Available online at www.sciencedirect.comProceedings

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Proceedings of the Combustion Institute 35 (2015) 2415–2422

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of the

CombustionInstitute

Study on one-dimensional steady combustionof highly densified biomass briquette (bio-coke)

in air flow

Takero Nakahara, Hui Yan, Hiroyuki Ito, Osamu Fujita ⇑

Division of Mechanical and Space Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido, Japan

Available online 8 September 2014

Abstract

Combustion experiments on cylindrical bio-coke (BIC), a highly densified biomass briquette, have beenconducted to observe whether quasi-one-dimensional steady combustion can be attained in room temper-ature air flow. In the experiments, the air flow velocity was the main test condition and the fuel consump-tion rate when the bottom surface of the BIC sample burned was evaluated as the regression rate of thecombustion zone at the bottom surface. In addition, one-dimensional calculations based on an energyequation at the combustion zone were conducted to understand the mechanism that results in steady com-bustion and predict the effect of water and volatile matter content in BIC on the extinction limit. Theresults showed that steady combustion of the BIC sample could be attained in 4.67 m/s or more, and,in contrast, extinction was observed in 3.82 m/s or less. The critical regression rate explained by the com-bustion zone temperature was shown, and the reason combustion becomes unsteady could be explained bythe energy balance at the combustion zone. Though the main reason for extinction was radiation heat loss,the heat loss by water and volatile matter was not negligible. Therefore, the effect of water and volatile mat-ter content on steady combustion must be considered.� 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Biomass briquette; Solid combustion; Surface combustion; Steady flame propagation; Extinction limit

1. Introduction

Presently, biomass fuels are an increasinglyattractive primary energy source because of theirintrinsically renewable nature and potentially lim-ited generation of pollutants. Biomass usuallycontains negligible sulfur and low concentrations

http://dx.doi.org/10.1016/j.proci.2014.08.0131540-7489/� 2014 The Combustion Institute. Published by El

⇑ Corresponding author. Tel.+81 11 706 6385, fax:+81 11 706 7841.

E-mail address: ofujita@eng.hokudai.ac.jp (O. Fujita).

of nitrogen and heavy metals and is neutral withrespect to greenhouse gases. However, there areissues in utilizing biomass as fuels, including thelow calorific value per unit volume and specialtreatment required before utilization for econom-ical transport and storage. Moreover, controllingthe burning rate in the combustion equipment isdifficult since raw biomass fuel may be inhomoge-neous and have a remarkable burning rate. Toovercome these disadvantages, a newly developedbiomass-type fuel has been proposed, “Bio-coke(BIC)” [1–4]. BIC, highly densified biomass

sevier Inc. All rights reserved.

Nomenclature

A frequency factorA0 theoretical amount of aircp specific heatD diffusion coefficientdn nozzle diameterE activation energyk reaction rate constantL latent heatNud Nusselt number for substance

diffusionQ calorific valueR universal gas constantSB regression rateT temperaturetd thickness of diffusion layer

Greek symbolsb substance diffusion rate

e emissivityk excess air factorm stoichiometric coefficientq densityr Stefan–Boltzmann constant

Subscriptsa ambientchar charCO2 carbon dioxidei carbon dioxide, nitrogen, oxygenO2 oxygens surfacev volatile matterw water

2416 T. Nakahara et al. / Proceedings of the Combustion Institute 35 (2015) 2415–2422

briquettes, is manufactured by high compressionat moderate temperatures, and this alternativefuel has been investigated in the present study.BIC has high mechanical strength, is able to with-stand the compressive stress in melting furnaces,and has been shown to be a potential alternativeto coal coke. BIC can be manufactured in a widerange of sizes, large dimensions (48 mm in diame-ter and 85 mm in length in this study) and densi-ties higher (1300 kg/m3) than ordinary woodpellets (600 kg/m3) [5]. This means the combus-tion time of the fuel enables a longer heat releasewith a smaller number of fuel loadings, resultingin a better overall performance compared withexisting biomass fuels. Additionally, BIC has rela-tively low transport and storage costs because ofits high volumetric calorific value and mechanicalstrength, and this new fuel is expected to be usedin melting furnaces, stoves, and boilers.

As biomass fuel is a popular topic in the fieldof combustion, many researchers have investi-gated the combustion characteristics of biomassfuel [6–9], but few studies are available on thesteady combustion mechanism of large blocks ofbiomass. In the combustion process of a large fuelblock, steady combustion is defined as the steadyregression rate of the fuel block during combus-tion. Discussing the mechanism of steady combus-tion is an important topic, and allows us to gain abetter understanding of the combustion character-istics of the newly developed biomass fuel andimprove the knowledge of combustion furnacedesign. Steady combustion can be obtained onlywhen a steady temperature distribution insideBIC is achieved. The temperature distributioninside the fuel was assumed to be dependent on

the surface char oxidative reaction rate and heattransfer process, and the combustion behaviorwould be influenced by these factors. In thisregard, it is important to investigate the effectsof the char combustion rate, water content, andvolatile matter content on the criteria of steadycombustion. In the existing literature, Makinoperformed experiments and analytical investiga-tions on carbon combustion in air flow. Theyshowed that a steady combustion rate does existand depends on the Damkohler number for thesurface reaction. However, they ignored the effectof water and volatile matter content on steadycombustion [10]. While there are studies on pyro-lysis of biomass fuels [11–13], little investigationof the combustion processes has been involved.One such investigation was reported by Oue-draogo, who presented a “shrinking core model”for pyrolysis, devolatilization, and char oxidationof solid biomass fuel under quasi-steady-statecombustion [14]. However, their study wasdirectly based on a steady-state assumption,without explaining why the steady state wasattained.

This study investigated the mechanism ofsteady combustion in convective air flow throughboth experiments and analytical calculations.One-dimensional combustion of BIC was per-formed based on end-face combustion method,which will be described further in the followingsection. The evidence of steady combustion isshown by regression rate and temperature distri-bution inside BIC. Meanwhile, the effects of airflow velocity, water content, and volatile matterinside BIC on the combustion behavior wereexamined.

Pulley

Combustion zone

Counterweight

Electric heater

Blower

Stage

Data logger

Air

Bio-coke sample

Thermocouples

Flow meter

Flow meter

Outer tube

PC

Germanium glass windowIR camera

DV camera

Window

Inner tube

Stainless steel wire

Mechanicalscale

Fig. 1. Experimental setup.

T. Nakahara et al. / Proceedings of the Combustion Institute 35 (2015) 2415–2422 2417

2. Experimental method

2.1. Test sample

Cylindrical BIC samples used in this studywere made from Japanese knotweed, a perennialplant abundant in Hokkaido, Japan. The manu-facture procedure for BIC is reported in detailelsewhere [4]. Briefly, pulverized Japanese knot-weed is stuffed into cylindrical molds, pressedand heated simultaneously, and subsequentlycooled to room temperature. After cooling, thecylindrical BIC briquette is removed from themold. Properties of the BIC sample are shown inTable 1. Dimensions of the BIC sample are48 mm in diameter and 85 mm in length. Compar-ing BIC and wood pellets, no obvious distinctionwas found in the gross calorific value (17.8 MJ/kgand 20.3 MJ/kg [13], respectively). However,because of the special treatment during manufac-turing, BIC has a much higher density than woodpellets (1300 kg/m3 and 600 kg/m3, respectively),leading to a remarkable energy density withinBIC. Furthermore, according to industrial analy-sis, a large amount of volatile matter (70%) andwater content (10%) are retained in the originalmaterial. This is the major difference in BIC fromprevious research on carbon combustion [10].

2.2. Experimental setup and test conditions

A schematic of the experimental setup is shownin Fig. 1. It consists of a (a) combustion chamber,including the outer and inner tubes (100 and49 mm in diameter, respectively), (b) counterweight on a stage, (c) air supply system, (d) aircooling system inside the inner tube, (e) an infrared(IR) camera (NEC Sanei, TH6200R, measurementrange of wavelength: 8–14 lm, measurement rangeof temperature: 523–1273 K) for monitoring thebottom surface temperature of the BIC sample,(f) thermocouples for measuring temperature dis-tributions inside the BIC sample, and (g) digital

Table 1Properties of the BIC sample.

Mass [kg] 0.2Density [kg/m3] 1300Calorific value [MJ/kg] (HHV) 17.9

Elemental analysis(dry basis)C [wt%] 47.72H [wt%] 5.95O [wt%] 32.75N [wt%] 0.47

Industrial analysis(as received basis)Char [wt%] 13Volatile matter [wt%] 70Water [wt%] 10Ash [wt%] 7

video (DV) camera (SONY, HCR-HC7). TheBIC sample is located inside the inner tube andconnected to the counter weight on the stage bya wire. Initially, the air is metered (4.67 m/s) andsubsequently heated (873 K) by the air supply sys-tem and blown onto the bottom surface of the BICsample. In order to obtain one-dimensionalcombustion behavior, we strategically plannedthe experiment (end face combustion). After thebottom face of the BIC sample ignites, the air tem-perature is switched to room temperature. Simul-taneously, the air cooling system between theinner tube and BIC sample plays a crucial role inpreventing the side wall and top surface of theBIC sample from being heated. Based on thisend-face combustion method, quasi-one-dimensional combustion behavior of BIC isachieved. [15]. Five K-type thermocouples(0.3 mm in diameter, wire) were installed insidethe BIC sample, and they were on the circumfer-ence of the 24-mm-diameter circle and at every10 mm in an axial direction from the initial loca-tion of the bottom surface. The IR camera andDV camera were employed to capture the maxi-mum surface temperature and direct surface com-bustion images, respectively. This experiment wasconducted three times under every air flow velocitycondition, which were 2.97, 3.82, 4.67, 5.52, and6.37 m/s, and the experimental data was averaged.

3. Calculation method

In this study, the energy balance in a combus-tion zone of BIC is proposed to describe theone-dimensional and steady phenomena, and thefollowing assumptions are invoked:

� Pyrolysis gas is composed of CO, CO2, andCH4 [11,12].

� Emissivity is 0.8.� Thermal properties are independent of

temperature.� The effect of ash is ignored.

2418 T. Nakahara et al. / Proceedings of the Combustion Institute 35 (2015) 2415–2422

The last assumption is according to the factthat the burning char is always visible and isexposed to the opposed air flow as seen in Fig. 2.

An energy balance equation is given as Eq. (1)on the basis of the above assumptions.

SB T s � T að ÞX

i

qcharmicpi þ qvcpv þ qwcpw

!

þ SBðqvQv þ qwLwÞ þ erðT 4s � T 4

aÞ¼ SBqcharQchar: ð1Þ

In Eq. (1), the energy balance among the absorp-tion of heat is considered, which consists of sensi-ble heat, decomposition heat of the volatilematter, latent heat of vaporization, and radiationheat, and the amount of generated heat from car-bon combustion(carbon in the char).

The first term that appears in the left-hand sideof Eq. (1) represents the sensible heat increasingfrom the original temperature to the combustionzone temperature of nitrogen and excess oxidizerby air flow, pyrolysis gas, water vapor, and CO2

generated by char oxidation inside the control vol-ume. The ratio of composition of pyrolysis gas isCO:CO2:CH4 = 2:2:1 [11,12]. The second term inthe left-hand side of Eq. (1) is the latent heat ofwater vaporization and heat for the pyrolysis reac-tion based on results of differential thermal analy-sis inside the control volume. The radiation heatin Eq. (1) is thought to approximate the combus-tion zone gray body in which emissivity is 0.8.This approximation is based on experimental dataand calibrating temperatures measured by IRcamera and thermocouple. The right-hand termin Eq. (1) is the char oxidative reaction insidethe control volume. The chemical reaction for-mula with nitrogen and excess oxidizer in the sup-plied air is expressed by Eq. (2).

Cþ 0:23kA0O2 þ 0:77kA0N2 ! mCO2CO2

þ 0:23ðk� 1ÞA0O2 þ 0:77kA0N2: ð2Þ

Both drying and pyrolysis are thermallycontrolled, while char oxidation can either bekinetically controlled or diffusion controlleddepending on the operating conditions. In Eq.

Air

BIC sample

Fig. 2. Direct image of end-face combustion.

(2), the excess air factor is defined as k ¼ 1þ b=k.The surface reaction rate constant k and diffusionrate of the oxidizer b are defined ask ¼ A expð�E=RT sÞ and b ¼ NudD=td , which indi-cate chemical control effect and diffusion controleffect, respectively. The excess air factor meansmass flux of diffused oxygen to the mass flux ofreacted oxygen in the case of equivalence or fuellean, and if all the supplied oxidizer is consumedby char combustion, the excess air factor is equalto unity. The activation energy and frequency fac-tor are determined by referencing a value withinthe range of the char oxidative reaction in stag-nate flow and combustion of woody biomass[16–19], A ¼ 1:9� 107 m=s and E ¼ 150 kJ=kg.The diffusion part is regulated with the experimen-tal correlations by J. Lee [20] for the Nusselt num-ber, which is defined as Nud ¼0:661Re0:566 td

dn

� ��0:078

, because they were dealing

with similar configuration as ours except the cool-ing system. In the present research, we assumedthe effect of cooling air flow on the expression ofNusselt number is negligible, because the coolingair is just supplied to cool the circumference ofBIC and evacuates outward as described inFig. 1. The regression rate of the combustion zoneis given by Eq. (3) because the mass of char sup-plied from unburnt fuel to the combustion zoneis equal to that of the oxidizer supplied to thecombustion zone.

SB ¼qO2

qcharmO2ð1=k þ 1=bÞ : ð3Þ

In this research, extinction limit is defined asthe limit of regression rate dropping to 0.

Air flow velocity conditions for this calculationwere from 0.00 to 6.37 m/s.

4. Results and discussion

4.1. Combustion behavior of BIC in convective flow

A direct image of quasi-one-dimensional com-bustion is shown in Fig. 2. Surface combustionwas observed at the bottom surface of the BIC sam-ple, but no visible flame appeared in the gas phaseduring any experiment because of the large stretchrate near the stagnation point. However, pyrolysisgas may exist there. Hence, only char combustionis treated in this, and gas phase combustion wasignored.

4.2. Steadiness of BIC combustion

Temperature histories inside the BIC samplewith 6.37 m/s air flow are shown in Fig. 3 as afunction of elapsed time. Here elapsed time meansthe time after the start of the air supply, which isheated to 873 K only at the initial moment of air

200300400500600700800900

100011001200

0 10 20 30 40 50 60 70Elapsed time [min.]

Tem

pera

ture

[K]

10mm

20mm

30mm

40mm50mm

Fig. 3. Temperature history inside the BIC sample with6.37 m/s air flow.

0

10

20

30

40

50

60

0 20 40 60 80 100 120 140

Loca

tiom

of b

otto

m s

urfa

ce [m

m]

Elapsed time [min]

2.97m/s 3.82m/s 4.67m/s

5.52m/s 6.37m/s

Fig. 4. Temporal change of the end-face location.

200

300

400

500

600

700

800

900

1000

0 2 4 6 8 10Distance from bottom surface [mm]

Tem

pera

ture

[K]

Regression length: 20mm

Regression length: 40mm

Fig. 5. Temporal change of the temperature distributioninside the BIC sample with 6.37 m/s air flow.

T. Nakahara et al. / Proceedings of the Combustion Institute 35 (2015) 2415–2422 2419

supply for ignition. The temperature measurementwas conducted with thermocouples set at differentdepths of the BIC sample, as described in Fig. 1.At any measuring point, the temperature increasesgradually to �373 K and maintained at 373 K fora while. This change in temperature is explainedby the water content in the BIC sample after theconductive supply from the combustion zone.Subsequently, the temperature quickly increasesto 800–1000 K when the combustion zone comescloser to the position of the individual thermocou-ples. The time to reach the maximum temperatureindicates the moment the flame front arrived atthe thermocouple position. Using this informa-tion, the propagation characteristics of the com-bustion zone can be determined. As shown inFig. 3, the temperature peak for 30 mm is quitelow, this is due to large fragment of BIC nearthe thermocouple drops. Temperature data’s insuch case are omitted in the following discussion.

Temporal changes in the location of the com-bustion zone and displacement of the flame frontfrom the initial position are shown in Fig. 4 as afunction of elapsed time. The convective air flowvelocity is the experimental parameter in the fig-ure. In this study, the displacement rate of flamefront is called the regression rate and is given bythe slope gradient in Fig. 4. For every air flowvelocity case, we conducted three repeated tests.Plots of Fig. 4 are the average of repeated tests.Under high air flow velocity conditions (4.67–6.37 m/s), quasi-one-dimensional steady combus-tion is observed in all repeated tests. The locationof the bottom surface varied almost linearly,which means that the regression rate is almostconstant and larger as the air flow velocityincreases. In contrast, extinction is alwaysobserved before all parts of the BIC sample areconsumed under low air flow velocity conditions(2.97 m/s). As for 3.82 m/s case, in one experi-ment, extinction happens, while it survives inother two experiments. Based on these results, aminimum air flow velocity between 3.82 and4.67 m/s exists, which can sustain steady combus-tion. The regression rate in low air flow velocityconditions is smaller than that in high air flow

velocity conditions. Furthermore, the regressionrate decreases with time and flame, resulting inextinction. Based on these results, a minimumair flow velocity between 3.82 and 4.67 m/s exists,which can sustain steady combustion.

Temporal changes in the temperature distribu-tion inside the BIC sample at an air flow velocityof 6.37 m/s are shown in Fig. 5 as a function ofthe distance from the temporal bottom surfaceposition. A comparison of the temperature distri-butions when the regression length is 20 mm(dashed line) and 40 mm (solid line) is shown.According to this figure, the temperature distribu-tions are not significantly different. Hence, a sim-ilar temperature distribution can be assumed to bekept nearly constant for the entire flame propaga-tion period.

From the above observation, steady combus-tion can be attained under high air flow velocityconditions. However, this is not possible underlow air flow velocity conditions. In the followingsection, the mechanism for determining the crite-ria for steady combustion is discussed on the basisof comparison between calculation and experi-mental results.

4.3. Dependence of air flow velocity on regressionrate and temperature

The effects of air flow velocity on regressionrate and surface temperature are shown in Fig. 6

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 Air flow velocity [m/s]

Regr

essi

on ra

te [1

0 m

/s]

-5

Solid line : CalculationDots : Experiment

Fig. 6. Effect of air flow velocity on regression rate.

300

400

500

600

700

800

900

1000

1100

1200

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Regression rate [10 m/s]-5

Solid line : CalculationDots : Experiment

4.67m/s

5.52m/s

2.97m/s

3.82m/s

6.37m/s

Tem

pera

ture

of c

ombu

stio

n zo

ne [K

]

Fig. 8. Relationship between the regression rate andcombustion zone temperature.

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

Air flow velocity [m/s]

Hea

t los

s [k

J/m

s]

2

Decomposition heat of volatile matter

Radiation

Sensible heat

Latent heat of vaporization

Fig. 9. Comparison between heat losses.

2420 T. Nakahara et al. / Proceedings of the Combustion Institute 35 (2015) 2415–2422

and Fig. 7, respectively. The experimental data areobtained every 10 mm in the combustion zone.The regression rate and temperature increase asthe air flow velocity increases for both experi-ments and calculations, of course in the conditionextremely close to extinction limit, the effect ofchemical part turns to be controlling step, butgeneral trend in Fig 6 is controlled by diffusionprocess according to the Nusselt number andchemical parameter, but the calculation resultsare larger than the experiments results. In Fig. 6,the calculated regression rate goes to zero at aflow velocity less than 3.00 m/s, while its steadyvalues exist at a flow velocity higher than3.00 m/s. The same criterion is observed inFig. 7, above that flow velocity a steady surfacetemperature is attained. The reason the calcula-tion results are larger is because of the assumptionof a fresh air supply to the bottom surface of theBIC sample. However, the oxygen concentrationin the supplied air near the surface may decreasebecause of oxygen consumption in the gas phase.As a result, the minimum air flow velocity to sus-tain steady combustion is larger in the experi-ments (between 3.82 and 4.67 m/s) than in thecalculations (3.00 m/s). Figure 8 shows the rela-tionship between regression rate and surface tem-perature from experiments and calculations. Thesurface temperature decreases with a decrease inthe regression rate for both experiments and cal-culations. According to the calculations, the

300 400 500 600 700 800 900

1000 1100 1200

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 Air flow velocity [m/s]

Tem

pera

ture

of c

ombu

stio

n zo

ne [K

]

Solid line : CalculationDots : Experiment

Fig. 7. Effect of air flow velocity on the combustionzone temperature.

decrease in the temperature is explained by adecrease in the heat release rate because of adecreased regression rate, that is, the ratio of heatrelease to heat loss increases. This temperaturedecrease is the mechanism of extinction becausethe chemical reaction rate changes nonlinearly,and it suddenly decreases when the temperatureis lower than a critical value.

As discussed in the previous section, the regres-sion rate is controlled by the air flow velocity andthe combustion is diffusion-controlled. Therefore,there is minimum air flow velocity to sustainsteady combustion.

4.4. Discussion on extinction limit

Heat losses corresponding to each term in Eq.(1) are shown in Fig. 9 as a function of air flowvelocity. Any heat loss monotonically increasesas the air flow velocity increases. Of all the heatloss terms, radiation heat loss is the largest factor,accounting for 48% of the total heat loss at theextinction limit. The decomposition heat of vola-tile matter and latent heat of vaporization accountfor 12% and 5% of the total heat loss, respectively.

Consequently, extinction at low air flow veloc-ity is mainly due to radiation heat loss. However,the effects of the decomposition heat of volatilematter and latent heat of vaporization are notnegligible, which results in the extinction limitshifting to a higher air flow velocity.

1.00

1.50

2.00

2.50

3.00

0 5 10 15 20 25

Reg

ress

ion

rate

[10

m/s

]-5

Water content [%]

6.37m/s

3.82m/s

4.67m/s

5.52m/s

2.97m/s

Unsteady region

Fig. 10. Effect of water content on the regression rateand extinction limit.

Volatile matter content [%]

Reg

ress

ion

rate

[10

m/s

]-5

6.37m/s

3.82m/s4.67m/s

5.52m/s

2.97m/s

1.00

1.50

2.00

2.50

3.00

3.50

60 65 70 75 80

Unsteady region

Fig. 11. Effect of volatile matter content on the regres-sion rate and extinction limit.

T. Nakahara et al. / Proceedings of the Combustion Institute 35 (2015) 2415–2422 2421

4.5. Effects of water and volatile matter content onsteady one-dimensional combustion

The effects of water content on the regressionrate and extinction limit are shown in Fig. 10.One thing we should mention here is that the effectof water and VM on steady combustion isaddressed by modeling and not through experi-ment. The regression rates when the water contentis varied are shown for every air flow velocity bysolid and dashed lines. The extinction limit isshown by dots, and the region under the dots iswhere one-dimensional combustion becomesunsteady. The regression rate gradually increasesas the water content increases and reaches a max-imum value, which is followed by a decrease andextinction. The extinction limit also increases asthe water content increases. In this study,increases and decreases in water content are trea-ted as increases and decreases in the mass fractionof char. As the mass fraction of char increases,surface combustion can be sustained, but theregression rate decreases. For additional increasesin the water content, however, the extinction limitappears because the ratio of heat loss from water,latent heat, and heat release from char combus-tion increases. For large water content, the heatloss significantly affects the heat balance at thesurface relative to the generated heat, and the

extinction limit appears after passing through amaximum value.

The effect of volatile matter content on theextinction limit is shown in Fig. 11. The tendencyis the same as that shown in the previous section.Briefly, heat loss through volatile matter anddecomposition heat influences the heat balanceat the surface, and extinction appears based onthe volatile matter content.

Both water and volatile matter content are fac-tors that can result in unstable one-dimensionalcombustion. The effect of water and volatile mat-ter content on the extinction limit is importantwhen considering steady combustion.

5. Concluding remarks

Experiments with BIC, a highly densified bio-mass briquette, were conducted by burning theBIC quasi-one-dimensionally to observe whetherthe combustion reaches a steady state in roomtemperature air flow. In addition, one-dimen-sional calculations were done to help understandthe mechanism that allows for steady combustionand predict the effect of water and volatile mattercontent in BIC. The results thus obtained may besummarized as follows:

(1) It was confirmed that quasi-one-dimensionalcombustion of BIC is attained in roomtemperature air flow. The combustionreaches a steady state in high air flowvelocity (4.67 m/s or larger), but extinctionis observed in low air flow velocity (3.82 m/s or lower).

(2) According to the one-dimensional analysis,there is a critical regression rate to sustain asteady state. The existence of the criticalregression rate is explained by the bottomsurface temperature change, which is givenby the ratio of heat loss to heat releasedby char combustion.

(3) The heat loss caused by water and volatilematter content included in the BIC is notnegligible, but the main reason for extinc-tion in low air flow velocity is radiation heatloss. Moreover, the extinction limits shiftsto a higher regression rate as water and vol-atile matter content increase. Therefore, theeffects of water and volatile matter contenton steady combustion are important.

Acknowledgements

This research was supported by Grants-Aid forScientific Research (Houga, Subject #22656050)from MEXT Japan in 2010–2011 and was par-tially supported by Grants-Aid for ScientificResearch (Kiban (C), Subject #26420160) fromMEXT Japan in 2014.

2422 T. Nakahara et al. / Proceedings of the Combustion Institute 35 (2015) 2415–2422

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