Study on the combustion behaviours of two high-volatile coal particle streams with high-speed OH-PLIF

Fuel 265 (2020) 116956

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Study on the combustion behaviours of two high-volatile coal particle streams with high-speed OH-PLIF ⁎

T

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Wenkun Zhua, Xiaohui Lib, Jiangbo Pengb, , Zhuozhi Wangc, Rui Suna, , Lei Zhanga, Xin Yub, Yang Yub a

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150001, China c School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300401, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Devolatilization Volatiles combustion Particle stream OH-PLIF diagnose

An optical entrained flow reactor, based on a downward flat-flame, was established to explore the devolatilization and volatile combustion behaviours of coal particle streams with surrounding temperatures from 1600 K to 1800 K and oxygen mole fractions in the range of 10%–30%. The coal particle streams with particle sizes of 53–80 μm are two kinds of high-volatile pulverized bituminous coal and lignite. Using High-speed Planar LaserInduced Fluorescence to diagnose OH (OH-PLIF), the planar sheet-imaging of OH-radicals was obtained to visualize the coal particle flames, and then, the normalized fluorescence signal intensity along the distance downstream from the burner and along the radial distance was obtained to investigate the volatiles combustion process. From the planar sheet-imaging, the ignition of tested high-volatile bituminous coal and lignite is dominated by the combustion of their volatiles. The combustion processes of volatiles can be divided into 4 stages: ignition, accelerating burning, stable burning and feeble burning, to investigate the intrinsic combustion correlation phenomena. The relative standard deviation (RSD) changes of the OH radial profiles can provide a new insight into flame stabilization, wherein the flame stability of the bituminous coal particle stream is higher than that of lignite under the same combustion conditions. The volatiles combustion of lignite is more sensitive to the oxygen concentration and temperature than that of bituminous coal, and the volatile releasing ratio of bituminous coal (65%–81%) was substantially less than that of lignite (92%–100%) by the analysis of the char residues.

1. Introduction

significance for energy conservation and utilization. Combustion cannot proceed without steady and continuous ignition. The ignition and combustion characteristics of PF, which directly affect the safety, stabilization and efficiency of boilers, play an important role in the clean and efficient utilization of coal [2–4]. PF combustion is an extremely complex gas-solid two-phase combustion, wherein it contains two processes: the homogeneous gas reaction and the heterogeneous char solid surface reaction. Researchers have paid great attention to whether the PF ignites first in the gas phase or on the solid phase surface of the monodispersed particles [5–9] or particle streams/clouds [10–16]. The ignition mode depends on competition between the heating rate of the particle surface and the release rate of the volatiles under certain conditions. Previous studies on the ignition mode have been extensively reported, where the homogeneous ignition was usually related to large particles with low heating rate (less than 100 K/s), while heterogeneous ignition occurred for small particles (no

Coal has been, and will continue to be, one of the world’s most important sources of energy, especially for electricity power generation, because of its abundant reserves and low prices. The share of coal in the world’s energy structure was 27.6% in 2017, with a corresponding 23.4% for natural gas and 34.2% for oil. Simultaneously, an amount of coal equivalent to 1.89 billion tons of oil, which accounted for 50.7% of the world’s coal consumption and 60.4% of China's energy consumption, was produced in China, which is much higher than the average worldwide level [1]. Lignite and bituminous coal have abundant reserves in China, especially in the north, and are chiefly yielded in power generation and industrial boiler combustion. Pulverized fuel (PF) combustion technology is the most widely used coal combustion technology in China and will still remain dominant for the next 20–30 years. Therefore, the study of the PF combustion mechanism is of great

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Corresponding authors. E-mail addresses: [email protected] (J. Peng), [email protected] (R. Sun).

https://doi.org/10.1016/j.fuel.2019.116956 Received 7 October 2019; Received in revised form 21 December 2019; Accepted 25 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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flame burner. The standoff distance from the coal flame to the burner was used as a metric of ignition delay and the variation of the flame location in time was as an indication of flame stability. The experiments show that ignition delay decreased and the flame stability increased, as oxygen concentration increased. In our experiment, the variation of OH signals with time was used to study the flame stability, because the OH signal directly reflected the volatiles burning intensity. In general, the formation mechanism of the electronically excited species OH, CH and C2, which are the main intermediate reactive substance of the gas phase flame contributing to flame luminescence, is as follows [28–29]:

more than 100 μm) with high heating rate [17]. Considering the surface oxidation reaction of coal char, the homogeneous ignition mode was further developed by correcting the prediction of a low ignition temperature in the original homogeneous model in low ambient temperature [18]. Afterwards, in some specific cases, the PF mixture ignited first in the surface of char, and then, researchers proposed a mode of heterogeneous ignition: the small surface flux of volatiles would cause simultaneous homogeneous and heterogeneous combustion on the surface of particles. Conversely, a large surface flux forced the reaction away from the coal surface [19]. In addition, Annamalai provided a transient-state ignition theory that could explain the influence of the ambient temperature, particle size, volatile content and kinetic parameters on the ignition, but only in the spherical particle [5]. Ignition of the particle stream below diameters of 15 μ m was mostly heterogeneous ignition in the experimental verification, which has more advantages in explaining the influence of many factors on ignition. For example, some experimental results have shown that non-homogeneous ignition occurs in anthracite, bituminous coal and derived fuels, based on the Semenov's thermal ignition theory [8,20]. These results and theories further illustrate the complexity and variability of coal combustion. In short, there is still no adequate theory to fully explain the ignition and combustion behaviours of the PF stream. Optical apparatuses have been introduced to study combustion reaction kinetics with the development of optical technology [21–28], such as multi-colour pyrometers, laser-induced breakdown spectroscopy, high-speed cameras, planar laser-induced fluorescence (PLIF). Levendis [6,7] studied the process of ignition and combustion of a single particle in an drop-tube furnace, and gave particle temperature variation by a three-colour pyrometer. The results show that high-rank bituminous coal particles with particle sizes of 75–90 μm underwent homogeneous ignition in the environment of O2/N2. However, when low-rank lignite particles fragment before ignition, they were more likely to ignite heterogeneously, whereas when they fragment after ignition, there was evidence that at times, their ignition took place in gas phase. Seung and Hayashi et al. [21–22] built an optical entrained flow reactor using a pyrometer, where the volatile combustion was obtained by OH-PLIF. The results show that combustion reaction only occurred in the upstream edge of the PF stream, and then could be observed in the downstream inside and edge of the stream. Balusamy et al. [23] performed another study on PF burners using methane combustion as the blending heat source, and OH-PLIF technology was used to monitor the combustion reaction zone. The increasing feeding rate would accelerate the combustion reaction and shorten the length of the jet flame to a certain extent, while soot might be formed in discrete regions in the wake of burning coal particles. Koser et al. [24] continued to explore the PF combustion in a premixed flat flame burner, where the OH transient sheet-imaging of a single particle was captured by 10 kHz OH-PLIF technology. Volatile combustion was fiercely influenced by the O2 mole fraction, which provided the experimental images of the homogeneous ignition for single particle of bituminous coal. Compared to traditional spontaneous emission images, sheetimaging showed the starting point of ignition to be determined and the internal structures of the flames surrounding the coal particles to be resolved. However, the combustion variation of volatiles and flame stability for group particles is still unclear in the high space-time resolution. The ignition characteristics of group particles are closely related to flame stability, while isolated particles and interactive particles agglomerate in turn to affect the ignition. The flame stability is dependent on the oxygen concentration in the gas stream from burners, which is to introduce sufficient oxygen into the PF flame in order to ensure satisfactory ignition and flame stability [25]. Kiga [26] investigated the ignition and flame stability of pulverised coal by measuring the flame propagation speed, where the flame propagation speed increased as the oxygen concentration increases in all cases. Molina et al. [27] studied ignition of particle group of a high-volatile bituminous coal in the flat

CH + O2 = CO + OH

(1)

C2 + OH = CO + CH

(2)

C2H + O = CO + CH

(3)

C + CH = C2 + H

(4)

In particular, the OH density profiles were the same as those of CH ad C2 across the reaction region where the contents of CH and C2 were high, while OH radiation decaying rate was abruptly slow in the downstream of the reaction zone where the decreased decaying rate did not occur until the CH and C2 emissions decreased to 0.5% of their peak values [30]. According to previous spectrophotometric results [28–30], the emissions of OH, CH and C2 closely coincide with the reaction region, which has a good correlation in the main reaction region as a tracer of the volatile gases flame. Therefore, OH can be selected to explore the combustion behaviour of the volatiles. The objective of this work is to evaluate the effect of ambient O2 mole fraction (10%–30%) and temperature from 1600 K to 1800 K on volatiles combustion of two high-volatile coals (one bituminous coal, one lignite coal). The OH-PLIF technique is employed to study the volatile combustion in an optical laminar premixed entrained flow reactor with a transparent quartz chamber. The relative standard deviation (RSD) changes of the OH radial profiles can provide a new insight into flame stabilization. The char burnout ratio was also analysed by collecting char residues with a probe at the reactor bottom, and then, the pore structures were measured and compared for bituminous coal and lignite. 2. Experimental set-up 2.1. Material preparation Bituminous coal and lignite are widely used as a major energy source in China and are employed as samples for this investigation. Two typical Chinese coals of different rank were tested: a high-volatile bituminous coal from Shanxi (SH), and an even more high-volatile lignite coal from the east area of inner Mongolia (MD). The coals were ground and sieved to a particle size cut of 53–80 µm. To reduce the agglomeration of PF particles, the moisture in samples was removed with a dry oven (105 °C, 150 min in air) before the experiments. Thus, the composition analysis was given on the dry basis. The proximate analyses referred to the Chinese National Standard GB/T 212-2008, and the ultimate analyses referred to the Chinese National Standard GB/T 30733-2014 (C, H and N), GB/T 214-2007 (S), and GB/T 3558-1996 (Cl). The physical and chemical analyses of the tested coal samples are shown in Table 1. The coal char collected in the experiments was also analysed according to Chinese National standard GB/T 212-2008. The entire pore structure of the sampled char was determined by the Mercury Porosimetry (Autopore II 9220, Micrometritics). The repeatability of the analysis results was verified with parallel experiments. 2.2. Combustion setup The experiments were performed in an optical entrained flow 2

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Table 1 Ultimate analysis and proximate analysis of the tested coal. Ultimate analysis (wt.%, dry ash free basis) C H N SH 80.19 4.46 2.01 MD 55.57 5.38 0.96 Proximate analysis (wt.%, dry basis) Ash Volatile SH 2.94 31.42 MD 13.86 44.34

Table 2 Operational conditions. Exhaust conditions

S 0.22 0.47

Fixed Carbon 65.64 41.80

Cl (mg/g, dry basis) SH 0.91 MD 0.68

* Determined by difference.

burner based on a downward flat-flame burner. The burner, namely, a laminar premixed flow reactor, was designed, installed, optimized and operated, and can simulate the combustion environment in an actual furnace. The setup is schematically shown in Fig. 1. The core part of installations was a downward flat-flame burner, consisting of a burner brass disk with a cylindrical cross-sectional area. There was a central circular hole with a diameter of 10 mm in the disk, which has two layers of borehole outside, including the premixed mixtures region and

Temperature (K)

O2 (%)

O2

CH4

N2

O2

N2

a1 a2 a3 b1 b2 b3 c1 c2 c3

1600

10 20 30 10 20 30 10 20 30

3.52 5.07 6.62 3.90 5.43 6.73 4.12 5.57 7.03

1.03 1.03 1.03 1.18 1.18 1.18 1.33 1.33 1.33

10.45 8.90 7.35 9.92 8.39 7.09 9.55 8.10 6.64

0.5 1 1.5 0.5 1 1.5 0.5 1 1.5

4.5 4 3.5 4.5 4 3.5 4.5 4 3.5

1700

1800

the protective air region. The premixed mixtures (CH4/O2/N2) were introduced in the premixed gas region of more than 2000 circular holes (diameter: 0.5 mm), which resulted in a laminar flame several millimetres thick, simulating the high temperature combustion environment of the PF boilers. The centre line temperature of the chamber has been calibrated by a B-type thermocouple, and the temperature field remains

MFC

MFC

Premixt ure

Rota

Premi xture

O2 N2 O2

SHG

Feeder

MFC

CH4

Mirror Filter

N2 Lens1

Apertures

ND:YAG laser

Protective mixtures (L/ min)

Items

N2

Dye Laser

Premixed mixtures (L/min)

O* 10.17 23.75

Mirror Concave lens

Filter

Digital signal generation

Optical collector

Lens2

Coal flame

Flat Flame

ICMOS

Percolator

Exhaust gas

Drying

Active Carbon

Pump

Gas analysis Fig. 1. Schematics of an optical entrained flow PF reactor system. 3

Collector

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certain radical. Therefore, PLIF is more sensitive for judging the ignition and combustion. The OH radicals in volatile flame can be selectively excited to reflect the inner structure of PF flame, where sheetimages of the particle stream can be more accurately identified for the signal fluctuations. The OH radicals are selectively excited without interference from other substances, and its changes can more accurately correspond to the volatiles combustion process.

essentially constant within 0–80 mm. For more details, please refer to our previous studies [12,13]. Protective air (O2/N2), where the oxygen concentration remained constant with the flue gas, could closely inhibit the tested flame washing to quartz glass window. A PF pipe with an inner diameter of 1.5 mm was nested in the central circular hole, through which coal particles were delivered by a carrier gas (0.2 sL/ min N2). During the experiment, the PF coal feeding rate was 1.9 g/h with a good variance of less than ± 5% for all conditions. The proportion of premixed mixtures (total constant flow) can be flexibly adjusted to meet the demands of different temperatures and O2 mole fractions. The operational conditions are listed in Table 2. The exhaust gas generated after combustion is a laminar flow (Hot State: Re < 40); therefore, the fluidity of the PF particles stream can maintain consistency to undergo the same heating and combustion process. The experimental setup is very helpful for optical detectors to measure the in-site combustion characteristics of solid fuels and to collect and analyse the physical and chemical properties of coal char.

3. Results and discussions 3.1. Velocity along the axial distance and the time history of coal particles The velocity of the particles stream could be measured by using the timing diagram of the OH-PLIF images, because the ICMOS camera continued to record the time-resolved particles flame. The interval time of successive images was 0.01 s, and the particle locational difference on the axis between the two pictures was just the particle displacement. The residence time of a PF fireball along the distance from downstream burner should be integrated by t = ∫ 1/ vl dl , where vl is the PF particle fireball velocity along the axial distance of the downstream burner. The averaged velocities of the fireball, where the averaged velocity was also the instantaneous velocity of the middle position of the displacement, could be identified in different locations. Numerous velocities from many different locations could be fitted in the velocity curves, as shown in the polynomial fitting velocity results in Fig. 3. The theoretical calculated velocity value of free fall of the initial cool particles at the burner entrance was 0.63 m/s, and the gas blowing could also promote particle movement, wherein the particle mass lessened. Pyrolysis and combustion have small impacts on spatial and temporal variations of particle velocity, so it can be considered that the particles essentially undergo uniformly accelerated linear motion downstream of the burner. The results of the fitting curves also prove that the particle velocity elevated approximately linearly. The different speeds at different temperatures are mainly caused by the expansion and contraction properties of the flue gases.

2.3. Planar laser-induced fluorescence of OH The optical system for OH-PLIF is similar to that used in the literature [15] and is shown in Fig. 1. The PLIF system consists of a laser module, a beam shaping module and a signal acquisition module. The laser module includes a pulsed Nd: YAG laser pumping a dye laser circulating rhodamine 590 dye in methanol. The dye laser light was frequency doubled to yield 0.7 mJ/pulse with a good variance of less than ± 3% at a frequency of 100 Hz. The laser was tuned to 283 nm to excite the Q1(6) line of the A-X (1–0) transition of OH. A digital signal generator synchronized a camera and LIF to capture the OH fluorescence. Fluorescence was recorded at 310 nm by an ICMOS camera equipped with a UV-achromat lens (Coastal Optics, f = 105 mm, f/4.5) and a 30-nm bandwidth filter centred at 310 nm. The OH-PLIF signal was captured with the gate width of 50 ns and the frequency of 100 Hz, while the OH chemiluminescence emission and integral light emission were captured with the gate width of 100 µs and the frequency of 20 Hz, with and without the UV-achromat lens and the bandwidth filter, respectively. In addition, the shorthand notations represent as following: sh/md is SH and MD; chemi/integ/plif is OH chemiluminescent emission, integral signals intensity (including black emission, chemiluminescent emission, etc.) and OH-PLIF, respectively; see Table 2 for details on a1-c3. The normalized fluorescence signal intensity curves removed the signal background noise in Fig. 2 were the first to appear at the intersection with the line y = 0.2, when y is the normalized signal intensity. The integral signals and OH chemiluminescent signals are passively optical signal, while the PLIF signals are active and selectable for

3.2. Sheet-imaging of PF volatiles combustion In tested PF particle system, the particle spacing ratio is expressed as

−

−

lr / dP = (π /6c )1/3 where lr is the distance between two particles, dP is monodisperse mean size, c is the particle volume fraction [15]. The ignition and combustion behaviours of the coal particle stream can be regarded as a statistical summation of all monodisperse coal particles at −

the large lr / dP . The particle volume fractions for SH and MD in the carrier gas are estimated as 2.53 × 10−5 and 2.04 × 10−5 at 1700 K, and the particle spacing ratios are also estimated as 27.4 and 29.5,

Fig. 2. Normalized signal intensity of chemiluminescent emission, integral signal emission, and OH fluorescence of SH coal (left) and MD coal (right). 4

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particle stream was 1700 K–1900 K with an increasing O2 concentration [13], which might also burn at the surface of the char particles as the diffusion of oxygen increased. However, it affects the volatiles combustion to a small extent. The devolatilization rate was forcefully dependent on the particle temperature and heating rate in the endothermic process [3], and the onset points of devolatilization could expect an approximately similar onset time under all ambient conditions. The homogeneous ignition was dominated by the local fuel-oxidizer ratio, the exothermicity rate and the transport characteristics of the surrounding gas. Ignition generally occurs in a location where the hot oxidant gas and volatiles first combine. As a result, ignition usually happens in the leaner point of the isothermal fuel-oxidizer mixture [31]. The overlaps of the volatile flames resulted in a continuous gasphase reaction zone, which strongly affected the stabilization of PF. The ignition of the particle stream affects the subsequent combustion reaction, but the continuous and stable combustion of volatiles directly affected the flame safety and stability for the operating burner. The OH sheet-imaging of the flame structure may have great benefits for verifying the CFD numerical simulation results, which are transient, highresolution and quantitative.

0.75

Particles velocity (m/s)

R2 0.85277

R2 0.76147

0.70

R2 0.91183

a2 b2 c2 Polynomial Fit of a2 Polynomial Fit of b2 Polynomial Fit of c2

0.65

0.60 0

2

4

6

8

10

12

14

16

18

20

22

Distance downstream from the burner (mm) Fig. 3. Particle velocity profiles of SH coal along the axial distance downstream from the burner and fitting lines.

respectively. Thus, the ignition and combustion of a single particle can be used to characterize the volatiles combustion of the particle streams at the large space ratio. From the sheet-images of the particle stream in Fig. 4, the volatile flames overlapped for tested coal particle stream, and then formed a well-developed flame beam. Following the heat up of the initial coal particles, the onset of the homogeneous or heterogeneous ignition occurs [9,10,19], whereas the burst of volatiles may quench the initial heterogeneous ignition [10]. However, there is no discernible surface oxidation of coal char prior to volatiles combustion in our experiments. From the sheet-images of the flame structure of a single particle in Fig. 4, well-dispersed coal particles are heated by surrounding gas flames, in which ignition is dominated by the volatiles, consistent with the image results of Molina et al. [3,11] and Mclean et al. [16]. The average flame temperature of the bituminous coal

3.3. Normalized OH fluorescence intensity curves of volatiles combustion The signal intensities of the images removed the background noise were extracted, and normalized by the peak intensity, and then the normalized fluorescence signal intensity curves are shown in Figs. 5 and 6 along the distance downstream, and in Fig. 7 along the radial distance, wherein there are 4 corresponding dividing lines (as shown in Fig. 5). To reduce the experimental error, 500 individual images were averaged and normalized. For the high-volatile bituminous and lignite coal, volatiles combustion of the particle stream guides the ignition process, and the heat release can provide the necessary conditions for char particles to burn [7,10,14]. Since volatile clouds determine the ignition and combustion stability of the particle stream, an in-depth

Fig. 4. Fluorescence signal sheet-imaging of a coal particle stream (left) and a single coal particle (right) at the b2 condition for the SH coal. 5

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Fig. 5. The normalized fluorescence signal intensity of SH along the distance downstream from the burner under different ambient conditions.

experimental measurement methods, it indicates a tendency for weakening and further volatile burnout. From Fig. 3, the residence time of four stages is calculated by the particle velocity profiles. In Figs. 5 and 6, the rapid pyrolysis of volatiles improves the combustion and heat of the particle streams in regions I and II; then, the pyrolysis process becomes weak and the volatiles combustion burns stably in region III; Finally, volatiles are consumed too much to maintain intense combustion, and the OH signal intensity sharply reduces in region IV. The normalized OH signal intensity continues to increase until it plateaus and then decreases in Fig. 5. The volatiles combustion linearly increases within regions I and II, while the signal intensity fluctuated around the maximum intensity value in region III, indicating that the release and combustion of volatiles gradually reaches a relatively stable state. Finally, the signal intensity essentially decreases linearly in region IV. The elapsed time from the onset of ignition until forming a fully developed volatile flame is only in 1–2 ms for single particle [24], while the time of the particle stream from the onset until forming a stable volatile flame is relatively longer. It should be noticed that the heating rate of the group particles may be lower than that of the single particle, with differences in the overlapping flame, fuel-oxidizer ratio, and gas mass transport. In the plateau, devolatilization begins to weaken, and the volatiles generated continue to be consumed. The dropping rate of the OH signal intensity in region IV was similar to the rising rate of the signal intensity in region II. The volatiles combustion of SH coal does

analysis needs to be performed on the characteristics of the volatiles combustion by exploring the time-resolved change of the reacting region of the volatile. The normalized OH signal intensity curves of SH coal are shown in Fig. 5. The curves are divided by 4 straight lines (line0.05, line0.2, line0.8, line0.45) at Y = 0.05, Y = 0.2, Y = 0.8, and Y = 0.45, respectively, where Y is the normalized OH fluorescence signal intensity, to further analyse the volatiles combustion process. Thus, the volatiles combustion process can be divided into 4 stages: ignition, accelerating burning, stable burning and feeble burning, which correspond to curves I, II, III, and IV, and the ignition time, acceleration time, stabilization time, and feebleness time, respectively. In order to study the unification and reduce error signal interference, the location of the Line0.05 is chosen as the starting point. The criterion that the signal intensity just reached 20% of the peak intensity is defined as the ignition point, because the definition approximately sets the point where the ignition tendency can be ensured to be stable [12–15]. From the test observations, the signal intensity reaching approximately 80% of the peak intensity can be considered to achieve a stable combustion status, wherein the flame burns steadily and continuously. Line0.8 crosses the normalized OH signal intensity curves at two points: line0.8F and line0.8L. The signal intensity decreasing from 80% to 45% of the peak value can be considered to be feeble burning. Although it could not accurately distinguish the combustion of volatiles and char with the limitation of

Fig. 6. The normalized fluorescence signal intensity of MD along the distance downstream from the burner under different ambient conditions. 6

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0.9

Line0.2 Line0.8F Line0.8L Line0.45

md-b2 0.9

0.8

0.8

0.7

0.7

0.6

0.6

Intensity (AU)

Intensity (AU)

1.0

Line0.2 Line0.8F Line0.8L Line0.45

sh-b2

0.5

0.5

0.4

0.4

6.5mm

0.3

9.1mm

0.2

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PF pipe

0.1

9.3mm

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PF pipe

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0.0

0.0 -10

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0

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0

5

10

Radial distance (mm)

(a) 0.7

sh-b2

0.7

Line0.2 Line0.8F Line0.8L Line0.45

0.6

md-b2

Line0.2 Line0.8F Line0.8L Line0.45

0.6

0.5

RSD

RSD

0.5

0.4

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0.2

0.2

-10

-5

0

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Radial distance (mm)

-10

-5

0

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Radial distance (mm)

(b) Fig. 7. Radial distribution of luminescence intensity (a) and its RSD (b) and (c) at different intersection points of Line0.2, Line0.8F, Line0.8L, and Line0.45 under different oxygen mole fractions.

intensity, which enhances again in region III, obviously fluctuates. Volatiles dominate the gas reaction in region II, where the release and diffusion of volatiles are more affected by the temperature and oxygen concentration. The curves of MD change faster and more violently in Fig. 6. The radial reaction region spans of the OH signals of MD are larger than those of SH, as shown in Fig. 7, which means the lignite

not obviously change under our experimental conditions, and one possible explanation is that the devolatilization rate change little according to the external ambient test conditions. The volatile combustion of the particle stream of MD are more complex and variable in Fig. 6. The normalized signal curves under different ambient conditions are largely dispersed, while the signal 7

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0.7

0.7

sh-b1-10% O2 sh-b2-20% O2 sh-b3-30% O2

Line0.8F

md-b1-10% O2 md-b2-20% O2 md-b3-30% O2

Line0.8F

0.6

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RSD

0.6

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-5

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(c) Fig. 7. (continued)

while the RSD at the other lines have two peaks for the SH coal. The lines (Line0.8F, Line0.8L, Line0.45) appear at a valley around the axis region (radius: 2 mm), and the valley value is much lower than those in the exterior regions. The RSD at all position lines of the MD coal only has a single peak, and the signal fluctuation increases with the OH concentration in all lines. The RSD peak values at measuring position lines of SH and MD coal decrease in order as: Line0.2 > Line0.45 > Line0.8L > Line0.8F. The results are consistent with the combustion uncertainty in regions I and IV, and support the combustion stabilization in region III. The surrounding high-oxygen atmosphere helps to reduce the RSD value for SH and MD, as shown in Fig. 7 (c), where the RSD around the axis drop greatly. The results show that the surrounding high-oxygen atmosphere improve the flame stability of two high-volatile coal particle streams. The flame propagation speed and flame stability are enhanced because a high-oxygen gas increases the transport of oxygen molecules [25,26]. whereas, the relationship between gas temperature and flame stability reflected by the RSD is very weak [27] and requires for further study. The RSD value around the axis for SH decreased with oxygen mole fractions and much lower than those in the exterior regions. The molecular diffusion rate of volatiles and the flame propagation speed near the centre of the particle stream may keep a better and longer equilibrium at relatively higher oxygen concentrations for O2 = 20% and O2 = 30% cases. The SH coal maintains a stable devolatilization rate until the volatiles are removed completely, which keeps ignition of its volatile flame more stable. However, the RSD around the axis for MD decreased with the oxygen concentrations, and higher than those in the exterior regions. large amounts of the explosive volatiles released and fragmentation may cause instability of the devolatilization rate. The range of the RSD value is 0.25–0.64 of MD coal, and is much higher than 0.18–0.55 of SH coal. Therefore, the gas flame of SH is more stable in the combustion zone.

particle stream had a larger combustion reaction region. Possible reasons are that lignite has a higher volatile content (44.34%) and a better combustion reactivity, and then the combustion reaction region may expand for the high-volatile concentration gradient. And, inherent physical and chemical characteristics of lignite lead to particle fragmentation and coal char structure destruction during the devolatilization and volatiles combustion [6,32], which accelerates the devolatilization rate. We can speculate that the fragmentation behaviour of lignite mainly happened in region III, because the change in MD was similar to that of SH in regions I and II, while the variations were dramatically different in regions III and IV. After volatiles of the particle stream burst and burn, the char particles are heated, and the internal structures of particles begin to be destroyed and then begin to fragment. The OH signal intensity dropped sharply in region IV at a much faster rate than the rise of the signal intensity in region II, indicating that fragmentation may promote the release and burnout of volatiles. In Fig. 7 (a), the radial luminescence intensity profiles of two kinds of coal in the b2 condition are shown at curve cross positions at Line0.2, Line0.8F (F: first), Line0.8L (L: last), and Line0.45, where the OH radical signal does not go to zero until it is stabilized because of the gas flame diffusion and laser energy attenuation. The signal variations of all lines are approximately consistent, as is lignite. The OH radical profile spans of MD are larger than the spans of SH at 20% and 30% of the peak value, and the volatile reaction region clearly envelops the particle stream by the homogeneous gas-phase reaction, consistent with Yang Xu et al. [33]. MD coal has a larger span of the gas combustion region, and its volatiles are found to diffuse more easily out of particle stream. The homogenous flame fluctuations can be represented as the relative standard deviation (RSD) of the sequential images: − − − n RSD = s / x × 100% = ∑i = 1 (x i − x )2 /(n − 1) / x × 100%, where s is the −

standard deviation, x is the sample mean, x i is a single sample (a single picture, in the case), and n is the total number of samples [27]. The values of RSD with 4 lines are given to show the volatile flame stability, as shown in Fig. 7 (b). The RSD at Line0.2 only occurs as a single peak,

3.4. Time period of the volatile combustion stages The time periods of volatile combustion stages for SH coal obtained 8

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--

Feebleness

Ambient conditions

c3 2.06

Stabilization

10.06

Acceleration 16.05

4.93

c2 2.50

10.09

17.01

c1

9.61

18.08

2.71

b3 2.24

9.78

Ignition

5.18 4.64

16.66

5.15

b2 2.59

10.13

17.50

5.31

b1 2.63

9.82

17.74

5.58

a3 2.07

10.52

a2 2.56

10.63

a1

10.43

2.86

0

4

8

16.41

5.60

17.89

5.47

19.02

12

16

20

24

4.78

28

32

36

Combustion time (ms) Fig. 8. Time period of the volatile combustion stages for SH under different ambient conditions.

transfer number, B , is provided by B = [Cp (T∞ − Ts ) + (Y0, ∞/OF) hc ]/ h v where T∞ is the bulk gas temperature, Ts is the particle surface temperature, Y0, ∞ is the mass fraction of oxygen in the bulk gas, OF is the stoichiometric mass oxygen-fuel ratio, hc is the mass heat of the combustion of the fuel, h v is the mass heat of the vaporization of the drop (or the heat of devolatilization, in the case of coal), and the relevant specific heat, Cp , is that of the volatilized fuel. Two kinds of coals have very different devolatilization rates that are influenced by the volatiles, coal rank, pore structure, exothermicity rate, etc. The percentage of the sum of the ignition time and acceleration time from a1 to c3 varies between 35% and 37% of SH and 33% and 68% of MD, respectively. T∞ and Y0, ∞ in our experiments are not the main factors that affect the devolatilization rate of SH coal, while they can strongly control the devolatilization process of MD coal. The processes can also further affect the volatile flame stabilization. Thus, different local fuel-oxidizer mixing locations emerged as a result of the fact that the ignition and combustion behaviours differed for the two coals.

from Fig. 5 are shown in Fig. 8. The high concentration of oxygen molecules easily diffused into the particle stream, where the concentration gradient of the volatiles tends to promote the movement out of the particle stream. Since the diffusion motions increase the mixing degree of the fuel-oxidizer mixtures, the particle stream combustion reactivity was greatly enhanced and then the ignition delay time and acceleration time were both reduced. The elapsed time of the PF particle stream from the onset of ignition until forming a stable volatile flame was in the sequence of 12–14 ms (sum of the ignition time and acceleration time). With an increase in the oxygen concentration (a1a3, b1-b3, c1-c3), the ignition, stabilization, acceleration, and feebleness time periods fluctuated slightly. The time portion of each stage was relatively constant for SH coal, wherein the stabilization time accounted for approximately half of the total combustion residence time, and the sum of the ignition and acceleration times accounted for 35%–37% of the residence time. The ignition time decreases slightly, the acceleration time decreases obviously, and the stabilization time increases obviously with an increase in the oxygen concentration in Fig. 9. The sum of the ignition and acceleration times of MD accounted for 33%–68% of the total combustion time, and the time was significantly higher than that of SH, wherein the time of b3 is only roughly the same as that of SH. An increasing oxygen concentrations and temperatures lead to a reduction in the acceleration time, while the oxygen mole fractions promote their opposite tendency in the stabilization time with the elevating thermal diffusivity and consuming volatiles. Every process of the combustion time of the SH coal changes very little with the surrounding oxygen concentrations and temperatures, while the time of each process of MD fluctuates widely. As analysed above, the temperatures and oxygen concentrations do not significantly inhibit or promote devolatilization and volatiles combustion of SH, but they can greatly influence the volatiles combustion of MD, especially for the oxygen mole fractions. The effect on the volatile flame in the different surrounding gas can be explained by the quasi-steady droplet combustion theory [34]. The mass consumption rate is conveyed as ṁ = (ρs D)(4πrs )ln(1 + B ) where ρs is the fuel vapor density at the droplet surface, D is the mass diffusivity of the fuel vapour, rs is the droplet radius, and the Spalding

3.5. Characteristics analysis of collected coal char The design of the experimental device is beneficial for collecting coal char, of which char burnout ratios can be analysed with char proximate analysis, and the pore characteristics of the residual chars is also measured. The mass of ash in collected chars is supposed to remain consistent in the raw coals, and in the char residues during combustion, which is called the Ash Tracer Method. The volatiles releasing ratio/ char burnout ratio can be calculated using Formula (1) with the Ash Tracer Method [12].

Ri =

I0 − Ii × A0 / A × 100%(i : volatiles, fixed carbon) I0

(1)

Ri is the volatile releasing ratio or char conversion ratio, %, i represents the volatile or fixed carbon; I0 is the mass percentage of volatile or fixed carbon in raw coal, %; Ii is the mass percentage of volatile or fixed carbon in sampled coal char, %; A0 is the mass percentage of ash in the raw coal, %; and A is the mass percentage of ash in sampled coal char, %. When collecting the char residues, we make sure that all 9

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Ambient conditions

--

Feebleness

c3

2.64

c2

3.09

c1

3.57

b3

3.08

b2

3.33

b1

3.92

a3

3.56

a2

3.92

a1

4.24

0

Stabilization

8.49

Acceleration 20.12

11.72

2.79 18.07

19.03 18.52 13.59

1.26 2.75

15.90 19.18

1.51 9.97

13.32

1.48

15.57 18.52

12

2.81 11.45

19.71

8

1.29 10.34

9.94

4

Ignition

1.50

9.51

16

20

24

28

1.91

32

36

Combustion time (ms) Fig. 9. Time period of the volatile combustion stages for MD under different ambient conditions.

Fig. 10. The volatile releasing ratio and char burnout ratio with different oxygen mole fractions (left) and temperatures (right).

lignite leads to a relatively active combustion reactivity. In fact, the combustion of volatiles of MD coal can be more violent than that of SH coal. The pore structure of the high-volatile raw coal may collapse with Autopore II 9220 under high pressure (Max: 200 MPa), and thus, the variation of pores with diameters greater than 50 nm for raw coal and coal char is only considered, wherein pores (larger than 50 nm) are macropores according to the classification of the pore size [35]. The specific surface area and the total pore volume of MD change greatly, while that of SH changes little under the same experimental conditions from Fig. 11. The macropore structure of coal char is mainly formed in the volatile release and surface oxidation of coal char under high temperature and high oxygen concentration conditions. As shown in Fig. 12, the surface of the MD coal is more uneven and has a larger specific surface area than that of SH coal. The formation of large pores increases the O2 mass transportation inside the char particles and

solid residues were captured. Based on the proximate analyses data, Ri can be calculated, and the result is shown in Fig. 10. Under the b3 condition (1700 K, 30% O2), the burnout ratio of MD is too high to collect any unburned mass in the coal char, so the volatiles releasing ratio/char burnout ratio is defined as 100%. For the SH coal, the increment of the volatiles releasing ratio is the same as that of the MD lignite in the range of 10%–30% O2, but the releasing ratio (65%–81%) is substantially less than that of lignite (92%–100%). Moreover, the temperature (1600 K-1800 K) has the same effect on the volatiles releasing both SH and MD, but it has a small effect on the volatile releasing ratio. The char burnout ratio of MD is lower than the percent of SH at the beginning, and then gradually becomes higher than that of SH with the oxygen concentration and temperature. However, the oxygen mole fractions are obviously more important. This also indicates that the oxygen concentration has a more obvious effect on the char burnout of lignite. The intrinsic soft property and high volatile contents of 10

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Fig. 11. Specific surface area and total pore volume of coal char under different ambient conditions.

flame surrounding the particle stream to be resolved. However, a larger view field and higher resolution, bound by advanced laser techniques, may completely include the entire combustion zone and achieve more accurate flame structure information for PF particle streams. In addition, CH2O/NO-PLIF should also be introduced into the research of coal streams, which is employed to explore the phenomenon of low-temperature reaction and NOx formation. (2) The normalized fluorescence signal intensity curves of bituminous coal are more concentrated, and the volatile release and combustion are not very sensitive to the influence of external conditions. However, the signal curve of lignite is more dispersed, and the volatile release and combustion are greatly affected by the temperature/O2 concentration. Lignite particles fragmentation may mainly happen in the stabilization stage. The RSD of radial OH profiles with time may represent flame stabilization. The results show that the particle stream of bituminous coal has a more stable flame than that of the lignite particle stream under the same conditions. (3) The volatiles combustion can be analysed more intuitively by dividing the combustion into 4 processes (ignition, acceleration, stabilization and feebleness) with the normalized fluorescence signal intensity. With an increase in the oxygen concentration, the combustion time of each process of bituminous coal fluctuates less than that of lignite with elevating temperatures. The devolatilization rate of bituminous coal changes much less than that of lignite in our experimental conditions. Lignite is more sensitive to oxygen concentration and owns a larger combustion reaction zone than that of bituminous coal. (4) Based on the analysis of the collected residual char, the volatile releasing ratio of lignite (92%–100%) is higher than that of

enhances the surface oxidation reaction. Softening, melting, hole collapse and ash melting occurs on the surface of coal char during a long period of surface heating and combustion, which led to a sharp increase and then a decline in the pore structure [35,36]. SH coal, of which carbonization is significantly higher than that of MD coal, has a more stable coal char structure. Thus, the specific surface area and total pore volume has a small variation and the residence time of the volatile combustion stages fluctuates little with the ambient conditions from Figs. 5 and 8. MD coal, which occurs in the maximum specific surface area in the 10% of O2 or 1600 K, inherits a more unstable char structure for low carbonization [37], and the residence time of the volatile combustion fluctuates largely with the ambient conditions from Figs. 6 and 9. 4. Conclusions In this work, an optical entrained flow system, based on a downstream flat-flame burner, was established to explore the collective combustion behaviours of the particle stream. For this purpose, we focused on an in-site experimental analysis of devolatilization and volatile combustion, combined by off-line analyses of coal char. The effects of the ambient temperature (1600 K-1800 K), O2 mole fraction (10%–30%) and the coal rank (bituminous coal and lignite) on the particle streams are considered. The following conclusions are summarized as follows: (1) OH-PLIF is a powerful tool for studying the ignition and volatiles combustion characteristics of coal particle streams. The gas homogeneous reaction controls the combustion of the high-volatile coal streams. Sheet-imaging allows for the internal structure of the

Fig. 12. SEM micrographs of particles of the two fuels used in the b2 condition: SH coal (left) and MD coal (right). 11

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bituminous coal (65%–81%). The char burnout ratio of lignite is more sensitive to the ambient oxygen concentration. The specific surface area and total pore volume of the lignite residual char are larger and change more violently than those of bituminous coal because of its low carbonization and loose char structure.

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CRediT authorship contribution statement Wenkun Zhu: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Xiaohui Li: Formal analysis, Software, Validation. Jiangbo Peng: Resources, Writing - review & editing, Supervision, Data curation. Zhuozhi Wang: Writing - review & editing, Formal analysis, Validation. Rui Sun: Resources, Methodology, Writing - review & editing, Supervision, Data curation. Lei Zhang: Software. Xin Yu: Project administration, Funding acquisition. Yang Yu: Investigation. 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. Acknowledgments This research is funded by the National Key Research and Development Program of China (NO. 2017YFB0602002) and National Natural Science Foundation of China (grant NO. 51536002). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116956. References [1] BP. Statistical Review of. World Energy; 2018. [2] Hao Jiming WJ. Technology roadmap on air pollution control for coal utilization in medium and long term. Chin Acad Eng 2015;17(09):42–8. [3] Molina A, Shaddix CR. Ignition and devolatilization of pulverized bituminous coal particles during oxygen/carbon dioxide coal combustion. Proc Combust Inst 2007;31(2):1905–12. [4] A study of gas composition profiles for low NOx pulverized coal combustion and burner scale-up [J]. Symposium (International) on Combustion, 1988, 21(1):1199–1206. [5] Annamalai K, Durbetaki P. A theory on transition of ignition phase of coal particles. Combust Flame 1977;29(2):193–208. [6] Levendis YA, et al. Combustion behavior in air of single particles from three different coal ranks and from sugarcane bagasse. Combust Flame 2011;158(3):452–65. [7] KHATAMI, et al. Ignition characteristics of single coal particles from three different ranks in O2/N2 and O2/CO2 atmospheres. Combust Flame 2012;159(12):3554–68. [8] Howard JB, Essenhigh RH. The mechanism of ignition of pulverized coal. Combust Flame 1965;9(3):337–9. [9] Du X, Annamalai K. The transient ignition of isolated coal particle ☆. Combust Flame 1994;97(3–4):339–54. [10] Essenhigh RH, Misra MK, Shaw DW. Ignition of coal particles: a review. Combust Flame 1989;77(1):3–30. [11] Shaddix CR, Molina A. Particle imaging of ignition and devolatilization of pulverized coal during oxy-fuel combustion. Proc Combust Inst 2009;32(2):2091–8.

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