Experimental Research on Spontaneous Combustion Tendency of High Volatile Blended Coals

doi:10.4236/eng.2013.53042

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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

Email: [email protected]

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

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

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

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







 (1)

Using non-isothermal method, increasing temperature

with constant heating rate, and setting up the heating rate



, Equation (1) can be transformed into

 

dexp d

AERT T





 (2)

Definition

 



 (3)

Coats and Redfern, according to Equations (1)-(3), that

are derived and simplified to Coats-Redfern integral

French program is as follow,





ln ln

ERT













 (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,



ln ln









 (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

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;

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;







and





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

 





 

111

















Reaction mechanism function is reaction model of n =

3 A-E in oxygen absorption and weight gain stage.

where,

 

11ln1

 







 

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

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

## ##

HLH 56TF1HLH2FSH

ln lnlnlnE EEEAAAA ,

4HLH3

ln lnlnln

AAA,

HLH 56TF

lnln ln ln

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

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

AA A,

FX HLH 34

lnlnln ln

AAA

it can achieve an explosion proof effect of blended coal

eration of

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