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Characterization on ignition and volatile combustion of dispersed coal particle streams: in-situ diagnostics and transient modelling Yang Xu, Shuiqing Li, Qi Gao, Qiang Yao, and Jianmin Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01322 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018
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Characterization on ignition and volatile combustion of dispersed coal particle
2
streams: in-situ diagnostics and transient modelling
3 4
Yang Xua, Shuiqing Lia,*, Qi Gaoa, Qiang Yaoa, Jianmin Liub
5
a
6
Power Engineering, Tsinghua University, Beijing, 100084, China
7
b
8 9
Key laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and
Guodian Science and Technology Research Institute, Nanjing, 210023, PR China
Mailing Address:
10
Department of Energy and Power Engineering, Tsinghua University, Beijing, 100084, China
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Fax: +86-10-62794068; Tel: +86-10-62788506
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Email: lishuiqing@ tsinghua.edu.cn (Prof. Shuiqing LI)
13
Abstract
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In this paper, applying OH-PLIF diagnostics, we explored the ignition and volatile combustion of dispersed coal
15
particle streams in a Hencken flat-flame burner. The effects of ambient temperature and oxygen mole fraction on
16
coal combustion behaviors have been systematically examined. First, different coal combustion stages and ignition
17
modes (i.e. heterogeneous ignition, homogeneous ignition and hetero-homogeneous joint ignition) in different
18
ambiences are characterized by the OH-profiles within the coal flame domain, with ignition delay time and volatile
19
combustion time being quantitatively determined. Then, an improved transient model, integrating the gas phase
20
flame-sheet reaction model and the CPD (Chemical Percolation Devolatilization) model, is established to reveal the
21
underlying physicochemical mechanisms during the early stage of coal combustion. This transient model well
22
captures the effects of ambiences on both the characteristic ignition delay time and volatile combustion time.
23
Finally, based on experimental observations and simulation results, a novel criterion is proposed to identify ignition
24
modes by comparing characteristic heterogeneous/homogeneous ignition time.
25 26
Key words: coal combustion, ignition, volatile combustion, PLIF diagnostics, transient modelling. 1
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1. Introduction
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Coal is continuing to be one of the most significant primary energy carriers for power generation in long term,
3
because of its enormous reserves and low costs [1]. Whereas, the environmental concerns caused by emissions of
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NOx, particulate matters and even greenhouse gases from coal combustion have received considerable attention [1].
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Effective strategies for clean coal utilization are in urgent need, including low NOx combustion technology [2],
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oxy-coal combustion technology [3], hybrid solar-combustion technology [4], etc. All these concerns require a
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deeper insight into the underlying physicochemical mechanisms of coal combustion process, especially the ignition
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and volatile combustion in the initial combustion stage, as they determine the stability and safety during the
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optimization of combustion systems [5].
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Coal combustion is a complicated process involving high temperatures, transient conditions, the presence of
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radical species and multiple phases, which brings great challenge to the accurate measurement using traditional
12
intrusive diagnostic approaches. Therefore, optical diagnostics techniques, which allow an in-situ noninvasive
13
measurement of velocity, density, temperature, pressure, and species concentration, recently stimulate a major
14
interest. For the study of ignition during the initial stage, Costa and co-workers [6-7] investigated the ignition mode
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and ignition delay time of both coal and biomass particles by high-speed imaging techniques. Levendis and
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co-workers [8-9] also studied the ignition behaviors of different ranked coals in a drop tube furnace by high-speed
17
cinematography. Particularly, the optical emission signatures from important intermediate products of reactions
18
between hydrocarbons and atomic or molecular oxygen (i.e. CH and OH), with pathways described as C2H+O →
19
CO+CH*, C2H+O2 → CO2+CH*, CH+O2 → CO+OH*, can provide a convenient chemically based diagnostic for
20
the detailed kinetics during the combustion process, especially in the initial stage of devolatilization [10-11]. The
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most widely used optical techniques for the detection of CH and OH signal can be generally divided into two
22
categories. One is the spontaneous spectrum, which basically results from energy state relaxation from an excited 2
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electronic state and is manifested by the energy release in a chemiluminescent reaction without the need for an
2
external laser inducement. For instance, Shaddix et al. [12-13] used an intensified CCD camera coupled with a
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431-nm filter for the imaging of CH* chemiluminescence in both the conventional and oxy-fuel combustion
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conditions to study the ignition delay of pulverized coal streams. Moon et al. [14] adopted a similar methodology to
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investigate thermochemical and combustion behaviors of different ranks of coals as well as their blends. However,
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one main drawback of this method is the contamination of CH* signal by CO2* as well as thermal radiation from
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hot soot and char particles [12,14]. Recently, a multi-filter methodology, first proposed and validated by Karnani
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[15] et al. in a sooting flame, has been extended to obtain pure CH* signal in coal combustion system by Yuan et al.
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[16]. Nevertheless, the strict requirement for the synchronization of the multi-channel measurements and the
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stability of the particle feeding system may still restrict its application in coal combustion diagnostics. The second
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kind of technique, laser-based diagnostics, featured by their superior accuracy and versatility, has received renewed
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attention in current combustion researches. Laser-induced fluorescence (LIF) technique provides another reliable
13
option in the detection of unstable intermediates like OH, NO, CH, etc. [17-18]. Most importantly, LIF technique
14
enables us to get relatively pure CH and OH signals exempting the aforementioned interference in sooting flames
15
by selecting a reasonable laser fluence and detector parameter [10, 19]. Koser et al. [10] presented a planar LIF
16
(PLIF) measurement of OH radical in the boundary layer of single coal particle, which enables one to identify the
17
reaction and post-flame zones and gives an access to track the temporal evolution of a single burning particle.
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Hwang et al. [19] applied a similar method to investigate the spatial relation of combustion reaction zones of
19
pulverized coal particles and found it effective for evaluating the detailed flame structure. Despite a great effort that
20
has been made in coal flame investigation with OH-PLIF, a detailed characterization on the ignition and volatile
21
combustion process, e.g. the distinction of ignition mode, determination of ignition delay time and volatile
22
combustion time, is still lacking. 3
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The challenge also exists in modelling the coal devolatilization and combustion, which originates from the
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complex coupling of sub-processes and the strong dependence of properties and chemical structures of the coal
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samples. The single-rate [20] and two-step [21] models use one and two simple Arrhenius expression(s) to describe
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the devolatilization process, respectively. The Distributed Activation Energy (DAE) model proposed by Anthony et
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al. [22] introduced string statistics to predict the production of monomer species that play a dual role in the
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formation of both volatile tar and char. By taking the effect of the chemical structure of coal into account, Fletcher
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and co-workers [23] put forward the Chemical Percolation Devolatilization (CPD) model, in which coal is
8
represented as a collection of functional groups including aromatic rings, aliphatic chains and bridges and
9
oxygen-carrying groups. With further development and refinement [24-26], CPD has become one of the most
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sophisticated models for coal devolatilization. Tufano et al. [27] and Goshayeshi et al. [28-29] have performed
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series numerical simulations on gas phase ignition of coal particles by incorporating CPD model and the detailed
12
kinetics. Although large amount of refined work has been done, most of them mainly focus on the gas phase
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homogeneous ignition. The potential competition between the two ignition modes (heterogeneous ignition and
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homogeneous ignition) is rarely considered. Therefore, instead of focusing on the gas phase kinetics, this work
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combines the CPD model and a simple gas phase flame-sheet model, and intends to specifically investigate the
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underlying mechanisms of ignition mode transition with the ambient conditions.
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This work aims at investigating the ignition and the volatile combustion of dispersed coal particle streams by
18
the specially developed OH-PLIF diagnostics. First, we discuss the effects of both ambient temperature and oxygen
19
mole fraction on the ignition mode, the ignition delay time as well as the volatile combustion time based on
20
experiments. Then, a transient model, combing the CPD model and gas phase flame-sheet reaction kinetics, is
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further established to elaborate the underlying mechanisms. Finally, based on experimental observations and
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simulation results, we proposed a reasonable criterion for the identification of the ignition modes in different 4
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ambiences.
2
2. Experimental set-up
3
2.1 Fuel properties
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One typical Chinese Shenhua High Ash Fusion (HAF) bituminous coal and one kind of low-volatile
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bituminous char were used in this work. The fuel properties of the coal and char samples are presented in Table 1.
6
All the particle samples were screened to 65-74 µm and dried to air-dry basis before each experiment.
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Table 1. Proximate and ultimate analysis of fuel samples HAF Bituminous coal
Fuel type Proximate analysis (wt%, on a dry basis) Fixed Carbon (%) Volatile (%) Ash(%) Ultimate analysis (wt%, on a dry-ash-free basis) Carbon (%) Hydrogen (%) Oxygen (%) Nitrogen (%) Sulfur (%) Ash Composition(wt%, dry basis) SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 P2O5 K2O Na2O
Char
55.5 24.1 20.4
78.1 3.1 18.8
82.53 4.39 11.26 0.89 0.93
93.66 1.41 2.54 1.50 0.89
56.83 26.17 6.98 2.68 0.7 1.2 2.1 0.48 1.02 0.2
26.11 12.26 22.24 25.44 1.52 0.83 6.42 0.35 0.72 1.5
8 9
2.2 OH-PLIF experimental set-up
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5
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Figure 1. Schematics of the OH-PLIF measurement system
3 4
The experimental set-up is schematically shown in Fig. 1. The coal combustion experiments were
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performed on the optical-accessible flat-flame Hencken burner. With hundreds of 1.5 mm inner diameter stainless
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steel tubes embedded in a 64 mm outer diameter super-alloy honeycomb, the Hencken burner can provide a
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uniform hot-gas environment for coal combustion. CO, with a small amount of CH4 (~10 ml/min), was used as fuel
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gas. We particularly used CO (lack of H) to suppress the OH signal in the background gas flame. A mixture of O2
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and N2 was injected as oxidant and dilution gas. A novel feeder, based on the principle of de-agglomeration via
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high-frequency vibration, was utilized to offer a well-dispersed coal particle stream with a steady flow rate of 0.07
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g/min [30]. In this case, the ignition of coal particle streams can be regarded as a statistical summation of ignitions
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of all isolated particles [31]. The coal particles, carried by N2, were injected into the burner through a 2.5 mm
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stainless steel tube located in the center of the honeycomb. The burner can flexibly reach different working
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conditions by adjusting the flow rates of the fuel gas and diluents, and thus offers a reasonable single variable
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method to investigate the influence of ambience on coal ignition and volatile combustion characteristics [30-32].
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Three typical ambient temperatures, 1200 K, 1500K, and 1800K, with different mole fractions, 0.1, 0.2, and 0.3,
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were selected to examine the ambient effect. Notably, the mole fraction of O2 here refers to the concentration in the
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combustion product gas of Hencken burner. The gas flow rates of CO, O2 and N2 in different ambiences are
19
tabulated in Table 2. The temperature along the height above the burner was further measured by a 100 µm B-type 6
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thermocouple, of which the radiation heat loss had been corrected. The ambient temperature almost keeps constant
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in the region of 10 mm to 80 mm above the burner, providing a stable high temperature environment for
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experimental investigation [32]. Particle velocity along the centerline in different conditions has been measured by
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the Phase Doppler Anemometry (BSA P60 from DANTEC dynamics). After the transformation using the formula
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τ = ∫ 1 / v p dl , the residence time as a function of height above the burner can be obtained [30]. More detailed
6
information about the Hencken burner can be found in our previous work [30-32].
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Table 2. Gas flow rate in different ambiences
Case
T(K)
CO
N2
O2
(L/min)
(L/min)
(L/min)
Oxygen
A1
1200
0.1
5.53
54.47
10.50
A2
1200
0.2
5.56
47.56
17.37
A3
1200
0.3
5.59
40.66
24.26
B1
1500
0.1
6.20
41.23
9.57
B2
1500
0.2
6.24
35.70
15.08
B3
1500
0.3
6.27
30.16
20.59
C1
1800
0.1
6.78
32.24
9.03
C2
1800
0.2
6.82
27.62
13.63
C3
1800
0.3
6.85
22.99
18.23
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A frequency-doubled dye laser (Sirah) pumped at 532 nm by a Nd:YAG laser was used to produce UV pulses
10
of ~25 mJ at 5Hz. The laser was tuned to 283.55 nm to excite the Q1(8) line of the A-X (1-0) transition of OH
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radical, which was calculated and verified to vary less than 5% over the temperature range of 1200 K-1800 K [33].
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Micro-cylindrical lens array (MCLQ(S)-509) was utilized to form a 0.1-mm-thick laser sheet beam with a height of
13
88 mm. The fluorescence was detected using an intensified charge-coupled device (ICCD, QE, Lavision) equipped
14
with a UV-achromat lens and a 10-nm bandwidth filter centered at 307 nm. The gate and gain of ICCD were set as 7
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55 ns and 99. Assuming 100 % laser absorption and no heat losses from the particle to the surroundings, particle
2
heating was estimated to be about 67 K when coal particles moving through the laser sheet. To get rid of
3
irregularities resulting from instantaneous variation of coal feeding rate, 100 pictures were taken and averaged in
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each experiment.
5
3. Model development
6
A simple gas phase flame-sheet model is incorporated with the CPD model to systematically study the detailed
7
reaction mechanisms of single coal particle in the early stage of coal combustion.
8
3.1 Governing equations
9
Assuming a spherical symmetry in a one-dimensional framework and no relative motion between the particle
10
and the surrounding gas, the conservation equations of mass, species and energy for the gas phase are provided as:
11
Mass:
4π r 2
12
13
where ρ is the gas phase density,
14
phase mass exchange.
15
Species:
∂ρYi ∂Y ∂ ∂Y + m& i − (4π r 2 ρ D i ) = 4π r 2ωi + SPYi , ∂t ∂r ∂r ∂r
17
where , and are the mass fraction, diffusion coefficient, and reaction source term, respectively;
18
the release rate of species i from particle the into the gas phase, which is detailed as below.
19
Energy:
20
(1)
& is the mass flow rate, S p m is the source term accounting for the particle-gas m
4π r 2
16
∂ρ ∂m& + = S pm , ∂t ∂r
4π r 2
∂ρhT ∂h ∂ ∂h + m& T − (4π r 2 ρ D T ) = 4π r 2ωh , ∂t ∂r ∂r ∂r
8
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(2)
S P Y i is
(3)
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where ℎ is the enthalpy; is the energy source term from the reaction. Closure of the system is achieved by
2
equation of state for ideal gas P=ρRT/M.
3
The source terms in Eq. (1) and Eq. (2) are given by =
, which is also displayed in Eq. (4), and
4
=
5
devolatilization and oxidation reaction, respectively, which will be detailed in section 3.2.
+
. Here,
and
represent the mass source terms for species i from coal
6
For the particle phase, the particle mass mp, diameter dp, and temperature Tp are used to describe the particle
7
evolution during the combustion process. Within this model, a constant external diameter dp=dp0 is assumed during
8
the devolatilization stage of coal combustion. Then we have the governing equations:
9
Mass:
dm C dm V dm P , = + dt dt dt
(4)
dm V d ρP )=− , dt dt
(5)
dm dm dTP A h(Tgas − TP ) + εσ (Twall 4 − TP4 ) + C hC − V hV , = dt mPCP dt dt
(6)
10 11
Density:
(π d P 3 )(
12 13
14
Energy:
15
where mp, mc and mv are the masses of coal, char and volatile species; ρp is the particle density; Cp, A, and ε are the
16
particle heat capacity, surface area, and emissivity, respectively. Tgas and Twall are the ambient temperature and wall
17
temperature. σ is the Stefan-Boltzmann constant. hC is the heating value for the heterogeneous reaction and hV is
18
the heating value for volatile decomposition, respectively. It should be noted, the assumption that there is no
19
relative motion between the particle and surrounding gas may underestimate the particle heating, but this effect
20
may be less prominent as the particle slip velocity in Hencken burner was reported to be less than 1 m/s [27].
21
3.2 Chemical reaction model
22
Four parameters from
13
C NMR (Nuclear Magnetic Resonance) measurement, including Mcl (the average 9
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molecular weight per aromatic cluster), Mδ (the average side-chain molecular weight), σ+1 (the average
2
coordination number per cluster), and p0 (the fraction of intact bridges) are required as the input of the CPD model.
3
As the experimental data of
4
reasonably estimated by regression results proposed by Jupudi et al [34], as shown in Table 3.
13
C NMR are not available, these four parameters for HAF coal in this work are
5 6
Table 3. Input parameters for the CPD model Coal
Mcl
Mdel
p0
Sig+1
c0
HAF
300
18
0.66
5.1
0
7 8
To verify the parameters in our CPD modeling, we have conducted a pyrolysis experiment on TGA in inert
9
ambience (N2) with a relatively low heating rate (30 K/min) and compared the results with CPD prediction under
10
the same heating rate. As shown in Fig. 2, the results show a satisfactory consistency, implying that the input
11
parameters obtained from the correlation can well reveal the devolatilization properties of Chinese HAF bituminous
12
coal under this circumstance. As it has been verified that the CPD is valid in a wide range of heating rate [24], we
13
reasonably conjecture that CPD model can also well predict the coal devolatilization behaviors under high heating
14
rate on the Hencken burner despite the potential uncertainties caused by the heating rate difference.
15 16
Figure 2. Comparisons of TGA results and CPD prediction with the parameters listed in Table 3.
17 18
The volatile species produced by the CPD model are: CO2, CO, CH4, C2H2, and H2O. Here, C2H2 is used to
19
present the tar species due to similar H/C ratio [27,29]. It should be noted that this assumption is reasonably made 10
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for the simplification of gas phase oxidation, although the ignition behavior of C2-hydrocarbons may be different
2
from larger tar species. The oxidation of combustible volatiles (CO, CH4, and C2H2) in the gas phase is described
3
by global kinetics expressions:
ωi = M V AV [VM ]
nV
4
[O2 ]
nO2
exp(− EV / RTP ) .
(7)
5
MV is the molecular weight of volatile species; [VM] and [O2] are the mole concentration of volatile species and
6
oxygen. AV, EV, nV and are tabulated in Table 4. The oxidation products are assumed to be CO2 and H2O, and
7
this assumption may to certain extent overpredict the rate of release of chemical energy [35].
8
Table 4. Reaction parameters for the homogeneous gas phase reaction [35-37] Av Species CH4 CO C2H2
Ev
1 !" %&' &' ( # ) ( ) 5.2E11 3.2E12 2.1E8
*+
-
203 167 125
0.7 1.5 0.5
0.8 0.25 1.25
,
9 10
Kinetics of surface carbon reaction is first-order in oxygen concentration, which can be depicted as:
.
11 12
&8
= /0 1 23 exp ( 9. )
:;. :;8.75 mm in 1800 K ambience, to 6.25 mm in 1500 K ambience, and then to 3.75 mm in 1200 K
9
ambience, with the same oxygen mole fraction of 0.2. It is noted that the characteristic radial displacement of OH
10
radical in 1200 K case almost approaches the radius of the coal particle stream (1.25 mm, as shown in grey
11
rectangle in Fig. 6), indicating the direct oxidation of char and in-situ volatiles at the particle surface. This further
12
implies a heterogeneous-dominant ignition (HI)/combustion induced the by the slow volatile release and direct
13
attack of the oxygen on the particle surface [30,38-40]. However, in 1500 K and 1800 K, an expanded combustion
14
zone in the gas phase is observed due to the dramatic increase of the volatile releasing rate. The OH signal is found
15
to be intense both in the gas phase and in the particle stream in the case of B1-B3 and C2-C3. It suggests an
16
ignition mode transition from the heterogeneous-dominant (HI) to the hetero-homogeneous joint (HGI) ignition,
17
where the particle surface oxidation and gas phase reaction both exert comparable influences on the ignition and
18
combustion process. Contrarily, in the case of 1800 K-0.1 O2, the OH radical profile highlights a
19
significantly-weakened OH signal intensity (only 2/3 of those in high oxygen ambience, see Fig. 6 (b)) in the
20
central particle stream. It is mostly attributed to the suppression of the heterogeneous oxidation when most of the
21
oxygen is consumed by the bulk volatile species in the gas phase before it reaches the particle surface [38-40]. In
22
this case, the gas phase homogeneous reaction instead plays a dominant role in particle heating and ignition process, 15
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1
leading to a homogenous-dominant ignition (GI). Based on the analysis, we can roughly judge the dominant
2
ignition/combustion mode in different ambiences from the OH profile. A concentrated OH radial profile in the
3
particle stream indicates a heterogeneous dominant ignition mode, which usually happens in low temperature,
4
i.e.1200 K. When temperature increases (1500 K or 1800 K), the OH radial profile gets expanded and coal particles
5
undergo a hetero-homogeneous joint ignition. It will further transfer to homogeneous dominant ignition when there
6
exists no obvious concentrated OH radical around the particle stream in the ambience with insufficient oxygen
7
(1800 K-0.1O2).
8
9 10 11 12
Figure 6. Radial distribution of OH signals at a residence time of 20 ms in different ambiences.
4.3 Characteristic ignition delay time and volatile combustion time
13
The OH/CH signal was also reported as a good indicator for the onset of ignition [17,29]. Moreover, as
14
discussed above, OH signal can be used to distinguish stages of volatile combustion and heterogeneous char
15
combustion during coal combustion. Therefore, we estimate the characteristic ignition delay time as well as the
16
volatile combustion time based on OH-PLIF results.
16
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Figure 7. Definition of ignition delay time and volatile combustion time
4
Figure 7 illustrates the OH signal intensity (raw data and fitting line) in the centerline as a function of the
5
height above the burner in the case of 1500 K-0.2O2. The image is binarized for better understanding of the OH
6
centerline profile. Ignition is considered to happen at the location with the maximal upslope of OH signal intensity
7
[12,14,17]. Volatile combustion ends when the OH signal intensity decreases to the same value as that of the
8
ignition point [12, 14]. The ignition delay time tig and volatile combustion time tvol can thus be determined, as
9
shown in Fig. 7. The polynomial fit line can well capture the sharp intensity rise when referring to the recorded
10
image. However, this may still cause inevitable variations to the results due to the fitting uncertainties. It should be
11
noted, in 1200 K within heterogeneous-dominant domain, a clear identification of volatile combustion zone is
12
difficult because of the slow release of volatile and close incorporation with char combustion [30]. For this reason,
13
the volatile combustion time in 1200 K cases will not be discussed hereafter. Figure 8 shows the experimental
14
results of tig and tvol for HAF bituminous coal in different ambiences. The error bars come from the parallel
15
experiments. The characteristic time deduced based on OH profile is found to be quite reasonable when compared
16
with data from literature [6-7,12]. Generally, tig decreases with increasing ambient temperature and oxygen mole
17
fraction. In cases of A1-A3 where heterogeneous ignition dominates, tig drops remarkably from 18.0 ms at 0.1 O2 to
18
15.3 ms at 0.3 O2 due to the enhanced surface oxidation of coal particles. Whereas, in joint-ignition cases of B1-B3
19
and C2-C3, the enhancement of ignition with increasing O2 becomes less prominent. This can be attributed to a fast
20
release of the volatile which to some extent quenches the heterogeneous reactions [38-40]. Further increment in 17
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ambient temperature and decrease in O2 concentration will enhance this volatile-barrier effect and result in the
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homogeneous-dominant ignition in case C1, which has a delay time of 13.5 ms. In addition, tvol shows a similar
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“dropping” trend with increasing ambient temperature and oxygen concentration in 1500 K and 1800 K cases.
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Similar to the classical droplet combustion, a high temperature accelerates both the devolatilization (vaporization,
5
in the case of droplet) and the consumption of volatile, while an elevated oxygen concentration mainly enhances
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the latter. These combined effects commonly lead to a decrease of tvol from 15.4 ms (B1) to 12.1 ms (B3) in 1500 K
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and from 12.2 ms (C1) to 10.2 ms (C3) in 1800 K.
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9 10 11
Figure 8. Effect of ambient temperature and oxygen mole fraction on ignition delay time and volatile combustion time for HAF bituminous coal
12 13
4.4 Model interpretation
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We further use the developed transient model to predict the ignition and volatile combustion process. In this
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model, the heterogeneous ignition is assumed to occur at the inflection point of the particle temperature profile.
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Compared with the criterion based on the maximum upslope of dmc/dt, the inflection point is more physically
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reasonable, as it represents the point where the heating of the coal particle by the surroundings is overtaken by the
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chemical reactions [29,38]. As for homogeneous ignition, it starts when the gas-phase temperature of a shell (from r
19
to r+∆r) is greater than the temperature of both its adjacent shells [30,38], which also indicates the rapid increment 18
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of local gas phase oxidation rate ( ). As a consequence, the overall ignition time is defined as tig=min[thetero, thomo],
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which implies an ignition mechanism that occurs first. For example, Figure 9(a) and Figure 9(b) individually show
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the evolution of radial gas temperature profile as time proceeds in 1500 K-0.2O2 and 1800 K-0.1O2. The inset
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shows the temperature history of coal particles in these cases. It can be found that the heterogeneous ignition (the
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inflection point) occurs first at about 14 ms in the 1500 K-0.2O2 condition, which is about 3 ms before the
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homogeneous ignition (dramatic increase of gas phase reaction rate) taking place. However, in the 1800 K-0.1O2
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condition, the homogeneous ignition occurs earlier at 13 ms and dominates the ignition process. The tig in these two
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cases, therefore, is determined to be 14 ms and 13 ms, respectively. The volatile combustion time tvol can be further
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obtained from the ignition point to the point when the gas phase volatile combustion ends (no peak in the radial
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temperature profile).
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12 13 14 15
Figure 9. Heterogeneous ignition and homogeneous ignition calculated by the transient model in 1500 K 0.2 O2 and 1800 K 0.1 O2 For better comparison, the calculated results of tig and tvol in different cases are shown in Fig. 10 (a) and (b), 19
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which are found to well capture the experimental trends in all cases (with relative deviations
