Small diffusion flames over a wood particle

Pergamon

SMALL

Energy Vol 20, No 10, pp 1037-1040, 1995 Copynght© 1995 ElsewerScienceLtd Pnnted m GreatBritain All rightsreserved 0360-5442/95 $9 50 + 0 00

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DIFFUSION

FLAMES

OVER

A WOOD

PARTICLE

BUI TUYEN,t:~ R. LOOF,§ and S. C. BHATI'ACHARYAI" tEnergy Technology Program, School of Environment, Resources and Development and §Water Resources Englneenng Program, School of Civd Engmeenng, Asian Institute of Technolog), G P O Box 2754, Bangkok 10501. Thadand (Recewed 13 December 1994) Abstract--Flames over a wood element m qmescent air may be classified as (a) near-extinction, (b) stable-smokeless, and (c) smoky. The direct connection of these forms of flames with the surface gas flux (SGF) emitted from a pyrolyzmg sohd was investigated. Measurements were made on flames of wood pyrolysis gas issuing out of an 8.5-mm tube For comparison, the same device and an 8 5-mm wick were used with kerosene, diesel and vegetable cooking o11. The ranges of the SGF for stable-smokeless flames were found to be qmte narrow, indicating that improved combustion reqmres an increasing flame surface or keeping the SGF within a well defined range

INTRODUCTION When wood is rapidly heated in air, it decomposes and releases combustible volatile gases that burn in a diffusion flame. Once the flame is established, it supphes the sohd with the heat necessary for pyrolysis of interior layers, thus generatmg combustible gases that feed the flame. ~.2 Flaming combustion of wood involves strongly interacting physical and chemical processes of the two major stages: pyrolysis of the solid and gas-phase combustion. 3'4 Initiation of flaming combustion Involves heating and pilot ignition by an external source. Termination occurs when the pyrolysis gas flux is too weak to sustain the flame. In many practical situations, it is necessary to know when a flame is stable or ~s going to be extmgmshed and when pilot ignition ~s possible. In this study an attempt has been made to estabhsh a phenomenological hnk between the three specified forms of flame and the fuel supply, which is characterized by the SGF (in kg-m-2-sec -~). A minimum condition for pilot ignition of a gas phase near a wood surface IS the presence of fuel concentration above the lean limit. To maintam the flame after ignition, the pyrolysis gas flux must be in balance with the fuel-consumption rate of the flame. 2 If feed-back heat from the flame to the solid can generate an increasing SGF, then the flame grows and expands its size. At a particular value of the SGF, sooting may start at the tip. 2'5 When the SGF decreases for some reason, the flame also decreases in size and eventually extinguishes. The experimental observations and measurements were conducted m quiescent air at an ambient temperature of 30 +__2°C. EXPERIMENTS Experimental procedure

Three sets of measurements were obtained on flames of variable pyrolysis-gas fluxes. The first set was obtained on gas flames on a metal tube with a diameter of 8.5 mm. W o o d sticks (or liquid fuels) were gasified by heating the tube with an external electric heater. The gas-production rate and the flame size were controlled by changing the voltage applied to the heater. W o o d (casuarlna species) was airdried to a moisture content of 10.4%. Other measured fuel properties are shown in Table 1. The experimental arrangement and the three flame forms of interest are shown schematically in Fig. 1. The amounts of fuel loaded for a measurement were a 2-g (approximately) wood stick or 1.5 g of liquid. The tube was next heated up at a rate of 1-3°C-sec -l. A small pilot flame was brought to the tube mouth every 5 sec to check whether pilot ignition was possible. After lgmtlon, the flame was kept

:~To whom all correspondence should be addressed rGYzo/lo-r

1037

1038

Bui Tuyen et al Table 1 Surface gas flux (SGF) for flames of different fuels.

Fuel

Diesel Kerosene

Vegetable

Flame forms and corresponding SGF, 10"3 kg-m'2-6"1

Density, HHV, kg-m" MJ.kg-I at 30"C

NearStableextinction smokeless

Smoky flames

Maximum flame height, mm

Average rate of fuel consumption, g.cm-~.s4

825 790 880

44.3 42.4 39.2

3.3 to 5 3.7 - 5.5 4.5 - 7.5

7 - 11.2 7 - 15 10- 35

> 11.2 > 15 > 35

28 40 50

17 15 29

710

18.1

5-9

15 - 50

> 50

70

30

cooking oil

Wood stick

/~Flame o o o

= -

o

o

~-~ ~ D a r k

zone Flame

r7 7

./ 4 / 4 II / 4 II I i I I

/

•

II II / II 7 I r7 /

Q Variable Q AC p o w e r IEleetrie IMetal

Dark

body

zone

heater tube

1

a

2

3

b

Fig. 1. (a) Schematm of the experimental setup. (b) Three dfffusmn-flameconditions: 1, near-extinction; 2, stable-smokeless; 3, smoky. in a desired form by controlling external heating until the fuel was consumed. The flame-duration time and the amount of fuel consumed (estimated as the difference between the original weight of wood and the remaining char or the amount of liquid vaporized) were used to calculate the SGF as follows: SGF = w/At,

(1)

where w is the weight (kg) of fuel consumed, A the cross-sectional area (m 2) of the tube and t the flame duration (see). The second set of measurements was performed on a cotton wick enclosed in an 8.5-mm metal tube mounted on the cover of a small fuel container. The flame size and the feeding-fuel flux depended on the position of the wick top relative to the enclosure rim. After a desired steady flame was established by adjusting the wick top position, 6 the weight of the whole burner was monitored continuously. The SGF corresponding to a particular form of flame was calculated from weight-loss histories and Eq. (1). The third set of measurements was conducted using 400 m m long wood sticks of square cross section ranging from 5 x 5 to 7 x 7 m m 2. The flame over a stick was varied by changing the angle ot of the stick with respect to the horizontal. 7.8 The flame is characterized by its length L and height H, as shown schematically in Fig. 2. Measurements of the pyrolyzing solid surface beneath the flame on a wood stick were made for steady burning, i.e. when the flame size had stabilized and the whole flame moved along the stick steadily. An asbestos-cloth socket was used to trail the flame to prevent glowing combustion of char behind the flame. The SGF for the wood surface was estimated by dividing the amount of wood pyrolyzed by the surface area beneath the flame and the time duration for burning the section, i.e.

Small diffusmn flames over a wood particle

istine

1039

wood

Char I

Fig 2 Schematic of a flame over a wood stick (2) SGF = w/4aLt, where w is the amount of wood pyrolyzed, a stick width, L flame length, and t time for flame passage.

Observations Flames of gas issuing from the tube and the wick were very useful for observations of the effect of the SGF on flame appearance since the flame size was directly determined by the fuel flux. Weak or near-extinction flames were those with a height less than 5 mm and could be easily blown off by slight air movements. Further reduction of the SGF resulted in a weaker flame and eventually a minimum value (SGF)m,n for flame existence was reached. The flame was extinguished when attempts were made to reduce the SGF below (SGF)m,n. On the other hand, as the SGF was increased, the flame became longer and stronger. Stable-smokeless flames had heights greater than 5 mm but were without visible smoke at the tip. The appearance of very thin black smoke at the tip served as the indicator that the smoke limit had been reached. The flame height was a maximum at this point. A further increase of the SGF only resulted in a thicker black tip of smoke and even some reduction of the flame height. The flame surface was smooth in all cases; no sign of turbulence was observed. There was a dark zone at the flame base for all of the flame forms studied. When the wick was fed by kerosene and diesel, flames were rehably reproduced for any desired form, but the use of vegetable cooking oil produced only flames up to the limit stable-smokeless. Flame over a wood stick oscillated all of the time and, therefore, mean values had to be estimated. RESULTS AND DISCUSSION The first set of measurements yielded values of the SGF corresponding to the three forms of flames for all tested fuels. The second set gave results on liquid fuels only, and these results were practically identical to those of the first set. The third set yielded a few values of the SGF that produced stablesmokeless flames; these values of the SGF fell within the range established in the first set. Ranges of the SGF corresponding to the three forms of flames for tested fuels are listed in Table 1. For forced ignition to occur, wood must first be heated to emit volatiles. As the fuel concentration near the surface increases, the fuel-dissipation flux to the bulk air must also increase. Pilot ignition of the gas mixture at the boundary is possible only when the fuel concentration is above the lean limit. At this concentration, there must be an SGF which is in balance with the fuel-dissipation flux. This SGF is called the pilot-ignition threshold. For practical purposes, the lower value of (SGF)mm for the range of near-extinction flames from Table 1 may be taken as the pilot-ignition threshold (or extinction limit) of the corresponding fuel. In the region from the extinction to the smoke limit, the flame height was nearly directly proportional

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Bm Tuyen et al

to the SGF; therefore, fuel consumption by flame is proportional to the flame-surface area, where the combustion reactions actually take place in a skin-deep layer) .2.5 The average rate of consumption of fuel RAvc (g-cm-3-sec-~) in the flame-reaction zone defined by Glassman 2 was estimated by using the following expression with measurements made for flames just before reaching the smoke point when the flame shape was conical: Rave = (SGF A)/[O.5¢fdS(d2/4 +/-/2) 1/2] ,

(3)

where d is the tube diameter, H flame height, and 8 thickness of reaction zone which was assumed equal to 1 mm for all fuels. The values of RAve are also listed in Table 1. Generally, the flame appearance depends not only on the SGF and fuel but also on the ambient temperature, composition, flow condition, and system geometry: For some wood burning systems if the ambient air flows by forced convection, the (SGF)mm is expected to be greater than for stagnant air. If the ambient is oxygen-rich and at high temperatures, then even a small combustible gas flux will react: But if oxygen is present in less than the stoichiometric proportion, then part of pyrolysis gas will remain unburnt and pass through the reactor. Stable-smokeless burning in the whole reactor space is the desired operational mode for combustion systems using large pyrolyzing solids. However, stable-smokeless clean combustion is possible only in a narrow range of SGF as shown in Table 1. There always exist operation regimes for the whole reactor or locally at which the flame will assume one of two undesirable forms that are weak or smoky. The space enclosed in the flame cone is not used for combustion. Therefore, a procedure for improving the combustion rate is to reduce this unused space by breaking a large diffusion flame into smaller flames or changing its shape to maximize the surface to volume ratio. 6 Another procedure is to maintain the SGF for particles within a desired range by controlling the operating temperature or choosing the particle size selectively. A suitable air-distribution arrangement will always be useful. CONCLUSIONS In quiescent air, the SGF determines the form of a small diffusion flame over a pyrolyzing solid. The flame is weak for small values of the SGF and extinguishes at the minimum (SGF)m,n but it is smoky for SGF values greater than those corresponding to the smoke limit. For wood, the (SGF)m,, and smoke limit were found to be - 5 x 10-3 and ~50 × 10- 3 kg-m-2-sec-~, respectively. The ( S G F ) m m may be used as a pilot-ignition threshold. The narrow range of the SGF (from 15 x 10-3 to 50 x 10- 3 kgm-2-sec-~) in which stable-smokeless flames are possible indicates difficulty in achieving smoke-free operation of simple stoves using solid fuels of inferior quality. The average rate of consumption of wood-pyrolysis gas in the reaction zone of the flame was ~30 x 10-3 g cm-3-sec-l. Acknowledgement--This work was supported by the Deutsche Gesellschaftfur Techmsche Zusammenarbelt(GTZ), GmbH.

REFERENCES 1. A. M. Kanury, Introduction to Combustion Phenomena, Chaps. 5 & 6, pp. 142-216, Gordon & Breach, New York, NY (1982) 2. I. Glassman, Combustion, pp. 158-173 & 240, Academic Press, New York, NY (1977) 3. A. M. Kanury, Combust. Flame 18, 75 (1972) 4. C. Di Blasi, Prog. Energy Combust. Sci. 19, 71 (1993) 5. A. G. Gaydon and H. G. Wolfhard, Flames: Their Structure, Radtatton and Temperature, Chaps. 6 & 8, pp. 146175 & 195-237, Chapman & Hall, London (1979) 6. L. Nagle and D. Probert, Appl. Energy 41, 309 (1992) 7. R. O. Weber and N. J. de Mestre, Combust. Sci. Tech. 70, 17 (1990). 8. M. Sibulkin and C. K. Lee, Combust. Sci. Tech. 9, 137 (1974).