Self-ignition risk classification for coal dust layers of three coal typeson a hot surface
Bei Li a, b, Gang Liu a, **, Ming-Shu Bi a, Zhen-Bao Li c, Bing Han b, Chi-Min Shu d, *
a School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning, 116024, PR Chinab Inspection and Research Institute of Boiler and Pressure Vessel, Dalian, Liaoning, 116000, PR Chinac School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou, 730050, PR Chinad Department of Safety, Health, and Environmental Engineering, National Yunlin University of Science and Technology, Douliu, Yunlin, 64002, Taiwan, ROC
a r t i c l e i n f o
Article history:Received 27 February 2020Received in revised form26 October 2020Accepted 28 October 2020Available online 30 October 2020
Pulverised coal in industrial sites and their dust can experience spontaneous combustion and self-heating, increasing the risk of fire and dust explosion. The main objective of the present study was toresolve thermal combustibility (as reflected by comprehensive combustibility index [Sn] and kineticproperties) for three types of coal (S1-BN, S2-CY, and S3-JM) through thermal analysis. The Sn values ofthe samples indicated a degradation in the quality of comprehensive combustibility. Apparent activationenergies (Ea) at the initial stage of spontaneous coal combustion (130e300 �C) were decided throughAchar and CoatseRedfern methods. Moreover, thermal susceptibility (minimum auto-ignition temper-ature [MAIT] and thermodynamic parameters) was evaluated using the hot plate method. The MAITvalues for the three coal dust layers were 210, 220, and 300 �C. The results exhibited that heat conductionwas the dominant heat transfer mode that originated the temperature distribution within the coal dustlayer under the subcritical conditions for ignition; while it converted chemical reaction controlled-modeafter thermal runaway. Furthermore, the results based on an improved risk matrix approach showed theS1-BN and S2-CY samples had a high self-ignition risk, whereas the S3-JM sample had a moderateignition risk.
Coal chemical technology is a preferred approach to generatingpetrochemical feedstock and clean fuels as target products by usingraw coal [1]. The utilisation of chemically processed coal has severaladvantages, including ecofriendliness and favourable operability,over the direct combustion of coal for reducing the emissions ofsulphur dioxide, nitrogen oxide, suspended dust, harmful heavymetals, and other pollutants in the related applications [2,3].Accumulated pulverised coal dust generally takes place in diversi-fied steps of the production process, including coal preparation,milling, transportation, storage, and pyrolysation [1,4,5]. Contact ofthe accumulated dust with air (or other oxidisers) and an ignitionsource may bring about thermal runaway, which may ignite
smouldering fires that further generate hot nests, triggering coaldust explosions under certain conditions [6]. Thermal runaway is aphenomenon of supercritical self-heating and spontaneous ignitionfor dust from solid fuel. The European Standard of EN 1127e1e2011for the identification and appraisal of hazardous situations leadingto explosion classifies 13 types of possible ignition sources, ofwhich hot surfaces are listed as a typical ignition source responsiblefor causing fire and explosion risks in the aforementioned indus-trial production processes and locations. However, numerous typesof high-temperature surfaces are inevitably prevalent in the coalchemical industry, including superheated exterior surface, steamboiler heat metre, steam pipe, iron sheet chimney, heated metalparts, high-temperature reaction vessel, and high-temperaturedrying device surface [7,8]. Ignition of dust layers through ther-mal runaway may occur when sufficient energy from a hot surfaceis supplied to the dust, which triggers exothermic reactions,causing a notable increase in temperature [6,9].
For safety management, various methods, such as hot plate testand thermal analysis, have been introduced to probe the predi-lection of thermal combustibility and thermal susceptibility for
d50 median diameter, mmd diameter of the coal dust layer, mEa apparent activation energy, kJ/molht effective total heat transfer coefficient, W/(m2 K)hc effective convective coefficient, W/(m2 K)hr effective radiant coefficient, W/(m2 K)ke effective thermal conductivityL thickness of the dust layer, mmt mass at time t, mgm0 initial mass, mgmf final mass, mgn reaction orderQ heat of reaction, JSn comprehensive combustibility index, %2/(�C3 min2)S specific surface area, m2/gTa ambient temperature, �CTi ignition temperature, �CTmax peak temperature, �CTb burnout temperature, �CTp hot plate temperature, �CTs temperature of the surface of the dust layer, �Cva kinematic viscosity, m2/s
(dw/dt)ave mean mass loss rate, (%/min)g(a) integral function based on conversionf(a) reaction model based on conversiontb corresponding time at burnout, mintd ignition delay time, minth time at whichmaximum temperature is reached, minti corresponding time at ignition, mintmax corresponding time at peak, minR universal gas constant, 8.314 kJ/(mol K)Ra Rayleigh numberR2 correlation coefficientW/(m K) Greek symbolsa conversion degree, %b heating rate, �C/minε emissivitys StefaneBoltzmann constant, W/(m2 K4)
AbbreviationASTM American Society for Testing and MaterialsTG thermogravimetric, %DTG mass loss derivative rate, %/minHTL highest temperature in the layer, �CIEC International Electrotechnical CommissionMAIT minimum auto-ignition temperature of dust layer, �CNFPA National Fire Protection AssociationPSD particle size distributionSEM scanning electron microscope(dw/dt)max maximum mass loss rate, (%/min)
B. Li, G. Liu, M.-S. Bi et al. Energy 216 (2021) 119197
dust materials [5e13]. The minimum auto-ignition temperature(MAIT) of the coal dust layer has been determined with differentparticle sizes [8,9] and different angles of wedges [12] at a constantsurface temperature [7] and a constant heat flux [10] by the hotplate method. The ignition dynamics of coal dust deposits was alsoexplored to estimate the self-ignition risk in the oxy-fuel com-bustion environments [11]. In addition, several studies [13e16]have provided thermogravimetric (TG) results for the assessment ofthe self-ignition risk based upon kinetics (apparent activation en-ergy [Ea], reaction order [n], and pre-exponential factor [A]). Theignition temperature and spontaneous combustion risk of solidfuels, which are affected by fuel type, heating rate, atmospherecomposition, and particle size, have been examined under differentthermal conditions [16]. Moreover, the results from Chen et al. [17]revealed that the ignition models varied from homogeneous igni-tion to heteroehomogeneous ignition and then to heterogenousignition with an increase in coal rank. The thermal combustibilityand thermal susceptibility of dust materials are assessed to inves-tigate fire and explosion risks in the process industries. In addition,the US Standard of NFPA 652e2019 has provided a procedure fordust hazard analysis. This method can assess fire, deflagration, andexplosion hazards and is significant to dust management in in-dustrial sites with coal and accumulated combustible dust, butbasic experimental data and an index for specific dust risk assess-ment are lacking. Recently, Ramírez et al. [18] proposed a methodfor ranking the relative ignition risk of biomass dust. This methodwas applied by Chin et al. [19] and Jones et al. [20] for assessing theignition risk of solid fuels. Based upon Thomas’s thermal runawaymodel [21e23], Park et al. [21] developed a heat transfer model of adust layer on a hot surface. They obtained several parameters, suchas Ea, effective thermal conductivity (ke), and heat of reaction (Q),for the assessment of the thermal susceptibility of a coal dust layer.
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Evaluation of the self-ignition features of accumulated coal dust isbeneficial for equipment design and prudent selection as well as forpreventing fire and explosion hazards caused by a hot surface.However, research evaluating the self-ignition risks of the dustlayer on a hot surface based upon thermal combustibility andthermal sensitivity is insufficient. In practice, TG and the hot platetest are broadly applied techniques that are used to obtain ther-mokinetic characteristics and self-ignition properties under variousthermal conditions, but these methods have seldom been used incombination, as in this study.
This study explored the self-ignition and spontaneous com-bustion behaviours of three types of coal dust in ambient air usingboth the TG technique and hot plate tests. By varying the heatingconditions, we determined the key self-ignition parameters, suchas MAIT, ignition delay time (td), and thermodynamic parameters(ke, Ea, and A), for the coal samples with a single thickness by usingthe hot plate test. The measured self-ignition characteristic pa-rameters were compared for evaluation and identification thermalcombustibility and thermal sensitivity for the coal dust samplesbased upon multiple experimental methods. The methods can beconsidered approaches for estimating the self-ignition risk and canbe considered feasible for evaluating the relative fire and explosionrisks of industrial accumulated coal dust.
2. Methodology and technique
2.1. Determination of physicalechemical properties of coal dust
2.1.1. Sample preparationThree typical coal samples with different carbon contents were
gleaned for this study, as shown in Fig. 1. They were labelled S1-BN(noncoking coal from the Daliuta coal mine in Yulin City, Shaanxi
Fig. 1. Location of collected samples and scanning electron microscope images.
B. Li, G. Liu, M.-S. Bi et al. Energy 216 (2021) 119197
Province, China), S2-CY (long-flame coal from the Daliuta coal minein Yulin City, Shaanxi, China), and S3-JM (coking coal from theGubei coal mine in Huainan City, Anhui Province, China). Thesetypes of coal are vastly used as the starting materials in the coalchemical industry as well as primary solid fuels for power gener-ation. The “mine-size coal” was deemed to coal dust passingthrough a 20-mesh sieve (850 mm), at which 20%was less than 200-mesh (75 mm) [24]. The samples were milled to a particle size of<212 mm; these samples were purposely selected for analysis in thepresent study.
Changsha, Hunan Province, China) was applied to determine thevolatile, fixed carbon, and ash contents of the three coal samples byfollowing the Chinese National Standards of GB/T212e2008. Theresults of proximate analysis for all samples are listed in Table 1.
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According to these results, the samples were arranged in increasingorder of volatile contents (27.59%e34.04%). Furthermore, the ashcontent of coal was in the range of 5.31%e11.75%. The ash contentsof the S1-BN (5.31%) and S2-CY (9.74%) samples were lower thanthose of the S3-JM sample (11.75%). However, among the preparedsamples, the highest fixed carbon (56.37%) and moisture (10.73%)contents were found in the S1-BN sample.
2.1.3. Ultimate analysisThe Vario EL III cube (Elementar, Frankfurt, Germany) was used
to measure the chemical composition of the samples in the air at-mosphere. The results of carbon, hydrogen, nitrogen, and sulphurcontents are summarised in Table 1. According to ISO 17247�2013,the oxygen content of the coal samples in air dried bases wascalculated by 100% minus the percentage mass fractions of carbon,hydrogen, nitrogen, sulphur, ash, and moisture. The carbon andhydrogen contents of the coal samples ranged from 65.80% to
Table 1Preparatory analysis of the three coal dust tested.
Notes: * Oxygen content was calculated by difference; V, Volatile; FC, Fixed carbon; H, Hydrogen; C, Carbon.
B. Li, G. Liu, M.-S. Bi et al. Energy 216 (2021) 119197
71.61% and from 4.11% to 4.79%, respectively. Among all the threecoal samples, the S3-JM sample had the highest oxygen content of14.04%. Moreover, all three samples had low sulphur content(<0.5%).
2.1.4. Morphology analysisScanning electron microscope (SEM) images of all coal dust
samples were magnified 3000 times by a tungsten lamp SEM (FEI,Model Quanta 450, Hillsboro, Oregon, USA) for observation of sur-face morphology. According to the SEM results shown in Fig. 1, aftercrushing, the prepared coal particles showed irregular and roughsurfaces with numerous pores.
2.1.5. Particle size distribution analysisThe particle size distribution (PSD) of the prepared samples was
evaluated using the Mastersizer laser particle size analyser (version2.15, Malvern Instruments Ltd., Malvern, UK). From the PSD results,the median diameter (d50) of the samples ranged from 73.59 to183.49 mm. Dust particles finer than 75 mm were found in pro-portions of 37.56%, 59.19%, and 22.78%. Therefore, the prepared coaldust containing 20% through 75-mm mesh at least was consideredto be typical “mine-size dust” for the hot plate ignition test.Detailed information is provided in Table 2.
2.2. Determination of thermal combustibility for coal dust
USA) was applied for obtaining thermal combustibility character-istics during the entire combustion process, such as TG and dif-ferential thermogravimetric (DTG). The thermal gradients andtransport effects during combustion were negligible under a smallparticle size. Approximately 6e7 mg of the prepared coal samplewas conducted from 30 to 850 �C at a heating rate of 20 �C/min. Airwith a flow rate of 50 mL/min was injected as a gas carrier at at-mospheric pressure. Subsequently, mass loss and mass loss deriv-ative with respect to temperature and corresponding characteristictemperatures was determined.
2.2.2. Combustion performance methodsIn present study, the ignition temperature (Ti, �C), peak tem-
perature (Tmax, �C), and burnout temperature (Tb, �C) were derivedfrom the TG and DTG curves obtained from previous studies [13,14].The ignition index (Di, %/min3), burnout index (Db, %/min4), andcomprehensive combustibility index (Sn, %2/[�C3 min2]) werecalculated for assessing the combustion performance of the three
Table 2Characteristic parameters of the three coal particles.
coal samples [15]. On the basis of the TG results, these indexes areexpressed in Eqs. (1)e(3):
Di ¼ðdw=dtÞmax
tmaxti(1)
Db ¼ðdw=dtÞmaxDt1=2tmaxtb
(2)
Sn ¼ðdw=dtÞmaxðdw=dtÞaveT2i Tb
(3)
where (dw/dt)max and (dw/dt)ave are the maximum and meanmass loss derivatives, respectively (%/min); ti, tmax, and tb are thecorresponding times at ignition, peak, and burnout temperatures(min); and Dt1=2 is the time zone of DTG/DTGmax ¼ 0.5 (min).
2.2.3. Kinetic analysisThe TG results were applied for estimating chemical kinetic
parameters (Ea and A). The reaction rate of coal sample decompo-sition is expressed in Eqs. (4)e(6) [13e15]:
dadt
¼KðTÞf ðaÞ (4)
a¼m0 �mim0 �mt
(5)
KðTÞ¼A exp��Ea
RT
�(6)
where a is the conversion degree of the sample; t is the time; m0,mi, and mt are initial, final, and instantaneous mass, respectively; Ris the universal gas constant; and T represents the absolutetemperature.
2.3. Determination of thermal susceptibility for the coal dust layer
2.3.1. Hot plate ignition testThe ignition properties of the coal dust layer were measured
using the hot plate device [7,9]. The hot plate was made of a200 � 200-mm2 aluminium square. According to ASTM E2021e09e2013 (i.e., Standard Test Method for Hot-Surface IgnitionTemperature of Dust Layers), a plate temperature distributionwithin ±5 �C can be considered within the acceptable error range.
B. Li, G. Liu, M.-S. Bi et al. Energy 216 (2021) 119197
In this study, the temperature controller (AntaiX, Shenzhen,Guangdong Province, China) was connected to the hot plate tocontrol the surface temperature of the plate, with a thermal sta-bility of ±3 �C. A stainless ring of 10-mm height (L), 100-mm innerdiameter (d), and 4-mm wall thickness was placed on the platecentre. Four pairs of diametrically opposite slots were cut into thering wall to accommodate the thermocouples inserted into the dustlayer for measuring the internal temperatures of the dust. Thethermocouples (Omega K-type) with a diameter of 0.51-mm werestretched through the slots in the dust layer; the thermocoupleswere placed into the ring parallel to the surface of the plate and hadno influence on the structure for the dust layer. Ceramic pipes wereused to insulate the thermocouples and metal ring to avoid contactbetween them. As demonstrated in Fig. 2, the junctions of thethermocouple were positioned at the geometric centre of the ringat 1.5, 4.0, 6.8, and 9.0 mm above the hot surface. During the self-heating process, a camera connected to a personal computercaptured the surface changes of the dust layer.
The hot plate ignition test met the performance requirementsspecified of the Chinese National Standards of GB/T 16,430e1996.Once the surface temperature of the hot plate reached the setting,the cavity of the ring was carefully filled with the prepared sample,which was then smoothed gently with a shovel. The duration ofeach test was no less than 60 min. The coal dust layer was heated ata constant plate temperature, and ignition was determined byexamining the temperature of the layer and observing the surfaceof the layer for visual signs of smouldering [25]. According toprevious studies [7,11,25], ignition occurred if any of the followingconditions weremet: (i) visible flame or the evolution of smoke andthe formation of white coloured ash due to combustion, (ii) thepeak temperature of the dust layer reached 450 �C, and (iii) thetemperature of the dust layer increased to 250 �C, which is abovethe set plate temperature. In this study, ignition was determinedonce a visible glow or white coloured ash was observed. MAIT wasdefined as the critical temperature of the hot plate resulting inignition in the dust layer. If no ignition occurred in the dust layerafter a given heating time, this was considered as non-ignition. Theplate temperature increased at increments of 10 �C for a freshsample until thermal runaway of the dust layer was clearly andevidently observed.
2.3.2. Determination of the self-ignition index for the coal dustlayer
Thermal susceptibility can be determined by investigating theparameters related to the ease of self-ignition, such as MAIT,highest temperature in a dust layer (HTL), ignition delay time (td),
Fig. 2. Schematic of t
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and time to the peak temperature (th). Ignition tests were con-ducted to determine these thermal sensitivity parameters using thehot plate apparatus in this study. When the hot plate temperaturewas sufficiently low, the chemical reactions of accumulated coaldust were negligible. The thermokinetic parameters of accumu-lated dust were computed based upon the hot plate results ob-tained under the steady-state condition.
3. Results and discussion
3.1. Analysis of thermal combustibility of the samples
3.1.1. Thermal behaviours of the prepared coal dust duringcombustion
The TG and DTG of the three samples with increases in tem-perature (b ¼ 10 �C/min) is depicted in Fig. 3. On account of the TGresults, the residues of the samples were 5.07%, 10.31%, and 16.70%,which were consistent with their ash contents. Coaleoxygen re-actions are complicated because they involve water evaporation,gas desorption, oxygen adsorption, and pyrolysis of carbonaceoussubstances [26]. Fig. 3a presents the thermal degradation curves ofthe S1-BN sample; the curves illustrate that the mass of the S1-BNsample first decreased at a temperature less than 105 �C, thenincreased to the maximum point at 285 �C, and then lessened againuntil the end of the experiment. In view of the TG results, thethermal degradation process was divided into three stages. Stage 1occurred from 24 to 285 �C for the S1-BN samplewith amass loss of2.73%, which was accredited to water evaporation, gas desorption,and oxygen absorption [27].
The decomposition of small molecular components in coaloccurred more readily than that of highly cross-linked aromaticswith stronger conjugated double bonds. Liu and co-writers [28]reported that the bond-breaking reactions occurred in pulverisedcoal when the heated temperature was gradual up to 300 �C. In thepresent study, bond-breaking chemical reactions producednumerous types of radical fragments, which were associated withthe decomposition of cross-linked aromatics in coal. Stage 2occurred in the temperature range of 285e582 �C, with a mass lossof 90.54%. The mass loss of the S1-BN sample in this stage wasmainly responsible for the dissociation of strong chemical bonds,such as those in benzene rings and aromatic chains, and strong charoxidation [15]. Moreover, a sharp peak was observed at 468 �C inthe DTG curve in stage 2. From stage 2e3, the mass loss of the S1-BN sample transformed, with the total mass decreasing from97.27% to 6.69%. Stage 3 emerged in the temperature range of582e911 �C, with a residue of 5.07%. The results of proximate
he hot plate test.
Fig. 3. TG/DTG curves at air gas flow with a heating rate of 10 �C/min.
B. Li, G. Liu, M.-S. Bi et al. Energy 216 (2021) 119197
analysis (Table 1) revealed that the S1-BN sample had higher car-bon and fixed carbon contents than the S2-CY and S3-JM samples,indicating its higher reactivity and calorific value. In addition,among the three samples, the S1-BN sample contained fewer im-purities due to its lowest ash content of 5.31%; its combustionduring oxidation may be more complete. These results were inaccordancewith those in Fig. 3b and c. According to DTG curves, themaximum rate of oxidation reactions (dw/dt)max of the threesamples was �10.74%/min, �9.12%/min, and �5.81%/min. The re-sults revealed that the S1-BN sample was the most thermally labile,followed by S2-CY and S3-JM. More detailed information is pro-vided in Table 3.
3.1.2. Determination of combustion characteristic index for differenttypes of coal dust
The combustion characteristic index of the three coal samplesare presented in Table 4 and Fig. 4. Zhou et al. [29] reported that thevolatile fuel ratio [V/(V þ FC)] was an essential factor contributingto ignition performance. The volatile fuel ratio of the S2-CY sample
Table 3Temperature interval and mass loss in different regions for all three samples at a heatin
Notes: Stage 1 is the water evaporation and gas adsorption stage; Stage 2 is the decomreaction rate; (dw/dt)max is the maximum rate of the oxidation reaction.
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was 0.40, which was higher than that of the S1-BN (0.33) and S3-JM(039) samples. Furthermore, the ignition temperatures of the S1-BN, S2-CY, and S3-JM samples were 427, 415, and 422 �C, respec-tively, implying that the S2-CY sample was more readily ignitedthan the other two samples. The result for the ignition temperaturewas in agreement with the value of volatile fuel ratio obtained inthis study. However, the ignition characteristics of three coalsamples were assessed based upon the ignition temperature, andignition performance was related to the maximum oxidation re-action rate. The ignition index (Di) of the S1-BN sample (61.65%/min3) was higher than that of the S2-CY (54.13%/min3) and S3-JM(33.33%/min3) samples, expressing that the S2-CY sample exhibi-ted the highest reactivity and thermal susceptibility. The burnoutindex (Db) of the three samples was 7.13%/min4, 5.15%/min4, and1.88%/min4, suggesting that the S1-BN sample had the highestburnout performance. In addition, higher Tb was found, whichrepresented a lengthened combustion process, implying lowerthermal susceptibility. The result for the burnout temperature wasin accordance with that of the burnout index. Generally, coal with a
Notes: Ti is the ignition point temperature; Tmax is the peak temperature; Tb is the burnout point temperature; tmax, ti, tb, and Dt1/2 are the peak time, ignition time, burnouttime, and time zone at (dw/dt)/(dw/dt)max ¼ 0.5.
Fig. 4. Values of Sn, Dh, and Di for the three coal samples.
B. Li, G. Liu, M.-S. Bi et al. Energy 216 (2021) 119197
higher Sn value has more satisfactory combustion performancethan the lower one [30]. The Sn of the S1-BN, S2-CY, and S3-JMsamples decreased from 27.88 � 10�9 to 12.35 � 10�9%2/(min2
K3), indicating a degradation in the quality of the comprehensivecombustibility of the three samples.
3.2. Analysis of thermal susceptibility of the coal dust layer
3.2.1. Self-ignition behaviours of the coal dust layer on a hot plateFig. 5 presents the appearances of the layers for the S2-CY
sample with plate temperature of 210 and 220 �C. During the testperiod, non-ignition was observed for the dust layer on the hotsurface at 210 �C. From Fig. 5a, it can be seen that the coal dust layerhad no significant change, but several little fissures caused byheating were visible on the top surface of the dust layer during theheating period. However, ignitionwas observed in the dust layer onthe hot plate at 220 �C. White coloured water vapour and smokeemanated from the coal surface in the initial heating stage (before20 min). These gaseous products adsorbed moisture from evapo-ration and contained some by-combustion products, such as carbondioxide and carbon monoxide, from pulverised coal particles. Wa-ter evaporation and the self-heating reaction could be evidenced bythe mass loss stage 1 in the TG curve in Section 3.1.1. As shown inFig. 5b, after heating for 30 min, some fine cracks emerged on thecoal dust layer in the marked area, which partitioned the dust layerinto various areas. These cracks gradually became longer, deeper,and wider and more apparent as the heating time increased. Theywere witnessed on the entire surface of the dust layer after 50 minof heating. At the end of the experiment, the coal dust layer wasdivided into several subregions with various areas due to the for-mation of cracks. Therefore, the MAIT of the S2-CY sample was220 �C, and it was the supercritical temperature at which the coal
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dust layer ignited under the experimental conditions.
3.2.2. Temperature distribution of the coal dust layer on a hot plateTemperature profiles of the coal dust layer for the S2-CY sample
at specific times (10, 20, 30, 40, 50, and 60 min) are shown in Fig. 6.For the non-ignition case in Fig. 6a, the curves showed anapproximate linear temperature distribution for the coal dust layer,signifying the steady-state condition with a relatively minuteamount of heat generation for the hot surface at 210 �C. Heatconduction was the dominant heat transfer mode that generatedthe temperature distribution within the coal dust layer under thesubcritical conditions for ignition. For the ignition case in Fig. 6b,variation was found in the temperature distribution over 30 and20 min of heating, which clearly showed that the higher temper-ature zone moved to the upper surface of the dust layer because ofmore oxygen supply near the open boundary. This finding alsosuggested that thermal runaway occurred in this duration. Thetemperature rise after thermal runaway depends on the rate of heatgeneration by coal-oxidation reactions [21]. When thermalrunaway occurred, the temperature distribution was primarilydetermined by the chemical reaction mode in the coal dust layer.
3.2.3. Temperature evolutions of the coal dust layer on the hot plateFig. 7 shows the typical temperatureetime profiles of the coal
dust layer for the S2-CY sample on the hot plate under critical andsubcritical conditions. As shown in Fig. 7a, the coal temperature atthe four measuring positions first increased to the highest valuesand then slightly decreased until the end of the experiment.However, the temperatures of the coal dust layer were lower thanthe plate temperature of 210 �C. In addition, a temperature gradientwas apparent at measurement points 1e4 in the coal dust layer.Distinct trends were obtained for the temperatures at the fourmeasurement points, which decreased from the bottom to surfaceof the dust layer. For the ignition case in Fig. 7b, the temperature atmeasurement point 4 first reached 222 �C within 24 min andexceeded the set temperature of the hot plate (220 �C), followed thepeak temperatures at the measurement point 3 within 26 min(225 �C), measurement point 1 within 27 min (253 �C), and mea-surement point 2 within 28 min (235 �C). According to the DTGresults delineated in Fig. 3b, the improvement of oxygen absorptionability for the S2-CY sample in the temperature range of200e274 �C reflected in a slight increase in mass, which enhancedthe coal-oxidation reaction rate in this stage. Subsequently, thermalrunaway transpired when the supercritical temperature of the dustlayer was achieved for ignition at set temperature of 220 �C. Thetemperature at measurement points 1 to 4 successively reachedHTL within 33 min (379 �C), 35 min (413 �C), 39 min (361 �C), and40 min (285 �C), respectively. Based upon the aforementionedfindings, the region between measurement points 1 and 2 was themost sensitive ignition location in the coal dust layer. This isbecause sufficient oxygen supply near the surface facilitated moreradical exothermic reactions at the upper locations of dust layerthan those of lower regions. This result was consistent with that
Fig. 5. Coal dust layer for S2-CY during heating process (a) non-ignition occurred at a hot plate of 210 �C and (b) an ignition took place at a hot plate of 220 �C.
Fig. 6. Temperature distribution with respect to distance from the hot surface at corresponding times, (a) for Tp ¼ 210 �C and (b) for Tp ¼ 220 �C.
B. Li, G. Liu, M.-S. Bi et al. Energy 216 (2021) 119197
obtained by Park et al. [21]. Measurement point 2 had the highestHTL value in the dust layer because it was less affected by the heatconvection with the environment than measurement point 1. Thevalue of HTL (413 �C) at measurement point 2 was close to theignition point temperature (Ti ¼ 415 �C) determined because of theTG results. Therefore, the following ignition parameters obtainedfrom the measurement point 2 were used to represent the ignitionproperties of each pulverised coal dust layer.
3.2.4. Determination of thermal susceptibility parameters fordifferent types of coal dust layers on hot plate
The temperatureetime profiles of the prepared coal dust layers
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at the measurement point 2 are delineated in Fig. 8, and thedetailed self-ignition parameters are provided in Table 5. On thebasis, the hot plate test results, the MAIT of the dust layers for threecoal types was 210, 220, and 300 �C. For both the S1-BN and S2-CYsamples, each temperature evolution curve of the coal dust layersunder the supercritical (ignition) condition showed an apparentinflexion point within 26.7 and 26.1min. However, the temperatureinflexion point of the coal dust layer for the S3-JM sample under thesupercritical conditionwas not obvious, but a slight fluctuation stillexisted at 18.9 min (298 �C). The S3-JM sample had the leastmoisture content (1.55%) but the highest volatile content (34.04%)and oxygen content (14.04%). Accordingly, the devolatilisation
Fig. 7. Temperatureetime profiles of the coal dust layer for the S2-CY sample under (a) subcritical and (b) supercritical conditions for ignition.
Fig. 8. Temperatureetime profiles for the prepared coal dust layer at the measurement point 2.
Table 5Thermal stability parameters of the coal dust layer at the measuring point no. 2.
Sample Hot surface temperature (�C) HTL (�C) △T (�C) th (min) td (min) Result MAIT (�C)
Notes: HTL stands for the highest temperature in the layer; td annotates as ignition delay time; th is time at which maximum temperature reached; △T represents thetemperature difference between the peak temperature with the hot plate temperature.
B. Li, G. Liu, M.-S. Bi et al. Energy 216 (2021) 119197
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appeared for the S3-JM sample and was accompanied by moistureevaporation from the initial heating treatment.
Previous studies [27,31] have reported that the functionalgroups containing oxygen possibly constituted most of the activesites and proportionately increased with the oxygen content. In thepresent study, after baking on the hot plate at 300 �C for 18.9 min, aslight quantity of greyish smoke was clearly observed for the layerfor the S3-JM sample (Fig. 8c). This indicates that the S3-JM samplemay undergo noticeable chemical changes, during which bondbreakage was initiated and the branching of the molecular struc-ture was changed [32]. For the S3-JM sample, the number of activecentres increased with elevated temperatures, which enhanced theoxygen chemical absorption capacity. This was confirmed by amassgain of 0.75% at the end of the first stage in the TG curve (Fig. 3c).With increasing temperature, the oxidation process may have atransit from volatile combustion to char combustion, and thetemperature increased from the first inflexion point (298 �C) to HTL(410 �C) within 27 min. The corresponding td for the three samplesunder the supercritical condition was 29.5, 30, and 20 min, indi-cating less time being taken by the S3-JM sample than the S1-BNand S2-CY samples to reach the critical ignition point.
3.3. Estimation of thermokinetic parameters for the coal dust
3.3.1. Evaluation of effective thermal conductivityThe heat transfer process in the coal dust layer is a complex
phenomenon involving transient mass and heat transfer [9], reac-tion kinetics, and fluid dynamics [33]. The effective thermal con-ductivity (ke) of the accumulated dust is a vital parameter forevaluating the self-ignition and spontaneous combustion risks [34].The dust layer can be considered an infinite plane slab in idealthermal contact with a hot surface at temperature Tp, and theexposed surface loses heat to the environment through the New-tonian cooling process. Using the samemethod in the study by Parket al. [21], ke is expressed in Eq. (7):
where Ts, Ta, and Tp are the temperature of the surface of the coaldust layer, ambient temperature, and hot plate temperature,respectively; L is the thickness of the dust layer (0.01 m); and ht, hc,and hr are effective total heat transfer, convective, and radiant co-efficients, respectively. hc and hr are introduced in Eqs. (8) and (9)[11]:
8>>><>>>:
hc ¼ 0:54R0:25a kad
; for 105z<Raz<107
Ra ¼ gdðTs � TaÞd3yaaa
(8)
hr ¼ εs�T2s þ T2a
�ðTs þ TaÞ (9)
where ka, va, and aa are the thermal conductivity (0.026 W/[m2 K][21]), kinematic viscosity (12.71 � 10�6 m2/s [35]), and thermaldiffusivity of air (2.77 � 102 m2/s [35]), respectively; d is thediameter of the coal dust layer (0.10 m in this study); g is thegravitational constant (9.81 m/s2); d ¼ 2/(Ts þ Ta); and ε and s arethe coal emissivity (0.90 [11]) and the StefaneBoltzmann constant(5.67 � 10�8 W/[m2 K4] [11]), respectively.
Input parameters for experimentally determining thermody-namic parameters were measured by four thermocouples placed atheights of 1.5, 4.0, 6.8, and 9.0 mm in the 10.0-mm-thick coal dust
10
layer on the hot plate at 50 �C. There was little temperature changefrom 40 to 60 min, and the temperature distribution of dust layerswas almost stable during this stage (Fig. 9aec). For this reason, theaveraged temperatures at each measurement point from 40 to60 minwere used for the calculation of Ts with linear extrapolation.As shown in Fig. 9def, Ts values estimated from the linear fit for theS1-BN, S2-CY, and S3-JM samples were 38, 37, and 40 �C, corre-spondingly. Due to an error rate of ±3 �C for thermal stability of thelayer on the hot surface, the Tp value obtained from linear extrap-olation was 52 �C. The parameters obtained from the experimentsand calculations are tabulated in Table 6. Based upon the calcula-tion results, the Ra values of the three samples (2.15 � 106,2.03 � 106, and 2.38 � 106) were located within the useable rangein Eq. (5). From the summation of hc and hr, ht values were 11.01,10.91, and 11.19 W/(m2 K). The effective thermal conductivity (ke)values of the coal dust layers for the three coal types calculatedfrom Eq. (7) were 0.14, 0.12, and 0.19 W/(m K). Kosowska-Golachowska et al. [36] reported that thermal conductivityranged from 0.088 to 0.228 W/(m K) with different ranks of coal(brown coal, hard coal, and anthracite coal) at room temperature(25 �C). The present results of ke fell within this range. Furthermore,the ke values of coals from South Africa; Sebuku, Indonesia; andPittsburgh (0.10, 0.11, and 0.10 W/(m K)) that were published byPark et al. [21], Wu et al. [11], and Reddy et al. [37] using the hotplate method were also close to our results.
3.3.2. Determination of kinetics parameters for different types ofcoal dust
The kinetic parameters (Ea and A) are significant indexes for theassessment of heterogeneous solid-state reactions [38]. From theTG results in Fig. 3, themass of the tested samples slightly increasedwithin the temperature range of 130e300 �C. Wang et al. [39] andDeng et al. [21] have indicated that the minute mass gain at lowtemperature is mainly related to oxygen chemical absorptionaccompanied by the generation of carbon-oxygen complexes. Thefirst temperature inflexion point of the dust layer obtained from thehot plate test results also fell within this range, as seen in Fig. 7.Thermal runaway occurred after the first temperature inflexion ofthe accumulated dust, followed by ignition thatmay lead to fire andexplosion accidents at industrial sites. From a loss preventionmanagement perspective, the study of kinetic parameters in a low-temperature range (before ignition) is more instructive for poten-tial hazard prevention. In this study, both differential and integralmethods obtained from a previous study [13] were applied toanalyse the thermodynamic characteristics of the three samples attemperatures of 130e300 �C, as expressed in Eqs. (10) and (11):
Achar differential: ln�
dxdTfðxÞ
�¼ ln
Ab� EaRT
(10)
Coats�Redfern integral: ln�gðxÞT2
¼ ln
�ARbEa
� EaRT
(11)
where b is the heating rate, and f(x) and g(x) are the hypotheticalmodels of reaction mechanisms in differential and integral forms,respectively. The kinetic parameters Ea and A can be obtained fromthe Arrhenius plot of ln((dx/dt)/f(x)) or ln(g(x)/T2) versus �1/T atthe selected conversion value.
Previous studies [40,41] have revealed 16 types of reactionmechanisms for solid-state fuels, which are summarised inTable S1; using these mechanisms, Ea was determined using Acharand CoatseRedfern methods. Detailed information is listed inTable S2. Thus, the mechanisms of the reaction models weredetermined using the procedure of Bagchi [41]. If Ea obtained from
Fig. 9. Temperature profiles of 10-mm-thick coal dust layers at various heights on a hot plate at 50 �C; temperatureetime profiles of the (a) S1-BN, (b) S2-CY, and (c) S3-JM samples;equilibrium temperature distribution and its linear trends for the (d) S1-BN, (e) S2-CY, and (f) S3-JM samples.
Table 6Summary for determination of thermokinetic parameters for S1-BN, S2-CY, and S3-JM of coal samples.
B. Li, G. Liu, M.-S. Bi et al. Energy 216 (2021) 119197
the samemechanism of differential and integral methods was closeto the high linear correlation coefficients, the corresponding modelwas considered as the optimal mechanism for describing the
11
spontaneous coal combustion in the initial stage. After comparingcalculation results from different methods (as displayed in Fig. 10and Table 7) and taking the correlation coefficient (R2) into ac-count, a 3D diffusion model (g(a) ¼ (1 � (1 � a)1/3)2,f(a) ¼ [3(1 � a)2/3]/[2(1 � (1 � a)1/3)]) was determined as theoptimal model to explain the initial chemical reaction process ofthe coal samples. The Ea values of the three samples calculatedusing the Achar method were 95.40, 113.13, and 145.53 kJ/mol. Thevalues calculated using the CoatseRedfern method were 97.50,112.53, and 140.66 kJ/mol. The values of Ea calculated using theAchar method were almost similar to those calculated using theCoatseRedfern method. Ma et al. [41] investigated the kineticproperties of four coal types by using the same two methods in thetemperature range of 69e285 �C. Their values of apparent activa-tion energies in the range of 119.04e263.30 kJ/mol were similar tothe results in this study. In addition, the same reaction mechanismwas found by Yang et al. [42] in the co-pyrolysis process of walnut
Fig. 10. Plots of the three coal samples calculated using the (a) Achar differential method and (b) CoatseRedfern integral method.
Table 7The most optimal kinetic parameters for the three coal samples.
B. Li, G. Liu, M.-S. Bi et al. Energy 216 (2021) 119197
shell and coal when the heating rate was 10 �C/min. They reportedthat the reaction models of co-pyrolysis for blends followed 3Ddiffusion. This was because fragment diffusion in coal particleswould encounter resistance, and relatively few pores were formedafter coal devolatilisation.
Fig. 11. Self-ignition risk ranking of the coal dust samples based upon the MAIT and Ea.
3.4. Ranking of self-ignition risk of the coal samples
According to requirements in the standards of IEC60079e0e2011 and 60,079e14e2013 on safety regulation, theheated external surface temperature of equipment in dust defla-gration hazard areas should be at least 75 �C below the minimumtemperature of the dust layer. Evaluation of the self-ignitionproperties of the dust materials provides prominent guidance forequipment design and selection. Ramírez et al. [18] estimated theignition risks based on the characteristic temperature (peak tem-perature during combustion in an oxygen stream) and Ea for py-rolysis. In addition, Chin et al. [19] and Magalh~aes et al. [43] haveranked the ignition risk based on the parameters of Ea and the firstDTG peak temperature obtained from TG curves. According to thestandard of IEC 60079e14e2013 for the surface temperature ofelectrical equipment of Group III (equipment employed in scenarioswith explosive dust atmosphere other than mines susceptible tomethane), the maximum allowable surface temperature for thefacility should be based upon the examined MAIT of the dust layer.For the coal dust deposited at coal chemical industry sites, it is moreappropriate to use the MAIT for the assessment of ignition risks; itcan be considered a crucial indicator for determining the surfacetemperature of industrial equipment and for loss preventionmanagement. On the basis of Ramírez’s risk classification approach,improved risk matrix zones with respect to the MAIT and oxidatingEa were generated to assess the self-ignition properties of the threetypes of coal, as given in Fig. 11. Ea shown in Fig. 11 is the arithmeticmean value of Ea obtained using Achar and CoatseRedfernmethods. The results showed that the S1-BN and S2-CY sampleshad high self-ignition risks, and the S3-JM sample had moderate
12
self-ignition risk. Semi-anthracite coal, bituminous coal (data froma previous study [18]), and Colombian coal UNC-10 (data from aprevious study [44]) were found to have low, moderate, and highrisks, respectively.
3.5. Enlightenment for risk assessment of fire and explosion hazardsin industrial sites
The ignition process of a typical dust accumulation case (IEC2874/13) in the industrial scene based upon IEC 60079e14e2013was analysed in the present work. Fig. 12 depicts the trans-formation of self-ignition for coal dust layer on a hot surfaceequipment. As can be seen, a thermal runaway occurred when thehot surface temperature tp of the equipment reached the MAITpoint. At this moment, the heat from exothermic oxidation re-actions made more contributions to igniting the dust layer thanthat of heat from the hot surface of equipment. The occurrence ofthermal runaway meant the transformation of the self-ignitionprocess from physical control mode (heat conduction) to chemi-cal control mode (exothermic oxidation reaction). This was thereason that caused a sudden fast-increasing change in temperaturein the inner layer of coal dust. The temperature field of the dustlayer displayed a nonlinear distribution, as shown in Fig. 7b. The
Fig. 12. Transformation of self-ignition process for coal dust deposits at industrial sites.
B. Li, G. Liu, M.-S. Bi et al. Energy 216 (2021) 119197
accumulation form, particle size, and layer thickness are the vitalexternal factors which affected the heat transfer process and self-ignition properties of dust layer under the subcritical conditions.However, in the practice of dust risk assessment of industrial sites,the intrinsic properties of dust materials, such as the effectivethermal conductivity, reaction mechanism, and the apparent acti-vation energy, should also be considered for forestalling fire andexplosion hazards in industrial sites.
4. Conclusions
This study provided crucial information for evaluating the self-ignition risk for the dust of three coal samples. The reactivity ofthe coal samples was investigated using TGA and the hot surfacetest. Differences in combustion properties, thermodynamic per-formance, and thermokinetic parameters were determined forcharacterising the spontaneous ignition tendency of the coal dust.The MAIT and Ea can be used as two key indicators for determiningthe permissible surface temperature of industrial equipment forsafety management. The results on account of an improved riskmatrix approach showed the S1-BN and S2-CY samples had a highself-ignition risk, and the S3-JM sample had a moderate ignitionrisk. Thermal combustibility analysis (TG tests) offers the advan-tages of shorter test times and higher measurement accuracy. Thethermokinetic parameter Ea in the initial stage of spontaneous coalcombustion (130e300 �C) was determined using the Achar methodand was 95.40, 113.13, and 145.53 kJ/mol. Using the CoatseRedfernmethod, the values were 97.50, 112.53, and 140.66 kJ/mol. Thermalsusceptibility analysis (hot plate test) may be useful for equipmentdesign and loss pevention management. The MAIT values of 10.0-mm coal dust layers were 210, 220, and 300 �C. The resultsshowed that heat conductionwas the dominant heat transfer modethat generated the temperature distribution within the coal dustlayer under the subcritical conditions for ignition. However, thetemperature distribution was primarily determined by the chemi-cal reaction mode in the coal dust layer after thermal runaway.
Authorship contribution statement
Bei Li: Writing - original draft & investigation. Gang Liu:Writing - review, Editing & data analysis. Ming-Shu Bi: Supervi-sion, Writing - review. Zhen-Bao Li: Writing - review & editing.Bing Han: Resources, Review. Chi-Min Shu: Formal analysis,Writing - review & editing.
13
Declaration of competing interest
The authors declare that they have no known competingfinancial interests or personal relationships that could haveappeared to influence the work reported in this paper.
Acknowledgements
This study was supported by National Key Research andDevelopment Program of China (No. 2018-YFC-0807900), by Fundof the National Natural Science Foundation of China (No. 5190-4054), and by Fundamental Research Funds for the Central Uni-versities of China (No. DUT19RC(4)002).
Appendix A. Supplementary data
Supplementary data to this article can be found online athttps://doi.org/10.1016/j.energy.2020.119197.
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