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An Experimental Study of Ignition and Combustion Characteristics of Single Particles of Zhundong Lignite Zhezi Zhang, Mingming Zhu, Jianbo Li, Kai Zhang, Gang Xu, and Dongke Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03145 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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Energy & Fuels

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An Experimental Study of Ignition and Combustion Characteristics of

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Single Particles of Zhundong Lignite

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Zhezi Zhang1*, Mingming Zhu1, Jianbo Li1,3, Kai Zhang2, Gang Xu2 and Dongke Zhang1

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1

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Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

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2

Beijing Key Laboratory of Emission Surveillance and Control for Thermal Power Generation, North China Electric Power University, Beijing 102206, China

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3

Key Laboratory of Low-grade Energy Utilization Technologies and Systems of the Ministry of Education of China, Chongqing University, Chongqing 400044, China

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(A manuscript offered to the “6th Sino-Australian Symposium on advanced coal and biomass

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utilisation technologies” Special Issue of “Energy & Fuels”)

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* Corresponding author:

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Zhezi Zhang

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Email: [email protected]

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Phone: +61 8 6488 7602

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Fax:

+61 8 6488 7622

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Abstract

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The ignition and combustion behaviour of single particles of Zhundong lignite was experimentally

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investigated. Single particles of Zhundong lignite with diameter varying from 2 to 3 mm were

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suspended on a SiC fibre and their burning in air in a horizontal tube furnace operating at 1023,

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1073, 1123, 1173 and 1223 K observed, aided with a CCD camera at 25 fps. By analysing the

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captured images, the ignition delay time, flame displacement, volatile flame duration, volatile

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extinction time, total burnout time and the ignition mechanism were determined. The typical

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ignition and combustion process of Zhundong lignite consisted of four sequential but overlapping

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stages: 1) pre-ignition stage involving drying, devolatilisation and oxidation at the particle surface;

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2) heterogeneous ignition and combustion; 3) ignition and combustion of volatile matter in the gas

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phase; and 4) combustion of the remaining char residue. Surprisingly, the ignition of Zhundong

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lignite followed a joint hetero-homogeneous mechanism under all conditions studied. Upon the

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volatile matter ignition, a soot-free yellowish translucent flame was formed surrounding the particle

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and the volatile flame duration was noticeably long-lasting. The high sodium content in Zhundong

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lignite was believed to be responsible for such a phenomenon. The ignition and combustion

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characteristics, such as ignition delay time, volatile flame extinction time and the total combustion

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time of Zhundong lignite, decreased with increasing furnace temperature and decreasing particle

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size.

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Keywords: Char combustion; Devolatilisation; Ignition; Single particles; Soot-free flame;

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Zhundong lignite

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1. Introduction

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As the major coal production base in China, Zhundong lignite plays a vital role in China’s energy

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security1. A scientific understanding of the ignition and combustion behaviour of this low-rank coal

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is crucial for more efficient utilisation of this resource2-4. While an enormous amount of attention

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has been paid to the understandings of ash slagging, fouling and corrosion problems due to the high

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sodium content in Zhundong lignite1, 5, 6, information on its fundamental ignition and combustion

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characteristics is very scarce. Weng et al.

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characteristics of Zhundong lignite using the thermogravimetric technique, focusing on the effect of

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sodium and alkali metals. They reported that ignition and combustion behaviour of Zhundong

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lignite was affected by removing the water-soluble and organically bonded alkali metal in the

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lignite. It was concluded that the water-soluble and organically-bonded alkali metal had some

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catalytic effects, promoting ignition and increasing burning rate of the lignite. However,

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contradictory results were reported by Chen et al9 , who found that only the organically-bonded

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sodium promoted the combustion of Zhundong lignite in their TGA experiments. Nevertheless, the

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slow heating rates used in TGA experimentation were considerably different from the actual

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combustion process in a boiler. Therefore, the understanding of the fundamental ignition and

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combustion behaviours of Zhundong lignite in an environment that simulates the real heating rate in

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the boiler is essential.

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and Wang et al.

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studied the ignition and combustion

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Our previous studies focussed on the understanding of the effect of washing treatment10 on the

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ignition and combustion characteristics of Zhundong lignite. As a continuation effort of these

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studies, the present study examined the effect of furnace temperature (1023 – 1223K) and particle

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size (2 – 3 mm) on the ignition mechanisms and combustion phenomena of Zhundong lignite using

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the suspended single particle experimentation approach11-13. The selected furnace temperature and

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particle size represented typical fluidised bed combustion conditions 14.

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2. Experimental

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The proximate, ultimate and ash composition data of the Zhundong lignite used in this study are

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shown in Tables 1 and 2. The surface functional groups of Zhundong lignite were analysed using

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FTIR (Nicolet 6700, Thermofisher) with the results presented in Figure 1. The as-received lignite

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was first crushed into small chunks, which were then carefully filed into spherical particles of 2, 2.5

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and 3 mm in diameter. A small hole (ca. 0.25 mm in diameter) was carefully drilled into the centre

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of the particle for attaching the particle to a SiC fibre. The samples were then dried in an oven at

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378 K overnight and stored in a desiccator prior to experimentation.

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Figure 2 shows the schematics of the experimental setup of the single particle ignition and

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combustion 12. It consisted of a horizontal tube furnace (800 mm in length and 40 mm in diameter)

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to provide a hot air environment, a particle suspension system, a linear stepper - motor for

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delivering the lignite particle into the furnace and a CCD camera (Basler PIA-210gc) for capturing

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the combustion process. A LED backlight was used to illuminate the lignite particle before the start

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of each experiment to identify the time at which the particle arrived at the centre of the furnace. To

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start the experiment, the linear stepper motor, the data acquisition card, the backlight and the CCD

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camera were triggered simultaneously, using a Programmable Logic Controller, after the furnace

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reached the desired temperatures (1023, 1073, 1123, 1173 or 1223K).

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Key ignition and combustion characteristics, such as ignition delay time, volatile ignition time (tvi),

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surface ignition time (thi), volatile flame duration (tf), volatile flame extinction time (tve), burnout

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time (tb), total combustion time (tt), the flame displacement (xf), ignition mechanism and temporal

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variation of the particle size, were obtained by processing the images captured by the camera. The

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definitions of the aforementioned ignition and combustion characteristics were schematically shown

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in Fig. 3 and more details can be found in authors’ previous publication10. In the present work, the

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flame displacement was determined using the following method. Firstly, a vertical line (488 pixel 4 Environment ACS Paragon Plus

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length and one pixel width) was drawn passing through the centre of the particle. Then the pixel

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greyscale intensity along the line was taken using Matlab and plotted against the number of pixels

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along the line as shown in Figure 4 (a). Secondly, the first derivative of the pixel greyscale along

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the line was calculated and plotted as shown in Figure 4(b). It is evident that there were two

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distinctive peaks, namely, peaks 1 and 2, which indicate the volatile flame front and the particle

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surface, respectively. Thus, the distance between these two peaks was taken as the flame

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displacement.

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3. Results and Discussion

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Figure 1 shows the FTIR spectra of the Zhundong lignite. The peaks at 2920 cm-1 and 2850 cm-1

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correspond to CH3 and CH2 (stretching in the aliphatic group)15-18. The peaks near 1435 cm-1 and

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1370 cm-1 correspond to the absorption of the methyl group (–CH3)15-18. In the region from 1300 to

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1000 cm-1 various oxygenated compounds are responsible, including aliphatic ethers/alcohols in

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1100 - 1000 cm-1 region, C-O stretch and O–H bend in phenoxy structures in 1300 – 1100 cm-1

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region16-18. The peak at 3060 cm-1 corresponds to an aromatic C-H stretching absorption band and

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the peaks near 910, 810 and 750 cm-1 are related to the C-H bending adjacent to an aromatic ring16-

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18

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peak at 1576 cm-1 is related to the carboxyl group in the salt from (–COOM)16. This is also in

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agreement of the ash chemistry data presented in Table 2 and literature findings that a large portion

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of the sodium in the raw Zhundong lignite exists as carboxylate sodium salts6, 19, 20.

. The peak at 1700 cm-1 corresponds to the C=O stretches in carboxylic groups (-COOH)15-18. The

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Figure 5 shows the typical time sequenced images of ignition and combustion of Zhundong lignite

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in air. It is clear that the ignition of Zhundong lignite conformed the joint hetero-homogeneous

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ignition mechanism, with the ignition on the surface occurred prior to the ignition of the volatile

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matter. As shown in Figure 5, the ignition and combustion processes of Zhundong lignite consisted

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of four distinctive stages: (1) Pre-ignition stage: when the particle arrived at the centre of the 5 Environment ACS Paragon Plus

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furnace, a rapid increase in the particle temperature was induced by radiation heat transfer from the

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furnace wall and, to a lesser extent, conduction and convection heat transfer from the hot air. Once

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the heterogeneous ignition criterion was met, the ignition occurred on the ignite surface; (2)

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Heterogeneous ignition and combustion stage: as can be seen in Figure 5, at 6.7 s a bright dot

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appeared on the surface of the particle, indicating the heterogeneous ignition of the particle. Upon

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ignition, the combustion of the particle continued as demonstrated by the development of the bright

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area on the particle surface; (3) Volatile ignition and combustion stage: as combustion continued, at

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9.02 s, a yellowish translucent flame surrounding the particle was observed. This suggests that the

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volatile matter had ignited and was burning. The yellowish translucent flame also indicated that the

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burning of the volatile matter was almost soot-free

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noticeably long-lasting, ca. 54 s, which was unusual compared with the combustion phenomena of

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other low rank coals with low sodium contents as reported in the literature 11-13, 22. It is believed that

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the water-soluble form of sodium (e.g. NaCl) and the organically-bonded form of sodium (e.g.

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sodium carboxylic functional groups) in Zhundong lignite are converted to carbon matrix anchored

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sodium containing surface complex during the particle heating, devolatilisation and combustion

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processes 23, 24. Some of the large fragments of volatile matter are deposited on to these active sites,

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via condensation, recombination and/or polymerisation reactions, and become catalytically cracked

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to the formation of smaller particles. On the other hand, due to the metal-induced catalytic cracking,

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the OH radical concentration is increased by the presence of Na species. As a result, the flame

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temperature is increased, further accelerates the oxidation of the soot precursors and nascent soot

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particles

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after the volatile flame extinguished. The brightness of the burning char decreased towards the end

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of the combustion and finally extinction occurred at 77.06 s.

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. In addition, the volatile flame duration was

. The release of the sodium species into the gas phase suppresses particulate amalgamation leading

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; and (4) Residue char combustion stage: the burning of the char residue continued

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The four-stage combustion phenomena were observed for all particles examined in the present

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study. However, the behaviour of volatile matter combustion was significantly affected by

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temperature and particle size. As shown in Figure 5 (b) and (c), in the first a few seconds following

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ignition, the volatile matter flame was seen to consist of two layers, namely, a luminous yellowish

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inner layer and a translucent outer layer. This dual layer flame structure became more profound at

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either a higher furnace temperature or for a smaller particle and only lasted for a very short period,

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in the order of a few seconds. The bright yellowish flame was an indication of the formation of soot

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particles. The ignition process of large lignite particles is generally controlled by the external heat

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transfer to the particle29, 30. The external heat transfer to the particle can be expressed in terms of Eq

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(1), according to which the external heat transfer is significantly enhanced at a higher furnace

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temperature and for a smaller particle, leading to a higher heating rate of the particle. The higher

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heating rate would lead to more intense volatile release into the boundary layer which was also

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deprived of oxygen. As a result, a greater amount of soot was formed for smaller particles at higher

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furnace temperatures.

λc 171

∂Tp ∂r

r=

D 2

= εσ (Tw4 − Tp4 ) +

2λa  ln(1 + B )  (T∞ − Tp )  D B  

(1)

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where λc and λa are the thermal conductivity of lignite and air (kWm-1·K-1), r is the radial distance

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from the particle center (m), D is the diameter of the lignite particle (m), ε is the lignite particle

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emissivity, σ is the Stefan-Boltzmann Constant (5.6703×10-8 Wm-2K-4), Tw, T∞ and Tp are the

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furnace wall temperature, gas temperature and particle temperature (K) and B is the Spalding heat

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transfer number.

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Figure 6 shows the time-dependent variation in the volatile flame displacement for the burning

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particle that is presented in Figure 5(a). It can be seen that the flame front initially expanded to ca. 2

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mm once the flame was formed but then quickly contracted back to ca. 0.8 mm from the particle

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surface at around 14 s. During the ignition delay period, a large amount of volatile matter was 7 Environment ACS Paragon Plus

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released and accumulated around the particle, which, upon ignition, demanded a great amount of

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oxygen, resulting in a rapidly increasing flame size to reach more oxygen. Once the flame was

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stabilised, the flame size remained almost constant till about 63 s.

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Figure 7 shows the d2–t plots31 for the burning of Zhundong lignite for a) 3 mm single particles at

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various furnace temperatures and b) 2, 2.5 and 3 mm single particles at a fixed furnace temperature

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of 1123 K. It is evident that the particle sizes remained invariant from arrival at the furnace centre

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till the ignition of volatile matter. This indicates that the release of volatile matter from the particles

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and particle surface reactions did not cause noticeable changes to the particle size prior to ignition

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for Zhundong lignite. After ignition, the particle size decreased almost linearly as a function of time,

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suggesting the char oxidation process followed the classic d2-law 32 , controlled by oxygen diffusion

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in the boundary layer to the particle surface

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combustion process, an ash shell was observed while the char oxidation continued within the shell.

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The particle size became almost constant while the char oxidation continued, suggesting that the

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burning of the residue char in this period followed the classic shrinking core model30, 33.

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The tvi, thi, tve and tt of Zhundong lignite as a function of furnace temperature and particle size are

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shown in Figures 8 and 9. Under all test conditions, the heterogeneous ignition consistently

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occurred before the ignition of volatiles in the boundary layer. As expected, tvi, thi, tve and tt all

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decreased with increasing furnace temperature and decreasing particle size. The trends for

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variations in the heterogeneous ignition time and homogeneous ignition time are in agreement with

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the theoretical calculations that the ignition process of large lignite particle is controlled by the

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external heat transfer from ambient to the particle29, 30. The external heat transfer was enhanced at

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higher temperature and smaller particle size and ultimately reduced the time required for ignition.

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As the combustion process is controlled by the diffusion of oxygen to the particle surface as

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discussed above, at the same furnace temperature, the time required for the complete combustion of

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single particles decreased with decreasing particle size. However, an increase in the furnace

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. However, in the final few seconds of the char


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temperature only induced a small decrease in the total combustion time for particles of the same

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size.

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4. Conclusions

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As a continuation effort of our previous studies, the effect of particle size and furnace temperature

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on the ignition and combustion characteristics of single particles of Zhundong lignite were studied

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in the present paper. Under the tested conditions, the ignition of Zhundong lignite followed the joint

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hetero-homogeneous mechanism and the combustion of the Zhundong lignite consisted of pre-

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ignition heating, heterogeneous ignition and combustion, volatile matter ignition and combustion,

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followed by the combustion of solid residue. The particle size was almost constant during the pre-

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ignition and the heterogeneous ignition and combustion stages while decreased almost linearly with

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time upon the volatile matter ignition. The volatile flame was virtually soot-free and lasted a

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lengthy period of time. However, at higher temperatures, a layer of sooty flame was formed close to

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the particle surface but only lasted a very short period. In the final stage of the combustion of solid

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residue, the particle size decreased with time initially but then remained invariant towards the end

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of the combustion, following the shrinking core model. Generally speaking, the key ignition and

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combustion characteristic times decrease with increasing furnace temperature and decreasing

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particle size.

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Acknowledgement

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Financial and other supports have been received from the Australian Research Council under the

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ARC Discovery Project scheme (DP110103699) and the ARC Linkage Project scheme

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(LP100200135).


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Wu, L.; Qiao, Y.; Gui, B.; Wang, C.; Xu, J.; Yao, H.; Xu, M., Effects of Chemical Forms of

van Eyk, P.; Ashman, P.; Nathan, G., Mechanism and kinetics of sodium release from

Li, C. Z.; Sathe, C.; Kershaw, J. R.; Pang, Y., Fates and roles of alkali and alkaline earth

Ndubizu, C. C.; Zinn, B. T., Effects of metallic additives upon soot formation in polymer

Bonczyk, P. A., The influence of alkaline-earth additives on soot and hydroxyl radicals in

Bonczyk, P. A., Effects of metal additives on soot precursors and particulates in a

Smith, I. W., The combustion rates of coal chars: A review. Symposium (International) on

Chern, J.-S.; Hayhurst, A. N., A model for the devolatilization of a coal particle sufficiently


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Energy & Fuels

307

31.

Law, C. K., Recent advances in droplet vaporization and combustion. Progress in Energy

308

and Combustion Science 1982, 8, (3), 171-201.

309

32.

310

Particles. Journal of Engineering for Power 1963, 85, (3), 183-188.

311

33.

312

Combustion and Flame 2007, 151, (3), 426-436.

Essenhigh, R. H., The Influence of Coal Rank on the Burning Times of Single Captive

Mitchell, R. E.; Ma, L.; Kim, B., On the burning behavior of pulverized coal chars.

313


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

314

Figure Captions

315

Figure 1

FTIR spectrum of Zhundong lignite

316

Figure 2

Schematic diagram of the single particle ignition rig

317

Figure 3

A schematic diagram showing the definitions of key ignition and combustion characteristics of single lignite particles

318

319

Figure 4

(a) greyscale intensity and (b) first-order derivative of the greyscale intensity as a

320

function of the numbers of vertical pixels of a burning 3 mm Zhundong lignite particle

321

at 1073 K.

322

Figure 5

Typical sequences of images of (a) 3 mm Zhundong lignite burning at 1073 K, (b) 2.5

323

mm Zhundong lignite burning at 1123 K and (c) 3 mm Zhundong lignite burning at

324

1273 K in air

325

Figure 6

5(a)

326

327

Figure 7

Temporal size variations of (a) 3 mm particles burning at various furnace temperatures and (b) 2, 2.5 and 3 mm particles burning at the fixed temperature of 1123 K.

328

329

Temporal variations of the flame displacement of the burning particle shown in Figure

Figure 8

Variations of the heterogeneous ignition time, homogeneous ignition time, volatile

330

flame extinction time and the total combustion time of single Zhundong lignite particles

331

of 3 mm in diameter burning in air as a function of furnace temperature

332

Figure 9


333

flame extinction time and the total combustion time of single Zhundong lignite particles

334

of 2, 2.5 and 3 mm in diameter burning in air at 1123 K

335


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336

Energy & Fuels

Figure 1

337 338

Figure 1

FTIR spectra of Zhundong lignite

339


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

340

Page 16 of 26

Figure 2

Linear Stepper Motor

Coal Particle

Backlight

CCD Camera PLC

Motor Controller

Temperature Controller

SiC fibre Furnace Computer

341 342

Figure 2

Schematic diagram of the single particle ignition rig

343


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344

Energy & Fuels

Figure 3

345

346 347 348

Figure 3

A schematic diagram showing the definitions of key ignition and combustion characteristics of single lignite particles

349


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

350

Figure 4

351 352

Figure 4

(a) greyscale intensity and (b) first-order derivative of the greyscale intensity as a

353

function of the numbers of vertical pixels of a burning 3 mm Zhundong lignite

354

particle at 1073 K.

355


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356

Energy & Fuels

Figure 5

Ignition

(a)

Image No.

1

2

3

4

5

6

7

(Time, s)

(t=0)

(t=6.7)

(t=9.02)

(t=11)

(t=63)

(t=77)

(t=77.06)

Ignition

(b)

Image No.

1

2

3

4

5

6

7

(Time, s)

(t=0)

(t=4.54)

(t=4.84)

(t=6)

(t=50)

(t=60)

(t=60.4)

Ignition

(c)

Image No.

1

2

3

4

5

6

7

(Time, s)

(t=0)

(t=4.24)

(t=4.76)

(t=6.5)

(t=52)

(t=69)

(t=69.3)

357 358

Figure 5

Typical sequences of images of (a) 3 mm Zhundong lignite burning at 1073 K, (b)

359

2.5 mm Zhundong lignite burning at 1123 K and (c) 3 mm Zhundong lignite burning

360

at 1273 K in air

361


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

362

Figure 6

363 364 365

Figure 6

Temporal variations of the flame displacement of the burning particle shown in Figure 5(a)

366


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Energy & Fuels

Figure 7

368

369 370 371

Figure 7

Temporal size variations of (a) 3 mm particles burning at various furnace temperatures and (b) 2, 2.5 and 3 mm particles burning at the fixed temperature of 1123 K.

372


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

373

Figure 8

374 375

Figure 8


376

flame extinction time and the total combustion time of single Zhundong lignite

377

particles of 3 mm in diameter burning in air as a function of furnace temperature

378


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379

Energy & Fuels

Figure 8

380 381

Figure 8


382

flame extinction time and the total combustion time of single Zhundong lignite

383

particles of 2, 2.5 and 3 mm in diameter burning in air at 1123 K

384


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

385

Table Caption

386

Table 1

Proximate and ultimate analyses of the Zhundong lignite

387

Table 2

Ash chemistry of the Zhundong lignite

388


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389

390

Energy & Fuels

Table 1

Proximate and ultimate analyses of the Zhundong lignite Proximate analysis

Ultimate analysis

(wt% d.b.1)

(wt% d.a.f.2)

Ash

Fixed carbon

Volatile matter

C

H

O3

N

S

3.4

59.7

36.9

70.5

2.6

25.3

0.6

1.0

Note: 1. dry basis; 2. dry ash free basis; 3. oxygen content was calculated by difference.

391


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

392

Table 2

Page 26 of 26

Ash composition of the Zhundong lignite Ash composition (wt%) SiO2

Al2O3

CaO

Fe2O3

K2O

MgO

Na2O

P2O5

TiO2

5.42

6.39

40.7

3.06

0.55

7.62

6.08

0.048

0.30

393


Experimental Study of Ignition and Combustion Characteristics of

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