Flash pyrolysis of wood in a cyclone reactor

Flash pyrolysis of wood in a cyclone reactor

309 Flash Pyrolysis of Wood in a Cyclone Reactor Pyrolyse Eclair du Bois dam un Rkacteur Cyclone J. LEDE, F. VERZARO, B. ANTOINE and J. VILLERMAUX ...

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309

Flash Pyrolysis of Wood in a Cyclone Reactor Pyrolyse Eclair du Bois dam un Rkacteur Cyclone J. LEDE, F. VERZARO,

B. ANTOINE

and J. VILLERMAUX

Laboratoire des Sciences du Genie Chimique, ChJRS-ENSIC, 1 rue Grandville, 54042 Nancy (France) (Received

December

16, 1985; in final form July 1, 1986)

Abstract This paper reports the first results of an experimental study of the continuous flash pyrolysis of wood sawdust in a cyclone reactor between 893 and 1330 K. The reaction produces low fractions of char (4%) and the gasification yield increases from 0% at about 800 K to 90% at around 1330 K with a constant volume fraction of CO and Hz (-73%) and an increasing fraction of light hydrocarbons (up to 50% mass fraction). The heating value of the gas reaches 19 000 kJ me3 STP for the highest temperatures. The wood particles mainly heated by radiation and solid convection react in less than 1 s while the carrier gas (residence time of the order of 0.05 s) seems to be only weakly heated. The 46.2 X lop6 m3 cyclone reactor can operate with excellent stability for wood flow rates up to 0.35 kg h-r at a W& temperature of 1330 K. The cyclone seems to be very efficient for carrying out reactions of the solid + fluids type but more accurate determination of process parameters such as gas and solid residence times and heat transfer efficiencies are required to gain a better understanding of the behaviour of such a high temperature reactor.

Resume Cet article presente les premiers resultats dune etude experimentale portant sur la pyrolyse eclair de sciure de bois dans un reacteur cyclone entre 893 et 1330 K. La reaction produit de faibles quantites de charbon de bois (4%) et le taux de gazeification augmente de 0% a -800 K a 90% a -1330 K avec une fraction volumique constante de CO et Ha (-73%) et une fraction croissante d’hydrocarbures legers (jusqu’a 50% de fraction massique). La capacite calorifique du gag atteint 19 000 kJ N m-3 pour les plus hautes temperatures. Les particules de bois chauffees principalement par rayonnement et convection solide reagissent en moins d’une seconde alors que le gaz vecteur (temps de passage de l’ordre de 0.05 s) semble ne s’echauffer que faiblement. Le reacteur cyclone d’un volume de 46,2 X 10e6 m3 peut fonctionner dans des conditions d’excellente stabilite avec des debits de bois pouvant atteindre 0,35 kg h-’ pour une temperature de paroi de 1330 K. Le cyclone semble t&s efficace pour mettre en oeuvre des reactions du type solide -+ fluides mais la determination plus rigoureuse des parametres du pro&de comme le temps de sejour du gaz et des particules, ainsi que de l’efficacitt des transferts de chaleur, est ntcessaire si l’on desire mieux comprendre le comportement d’un tel reacteur haute temperature.

Synopse L ‘objet de cet article est de montrer les avantages du cyclone pour mettre en oeuvre d haute temperature des rt!actions rapides (inferieures d quelques secondes) du type solide -f fluides + (sohiies). L’etude experimentale d&rite est effectuee sur kz reaction de pyrolyse eclair de sciure de bol;s. Le cyclone, du type Lapple, d ‘un volume de 42,6 X IO” m3 (Fig. 1) en acier inox est chauffe au foyer du four solaire de 6 kW d’Odeillo, d des temperatures variant entre 893 et 1330 K. La sciure de h&e (granulomdtrie 200-400 m) est transport&e en continu darts le cyclone par un courant de vapeur d’eau. Les residus solides de la r&&on sont s&pares et r&up&es d la base du cyclone, tandis que les volatiles condensables sont pieges darts une batterie

de condenseurs avant la mesure du debit gazeux forme dont la composition est p&is&e par analyse chromatographique. Le pnncipe du montage experimental est sch&natisd sur la Fig. 2. L’article d&it les resultats detaill& dun bilan de matiere (verifie a 0,6% p&s) effectue sur une experience de plus d ‘une heure avec des parois a 1321 K et un debit de bois de 9 X 10e5 kg s-‘. Le bilan conduit aux resultats suivants: gaz set 79,5%; liquides condenses 15,3%;solides s&pares 4,0%; aerosols 1,2%. La composition volumique du gas set est la suivante: Hz 27,9%; CO 42.6%; CH4 10,6%; COa 10,5%; C,H, 6,3%; C,H, 1.1%; &He. 0,8%; C,H, 0,2%. D’une maniere g&&ale, la fraction de soltiies produits et separes (cendres minerales et charbon de bois) est proche de 4% quelles que soient les conditions expeni

310 mentales (Tablemc 1). L ‘analyse du gaz r&v$le (Tableau 2) une proportion vokmique importante de CO et HZ (73% du total) dt$endant peu de Tw, alors que la proportion d’hydrocarbures -lkgers aug&ente av& Tw (jusqu% 50% en masse). Le pouvoir calorifque du gaz peut atteindre 19 000 kJ N rn-’ alors que sa densite’ est approximativement constante et &gale ri 1. Le taux de gaztification de la r&action augmente avec T, pour tendre vers 1 pour Tw s 1500 K (Fig. 4). L’expdrience rrwntre d’autre part que la r&&ion ne semble se manifester de mani&e sensible que pour des valeurs de Tw sup&ieures approximativement d 750-800 K (Fig. 4). Cette observation est en accord avec d inctres travaux /IO] r&tYant que In tempdrature de la surface de pi&es de bois r&gissant dam des conditions de pyrolyse &lair en regime d’ablation est approximativement constante et @gale d 739 K quelles que soient les conditions extemes (‘modt’le de fision’). En d&pit d’un volume relativement modeste (42.6 X 1O-6 m3) le cyclone utilist! pemtet de convertir des chalges de bois rekztivement importantes: jusqu’ri 0,3S kg h-l d 1321 K. Au de& de cette charge un phdnomt?ne de bouchage est observe (Fig. 5). Celui-ci peut cependant dtre tlimine en interrompant momentaniment l’alimentation en bois tout en maintenant celle en vapeur d’eau, sugg&ant une possible marche alter&e du r&acteur. Une &ude de l’efficacitd du transfer-t de chaleur gaz vecteur-paroi en [‘absence de particules montre que, dans nos conditions, le gaz vecteur n ‘est que modestement chuff& lors de son passage dans le rkacteur. Cette efficacitk de chauffage &ant encore minimisge par l’effet d’icran dti d la prt?sence de particules /3/ il y a tout lieu de penser que les &changes convectifs gaz vecteur-particules de bois sont ndgligeables devant les &changes radiatifs et conductifs avec les parois. Il s;zgit Id d’ailleuls d’un nouvel avantage potentiel du rkacteur cyclone de permettre ri la fois une d&omposition rapide de particules solides et un refroidissement rapide des jluides form&. Un calcul estimatif montre que la den&P de jlux de chaleur transf&e aux particules dans ces conditions serait sup&ieure ci IO5 W rne2. Pour une exp&ience typique don&e, le temps de passage du gaz dans le r&cteur a pu &tre estimd d 0,045 s et le temps de riaction nwyen d’une particule ti 0,53 s. Ce dernier, inf&eur au temps de @our moyen theorique des particules /I,4 s) est en accord avec le fait que la d&composition du bois est complt?te en sortie de r&acteur. Une &de comparative approchee des temps caracte’ristiques de r&action chimique du bois et de transfert de chaleur dans la particule montre que la rgaction de ddcomposition s’effectue selon un r&ime intermkdiaire entre un r&me d’ablation et un r&ime chimique. La plupart des calculs dheloppe’s dam cet article ne sont qu ‘approximatifs e’tant donnt le manque d Ynformations disponibles dans la littdrature sur la distribution des temps de s.@jourdu gaz et des particules dam un cyclone d haute tempkrature et sur l’ef~cacit~ des transferts de chaleur paroi-gaz-particules. De telles e’tudes likes ri une exp&imentation d diverses khelles sont fortement recdmmandCes &ant don& les qualit& potentielles d’un tel reacteur pour la mise en oeuvre de

r&actions (solides).

rapides

(
s) du

type

solide -+ fhtides +

1. Introduction Gas-solid reactors are usually divided into three classes according to the size of the particles and their characteristic reaction times [ 1,2] : (1) fixed beds, rotary kilns, moving grates, multiple hearths, . . . , adapted to large particles (1 0V3 to 3 X 10-l m) and slow reactions (reaction times from lo3 to IO5 s); (2) fluidized bed, spouted bed and free falling bed reactors for medium sized particles (10’ to lop2 m) with reaction times ranging from IO2 to lo4 s; (3) pneumatic transport and vortex reactors for small particles (1O-6 to lo-’ m) and short reaction times (5 x 10-S to 10 s). Reactors of the first two types are used in most industrial applications and, consequently, many studies have been published in that field. Reactions having short characteristic times are more unusual and the corresponding reactors are less well known. This is the case for cyclone reactors, the most representative of vortex devices, which are extensively used for cleaning purposes and for which very few predictive correlations have been proposed to represent their performance as chemical reactors. The first studies by Szekelly and Carr [3] have shown that cyclones seem to ensure excellent heat transfer efficiencies between carrier gas, solid particles and heated walls. Moreover, residence times in cyclones are generally of the order of or lower than a few seconds. Consequently, it can be anticipated that cyclones would be well suited for carrying out fast reactions of solid particles requiring high heat fluxes. Moreover, it could be expected that, in the same time, solid byproducts and unreacted particles would be automatically separated from gaseous products. Among the possible processes known for the thermal upgrading of biomass (combustion, gasification with air and 02, slow and flash pyrolysis), flash pyrolysis appears to be very attractive because of the large fractions of fluids produced and the highly minimized char fraction, compared with the usual case of slow pyrolysis [4]. One of the required conditions to achieve flash pyrolysis is to heat up biomass under high heat fluxes. It thus seems that the cyclone is well adapted to such a reaction, five main functions being fulfilled in the same vessel: fast heating of the particles, chemical decomposition, efficient friction of the particles against the walls which eliminates the products, further reactions of the primary products (at the walls or in the gas phase), and cleaning of the evolved gas (char and mineral ashes being automatically separated at the bottom of the reactor). The aim of the present paper is to study the behaviour of the cyclone as a chemical reactor for the flash pyrolysis of wood sawdust and to estimate some of its performances, 2. Experimental

apparatus and procedures

The use of high temperature heat provided by a solar concentrator has often been recommended for carrying

311

out chemical reactions [S, 61, especially flash pyrolysis reactions [7,8]. In this study the reaction was carried out in a cyclone of the conventional Lappel type (diameterD=2.8 X10-2m;insidesurfaceS=8.06X10-3m2; volume V= 46.2 X 10V6 m3) built with NS 30 type refractory stainless steel (Fig. 1). The cyclone was placed at the focus of a 6 kW vertical solar concentrator at the Odeillo CNRS laboratories [9]. The experimental set-up (Fig. 2) was such that solids, liquids and gas fractions of the products could be recovered, weighed and/or analysed in order to establish the mass balances of the process. Sawdust was stored in a vigorously and uniformly stirred hopper (maximum capacity 3 kg) connected to a screw feeder of 188 X 1O-6 m3 which could deliver a constant and known flow rate of beech sawdust (200400 m granulometry , 11.8%humidity) up to a maximum value of 6 kg h-l. Particles were then carried away by a small secondary flow of helium and afterwards by a flow of steam delivered by a generator (a stainless steel cylinder settled inside an electric furnace and fed by a peristaltic pump). Only demineralized water was used. The mixture of steam, sawdust and helium entered

Sizes in mm Wm)

Fig. 1. Dimensions (in mm) of the stainless steel cyclone reactor used for solar flash pyrolysis of wood sawdust with location of thermocouples for gas inlet temperature Te and wall temperature measurements TW = (Twl+ Tw2 + Tws)/3.

n SOLAR

\

FURNACE

/

\

Sawdust hoPF

Motor

U-

/ \

Concentrated f

1

Sthring

axis

Sapw

feeder r

SOLID RESIDUES

Fig. 2. Experimental set-up.

IJ

Pressure

312 tangentially into the cyclone reactor which was fitted inside an alumina cavity placed at the focus of the solar concentrator. The radiant flux entering the cavity could be adjusted to obtain a desired reactor wall temperature by modulating the aperture of a diaphragm located between the focus and the parabolic miror. A thermocouple measured the temperature of the reactants at the inlet of the reactor while three others, welded on the walls, measured wall temperature. The location of these thermocouples is shown in Fig. 1. The wall temperature Tw was chosen as the arithmetic mean of the indications of these three thermocouples. The accuracy was better than +-SO K. No accurate measurement of the gas temperature at the reactor exit was possible because of the perturbing influence of incident concentrated radiation. Solid byproducts were automatically separated and recovered in an easily removable hopper at the bottom of the cyclone before being weighed. A mixture of gas, vapours and aerosols free from dust escaped from the top of the cyclone and flowed through a series of condensers. Water and condensable products were recovered at the bottom of each condenser and weighed. Aerosols were then trapped by passage of the gases through a bed of cotton wool, whose weight increase was measured for a given duration of the experiment. The purified gases then flowed through a glass flask from which gas samples were taken for gas chromatog raphy. The analysis was performed in two stages: (1) by catharometer detection-separation at room temperature of Hz, CO, CO2 and C& in a stainless steel column packed with Porapack Q and at 333 K of Hz and He in a stainless steel column packed with active carbon; (2) by flame ionization detector-separation of CH4, &Hz, C,H, and C,H, at 453 K in a stainless steel column packed with 13 X molecular sieve. At the end of the chain, the instantaneous flow rate and the total volume of gas evolved by the reaction were measured with a volumetric flow meter. The total mass of gas evolved was calculated from the volume evolved and from the composition obtained by chromatographic analysis. The total quantity of sawdust pyrolysed during a given time was calculated from the calibration curve of the screw feeder and from measurement of the weight of the hopper before and after the test. The flow rate of the steam was obtained from calibration curves of the peristaltic pump, while the helium flow rate was measured by the volumetric flow meter in the absence of reaction before and after the test.

3. Results and discussion 3.1. Mass balance Experimental conditions in a typical experiment. Room temperature: 291 K; room pressure: 8.38 X lo4 Pa; inlet temperature of reactants: 454 K; wall temperature: 1321 K; duration of passage of steam in the hot cyclone: 5900 s; duration of passage of wood in the hot cyclone: 4304 s; water flow rate from the steam generator: 2.23 X 10e4 kg s-l; helium flow rate: 9.3 X

10F5 m3 s-l STP; total quantity of wood pyrolysed: 0.387 kg; wood flow rate: 9 X 107’ kg s-‘; dry wood flow rate: 8 X 1O-5 kg s-l. Properties of the evolved gas. Dry gas composition (vol.%): Hz (27.9), CO (42.6), CI& (10.6), COz (10.5), C2H, (6.3), &Hz (;.l), C,I-r, (0.8), CsH, (0.2); gas density: 0.946 kg mm3 STP; total volume of pyrolysis gas evolved: 295.43 m3 STP. Overall mass balance. Reactants: water, 1.318 kg; wood, 0.387 kg; total, 1.705 kg. Products: solids separated at the bottom of the reactor, 1.39 X lo-’ kg; aerosols trapped, 4.3 X 10e3 kg; pyrolysis gases, 0.279 kg; liquids condensed, 1.409 kg; water escaped (steam from condensers at 283 K), 8.7 X 10m3 kg; total, 1.715 kg. It is concluded that the mass balance is verified within an error of only 0.6%. Mass percentage of each phase evolved by rhe reaction. Beside the aerosols, the quantity of liquids evolved by the reaction was: liquids condensed (1.409 kg) + steam escaped (8.7 X 10e3 kg) - water delivered by the steam generator (1.3 18 kg) - water contained as humidity in thewood(46X10~3kg)=53.8X10-3kg.Itcanbeseen that the total amount of products evolved by the reaction (0.351 kg) is close to the mass of dry wood pyrolysed (0.342 kg). Related to the total amount of products, the mass percentage of each phase evolved by the reaction was: dry gas, 79.5%; liquids condensed, 15.3%; solids separated, 4.0%; aerosols, 1.2Yc. Distillation under reduced pressure and at 308 K of the liquids trapped in the condensers yielded 17.2 X lop3 kg of viscous and strongly coloured liquids (‘heavy tars’, 32%). The difference between the total quantity of liquids and these heavy tars corresponds to volatiles whose nature has not yet been fully elucidated. 3.2. Char fraction Table 1 shows that the mass fraction of the solids seems to be independent of temperature with a mean value of 4.1%. This low fraction of char compares very favourably with the high quantities reported in the usual method of pyrolysis (more than 30%). These observations make it easy to deduce the mass fraction of tars formed if the quantity of gases evolved is known. TABLE

1. Yield of solid as a function

2-w (K) Char fraction

0%)

of wall

temperature TW

1065

1106

1321

1327

2.84

6.00

4.00

3.55

3.3. Gas composition and gasification yield Table 2 shows that H2 and CO concentrations are independent of Tw (their total fraction represents about 73% of the gas mixture) while the CzH4 fraction increases and CH, remains constant. For the highest temperatures tested, the mass fraction of hydrocarbons reaches about 20% of the pyrolysis gas. The gas heating value was calculated from the gas composition and the combustion cnthalpy of each component at 298 K (Hz: -285.84 kJ mol-‘; CO: -289.99 kJ mol.-‘; CEb:

313 TABLE 2. Experimental results of pyrolysis gas composition (vol.%) and gasification yield (Qsteam= 2.23 X 10m4 kg s-r; Qdrv wood = 3.5 x lo-s-lo-4kg s-1)

CzH2

(%I

-

1065 28.2 38.4 19.3 8.3 1.3 4.0 0.5

C3H6

(96)

_

-

_

_

_

0.955 13829

1.016 15444

1.057 13615

1.025 15750

0.914 16573

893

Tw 00 Hz

30.7 46.1 14.0 7.3

(%I

co (%) co2

(%I

CD4 (%) C2H6

(%I

-

CZH4 (%)

1.9

Density (kg me3 STP) Gas heating value (kJ me3 STP) Gasification yield X

0.17

0.36

1106 21.4 39.0 22.8 7.1 1.2 2.5 -

1163 20.3 58.8 8.4 9.2 3.3 -

1163 22.3 55.4 7.0 10.4 3.9

0.42

0.65

1125 16.7 57.8 9.9 9.2 0.9 4.8 0.6 0.1 1.079 17181

0.62

0.66

1278 23.0 48.0 10.0 11.0 1.1 6.6 0.9 0.2 1.007 19000

1321 21.9 42.6 10.5 10.6 0.8 6.3 1.1 0.2 0.945 18525

0.83

1327 29.9 40.8 10.8 10.4 0.6 6.0 1.2 0.3 0.926 18298

0.80

0.93

0.15

-890.35 kJ mol-‘; C2H6: -1559.88 kJ mol-‘; &I&: -1410.97 kJ mol-r; C2H2: -1299.63 kJ mol-‘; &I-&: -2056.51 kJ mol-r). The heating value increases with temperature up to 19 000 kJ me3 STP as a consequence of the increasing fraction of hydrocarbons, 50% of the total heating value being due to hydrocarbons for the highest Tw These values are much better than those reported for conventional air or O2 gasification processes. Figure 3 shows that, under given experimental conditions, the gasification yield is independent of the amount of dry wood pyrolysed per unit time. It is then possible to study the variations of the gasification yield as a function of temperature without taking into account the wood flow rate. Table 2 shows that the density of the pyrolysis gas is roughly constant within the range of conditions studied, with a mean value close to unity. Consequently, the STP volume of gas evolved referred to the amount of dry wood pyrolysed is, to a first approximation, equal to the gasification yield weight of dry gas x= weight of dry wood pyrolysed Figure

4 shows

that X is an increasing

function

of

Tw. Complete gasification (X+ 1) can be expected for Tw greater than about 1500 K, showing that steam has probably only a minor chemical effect on the reaction carried out under these conditions. Figure 4 shows also

a5

' 015

oicl 025

c!.m 035

a40

a45

DRY WC00 FLW RATE , (kgh-') 0.50 cl55 060

Fig. 3. Variation of the ratio pyrolysis gas flow rate/dry wood flow rate as a function of dry wood flow rate. TW = 1334 K and Qsteam = 2.28 x 1O-4 kg s-l.

X

t

500

,a/, , , . ,

T,, (K)

0 602

7ol

ml

900

lcm

1100

1200

1x0

1400

Fig. 4. Gasification yield X as a function of wall temperature T&.

that the reaction seems to occur only for wall temperatures greater than approximately 750-800 K. This result is in good agreement with experiments on the flash pyrolysis of wood rods in contact with a hot spinning disk [lo], showing that the surface of reacting wood is roughly at a constant temperature (Td = 739 K) whatever the disk temperature (for 773 < Two() < 1173) as would be observed in a melting process. For Tw < 739 K, wood reacts only very slowly, the overall rate being limited by chemical processes. For Tw > 739 K, the overall rate, limited by chemistry and heat transfer resistances, increases with Tw only as the heat flux densities transferred from the walls to the solid increase with Tw. In such a model, the decomposition temperature of particles being roughly constant, the increase of gas fraction with wall temperature would then result from further decomposition of primary products (mainly liquids) at the walls and/or in the gas phase, with an efficiency depending on Tw. These primary liquid products would be efficiently removed from the surface of the reacting solid by abrasion as the particles move rapidly against the inner wall of the cyclone. Such a model based on a ‘fusion’ concept of wood (with an apparently char-free reaction) is in agreement with the strong probability that cellulose and wood would pass through a liquid or plastic state during the fust stages of pyrolysis. Such a concept of a molten

314 ‘activated’ intermediate has been discussed recently and exploited by Diebold [ 111. Considering that the reaction time of a particle in the cyclone is about 0.5 s (see following sections) the corresponding heating rate would be of the order of 1000 K s-l. Modelling of cellulose pyrolysis by Diebold [12] shows that, under these conditions, the reaction would occur between 673 and 773 K with a conversion to ‘active intermediate’ of approximately 89%. These estimations agree with our assumption that the reaction occurs at T, = 739 K with the production of a very low char fraction (4%). 3.4. Maximum capacity of the cyclone reactor For high flow rates of wood, three phenomena are observed simultaneously after a certain time tQ: the wall temperature suddenly rises, the upper parts of the wall become covered with a rapidly thickening layer of black products, and the reaction stops by clogging of the whole reactor. Figure 5 shows that, for a given wall temperature, tp increases as the mass flow rate of wood decreases, to reach an infinite value for a given flow rate under which the cyclone can operate indefinitely Q wzl?&t any apparent damage and any reduction of the yield. For Tw= 1321 K, Q,, is of the order of 0.35 kg h-‘. In the case of clogging, the walls can be cleaned by stopping the flow of sawdust while the steam flow is maintained in order to gasify the solid layer (analysis of the gases then shows very high yields of Hz and CO), after which wood may be fed in again. Although the mass balances reported in the present paper have been established by continuous operation (mass flow rate of wood < Qmax), it is suggested that the reactor could be operated discontinuously with flow rates higher than Q nlax? in which case periods of wood pyrolysis and of wall cleaning by steam would succeed each other alternately. The overall gas composition would probably be slightly different (an increase of the fractions of CO and Hz is to be expected), but the total amount of wood pyrolysed would probably increase. Moreover, a few experiments performed under these conditions have

shown that the mass fraction of char recovered at the bottom was reduced to less than 1%. This clogging phenomenon has not been fully explained but it is likely to occur by agglomeration of sawdust particles covered by tar when their quantity exceeds a certain limit value in the gas phase, the observed increase of wall temperature resulting from insulation by the agglomerated particles deposited between the inner parts of the cyclone and external heat source. 3.5. Study of heat transfer processes inside the cyclone Three processes have to be considered to explain the heating of solid particles: gas convection, solid convection against the wall, and radiation. The overall heat flux density @transferred to the particles is I#J= h,(TG - Td) + (h, + h&T,

h,, h, and hR being the heat transfer coefficients corresponding to the three possible processes. Let us consider the experimental conditions of the complete mass balance experiment described previously. The residence time tG of the carrier gas in the cyclone was of the order of 0.045 s (see next section). The cyclone then works as a plug flow reactor for the gas [ 131. Under these conditions, the heat transfer efficiency between the wall and the carrier gas can be estimated from a relationship between the Nusselt and cyclone inlet Reynolds numbers Nu and Re, [ 141: Nu = 0.0033

Reo”05

&I Fig.

cl39 5. Tie

oil0

wood flow rate

050

of0

required for Cyclone (Tw = 1330 K).

fQ

clogging

as

a

function of

(1)

with

Reo= 4Q,lm& and

Q, = 2.4 X lop4 kg s-l is the mass flow rate of steam and helium; v. = 1.69 X 10u5 kg s-l m-l is the viscosity at inlet temperature TO= 454 K; Z, = 0.112 m is the length of an equivalent cylindrical tube having the same surface area as the cyclone and a diameter equal to the hydraulic diameter dH of the reactor (dH = 4V/S = 2.29 X lo-’ m); do = lop2 m is the diameter of an equivalent circle having the same surface area as the cyclone inlet. For these experimental conditions Re, = 1808 and, from (l), Nu = 8.7. For the steam and helium mixture used in the experiment, cP is close to 2300 J kg s-‘, while the following relationship is a good approximation with which to represent the variations of the thermal conductivity h of the mixture as a function of temperature [ 1.51:

h= 1.48 X 10-4(TWOO0FLOWRATE (kq.h-‘1 w

- Td)

For 773 K. This outlet Tw) is

48.4)

Tw= 1321 K, relations

W m-l

(4)

(3) and (4) lead to T, =

temperature reached by the carrier gas at the of the cyclone (under these conditions of high only slightly higher than the temperature Td =

315 739 K required for wood to decompose under flash pyrolysis [lo]. As the reactor works in the plug flow regime, the gas temperature is lower inside the reactor than at the outlet (between 4.54 and 773 K). Moreover, this temperature is probably still lower in the presence of particles, as Szekelly and Carr [3] have shown that solids reduce the rate of heat transfer from the wall to the gas. It can be seen that for the lowest wall temperatures explored in the present work (893 K), TS is lower than 600 K. These remarks show that speaking of ‘reaction temperature’ (referred, for instance, to exit gas temperature) in such a reactor would be meaningless since the gas temperature increases regularly inside the reactor (plug flow), while solid particles probably begin to react on the hot walls as they enter the cyclone. Moreover, the vapour products evolved by the reaction are very likely to undergo fast cooling as they are liberated in the main stream of the relatively low temperature carrier gas. This remark shows that secondary reactions accounting for the increase of gasification yield with Tw (Fig. 4) are likely to occur mainly by decomposition of liquid primary products at the surface of the hot wall and not in the gas phase. In conclusion, it is likely that particles are not efficiently heated by gas convection but mainly by solid convection and/or radiation. The heat transfer coefficient h between the walls and the particles may be calculated from an estimation of the partial coefficients h, (direct contact heat exchange) and hR (radiation heat exchange). The calculation of h, can be estimated from experiments on the flash pyrolysis of wood rods achieved by contact with a spinning disk heated between 773 and 1173 K [lo]: h, = 0.017 pc

W m-* K-r

where pe is the surfaces, expressed present experiments centrifugal forces particles against the

(6)

Of course, only a rough estimation of Pe can be made since the velocity v of the reacting particles in the hot medium is not known, nor is their contact surface s. In addition, the weight m of each particle diminishes as the reaction proceeds. Under inlet conditions, the velocity v of the particles will be assumed to be the same as the inlet velocity of the carrier gas (El0 m s-i). The mean weight of one sawdust particle (%10-s kg) was estimated by weighing a known number of particles (200 particles: 2 X 10m6 kg; 550 particles: 4.5 X lope’ kg; 100 particles: 1.7 X lop6 kg; 300 particles: 6 X 10F6 kg). D is the diameter of the cyclone (2.8 X lo-’ m). Then 1.21 x lo*

h,

W rnp2 s-l

s

The radiation sed as

heat transfer

= eu(Tw2 + Ta2)(Tw + Td)

hR=272e

W m-’ K-’

As a result, h=

1.21 x 10-6

+ 272e

W me2 K-’

S

(8)

For a rough estimation of h it can be expected that, for a wood particle surrounded with black decomposition products, e = 1. The choice for s is much more difficult. The particles will be assimilated to small cubes (of side 300 m), the contact surface at initial conditions being s = 9 X 10ms m. Then h, = 13 W me2 K-l hR = 272 W rn-’ K-r h = 285 W rn-’ K-’ It appears that radiation would play a major role, but it must be noticed that hn = 272 W me2 K-’ represents a maximum while h, = 13 W rn-’ K-’ ~ represents a minimum, s being probably lower than 9 X 10-s m. This order of magnitude for h is still in quite fair agreement with Szekelly and Carr’s [3] predicted and observed values for non-reactive particles of sand, iron and bronze in a cyclone (mean value 267 W m-’ K-l). Under such conditions, the total heat flux density transferred to the particles +=h(Tw

- Td)

(9)

would be close to 1.7 X lo5 W m-‘, in agreement other independent determinations [ 141.

with

(5)

contact pressure between the two in Pa. Let us suppose that in the the pressure pc results from created by the movement of the wall of the cyclone:

p. = 2mvZIDs

h, =

With the constant of Stephan-Boltzmann’s law, u = 5.76X10-s W me2 Km4, Tw = 1321 K, and a constant surface temperature of particles, T, = 739 K, we obtain

coefficient

hR is expres-

(7)

3.6. Residence time of gas and particles. Controlling step of the reaction The mean residence time tG of the carrier gas can be estimated from the volumetric flow rate of the mixture of steam and helium (Qe= 3.71 X 1O-4 m3 s-l STP) under experimental pressure (p = 8.38 X lo4 Pa) and mean temperature (T = (T, + T&2 = 630 K): t‘j = -

V

Qo

273 p = 7 = 0.045 T 10’

s

The mean residence time t, of wood sawdust particles in cyclones has been measured as a function of tG at room temperature by two independent methods. In the first, the time t, required for one particle to pass through the cyclone is measured by two photocells located at the inlet and outlet of the cyclone [8]. In the second method, t, is calculated from photographic measurements of the vertical component of the velocity of the particles as a function of the vertical abscissa in the cyclone [14]. Both approaches lead to the same result: t, increases when tG decreases. The ratio t&G is much larger than that for low gas residence times. Such results agree with Szekelly and Carr’s hold-up measurements [3], showing that for tG ZE0.05 s t, varies from 0.86 to 1.05 s. Assuming that these estimations are still

316 valid at high temperature whose diameter diminishes canestimate [ 141

and with reacting particles as the reaction proceeds, one

t s -1.4s

(111

Let us write a local heat balance hs(T,

-

T,) = ;

[c,(Td

-

at the particle level:

To) + AH]

(12)

where s is the contact surface as defined before, supposed to be constant, and t, is the time required for total consumption of one particle of mass m. AH, the enthalpy of the reaction, may be neglected compared with cp(Td - To) [lo]. It is then possible to calculate an order of magnitude for tF from the estimated h (285 W mu2 K-r), m (lo-* kg) and c,, (2800 J kg-’ K -‘):

tp = 0.53 s

(13)

This order of magnitude, slightly lower than the expected residence time of particles (ts = 1.4 s), is in agreement with the fact that no unreacted wood particle was found at the bottom of the cyclone. Let us now compare two characteristic times:

t,=

LO2

-

and

(Y

tn=

1 -

(14)

k

tT is a characteristic time of heat penetration into the solid by conduction (here tTs 0.23 s) and t, is a characteristic chemical reaction time, the inverse of a pseudo first-order kinetic constant for wood decomposition. The values of the activation energy and frequency factor proposed in the literature cover very large intervals [ 16, 171. But considering that under our conditions the rate limiting reaction is the primary step to be taken into account, k

wood --+

liquid ‘active wood’

(15)

and as these liquids are probably rapidly eliminated by efficient friction of the particles at the wall, we may select Bradbury’s expression [ 181 for k : SC’

(16)

leading at 739 K to tn=3.9x10-as

(17)

It appears then that tR is less than tT, meaning that the reaction is limited by both chemistry and heat conduction into the solid. As the reaction rate is also limited by external heat transfer, it is important to notice that the estimated time t, cannot be simply and directly related to a pure chemical time for wood decomposition. In a recent publication [ 191, it has been shown that the thermal volatilization of a solid may be represented by two possible limiting regimes: the chemical and the ablation regime. In the first case, there is no limitation by heat transfer resistances but only by chemical kinetics. In the second, where heat transfer and chemical resistances are both rate determining, the reaction takes place in a very thin layer (e, +ZLo) close

to the outer surface of the particle, which shrinks at a constant linear velocity u, (this extreme case corresponds to t, s tR). The correlations derived from this model could be used in the present problem to calculate tF if accurate values of the parameters tR and h were known. Of course, an experimental determination of tF combined with a better knowledge of h would also be a suitable way to determine tR and hence the chemical kinetic parameters. Unfortunately, none of these parameters are known with sufficient accuracy and these theoretical predictions would lead only to simple orders of magnitude at the present time. It is, however, possible to make a rough and simple estimation of the nature of the controlling step. Under the assumption of a pure ablation regime, the time for total consumption of one particle is related to the constant linear shrinking ablation velocity u, by the expression tF

=-bt%

With L,, = 150 pm and tF = 0.53 s, the velocity would be ug = 2.8 X 10H4 m s’-‘. As, to a first approximation [lo],

e,~,~:~lO-~ m2 s ’

theoretical

(19)

a reasonable estimation of e, would be 353 ,um. It appears then that e, is of the same order of magnitude as the initial size of the particles (2L0= 300 pm), which is inconsistent with the ablation regime assumption imposing e, < Lo, It is thus concluded that the reaction occurs in the intermediate regime between the chemical and ablation regimes for the inlet conditions. Of course, the conditions for the chemical regime would be better achieved for lower wall temperatures Tw, for smaller granulometry and, in any case, as the particle size diminishes by reaction in the reactor. Pyrolysis carried out with particles at least ten times smaller would be such that tF = t, (making it possible to determine the chemical kinetic parameters easily from rR if the hydrodynamic conditions in the reactor were adjusted in such a way that t, = tF: no unreacted particle separated at the bottom of the cyclone). The consumption rate in operations carried out with bigger particles (10-2-2 X 10m2 III in diameter) would probably be severely limited by heat transfer into the wood matrix. 4. Conclusion The flash pyrolysis of wood sawdust has been carried out in a cyclone reactor between 893 and 1330 K with steam as carrier gas. The reaction produces very low fractions of char (~4%), while the gasification yield increases with wall temperature and is probably complete above Tw E 1500 K. The gas evolved contains mainly Hz and O2 (73% vol. fraction, a roughly constant value), while hydrocarbons (mainly C&, CsH4 and &He) sometimes represent a mass fraction close to 50%. The calorific value of the gas, greater than 18 000 kJ n-3 STP, is much better than the values reported for air or oxygen gasitiers.

317 The cyclone has proved to be very efficient for carrying out such a fast reaction where heating, reaction and gas cleaning (gas-solid separation) occur in the same vessel. The residence and decomposition times of the wood particles of diameter 300 m are of the order of 1 s, while the residence time of the carrier gas is of the order of 50 X 10m3 s. The efficient friction of the particles against the walls has two advantages: cleaning of the walls and continuous elimination of the primary products from the wood surface. The particles seem to be mainly heated by direct heat transfer from the walls by radiation and solid convection (the reaction takes place under intermediate conditions between the pure chemical and ablation regimes), while the carrier gas is only weakly heated. Most calculations reported in this paper are only rough estimations because of the present lack of quantitative information about gas and solid residence times in a hot cyclone and about the efficiency of heat transfer processes between wall, gas and particles. Because of the potential qualities of such a reactor for carrying out fast reactions (< 1 s) of the solid + fluids (+ solids) type, typical reactor studies are greatly needed and recommended. The high capacity of such a reactor (0.35 kg h-l) compared to its small size (volume 42.6 X 10m6 m3) must also be emphasized. Experiments with cyclones of different sizes are required for scale-up to the industrial scale.

carrier gas mass flow rate, kg s-l carrier gas volume flow rate, m3 s-l STP Rea Reynolds number (inlet conditions) contact surface of one particle, m2 ; cyclone inner surface, m2 time for total consumption of one particle, s tF carrier gas residence time, s tG time necessary for reactor clogging, s trd chemical kinetic characteristic time, s tR residence time of particles, s t, heat penetration characteristic time, s tT Td = 739 K, constant temperature of wood decomposition TG carrier gas temperature, K To inlet temperature of gas and particles, K T, outlet temperature of carrier gas, K Tw wall temperature, K V inlet velocity of gas and particles, m s-l ablation rate, m s-l Vi3 V cyclone volume, m3 x gasification yield

Acknowledgements

References

a: E 90 x

i

The authors wish to thank AFME (Agence Francaise pour la Maitrise de 1’Energie) for financial support of this investigation, under contract COMES 8075 115. The authors also express their thanks to Drs. A. Vialaron, C. Roy&e and M. Rivot of Odeillo CNRS Laboratories, France, for helpful discussions and efficient assistance during the experimental phase of this work. Nomenclature CP

do

‘hi D

e, h

hP h, AH k

1, Lo

El P PC

heat capacity of wood, J kg s-l diameter of equivalent circle of same surface as cyclone inlet, m hydraulic diameter of cyclone, m cyclone diameter, m ablation depth, m overall external heat transfer coefficient, W m-’ K-l heat transfer coefficient by wall to particle contact, W me2 K-r radiation heat transfer coefficient, W rn-’ K-r enthalpy of reaction, J kg-’ first-order chemical kinetic constant, se1 length of cylindrical tube of same surface as cyclone, m half initial size of one particle, m mass of one particle, kg Nusselt number atmospheric pressure, Pa contact pressure, Pa

9 10 11 12

13 14 15 16 17 18 19

thermal diffusivity of wood, m2 s-l emissivity carrier gas viscosity for inlet conditions, kg s-r m-l thermal conductivity, W m-l constant of Stephan-Boltzmann law, W rn-’ Ke4 heat flux density, W mm2

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