Operational Limits of Ignition Front Propagation ... - ACS Publications

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Energy & Fuels 2002, 16, 676-686

Operational Limits of Ignition Front Propagation against Airflow in Packed Beds of Different Wood Fuels M. Horttanainen,*,† J. Saastamoinen,‡ and P. Sarkomaa† Lappeenranta University of Technology, Department of Energy and Environmental Technology, P.O. Box 20, FIN-53851 Lappeenranta, Finland, and VTT Processes, P.O. Box 1603, FIN-40101 Jyva¨ skyla¨ , Finland Received August 8, 2001. Revised Manuscript Received January 30, 2002

Propagation of the ignition front against airflow in packed beds of different wood fuels has been studied. The results of experiments carried out with pellets and mixtures of wood chips and sawdust are presented and compared with earlier experiments with different wood fuels. Increase in particle density and size was found out to widen the range of possible airflow rates, and transfer the maximum rate of ignition front propagation toward fuel lean conditions. Increase in the average sphericity of particles decreases the porosity of the bed. Mixing of small and large particles seems to be advantageous for combustion so that small particles change the optimum airflow rate to fuel rich conditions and large particles widen the usable range of airflow rates. A correlation was found for the maximum rate of ignition front propagation in beds of wood fuels.

Introduction Different fuel bed models and measurements have been recently presented by several researchers1-9 for the case of opposite airflow and flame propagation directions. By ignition front we mean here the propagating plane in the fuel bed where the ignition of the volatile gases and/or particle surfaces occurs. The velocity of the ignition front depends on the flow rate of air to the fuel bed, fuel moisture content, fuel composition, particle density, particle size and shape, and distributions of size and shape. In this study we have tried to find dependencies between fuel properties and the rate (velocity and mass flow) of ignition front propagation. Especially the optimal conditions of ignition front * Corresponding author: Lappeenranta University of Technology, Department of Energy and Environmental Technology, P.O. Box 20, FIN-53851 Lappeenranta, Finland. E-mail: [email protected]. Fax: +358 5 621 2799. † Lappeenranta University of Technology. ‡ VTT Processes. (1) Saastamoinen, J. J.; Horttanainen, M.; Taipale, R.; Sarkomaa, P. Combust. Flame 2000, 123, 214-226. (2) Saastamoinen, J. J.; Horttanainen, M.; Sarkomaa, P. Combust. Sci. Technol. 2001, 165, 41-60. (3) Horttanainen, M.; Saastamoinen, J.; Sarkomaa, P. Proceedings of the 5th European Conference on Industrial Furnaces and Boilers, Porto, Portugal, 2000; pp 89-99. (4) Horttanainen, M. V. A.; Saastamoinen, J. J.; Sarkomaa, P. J. IFRF Combust. J. 2000, 21 pp (Article. No. 200003). http://www.journal.ifrf.net/articles.html, an electronic journal. (5) Friberg, R. A New Measurement Method to Analyse the Thermochemical Conversion of Solid Fuels. Academic Dissertation, Royal Institute of Technology, 2000. (6) Axell, M. Fo¨rbra¨nningsfo¨rlopp i en bed av biobra¨nsle. Uppsats fo¨r licentiatexamen, ChalmersTekniska Ho¨gskola, Go¨teborg, Sweden, 2000. (In Swedish.) (7) Shin, D.; Choi, S. Combust. Flame, 2000, 121, 167-180. (8) Gort, R. On the Propagation of a Reaction Front in a Packed Bed, Thermal Conversion of Municipal Solid Waste and Biomass. Academic Dissertation, University of Twente, Enschede, Netherlands, 1995, 193 pp. (9) Thunman, H.; Leckner, B. Fuel 2001, 80, 473-481.

propagation are considered. The maximum velocity of ignition front propagation using a certain fuel is reached when the conditions are optimal. The theoretical background of the ignition front propagation in packed beds of wood particles is presented in the former articles of the present authors.1,2 The knowledge of ignition front propagation against the airflow can be applied to several practical combustion or gasification technologies. It has been applied to grate combustion even though the applicability has been critisised.9 More evidently, this kind of research can be utilized in underfeed (or co-current) combustion and gasification where the fuel and primary air are supplied in the same direction. These kinds of techniques are widely used in small scale (10-2000 kW) combustion and gasification facilities. It is generally not possible to change only one property of any real fuel without changing some of the others at the same time. The porosity of the bed, particle size distribution, and particle shape are usually changed at the same time. If the particle shape is changed, also the porosity changes even though the particles had a uniform size. The only way to change one bed property at a time would be to build up a structured grid of the particles but it would be very laborious. For example, Rothermel10 and Catchpole11 have used this method. These experiments were carried out, however, to model the propagation of the ignition front in forest fires that are enhanced by wind (air and fire propagation to the same direction in a wind tunnel). (10) Rorthermel, R. A Mathematical Model for Fire Spread in Wildland Fuels. USDA Forest Service Research Paper INT. 1972, 115, 40 pp. (11) Catchpole, W. R.; Catchpole, E. A.; Butler, B. W.; Rothermel, R. C.; Morris, G. A.; Latham, D. J. Combust. Sci. Technol., 1998, 131, 1-37.

10.1021/ef010209d CCC: $22.00 © 2002 American Chemical Society Published on Web 04/06/2002

Airflow in Packed Beds of Wood Fuels

Gort8 has presented that particle size does not affect ignition front propagation remarkably in a fuel that contains a lot of volatile compounds. The conclusion is based on experiments with two different sizes of cubic wood blocks. He considered that devolatilization starts at the surface of the particles and the ignition front can progress downward, creeping along the surface of the particles. Axell6 has also concluded that change in particle size does not have a great effect on the rate of the ignition front propagation but it affects the location and wideness of the different combustion regimes of the bed. Increase in particle size was found to transfer the regime of fuel rich combustion with total consumption of oxygen to higher airflow rates. It was also found to narrow down the possible achievable airflow region of this combustion regime. Increase in particle density was found to widen the airflow region of this regime. Stubington and Fenton12 have studied experimentally the rate of ignition front propagation and combustion of dried bagasse in forms of loose fiber (Fb ) 46.4 kgm-3), dense fiber (Fb ) 77.4 kgm-3), small pellets (Fb ) 366 kgm-3), and large pellets (Fb ) 350 kgm-3). They noticed that the maximum combustion rate (change of the fuel mass kg m-2 s-1) decreased and the corresponding airflow rate increased when the bulk density of the fuel increased from loose fiber to dense fiber and to small pellets. The maximum rate of ignition front propagation did not have as clear logic behavior as combustion rate when the bulk density or particle density was changed. The corresponding airflow rate increased with the increase of bulk density and particle density. Increase in particle size decreases the surface area of particles in the bed volume. Also shape change toward spherical shape decreases the surface area of particles per bed volume. Huff13 has found in his experimental study concerning burning times of single particles that the flame time (pyrolysis time) of the particle is linearly dependent on sphericity, particle density and moisture content, respectively. Flame time was increased in his tests with the square of particle size. Thunman and Leckner9 have studied ignition front propagation in the cross current bed combustion of wood. They have concluded that particles, at the certain distance from the beginning of the grate, first ignite on the surface of the grate instead of the bed surface. Thus, grate combustion can be considered and modeled partly as counter-current combustion and not as cocurrent combustion as is often done. Ignition front velocity has found to be increasing when the porosity of the bed increases1. This does not apply straight for ignition mass flux (ignited mass of dry fuel/ unit area of grate and time). Experiments The experiments were carried out in a pot furnace located at Lappeenranta University of Technology (LUT). The furnace (Figure 1) has a square cross section (inner side 150 mm) and the distance between the grate and flue gas outlet is about (12) Stubington, J.; Fenton, H. Combust. Sci. Technol. 1984, 37, 285-299. (13) Huff, E. R. Effect of Size, Shape, Density, Moisture and Furnace Wall Temperature on Burning Times of Wood Pieces. In Fundamentals of Thermochemical Biomass Conversion; Overend, R. P., Milne, A., Mudge, L. K., Eds.; Elsevier: New York 1985; pp 761-775.

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Figure 1. The pot furnace at LUT. 900 mm. The furnace rests on a weight balance so that changes of the fuel mass can be measured during the tests. Flame spread was measured with different fuel types, different moisture contents of some fuel types, and different airflow rates for each fuel type and moisture content. The velocity of the ignition front propagation was determined calculating the time between the moments when temperature waves of 673, 773, and 873 K reached successive, or a couple of chosen, thermocouples located in the bed. The average of these values was used as the flame spread velocity. The fuels used in the experiments were mainly real byproducts of forest industry that are often burned to produce heat and/or electricity for the process or for selling. The properties of the fuels are presented in Table 1, and the main fuels are shown in Figures 2 and 3. The properties shown are average values of different samples. The hydraulic diameter (dh) and sphericity (Ff) of particles are mass average values for the whole fuel type that contains a wide variety of different size and shape particles. The definition of the hydraulic diameter of a particle is dh )4Vp/Ap, where Vp is the volume and Ap the surface area of a particle. Sphericity is determined as the ratio of the surface area of a spherical particle with hydraulic diameter dh and the surface area of a real particle with the same hydraulic diameter. The porosity of the bed is calculated from measured properties as follows: ) 1 - Fwb/Fwp, where Fwb is the density of the moist bed and Fwp is the density of moist particles.

Results The common way of presenting flame propagation in packed beds is to show the results as ignition front propagation velocity or ignition mass flux versus air mass flow rate in the furnace/grate area. The latter kind of relation is presented for two very different fuels in Figure 4.

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Table 1. Fuels Used in the Experiments type thin wood chips

(twc)a

spruce and pine shavings (sh)b

pine sawdust (sd)b pine wood chips (wc)c mixture of twc and sd (50% each)c pellets (rated diameters: 5, 8, and 10 mm)d mixed wc and sd:d 100% wc 75% wc 50% wc 25% wc 100% sd birch wood chips a

Reference 1. b Reference 2. c Reference 3.

uw %

Fdb kg m-3

Fwp kg m-3

dh (mm)

Ff

9 17 30 8.5 14 18 26 12 21 25 8.6 25 30 7.0 6.3 7.6 9.0

50-67 56-63 74-102 118-122 102-112 106-115 113-121 174-177 163-166 151-158 130-164 131-148 132-144 102-115 606-677 522-614 509-603

498 529 564 497 517 532 562 510 542 558 561 629 649 555 1000 1100 1030

2.7

0.035

1.1

0.052

1.7

0.22

6.8

0.15

2.2 5.1 7.2 9.1

0.13 0.60 0.44 0.44

0.87 0.87 0.81 0.74 0.76 0.74 0.72 0.61 0.62 0.63 0.71 0.70 0.70 0.79 0.32 0.44 0.41

11 10 10 11 10 9.8

144-153 152-165 172-181 176-185 159-161 203-212

570 570 570 570 570 630

6.8 5.5 4.3 3.0 1.7 5.9

0.15 0.17 0.19 0.21 0.22 0.16

0.71 0.69 0.65 0.65 0.69 0.63

d

Present study.

Figure 2. Samples of pine wood chips, birch wood chips, sawdust, thin wood chips, and shavings. Height of the capital letters in the fuel titles is 7 mm.

Figure 3. Samples of different pellets.

Pellets and Thin Wood Chips. Thin wood chips are a very porous fuel and its particle size varies in a wide range. Experimental results on thin wood chips have been presented earlier.2,3 The pellets are quite uniformly

sized (although the length varies) dense particles that form a bed with low porosity. Figure 4 shows that it is possible to reach higher ignition mass fluxes with thin wood chips, but the range of usable airflow rates is wider for pellets. Increase in moisture content decreases the rate of ignition front propagation considerably and it also narrows down the range of possible airflow rates. The smallest pellets have produced the highest ignition mass fluxes of pellets. Increasing of particle size seems to make the usable range of airflow rates wider. The straight line in Figure 4 shows the stoichiometric airflow rate for the corresponding mass flux of totally combusted fuel. The line has been determined for pine wood but it does not change a lot if the wood species changes. It can be seen that the airflow rate that causes the highest rates of ignition front propagation (optimal airflow rate) with thin chips is clearly not enough to burn out all the fuel ignited in time unit. The corresponding airflow rates for pellets are considerably higher and more than enough for complete combustion. The curves of the pellets are quite flat so that there is a range of airflow rates that cause nearly constant rate of ignition front propagation. Mixtures. Experimental results of wood chips (uw = 11%), sawdust (uw = 10%), and mixtures of them give information of the effect of particle size and shape to the ignition front propagation. The results of these tests are presented in Figure 5. Also measurements with a mixture of thin wood chips and sawdust are presented in the same figure. It can be seen that the highest ignition mass flux values of the first mixture type are reached with birch wood chips, sawdust, and a mixture that contains 75% sawdust and 25% wood chips. Pure sawdust did not burn when the airflow rate was increased over 0.8 kg m-2 s-1. The rate of ignition front propagation of wood chips decreased clearly when the airflow rate was increased over the optimum flow rate.



Figure 4. Effect of airflow rate/area on ignition mass flux of pellets and thin wood chips with different moisture contents. The straight line shows the mass flux of pine wood that can be burned stoichiometricly with the corresponding airflow rate. The curves are polynomial fits of the experimental points meant to help the reading of the results.

Figure 5. Ignition mass flux (as dry solids) of wood chips, sawdust, and different mixtures of them, and ignition mass flux of mixed thin wood chips and sawdust (50% each). The numbers in the legend mean the mass percentages of the fuels in the mixture.

Mixing sawdust to the wood chips raised the average level of the ignition mass flux values. With mixtures it was also possible to supply high airflow rates to the bed without quenching of the flame. The average level of the ignition mass flux was the highest with 75% sawdust content. It can be seen from Figure 6 that the dry bed density of the bed was also highest for that mixture. The optimal airflow rate that produced the maximum propagation rate of ignition front was lower with fuels that contained also small particles. It can be seen from Figure 5 that the highest ignition front propagation rates were achieved in fuel rich conditions when the

Figure 6. Effect of sawdust mass fraction on dry bed density of the mixtures of sawdust and wood chips.

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Figure 7. (a) Change of the relative mass of the fuel sample during an experiment. The fuel contained 25% pine wood chips and 75% pine sawdust. The airflow rate was 0.25 kgm-2s-1 and the average ignition front propagation rate 0.074 kg m-2s-1. (b) Change of the bed height for the same case. c) Change of relative mass, when the airflow rate was 1.4 kgm-2s-1 and the ignition mass flux 0.063 kg m-2s-1. (d) Bed height in case c.

fraction of sawdust was more than 50%. Also birch wood chips were ignited fastest in fuel rich conditions. The highest values of ignition mass flux are near the corresponding values of pellets. The highest rates of ignition front propagation of all were reached with the mixture that contained 50 mass-% thin wood chips and 50% sawdust. Only one experimental point of pure thin wood chips was higher than the maximum rates of ignition front propagation of this mixture. The range of possible airflow rates was, however, rather narrow. This fuel is very porous ( ) 0.79) but not as porous as thin wood chips alone. All the particles were small or at least thin and so the burning zone is also thin. This makes the fuel to quench easily when the airflow rate is increased. In Figure 7 we can see the change of the relative mass and the relative bed height of a fuel sample during two experiments. The flow rate of air was considerably lower in the case of Figure 7a. The ignition front propagated faster in this case and also the change of mass was a bit quicker. We can see that the change of bed height was, for most of the test time, faster when the airflow rate was higher.

In the case of low airflow rate there was not enough oxygen to burn out the char after the volatile gases had reacted near the ignition front. The mass proportion of char is, however, small in wood and so the mass change was still quite fast. The airflow rate was significantly higher in the case of Figure 7b. It was near the highest possible flow rate. In this case there was enough oxygen in the gas flow to burn out all the fuel near the ignition front. The surface of the bed lowered with the same speed as the ignition front propagated and was located near the ignition front. The flow rate of air was so high that it diluted the volatiles and cooled the combustion zone so that the rate of ignition front propagation was decreased from that of fuel rich conditions. The optimal airflow rate was between these two airflow rates for this fuel. This kind of phenomenon was seen with all the different fuels. Other Tested Fuel Types. The experimental results of sawdust, shavings, and wood chips are presented in Figures 8 and 9. The effects of different factors have also been discussed earlier.1-4 It is obvious that an increase in the moisture content of wood decreases the maximum ignition mass fluxes. It also narrows down



Figure 8. Ignition mass flux of shavings (filled symbols) and sawdust (open symbols) when the airflow rate and moisture content of fuel are varied. The data are from ref 4.

Figure 9. Ignition mass flux of pine wood chips when the airflow rate and moisture content of fuel are varied. The effect of airflow rate on the ignition mass flux of a dry mixture of thin wood chips and sawdust. The mixture of thin wood chips and sawdust is the same as in Figure 5. The data are from ref 4.

the range of possible airflow rates. The effect of moisture in the optimum airflow rate has been shown to be unclear. The optimum airflow rates of these fuels were all smaller than the stoichiometric airflow rate. The values of optimum airflow rate were the smallest with shavings and sawdust. It was not possible to test the highest possible airflow rates with the fuels of Figures 8 and 9 because of problems in airflow rate measuring. Discussion Pellets and Thin Chips. The density of pellets is about twice the density of thin wood chip particles. This

causes slower pyrolysis of pellets. The average particle size of pellets is bigger, which also increases the pyrolysis time of single particles. These differences would explain the fact that the ignition mass flux of thin wood chips is higher than that of pellets, but we can see from the results of other fuels that the dependence is not so simple. The higher density and the size of the particles can explain the difference in the wideness of the possible airflow rates. The combustion is kept on mainly by the burning of char particles when the airflow rate is high. The gas flame is extended when the rate of airflow is

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Figure 10. Effect of air excess number of volatiles on ignition mass flux with thin wood chips and pellets. The data are from ref 16.

increased, and the heat flux from the flame is decreased. When the particles are small the combustion time of char particles is short and the layer of burning char particles above the ignition front is relatively thin. Thus the intensity of the heat flux to the ignition plane is low and quenching occurs easily when the airflow rate is increased. Small and light particles also entrain with the airflow more easily than big and dense particles. The entraining of particles also decreases the heat flux to the virgin fuel. The porosity of the bed influences the ignition front propagation by changing the fractions of radiated heat flux from the gaseous flame and the burning char surfaces. The higher the porosity is the smaller proportion of radiation, coming to the ignition plane, originates from the char. With high airflow rates the intensity of flame radiation decreases. The total intensity of radiation in very porous beds decreases more than that in denser beds. One reason for the higher rates of ignition front propagation of thin wood chips can be a different way of ignition front propagation. Thin chips contains a lot of particles such as sticks. The flame is able to creep along sticks against the airflow. The flame on the surface of a stick can then ignite the shorter particles around it. So the heat transfer to the virgin fuel is not only radiation or conduction in the contact points of particles, but also convective heat transfer from flames around the burning sticks. Heat is transferred also by conduction and mixing in the gas in the boundary layer of particles opposed by cool air moving against. In addition, processes very near the ignition front that is creeping along the surfaces of the particles are important. The heat conduction and mixing of released pyrolysis gas with the opposing cool air in the boundary layer and conduction and devolatilization in the solid in the vicinity of the ignition front are important factors for the flame propagation. There are very great temperature differences in a single fuel particle in a short distance due to asymmetric heating conditions leading

to asymmetric burning. This resembles burning of a matchstick, which is cool at one end and flaming at the other end with a very narrow transition region between the two. Thus, the processes in this thin region are significant for the propagation of the flame. This asymmetric progress of burning along the surfaces in the crack-like passages formed by the bed of particles has been visually observed also for compact cube formed large particles. Downward flame spread along thermally thin14 and thick15 fuel has been much discussed in the literature, but these models are not well applicable for real fuel beds. The stoichiometry of the combustion of volatiles in the ignition front is illustrated with Figure 10. We have used the air excess number of volatiles to describe the ratio of airflow through the fuel bed and the theoretical airflow needed to burn the released volatiles completely. Air excess number of volatiles is determined with the equation

λv )

m ˘ ′′air fvYO2,0 m ˘ ′′airfvYO2,0 ) m ˘ ′′v vwigFb

(1)

In eq 1, m ˘ ′′air is the air mass flux into the bed, fv is the stoichiometric coefficient (mass of volatiles/mass of oxygen) (fv ) 0.828 for pine), YO2,0 ) 0.232 is the mass fraction of oxygen in air, m ˘ ′′v is the mass flow of volatiles generated/bed area, v is the mass fraction of volatiles of dry fuel (assumed to equal 0.85), wig is the ignition front velocity, and Fb is the dry bed density. Figure 10 shows clearly that the highest values of the ignition mass flux with thin wood chips are achieved when the airflow is not high enough to burn off all the volatiles (λv ) 0.47 (8.9% moisture content), λv ) 0.89 (17%), and λv ) 0.31 (26%)). With the highest moisture content it was not possible at all to use the stoichio(14) Duh, F.-C.; Chen, C.-H. Combust. Sci. Technol. 1991, 77, 291305. (15) Higuera, F. J. Combust. Theory Modell. 1999, 3, 147-158.


metric airflow rate without quenching the flame. With pellets the highest ignition mass flux was reached with air excess numbers of volatiles varying between 1.7 and 3.5 depending on the size of the pellet. Thus, fuel lean conditions are needed for the maximum rate of ignition front propagation in beds of pellets. Mixtures. Mixing small and big particles seems to be advantageous for packed bed combustion. The usable range of airflow rates is wider for mixtures than for pure sawdust. The optimum airflow rate is transferred to the fuel rich direction when small particles (sawdust) are added among bigger particles (wood chips). Fuel rich conditions are preferred in packed bed combustion because NOx emissions are known to increase considerably in oxygen rich conditions,17 and particle entrainment is enhanced when the air velocity in the bed is increased. The highest rates of ignition front propagation of mixed wood chips and sawdust are about the same as the highest rates with a single particle type. With the mixture of thin wood chips and sawdust even higher ignition mass fluxes were reached. Small particles ignite with small ignition energy. Heat flow from these particles enhances the ignition of bigger particles. The mixing of different fuel types affects also the porosity of the bed. The particles of pure sawdust have a relatively uniform size and shape, and they usually form a bed with a rather low porosity. The porosity of the mixture of thin wood chips and sawdust is higher than the average of the porosity of these fuels. With normal wood chips and sawdust the effect of mixing was opposite for the case of thin wood chips and saw dust. Small particles among the bigger can fill part of the gaps between particles and thus decrease the porosity. The sticks in the thin wood chips often form rather large gaps between particles, thus increasing the porosity of the bed. Maximum Rate of Ignition Front Propagation for All the Tested Fuels. Here we have tried to find out such dependencies as those between fuel properties, airflow rate, and ignition front propagation rate that are valid for all of the tested fuel types. Especially, the optimum conditions for reaction front propagation that produces the maximum propagation rate are considered. It can be noticed (see Figures 4, 5, and 8) that, for example, particle size or porosity of the bed alone cannot explain the difference in maximum ignition mass flux or optimum airflow rate. As an example, let us compare sawdust, thin wood chips, and the smallest pellets (Table 2). There does not seem to be any simple logic dependence between the separate variables and ignition mass flux or optimum airflow rate. There is dispersion in the experimental results because of the random arrangement of particles and distribution of particle size but the basic trends of ignition mass flux and optimum airflow rate can be approximated from the experiments. It can be seen from Figure 11 that particle shape affects the porosity of the bed. Porosity decreases when (16) Horttanainen, M.; Saastamoinen, J.; Sarkomaa, P. The Effect of Reaction Stoichiometry on the Propagation Rate and Temperature of Ignition Front in Packed Beds of Wood Particles. In Proceedings of Nordic Seminar on Thermochemical Conversion of Biofuels, Trondheim, Norway, Nov 21, 2000; pp 93-108. (http://www.tev.ntnu.no/Oyvind.Skreiberg/ NS001121.pdf.) (17) Saastamoinen, J. J.; Taipale, R. Environ. Combust. Technol. 2001. In press.


Figure 11. Porosity of the bed as function of average sphericity of particles, for all the tested fuels.

Figure 12. Comparison of measured and predicted values of the maximum ignition front velocity with different wood fuels. The line shows the perfect (ideal) correlation. Table 2. Comparison of Optimal Conditions of Different Wood Fuels property

sawdust

thin chips

pellets 5 mm

dh (mm) e uw m ˘ tg,max ′′ (kg m-2s-1) m ˘ ′′air,opt (kg m-2s-1)

1.7 0.69 0.10 0.075 0.26

2.7 0.87 0.09 0.13 0.25

5.1 0.32 0.06 0.072 0.9

the average sphericity increases. The best fit for these experimental results including all the fuels used is

) 0.87 exp(- 1.5Ff)

(2)

The fit is valid for average sphericities between 0.03 and 0.66. In the following a suitable dimensionless presentation for the maximum ignition velocity is looked for. Ignition velocity depends on the heat flux to the ignition front of the flame (q′′) and on the energy (Q′′′ ig) required for the heating of the material to the ignition temperature by the approximate relation1

wig ) q′′/Q′′′ ig

(3)

The heat flux from burning particles to the nonignited ones is q′′ ∼ (kb/dh)∆Tf, where the effective conductivity of thermal radiation kb ∼ dhTf 3. The energy for ignition can be expressed approximately by

Q′′′ ig ) (1 - )(1 + 3.4u)Fpcp∆Tig

(4)

when we assume ∆Tig ≈ 330 K and cp ≈ 2 kJ kg-1K-1. Then we obtain

wig ∼

(kb/dh)(∆Tf/∆Tig) (1 - )(1 + 3.4u)Fpcp

∼

σTf3(∆Tf/∆Tig) (1 - )(1 + 3.4u)Fpcp

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Figure 13. Effect of sphericity of particles (u ) 0.12) on the ratio m ˘ ′′ig/m ˘ ′′air at the maximum ignition velocity ((: twc, sh, pwc, bwc, pe 5 mm, pe 8 mm and pe 10 mm. 0: wc+sd. O: pwc+sd).

The temperatures in the equation above are not exactly known, but they are of the same order of magnitude in the optimal conditions for different fuels.1,2 Here we assume σTf 3 ∼ href ≈ constant and ∆Tf/∆Tig ≈ constant or their effect will be hidden in the developed correlation. Thus, a suitable dimensionless parameter for correlating the maximum ignition velocity is

Π1 )

href wig(1 + 3.4u)Fdpcdp

(5)

hrefFf0.24 m ˘ ′′ig,max = 138 (1 + 3.4u)cdp(1 - )0.80

where we choose href ) 1 W m-2 K-1. For another dimensionless parameter we choose

Π2 ) (1 - )

(6)

We find a correlation for these parameters using the maximum ignition front velocities of all the tested fuels. The values used as the maximal ignition velocities were the highest velocities measured. The real maximum velocities can deviate a little from these values, but not very much. The correlation between Π1 and Π2 was relatively good (R2 ) 0.96), but it could still be improved with a third dimensionless parameter Π3 ) Ff. Finally, we obtain the correlation

Π1Π30.24 ) 0.00723Π21.80

(7)

The correlation coefficient of this fit is R2 ) 0.985. From the correlation we obtain the equation for maximum velocity

hrefFf0.24 wig,max ) 138 (1 + 3.4u)Fdpcdp(1 - )1.80

to this fit, an increase in the moisture content decreases the maximum ignition velocity linearly. An increase in average sphericity of the particles seems to accelerate the reaction front propagation a little. It has to be remembered that an increase in sphericity decreases the porosity of the bed. Correspondingly, an increase in porosity increases the ignition velocity. The joint effect of porosity and sphericity can be modeled approximately like in eq 8. The maximum ignition velocity seems to be inversely proportional to the density of the particle. The equation of ignition mass flux is

(8)

The error of the prediction of wig,max varies in the range -20 to 17%. The measured and predicted values of the ignition velocity are compared in Figure 12. According

(9)

The ignition mass flux is not proportional to particle density. Fdp(1 - ) ) Fdb, used in the derivation of eq 9, is not exactly valid because the moisture of the particles affects the volume of the particles a little. Since it is not easy to determine the value of Ff, it can be expressed as a function of the porosity using eq 2 as Ff ) -ln(/0.87)/1.5. By using this, eq 7 is changed to

[{(

Π1 ) 0.00723Π21.80 - ln

)}

]

1 - Π2 /{1.5} 0.87

-0.24

(10)

and the correlation coefficient for this is R2 ) 0.96. For this correlation the porosity of the bed should be lower than 0.87. The effect of particle size was also tested using the dimensionless number Π4 ) wig,max /dhkvol, where kvol (frequency factor of the volatilization) was assumed to be constant. This did not improve the prediction. About Optimum Airflow Rate. When we compare all the maximum ignition mass flux values of the tested fuels (Table 2) we can see that most of the values of the relatively dry fuels (u < 0.13) are in the range 0.060.08 kg m-2 s-1. The lowest maximum ignition mass flux of the moist fuels is 0.036 kg m-2 s-1 and the highest of



Table 3. Maximum Ignition Mass Flux and the Optimum Airflow Rate for Each Tested Fuel Sample: Thin Wood Chips (twc), Shavings (sh), Pine Wood Chips (pwc), Saw Dust (sd), Pellets (pe), Birch Wood Chips (bwc), Mixtures (pwc+sd), where Values Mean the Mass Percentage of the Fuel in Mixture) fuel type and moisture content

air flow rate/area (kg m-2 s-1)

ignition mass flux (kg m-2 s-1)

twc, u ) 0.098 sh, u ) 0.093 pwc, u ) 0.094 twc+sd, u ) 0.075 pe 5 mm, u ) 0.067 pe 8 mm, u ) 0.082 pe 10 mm, u ) 0.099 pwc, u ) 0.12 pwc75+sd25, u)0.11 pwc50+sd50, u)0.11 pwc25+sd75, u)0.11 sd, u ) 0.10 bwc, u ) 0.11 sh, u ) 0.17 sd, u ) 0.14 twc, u ) 0.21 sh, u ) 0.22 sd, u ) 0.26 twc, u ) 0.35 sh, u ) 0.35 sd, u ) 0.33 pwc, u ) 0.33 pwc, u ) 0.40

0.274 0.110 0.198 0.299 0.921 0.467 0.926 0.469 0.474 0.468 0.366 0.255 0.363 0.142 0.156 0.337 0.146 0.097 0.076 0.131 0.133 0.291 0.248

0.131 0.068 0.082 0.114 0.072 0.061 0.059 0.067 0.070 0.077 0.078 0.075 0.079 0.061 0.062 0.086 0.049 0.050 0.056 0.039 0.044 0.053 0.046

Conclusions

all the fuels is 0.131 kg m-2 s-1 (3.6 times higher than the lowest). The corresponding range of airflow rates is considerably wider (0.11-0.93 kg m-2 s-1 for dry fuels and 0.076-0.34 kg m-2 s-1 for moist fuels). The highest is about 12 times the lowest air flow rate. It would be more important to find the way to predict the optimum airflow rate for a fuel type with a certain moisture content. It seems that the sphericity has a great effect on the ratio of the ignition mass flux and air mass flux at the optimum conditions for relatively dry fuels as shown in Figure 13. The effect of sphericity can be presented as a correlation for a single fuel

(m ˘ ′′ig/m ˘ ′′air)opt ) -0.1771ln(Ff) - 0.0334

dilution of volatiles and lengthening of the flame. There is a clear physical reason for the maximum possible air rate beyond which the flame is quenched. Let us consider two adjacent particle layers; one is burning and the other is not yet ignited. The burning time decreases as the air velocity increases according to the diffusion theory of particle combustion. On the other hand the net heat flux (radiation from burning particles minus convective losses) to the nonignited layer will be reduced at higher air rates, since the radiation from (and temperature of) burning char is not much influenced, but the convective loss from the nonignited particles increases. When the air rate is increased, the combustion time is reduced, but the time needed for ignition is increased. Then at some critical air rate, the burning time is not sufficiently long to heat up the nonignited particles.

(11)

where m ˘ air ′′ is the airflow rate /area of the grate. For the mixture of wood chips and sawdust the decreasing trend with increasing Ff is not shown, but an increasing trend is seen in the rather limited range of Ff. We can see that particle density also affected the optimum airflow rate. Pellets needing the highest airflow rates for the maximum ignition mass flux though their curves (airflow rate vs ignition mass flux) were rather flat (Figure 4). The optimum airflow rates of the tested fuels are also presented in Table 3. Discussion of the Minimum and Maximum Air Flow Rates. The lowest possible airflow rate is enough to cause a heat release rate of combustion that can rise the fuel to ignition conditions. It was seen from the temperature measurements of the earlier studies that the maximum temperatures were often near adiabatic temperatures with low airflow rates.1 The lowest maximum temperatures were about 900 K with low airflow rates. This information can be used in further studies for prediction of the lowest possible airflow rates. When the airflow rate is increased after the optimum airflow rate the combustion region is cooled due to

The propagation rate of the ignition front was studied experimentally with different wood fuels. The ignition mass flux was found to be the highest with thin wood chips and a mixture of thin wood chips and sawdust. Experiments with pellets showed that the range of usable airflow rates increases when the density and size of particles is increased. At the same time the optimum airflow rate moves toward the region of excess air combustion. The mixing of small and large particles seems to be advantageous in respect of NOx emission reduction and improvement of combustion stability. Small particles make it possible to reach the highest rates of ignition front propagation with substoichiometric airflow and large particles widen the possible range of airflow rates. A correlation was found for the maximum rate of ignition front propagation. It predicts that the ignition mass flux is increased with the increase of the porosity and sphericity of particles and decrease of the moisture content of the fuel. Increase in the average sphericity of particles was found to decrease the porosity of the bed. The most challenging subject to be studied in the future is the airflow rate that causes the maximum rate of ignition front. Nomenclature A ) surface area, m2 cp ) specific heat, J kg-1 K-1 dh ) hydraulic diameter, m Ff ) sphericity of a particle fv ) stoichiometric coefficient (mass of volatiles/mass of oxygen) href ) reference heat transfer coefficient, W m-2 K-1 kb) effective conductivity in the bed, W m-1 K-1 kvol ) global frequency factor for volatilization reactions, s-1 m ˘ ′′air ) air flow rate/unit area of grate, kg m-2 s-1 m ˘ ′′ig ) ignition mass flux (ignited mass of dry fuel/unit area of grate and time), kg m-2 s-1 m ˘ ′′v ) mass flow of volatiles generated/bed area, kg m-2 s-1 Q′′′ig ) energy needed for heating a volume unit of the fuel to the ignition conditions, J m-3 q′′ ) heat flux from the flame to the ignition front, W m-2 R2 ) correlation coefficient T ) temperature, K ∆Tf ) temperature difference between the flame and the initial state (environment), K

686


∆Tig ) temperature difference between the ignition and initial state, K u ) moisture content of the fuel in dry basis uw ) moisture content of the fuel in wet basis V ) volume, m3 wig ) ignition velocity (velocity of the ignition front in the bed), m s-1 YO2,0 ) mass fraction of oxygen in air ) 0.232 Greek Letters ) porosity of the bed λv ) air excess number of volatiles v ) mass fraction of volatiles of dry fuel (assumed to equal 0.85) Π ) dimensionless parameter F ) density, kg m-3 σ ) Stefan-Boltzmann’s constant, 5.67 × 10-8 W m-2 K-4 Subscripts b ) bed of particles d ) dry f ) flame max ) maximum opt ) optimum

Horttanainen et al. p ) particle v ) volatiles w ) moist Abbreviations bwc ) birch wood chips pe ) pellets pwc ) pine wood chips sd ) saw dust sh ) shavings twc ) thin wood chips

Acknowledgment. The financial support from the National Technology Agency of Finland (TEKES) and its research program CODE are acknowledged. Also the scholarships awarded by Lappeenranta University of Technology and the foundations Tekniikan edista¨missa¨a¨tio¨ and Finnish Cultural Foundation have been essential for the realization of the study, and the donors are gratefully acknowledged. The fuels for the experiments were kindly given by VAPO Oy and UPMKymmene Oy. EF010209D

Operational Limits of Ignition Front Propagation ... - ACS Publications

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