Combustion of a Single Particle of Biomass - Energy & Fuels (ACS

306

Energy & Fuels 2008, 22, 306–316

Combustion of a Single Particle of Biomass Yao B. Yang,† Vida N. Sharifi,† Jim Swithenbank,† Lin Ma,‡ Leilani I. Darvell,‡ Jenny M. Jones,‡ Mohamed Pourkashanian,‡ and Alan Williams*,‡ Department of Chemical and Process Engineering, UniVersity of Sheffield, Sheffield, U.K.; Energy and Resources Research Institute, UniVersity of Leeds, Leeds LS2 9JT, U.K. ReceiVed May 28, 2007. ReVised Manuscript ReceiVed September 26, 2007

Biomass is one of the important renewable energy sources. Biomass fuels exhibit a range of chemical and physical properties, particularly size and shape. Investigations of the behavior of a single biomass particle are fundamental to all practical applications, including both packed and fluidized-bed combustion, as well as suspended and pulverized fuel (pf) combustion. In this paper, both experimental and mathematical modeling approaches are employed to study the combustion characteristics of a single biomass particle ranging in size from 10 µm to 20 mm. Different subprocesses such as moisture evaporation, devolatilization, tar cracking, gas-phase reactions, and char gasification are examined. The sensitivity to the variation in model parameters, especially the particle size and heating rates, is investigated. The results obtained from this study are useful in assessing different combustion systems using biomass as a fuel. It helps to clarify the situations where the thermally thin and thermally thick cases interface. It is clear that simple models of particle combustion assuming constant particle temperature are sometimes inadequate and that for large particles a more detailed mathematical representation should be applied.

Introduction As one of the important renewable energy resources, biomass fuels are attracting more attention in order to improve our understanding of their process characteristics during mass-toenergy conversion. The key equipment type used for thermal conversion of biomass includes packed-bed, fluidized-bed, and pulverized fuel (pf) suspended furnaces. Different particle sizes are required for these systems. For pf suspended systems, biomass fuel particles are normally reduced to a wide size range of 10-1000 µm in an attempt to ensure complete burnout in only a few seconds of residence time. For fluidized-bed systems, biomass fuels are pelletized or chipped to 2–5 mm or larger satisfy the fluidization requirements. For packed beds, biomass fuels are fired either as received or with minimum preprocessing and the size ranges are much wider, from 5–100 mm or even higher.1,2 Because of the coupling between chemistry and heat and mass transfer during particle conversion, fuel size is expected to have noticeable effects on process characteristics. There has been considerable discussion for many years about the relative roles of the controlling mechanism for both pyrolysis and oxidation, whether it is chemical or physical, and the general features are well-understood. In the case of pyrolysis, it is a question of chemical kinetic rate control versus internal heat transport control, and in the case of char oxidation, it is chemical kinetic rate control versus internal or external diffusion control.3–5 The reaction regimes for pyrolysis, char formation, and char * Corresponding author. Tel.: ++44 113 3432507. Fax: ++ 44 113 2467310. E-mail: [email protected]. † University of Sheffield. ‡ University of Leeds. (1) Williams, A.; Pourkashanian, M.; Jones, J. M. Prog. Combust. Energy. 2001, 27, 587–610. (2) Williams, A.; Pourkashanian, M.; Jones, J. M. Proc. Combust. Inst. 2000, 28, 2141–2162. (3) Zielinski, E. Fuel 1967, 46, 329–340.

combustion can be defined as thermally thin, thermally thick, and thermal wave regimes. In the first case, the temperature is assumed to be constant across the particle, and this is the situation normally assumed to be the case in the heating-up step for small pf particles and applies to the drying and pyrolysis steps. In larger particles in fluidized or fixed beds, a thermal wave initially causes drying and pyrolysis followed by char formation and then combustion as the wave passes through the particle. In this situation all, the regimes of drying, pyrolysis, and combustion coexist. The thermally thick case predominantly applies when there is large-sized material involved and considerable thermal gradients, such as those found in fires of flammable materials or in underground combustion situations. Backreedy et al.6 and Ma et al.7 have applied numerical modeling to describe a pf biomass combustion furnace of particle sizes under 1000 µm, and a number of other authors have made similar simulations, for example in ref 8. These simulations have treated the whole assembly of biomass particles on a macroscale which considers the particles either as a continuous porous medium or having uniform temperature across their diameters, regardless of their individual shapes and sizes, although equivalent shapes have been used for the particles with high aspect ratios. Gera et al.8 considered the combustion (4) Herzberg, M.; Zlochower, I. A.; Edwards J. C. Coal particle pyrolysis mechanisms and temperatures; Report RI 9169, Bureau of Mines: Springfield, VA, 1988. (5) Bryden, K. M.; Hagge, M. J. Fuel 2003, 82, 1633–1644. (6) Backreedy, R. I.; Fletcher, L. M.; Jones, J. M.; Ma, L.; Pourkashanian, M.; Williams, A. Proc. Combust. Inst. 2005, 30, 2955–2964. (7) Ma, L.; Pourkashanian, M.; Williams, A.; Jones, J. M. A numerical model for predicting biomass particle depositions in a pf furnace. Proceedings of ASME Turbo Expo 2006 Power for Land, Sea, and Air; Barcelona, Spain, May 8–11 2006. (8) Gera, D.; Mathur, M. P.; Freeman, M. C. Energy Fuels 2002, 16, 1523–1532. (9) Bruch, C.; Peters, B.; Nussbaumer, T. Fuel 2003, 82, 729–738. (10) Yang, Y. B.; Yamauchi, H.; Sharifi, V. N.; Swithenbank, J. J. Inst. Energy 2003, 76, 105–115.

10.1021/ef700305r CCC: $40.75  2008 American Chemical Society Published on Web 12/06/2007

Combustion of a Single Biomass Particle

of biomass particles with a high aspect ratio and concluded that if temperature gradients are ignored in such particles then the particle temperature is underestimated. Reactor simulations of biomass conversion processes have been carried out by many researchers, for example those in refs 5-13. A number of papers have considered the pyrolysis step alone, especially fast pyrolysis because this is dependent on the rapid heating of the particle. For example, Di Felice et al.14 and Lathouwers et al.15 simulated the biomass pyrolyses processes in a fluidized bed reactor. Janse et al.16 applied numerical modeling to the flash pyrolysis of a single wood particle with size ranging from 200-1000 µm at a fixed external surface temperature of 823 K. The flow of vapors has been described using the dusty gas model,17 and a parallel reaction model was employed to describe the devolatilisation process. The bio-oil yield (approximately 77%) was found to be hardly affected by the particle sizes at the temperatures used, which indicated that little tar cracking occurred in this small particle size range. Both particle volume and internal porosity were assumed to be unchanged during conversion, and particle interaction with the main flows was omitted. While this greatly simplified the computation process, the assumptions may not be valid in actual applications where biomass particles shrink and porosity varies significantly during heating-up. Peters18 developed a discrete particle model (DPM) based on a system of one-dimensional transient conservation equations for mass and energy inside a spherical particle, which is coupled to the surrounding gaseous phase by heat and mass transfer processes. Wood particles from 8 to 17 mm were modeled assuming the reacting core was either shrinking or nonshrinking. The boundary conditions at the particle external surface were assumed to be uniform, and so, this model cannot reflect the threedimensional details in an irregular-shaped biomass particle. Di Blasi19 modeled the heat, momentum, and mass transport through a shrinking biomass particle exposed to uniform thermal radiation. The volume occupied by the solid was assumed to decrease linearly with the wood mass and to increase with the char mass by a chosen shrinkage factor. The initial particle halfthickness was 25 mm. None of these investigations considered interaction with the main gas flow. This limits the direct application of the results to a practical reactor where the main flow acts as the driving force for the solid conversion processes. In later research, Di Blasi20 improved the model, with the particle coupled with a plug-flow assumption for extraparticle processes of tar cracking, in order to predict the fast pyrolysis of wood in a fluid-bed reactor for liquid-fuel production. Particle sizes ranged from 0.1 to 6 mm, and particle dynamics were affected strongly by the convective transport of volatile products. Bryden and Hagge5 considered the combined impact of moisture and char shrinkage (11) Yang, Y. B.; Swithenbank, J.; and Sharifi, V. N. Fuel 2004, 83, 1553–1562. (12) Yang, Y. B.; Ryu, C.; Khor, A.; Yates, N. E.; Sharifi, V. N.; Swithenbank, J. Fuel 2005, 84, 2116–2130. (13) Yang, Y. B.; Sharifi, V. N.; Swithenbank, J. AIChE J. 2006, 52, 809–817. (14) Di Felice, R.; Coppola, G.; Rapagna, S.; Jand, N. Can. J. Chem. Eng. 1999, 77, 325–332. (15) Lathouwers, D.; Bellan, J. Int. J. Multiphase Flow 2001, 27, 2155– 2187. (16) Janse, A. M. C.; Westerhout, R. W. J.; Prins, W. Chem. Eng. Process. 2000, 39, 239–252. (17) Bliek, A.; van Poelje, W. N.; van Swaaij, W. P. N.; van Beckum, F P H. AIChE J. 1985, 31, 1666–1681. (18) Peters, B. Combust. Flame 2002, 131, 132–146. (19) Di Blasi, C. Chem. Eng. Sci. 1996, 51, 1121–1132. (20) Di Blasi, C. AIChE J. 2002, 48, 2386–2397.

Energy & Fuels, Vol. 22, No. 1, 2008 307

on the pyrolysis of a wood particle and concluded that the predictions were different than if they had been considered separately. Simulations of the whole combustion process of a single biomass particle, including the char burnout stage, are rare. Under combustion conditions, the temperature gradient inside a particle is much higher, an ash layer forms, and the burning processes are never one-dimensional in terms of the particle radius. In this paper, a two-dimensional model is proposed for a cylindrical particle of biomass surrounded by a passing gas stream. A single particle model is fundamental for a packedbed where multiple particles are present, and this work helps in the understanding and characterization of the packed-bed combustion. Compared to previous simulations, the flow boundary layer will be modeled for the first time to reveal the microstructure of the fluid flow surrounding the particle. Particle size will range from under 1 to 20 mm to cover most of the actual situations. Experiments with Single Particles Numerous experimental studies of the combustion of small single particles of solid fuels have been undertaken. In the case of small particles, three cases are considered: thermogravimetric analysis (TGA), entrained particle combustion in a supporting flame, and suspended particle combustion. Many of the earlier experiments were made using TGA experiments where the particles are in the form of a distributed bed and the particle size is typically between 20 and 200 µm (diameter). Since the heating rate is low and the mass is small, the particles are considered to be isothermal. However, there can be severe consequences in the determination of kinetic data arising from secondary reactions especially in the larger particles. The correct choice of kinetic data is important in all applications, but especially in the case of a pulverized biomass flame, because most of the biomass components are released as volatiles and the correct kinetic parameters are essential for modeling applications.21 Depending upon the nature of the experiment, the measured apparent kinetic parameters are a composite of the fundamental reaction kinetics and the secondary cracking and condensation reactions. The fundamental decomposition reaction consists of multiple steps each described by a first-order reaction. These can be mathematically reduced to a smaller number of equations, typically one, two, or three. In this paper, we will use the single first-order form of the equations. We have investigated the combustion of particles of willow coppice in a methane-air flat flame using high speed video photography.22,23 By this method, the rate of burn-out could be observed as well as phenomena such as particles accelerated by a jet of volatiles and also particle rotation. Particles could be captured for investigations by scanning electron microscopy. The particles that were passed through the flame ranged from spherical particles 0.5-1.0 mm in diameter to cylindrical particles typically 0.5 mm × 1.5 mm (with an equivalent diameter on a volume basis of 0.825 mm). Previous calculations using the FLUENT code2 gave the temperature-devolatilisation history of the particle, but it also used the assumption of uniform particle temperature. Using this assumption, Figures 1a and b respectively shows the progress of a 0.5 and 1.0 mm particle being heated in both cases in a hot gas stream to final temperatures of 1500 and 2200 K. The effects of particle size and temperature are very marked, and particle temperatures and (21) Jones, J. M.; Pourkashanian, M.; Williams, A.; Hainsworth, D. A. J. Renewable Energy 2000, 19, 229–234. (22) Darvell, L. I.; Hrycko, P.; Jones, J. M.; Nowakowski, D.; Pourkashaniam, M.; Williams, A. Impact of minerals and alkali metals on willow combustion properties. World Renewable Energy Congress, Regional Meeting; Aberdeen, May 2005. (23) Jones, J. M.; Darvell, L. I.; Bridgeman, T. G.; Pourkashanian, M.; Williams, A. Proc. Combust. Inst. 2007, 31, 1955–1963.

308 Energy & Fuels, Vol. 22, No. 1, 2008

Yang et al.

Figure 3. SEM of biomass char: (a) hard wood; (b) strawsearly stage of combustion; (c), strawslate stage of combustion; (d) palm oil plant.

Figure 1. Predicted particle temperature and mass loss (volatile release) of (a) 0.5 mm and (b) 1 mm spherical pine wood particles heated to two different final temperatures, 1500 and 2200 K.

Figure 2. Predicted particle temperature and volatile loss for a 0.5 mm × 1.5 mm pine wood particle (LHS) in a 1 MW pf furnace (furnace temperature 1800 K and exit temperature of 1500 K) compared with a spherical particle (RHS) of equivalent size (0.825 mm with a particle shape factor of 0.78).

devolatilisation rates are dictated by particle size. Figure 2 demonstrates the effect of particle shape in a furnace at 1800 K where the particle is heated by both radiation and convection. A cylindrical 0.5 × 1.5 mm particle and the equivalent spherical particle on a volume basis, 0.825 mm diameter for particles, are shown. Particles having an equivalent volume are used for aerodynamic modeling, but it is seen that the heating-up rate of the equivalent particle is much slower. In practice, while many particles are approximately spherical or cylindrical, this is not always the case and equivalence to the closest geometry has to be employed. Examples of biomass char types produced from different types of milled biomass passing through a methane flame are shown in Figure 3. They include particles of hard wood, straw, and palm waste. Unlike coal or plastic materials which can melt or soften and form spherical droplets, biomass char particles can be very irregular and to a large extent are determined

by the combined influence of the lignin structure of the original biomass and the mechanical process by which the particles are formed. For example, pulverized wood may have either have a spherical or a cylindrical structure whereas straws only have a cylindrical structure where the aspect ratio is determined by the degree of milling. Studies using individual single particles, of whatever shape, can be made using supported particles ignited either by a flame or laser, e.g. see ref 24. The results quoted here are based on single suspended particles of a wood, willow, which are easily suspended in a gas flame.23 In the case of other more fragile woods and straws, special suspension techniques can be used, such as a fine platinum mesh basket. A typical set of video images are shown in Figure 4a where the 0.5 mm × 1.5 mm particle goes through the stages of heating-up, ignition, devolatilisation, char burn-out, and disintegration of the residual ash. In order to study the combustion of larger particle sizes of 5–35 mm, a stationary packed-bed reactor was used. It is a vertical cylindrical combustion chamber with an inner diameter of 200 mm, and a grate is located at the bottom of the chamber with primary air fed from the bottom. The mass loss rate of the burning bed is determined gravimetrically. The fuel was pine wood chips cut into four different sizes: 5 mm × 5 mm × 5 mm, 10 mm × 10 mm × 10 mm, 20 mm × 20 mm × 20 mm, and 35 mm × 35 mm × 35 mm. For each run, 3.8 kg of fuel was used and the initial bed height was around 410 mm. The primary air employed was 0.1 kg m-2 s-1 at room temperature. The mass loss histories for different particle sizes are shown in Figure 5, and it is clear that the larger particles react more slowly.

Modeling Approach Heat and Mass Transfer. The modeling configuration is illustrated in Figure 6. A cylindrical particle of diameter d0 and height h0 is positioned stationary, with the main gas flow passing around it. The direction of the inlet main flow is parallel to the vertical axis of the particle. The computation domain is larger than the particle volume and also in the cylindrical shape of (24) Tidoda, R. J. Combust. Flame 1980, 38, 335–337.



Figure 4. Experimental measurement and simulation of a burning suspended biomass particle. (a) Experimental video images. (b) Simulated solid temperature. (c) Reaction front propagations: moisture evaporation, devolatilisation, and char burnout. (d) Simulated gas temperature. (e) Simulated tar concentration. (f) Simulated CO concentration.


Yang et al.

7. The particle structure is isotropic, and the particle volume shrinks as the conversion processes proceed. The direction of the shrinkage is the same as the direction of the propagation of a mass-loss front inside the particle. 8. The solid species components are moisture, volatile matter, fixed carbon, and ash. The components are immobilized inside the particle structure, while Fick’s law of diffusion is valid for gaseous species transport. The transport equations can be written,25 for the gas phase as following: Continuity Figure 5. Mass loss history obtained from packed-bed experiments for particle sizes from 5 to 35 mm: fuel pine wood; air velocity 0.1 kg m-2 s-1.

∂ (φFg) ∂ (φFgux) ∂ (rφFgur) + + ) r˙M + r˙V + r˙C ∂t ∂x r∂r x-Momentum

(1)

∂ (φpg) µ ∂ (φFgux) ∂ (φFguxux) ∂ (rφFgurux) + + )- ux + ∂t ∂x r∂r ∂x K ∂ux ∂ur ∂ux ∂ ∂ + 2 µ + rµ (2) ∂x ∂x r∂r ∂r ∂x y-Momentum

( )

((

))

∂ (φpg) µ ∂ (φFgur) ∂ (φFguxur) ∂ (rφFgurur) + + )- ur + ∂t ∂x r∂r ∂r K ur ∂ur ∂ux ∂ur ∂ ∂ + rµ + µ - 2µ 2 (3) 2 r∂r ∂r ∂x ∂r ∂x r

( ( )) ( (

))

Gaseous species ∂ (φFgYi) ∂ (φFguxYi) ∂ (rφFgurYi) + + ) ∂t ∂x r∂r j)N ∂ (φFgYi) ∂ ∂ (φFgYi) ∂ D/g rD/g + r˙ij (4) ∂x ∂x r∂r ∂r j)1

(

) (

)

∑

Energy Figure 6. Modelling configuration.

diameter D and height H. The configuration is designed in such a way that 2D cylindrical coordinates can be employed. The properties of the inlet main flow include temperature Tgi, uniform velocity Ui, and species concentrations, such as O2, CO2, N2 etc. The orientation of the particle may be not necessarily parallel to the main flow direction in an actual reactor, but such simplification is necessary at this stage of the study for ease of calculations. For a similar reason, the computation domain size is intentionally set significantly larger than the particle size to reduce the effect of the particle existence to the far-field flow. The following assumptions are made: 1. The main flow passing the particle is laminar. 2. The energy exchange and the exchange of mass species to and from the particle exist through the boundary layer. 3. Radiation heat exchange exists between the particle external surface and surrounding environment. 4. Initially, the particle has closed-off pores, but the pores become open and connected after a mass-loss front (moisture evaporation or devolatilisation) propagates past the local pores. 5. Gas flow inside the particle is similar to the gas flow through a porous medium, and the general transport equations for continuous medium can be applied inside the particle. 6. Gaseous and solid phases have the same temperature inside the particle.

∂Hg ∂ (φFguxHg) ∂ (rφFgurHg) + + ) ∂t ∂x r∂r j)N ∂Tg ∂Tg ∂ ∂ keffφFgCpg + rkeffφFgCpg + r˙j∆Hj + ∂x ∂x r∂r ∂r j)1

φFg

(

)

(

)

∑

(1 - φ)hsgSa(Ts - Tg) (5) For permeability K in the second term on the right-hand side of eqs 2 and 3, Darcy’s law is employed26 where K)

lpore2φ3 150(1 - φ)2

(6)

Here, lpore is taken as the local pore size inside the particle. The solid-to-gas heat transfer surface area Sa in eq 5 is assigned an artificially very high value to get an equal temperature between the solid phase and gas phase inside the particle. The above equations are applied to regions both inside the particle where 0 e φ < 1 as well as outside the particle where φ ) 1. For the solid phase, the mass loss rate is described by d((1 - φ)FsVp) ) -Vp(r˙M + r˙V + r˙C) dt

(7)

where Vp is the particle volume. For solid components of moisture, volatile matter, and fixed carbon, the conservation equations are (25) Yang, Y. B.; Sharifi, V. N.; Swithenbank, J. Trans. Inst. Chem. Eng. Part B 2005, 83, 549–558. (26) Vafai, K.; Sozen, M. J. Heat Transfer 1990, 112, 690–699.


Energy & Fuels, Vol. 22, No. 1, 2008 311 s ) (1 - φ)(XMkM + XVkV + XCkC + XAkA) + 13.5σTs3lpore/ω keff (15)

The last term on the right-hand side represents radiation contribution as given in the work of Chan et al.27 The initial and boundary conditions for the above equations are At t ) 0,

Ts ) 300 K,

XM ) M,

XV ) VM, XC ) FC, XA ) Ash

∂Tg ∂Ts ∂Yi ∂ux ) ) ) 0, ∂r ∂r ∂r ∂r ∂Tg ∂Yi ∂ux At r ) 1/2D, ) ) ) 0, ∂r ∂r ∂r

At r ) 0, Figure 7. Particle volume shrinkage during conversion.

d((1 - φ)FsXj) ) -r˙j dt

(8)

where j ) M, V, or C representing solid moisture, volatile matter, and fixed carbon, respectively. For solid-phase energy conservation, (1 - φ)Fs

(

)

(

where Qext represents radiation absorption from environment and heat conduction from the gas phase through the boundary layer at the particle external surface. Qext ) 0 elsewhere. Local porosity variation is related to moisture evaporation, volatile release, and fixed-carbon combustion processes by a linear relationship: φ ) φ0 + (1 - φ0)(RM(XM0 - XM) + RV(XV0 - XV) + RC(XC0 - XC)) (10) The three parameters (RM, RV, RC) represent the extent of particle shrinkage during each of the three solid-phase processes. A value of 0 means no change in the internal porosity and the particle volume shrinks proportionally to mass loss, while a value of 1 means an increasing particle internal porosity and that no shrinkage takes place. From (eq 7), the actual particle volume variation during conversion can be deduced as 1 dφ 1 1 dVp ) (r˙ + r˙V + r˙C) Vp dt 1 - φ dt (1 - φ)Fs M

(11)

The direction of the particle volume shrinkage is the same as the propagation direction of a mass-loss front, as illustrated in Figure 7. The volume shrinkage is converted to the dimensional shrinkage in the x and r directions, respectively, in the related cell as (12)

where ∆L is the cell dimension and n the direction factor. The index k ) x or r represents either one of the two coordinates. The gaseous species diffusion coefficient in the particle D/g is calculated as D/g ) Dgφ

(13)

and the effective thermal heat transfer coefficient for the gaseous phase, keff ) kgφ For the solid-phase,

Tg ) Tg0,

(14)

ur ) 0,

At the particle external surface, Qext )

)

∂Hs ∂ s ∂Ts ∂Ts ∂ ) k + rks + r˙M∆HM + ∂t ∂x eff ∂x r ∂ r eff ∂r r˙V∆HV + r˙C∆HC + (1 - φ)hsgSa(Tg - Ts) + Qext (9)

1 dVp 1 d∆Lk ) n ∆Lk dt Vp dt k

At x ) 0 (main flow inlet),

ur ) 0 ur ) 0 ux ) U, Yi ) Yi0

Sext ∂T + k Vext eff ∂n

(

)

ωσ(Tenv4 - Ts4) (16) Moisture Evaporation. It is assumed that the progress of drying is limited by the transport of heat inside the particle, and the moisture evaporation rate is approximated by:28 r˙M )

{

fM

(Ts - Tevap)FMCpm ∆HMδt 0

if Ts g Tevap

(17)

if Ts < Tevap

where fM ) 1. To overcome potential numerical instability during calculations, the above equation is modified with fM ) XM where M is the initial moisture content in the biomass fuel. Devolatilisation. The general form of the devolatilisation equation is biomass f volatiles + char and it is assumed that the rate can be represented by an apparent first order Arrhenius equation. The assumptions in this representation are first that the particle is thermally thin so that reaction temperatures are uniform throughout and second that there is no secondary reaction leading to the deposition of cracked products. This has implications in the correct interpretation of the experimental results used to determine the kinetic constants. If the reaction takes place in a situation outside the thermally thin regime, the kinetic data derived is a composite of the fundamental kinetic data, the effect of tar cracking, and the consequences of inadequate heat transfer. Consequently, fundamental kinetic data is only applicable to thermally thin situations, i.e. small particles. For larger particles, the reaction is considered to propagate by a thermal wave and a correction to the pyrolysis kinetic parameters will be needed to account for the secondary reactions depending on the time scale and the thermal thickness. The fundamental first-order rate constants for the first stage of willow devolatilisation is29 A ) 2.2 × 1013 s-1 and E ) 170 kJ kg-1. However, the combustion processes studied here include secondary decomposition reactions. Thus, it is appropriate to use data from TGA experiments made over the whole of the decomposition regime, and thus, we used values (27) Chan, W. R.; Kelbon, M.; Krieger, B. B. Fuel 1985, 64, 1505– 1513. (28) Peters, B. Thermal conVersion of solid fuels; WIT Press: Southampton, Boston, 2003. (29) Unpublished work in conjunction with Advanced Fuel Research, Hartford, CT.


Yang et al. Table 1. Model Parameters and Fuel Properties Fuel: Willow Chips

fixed carbon %

moisture %

ash %

volatile matter %

C%

H%

O%

N%

S%

LCV MJ/kg

6

6

74

46.2

5.3

36.2

0.22

N/A

17.05

14 RM

RV

RC

φ0

Fs

lpore

KM

KV

KC

KA

0.6

0.6

0.8

0.5

820 kg/m3

5 × 10-5 m

0.653 W/mK

0.2 W/mK

0.15 W/mK

0.1 W/mK

of A ) 1.63 × 105 s-1 and E ) 90. 4 kJ mol-1 in the model which are consistent with those obtained by Heikkinen30 and with values for willow coppice previously obtained by us.22 Char Combustion. Char combustion has been outlined by others2,31 There are still some remaining issues, and in particular, there are uncertainties in the reaction order with respect to the oxygen concentration; here, we adopted a reaction order of one which has been used by many groups, e.g. see ref 32. In addition, char combustion is complicated by the porous nature of the char especially for particles with nonspherical geometry. The model presented here does not have explicit inclusion of the diffusion of O2 from the main stream to the particle. Diffusion is implicitly included because the structure of the boundary layer is calculated, and so, there is no need to include the traditional term for the diffusion rate constant in the charburnout rate expression. Only the chemical kinetic rate constant is needed which is based32 on the internal surface area and the intrinsic pre-exponential constant and activation energy where kr ) ArT exp(-Er/RT), m s-1. We used values consistent with willow coppice22 where the internal surface area (N2 BET) ) 6.3 m2/g and values of Ar )10.3 m/s and Er ) 74.9 kJ mol-1. These values will vary with the potassium content of the biomass, but the model presented here is only weakly dependent on this value. Combustion of Volatile Gases. A representative species, CmHnOl, was assumed for the devolatilisation products. It is oxidized to produce CO and H2: CmHnOl + (m/2 - l/2)O2 f mCO + n/2H2

(R2)

CO is then burned to form CO2 CO + 0.5O2 f CO2

(R3)

H2 + 0.5O2 ) H2O

(R4)

and H2 forms H2O

The rate for CmHnOl oxidation, RCmHnOl, is assumed to be the same value as for33 CmHn RCmHnOl ) 59.8TgP0.3 exp(-12200/Tg)CCmHnOl0.5CO2

RH2 ) 3.9 × 1017 exp(-20500/Tg)RH20.85CO21.42CCmHnOl-0.56 (20) where the original CC2H4 was replaced with CCmHnOl, and this rate was also used for tar combustion. Evaporation of Metals. A number of investigations have been undertaken about the mechanism of metal release from a biomass particle during the devolatilisation and the char burning stages, since these can be catalyzed by the presence of potassium. This catalytic effect is particularly marked in char combustion where the rate of char combustion is 100 times different between washed and unwashed willow at temperatures of 400 °C although less at high temperature char combustion where a greater fraction of the potassium is lost during devolatilisation. The effect obviously varies enormously from biomass to biomass but becomes less important in large particles, as in fixed or fluidized-bed combustion where the rate is determined by transport processes and not by the chemical reactivity. The sample studied in ref 23 is similar to that shown in Figure 4, and the model used measured devolatilisation and char burning rates (i.e., incorporating the impact of inherent catalytic metals) but did not account for the changing char burning rate as the metal evaporates from the particle. Tar Cracking Inside the Particle. Tar is a major product of the devolatilization process. Secondary cracking of tar occurs inside the particle, and a major issue is whether this tar evaporates or remains as a carbonaceous product in the biomass particle, the latter route influencing the kinetics obtained by TGA experiments. The kinetic rate also depends on the mineral content in the original fuel.36 Wood has a strong ability to crack the tar into lighter molecules at elevated temperatures. Available data are rare though in this aspect and in this paper the reaction kinetics of Liden et al.37 as quoted by Yang et al.36 were adopted. The kinetic rate constant for tar-char is ktar-char ) 1.0 × 106 exp(-12870/Tg) and ktar-gas ) 4.28 × 106 exp(-12870/Tg). Other model parameters and fuel properties are listed in Table 1.

(18)

and the rate for CO oxidation, Rco, is given by the following:34

Results of Mathematical Modeling

RCO ) 1.3 × 1011 exp(-62700/Tg)CCOCH2O0.5CO20.5

Figure 4 shows a comparison between the video images of the burning of a biomass particle suspended in a methane flame against computed results. The simulation depicts the whole combustion process, from initial heating up of the particle to final char burnout, with continuous shrinking of the particle volume and change of the shape. It shows that the particle first heats up at the bottom edge where the boundary layer is the thinnest. A thin moisture-evaporation front is formed, propagating inward as well as upward. Shortly after, the devolatilisation process begins, also starting from the bottom of the particle. The released volatile gases ignite and form a flame envelope

(19)

The kinetic rate for H2, RH2, was adopted from ref 35 as (30) Heikkinen, J. M.; Hordijk, J. C.; Jong, W. de; Spliethoff, H. J. Anal. Appl. Pyrolysis 2004, 71, 883–900. (31) Hurt, R. H. Proc. Combust. Inst. 1998, 27, 2887–2904. (32) Hobbs, M. L.; Radulovic, P. T.; Smoot, L. D. AIChE J. 1992, 38, 681–702. (33) Siminski, V. J.; Wright, F. J.; Edelman, R. B.; Economos, C.; Fortune, O. F. Research on methods of improVing the combustion characteristics of liquid hydrocarbon fuels; AFAPL TR 72-74, Air Force Aeropropulsion Laboratory: Wright Patterson Air Force Base, OH, 1972; Vols. I and II. (34) Howard, J. B.; William, G. C.; Fine, D. H. Proc. Combust. Inst. 1973, 14, 975–986. (35) Hautman, A. N.; Dryer, F. L.; Schlug, K. P.; Glassman, I. Combust. Sci. Technol. 1981, 25, 219–235.

(36) Yang, Y. B.; Phan, N. A.; Ryu, C.; Sharifi, V. N.; Swithenbank, J. Fuel 2007, 86, 169–180. (37) Liden, A. G.; Berruti, F.; Scott, D. S. Chem. Eng. Commun. 1988, 65, 207–221.


surrounding the particle. As the combustion proceeds, the particle volume shrinks, and the devolatilisation is also initiated at the top of the particle as the local solid temperature exceeds the threshold of around 200 °C. The devolatilisation zone is much thicker than the evaporation, presumably because of the high volatile content and relatively low moisture content. The char starts to burn only when the devolatilsation approaches the final stage, and a circular char-burnout front is formed, which travels inward and leaves an ash “shell” behind. The figure also demonstrates the evolution of the tar and CO concentrations both inside and outside the particle. Figure 8 shows the effect of particle size in terms of mass loss history, maximum and minimum solid temperatures, and individual process rates. For the 0.5 mm × 1.5 mm particle (Figure 8a), there is a steady mass loss and particle temperature increase until t ) 2.5 s when the released volatile gases are ignited and a jump in particle temperature occurs. Because of the jump in temperature, the volatile release and char burning rates also increase. However, the moisture evaporation completes much earlier, in less than 1 s from the start of the process. Char burnout is the slowest stage and takes more than 5 s. Obviously, ignition of the released volatile gases greatly accelerates the combustion processes. For the particle of 2 mm × 6 mm (Figure 8b), ignition of the released volatile gases occurs after the same time interval as the 0.5 mm × 1.5 mm particle, at t ) 2.5 s, although the particle heating rate is slower. The mass loss curve is much more like an ordinary TGA pyrolysis curve. However, there are overlaps of the three individual processes, and the maximum solid temperature during the whole combustion process decreases from 1560 K for the 0.5 mm × 1.5 mm particle to 1500 K for the 2.0 mm × 6.0 mm particle. As the particle size further increases to 6 mm × 18 mm, ignition of the volatile gases takes place much later, at t ) 15 s when a thin layer of flame from gas-phase volatile combustion is observed in the model simulation (not shown). The subsequent mass loss is virtually linear vs time, until t ) 40 s when devolatilisation is complete. The moisture evaporation overlaps completely with the volatile release stage and even partially with the char burnout stage. The maximum solid temperature decreases further to 1420 K. For the largest particle simulated (20 mm × 20 mm), gasphase ignition of the released volatiles takes place at t ) 70 s, more than 1 min after the particle is placed in the hot flue-gas stream from methane combustion. After an initial jump in the devolatilisation rate, the particle combustion processes slow down gradually. This is quite different from the behavior observed in the simulation of the combustion of smaller particles and is due to the slower internal heat transfer process which inhibits the release of volatile matter from inside the particle. For the char reaction spike during devolatilization, the spike occurs as the devolatilization approaches its end, and part of the oxygen can penetrate the particle surface. At the same time, ignition of the released volatile matter occurs outside the particle surface, which raises the particle temperature near the surface significantly. The result is a spike of char rate, indicating the start of the char burnout process. Figure 9 shows the effect of particle size and particle-gasflow relative velocity on the characteristic combustion time. The latter is defined as the time when 90% of the combustibles (including volatiles and char) are burnt. The particle with 0.5 mm diameter (0.5 mm × 1.5 mm) has a characteristic combustion time of 3–4 s, and this time decreases slightly as the gas velocity increases. For a 2 mm diameter particle (2 mm × 6


mm), the combustion time increases to 10–14 s, which further increases to 40–80 s for the 6 mm diameter particle (6 mm × 18 mm). For the largest particle simulated (20 mm × 20 mm), the characteristic combustion time rises significantly to between 220 s at Ui ) 2.0 m s-1 to 360 s at Ui ) 0.8 m s-1. Figure 10 shows the effect of particle size on the temperature gradient inside the particle during combustion. It is surprising to see that, even with the smallest particle simulated (0.5 mm × 1.5 mm), there is still temperature difference up to a maximum of 600 K inside the particle. For the other three larger particles, the maximum temperature difference is over 1000 K. This large temperature gradient occurs during the stage where the three individual processes overlap: char is burning at the bottom section of the particle, producing the highest surface temperature; devolatilization is occurring in the middle and upper sections of the particle, where the temperature is mainly in the range 573-1073 K; and in the innermost part of the particle, residual moisture is still being evaporated and the dominant temperature is around 373-423 K. Figure 11 shows the heating rates of the particles achieved during the combustion process. The maximum heating rate is achieved when the released volatile combustibles begin to burn in the gas phase, thus providing extra heat to the particle to accelerate the whole process. The predicted heating rates for different size particles are plotted and vary from around 5000 K s-1 for the 0.5 mm × 1.5 mm particle to 15 K s-1 for the 20 mm × 20 mm particle. Figure 12 demonstrates the tar cracked inside the biomass particle plotted versus particle size. For the 0.5 mm × 1.5 mm particle, only 0.1% of the released tar is cracked. But as the particle size increases to 20 mm, 11% of the tar is cracked inside the particle.

Discussion It has been recognized for a considerable time that particle size is important in the combustion of solid fuels. Generally, it has been assumed that, at least for coal combustion, pf combustion behaves as if the thermally thin small particle assumption held and that, in fluidized and fixed beds, combustion behaves according to thermally thick particles. With the advent of pf biomass combustion of particles of irregular shape, it is clear that a new approach has to be developed. From the results presented here, the following points are apparent. In the combustion of pulverized biomass, the particles in the upper size range are not thermally thin. In the cases shown in a gas flame of uniform temperature (Figure 1a and b) and for an industrial furnace (Figure 2) with a high flame temperature (1800 K) and an exit temperature of 1500 K, it is clear that the heating-up times are significant in both flame and furnace conditions for particles of that size. Rapid reaction takes place at about 1000 K, giving characteristic reaction times (90% reaction) for the 0.5 mm particle of 0.05 s at 2000 K and 0.18 s at 1500 K. In the case of the 1 mm particle, they are 0.11 and 0.41 s, respectively. This is still the case for the cylindrical particle in Figure 2 even though the reaction conditions are more severe. The reaction time for the equivalent sphere is longer than the cylinder indicating that it is not a good approximation in computational fluid dynamics (CFD) models.


Yang et al.

Figure 8. Effect of particle size in terms of mass loss history, maximum and minimum solid temperatures inside the particle, and process rates of moisture evaporation, devolatilisation, and char burnout. The arrow indicates the ignition point of volatile gases.

Indeed, as shown in Figure 8a, there is a significant temperature difference between the outside and the center for a 0.5 mm × 1.5 mm particle, and so, the uniform temperature assumption as perceived previously is not correct. This is

manifest in the much longer reaction times shown in Figure 8a compared with Figure 2. The Biot number has been used traditionally to assess the temperature uniformity within a particle, and thermally thick particles are defined where the


Figure 9. Effect of particle size and particle-gas-flow relative velocity in terms of characteristic combustion time (defined as the time when 90% of the combustible is burnt, including volatile matter and fixed carbon).


Figure 12. Tar cracking inside the particle vs particle diameter during combustion.

material). This conclusion has considerable implications for CFD modeling of biomass particles in pf furnaces. In the case of the suspended particle (cf. Figure 4), there is a convective flow of gas and this controls the onset of a combustion wave and the orientation of the wave. The model is able to predict these characteristic features. It should be noted however that the prediction is dependent on the choice of the devolatilisation kinetics, which can vary with the type of biomass, and the value chosen has to incorporate the effects of secondary reactions. In all the cases studied here and especially in the case of larger particles, the theoretical model describes the experimental observations for progression of the combustion wave. Figure 10. Effect of particle size in terms of maximum temperature gradient inside the particle during combustion.

Conclusions A mathematical model has been presented that predicts the behavior of a range of particles of different sizes both moving and suspended. In particular in the case of the larger suspended particles, it demonstrates the occurrence of the general features of combustion and the occurrence of a combustion wave. It also demonstrates the release of tar and its combustion. These observations are consistent with experimental observations made using suspended biomass particles. It is concluded that these effects occur in particles which are greater than at 200–250 µm in diameter for spherical biomass particles and 150–200 µm for cylindrical biomass particles depending on the exact heat transfer conditions. This has considerable implications for the CFD modeling of biomass particles in pf furnaces.

Figure 11. Maximum particle heating rate vs particle diameter during combustion.

temperature gradient inside a particle can not be neglected (Bi > 1). Combining with the Nusselt number, we can deduce Bi ) Nu · k(gas)/k(solid), and radiation heat transfer has to be included as it is the major heat transfer mode between a fuel particle and its surroundings in the combustion environment. Calculations suggest that biomass particles of about 250 µm in diameter are the upper limit for a uniform temperature (Bi < 1), and thus, in most fixed-bed situations, the particles are thermally thick (where particle sizes are in the range of 5–50 mm). In a pf furnace, the transition from thin to thick particles takes place for spherical particles 200–250 µm in diameter and for cylindrical particles 150–200 µm depending on the exact heat transfer conditions (i.e., the highest flame temperature experienced by the particle and the actual thermal conductivity of the fuel

Acknowledgment. We wish to acknowledge support from EPSRC under the Supergen Biomass, Biofuels, and Energy Crops Consortium Project GR/S28204.

Nomenclature C ) species concentration, kmol m-3 Cp ) specific heat capacity, J kg-1 K-1 D ) diffusion coefficient, m2 s-1 K ) permeability, darcy (9.86923 × 10-13 m2) Hg ) enthalpy of gas phase, J kg-1 Hs ) enthalpy of solid phase, J kg-1 hsg ) gas-to-solid heat transfer coefficient, W m-2 K-1 lpore ) particle internal pore size, m k ) thermal conductivity, W m-1 K-1 p ) pressure, Pa r ) radial coordinate, m r ) process rate, kg m-3 s-1 t ) time, s

316 Energy & Fuels, Vol. 22, No. 1, 2008 T ) temperature, K Sa ) surface area, m2 Qext ) external radiation heat transfer to the particle surface u ) velocity, m s-1 V ) volume, m-3 x ) axial coordinate, m X ) solid component mass fraction Y ) species mass fraction F ) density, kg m-3 φ ) porosity inside a particle µ ) gas viscosity ∆H ) reaction heat, J kg-1 ω ) emissivity

Yang et al. Subscripts 0 ) initial C ) char burnout evap ) evaporation g ) gas phase M ) moisture evaporation p ) particle r ) radial direction R ) gas-phase reaction rates, kg m-3 s-1 s ) solid phase V ) volatile release x ) axial direction EF700305R

Combustion of a Single Particle of Biomass - Energy & Fuels (ACS

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