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Energy & Fuels 2005, 19, 2556-2561
Quality Improvement of Pyrolysis Liquid: Effect of Light Volatiles on the Stability of Pyrolysis Liquids Anja Oasmaa,* Kai Sipila¨, Yrjo¨ Solantausta, and Eeva Kuoppala VTT Processes, P.O. Box 1601, 02044 VTT, Biologinkuja 3-5, Espoo, Finland Received November 23, 2004. Revised Manuscript Received August 22, 2005
The concentration method was developed for improving the storage stability of pyrolysis liquids without significantly decreasing the flash point of the liquid. This method, by which most of the water and part of the light reactive volatiles of fast pyrolysis liquid are replaced by alcohol, proved in laboratory-scale experiments to possess excellent potential to produce a high-quality (homogeneous, viscosity similar to light fuel oil, stable) liquid product, remove the unpleasant odor of pyrolysis, remove a part of the acidic content, and increase the heating value of the liquid by removing water. Preliminary tests at our companies’ process development unit PDU-scale pyrolyser (20 kg/h) showed that similar product upgrading could also be achieved on a larger scale. The kinds of compounds removed were similar. The condenser temperature was optimized to 50 ( 4 °C. Within these conditions, up to 23 wt % of the feed and pyrolysis water can be eliminated from the condensers. This fraction, which can contain up to 9 wt % of the organic yield, can be used for process heat. An overall liquid recovery of 44.3 wt % (compared to typical 48.9 wt %) and a pyrolysis water yield of 9.9 wt % (compared to typical 12 wt %) were measured for dry forest-residue feedstock.
Introduction Pyrolysis liquid (e.g., pyrolysis oil, bio-oil, bio-fuel oil) has been estimated to be the lowest-cost liquid biomassbased fuel in both IEA (International Energy Agency) Bioenergy studies1 and work performed in Finland.2 Its use as fuel in boilers3-9 and engines10-17 has been tested. However, before pyrolysis liquid can be commercialized, the estimated production costs have to be further decreased, its quality18 has to be improved, and * To whom correspondence should be addressed. Fax: +358-9-460 493. E-mail:
[email protected]. (1) McKeough, P.; Nissila¨, M.; Solantausta, Y.; Beckman, D.; O ¨ stman, A. Techno-Economic Assessment of Direct Biomass Liquefaction Processes; Final Report of IEA Cooperative Project Biomass Liquefaction Test Facility, VTT Research Report 337; VTT: Espoo, Finland, 1985; p 139. (2) Gust, S. Economics, Net Energy & CO2 Balance of Liquid Biofuels Produced in Finland; Final report to the SIHTI Programme; SIHTI: Neste Oy, Finland, 1992. (3) Wornat, M. J.; Porter, B. J.; Yang, N. Y. C. Single Droplet Combustion of Biomass Pyrolysis Oils. Energy Fuels 1994, 8, 11311142. (4) Hallgren, B. Test Report of Metlab Miljo¨ Ab; Metlab Miljo¨ Ab: Skelleftehamn, Sweden, 1997; p 17. (5) Shaddix, C. R.; Huey, S. P. Combustion Characteristics of Fast Pyrolysis Oils Derived from Hybrid Poplar. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, U.K., 1997; Vol. 1, pp 465-480. (6) Gust, S. Combustion Experiences of Flash Pyrolysis Fuel in Intermediate Size Boilers. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, U.K., 1997; Vol. 1, pp 481-488. (7) Oasmaa, A.; Kyto¨, M.; Sipila¨, K. Pyrolysis Liquid Combustion Tests in an Industrial Boiler. In Progress in Thermochemical Biomass Conversion; Bridgwater, A., Ed.; Blackwell Science: Oxford, U.K., 2001; Vol. 2, pp 1468-1481. (8) Gust, S.; Nieminen, J.-P.; Nyro¨nen, T. Forestera (TM)-Liquefied Wood Fuel Pilot Plant. In Pyrolysis and Gasification of Biomass and Waste-The Future for Pyrolysis and Gasification of Biomass and Waste: Status, Opportunities and Policies for Europe; Bridgwater, A. V., Ed.; CPL Press: Newbury, U.K., 2003; pp 169-174.
the production of a homogeneous liquid has to be demonstrated industrially. The most important criteria for fuel oil quality are (i) low solids content, (ii) good homogeneity and stability, and (iii) a reasonably high flash point.19,20 Alcohol (9) Kyto¨, M.; Martin, P.; Gust, S. Development of Combustors for Pyrolysis Liquids. In Pyrolysis and Gasification of Biomass and WasteThe Future for Pyrolysis and Gasification of Biomass and Waste: Status, Opportunities and Policies for Europe; Bridgwater, A. V., Ed.; CPL Press: Newbury, U.K., 2003; pp 187-190. (10) Solantausta, Y.; Nylund, N.-O.; Westerholm, M.; Koljonen, T.; Oasmaa, A. Wood Pyrolysis Liquid as Fuel in a Diesel Power Plant. Bioresour. Technol. 1993, 1-2, 177-188. (11) Solantausta, Y.; Nylund, N.-O.; Gust, S. Use of Pyrolysis Oil in a Test Diesel Engine to Study the Feasibility of a Diesel Power Plant Concept. Biomass Bioenergy 1994, 7, 297-306. (12) Gros, S. Pyrolysis Oil as Diesel Fuel. In Power Production from Biomass II with Special Emphasis on Gasification and Pyrolysis R&D; Sipila¨, K., Korhonen, M., Eds.; VTT Symposium 164; VTT: Espoo, Finland, 1996; pp 225-238. (13) Jay, D. C.; Rantanen, O. A.; Sipila¨, K.; Nylund, N.-O. Wood Pyrolysis Oil for Diesel Engines. In New Technology and Design, Proceedings of the 17th Annual Fall Technical Conference, Milwaukee, WI, Sept 24-27, 1995; Caton, J. A., Ed.; ASME: New York, 1995. (14) Sipila¨, K.; Oasmaa, A.; Arpiainen, V.; Westerholm, M.; Solantausta, Y.; Angher, A.; Gros, S.; Nyro¨nen, T.; Gust, S. Pyrolysis Oils for Power Plants and Boilers. In Biomass for Energy and the Environment; Chartier, P. et al., Eds.; Elsevier Science: Oxford, U.K., 1996; Vol. 1, pp 302-307. (15) Suppes, G. J.; Natarajan, V. P.; Chen, Z. Autoignition of Select Oxygenated Fuels in a Simulated Diesel Engine Environment. AICHE 1996 National Meeting, New Orleans, LA, Feb 26, 1996; AICHE: New York, 1996; Paper No. 74e, p 9. (16) Shihadeh, A. L. Rural Electrification from Local Resources: Biomass Pyrolysis Oil Combustion in a Direct Injection Diesel Engine. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1998. (17) Andrews, R. G.; Zukowski, S.; Patnaik, P. C. Feasibility of Firing an Industrial Gas Turbine Using a Biomass Derived Fuel. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, 1997; Vol. 1, pp 495-506. (18) Oasmaa, A.; Kuoppala, E. Fast Pyrolysis of Forestry Residue. 3. Storage Stability of Liquid Fuel. Energy Fuels 2003, 17, 1075-1084.
10.1021/ef0400924 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/07/2005
Stability of Pyrolysis Liquids
Figure 1. Flow sheet of VTT PDU (20 kg/h) pyrolysis unit.
addition has been suggested21-24 as one of the cheapest methods for quality improvement. However, it also lowers the flash point.24 It has been suggested25 that removing light compounds which participate in aging reactions from pyrolysis liquid would improve its stability. These light compounds also cause its unpleasant smell and lower the flash point. The objective of this study was to combine the benefits of alcohol addition and removal of unwanted compounds (excess water, acids, the reactive compounds which cause the instability, and the compounds causing the unpleasant smell). In the study, the water and organics (i.e., low molecular-mass acids, aldehydes, ketones) removed were replaced by alcohol, and the properties of the resulting liquid were compared to those of the original liquid. In laboratory tests, this was carried out by evaporating the pyrolysis liquid under mild conditions (low temperature, low vacuum) in a rotavapor. In a large-scale pyrolysis plant, the removal of water and organics would be carried out by raising the temperature of condensers and, hence, by evaporating the light compounds out of the liquid. Experimental Section Liquid Production. Pyrolysis was carried out in a 20 kg/h capacity process development unit (PDU) at VTT26 (Figure 1). (19) Oasmaa, A. Fuel Oil Quality Properties of Wood-based 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). (20) Peacocke, C.; Meier, D.; Gust, S.; Webster, A.; Oasmaa, A.; McLellan, R. Determination of Norms and Standards for Biomass Derived Pyrolysis Liquids; Final Report, Commission of the European Communities, Contract No. 4.1030/C/00-015/2000, 2003. (21) Czernik, S. Storage of Biomass Pyrolysis Oils. In Biomass Pyrolysis Liquid Properties and Combustion Meeting; Estes Park, CO, Sept. 26-28, 1994; NREL: Washington, D.C., 1994; Paper No. CP430-7215, pp 67-76. (22) Diebold, J. P.; Czernik, S. Additives to Lower and Stabilize the Viscosity of Pyrolysis Oils during Storage. Energy Fuels 1997, 11, 1081-1091. (23) 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, p 424. (24) Oasmaa, A.; Kuoppala, E.; Selin, J.-F.; Gust, S.; Solantausta, Y. Fast Pyrolysis of Forestry Residue and Pine. 4. Improvement of the Product Quality by Solvent Addition. Energy Fuels 2004, 18, 15781583. (25) Sipila¨, K.; Oasmaa, A. Improvement of Liquid Quality. Patent Application FI 19992181, Helsinki, Finland, 1999. (26) Solantausta, Y.; Oasmaa, A.; Sipila¨, K. Fast Pyrolysis of Forestry Residues. In Pyrolysis and Gasification of Biomass and Waste-The Future for Pyrolysis and Gasification of Biomass and Waste: Status, Opportunities and Policies for Europe; Bridgwater, A. V., Ed.;. CPL Press: Newbury, U.K., 2003; pp 271-276.
Energy & Fuels, Vol. 19, No. 6, 2005 2557 The transport-bed reactor, designed and delivered by Ensyn Technology in 1995, has been subsequently modified by VTT. The ground, sieved, and dried feedstock (95 20-22
0 water (wt %) viscosity at 40 °C (cSt) viscosity increase 6 h 80 °C (%) flash point (°C) LHV (MJ/kg)
liquid 15-30 15-50 100-175 >40 15-20
liquid + 10 wt % MeOH viscosity at 40 °C (cSt) 5-10 50-100 viscosity increase 6h 80 °C (%) 10-20 0-25 flash point (°C) NDb 35-55
50-350 0-25 50-55
a
Water was removed to the lowest possible level in the system used. b ND ) Not determined
Terminology.19,31 In this part of the study the following definitions are used. Condenser temperature means the liquid temperature in the 2nd liquid scrubber. Extractives are the n-hexane-soluble compounds of the pyrolysis liquid. In the fractionation scheme, they dissolve in dichloromethane with LMM lignin material. Their amount is determined by n-hexane extraction from the original pyrolysis liquid. Forestry-residue liquid is the bottom phase of the product liquid which is the majority (80-90 wt %) of pyrolysis liquid produced from fresh green (G) or stored brown (B) forestry residue. High molecular-mass (HMM) lignin material is the dichloromethane-insoluble fraction of water-insolubles. This fraction is powderlike lignin-derived (Mw ) 1050 Da, pd 2.3) material and solids (char). There are no GC-eluted compounds. Except for the solids this fraction dissolves in methanol. Low molecular-mass (LMM) lignin material is the dichloromethane-soluble, lignin-derived (Mw ) 400 Da, pd 1.7) material of water-insolubles. GC-eluted compounds of this fraction are poorly water-soluble lignin monomers (guaiacol and catechol derivatives) and lignin dimers (e.g., stilbenes). Organic yield (wt % of dry feed) is obtained by dividing the total mass of organic liquid product (liquid product - pyrolysis water and feedstock moisture) by the total mass of dry-biomass feedstock fed to the reactor and multiplied by 100%. Pyrolysis liquid is a liquid product obtained from the rapid heating of biomass in the absence of air. Synonyms: pyrolysis oil, bio-oil, bio-fuel, BFO The stability of pyrolysis liquids is measured as an absolute increase in its viscosity or by using an accelerated stability test. In the latter tests, the pyrolysis liquid is kept at 80 °C for 24 h, and the increase in viscosity (measured at 40 °C) is determined.28,29 A high-quality pyrolysis liquid is defined as one which remains as a homogeneous single-phase liquid for a minimum of six months storage at room temperature.
Figure 2. Mass balance of concentration and chemical compound groups of a pyrolysis liquid. A poor-quality pyrolysis liquid is one which separates into two or more phases during six months of storage at room temperature. Sugar constituents are the ether-insoluble materials of the water-soluble fraction of the product liquid. They are a syruplike fraction containing anhydrosugars, polysaccharides, and aliphatic hydroxy carboxylic acids. The amount of GCeluted compounds of the fraction is low. The top phase of the forestry-residue liquid is the extractiverich fraction of the total product (10-20 wt %), which separates out of the product liquid after condensation. Water-insolubles are the water-insoluble fraction of the pyrolysis liquid. They are composed of degraded lignin (HMM and LMM lignin materials), extractives, and solids (char).
Results Laboratory Tests. The main changes in physical properties when employing this method are a decrease in water and increases in (i) viscosity, (ii) heating value, and (iii) flash point. The lowest water content obtained (Table 1) was 3.8 wt %, when the loss of organics, mostly acids (Table 2), aldehydes, and ketones, was 7.5 wt %. At very low concentrations of water, the sample is almost solid, as can be seen from the viscosity. No chemical changes (Figure 2) took place during the concentration step. The correlation of the loss of water and organics in Figure 3 shows that the proportion of water and organics removed is relatively constant. Figure 4 shows the effect of concentration on the viscosity of the liquid. It can also be seen that heating the liquid diminishes the differences in viscosity. When the water removed was replaced with methanol or a methanol-water (1:1) mixture, the viscosity of the resulting liquid was similar to that of the original (Figure 5). The pH of the mixtures was 10 (methanolwater) to 20% (methanol) higher than of the original
Table 2. Removal of Volatile Carboxylic Acids from Pine Pyrolysis Liquid in Concentrationa acid
acid in pyrolysis liquid wt %
acid in distillate wt % of pyrolysis liquid
acid in concentrate wt % of pyrolysis liquid
formic acetic propionic iso-butyric glycolic n-butyric lactic
1.90 4.17 0.21 0.02 0.22 0.08 0.02
0.30 1.73 0.09 0.01 0.01 0.02 0.00
1.60 2.44 0.12 0.01 0.22 0.07 0.02
a The temperature of water bath of the rotavapor was 40 °C; evaporation started at air pressure (760 Torr), and the pressure was decreased gradually to 20 Torr, when the formation of distillate ceased (compounds boiling below 120 °C evaporated). Mass balance of concentration i shown Figure 2.
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Figure 3. Correlation of loss of water to loss of organics in pyrolysis liquid concentration. Figure 6. Chemical composition (wt %) of original and treated pyrolysis liquids as fresh and aged (24 h 80 °C). IPA ) 2-Propanol.
Figure 4. Effect of concentration and temperature on the viscosity of pyrolysis liquid (PL). MeOH ) Methanol.
Figure 5. Effect of concentration and solvent addition on the viscosity of pyrolysis liquid.
pyrolysis liquid because of the pH of the solvents (pH of methanol 7.6 and water 6.6) and removed acids. The heating value, based on the dry basis, did not change significantly in the treatment, but because of the removal of water, the heating value on the wet basis increased. The stability of the liquid improves significantly (Figure 6), which can be seen as smaller increase in HMM lignin in an accelerated stability test. The effects of methanol, ethanol, and 2-propanol on the viscosity of the pyrolysis liquid are quite similar.24 They all also improve the storage stability of pyrolysis liquids; methanol is the most powerful.22,24 The effect of methanol addition is greatest when added to fresh pyrolysis liquid.24 When 10 wt % methanol was added to the concentrated pyrolysis liquid, the viscosity of the liquid at 50 °C did not change in the stability test. Most reactive-volatile aldehydes are removed with water. When removing 50-90 wt % of the original water, 20-25 wt % of volatile acids (20-35 wt % of acetic and 15-20 wt % of formic acid), 20-50 wt % of furans (1.5-2 wt % in original liquid), 35-80 wt % of
ketones (about 6 wt % in original liquid), about 90 wt % of aldehydes (7-8 wt % in original liquid), and 1-6 wt % of lignin monomers (3-4 wt % in original liquid) were also removed. The main compounds in the distillate (pH about 2) were acetic and formic acids, straightchain ketones (5 wt % 1-hydroxy-2-propanone of original liquid), and aldehydes (7 wt % hydroxy-acetaldehyde of the original liquid). The distillate was composed mainly of compounds similar to those in the diethyl ethersoluble fraction31 of the water-soluble part of the pyrolysis liquid. The major portion of the volatile compounds evaporated as azeotropes with water and, hence, condensed in the distillate. The lowest-boiling compounds of the distillate (2 wt % of pyrolysis liquid) were recovered in a separate cooling trap (“lightest distillate”). This fraction was a clear yellowish-green liquid with a sharp unpleasant odor. The unpleasant odor of pyrolysis liquid disappeared in the concentration and was localized in this lightest distillate. Compounds identified in this fraction were low molecular-mass ketones (2-propanone, 2-butanone, 3-methyl-2-butanone, 2-methyl-1-buten-3-one, 2.3-butanedione, but-3-enal-2-one, 2-pentanone, pent3-en-2-one, 4-methyl-2-pentanone, 2,3-pentanedione, 2-hexanone, furan, 2-methylfuran, 3-methylfuran, (5H)furan-2-one, (3H)-furan-2-one, tetrahydrofuran-3-one), aldehydes (formaldehyde, acetaldehyde, 2-propenal, 2-hydroxypropanal, 2-butenal cis/trans, but-2-enal, 2-pentenal, pent-2-enal, 2-hexenal, 2-furaldehyde), benzene derivatives (methylbenzene, dimethyl benzenes, trimethyl benzenes, ethyl benzene, propyl benzene), acid esters (formic acid methyl ester, acetic acid methyl ester), monoterpenes (C10H16, 4-carene, d-limonene), 1.1diethoxyethane, and 2-ethoxypropane. One of the main compounds causing the odor was 2-butenal (bp 100-102 °C), which was the main component of the lightest fraction. Other ill-smelling compounds included low molecular-mass (C3-C6) ketones, aldehydes, and aromatic hydrocarbons. The main fraction of distillate (water fraction) contained compounds similar to the light distillate, but in particular, the proportion of straight-chain aldehydes and ketones was lower. The main compounds were acetic and formic acids, furfurals (2-furaldehyde, 5-methyl-2-furaldehyde), ketones (hydroxyl-propanone), and lignin monomers (guaiacol, 4-methylguaiacol, 4-eth-
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ylguaiacol). The fraction was a light greenish-yellow liquid with a pH of 2, water content of 80 wt %, and 0.9-1.3 wt % organic carbon. The volatile organic material was in the range of 5.5-7.5 wt % of the original pyrolysis liquid. PDU Tests. The removal of water and volatiles was controlled by increasing the temperature of liquid condensers. When the condenser temperature was 55 °C, the product liquid phase separated within a few days because of the high amount of water in the liquid. On the basis of product composition (high in water-insolubles and water, low in carbohydrates compared to typical liquid product19,31), it was concluded that the liquid product had reacted in the condensers and water had been formed as a degradation product of sugars. When the condenser temperature was 45 °C, there were no problems with the product quality but only limited removal of unwanted compounds was achieved; 15 wt % of the product liquid water was carried to the demisters. The compounds removed in the PDU tests were the same as those demonstrated in laboratoryscale experiments; however, the amount removed was smaller. The products removed from the liquid were collected in the demisters of the PDU.
Oasmaa et al.
Figure 7. Recovery of organics and water in demisters as a function of condenser temperature.
Discussion In the laboratory-scale concentration, the removal of water also removes organics from the liquid. This causes an increase in viscosity and flash point. When 80% of the water was removed, the stability of the liquid improved significantly. This is caused by simultaneous removal of reactive volatile aldehydes and ketones, which participate in the aging reactions.18 The addition of solvent further improves the fuel-oil quality of the liquid product. The addition of alcohol can decrease the viscosity of the liquid to the same level as that of the original pyrolysis liquid. Methanol improved the stability more than a methanol-water mixture (1: 1). Compared to the original liquid, the methanol-water mixture also improved the stability. When methanol was added to the pyrolysis liquid, the flash point of the liquid decreased from 62-95 °C to 52-54 °C, and with methanol-water addition to 56 °C. The flash point is then close to the 2nd (21-56 °C) and 3rd (>56 °C‚‚‚
