Combustion of Spanish coals under simulatedpressurized-¯uidized-bed-combustion conditions

Elena Alvarez*, Juan F. GonzaÂlez

ENDESA, PrõÂncipe de Vergara 187, E-28002 Madrid, Spain

Received 20 April 1998; accepted 24 August 1998

Abstract

Coal reactivity is an important parameter related to ef®ciency of boilers, and learning how to predict it can reduce costs when screening for

new fuels. We have measured the reactivity of Spanish coals in a high pressure thermogravimetric analyzer, under operational conditions

similar to those found in Pressurized-Fluidized-Bed-Combustion (PFBC). The results obtained from non-isothermal tests were used to look

for correlations between the reactivity and the physical and chemical properties of the coals. Reactivity increased with increasing rank, but no

relationship was found with the initial pore or surface characteristics of either the coals or the chars. We found a correlation between the true

density of the chars and their reactivity. We also observed a decrease in the reactivity, calculated from isothermal tests, as the pressure was

increased, while there was a slight increase in reactivity with increasing temperature. q 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Coal combustion; High pressure; Thermogravimetric analysis

1. Introduction

The production of steam for electric power by burning

coal seems likely to remain an important energy conversion

route until well into the next century. Coals are heteroge-

neous solids that can vary widely in their properties; hence

their combustion behavior in power stations can differ

signi®cantly. This has been particularly accentuated with

the increasing trade in thermal coals. The international

market has provided utility operators with a wide selection

of coals. In many cases, expensive full-scale test burns are

required to con®rm the suitability of a coal. There is there-

fore a need to develop methods for assessing coal character-

istics so that combustion plant performance can be predicted

more effectively.

A common technique for coal characterization is thermal

analysis [1,2]. This is a fast, cheap and reliable technique for

reactivity measurement. The usual method in thermal analy-

sis involves continuously measuring a physical or chemical

change in a substance whilst it is being subjected to a

controlled temperature program. The measured variable in

thermogravimetric analysis (TG) is the change in weight of

the sample, which is plotted as a function of furnace

temperature (non-isothermal mode) or time (isothermal

mode). With the derivative thermogravimetric analysis

(DTG), a plot of the rate of weight change is produced.

When the coal is heated in an atmosphere of ¯owing air,

the graphical plot produced is known as the burning pro®le,

which can provide a comparative evaluation of the charac-

teristics of the coal. The order of reactivity of the coal is

assessed on the peak temperature, with less reactive coals

showing higher peak temperature.

The reactivity of coals has been widely studied because of

its importance for the ef®cient combustion of coals and for

the screening of candidate fuels [3,4]. The ability to screen

candidate fuels quickly and cheaply is essential when the

construction of a new plant is considered, or in order to

reduce fuel costs or emission in an existing one.

General relationships between the physical and chemical

characteristics and reactivity of coals and the nature of the

combustion process are usually established based on the

study of the thermal behavior of coals in atmospheric condi-

tions [5]. However, the reactivity values obtained would be

specially meaningful if the conditions in the simulated test

could mimic those in the plant. One of the reasons for

concern is that pressure and temperature in¯uence substan-

tially the extent and the control of the combustion process.

Because of the complexity of the reactions involved,

combustion is considered as two separate stages: devolati-

lization, and the subsequent char combustion. Since char

Fuel 78 (1999) 335±340

0016-2361/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved.

PII: S0016-2361(98)00160-4

* Corresponding author. Present address: Energy Science Laboratories,

Inc., 6888 Nancy Ridge Dr., San Diego, CA 92121, USA. Tel.: 1 1-619-

552-2033; Fax: 1 1-619-587-7092; e-mail: ealvarez@fda.net.

gasi®cation or combustion is the slower step, it usually

controls the overall conversion process. A better under-

standing of the kinetics of this step under plant conditions

is therefore essential for optimum design and operation of

coal gasi®ers.

In this work, we have focused on the study of the combus-

tion of six Spanish coals at high pressure and high tempera-

ture conditions, using a thermogravimetric analyzer. All of

these coals are under consideration for usage at the Puertol-

lano station, which generates electric power through the

gasi®cation in combined cycle technology, or at the pressur-

ized ¯uidized bed combustion (PFBC) station of EscatroÂn.

We have been able to establish correlations between the true

density of the coals and their reactivity in plant conditions.

Furthermore, we have investigated the nature of the reaction

controlling the combustion process and the effect of varying

pressure and temperature.

2. Experimental

2.1. Coal samples and analysis

Seven coal samples of different rank and origin were

chosen for this study. Five of them (Virgen del Pilar, Opor-

tuna, Segre, Luengo and Barrabasa) are sub-bituminous

Type C coals from the Teruel and Mequinenza areas

(Spain); the antracitic Compostilla (LeoÂn, Spain) and the

imported bituminous SudaÂfrica coals are being used in

different pulverized fuel (PF) power plants.

The as received coal was crushed and screened to less

than 200 mm for analysis. The ultimate and proximate

analyses of coals are presented in Table 1. Char samples

were prepared by crushing and screening the coal to 100±

200 mm diameter and heating 20 g of each coal at 8508C in a

¯ow of inert gas N2 during 30 min in a vertical reactor.

The surface area of coals and chars was determined by the

single-point Brunnauer±Emmet±Teller (BET) method

using liquid N2 in a Micromeritics Flowsorb 2200. Porosity

and pore size distribution were measured by a Micromeritics

PoreSizer 9300 mercury porosimeter and corrected for coal

compressibility effect [6]. Helium density was obtained by

means of a picnometer Micromeritics Accupyc 1300.

Samples for textural analysis were ®rst degassed at 1408Cfor 12 h under vacuum. Surface areas, density and porosity

results are reported in Table 2.

Coals were screened to 100±200 mm diameter for ther-

mogravimetric tests.

2.2. Thermobalance

The pressurized thermogravimetric apparatus used in this

study has been previously described [7]. The main parts are

the microbalance, housed in a steel block, and the reactor.

Both are constructed for a maximum working pressure of

100 bar. The sample holder, connected to the balance with a

chain, was made of Incoloy net and can be smoothly

lowered and lifted up to the hot chamber of the reactor by

a driven winch system. The reaction tube can be heated

electrically up to 11008C. As reaction atmosphere, air and

N2 were used for combustion tests, as indicated. The

temperature is measured by means of a Pt±Rh thermocouple

located below the sample holder, thus representing the

furnace temperature. The data acquisition system allows

E. Alvarez, J.F. GonzaÂlez / Fuel 78 (1999) 335±340336

Table 1

Immediate and ultimate analysis of the coals used in this study

Coal Moisture (%) Volatile matter (%,d) Fixed carbon (%,d) Ash (%,d) C (%,d) S (%,d) N (%,d) H (%,d) O (%,d)

Virgen del Pilar 20.3 49.8 25.0 25.3 52.7 10.7 0.8 4.0 6.4

Oportuna 14.0 31.9 30.5 37.6 42.8 9.8 0.4 2.9 6.5

Barrabasa 14.8 34.6 27.4 38.0 41.3 8.2 0.4 3.1 9.0

Luengo 13.2 32.3 33.0 34.7 45.4 7.1 0.6 3.2 9.0

Segre 14.4 49.8 16.5 33.7 48.0 9.6 0.6 3.6 4.5

Compostilla 1.0 10.4 64.4 25.2 68.8 1.6 1.0 2.0 1.4

SudaÂfrica 4.0 29.5 57.7 12.7 72.8 0.6 1.8 4.1 7.9

Table 2

Textural analysis of coals and chars

Coal Density (g cm23) Surface (m2 g21) Vp (cm3 g21)

Original Pyrolized Original Pyrolized Original Pyrolized

Virgen del Pilar 1.62 2.11 12.6 83 0.110 0.118

Oportuna 1.77 2.29 6.7 54 0.108 0.159

Barrabasa 1.82 2.31 11 61 0.156 0.193

Luengo 1.70 2.23 36 29 0.104 1.125

Segre 1.61 2.20 5.5 100 0.146 0.176

Compostilla 1.71 1.96 4.2 2.2 0.030 0.039

SudaÂfrica 1.53 1.97 3.6 4.9 0.069 0.084

storing of the signals of the microbalance and the thermo-

couple in preset intervals of time. The measured weight

values are further corrected for the buoyancy.

2.3. Coal combustion tests

To monitor coal combustion reactivity, isothermal and

non-isothermal thermogravimetric methods were used.

Mass transfer effects were eliminated by using a suf®ciently

high ¯ow rate of gas and a small sample size. Preliminary

tests showed that when sample weights were lower than

25 mg and gas ¯ow rate was about 1 l min21, the reaction

rate remained unchanged with the experimental uncertainty.

Thus, under these conditions, the effect of external mass

transfer is negligible.

In the non-isothermal tests, a sample of 25 mg of coal was

heated in a ¯owing atmosphere of 2 l min21 of a mixture of

composition 4.5% O2 in N2 at a heating rate of 108C min21.

The weight loss of the sample was continuously monitored

as a function of temperature. Fig. 1 represents a typical non-

isothermal thermogram and the plot of the rate of weight

loss versus temperature, i.e. the burning pro®le. The differ-

ent regions in the curve represent: (1) loss of moisture at

temperature below 1008C; (2) volatile loss; (3) abrupt

increase of slope as the combustion becomes the major

mechanism; (4) maximum of weight loss at the peak

temperature (Tm), which is the main characteristic point of

the curve; (5) rate decrease to zero at another characteristic

point of the burning pro®le, the burnout temperature (Tb).

This point represents the temperature at which the rate of

weight loss is 1% min21, and the oxidation of the sample is

considered to be complete [8].

Arrhenius plots obtained from burning pro®les have been

used to derive apparent activation energies. The intrinsic

reaction rate, k, is given by the relationship:

k � Ae2E=RgT �1�

where E is the activation energy (J mol21), A the pre-expo-

nential factor (mol min21), T the temperature (K) and Rg the

universal gas constant.

In the isothermal combustion tests, samples of 25 mg of

coal were quickly introduced into the reaction tube at the

reaction temperature in a ¯ow of 1 l min21 of N2 until devo-

latilization was ®nished. Once the reaction temperature was

attained under N2, combustion was performed in a ¯ow of

1 l min21 of air until constant weight. A typical weight loss

history and the rate of weight loss for a coal are shown in

Fig. 2. There are four distinct regions in the combustion

curve, that represent: (1) devolatilization; (2) built up of

oxygen complexes on the surface of the char, as an increase

in weight is observed and an increase in reaction rate as the

char undergoes activation; (3) rectilinear portion of the

burn-off plot, in which about 40% of the material is burned;

(4) reduction of reaction rate as the surface per unit of

weight decreases. In zone 3, the increase in burn-off is

directly proportional to time. It was from this portion of

the plots that the reactivity parameter was calculated. The

maximum rate of char weight loss, calculated by;

Rmax � 21

W0

dW

dt

� �max

�mg mg21 h21� �2�

was used as an index of coal reactivity [9,10], where W0 is

the initial weight of char in a dry-ash free basis (mg) and

(dW/dt)max is the maximum rate of weight loss in a dry-ash

free basis (mg h21).

Non-isothermal reactivity measurements were performed

to determine the relative reactivities of the different coals

used in this study in order to identify possible relationships

between combustibility and characteristics of coals. The

isothermal reactivity parameter was used to evaluate the

effect of varying operating conditions (temperature,

pressure) on coal behavior.

E. Alvarez, J.F. GonzaÂlez / Fuel 78 (1999) 335±340 337

Fig. 1. Non-isothermal weight loss curve (Ð) and burning pro®le, R, (± ±)

of the sub-bituminous coal Virgen del Pilar. R values are normalized by W0.

The peak temperature is indicated by Tm and the burnout temperature by Tb.

Fig. 2. Isothermal weight loss (Ð) and rate of weight loss, R, curves (± ±)

of the combustion of the sub-bituminous coal Virgen del Pilar. R values are

normalized by W0. The maximum rate of weight loss is indicated by Rmax.

3. Results and discussion

The ®rst objective of this study was to investigate the

correlation between the physical and chemical properties

of several coals (shown in Tables 1 and 2) and their reactiv-

ity at high pressure and temperature. The reactivity para-

meters for each coal are listed in Table 3. Multiple

determinations were made for the Virgen del Pilar coal at

12 bar and 108C min21. The results were found reproducible

within a maximum spread of 0.2% and 0.8% for the Tm and

Tb parameters, respectively.

The parameter burnout temperature, Tb, could not be

correlated with any characteristics of coals or chars. As

expected, peak temperature of combustion, Tm, increased

with increasing rank: coals with a lower carbon content

and a high content of volatile matter were more reactive.

The same relationship was observed for chars.

No apparent relationship was found between reactivity

and initial pore or surface characteristics of parent coals

or chars, although the changes in surface area were not

determined as a function burn-off. No conclusions can be

inferred on how the development of porosity could affect

reactivity for a particular char, or if correlations might exist

for conversions other than 0%. On the other hand, a good

correlation was obtained with the initial absolute He-density

of chars, as shown in Fig. 3. Helium displacement is often

used to determine the true densities of solids, since this gas

is not easily absorbed on the internal surfaces of coal at

room temperature, and it is believed to penetrate pores

greater than 3±4 AÊ . The correlation with Tm could indicate

that the structure at the micropore level plays an important

role in combustion rates, although could indicate a relation

between reactivity and mineral matter content, since the true

density reported in Table 2 comprises both density of the

carbon and the mineral matter.

To express density in ash-free basis, the following rela-

tionship is used:

r 0He � rHe

1 2 XASH

ÿ � 1rASH

XASH

�3�

where r 0He is the speci®c volume of coal or coal char solids

(cm3 g21), XASH the ash mass fraction, rASH the true density

of ash, and rHe the speci®c volume in ash-free basis

(cm3 g21). Since there is no available data on the true

density of the ashes contained in the different coals, an

average value of 2.7 g cm23 has been used [11]. When the

recalculated true densities were plotted vs. the reactivities,

no correlation could be found, which might indicate that

reactivity would depend on a property of the mineral matter.

However, if this was the case, the lack of correlation

between the reactivity and total ash would suggest that

only a certain type of impurity is involved in the catalytic

effect. For low rank coals (C , 80%), a catalytic effect of

Ca, Mg, Na, and K has been reported [12]. In that study, not

only the amount of the coal minerals was important, but also

their state and distribution. For example, the catalytic prop-

erties of Ca can be masked in coals with high sulfur content,

due to the formation of inactive sul®des [13].

To study the combustion reaction under high pressure

conditions, we ®rst identi®ed the different processes

controlling reaction and estimated the activation energies

for each of them. Then we investigated the in¯uence of

different operational parameters, temperature and pressure,

on coal combustion reactivity. These parameters were modi-

®ed within the workable levels in a PFBC combustor.

E. Alvarez, J.F. GonzaÂlez / Fuel 78 (1999) 335±340338

Table 3

Peak (Tm) and burnout (Tb) temperatures derived from the burning pro®le of

the coals

Coal Tm (8C) Tb (8C)

Virgen del Pilar 387 495

Oportuna 380 524

Barrabasa 355 492

Luengo 372 473

Segre 387 487

Compostilla 459 549

SudaÂfrica 460 554

Fig. 3. Correlation between the He-density of the chars studied (rHe) and

the peak temperature (Tm) obtained from the burning pro®le.

Fig. 4. Arrhenius plot for the combustion of the coal Virgen del Pilar.

A typical Arrhenius plot is shown in Fig. 4. Different

linear portions can be drawn in this curve, which can be

interpreted as a series of separate events. The linear fraction

number 1 was found at low temperatures and is related to the

chemical control of the reaction rate [14]. As the tempera-

ture rises, the consumption rate exceeds the rate of diffusion,

and the rate is controlled by diffusion effects [14]. The ener-

gies of activation corresponding to each linear portion for

each coal are indicated in Table 4.

The values obtained at high pressure are in accordance

with the ones obtained by other authors for the chemical

reaction [15,16] and the diffusion process [14] at atmo-

spheric pressure. In most of the cases, the energy E2 is

approximately half the value of energy E1, which indicates

a transition to pore diffusion control [14], being the transi-

tion point around 5208C.

Different isothermal combustion tests were performed

under various temperatures between 800 and 9258C. The

effect of temperature on the reactivity at a pressure of

12 bar for one of the coals tested is shown in Fig. 5. The

evolution of the reaction rate is represented as a function of

conversion. After an initial phase during which char activa-

tion occurs, the reaction rate increased linearly with time.

The reaction rate reached a maximum level which is usually

interpreted as the point that corresponds to the highest value

of surface area in the burning char [17]. The conversion at

that point varied between 25% and 50% depending on the

coal studied. The rates showed little change with conversion

until reaching high levels of conversion after which they

dropped to zero. This decrease in rate could be due to loss

in the surface area. Table 5 lists the reactivity result for all

coals examined in these conditions. All the samples were

found to exhibit similar behavior, showing an increase in

reactivity with increasing combustion temperature.

E. Alvarez, J.F. GonzaÂlez / Fuel 78 (1999) 335±340 339

Table 4

Apparent activation energies (kJ mol21) for the chemical (E1) and diffusion

(E2) reaction rate control

Coal E1 E2

Virgen del Pilar 110.2 61.7

Oportuna 143.7 98.6

Barrabasa 107.0 91.1

Luengo 167.9 107.4

Segre 174.0 87.3

Compostilla 246.0 86.5

SudaÂfrica 156.0 88.2

Fig. 5. Isothermal combustion of the coal Oportuna at 12 bar and tempera-

tures (Ð) 8008C, (± ±) 8508C and (´´´) 9258C.

Table 5

In¯uence of temperature on the reactivity of different coals

Coal Rmax (mg mg21 h21)a

8008C 8508C 9258C

Virgen del Pilar 80.5 82.9 100.9

Oportuna 74.3 72.8 78.1

Barrabasa 81.7 82.2 94.8

Luengo 68.7 66.2 84.8

Segre 86.2 87.0 99.2

Compostilla 49.5 51.2 54.5

SudaÂfrica 50.6 50.2 49.2

a The results are the average obtained from three different runs.

Table 6

Infuence of pressure on the reactivity of different coals

Rmax (mg mg21 h21)a

Coal 12 bar 20 bar 25 bar

Virgen del Pilar 82.9 70.4 59.5

Oportuna 72.8 56.9 49.8

Barrabasa 82.2 64.8 57.0

Luengo 66.2 58.9 51.8

Segre 87.0 62.6 64.4

Compostilla 51.2 36.8 37.2

SudaÂfrica 50.2 40.2 34.3

a The results are the average obtained from three different runs.

Fig. 6. Isothermal combustion of the coal Oportuna at 8508C and pressures

(Ð) 12 bar, (± ±) 20 bar and (´´´) 25 bar.

The effect of pressure on the reactivity of the same coal as

above at a temperature of 8508C is shown in Fig. 6. The total

pressure was varied between 12 and 25 bar in different

isothermal combustion tests with air. Table 6 lists the reac-

tivity results for all coals. A decrease in reactivity was

observed as the pressure increased within the limits and

conditions studied. To explain this effect, two events have

to be considered: the increase in the O2 partial pressure, and

the increase in total pressure.

The oxygen partial pressure is the major driving force for

the O2 transfer from bulk gas to particle surface and for the

chemical reaction. Kinetic studies of the reaction between

char and oxygen have yielded orders of reaction of 1 to 1/2

during pulverized coal combustion, and 1 during AFBC

conditions [18]. According to these reports, a positive effect

of the increasing pressure on the reactivity should be

expected. However, the true reaction order has been found

to be close to 0 during combustion at high pressure under

high oxygen partial pressure in experiments using a shock

tube [19], and between 0.3 and 0.9 during laboratory-scale

PFBC studies [20], which would explain why the reactivity

did not increase with increasing pressure in our studies.

The most likely explanation for the reduced reactivity at

higher pressures is the reduction in the oxygen diffusion

coef®cient, as well as the increase in the resistance of

gaseous products arising from the char. Since the reaction

is under diffusional control, small changes in pressure are

likely to affect reactivity in an inversely proportional

manner. It has been recognized that the apparent order of

reaction respect O2 under pore diffusion control is 0.5, lead-

ing to a true order of reaction of zero [14,21]. As this order is

lower than the unity, the increase in O2 concentration can

not compensate for the decreasing diffusion coef®cients and

thus the observed rate decrease. On the other hand, a result

found in studies of combustion of pulverized coal, and that

is common also to gasi®cation processes, is that pressure

can also affect the reaction by reducing the heat transfer and

heating rate, so that the total ef®ciency of combustion drops

as pressure increases [14].

Acknowledgements

This investigation was carried out under ECSC funds

(project 7220-EC-761) in the facilities of TecnologõÂa y

GestioÂn de la InnovacioÂn, S.A. (Madrid, Spain). We thank

J.A. Pitarch for excellent technical assistance, and M.A.

BlaÂzquez for critical reading of the manuscript.

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