ARTICLE pubs.acs.org/EF
Further Separation, Characterization, and Upgrading for Upper and Bottom Layers from Phase Separation of Biomass Pyrolysis Oils Hong-Wei Chen,† Qin-Hua Song,*,†,‡ Bing Liao,§ and Qing-Xiang Guo†,‡,§ †
Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China Anhui Province Key Laboratory of Biomass Clean Energy, Hefei 230026, People’s Republic of China § Key Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, People’s Republic of China ‡
ABSTRACT: Effective separation methods must be developed before bio-oils become a source of chemical products or liquid fuels by further upgrading. Phase separation is one effective pathway to realize an initial isolation of bio-oils. When aqueous salt solutions are added, phase separation of the pyrolysis bio-oil can occur to form two different phases: the upper layer with high contents of water, acetic acid, and water-soluble compounds and the bottom layers with low water content and high lignin pyrolysis compounds [Song, Q.-H.; Nie, J.-Q.; Ren, M.-G.; Guo, Q.-X. Effective phase separation of biomass pyrolysis oils by adding aqueous salt solutions. Energy Fuels 2009, 23 (6), 33073312]. In this paper, the salt-induced phase separation of bio-oils with 20 kinds of salt solutions was investigated. On the basis of the dependence of the mass ratio of the bio-oil bottom layer to the whole bio-oil sample upon the salt solution concentrations, it has been demonstrated that the phase separation derives from a normal ionic strength effect as well as chemical properties for some metal ions. Solvent fractionations of the two-phase bio-oils were performed, and fractions were characterized by elemental and gas chromatography/mass spectrometry (GC/MS) analyses. Among the four bottom-layer bio-oil fractions, fraction A contains insoluble macromolecular substances, such as cellulose, and fractions, B, C, and D are mainly phenolic compounds. Furthermore, O-methylation of the mixtures of fractions B, C, and D with dimethyl carbonate (DMC) has been performed in ionic liquid [Bmin]Cl, and the reaction is highly effective and recyclable for the ionic liquid. The modified mixture of the fractions reveals a large elevation in the heating value.
1. INTRODUCTION Technologies to more efficiently use biomass for energy have been developed for reducing environmental problems and obtaining value-added chemical products over the past 20 years. Many efforts have been made to convert biomass to liquid fuels and chemicals since the oil crisis in the mid-1970s.1,2 Fastpyrolysis-derived bio-oils have potential as feedstocks for chemical production38 and as a promising route to liquid fuels.1,811 However, many problems arise in their handing and use, resulting from some special properties of pyrolysis oils. Bio-oils are complex colloidal multidispersed systems, which contain a large amount of water, carboxylic acids, carbohydrates, and lignin-derived substances.12,13 As a result, bio-oils exhibit some undesired properties, such as acidity, thermal instability, high oxygen content (3540%), low heating value, high viscosity, corrosiveness, and chemical instability.14 These drawbacks limit the applications of bio-oils for vehicle fuels.1517 Therefore, many upgrading techniques have been developed, such as esterification,1823 hydrotreatment,24,25 blending,26 and catalytic conversion of acidic components.27,28 On the other hand, if biooils are to become a source of chemical products (as opposed to fuels), separation technology will be important. Phase separation of bio-oils can occur spontaneously2931 or by adding water32 or salts33 to form two different phases in their physcochemical properties and to realize effective initial separation of bio-oils. In our previous paper,28 two phases from phase separation by adding aqueous salt solutions demonstrated large differences in physcochemical properties. The upper layers exhibited high r 2011 American Chemical Society
water contents, acetic acid, and water-soluble compounds, low density, viscosity, and calorific values, and high distillable substances. The bottom layers contained low contents of water, high lignin-pyrolysis compound contents, high viscosity and calorific values, and low distillable substances. In this paper, the origin of the phase separation induced by salt solutions has been investigated, solvent fractionation of the two phases has been performed, and the fractions have been characterized. Furthermore, upgrading of three fractions has been performed through O-methylation with dimethyl carbonate (DMC) in an ionic liquid.
2. EXPERIMENTAL SECTION 2.1. Bio-oil Production. In this study, bio-oil was obtained by fast pyrolysis of rice husk in an autothermal fluidized-bed pyrolyzer with a capacity of 120 kg of oil/h at our laboratory. The pyrolysis device mainly consists of a hopper, two screw feeders, an electric heater, a fluidized-bed reactor, two cyclones, a condenser, and an oil pump, as well as some thermocouples and pressure meters. The hopper contains the feedstock, such as rice husks, sawdust, or their mixture. The two screw feeders have the same configuration and size; the first one controls the feeding rate, and the second operates at a relatively high speed to prevent jamming of the feeding system. The fluidized-bed reactor has a height of 2 m and a diameter of 0.7 m, in which rice husks or sawdust are rapidly heated for pyrolysis. Nitrogen is fed as an inert gas to reduce air input into the Received: May 13, 2011 Revised: September 1, 2011 Published: September 03, 2011 4655
dx.doi.org/10.1021/ef201016a | Energy Fuels 2011, 25, 4655–4661
Energy & Fuels
ARTICLE
Table 1. Physiochemical Properties of Bio-oil in This Work property water content (wt %)
bio-oil
Scheme 1. Solvent Fractionation for the Separation of the Bottom-Layer Bio-oil
30
density (kg/L)
1.17
viscoisity at 40 °C (cSt)
29.9
heating value (MJ/kg)
17.1
pH
2.9
ash (wt %)
0.086
element composition (wt %) C H
40.9 7.55
N
0.95
O (by difference)
50.5
reactor. The electric heater can preheat the nitrogen to the temperature range of 450550 °C before entering into the fluidized-bed reactor. The two cyclones separate solid particles, such as charcoal and ash, from the hot gas. The condenser is equipped with nozzles and a heat exchanger. The condenser can quickly cool the cleaned hot gas into a liquid. An oil pump transfers the condensed liquid from the bottom of the condenser to the nozzles at the top of the condenser. Pumping the cooled liquid back into the condenser assists in the scrubbing and condensation process. Thermocouples and pressure meters monitor and control the pyrolysis system. More characteristics of the pyrolysis reactor have been described elsewhere.34 Physiochemical properties of bio-oils used in this work are listed in Table 1. 2.2. Phase Separation of Bio-oils. Phase separation of bio-oils was performed by adding various inorganic aqueous salt solutions. Samples of bio-oil (10 g) were placed in glass tubes (15 mm in diameter, with a capacity of 15 mL), and various molar concentration salt solutions (1.0 g) were added with stirring and sonication in a water-cooled bath below 15 °C. Afterward, the tubes were sealed with parafilm and stored for 10 h at room temperature, forming two phases (upper and bottom layers). The upper layer was removed through pouring out from the tube because of a large difference in viscosity of two phases. Following separation, the two phases were weighed. 2.3. Physicochemical Characterization. Physicochemical properties, such as density, pH value, water content, gross calorific value, and viscosity were determined by standard American Society for Testing and Materials (ASTM) methods. The elemental composition (carbon, hydrogen, and nitrogen) was determined in an Elementar Vario El-III analyzer. The oxygen content was calculated by difference.
2.4. Solvent Fractionations of the Upper and Bottom Layers. A total of 10 g of the upper layer from phase separation was
extracted with ethyl acetate (EA) in three replicates (3 15 mL). The organic phase combined was dried with anhydrous MgSO4 and filtered, and the solvent was removed in vacuo (
