
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12269-4
10.1016/j.heliyon.2024.e36238
e36238
Research Article
Combustion visualization analysis of alternative fuels in the pulverized coal injection raceway through laminar flow reactor
Lee Dae-Gyun a
Kim Min-Woo b
Ku Min-Jong b
Bae Yoon-Ho b
Kim Kang-Min c
Kim Gyeong-Min d
Jeon Chung-Hwan chjeon@pusan.ac.kr
bd⁎
a Korea Institute of Energy Research (KIER), 152, Gajeong-ro, Yuseong-gu, Daejeon, 34129, Republic of Korea
b School of Mechanical Engineering, Pusan Nat'l Univ., 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan, 46241, Republic of Korea
c Korea Electric Power Corporation (KEPCO) Research Institute, 105, Munji-Ro, Yuseong-gu, Daejeon, 34056, Republic of Korea
d Pusan Clean Energy Research Institute, Pusan Nat'l Univ., 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan, 46241, Republic of Korea
⁎ Corresponding author. School of Mechanical Engineering, Pusan Nat'l Univ., 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan, 46241, Republic of Korea. chjeon@pusan.ac.kr
30 8 2024
15 9 2024
30 8 2024
10 17 e362385 8 2024
12 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Currently, the steelmaking process uses a pulverized coal injection (PCI) system that serves as the heat source and reductant for ironmaking (blast furnace and FINEX) where system uses expensive high-grade coal and high operating costs. Hydrogen steelmaking is currently being developed to achieve carbon-free operation. To achieve a soft-landing during this phase of rapid change, the use of biomass and inexpensive, thermal coal, and coke dust is necessary. Research on their combustion characteristics is necessary to apply these alternative fuels to PCI. Therefore, this study analyzed the combustion characteristics of ignition delay, devolatilization, and char combustion using a laminar flow reactor visualization equipment that simulates blast furnace (BF) and FINEX PCI tuyere, using flame image data processing. The ignition time were generally longer in BF than in FINEX, and the char combustion length and time also showed the same trend due to the high oxygen rate which indicate under 2 ms on ignition delay, under 16 ms on char combustion. Also, the volatile cloud was qualitatively shown in the image to be highest in thermal coal and biomass with high volatile matter. Based on the correlation and theoretical calculation with proximate analysis and the results, ignition delay time had a combined effect of volatile matter and moisture except coke dust, and char combustion time affected unburned carbon. The combustion chemical characteristics were discussed with chemical percolation devolatilization (CPD) model parameter. Through SEM image and BET analysis, the surface area has been increased more than 10 times after combustion. Consequently, the biomass and high moisture thermal coal could cofired within 10 % and coke dust could be cofired within 9 %, respectively.

Highlights

• Suitability of different fuels were estimated in blast furnace and FINEX PCI raceway.

• Ignition delay, volatile and char combustion were determined with LFR visual analysis.

• Above 90 % of thermal coal and biomass burned out, while 50 % of coke dust in BF.

• Thermal coal, biomass coke dust can be cofired within 100, 7, 3 % each in BF.

• Thermal coal, biomass and coke dust can be cofired within 54, 8, 9 % each in FINEX.

Keywords

Combustion visualization
Raceway
Blast furnace
FINEX
Laminar flow reactor
Unburned carbon
Alternative fuel
==== Body
pmcList of abbreviations

PCI Pulverized Coal Injection

LFR Laminar Flow Reactor

UBC Unburned Carbon

PA Pulverized Coal Injection Coal A

PB Pulverized Coal Injection Coal B

TA Thermal Coal A

TB Thermal Coal B

WP Wood Pellet

TWP Torrefied Wood Pellet

CCD Charge Coupled Device

1 Introduction

During iron production, sintered ore and coke are charged into the upper part of the blast furnace in a layered structure. At the blast furnace bottom, air preheated to 1200 °C is injected at high pressure and speed through the tuyere; the injected coke reacts with oxygen to produce CO gas, which penetrates the formed coke layer and rises from the bottom to the top. The final molten iron is discharged at approximately 1400 °C, and the sintered ore undergoes extensive temperature changes and various reduction reactions. Here, CO gas plays an important role as a reducing agent for iron oxide inside the blast furnace. Therefore, the pulverized coal injection (PCI) system was introduced into the blast furnace to quantitatively control the CO gas and temperature. The PCI serves as a supplemental heat source, reducing the amount of coke used and acting as a reducing agent for iron oxide, ultimately lowering production costs and reducing energy use and emissions [1,2].

The traditional steelmaking process using blast furnaces relies on coke from bituminous coal, generating hazardous substances and requiring additional facilities. The FINEX process offers a more environmentally friendly alternative by using fluidized bed reactors and pulverized iron, avoiding the need for coke and allowing for the use of cheaper coal. This process also requires less initial capital investment. Despite its early development, the PCI process gained popularity due to increased iron production costs in the 1970s. However, challenges with coal combustibility and efficiency in blast furnaces remain, necessitating further research to meet the rising demand for high-quality iron [[3], [4], [5]].

PCI is currently used in blast furnaces and FINEX, and it can be confirmed that the fuel and air conditions input through the tuyere differ [3]. In a blast furnace, pulverized coal is fed into the center of the tuyere through a lance with N2 carrier gas. Simultaneously, high-temperature air of 1200 °C and oxygen are premixed together, forming a raceway. In contrast, in the FINEX process, pulverized coal is injected with N2 and carrier gas, and it is not injected into the center of the tuyere but rather at the outlet under the pure oxygen condition at 25 °C. Therefore, to compare and analyze the combustibility of alternative fuels, the blast furnace and FINEX conditions must be considered to determine which process to use. Pulverized coal injection technology uses up to 250 kg per ton of hot metal and is currently optimized to the range of 150–180 kg per ton of hot metal, similar to that of the blast furnace [2,3].

Additionally, considerable research is being conducted on using biomass as an alternative fuel to PCI. According to references, bio-oil, wood pellets, and charcoal can be replaced for PCI in amounts of 140, 50–150, and 200 kg/tHM, respectively [6]. Additionally, coke dust can be recycled into the PCI. Coke dust is a by-product left over from the process of manufacturing coke, not PCI, and contains heat, so it can play a role in supplying the heat source inside PCI with pulverizing. Research related to coke dust is in progress, and many studies are being conducted on recycling coke dust that is currently discarded as waste [7]. In this study, we analyzed the combustion characteristics generated by PCI using biomass from an environmental perspective and the relatively inexpensive thermal coal and coke dust from an economic perspective.

Research on the flame characteristics of solid fuel combustion has been conducted using several apparatuses. Among them, the most commonly used is the laminar flow reactor (LFR). This apparatus operates the gas in a laminar state and the solid fuel particles move at a constant speed, causing combustion and thermal decomposition. It has the advantage of visually analyzing the combustion characteristics [[8], [9], [10]], such as ignition, devolatilization, and char combustion, and use various analysis methods to derive the combustion speed and combustibility. As shown in Fig. 1, the combustion process of solid fuel is as follows. Solid fuel contains moisture, volatile matter, fixed carbon, and ash. When combustion begins, demoisturizing occurs, in which moisture is removed. At this time, heat is absorbed from the outside, and both external moisture and inherent moisture contained in the solid fuel evaporate. At this time, the next steps, ignition and devolatilization, occur. Previous studies have shown the relationship between moisture and ignition, and confirmed that ignition delay increases as the moisture content increases. There are also results showing that the content of volatile matter affects ignition [8]. And when the temperature increases above a certain level, a devolatilization reaction occurs, and fragmentation of some PCI coals occurs due to the high heating rate. According to existing research, combustion of solid fuel progresses primarily through homogeneous ignition, followed by heterogeneous ignition. Homogeneous reactions occur when combustion is dominated by volatile matter such as light gas, and heterogeneous ignition occurs in the reaction of char from which volatile matter has been removed [10]. And when char combustion is completed, it exists as ash, and at this time, the ash component plays a major role in blast furnaces and FINEX. There is also a reference that conducted a macroscopic study on this combustion process [9], through which LFR can be said to be a device that can determine all combustion characteristics of solid fuel. In addition, it can simulate an actual PCI system by generating a direct flame through a burner.Fig. 1 Combustion process of solid fuels with LFR visualization analysis.

Fig. 1

The purpose of this study is to review the fuel applicability of alternative fuels within the PCI of BF and FINEX. To achieve this, the combustion characteristics through flame visualization analysis of alternative fuels using LFR has been conducted. First, the combustion characteristics of solid fuel, such as ignition delay, devolatilization, and char combustion flame, were measured qualitatively and quantitatively. In the case of ignition delay, the actual time was compared and analyzed, based on the theoretical equation calculated from the reference, and char combustion was also compared to unburned carbon to derive the correlation between the length of the flame and UBC. The chemical structure of the chemical percolation devolatilization (CPD) model parameters was analyzed, based on proximate and ultimate analysis, and the resulting influence on combustibility was derived. The structural changes have been investigated using SEM images of the samples before and after combustion. And a BET experiment was performed to determine the change in surface area of before and after combustion. In addition, using combustion visualization analysis, a comparative analysis of the combustion characteristics of existing PCI high-grade coal helped determine the applicability of alternative fuels. Based on the analyzed results, the maximum co-firing amount that can be applied to the PCI system of alternative fuels was derived.

2 Experimental

2.1 Sample preparation and their properties

Seven samples were prepared for the PCI process. Currently, the following types of coal are used in PCI: PCI coal A(PA), PCI coal B(PB) Wood pellets (WP), a biomass fuel, torrefied WP (TWP), and thermal coal A(TA), thermal coal B(TB), used in coal-fired power plants. Coke dust is generated when manufacturing coke from coals. We conducted proximate analysis was conducted using a TGA-701 thermogravimeter (LECO Co., St. Joseph, MI, USA), and ultimate analysis using TruSpecific and SC-432DR sulfur analyzers (LECO). A Parr 6320 EF calorimeter was used to measure calorific values. The ash properties were determined by X-ray fluorescence (XRF) analysis using a Primus II (RIGAKU Co.). Each analysis was performed according to ASTM standards [[11], [12], [13], [14], [15], [16]]. The proximate, ultimate, and calorific values of each sample are presented in Table 1.Table 1 Basic properties of samples.

Table 1Samples	PCI Coal	Biomass	Thermal Coal	Cokes
Dust	
PA	PB	WP	TWP	TA	TB	
Proximate Analysis (ad)	
Inherent Moisture (%)	1.19	0.41	2.76	1.09	2.86	14.81	0.21	
Ash (%)	7.77	7.85	2.18	8.07	7.60	4.69	11.60	
Volatile Matter (%)	11.01	10.84	75.65	49.98	38.47	36.95	4.88	
Fixed Carbon (%)	80.03	80.90	19.41	40.86	51.07	43.55	83.31	
Ultimate Analysis (DAF)	
C (%)	90.87	89.73	48.10	64.12	75.64	69.48	83.31	
H (%)	3.94	4.01	6.20	5.79	5.82	5.37	0.02	
O (%)	2.73	2.82	45.10	29.60	16.68	23.78	14.69	
N (%)	1.91	2.97	0.60	0.49	1.43	1.16	1.10	
S (%)	0.55	0.47	0	0.00	0.43	0.21	0.88	
Heating Value (MJ/kg)	31.68	31.53	20.73	23.12	29.76	25.53	28.33	

The PCI coal was selected from the coal currently used in steel mills and had a calorific value of approximately 31.53–31.68 MJ/kg. The fixed carbon and volatile fractions were approximately 80 and 10 wt%, respectively. For biomass, we used the most widely used WP and torrefied biomass, produced at about 270 °C for 30 min. Wood-based WP has a high volatile content of 75 % and a fixed carbon content of approximately 20 %. After torrefaction, the moisture and volatile contents were slightly reduced, resulting in a fixed carbon and volatile contents of 26 % and 70 %, respectively. Two types of thermal coal are used in power plants: sub-bituminous coal and high-moisture coal, with heating values of approximately 29.76 and 25.53 MJ/kg, respectively. Additionally, coke dust, having the highest fixed carbon and lowest volatile content, was used.

In order to classify the samples, the Atomic H:C and Atomic O:C ratios of each sample were calculated and displayed on the Van-Krevelen graph, which is the indication of the coal rank. As shown in Fig. 2(a), PA and PB belong to the bituminous coal area. In the case of TA, it was included between sub-bituminous and bituminous, and TB was included in sub-bituminous. In the case of WP, it was confirmed that it was included in the biomass area, and TWP was carbonized between lignite and sub-bituminous coal after being torrefied in WP, showing a high-quality state. However, coke dust does not belong to any region, which is thought to be because there is almost no hydrogen contained within it, as shown in Table 1.Fig. 2 Sample basic characteristics: (a) Van-Krevelen diagram of the samples (b) Particle size distribution of pulverized samples.

Fig. 2

Additionally, the particle size distribution of the differentiated samples was analyzed using a particle size analyzer, model LS 13 320 (Beckman, USA). This device measured particle size, diameter, and fine particles. The distribution is shown in Fig. 2(b), and the results are as follows. For PA and PB, the results are almost identical. They both have a peak around 85–90 μm, with the overall average particle size also falling within the 75–90 μm range. WP and TWP show peaks at a relatively lower diameter of 60 μm, indicating that fine particles were not sufficiently filtered during the crushing process. TA exhibits a distribution similar to TWP, which is likely due to the same reason mentioned for TWP. However, TB displays a relatively higher peak. This can be attributed to the high moisture content, causing the fine particles to stick together, resulting in relatively larger particle sizes and a less smooth peak distribution. In the case of coke dust, the distribution is the most ideal, with a peak at around 88 μm, which is within the 75–90 μm range.

2.2 Laminar flow reactor

In this study, the combustion characteristics of each fuel type (ignition delay and char combustion) were visually measured using LFR [8]. This apparatus provides a combustion heat source for the coal particles through a laminar diffusion plate flame, enabling coal ignition and flame analysis. The LFR consists of a flat flame burner, coal supply component, and gas supply component; a schematic of the device is shown in Fig. 3. A two-axis feeder is installed to position the reactor. A stepper motor connected to the two axes is moved through the control unit using computer software to transfer the reactor to the desired position.Fig. 3 Schematic of Laminar Flow Reactor and its applicability of LFR to describe the PCI system.

Fig. 3

In the flat flame burner unit, stainless steel capillaries (inner diameter, 0.7 mm) are inserted at regular intervals into a hexagonal honeycomb-shaped cell matrix to form a flat flame. The entire flat flame surface is housed in a circular shape within a rectangular cross-section (48 × 48 mm). The capillaries are the conduits through which the oxidizer, gases, and fuel required to create the flat flame are supplied. A quartz tube with a rectangular cross-sectional shape serving as a reactor is erected on the burner based on the rectangular cross-section around the burner. In this experiment, a flat-plate laminar diffusion flame was formed by the combustion of CO, H2, N2, and O2. The gas concentration was adjusted so that the oxygen concentration after the flat flame was simulated the BF condition and FINEX condition respectively.

The challenge in coal particle combustion experiments is quantitatively supplying the microsized particles. When the size of the coal particles is below 100 μm, the particles are easily attached to the tube wall due to electrostatic charging, making clogging or transporting challenging. Various particle supply devices have been developed and used for combustion experiments of fine coal particles. The device consists of a sealed syringe that serves as a container for coal particles, a syringe pump that pushes up a syringe plunger at a rate corresponding to the flow rate of the coal to be supplied, two capillary tubes and a vibrator. Two capillaries are inserted into the syringe using a urethane rubber stopper to seal the tip. The vibrator attached to a urethane rubber stopper, applies vibration to the syringe to cause the coal particles to float slightly from the surface. After injecting the coal particles into the syringe, a plunger is inserted and attached to the syringe pump such that the plunger advances at a constant speed corresponding to the flow rate of the coal particles to be supplied. The transport flow rate of coal was checked by measuring the weight of the coal particles supplied for a certain period to confirm sufficient supply [17]. The coal feeding mass flow rate is monitored by measuring the mass of the coal being fed. The feed rate of the coal particles, controlled by the syringe pump plunger, was adjusted to correspond to a supply rate of 7 mg/min. The sample particle size was classified into 75-90 μm and the heating rate was 106 K/s. Residence time is up to about 20 ms.

The gas supplied to the burner is classified by use into fuel and oxidizer for creating a flat-spread flame and transport gas for transporting coal particles. This device selects and mixes the fuel and oxidizer to be used according to the formation conditions of the atmospheric gas for burning coal particles. In other words, Fuel gas forms a flat flame and serves as a heat source for the solid particles to be injected. And the oxydizer controls the oxygen concentration after the flat flame is formed. Each gas was controlled using an individual mass flow controller (MFC), and each MFC was connected to the channel of the entire control device to set the value to be controlled. CO and H2 were used to generate a flat flame, and N2 was used for concentration dilution. O2 and N2 were used as oxidizing agents, and N2 was used as the carrier gas for coal particle injection. The flow rates of all gases are summarized in Table 2. The coal particles injected into the flat flame generated under the above conditions were burned under oxygen-loaded blast furnace and FINEX conditions, respectively. The flat flame generated under these conditions had a temperature distribution of 2000–2250 K. These conditions were created by simulating actual operating conditions. In the case of BF, air and oxygen are input in proportion, and in the case of FINEX, operation is performed under pure oxygen conditions. Accordingly, as shown in the oxidizer, BF is injected with a certain amount of O2 and N2, and FINEX is mostly composed of oxygen and injected in the oxidizer.Table 2 Experimental condition of LFR.

Table 2Actual Flow rate (SLPM) [L/min]	
Blast Furnace	Fuel	Oxidizer	
N2 (que)	N2 (car)	CO	H2	N2	O2	N2	
15	0.367	3.04	0.74	2.64	6.20	3.54	
FINEX	Fuel	Oxidizer	
N2 (que)	N2 (car)	CO	H2	N2	O2	N2	
15	0.367	3.04	0.74	0.64	11.20	0.62	

A PCI system simulation of the LFR experimental device was performed to justify using LFR not only in reference [10], but also in this study. The finely divided particles are sprayed into the Tuyere through the lance to generate raceway flames [18]. This is also applied in LFR, and the premixed fuel forms a flat flame; pulverized coal is injected accordingly, and the raceway of coal occurs. Thus, the flame generation part of the actual PCI system can be accurately implemented through LFR. Hence, in this study, the combustion mechanism of coal was visually analyzed by focusing on the injection parts of the PCI and LFR to simulate the raceway.

2.3 Combustion image data processing

The combustion behavior of the alternative fuels was visually analyzed using a CCD camera, and the combustion range was determined by quantifying the image's intensity [19]. The measurement method is shown in Fig. 4 and can be explained as follows: Three combustion characteristics were confirmed experimentally. First, during solid fuel combustion, ignition delay occurs. This is the period during which heat penetrates and increases the temperature of the solid fuel until the ignition point of the solid fuel is reached. Second, is the volatile combustion, where volatile substances combust within a solid fuel through contact with external oxygen via a devolatilization process. This represents the formation of a translucent cloud, forming the flame of an actual combustion reaction between liquid and gas. Finally, char combustion of the solid fuel occurs, which emits light with a higher intensity than that of the cloud. Image data processing was performed to analyze combustion behavior. The appearance of the flame was converted into a black-and-white photo, expressed in intensity from 0 to 255, and analyzed. After selecting the burner and the part where the ignition starts as 50 % of the maximum intensity, the area where each combustion occurred was quantified. After measuring the actual length, the actual length per pixel was compared, and arithmetic mean was used to calculate the ignition delay length and combustion length of the solid fuel combustion. Thus, the ignition delay of coal and char combustion length can be confirmed, and devolatilization combustion can be determined qualitatively.Fig. 4 Flat flame of LFR and Image data processing of solid fuel combustion.

Fig. 4

2.4 Combustion characteristics and SEM image analysis

The ignition delay phenomenon of coal flames significantly affects the size and shape of the raceway when combustion occurs inside the PCI. Therefore, analyzing the ignition delay of each sample is important. Additionally, the ignition delay is determined by volatile matter and internal moisture [20]. Because volatile ignition occurs owing to instantaneous heat transfer from a surrounding heat source, heat loss can be ignored compared to a chemical reaction [21]. Assuming that the volatile content of coal is methane, a representative hydrocarbon [22], we simplify this to an adiabatic thermal explosion problem and derive the ignition delay time (τi)as follows [21]:(1) τi=cv(Tg2Ta)qcYF,OAexp(−TaTg)

Because the amount of coal supplied is constant, the combustion heat release of volatile matter (qc) is constant. The static specific heat (cv) is a function of the reaction area temperature and is derived through linear interpolation using the reaction area temperature according to the moisture content in Ref. [8] and the initial mass fraction of the volatile matter (YF,0) decreases. In addition, at the reaction zone temperature (Tg) changes depending on the moisture content, and the kinetic parameters of volatile matter (A, Ta) are adopted as the reaction rate constants for the single combustion stage of CH4. According to previous research, ignition delay is defined as a total of three phenomena [10,23]. (1) Homogeneous ignition refers to the ignition of the volatile substances emitted from coal. (2) Heterogeneous ignition involves the ignition of the coal particle surface.(3) Hetero-homogeneous ignition occurs when both the volatile substances and the coal particle surface ignite simultaneously. In reference [23], this ignition phenomenon was implemented through TGA analysis. Therefore, in this study, we investigated this phenomenon using TGA and discussed how the ignition occurs with the condition of pyrolysis and combustion. The pyrolysis has been conducted in only N2 gas and 5 K/min heating rate and combustion in air(O2 21 %) condition and 5 K/min heating rate which is same as reference [23].

Here, the reaction area temperature was measured using an R-type thermocouple at 70 μm intervals of 5 mm from the coal supply nozzle in the vertical direction along the central tube where coal particles are supplied, as shown in Fig. 5. Here, as shown in temperature profile under the blast furnace condition and the FINEX condition, it is slightly higher in the FINEX condition but shows a similar trend. This is because the amount of fuel gas is the same, forming a similar flat flame, as can be seen in Table 2. In addition, to compare and analyze the calculated and measured ignition delay, the actual time was calculated through actual length analysis. In actual time, as shown in Fig. 5, assuming that the fuel particles rise with the gas in a laminar state owing to buoyancy at 30 L/min, the particle speed can be calculated by dividing the area accordingly. The ignition delay of the particle and the char combustion time can be calculated by L dividing the particle speed, which is the actual length measured at that time.Fig. 5 Temperature profile and calculation of particle combustion time.

Fig. 5

In addition, the char combustion of alternative fuels considerably influences raceway formation. The char was analyzed after combustion using a char collector in the LFR combustion collection unit. For the collected char, the Unburned Carbon(UBC) was calculated using Eq. (2) and the resulting combustibility was analyzed using the ash tracer method [24] by comparing it with the actual char combustion length. Here, A0 refers to the ash content of raw coal, and Achar refers to the ash content of char collected after reaction. In addition, structural analysis of the char after combustion was performed using SEM images.(2) UBC(%)=[A0(100−Achar)Achar(100−A0)]×100

2.5 Chemical structure analysis with CPD model parameters

Devolatilization models used in the gasification process include Badzioch's single step model, Kobayashi's two step model, and CPD model [[25], [26], [27]]. The single step model is a model that assumes the overall reaction as one part and is analyzed through reaction rate constants, and the two step model obtains the reaction rate constants of the two reaction regions, low temperature and high temperature, to complement the simple single step model. It allows accurate prediction of both devolatilization amount and reaction rate over a wider reaction area. However, the above two models have the disadvantage of not being able to predict the various products released during the devolatilization process and not being able to take pressure into account. In contrast, the CPD model is based on coal chemical structure characteristics and reaction rate theory analyzed by 13-C Nuclear Magnetic Resonance spectroscopy. Because it predicts well the changes in volatile components (tar, CO, CO2, H2O, CH4) depending on temperature, residence time, and pressure during the devolatilization process, it is highly evaluated for reliability and is widely used. The CPD model was largely developed based on the chemical structure of coal, percolation theory, and chemical mechanism.

The CPD model shows volatile components according to temperature, residence time, and pressure during the devolatilization process, based on the characteristics of the chemical structure analyzed by 13-C NMR spectroscopy. During devolatilization of a single coal particle, the lattice structure of the chemical structure is broken, and the relatively more bonded aromatic clusters become char, the less bonded ones become tar, and the side chains become gas. Mcl, Mdel, σ+1, and p0 data obtained through 13-C NMR spectroscopy experiments are input values to the CPD model. However, in order to compensate for the disadvantages of 13-C NMR spectroscopy experiments being expensive and time-consuming, Fletcher calculated Mcl, Mdel, σ+1, p0 using only proximate analysis and ultimate analysis of solid fuel [28]. Mdel refers to the quantitative molar mass of the side chain, indicating its conversion into a light gas as the temperature increases during the devolatilization process. Mcl denotes the quantitative molar mass of aromatics, which can transform into coal char or tar with rising temperatures during the devolatilization process. The term p0 indicates the ratio of coal char to tar during their formation; a value closer to 1 suggests a higher tendency to form coal char. σ+1 represents the number of bridges connected to an aromatic site.

3 Results and discussion

3.1 Combustion flame image analysis of alternative solid fuels

The solid combustion flame images, ignition points, and char combustion lengths of the seven samples were quantitatively analyzed using LFR. The flame shape and characteristics of solid combustion are qualitatively shown in Fig. 6.Fig. 6 Solid fuel combustion flame images of alternative fuels and their combustion characteristics (a) Blast furnace condition (b) FINEX condition.

Fig. 6

Coal for PCI in steel mills exhibits a small flame cloud phenomenon resulting from the volatile matter combustion under blast furnace conditions [29]. In addition, fragmentation, splitting of coal particles, occur in PB. Under FINEX conditions, fragmentation occurs actively in PA and PB cases. Afterwards, homogeneous ignition of the separated fragmented char containing a trace amount of volatile matter occurs, and heterogeneous ignition with the volatile matter removed occurs [10]. When the amount of oxygen was increased, the char combustion length decreased compared to the blast furnace conditions. It was also observed that ignition delay was reduced which means the green line indicates the ignition point. In other words, the longer the ignition point, the longer the ignition delay phenomenon [8]. In the case of WP, as shown in the proximate analysis in Table 1, a large amount of volatile combustion clouds is generated because of the large amount of volatile matter, and char combustion also showed a significantly shorter flame length than the other samples. Under FINEX conditions, the char combustion length was shortened. Because the amount of fixed carbon was less than the amount of volatile matter, it was confirmed that the combustion of volatile matter continued even after char combustion was completed, which means a larger amount of homogeneous ignition than heterogeneous combustion occurs [10]. With TWP, volatile matter was released below 270 °C during torrefaction, confirming that volatile cloud formation did not occur compared to existing WP, and the char combustion length was also relatively increased. This sample also confirmed that the combustion length and ignition start point were shortened under the FINEX conditions. In addition, in the case of thermal coal used in power plants, it was confirmed that a volatile cloud occurred because of the relatively larger amount of volatile matter than in PA and PB and that the char combustion length was longer than that of biomass. Coke dust exhibits a flame length exceeding the maximum measurable length under blast furnace conditions. In addition, as it contained almost no volatile matter, cloud formation could not be confirmed which means only heterogeneous ignition occurs [10]. Moreover, it exhibits the highest ignition point. Under FINEX conditions, char combustion was completed within the maximum measurable length range.

These photographs were quantitatively analyzed for the ignition point and char combustion length through image processing, as shown in Fig. 4 and shown in Fig. 7 [8,9]. In all the cases, the oxygen concentration increased, while the ignition point and char combustion length decreased. The ignition point shows the lowest values for WP and TA due to the high volatile and low moisture content in each sample. In addition, one of the salient features is the relatively large error bars in the WP, TWP, and TB results. This is because the biomass and TB were not supplied smoothly and irregularly during feeding. In addition, the char combustion length shows a relatively large error bar in PA and PB, indicating irregular combustion length owing to the fragmentation phenomenon, and the sample particle size rapidly decreases [9,28]. Therefore, we analyzed the reasons for these combustion characteristics through quantitative analysis.Fig. 7 Alternative fuel combustion image data processing of (a) ignition point and (b) char combustion length.

Fig. 7

3.2 Ignition characteristics

After quantitatively analyzing the previous ignition point and char combustion length, the corresponding ignition delay and char combustion times were analyzed using the calculations in Fig. 5. First, to derive a correlation with the internal moisture in the case of ignition time, the inherent moisture and ignition delay conducted in the proximate analysis were compared, as shown in Fig. 8(a). When the indirect moisture content is less than 4 %, the blast furnace and FINEX conditions showed a slight decreasing trend; however, in the case of TB with a high moisture content, the decreasing trend showed different characteristics. As shown in Fig. 8(b), a comparison of the volatile matter and ignition delay time shows a decreasing trend under the blast furnace condition; however, in the FINEX, the time is lower than 2 ms in all cases, which means that ignition occurs faster overall at high oxygen concentrations. This implies that volatile matter affects the ignition delay time.Fig. 8 Correlation between actual ignition delay time with (a) inherent moisture and (b) volatile matter.

Fig. 8

Based on previous results, moisture and volatile matter can influence ignition delay. To verify this, the theoretical ignition time specified in Eq. (1) [8], and the actual ignition delay time obtained from the experiment are shown in Fig. 9. In this graph, the closer actual ignition delay time is to the coincidence line, the more accurate are the theoretical values. All samples except one are similarly distributed along the coincidence line. However, the coke dust sample deviates from the corresponding line, which is believed to show a different tendency from existing fuels, as the combustibility was reduced because considerable carbonization occurred in the coke dust [7].Fig. 9 Comparison between theoretical and actual ignition delay time.

Fig. 9

Additionally, to understand the principle behind the ignition phenomenon in detail, TGA was used to measure the weight loss with temperature under combustion and pyrolysis conditions [23], and the results are shown in Fig. 10. As seen in Fig. 10(a), PA and PB exhibit similar trends under pyrolysis and combustion conditions, as they have similar properties according to the basic properties listed in Table 1. The ignition temperatures are 699.3 K for PA and 706.5 K for PB, with a difference of about 7 K, and coke dust ignites at 750.8 K. For coke dust, the initiation temperatures for pyrolysis and combustion occur at approximately 50 K higher, and the slope is gentler due to its lower reactivity. Additionally, all three samples, PA, PB, and coke dust, show minimal changes in the pyrolysis graph due to the low amount of volatile matter contained within. Fig. 10(b) shows the weight loss of biomass. For WP, pyrolysis begins at about 531.4 K, and ignition starts at 552.5 K for TWP. The devolatilization completes at 655 K and 670 K, respectively, after which char combustion occurs. Fig. 10(c) depicts the weight loss graph for the thermal coals, TA and TB. For TA, devolatilization occurs simultaneously, and combustion begins at 680 K. It was also observed that due to the high volatile matter content, the uniformity of pyrolysis is slightly lower compared to other samples. When comparing ignition temperatures, the order is WP < TB < TWP < TA < PA < PB < Coke dust, which is similar to the trend seen in ignition delay time. Although TB and TWP exhibit higher ignition delays than TA, the corresponding temperatures are opposite. This is likely due to the different heating rates between TGA and LFR. However, the overall trends are similar.Fig. 10 TGA analysis and ignition point temperature of the alternative fuels; (a) PA, PB and coke dust,(b) WP and TWP, (c) TA and TB.

Fig. 10

Based on the LFR images and TGA results, the ignition behaviour characteristics of each sample were determined. According to Ref. [23], the point where the weight loss of pyrolysis and combustion diverges is defined as the combustion initiation point. As shown in Fig. 10, WP, TA, and TB exhibit homogeneous ignition first because the points where the slopes of pyrolysis and combustion change correspond to the amount of volatile matter. After the volatile matter combustion is complete, heterogeneous combustion of the char occurs. In the case of TWP, simultaneous combustion of volatile matter and char results in a steeper slope. This indicates hetero-homogeneous ignition, where volatile matter combustion is followed by char combustion. For PA and PB, char combustion occurs before the volatile matter decreases, indicating that heterogeneous ignition predominantly occurs. In the flame images, fragmentation is observed for PA and PB. The summarized results are shown in Fig. 11. To summarize, for PCI coals PA and PB, fragmentation occurs as the coal heats up and de-moisturizes. Initially, a slight homogeneous ignition occurs, quickly followed by heterogeneous ignition. This steps could be defined similarly in Ref. [10]. For WP, homogeneous ignition occurs first as it heats up, followed by heterogeneous ignition of the char. TWP differs from WP in that hetero-homogeneous ignition occurs first, followed by char combustion through heterogeneous ignition. For TA and TB, homogeneous ignition occurs slightly later than for biomass but in the same order as WP. For coke dust, due to its low VM, only heterogeneous ignition occurs directly.Fig. 11 Ignition behaviour of alternative fuel samples.

Fig. 11

3.3 Char combustion characteristics

Parameters for applying the CPD model to the samples were derived and shown in Table 3. First, the CPD model parameters of the solid fuels were compared except coke dust. Biomass, WP and TWP show relatively high Mdel values, followed by thermal coal TA and TB, and PCI coal PA and PB show the lowest values. This can be determined from the fact that the higher the grade of fuel, the smaller the amount of side chains. And Mcl also showed the amount in the order of biomass, thermal coal, and PCI coal, which means that it is the same as the order of aromatic cluster amount. And in the case of p0, which is the fraction of bridge attachments, WP exists in the largest amount and shows similar amounts to PA and PB. TWP, TA, and TB showed the most similar results, but TA had the lowest value. In the case of σ+1, which represents the average number of attachments per cluster, WP shows the lowest value, and TWP occurs the next most frequently. The types of coal, PA, PB, TA, and TB, all show similar results. In particular, coke dust is located in the range between Mdel thermal coal and PCI coal. In particular, it was confirmed that the amount of Mcl, or aromatic cluster, occurred significantly. And it was derived that σ+1 also has a large number [[25], [26], [27]].Table 3 CPD parameter of alternative fuel.

Table 3Sample	Mdel	Mcl	p0	σ+1	
PCI coal	PA	12.83	238.25	0.83	4.25	
PB	13.17	254.07	0.82	4.55	
Biomass	WP	72.86	434.23	0.88	1.18	
TWP	55.74	430.22	0.59	3.66	
Thermal coal	TA	43.04	426.79	0.48	4.89	
TB	48.34	383.79	0.58	4.51	
Coke dust	31.45	1225.95	0.74	8.21	

To confirm the effect of char combustion time, unburned carbon analysis was performed using Eq. (2) of the ash tracer method on the samples generated in the char collector at the rear end of the LFR [24]. The results are shown in Fig. 12(a) and are as follows. PA and PB contain approximately 10 % unburned carbon under blast furnace conditions and approximately 4 % unburned carbon under FINEX conditions. With increase in oxygen, carbon activates the combustion reaction and combustibility increases. In the case of WP and TWP, there is almost no unburned carbon, and in the case of TWP, less than 2 % unburned carbon is present. In the cases of TA and TB, considerable unburned carbon remains than that of WP. However, combustion is completed in WP, TWP, TA, and TB under the FINEX conditions. As shown in Fig. 6, coke dust exhibits a flame length exceeding the measurable range, consistent with the unburned carbon results. Under blast furnace conditions, more than half of the carbon was not consumed, and the combustibility was judged to be significantly low. However, under FINEX conditions, approximately 14 % is similar to the results under the PA and PB blast furnace conditions, and this can be used to some extent in FINEX operations. Fig. 12(b) plots the char combustion time vs. unburned carbon. As the amount of unburned carbon increases, the char combustion time increases; that is, the flame length is a criterion for judging the combustibility of the samples.Fig. 12 (a) Unburned carbon analysis after LFR combustion and (b) correlation of unburned carbon and char combustion time.

Fig. 12

Fig. 13 is showing the correlation between the unburned carbon of char and the chemical structure of the sample in BF condition. Under FINEX conditions, it was difficult to confirm relative trends because all samples except coke dust burned within 5 %. As shown in Fig. 13(a), Mdel and Mcl show a linear trend with unburned carbon, which indicated that Mdel and Mcl increase, unburned carbon decreases. This shows that the combustibility increases in samples with larger amounts of the average molecular weight per side chain and the average molecular weight per aromatic cluster, which means that these two parameters have an effect on combustibility. And as can be seen in Fig. 13(b), which shows the remaining parameters, it was difficult to confirm any significant trends in the case of p0 and σ+1. Therefore, the fraction of bridge attachments and the average number of attachments per cluster indicate that they have relatively little effect on combustibility.Fig. 13 Correlation between CPD parameters of alternative fuel and Unburned Carbon in BF condition (a) Mdel and Mcl (b) p0 and σ+1 (except coke dust).

Fig. 13

3.4 SEM image and surface analysis

The samples that could be used under blast furnace and FINEX conditions were confirmed by analyzing the ignition delay time, char combustion time, and combustion characteristic scale, and the reason for the phenomenon was examined through SEM images of the char of the sample collected with a char collector [30]. Structural causes were analyzed in this study. In the FINEX conditions, almost no unburned carbon char was collected through the char collector, and as it existed in an ash state, there were limitations in extracting SEM images due to structural changes caused by collisions during transportation. Therefore, the char was collected only under blast furnace conditions, and SEM images were analyzed.

The SEM images of the samples before and after combustion in the blast furnace are shown in Fig. 14. The characteristics of each sample are as follows: PA and PB showed the same crystalline form as general fine coal and a hollow structure was formed after combustion [30]. Additionally, a shape in which one large particle was fragmented is observed. WP was torn and ground before combustion owing to its low grindability and all char was lost because of its high combustability, making it difficult to obtain SEM images. With TWP, each particle is finely divided owing to its relatively high grindability through torrefaction, and the original shape is maintained after combustion; however, a slight porosity is confirmed which is similar structure of biochar [31]. In the case of thermal coal, the hollow structure occurred actively due to the combustion of volatile substances in a circular body before and after combustion, and the specific surface area increases accordingly [30]. The coke dust is finely divided into a thin plate-like shape. After combustion, few hollows are formed owing to the low volatile content, and their shape did not change significantly [32].Fig. 14 SEM images of pulverized alternative fuels before and after LFR combustion in blast furnace condition.

Fig. 14

The BET was used to determine surface area changes due to structural changes before and after combustion [29]. The resulting BET surface area is shown in Table 4, and the results are as follows. In the case of PA and PB, it is about 1.52 and 1,78 m2/g, and after combustion, it increased about 6–7 times to about 9.24 and 9.29 m2/g. In the case of WP, as previously explained, UBC was almost lost, making analysis difficult, and in the case of TWP, it increased from 1.45 m2/g to 38.75 m2/g. This increased by about 25 times, which can explain why the UBC results in Fig. 12(a) show relatively lower values than PA PB [33]. TA and TB were 30.24 and 27.18 m2/g, respectively, compared to 1.2 and 3.53 m2/g before combustion. The before combustion surface area of TB has a relatively high value. Due to the high moisture content, complete drying occurred during BET analysis, and the specific surface area increased due to the space where the moisture inside was evaporated. TA and TB are estimated to have a high porous form due to the occurrence of many internal pores, which increases the surface area [34]. In the case of coke dust, since it was a sample that had already been carbonized once, there was no significant difference as shown in the SEM image results [32].Table 4 BET surface area changes of alternative fuels.

Table 4BET surface area(m2/g)	Before Combustion	After Combustion	
PCI coal	PA	1.52	9.24	
PB	1.78	9.29	
Biomass	WP	0.8	N.D.	
TWP	1.54	38.75	
Thermal coal	TA	1.2	30.24	
TB	3.53	27.18	
Coke dust	0.92	2.67	

3.5 Maximum amount of alternative fuel co-firing

When co-firing to review the PCI applicability of the fuel, it is also important to calculate the limit co-firing amount of the alternative fuel. Many other requirements are required to apply real fuel to blast furnaces and FINEX, but the most important requirements are the ash content in the raceway heating supply (RHS) and slag formation. However, in the case of overall alternative fuel, it has ash similar to the corresponding fuel or has a smaller amount of ash, so it can be used sufficiently in terms of ash content. To calculate the raceway heating supply, the calorific value of the fuel is required, and the conversion rate is calculated through the UBC analyzed in this study eq. (3). Here, in order to calculate the maximum co-firing rate (x), it is necessary to calculate the standard ratio of PCI coal of RHS (RHSPCI) and alternative fuel RHS under Blast furnace and FINEX (RHSBF or FINEX) conditions as RHSstd, as shown in eq. (4).(3) RHS(RacewayHeatingSupply,Mj/Kg)=HHV(calorificvalue,Mj/kg)×Conversionrate(1−UBC,%)

(4) (1−x)RHSPCI+xRHSBForFINEX=RHSstd

To obtain RHSstd, it was calculated using the PCI coal calorific value standard of 31.2Mj/kg in Ref. [35]. The maximum co-firing rate of each sample is shown in Table 5 and the results are as follows. When WP is co-fired with PA, co-firing is possible by about 5–6 %, either under blast furnace conditions or FINEX conditions. TWP is capable of co-firing at about 7 %, and TA is judged to be sufficiently useable as an alternative fuel in blast furnaces. However, under FINEX conditions, it can be used by mixing about 55 %. And in the case of TB, blending can be done at about 10–11 %, and coke dust can be used at about 3 % in Blast furnace and up to 9 % in FINEX due to its low combustibility. The maximum co-firing rate with PB coal shows similar results to PA. However, due to its relatively lower calorific value than PA, the overall co-firing rate decreased, but it was confirmed that TA can still be fully utilized as a substitute.Table 5 Calculated maximum cofiring rate of alternative fuels.

Table 5Maximum Co-firing Rate (%)	Blending with PA	Blending with PB	
BF	FINEX	BF	FINEX	
Biomass	WP	5.46	5.67	3.82	3.31	
TWP	7.42	7.45	5.22	4.39	
Thermal coal	TA	100	54.77	100	40.83	
TB	10.09	10.50	7.16	6.27	
Coke dust	2.88	8.79	2.00	5.20	

4 Conclusion and summary

This study analyzed the flame shape of each sample using an LFR in a blast furnace and FINEX to determine the suitability of alternative fuels in the PCI raceway. To qualitatively and quantitatively confirm the flame shape, the ignition delay, volatile matter combustion, and char combustion of the solid fuel flame were visually analyzed using a general digital camera and an CCD camera. The ignition point and char combustion length were analyzed through image data processing. Based on these results, the ignition delay and char combustion times were calculated, and the factors influencing the characteristics were determined by comparing the calculated combustion characteristics with the theoretical ignition delay and unburned carbon. In addition, any changes in shape after combustion were investigated microscopically by SEM imaging of the unburned char extracted with a char collector. The main results are as follows.(1) Visual analysis of the imaged flame of the solid fuel showed fragmentation in the case of PCI coal owing to the high-temperature gradient. In particular, combustion and fragmentation occurred more actively when the oxygen concentration increased under FINEX conditions than in blast furnace conditions. For biomass, TA, and TB, a volatile matter combustion cloud occurred because of the high volatile matter content, and depended on the amount. With coke dust, the char combustion length was long and could not be measured under blast furnace conditions. Unlike other flames, FINEX also showed the highest char combustion phenomenon.

(2) In the case of the ignition point and ignition delay time, PCI coal with a low volatile content, TWP from which the volatile content was extracted, and TB with a high moisture content showed a high ignition delay. This decreased under FINEX conditions, but the overall trends were similar. Additionally, homogeneous ignition is dominant in the case of WP, TA, and TB, heterogenous ignition is dominant in the case of PA, PB, and Coke dust, and hetero-homogenous ignition occurs in the case of TWB. In the case of char combustion length and time, highly reactive biomass showed the lowest values, followed by thermal coal < PCI coal < coke dust. In this study the ignition delay has a complex effect on the amount of moisture and volatile matter and char combustion also influence unburned carbon. The combustibility increases in samples with larger amounts of the average molecular weight per side chain and the average molecular weight per aromatic cluster, which means that these two parameters have an effect on combustibility.

(3) The structural changes after combustion were examined using SEM image analysis of the samples before and after combustion. In the case of PCI and thermal coal, hollow structures, and fragmentation were qualitatively confirmed in large quantities, and structural changes were observed in TWP. In addition, several small hollow structures were observed. Moreover, the coke dust had no significant structural changes before and after combustion and had a sharp, flat shape.

Consequently, the maximum co-firing amount of alternative fuel was calculated using LFR experiments. Biomass can be used by blending about 4–7 %. In the case of thermal coal, TA can be replaced with PCI coal in blast furnaces due to its high calorific value and combustibility. However, in FINEX, about half can be mixed and used. And coke dust can be utilized within 10 % due to its high combustibility.

CRediT authorship contribution statement

Dae-Gyun Lee: Conceptualization, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing – original draft, Writing – review & editing. Min-Woo Kim: Data curation, Investigation, Visualization. Min-Jong Ku: Formal analysis, Investigation, Visualization. Yoon-Ho Bae: Methodology, Validation. Kang-Min Kim: Methodology, Resources. Gyeong-Min Kim: Data curation, Investigation. Chung-Hwan Jeon: Project administration, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This research was supported by the 10.13039/501100003565 Ministry of Land, Infrastructure and Transport of the Korean government (grant number 21PCHG-C163217-01 ) and the “National Research Foundation of Korea” funded by the 10.13039/501100014188 Ministry of Science, ICT, and Future Planning (grant number 2022K1A4A8A01080312 ).
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