Adsorption Characteristics of Bio-oil on Biochar in Bioslurry Fuels

Aug 18, 2017 - Fast pyrolysis(1-4) converts biomass that is bulky and has poor grindability into high-energy-density bio-oil and biochar for various a...
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Adsorption Characteristics of Bio-oil on Biochar in Bioslurry Fuels Mingming Zhang, Qiqing Shen, and Hongwei Wu* Department of Chemical Engineering, Curtin University, General Post Office Box U1987, Perth, Western Australia 6845, Australia ABSTRACT: This study reports the adsorption characteristics of bio-oil on biochar in biochar/bio-oil slurry (i.e., bioslurry) systems at three biochar loading levels (2.4, 4.8, and 16.7%, respectively). At low biochar loading levels (2.4 or 4.8%), heavy organic compounds (particularly those containing fused aromatic systems) in bio-oil was selectively adsorbed onto the biochar. Such selective adsorption also resulted in reduced concentrations of alkali and alkaline earth metallic (AAEM) species in the adsorbed bio-oil phase compared to that in the bio-oil blank. A comparison between the experimental and calculated distributions of AAEM species in bio-oil indicates that at least part of the AAEM species adsorbed on biochar is associated with heavy organic compounds. Consequently, such selective adsorption results in changes in the unadsorbed bio-oil phase that has decreased viscosity and aromaticity and increased polarity. It is interesting to note that the selective adsorption was weakened with an increasing biochar loading level in the bioslurry, and such a phenomenon was not observed in the bio-oil−water-soluble fraction (WSF)/biochar system because of the absence of fused aromatic compounds in the WSF.

1. INTRODUCTION Fast pyrolysis1−4 converts biomass that is bulky and has poor grindability into high-energy-density bio-oil and biochar for various applications.5−10 Suspending biochar into bio-oil can produce biochar/bio-oil slurry (i.e., bioslurry),11−14 which is a new class of fuels. Previous studies15−18 on preparation and characterization of bioslurry demonstrated the suitable rheological and fuel properties of bioslurry fuels in terms of the heating value, acidity, density, viscosity, surface tension, etc. Bioslurry can be suitable for gasification19−21 and combustion22 in stationary systems. Spray characteristics are crucial for liquid/slurry fuels in stationary combustion/gasification applications23 and determined by properties such as viscosity, surface tension, and density.24,25 The spray of bioslurry is significantly influenced by fuel viscosity and biochar loading level.23 It is also known that biochar as a porous material can soak bio-oil.15,21,26 However, thus far, biochar soakability of biooil was only considered as a parameter in bioslurry preparation for maximum biochar loading. There is no systematic study on the adsorption behavior of bio-oil on biochar in a bioslurry system, which can be an important factor to consider during bioslurry applications. Biochar as an adsorbent for removal of organic and inorganic contaminants has wide applications in soil remediation and water treatment.27,28 Strong affinity of aromatic compounds to char materials is well-recognized.29,30 Biochar was also used as adsorbent for tar removal31 or pyrolysis wastewater treatment.32 Selective adsorption of polycyclic aromatic hydrocarbons in soil washing solutions on biochar has also been reported.33 As a porous material, biochar can also be used as a catalyst/support for various reactions (e.g., hydrogen production,34 syngas methanation,25 and biodiesel production35). Advances of biochar applications can be found in recent review papers.36,37 Bio-oil has a complex composition, including water, carboxylic acid, ketone, aldehyde, aromatic compounds, etc.38 It is not clear if selective adsorption of the bio-oil component on biochar will occur in a bioslurry system. In addition, alkali and alkaline earth metallic (AAEM) species are known to be © XXXX American Chemical Society

associated with ash-related issues during stationary application.39,40 How the adsorption of bio-oil on biochar affects the distribution of AAEM species in the bioslurry system is also unknown. Therefore, this study aims to carry out an experimental investigation into the adsorption behavior of bio-oil on biochar in bioslurry systems, focusing on the characterization of adsorbed and unadsorbed fractions of biooil.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The raw biochar was produced by pyrolysis of pine wood biomass at around 500 °C in a laboratory-scale drop-tube/fixed-bed reactor (see detailed description given elsewhere26). The raw biochar was then sieved to a size fraction of 38−75 μm and washed with methanol and 0.01 M hydrochloric acid solution for 24 h at room temperature, separately, to yield the biochar sample for adsorption experiments in this study. Methanol washing was employed to remove any organic matter in biochar that can be washed out. The use of methanol was to wash off the adsorbed bio-oil on biochar in subsequent experiments. Washing the char with 0.01 M hydrochloric acid solution is to eliminate the leaching of AAEM species from biochar into bio-oil during the adsorption process. The washed biochar sample was flushed with deionized water on a Büchner funnel until the filtrate was neutral. The flushed biochar was then oven-dried at 60 °C and stored in a sealed container for subsequent adsorption experiments in this study. Unless otherwise stated, the biochar hereafter refers to the pretreated biochar. The raw bio-oil was sourced from a commercial supplier, which was produced from fast pyrolysis of pine wood available to the supplier at 500 °C. The raw bio-oil was filtered through a 0.45 μm syringe filter to prepare the biooil sample for adsorption experiments in this study. Hereafter, bio-oil refers to filtered bio-oil, unless otherwise stated. The bio-oil−watersoluble fraction (WSF) was prepared by adding water to bio-oil in a mass ratio of 1:1.41 2.2. Adsorption Experiment. The bio-oil and biochar samples were mixed in a capped centrifuge tube. For good mixing, the mixture was placed on an electronic shaker at room temperature for 24 h to Received: July 13, 2017 Revised: August 17, 2017 Published: August 18, 2017 A

DOI: 10.1021/acs.energyfuels.7b02041 Energy Fuels XXXX, XXX, XXX−XXX

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Table 1. Properties of the Raw Bio-oil, Raw Biochar, Bio-oil (after Filtration), and Biochar (after Pretreatment) Samples Used in This Study sample

raw bio-oil

proximate analysis (wt %) water contenta 24.0 ashb 0.03 solids (as 0.01 methanol a insoluble) volatile matterb ndd fixed carbonb ndd e elemental analysis (wt %) C 42.53 H 8.42 N 0.09 Of 48.96 AAEM species (wt %)e Na 0.0001 K 0.0012 Mg 0.0002 Ca 0.0015 viscosity at 20 °C 178.0 (mPa s) BET surface area nac (m2/g)

bio-oil

raw biochar

biochar

24.4 0.03 0.00

1.3 1.6 nac

0.9 0.9 nac

ndd ndd

20.5 76.6

21.2 77.0

42.34 7.99 0.11 49.56

81.28 3.41 0.16 15.15

81.01 3.40 0.20 15.39

0.0001 0.0012 0.0002 0.0015 184.0 nac

0.0098 0.3204 0.1076 0.3132 nac 107

0.0037 0.0227 0.0911 0.2660 nac 120

a

As-received basis. bDry basis. cNot applicable. dNot determined. eAsreceived basis for bio-oil and dry basis for biochars. fBy difference.

Figure 1. Experimental procedure used in this study, including sample pretreatment, adsorption, and subsequent analysis on adsorbed and unadsorbed fractions of bio-oil. HCl = hydrochloric acid solution.

V30), respectively, following methods detailed elsewhere.43,44 The Brunauer−Emmett−Teller (BET) surface area of biochar samples was analyzed on Tristar using a CO2 sorption isotherm. The typical properties of the biochar and bio-oil samples used in this study are listed in Table 1.

allow for adsorption equilibrium.26,32 Afterward, the bioslurry was separated via repeated centrifugations at 4700 rpm until no liquid was further separated. Therefore, in this study, the adsorption of bio-oil on biochar is defined by those strongly adsorbed on biochar that cannot be separated out by intensive centrifugation. Unadsorbed bio-oil was syringe-filtered to avoid the suspending of fine biochar particles in this fraction and then collected for analysis of viscosity, water content, and elemental composition. Adsorbed bio-oil on biochar was desorbed by methanol wash until colorless. Further methanol washing of pretreated biochar was conducted in parallel as the methanol desorption blank. The desorption solutions were then collected for subsequent analyses. All of the analyses were conducted on bio-oil without subjection to adsorption (referred to as the “bio-oil blank”) as benchmarking. The experimental procedure is illustrated in Figure 1. 2.3. Sample Characterization. For liquid samples, ultraviolet (UV) fluorescence analysis was conducted on a spectrometer (PerkinElmer LS55) using synchronous scan mode following a method detailed elsewhere.42 Gel permeation chromatography (GPC) analysis was carried out on a liquid chromatography system (Varian 380) equipped with a PLgel column (3 μm, 100 Å, 300 × 7.5 mm) and an UV detector. Tetrahydrofuran was used as eluent at a flow rate of 1.0 mL/min. Phenol and polystyrene in a range of 94− 4120 were used as standards for calibration of the molecular weight. Quantification of AAEM species in desorption solution, bio-oil, and bio-oil−WSF were conducted in accordance with a method developed previously.43 Briefly, the samples were combusted in a Muffle furnace according to a specifically designed temperature−time program. Obtained ash was then digested using nitric acid for at least 16 h, and then the residue was dissolved in 0.001 M nitric acid solution, followed by ion chromatography (IC) analysis. For biochar and bio-oil samples, a PerkinElmer CHN/O analyzer (2400 Series II) and a Mettler thermogravimetric analyzer (TGA/DSC 1 STAR) were used for elemental and proximate analyses, respectively. Viscosity and water content of the bio-oil samples were analyzed using a rheometer (Haake Mars II) and a Karl−Fisher titrator (Mettler

3. RESULTS AND DISCUSSION 3.1. Adsorption of Bio-oil by Biochar Based on Weight. Figure 2 presents the data on bio-oil adsorption onto biochar in bioslurry samples at different biochar loading levels. It can be seen from Figure 2a that increasing the biochar loading level in bioslurry from 2.4 to 16.7% leads to an increase in the percentage of bio-oil adsorbed on biochar from ∼7 to 39%. This is expected because more biochar in bioslurry can adsorb more bio-oil. It should be pointed out that the biochar used in this study was washed with methanol and 0.01 M hydrochloric acid solution. There is a slight increase in the surface area (from 107 to 120 m2/g; see Table 1) of biochar after such pretreatment. This is likely due to the increase in char pore availability (that is known for char after washing using a solvent, such as methanol41). However, char washing with both acid and methanol has little influence on the aromatic structure of biochar and surface functional groups.45,46 Therefore, such biochar pretreatment may have slightly enhanced adsorption capability but little influence on adsorption selectivity. Furthermore, Figure 2b shows that the amount of bio-oil adsorbed onto unit mass of biochar (i.e., biochar adsorption capacity for bio-oil) decreases slightly with an increasing biochar loading level in bioslurry. For example, as the biochar loading level increases from 2.4 to 16.7%, the biochar adsorption capacity for bio-oil decreases from 2.8 to 2.4 g of bio-oil/g of biochar. The results suggest that selective adsorption takes place in the bioslurry systems. It appears B

DOI: 10.1021/acs.energyfuels.7b02041 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. (a) Molecular weight distribution and (b) UV fluorescence spectra of bio-oil adsorbed on biochar in bioslurries with different char loading levels, benchmarking with the bio-oil blank. BO, bio-oil; ABO, adsorbed bio-oil; and x% BS, bioslurry with x% of biochar loading. The bio-oil samples were diluted in methanol at about 8 ppm to avoid selfabsorption in UV fluorescence analysis and about 0.3% for GPC analysis.

Figure 2. (a) Distribution of bio-oil after biochar adsorption and (b) biochar adsorption capacity for bio-oil in bioslurry samples at different biochar loading levels.

that, at a low biochar loading level (e.g., 2.4%), some heavy components of bio-oil were selectively adsorbed onto biochar, while at a high biochar loading level (e.g., 16.7%), the biochar adsorption capacity for bio-oil is close to the soakability of the raw biochar (2.3 g of bio-oil/g of biochar26). The results suggest that, when there are abundant biochar in the system, adsorption of bio-oil onto biochar becomes less selective because there are excessive biochar available for bio-oil adsorption. Therefore, efforts were then made to characterize the organic matter selectively adsorbed on biochar at various biochar loading levels. 3.2. Characteristics of Organics of Bio-oil Adsorbed by Biochar. On the basis of the similar convention in the literature,47 the molecular weight distributions of the bio-oil samples can be derived from the corresponding GPC spectrum. Figure 3a shows that the molecular weight distribution curves for the bio-oil blank and adsorbed bio-oil on biochar have similar shapes (which are also consistent with that previously reported for a bio-oil sample47). However, the proportion of heavy organic compounds (with a molecular weight in the range from log 2.6 to log 3.5) in adsorbed bio-oil from bioslurry at a low biochar loading level (e.g., 2.4 or 4.8%) is considerably higher than adsorbed bio-oil from bioslurry at a high biochar loading level (e.g., 16.7%) and the bio-oil blank. Such results provide direct evidence for the selective adsorption of some heavy components of bio-oil in bioslurry at a low biochar

Figure 4. UV fluorescence spectra of the adsorbed WSF on biochar in a bioslurry with 4.8% biochar, benchmarking with the WSF blank. WSF, water-soluble fraction separated from bio-oil at a water/bio-oil ratio of 1:1; 4.8% BS, bioslurry prepared from WSF and 4.8% biochar. The WSF samples were diluted in methanol at about 0.01% to avoid self-absorption in UV fluorescence analysis.

C

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Figure 6. Concentration of AAEM species in the adsorbed WSF, benchmarking with the WSF blank. WSF, water-soluble fraction separated from bio-oil at a water/bio-oil ratio of 1:1; 4.8% bioslurry, bioslurry prepared from WSF and 4.8% biochar.

the aromatic ring structures in bio-oil onto biochar can be mainly attributed to the π−π interaction of the aromatic compounds in bio-oil with the aromatic carbon structure of biochar.29 The selectivity over the fused aromatic ring is likely due to the porosity-selective effect.49−51 It appears that, in comparison to an aromatic ring structure with a single ring, the fused rings in bio-oil match better with the pore sizes of biochar, thereby being strongly associated with the biochar structure rather than being separated out during intensive centrifugation. Figure 3 also shows that the preferential adsorption of large aromatic ring structures becomes weakened when there is a large amount of biochar loaded into the system. Both the molecular weight distribution and the UV fluorescence spectrum of adsorbed bio-oil from bioslurry at a high biochar loading level are very similar to those of the bio-oil blank. This is consistent with the observation of the decrease in the biochar adsorption capacity for bio-oil (see Figure 2b). It appears that, in the presence of a large quantity of biochar in the system, the surface or pores of biochar became excessive for fused aromatic rings, so that adsorption of both single and fused aromatic structures takes place. It is further interesting to note that, unlike bio-oil, the WSF of bio-oil only has one peak corresponding to single aromatic structures in the UV fluorescence spectrum (see Figure 4). Efforts were then made to investigate whether selective adsorption is also notable in such a WSF/biochar system at 4.8% biochar loading level. As shown in Figure 4, the UV fluorescence spectrum of the adsorbed WSF is almost the same as that of the blank WSF. This indicates that, in absence of fused aromatic ring structures, little selective adsorption takes place in the system. In such a system, it is mostly likely that hydrophobicity or functional groups of biochar play an important role in the adsorption of the WSF of bio-oil on biochar (similar to water uptake in biochar as reported previously52). 3.3. Changes in the Characteristics of AAEM Species in Bio-oil as a Result of Biochar Adsorption. Figure 5 shows that the concentrations of AAEM species in adsorbed bio-oil on biochar are lower than those in the bio-oil blank, but the difference becomes smaller with an increasing biochar

Figure 5. Concentrations of AAEM species in bio-oil adsorbed by biochar in bioslurry fuels at different biochar loading levels.

loading level. Figure 3b presents the UV fluorescence spectra of the samples. It can be seen that bio-oil has two peaks in the UV fluorescence spectrum (consistent with our previous study41): one peak at 270−290 nm and the other peak at 290−400 nm (corresponding to single aromatic ring structures and two or more condensed aromatic ring structures in bio-oil, respectively42,48). While the spectrum for adsorbed bio-oil also has two peaks at similar wavelengths, the intensities of these two peaks (especially the second peak corresponding to two or more condensed aromatic ring structures) are higher than those for the bio-oil blank. Such differences are considerable for the adsorbed bio-oil from bioslurry fuels at low biochar loading levels (e.g., 2.4 or 4.8%). The results clearly show that the aromatic ring structures with two or more fused rings in bio-oil can be preferentially adsorbed on biochar. The adsorption of D

DOI: 10.1021/acs.energyfuels.7b02041 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 7. (a−c) Distribution of AAEM species in bio-oil after adsorption in bioslurry fuels at different biochar loading levels, benchmarking against the calculated distribution based on WSF assumption or WISF assumption, and (d−f) difference values between experimental (exp.) and calculated (calc.) values. WSF stands for the water-soluble fraction separated from bio-oil at a water/bio-oil ratio of 1:1. WSF assumption refers to the assumption that only WSF is adsorbed onto biochar. WISF stands for the water-insoluble fraction of bio-oil, with WSF not being included. WISF assumption refers to the assumption that only WISF is adsorbed onto biochar. BO, ABO, UABO, and x% BS stand for bio-oil, adsorbed bio-oil, unadsorbed bio-oil, and bioslurry at x% of biochar loading level, respectively.

becomes excessive and a large amount of bio-oil is adsorbed on biochar, leading to less selective adsorption (see the discussion in previous sections), so that the composition of AAEM species in adsorbed bio-oil becomes closer to that in bio-oil. Similarly, such selective adsorption was not observed in the WSF/biochar system, as evidenced in Figure 6 that clearly shows negligible

loading level in the system. The concentrations of Mg and Ca in adsorbed bio-oil from bioslurry at 2.4% biochar loading level are ∼0.69 and 6.84 ppm, accounting for only 27 and 43% of those in bio-oil (∼2.56 and 14.76 ppm), respectively. This further supports the conclusion of selective adsorption of biooil onto biochar taking place at low biochar loading levels in a bioslurry system. At a high biochar loading level, biochar E

DOI: 10.1021/acs.energyfuels.7b02041 Energy Fuels XXXX, XXX, XXX−XXX

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assumptions corresponding to two extreme cases, i.e., adsorbed bio-oil on biochar being only from either the water-soluble fraction (referred to as “WSF assumption”) or water-insoluble fraction (referred to as “WISF assumption”). Figure 7 presents the comparisons between the calculated results based on these assumptions and the experimental data. Panels a−c of Figure 7 show that the experimental results on the AAEM species distributed in the adsorbed bio-oil phase are between the calculated value based on WISF assumption and that based on the WSF assumption. For bioslurry at a low biochar loading level of 2.4%, the experimental results are very close to the calculated results based on WISF assumption, with the difference being