2), the NOx emission significantly reduced to a quite low level. However, the heat pattern and sinter strength were badly deteriorated in Case 2. Therefore, the methane injection (Case 3) was employed to enhance the sintering performance, whose NOx emission slightly increased. At lower solid fuel consumption (Case 4), NOx emission reduced in comparison with Case 3. Furthermore, by employing the methane segregation injection method (Case 8), NOx emission increased at initial period due to the higher peak temperature [27] caused by the higher methane concentration in the upper bed. Then, there was a small drop of NOx concentration because of the decreasing methane concentration in the final period. Generally, the NOx emission significantly reduced by the combined usage of charcoal substitution and methane injection methods, especially in Case 6 and Case 8. In this section, gaseous fuel segregation method was proposed and confirmed for improving the imbalance of heat distribution. Fig. 13 presents the improvement of the red-hot zone distribution by employing the gaseous fuel segregation method. Increasing the injecting gaseous fuel concentration in the upper bed is able to

Fuel concentration in segregation case

Fuel concentration in non-segregation case Gaseous fuel injection

Ignition point Expanded Up

Non-Segregation Segregation Ideal case

Middle Narrowed Bottom Fig. 13. Revolution of red-hot zone at different cases in the moving strand.

Fig. 14. Real application of gaseous fuel segregation on the sintering machine.

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

12

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx

expand the red-hot zone, enhancing the weak heat pattern and sinter strength. Reducing the injecting gaseous fuel concentration in the lower bed could narrow the red-hot zone, avoiding the excessive heat supply. Actually, the completely uniform red-hot zone distribution is the ultimate goal for most operations, as shown in Fig. 13. However, the gaseous fuel segregation study in this paper was not able to achieve that due to the different energy utilization efficiency, the random distribution of solid fuel particles and the limited experiment conditions. The authors believed that it is promising to achieve the ultimate goal, if the operators turn to the gaseous fuel segregation, oxygen enrichment and on-line control technologies simultaneously which is our follow-on work. In the future, the real application of the gaseous fuel segregation is illustrated in Fig. 14. As indicated in Fig. 14, several gaseous fuel injection systems are installed independently above the moving strand. The number of the gaseous fuel injection system is relied on the length of the sintering bed and the actual need. The fuel injecting flowrate is controlled by the mass flow controller according to the designed fuel concentration and the reading of air flowrate. Actually, the added system would not bring heavy burden on the capital investment or challenges of operation safety. 4. Conclusions Researchers employed the biomass to partially replace the fossil fuels for controlling the emissions of CO2 and pollutants in the sintering process. In this paper, aiming at improving the unreasonable combustion organization and cooling condition in the co-fire of coke breeze and charcoal in iron ore sintering, the authors proposed the ultra-lean gaseous fuel injection method in this paper. It was adopted to generate a secondary combustion zone for expanding the burning area and reducing the cooling rate. Actually, the gaseous fuel with ultra-low concentration can be derived from steelworks as by-product at low cost. This method offers a technical support for the sustainable sinter production using carbon neutral solid fuel. More importantly, this method can simultaneously reduce solid fuels (the main contributor of contamination dust during the production, transportation and storage) consumption which are the urgent needs for the sinter plant. Furthermore, the imbalance of heat distribution has been considered as one of the main contributors to the low energy efficiency in the sinter production. Based on the influencing mechanism of gaseous fuel concentration on the heat pattern in the sintering bed, the gaseous fuel segregation method was designed to balance the unevenly distributed heat at higher accuracy and versatility. Actually, the gaseous fuel segregation method was firstly proposed in this field. In comparison with other approaches, the gaseous fuel segregation method is easy to achieve on-line control in the future at much lower cost and higher efficiency. The major findings were summarized as follows: (1) The gaseous fuel injection method provided an effective way to enhance the heat pattern and sinter strength when using low grade solid fuels. It was observed that a secondary combustion zone of gaseous fuel was generated and selfsustained above the solid fuels combustion zone, which was able to improve the heat pattern and sinter strength obviously at the equivalent calorific heat input. The selfsustained secondary combustion zone could prevent the original melting zone from being cooled directly by the fresh air. Besides, its own calorific heat release further expanded the melting zone. Consequently, the sinter strength grew observably from 48.86% in Case 2 to 72.13% in Case 3, even higher than that in Case 1. Moreover, the results indicated that even the total calorific heat input reduced 4% at the

methane injection concentration of 0.5% vol. (Case 4), the heat pattern and sinter strength can be still maintained higher (1.44%) than those in the coke sintering. In Case 4, the solid fuel decreased 11.52%, consisting of 4% cut of the calorific heat and 7.52% replacement by the ultra-lean (0.5%) methane. (2) The gaseous fuel segregation was a feasible solution for improving the imbalance of heat distribution in the sintering bed. The results of thermocouple and visual experiments indicated that gaseous fuel segregation method had significantly positive impacts on the heat distribution and average sinter strength, compared with the non-segregation case. With the view of balancing the heat distribution, high gaseous fuel segregation degree was preferred in this paper. However, the average sinter strength stopped increasing, even began to decrease slightly at excessively high segregation degree. The optimum segregation degree of 1.0%/mm was recommended in the present study where the sinter strength grew 2.31% at the same energy consumption (Case 4 and Case 8). The approaches proposed in this paper offer some guidance for utilizing the low grade charcoal, reducing fuel the consumption rate, and improving the unevenly distributed heat in the sintering bed, which have positive influences on the improvement of energy efficiency. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant No. 51536007). References [1] Chen WY, Yin X, Ma D. A bottom-up analysis of China’s iron and steel industrial energy consumption and CO2 emissions. Appl Energy 2014;136:1174–83. [2] Cheng ZL, Yang J, Zhou L, Liu Y, Wang QW. Characteristics of charcoal combustion and its effects on iron-ore sintering performance. Appl Energy 2016;161:364–74. [3] Chen HX, Rein G, Liu NA. Numerical investigation of downward smoldering combustion in an organic soil column. Int J Heat Mass Transf 2015;84:253–61. [4] Sennoune M, Salvador S, Quintard M. Reducing CO2 emissions from oil shale semicoke smoldering combustion by varying the carbonate and fixed carbon contents. Combust Flame 2011;158:2272–82. [5] Khusid BM, Kulebyakin VV, Bashtovaya EA, Khina BB. Mathematical and experimental modelling of quenching a self-propagating high-temperature synthesis process. Int J Heat Mass Transf 2009;42:4235–52. [6] Roy NC, Hossain A, Nakamura Y. A universal model of opposed flow combustion of solid fuel over an inert porous medium. Combust Flame 2014;161:1654–8. [7] Roy MM, Dutta A, Corscadden K. An experimental study of combustion and emissions of biomass pellets in a prototype pellet furnace. Appl Energy 2013;108:298–307. [8] Miedema JH, Benders RMJ, Moll HC, Pierie F. Renew, reduce or become more efficient? The climate contribution of biomass co-combustion in a coal-fired power plant. Appl Energy 2017;187:873–85. [9] Zhao JP, Loo CE, Dukino RD. Modelling fuel combustion in iron ore sintering. Combust Flame 2015;162:1019–34. [10] Yang W, Choi S, Choi ES, Ri DW, Kim S. Combustion characteristics in an iron ore sintering bed-evaluation of fuel substitution. Combust Flame 2006;145:447–63. [11] Luo YH, Zhu DQ, Pan J, Zhou XL. Utilization of semi-coke as by-product derived from coal-based direct reduction process in iron ore sintering. Ironmaking Steelmaking 2016;43:628–34. [12] Ooi TC, Thompson D, Anderson DR, Fisher R, Fray T, Zandi M. The effect of charcoal combustion on iron-ore sintering performance and emission of persistent organic pollutants. Combust Flame 2011;158:979–87. [13] Nunes LJR, Matias JCO, Catalao JPS. Mixed biomass pellets for thermal energy production: a review of combustion models. Appl Energy 2014;127:135–40. [14] Cheng ZL, Yang J, Zhou L, Liu Y, Guo ZG, Wang QW. Experimental study of commercial charcoal as alternative fuel for coke breeze in iron ore sintering process. Energy Convers Manage 2016;125:254–63. [15] Lu L, Adam M, Kilburn M, Hapugoda S, Somerville M, Jahanshahi S, Mathieson JG. Substitution of charcoal for coke breeze in iron ore sintering. ISIJ Int 2013;53:1607–16.

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024

Z. Cheng et al. / Applied Energy xxx (2017) xxx–xxx [16] Wang G, Wen Z, Lou GF, Dou RF, Li XW, Liu XL, Su FY. Mathematical modeling and combustion characteristic evaluation of a flue gas recirculation iron ore sintering process. Int J Heat Mass Transf 2016;97:964–74. [17] Ahn H, Choi S, Cho B. Process simulation of iron ore sintering bed with flue gas recirculation Part 2 – Parametric variation of gas conditions. Ironmaking Steelmaking 2013;40:128–37. [18] de Castro A. Model predictions for new iron ore sintering process technology based on biomass and gaseous fuels. Adv Mater Res 2014;918:136–44. [19] Loo CE, Leaney JCM. Characterizing the contribution of the high-temperature zone to iron ore sinter bed permeability. Miner Process Extractive Metall 2002;111:11–7. [20] Zhou H, Zhou MX, Cheng M, Guo WS, Cen KF. Experimental study and X-ray microtomography based CFD simulation for the characterization of pressure drop in sinter bed. Appl Therm Eng 2017;112:811–9. [21] Aldushin AP, Rumanov IE, Matkowsky BJ. Maximal energy accumulation in a superadiabatic filtration combustion wave. Combust Flame 1999;118:76–90.

13

[22] Cheng ZL, Yang J, Zhou L, Liu Y, Wang QW. Sinter strength evaluation using process parameters under different conditions in iron ore sintering process. Appl Therm Eng 2016;105:894–904. [23] Zhao J, Loo CE, Ellis BG. Improving energy efficiency in iron ore sintering through segregation: a theoretical investigation. ISIJ Int 2016;56:1148–56. [24] Kang HJ, Choi S, Yang W, Cho B. Influence of oxygen supply in an iron ore sintering process. ISIJ Int 2011;51:1065–71. [25] Nath NK, Mitra K. Mathematical modeling and optimization of two-layer sintering process for sinter quality and fuel efficiency using genetic algorithm. Mater Manufact Process 2005;20:335–49. [26] Machida S, Higuchi T, Oyama N, Sato H, Takeda K, Yamashita K, Tamura K. Optimization of coke breeze segregation in sintering bed under high pisolite ore ratio. ISIJ Int 2009;49:667–75. [27] Zhou H, Cheng M, Zhou MX, Liu ZH, Liu RP, Cen KF. Influence of sintering parameters of different layers on NOx emission in iron ore sintering process. Appl Therm Eng 2016;94:786–98.

Please cite this article in press as: Cheng Z et al. Optimization of gaseous fuel injection for saving energy consumption and improving imbalance of heat distribution in iron ore sintering. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.06.024