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Proceedings of the Combustion Institute 31 (2007) 1947–1954

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Char characteristics and particulate matterformation during Chinese bituminous coal combustion

Yun Yu, Minghou Xu *, Hong Yao, Dunxi Yu, Yu Qiao, Jiancai Sui,Xiaowei Liu, Qian Cao

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China

Abstract

The characteristics of char particles and their effects on the emission of particulate matter (PM) from thecombustion of a Chinese bituminous coal were studied in a laboratory-scale drop tube furnace. The rawcoal was pulverized and divided into three sizes, <63, 63–100, and 100–200 lm. These coal samples weresubjected to pyrolysis in N2 and combusted in 20 and 50% O2 at 1373, 1523, and 1673 K, respectively. Charsamples were obtained by glass fiber filters with a pore size of 0.3 lm, and combustion-derived PM wassize-segregated by a low pressure impactor (LPI) into different sizes ranging from 10.0 to 0.3 lm. The char-acteristics of char particles, including particle size distribution, surface area, pore size distribution, swellingbehavior and morphology property, were studied. The results show that, coal particle size and pyrolysistemperature have significant influence on the char characteristics. The swelling ratios of char samplesincrease with temperature increasing from 1373 to 1523 K, then decrease when the temperature furtherincreases to 1623 K. At the same temperature, the swelling ratios of the three size fractions are markedlydifferent. The finer the particle size, the higher the swelling ratio. The decrease of swelling ratio at high tem-perature is mainly attributed to the high heating rate, but char fragmentation at high temperature may alsoaccount for the decrease of swelling ratio. The supermicron particles (1–10 lm) are primarily spherical, andmost of them have smooth surfaces. Decreasing coal particle size and increasing the oxygen concentrationlead to more supermicron-sized PM formation. The influence of combustion temperature on supermicron-sized PM emission greatly depends on the oxygen concentration.� 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Coal combustion; Particulate matter; Char characteristic

1. Introduction

China, as a rapidly developing country, is nowfacing very serious health risks from fine particlesresulting from various sources including coal-firedindustrial boilers, diesel motor vehicles, tobaccosmoking, and so on. Among the cities which have

1540-7489/$ - see front matter � 2006 The Combustion Institdoi:10.1016/j.proci.2006.07.116

* Corresponding author. Fax: +86 27 87545526.E-mail address: mhxu@mail.hust.edu.cn (M. Xu).

exceeded the standard of air quality in China, 68%of them have serious problem of particulate mat-ter pollution. Particulate matter (PM) generatedfrom coal combustion was considered as one mainsource of fine particles in the atmosphere. Espe-cially in China, due to the low thermal efficiencyof coal-fired boiler and prevalence of low-rankcoal, PM emission from coal combustion accountsfor about 1.2–1.5% of the consumed coal in thepower generation plant, causing the most severeenvironmental pollution there [1].

ute. Published by Elsevier Inc. All rights reserved.

1948 Y. Yu et al. / Proceedings of the Combustion Institute 31 (2007) 1947–1954

Coal-derived PM has a bimodal particle sizedistribution and is formed from the inorganicmatter present in coal, which occurs in the formof minerals of various types and sizes. Theseminerals can be closely associated with theorganic matter (included minerals), or separatefrom the organic matter (excluded minerals).The mechanisms of PM formation have beenstudied extensively in the previous studies, andare affected by the combustion conditions andthe coal characteristics [2–4]. The majority ofPM is formed from four processes: (1) includedmineral coalescence, (2) char fragmentation, (3)excluded mineral fragmentation and (4) vapori-zation and subsequent condensation of inorganicmatter. PM10 (PM with an aerodynamic diameterless than 10 lm) generated during coal combus-tion can be divided into supermicron-sized PMwith an aerodynamic diameter of 1–10 lm, whichis mainly formed by fragmentation and coales-cence mechanisms, and submicron-sized PM withan aerodynamic diameter less than 1 lm, whichis mainly formed by the vaporization and con-densation mechanism [5]. Char fragmentationand mineral coalescence are the key processesinfluencing the transformation of included miner-als during pulverized coal combustion, and supe-rmicron PM formation during pulverized coalcombustion is the consequence of the competi-tion between char fragmentation and includedminerals coalescence.

Char particles with various structures have sig-nificantly different behavior during combustion.Baxter [6] indicated that the fragmentation of charis strongly rank dependent, which is affected bydifferences in the char structure. Wu et al. [7] con-firmed that char particles of different structuraland morphological properties exhibited differentfragmentation behavior, and ash formation mech-anism. Highly porous char particles fragmentextensively and produce fine ash particles. Charparticles with a low porosity exhibit the low extentof fragmentation, leading to the formation ofcoarse ash particles. For bituminous coals, as theyundergo swelling upon heating, then swellingbehavior becomes an important factor which willgreatly affect the char structure. As a consequenceof swelling, a large portion of char particles areproduced with highly porous or cenosphericalstructures, which tend to undergo extensive frag-mentation during combustion, resulting in thedecrease of the extent of coalescence of includedmineral matter. Gale et al. [8] studied the swellingand porosity of several bituminous coals duringdevolatilization at high heating rates. Hamilton[9] found that vitrinite was primarily responsiblefor the plastic behavior in the swelling bituminouscoals. Yu et al. [10] used sink–float techniques toobtain pulverized coals of different maceral con-centrations through density separation, and inves-tigated the swelling behavior of char particles in a

drop tube furnace (DTF). However, there is littledirect work on the influence of char characteristicson PM formation up to now.

This paper aims to investigate the influence ofchar characteristics on PM formation by experi-ments. Char samples were prepared from threesize fractions of a bituminous coal at differenttemperatures in a DTF. Char characteristics,including particle size distribution, surface area,pore size distribution, swelling behavior and mor-phology property, were studied. PM was collectedby a Deketi low pressure impactor (DLPI) toobtain the PM emission under various conditions.By relating char characteristics to PM emissionduring coal combustion, the PM formation pro-cesses during pulverized coal combustion can bebetter understood.

2. Experimental

2.1. Coal samples

A Chinese bituminous coal (Pingdingshan,Henan Province, China) was used in this study.The pulverized coal was dried and divided intothree sizes: <63, 63–100, and 100–200 lm, indicat-ed as C01, C02, and C03, respectively. The prox-imate and ultimate properties, as well aspetrographic composition of the size-segregatedfractions are shown in Table 1. It can be seen thatcoal particle size has slight effect on coal proper-ties. Both fixed carbon and inertinite contentincrease as coal particle size increases, while vola-tile matter and vitrinite content decrease withincreasing coal particle size.

2.2. Char and PM samples preparation

The DTF used in this study consists of a coalfeeding system, high temperature furnace and asampling probe, as shown in Fig. 1. Coal particlesentrained in the gas were fed into the DTF by aSankyo Piotech Micro Feeder (Model MFEV-10), and the feeding rate of coal particles is0.2 g/min. The furnace has a length of 2 m andan inner diameter of 56 mm. It is electrically heat-ed, and the temperature of the inner wall mea-sured with thermocouples is displayed on amonitor. The sampling probe is water-cooledand to prevent samples from further reactions,high-purity nitrogen gas is supplied from the topof the probe.

Chars were prepared from the three size-classi-fied coal samples in the DTF at the temperaturesof 1373, 1523, and 1673 K, respectively. The pyro-lysis experiments were carried out at atmospherepressure in nitrogen gas. Pyrolyzed residues werecollected at the bottom of the furnace with thewater-cooled, nitrogen-quenched sampling probethat was fixed in position during all experiments.

Table 1Properties of raw coal samples

Coal sample Size (lm) Proximate analysisa % (wt, ad) Ultimate analysis % (wt, ad)

M VM A FC C H N S + Ob

C01 <63 0.77 41.49 18.51 39.23 68.95 5.14 1.11 5.52C02 63–100 1.16 37.91 17.30 43.63 68.88 5.45 0.13 7.08C03 100–200 1.05 35.98 18.75 44.22 67.03 5.34 1.09 6.74

Coal sample Size (lm) Petrographic analysisc % (vol) Rod (%)

L V I MM

C01 <63 1.1 80.2 14.1 4.6 1.04C02 63–100 1.8 79.0 15.0 4.2 0.89C03 100–200 0.3 70.4 17.6 11.7 0.97

a M, moisture; VM, volatile matter; A, ash; FC, fixed carbon.b By difference.c L, liptinite; V, vitrinite; I, inertinite; MM, mineral matter.

Fig. 1. Schematic diagram of experimental setup.

Y. Yu et al. / Proceedings of the Combustion Institute 31 (2007) 1947–1954 1949

Char particles were finally collected on glass fiberfilters with a pore size of 0.3 lm, which is shownas collection device I in Fig. 1.

Combustion experiments of three size-classi-fied coal samples were also performed at the tem-peratures of 1373, 1523, and 1673 K, respectively.As seen in device II in Fig. 1, the combustionproducts were collected through the water-cooledsampling probe by a Dekati cyclone (Model SAC-65) and the DLPI. The used gas was a mixture ofcompressed oxygen and nitrogen and two oxygenvolume fractions of 20 and 50% were selected.For all combustion experiments, the char burnout

was measured, with values higher than 99%. Theparticles larger than 10 lm (in aerodynamic diam-eter) were captured by the cyclone, while PM10

was collected and size classified by the 13-stageDLPI. The 50% cutpoints of Stages 1–13 are0.0281, 0.0565, 0.0944, 0.154, 0.258, 0.377, 0.605,0.936, 1.58, 2.36, 3.95, 6.6 and 9.8 lm (in aerody-namic diameter), respectively. To reduce conden-sation of particles on the inner shell of thesampling probe, high-purity gas is also suppliedfrom the top of the probe as quench medium.

2.3. Sample analysis

Malvern particle-size analyzer (MAM 5004)and Micromeritics surface area and pore size ana-lyzer (ASAP 2000) were used to respectively mea-sure the particle size distributions, surface areaand pore size distribution of the coal samples, thatis, C01, C02, C03, and their corresponding charresidues prepared at different pyrolysis tempera-tures. For the convenience of the following discus-sion, swelling ratio used in this study is defined asthe average particle diameter of resultant charsample over that of the original coal sample. Sur-face and cross-sectional characterization of resul-tant chars were conducted on a FEI Sirion 200field emission Scanning Electron Microscope(FESEM).

Aluminum foils were used to collect PM10 pro-duced under different combustion conditions. Par-ticle mass on each stage were obtained bysubtracting the weight of the aluminum foil fromthe total weight of the aluminum foil and the col-lected particles. Gravimetric analysis was con-ducted with a Sartorius M2P Microbalance(readability, 0.001 mg). Teflon filters were alsoused in this study to collect particles for the pur-pose of particle morphology characterization withFESEM equipped with an energy dispersive X-rayspectrometer (EDX).

1950 Y. Yu et al. / Proceedings of the Combustion Institute 31 (2007) 1947–1954

3. Results and discussion

3.1. Coal and char sample characteristics

3.1.1. Pore structure and surface area of coalsamples and resultant chars

Generally, the pores in char particles are divid-ed into three classes: micropores, mesopores andmacropores. According to the studies of Kantoro-vich and Bar-Ziv [11,12], the porous structure of achar is constructed from solid microcrystals, ran-domly oriented and randomly interconnected.Intercrystal voids are referred to as micropores.The skeleton of the microcrystals, together withmicropores form the microstructure. Microstruc-ture is bounded by macropores, with dimensionsmuch larger than the mean dimensions of themicrocrystals. As the mean length of the micro-crystals is about 4 nm [13], we define the boundarybetween micropores and mesopores as 10 nm,while the boundary of mesopores and macroporesis defined as 50 nm.

The pore structure and BET surface area of thethree coal samples and their corresponding charresidues prepared at 1373, 1523 and 1673 K arecompared in Table 2. First, it can be seen that coalparticle size has a great effect on its pore structureand BET surface area. The finer the particle size,the larger the total pore volume and BET surfacearea. For three size-classified coal samples, theirpore structures are mainly macropores and mes-opores, which account for more than 90% of thetotal pore volume. Second, in comparison withtheir corresponding raw coal samples, the totalpore volume and BET surface area of char sam-ples all increase greatly, which is caused by thevolatile yield during devolatilization. Third, pyro-lysis temperature has a distinct effect on the porestructure of char samples. For example, at 1373and 1523 K, macropores and mesopores of allchar samples account for more than 90% of the

Table 2Pore structure and BET surface area of raw coal samples and

Sample Macroporesa

% (vol)(dp > 50 nm)

Mesoporesa

% (vol)(10 nm < dp < 50

C01 Raw coal 85.2 14.8Char(1373 K) 71.5 24.8Char(1523 K) 71.9 20.6Char(1673 K) 53.6 27.4

C02 Raw coal 78.3 16.0Char(1373 K) 84.3 8.6Char(1523 K) 71.8 26.6Char(1673 K) 56.3 19.4

C03 Raw coal 71.7 24.7Char(1373 K) — —Char(1523 K) 95.9 3.4Char(1673 K) 97.3 1.35

a Percentage which different size pore volumes account for o

total pore volume. However, the percentage ofmacropores decreases, and that of microporesincreases at 1673 K. The increase of microporesat high temperature also leads to the increase ofBET surface area.

3.1.2. Particle size distribution of coal samples andresultant chars

The cumulative volume particle size distribu-tions of the three coal samples and their corre-sponding char residues prepared at 1373, 1523,and 1673 K are compared in Fig. 2. From the fig-ure, it can be clearly seen that all these char sam-ples produced at different temperatures havemuch larger sizes than their corresponding coalsamples. It is indicated that coal particles undergosignificant swelling during pyrolysis. It can also benoted in Fig. 2 that, for each size coal, the charprepared at 1523 K is coarser than those preparedat 1373 and 1673 K. It is indicated that a signifi-cant reduction in particle size occurs during pyro-lysis at high temperature, which was also observedby Maloney [14].

3.1.3. Swelling behavior of resultant charsThe relationship between swelling ratio and

temperature is shown in Fig. 3. It can be seen thattemperature has a great influence on the swellingratio for three char samples. For each size frac-tion, the swelling ratio at the temperature of1523 K is always larger than that at other twotemperatures. It seems that the effect of tempera-ture on increasing swelling of coal particles has alimited range, and high temperature can lead toa decline of swelling. Gale [8] stated that, thechemical release rate of volatiles is believed to befaster than the relaxation time involved in expan-sion of the particle shell at high heating rate andthe viscosity of the swelling bubble film is to below before the formation of resolidified shells,which lead to more frequent bubble ruptures.

resultant chars

nm)

Microporesa

% (vol)(dp < 10 nm)

Total porevolume(cm3/g)

BETsurface area(m2/g)

0 5.59e�3 2.193.7 3.23e�2 2.897.5 8.48e�2 3.43

19.0 7.91e�2 5.29

5.7 2.60e�3 1.477.1 1.13e�2 4.651.7 4.89e�2 4.12

24.3 3.06e�2 17.20

3.6 1.23e�3 0.52— — 3.520.7 1.31e�2 3.501.35 3.55e�2 6.23

f the total pore volume.

00

20

40

60

80

100

Cu

mu

lati

ve v

olu

me

frac

tio

n, %

Cu

mu

lati

ve v

olu

me

frac

tio

n, %

Cu

mu

lati

ve v

olu

me

frac

tio

n, %

Particle diameter, µm

C01Char(1373K)Char(1523K)Char(1673K)

00

20

40

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80

100

Particle diameter, µm

C02Char(1373K)Char(1523K)Char(1673K)

b

00

20

40

60

80

100

Particle diameter, µm

C03Char(1373K)Char(1523K)Char(1673K)

c

800600400200 400300200100 400300200100

a

Fig. 2. The cumulative volume particle size distribution of coal samples and chars prepared at different temperatures. (a)Coal C01 and its resultant chars; (b) coal C02 and its resultant chars; (c) coal C03 and its resultant chars.

1350 1500 1650

1

10

Sw

ellin

g R

atio

Temperature (K)

C01C02C03

Fig. 3. The relationship between swelling ratio andtemperature of three resultant chars.

Fig. 4. Typical SEM micrographs of resultant charsfrom sample C01. (a) 1373 K, surface; (b) 1523 K,surface; (c) 1673 K, surface; (d) 1373 K, cross-section;(e) 1523 K, cross-section; (f) 1673 K, cross-section.

Y. Yu et al. / Proceedings of the Combustion Institute 31 (2007) 1947–1954 1951

Therefore, the decrease of swelling ratio and thereduction of char particle size at high temperatureare probably attributed to the high heating rate athigh temperature. The influence of heating rate onswelling behavior of char particle has been dis-cussed in detail in our newly published paper[15]. It can also be seen from Fig. 3 that coal par-ticle size also has a significant effect on the swell-ing ratio of char samples. The finer the particlesize, the higher the swelling ratio.

3.1.4. Morphology property of resultant charsImage analysis techniques were used to study

the morphologies of resultant chars from differentconditions. Derived SEM micrographs show thata large number of char cenospheres are formedduring pyrolysis, as shown in Fig. 4, which aretypical SEM micrographs of resultant chars fromC01 under different temperatures. These ceno-spherical chars have a large central void in theparticle surround by a thin shell with a non-uni-form distribution of macropores. They arebelieved to the results of swelling of coal particlesdue to the rapid release of volatiles upon heating.Another reason for char swelling is that the coalsamples contain high content of vitrinite (80.2%for C01, 79.0% for C02, and 70.4 for C03, as indi-

cated in Table 1). It is known that vitrinite devel-ops more fluidity upon heating, and bituminouscoal particles easily undergo softening, passingthrough a plastic stage. During the stage swellingwill occur, resulting in these types of charstructure.

From Fig. 4, it can be verified that temperaturehas a significant effect on the formation of char.With the increase of temperature, the porosity ofresultant char increases and more highly porouschar cenospheres are generated. That is becausethat high furnace temperature leads to a higherheating rate during devolatilization, and also ahigher volatile yield. The rapidly released volatilematter has insufficient time to escape throughsmall pores in the particle surface, resulting inpressure buildup and intensive bubbling phenom-ena within the particle, which will contribute tothe swelling [15].

Moreover, lots of fragments are generated athigh temperature, which may reduce the averageparticle size and swelling ratio of char samples.As more highly porous char cenospheres areformed at high temperature during pyrolysis andthey are easily to fragment due to the explosiveejection process [16], char fragmentation duringpyrolysis is also a possible reason for the reduc-tion of char particle size and the decline of the

Fig. 6. The SEM micrographs of typical supermicronparticles.

1952 Y. Yu et al. / Proceedings of the Combustion Institute 31 (2007) 1947–1954

swelling ratio of char samples, besides the reasonof the high heating rate discussed above.

3.2. PM results

The PM collected by DLPI has a bimodal sizedistribution, with a large mode of about 4 lm anda fine mode of approximately 0.1 lm, as seen inFig. 5. As indicated in the Introduction section,char characteristics mainly influence the forma-tion of supermicron-sized PM. This paper focusedon the properties of supermicron-sized PM, suchas the morphology and the emissions under differ-ent oxygen concentrations.

3.2.1. Morphology of supermicron-sized PMPM collected on Stage 11, with an aerodynam-

ic 50% cutoff diameter of 3.95 lm, was qualita-tively examined by the Sirion 200 FESEM. Thetypical morphologies of supermicron-sized PMare shown in Fig. 6. The supermicron particlesare primarily spherical in shape, as shown inFig. 6a. This is consistent with the observationin the literature [17,18]. Most of these sphereshave smooth surface and a size of approximately2 lm (Fig. 6b). However, some may contain dis-crete submicron particles sintered onto the parti-cle surface. The EDX indicates that thesesmooth spherical particles are mainly aluminosili-cate structure. In addition, there are some crystal-lized particles indicated in Fig. 6c, and the EDXshows that they have high concentration of Fe,which is believed to result from pyrite present inthe raw coal. Only a very small fraction of the par-ticles showed any evidence of deformation orirregularities (Fig. 6d).

The temperature and oxygen concentration arefound to have obvious influence on the morphol-ogies of supermicron particles. Increasing the tem-perature and oxygen concentration all lead tomore spherical particles formation. More irregu-lar shape particles are formed at low temperature,

Fig. 5. Typical particle size distribution of PM10 (C011523 K).

,

and this could be the consequence of fragmenta-tion mechanisms involving particle inflation,cracking, and material shedding [18].

3.2.2. Effect of combustion temperature on supe-rmicron-sized PM emission

The mass concentrations of supermicron-sizedPM obtained in 20 and 50% O2 at differenttemperatures are illustrated in Fig. 7. ComparingFig. 7a with Fig. 7b, it can be found that temper-ature has different effects on supermicron-sizedPM formation. For supermicron-sized PMobtained in 20% O2, the influence of tempera-ture on supermicron-sized PM concentrationis not obvious, while for supermicron-sized PMobtained in 50% O2, the mass concentration ofsupermicron-sized PM increases with increasingthe temperature.

As discussed above, the supermicron particlesare mainly formed by char fragmentation andincluded mineral coalescence, which are two com-petitive processes. However, char fragmentationstrongly depends on pore structure of char. Helbleand Sarofim [19] observed perimeter fragmentationof chars with different dominant modes of porosityin regime II, and concluded that macroporosityplayed the controlling role. Bar-Ziv and Kantoro-vich [12] indicated that char fragmentation wasnot affected merely by the total porosity, but ratherby macroporosity in regime I. Furthermore, ifreaction occurred on the external surface of charparticle, the pore structure changes in the externaldimensions are obvious. In this case, included min-eral coalescence is the primary mechanism to formsupermicron-sized PM. When the porosity of charor the oxygen concentration is large enough foroxygen to penetration into the char particle, charis mainly oxidized on the internal surface, whicheasily causes fragmentation due to the increase ofmacroporosity. The reason is that the internal sur-

0

10

20

30

40

50a

C03C02C01

m/g

m,cnoc

MP

dezis-norci

mrep

uS

3 N1373K1523K1673K

0

20

40

60

80

100

120

140b

C03C02C01

m/gm,c

nocM

Pdezis-norci

mrepuS

3 N 1373K1523K1673K

Fig. 7. The supermicron-sized PM mass concentrations. (a) 20% O2; (b) 50% O2.

Y. Yu et al. / Proceedings of the Combustion Institute 31 (2007) 1947–1954 1953

face area of char is very large in comparison withthe external area (often as much as 1000 times)and oxygen is available within the particle to theentire internal surface area. Therefore, char frag-mentation is the most important mechanism toproduce supermicron-sized PM in 50% O2.

From the study of char characteristics, theinfluence of temperature on supermicron-sizedPM formation can be explained to some extent.When the oxygen concentration is 20%, char par-ticles are probably oxidized mainly on the externalsurface, then the supermicron-sized PM is primar-ily formed by included mineral coalescence. How-ever, the coalescence process is strongly affectedby the mineral matter distribution inside char par-ticles, which is much different from that in coalparticles. This may lead to the uncertainty ofsupermicron-sized PM concentration in 20% O2

at different temperatures. While in 50% O2, charoxidation reaction probably mainly happensinside the char particle, and char fragmentationis thought as the main mechanism to form supe-rmicron-sized PM. As the total porosity and sur-face area of char particles at high temperatureall increase greatly, they provide enough surfaceson which reactions of oxygen with carbon occur.The increase in char porosity can enhance charfragmentation, hence more supermicron-sizedPM formation. Also, increasing oxygen concen-tration leads to a higher particle heating rate,which may induce more char fragmentation dur-ing devolatilization, as observed in this study.These small fragments will form small particlesafter their burnout, and also lead to an increaseof supermicron-sized PM.

3.2.3. Effect of coal particle size on supermicron-sized PM emission

From Fig. 7, coal particle size is found to havegreat influence on supermicron-sized PM massconcentration, which increases with decreasingcoal particle size in both 20 and 50% O2. This isbecause the swelling ratio and porosity of charfrom small coal particle are greater than thosefrom large coal particle, as discussed above. The

increase of swelling ratio and porosity enhancesthe char fragmentation, and leads to more supe-rmicron-sized PM formation. However, the trendof supermicron-sized PM concentration for C02and C03 is not very obvious. As the swelling ratiosfor these two coal samples are not high, the supe-rmicron-sized PM formation is controlled by thecompetition of char fragmentation and includedmineral coalescence, which is more influenced bymineral matter distribution inside char particles.

3.2.4. Effect of oxygen concentration on supermi-cron-sized PM emission

The effect of oxygen concentration on supermi-cron-sized PM mass concentration is distinct, asindicated in Fig. 7. With the increase of the oxygenconcentration, supermicron-sized PM mass con-centration increases obviously. For example, supe-rmicron-sized PM mass concentration for C01 at1673 K is 43.55 mg/m3 in 20% O2, while increasesto 126.73 mg/m3 in 50% O2. The reason is that oxy-gen concentration determines where char oxidationreaction happens. The higher the oxygen concen-tration, the more oxygen comes into the char parti-cles, and reaction rapidly occurs within the charparticles. Therefore, char fragmentation becomeseasier and more supermicron-sized PM is formed.

4. Conclusions

Char characteristics and PM formation of aChinese bituminous coal were studied by the com-bustion of three size-classified coal samples in aDTF at different temperatures. Char characteris-tics, including pore structure, BET surface area,particle size distribution, swelling ratio, and mor-phology property, were related to the formationof supermicron-sized PM. Effects of coal particlesize, temperature and oxygen concentration onsupermicron-sized PM emission were discussed.

From the supermicron-sized PM obtained in20% O2, it is found that the PM is mainly formedby included mineral coalescence and the influence

1954 Y. Yu et al. / Proceedings of the Combustion Institute 31 (2007) 1947–1954

of temperature is not obvious. While from thesupermicron-sized PM obtained in 50% O2, thePM emission increases with the increasing of tem-perature, which is probably attributed to theincreasing extent of char fragmentation at hightemperatures. Coal particle size is found to havegreat influence on supermicron-sized PM emis-sion. The PM emission increases with decreasingcoal particle size in both 20 and 50% O2. This isbecause that the swelling ratio and the porosityof char from small coal particles are greater thanthose from large coal particles, and this makes thechar particle fragment easily. With the increase ofoxygen concentration, the supermicron-sized PMemission increases obviously. The reason is thatwith higher oxygen concentration, more oxygenwill penetrate into the char particles and hencemore extensive char fragmentation will occur.

Acknowledgments

The authors thank the financial support pro-vided by the National Natural Science Founda-tion of China (Grant No. 50325621) and theNational Key Basic Research and DevelopmentProgram of China (Grant No. 2002CB211602).

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Comments

Paul Fennell, Cambridge University, UK. Did youconfirm your pore-size information from gas-absorptionanalysis, which detects pores <200 nm by, for example,comparison with mercury porosimetry, which will allowyou to probe pores up to �1 nm? The bigger poresdetectable only with Mercury porosimetry will contrib-ute most to the pore volume.

Reply. As indicated in the paper, the pore-size infor-mation is obtained by Micromeritics surface area andporosimetry analyzer (ASAP 2000) which uses the nitro-gen adsorption. We agree that mercury porosimetry candetect much larger pores that are out of the detectionrange of nitrogen adsorption. However, with nitrogenadsorption, the smallest pores (up to 0.3 nm) that areout of range of mercury porosimetry, can be determined.In this study, we pay more attention to the smaller pores,because they contribute more to the surface area. Due tothe absence of mercury porosimetry, we did not compare

our results with those from mercury porosimetry. In thefuture work, we will consider using different methods toobtain a broad feature of the pore structures.

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Alejandro Molina, National University of Colombia,

Medellia, Colombia. For the 10 swelling ratio data, didyou see any particle agglomeration in your data?

Reply. We did not see any char particle agglomera-tion under SEM. We believe that such high ratio dataare mainly due to particle swelling during devolatiliza-tion. As indicated in Table 1, the coal sample C01, whichhas the smallest particle size, contains the highest con-tent of vitrinite (80.2%). Therefore, a large number ofhighly porous char particles (Fig. 4) were produceddue to swelling, which reasonably results in high swellingratios for C01.