A laboratory study of spontaneous combustion of coal: the influence ofinorganic matter and reactor size

Wiwik Sujanti, Dong-ke Zhang*

CRC for New Technologies for Power Generation from Low-rank Coal, Department of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005,Australia

Received 8 May 1998; received in revised form 30 September 1998; accepted 9 October 1998

Abstract

A laboratory investigation into the effect of inorganic matter on spontaneous combustion behaviour of a Victorian brown coal. Fourteensamples were prepared, namely, the raw coal, water-washed coal, acid-washed coal, and acid-washed coal doped with 11 additives. Each ofthe samples was then tested in an isothermal reactor to obtain its critical ambient temperature, above which spontaneous combustion occurs.The relative effectiveness of the additives was determined by comparing their critical ambient temperatures with those of the raw coal and theacid-washed coal. Potassium chloride, Montan powder, and sodium chloride were found to be the most effective inhibitors, followed bymagnesium acetate, and calcium chloride. The presence of sodium nitrate and ammonium chloride in the coal samples did not show anysignificant influence on the spontaneous combustion. However, calcium carbonate, sodium acetate, potassium acetate, and pyrite promotedthe spontaneous combustion. The effect of additive loading was also investigated for an inhibition agent (KCl) and a promotion agent(NaAc). It was revealed that the effectiveness of these promotion and inhibition agents was enhanced with an increase in the additive loading.Low-temperature oxidation kinetics were also estimated by an energy balance approach and compared with the self-heating potential of thesesamples. The effects of reactor size and reactor specific surface area on the critical ambient temperatures are also discussed.q 1999 ElsevierScience Ltd. All rights reserved.

Keywords:Additives; Coal combustion; Reactivity; Inorganic matter; Self-heating; Spontaneous combustion

1. Introduction

Self-heating and spontaneous combustion of coal havelong been known to pose a serious problem for coal produ-cers and users [1]. This is because spontaneous combustionof coal can lead to loss of desirable coal properties andproduct, and raise concerns about safety, especially incoal stockpiles, transportation over long distances, and inunderground mining [1–3].

A number of studies were carried out to determine thebest method of preventing the spontaneous combustion ofcoal [4–6]. Compaction of coal piles and inerting the atmo-sphere were applied [4]. Additives, such as carbamide,diammonium phosphate, gel solution from water glass [5],aqueous solutions of inorganic salts, and salt–clay mixtureswere also examined as potential inhibition agents to spon-taneous combustion [6]. Smith, et al. [4] evaluated the effec-tiveness of 10 additives to suppress the self-heating processand revealed that sodium nitrate, sodium chloride, and

calcium carbonate inhibited the spontaneous combustion,whereas sodium formate and sodium phosphate stimulatedthe self-heating process.

The main aim of the work described in this paper was toexamine the effect of inherent inorganic matter and differentadditives on spontaneous combustion so as to obtain a betterunderstanding of the role of the inorganic matter in the low-temperature oxidation processes. An isothermal reactor wasused to determine the critical ambient temperature, abovewhich spontaneous ignition (thermal runaway) occurs. Thecritical ambient temperatures were then used to compare thetendency of various coal samples toward spontaneouscombustion. An increase in the critical ambient temperaturedue to the use of an additive may indicate the effectivenessof the additive to suppress the self-heating potential of thecoal. The effect of additive loading on the critical ambienttemperature for spontaneous combustion was investigatedfor typical promotion and inhibition additives. Low-temperature oxidation kinetics of these coal samples arealso estimated from the temperature measurements. Theeffects of reactor size on the critical ambient temperatures

Fuel 78 (1999) 549–556

0016-2361/99/$ - see front matterq 1999 Elsevier Science Ltd. All rights reserved.PII: S0016-2361(98)00188-4

* Corresponding author. Tel.:161-8-8303-5085; fax.:161-8-8303-6081.

and the estimated low-temperature oxidation kinetics arealso examined.

2. Experimental

2.1. Coal samples

A Victorian brown coal was selected for this study, Table1 features the proximate, ultimate and inorganic analyses ofthe coal. The freshly picked raw coal was crushed andsieved and the 0.25–1.00 mm size fraction was retainedfor preparation of all other samples.

Double-deionised water-washed and acid-washedsamples [7,8] were prepared to investigate the effect ofinherent inorganic matter on the spontaneous combustion.Double-deionised waterwashing removed the water-solubleinorganic matter and acid-washing (37% HCl acid solution)removed most of the inorganic matter from the raw coal

To investigate the effect of individual inorganic compo-nents in coal on spontaneous combustion, 11 additives wereselected and, respectively, added into the acid-washed coal.They were pyrite (FeS2), potassium acetate (KAc), sodiumacetate (NaAc), calcium carbonate (CaCO3), sodium nitrate(NaNO3), ammonium chloride (NH4Cl), calcium chloride(CaCl2), magnesium acetate [Mg(Ac)2], sodium chloride(NaCl), potassium chloride (KCl), and Montan powder[CaCl2 powder 1 3% sodium dodecyl sulphate (SDS)].Water soluble additives [KAc, NaAc, NaNO3, NH4Cl,CaCl2, Mg(Ac)2, NaCl, and KG] were dissolved in double-deionised water to form solutions with a concentration of

4.76 wt%. 2.5 l of each solution and 600 g of acid-washedcoal were mixed to form slurry. The mass ratio of the addi-tive in a solution to the acid-washed coal was 25%. Theslurry was continuously stirred for 24 h. The liquid in theslurry was then allowed to drain through a filter, and theresulting paste was dried in nitrogen at 1058C for 8–10 h.Chemical analysis confirmed that the samples prepared insuch a way contain about 5 wt.% additive. Water insolubleadditives (FeS2, CaCO3, and Montan powder) were added tothe coal as solid fine powder and the mixtures were wellmixed. The concentration of water insoluble additives in thecoal samples was 5 wt.%. All the samples were stored intightly sealed plastic bags to avoid pre-oxidation beforeexperimentation.

2.2. Experimental procedures

A sample to be tested was loosely placed in an isothermalreactor, a stainless-steel cylindrical reactor of 8 cm heightand 4 cm diameter. Four thermocouples were inserted insidethe reactor to monitor temperatures along the reactor axis.The thermocouples had a precision of^ 0.18C. A gas (N2 orair) was preheated in an 18 m long copper tube to the oventemperature, and then passed through the coal sample fromthe bottom of the reactor.

The oven was first preheated to a preselected temperature.During this stage, nitrogen gas passed through the reactor at100 ml min21 (N.T.P.) to minimise any pre-oxidation. Oncethe desired temperature was reached, air then replaced thenitrogen at the same flow rate and was maintained through-out the experiment. A data acquisition system was activatedto record the temperatures. A schematic diagram of experi-mental apparatus used in this work is shown in Fig. 1.

In preliminary experiments, attempts were also made tofollow the gas products in the exiting gas from the reactorusing a gas chromatograph. However, no detectable CO orCO2 was noticed, nor was there any depletion of oxygen attemperatures of interest. This was apparently due to the lowreaction temperatures (thus slow reaction rates) and thesmall quantity of coal samples used. Therefore, temperaturemeasurement was chosen as the sole technique to monitordevelopment of spontaneous combustion in these experi-ments. It was also confirmed that the four thermocouplesgave exactly the same temperature reading.

The experiment was terminated either when a thermalrunaway occurred or when the temperature of the samplecontinued to level off towards the oven temperature. Typicaltemperature profiles of the Mg(Ac)2-added-coal are shownin Fig. 2. It can be seen that at temperatures below 1558C,thermal runaway did not occur, so that the experiments wereterminated when the average temperature along the reactorshowed a trend to level off. However, at 1558C, thermalrunaway occurred. Experiments were repeated for differentoven temperatures, at a 58C interval, until the critical ambi-ent temperature,Tcrit, of a sample was determined. Repeat-ability of the determinedTcrit values was confirmed by

W. Sujanti, D. Zhang / Fuel 78 (1999) 549–556550

Table 1Proximate and ultimate of coal used [7]

Proximate analysis (wt%)Moist VM (db) F.C. (db) Ash (db)61.3 48. 3 48.7 3.0Ultimate analysis (wt% db)C H N O Organic sulfur STotal

67.4 4.7 0.7 26.9 0.26 0.26

Fig. 1. Schematic diagram of the experimental apparatus.

repeating the experiments and essentially the sameTcrit

value was obtained. Thus, for each sample, the criticalambient temperature was taken as the average of the lowesttemperature at which the thermal runaway occurred and thehighest temperature at which the thermal runaway did notoccur, with an accuracy of̂ 2.58C.

3. Results and discussion

3.1. The effect of inherent inorganic matter

The presence of inorganic matter in coal may catalyse theoxidation of coal [7], and removing the inorganic mattermay change the coal reactivity. Table 2 shows the inorganicanalysis of raw, water-washed, and acid-washed coal. It isshown that water-washing only removes small amount ofNa, Fe and K, while acid-washing removes most of thisinorganic matter. Therefore, in this instance, the influenceof inherent inorganic matter on the spontaneous combustioncan be substantially eliminated by treating the raw coal withan acid. Table 3 shows that the critical ambient temperaturesof the water-washed sample (1458C) and the acid-washedsample (147.58C) are higher than that of the raw coal(142.58C). The differences are small in relation to the accu-racy of determination but the increase in critical ambienttemperature indicates that removing the inherent inorganicmatter from coal helps reduce its self-heating potential.

3.2. The effect of additives

Table 3 also shows that among the 11 additives used,FeS2, KAc, NaAc, and CaCO3 promote the spontaneouscombustion of the coal used as the relevant critical ambienttemperatures are considerably lower than that of the acid-washed coal. NH4Cl and NaNO3 are effectively neutral, butCaCl2 and Mg(Ac)2 increase the critical ambient tempera-ture for spontaneous combustion by 58C–152.58C, andMontan powder, KCl, and NaCl push the critical ambienttemperature to 157.58C, indicating that these additives inhi-bit the spontaneous combustion. For additives which showthe same critical ambient temperatures of the samples, suchas Montan powder, KCl, and NaCl, or CaCl2 and Mg(Ac)2,their abilities to suppress the spontaneous combustion arecompared, based on the rates of temperature rise at theircritical ambient temperatures.

It has been acknowledged [1,2,7,9] that FeS2 in coal cata-lyses the oxidation reaction. In addition, in a moist air, FeS2

itself also experiences an exothermic oxidation reactionwhich helps accelerate the coal self-heating [1,2,7,9].Sodium and potassium also catalyse the spontaneouscombustion especially when they are present as organicsalts [7,9]. The results for FeS2, KAc and NaAc addedsamples used in the present study are in agreement withpreviously published work.

The addition of CaCO3 to the acid-washed coal decreasesthe critical ambient temperature of the acid-washed coal by108C to 137.58C, which could be due to the catalytic effect

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Fig. 2. Temperature histories of Mg(Ac)2-added-coal sample at several oven temperatures.

Table 2Inorganic analyses of raw coal, water-washed coal, and acid-washed coal

Samples Inorganic analysis (wt% db)

Na Mg Fe Si Al K Ca

Raw coal 0.09 0.28 0.29 , 0.05 , 0.05 0.007 0.62Water-washed coal 0.06 0.28 0.22 , 0.05 , 0.05 0.006 0.62Acid-washed coal , 0.001 0.001 0.03 , 0.05 , 0.05 , 0.002 0.004

of the calcium on the spontaneous combustion. NaNO3 doesnot show any effect, suggesting that sodium in an inorganicsalt would have little catalytic effect when compared with itsorganic salts. However, these results do not agree with theprevious findings [4] that NaNO3 and CaCO3 inhibited spon-taneous combustion. NH4Cl was also found to have nosignificant effect on the spontaneous combustion behaviourof the coal, but this may be a consequence of the decom-position of the salt at 1008C, which would eliminate anycatalytic activity of the NH14 .

In general, alkali and alkaline-earth metals in organicsalts tend to promote the spontaneous combustion whileinorganic salts would inhibit spontaneous combustion.However, Mg(Ac)2 seems to be an exception, as it inhibitsthe spontaneous combustion. It was suggested [10,11] thatMg can enhance oxidation of carbon, probably at hightemperatures. However, Mg21 may have no catalytic effector be a negative catalyst for the low-temperature (, 2008C)oxidation reaction of coal.

3.3. The effect of additive loading

To investigate the effect of additive loading on promotion

or inhibition of spontaneous combustion, samples with 1, 5,and 10 wt% of NaAc and KCl were prepared and tested.Table 4 presents the critical ambient temperatures of thesesamples. In general, the critical ambient temperaturedecreases with increasing NaAc loading, but increaseswith KAc loading, indicating that promotion and inhibitioneffects of these additives on spontaneous combustiondepend on their loading levels. Although 1% and 5 wt%NaAc loadings in the samples offered a similar criticalambient temperature, within the accuracy (^ 2.58C) ofthe experiments, a closer examination of the temperatureprofiles during the self-heating revealed that for the 5 wt%NaAc loading a higher rate of temperature rise was experi-enced. Similarly, in the presence of 1 wt% KCl the sampleshowed the same critical ambient temperature as the acid-washed coal, but its rate of temperature rise was lower dueto the inhibition effect of KCl.

3.4. Estimation of low-temperature oxidation kinetics

To compare reactivities of the samples and correlate thereactivities with their self-heating behaviour observed inthe experiments, low-temperature oxidation kinetics were

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Table 3Summary of critical ambient temperatures, heat of oxidation reaction, and apparent kinetic constants estimated for various samples

Sample Tcrit (8C) Q (MJ kg21) E (kJ mol21) A (kg m-3 s21 Pa21)

FeS2 1 coal 110.0̂ 2.5 18.8 79.6 4.91× 103

KAc 1 coal 125.0̂ 2.5 21.8 111.9 3.53× 107

NaAc 1 coal 137.5̂ 2.5 22.2 109.5 1.79× 107

CaCO3 1 coal 137.5̂ 2.5 21.2 189.6 1.83× 1017

Raw coal 142.5̂ 2.5 22.9 144.5 9.84× 1010

Water-washed coal 145.0̂ 2.5 23.7 239.7 1.55× 1022

Acid-washed coal 147.5̂ 2.5 24.2 164.4 2.43× 1013

NH4Cl 1 coal 147.5̂ 2.5 22.6 160.2 4.21× 1012

NaNO3 1 coal 147.5̂ 2.5 22.3 203.5 1.42× 1018

CaCl2 1 coal 152.5̂ 2.5 20.3 145.6 6.53× 1010

Mg(Ac)2 1 coal 152.5̂ 2.5 22.6 103.2 5.07× 105

Montan powder1coal

157.5^ 2.5 20.4 102.9 3.52× 105

KCl 1 coal 157.5̂ 2.5 21.5 109.1 2.00× 106

NaCl 1 coal 157.5̂ 2.5 15.7 219.5 7.35× 1019

Table 4Summary of the critical ambient temperatures and kinetic constants estimated for samples with various additive loadings

Sample Tcrit (8C) Q (MJ kg21) E (kJ mol21) A (kg m-3 s21 Pa21)

Acid-washed coal 137.5̂ 2.5 24.2 103.6 1.62× 106

1% NaAc-added-coal 132.5̂ 2.5 23.1 180.1 8.92× 1015

5% NaAc-added-coal 132.5̂ 2.5 22.2 208.0 4.34× 1019

10% NaAc-added-coal 122.5̂ 2.5 21.0 256.5 5.78× 1026

1% KCl-added-coal 137.5̂ 2.5 22.4 164.0 9.03× 1013

5% KCl-added-coal 142.5̂ 2.5 21.5 179.5 5.64× 1015

10% KCl-added-coal 147.5̂ 2.5 20.4 179.1 2.45× 1015

estimated by an energy balance approach. The heat balancefor a sample in an isothermal reactor at a point when the coaltemperature is equal to the oven temperature (To), underwhich condition there is no heat loss from the sample tothe oven environment, can be written as:

csdTdt

� �o� QAe

2E

RTo PnO2

�1�

wherec is the specific heat capacity of coal [7] (J kg21 K21),1550 J kg21 K21; s is the bulk density of coal (kg m23),1350 kg m23; T is the coal temperature (K) andt is time(s); Q is the heat of oxidation reaction (J kg21) which wasdetermined experimentally for each of the samples.A is theapparent pre-exponential, factor (kg m23 s21 Pa21) andE isthe apparent activation energy (J mol21); R is the universalgas constant.PO2

is the partial pressure of oxygen gas,which is 21 kPa in the present case. The apparent reactionorder,n, is assumed to be 1.

According to Eq. (1), from a linear plot of ln[dT/dt]o

versus 1/To the apparent kinetic constants,A andE, can bedetermined. Fig. 3 shows typical plots of ln[dT/dt]o versus 1/To for the raw and acid-washed coal samples. Since it isevident that a linear relation between ln[dT/dt]o versus 1/To does exist, and that the temperature inside the reactor isnoticed to be uniform, Eq. (1) can be used as a technique forestimation of apparent kinetics of coal in such spontaneouscombustion tests.

Fig. 4 compares the Arrhenius plots of several samples inthe presence of promotion agents, raw coal, water-washed,and acid-washed coal. The presence of FeS2 in the coalresults in the highest reactivity which corresponds to thelowest critical ambient temperature observed. The reactivityof the samples with the presence of KAc and NaAc arelower than that of the sample with FeS2, but higher thanthose of raw coal, water-washed, and acid-washed coalsamples.

However, the reactivity of the water-washed coal ishigher than that of the raw coal, which is not in agreement

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Fig. 3. Typical linear plots of ln[dT/dt] versus 1/To for the raw and acid-washed coal samples for estimation of apparent oxidation kinetics of coal.

Fig. 4. Comparison of reaction rates for various samples in temperature range of 1108C–1508C, assuming an oxygen partial pressure of 21 kPa.

with their critical ambient temperature data. To further clar-ify this, the reaction rates estimated using the calculatedkinetics are plotted against the critical ambient temperaturesfor all samples studied in Fig. 5. In general, Fig. 5 shows anoverall trend that the lower the critical ambient temperatureof a sample, the higher its oxidation reactivity. However, thedata are more scattered for those samples containing theinhibition agents. The reactivities of samples with the inhi-bition agents do not show a clear trend with their criticalambient temperatures. It is possible that the assumed sampledensities and specific heat capacities of the samples used inEq. (1) may have contributed to the inconsistency in thereactivity–critical ambient temperature correlation. Thevarying packing density of the samples in the reactorscould also have contributed to this inconsistency. However,reproducible critical ambient temperature measurements

reject this argument, although the effect of packing densityis of interest on its own, which will be further investigated infuture studies. Further, the coal treatments, i.e. washing andadditive loading, may have changed the coal structure, andthus the coal reactivity.

Fig. 6 compares the Arrhenius plots of several sampleswith the presence of NaAc and KCl in various loadings. Fig.6 shows that 10 wt% of NaAc in the coal sample leads to thehighest reactivity of the NaAc loaded samples, which corre-sponds to the critical ambient temperatures observed. Fig. 6also shows that although, within the experimental accuracy,both 1 and 5 wt% NaAc-added-coal samples have the samecritical ambient temperature, the 5 wt% NaAc-added-coalsample is more reactive. The reactivities of the samples withKCl in them show the opposite trend; the higher KCl load-ing, the lower its reactivity.

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Fig. 5. Low-temperature oxidation rates versus the critical temperature for various samples in air at 1408C.

Fig. 6. The effect of additive loading on the estimated oxidation rates for NaAc and KCl added coal samples.

4. The effect of reactor sizes

To examine the effect of reactor size on the self-heatingprocess, three larger isothermal reactors were constructed,with 27, 15 and 12 cm height and 9.8, 8 and 6 cm diameter,respectively. The four isothermal reactors, R-1, R-2, R-3,and R-4, respectively, were filled with 1000, 350, 140 and30 g of coal, and the critical ambient temperature of eachreactor was determined. In general, it was observed that thecritical ambient temperatures decreased with increasingcoal mass. However, the correlation between the criticalambient temperatures and the coal mass is not rectilinear.Whether a spontaneous combustion occurs or not is deter-mined by two competing processes: the rate of heat genera-tion due to oxidation, and the rate of heat loss to thesurroundings which depends on the external surface area.Thus, Fig. 7 shows that the critical ambient temperatureincreases linearly with specific external surface area. Thissuggests that for a given coal, the spontaneous combustion

behaviour depends on the external heat transfer environ-ment.

The apparent kinetic parameters,E andA, from these fourreactors (R-1, R-2, R-3 and R-4) were also determined. Itwas found that the values ofE obtained from these reactorsare similar, i.e. 106.16, 105.49, 107.37 and 107.10 kJ mol21,respectively. However, the values ofA are significantlydifferent, i.e. 2.99× 108, 1.97 × 108, 1.09 × 108 and1.60 × 107 kg m23 s21 Pa21, respectively. Fig. 8 showsthat the smaller the reactor, thus the larger the specific exter-nal surface area, the lower the reactivities. As summarisedby Chen [12] the value ofA varies with temperature, andextent of reaction. It was observed during the experimentsthat the smaller the reactor, the higher the criticalambient temperature required for the thermal runaway tooccur, and thus the more rapid the oxidation of the coal.The pre-oxidation before the thermal runaway is detected,could have caused a loss of the reactivity for smallerreactors.

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Fig. 7. A plot of critical temperature versus reactor specific external surface area.

Fig. 8. The effect of reactor size on the estimated reactivity of coal oxidation in air between 708C and 1408C.

5. Conclusions

Water-washing only removes a small amount of inherentinorganic matter, while acid-washing removes most of theinorganic matter in this coal. The critical ambient tempera-ture of acid-washed coal is higher than those of raw andwater-washed coals, indicating the catalytic effect of inher-ent inorganic matter in the coal. Of the eleven additivesapplied to the acid-washed coal, FeS2, KAc, NaAc andCaCO3 were found to promote the spontaneous combustion,while Montan powder, KCl, NaCl, CaCl2, and Mg(Ac)2inhibit the spontaneous combustion. NaNO3 and NH4Cl,have very little effect. The low-temperature oxidationkinetics of the samples were also estimated, and whilst thereactivities show a general trend of lower critical ambienttemperature, the higher the reactivity of the sample, furtherinvestigation is required in order to improve the accuracy ofthe kinetic estimation. The promotion and inhibition effectsof NaAc and KCl, respectively, on spontaneous combustionare enhanced with an increase in their loadings. The criticalambient temperatures were also determined for variousreactor sizes. The critical ambient temperature showed alinear correlation with the reactor specific external surfacearea, indicating the important role of external heat transferin the spontaneous combustion process.

Acknowledgements

The authors gratefully acknowledge the financial andother support received for this research from the Coopera-tive Research Centre (CRC) for New Technologies for

Power Generation from Low-rank Coal, which is estab-lished and supported under the Australian Government’sCooperative Research Centres program. Sujanti wouldalso like to thank the CRC for a postgraduate scholarship.The authors also wish to thank Prof. Chuguang Zheng ofHuazhong University of Science and Technology, China,for measurements of the heating values of the coal samples.

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