Experimental Investigation of Coal Combustionin Coal-Laden Methane JetsRajavasanth Rajasegar1 and Dimitrios C. Kyritsis2

Abstract: Combustion characteristics of pulverized coal were studied in a methane jet that entrained pulverized coal particles usingthe Venturi effect. The dependence of entrainment rate on the size of coal particles was studied as a function of the flow rate. Particlestreak velocimetry performed on coal-laden jets provided valuable insight into the relative velocity of entrained coal particles with respectto the fluid velocity. High-resolution still images and high-speed videos of laser-sheet light scattered by the coal particles were used inorder to determine the mode of interaction of entrained coal particles with the flame front. The effect of combustion on the entrained coalparticles was analyzed both in terms of macrostructure and microstructure using a combination of loose density measurement, Fraunhofer-diffraction-based particle-size distribution measurements, and scanned electron microscopy. It was established that the combustionprocess did not have any significant effect on the macrostructure of the coal particles. At the same time, remarkable changes were observedin particle microstructure. Based on these findings, it was established that the coal particles underwent only partial devolatilization duringtheir passage through the flame due to the small residence time. Hence, it was concluded that oxidation was basically a surface phenomenon.The effect of oxidizer composition on the combustion of coal particles was studied by comparing the measured particle-size distributions forCH4=air, CH4=O2=CO2, and CH4=O2 flames. DOI: 10.1061/(ASCE)EY.1943-7897.0000228.© 2014 American Society of Civil Engineers.

Author keywords: Pulverized coal; Coal-laden methane jets; Particle streak velocimetry; Fraunhofer diffraction; Scanned electronmicroscopy; Devolatilization.

Introduction

Energy generation from combustion of fossil fuels results insignificant emission of greenhouse gases, predominantly CO2

[Intergovermental Panel on Climate Change (IPCC) 2007; Wangand Tseng 2012]. The increasing awareness about the need to re-duce greenhouse gases has renewed interest in coal combustiontechnologies and has led to recent developments in oxy-coal com-bustion that can produce a steady stream of nitrogen-free flue gasreadily available for carbon sequestration [International EnergyAgency (IEA) 2004; World Energy Council (WEC) 2004; Buhreet al. 2005]. Due to the change in the oxidant used for combustion,oxy-coal technology affects the combustion process of pulverizedcoal and the associated processes like heat transfer and combus-tion chemistry (Scheffknecht et al. 2011). In addition to this, thecombustion of pulverized mixtures inherently poses a series of in-triguing theoretical challenges. The basic theory underlying coal-particle combustion has been well established (Williams et al.2001; Sampath et al. 1998; Gibbins et al. 1999; Russel et al. 1998)and considerable amount of work has been focused on studyingthe effect of various parameters such as particle size (Cashdollar1996; Amyotte et al. 1993), concentration (Hertzberg et al. 1982),and volatility (Horton et al. 1997) on the burning velocity (Slezaket al. 1983; Liu et al. 2007) in coal dust–air flames.

Preliminary studies carried out on the combustion of coal par-ticles entrained in premixed methane-air flames (Xie et al. 2012;Rockwell and Rangwala 2013) was focused on studying theeffect of coal dust concentration on the burning velocity of themethane-air premixed flames. Two competing effects—volatilerelease and heat sink effect of the coal particles—affected theflame temperature and the burning velocity (Xie et al. 2012). Ithas also been reported that smaller particle size and large con-centrations increase the turbulent burning velocity significantlywhen compared to larger particle sizes and lower concentrations(Rockwell and Rangwala 2013). Despite some pioneering efforts(e.g., Shaddix and Molina 2009), what is largely missing from thestate of the art is the utilization of modern diagnostic tools, such aslaser and optical diagnostics and surface characterization tech-niques that will confirm models that have been proposed for coalparticle combustion. Much research has focused on the effect ofentrained particles on flame speed, but the related fluid mechanicshave received less attention. The detailed structure of the multi-phase and reactive flows that are involved has not yet been studiedwith the use of either high-fidelity computation or experimentaltechniques with high spatial and temporal resolution.

The overarching goal for this work was to introduce a combi-nation of laser/optical diagnostics techniques and surface charac-terization techniques in the study of combustion of coal-particleladen jets and thus provide seminal data on the effect of reactiveflow field on particle properties and structure. In particular, high-speed flow visualization was performed with particle streakvelocimetry (PSV). The effect of combustion was quantitativelyanalyzed by measuring the particle-size distribution before andafter combustion using Fraunhofer diffraction particle sizing(Malvern, RT 97 Spraytec, Malvern Instruments, Worcester, U.K.).Also, the effect of combustion on particle morphology wasanalyzed using scanning electron microscopy (SEM) before andafter combustion. The results were rationalized based on residence

1Dept. of Mechanical Science and Engineering, Univ. of Illinois atUrbana-Champaign, Urbana, IL 61801.

2Professor, Dept. of Mechanical Engineering, Khalifa Univ. of Science,Technology and Research, Al Saada St., P.O. Box 127788, Abu Dhabi,UAE (corresponding author). E-mail: dimitrios.kyritsis@kustar.ac.ae

Note. This manuscript was submitted on May 3, 2014; approved on July18, 2014; published online on October 14, 2014. Discussion period openuntil March 14, 2015; separate discussions must be submitted for individualpapers. This paper is part of the Journal of Energy Engineering, © ASCE,ISSN 0733-9402/C4014012(8)/$25.00.

© ASCE C4014012-1 J. Energy Eng.

J. Energy Eng.

Dow

nloa

ded

from

asc

elib

rary

.org

by

Uni

vers

ity o

f Il

linoi

s A

t Urb

ana

on 1

2/07

/14.

Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

time estimates that were based on the PSV results. The general areawhere the paper aspires to contribute is that of solid particle/flameinteraction. The objective was to provide a quantitative assessmentof the effect that passage through the flame has on the solid particlewith a particular emphasis on the distinction between volumetricand surface processes. This can have impactful applications asfar as the validation of models is concerned that have been pro-posed for coal combustion, especially in view of the emergenceof novel technologies such as oxy-coal.

Experimental Apparatus

Coal samples were procured from southern Illinois PowerCooperative where they are typically used for cyclone boilers(milled coal). Detailed information about the composition of Illi-nois coal is provided in the Keystone Coal Industry Manual(2010). These are in general high-sulfur content coals. The resultsreported herein relate to the nature of the oxidation of the bulk ofthe coal particles, so they should not depend heavily on the detailsof the particle composition. The coal samples were graded accord-ing to particle size by passing them sequentially through USAStandard 3” diameter sieves (Dual Manufacturing, Franklin Park,Illinois). Eleven different samples with nominal particle size rang-ing between 1,000 and 178 μm were selected for investigation.Nominal particle size refers to the characteristic dimension of anindividual sieve cell that was used to generate a particular sample.The uncompacted density of the coal samples, also referred to asthe loose density, was determined by accurately measuring theweight of 100 cm3 of coal samples using an JR120 Precision Stan-dard Electronic balance (Ohaus Corporation, Newark, New Jersey).

A schematic of the experimental apparatus used for coal-particle-laden methane jet combustion is shown in Fig. 1(a). Avertically oriented solid particle injector based on the Venturi effectas shown in Fig. 1(b) was designed in order to create a coal-particle-laden jet. The particles are basically entrained by thepressure differential that the acceleration of the gas causes acrossa converging-diverging nozzle (Xie et al. 2011). A small orificeplate with a 1-mm-diameter hole in the center was mounted ontoa 300-mm long, 12-mm outer diameter (OD), and 11-mm innerdiameter (ID) steel tube using compression springs. Three radiallyequally spaced circular holes 5 mm in diameter were drilled on theperiphery of the steel tube just above the orifice plate. A hopperarrangement attached to the steel tube was used to feed the coalparticles. The entrainment rate was controlled by adjusting the flowrate and thereby modifying the pressure drop associated with theflow that drove the entrainment phenomena. Measurements ofparticle-size distribution before and after combustion were per-formed using a Malvern RT 97 Spraytec particle-size measurementinstrument that will be referred to as Malvern for the purpose ofbrevity. The Malvern uses a 670-nm He-Ne laser beam and recordsthe scattered light signal on a set of 31 concentric circles on thedetector system. It then determines the probability density of theparticles’ size distribution based either on the complete Mie scat-tering or the Fraunhofer approximation. Time-averaged data wereacquired at a rate of 500 Hz over a time period of 2 s.

High-speed visualization was performed using a Phantom v.7.0high-speed camera (Vision Research, Wayne, New Jersey) at a rateof 4,800 frames per second and a resolution of 800 × 600 pixels. Alaser sheet (50 mm tall and 2 mm thick at the image plane) createdusing a Stabilite 2017 Argon Ion 6W (Spectra Physics, Palo Alto,California) laser was used for illumination. All Optics used forcollection were by Nikon, Melville, New York. In particular a50-mm f/1.8D Nikkor lens along with a Fotodiox Canon (Canon,

New York) EOS Macro Extension Tube Set Kit for ExtremeClose-up gave sufficient magnification of the image plane (∼1∶1.5)for high-speed videos. To perform particle streak velocimetry(PSV), a Nikon D5100 camera was used to capture high-resolutionstill images at [4,928 × 3,264 pixels—16 MP (megapixel)]. Thecamera was controlled remotely using the software Control-My-Nikon v.4.0. By using suitably long exposure rates, the particleswere allowed to travel spatially within a single frame, resultingin a streakline that defined the path traveled by a single particleover a known period of time equal to the exposure rate. Velocitydata were extracted by using the open-source Java-based softwareImageJ developed by the National Institutes of Health.

SEM analysis of the coal particles before and after combustionwas carried out in order to reveal any structural changes in the coalparticle as a result of combustion.A Jeol 6060 LV (low voltage) SEMwas capable of magnifications ranging from 5× to 200,000×.It was typically operated at high vacuum (10–5 Torr) with the electronbeam voltages varying from a few hundred V up to 30 kV. The coalsamples were prepared by applying a thin coating of around 10 nm ofgold-palladium alloy (Au-Pd) using a sputtering device. This Au-Pdcoating made the sample more conductive and permitted higherelectron beam voltages for better resolution at high magnifications.

Results and Discussion

Effect of Particle Size on Entrainment Rate

The entrainment rate of the coal particles was calculated by collect-ing the entrained coal particles on a collection tray fitted on thehopper arrangement over a fixed period of time of 20 s. Fig. 2shows the entrainment rate of coal particles (in g=min) as functionof the air mass flow rate for various sizes. In general, the particleentrainment rate increased almost linearly with the flow rate of airdue to the increased Venturi effect. This was in good agreementwith similar studies carried by Xie et al. (2011, 2012).

It was also interesting to note that the minimum flow rate atwhich entrainment occurred (which is indicated by the data pointwith the lowest flow rate for each sieve size) decreased considerablywith decreasing particle size. For particles with a larger mean diam-eter, a larger pressure drop was required to cause significant entrain-ment of particles into the flow due to increased frictional and inertiaeffects. Beyond this point of minimum flow rate for significant en-trainment, the entrainment rate for all the cases increased linearly.

Effect of Combustion on Loose Density andParticle-Size Distribution

A first interesting result on the interaction of coal particles with theflame is shown in Fig. 3, which presents the effect of combustionon the loose density (uncompacted density) of the coal particlesbefore and after their passage through the flame. In both cases,the loose density of coal particles increased monotonically with de-creasing particle size. Coarser particles tend to form bigger voidswithin the sample, leading to an increase in the void fraction, whichresults in decreased loose density of coarser coal samples. Perhapsmore importantly, it is also clear that there was no significantchange in the loose density of coal particles as a result of combus-tion. This generated the need to study the structure of the reactiveflow-field in order to compare residence times with the time fordevolatilization of coal particles. The loose density of finer coalsamples was affected to a relatively larger extent by the combustionprocess than the one of coarser coal samples, although the relativechange is in any case smaller than 1%. Notably, the loose density ofthe coal particles after combustion was not dependent on flame

© ASCE C4014012-2 J. Energy Eng.

J. Energy Eng.

Dow

nloa

ded

from

asc

elib

rary

.org

by

Uni

vers

ity o

f Il

linoi

s A

t Urb

ana

on 1

2/07

/14.

Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

configuration, i.e., whether this was a premixed or non-premixedflame, what the precise content in diluent was, etc.

Particle-size distributions before and after combustion for twonominal particle sizes (388 and 927 μm) are presented in Fig. 4 fornon-premixed flames. In these figures, the probability density func-tion of particle volume is presented as a function of the correspond-ing particle diameter. Particle-size distributions before and aftercombustion were unimodal with substantial dispersion. The mea-sured median particle size corresponded closely to the nominalsieve rating. The particle-size distributions before and after passageof the particles through the flame were almost identical for bothnominal particle sizes. The combustion process had a morepronounced effect on smaller coal particles than on larger ones.Passage through the flame decreased the fraction of total volume

of the solid phase that resided in both the smallest-size and thelargest-size coal particles in the sample, as evidenced by the post-combustion distributions being narrower. The decrease in thevolume fraction of smaller coal particles was significant when com-pared to those of larger coal particles. There was always an increasein the volume fraction of the midsized particles in each of thesedistributions. This can be attributed to two different phenomena,namely, the larger coal particles started to sinter and continuedto burn as they crossed the flame front, leading to a decreased par-ticle diameter; meanwhile some of the finer coal particles wereeither burnt completely or agglomerated, thus generating larger-diameter particles. The combustion process hardly changed themean particle diameter, which is in good agreement with the smallchanges in loose density shown in Fig. 3.

Fig. 1. (a) Experimental apparatus for combustion of coal-particle-laden CH4 jets; (b) schematic layout of the solid particle injector

© ASCE C4014012-3 J. Energy Eng.

J. Energy Eng.

Dow

nloa

ded

from

asc

elib

rary

.org

by

Uni

vers

ity o

f Il

linoi

s A

t Urb

ana

on 1

2/07

/14.

Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

In order to compare the effect of flame configuration on particle-size distribution before and after combustion, premixed, stoichio-metric CH4=air, CH4=O2=CO2, CH4=O2 premixed flames wereconsidered in addition to the non-premixed air flames discussedearlier. The relative proportion of the gases in the diluted combus-tible mixture CH4=O2=CO2 was determined so that a constant adia-batic flame temperature equal to 2226 K (stoichiometric adiabaticflame temperature for premixed CH4=air) was maintained. The cal-culation was done with GASEQ (www.gaseq.co.uk), the chemicalequilibrium solver. From Fig. 5, it is clear that the particle-size dis-tributions obtained with the premixed oxy-flames (even when thesewere diluted with CO2) were qualitatively similar to those obtainedin the non-premixed flames, so transition to oxy-fuel would notchange the substance of the results reported here. The particle-sizedistributions for the premixed CH4=air and CH4=CO2=O2 flames

were almost identical [by construction these two flames had thesame adiabatic flame temperature (2226 K)]. This provided a strongindication that what really determined the fate of coal particles wasa combination of adiabatic flame temperature and residence time,independently of the particular diluent. The CH4=O2 premixedflame had a higher adiabatic flame temperature (3053 K) and gen-erated the narrowest size distribution of postcombustion particles,but as shown in Fig. 5, the difference with the diluted flames wasminute.

Flow Field Visualization and ParticleStreak Velocimetry

The reasons for the evidently minor effect of the passage of coalparticles through the flame on loose density and particle size can berationalized in terms of the structure of the flow field and throughSEM investigation of particle morphology. Fig. 6 conveys the ma-jor feature of this flow. Specifically, despite the strong expansionof the gaseous flow across the flame, the particle trajectories basi-cally remain unaffected. The coal particles are not diverted by theflow and there is basically no effect on their speed as they go

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40

Ent

rain

men

t of

coa

l par

ticl

es (

g/m

in)

Air Flow Rate (g/min)

Sieve cell size: 927µm

Sieve cell size: 388 µm

Sieve cell size: 178 µm

Fig. 2. Coal particle entrainment rate as a function of airflow rate forseveral particle sizes

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.73

0.74

0.75

0.76

0.77

0.78

0.79

0 200 400 600 800 1000

% C

hang

e in

loos

e de

nsit

y

Loo

se D

ensi

ty (

g/cc

)

Characteristic sieve cell size (µm)

Pre-CombustionPost-Combustion% change in loose density

Fig. 3. Comparison of loose density of coal particles before and aftercombustion as a function of characteristic sieve cell size

(a)

(b)

0

5

10

15

20

25

30

35

% V

olum

e in

a p

arti

cula

r bi

n si

ze

Average particle diameter based on Malvern bin size (µm)

Pre-Combustion

Post-Combustion

0

5

10

15

20

25

30

35

0 200 400 600 800 1000

0 200 400 600 800 1000

% V

olum

e in

a p

arti

cula

r bi

n si

ze

Average particle diameter based on Malvern bin size (µm)

Pre-Combustion

Post-Combustion

Fig. 4. Particle-size distribution of coal particles with a nominal sievecell sizes of (a) 927 μm and (b) 388 μm before and after combustion ina non-premixed, coal-laden, CH4 jet flame

© ASCE C4014012-4 J. Energy Eng.

J. Energy Eng.

Dow

nloa

ded

from

asc

elib

rary

.org

by

Uni

vers

ity o

f Il

linoi

s A

t Urb

ana

on 1

2/07

/14.

Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

through the flame. This can be understood if we consider the stop-page time (τp) and the Stokes number (Stk) (Friedlander 2000) ofthe particles

τp ¼ ρd2p=18μ Stk ¼ τpU=L

where ρ = particle density, which was approximated as 750 kg=m3

from the data in Speight (2005), and dp was approximated by thenominal particle diameter. The mean flow speed U is typically onthe order of a few m=s, and the dynamic viscosity of air μ wasapproximated as equal to the one of air at 25°C (18.2 μPa=s). L

was assumed equal to the jet diameter. Mean particle speed is re-ported as a function of Stokes number on Fig. 7 for a mean flowspeed of 2.5 m=s. The mean particle speed for a given sample witha given nominal particle diameter was computed by averaging thevelocity information obtained by measuring the length of the indi-vidual streaks in PSV and dividing by the corresponding imageexposure. The estimated Stokes numbers were substantially largerthan unity; therefore, the particle velocities differed significantlyfrom the one of the flow. Notably, typical coal particles have veryhigh Stokes numbers even for speeds as low as a fraction of m=s.So, if a coal particle approaches a flame front with any sensiblespeed, it will simply cross the flame unaffected by the local flowfield. In order to keep the particles in the narrow high-temperatureregion of the flame, it is necessary to drive them to actual stagna-tion. The residence time in the high-temperature region of the flamecan be approximated as the time needed for the incandescent par-ticles in Fig. 6 to cross the luminous zone of the flame. The relatedparticle speeds are on the order of 1 m=s, whereas the thickness ofthe luminous zone is conservatively estimated to be on the order ofa few mm. This yields residence times on the order of a few ms,which is about 1 order of magnitude less than the time required forcomplete devolatilization of pulverized coal particles as reported inSmith (1982). As a result, there is not enough time for devolatili-zation, which explains the minor effect of passage through theflame on loose density and particle size.

Effects on Particle Surface

This result is supported further by analysis the macrostructureand microstructure of the coal particles using SEM imaging. InFig. 8, the macrostructure of the particles before and after combus-tion is compared. Specifically, coal samples with nominal particlesize of 927, 388, and 178 μm are compared before (a, b, c) and aftercombustion (d, e, f). A characteristic length scale is shown on thefigure. Clearly, the small residence time prevented any permanentnoticeable change on the macrostructure of these particles aftercombustion. The particles basically retained their sharp edgesand angular features similar to that of unburned coal particles.Only relatively small particles (smaller than 500 μm) had fewersurface irregularities after combustion, whereas the SEM data seem

0

5

10

15

20

25

30

35

0 200 400 600 800 1000

% V

olum

e in

a p

arti

cula

r bi

n si

ze

Average particle diameter based on Malvern bin size (µm)

Pre-Combustion

Diffusion Flame

Premixed CH4/Air Flame

Premixed CH4/O2/CO2 Flame

Premixed CH4/O2 Flame

Fig. 5. Particle-size distribution of coal particles with a nominal sievecell size of 388 μm before and after combustion for several configura-tions of a CH4-jet flame

Fig. 6.High-speed snapshots showing the trajectory of coal particles asthey shoot through the flame (image by R. Rajasegar)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 100 200 300 400 500

Vel

ocit

y (m

/s)

Stokes Number

Particle Mean Velocity

Fig. 7. Average particle velocity measured by PSV as a function ofStokes number for a mean flow velocity of 2.5 m=s

© ASCE C4014012-5 J. Energy Eng.

J. Energy Eng.

Dow

nloa

ded

from

asc

elib

rary

.org

by

Uni

vers

ity o

f Il

linoi

s A

t Urb

ana

on 1

2/07

/14.

Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

to indicate that the smallest particles (smaller than 200 μm) havepractically disappeared either through complete volatilization orsintering.

SEM images comparing the microstructure of the coal particlesin Fig. 9 reveal a completely different picture in terms of thedrastic changes that are evident as a result of combustion. Specifi-cally, the surface of the coal particles after combustion is pock-marked with blow holes that are caused as a result of rapiddevolatilization of volatiles as the coal particles interact with theflame front. In addition to the formation of blow holes, some par-ticles also showed signs of plastic deformation as result of combus-tion. These findings are in good agreement with the observations ofHertzberg et al. (1982). For an estimated residence time τ of theparticles in the flame on the order of 0.01 s and a characteristic

particle size of 500 μm, the Fourier number is estimated to beon the order of 0.01. [Fo ¼ ατ=L2, where α ¼ k=ρcp; α (thermaldiffusivity) = 3 × 10−7 m2=s;L: characteristic particle size, k (ther-mal conductivity) = 0.3 W=m · K, cp (specific heat capacity) =1,400 J=kg · K, ρ (particle density) = 750 kg=m3 (Speight 2005;Herrin and Deming 1996)].

This finding suggested that the effect of combustion on the par-ticles was only confined in a very small depth from the surface ofthe coal particles. During the small residence time in the high-temperature zone, the devolatilization process only occurred nearthe surface. In this layer, the devolatilization advanced through thegeneration of blow holes typically 20–50 μm in size through whichthe volatiles were exhausted to the ambient air. It is to be noted thatalthough devolatilization depends on volatile content, this study’s

Fig. 8. Macrostructure of coal particles before (a, b, c) and after (d, e, f) combustion for nominal particle sizes of (a, d) 927 μm; (b, e) 388 μm; and(c, f) 178 μm (image by R. Rajasegar)

© ASCE C4014012-6 J. Energy Eng.

J. Energy Eng.

Dow

nloa

ded

from

asc

elib

rary

.org

by

Uni

vers

ity o

f Il

linoi

s A

t Urb

ana

on 1

2/07

/14.

Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

main conclusion that the particle combustion process is a surfacephenomenon for the particular residence times can be argued forany type of coal. Char burnout and the associated stages of com-bustion that are highly dependent on coal composition do not havethe time to develop during the residence time of the particles in thevicinity of the flame.

Conclusions

When coal particles are entrained in either premixed or non-premixed gaseous fuel jets with gaseous speeds on the order of1–10 m=s, a multiphase flow with very large Stokes numbers

(on the order of 50–500) ensues. As a result, the coal particles crossthe flame front pretty much unaffected by the flame without anysensible effect on their trajectory or speed. The residence timesin the vicinity of the high-temperature regions of the flame areat least 1 order of magnitude lower than the characteristic timesthat have been reported for complete particle devolatilization. Asa result, there is only a minor effect on particle loose densityand particle-size distribution that mainly affects particles that aresmaller than 200 μm in size. The Fourier numbers for conductionin the coal particle that correspond to the residence time in the high-temperature region of the flame are on the order of 0.01, whichmeans that devolatilization is basically a surface phenomenon.In fact, SEM microscopy of the particles reveals devolatilization

Fig. 9. Microstructure of coal particles before (a, b, c) and after (d, e, f) combustion for nominal particle sizes of (a, d) 927 μm; (b, e) 388 μm; and(c, f) 178 μm (image by R. Rajasegar)

© ASCE C4014012-7 J. Energy Eng.

J. Energy Eng.

Dow

nloa

ded

from

asc

elib

rary

.org

by

Uni

vers

ity o

f Il

linoi

s A

t Urb

ana

on 1

2/07

/14.

Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

occurs through blow holes with diameters on the order of10–20 μm. The substance of these results is unaffected by flameconfiguration, i.e., it is the same for premixed and non-premixedflames, oxy-flames, as well as flames diluted with CO2.

Notation

The following symbols are used in this paper:Cp = specific heat capacity of coal (J=kg · K).dp = particle diameter (m);Fo = Fourier number;k = thermal conductivity of coal (W=m · K);L = characteristic length scale (m);U = mean flow speed (m=s);α = thermal diffusivity of coal (m2=s);μ = dynamic viscosity of air (Pa=s);ρp = particle density (kg=m3); andτp = stoppage times;

References

Amyotte, P. R., Mintz, K. J., Pegg, M. J., and Sun, Y. H. (1993). “Theignitability of coal dust-air and methane-coal dust-air mixtures.” Fuel,72(5), 671–679.

Buhre, B. J. P., Elliott, L. K., Sheng, C. D., Gupta, R. P., and Wall, T. F.(2005). “Oxy-fuel combustion technology for coal-fired power gener-ation.” Prog. Energy Combust. Sci., 31(4), 283–307.

Cashdollar, K. L. (1996). “Coal dust explosibility.” J. Loss Prev. ProcessInd., 9(1), 65–76.

Friedlander, S. K. (2000). Smoke, dust, and haze: Fundamentals of aerosoldynamics, 2nd Ed., Oxford University Press, New York.

Gibbins, J., Zhang, J. B., Afzal, H., Riley, G., and Beeley, T. (1999). “Pros-pects for coal science in the 21st century.” Proc., 10th Int. Conf. on CoalScience, Vol. 1, Elsevier, Philadelphia, 459–462.

Herrin, J. M., and Deming, D. (1996). “Thermal conductivity of U.S.coals.” J. Geophys. Res.: Solid Earth, 101(B11), 25381–25386.

Hertzberg, M., Cashdollar, K. L., Ng, D. L., and Conti, R. S. (1982).“Domains of flammability and thermal ignitability for pulverized coalsand other dusts: Particle size dependences and microscopic residueanalyses.” Proc. Combust. Inst., 19(1), 1169–1180.

Horton, M. D., Goodson, F. P., and Smoot, L. D. (1997). “Characteristics offlat, laminar coal-dust flames.” Combust. Flame, 28(1), 187–195.

Intergovermental Panel on Climate Change (IPCC). (2007). “Fourth assess-ment report—Climate change.” (AR4) ⟨http://www.ipcc.ch⟩ (Nov. 4,2013).

International Energy Agency (IEA). (2004). World energy outlook, ⟨http://www.iea.org⟩ (Nov. 12, 2013).

Keystone Coal Industry Manual. (2010). ⟨http://isgs.illinois.edu/sites/isgs/files/files/coal-maps/Illinois-coalgeology.pdf⟩ (Jun. 1, 2010).

Liu, Y., Sun, J., and Chen, D. (2007). “Flame propagation in hybrid mixtureof coal dust and methane.” J. Loss Prev. Process Ind., 20(4–6),691–697.

Rockwell, S. R., and Rangwala, A. S. (2013). “Influence of coal duston premixed turbulent methane-air flames.” Combust. Flame, 160(3),635–640.

Russel, N. V., Beeley, T. J., Man, C. K., Gibbins, J. R., and Williamson, J.(1998). “Development of TG measurements of intrinsic char combus-tion reactivity for industrial and research purposes.” Fuel Process.Technol., 57(2), 113–130.

Sampath, R., Maloney, D. J., and Zondlo, J. W. (1998). “Evaluation of ther-mophysical and thermochemical heat requirements for coals at combus-tion level heat fluxes.” Proc. Combust. Inst., 27(2), 2915–2923.

Scheffknecht, G., Al-Makhadmeh, L., Schnell, U., and Maier, J. (2011).“Oxy-fuel coal combustion—A review of the current state-of-the-art.” Int. J. Greenhouse Gas Control, 5(1), S16–S35.

Shaddix, C. R., and Molina, A. (2009). “Particle imaging of ignition anddevolatilization of pulverized coal during oxy-fuel combustion.” Proc.Combust. Inst., 32(2), 2091–2098.

Slezak, S. E., Fitch, D. J., Krier, H., and Buckius, R. O. (1983). “Coal dustflame propagation in a laboratory flammability tube.” Combust. Flame,54(1–3), 103–119.

Smith, I. W. (1982). “The combustion rates of coal chars: A review.” Proc.Combust. Inst., 19(1), 1045–1065.

Speight, J. G. (2005). Handbook of coal analysis, Vol. 166, Wiley,Hoboken, NJ.

Wang, L., and Tseng, C. L. (2012). “Special issue on challenges andopportunities in the 21st century energy infrastructure (editorial).”J. Energy Eng., 10.1061/(ASCE)EY.1943-7897.0000072, 31–32.

Williams, A., Pourkashanian, M., and Jones, J. M. (2001). “Combustionof pulverized coal and biomass.” Prog. Energy Combust. Sci., 27(6),587–610.

World Energy Council (WEC). (2004). Sustainable global energy develop-ment: The case for coal, ⟨http://www.worldenergy.org/documents/globalcoal2004.pdf⟩ (Jul. 1, 2014).

Xie, Y., Raghavan, V., and Rangwala, A. S. (2011). “Naturally entrainingsolid particle injector.” Powder Technol., 213(1–3), 199–201.

Xie, Y., Raghavan, V., and Rangwala, A. S. (2012). “Study of interaction ofentrained coal dust particles in lean methane-air premixed flames.”Combust. Flame, 159(7), 2449–2456.

© ASCE C4014012-8 J. Energy Eng.

J. Energy Eng.

Dow

nloa

ded

from

asc

elib

rary

.org

by

Uni

vers

ity o

f Il

linoi

s A

t Urb

ana

on 1

2/07

/14.

Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.