Study on Co-combustion Characteristics of Superfine Coal with Conventional Size Coal in O<sub>2</sub>/CO<sub>2</sub> Atmosphere

doi:10.4236/epe.2013.54B007

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Energy and Power Engineering, 2013, 5, 36-40

doi:10.4236/epe.2013.54B007 Published Online July 2013 (http://www.scirp.org/journal/epe)

Study on Co-combustion Characteristics of Superfine Coal

with Conventional Size Coal in O2/CO2 Atmosphere

Yuhang Zhang, Mingyan Gu*, Biao Ma, Huaqiang Chu

Department of Metallurgy and resource, Anhui University of Technology, Ma’an shan, China

Email: seveny1110@foxmail.com, [email protected]

Received March, 2013

ABSTRACT

The pulverized coal combustion in O2/CO2 atmosphere is one of the promising new technologies which can reduce the

emission of carbon dioxide and NOx. In this study, the combustion behaviors of different mixing ratio of Shenhua coal

with 20 μm and 74 μm particle size in the O2/CO2 atmosphere and air atmosphere were studied by using a ther-

mal-gravimetric analyzer. The combustion characteristics such as ignition and burnout behavior were investigated in the

temperature from 20℃ to 850℃. The influence of mixing ratio on combustion characteristics was conduced. The re-

sults obtained showed that the ignition temperature of the two kinds of particle size in O2/CO2 atmosphere is higher than

in the air, while the activation energy in O2/CO2 atmosphere is lower. With the increasing ratio of 20 μm superfine pul-

verized coals, the ignition temperature and the activation energy decreased, while the DTG peak value increased, the

maximum burning rate position advanced. There were three trends for the ignition temperature curve with the increas-

ing of superfine coal ratio: the ignition of the mixed coal decreased rapidly, then changed less, at last reduced quickly.

Keywords: Coal Particle Size; Co-combustion; Thermo-gravimetric; Heat of Combustion

1. Introduction

Coal makes a significant contribution to energy produc-

tion and because of its availability and flexibility，it is

likely to continue to be a major part of the energy scene

for some time. However, this fuel has the disadvantage of

producing more CO2 per unit of energy produced than

the other fossil fuels which will in time require the ap-

plication of carbon capture and storage technologies.

Coal combustion in O2/CO2 is considered as a new gen-

eration of combustion technology that both direct access

to high concentrations of CO2 and comprehensively con-

trol the coal-fired pollutant emission [1-3]. During the

O2/CO2 combustion, coal is burnt in a mixture of O2 and

recycled flue gas (mainly CO2 and H2O), to yield a rich

CO2 stream. Previous results have shown that the re-

placement of N2 by CO2 can cause significant differences

in the area such as burning stability, char burnout, heat

transfer and gas temperature profiles [4-6]. The flame

propagation speed, flame stability and gas temperature in

O2/CO2 environment are lower and the unburned carbon

content is higher than those in air environment. Experi-

mental studies on coal-fired O2/CO2 combustion have

been performed in laboratory and semi-industrial scale

test facilities of different size [7-10]. The coal combus-

tion appears ignition delay and combustion instability in

O2/CO2 [11], improving the oxygen concentration [12]

and using the superfine pulverized coal combustion can

improve the combustion characteristics, and the particle

size has a great influence on the combustion characteris-

tics [13-14].

In this study, the combustion process of mixed coal

with 20 μm and 74 μm of Shenhua coal under both air

and O2/CO2 atmosphere were employed by a thermal-gra-

vimetric analyzer (TGA). The influence of superfine

pulverized coal on the combustion characteristics was

analyzed in the atmosphere of O2/CO2 and the ignition

temperature and kinetic parameters were obtained. The

results obtained provide a better understanding of coal

combustion in O2/CO2 atmosphere.

2. Experimental

A NETZSCH STA449C simultaneous thermal analyzer

was used in this work. The temperature was increased

from 20℃ to 850℃ linearly at the heating rate of 10

K·min-1. The flow rate of evolved gas was fixed at 80

ml·min-1, 7.5 mg of fuel sample with 20 μm and 74 μm

sizes were used for each test. Two kinds of different par-

ticle size with Shenhua coal and their mixture were used

in this study and their proximate analysis was listed in

Table 1. The sizes of coal are 20μm and 74μm respec-

tively.

Y. H. ZHANG ET AL. 37

3. Result and Discussion

3.1. The Coal Combustion under Different

Atmospheres

The TG and DTG profiles for all cases were obtained to

analyze the combustion characteristic parameters. For all

TG curves, it was shown that after an initial moisture

removal, the volatile matter started to release and then a

rapid weight loss were observed. Temperature at which

the DTG curve showed peak value was denoted as the

maximum weight loss temperature. The ignition tem-

perature (Ti) was obtained by co-analyzed TG-DTG

curves, at which the DTG has its peak value and the cor-

responding slope to the intersection with respect to the

TG profile. Burnout temperature (Tb) was determined

when the mass loss reaches 99% of final weight loss at

the temperature of 850℃.

The TG-DTG curves for 20μm and 74μm coal in

O2/CO2 and air atmosphere were shown in Figure 1.

From these TG curves, it can be seen that, there were

three stages of weight loss during the coal burning. First

stage corresponds to the removal of inherent moisture

until the temperature exceeds 100℃. Following that was

the volatile matter release in the second stage; finally, the

char combustion was observed. Compared with that un-

der air atmosphere, the coal DTG curve moved to the

right and the maximum weight loss temperature in-

creased while the maximum weight loss values decreased

in same concentration of oxygen under O2/CO2 atmos-

phere.

Table 2 showed the parameters of two particle size

coal combustion characteristics in air and O2/CO2 at-

mospheres. For 20 μm coal in air atmosphere, the igni-

tion temperature increased from 375℃ to 384℃ and the

burnout temperature increased from 658℃ to 796.1℃

under O2/CO2 atmosphere. For 74 μm coal, the ignition

increased from 396℃ to 401℃ and the burnout tem-

perature increased from 648.6℃ to 784.5℃ in O2/CO2

atmosphere. The ignition temperature of coal with 20 μm

was lower than the one with 74 μm, even lower than the

ignition temperature in air atmosphere. In addition, under

O2/CO2 atmosphere, for 20μm coal, the maximum weight

loss decreased from 13.72% to 13.62%, while the maxi-

mum weight loss temperature increased from405℃ to

412℃ and the burnout rate increased from95.52% to

97.18%. For 74μm coal, the maximum weight loss de-

creased from 12.67% to 12.36%, while the maximum

weight loss temperature increased from 437℃ to 441℃

and the burnout rate increased from 99.39% to 99.96%.

Table 1. Proximate analysis (mass/%).

Fuel M A V FC

ShenHua coal(SH) 4.80 5.00 31.90 58.30

Figure 1. TG and DTG curves under O2/CO2 and air at-

mospheres.

Table 2. Combustion characteristic parameters of two par-

ticle size coals.

coal atmosphereTi(℃)T

b(℃) (dW/dt)max

(%/min)

Tmax

(℃)η(%)

Air 375 658.0 13.72 40595.52

20 μm

O2/CO2 384 796.1 13.62 41297.18

Air 396 648.6 12.67 43799.39

74 μm

O2/CO2 401 784.5 12.36 44199.96

Arrhenius kinetic parameters were obtained by ana-

lyzed the TG-DTG curves, as shown in Table 3. A sim-

ple kinetic analysis was performed which a single step

reaction process is supposed.

The kinetic parameters of two particle size coals were

listed in Table 3. It was clearly that in O2/CO2 atmos-

phere, the reaction activation energy of coal with both 20

μm and 74 μm diameters were lower than that in air en-

vironment, and the activation energy of 20 μm coals were

lower than 74 μm coal.

Y. H. ZHANG ET AL.

Figure 2 showed the TG and DTG curves for blending

coals with 20 μm and 74 μm Shenhua coal with different

ratio under O2/CO 2 atmosphere. From the figures, it can

be seen that, blending the 20 μm coal into 74 μm coal

moved the coal DTG curve to the left, increase the

maximum weight loss and the maximum burning rate.

Combustion characteristic parameters of blending

coals were listed in Table 4. From the figures, it can be

seen that for the 20 μm and 74 μm blending coals, with

the increasing ratio of 20 μm coal from 30% to 70%, the

Table 3. Kinetic parameters of two particle coals.

coal atmosphere

E/(kJ﹒mol-1) A/s-1 R

Air 101.46 164737.3 0.9666

20μm

O2/CO2 81.44 3246 0.9822

Air 122.94 4578628.6 0.9878

74μm

O2/CO2 91.87 17248.9 0.9788

Figure 2. TG and DTG curves for different superfine coal

ratio.

Table 4 .Combustion characteristic parameters of blending

coals.

Coal Ti/°C Tb/°C (dW/dt)max

/%·min-1

Tmax

/°C η/% Cb

/×10-5

30%20μm

+70%74μm392782.612.27 438 99.697.98

70%20μm

+30%74μm388779.612.78 425.5 98.998.49

80%20μm

+20%74μm386778.113.25 415.5 97.828.89

Table 5. Kinetic parameters of blending coals.

coal E/(kJ﹒mol-1 ) A/s-1 R

30%20μm + 70%74μm 87.85 8994.2 0.9787

70%20μm + 30%74μm 82.73 3652.3 0.992

80%20μm + 20%74μm 81.79 2352.7 0.9821

maximum weight loss temperature decreased from 438

℃ to 425.5℃, increasing the ratio of 20 μm coal to 80%,

the maximum weight loss rate appears at 415.5℃; With

the increasing ratio of superfine coal, the peak value of

DTG curves increased, and the maximum weight loss

was from 12.27% to 13.25%. Increasing the ratio of

20μm coal lower the ignition temperature from 392℃ to

386 ℃ and the burnout temperature decreased from

782.6℃ to 778.1℃. From Table 4, it also can be seen

that with the increasing ratio of superfine coal, the burn-

out rate decreased which may be caused by the small

coal particle size and packing density.

This paper used the flammability index to compare the

combustion characteristics of different blending coals. It

is defined as Cb= (dW/dt)max/ Ti

2 ;Cb mainly reflect the

coal combustion reaction capability which the higher the

value, the better the flammability of coal. For the blend-

ing coals, with the increasing of 20 μm coal, the flamma-

bility index of mixed coals increased from 7.98 to

8.89(×10-5).

Table 5 showed the kinetic parameters of blending

coals. It can be seen that increasing the ratio of 20 μm

coal decreased the reaction activation energy from 87.85

to 81.79 (kJ﹒mol-1). Higher the ratio of 20 μm coal, the

lower the activation energy value.

Figure 3 showed the ignition temperature of different

ratios of blending coals. It can be seen that with the in-

creasing of 20 μm coal, the ignition temperature tends to

a downward trend. When the ratio of 20 μm was from 0

to 20%, the ignition temperature decreased rapidly, while

the ratio was 20% to 60%, the ignition temperature

Y. H. ZHANG ET AL. 39

Figure 3. The ignition temperature of blending coals.

changed less. Increasing the ratio of 20 μm coal further,

the ignition temperature decreased rapidly again.

4. Conclusions

In this paper, the combustion characteristics of ShenHua

coals with diameters of 20 μm and 74 μm and their mix-

ture under air and O2/CO2 atmosphere had been investi-

gated using the TG analysis. The conclusions were listed

as follows.

1) The coal burning rates decreased and the ignition

temperature lowered under O2/CO2 atmosphere compared

with coal combustion in the air environment.

2) For coal burning both in O2/CO2 atmosphere and air,

the ignition temperature with superfine size coal was

lower than that of large particle size, and the maximum

weight loss rate was higher than the large particle size

coal.

3) The addition of 20 μm coal into 74 μm lowered the

ignition temperature under O2/CO2 atmosphere. With the

decrease of coal particle size, the DTG curve moves to

the left, the ignition temperature and the burnout tem-

perature decreased which indicating that the coal ignition

advanced and it's easy to reach the ignition temperature

and combustion.

4) Activation energy of blending coals combustion va-

ried with the ratio of 20 μm for blending coals. For 20

μm - 74 μm blending coals, the activation energy de-

creased with the increasing ratio of 20 μm coal.

5. Acknowledgements

The present work was supported by National key scien-

tific instrument and equipment development project

(2012YQ22011 9).

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