Article pubs.acs.org/IECR
Influence of Microwave Heating on Chemical Properties of Liquefied Lignocellulosic Residues Rodrigo Briones, Luis Serrano,* Ane Sequeiros, and Jalel Labidi Chemical and Environmental Engineering Department, University of the Basque Country, Plaza Europa 1, 20018, Donostia-San Sebastián, Spain S Supporting Information *
ABSTRACT: The chemical composition of liquid products obtained from liquefaction of several agro-industrial wastes was analyzed and the effects of microwave treatments were investigated, evaluating changes in chemical composition, structure, and physical properties, looking forward to exploring new utilization routes or processing paths for these liquid products. Gas chromatography−mass spectrometry (GC-MS) showed that the chemical components of liquefied products can be divided into several groups: furans, alcohols, and esters and acid derivatives. The microwave evaluation showed that irradiation at constant power levels affects the compositions of liquefied products. Fourier transform infrared (FT-IR) spectroscopic analysis showed increases in the absorption bands of carbonyl and CH groups, which suggests that microwaves induced more intensive oxidation of hydroxyl groups into carbonyl groups. Elemental analysis indicated higher carbon and lower oxygen contents and higher heat heating value (20 MJ/kg) in treated products with respect to untreated samples. The use of liquefied products as a new energy source has advantages such as their liquid state, convenient energy value, and renewability.
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tions.9−11 In the majority of cases, this is a consequence of the high reaction temperatures that can rapidly be attained when polar materials are irradiated in a microwave field; that is, a purely thermal/kinetic effect. Following this trend, microwave irradiation for polymerization reactions focusing on specific products has gained progressive attention. In that context, microwave-assisted reaction has been used to convert several lignocellulosic materials in biomass-derived chemicals, such as furfural or their derivative products,12−14 that can be used to replace petrochemicals. The encouraging results obtained by these authors justify the interest in exploring microwave activation that could change the chemical structure of liquefied products, looking forward to new challenges. The aim of this work was to investigate the chemical composition of liquefied products obtained from some lignocellulosic agro-industrial residues and to evaluate the effect of microwave treatments of different power levels on the properties of liquefied products, looking forward to exploring new utilization routes or processing paths of these liquid products. The chemical composition of the samples was analyzed by a combination of chromatographic and spectroscopic techniques, such as Fourier transform infrared (FTIR) spectroscopy, gas chromatography−mass spectrometry (GCMS), and elemental analysis. In addition, physical and chemical properties were also measured.
INTRODUCTION Lignocellulosic biomass is a renewable, abundant, and cheap source of raw materials for the chemical industry.1 Nowadays, among the numerous novel chemical products investigated, pure chemicals and precursors from natural polymeric substrates have attracted considerable attention.2 In our recent works,3,4 we have successfully reported the solvolytic liquefaction of several lignocellulosic residues from agricultural or industrial activities, which are mainly discarded or used as animal feed or for energy requirements, into polyols, multifunctional liquids with suitable physical and chemical features for polyurethane synthesis. The liquefied products obtained were highly reactive black liquors that contain plentiful hydroxyl groups, mainly cellulose, hemicelluloses, and lignin degradation products, in addition to blended liquefying solvents. To search for novel applications for those byproducts, it is necessary to deepen their chemical characterization and identify ingredients, taking into account the possibility of modifying their structure through novel techniques to open new application areas for the resulting products. In that sense, the application of several novel methods, such as microwave sample treatment or chemical reactions, has recently attracted significant attention because it could be a promising option for chemical processing industry in terms of the minimization of energy and optimization of reaction control. The application of microwaves for organic synthesis goes along with a decrease of reaction time while increasing the yield due to specific reaction conditions.5,6 This would lead to better yields in a short time and energy savings in comparison with classical thermal methods.7,8 Moreover, in recent years microwave irradiation is gaining importance not only as an alternative to conventional heating but also because it can cause rate enhancements and sometimes altered product distribu© 2013 American Chemical Society
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MATERIALS AND METHODS Liquefied Products. The liquefied products evaluated in this study were produced by solvolytic liquefaction of
Received: Revised: Accepted: Published: 2755
September 25, 2012 December 19, 2012 January 28, 2013 February 14, 2013 dx.doi.org/10.1021/ie3026136 | Ind. Eng. Chem. Res. 2013, 52, 2755−2761
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(FTIR) spectrometer by direct transmittance in a singlereflection attenuated total reflectance (ATR) system (ATR top plate fixed to an optical beam condensing unit with ZnSe lens) with an MKII Golden Gate SPECAC instrument. Each spectrum was recorded over 20 scans in the range from 6500 to 800 cm−1, with a resolution of 2 cm−1. Physical and Chemical Properties. Elemental analysis based on carbon, hydrogen, and nitrogen content for each viscous liquid product was carried out on a Euro EA 3000 series elemental analyzer from EuroVector SpA, Milano, Italy. Total oxidation of the sample (10 mg) was conducted at 1020 °C. Combustion products were separated in a chromatographic column [poly(tetrafluoroethylene) (PTFE) column for C, H, N, and S, 2 m; carrier gas, He, 70 kPa; purge flow, 80 mL/min; oxygen pressure, 35 kPa]. The oxygen content was calculated by difference. Gel-permeation chromatography (GPC) was used to determine the average molecular weight of the samples. A Jasco instrument equipped with an interface (LC-NetII/ADC) and a refractive index detector (RI-2031Plus) was used. Two PolarGel-M columns (300 × 7.5 mm) and PolarGel-M guard (50 × 7.5 mm) were employed. Dimethylformamide +0.1% (wt) lithium bromide solution was used as eluent, the flow rate was 0.7 mL/min, and the analyses were carried out at 40 °C. Calibration was made with polystyrene standards (Sigma− Aldrich). The heat heating value (HHV) of the samples was calculated by the Lloyd and Davenport equation.15 The pH was determined in water by a procedure similar to the methods used for wood or soil as follows: a mixture of 1 g of the sample of liquefied product and 50 mL of water was stirred, and the pH of the water was recorded on a calibrated pH-meter (Fisher Scientific Accumet Research AR 15). In addition, several features such as density and moisture content obtained at the conditions of the maximum liquefied products yield were determined according to ASTM standard methods.16
lignocellulosic agricultural and industrial residues from renewable resources (olive stone, corncob, apple pomace, date seeds, and rapeseed cake) by using a mixture of poly(ethylene glycol) [PEG, with average molecular weight (Mw) 400] and glycerol (G) as solvents and 98% sulfuric acid (SA) as catalyst. The chemical composition in terms of lignocellulosic components, following standard methods and procedures cited in literature, and FTIR analysis of the raw materials are presented in the Supporting Information section (Table S1 and Figure S1, respectively). Accordingly, the raw materials evaluated confirm the availability of hydroxyl groups, and thus are potentially suitable for liquefaction techniques, and they could be an attractive choice for a renewable source of biopolyols. The liquefaction conditions were selected by considering the optimized results obtained in the recent work by our group3,4 and were as follows: mass/liquefying solvent ratio 0.25 (0.2 for rapeseed cake); weight ratio PEG:G:SA 80:20:3; temperature reaction 160 °C (170 °C for rapeseed cake); and reaction time 60 min (80 min for rapeseed cake). The liquid samples are identified as LOS (liquefied olive stone), LCC (liquefied corn cob), LAP (liquefied apple pomace), LDS (liquefied date seeds), and LRC (liquefied rapeseed cake) All chemicals used were reagent-grade and were supplied by Panreac Co. Microwave Treatment. Samples (2 ± 0.05 g) were weighed out into a microwave tube containing a magnetic stirrer and then sealed with the microwave tube lid. The microwave tube was placed in a CEM microwave discover system model. The microwave system was a temperaturecontrolled instrument (infrared), having an internal temperature sensor. A microwave program with the appropriate temperature, residence time, and cooling time was defined by use of the microwave system software. The range of experiments was carried out with varying microwave power (175, 200, 225, 250, 275, and 300 W). There was constant stirring during the time of the microwave treatment. At the end of the residence time for each test, the microwave power was automatically turned off by the system, and the vessels were cooled down for 10 min to stop possible reactions. The liquid product was diluted with acetone and the resultant solution was filtered. The solid fraction was washed with acetone, dried at 105 °C for 12 h in an oven, and then weighed to calculate the amount of possible residue present in the sample after the treatment. Then the acetone was evaporated from the diluted fraction and the liquefied product was collected for later use and analysis. All treatments were performed at least in triplicate. Characterization of Samples. Gas Chromatography and Mass Spectrometry. The components of the liquefied products were analyzed on an Agilent 7890A/5975MSD GC-MS. The GC was fitted with a 30 m × 0.25 mm × 0.25 μm fused quartz capillary column and coated with HP-5MS. Helium (99.999%) was used as the carrier gas with a constant flow of 1.0 mL/min. In this study, the samples were extracted with dichloromethane that allows qualitative analysis of contained organic compounds by GC-MS. The oven was programmed with a 10 °C/min increase to a final temperature of 280 °C and held for 10 min. After a solvent delay of 5 min, full-scan mass spectra were acquired from 50 to 650 m/z. The injection size was 1 μL. The identification of the peaks was based on computer matching of the mass spectra with the National Institute of Standards and Technology (NIST) 2007 library. Fourier Transform Infrared Spectroscopy. The changes of functional groups of the liquefied products during microwave treatment were analyzed on a Fourier transform infrared
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RESULTS AND DISCUSSION Gas Chromatography and Mass Spectrometry. Gas chromatography and mass spectrometry (GC-MS) was used to obtain more detailed information on the liquefied products obtained from agro-industrial residues, specifically those components with lower boiling points and lower molecular weights. In our case, more than 100 peaks were displayed in the GC-MS chromatograms for each sample (Figure 1), which confirms the complex composition of liquefied products. In general, liquefied products derived from lignocellulosic materials have a large number of oxygen-containing reactive functional groups that are fragments of cellulose, hemicellulose, and lignin constituents. It is expected that liquefied products consist of mainly carboxylic acids, carbohydrates, and ligninderived substances. Perfect separation of all peaks was not possible due to the complex composition of samples. Only those separated products that arose in considerable amounts were semiquantitatively evaluated. In this study, only those compounds that had a probability above 75% and that presented 0.1% or higher of the total area were selected. The organic compounds identified for liquefied products are presented in Table 1. The liquefied products obtained from solvolytic liquefaction of agro-industrial residues by use of glycols as solvent mixture and sulfuric acid as catalyst have similar chemical constitutions, 2756
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fatty acids present in animal and plants, were also detected in the liquefied products. The chemical compounds identified, which are repeated between liquefied products, could be associated with liquefaction conditions and different raw material compositions, in particular, the difference in the contents of its main components (cellulose, hemicelluloses, and lignin). In this study all the products evaluated presented liquefied components derived from the decomposition of cellulose; however, we did not observe a pattern of the quantity of these derivatives associated with the cellulose content in the raw materials. Furthermore, because mainly aromatic compounds are derived from the decomposition of lignin,17 one would assume that the more lignin contained in the raw material, the more aromatic components would be obtained in the final product. The results did not permit us to validate this affirmation due to the low amount of aromatic compounds identified in the final product. On one hand, this could be explained by considering that the applied liquefaction conditions led to a severe decomposition of lignin, primarily toward the production of acids; on the other hand, it could be that that the liquefaction conditions tend to decompose the lignin part of high molecular weight compounds that cannot be detected by GC-MS. It is noteworthy that higher lignin content was observed in the olive stones (25%) and date seeds (23%). Consequently, liquefied products from olive stones and date seeds presented aromatic derivatives. In general, the chromatographic analysis showed that the chemical components of liquefied products could be divided into groups, mainly emphasizing the initial section of furan derivatives, then the section of derivatives of alcohols, and finally the section of acids and esters with longer acid chains greater than C12. Microwave Treatment. For evaluating the possibilities of microwave activation in the liquefied products, the effect of microwave power on the presence of insoluble residues in the final liquid sample was preliminarily investigated. In the context of this study, the presence of insoluble residues in the liquefied product after activation by microwaves is undesirable because it would require further processing to separate the residue for recovery and use of the final product. A series of tests were carried out once the power level of microwave energy applied to each sample was defined and the data were incorporated into the software. At the beginning of each microwave test, the temperature was raised in less than 1 min up to a maximum temperature (>200 °C) that is maintained until the end of the residence time (3 min). Finally, at the end of the period of constant microwave radiation, the sample is rapidly cooled to a temperature of 50 °C by an air stream. In each experiment, the maximum temperature value depended on the power level of microwave irradiation. The maximum temperatures and the presence of insoluble residue in the samples obtained after treatment at microwave power levels (175, 200, 225, 250, 275, and 300 W) are shown in Table S2 in the Supporting Information’section. Obviously, the higher the microwave power, the higher final temperature in the samples was obtained. In our tests, there were cases where the microwave treatment led to the production of undesired products in the sample due to the high temperatures reached. It was found that, around 200 °C, liquefied products started producing an insoluble residue in the liquid fraction. We have found from our work that activating the sample to elevated power levels (250, 275, or 300 W) has no positive influence on the composition of liquid products
Figure 1. GC-MS spectra of liquefied products from rapeseed cake (LRC), date seeds, (LDS), olive stone (LOS), and apple pomace (LAP).
with a predominance of derivative compounds of alcohols and acids, showing a high proportion of oxygenated compounds. Several alcohol derivatives could be obtained from degradation of cellulose in acids, which further hydrolyzed cellulose to generate alcohol derivatives.17 In this study, furan derivatives originated from cellulose decomposition,18 such as 5-methyl-2(3H)furanone and dihydro-2(3H)-furanone in LRC and LAP, tetrahydro-3-furanol in LDS, and furfural in LOS, were identified in the samples. Further decompositions of these compounds might contribute to the formation of acids, such as propanoic acid,17 and esters. Dodecanoic acid and hexadecanoic acid, the most common 2757
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Table 1. Chemical Compounds Identified in Liquefied Products by GC-MS liquefied product rapeseed cake (LRC) 5-methyl-2(3H)-furanone dihydro-2(3H)-furanone tri(ethylene glycol) di(ethylene glycol) monovinyl ether tetra(ethylene glycol)
date seeds (LDS)
olive stone (LOS)
Furan Derivatives furfural 5-methyl-2(3H)-furanone Alcohols and Derivatives 1-(2 methylpropoxy)-2-propanol 3-methoxy-1,2-propanediol 1,2,4-butanetriol 1,2,3-propanetriol monoacetate tetrahydro-3-furanol
2-butoxyethyl oleate
1-propoxy-2-propanol 1,2,3-butanetriol tri(ethylene glycol) penta(ethylene glycol) di(ethylene glycol) monovinyl ether hepta(ethylene glycol) Acids, Esters, and Derivatives 3-methoxypropanoic acid, methyl 3-ethoxypropanoic acid, ethyl ester ester butanoic acid, 2,2-dimethylpropyl 3-hydroxybutanoic acid ester dodecanoic acid (lauric acid) acetic acid, 3,4-dihydroxy-3-methylbutyl ester palmitic acid, ethyl ester hexadecanoic acid (palmitic acid)
2-isopropoxyethyl propionate
oleic acid, ethyl ester 2-isopropoxyethyl propionate
2-hydroxypropanoic acid, ethyl ester dodecanoic acid (lauric acid) hexadecanoic acid (palmitic acid)
2,2′-methylenebis(6-tert-butyl-pcresol)
oleic acid pentacosanoic acid, methyl ester
apple pomace (LAP) 5-methyl-2(3H)-furanone dihydro-2(3H)-furanone 1,5-pentanediol methyltriglycol acetate tri(ethylene glycol) tetra(ethylene glycol) hepta(ethylene glycol) monododecyl ether 3-ethoxypropanoic acid, ethyl ester 3-hydroxybutanoic acid acetic acid, monoglyceraldehyde acetic acid, 3,4-dihydroxy-3-methylbutyl ester butyric acid, 2-ethylallyl ester 16-octadecenoic acid, methyl ester hexadecanoic acid, ethyl ester pentacosanoic acid methyl ester palmitic acid, ethyl ester
Phenolic Derivatives 2,2′-methylenebis(6-tert-butyl-p-cresol)
Furthermore, stronger intensities of several bands associated with carboxyl groups were observed when the microwave power level was increased. This could mean changes in the structure of liquefied products, in particular, an increase in the content of carbonyls. FTIR spectra of samples treated at 175, 200, 225, and 250 W, indicating the increase in the carbonyl absorption, are provided in the Supporting Information section (Figure S2). It is noteworthy that functional groups such as carbonyl, carboxyl, and phenolic are related to the degree of reactivity of a lignocellulosic substance (in this case the liquefied product)’ this is a factor to be considered for possible applications of the viscous liquid in view of its use as a chemical precursor. Considering the results, it was decided to continue the evaluation of treated samples coming from date seeds, olive stones, and corncob treated at microwave power level of 225 W and those from rapeseed cake and apple pomace treated at 250 W. Table S3 in the Supporting Information section presents the relative absorbance and assignment of typical bands with their frequency range, functional groups, and possible compounds of liquefied products in both states, conventional and with microwave activation. The FTIR analysis indicated that there were differences in the relative absorbance associated with hydroxyl and carbonyl group contents in liquefied products with microwave activation compared to conventional ones. Conventional liquefied products presented higher absorbance of bands at 3300 cm−1 (O−H stretching) and 1046 cm−1 (C−O stretching). On the other hand, liquefied products with microwave activation showed strong absorbance in the bands at 1730 cm−1 (C=O stretching). This indicates a higher content of carbonyl in the treated products and higher hydroxyl content in the conventional ones.
from date seeds, olive stone, and corncob. In the case of liquefied product from rapeseed cake and apple pomace, the presence of residue was detected at power levels of 275 and 300 W. The high microwave power levels are associated with high temperatures. It is estimated that when the temperature is above 200 °C, repolymerization is the predominant phenomenon that leads to the generation of insoluble material, increasing the fraction of residues in the sample.19 Repolymerization reactions involve reactions between unstable fractions of the liquefied product and the original liquefied product, producing a repolymerizated fraction in the original product or high molecular weight products, in this case insoluble residue. In our case, lower temperature not only allows us to obtain a liquefied product without the undesirable presence of residue but also could lead to a reduction in the energy requirement of the process. Considering the above results, only those samples that did not present residue after microwave activation were evaluated by infrared spectroscopic analysis. In this sense, the study continued with the evaluation of liquefied products from date seeds, olive stone, and corncob treated with microwaves at 175, 200, and 225 W, and the viscous liquids from rapeseed cake and apple pomace were evaluated at the levels mentioned and 250 W. Fourier Transform Infrared Spectroscopy. Infrared spectroscopic analysis (FTIR) was used to study the components of agro-industrial lignocellulosic liquefied products treated with different levels of microwaves, and they were compared to samples without treatment. From analysis of the spectra of liquefied products treated with microwaves, it was observed that, for higher power levels, higher intensities of some bands associated with carbonyl groups are obtained (wavelength range 1730−1710 cm−1). 2758
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Table 2. Physical and Chemical Features of Liquefied Products before and after Microwave Treatmenta rapeseed cake properties elem anal. C, % H, % N, % Ob, % H/C ratio O/C ratio HHV, MJ/kg Mw, g/mol density, g/cm3 water content, % pH a
date seeds
olive stone
apple pomace
corncob
L
250 W
L
225 W
L
225 W
L
250 W
L
225 W
48.52 8.58 0.23 42.67 2.12 0.66 19.4 1240 1.125 10.12 2.20
51.87 8.72 0.22 39.19 2.02 0.57 20.9 5771 1.128 10.15 2.21
51.02 9.32 0.3 39.36 2.19 0.58 21.0 14728 1.120 10.29 2.30
51.98 8.67 0.22 39.13 2.00 0.56 21.8 15370 1.125 10.30 2.35
48.01 8.08 0.13 43.78 2.02 0.68 18.8 6950 1.159 10.20 2.25
50.62 8.05 0.14 41.19 1.91 0.61 19.8 7407 1.167 10.24 2.28
50.29 8.48 0.26 40.97 2.02 0.61 20.0 3510 1.145 10.22 2.42
52.08 8.34 0.25 39.33 1.92 0.57 20.6 8818 1.153 10.33 2.43
48.05 8.20 0.16 43.59 2.05 0.68 18.7 3857 1.140 10.88 2.43
50.00 8.37 0.18 41.45 2.01 0.62 19.6 6938 1.144 10.92 2.47
L, conventional liquefied product without treatment; microwave treatment, 225 or 250 W. bCalculated by difference (Table 2).
microwave-treated products had higher values of this property than conventional products, with the values varying in a range between 20.7 and 21.9 MJ/kg. The liquefaction tests were performed with raw material in anhydrous state; therefore, water in the liquefied products coming from the dehydration reactions occurredg during the conversion process. Minimal increases were observed in the moisture content of the samples treated with microwaves compared to conventional ones, which allowed us to estimate that the increase in the high heating value of samples treated with microwaves with respect to untreated samples was due to the change in chemical composition in the viscous liquids. The molecular weights (Mw) of products treated with microwaves had higher values compared to the values obtained for untreated samples. This situation suggests longer carbon chains in treated samples than in those not treated with microwaves. In addition, the molecular weight distributions for the liquefied products activated by microwaves were wider than those without treatment, indicating a higher Mw/Mn ratio of the final products, which could have a negative effect for use of liquefied products as polymer precursors due to their higher polydispersity index. The microwave power levels assessed in this study are associated with temperatures that favor repolymerization reactions in the treated product. This situation leads to increased fractions of higher molecular weight in the final sample. Repolymerization consist of reactions between unstable liquefied fractions and other fragments of the same liquefied product, producing a repolymerized liquid with a molecular weight higher than the original product, resulting in an increase in molecular weight of the sample tested. This is in accordance with the stronger absorption in several bands (2890 cm−1) reported in the infrared spectroscopy analysis discussed earlier, which indicated the presence of long carbon chains in the product treated with microwaves compared to untreated samples. All liquefied products showed similar values of density, with a slight increase for those samples with microwave treatment, which may be explained by the increase in molecular weight observed for treated samples. In general, the pH values obtained from the analysis performed on products processed via microwaves were very similar to the pH values obtained in their conventional states. However, slight increases in the moisture of liquefied products could explain the higher values of pH for the treated products. It is noteworthy that liquefied
This indicated higher hydroxyl content in conventional liquefied products but higher carbonyl content in those activated by microwaves. It can be deduced that microwave activation induced more intensive oxidation of hydroxyl groups into carbonyl groups. This can be explained by the oxidation of alcohols to carbonyl compounds following one of the basic reactions in organic chemistry.20,21 Furthermore, the intensity of the band at 2890 cm−1 (CH stretching) suggests the presence of longer carbon chains in the treated products compared to untreated ones. Physical and Chemical Properties. Table 2 shows the elemental compositions of liquefied products with microwaves and properties that were measured and compared with those of untreated liquefied products. The liquids in both cases are carbon- and hydrogen-rich feedstock, containing nitrogen in low amounts. However, the carbon and hydrogen contents of liquid products treated by microwaves are greater than conventional ones, varying in a range between 50% and 52.08%. On the other hand, the conventional liquefied products had higher oxygen content than treated products, which mainly presented oxygen functional groups such as alcohol, acid, and ester derivatives. The oxygen content of liquefied products treated with microwaves varied in a range from 39.13% to 41.45%. The H/C and O/C molar ratios of treated liquefied products were in ranges from 1.91 to 2.02 and from 0.56 to 0.62, respectively. The microwave-liquefied products from rapeseed cake, date seeds, and corncob presented interesting H/C molar ratios around 2. In addition, the largest percentage decrease in the amount of oxygen content for a treated sample with respect to conventional ones occurred in the sample from rapeseed cake (3.48%). Indeed, among the liquefied products treated with microwaves, the liquefied rapeseed cake was identified as the sample with the highest H/C molar ratio. The results indicated that the products treated by microwaves had a higher energy density compared to conventional ones. Microwave irradiation could produce dehydration reactions in the sample during the test, which could diminish the oxygen proportion in the liquefied product due to generation of water, and then increasing the heating value of the sample. In this sense, the higher heating values of treated samples were slightly higher than the values reported for conventional products, varying in a range between 18.7 and 21 MJ/kg. By comparing the estimated values of the samples obtained through the Lloyd equation with those calculated with the equation of Friedl et al.,22 we obtained the same tendency; 2759
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avoided by appropriate storage containers. Moreover, liquefied products have characteristics very favorable in view of their use as liquid fuel in boilers or in the coprocessing in a refinery due to their low viscosity (around 1 Pa·s) and suitable calorific value, in which they can play the role of unconventional raw material, similar to that of heavy oils.
products, independent of their state, showed acidity, low pH value due to the high presence of organic acids characteristic of modified raw materials, which were identified in GC-MS analysis. The results indicated changes in elemental composition and properties of products treated with microwaves compared to original ones, indicating that the treatments had an effect on the chemical composition of the liquefied products. The microwave treatments caused rapid and volumetric heating, as too-rapid cooling of samples could lead to changes observed in the treated samples compared with conventional ones. In this sense, it is estimated that the modifications are the result of thermal-kinetic effects that are predominant during the test, being a consequence of the temperatures achieved when the microwaves were irradiated in the liquid, which is in agreement with some other authors.23,24 Applications. One of the possible applications of a liquid from the biomass is as biofuel, and depending on their properties, they may be used as an energy source for engines and/or steam boilers for electric generation and/or heat. In general, the key to convert biomass into liquid fuel is to eliminate or reduce the oxygen of the biomass, considering that biomass contains much more oxygen and less carbon with a lower calorific value compared with oil. Liquid produced from liquefaction and/or pyrolysis always contains large amounts of aldehydes, ketones, esters, and ethers25 with a high oxygen content and low calorific value.26 The H/C molar ratio in the fossil fuel is in the range 1.5−2.0 and the O/C molar ratio is less than 0.06, while in timber the O/C molar ratio is greater than 0.3.27 Therefore, the higher oxygen content is responsible for the low calorific value. In this study, the H/C ratios of liquefied products are in the range mentioned for a biofuel, but the O/C is much higher than those mentioned. Although microwave treatment is displayed as a suitable method for reducing the amount of oxygen in the liquefied product, the presence of oxygen in the sample is high, affecting the HHV compared with other highperformance biofuels. In general terms, the calorific values of liquefied products with and without microwave treatment are similar to the original biomass source and 40−45% lower than those of fossil fuels (heavy fuel oil 40 MJ/kg)28,29 due to water content and mainly to the oxygen content of the liquefied products. Liquefied products, with and without microwave treatment, have properties and characteristics (hydrocarbon chains, heating value, elemental composition, water content, and density) similar to bio-oils obtained from biomass conventional pyrolysis or hydroliquefaction. The obtained products in this study are similar to bio-oils from timber, containing approximately 35−40% oxygen, 55−60% carbon, acid pH, density of ∼1.2 g/cm3, and more than 15% moisture,30 which defines combustion properties that differ from those of petroleum derivatives. Liquefied products evaluated in this study present high heating values from 18.7 to 21.0 MJ/kg, relatively higher values compared to lignite (10−20 MJ/kg) and wood in the dry state (14−20 MJ/kg).31 In this sense, these products might represent an interesting alternative energy source for boilers in power plants or thermoelectric heat or ovens that use lower rank fuels (wood or lignite), due to ease of handling, high energy value, high biomass content, and renewability. On the other hand, the liquefied products have the disadvantage of being corrosive due to the presence of acids; however, this situation can be easily
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CONCLUSIONS To open new avenues of exploitation of liquefied products, detailed chemical compositions were determined. Moreover, microwave activation was evaluated to study the effect that the treatment had on the characteristics of liquefied products. GCMS analysis shows that the liquefied products are complicated organic compounds that mainly consist of alcohol, acid, and ester derivatives. Microwaves are a promising route to enhance liquefied products. Indeed, microwave treatment at different power levels produced changes in the compositions and characteristics of the samples. FTIR analysis indicated increases in the absorption of carbonyl bands and CH groups, which suggests that microwave activation induced more intensive oxidation of hydroxyl groups into carbonyl groups, in addition to the presence of longer carbon chains in the treated products compared to conventional ones. In addition, elemental analysis verified an increase in carbon content and a decrease in the amount of oxygen present in the activated samples, resulting in a higher energy density. Moreover, microwave-treated liquefied products had higher HHV than conventional samples. There were smooth increments in density, water content, and pH values for the microwave-treated samples with respect to conventional ones. It is thought that the rapid and volumetric heating and subsequent cooling of the treated samples could lead to alterations in their chemical compositions. It is supposed that these modifications are principally the result of thermal-kinetic effects, being a consequence of the temperatures reached when the microwaves were irradiated into liquids. There are interesting possibilities of using the treated products as fuel due to their interesting chemical constitutions and calorific properties. The presence of microwaves had a positive effect on the characteristics of liquefied product, if the liquefied product is going to be used as a biofuel.
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ASSOCIATED CONTENT
S Supporting Information *
Three tables, listing chemical composition of raw materials, maximum temperatures during tests under different microwave power levels, and absorbance and assignments by FTIR of liquefied products; and two figures, showing FTIR of raw materials and liquefied products treated with microwaves. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: +34-943017178. Fax: +34-943017140. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the University of the Basque Country (GIU11/10), the Spanish Agency for the International Cooperation for Development (doctoral fellowship MAEC-AECID), and 2760
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Industrial & Engineering Chemistry Research
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Spanish Ministry of Economy and Competitiveness (Juan de la Cierva Contract JCI-2011-09399) for financially supporting this research project.
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dx.doi.org/10.1021/ie3026136 | Ind. Eng. Chem. Res. 2013, 52, 2755−2761
