Product Distribution from Pyrolysis of Wood and Agricultural Residues

May 13, 1999 - The pyrolysis characteristics of agricultural residues (wheat straw, olive husks, grape residues, and rice husks) and wood chips have b...
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Ind. Eng. Chem. Res. 1999, 38, 2216-2224

Product Distribution from Pyrolysis of Wood and Agricultural Residues Colomba Di Blasi,* Gabriella Signorelli, Carlo Di Russo, and Gennaro Rea Dipartimento di Ingegneria Chimica, Universita´ degli Studi di Napoli “Federico II”, P.le V. Tecchio, 80125 Napoli, Italy

The pyrolysis characteristics of agricultural residues (wheat straw, olive husks, grape residues, and rice husks) and wood chips have been investigated on a bench scale. The experimental system establishes the conditions encountered by a thin (4 × 10-2 m diameter) packed bed of biomass particles suddenly exposed in a high-temperature environment, simulated by a radiant furnace. Product yields (gases, liquids, and char) and gas composition, measured for surface bed temperatures in the range 650-1000 K, reproduce trends already observed for wood. However, differences are quantitatively large. Pyrolysis of agricultural residues is always associated with much higher solid yields (up to a factor of 2) and lower liquid yields. Differences are lower for the total gas, and approximate relationships exist among the ratios of the main gas species yields, indicating comparable activation energies for the corresponding apparent kinetics of formation. However, while the ratios are about the same for wood chips, rice husks, and straw, much lower values are shown by olive and grape residues. Large differences have also been found in the average values of the specific devolatilization rates. The fastest (up to factors of about 1.5 with respect to wood) have been observed for wheat straw and the slowest (up to factors of 2) for grape residues. Introduction Pyrolysis and gasification are thermochemical conversion routes to recover energy from biomass and waste fuels. Pyrolysis is not only an independent conversion technology but also part of the gasification process, which can be broadly separated into two main stages, solid devolatilization (pyrolysis) and char conversion (combustion and gasification). Given a certain biomass, the ratio between the yields of solid char and volatile pyrolysis products depends on temperature, pressure, and heating rate. Char reactivity in the second stage is dependent upon the formation conditions (essentially temperature and heating rate) and the amount and composition of the inorganic content. In addition, the type of biomass (chemical composition and physical properties) also largely affects both biomass devolatilization and char conversion. Given the key role played by the devolatilization stage in the conversion processes, numerous studies have been carried out in order to determine product distribution and gas, and sometimes liquid, composition. Extensive information is available on product distribution from wood (poplar, beech, oak, maple, etc.) for flash pyrolysis carried out through fluid-bed reactors1-5 or other devices6 and conventional pyrolysis in relation to single (large) particles/samples7-9 and packed beds.10 The pyrolysis of some agricultural residues has also been examined under different experimental conditions.11-14 However, apart from the limited variety of residues examined, for the investigations carried through packed beds,12,14 the temperature range is very narrow or the thermal conditions are not exactly known. Furthermore, the different experimental conditions do not allow the results to be compared in relation to both different * Corresponding author. Tel: 39-081-7682232. Fax: 39-0812391800. E-mail: [email protected].

agricultural residues and wood. Therefore, basic information is not currently available for the understanding of the complex interaction between chemistry and transport phenomena in chemical reactors and thus for the selection of the most appropriate fuels and the optimal conditions for pyrolysis/gasification. Devolatilization characteristics are also needed for the formulation of reliable computer models of the gasification process. Indeed, both the temperature dependence of pyrolysis product classes (char, gases, and liquids), due to reaction selectivity,15,16 and the gas composition should be taken into account. The use of consistent data, valid over wide temperature ranges and for different fuels, is particularly important because, contrary to coal gasification where the devolatilization stage contributes only for 20-40% of the total volatiles released, in biomass gasification this contribution increases up to 60-80%. On the other hand, the pyrolysis characteristics influence the predictions of both the producer gas quality and activity of gasification reactions, through hydrogen, carbon dioxide, and steam concentrations. In this study, the devolatilization behavior of several agricultural residues typical of Mediterranean countries, such as olive husks, grape residues, wheat straw, and rice husks,17 is investigated and comparisons are made with (fir) wood chips. The external heating conditions are the same for all biomasses and correspond to those of a relatively thin packed bed of particles suddenly exposed in a high-temperature environment, with the purpose of reproducing the devolatilization stage of fixed-bed countercurrent and concurrent reactors (pyrolyzers and gasifiers). The heating rates are those of conventional pyrolysis because of the coarse particles usually required and/or the indirect heating modalities applied. For gasifiers, the heating rates are slower for the countercurrent configuration, where, given the stratification of the different processes along the reactor

10.1021/ie980711u CCC: $18.00 © 1999 American Chemical Society Published on Web 05/13/1999

Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2217

Figure 1. Schematic representation of the bench-scale pyrolysis system: (1) radiant heater, (2) quartz reactor, (3) sample holder, (4) flowmeter, (5) data acquisition, (6) condensers, (7) filters.

axis, the sections of combustion (high temperatures) and drying/devolatilization are located at the two extremes. On the contrary, in concurrent gasifiers, devolatilization and combustion occur roughly at the same location; therefore, temperatures and heating rates are higher. The focus of the work is mainly on the temperature dependence of the product yields and gas composition. The characterization of liquid products, which is important for their successive utilization (pyrolysis) or for the design of effective gas cleaning procedures (gasification), is currently underway. Experimental Section Pyrolysis of packed biomass beds has been carried out through a bench-scale plant (Figure 1). Biomass particles are packed in a stainless steel mesh, cylindricalshaped holder, so that the desired bed density can be achieved. The bed is vertically positioned in the uniformly heated zone of a radiant furnace. This, manufactured by Research Inc., presents four tubular quartz infrared lamps with a tungsten wire filament that emits radiant energy in proportion to the applied voltage. Elliptical, polished-aluminum, water-cooled reflectors focus the high-density infrared energy, emitted by lamps, onto a cylindrical-shaped target area (diameter 6.5 x 10-2 m). To avoid interaction between the volatile pyrolysis products and the lamps, a quartz tube (ID 6 x 10-2 m and length 22 x 10-2 m), transparent to infrared radiation, is located inside the furnace and used as a reaction chamber. A nitrogen flow is applied at the top of the quartz tube in order to reduce the extra-bed residence time of volatile pyrolysis products and to establish the proper reaction environment. For each chosen radiation intensity, steady temperatures of the radiant heater are achieved within a couple of minutes, but, given the bed depth, pyrolysis takes place under heat-transfer control. For each test, a fixed amount of biomass particles is uniformly distributed inside the sample holder and positioned in the constant radiation zone of the reactor (bed length 5 × 10-2 m). A nitrogen flow is established and maintained for 30

min in order to remove the air from the reaction chamber, and then the test begins. Temperatures along the bed radius at the median section are continuously monitored (0.5 mm bead chromel-alumel thermocouples), while the composition of the gas is analyzed, at selected times. This is carried out through a gas chromatograph (Perkin-Elmer Auto-System XL), equipped with a thermal conductivity detector (TCD) and a packed column (Supelco 60-80 Carboxen 1000, 15 ft) with helium as the carrier gas. The oven program is as follows: an isothermal stage of 403 K for 3.2 min followed by a rate of 25 K/min up to 513 K (injector temperature 423 K). An external standard method is applied with two-point calibration of a mixture of seven gaseous pyrolysis products. Steady global mass balances over the system allow the exit volumetric flow rate and mass of each gaseous species to be determined, given the inlet conditions (nitrogen flow rate, temperature, and pressure) and the measurement of species molar fractions at the exit (assumed to be at ambient pressure and temperature). More precisely, the mass balance on nitrogen is used to evaluate the volumetric flow rate at the exit, whereas the other mass balances (upon integration) give the gas species mass as a function of time (in particular, also the final total mass indicated as “gas”). After complete conversion, the power is turned off and the solid residual is left under a nitrogen flow until its temperature lowers to ambient values. This residual is indicated as “char”. The liquids are collected through a condenser train, consisting of water-cooled traps, cotton wool demisters, and a silica gel bed. All of the condensable products collected and weighed from the traps (organic compounds and product water formed) are indicated as “liquids”. Results Pyrolysis tests, through the experimental system previously described, have been carried out for (fir) wood chips, olive husks, grape residues, rice husks, and wheat straw. The elemental and chemical analyses of the feedstocks are reported in Table 1. For the determination of C, H, and N (CHNO-Rapid, Heraeus-Foss Elementar GmbH analyzer) the sample is burned in pure oxygen at 1223 K. The carrier gas (helium) and the combustion products, after complete oxidation, are passed over heated copper to remove excess oxygen and to reduce NO2 to N2. Gases are then passed through a heated chromatographic column to separate and elute N2, CO2, and H2O. Concentration of each eluted gas is measured by TCD. The same analyzer is used for O. The sample is pyrolyzed in the presence of pure carbon at 1423 K and helium. All of the oxygen is then present in the form of CO. The gaseous pyrolysis products are passed over heated copper and copper oxide to remove sulfur containing gas and to oxidize CO to CO2, which is then measured by TCD. For sulfur determination (Sulmhomat-Wosthoff analyzer) the sample is burned in an oxygen stream at 1573 K. The reaction gases are

Table 1. Elemental Analysis and Chemical Composition of Wood Chips and Agricultural Residues biomass

C

H

N

S

extractives (%)

hemicellulose (%)

cellulose (%)

lignin (%)

ash (%)

wood chips wheat straw olive husks grape residues rice husks

46.4 43.6 50.9 47.9 40.3

5.9 6.2 6.3 6.2 5.7

0.085 0.30 1.37 2.11 0.30

0 0.08 0.03 0.09 0.03

4.6 7.4 8.7 15.6 8.0

31.8 27.3 18.5 17.2 24.3

31.8 27.3 18.5 17.2 24.3

19.0 16.4 28.0 30.4 14.3

0.45 5.5 2.8 5.1 15.3

2218 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 Table 2. Characteristics of the Packed Beds for Wood Chips and Agricultural Residues (Moisture Content on a Dry Basis)

biomass

bulk density (kg/m3)

% moisture (kg/kg)

particle size (mm)

particle size (mm)

mi (kg)

wood chips wheat straw olive husks grape residues rice husks

270 160 515 325 150

8 7 8.5 9 7

1-3 0.5-1 0.5-3 1-3 0.2-1

1-3 0.5-1 0.5-3 1-3 0.2-1

0.017 0.010 0.032 0.020 0.009

purified, and the sulfur dioxide is absorbed in an electrical conductivity cell, which contains diluted sulfuric acid and hydrogen peroxide. The SO2 reacts to form sulfuric acid and causes changes in the thermal conductivity (a linear function of the absorbed sulfur dioxide). Extractives and holocellulose are determined by means of the extractor Soxhtec HT2. Acetone (60 mL) is used as the solvent for extractives (1 g of biomass) with residence times for the boiling and rising stages equal to 90 and 20 min, respectively (temperature equal to 363 K). Holocellulose (extractive-free basis) is determined according to the Kurschener-Hoffer method. A solution of 40 mL of ethyl alcohol and 10 mL of nitric acid is used as the solvent for 1 g of biomass with residence times of 3 h for the boiling stage and 20 min for the rising stage (solvent temperature equal to 363 K). Hemicellulose is extracted from holocellulose by means of a sodium idrate solution (1% mol) with a temperature of 353 K and the same residence times as those for holocellulose extraction. Filtration, washing (hot water, a solution of acetic acid of 10% mol, and again water), and drying of the residue (cellulose) allow the hemicellulose fraction to be determined by difference. Lignin is determined according to the Klason method. Table 2 reports the main physical characteristics of the biomasses, that is, density of the bed, particle size, initial moisture content (dry basis), and initial sample mass. The woody biomass (fir) is a forestry residue, which also retains its bark and has been cut in more or less uniform pieces of typical size (thickness) of 1-3 mm. No pretreatment is applied before pyrolysis of agricultural residues, so that significant differences are seen in the bed characteristics. Tests have been carried out by varying the intensity of the applied radiative heat flux. As pyrolysis takes place in the presence of spatial (radius) gradients, the steady temperature attained by the char bed, about 2 mm below the heat exposed surface, Tb, is taken as the characteristic process temperature. For the range of conditions investigated, this varies from about 650 to 1000 K. Though the external heating conditions are the same for all biomasses, the different chemical and physical properties of the bed give rise to different heating rates, as expected in practical applications. Therefore, the pyrolysis characteristics (conversion times, product yields, and gas composition) determined here reflect both the differences in the biomass intrinsic reactivities and the different thermal conditions undergone during pyrolysis. Preliminary tests have also been made to evaluate the influence of the nitrogen flow rate, bed depth, and particle size on the process characteristics. These effects have been investigated for wood chips and high temperature (Tb ) 800 K), when both primary and second-

ary pyrolysis reactions take place (mass closure comprised betwen 95 and 99%). For a bed diameter of 4 × 10-2 m, the nitrogen flow rate has been varied from 1 × 10-3 to 6 × 10-3 m3/min, corresponding to a nominal gas velocity (ambient conditions and cross section of the quartz tube) in the range 0.0042-0.025 m/s. Char yields are independent of this parameter, whereas variations in the volatile yields are also small for nitrogen flow rates of 1.5 × 10-3 m3/min and above. At lower values, the liquid yields slightly increase (correspondingly gas yields decrease), as a consequence of the activity of secondary tar cracking reactions. Maximum variations are, however, small and correspond to about 5% (liquids) for the lowest nitrogen flow rate. Nitrogen flow rates below 1 × 10-3 m3/min have not been considered because, at these low gas flow rates, the liquid collection train introduces a significant delay in the gas collection and analysis, so that temperature and species evolution become two separate processes. All of the tests have been made for a nitrogen flow rate equal to 6 × 10-3 m3/min (nominal gas velocity of 0.025 m/s and nominal residence times of volatiles along the heated section of about 3 s). It should be noted that the true residence times are much shorter, given the gas expansion at the pyrolysis temperatures and the presence of the sample, which reduces the void section of the quartz tube. A bed diameter of 4 × 10-2 m is chosen for all of the tests, but the results are not truly independent of this parameter. Char yields increase by about 6% when the bed diameter is varied from 2 to 4 × 10-2 m, while the gas yields decrease by about 15% (with an increase in the liquids), as a consequence of primary reactions taking place at lower temperatures and, probably, of the shorter volatile residence times and reduced activity of secondary reactions. Finally, in agreement with previous studies (for instance, ref 10), it has been found that, for a chosen bed density, particle sizes do not influence the pyrolysis characteristics for the range of values used in this study (0.5-3 mm), indicating that the heattransfer resistance across the bed predominates over that across the particles. Indeed, the bed radius is from 1 to 2 orders of magnitude larger than the particle sizes. Comparison with Literature Data. In order to compare the experimental system and procedure chosen in this study with conditions previously employed in the literature, product yields (percent of the initial moisturefree (mf) solid mass) measured for wood chips have been reported in Figure 2A,B as functions of temperature together with other data obtained with fluid-bed3 (maple wood particle 0.6 mm thick, volatile residence times of about 0.5 s) and packed-bed10 (8 g of holm-oak particles 0.4-2 mm thick) reactors. In principle, differences can be due to reactor configurations (external heat-transfer rates), operating conditions (volatile residence times and particle characteristics), and wood type. However, it is believed that the first two factors are predominant, as variations in the wood chemical composition cannot account for differences as large as those shown in Figure 2A,B. As the temperature increases, the final solid residual initially decreases, as a result of the competition between charring and devolatilization reactions, which become successively more favored. Then, at high temperatures, the char yield tends to become constant, because the variation in the actual wood degradation temperature (in relation to the external heating temperature) becomes very low, because of the narrow range

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Figure 3. Radial profiles of temperature for wheat straw at several times for Tb ) 900 K.

Figure 2. (A) Char yields as functions of temperature for wood chips. (B) Liquid and gas yields as functions of temperature for wood chips. Literature data are also enclosed for comparison purposes.

of temperatures characteristic of biomass pyrolysis and heat-transfer resistance through the packed bed (this study and ref 10) or the particle.3 The char yields are about the same for both the two packed-bed configurations (slightly larger in this study, presumably as a result of a thicker bed and/or the presence of bark)11 while, for temperatures above 700 K, they are much lower for the thin particles (600 µm) pyrolyzed in a fluidbed reactor. Also, it is worth noting that, at low temperatures, when chemical reaction is controlling, the char yields tend to become independent of the conversion unit. As expected, liquid yields are maximized under flash pyrolysis conditions3 and, in all cases, they present a maximum as a function of temperature. This results from both primary volatile formation and secondary degradation of tar vapors becoming successively more favored by higher temperatures.15 However, the rates of increase (low temperature) and decrease (high temperature) are much larger for fluid-bed conversion, because the distribution of volatile products is mainly dictated by extraparticle secondary reactions, which occur in a nearly isothermal environment. On the contrary, for (laboratory) packed-bed reactors, secondary reactions take place both across the bed (intraparticle activity may also be significant for large particle sizes) and in the heated extra-bed environment. However, given the larger volume and the higher temperature, the extra-bed zone is the main reason for tar cracking, unless the residence times of volatiles are made short through forced (inert) convection and a relatively small heated zone. It is plausible that the extra-bed residence times accomplished in the packed-bed system of this study are much shorter than those of ref 10. Consequently, the corresponding liquid yields are higher (and gas yields lower). Moreover, at very high temperatures (above 950 K) they are also higher than those of fluid-

bed conversion. Indeed, apart from the short extra-bed vapor residence times, the existence of radial gradients across the bed gives rise to temperatures on the average lower; hence, secondary reactions occur at a less extent. Process Dynamics. The macroscopic behavior of the different biomasses while degrading is qualitatively similar, with quantitative differences in the heating times caused by the different bed properties (in particular, density). Figure 3 shows the radial temperature profiles at several times, for a radiation intensity corresponding to Tb ) 900 K, as measured for wheat straw. For all radiation intensities, significant spatial gradients are observed, so that the temperature increases more slowly as the distance from the heated surface increases, but they become successively higher as more severe heating conditions are established. Moisture evaporation is responsible for the flat profiles observed close to the sample centerline for temperatures below the normal boiling point of water, while gas release curves indicate that devolatilization begins for temperatures above 550-575 K. The temperature profiles show that initially only a thin layer beyond the surface is heated and undergoes a degradation process. Successively, the thermal (and reactive) front propagates along the bed radius, with an increasing thickness, until degradation is complete. For Tb > 750 K, the temperature continuously increases, attaining a maximum at the completion of the process, as a consequence of inert char heating. For lower Tb, temperatures slightly higher than the final steady value are detected in the last part of the degradation process. Maximum temperature differences are about 15-50 K for the different biomasses and are probably associated with exothermic lignin degradation.18-20 Finally, temperature profiles show maximum values significantly dependent on the external heating conditions. However, they are attained only after solid devolatilization, which takes place for comparable thermal conditions along the whole bed radius, as the temperatures typical of biomass degradation are rather low. Thus, for high furnace temperatures, primary degradation is only weakly affected by the variation in the spatial temperature profiles. The dynamics of gas-phase species are shown through the time profiles corresponding to Tb ) 900 K (Figure 4), as measured for wheat straw. From the qualitative point of view, all gas-phase species profiles are similar, that is, they present a maximum as a function of the release time, which becomes successively shorter, as the reaction temperature increases. The position of the maximum is, however, dependent on both the temperature and the component. The species CO2 and CO are

2220 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999

Figure 4. Time-species profiles for wheat straw and Tb ) 900 K.

released at shorter times, as compared with the other components (H2, CH4, C2H4, and C2H6). At low temperatures, the evolution of CO (and CO2 and water vapor) is due mainly to the degradation of hemicellulose and the activity of the first path in cellulose degradation, leading to gas and char formation.21 As the temperature increases, the formation of tar vapor from cellulose becomes predominant, while also lignin degradation begins. This process is largely responsible for char formation and also for the evolution of CO2, CO, CH4, and H2 (the last two species at high temperatures). On the other hand, secondary tar degradation reactions become active for temperatures above 700 K (wood22), with CO as the main product, followed by C2H4 and CH4.23 Pyrolysis of Agricultural Residues. For a chosen radiation intensity (temperature) and bed depth, the two main parameters which determine product distributions and conversion times are the density of the bed, largely responsible for the heating characteristics, and the intrinsic reactivity of the biomass. Conversion times (not shown), defined as the times when 95% of the total gas mass has been released, show a first region, where the temperature dependence is very strong, followed by a tendency toward constant values. This can be explained by the relatively narrow temperature range where biomass undergoes pyrolysis. Thus, despite the increase in the temperature of the heating system and the average bed temperature, primary degradation characteristics (in particular, gas release time and char yields) tend to become constant. Conversion times are about the same for olive husks and grape residues (variations from about 23 to 7.5 min, for the range of temperatures considered) and are longer than those for wood (up to factors of about 2). On the other hand, they are shorter for rice husks and mainly for wheat straw (factors of 1.5 with respect to wood). This behavior can be, in part, explained by the almost linear increase of the conversion time with density, given the successively slower particle heating rates,24-25 whereas the surprisingly long times for grape residues, whose bed densities are comparable to those of wood chips, could be due to a lower reactivity. This seems to be confirmed by Figure 5, where the temperature dependence is shown of the average devolatilization rates, for the different biomasses. The average devolatilization rate is defined as the ratio between the total mass fraction of volatiles (including the moisture content) and the conversion time. The lowest values, measured for grape residues, indicate that both the amount of volatiles released is low and their release time is long. Wood chips show intermediate values (almost coincident with rice husks) comprised

Figure 5. Average devolatilization rates (ratio of volatile mass fraction and conversion time) as functions of temperature for wood chips, rice husks, wheat straw, grape residues, and olive husks.

between those of wheat straw (higher up to factors of 1.4-1.2) and olive and grape waste (lower by factors of 1.2-2). Also, Figure 5 shows that biomasses under study can be roughly classified into two categories. The first (rice husks, wheat straw, and wood) presents average degradation rates highly increasing with temperature (with values up to 5 times higher), while the dependence is weaker for the second category (rates for olive and grape waste rates only increase by factors of 3-3.5). For all of the biomasses, final product yields (expressed on a moisture-free basis) as functions of temperature (Figure 6A-C) show the same trends as those already discussed for wood, but differences are quantitatively large. The mass closure is very good for wood (95.5-99%) and olive husks (95-100%), while errors in the liquid collection are larger for straw (92.5-98%), grape residues (92-97%), and rice husks (90-96%). Therefore, for a better presentation of the data, dashed lines have also been included in Figure 6B corresponding to liquid yields computed by difference. All of the char yields show the same qualitative dependence on temperature, but differences are quantitatively large. In comparison with wood, agricultural residues always give rise to larger char yields. These findings are in qualitative agreement with previous studies, where the higher char yields observed for agricultural residues were attributed to the higher lignin (and carbon) contents13 or to the presence of large amounts of inorganics which favor charring reactions.11 Another difference (Figure 6A) is that the char yield curves of agricultural residues show a tendency toward a constant value at temperatures slightly lower than those of wood (800-850 K against 900 K), probably as a consequence of higher degradation rates at lower temperatures, often associated with the presence of extractives and ashes. The chemical composition of the chars (Table 3), obtained for intermediate values of the radiation intensity (Tb ) 850 K), shows that the carbon content increases in the order of biomasses with a successively lower ash content. As a consequence, the calorific value of the solid residual also increases, given a smaller contribution of inorganics in its composition. Therefore, in view of char utilization as fuel, what is really needed is a comparison of the different yields on a moistureand ash-free (maf) basis, as shown in Figure 7. This plot confirms that the pyrolysis of agricultural residues is associated with solid yields much higher than those from wood, but grape residues give the highest values, instead of rice husks. Apart from low temperatures, they are followed by olive and rice husks and straw.

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Figure 7. Char yields (maf basis) from pyrolysis of wood chips and agricultural residues as functions of temperature.

Figure 6. (A) Char yields from pyrolysis of wood chips and agricultural residues as functions of temperature. (B) Liquid yields from pyrolysis of wood chips and agricultural residues as functions of temperature. (C) Gas yields from pyrolysis of wood chips and agricultural residues as functions of temperature. Table 3. Elemental Analysis of Chars from Wood Chips and Agricultural Residues (Tb ) 850 K) biomass char

C

H

N

O

S

HHV (MJ/kg)

wood chips wheat straw olive husks grape residues rice husks

83.3 64.27 78.68 68.48 51.52

3.1 2.39 2.83 2.62 2.14

0.21 0.48 0.98 2.05 0.46

11.38 14.31 12.88 16.53 9.77

0.015 0.30 0.032 0.11 0.021

30.53 22.66 28.26 23.83 18.73

As a consequence of the large char yields, the lowest liquid and gas yields (Figure 6B,C) are observed for rice husks followed by grape residues. Wood gives the highest liquid yields, with a maximum of about 51%, but it is straw which is associated with the highest gas yields (up to 31%), whereas olive husks present intermediate values of the liquid yields. It is worth noting that the liquid yields, measured by difference, though they still retain the characteristics previously illustrated, indicate that rice husks and grape residue yields on the one side and straw and olive husks on the other give comparable values, so that differences between residues are reduced. A comparison on a maf basis (not

shown) indicates that the differences in the gas yields are small and maximum liquid yields are associated with minimum char yields (wood) (conversely minimum liquid values are associated with maximum char values in the case of grape residues). This finding confirms that the main route for char formation is the conversion of monomers and oligomers into char and light volatiles.26 The pyrolysis gas consists (mf basis) mainly of CO2 (the largest contribution), CO, CH4, and lower amounts of H2 and C2 hydrocarbons. For Tb ) 850 K, CO2 contributes to the total gas yield for about 59% (wood), 60% (rice husks), 62% (straw), 67% (olive husks), and 76% (grape residues). Contributions are lower for CO and are about 14% (grape residues), 23% (olive husks), 28% (straw), 32% (rice husks), and 34% (wood). The contribution of CH4 varies from about 4% (straw) to 6% (olive husks) and that of C2 hydrocarbons from 1.6% (rice husks) to 3% (grape residues), whereas hydrogen is always very low (from 0.1% for straw to 0.5% for grape residues). On the whole, the yields of all of the gas components increase with the reaction temperature, as shown by Figure 8A-E, though some differences exist between the species CO2 and the others. Indeed, these show a continuous increase with temperature, whereas, for temperatures below 850 K, the yields of CO2 only slightly increase or remain almost constant and then, apart from wood chips, start to increase with a rather high rate. The existence of two different regions in the CO2 yields is in qualitative agreement with previous literature (for instance, see ref 3). This was attributed to the fact that CO2 is a product of the primary pyrolysis of cellulose and hemicellulose by a pathway that becomes less favored as the temperature increases, resulting in a plateau for values of about 723 K (fluid-bed reactor). Then, the successive increase is attributed to secondary reactions. The measurements carried out in this study indicate that CO2 yields from wood remain roughly constant (values of about 13%) while the other gas species increase. It can be argued that this is essentially due to primary reactions and to a competition between gas and char formation in the degradation of the lignin fraction, with gas becoming successively more favored with high temperatures. This explanation is also consistent with the continuous char reduction even at relatively high temperatures, while the variations in the liquid yield are small. Agricultural residues present a significant difference with wood in that the second region of significant increase in the CO2 yields is clearly visible for temperatures above 850 K (olive husks, straw, grape residues) or 950 K (rice husks). Because the thermal conditions of the bed are roughly the same for both wood and agricultural residues, it can

2222 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999

Figure 8. (A) CO, (B) CO2, (C) H2, (D) CH4, (E) C2 (C2H4 and C2H6) yields for wood chips and agricultural residues as functions of temperature.

be argued that vapors produced in the latter case are more reactive than those of wood and thus are more prone to undergo cracking. This behavior also appears to be consistent with the char yield which tends to become constant at lower temperatures compared to wood (that is, the ratio between primary volatiles and char becomes constant). A comparison between the gas composition measured for wood and that for agricultural residues also indicates that the latter, apart from rice husks, are characterized by a higher CO2 production (yields up to 17.5-18.7%). Few tests carried out with wood chips, after bark elimination, give rise to a reduction in the gas yields and in particular in the CO2 yields (for instance, for Tb ) 735 K, they decrease from 12% to 8.6%), which become lower than those measured for rice husks. Again, these findings, in relation to both CO2 yields and bark influence, are in qualitative agreement with previous studies carried out under fluidized-bed conditions.11-13 Wood, straw, and rice husks present comparable yields of CO at low temperature (5-6%), but the increase with temperature is significantly higher for wood (values up to about 13%). The CO yields are lower for olive husks and especially for grape residues (values from 2 to 6.5%).

At low temperatures (below 800 K) the production of hydrogen is negligible for all biomasses. A rapid increase is then observed for temperatures above 850-900 K. Olive and mainly grape residues, presumably as a consequence of their higher lignin content, are characterized by the highest C2 yields, followed by the comparable values for wood and straw and the low values of rice husks. Finally, though the CH4 yields are comparable for all of the biomasses, on the average, the highest values are observed for olive husks. Flash pyrolysis studies report very general approximate relationships among the ratios of the major gasphase products, CO, CH4, CO2, and C2 hydrocarbons, over wide ranges of experimental conditions (temperatures). These are explained to be due to comparable values of the activation energies for the formation of different species,3,11,20 with models usually representative of apparent kinetics, which include both primary and secondary reactions. Because one of the motivations for this study was to provide data on the gas characteristics to be used for reactor modeling, it is important to understand if the gas yields, previously presented, obey the same apparent kinetics in relation to different species and biomasses. Thus, a plot is constructed of the logarithm of the ratio of yields of major gas-phase

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Figure 9. Logarithm of the ratio CO/CO2 and CO/C2 yields on the dependence of the logarithm of temperature for wood chips and agricultural residues.

species on the logarithm of temperature. An example for CO/CO2 and CO/C2, is shown in Figure 9. In qualitative agreement with previous studies carried out for flash pyrolysis,3,11 it can be seen that a linear dependence is roughly established, confirming that major gas species present comparable kinetic constants (in particular, activation energies) also for conventional pyrolysis. As expected, there are some differences between the ratio of values evaluated in this study and those reported for flash pyrolysis, because of the different rates of gas formation. Indeed, in the first place, the actual degradation temperature is lower than the surface temperature (here used as the characteristic temperature), especially at high thermal severity. Moreover, the increase in the gas yields at high temperatures is due mainly to secondary reactions which, as already observed, occur at a larger extent in flash pyrolysis units. Figure 9 also indicates two different classes of biomass fuels with reference to gas release behavior, that is, wood, straw, and rice husks on the one side and grape and olive residues on the other (though with differences between the last two). More precisely, the latter species present always lower values of the logarithms, because of the lower yields of CO and the higher yields of CH4, C2, and CO2 species, probably as a result of the higher lignin content. On the basis of these results, it can be concluded that the gas release characteristics are significantly dependent on the type of biomass and, probably, a correlation can be suggested with the chemical composition (that is, lignin and holocellulose). Conclusions A bench-scale system has been applied to investigate the pyrolysis characteristics of wood chips and some agricultural residues, through packed beds (conventional pyrolysis). Contrary to previous studies conducted for very slow external (furnace) heating rates, the pyrolysis conditions established in this study are essentially those determined by the physicochemical properties of the packed bed. The mechanism of external heating is radiation, and different intensities have been applied, which correspond to maximum bed temperatures in the range 650-1000 K. Also, extra-bed secondary reactions are minimized through a reduced length of the heated zone and short residence times of volatile products. Product yields and gas composition have been determined over the above temperature range. From the qualitative point of view, the dependence of the product yields on temperature, for all fuels, is the

same as that observed in flash pyrolysis, with quantitative differences resulting from the characteristic process length (bed depth) being about 1 order of magnitude higher and the reduced activity of secondary reactions. For conventional pyrolysis, wood chips show intermediate values of the average devolatilization rate (almost coincident with rice husks), comprised between those of wheat straw (higher up to factors of 1.4-1.2) and olive and grape waste (lower by factors of 1.2-2). In comparison with wood, char yields from agricultural residues are higher up to factors of 2, with corresponding lower liquid yields. In particular, on a maf basis, grape waste gives rise to the highest char yield. The total gas yields present small differences, with comparable activation energies of the apparent kinetics of the main species release rates. However, the differences in the composition are again significant. Thus, while the behavior of gases from straw and rice husks can roughly be assimilated to wood, olive and grape residues trends are highly affected by the lower carbon monoxide yields and the higher carbon dioxide, methane, and C2 hydrocarbon yields. Acknowledgment The research was funded in part by the European Commission in the framework of the Non Nuclear Energy Programme (JOULE III; Contract JOR3-CT950021). Literature Cited (1) Scott, D. S.; Piskorz, J. The flash pyrolysis of aspen-poplar wood. Can. J. Chem. Eng. 1982, 60, 666. (2) Scott, D. S.; Piskorz, J. The continuous flash pyrolysis of biomass. Can. J. Chem. Eng. 1984, 62, 404. (3) Scott, D. S.; Piskorz, J.; Bergougnou, M. A.; Graham, R.; Overend, R. P. The role of temperature in the fast pyrolysis of cellulose and wood. Ind. Eng. Chem. Res. 1988, 27, 8. (4) Beaumont, O.; Schwob, Y. Influence of physical and chemical parameters on wood pyrolysis. Ind. Eng. Chem. Res. 1984, 23, 637. (5) Gray, M. R.; Corcoran, W. H.; Gavalas, G. R. Pyrolysis of a wood-derived material. Effects of moisture and ash content. Ind. Eng. Chem. Res. 1985, 24, 646. (6) Nunn, T. R.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Product Compositions and kinetics in the rapid pyrolysis of sweet gum hardwood. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 836. (7) Kashiwagi, T.; Ohlemiller, T.; Werner, K. Effects of external radiant flux and ambient oxygen concentration on nonflaming gasification rates and evolved products of white pine. Combust. Flame 1987, 69, 331. (8) Chan, W. R.; Kelbon, M.; Krieger-Brockett, B. Single-particle pyrolysis: correlations of reaction products with process conditions. Ind. Eng. Chem. Res. 1988, 27, 2261. (9) Bilbao, R.; Millera A.; Murillo, M. B. Temperature profiles and weight loss in the thermal decomposition of large spherical wood particles. Ind. Eng. Chem. 1993, 32, 1811. (10) Figueiredo, J. L.; Valenzuela, C.; Bernalte, A.; Encinar, J. M. Pyrolysis of holm-oak wood: influence of temperature and particle size. Fuel 1989, 68, 1012. (11) Scott, D. S.; Piskorz, J.; Radlein, D. Liquid products from the continuous flash pyrolysis of biomass. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 581. (12) Williams, P. T.; Besler, S. The pyrolysis of rice husks in a thermogravimetric analyser and static batch reactor. Fuel 1993, 72, 151. (13) Zanzi, R.; Sjostrom, K.; Bjornbom, E. Rapid high-temperature pyrolysis of biomass in a free-fall reactor. Fuel 1996, 75, 545. (14) Encinar, J. M.; Beltran, F. J.; Bernalte, A.; Ramiro, A.; Gonzalez, J. F. Pyrolysis of two agricultural residues: olive and grape bagasse, influence of particle size and temperature. Biomass and Bioenergy 1996, 11, 397.

2224 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 (15) Di Blasi, C. Modeling and simulation of combustion processes of charring and noncharring solid fuels. Progr. Energy Combust. Sci. 1993, 19, 71. (16) Di Blasi, C. Kinetic and heat transfer control in the slow and flash pyrolysis of solids. Ind. Eng. Chem. Res. 1996, 35, 37. (17) Di Blasi, C.; Tanzi, V.; Lanzetta, M. A study on the production of agricultural residues in Italy. Biomass Bioenergy, 1997, 12, 321. (18) Shafizadeh, F. Pyrolytic reactions and products of biomass. In Fundamentals of Thermochemical Biomass Conversion; Overend, R. P.; Milne, T. A., Mudge, L. K., Eds.; Elsevier: London, 1985; p 183. (19) Antal, M. J. Biomass pyrolysis: a review of the literature. Part I. Carbohydrate Pyrolysis. In Advances in Solar Energy; Baer, K. W., Duffie, J. A., Eds.; American Solar Energy Society: Boulder, CO, 1982, p 61. (20) Antal, M. J. Biomass pyrolysis: a review of the literature. Part II. Lignocellulose Pyrolysis. In Advances in Solar Energy; Baer K. W., Duffie, J. A., Eds.; Americam Solar Energy Society: Boulder, CO, 1985; p 175.

(21) Antal, M. J.; Varhegyi, G. Cellulose pyrolysis kinetics: the current state of knowledge. Ind. Eng. Chem. Res. 1995, 34, 703. (22) Liden, A. G.; Berruti, F.; Scott, D. S. A kinetic model for the production of liquids from the flash pyrolysis of biomass. Chem. Eng. Commun. 1988, 65, 207. (23) Boroson, M. L.; Howard, J. B.; Longwell, J. P.; Peters, A. W. Products yields and kinetics from the vapor phase cracking of wood pyrolysis tars. AIChE J. 1989, 35, 120. (24) Kanury, A. M. Combustion characteristics of biomass fuels. Combust. Sci. Technol. 1994, 97, 469. (25) Di Blasi, C. Influences of physical properties on biomass devolatilization characteristics. Fuel 1997, 76, 957. (26) Mok, W. S. L.; Antal, M. J.; Szabo, P.; Varhegyi, G.; Zelei, B. Formation of charcoal from biomass in a sealed reactor. Ind. Eng. Chem. Res. 1992, 31, 1162.

Received for review November 10, 1998 Revised manuscript received February 24, 1999 Accepted March 23, 1999 IE980711U