1578
Energy & Fuels 2004, 18, 1578-1583
Fast Pyrolysis of Forestry Residue and Pine. 4. Improvement of the Product Quality by Solvent Addition Anja Oasmaa,*,† Eeva Kuoppala,† Johan-Fredrik Selin,‡ Steven Gust,‡ and Yrjo¨ Solantausta† VTT Processes, P. O. Box 1601, 02044 VTT, Biologinkuja 3-5, Espoo, Finland, and Fortum Oyj, P.O. Box 310, 06101 Porvoo, Finland Received March 30, 2004
Addition of alcohols was found to improve the homogeneity, decrease the viscosity and density, lower the flash point, and increase the heating value of pyrolysis liquids. Alcohol addition also lowered the viscosity and molecular mass increase during the aging of pyrolysis liquids. The reduction in the viscosity change was primarily due to a stabilizing effect of alcohols on the waterinsoluble high molecular mass lignin-derived fraction. Other effects include the formation of acetals in reactions of alcohols with aldehydes, ketones, and anhydrosugars. Low (e5 wt %) alcohol additions prevented aging reactions by a few months, while the higher (g10 wt %) ones retarded them by almost a year. Methanol was the most effective alcohol of those tested (methanol, ethanol, isopropanol). By improving solubility, the alcohols also enhanced the separation of the extractiverich top layer in the pyrolysis of forestry residue by decreasing its volume and increasing the concentration of extractives and solids in the top layer.
Introduction The advantage of pyrolysis liquids compared to pellets, wood chips, or other solid biomass will be their ease of use. All the customer has to do is to order the fuel and perform annual maintenance. This in turn requires that the fuel is stable. Thus, to achieve customer acceptance, heating oils have to tolerate a storage period of at least six months with the customer. Pyrolysis liquids are unstable1,2 due to their high amount of reactive oxygen-containing compounds. The instability is seen as a viscosity increase during storage,3,4 which, connected with the formation of water, finally leads to the separation of a lignin-rich bottom sludge.4 The pyrolysis liquids consist of water, acids, alcohols, aldehydes, ketones, carbohydrates, extractives, and degraded lignin.5 The main changes6 upon aging of pyrolysis liquids take place during the first six months * To whom correspondence should be addressed. Fax: +358-9-460 493. E-mail:
[email protected]. † VTT Processes. ‡ Fortum Oyj. (1) Diebold, J. P. A Review of the Chemical and Physical Mechanisms of the Storage Stability of Fast Pyrolysis Bio-oils. In Fast Pyrolysis of Biomass: A Handbook; Bridgwater, A., Ed.; CPL Press: Newbury, U.K., 2002; Vol. 2, pp 243-292. (2) Oasmaa 2003: Oasmaa, A. Fuel Oil Quality Properties of Woodbased Pyrolysis Liquids. Academic dissertation. Research Report Series, Report: 99. Department of Chemistry, University of Jyva¨skyla¨: Jyva¨skyla¨, Finland, 2003; 32 pp + appendices (251 pp). (3) Czernik, S. Storage of Biomass Pyrolysis Oils. In Proceedings of Specialist Workshop on Biomass Pyrolysis Oil Properties and Combustion; Estes Park, CO, Sept 26-28, 1994; NREL Paper No. CP-4307215, pp 67-76. (4) Oasmaa, A.; Leppa¨ma¨ki, E.; Koponen, P.; Levander, J.; Tapola, E. Physical Characterisation of Biomass-Based Pyrolysis Liquids. Application of Standard Fuel Oil Analyses; VTT Publication 306; VTT: Espoo, Finland, 1997; 46 pp + appendices (30 pp). (5) Oasmaa, A.; Kuoppala, E.; Solantausta, Y. Fast Pyrolysis of Forestry Residue. 2. Physicochemical Composition of Product Liquid. Energy Fuels 2003, 17 (2), 433-443.
of storage. The amount of water-insoluble fraction increases mainly due to polymerization and condensation reactions.6 This causes an increase in the average molecular mass of the liquid and in viscosity.3 The water content increases due to condensation reactions. The decrease in volatile aldehydes and ketones raises the flash and pour points of the liquid.6 An increase in water decreases the heating value.6 Polar solvents have been used to homogenize pyrolysis liquids4,7 and to reduce the increase in their viscosity.8,9 Quantitative data is reported on the effects of water,10,11 methanol8,12 and furfural.13 The addition of solvents, (6) Oasmaa, A.; Kuoppala, E. Fast Pyrolysis of Forestry Residue. 3. Storage Stability of Liquid Fuel. Energy Fuels 2003, 17 (3), 10751084. (7) Oasmaa, A.; Kyto¨, M.; Sipila¨, K. Pyrolysis Liquid Combustion Tests in an Industrial Boiler. In Progress in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackwell Science, U.K., 2001; Vol. 2, 1468-1481. (8) Diebold, J. P.; Czernik, S. Additives to Lower and Stabilize the Viscosity of Pyrolysis Oils during Storage. Energy Fuels 1997, 11, 1081-1091 (9) Czernik, S.; Maggi, R.; Peacocke, G. V. C. Review of Methods for Upgrading Biomass-derived Fast Pyrolysis Oils. In Fast Pyrolysis of Biomass: A Handbook; Bridgwater, A., Ed.; CPL Press: Newbury, U.K., 2002; Vol. 2, p 424. (10) Solantausta, Y.; Diebold, J.; Elliott, D. C.; Bridgwater, T.; Beckman, D. Assessment of Liquefaction and Pyrolysis Systems. VTT Research Notes 1573; VTT: Espoo, Finland, 1994. (11) Tiplady, I. R.; Peacocke, G. V. C.; Bridgwater, A. V. Physical Properties of Fast Pyrolysis Liquids from the Union Fenosa Pilot Plant. In Proceedings of the Second EC/Canada Workshop on Thermal Biomass Processing; Bridgwater, A. V., Hogan, E., Eds.; CPL Scientific Information Services, Ltd.: Newbury, U.K., 1996; pp 164-174. (12) Casanova, J. Comparative Study of Various Physical and Chemical Aspects of Pyrolysis Bio-Oils Versus Conventional Fuels Regarding Their Use in Engines. In Proceedings of Specialist Workshop on Biomass Pyrolysis Oil Properties and Combustion; Estes Park, CO, Sept 26-28, 1994, NREL CP-430-7215, pp 343-354. (13) Salvi, G.; Salvi, G., Jr. Pyrolytic Products Utilization Assessment Study, Commission of European Communities, Contract No. EN3B0191-1(CH), 1991.
10.1021/ef040038n CCC: $27.50 © 2004 American Chemical Society Published on Web 08/31/2004
Fast Pyrolysis of Forestry Residue and Pine Scheme 1. Esterification of Aldehydes and Ketones15
Energy & Fuels, Vol. 18, No. 5, 2004 1579 Table 1. Properties of Alcohols Compared to Those of Pyrolysis Liquids pyrolysis liquid methanol ethanol isopropanol
Scheme 2. Hemiacetal and Acetal Formation from Aldehydes and Ketones15
water, wt % viscosity at 20 °C, cSt LHV, MJ/kg density at 20 °C, kg/dm3 boiling point, °C flash point, °C pour point, °C a
20-30 100-200
max 0.05 0.75
max 6 1.5
max 0.1 2.8
13-18 1.10-1.30
19.9 0.791
26.6 0.800
28.7 0.785
40-110 27
64.6 11 DNAb
78.0 12 -111a
82.2 12 -89.5
Freezing point. b DNA ) data not available.
Experimental Section
Scheme 3. Acetal Formation from Monosaccharides15
especially methanol, showed a significant effect on the stabilization. The rate of viscosity increase for the liquid with 10 wt % of methanol was almost 20 times less than for the liquid without additives.8 It has been postulated8,14 that solvent addition can impact the liquid viscosity by three mechanisms: (1) physical dilution without affecting the chemical reaction rates, (2) reducing the reaction rate by molecular dilution or by changing the liquid microstructure, and (3) chemical reactions between the solvent and the liquid components that prevent further chain growth. The chemical reactions1,8 that have been suggested to occur between the pyrolysis liquid compounds and methanol or ethanol include esterification (Scheme 1) and acetalization (Schemes 2 and 3).15 Acetals serve as protecting groups for aldehydes and ketones.15 Considering the simplicity, the low cost of some solvents, and their beneficial effects on the liquid properties, alcohol addition has been suggested to be the most practical approach for pyrolysis liquid quality upgrading.14 In the present study, confirmation of the suggested reactions as well as new data on the effect of alcohol addition on physicochemical changes of softwood liquids during storage are presented. This is the fourth part of a series of publications that have focused on fuel oil properties of fast pyrolysis liquids of forestry residue: optimization of separation of extractives,16 physicochemical properties,5 and storage stability.6 (14) Oasmaa, A.; Czernik, S. Fuel Oil Quality of Biomass Pyrolysis Oils - State of the Art for the End Users. Energy Fuels 1999, 13 (4), 914-921. (15) McMurry, J. Fundamentals of Organic Chemistry. Brooks/Cole Publishing Co.: Pacific Grove, Albany, 1998; pp 566 + appendices. (16) Oasmaa, A.; Kuoppala, E.; Gust, S.; Solantausta, Y. Fast Pyrolysis of Forestry Residue. 1. Effect of Extractives on Phase Separation of Pyrolysis Liquids. Energy Fuels 2003, 17 (1), 1-12.
Liquid Production. Pyrolysis was carried out by employing a 20 kg/h capacity process development unit (PDU) at VTT.16,17 This transport bed reactor was initially designed and delivered by Ensyn Technology in 1995, and it has been subsequently modified by VTT. The ground, sieved (50 wt %. In addition, the solid content of the top phase increased. After the water content of the top phase decreased to about 6-7 wt %, no further reduction of the top phase in any pyrolysis liquid was achieved by further alcohol additions. This water content was typically achieved by 5 wt % alcohol addition. There was no significant difference between ethanol (EtOH) and isopropanol (IPA) in this respect. Effect of Alcohol Addition on Physicochemical Properties of Pyrolysis Liquids. Addition of alcohol (Table 1) improved the homogeneity/solubility of hydrophobic compounds (Figure 2) of the liquid; decreased its
Fast Pyrolysis of Forestry Residue and Pine
Figure 3. Effect of alcohol addition on viscosity of a brown forestry residue pyrolysis liquid (bottom phase): [, methanol; 0, ethanol; 4, IPA.
Figure 4. Effect of solvent addition on the flash point of a brown forestry residue pyrolysis liquid (bottom phase): [, methanol; 0, ethanol; 4, IPA.
Figure 5. Effect of solvent addition on lower heating value (LHV) of a brown forestry residue pyrolysis liquid: [, methanol; 0, ethanol; 4, IPA.
viscosity (Figure 3), density, and flash point (Figure 4); and increased its heating value (Figure 5). Homogeneity/Solubility. Alcohols are known4 to be efficient solvents for pyrolysis liquids. Figure 2 shows how another liquid phase (extractive material) dissolves in the pyrolysis liquid matrix. The amount of alcohol addition required for the dissolution of the extractives varied depending on the chemical composition of the pyrolysis liquid. As presented previously,6,19 the polar pyrolysis liquid compounds, like acids and alcohols, dissolved the hydrophobic lignin-derived and extractive material in the polar pyrolysis liquid matrix. If the ratio of hydrophobic to hydrophilic material increased significantly, the hydrophobic material, like lignin, separated out of the matrix. In aging, the increase in molecular mass of the liquid caused the separation of (19) Radlein, D. The Production of Chemicals from Fast Pyrolysis Bio-Oils. In Fast Pyrolysis of Biomass: A Handbook; Bridgwater, A., Ed.; CPL Press: Newbury, U.K., 1999; Vol. 1, pp 164-188.
Energy & Fuels, Vol. 18, No. 5, 2004 1581
Figure 6. Effect of alcohol on viscosity increase with roomtemperature storage: 9, pine; 0, pine + 5 wt % EtOH; [, brown forestry residue (BFR); ], BFR + 5 wt % IPA; /, BFR + 10 wt % IPA.
Figure 7. Viscosity increase (aging test for 6 h at 80 °C, which correlates to 3-4 months storage at room temperature6) versus added alcohol: [, methanol; 0, ethanol; 4, IPA.
the lignin-derived material as a heavy bottom sludge.4,18 Addition of alcohols prevented the phase separation longer. In the pyrolysis of forestry residue with a high extractive content, this phenomenon was seen as a separation of an extractive-rich top phase.16 Viscosity. Alcohol addition decreased the viscosity of the liquid (Figure 3) depending on the viscosity of the added solvent (Table 1). Pyrolysis liquids are typically Newtonian ones.16,18 The extractive-rich viscous top phase of forestry residue liquid caused a slight nonNewtonian behavior19 at lower temperatures, but the behavior disappeared when heated, as the extractives melted.16 Density. Addition of alcohol into pyrolysis liquid decreased expectedly its density. The difference between the samples was marginal due to minor differences in the densities of the alcohols (Table 1). Flash Point. Addition of alcohols (Table 1) into pyrolysis liquids had a beneficial effect on the pour point, but it also decreased the flash point (Figure 4) of the liquid. The relative change in the flash point diminished when the amount of added alcohol was increased. Chemical legislation in Finland limits the flash point of a flammable liquid to 21 and 55 °C. In transportation legislation the limit is 62 °C. The flash point of pyrolysis liquids typically falls into this category,4,18 if the product has been condensed at low temperature. The flash point of forestry residue liquids is lower than that of pine sawdust pyrolysis liquids, due to a larger amount of volatile compounds in the forestry residue feedstock (spruce, needles, bark). The flash point of pyrolysis
1582 Energy & Fuels, Vol. 18, No. 5, 2004
Oasmaa et al.
Figure 8. Aging of brown (stored) forestry residue liquid with various amounts of ethanol. Table 2. Change in Alcohols during Aging of a Brown Forestry Residue Liquid aged (24 h at 80 °Ca) liquid, wt %
fresh liquid, wt % BFRb BFR + 5 wt % EtOH BFR + 10 wt % EtOH BFR + 5 wt % IPA BFR + 10 wt % IPA a
change, wt %
water
MeOH
EtOH
IPA
water
MeOH
EtOH
IPA
water
MeOH
EtOH
IPA
26.7 25.5 24.3 25.5 24.3
1.04 0.94 0.91 0.94 0.89
0 4.88 9.34 0 0
0 0 0 5.19 10.3
28 26.5 25.4 26.6 25.4
0.82 0.81 0.77 0.75 0.71
0 3.98 7.73 0 0
0 0 0 4.44 8.74
28 26 25 26 25
-27 -16 -18 -25 -25
0 -23 -21 0 0
0 0 0 -17 -18
Correlates with changes upon 1 year of storage at 20 °C.6
b
liquid can be affected both by feedstock choice and by changing the liquid condensation conditions.5 Heating Value. Addition of alcohols increased the heating value of pyrolysis liquid (Figure 5), which can also be noticed by arithmetic calculations. The amount of the increase depended on the heating value of added alcohol (Table 1). With forestry residue liquids, the solvents also improved the solubility of high energy value16 extractives in pyrolysis liquids from the top phase to the bottom phase (Figure 1), which caused an additional increase in the heating value. Effect of Alcohol Amount on Aging. Addition of alcohol diminished the increase in viscosity of pyrolysis liquids (Figure 6). Methanol was the most efficient agent for decreasing aging, even though the differences between the used alcohols were small (Figure 7). There were some differences in chemical changes during the storage of pyrolysis liquids with or without6 alcohol addition (Figure 8). Addition of 2 wt % alcohol did not have any significant effect on stability improvement, but addition of 5 wt % alcohol prevented or retarded the aging reactions by a few months, especially the reactions of water-soluble compound groups. With 5 wt % alcohol addition, after 3-4 months storage the changes were fast, and within 6-7 months they reached the same level as without alcohol addition. Addition of 10 wt % alcohol prevented clearly the aging reactions (Figure 8). The efficiency of alcohols decreased in the following order: methanol, ethanol, isopropanol. Diebold and Czernik8 drew a similar conclusion: a decrease in efficiency in the order methanol, acetone, methanolacetone (1:1), ethanol, methyl isobutyl ketone-methanol (1:1), ethyl acetate.
Brown forestry residue.
Figure 9. Change in ether-solubles (ESs, dry matter) with and without alcohol during storage for pine pyrolysis liquids: [, pine 1; ], pine 1 + 5 wt % IPA; 9, pine 2; 0, pine 2 + 5 wt % EtOH.
Chemical Changes in Water-Soluble Fraction during Storage. Acids. There was no significant change in the amount of the main acids, acetic and formic acids, during 4-month storage of pyrolysis liquids with or without6 alcohol addition. Alcohols. The amount of alcohol in pyrolysis liquids is very low (
