Scholarly article on topic 'Experimental investigation of ignition behavior for coal rank using a flat flame burner at a high heating rate'

Experimental investigation of ignition behavior for coal rank using a flat flame burner at a high heating rate Academic research paper on "Nano-technology"

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Abstract of research paper on Nano-technology, author of scientific article — Ryang-Gyoon Kim, Dongfang Li, Chung-Hwan Jeon

Abstract The ignition behavior of pulverized coal particles was investigated as a function of different ranks and sizes using a flat flame burner at high heating rate conditions (>105 K/s). A high-speed camera was used to image the ignition process. Five coal types (anthracite, medium-volatile bituminous, high-volatile bituminous, subbituminous, and lignite coals) with particle sizes 150–200, 75–90, and <45μm were tested. The released volatile matter of medium- and high-volatile bituminous coal in the size ranges 150–200 and 75–90μm underwent homogeneous ignition. When the particle size is <45μm, high-volatile bituminous coal underwent homogeneous ignition, while medium-volatile bituminous coal underwent heterogeneous ignition. For particle sizes in the range 150–200μm, anthracite coal exhibited homogeneous ignition after primary fragmentation, whereas lignite coal underwent direct fragmentation and homogeneous ignition prior to ignition without primary fragmentation.

Academic research paper on topic "Experimental investigation of ignition behavior for coal rank using a flat flame burner at a high heating rate"

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Experimental Thermal and tei1* Fluid Science

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Experimental Thermal and Fluid Science

journal homepage: www.elsevier.com/locate/etfs

Experimental investigation of ignition behavior for coal rank using a flat flame burner at a high heating rate

Ryang-Gyoon Kim1, Dongfang Li1, Chung-Hwan Jeon *

School of Mechanical Engineering, Pusan Clean Coal Center, Pusan National University, Republic of Korea

ARTICLE INFO ABSTRACT

The ignition behavior of pulverized coal particles was investigated as a function of different ranks and sizes using a flat flame burner at high heating rate conditions (>105 K/s). A high-speed camera was used to image the ignition process. Five coal types (anthracite, medium-volatile bituminous, high-volatile bituminous, subbituminous, and lignite coals) with particle sizes 150-200, 75-90, and <45 im were tested. The released volatile matter of medium- and high-volatile bituminous coal in the size ranges 150-200 and 75-90 im underwent homogeneous ignition. When the particle size is <45 im, high-volatile bituminous coal underwent homogeneous ignition, while medium-volatile bituminous coal underwent heterogeneous ignition. For particle sizes in the range 150-200 im, anthracite coal exhibited homogeneous ignition after primary fragmentation, whereas lignite coal underwent direct fragmentation and homogeneous ignition prior to ignition without primary fragmentation.

Crown Copyright © 2014 Published by Elsevier Inc. All rights reserved.

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Article history:

Received 5 September 2013

Received in revised form 23 November 2013

Accepted 21 December 2013

Available online 8 January 2014

Keywords:

Pulverized coal particle High heating rate Ignition Fragmentation Coal rank Particle size

1. Introduction

Worldwide, coal continues to be one of the most widely used primary fuel sources owing to its abundance, low cost, and easy availability with respect to safety and stability. Applications utilizing coal have been under investigation for centuries; numerous types of coal technologies have been developed to produce electricity energy using a pulverized coal boiler and iron using a blast furnace. The pulverized coal boiler technology remains the most widely used to date and is expected to be the dominant method for coal combustion over the next two or three decades, especially in the field of power production. The blast furnace process is still the most widely used technology to produce iron. Its major advantages are a very high production rate and a high degree of heat utilization [1]. However, the pulverized coal boiler and blast furnace use different coal ranks. In a pulverized coal boiler, middle-rank coal is used, with a heating value of approximately 6080 kcal/kg. Pulverized coal injection (PCI) in the case of a blast furnace requires high-rank coal, whose heating value is above 7500 kcal/kg. Most power plants and iron making companies in South Korea produce power and iron using poor-quality imported non-designed coal, because of the recent increase in the price of high-rank coal.

Abbreviations: PC, pulverized coal; VM, volatile matter. * Corresponding author. Tel.: +82 51 510 3051; fax: +82 51 582 9818.

E-mail address: chjeon@pusan.ac.kr (C.-H. Jeon).

1 These authors contributed equally to this work.

However, combustion problems occur when non-designed coal is used in a pulverized coal boiler or a blast furnace, owing to its fuel characteristics that make it difficult to obtain the optimal operating conditions. Moreover, the problem of burner damage must be considered, which occurs because of the ignition mechanisms involved in high-volatile low-rank coal. Therefore, to avoid burner damage, the ignition behavior as a function of coal rank should be investigated as fundamental data in order to take advantage of the new operating procedure of the burner.

Most practical systems deal with combustion under turbulent conditions, in which the heat release is much faster than that under laminar conditions. At very high Reynolds numbers, the flow becomes turbulent. In a turbulent flow, eddies move randomly back and forth and across the adjacent fluid layers. The flow no longer remains smooth and orderly. Even chemically non-reacting turbulent flows are highly challenging because of the above mentioned characteristics. The problems become even more complex when chemical reactions occur, since the turbulent fluid flow is further coupled with chemical kinetics, and quite often, with phase changes. For laminar flows, however, the adjacent layers of fluid slide past one another in a smooth, orderly manner. The velocity, temperature, and concentration profiles measured in a laminar flow will be quite smooth [2]. Therefore, many investigators have preferentially studied the coal combustion and ignition behaviors under laminar flow conditions before studying these under turbulent conditions, in order to solve the chemically reacting turbulent

0894-1777/$ - see front matter Crown Copyright © 2014 Published by Elsevier Inc. All rights reserved. http://dx.doi.Org/10.1016/j.expthermflusci.2013.12.017

flow. The mechanism of coal combustion on laminar flow condition has been elucidated by previous investigations. The combustion of most solid fuels involves two major steps: (i) devolatilization that occurs during the initial heating; (ii) subsequent combustion of the released volatile matter and porous solid residue (char) of the first step. Ignition occurs between these two steps. Many previous studies [3] have investigated the ignition process of pulverized coal (PC) particles, which may occur by either homogeneous ignition of the released volatile matter or heterogeneous ignition on the char surface depending on the coal rank [4-9], particle size [10], coal-feeding rate [11], heating rate [9], and ambient gas conditions, i.e., gas composition [6,7,12,13], oxygen concentration [6], and temperature [11].

Ignition behaviors based on the particle size and coal type under low heating-rate conditions have been studied By Chen et al. [9], who examined the ignition of anthracite, bituminous coal, and lignite by thermogravimetry (TG) and differential thermal (DT) analyses, with PC sizes ranging from 37 to 4000 im. They proposed that with an increase in the coal quality (from lignite to bituminous to anthracite), the ignition type changed from homogeneous to hetero-homogeneous to heterogeneous, respectively. Moreover, the ignition behavior of bituminous coal changed from a hetero-homogeneous to homogeneous process with increasing coal particle sizes, whereas in anthracite and lignite coal, the behavior did not change with change in the range of the particle sizes examined. However, Chen et al. [9] conducted their study under low heating-rate conditions(<101 K/s). Therefore, the investigation of reactors with heating rates similar to that of PC boilers is required. McLean et al. [4] used a flat flame burner designed for high heating rates (>105K/s) and a shadowgraph to observe the condensed matter surrounding the coal particles with a mean mass size of 65 im corresponding to the ignition of bituminous coal—suggesting that homogeneous ignition initially occurs. The shadowgraph technique offers the advantage of detecting the soot cloud. Molina and Shaddix [13] used CCD (Charged-Coupled Device) camera images with LFR to capture excited CH* emissions from the coal flame to determine the ignition point. These results showed a significant ignition delay in char compared to coal particle ignition, using coal from the same seam and under similar experimental conditions as the McLean study [4]. Shaddix and Molina [12] also used a CCD camera with a very short exposure time (20 is) to investigate the ignition and devolatilization behavior of single particles of bituminous and subbituminous coals by changing the ambient gas conditions. The results showed that highly volatile bituminous coal ignited with a high-temperature soot cloud; in contrast, this cloud was not obvious during the ignition of subbituminous coal. All the aforementioned studies describe how coal rank and particle sizes affect PC particle ignition at various conditions; however, the ignition phenomenon of PC particles, in a wide size range, at very high heating rates (>105 K/s) as a function of coal rank is yet to be thoroughly investigated.

Herein, we report our investigations on the ignition behavior of PC particles of different ranks and sizes using a high-speed camera at high heating-rate conditions (>105 K/s) in a flat flame burner which is developed for setting up a similar heating rate of large industrial scale. Five types of coal were examined, namely anthracite, medium-volatile bituminous, high-volatile bituminous, subbituminous, and lignite coals.

2. Experimental method

Five coal types of different ranks were investigated in this

study; Table 1 shows their properties. Coal rank was defined

using the ASTM method. Low-rank coals, which are lignite,

subbituminous coals, and high-volatile bituminous coals, are classified according to the heating value and not the volatile content. Further, the basis of the volatiles content when comparing high rank coals which are medium volatile, low volatile bituminous coals and anthracite are compared on a dry ash-free basis (Table 1). Experiments were carried out in an entrained-type flat flame burner at Pusan Clean Coal Center, South Korea. A schematic of this system is shown in Fig. 1. The burner was designed with a honeycomb structure and a circular cross section in order to ensure symmetry in temperature and gas-composition conditions along the horizontal plane. A quartz tube with a rectangular cross section was set above the burner to isolate the inner reacting zone and decrease the heat loss. A non-premixed flat flame was created using CO and H2 as a fuel and O2 as an oxidizer to provide a high-temperature condition above the burner. N2 was also used to control the temperature, oxygen concentration, and velocity of post flame gas flows. The PC particle inlet was designed at the center of the burner using N2 as a carrier gas. The calculated composition of the post flame gas is shown in Table 2. The gas-temperature profile along the burner centerline was measured with a 125 im type R thermocouple and corrected for radiation losses. The corrected gas temperature as a function of the height above the burner is shown in Fig. 2. In a blast furnace, pulverized coal particles are injected into the hot blast from a lance with a small diameter. The temperature and oxidant gas of the hot blast are 1300-1500 K and air, respectively [14]. In a pulverized coal boiler, the coal particle that passes through the burner nozzle encounters a 1700 K flame zone [15]. Therefore, the experimental conditions were chosen as 21.6 mol% O2 and 1600 K.

PC particles were fed into the burner at a feeding rate of <1 mg/ min to ensure a single-particle feeding condition and to guarantee that the reacting zone conditions would not be affected by the heat of combustion of the PC particles. The ignition behavior of the PC particles was captured via a Photron FASTCAM SA4 high-speed camera. Because the ignition process occurs at the speed of 101 ms (including gas-phase and heterogeneous reactions under high heating-rate conditions), a frame rate of 10,000 frames/s was selected. In order to capture the PC particles before ignition when no light is released, a backlight using a Photron HVC-UL device was oriented toward the high-speed camera.

3. Results and discussion

3.1. Effect of coal rank: homogeneous and heterogeneous ignition behavior

The effect of coal rank for particles in the size range 150-200 im are shown in Fig. 3. Coal particles are injected into the burner, and they are preheated by the flame in the downstream direction. Then, volatile matter is evolved, and ignition occurs on the particle surface or the gas phase. After almost all the volatile matter is evolved, char combustion becomes dominant. Finally, ash is produced after the char combustion is complete. The mass of the coal particle decreases with an increase in the residence time during the combustion progress. As mentioned in the Introduction section, PC particle ignition can occur via either gas-phase combustion of the released VM (homogenous ignition) or heterogeneous combustion of the particle surface (heterogeneous ignition). Bituminous (coals A, D) and subbituminous coals (Adaro coal) showed a typical combustion behavior as seen in Fig. 3. As the coal particle heated up after injection into the burner, homogenous ignition occurred after the gas-phase VM was released from the particle. As more gas-phase VM was released, the corresponding combustion flame became bigger and surrounded the particle. The solid coal particles remained black prior to the extinction of the

Table 1

Proximate and ultimate analysis of different coals.

Coal Rank FX Semi-anthracite A Medium-volatile bituminous D High-volatile bituminous Adaro Subbituminous Kideco Lignite

Proximate analysis (as received) (%) Moisture 1.2 1 2.5 5.2 32.2

Volatile matter 12.4 23 34.7 50.7 40.8

Fixed carbon 78.6 66.9 54.4 39.9 24.7

Ash 7.8 9.1 8.4 4.2 2.3

Ultimate analysis (dry ash-free basis) (%) C 92.3 90.5 84 70.7 67.8

H 4.5 5.1 5.7 4.9 6.1

O 0.9 2.8 7.9 23.4 24.3

N 1.8 1.2 1.8 0.9 1.1

S 0.5 0.4 0.6 0.1 0.7

Heating value (dry ash-free basis) (kcal/kg) 8109 7379 6914 6013 4200

Feeder

Fig. 1. Schematic of flat flame burner and high-speed camera imaging system.

Table 2

Calculated compositions of post flame gases (mol%).

CO2 H2O O2 N2

20.9% 8.4% 21.6% 49.1%

gas-phase volatile combustion; however, the particles became luminous at the end of that the combustion process, indicating the initiation of heterogeneous ignition. Thus, bituminous and subbituminous coals could undergo both homogeneous and heterogeneous ignition processes.

A comparison of bituminous (coal A, D) and subbituminous (Adaro) coal samples revealed different volatile combustion behaviors. In bituminous coal, prior to the extinction of the gas-phase volatile combustion, a condensed tail with its direction oriented along the gas flow streamline formed above the particles (Fig. 3

(b) images 5, 6, (c) images 5, 6), assumed to be tar combustion [4,16]. Tar combustion continued even after the occurrence heterogeneous ignition. This result is in agreement with the schematic representation of the formation of soot tails as proposed by Fletcher [17]. However, in subbituminous (Adaro) coal, a long tar combustion tail was not observed during volatile combustion. Previous studies [12,18-21] have shown that most subbituminous coals have a relatively low content of tar during the devolatiliza-tion process, while bituminous coals have a larger tar content.

3.2. Effect of coal rank: fragmentation ignition behavior

Anthracite (FX) and lignite (Kideco) coals exhibited an entirely different phenomenon compared to that seen in bituminous and subbituminous coal samples (Fig. 3). Prior to ignition, most of the coal particles underwent the fragmentation. At the start of the ignition, the particle fragmented into two or more pieces - a process

Height (mm)

Fig. 2. Temperature distribution in vertical direction above burner along centerline.

defined as primary fragmentation. Some of the primary fragments separated from each other at very high velocities (~4 m/s), while others rotated and accelerated owing to the momentum changes caused by separation during primary fragmentation (Fig. 4). VM was released and ignited homogeneously after primary fragmentation. Homogeneous ignition occurred in a manner similar to that of bituminous and subbituminous coals, although its diffusion flame was much smaller compared to that of bituminous and subbitumi-nous coals because of the latter's lower VM content. Most particles of lignite coal underwent fragmentation directly prior to ignition without undergoing primary fragmentation; this coal fragmentation was shorter than that for anthracite coal.

3.3. Effects of particle size

In order to show the effect of the particle size, medium-volatile (A) and high-volatile (D) bituminous coals were selected (Fig. 5). Microscopic images of the particles showed that both

(a) Anthracite coal (FX)

(1) (2) (3) (4) (5)

(b) Medium-volatile bituminous coal (A)

(1) (2) (3) ■ (4) n (5) (6) (7)

(c) High-volatile bituminous coal (D)

(1) (2) (3) (4) (5) (6) (7)

(d) High-volatile subbituminous coal (Adaro)

(1) (2) (3) (4) (5) (6) (7)

(e) Lignite coal (Kideco)

(1) (2) (3) (4)

._ ___

Fig. 3. Ignition behavior as a function of coal rank with particles sizes 150-200 im.

(a) 150-200 pm

Coal A i i

Coal D

(b) 75-120 pm

Coal A •

Coal D $ 0 n ^J *

(c) <45 pm

Coal A

Coal D 1 • 1 -

Fig. 5. Ignition behavior of bituminous coal as a function of particle size using medium-volatile bituminous coal (Coal A) and high-volatile bituminous coal (Coal D).

Coal rank

Anthracite (FX)

MVMB (А)

HVMB (D)

Subbituminous (Adaro)

Lignite (Kideco)

< 45 150-200

45-200

150-200

150-200 Size[pm]

Ignition behavior

Heated-up particle oQ pr|mary fragmentation

Homogeneous ignition

Heterogeneous ignition f'

Fig. 6. Ignition behavior as a function of coal rank and size.

Fragmented homogeneous ignition Tar tail combustion

medium- and high-volatile bituminous coal particles with size 150-200 im exhibited similar phenomena, with homogeneous and heterogeneous ignition occurring in sequence (Fig. 5a). For the medium- and high-volatile bituminous coal particles 75-90 im in size, delay times for both homogeneous and heterogeneous ignition were shorter than that exhibited by 150-200 im sized particles (Fig. 5b). However, for particles <45 im, no volatile combustion flame was detected for medium-volatile bituminous coal prior to heterogeneous ignition. Initially, the solid particle was partially ignited, exhibiting a dark red color, which subsequently spread over the entire surface and made it luminous (Fig. 5c). These observations indicate that medium-volatile bituminous coal particles <45 im are directly ignited in a heterogeneous manner without homogeneous ignition; however, in comparison, high-volatile bituminous samples exhibited distinctly different phenomena. For particles <45 im, significant volatile combustion was observed prior to heterogeneous ignition, with homogeneous ignition occurring as the initial ignition event. These results can be attributed to the fact that the high-volatile bituminous coal (D) has a more volatile content comparing with the medium-volatile bituminous coal (A).

Finally, through careful observation of the ignition behavior, a summary of the ignition behavior as a function of coal rank and size can be proposed, as illustrated in Fig. 6. As mentioned before, the ignition behavior is described through homogeneous/heterogeneous ignition and primary fragmentation/fragmented homogeneous ignition based on the effect of coal rank and size.

4. Conclusion

This study investigates the ignition behavior of five coals varying in ranks and sizes under high heating-rate conditions. Bituminous (A, D) and subbituminous (Adaro) coals with particle sizes 150-200 im underwent homogeneous ignition of the

released VM and heterogeneous ignition of char particles. Anthracite coal (FX) underwent homogeneous ignition after primary fragmentation, whereas lignite coal (Kideco) simultaneously underwent direct fragmentation and homogeneous ignition prior to ignition, but not primary fragmentation. Both medium- and high-volatile bituminous coal samples (coals A, D, respectively) with particle sizes 150-200 and 75-90 im exhibited similar phenomena, with homogeneous and heterogeneous ignition occurring in sequence. For particles <45 im, high-volatile bituminous samples underwent homogeneous ignition, while medium-volatile bituminous coal exhibited heterogeneous ignition owing to a reduced volatile content.

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