Carbon 42 (2004) 351–359

www.elsevier.com/locate/carbon

Combustion behavior of xylite/lignite mixtures

D. Vamvuka *, E. Kastanaki, M. Lasithiotakis, C. Papanicolaou

Department of Mineral Resources Engineering, Technical University of Crete, Kounoupidiana, 73100 Chania, Greece

Received 23 May 2003; accepted 4 November 2003

Abstract

The behavior and the kinetics in nitrogen and air of two low-rank coals (lignite and xylite) and their blends, as well as the

compatibility of the component coals in the blends were evaluated, in an effort for the rational use of poor coals.

The experiments were conducted in a thermobalance system, at non-isothermal heating conditions, with heating rates of 20 and

100 �C/min, in the temperature range of 25–850 �C. Material particle size was )100 lm.

A first-order parallel independent reactions model and a power law model fitted successfully the rate data of pyrolysis and

combustion, respectively. Activation energy values and reaction orders ranged from 23 to 182 kJ/mol and 0.8 to 2 respectively. The

heating rate did not affect the kinetic parameters considerably, however when this was increased the reactions were shifted to higher

temperatures and the rates were greater.

The pyrolysis kinetics of lignite/xylite blends could be sufficiently predicted, based on the data of the individual fuels. However,

this was not true in the case of char combustion. Blending of lignite with xylite, in any proportion, seemed to cause some interactions

between the component coals in air.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Coal; B. Pyrolysis, combustion; C. Thermal analysis; D. Reaction kinetics

1. Introduction

The problem of the rational use of low-rank coals has

not yet been solved. In Greece, where lignites constitute

the major energy resource, covering 70% of the demand

for electricity production, this problem is caused by the

significant share of non-combustibles and their un-favorable chemical properties, which results in a lower

availability and a higher running cost of the plants.

An efficient way of solving the above problem is

probably blending. Coals can be blended to produce a

usable fuel from the coals available and increase the

number of supply options. Also, apart from reducing

plant costs, they can be blended to improve the com-

bustion performance and meet emission limits [1].However, as coal quality affects virtually every compo-

nent in the power plant, the impact of blending different

coals is not known and blending can result in serious

problems.

Since reaction rates are influenced by the composition

and the particle size of the coals, as well as the operating

*Corresponding author. Tel.: +30-821-37603; fax: +30-821-69554.

E-mail address: vamvuka@mred.tuc.gr (D. Vamvuka).

0008-6223/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2003.11.002

conditions and plant design, the knowledge of the re-

activities and the kinetics of the blends will provide

valuable information for the proper operation of the

combustion systems. The reactivities of carbonaceous

materials are commonly investigated by using iso-

thermal and dynamic thermogravimetric experiments.

There is an extensive literature on coal pyrolysis andcombustion kinetics [2–7], however this is limited as

concerns the kinetics of the blends [6,8]. Over-

simplifications, such as the description of the whole

process by a pseudo-first-order reaction, have been

widely used [3,4,7,9], however more sophisticated mod-

els, describing the pyrolysis/combustion process as a

series, consecutive or parallel first/nth-order processes,

are being developed recently, as being more accurate[5,10–12].

The overall aim of this work was to evaluate the

compatibility of the component coals in the blends, with

respect to pyrolysis or combustion and investigate pos-

sible interactions, which may or may not be beneficial, in

an effort for the rational use of poor coals. Thus, the

devolatilization and combustion reactivities of a lignite

and a xylite and their blends were studied under non-isothermal thermogravimetric experiments and kinetic

models, which could approximate reactivity changes

352 D. Vamvuka et al. / Carbon 42 (2004) 351–359

during conversion and could be used in mass balance

equations established in pulverized coal combustion

systems, have been developed.

2. Experimental

2.1. Materials

The materials, which were used in the present study,

were a lignite from the Ptolemais basin and a xylite from

the Vevi basin, in north Greece. Both are brown coals of

similar rank. However, the lignite is a lithotype having

matrix coal derived from marsh reeds, sedges, etc.,which are rich in cellulose, while the xylite is a lithotype

rich in woody material with over 10% stems and there-

fore rich in lignin. The chemical characterization of

these coals is shown in Table 1. Blends of the Ptolemais

lignite with the xylite, in proportions 80/20, 60/40, 40/60

and 20/80, were also prepared. All samples were grinded

to a particle size of )100 lm.

2.2. Thermogravimetric analysis

The experimental system used was a TGA/DTG

Perkin–Elmer thermobalance (precision of temperature

measurement ±2 �C, microbalance sensitivity <5 lg),with which the sample weight loss and rate of weight

loss as functions of time or temperature were recorded

continuously, under dynamic conditions, in the range

25–850 �C. The experiments were carried out at atmo-spheric pressure, under nitrogen/air atmosphere, with a

flow rate of 45 ml/min, at linear heating rates of 20 and

100 �C/min. Preliminary tests with different sample

masses and sizes and gas flow rates were carried out, in

order to check the influence of heat and mass transfer.

Small masses (20–25 mg) of each material, thinly dis-

tributed in the crucible and particle sizes of )100 lmwere found proper to be used in the experiments, inorder to eliminate the effects of eventual side reactions

and mass and heat transfer limitations.

The experimental procedure for obtaining a typical

thermogram was as follows: the sample under test was

uniformly spread and weighed directly into the crucible.

After flushing with nitrogen, the temperature was in-

creased to 110 �C in order to dry the sample. After a

drying period of 30 min, the temperature was increasedto the desired value of 850 �C at the pre-selected heating

Table 1

Proximate analysis, ultimate analysis and calorific values of coal samples, on

Sample Volatile

matter (%)

Fixed carbon

(%)

Ash (%) C (%)

Ptolemais lignite 45.19 25.95 28.86 40.60

Vevi xylite 55.39 38.65 5.96 57.10

rate and held constant at this value, until steady con-

ditions were obtained. The furnace was then cooled

down to ambient temperature, the nitrogen stream was

switched to air at a flow rate of 45 ml/min and thetemperature was again increased to 850 �C, until the

coal mass was completely burned.

The experiments were replicated at least twice to de-

termine their reproducibility, which was found to be

very good.

3. Kinetic model

The pyrolysis process was described by the in-

dependent parallel first-order reactions’ model [5,10–13].

According to this model, the decomposition of coal is

described by a number of independent parallel reactions.

The overall rate of conversion for N reactions and the

thermal decomposition of the individual components

can be expressed as:

� dmdt

¼X

i

cidaidt

i ¼ 1; 2; 3; . . . ;N ð1Þ

daidt

¼ Ai expð�Ei=RT Þð1� aiÞ ð2Þ

where:

dm=dtmass loss rate

i component

ai conversion rate

ci contribution of the partial process to the overall

mass loss, m0 � mchar

Ai pre-exponential factor

Ei activation energyR gas constant

T temperature

The model equations and the variation coefficient are

reported in detail by Gronli et al. [13].

Combustion of the coal chars was described by a

power law model. To include the char heterogeneity into

the model, it was assumed that a coal char sample couldbe a mixture of components with different reactivities

[11]:

mðtÞ ¼Xn

j¼1

cj½1� ajðtÞ� þ m1 ðmð0Þ ¼ 1Þ ð3Þ

a dry basis

H (%) N (%) O (%) S (%) HHV

(MJ/kg)

4.11 1.57 24.02 0.84 14.9

5.36 0.94 29.78 0.86 22.7

D. Vamvuka et al. / Carbon 42 (2004) 351–359 353

where, m is the sample mass normalized by the initial

sample mass, n is the number of components, cj is the

fraction of combustibles in component j, ajðtÞ is the

reacted fraction of component j in time t and m1 isthe normalized amount of the solid residues (minerals)

at the end of the experiment.

A separate equation was used for each component to

describe the dependence of the reaction rate on the

temperature and fractional burn-off:

dajdt

¼ Aj expð�Ej=RT ÞgðPO2Þf ðajÞ ð4Þ

where, Aj is the pre-exponential factor of component j,Ej is the activation energy of component j, g expresses

the effect of ambient gas composition and f describes

the change of surface reactivity as a function of the

fractional burn-off.

As the function gðPO2Þ represented the partial pres-

sure of oxygen in air, its value was included into the

pre-exponential factor, while the f ðajÞ function wasdescribed by:

dajdt

¼ ð1� ajÞnj ð5Þ

where nj is the reaction order.

4. Results and discussion

4.1. Thermal decomposition characteristics

A comparison between the DTG pyrolysis and com-

bustion curves of the coals, obtained at a heating rate of

20 �C/min, is illustrated in Fig. 1. As can be seen, the

bulk of the volatiles was released between 350 and 500�C. Both samples present two overlapping peaks in this

temperature region. On the other hand, the bulk of the

burning process occurred mainly between 450 and 600

�C. Lignite shows a bimodal combination of chars with

differing reactivity, in this temperature region, whereas

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

110 210 310 410 51

Temperature [

-d(m

/mo)/

dt

[1/m

in]

xylite (pyrolysis)

xylite (combustion)

lignite (combustion)

lignite (pyrolysis)

Fig. 1. DTG profiles of coal samp

xylite shows one single broad peak. The peak of lignite

occurring at temperatures higher than 700 �C is con-

sistent with the decomposition of calcium carbonate, as

shown in a previous work by the author [14]. Takinginto consideration that the peak height can provide a

relative measure of the reactivity, the xylite seemed to be

more reactive, as its decomposition rates were 2–3 times

higher than those corresponding to the lignite.

Heating rate may affect the devolatilization and

burnout processes. An increase of the heating rate from

20 to 100 �C/min led to the evolution of slightly higher

yields of total volatiles and therefore lower yields ofchar. Also, a higher heating rate of 100 �C/min increased

the reaction rates by an order of magnitude and shifted

the reactions towards much higher temperatures. This

shift was more pronounced in air (Fig. 2) and has been

assigned by several authors [4,15] as being due to the

combined effects of the heat transfer and the kinetics of

the decomposition.

The behavior of lignite/xylite blends was studiedunder the same experimental conditions. Maximum

pyrolysis and char combustion rates, as well as peak

temperatures for blending ratios 0:100, 20:80, 40:60,

80:20 and 100:0 are compared in Table 2. It can be

observed that when a blend in any proportion of xylite

to lignite was pyrolysed, an increase in maximum rate

and a shift in peak temperature towards that of xylite,

with the increase in its content of the sample was ob-served.

Furthermore, from Fig. 3, which represents the

burning profiles of lignite/xylite mixtures, it can be seen

that in the region of primary combustion the bimodal

combination of lignite char and the broad peak of xylite

char produced one single large peak, representing a

more homogeneous char. The maximum combustion

rate of the blends was increased and it was higher thanthat corresponding to the pyrolysis process, while the

maximum peak temperature was shifted as before to-

wards that of xylite. Therefore, it seems that some in-

teraction between lignite and xylite occurred, which

0 610 710 810oC]

0

0.02

0.04

0.06

0.08

0.1

-d(m

/mo)/

dt

[1/m

in]

les (heating rate 20 �C/min).

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

240 340 440 540 640 740 840

Temperature [oC]

-d(m

/mo)/

dt

[1/m

in]

0

0.04

0.08

0.12

0.16

0.2

-d(m

/mo)/

dt

[1/m

in]

100 °C/min(pyrolysis)

20°C/min (combustion) 100°C/min (combustion)

20°C/min (pyrolysis)

Fig. 2. Effect of heating rate on DTG profiles of Ptolemais lignite char, in air.

Table 2

Devolatilization and combustion characteristics of Ptolemais lignite/xylite blends (heating rate: 20 �C/min)

Sample Blending ratio Max decomposition

rate· 102 (min�1)

Temperature at max

decomposition rate

(�C)

Volatiles content (%

dry)

Char content (% dry)

Devolatilization

Vevi xylite/Ptole-

mais lignite

0:100 2.05 443 43.8 56.2

20:80 3.02 396 45.4 54.6

40:60 4.08 397 47.4 52.6

60:40 4.66 394 49.9 50.1

80:20 5.38 392 51.3 48.7

100:0 6.34 394 54.8 45.2

Char conversion up to 650 �C (%

dry)

Char combustion

Vevi xylite/Ptole-

mais lignite

0:100 4.93 460 85.3

20:80 5.43 519 89.3

40:60 6.07 518 93.2

60:40 7.19 518 95.3

80:20 7.90 534 96.9

100:0 9.05 522 99.4

354 D. Vamvuka et al. / Carbon 42 (2004) 351–359

increased the reactivity of the blends, due to the different

structure, petrographic and ash compositions of the

component coals.

As Fig. 4 shows, the huminitic subgroup of the two

coals is made up of different components. The lignite is

made of highly mashed and finely chopped organic

matter derived from marsh reeds, as it contains a high

percentage of humodetrinite, which is combusted atlower temperatures. On the other hand, the xylite is

made of lignin encountered in coniferous trees and when

resinite, a highly flammable material at low tempera-

tures, is absent from humotelinite, it is combusted at

higher temperatures [16]. Furthermore, low temperature

ashing and X-ray-diffraction ash analyses of Ptolemais

lignite and Vevi xylite samples [17] revealed that the

lignite was richer in 2:1 mixed layered silicates (whichhave larger surface area and could probably contribute

to the lowering of the combustion temperature), calcium

bearing minerals and quartz. In order to understand

better the effects of petrographic and ash compositions

during combustion, some more tests were undertaken.

Thus, the materials were acid washed to remove the

inorganics and then tested by thermogravimetric ana-

lysis, under the same experimental conditions. The

procedure of acid washing is described in detail else-where [14,18]. The results showed that the presence of

minerals increased the reactivity of the chars in air, by

lowering their peak combustion temperature. On the

other hand, as illustrated in Fig. 5, it can be seen that

even if the two coals were demineralized, the peak

temperature of their blend was very close to that of

xylite. Also, the calculated DTG curve for the blend (by

Eq. (6)) resembles the experimental one. Therefore, itseems that the different petrography and to a greater

Fig. 4. Plate 1: photomicrographs of matrix type brown coal, Pliocene age, of Ptolemais basin, Macedonia, north western Greece. Incident light, oil

immersion objectives. (a) Texto-ulminite (TU) associated with densinite (D). White light, 500·. (b) Highly fluoresent suberinite (Su) infilled with

corpohuminite (C). Blue light, 500·. Plate 2: photomicrographs of xylite type brown coals, Miocene age, of Florina basin, Macedonia, north western

Greece. Incident light, oil immersion objectives. (a) Longitudinal tangential section of textinite A. White light, 200·. (b) Texto-ulminite B with well

preserved cell lumen (Ce). White light 200·.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

250 350 450 550 650 750 850

Temperature [oC]

-d(m

/mo)/

dt

[1/m

in]

80% xylite

60% xylite

40% xylite

20% xylite

lignite

xylite

Fig. 3. DTG char burning profiles of lignite/xylite blends (heating rate 20 �C/min).

D. Vamvuka et al. / Carbon 42 (2004) 351–359 355

extent the different mineral matter content and compo-

sition of the component coals (xylite had a much lower

amount of mineral matter than lignite) affected the

combustion behavior of their blend. Similar interactions

between the components of the individual coals in

blends have been reported by other investigators [1,19].

4.2. Kinetics

The pyrolysis process was modeled by lumping the

numerous reactions into 3–5 first-order parallel reac-

tions, while the combustion process was described by a

power law model, adjusted to include char hetero-

geneity. From Fig. 6, which compares experimental and

calculated pyrolysis profiles of lignite, it can be observed

that the agreement between model predictions and ex-

perimental measurements is very good, with a deviaton

value of 1.98%. The best fit parameters estimated by the

model are tabulated in Table 3. It is clear that deviation

values for pyrolysis were below 2% and for combustionbelow 4%. In the region of primary devolatilization,

200–600 �C, (for heating rate 20 �C/min) the estimated

energy values of lignite vary between 19 and 107 kJ/mol.

However, those of xylite are higher (54–182 kJ/mol), as

the bulk of the devolatilization process occurred below

500 �C, in this case. In the case of char combustion, the

activation energies change in the range of 97–154 kJ/mol

between the two chars and the order of the reactions

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

250 350 450 550 650 750 850

Temperature [oC]

-d(m

/(m

o-m

ash))

/dt

[1/m

in]

demineralized xylite

demineralized lignite

demineralized blend ofxylite-lignite(60%-40%)experimental

demineralized blend ofxylite-lignite(60%-40%)calculated

Fig. 5. DTG char burning profiles of raw and demineralized samples (heating rate 20 �C/min).

0

0.01

0.02

0.03

0.04

0.05

0.06

200 300 400 500 600 700 800

Temperature [oC]

-d(m

/mo)/

dt

[1/m

in]

reaction 1

reaction 2

reaction 3

calculated

experimental

Fig. 6. Kinetic evaluation of Ptolemais lignite in nitrogen (heating rate 20 �C/min).

356 D. Vamvuka et al. / Carbon 42 (2004) 351–359

describing the combustion process varies between 0.75

and 2. These data are in accordance with those given in

the literature [2,4,5,11,20] and suggest that both pro-

cesses were chemically controlled.

The kinetic parameters obtained by the model at a

higher heating rate are also included in Table 3, for

comparison. As expected, the process becomes succes-

sively faster as the heating rate is increased and it waspreviously shown that the peaks attained by the rate of

weight loss were higher. This is a consequence of the

increase in the average reaction temperature. It can be

seen from the table that pre-exponential factors and

activation energies are similar between the chars tested

under low and high heating rates, revealing the absence

of heat transfer effects. Also, it can be observed that in

all cases the parameter n became higher when theheating rate was raised and took the value of 2.

In order to investigate whether the kinetics of the

blends could be predicted from the kinetic parameters

obtained for the isolated compounds, theoretical DTG

curves, representing the sum of the individual compo-

nents’ behavior in the mixture, were calculated:

ðdm=dtÞsum ¼ x1ðdm=dtÞlignite þ x2ðdm=dtÞxylite ð6Þ

where ðdm=dtÞlignite, ðdm=dtÞxylite are the normalized rates

of mass loss, as found from the individual experiments

and x1, x2 are the mass fractions of lignite and xylite in

the mixture, respectively.

Fig. 7, being representative among the samples,shows that the fit between experimental and calculated

DTG pyrolysis curves for xylite blends is very good,

with deviation values below 2%. Therefore, it seems that

no significant interactions occurred in the solid phase

during devolatilization and a cumulative model was

proposed, involving eight independent parallel reac-

tions, which are the sum of the reactions representing

the decomposition of each coal sample. However, Fig. 8shows that by a simple summation of the burning pro-

files of the component coal chars the fit between ex-

perimental and calculated DTG curves was not so good,

with deviation values ranging between 8.8% and 14.5%.

Similar fits were obtained for all mixtures. In order to

determine to which extent this non-additivity of the DTG

curves complicates the prediction of the combustion

Table 3

Calculated kinetic parameters for pyrolysis and char combustion (deviation (%)¼ 100{ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiO:F:ðz� NÞ

p=maxð�dm=dtÞexp}, where z is the number of data points and N is the number of parameters)

Reaction Atmosphere Heating rate (�C/min)

Sample

Ptolemais lignite Vevi xylite

Kinetic parametersa Kinetic parametersb

A (min�1, pyr.),

(min�1 MPa�1,

com.)

E (kJ/mole) c (%) n A (min�1, pyr.), (min�1 MPa�1,

com.)

E (kJ/mole) c (%) n

1 Nitrogen 20 1.5 · 103 47.7 12.2 1.7 · 1014 181.5 15.9

100 5.5 · 103 53.9 21.1 8.7 · 1014 180.9 9.0

Air 20 1.2 · 109 150.6 50.2 2 1.2 · 109 154.3 39.3 2

100 1.2 · 109 146.4 21.1 2 1.2 · 109 144.1 14.9 2

2 Nitrogen 20 2.6 19.1 60.9 3.4 · 103 53.6 37.1

100 2.4 · 101 25.8 45.5 1.2 · 105 63.1 42.7

Air 20 2.7 · 106 130.9 35.1 0.75 7.7 · 103 96.5 60.7 0.8

100 2.7 · 106 130.7 37.2 2 2.7 · 106 127.4 26.6 2

3 Nitrogen 20 1.8 · 107 107.0 5.1 2.8 23.1 47.0

100 1.8 · 107 109.1 2.2 2.8 23.1 47.0

Air 20

100 7.8 · 102 91.3 58.5 2

4 Nitrogen 20 5.3 · 1020 398.1 9.9

100 9.4 · 1015 318.6 7.2

Air 20

100

5 Nitrogen 20 1.8 · 105 119.9 11.9

100 1.8 · 105 121.0 24.0

Air 20

100

aNitrogen: at 20 �C/min dev¼ 1.98%; at 100 �C/min dev¼ 1.17%. Air: at 20 �C/min dev¼ 3.6%; at 100 �C/min dev¼ 2.9%.bNitrogen: at 20 �C/min dev¼ 0.96%; at 100 �C/min dev¼ 1.59%. Air: at 20 �C/min dev¼ 3.5%; at 100 �C/min dev¼ 3.8%.

D.Vamvukaet

al./Carbon42(2004)351–359

357

0

0.1

0.2

0.3

0.4

0.5

0.6

140 240 340 440 540 640 740 840

Temperature [oC]

-d(m

/mo)/

dt[

10-1

/min

]

20% lignite

80% xylite

blend calculated

blend experimental

Fig. 7. Additivity of DTG pyrolysis curves for lignite/xylite blends (heating rate 20 �C/min).

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

200 300 400 500 600 700 800

Temperature [oC]

-d(m

/mo)/

dt

[1/m

in]

60% lignite

40% xylite

blend calculated

blend experimental

Fig. 8. Additivity of char burning profiles for lignite/xylite blends (heating rate 20 �C/min).

358 D. Vamvuka et al. / Carbon 42 (2004) 351–359

reactivity of the blends, the power law model, previously

described, was also used to model the kinetics of the

mixtures. Some of the calculated results, at a blending

ratio of lignite to xylite 40:60 are represented in Table 4.As can be observed, the calculated values of the kinetic

parameters A, E and n for the blends were closer to the

values corresponding to xylite only. This information

will be useful for the co-firing of these low-rank coals. It

appears that although the devolatilization of the mixture

is sufficiently predicted by a simple summation of the

individual behavior of its components, in the case of

char combustion some interactions between the in-

Table 4

Calculated kinetic parameters for the combustion of coal char blends

at a blending ratio 60:40. (heating rate: 20 �C/min)

Reaction Vevi xylite/Ptolemais lignite

Kinetic parametersa

A (min1 MPa�1) E (kJ/mole) c (%) n

1 2.0 · 109 156.9 38.7 2

2 9.8 · 103 96.9 61.3 0.7

aDev¼ 3.9%.

dividual fuels occur. These interactions complicate the

prediction of the combustion behavior of blends and as

they influence combustion rates and heat-release, the

whole design of the boilers will be affected.

5. Conclusions

The thermochemical reactivity of xylite was higher

than that of lignite, in both nitrogen and air atmo-

spheres. When blending lignite with xylite, the max-

imum rate was increased and the maximum peak

temperature was shifted towards that of xylite, with the

increase in its content of the mixture.

A first-order parallel reactions model and a power

law model, adjusted to include char heterogeneity, fittedthe experimental results with great accuracy, for devo-

latilization and combustion, respectively. In the case of

devolatilization, the kinetic parameters of the blends

could be predicted accurately from those of the in-

dividual fuels, whereas this was not true in the case of

char combustion. It seems that some interaction be-

tween the component coals occurred, due to their dif-

D. Vamvuka et al. / Carbon 42 (2004) 351–359 359

ferent chemical compositions, which may complicate the

co-firing process.

A higher heating rate increased the reaction rates by

an order of magnitude and shifted the reactions towardsmuch higher temperatures. Activation energy values did

not change considerably.

Acknowledgement

The authors would like to thank Prof. A. Foscolos

for his contribution to this work.

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