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Effects of temperature and particle size on bio-char yield from pyrolysis of agricultural residues
J. Anal. Appl. Pyrolysis 72 (2004) 243–248 www.elsevier.com/locate/jaap
Effects of temperature and particle size on bio-char yield from pyrolysis of agricultural residues Ayhan Demirbas* Department of Chemical Engineering, Selcuk University, 42031 Konya, Turkey Accepted 8 July 2004 Available online 11 September 2004
Abstract This article deals with slow pyrolysis of agricultural residues such as olive husk, corncob and tea waste at high temperature (950–1250 K) in a cylindrical reactor batch reactor. The aim of this study was to experimentally investigate how different residues utilizing strategies affect the treatment conditions such as temperature, particle size, and lignin and inorganic matter contents on bio-char yield and reactivity. When the pyrolysis temperature is increased, the bio-char yield decreases. The bio-char yield increased with increasing particle size of the sample. A high temperature and smaller particles increase the heating rate resulting in a decreased bio-char yield. The higher lignin content in olive husk results in a higher bio-char yield comparison with corncob. Bio-char from olive husk was more reactive in gasification than bio-char from corncob because of the higher ash content. # 2004 Elsevier B.V. All rights reserved. Keywords: Agricultural residue; Slow pyrolysis; Bio-char; Char reactivity
1. Introduction Bio-char can be obtained from biomass pyrolysis. For a high bio-char production from biomass pyrolysis, a low temperature and low heating rate process would be chosen. The bio-char can be used in the preparation of active carbon when its pore structure and surface area are appropriate. The starting materials used in commercial production of activated carbons are those with high carbon contents such as wood, lignite, peat, and coal of different ranks or low-cost and abundantly available agricultural by-products. Active carbons can be manufactured from virtually any carbonaceous precursor, but the most commonly used materials are wood, coal and coconut shell [1]. The development of activated carbons from agricultural carbonaceous wastes will be advantageous for environmental problems. In water contamination, wastewater contains many traces of organic compounds, which is a serious environmental problem. Active carbons are carbonaceous materials with highly developed internal surface area and porosity. Activated * Tel.: +90 462 230 7831; fax: +90 462 248 8508. E-mail address: [email protected]. 0165-2370/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2004.07.003
carbon is widely used as an effective adsorbent in many applications such as air separation and purification, vehicle exhaust emission control, solvent recovery and catalyst support because of its high specific pore surface area, adequate pore size distribution and relatively high mechanical strength. The large surface area results in high capacity for adsorbing chemicals from gases and liquids [2]. Pyrolysis can be used for the production of bio-oil if flash pyrolysis processes are used and are currently at pilot stage [3]. Some problems in the conversion process and use of the oil need to be overcome; these include poor thermal stability and corrosivity of the oil. Upgrading by lowering the oxygen content and removing alkalis by means of hydrogenation and catalytic cracking of the oil may be required for certain applications [4]. Chemical additives (AlCl3, FeCl3, H3PO4, NH4Cl, KOH and ZnCl2) slightly affect the first step by inhibiting hemicelluloses decomposition and accelerating cellulose decomposition through the dehydration reaction. Phosphoric acid exhibited the largest influence on the pyrolysis process. At concentrations higher than 30% H3PO4, the two weight loss steps ascribed to hemicelluloses and cellulose decomposition overlapped. Bio-char with an alkaline character of
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the surface, tar and gas products are obtained by steam pyrolysis of biomass (almond shells, nut shells, apricot stones, cherry stones, grape seeds) [5]. The reaction mechanisms of biomass pyrolysis are complex but can be defined in three main steps: Biomass ! Water þ Unreacted residue
(1)
Unreacted residue ! ðVolatile þ GasesÞ1 þ ðCharÞ1
(2)
ðCharÞ1 ! ðVolatile þ GasesÞ2 þ ðCharÞ2
(3)
Pyrolysis proceeds in three steps: in the initial step moisture and some volatile loss (Eq. (1)). In the secondary step occurred primary bio-char (Eq. (2)). The last fast step follows by a slower step including some chemical rearrangement of the bio-char. During the third step, the bio-char decomposes at a very slow rate and carbon-rich residual solid forms. The formation of secondary charring (Eq. (3)) makes the char less reactive. The char gasification forms an important part of biomass gasification. The major thermochemical gasification reactions include the following:Carbon char to methane: C þ 2H2 @ CH4
(4)
Carbon char to oxides: C þ O2 ! CO2 and C þ CO2 @ 2CO
(5)
Carbon char to CO and H2: C þ CO2 þ H2 O @ CO þ H2
(6)
The hot combustion products (CO2 and H2O) are further reduced by the char. These endothermic reactions generate synthetic gas (syngas): CO and H2 (Eq. (6)), and the exit gas can be utilized as a gaseous fuel. The molecules in the biomass (primarily carbon, hydrogen and oxygen) and the molecules in the steam (hydrogen and oxygen) reorganize to form this syngas. The high reactivity of bio-char is higher when smaller biomass particle are subjected to pyrolysis. The reactions of CO2 and H2O with the char to produce CO and H2 are considerably slower than the drying, pyrolysis or combustion reactions. The bio-char samples obtained by rapid pyrolysis at higher temperatures are more reactive in steam gasification than those obtained at lower pyrolysis temperatures. This result is of practical interest for utilization of biomass as a raw material for gasification. The aim of this work is to study the effect of the treatment conditions such as temperature, particle size, and lignin and inorganic matter contents on bio-char yield.
2. Experimental In this study, olive husk, corncob and tea waste from East Black Sea region in Turkey were used as agricultural residues. The samples were ground and sieved to give particle size of between <0.5 and >2.2 mm. The particle
Table 1 Particle size distributions of agricultural residues Particle size (mm)
Dry olive husk (%)
Dry corncob (%)
Dry tea waste (%)
<0.5 0.5–1.0 1.0–1.5 1.5–2.2 >2.2
6.4 7.6 38.6 27.9 9.5
8.5 12.8 37.5 26.3 14.9
4.2 5.7 32.6 23.4 34.1
size distributions of the samples are presented in Table 1. The pyrolysis experiments were performed in a device designed for this purpose. The main element of this device was a cylindrical reactor of height 95.1 mm, i.d. 17.0 mm, and o.d. 19.0 mm heated externally by an electric furnace with the temperature being controlled by a thermocouple inside the reactor. The chemical analyses of the samples were carried out according to the ASTM D1103-80 and ASTM D1104-56 standard test methods. The standard test methods for biomass fuel analyses are: particle size distribution (ASTM E828), moisture (ASTM E871), ash (ASTM D1102), volatile matter (ASTM E 872), carbon and hydrogen (ASTM E 777), nitrogen (ASTM E 778), sulfur (ASTM E 775), chlorine (ASTM E776) and ash elemental (ASTM D3682, ASTM D2795, ASTM D4278, AOAC 14.7). For structural analyses, the wood samples were prepared according to TAPPI standard (TAPPI T 11 m-45). The ground sample was extracted with ethanol–benzene according to ASTM, and lignin was determined as the insoluble residue after hydrolysis with 72% sulfuric acid. The used materials are characterized by analytical methods. The samples were subjected to pyrolize for obtaining biochars at high temperature (450–1250 K) in a cylindrical reactor batch reactor. The pyrolysis processes were carried out with 10 K/s heating rate for obtaining the bio-char products from the samples at different temperatures: 470550, 650, 750, 850, 950 and 1050 K. All yields were expressed on a dry and ash-free (daf) basis, and the average yields from three experiments were presented within the experimental error of <0.5 wt.%.
3. Results and discussion The chemical analysis results of agricultural residues are given in Table 2. From Table 2, the corncob has the highest volatile matter content (84.6 wt.% daf). The structural analysis results of biomass samples are shown in Table 3. As seen from Table 3, the lignin content of olive husk was 50.6 wt.% daf. Fig. 1 shows the effect of temperature on the bio-char yield. The decrease for the olive husk was 56.4% (from 44.5 to 19.4 wt.% daf) for particle size between 1.5 and 2.2 mm when the temperature is increased from 450 to 1250 K. The decrease for the corncob is 81.4% (from 30.6 to 5.7 wt.% daf)
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Table 2 Chemical analysis results of agricultural residues (wt.% dry basis) Sample
C
H
N
O (diff.)
Ash
Volatile matter (wt.% daf)
Fixed carbon (wt.% daf)
Olive husk Corncob Tea waste
50.2 49.0 48.2
6.4 5.6 5.5
1.0 0.5 0.5
38.4 43.8 44.3
4.1 1.1 1.5
72.5 84.6 83.8
27.5 15.4 16.2
at the same conditions. Corncob has very high cellulose (52% by weight) and hemicelluloses (32.5% by weight) contents. The yield bio-char from corncob at lower temperatures was relatively high. However, the bio-char yield rapidly decreases with increasing of pyrolysis temperature. The destructive reaction of cellulose is started at temperatures lower than 325 K and is characterized by a decreasing polymerization degree. Thermal degradation of cellulose proceeds through two types of reaction: a gradual degradation, decomposition and charring on heating at lower temperatures, and a rapid volatilization accompanied by the formation of levoglucosan on pyrolysis at higher temperatures. The degradation of cellulose to a more stable anhydrocellulose, which gives higher bio-char yield, is the dominant reaction at temperature <575 K [7]. At temperatures >575 K, cellulose depolymerizes, producing volatiles. If the heating rate is very high, the residence time of the biomass at temperatures <575 K is insignificant. Thus, a high heating rate provides a shorter time for the dehydration reactions and the formation of less reactive anhydrocellulose, which gives higher yield of char [2]. The result is that the rapid heating of the biomass favors the polymerization of cellulose and the formation of volatiles and suppresses the dehydration to anhydrocellulose and char formation [6]. Hence the effect of heating rate is stronger in the pyrolysis of biomass than in that coal. The initial degradation reactions include depolymerization, hydrolysis, oxidation, dehydration, and decarboxylation [7]. The isothermal pyrolysis of cellulose in air and milder conditions, in the temperature range 623–643 K, was investigated [8]. Under these conditions, the pyrolysis reactions produced 62–72% aqueous distillate and left 10– 18% charred residue. After the pyrolysis, the residue was found to consist of some water-soluble materials, in addition to char and undecomposed cellulose. The hemicelluloses undergo thermal decomposition very readily. The hemicelluloses reacted more readily than cellulose during heating. The thermal degradation of hemicelluloses begins above 373 K during heating for 48 h; hemicelluloses and lignin are depolymerized by
steaming at high temperature for a short time. The metoxyl content of wet meals decreased at 493 K [9]. The stronger effect of the heating rate on the formation of bi-char from biomass than from coal may be attributed to the cellulose content of the biomass [10]. It is well known that heating rate has a significant effect on the pyrolysis of cellulose. Heating rate has a much greater effect on the pyrolysis of biomass than on that of coal. The quick devolatilization of the biomass in rapid pyrolysis favors the formation of char with high porosity and high reactivity [11]. The decreased formation of char at the higher heating rate was accompanied by an increased formation of tar. The net effect is a decrease in the volatile fuel production and an increased yield of bio-char cellulose converted to levoglucosan at above 535 K temperatures [12]. The inorganic properties of biomass samples are presented in Table 4. The different amount of inorganics may also affect the results. The yield of bio-char was calculated according to the equation (Eq. (7)) below [11]: Bio-char yield ðwt:% dafÞ ðAb =Ac Þ ðAb =100Þ ¼ 100 1 ðAb =100Þ
(7)
where Ab is wt.% ash in dry biomass and Ac is wt.% ash in dry bio-char. Fig. 2 shows the effect of particle size on bio-char yield in the conditions selected for the study. In the experiments with olive husk at 950 K the bio-char yield decreases 45.5% (from 35.6 to 19.4 wt.% daf) when the particle size reduced from 2.2 to 0.5 mm. An increase in particle size from 0.5 to
Table 3 Structural analysis results of agricultural residues (wt.% dry, ash and extractive free) Sample
Hemicelluloses
Cellulose
Lignin
Olive husk Corncob Tea waste
24.2 32.5 23.3
25.2 52.0 33.2
50.6 15.5 43.5
Fig. 1. Effect of temperature on bio-char yield. Particle size: 1.5–2.2 mm.
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Table 4 Inorganic properties of biomass samples (wt.% of the ash) Sample
Si2O Al2O2 Fe2O3 CaO MgO Na2O K2O SO3 P2O5
Olive husk 32.9 8.4 Corncob 52.9 9.1 Tea waste 44.4 7.2
6.3 6.8 5.2
14.5 4.2 9.4 3.3 12.6 3.8
26.4 1.6 1.8
4.3 0.6 5.3 4.9 18.8 1.4
2.5 6.6 4.8
2.2 mm for corncob at 950 K increases the solid residue from 5.7 to 16.6 wt.% after total pyrolysis, i.e. a 65.7% increase in amount of bio-char. Fig. 3 shows the effect of temperature on carbon content in char. Fig. 4 shows the effect of temperature on oxygen content in char. Fig. 5 shows the effect of temperature on hydrogen content in char. The results of the elemental analysis (Figs. 3–5) indicate that contents of carbon increase with pyrolysis temperature while these corresponding to hydrogen and oxygen decrease. Losses in hydrogen and oxygen correspond to the scission of weaker bonds within the bio-char structure favored by the higher temperature [13]. Fig. 6 shows relationships between the content of lignin and the bio-char yield from the samples. The higher lignin content in olive husk (Table 3) gives a higher bio-char yield comparison with oak wood and wheat straw. Lignin gives higher yields of charcoal and tar from wood although lignin has 3-fold methoxyl than that of wood [14–16]. If the purpose were to maximize the yield of liquid products resulting from biomass pyrolysis, a low temperature, high heating rate, short gas residence time process would be required. For a high char production, a low temperature, low heating rate process would be chosen. If the purpose were to maximize the yield of fuel gas resulting from pyrolysis, a high temperature, low heating rate, long gas residence time process would be preferred. Phenolics are derived from lignin by cracking of the phenyl–propane units of the macromolecule lattice. Pyrolysis
seems to produce the most substituted phenols on a selective basis. This phenomenon can be explained by the fact that the syringyl–propan units are not so linked to the lignin skeleton as the less substituted: gaiacyl–propane and phenyl–propane [17]. It has been compared to the DTA curves of different lignin preparations in vacuo and concluded that the degradation pattern was virtually the same, an endotherm extending from about 373–453 K and followed by an exotherm at about 675 K, as the results of thermal analysis of individual types of lignin made by many other investigators [18,19]. Lignin is broken down by extensive cleavage of b-aryl ether linkages during steaming of wood less than 488 K [20]. It has been found that on analysis of the metoxyl groups after isothermal heating of dry distilled wood, lignin decomposition begins at about 550 K with a maximum rate occurring between 625 and 725 K and the completion of the reaction occurs at 725 and 775 K [9].
Fig. 2. Effect of particle size on bio-char yield. Final temperature: 950 K.
Fig. 4. Effect of temperature on oxygen content in bio-char. Particle size: 1.5–2.2 mm.
Fig. 3. Effect of temperature on carbon content in bio-char. Particle size: 1.5–2.2 mm.
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4. Conclusion
Fig. 5. . Effect of temperature on hydrogen content in bio-char. Particle size: 1.5–2.2 mm.
The formation of char from lignin under mild reaction conditions is a result of the breaking of the relatively weak bonds, like the alkyl–aryl ether bond(s), and the consequent formation of more resistant condensed structures, as has already been noted by Domburg et al. [21]. One additional parameter, which may also have an effect on the char formation is the moisture content of the kraft lignin used. It has been found that the presence of moisture increased the yield of char from the pyrolysis of wood waste at temperatures between 660 and 730 K, while Stray et al. [22] found only a slight effect for water added on the hydrogenolysis of both hardwood and softwood lignins at temperatures between 470 and 675 K. The thermolysis reactions of the kraft lignin is comprised mainly of breaking the most reactive bonds like the methyl C–O bond of the methoxyl group and the condensation reactions to high molecular weight products (char) that follows.
Fig. 6. Effect of lignin content on yield of bio-char at 950 K final temperature. Particle size: 1.5–2.2 mm.
Bio-char can be obtained from biomass pyrolysis. If the purpose were to maximize the yield of bio-char resulting from pyrolysis, a low temperature, low heating rate process would be chosen. When the pyrolysis temperature increased the bio-char yield decreased. The bio-char yield increased with increasing particle size of the sample. The higher lignin content in olive husk results in a higher bio-char yield comparison with corncob. Bio-chars from olive husk are more reactive in gasification than bio-chars from corncob because of the higher ash content. Biomass converts by pyrolysis into liquid (bio-oil), biochar and gasses by heating the biomass to about 850 K in the absence of air. The process can be adjusted to favor bio-char, pyrolytic oil, gas, or methanol production with a 95.5% fuelto-feed efficiency. The heat flux is proportional to the driving force, the temperature difference between the particle and environment. At higher temperature, the heat flux is higher. The size of the particles affects the heating rate. The heat flux and the heating rate are higher in small particles than in large particles. The higher heating rate favors a decrease of the char yield [2].
References [1] W. Heschel, E. Klose, Fuel 74 (1995) 1786–1791. [2] R. Zanzi, Pyrolysis of biomass, dissertation, Royal Institute of Technology, Department of Chemical Engineering and Technology, Stockholm, 2001. [3] EUREC Agency, The Future for Renewable Energy Prospects and Directions, James and James Science Publishers Ltd., London, UK, 1996. [4] A. Demirbas, D. Gullu, Energy Edu. Sci. Technol. 2 (1998) 111– 115. [5] F. Shafizadeh, in: R.P. Overend, T.A. Milne, L.K. Mudge (Eds.), Fundamentals of Thermochemicals Biomass Conversion, Elsevier Applied Science, London, UK, 1985. [6] C. Ekstro¨ m, E. Rensfeld, in: J. Diebold (Ed.), Proceeding of the Specialists’ Workshop on Fast Pyrolysis of Biomass, Copper Mountai, CO, 1980 (SERI/CP-622-1076). [7] F. Shafizadeh, J. Anal. Appl. Pyrolysis 3 (1982) 283–305. [8] D. Fengel, G. Wegener, Wood Chemistry, Ultrastructure, Reactions, Walter de Gruyter, Berlin, 1983. [9] A. Demirbas, M.M. Kucuk, Cellul. Chem. Technol. 28 (1994) 85–94. [10] M.M. Kucuk, A. Demirbas, Cellul. Chem. Technol. 27 (1993) 679– 686. [11] R. Zanzi, K. Sjo¨ stro¨ m, E. Bjo¨ rnbom, Fuel 75 (1996) 545–550. [12] K. Freudenberg, A.C. Neish, Constitution and Biosynthesis of Lignin, Springer, New York, 1968. [13] D.A. Della Rocca, G.I. Horowitz, P.R. Bonelli, M.C. Cassanelo, A.L. Cukierman, Olive stones pyrolysis: chemical, textural and kinetic characterization, in: D.G.B. Boocock (Ed.), Development in Thermochemical Biomass Conversion, Blackie, London, UK, 1997. [14] A. Sakakibara, in: T. Higuchi, H.M. Chang, T.K. Kirk (Eds.), Recent Advances in Lignin Biodegradation Research, UNI Publisher, Tokyo, 1983, p. 125. [15] A. Demirbas, Energy Edu. Sci. Technol. 5 (2000) 21–45. [16] H.F.J. Wenzl, F.E. Brauns, D.A. Brauns, The Chemical Technology of Wood, Academic Press, New York, 1970.
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[17] F.L. Brown, Theories on the combustion of wood and its control. US Forest Product Lab Rep: 2136, 1958. [18] E.J. Soltes, T.J. Elder, in: I.S. Goldstein (Ed.), Organic Chemicals from Biomass, CRC Press, Boca Rotan, FL, 1981. [19] W.A. Pryor, Free Radicals, McGraw-Hill, New York, 1966. [20] J. March, Advanced Organic Chemistry: Reactions Mechanisms and Structure, McGraw-Hill, New York, 1977.
[21] G. Domburg, G. Rossinskaya, V. Sergseva, Study of thermal stability of b-ether bonds in lignin and its models, in: Proceedings of the 4th International Conference on Thermal Analysis, Budapest, 1974, p. 221. [22] C. Stray, P.J. Cassidy, W.R. Jackson, F.P. Larkins, J.F. Sutton, Fuel 65 (1986) 1524–1530.
Effects of temperature and particle size on bio-char yield from pyrolysis of agricultural residues Ayhan Demirbas* Department of Chemical Engineering, Selcuk University, 42031 Konya, Turkey Accepted 8 July 2004 Available online 11 September 2004
Abstract This article deals with slow pyrolysis of agricultural residues such as olive husk, corncob and tea waste at high temperature (950–1250 K) in a cylindrical reactor batch reactor. The aim of this study was to experimentally investigate how different residues utilizing strategies affect the treatment conditions such as temperature, particle size, and lignin and inorganic matter contents on bio-char yield and reactivity. When the pyrolysis temperature is increased, the bio-char yield decreases. The bio-char yield increased with increasing particle size of the sample. A high temperature and smaller particles increase the heating rate resulting in a decreased bio-char yield. The higher lignin content in olive husk results in a higher bio-char yield comparison with corncob. Bio-char from olive husk was more reactive in gasification than bio-char from corncob because of the higher ash content. # 2004 Elsevier B.V. All rights reserved. Keywords: Agricultural residue; Slow pyrolysis; Bio-char; Char reactivity
1. Introduction Bio-char can be obtained from biomass pyrolysis. For a high bio-char production from biomass pyrolysis, a low temperature and low heating rate process would be chosen. The bio-char can be used in the preparation of active carbon when its pore structure and surface area are appropriate. The starting materials used in commercial production of activated carbons are those with high carbon contents such as wood, lignite, peat, and coal of different ranks or low-cost and abundantly available agricultural by-products. Active carbons can be manufactured from virtually any carbonaceous precursor, but the most commonly used materials are wood, coal and coconut shell [1]. The development of activated carbons from agricultural carbonaceous wastes will be advantageous for environmental problems. In water contamination, wastewater contains many traces of organic compounds, which is a serious environmental problem. Active carbons are carbonaceous materials with highly developed internal surface area and porosity. Activated * Tel.: +90 462 230 7831; fax: +90 462 248 8508. E-mail address: [email protected]. 0165-2370/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2004.07.003
carbon is widely used as an effective adsorbent in many applications such as air separation and purification, vehicle exhaust emission control, solvent recovery and catalyst support because of its high specific pore surface area, adequate pore size distribution and relatively high mechanical strength. The large surface area results in high capacity for adsorbing chemicals from gases and liquids [2]. Pyrolysis can be used for the production of bio-oil if flash pyrolysis processes are used and are currently at pilot stage [3]. Some problems in the conversion process and use of the oil need to be overcome; these include poor thermal stability and corrosivity of the oil. Upgrading by lowering the oxygen content and removing alkalis by means of hydrogenation and catalytic cracking of the oil may be required for certain applications [4]. Chemical additives (AlCl3, FeCl3, H3PO4, NH4Cl, KOH and ZnCl2) slightly affect the first step by inhibiting hemicelluloses decomposition and accelerating cellulose decomposition through the dehydration reaction. Phosphoric acid exhibited the largest influence on the pyrolysis process. At concentrations higher than 30% H3PO4, the two weight loss steps ascribed to hemicelluloses and cellulose decomposition overlapped. Bio-char with an alkaline character of
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A. Demirbas / J. Anal. Appl. Pyrolysis 72 (2004) 243–248
the surface, tar and gas products are obtained by steam pyrolysis of biomass (almond shells, nut shells, apricot stones, cherry stones, grape seeds) [5]. The reaction mechanisms of biomass pyrolysis are complex but can be defined in three main steps: Biomass ! Water þ Unreacted residue
(1)
Unreacted residue ! ðVolatile þ GasesÞ1 þ ðCharÞ1
(2)
ðCharÞ1 ! ðVolatile þ GasesÞ2 þ ðCharÞ2
(3)
Pyrolysis proceeds in three steps: in the initial step moisture and some volatile loss (Eq. (1)). In the secondary step occurred primary bio-char (Eq. (2)). The last fast step follows by a slower step including some chemical rearrangement of the bio-char. During the third step, the bio-char decomposes at a very slow rate and carbon-rich residual solid forms. The formation of secondary charring (Eq. (3)) makes the char less reactive. The char gasification forms an important part of biomass gasification. The major thermochemical gasification reactions include the following:Carbon char to methane: C þ 2H2 @ CH4
(4)
Carbon char to oxides: C þ O2 ! CO2 and C þ CO2 @ 2CO
(5)
Carbon char to CO and H2: C þ CO2 þ H2 O @ CO þ H2
(6)
The hot combustion products (CO2 and H2O) are further reduced by the char. These endothermic reactions generate synthetic gas (syngas): CO and H2 (Eq. (6)), and the exit gas can be utilized as a gaseous fuel. The molecules in the biomass (primarily carbon, hydrogen and oxygen) and the molecules in the steam (hydrogen and oxygen) reorganize to form this syngas. The high reactivity of bio-char is higher when smaller biomass particle are subjected to pyrolysis. The reactions of CO2 and H2O with the char to produce CO and H2 are considerably slower than the drying, pyrolysis or combustion reactions. The bio-char samples obtained by rapid pyrolysis at higher temperatures are more reactive in steam gasification than those obtained at lower pyrolysis temperatures. This result is of practical interest for utilization of biomass as a raw material for gasification. The aim of this work is to study the effect of the treatment conditions such as temperature, particle size, and lignin and inorganic matter contents on bio-char yield.
2. Experimental In this study, olive husk, corncob and tea waste from East Black Sea region in Turkey were used as agricultural residues. The samples were ground and sieved to give particle size of between <0.5 and >2.2 mm. The particle
Table 1 Particle size distributions of agricultural residues Particle size (mm)
Dry olive husk (%)
Dry corncob (%)
Dry tea waste (%)
<0.5 0.5–1.0 1.0–1.5 1.5–2.2 >2.2
6.4 7.6 38.6 27.9 9.5
8.5 12.8 37.5 26.3 14.9
4.2 5.7 32.6 23.4 34.1
size distributions of the samples are presented in Table 1. The pyrolysis experiments were performed in a device designed for this purpose. The main element of this device was a cylindrical reactor of height 95.1 mm, i.d. 17.0 mm, and o.d. 19.0 mm heated externally by an electric furnace with the temperature being controlled by a thermocouple inside the reactor. The chemical analyses of the samples were carried out according to the ASTM D1103-80 and ASTM D1104-56 standard test methods. The standard test methods for biomass fuel analyses are: particle size distribution (ASTM E828), moisture (ASTM E871), ash (ASTM D1102), volatile matter (ASTM E 872), carbon and hydrogen (ASTM E 777), nitrogen (ASTM E 778), sulfur (ASTM E 775), chlorine (ASTM E776) and ash elemental (ASTM D3682, ASTM D2795, ASTM D4278, AOAC 14.7). For structural analyses, the wood samples were prepared according to TAPPI standard (TAPPI T 11 m-45). The ground sample was extracted with ethanol–benzene according to ASTM, and lignin was determined as the insoluble residue after hydrolysis with 72% sulfuric acid. The used materials are characterized by analytical methods. The samples were subjected to pyrolize for obtaining biochars at high temperature (450–1250 K) in a cylindrical reactor batch reactor. The pyrolysis processes were carried out with 10 K/s heating rate for obtaining the bio-char products from the samples at different temperatures: 470550, 650, 750, 850, 950 and 1050 K. All yields were expressed on a dry and ash-free (daf) basis, and the average yields from three experiments were presented within the experimental error of <0.5 wt.%.
3. Results and discussion The chemical analysis results of agricultural residues are given in Table 2. From Table 2, the corncob has the highest volatile matter content (84.6 wt.% daf). The structural analysis results of biomass samples are shown in Table 3. As seen from Table 3, the lignin content of olive husk was 50.6 wt.% daf. Fig. 1 shows the effect of temperature on the bio-char yield. The decrease for the olive husk was 56.4% (from 44.5 to 19.4 wt.% daf) for particle size between 1.5 and 2.2 mm when the temperature is increased from 450 to 1250 K. The decrease for the corncob is 81.4% (from 30.6 to 5.7 wt.% daf)
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Table 2 Chemical analysis results of agricultural residues (wt.% dry basis) Sample
C
H
N
O (diff.)
Ash
Volatile matter (wt.% daf)
Fixed carbon (wt.% daf)
Olive husk Corncob Tea waste
50.2 49.0 48.2
6.4 5.6 5.5
1.0 0.5 0.5
38.4 43.8 44.3
4.1 1.1 1.5
72.5 84.6 83.8
27.5 15.4 16.2
at the same conditions. Corncob has very high cellulose (52% by weight) and hemicelluloses (32.5% by weight) contents. The yield bio-char from corncob at lower temperatures was relatively high. However, the bio-char yield rapidly decreases with increasing of pyrolysis temperature. The destructive reaction of cellulose is started at temperatures lower than 325 K and is characterized by a decreasing polymerization degree. Thermal degradation of cellulose proceeds through two types of reaction: a gradual degradation, decomposition and charring on heating at lower temperatures, and a rapid volatilization accompanied by the formation of levoglucosan on pyrolysis at higher temperatures. The degradation of cellulose to a more stable anhydrocellulose, which gives higher bio-char yield, is the dominant reaction at temperature <575 K [7]. At temperatures >575 K, cellulose depolymerizes, producing volatiles. If the heating rate is very high, the residence time of the biomass at temperatures <575 K is insignificant. Thus, a high heating rate provides a shorter time for the dehydration reactions and the formation of less reactive anhydrocellulose, which gives higher yield of char [2]. The result is that the rapid heating of the biomass favors the polymerization of cellulose and the formation of volatiles and suppresses the dehydration to anhydrocellulose and char formation [6]. Hence the effect of heating rate is stronger in the pyrolysis of biomass than in that coal. The initial degradation reactions include depolymerization, hydrolysis, oxidation, dehydration, and decarboxylation [7]. The isothermal pyrolysis of cellulose in air and milder conditions, in the temperature range 623–643 K, was investigated [8]. Under these conditions, the pyrolysis reactions produced 62–72% aqueous distillate and left 10– 18% charred residue. After the pyrolysis, the residue was found to consist of some water-soluble materials, in addition to char and undecomposed cellulose. The hemicelluloses undergo thermal decomposition very readily. The hemicelluloses reacted more readily than cellulose during heating. The thermal degradation of hemicelluloses begins above 373 K during heating for 48 h; hemicelluloses and lignin are depolymerized by
steaming at high temperature for a short time. The metoxyl content of wet meals decreased at 493 K [9]. The stronger effect of the heating rate on the formation of bi-char from biomass than from coal may be attributed to the cellulose content of the biomass [10]. It is well known that heating rate has a significant effect on the pyrolysis of cellulose. Heating rate has a much greater effect on the pyrolysis of biomass than on that of coal. The quick devolatilization of the biomass in rapid pyrolysis favors the formation of char with high porosity and high reactivity [11]. The decreased formation of char at the higher heating rate was accompanied by an increased formation of tar. The net effect is a decrease in the volatile fuel production and an increased yield of bio-char cellulose converted to levoglucosan at above 535 K temperatures [12]. The inorganic properties of biomass samples are presented in Table 4. The different amount of inorganics may also affect the results. The yield of bio-char was calculated according to the equation (Eq. (7)) below [11]: Bio-char yield ðwt:% dafÞ ðAb =Ac Þ ðAb =100Þ ¼ 100 1 ðAb =100Þ
(7)
where Ab is wt.% ash in dry biomass and Ac is wt.% ash in dry bio-char. Fig. 2 shows the effect of particle size on bio-char yield in the conditions selected for the study. In the experiments with olive husk at 950 K the bio-char yield decreases 45.5% (from 35.6 to 19.4 wt.% daf) when the particle size reduced from 2.2 to 0.5 mm. An increase in particle size from 0.5 to
Table 3 Structural analysis results of agricultural residues (wt.% dry, ash and extractive free) Sample
Hemicelluloses
Cellulose
Lignin
Olive husk Corncob Tea waste
24.2 32.5 23.3
25.2 52.0 33.2
50.6 15.5 43.5
Fig. 1. Effect of temperature on bio-char yield. Particle size: 1.5–2.2 mm.
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Table 4 Inorganic properties of biomass samples (wt.% of the ash) Sample
Si2O Al2O2 Fe2O3 CaO MgO Na2O K2O SO3 P2O5
Olive husk 32.9 8.4 Corncob 52.9 9.1 Tea waste 44.4 7.2
6.3 6.8 5.2
14.5 4.2 9.4 3.3 12.6 3.8
26.4 1.6 1.8
4.3 0.6 5.3 4.9 18.8 1.4
2.5 6.6 4.8
2.2 mm for corncob at 950 K increases the solid residue from 5.7 to 16.6 wt.% after total pyrolysis, i.e. a 65.7% increase in amount of bio-char. Fig. 3 shows the effect of temperature on carbon content in char. Fig. 4 shows the effect of temperature on oxygen content in char. Fig. 5 shows the effect of temperature on hydrogen content in char. The results of the elemental analysis (Figs. 3–5) indicate that contents of carbon increase with pyrolysis temperature while these corresponding to hydrogen and oxygen decrease. Losses in hydrogen and oxygen correspond to the scission of weaker bonds within the bio-char structure favored by the higher temperature [13]. Fig. 6 shows relationships between the content of lignin and the bio-char yield from the samples. The higher lignin content in olive husk (Table 3) gives a higher bio-char yield comparison with oak wood and wheat straw. Lignin gives higher yields of charcoal and tar from wood although lignin has 3-fold methoxyl than that of wood [14–16]. If the purpose were to maximize the yield of liquid products resulting from biomass pyrolysis, a low temperature, high heating rate, short gas residence time process would be required. For a high char production, a low temperature, low heating rate process would be chosen. If the purpose were to maximize the yield of fuel gas resulting from pyrolysis, a high temperature, low heating rate, long gas residence time process would be preferred. Phenolics are derived from lignin by cracking of the phenyl–propane units of the macromolecule lattice. Pyrolysis
seems to produce the most substituted phenols on a selective basis. This phenomenon can be explained by the fact that the syringyl–propan units are not so linked to the lignin skeleton as the less substituted: gaiacyl–propane and phenyl–propane [17]. It has been compared to the DTA curves of different lignin preparations in vacuo and concluded that the degradation pattern was virtually the same, an endotherm extending from about 373–453 K and followed by an exotherm at about 675 K, as the results of thermal analysis of individual types of lignin made by many other investigators [18,19]. Lignin is broken down by extensive cleavage of b-aryl ether linkages during steaming of wood less than 488 K [20]. It has been found that on analysis of the metoxyl groups after isothermal heating of dry distilled wood, lignin decomposition begins at about 550 K with a maximum rate occurring between 625 and 725 K and the completion of the reaction occurs at 725 and 775 K [9].
Fig. 2. Effect of particle size on bio-char yield. Final temperature: 950 K.
Fig. 4. Effect of temperature on oxygen content in bio-char. Particle size: 1.5–2.2 mm.
Fig. 3. Effect of temperature on carbon content in bio-char. Particle size: 1.5–2.2 mm.
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4. Conclusion
Fig. 5. . Effect of temperature on hydrogen content in bio-char. Particle size: 1.5–2.2 mm.
The formation of char from lignin under mild reaction conditions is a result of the breaking of the relatively weak bonds, like the alkyl–aryl ether bond(s), and the consequent formation of more resistant condensed structures, as has already been noted by Domburg et al. [21]. One additional parameter, which may also have an effect on the char formation is the moisture content of the kraft lignin used. It has been found that the presence of moisture increased the yield of char from the pyrolysis of wood waste at temperatures between 660 and 730 K, while Stray et al. [22] found only a slight effect for water added on the hydrogenolysis of both hardwood and softwood lignins at temperatures between 470 and 675 K. The thermolysis reactions of the kraft lignin is comprised mainly of breaking the most reactive bonds like the methyl C–O bond of the methoxyl group and the condensation reactions to high molecular weight products (char) that follows.
Fig. 6. Effect of lignin content on yield of bio-char at 950 K final temperature. Particle size: 1.5–2.2 mm.
Bio-char can be obtained from biomass pyrolysis. If the purpose were to maximize the yield of bio-char resulting from pyrolysis, a low temperature, low heating rate process would be chosen. When the pyrolysis temperature increased the bio-char yield decreased. The bio-char yield increased with increasing particle size of the sample. The higher lignin content in olive husk results in a higher bio-char yield comparison with corncob. Bio-chars from olive husk are more reactive in gasification than bio-chars from corncob because of the higher ash content. Biomass converts by pyrolysis into liquid (bio-oil), biochar and gasses by heating the biomass to about 850 K in the absence of air. The process can be adjusted to favor bio-char, pyrolytic oil, gas, or methanol production with a 95.5% fuelto-feed efficiency. The heat flux is proportional to the driving force, the temperature difference between the particle and environment. At higher temperature, the heat flux is higher. The size of the particles affects the heating rate. The heat flux and the heating rate are higher in small particles than in large particles. The higher heating rate favors a decrease of the char yield [2].
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