Energy & Fuels 2003, 17, 1075-1084
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Fast Pyrolysis of Forestry Residue. 3. Storage Stability of Liquid Fuel A. Oasmaa* and E. Kuoppala VTT Processes, P.O. Box 1601, 02044 VTT, Finland
Energy Fuels 2003.17:1075-1084. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/03/19. For personal use only.
Received January 9, 2003. Revised Manuscript Received May 14, 2003
The storage properties of fuels are critical in regard to the introduction of a new fuel into markets. The fuel must be homogeneous, and the properties of the fuel should not change significantly during the storage at the customer’s facility. In this research, the storage stability of wood-based pyrolysis liquids was followed by analysis of the changes in the physical properties and chemical composition during storage. The main physicochemical changes took place during the first six months. The high-molecular-mass (HMM) fraction of water-insolubles, which were originally lignin-derived material, increased, because of polymerization and condensation reactions of carbohydrate constituents, aldehydes, and ketones. Therefore, the average molecular mass of pyrolysis liquids increased, which was also observed as an increase in viscosity. There was a clear correlation of the average molecular mass with the viscosity, water-insolubles, and the HMM fraction of water-insolubles. The chemical changes in the aging were similar to those which occurred during the accelerated aging test. The decrease in volatile aldehydes and ketones increased the flash point of the liquids. The increase in viscosity increased the pour point. Water was formed as a byproduct in various condensation reactions. Increases in the amount of water decreased the heating value. The density of the liquid increased, because the increase in HMM lignin fraction was more significant than the increase in water.
Introduction Pyrolysis liquids are not as stable as conventional petroleum fuels, because of their high amount of volatiles and nonvolatile oxygen-containing compounds. A typical pyrolysis liquid consists of water, carboxylic acids, alcohols, aldehydes, ketones, carbohydrates, and degraded lignin. The instability of pyrolysis liquids can be disclosed as1,2 (i) a slow increase in viscosity during storage (”aging”), (ii) a fast increase in viscosity by heating (resulting in, progressively, polymerization, phase separation, and gummy and coke formation), and (3) evaporation of volatile components and oxidation in air. Fast pyrolysis involves only a partial decomposition of biomass; therefore, the chemical composition of the resulting pyrolysis liquid is feedstock-dependent.3 Forestry residue, besides pine sawdust, is one of the most feasible biomass feedstocks for liquid fuel production in the Northern softwood forest zone. The majority of forestry residue in the Finnish market is comprised of spruce (50%-90%) and pine (10%-30%) from final end cutting. The green residue of spruce contains, on average, 40% wood, 23% bark, and 37% needles.4,5 * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Oasmaa, A.; Peacocke, C. A Guide to Physical Property Characterisation of Biomass-Derived Fast Pyrolysis Liquids; VTT Publication 450; VTT: Espoo, Finland, 2001; 65 pp + app. (34 pp). (2) 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 + app. (30 pp). (3) 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.
Scholze et al.6 determined that, in softwood pyrolysis liquids, more than 90% of the lignin units are guaiacyl units. In hardwood pyrolysis liquids, syringyl units dominate (60%-80%). It was also claimed that the lignin fraction in pyrolysis liquids consists mainly of trimers and tetramers of hydroxyphenyl/guaiacyl/syringyl (HGS) units and the average molecular mass of pyrolytic lignin varies between 650 and 1300 g mol-1. The phenolics in pyrolysis liquids are mainly derived from the lignin in the feedstock. The chemical structure of the lignin varies with the feedstock. In hardwood, the major amount of phenolics have two methoxy groups, i.e., similar to syringols. Softwood lignins mainly have only one methoxy group, i.e., similar to guaiacyls.7 The lignin in the inner bark is claimed to be similar to wood lignin, whereas the outerbark lignin is of unknown structure but significantly differs from the inner-bark lignin.7 Nokkosma¨ki et al.8 studied the quality improvement of pyrolysis liquid by a zinc oxide catalyst. They concluded that carbohydrates in pyrolysis liquid are (4) Oasmaa, A.; Kuoppala, E.; Solantausta, Y. Fast Pyrolysis of Forestry Residue. 2. Physicochemical Properties of Pyrolysis Liquids. Energy Fuels 2003, 17, (2), 433-443. (5) Arpiainen, V. Pyrolyysilaitoksen Metsa¨ta¨hdepohjaisen RaakaAineen Toivotut Ominaisuudet; Literature study; 2000. (In Finn.) (6) Scholze, B.; Hanser, C.; Meier, D. Characterisation of the WaterInsoluble Fraction from Fast Pyrolysis Liquids (Pyrolytic Lignin). Part II. GPC, Carbonyl Groups, and 13C NMR. J. Anal. Appl. Pyrolysis 2001, 58-59, 387-400. (7) Sjo¨stro¨m. E. Wood Chemistry. Fundamentals and Applications, 2nd ed.; Academic Press: San Diego, CA, 1993; 293 pp. (8) Nokkosma¨ki, M.; Kuoppala, E.; Leppa¨ma¨ki, E.; Krause, A. Catalytic Conversion of Biomass Pyrolysis Vapours with Zinc Oxide. J. Anal. Appl. Pyrolysis 2000, 55, 119-131.
10.1021/ef030011o CCC: $25.00 © 2003 American Chemical Society Published on Web 06/20/2003
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reactive and the stability of the liquid product can be significantly improved by catalytic treatment, which affects the sugar fraction. The carbohydrates (“sugars”) in pyrolysis liquids originated from the cellulose and hemicellulose constituents. In biomass, there are chemical linkages between lignin and carbohydrates.9 Lignin is thought to be chemically linked at least with a part of wood hemicelluloses; however, there are also indications of lignin and cellulose bonds. Chemical bonds have been reported between lignin and practically all the hemicellulose constituents. The chemical stability of such bonds and their resistance to acidic treatments are dependent not only on the type of linkage, but also, for example, on the chemical structures of the lignin and sugar units associated with the linkage. Pyrolysis liquids are complex solutions of various chemical compounds that have various polarities. Changes in the chemical composition during aging changes the mutual solubility of the pyrolysis liquid matrice. Radlein10 compared pyrolysis liquids to mineral oils. On the basis of that theory, the lignin-derived water-insoluble fraction would be suspended in micellar or microemulsion phases by the continuous aqueous phase, which acts as a bridging agent between the highmolecular-mass (HMM) lignin and the continuous aqueous phase. Dilution with water causes precipitation of the lignin fraction by dispersing the bridging components, which causes agglomerization and separation of the lignin micelles. A comprehensive overview on the stability of pyrolysis liquids has been given by Diebold.3 The reactions suggested to take place during the aging of pyrolysis liquids are mostly reactions of aldehydes to hydrates, hemiacetals, acetals (ethers), water, resins, and dimers. Oxygen in air is suspected to react with many of the organics present to form peroxides, which catalyze the polymerization of olefins and the addition of mercaptans to olefins. The organic acids and the elements in the char can act as catalysts for many of these reactions. Aging reactions result in undesirable changes in physical properties, such as an increase in viscosity and water content with a corresponding decrease in volatility.1,2,11 Diebold and Czernik11 have shown that the increase in viscosity during aging correlates very well with the increase in molecular mass. The aging rate is dependent on the feedstock, pyrolysis conditions, efficiency of solids/ash removal, and product collection. An accelerated aging test has been developed for a stability indicator of pyrolysis liquids.1,2,11,12 The advantage of this test is the short time required to demonstrate the aging properties of a particular pyrolysis liquid. Czernik12 concluded, based on correlations for the aging data, that the reactions in the temperature range of 37-90 °C are relatively similar. (9) Ale´n, R. Structure and Chemical Composition on Wood. In Forest Product Chemistry; Stenius, P., Ed.; Tappi Press: Jyva¨skyla¨, Finland, 2000; pp 11-57. (10) Radlein, D. Study of Levoglucosan ProductionsA Review. In Fast Pyrolysis of Biomass: A Handbook; Bridgwater, A., Ed.; CPL Press: Newbury, U.K., 2002; Vol. 2, pp 205-242. (11) Diebold, J. P.; Czernik, S. Additives to Lower and Stabilize the Viscosity of Pyrolysis Oils during Storage. Energy Fuels 1997, 11, (5), 1081-1091. (12) 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. CP430-7215, pp 67-76.
Oasmaa and Kuoppala
Czernik12 concluded that the increase in viscosity for a hardwood liquid at 37 °C for 3 months correlated to that after 4 days at 60 °C or to 6 h at 90 °C. The aging rate of softwood pyrolysis liquids is similar to that of hardwood liquids at 20 °C, with some possible differences at lower storage temperatures.2,3 For a pine pyrolysis liquid (water content of 21 wt %, viscosity of 30 cSt at 40 °C), the viscosity increase under the test conditions (24 h at 80 °C) correlated to the viscosity increase after one year of storage at room temperature. In this paper, the aging of forestry residue and pine pyrolysis liquids during typical storage conditions is discussed. This paper is the third part of a series of publications that have focused on the fast pyrolysis of forestry residue, the optimation of the separation of extractives,13 the physicochemical properties,4 and the storage stability of forestry residue liquids. The next publication will focus on the quality improvement of forestry residue liquids. Experimental Section 1. Feedstock Analyses. Feedstock analyses (Table 1) were performed using standard methods: moisture was measured according to DIN51718, volatiles was measured according to DIN51720, ash was measured according to EN7 at 815 °C, CHN elemental analysis was performed according to ASTM D 5373-93, and the heating value was determined according to DIN51900. Alkaline-metal analysis was conducted by applying wet oxidation for sample preparation and atomic absorption spectroscopy (AAS) for determination. Chlorine content was determined using the capillary electrophoresis technique after combusting the sample in an oxygen bomb, according to ASTM test method D 4239. Extractives were determined using acetone, and the extraction was continued using dichloromethane. 2. Liquid Production. Pyrolysis was conducted using a 20 kg/h-capacity process development unit (PDU) at VTT.13,14 Ensyn Technology initially designed and delivered the transport bed reactor in 1995, which has subsequently been modified by VTT. The ground, sieved, and dried (moisture content of 4-11 wt %) feedstock (Table 1) was fed into the reactor. The pyrolysis temperature was ∼520 °C and the residence time for pyrolysis vapors was 1-2 s in all experiments. The product vapors were condensed using liquid scrubbers. Liquid yields (organics and water) from forestry residue ranged from 6164 wt % of dry feed; this value was 68-75 wt % for pine sawdust.4 All liquids studied were produced by the VTT/PDU unit. The properties of product liquid from PDU has, in earlier studies,1,2 been shown to be similar to other pyrolysis liquids that are produced by different processes.1,2 The main variable affecting the quality of pyrolysis liquid was the feedstock. The top phase was separated from the bottom phase after 24 h of storage at 35 °C.13 3. Liquid Storage. Pyrolysis liquid samples were stored in firmly closed glass bottles in darkness at 9 °C or at room temperature under light. A reference sample was stored in deep-freeze conditions and analyzed after one year of storage. The viscosity of various pyrolysis liquids was followed at room temperature under light. (13) 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. (14) Solantausta, Y.; Oasmaa, A.; Sipila¨, K. Fast Pyrolysis of Forestry Residues. Presented at Pyrolysis and Gasification of Biomass and WastesThe Future for Pyrolysis and Gasification of Biomass and Waste: Status, Opportunities and Policies for Europe, Strasbourg, France, Sept. 30 - Oct. 1, 2002. Sponsored by Altener, IEA Bioenergy, Thermonet, PyNe, GasNet, Cirad Foret.
Fast Pyrolysis of Forestry Residue. 3
Energy & Fuels, Vol. 17, No. 4, 2003 1077 Table 1. Analyses of Feedstock Dry Matter Forestry Residue (FR)
moisture as received ash volatiles carbon hydrogen nitrogen oxygen sodium potassium calcium magnesium chlorine sulfur HHV LHVg LHV extractives
method
unit
DIN 51718 DIN 51719 DIN 51720 ASTM D 5373 ASTM D 5373 ASTM D 5373 as difference AAS AAS AAS AAS ASTM D4208 + CEF ASTM D 4239 DIN 51900 DIN 51900
wt % wt % wt % wt % wt % wt % wt % mg/kg mg/kg mg/kg mg/kg mg/kg wt % MJ/kg MJ/kg MJ/kg wt %
solvent extractionh
pine spruce (Pinus sylvestris)a (white spruce)b 3.3 0.2 82.5 50.3 6.0
