VOL. 10, NO.

15, AUGUST 2015 ISSN 1819-6608

ARPN Journal of Engineering and Applied Sciences
©2006-2015 Asian Research Publishing Network (ARPN). All rights reserved.

www.arpnjournals.com

THE EFFECT OF BLEND RATIO ON THERMAL BEHAVIOR AND ASH

COMPOSITIONS OF COAL/BARK CO-COMBUSTION
Theerapong Laongnual and Anusorn Chinsuwan
Department of Mechanical Engineering, Faculty of Engineering, KhonKaen University, KhonKaen, Thailand
E-Mail: theerapl@scg.co.th

ABSTRACT
The purpose of this work was to study the effect of blend ratio on thermal behavior and ash compositions of
coal/bark co-combustion. The study consists of two experiments, thermal behavior analysis and ash composition analysis
by using the non-isothermal thermo gravimetric method (TGA) under combustion conditions and X-ray fluorescence
(XRF) respectively. The results of the two experiments can show the interaction between coal and bark at three of blending
ratio30:70, 25:75, and 20:80 by weight. Bark shows a good interaction with coal leading to significant reduction in ignition
temperature of coal and this effect was more pronounced with higher blending ratio of bark. In addition, it was found that
the ash compositions were depended on the blend ratios.

Keywords: co-combustion, kinetics analysis.

1. INTRODUCTION and 20:80 by weight respectively. Proximate and ultimate

Coal is one of the most important fossil fuels in analyses of the fuels are shown in Table-1.
the world and widely used in industry. As per the report of
the International Energy Agency (IEA), world energy Table-1. Proximate and ultimate analysis of the fuels.
demand will increase by 1 to 3 times of current used
during 25 years from 2010 to 2035. In addition, the IEA's
report also shows that the demand of coal is still
increasing. Coal combustion generates CO2which causes
to global warming and makes the average temperature of
the world increase.
To minimize the global warming, using of
biomass as a fuel has become very important for the
production of clean thermal energy by combustion. This is
due to it neutral on the issue of the emission of CO2, the
most important gas causing global warming, so it does not
cause global warming.
Although the biomass has many advantages, it
also has disadvantage that cannot be avoided. Combustion
of biomass can result in ash deposition and high corrosion
(Davidsson et al. 2008) that effects to the thermal
efficiency, and expensive shutdown of boilers. In general,
biomass is composed of high alkali (Potassium, K and
Calcium, Ca) and Chlorine (Cl) and low sulphur (S) a
as received basis, bdry basis
content compare to coal. Alkali from co-combustion with
bark can cause low-melting-temperature ash which results
The materials were prepared for thermal analysis
in expensive shutdown of boilers. In this study, behavior
by Thermo gravimetric Analysis (TGA) method and Ash
of co-combustion and ash analysis between local
analysis by X-ray fluorescence (XRF).
eucalyptus bark and Indonesian coal at various blend
ratios will be investigated. The obtained results will be
b. Thermo gravimetric analysis (TGA)
useful information for bark and coal co-combustion.
Similar to several researchers (Cheoreon et al.
2013), (Gil et al. 2010), (Jillian et al. 2013), (Kaihua et al.
2. EXPERIMENTAL
2013), (Marisamy et al. 2010), (Munir et al. 2009), TGA
was used to investigate the thermal behavior of blending
a. Experimental setup
fuels. In this work, the experiments were carried out by the
The experiments were carried out for co-
simultaneous thermal analyzer, NETZSH model STA
combustion of two types of fuels: sub-bituminous from
449F3. The maximum weight loss temperature was
Indonesia and eucalyptus bark from KhonKaen, Thailand.
estimated as the temperature at which a derivative
There are five samples: Coal, Bark, C30B70, CB25B75,
thermogravity (DTG) curve showed the peak value.
and C20B80. The sample C30B70, CB25B75, and
Burnout temperature was detected based on the mass
C20B80 corresponds to the ratio of coal/bark 30:70, 25:75,

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 VOL. 10, NO. 15, AUGUST 2015 ISSN 1819-6608
ARPN Journal of Engineering and Applied Sciences
©2006-2015 Asian Research Publishing Network (ARPN). All rights reserved.

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stabilization. All of experiments were carried out at derivative thermo gravimetric analysis (DTG) were used
atmospheric pressure at a constant rate of heating up to investigate combustion behavior of the Coal/Bark
10°C/min from 30°C to 850°C which were oxidized by blends. The TG and DTG results in Figure-1 and Figure-2,
oxygen and carried out by placing about 200 mg of dried respectively. According to the results, Coal and Bark have
sample on a platinum pan. The TG and DTG curves are difference profiles so they will have difference burning
show in Figure-1 and Figure-2, respectively. performance. The profiles indicate that the process can be
divided into three stages: moisture evaporation,
c. Kinetic analysis decomposition of volatile organic compound and char
In a heterogeneous solid-state reaction, the initial combustion.
mass of the sample wo is decomposed to instantaneous Based on volatile matter to fixed carbon ratio, the
mass wt at time t, hence the rate change can be written as a bark is expected to be more reactive than the coal. After
function of weight conversion ratio as: initial moisture is removed, three characteristic
temperatures are investigated for this study: the first
wo  wt
 (1) initiation temperature where the weight loss first begins to
wo  w fall, the second is the peak temperature where the weight
loss reaches the maximum, and the third is the burn-out
Where w∞ is the residue mass at the end of process. The
temperature where the weight is constant indicating the
reaction rate may be described by Arrhenius equation:
completion of combustion.
d
 k (T ) f ( ) (2)
dt a) Bark
The first stage of weight loss of bark starts from
k(T) is the rate constant, according to the Arrhenius 36°C to 127 °C which is an initial moisture loss. It
correlation: corresponds to about 43% of the total weight. The second
k (T )  AeE/ RT (3)
stage or combustion stage starts from 127°C up to 487°C,
at this stage the decomposition and release of volatile
Where T is the reaction temperature, A is the pre- starts approximately at 170°C and there are two peaks of
exponential Arrhenius factor, E is the activation energy weight loss at the temperature of 306°C and 405°C. One is
and R is the gas constant. mainly due to volatile combustion and the other is char
f(α) is the function called the reaction model which combustion. The last stage is the stage of the low reactive
describes the dependence of the reaction rate on the extent components reaction or the burn-out stage which
of reaction. It is defined as: corresponds to temperature of 436 °C. This effect can be
clearly seen in the DTG profile for bark in Fig. 2. As it has
f ( )  (1   ) n (4) high volatile content, bark can be classified as a high
reactive material.
Where n is the reaction order. Substitution equations (3)
and (4) into equation (2): b) Indonesian coal
d The weight loss reduced from 100 to 70 % (wt
 Ae E / RT 1   
n
(5) %) during the initial moisture loss. After that, the weight is
dt
quite constant until the temperature of 240°C and then the
For non-isothermal TGA heating rate, β = dT/dt . Equation rate of weight loss starts to increase again due to pre-
(5) can be written as: combustion. In the case of coal, there is only single peak
of weight loss in the combustion stage and higher
d A
 (1   ) n e  E / RT (6) temperature is required for combustion to start as
dT  compared to the bark. The volatile release temperature for
coal is around 216°C compared to 170°C for bark. This
Equation (6) can be rearranged as:
indicates that coal requires a higher temperature to release
 d  its volatile matter, which makes it difficult to initiate
  A E (7)
ln  dT n   ln  combustion, and also the activation energy is higher than
 1      RT bark.
 

According to equation (7), a plot of ln [(dα/dT)/(1-α)n ]

versus 1/T corresponds to a straight line. For the value of n
which gives regression coefficient close to unity, A and E
can be determined from the intercept and slope of the line
respectively, which can show in Figure-3.

3. RESULTS AND DISCUSSIONS

Thermo gravimetric analysis (TGA) and

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ARPN Journal of Engineering and Applied Sciences
©2006-2015 Asian Research Publishing Network (ARPN). All rights reserved.

www.arpnjournals.com

the ignition temperature and the activation energy, but the

bark did not show a reduction in the burn out temperature.

Table-3. Thermal kinetic results of the samples.

Figure-1. TG curves of coal, bark, C30B70, C25B75, and

C20B80.
d) Ash analysis
Although the results from TGA can show that the
bark can improve devolatization of coal and also the
activation energy was reduced when the ratio of bark
increases. Another important thing except thermal
behavior is the ash composition of blending. The
combustion of biomass (for this study is bark) may cause
operational problems such as agglomeration of the bed
material and deposits on super heater tubes (Davidsson et
al. 2008). The result from XRF is necessary to know the
ash composition of any blends to avoid the problems as
mentioned.

Figure-2. DTG curves of Coal, Bark, C30B70, C25B75,

and C20B80.

c) Co-combustion behavior of blend materials

The Coal/Bark blends were prepared in three
ratios 30:70 (C30B70), 25:75 (C25B75), and 20:80
(C20B80). The characteristic parameters were given in
Table-2 and Table-3. The TG and DTG curves of the
blend materials show three regions of weight loss and two
peak temperatures same as the coal. The first region is
initial moisture loss, the second is volatile combustion,
and the last region is burnout.

Table-2. Thermal characteristic parameters of the

samples.

The mainly difference of blend materials compare

to coal is the temperature corresponding to maximum rate
of weight loss (Tmax) and the ignition temperature (Ti).
Impact of the high volatile contents in the bark can reduce

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 VOL. 10, NO. 15, AUGUST 2015 ISSN 1819-6608
ARPN Journal of Engineering and Applied Sciences
©2006-2015 Asian Research Publishing Network (ARPN). All rights reserved.

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Figure-4. Ash composition of coal (AsCoal), bark

(Asbark), C30B70 (C30B70), C25B75 (AsC25B75), and
C20B80 (AsC20B80).

After complete combustion process from TGA analysis,

ashes of all samples were investigated by Energy
Dispersive X-ray Fluorescence analyzer which is
illustrated in Figure-4. The element of ash composition is
expressed in oxide form. The coal in this study consists of
high silica content in the form of quartz (SiO2) compare to
the bark, this element can result in low-melting-
temperature ash when reacts with alkali from bark.
Furthermore, the bark contents of high Ca (in the presence
of CaO). Higher ratio of the bark can cause low-melting-
calcium-based silicates results in bed agglomeration in
CFB boilers (Elisabet et al. 2005).
Ca and K concentration in ash increased with an
increasing of the bark ratio. This may result in low-
melting-temperature ash as mentioned. On the other hand,
low-melting-temperature ash can also form with silica
from coal. In order to avoid the problem from interaction
between alkali (Ca and K) from the bark and silica from
the coal, some countermeasures should be concerned such
as minimize the bark ratio, change the type of coal (low
silica content) or type of biomass before co-combustion.

4. CONCLUSIONS
As the result of TGA and kinetic analysis, the
bark blending improves the devolatization and ignition
characteristics of the coal due to the high volatile contents
in the bark. The ignition temperature and the activation
energy of coal were reduced as the percent of the bark
ratio increases. Although the coal/bark blend enhances the
combustion rate of the coal, they did not show a reduction
in the burn out temperature, this is the effect from char
combustion of bark.
The results from XRF can show the ash
composition of individual fuels (coal and bark) and their
blends. The co-combustion of the coal/bark may cause low
Figure-3. Curves of fitting of kinetic model for: (A) Coal, melting temperature ash and results in expensive shutdown
(B) Bark, (C) C30B70, (D) C25B75, and (E) C20B80. of boilers.
Moreover, the results of this study can be applied
for another fuels to know the interaction between two (or
more than two) species of fuels which is the future works
of this study, and it will be useful for large scale or
commercial scale boilers to avoid expensive shutdown.

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©2006-2015 Asian Research Publishing Network (ARPN). All rights reserved.

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