Correlation between ash fusion temperatures

and chemical composition in Albanian coal ashesq

Saimir A. Lolja*, Hajri Haxhi, Rolanda Dhimitri, Spiro Drushku, Agim Malja

Department of Industrial Chemistry, University of Tirana, Tirana, Albania

Received 23 January 2002; revised 27 April 2002; accepted 14 June 2002; available online 17 July 2002

Abstract

This paper presents an analysis of the ash fusion temperatures (AFTs) for 17 Albanian coals. The contribution of oxides to AFTs is

investigated by considering oxides from various perspectives: as discrete species, acids and bases, crystal components, fluxing agents, and

cement constituents, and in accordance with the periodic table. Empirical correlations use simple and multiple linear equations and

polynomial equations, referring to both weight and molar percentages. The shift from the deformation temperature to the flow temperature is

accompanied by a shift of influence from basic oxides to acidic oxides. Many predictive correlative equations are obtained. q 2002 Elsevier

Science Ltd. All rights reserved.

Keywords: Ash fusion temperatures; Coals; Correlations; Albania

1. Introduction

The nature of ash produced during coal combustion is an

important engineering factor in terms of heat transfer

processes, work efficiency, ash removal and storage, utility

in the construction industry, operating and maintenance

costs, and time. It is important to be able to predict the

potential combustion products and, especially for coals with

high ash content, the extent to which a coal will slag or foul

upon combustion. Using different approaches, researchers

have shown that the chemical and mineral compositions of

coal ash determine its melting characteristics and fusion

temperatures (AFTs). The correlative equations, based on

this assumption, are still the most widely used techniques

for assessing the deposition characteristics of coal ashes

[1–7]; they express the mineral content as percentages of

oxides and use data from standard ash fusion tests. Such

tests do not simulate ash formation in real combustors

closely, but compensate for this by providing a consistent

set of test conditions.

For the first time, this research analyses through standard

means the characteristic melting temperatures of coal ashes

coming from 17 operating Albanian coal mines located in

three geological regions: Tirana (Krrabe, Mushqeta, Mezez,

Valias, Manze), Korca (Mborje-Drenove, Dardhas, Pet-

rushe, Alarup, Bezhan), and Memaliaj. Investigations of

these different types of coal and their ashes have been

reported previously [8,9].

Reliable correlative results would be useful for under-

standing coal combustion, because they would guide

operations and replace delicate laboratory tests. Especially

for correlations including a broad range of coal mines, such

results could provide a general frame of reference. In this

work, correlations proposed in previous research [1–11]

will be considered. Since the results of correlative

calculations are enormous, only important results will be

presented; in particular, those with a correlation coefficient

of R . 0:95: As to those relationships which will not be

discussed here, it will be implied that confident correlations

were not found out.

2. Experimental

The ash content and ash composition of the coals listed

above is presented in Table 1 on a wt% basis. The ash test

specimens were produced in the laboratory according to the

ASTM D1857 standard. They were prepared with the aid of

dextrin binder and had a conic profile with equilateral

triangle base. The ISO 540 standard with reducing

0016-2361/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.

PII: S0 01 6 -2 36 1 (0 2) 00 1 94 -1

Fuel 81 (2002) 2257–2261

www.fuelfirst.com

q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com

* Corresponding author. Tel.: þ355-4-225-454; fax: þ355-4-227-669.

E-mail addresses: slolja@hotmail.com (S.A. Lolja), hhaxhi@fshn.

tirana.al (H. Haxhi), rdhimitri@fshn.tirana.al (R. Dhimitri), spiro10@fshn.

tirana.al (S. Drushku), amalja@fshn.tirana.al (A. Malja).

atmosphere H2:CO2/20:80 was applied in the electrically

heated furnace to ash test specimens as an ash fusion test.

The experimental results are given in Table 2. Correlative

relations were sought in the following forms:

Y ¼ a0 þ a1X ð1Þ

Y ¼ a0 þ a1X þ a2X2 þ a3X3 ð2Þ

Y ¼Xn

i¼1

aiXi: ð3Þ

The deformation temperature (Td), hemisphere temperature

(Th), flow temperature (Tf), melting range (Th–Td), flow

range (Tf–Th), fusion range (Tf–Td), and their ratios are

combined in the dependent variable Y. The independent

variable X represents percentages of oxides taken from their

groupings as sole constituents, as acids and bases, as crystal

constituents, as fluxing agents, as constituents of cement

minerals, and according to the frame of the periodic table.

The correlative calculations considered the AFTs in 8C,

and the oxides either in wt% or mol%. When it was

necessary, the values of the mol% basis were obtained from

Table 1. Emphasis was put on the mol%, because it was not

considered by previous research, and because it gives better

representation of coal ash chemistry. The calculations

considered the samples as a complete set or according to

their geological regions as stated in each case. Separate

calculations were not performed for the Memaliaj region,

because there were only three experimental points repre-

senting it.

3. Results and discussions

The experimental data presented in Table 2 indicate that

the deformation temperature can range from 1040

(Mushqeta-2) to 1260 8C (Memaliaj-5), the hemisphere

temperature from 1070 (Krrabe-2) to 1290 8C (Memaliaj-5),

and the flow temperature from 1090 (Krrabe-2) to 1310 8C

(Memaliaj-5). That is, the overall variation has the same

value (220 8C) for the three AFTs when the mines are

considered as a single set. The variations in melting range,

flow range and fusion range are the highest for the Tirana

region and the lowest for the Memaliaj region. These

variations are expected, because as shown by Table 1, the

coals of the Tirana region vary more in their ash contents

than the coals of the Korca and Memaliaj regions [9].

From Table 2, the flow temperature tends to increase

linearly with both the deformation and the hemisphere

temperature (R ¼ 0:80; 0.92, respectively). The hemisphere

temperature increases linearly with the deformation tem-

perature ðR ¼ 0:96Þ; in the Korca region, the correlation

coefficient is R ¼ 0:98: These findings reconfirm that the

three fusion temperatures are related to each other in a

straightforward manner: the deformation temperature rep-

resents the starting point for the coal ash to loose its solid

phase behaviour, until reaching the flow temperature

beyond which the coal ash has liquid phase behaviour.

3.1. Oxides as discrete constituents

In terms of wt% and mol% bases, and both Eqs. (1) and

(2), no important correlations are observed between the

AFTs and the oxide contents or their ratios; separately, it is

observed that flow temperature decreases linearly with the

Na2O content ðR ¼ 0:94Þ: It explains that the oxide specimens

or their pair combinations alone do not determine the AFTs. In

addition, the percentage of a given oxide specimen in the coal

ash does not constitute a major factor in defining the AFTs.

These results reconfirm that the melting process of coal

ashes is a complex process involving physical and chemical

interaction among the gas, liquid, and solid phases.

Considering all mines as a single set and applying Eq. (3)

Table 1

The ash content and the composition of coals (wt%)

Mine-seam Ash SiO2 Fe2O3 Al2O3 TiO2 P2O5 CaO MgO NiO MnO Na2O K2O Total

1 Krrabe-2 6.71 30.73 19.73 17.15 0.61 0.37 19.02 7.22 0.13 0.07 3.16 1.81 100

2 Mushqeta-1 8.30 52.29 12.06 16.45 0.85 0.18 10.56 3.61 0.06 0.04 0.81 3.09 100

3 Mushqeta-2 7.91 39.95 23.51 13.40 0.71 0.29 10.99 5.88 0.12 0.07 2.46 2.62 100

4 Mezez-10 10.89 44.13 10.95 19.24 0.78 0.25 16.51 3.97 0.06 0.07 1.23 2.81 100

5 Valias-28 21.97 42.32 21.50 16.44 0.77 0.15 7.40 5.85 0.05 0.11 3.03 2.38 100

6 Manze-4 9.54 50.01 11.81 23.48 0.67 0.21 4.47 5.49 0.07 0.12 0.93 2.75 100

7 Manze-5 9.25 51.21 10.81 23.90 0.79 0.16 3.46 5.19 0.06 0.10 0.76 3.56 100

8 M-Drenove-1 4.10 48.43 17.54 12.71 0.34 0.19 3.44 15.51 0.11 0.08 0.93 0.72 100

9 M-Drenove-2 4.03 49.28 15.17 12.86 0.27 0.21 1.63 18.73 0.10 0.10 0.75 0.90 100

10 Dardhas-6 3.55 56.19 10.23 20.70 0.97 0.06 3.55 2.56 0.03 0.08 1.46 4.17 100

11 Petrushe-1 4.22 53.23 9.01 20.02 0.25 0.17 6.81 3.50 0.05 0.08 2.05 4.83 100

12 Alarup-1 22.88 53.03 12.96 13.56 0.61 0.30 12.29 4.88 0.01 0.15 0.95 1.26 100

13 Alarup-2 23.37 41.73 13.06 19.62 0.94 0.94 18.72 2.20 0.01 0.24 1.12 1.42 100

14 Bezhan-1 16.43 49.58 5.65 26.53 0.75 0.41 8.75 4.57 0.06 0.05 1.12 2.53 100

15 Memaliaj-5 10.12 44.57 8.26 6.46 0.46 0.17 27.09 7.06 0.05 0.02 4.44 1.42 100

16 Memaliaj-6 9.02 43.76 14.11 14.97 0.64 0.09 16.44 4.59 0.06 0.01 2.57 2.76 100

17 Memaliaj-7 8.01 50.41 8.84 16.66 0.61 0.15 12.95 4.79 0.05 0.03 2.71 2.80 100

S.A. Lolja et al. / Fuel 81 (2002) 2257–22612258

on a wt% basis, the deformation temperature, hemisphere

temperature, and flow temperature display correlations with

coefficients of R ¼ 0:95; 0.94, 0.93, respectively. The

application of Eq. (3) on a wt% basis for each region

gives correlations with coefficient R ¼ 1:0 for all AFTs,

melting range, flow range, fusion range, and their ratios.

Considering all the mines as a single set, but applying Eq.

(3) on a mol% basis, the correlation coefficients R are: for

deformation temperature 0.95, hemisphere temperature

0.94, flow temperature 0.93, melting range 0.90, flow

range 0.94, fusion range 0.96, ratio Td/Th 0.91, ratio Td/Tf

0.96, ratio Th/Tf 0.93, ratio Td=ðTf 2 TdÞ 0.95, ratio

Th=ðTf 2 TdÞ 0.95, and 0.95 for the ratio Tf=ðTf 2 TdÞ:From these results, the core and best correlative parameter is

the deformation temperature. Again, the application of Eq.

(3) to a given region gives correlations with coefficient R ¼

1 for each of the above parameters.

3.2. Oxides as acids and bases (mol%)

The oxides may be divided into three groups: acids

(SiO2, TiO2, P2O5); amphoteric oxides (Al2O3 and Fe2O3);

bases (Na2O, K2O, CaO, MgO, MnO, NiO). Alternatively, if

they were divided into two groups, Al2O3 would be among

the acids and Fe2O3 among the bases. There are no observed

correlations between the AFTs and the above groupings

when the mines are considered as a single set. These results

show again that acidic, amphoteric or basic oxides do not

determine the AFTs when they are considered as separate

entities. Obviously, it suggests that the coal ashes are

inherently composed of chemical complexes formed by

oxides with relatively opposite chemical nature. The above

results are emphasized for coal ashes from coal mines which

are not in close proximity.

When calculations are performed for specific regions, no

significant linear correlations are observed, but the

sequences of experimental data exhibit curves which are

sometimes well described by Eq. (2). For instance, the

deformation temperature, hemisphere temperature and

melting range for the Korca region are correlated ðR .

0:98Þ with the basic oxide content when the oxides are

divided into two groups. Similarly, the flow temperature for

the Tirana region is correlated ðR ¼ 0:99Þ with the ratio of

acids to amphoterics when the oxides are divided into three

groups as above. Considering all the mines as a single set,

the application of Eq. (3) does not exhibit any good

correlation. And for the Tirana region alone, Eq. (3)

correlates the ratio Td=ðTh 2 TdÞ with the acidic, amphoteric

and basic oxide contents ðR ¼ 0:94Þ:

3.3. Oxides as crystal components (mol%)

For the coal ashes of Table 1, the oxides with

coordination number four (SiO2, Al2O3) contribute 31.5–

80.7 mol%, the oxides with coordination number six

(Fe2O3, CaO, MgO, TiO2, NiO, MnO, Na2O) contribute

16.2–67.1 mol%, and the oxides with coordination number

eight (P2O5, K2O) contribute 0.6–3.7 mol% [9]. According

to Eq. (1), there is a linear correlation, but only for all mines

together ðR ¼ 0:93Þ; between the ratio Tf=ðTf 2 TdÞ and the

ratio of the content of oxides with coordination number six

to the content of those with coordination number eight. This

overall result reveals that the value of the flow temperature,

normalized by the value of the fusion range, correlates with

the ratio of the contents of the oxide groups with lesser

contents in the ash.

According to Eq. (2) and for the Korca region only, the

deformation temperature, the hemisphere temperature, and

the melting range exhibit correlations ðR . 0:97Þ with both

the content of oxides with coordination number four and the

content of those with coordination number six. The

application of Eq. (3) does not give any significant

Table 2

Experimental data obtained for AFTs

No. Mine-seam Deformation temperature (8C) Hemisphere temperature (8C) Flow temperature (8C)

1 Krrabe-2 1050 1070 1090

2 Mushqeta-1 1090 1150 1200

3 Mushqeta-2 1040 1070 1130

4 Mezez-10 1100 1150 1230

5 Valias-28 1070 1100 1130

6 Manze-4 1080 1120 1210

7 Manze-5 1080 1150 1240

8 M-Drenove-1 1170 1230 1270

9 M-Drenove-2 1230 1280 1300

10 Dardhas-6 1080 1160 1230

11 Petrushe-1 1110 1160 1230

12 Alarup-1 1060 1120 1200

13 Alarup-2 1100 1170 1260

14 Bezhan-1 1140 1190 1260

15 Memaliaj-5 1260 1290 1310

16 Memaliaj-6 1080 1110 1150

17 Memaliaj-7 1100 1120 1180

S.A. Lolja et al. / Fuel 81 (2002) 2257–2261 2259

correlation, except for a correlation concerning the

dependent variable Td=ðTh 2 TdÞ in the Tirana region.

3.4. Oxides as fluxing agents (mol%)

Applying Eq. (2) to the Korca region, the flow range

correlates ðR ¼ 0:98Þ with the ratio of non-fluxing acids

(Al2O3) to non-fluxing bases (CaO þ MgO). Similarly, the

melting range correlates with the proportion of non-fluxing

bases ðR ¼ 0:98Þ: For the Tirana region, the flow tempera-

ture displays polynomial correlation ðR . 0:97Þ with the

fraction of fluxing bases (Fe2O3 þ K2O þ Na2O þ NiO þ

MnO), with the ratio of fluxing to non-fluxing acids (SiO2 þ

TiO2 þ P2O5)/(Al2O3), and with the ratio of fluxing acids to

fluxing bases (SiO2 þ TiO2 þ P2O5)/(Fe2O3 þ K2O þ

Na2O þ NiO þ MnO). In addition, for the Tirana region,

the flow temperature and the flow range exhibit polynomial

correlation (R ¼ 0:99; 0.95, respectively) with the ratio

of non-fluxing acids to fluxing bases (Al2O3)/(Fe2O3 þ

K2O þ Na2O þ NiO þ MnO).

Applying Eq. (3) for all mines together, there is no

significant correlation in this distribution of oxides. But

for the Tirana region alone, Eq. (3) shows correl-

ations (R ¼ 0:92; 0.98, 0.94, respectively) for the hemi-

sphere temperature, the flow temperature, and the ratio

Td=ðTf 2 TdÞ:

3.5. Oxides as constituents of cement minerals (mol%)

Eqs. (1) and (2) are used to seek correlations in

connection with the slag coefficient (SiO2 þ Al2O3)/(Fe2-

O3 þ CaO þ MgO), the silica module (SiO2)/(Al2O3 þ

Fe2O3), the alumina module (Al2O3)/(Fe2O3), the hydraulic

module (CaO)/(SiO2 þ Al2O3 þ Fe2O3), the saturation

coefficient of silica with calcium oxide (CaO 21.65Al2O3

20.35Fe2O3)/(2.8SiO2), the basic module (CaO þ MgO)/

(SiO2 þ Al2O3), and the activity index. For the Tirana

region, the hemisphere temperature and the flow tempera-

ture hold polynomial correlations (R ¼ 0:99; 0.96, respect-

ively) with the silica module. In addition, for the Tirana

region, the flow range exhibits polynomial correlation with

both the alumina module and the hydraulic module

(R ¼ 0:99; 0.95, respectively). For the Korca region, the

deformation temperature and the melting range demonstrate

polynomial correlation with the basic module (R ¼ 0:96;0.99, respectively). Eq. (3) cannot be applied here.

3.6. Oxides according to periodic table (mol%)

For the Tirana region, the flow temperature exhibits

linear correlation ðR ¼ 0:97Þ with the ratio of transition

metal oxides to non-metal oxides (Fe2O3 þ NiO þ MnO þ

TiO2)/(SiO2 þ P2O5). For the Korca region, there are

polynomial correlations (R ¼ 0:98; 0.96, respectively)

between the melting range and the alkaline-earth metal

oxide content (CaO þ MgO), and between the flow range

and the metal oxide content (Al2O3). For the Korca region,

the deformation temperature, the hemisphere temperature

and the flow range show polynomial correlations (R ¼ 0:97;0.95, 0.99, respectively) with the ratio of alkaline-earth

metal oxides (CaO þ MgO) to metal oxide (Al2O3). In

addition for the Korca region, the flow range holds

polynomial correlation ðR ¼ 0:96Þ with both the content

of oxides of third and fourth period elements, and even with

their ratio. The application of Eq. (3) to the Korca region

displays good correlations (R ¼ 0:98; 0.95, respectively)

with the melting range and the ratio Td=ðTh 2 TdÞ; similarly,

for the Tirana region, there are good correlations (R ¼ 0:94;0.99, 0.95, 0.95, respectively) for the AFTs and the ratio

Td=ðTh 2 TdÞ:

4. Conclusions

In contrast to the good linear correlation between the

deformation temperature and hemisphere temperature, there

were no other significant correlations among the defor-

mation temperature, the hemisphere temperature, the flow

temperature, the melting range, the flow range, and the

fusion range. Better correlations were found with respect to

a specific geological region. In cases where a good simple

linear correlation was hard to find, the experimental data

sequences were often well described by a third-degree

polynomial equation. According to Eqs. (1) and (2), the

thermal parameters did not indicate any correlation with any

particular oxide content expressed either in wt% or mol%.

On the other hand, good correlative results were obtained by

applying Eq. (3). Although these results are in the form of a

mathematical model whose accuracy depends on the

availability of experimental data, they will be useful for

predicting thermal parameters for coal ashes.

It was reconfirmed that AFTs were decreased by an

increase in the basic oxide content [10]. In addition,

correlative results indicated that flow temperature often

had a better correlation with oxide content than deformation

temperature or hemisphere temperature [11]. This was

expected: once the flow temperature has been reached, the

melting path is of no further significance. Thus, the

influence of oxide species was stronger than that of their

mineral combinations in their respective melting paths.

Also, the move from deformation temperature to flow

temperature was associated with the shift in influence from

basic oxides to acidic oxides.

Acknowledgments

The authors take this opportunity to appreciate the

Organic Chemical Technology Laboratory at Faculty of

Natural Sciences of the University of Tirana and, in

particular, Mr M. Xhelepi.

S.A. Lolja et al. / Fuel 81 (2002) 2257–22612260

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