Engineering, 2013, 5, 309-315
http://dx.doi.org/10.4236/eng.2013.53042 Published Online March 2013 (http://www.scirp.org/journal/eng)
Experimental Research on Spontaneous Combustion
Tendency of High Volatile Blended Coals
Jinfeng Ma1, Fei Shao2, Junrui Shi1, Zhijia Xue1, Shuqun Wang1, Hongtao Li1, Huangxin Zhang1
1Department of Energy and Power, Shenyang Institute of Engineering, Shenyang, China
2MI Ninth Design & Research Institute Co., Ltd., Changchun, China
Received August 18, 2012; revised January 25, 2013; accepted February 3, 2013
ABSTRACT
Experiment of oxidation characteristics with slow velocity of high volatile blended coals was investigated in the sur-
roundings of low oxygen conditions, which gives three kinetic factors in the step of low temperature through the meth-
ods of thermo gravimetric analysis, the Coats-Redfern integration and Achar-Brindley-Sharp-Wendworth. The results
of calculation show that activation energy and pre-exponential factor increase with the reaction process, and tendency of
spontaneous combustion of the blended coal have changed. The experimental results show that the type of absorbing
oxygen is mainly physical adsorption and calorific value is very small during increasing weight. Volatile of blended
coal releases before single coal and combustion temperature of blended coal is between one and other single coals. If
the oxygen content is below 16% in the milling system, explosion-proof effect on high volatile blended coal can be ob-
tained.
Keywords: Thermogravimetric Analysis; High Volatile; Blended Coal; Kinetic Theory; Oxidation Characteristics
1. Introduction
Adding inert medium in power station boiler system can
reduce the oxygen concentration and realize the inhibi-
tion of combustion reaction. It can also eliminate the
pulverized coal spontaneous combustion and ignition
source. In a word, it is a very effective method of pre-
venting explosion. Referring to high volatile pulverized
coal explosion-proof standards of other countries, China
regulates that explosion protection index of oxygen vol-
ume share is less than 14%. Considering the drying re-
quirements of lignite, bituminous coal boiler burning
with lignite is difficult to meet explosion index above, so
research on oxidation characteristics of high volatile
blended coal in the surroundings of low oxygen atmos-
phere is carried on in this paper.
Long-term accumulation of coal powder in powder
system after slow oxidation may be spontaneous ignition.
Many domestic scholars have studied coal oxidation and
pyrolysis characteristics. Aiming at the particle size with
150 μm and oxygen concentration with 21%, research of
coal oxidation spontaneous combustion begins, which is
carried on in reference [1-3]. Coal powder of 20 - 60 μm
in the powder plant boiler is in the majority, reference
[4-8] studies about only the pulverized coal under the
inert condition of pyrolysis, the combustion characteris-
tics and burnout characteristics under the condition of
high temperature and oxidation on the particle size range
of coal powder, while slow oxidation spontaneous com-
bustion characteristics of blended coal under the condi-
tion of low temperature and low oxygen are not in-depth
study.
Aiming at high volatile Huolinhe lignite and three
kinds of typical northeast bituminous coal preparation of
mixed coal, according to high volatile coal explosion-
proof technology index at home and abroad [9,10], the
application of thermal analysis technology develops slow
oxidation experiment in the surroundings of low oxygen
atmosphere with 12%, 14% and 16% oxygen and dy-
namic analysis, and it is concluded that oxidation char-
acteristics parameters of coal and dynamic parameters of
characterization of pulverized coal self-ignition orienta-
tion, that is for slow oxidation experiment research in
oxygen atmosphere, providing theoretical basis for ex-
plosion-proof design and operation of bituminous coal
boiler burning with lignite.
2. Experimental Part
The experiment is conducting in the SMP/PF7548/MET/
600W thermal analysis instrument from Swiss MET-
TLER TOLEDO—United States (China) Company. Be-
fore pulverized coal in the coal preparation System go
into the boiler, the oxidation and pyrogenation process of
C
opyright © 2013 SciRes. ENG

J. F. MA ET AL. 310
the accumulated powder belongs to slow oxidation py-
rogenation process. In order to achieve coal oxidation
exothermic information, experiment carried on variable
heating rate with heating slowly in low temperature, and
heating fast in high temperature [3]. 25˚C - 200˚C at
heating rate of 2˚C·mi n –1 and 200˚C - 650˚C at heating
rate of 10˚C·min–1 were selected, the oxygen concentra-
tion was 12%, 14% and 16% in the experiment. Coal
samples are from Huolinhe lignite (HLH), Fuxin bitumi-
nous coal (FX), Fushun bituminous coal (FSH) and Tiefa
bituminous coal (TF).The average particle size is 53 μm.
The results of coal quality analysis are in Table 1. The
number and composition of blended coal are in Table 2.
3. The Experiment Results and Analysis
The trends of Thermogravimetric analysis (TG), Differ-
ential thermogravimetric analysis (DTG) and Differential
scanning calorimetry (DSC) curves are same. The chart 1
is an example of blended coal which gives experimental
results of heat analysis in 16% oxygen concentration
from Figure 1. Figure 2 shows variation diagram of coal
oxidation characteristic parameters with the change of
oxygen concentration.
According to the references [11,12], coal oxidation proc-
ess are divided into 5 stages, that are water evaporation
and desorption weightlessness stage (initial temperature
~T1); oxygen absorption and weight gain (T1 - Ts); ther-
mal decomposition (Ts - T
i); combustion (Ti - T2) and
burnout (T > T2); in which Tl is the completion of water
evaporation and desorption temperature; Ts is volatiliza-
tion separating temperature; Ti is ignition temperature; T2
is the corresponding temperature of complete combustion.
The experiment studies low temperature oxidation of
coal, so the 3 stages are not considered after the step.
From the Figure 1, in the step of water evaporation
and desorption, the environment temperature is quite low
and coal oxygen molecules between the collision and
contact in the slow condition and activated molecules in
coal are very few. Reaction rate between coal and oxy-
gen is very slow. Damage of the original key coal and
new material production become quite difficult. Moisture
and inherent moisture of coal have been evaporated, also
gas absorption in coal is desorption. Coal sample is ab-
sorbed in oxygen when water evaporats, owing to water
evaporation loss weight is greater than the oxygen gain
weight, TG curves showed a downward trend. DSC
curve is negative, and the stage of coal sample is in the
absorbing state.
In the step of oxygen absorption and weight increasing,
temperature of pulverized coal increases and the chance
of binding collisions between oxygen and coal increases
and activated molecule increases and chemical reaction
speeds up. Because of water evaporation and desorption,
the pulverized coal porosity and free surface increase,
and pulverized coal is absorbed in oxygen fast, TG curve
is a tendency of rising. If chemical oxygen and chemical
reaction of oxygen consumption are mainly in the stage,
heat will increase, DSC curves rises with oxidation tem-
perature rising. But in Figure 1, DSC curve drops first
with the increase of temperature, DSC curve showed a
Table 1. The analysis results of coal samples.
Element analysis (%) Industrial analysis (%)
Coal samples
Car H
ar O ar N ar S ar M ad A ad V ad F ad
HLH 37.73 2.64 10.37 0.64 0.31 7.28 21.93 34.32 36.47
FSH 38.11 2.39 9.51 0.73 0.3 1.69 31.50 34.50 32.31
FX 42.71 2.72 8.77 0.5 0.9 5.99 24.49 30.00 39.52
TF 42.24 2.61 10.0 0.61 0.39 3.95 33.80 27.52 34.73
Table 2. Composition and tag of blended coals.
Blended coal number HLH FSH TF FX
1# 70% 30% 0 0
2# 50% 50% 0 0
3# 70% 0 0 30%
4# 50% 0 0 50%
5# 70% 0 30% 0
6# 50% 0 50% 0

J. F. MA ET AL. 311
T
i
T
s
T
1
Figure 1. Thermal analysis curves of coal sample.
rising trend nearly at the end of oxygen increasing weigh.
It explains that the coal oxygen is mainly physical ad-
sorption, and oxidation heat of chemical adsorption and
reaction are little, indicating in pulverizing system in less
than 16% oxygen concentration can achieve an explosion
effect of high volatile blended coal.
Combustion and explosion of essence of pulverized
coal are mixed with oxygen that forms explosion, vola-
tilization separating temperature Ts has an important in-
fluence on spontaneous combustion and explosion of
coal. From Figure 2, with the oxygen concentration in-
creasing, variation of coal volatilization separating tem-
perature is inconsistent, this is because volatile devola-
tilization connects with the atmosphere condition under
oxidizing conditions, and it is mainly relation to coal
sample’s microstructure. Volatile devolatilization tem-
perature of blended coal basically is lower than the cor-
responding single coal, concluding that blended coal
volatile in advance compared with single coal.
Coal spontaneous combustion is the ignition tempera-
ture. This paper uses the commonly method of TG-DTG
to determine temperature of the pulverized coal ignition
[12]. From the Figure 2, the ignition temperature de-
creases with the oxygen concentration increasing. Igni-
tion temperature of blended coal is between one and
other corresponding single coals. With the proportion of
Huolinhe lignite increasing, ignition temperature of mixed
coal decreases. Compared with 14% oxygen concentra-
tion, volatilization separating temperature and ignition
temperature lower values of coal samples are below 9˚C
on the condition of 16% oxygen concentration.
4. Calculation of Kinetic Parameters
4.1. Dynamic Analysis Method
This paper uses Coats-Redfern integration and Achar-
Brindley-Sharp-Wendwort differential methods and makes
use of 18 kinds of kinetic mechanism function relying on
reference [13]. Compared with the calculation results of
the same mechanism function of differential method and
integral method, the mechanisms of the reaction func-
tions are inferred by Bagchi [14].
According to the law of mass action and Arrhenius
equation

dexp
dAERTf t



 (1)
Using non-isothermal method, increasing temperature
with constant heating rate, and setting up the heating rate
d
d
T
t
 , Equation (1) can be transformed into
 
dexp d AERT T
f


 (2)
Definition
 
0
d
Gf




 (3)
Coats and Redfern, according to Equations (1)-(3), that
are derived and simplified to Coats-Redfern integral
French program is as follow,


2
ln ln
G
A
RE
ERT
T








 (4)
Equation (1) is for separation of variables, on both
sides of the exponential Achar-Brindley-Sharp-Wend-
worth differencial French programs is as follow,

dd
ln ln
t
AE
f
RT






 (5)
where,

is conversion rate of reaction; t is reaction

J. F. MA ET AL. 312
12 13 14 15 16
200
210
220
230
240
250
260
Volatilization separating temperature /℃
Oxygen concentration /%
Huolinhe lignite
Fushun bituminous
1# blended coal
2# blended coal
12 13 14 15 16
200
210
220
230
240
250
260
2
7
0
Huolinhe lignite
Fuxin bituminous
3# blended coal
4# blended coal
Oxygen concentration /%
Volatilization separating temperature/℃
12 13 14 15 16
210
220
230
240
250
260
270
Oxygen concentration /%
Huolinhe lignite
Tiefa bituminous
5# blended coal
6# blended coal
Volatilization separating temperature/℃
12 13 14 15 16
300
320
340
360
380
400
420
440
460
480
500
Huolinhe lignite
Fushun bituminous
1# blended coal
2# blended coal
Oxygen concentration /%
Ignition temperature/℃
12 13 14 15 16
320
340
360
380
400
420
440
460
480
Oxygen concentration /%
Huolinhe lignite
Fuxin bituminous
3# blended coal
4# blended coal
Ignition temperature/℃
12 13 1415 16
320
340
360
380
400
420
440
460
480
Huolinhe lignite
Tiefa bituminous
5# blended coal
6# blended coal
Oxygen concentration /%
Ignition temperature/℃
Figure 2. Oxidation characteristics parameters with dif f erent oxygen concentration.

J. F. MA ET AL. 313
time, min; T is thermodynamic temperature, K;
A
is
apparent pre-exponential factor, min–1; is apparent
activation energy, J/mol; is molar gas constant, its
value is 8.31 J·K–1·mol–1;
E
R


f

and

G

are re-
spectively for the reaction mechanism of differential
form and integral form.
4.2. The Calculation Results
Apparent pre-exponential factor is parameters of the
combustion reaction speed. The greater the pre-exponen-
tial factor, the more intense reaction speed is showed.
The apparent activation energy is on behalf of the reac-
tant molecules from the initial steady state, which be-
comes activated molecules required for absorption of
minimum energy. Their numerical size reflects the re-
sponse of difficulty level [15]. Usually the smaller the
activation energy is, the less spontaneous combustion
tendency, the possibility of explosion reduces. The ref-
erence [16] makes the activation energy in low tempera-
ture oxidation stage from 45˚C to 70˚C as spontaneous
combustion tendency identification indexes of coal. The
reference [11] says after the loss of water, coal adsorbed
plenty of oxygen when generating complex physical and
chemical reaction in weight gain stage. The phase of the
activation energy is intrinsically connected with the
chemical structure and coal spontaneous combustion
mechanism of coal, which expresses difficulty level of
normal temperature oxidation and spontaneous combus-
tion of coal accurately.
On the basis of this paper, according to the thermal
analysis of experimental results in 16% oxygen concen-
tration, in view of water evaporation and desorption and
oxygen gain stage, conducting kinetic parameters calcu-
lation, the results in Table 3. To obtain kinetic mecha-
nism function of high volatile mixed coal.
Reaction mechanism function is reaction model of n =
3 in water evaporation and desorption stage.
where
 
3
1f

 
 
2
111
2
G 







Reaction mechanism function is reaction model of n =
3 A-E in oxygen absorption and weight gain stage.
where,
 
2 11ln1
3
f  



 
2
ln 1G 

 




The calculation results are in Table 3, the activation
energy and pre-exponential factor of Huolinhe lignite
increase when the reaction process is in depth, the activa-
tion energy and pre-exponential factor of three kinds of
bituminous coal decrease when the reaction process is in
depth.
In water evaporation and desorption stage,
HLH TF FX FSH
EEEE

 ,
HLH TF FXFSH
lnln lnln
A
AAA

 ,
namely the sort law of the activation energy and pre-
exponential factor are the same. But, the activation en-
ergy of Huolinhe lignite is obviously lower than three
kinds of bituminous coal. According to the view of ref-
erence [16], spontaneous combustion tendency of Huo-
linhe lignite is the largest. The activation energy E and
pre-exponential factor A sort law of blended coal are as
Table 3. Results of kinetics parameters.
Water evaporation and desorption stage Oxygen absorption and weight gain stage
Coal samples
Temperature range/˚C E/kJ·mol–1 lnA/min–1 r Temperature range/˚C E/kJ·mol–1 lnA/min–1r
HLH 25 - 123.23 161.36 58.42 0.9854123.23 - 220.99 263.30 64.90 0.9767
FSH 25 - 69.06 273.09 100.63 0.958469.06 - 283.50 119.04 26.18 0.9789
FX 25 - 80.29 250.59 94.08 0.978780.29 - 279.50 162.05 38.05 0.9847
TF 25 - 81.79 237.43 88.53 0.985581.79 - 285.15 149.28 34.37 0.9820
1# 25 - 157.67 87.14 32.27 0.9825157.67 - 237.86 357.96 89.92 0.9456
2# 25 - 95.51 196.92 71.66 0.969595.51 - 252.18 208.68 50.49 0.9599
3# 25 - 116.11 163.48 58.62 0.9880116.11 - 228.33 286.32 69.92 0.9558
4# 25 - 167.06 63.18 25.76 0.9870167.06 - 238.81 527.65 159.65 0.9821
5# 25 - 104.33 183.84 66.86 0.9857104.33 - 242.33 231.75 56.50 0.9538
6# 25 - 96.85 213.96 78.26 0.959996.85 - 256.03 201.50 48.57 0.9714

J. F. MA ET AL. 314
follows:
##
1HLH 2Fushun
EE EE , ,
##
4HLH3F
EE EE
X
FX
## ##
HLH 56TF1HLH2FSH
ln lnlnlnE EEEAAAA ,
##
4HLH3
ln lnlnln
A
AAA,
##
HLH 56TF
lnln ln ln
A
AAA .
Compared with bituminous coal, the activation energy
of blended coal burning that is composed of Fushun bi-
tuminous coal, Fuxin bituminous coal with Huolinhe
lignite reduces, according to the view of reference [16],
spontaneous combustion tendency of high volatile blended
coal increases.
In oxygen absorption and weight gain stage, the acti-
vation energy of Huolinhe lignite is obviously higher
than three kinds of bituminous coal, sort law of pre-ex-
ponential factor is the same, namely
FSHTF FXHLH
EEEE,
FSHTF FX HLH
lnln lnln
A
AAA .
According to the view of reference [11], spontaneous
combustion tendency of Fushun bituminous coal is the
largest. The activation energy E and pre-exponential fac-
tor A sort law of blended coal are as follows:
##
FSH2HLH 1
EEEE,
##
FX HLH 34
EE EE ,
##
TF 65HLH
EEEE ,
##
FSH2HLH 1
lnln lnln
A
AA A,
##
FX HLH 34
lnlnln ln
A
AAA
it can achieve an explosion proof effect of blended coal
eration of
th
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