Ind. Eng. Chem. Res. 2000, 39, 1925-1933
1925
Pyrolysis of Sawdust in a Conical Spouted Bed Reactor. Yields and Product Composition Roberto Aguado,* Martin Olazar, Marı´a Jose´ San Jose´ , Gorka Aguirre, and Javier Bilbao Departamento de Ingenierı´a Quı´mica, Universidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain
The performance of original equipment provided with a conical spouted bed reactor has been studied in flash pyrolysis of sawdust with an inert gas (N2) in the 350-700 °C range and with 50 ms of average gas residence time. The effect of pyrolysis temperature on the yields of gas, liquid, and char on gas and liquid composition and on char properties has been studied. The maximum yield of liquid (corresponding to 70 wt %) is obtained at 450 °C and its composition determined by GC/MS is similar to that reported in the literature for bubbling fluidized beds. Although temperatures above 600 °C are required for the development of the char porous structure, the yield of CO2 obtained under these conditions is unacceptable. Introduction Flash pyrolysis of biomass is one of the more promising techniques that may enable this raw material to become a viable alternative to oil.1,2 The liquid product has aroused great interest given its high heat value, its ease of transportation and handling, the interest in its individual components, and the consideration that it may be transformed into fuel and raw materials for the petrochemical industry.3 An additional advantage of this fuel lies in its low N and S content. The yield and composition of the liquid product depends on the pyrolysis conditions, mainly on temperature and on the residence time of the product stream in the reactor.4-7 The highest yields have been obtained in fluidized-bed reactors. Thus, Scott and Piskorz obtained a maximum production of the liquid fraction of 65 wt % at 480 °C by feeding maple timber,8,9 which has slightly higher production than that of Samolada and Vasalos.10 Subsequently, Scott et al. obtained a maximum yield of liquid of 80 wt % at 500 °C with the same material.11 With pine wood, a yield of 70 wt % at 480 °C has been reached.8,9 Nevertheless, the yields with other materials are lower, with wheat straw 47 wt % at 575 °C8,9 and with poplar bark 40 wt % at 500 °C,12 which is evidence of the great influence of the raw material on the yield of liquid. Nunn et al., operating in fixed bed, achieved a liquid yield of 55 wt % at 625 °C, which is an appreciably higher temperature than the optimum one in a fluidized bed and with the disadvantage that the yield of gases is of 40 wt % at 700 °C.13 Obviously, it must be taken into account that the results also depend on factors such as particle size, which is very small ( 80 a
The components above the dashed line are not present in the organic liquid fraction of pyrolysis.
Figure 5. Effect of temperature on the composition of the aqueous fraction.
nents.7,37 Figure 6 shows the evolution with storage time of the composition (according to groups) of the aqueous fraction obtained at 500 °C. The samples have been stored for 2 months in sealed containers, which were protected from direct solar radiation but maintained at the laboratory room temperature (20-30 °C). As is shown in Figure 6, the water content increases up to 60 molar % during the first 15 days whereupon it remains almost unchanged. This result may be attributed to the slow dehydration of organic components, whose concentration slowly decreases during the first 15 days of storage. This evolution is a consequence of secondary reactions and is not attributable to evaporation of certain volatile compounds such as formaldehyde and acetone, as their concentrations and those of the acids increase during the first 15 days. Subsequently, the concentration of formaldehyde decreases with storage time and those of the acids, levoglucosane, ethanol, and the other alcohols also decreases but does so very slowly.
Figure 6. Evolution with storage time (at room temperature) of the compositions of the aqueous liquid fraction obtained at 500 °C.
The organic liquid fraction has been analyzed by GC/ MS as it is collected in the pyrolysis equipment because of the fact that the acetone added has thinned it down. It has been proven that most of the compounds present are the same as those in the aqueous fraction. Nevertheless, there are two significant differences between these two fractions: (1) there is no water in the organic fraction; (2) likewise, there are components in the aqueous fraction which are not present in the organic fraction. In fact, only aqueous fraction components whose retention times in the chromatographic column are longer than 10 min are present in the organic fraction; these are shown in Table 1 below the dashed line and account for 35-40 wt % of the aqueous fraction. The composition of the organic fraction obtained at 500 °C is shown in Table 2 and 55 compounds have been determined. The results of the analysis ratify the literature view that the liquid composition is more dependent on
Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1931 Table 2. Results of Identification by GC/MS and Composition (% of Total Chromatographic Area) of the Compounds in the Organic Liquid Fraction Obtained at 500 °C compound 2-propanone, 1-hydroxy 2-propenoic acid formic acid, penthyl ester 2-propanone benzene, methyl 2-cyclopenten-1-one furfural 2-pentanone, 4-hydroxy-4-methyl 2-propanone 1-acetyloxy 2(5H)-furanone 2-methyl-2-cyclopenten-1-one 2(5H)-furanone, 3-methyl 2-furancarboxaldehyde, 5-methyl phenol 2-heptanone 1,2-cyclohexanedione 1,2-cyclopentanedione, 3-methyl 2,3-dimethyl-2-cyclopenten-1-one phenol, 2-methyl phenol, 4-methyl phenol, 2-methoxy 4H-pyran-4-one, 3-hydroxy-2-methyl cyclopropyl carbinol phenol, 2,4-dimethyl phenol, 3-ethyl benzaldehyde, 3-methoxy 1,2-benzenediol 1,3-benzenediol phenol, 2-methoxy-4-methyl phenol, 2,3,5-trimethyl phenol, 2-ethyl-6-methyl phenol, 4-ethyl-3-methyl 1,2-benezenediol, 3-methyl phenol, 4-ethyl, 2,6-dimethyl 1,2-benzenediol, 4-methyl phenol, 4-ethyl-2-methoxy phenol, p-allyl phenol, 2-methoxy-4-vynil phenol, 4-(2-propenyl) 1,3-benzenediol, 4,5-dimethyl phenol, 2-methoxy-4-(2-propenyl) benzaldehyde, 4-hydroxy-3-methoxy naphthalene, 1,7-dimethyl phenol, 2-methoxy-4-(1-propenyl) levoglucosane ethanone, 1,4(-hydroxy-3-methoxyphenyl) benzoic acid, 4-hydroxy-3-methoxy, methyl ester phenol, 2-methoxy-4-propyl (1,1′-biphenyl)-2-ol 1,4-benzenediol, 2-(2-propenyl) 2-(2-oxoethyl)cis-bicyclol(3.3.0)octane-3,7-dione 2-(1,2-epoxycycloheptyl)-1-pentene phenol, 4-(ethoxymethyl)-2-methoxy 2-naphthalenol, 3-methoxy 3-(p-hydroxy-m-methoxyphenyl) -2-propenal quality > 80
MW quality area % 74 72 116 58 92 82 96 116 116 84 96 98 110 94 114 112 112 110 108 108 124 126 72 122 122 136 110 110 138 136 136 136 124 150 124 152 134 151 134 138 164 152 158 164 162 166 196
90 94 80 80 97 83 91 86 80 86 96 80 93 94 83 56 93 86 97 96 97 78 50 97 97 87 91 47 96 91 78 91 91 81 94 96 69 87 65 60 97 96 35 96 86 95 59
1.27 0.74 0.46 2.02 0.25 0.55 1.09 1.07 1.28 1.73 1.14 0.86 1.88 1.44 0.45 0.54 3.12 1.11 1.54 3.03 2.67 0.96 0.76 1.89 1.96 0.91 2.19 0.82 5.74 1.28 1.37 1.47 2.45 0.38 2.83 2.52 0.72 1.37 1.07 0.86 5.94 2.67 0.35 8.94 12.34 1.98 0.62
166 170 150 138
60 95 58 86
1.47 0.44 0.74 0.70
180 182 175 179
78 87 78 91
0.34 1.62 0.33 1.75 90.43
pyrolysis conditions than on biomass composition. Thus, the results in Tables 1 and 2 are very similar to those obtained by Maggi and Delmon for the liquid product composition at 525 °C in an industrial flash pyrolysis plant with a residence time of 0.35 s and by the feeding of a mixture of sawdusts obtained from different woods with 5.4 wt % humidity.7 In the liquid product obtained, the concentration of polycyclic aromatics is very low as a consequence of the short gas residence time. The fact that the concentrations of aromatics and of condensed
Figure 7. Effect of pyrolysis temperature on the char average pore diameter.
Figure 8. Effect of pyrolysis temperature on the char micropore volume and on the surface area corresponding to micropores.
rings increase with residence time has been proven by Stiles and Kandiyoti.20 Composition and Porous Structure of the Char. The weight C/H ratio of the char increases with temperature: C/H ) 13.9 (6.8 for sawdust) at 350 °C, 19.1 at 400 °C, 21.2 at 450 °C, 25.9 at 500 °C, 30.5 at 600 °C, and 35.9 at 700 °C. The O content is ≈30 wt %, irrespective of the temperature. Figure 7 shows the effect of temperature on the char average pore diameter determined by N2 adsorptiondesorption. The initial value corresponding to the sawdust is 156 Å. Between 350 and 450 °C the average diameter decreases in a very pronounced way to a value of 15 Å, which is constant for higher temperatures. This fact reveals that the char microporous structure is created at 450 °C. Nevertheless, the total development of the char microporous structure requires the performance of pyrolysis at 600 °C or at higher temperatures, as is observed in Figure 8, where the effect of pyrolysis temperature on micropore volume and on the BET surface area of the char is shown. Once the microporous structure is developed, the micropore volume is 0.13 cm3 g-1 and its surface area is 330 m2 g-1. These results for the physical properties of the char are comparable to those obtained in the literature in the pyrolysis of other materials.38 Conclusions The performance of the plant used shows the interest of the technology based on the conical spouted bed for
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Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000
pyrolysis of biomass. The gas-solid contact in conical spouted beds has general characteristics that make them suitable for handling biomass materials: stability and versatility with different materials and in a wide range of biomass flow; reduced segregation; requirement of a small amount of inert solid; low cost of design, construction, and operation; small pressure drop; operation and control simplicity. Besides, this technology fulfills particularly interesting conditions for flash pyrolysis: short gas residence time, around 50 ms; temperature uniformity in the bed; good heat and mass transfer between phases; rapid heating of the feed. The results obtained, in addition to opening encouraging prospects for future studies, show that it may be an alternative technology to the bubbling fluidized bed, which is a relatively well-developed technology at present. A yield of liquid product of the order of 70 wt % is obtained at 450 °C, which is even a slightly lower temperature than that required in the fluidized bed, and the characteristics of the gas and char formed are very similar to those obtained in the fluidized bed. The yield of liquid is very sensitive to pyrolysis temperature in the 350-700 °C range, in which although the yield of gases (CO2, CO, C4-, H2) increases with temperature, those of char and liquid pass through a maximum at 450 °C. This temperature is also the minimum required for the char to have a microporous structure. On the other hand, at higher temperatures than 450 °C CO2 formation increases in a very pronounced way The pyrolysis liquid is obtained in two fractions: an aqueous one, which is slightly unstable for storage and has a high water concentration, and another exclusively organic one, which is immiscible in water. In the light fraction, the following compounds are present in a relatively significant concentration: formaldehyde, methanol, acetic acid, butenal, furfural, several phenols, and levoglucosane. In the organic fraction, 55 oxygenate compounds, whose molecular weights range from 55 to 196, have been identified. Acknowledgment This work was carried out with the financial support of the Ministry of Education and Culture of the Spanish Government (Project QUI98-1105) and of the University of the Basque Country (Project G34-98). Notation Ar ) Archimedes number, gdp3Fg(F - Fg)/µ2 Db, Dc, Di, D0 ) upper diameter of the stagnant bed, diameter of the column, diameter of the bed base, and diamer of the inlet, respectively, m dp ) particle diameter, mm Hc, HT ) height of the conical section and total height, m (Re0)ms ) Reynolds number, refers to D0, Fudp/µ u, ums ) velocity and minimum spouting velocity of the gas, m s-1 Greek Letters γ ) contactor angle, deg µ ) viscosity, kg m-1 s-1 F, Fg ) density of the solid and of the gas, respectively, kg m-3
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Received for review May 3, 1999 Revised manuscript received January 21, 2000 Accepted February 7, 2000 IE990309V
