Yields of oil products from thermochemical biomass conversion processes

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  • ~) Pergamon Energy Com,ers. Mgmt Vol. 39, No. 7, pp. 685 690, 1998

    ,~3 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain

    PII: S0196-8904(97)00047-2 0196-8904/98 $19.00 + 0.00

    Y IELDS OF O IL PRODUCTS FROM

    THERMOCHEMICAL B IOMASS CONVERSION PROCESSES

    A. DEMIRBA~ Department of Science Education, Karadeniz Technical University, 61335 Akcaabat, Trabzon, Turkiye

    (Received 19 August 1996)

    Abstract--Thermochemical biomass conversion processes, including mainly pyrolysis, liquefaction and supercritical fluid extraction, were applied non-catalytically and catalytically to obtain the maximum fuels from biomass. Different solvents and their mixtures were used in simple and supercritical extractions. Basic catalysts, such as NaOH, Na2CO3 and K2CO3, were used in catalytic conversion experiments. The experiments were performed within the 498-820 K range. 1998 Elsevier Science Ltd.

    Thermochemical conversion Liquefaction Pyrolysis

    Supercritical fluid extraction Thermocatalytic conversion

    INTRODUCTION

    Thermochemical biomass conversion processes are, mainly, pyrolysis, gasification, liquefaction and supercritical fluid extraction [1-4]. These processes aim at obtaining maximum fuel and chemical yields from biomass sources. The laboratories participating in the comparative tests represent a number of different approaches to biomass conversion and encompass a wide range in operating conditions. The different processes are grouped into atmospheric, low and high pressure categories [5].

    Biomass is converted to liquid fuels to increase the volumetric heat content and to decrease transportation costs. Fast- and flash-pyrolysis appear as the most promising methods for fuel-oil-substitute production from biomass [5, 6]. It has been verified that the highest liquid yields from novel biomass can be produced with flash pyrolysis [7]. An organic-liquid yield of 65-75 wt% of dry wood has been established as feasible, and it has been shown that the critical process features in maximising liquid yield are a rapid heating period and a short vapour-phase-residence time [8]. Previously, kinetic studies with cellulose have shown that under vacuum at 530-680 K, pyrolysis proceeds very rapidly and still provides good yields of tar and levoglucosan [9].

    Gasification of wood in molten salt has been described by Yosim and Barclay [10]. The possibility of converting black liquor into a combustible fuel gas was evaluated experimentally by Rockwell in 1978. The gasification process produces a hot, dirty gas stream containing organic aerosols, inorganic particulates, condensable organic vapours, noncondensable gases and water vapour. Detailed analyses of the raw producer gas streams generated by a downdraft gasifier and by a fluidized-bed gasifier were presented and discussed in terms of raw gas clean-up considerations and energy conversion efficiency [11].

    Conversion of wood to liquid products is based on early research [12]. These workers reported that a variety of biomass (wood products, agricultural and civic wastes) could be converted, in part, into a fuel oil by reaction with carbon monoxide and water at elevated temperatures in the presence of sodium carbonate which acts as a catalyst. The conversion of biomass to oils at high temperatures and pressures has been studied using CO/H2 in a water/cresol solvent with sodium carbonate and sodium formate catalysts. In the processes of biomass liquefaction earlier described, the amount of gaseous products released from biomass were high, and the liquid yields obtained were not higher than 50% by weight based on the original sample [13-15]. Aqueous liquefaction of lignocellulosic materials involves disaggregation of the wood ultrastructure, followed by partial

    685

  • 686 DEMIRBA~;: YIELDS OF OIL PRODUCTS

    Table 1. Chemical and structural analysis data of biomass samples (wt% of dry and extractive free basis)

    Sample Hemicelluloses Cellulose Lignin Ash C H O N

    Oriental beech 31.8 45.8 21.9 0.4 49.5 6.2 41.2 0.4 Oriental spruce 21.2 50.8 27.5 0.5 51.9 6.1 40.9 0.3 Ailanthus 26.6 46.7 26.2 0.5 49.5 6.2 41.0 0.3 Tea waste 19.9 30.2 40.0 3.4 48.6 5.5 39.5 0.5 Wheat straw 39.1 28.8 18.6 13.5 45.5 5.1 34.1 1.8 Corncob 32.0 52.0 15.0 1.0 49.0 5.4 44.6 0.4 Olive husk 23.6 24.0 48.4 4.0 50.0 6.2 42.2 1.6 Hazelnut shell 29.9 25.9 42.5 1.3 52.9 5.6 42.7 1.4 Hazelnut seedcoat 15.7 29.6 53.0 1.4 51.0 5.4 42.3 1.3 Corn stover 30.7 51.2 14.4 3.7 49.5 5.4 41.8 0.6 Tobacco stalk 28.2 42.4 27.0 2.4 49.3 5.5 42.3 0.5 Tobacco leaf 34.4 36.3 12.1 17.2 43.0 4.5 35.8 0.5

    depolymerization of the constitutive compounds. Solubilization of the depolymerized material is then possible [16].

    Supercritical fluid extraction of biomass is applied in an autoclave at above the critical temperature and pressure of the solvent. The yield of extractives increases with increasing pressure [17, 18], but under the extraction conditions, woody material undergoes highly thermal depolymerization at above 520 K. The process has been found to be a promising method for the recovery of compounds present in the wood [19, 20]. Commercial applications of supercritical fluid extraction include the use of supercritical gases, especially carbon dioxide, to isolate cocoa butter and to decaffeinate coffee [21, 22]. The supercritical extraction products, although produced in a high pressure system, clearly fall into the same category as the low pressure pyrolysis products. Thermal decomposition of biomass begins at 491 K, and the product group similarities can be explained more adequately when correlated with processing severity, including the reaction pressure and temperature as well as the time at conditions [5].

    The methods in the present study were based on conversion to oil of products of biomass pyrolysis, liquefaction and supercritical fluid extraction, and each process was applied as non-catalytic and catalytic. The yields of conversion obtained from these processes were concluded.

    EXPERIMENTAL

    The samples used in this study were supplied from different Turkish biomass sources. An air dried sample was chipped and then ground with a Thomas-Wiley mill to pass a 0.6 mm screen. The ground biomass was uniformly mixed prior to sampling.

    All the experiments were conducted in a 0.25 1 autoclave. The temperature was measured with a thermocouple and controlled to + 5 K, and pressures were measured to an accuracy of ___ 2%.

    Table 2. Yields of Soxhlet extractions of biomass samples (wt% of dry basis)

    Soxhlet extractives obtained by using

    Alcohol- Petroleum Diethyl Dichloro Acetone-- Sample benzene Acetone ether ether methane water

    Oriental beech 2.7 1.6 0.9 1.5 i.4 2.2 Oriental spruce 1.4 2.5 0.4 0.3 0.2 3.1 Ailanthus 2.1 2.6 0.3 0.3 0.3 2.6 Tea waste 7.8 8.6 1.8 2.2 2.5 11.7 Wheat straw 8.0 6.8 3.7 2.9 6.3 7.2 Corncob 1.2 1.3 0.6 0.8 1.0 1.4 Olive husk 8.1 7.5 3.6 5.3 6.4 8.0 Hazelnut shell 3.4 3.9 2.8 2.6 2.5 4.2 Hazelnut seedcoat 19.6 18.1 15.7 16.8 14.6 16.5 Corn stover 2.4 2.6 0.8 1.9 1.8 3.4 Tobacco stalk I 0.6 9.4 4.1 5.0 8.7 1 I. 1 Tobacco leaf 13.1 12.6 5.2 6.8 10.0 13.0

  • DEMIRBA~: YIELDS OF OIL PRODUCTS 687

    Table 3. The critical values for using solvents

    Solvent

    Critical Critical temperature pressure

    (K) (MPa)

    Water 647.3 22.12 Acetone 508.2 4.76 Ethanol 516.1 6.39 Methanol 513.1 7.97

    The sample was loaded via the bolt hole with M12 35 15 into the autocalve, and the hole was plugged with a screw bolt before each run [23]. In addition, the fast pyrolysis and flash pyrolysis of pulverized biomass particles were performed in a stainless-steel tubular reactor with 18 mm inside diameter. The tubular reactor was inserted into an electrically-heated tubular furnace. The experimental apparatus was placed on a balance in order to determine the weight loss of the sample.

    In a typical liquefaction process, 25 g of air-dried biomass powder and a solvent or solution containing the desired quantities of catalyst and/or water were employed.

    After each run, the gaseous products were captured in a beaker. All the rest of the oil and solids were removed from the autoclave by washing with used solvents.

    In a typical supercritical fluid extraction, 10.0 g of the sample and 100 g of solvent were fed to the 0.25 1 autoclave. After completion of the extraction the autoclave is cooled to room temperature. Catalytic supercritical fluid extraction was also performed in the 0.25 1 autoclave. In a typical run, the autoclave was loaded with 10.0 g of a biomass sample and a solution containing the desired quantities of catalyst and solvent.

    RESULTS AND DISCUSSION

    The chemical and structural analysis data of the biomass samples are given in Table 1. The yields of Soxhlet extraction of biomass are shown in Table 2. Table 3 shows the critical values of the used solvents. The yields of extracts obtained from supercritical fluid extraction are given in Table 4.

    The results obtained from two subclass pyrolyses, conventional pyrolysis and fast pyrolysis, are shown in Table 5. Data from non-catalytic and catalytic liquefaction runs of the biomass samples are given in Table 6. Data obtained from catalytic supercritical fluid extraction and catalytic pyrolysis runs are given in Table 7.

    Data from Tables 2 and 4 show that the yields of the supercritical fluid extractions were found to be higher compared to those obtained by simple Soxhlet extractions.

    Table 4. Yields from supercritical fluid extractions of biomass samples (wt% of dry, ash and extractive free)

    Extractives obtained by using

    Acetone Methanol Ethanol Water Temperature (K): 513 560 563 653

    Oriental beech 44.1 46.9 53.2 56.2 Oriental spruce 28.2 31.3 36.1 -- Ail

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