Comparative Study of Biomass Fast Pyrolysis and Direct Liquefaction

Article pubs.acs.org/EF

Comparative Study of Biomass Fast Pyrolysis and Direct Liquefaction for Bio-Oils Production: Products Yield and Characterizations Nicolas Doassans-Carrère,† Jean-Henry Ferrasse,*,† Olivier Boutin,† Guillain Mauviel,‡ and Jacques Lédé‡ †

Aix Marseille Université, CNRS, Centrale Marseille, M2P2 UMR 7340, 13451, Marseille, France CNRS, Université de Lorraine, LRGP UMR 7274, 54001 Nancy, France

‡

ABSTRACT: The objective of this work is to compare two biomass-to-oil processes: fast pyrolysis and direct liquefaction, using the same biomass (beech sawdust). Fast pyrolysis is conducted in a cyclone reactor (wall temperature between 870 and 1040 K) and direct liquefaction in a 150-mL-autoclave reactor (bulk temperature between 420 and 600 K). Three fractions of pyro-oil are obtained from fast pyrolysis (heavy oil, light oil, and aerosol), whereas two fractions of liq-oil (heavy oil and water-soluble organics) are obtained from direct liquefaction. The comparison of both processes is based on the product yields and their characterization (ultimate analysis for solid and oils, oil−water content, gas and oil molecular composition, 1H NMR for oils). For both processes, there is an optimal temperature at which the oil yield is maximum. Up to 62.6 wt % of pyro-oil are obtained at 970 K with the cyclone reactor (with 25.7 wt % of gas and 11.7 wt % of solid), whereas 47.0 wt % of liq-oil was obtained at 573 K with the batch-reactor (completed by 5.5 wt % of gas and 17.8 wt % of solid). Water content mainly explains the differences (mass yield and oxygen content) between oils from fast pyrolysis and direct liquefaction. Nevertheless, there are also some differences in organic composition: levoglucosane is a main component in pyro-oil, whereas levulinic acid is a main component in liq-oil. Finally, gas formed during direct liquefaction is mainly composed of CO2 (more than 99 wt %), whereas gas from fast pyrolysis is a mixture of CO, CO2, H2, CH4, and light hydrocarbons.

■

INTRODUCTION Global energy demand is increasing year after year mainly because of population growth and industrialization. Moreover, ecological considerations promote the use of CO2-neutral energies. Biomass contains one of the available renewable forms of energy and supplies today 12% of primary energy global demand and close to 50% in developing countries.1 Lignocellulosic biomass is mainly composed of cellulose (34−54 wt %) forming the skeleton, surrounded by hemicellulose (19−34 wt %) and lignin (11−30 wt %).2 This type of biomass contains also minor compounds like ashes (between 0.5 wt % in wood and 25 wt % in agricultural wastes).1,3 Biomass processing to produce biofuels is of a growing interest and could be economically viable in the future.4 Both fast pyrolysis5 and direct liquefaction6 technologies have been studied in order to obtain bio-oils. Fast pyrolysis consists in a thermal degradation of biomass in the absence of air which gives 60−75 wt % of bio-oil, 15−25 wt % of solids, and 10−20 wt % of gas. The key factors of fast pyrolysis are a high heat transfer coefficient between heat source and biomass, an estimated reaction temperature between 673 and 723 K (lower than bed or wall temperature), and efficient eliminations/recoveries of products.7 Direct liquefaction consists in a subcritical (≤647 K and ≤22.1 MPa) hydrothermal degradation of biomass.8 The maximum of bio-oil yield, in water solvent, is about 40 wt % for a temperature range between 570 and 600 K.9 Products from fast pyrolysis are recovered using one or several cyclones to separate solids from vapors and gas, followed by several condensation systems in order to recover bio-oils and cleaned gas.5 This bio-oil recovery step is quite the same for any pyrolysis process: it allows a direct comparison © XXXX American Chemical Society

between different fast pyrolysis reactors. In order to separate different bio-oils, solvent fractionation10 or fractional condensation can be used.11−14 For direct liquefaction, the use of water implies separation steps to recover bio-oils. Usually an organic solvent is used to clean the reactor and recover bio-oil. The solid is recovered by filtration and two liquid phases are obtained: an organic one and an aqueous one. The solvent can be ether,15,16 chlorine solvent,17,18 or acetone.19−21 Then, solvent and water are evaporated in order to obtain heavy oil from the organic phase and water-soluble organics from the aqueous phase.22,23 Bio-oils from fast pyrolysis are referred to in this article as pyro-oils and bio-oils from direct liquefaction as liq-oils. Both types of bio-oils have to be upgraded to serve as biofuels, e.g. using catalytic hydrodeoxygenation (HDO) to reduce oxygen content.24,25 That is the reason why products characterization is necessary to have a good prediction and scaling of the HDO step. The aim of this paper is a comparison between fast pyrolysis and direct liquefaction for the production of bio-oils, using the same biomass. This comparison is based not only on bio-oil, solid, and gas yields but also on their characterization with the same analysis apparatus. This comparison methodology is original in the literature.

■

MATERIALS AND METHODS

Raw Materials. In order to compare the two processes, experimentations are led with the same biomass, beech sawdust. Its Received: March 24, 2014 Revised: June 1, 2014

A

dx.doi.org/10.1021/ef500641c | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

Article

Figure 1. Fast pyrolysis setup scheme. Direct Liquefaction. Direct liquefaction (Figure 2) is achieved in a 150 cm3 autoclave. Six g of biomass are introduced prior to the locking

moisture content is 9 wt % on wet basis, and the ash content is 0.6 wt % on dry basis. Its elemental composition is 48.0 wt % of C, 5.9 wt % of H, and 46.1 wt % of O (calculated by difference) on dry ash-free basis. N and S contents are not quantified because they are below the detection limit (0.1 wt %). Granulometric classes used are 250−400 μm for fast pyrolysis in order to obtain high biomass conversion26,27 and 400−630 μm for direct liquefaction, since this class is easier to observe visually during the experiment. N2 is used as inert gas for both setups. For direct liquefaction, solvents used are distilled water as the reaction medium and pure acetone 99+% from Acros Organics to clean the reactor and extract oil from solids. Description of the Experimental Setups. Fast Pyrolysis. Fast pyrolysis is achieved using the cyclone reactor detailed elsewhere.14,26−28 It is a continuous reactor with an oil recovery system allowing the separation of three fractions of pyro-oil (Figure 1). Upstream of the cyclone, the installation is composed of the biomass feeding system (tank and screw conveyor) and the N2 supply (pressure regulator, mass flow controller). Biomass particles, carried by N2, are introduced tangentially into the cyclone and are pyrolyzed along the cyclone wall. The stainless steel cyclone is heated by induction and has a diameter, an internal surface, and a volume of respectively 6 cm, 370 cm2, and 470 cm3. A bypass system allows preheating and thermal equilibrium of the setup before biomass feeding. Temperature is controlled all along the setup thanks to a pyrometer for the wall temperature and several thermocouples placed along the installation. Using the separation properties of the cyclone, solids are recovered at the bottom into the solids collector. Gaseous products and carrier gas, evacuated through the upper part of the cyclone, pass through three water cooled heat exchangers in series in order to recover the first fraction of pyro-oil, “heavy oil” (PHO), in a liquid collector. Then the gas flows through refrigerated coils, maintained at 268 K, inside which the “light oil” (PLO) fraction is recovered. In order to recover the last fraction, “aerosols” (PAE), both electrostatic and membrane filters, are used in series. At the outlet, cleaned gas is finally sampled in a bag throughout the whole experiment duration. Experiment duration varies between 15 and 40 min in order to process around 130 g of biomass. Cyclone wall temperature, TW, is homogeneous on all the wall length with a maximum gradient of 25 K. With a wall temperature between 870 and 1040 K, T1 varies from 580 to 740 K, whereas T2 and T3 always equal 300 and 280 K, respectively. All setup devices are weighed before and after each run with a lab balance (std deviation 0.01 g). Thus, char and oil yields are calculated by mass differences. The gas yield is calculated from the N2 flow rate value and the concentration of gaseous products in the sampling bag.

Figure 2. Direct liquefaction setup scheme. of the reactor with the closing straps. Using a volumetric pump, N2 or distilled water can be introduced into the autoclave at a desired pressure. After inerting the reactor with N2 and setting the initial pressure (1 MPa), 100 mL of distilled water is added into the reactor heated by an electric resistance. The heating rate is 2.4 K·min−1 up to 373 K and then 3.5 K·min−1 until the final temperature. The reactor is cooled down by a refrigerating system using air (−2.9 K·min−1 to 473 K) and then water (−3.1 K·min−1). Once the reactor reaches ambient temperature, gases are sampled and analyzed by GC-TCD. The reactor is opened in order to collect the products (liquids and solids). Based on Sun et al.,29 pure acetone is used to rinse the reactor and wash solid. After filtration, solid is placed into a drying oven at 378 K. The liquid phase is then placed in a reduced pressure evaporator at 303 K in order to remove acetone from the mixture and separate the first fraction of liq-oils, ”heavy oils” (LHO), and the aqueous phase. The aqueous phase is then placed in a reduced pressure evaporator at 333 K in order to remove water and recover the second fraction, ”water-soluble organics” (LWSO). Figure 3 represents the recovery protocol. The mass of solid, heavy oil, and water-soluble organics are measured with a lab balance (std deviation 0.001 g). Analysis of Bioproducts. Gas from fast pyrolysis (H2, CO, CO2, CH4, C2H4, etc.) is analyzed by micro-GC Varian 490 equipped with four modules composed of two molecular sieves 5A, a PoraPlot U, and a Cp-Wax 52CB columns. Micro-GC-490 signal is calibrated using four standard cylinders (Air Liquide, France). Gas from direct B


Energy & Fuels

Article

Figure 3. Recovery protocol for direct liquefaction. liquefaction is analyzed by GC-TCD Varian CP-3800 equipped with two columns (HAYSEP Q and molecular sieves 5A) which are used alternatively in series and bypass. The mass of produced gas is then calculated by relative proportion according to N2 used in both processes. Major compounds of bio-oils are identified by GC-MS analysis and then quantified by GC-FID on a DB-1701 (60 m, ID 0.25 mm, df 0.25 μm) column. The GC heating program is as follows: 313 K for 5 min, 5 K·min−1 to 423 K, 10 K·min−1 to 543 K then hold 5 min. Quantification is done using an internal standard (1-undecene).30 1 H NMR is made with a 300 Hz NMR spectrometer (Bruker Avance 300). Solvent used is acetone-D6, 99.8%, Acroseal from Acros Organics. Ultimate analysis (Thermo Flash EA 1112) is done on solids and liquids recovered in order to measure mass content of C, H, N, and S. Oxygen mass content is calculated by difference. Water content of liquid is measured thanks to Karl Fischer analysis with HYDRANAL Medium K.

For both processes, the mass balance, i.e. the sum YS + YG + YL, should be equal to 1 if there is no mass loss. Fast Pyrolysis. Table 1 displays the operating conditions for each run. Two parameters are studied: cyclone wall temperature and biomass mass flow rate. Table 1. Fast Pyrolysis Runs

■

mG mB ·(1 − θB)

(2)

In fast pyrolysis, water from moisture and water produced during reaction are considered to be condensed in pyro-oil. Pyro-oil mass yield definition is given in eq 3, in which water from biomass moisture is subtracted. In this definition, there is no distinction between different fractions of pyro-oils, because water is included in all fractions. Fractions distinction will appear in the section ”Bio-Oil Composition”, with determination of water content in each fraction. mL refers to mass of all liquid fractions recovered. YL , Pyro =

mL − mB ·θB mB ·(1 − θB)

(3)

For direct liquefaction, there is no water in liq-oils since it is evaporated during the recovery protocol (cf. Figure 3). The liqoil mass yield is the sum of heavy oil and water-soluble organics mass yields (eqs 4−6). YL , Liq = YLHO + YWSO

(4)

mLHO mB ·(1 − θB)

(5)

YLHO = YLWSO =

mLWSO mB ·(1 − θB)

TW (K)

QN2 (10−4 kg·s−1)

QB (10−5 kg·s−1)

Γ (QN2/QB)

P1 P2 P3 P4 P5 P6

870 970 1040 920 920 920

9.6 9.6 9.6 9.6 9.6 9.6

13.1 13.1 13.1 13.1 8.42 5.1

7.3 7.3 7.3 7.3 11.4 18.8

Figure 4 shows influence of temperature and mass flow rate ratio on product yields. The overall mass balances range between 83 and 100 wt %. It can be noticed that mass balances are closer to 100 wt % when the sum of solid and gas mass yields are high: this loss may be due to an incomplete recovery of pyro-oils, which is confirmed in the section ”Atomic balances”. Moreover mass yield errors, evaluated by the repetition of P6 run, are respectively 0.5, 1.5, and 7 wt % for solid, gas, and pyro-oil. The liquid mass yield variability is mainly due to an incomplete recovery of the aerosols with a mass yield error of 5 wt %. In Figure 4(a), wall temperature influence on solid and gas mass yields is clear. The solid fraction strongly decreases with temperature increase up to 970 K and tends to a value around 8−10 wt % for higher temperature, confirmed by other studies performed with the same cyclone reactor.26,27 Gas formation continuously increases with temperature, indicating the existence of cracking reactions.14 Both of these evolutions entail the presence of a maximum liquid mass yield. For this study, this maximum appears at 970 K with 49.5 wt % of liquid, 11.7 wt % of solid, and 25.7 wt % of gas. These results confirm previous studies performed with this cyclone reactor that showed a linear dependency of gas mass yield with temperature (up to 75 wt % at 1220 K) and a maximum of liquid mass yield around 950 K (65 wt %).26,27 Similar observations can be made concerning the influence of the ratio of the mass flow rates (Figure 4(b)). With a decrease of this ratio which corresponds to an increase of biomass mass flow rate, solid mass yield increases and gas mass yield decreases, with a maximum liquid mass yield for a ratio of 11.4 for this study. The possible explanation for the increase of solid mass yields with biomass mass flow rate may be a shielding effect: some biomass particles may not be in direct contact with the hot wall because the solid hold-up is higher. Indeed the biomass mass flow rate should not modify the gas and solid residence time distribution.28 This shielding effet may also impact the gas temperature that would be less heated by the wall when biomass hold-up is higher. That may explain the lower gas yield since there would be less oil cracking in this

RESULTS AND DISCUSSION Product Yields. The mass yields are defined on dried biomass basis in order to avoid an overestimation of liquid mass yield in which all water (from biomass moisture and produced during reaction) is taken into account. For both processes, solid (YS) and gas (YG) mass yields are defined in eqs 1 and 2, taking into account the initial biomass moisture fraction θB = 0.09. mS YS = mB ·(1 − θB) (1) YG =

runs

(6) C


Energy & Fuels

Article

Figure 4. Fast pyrolysis product yields.

Figure 5. Product yields of (a) present study with runs P4 (920 K-7.8(Γ)) and P5 (920 K-11.4(Γ)), (b) fast pyrolysis of beech and oak in the cyclone reactor,14 (c) fast pyrolysis of beech in a fluidized bed,33 and (d) fast pyrolysis of pine in a fluidized bed.11

P6, explaining the mass balances misclosures. Results with fluidized bed fast pyrolysis of beech33 and pine11 show similar evolution of solid, gas, and liquid mass yields with temperature increase. The presence of a maximum liquid mass yield is also evidenced in the studies of Scott et al.34 and Demirbas et al.35 Direct Liquefaction. Table 2 lists bulk temperature and corresponding final pressure for direct liquefaction runs.

case. Similar results, with a maximum liquid mass yield, have been shown with a ratio close to 10 at 983 and 1143 K.27 Few authors have published works performed in similar cyclone reactors. The direct comparison of our results with other types of pyrolysis reactors presents several difficulties. Among them is the definition of temperature. The temperature is often assimilated to that of the heat source which can considerably differ from the real biomass reacting temperature.7,31 Heat transfer processes may also differ according to the reactor and type of heat source (hot walls, hot sand, hot gas, ...).32 In the present work, the comparison will be made on the basis of temperature ranges inside which the liquid mass yield is maximum. Furthermore, in many cases, mass yields are defined on wet basis instead of dry basis. In order to compare results on the same basis, mass yields from the literature are recalculated according to eqs 1, 2, and 3 with the biomass moisture supplied in each paper. Figure 5 compares our results with three other fast pyrolysis studies. Fast pyrolysis of beech and oak blend in the cyclone reactor,14 at 900 K and a ratio of 10.4, can be compared to runs P4−P5 which operate at 920 K and ratios of 7.8 and 11.4. This comparison shows similar solid and gas yields, even if gas mass yield is higher at 920 K. Moreover, liquid mass yield presented seems to confirm the partial recovery of liquid for runs P2 to

Table 2. Direct Liquefaction Runs runs

L1

L2

L3

L4

L5

L6

L7

L8

Tbulk (K) P (MPa)

523

553

503

483

603 25

573 19

513 8.5

528 13

Figure 6 shows the influence of bulk temperature on direct liquefaction mass balance. The overall mass balance ranges between 64 and 83 wt %. These results correspond to runs L5 to L8 for which all products (solid, gas, and liquids) are recovered. Moreover, mass yield errors, evaluated by the repetition of L6 run, are respectively 0.3, 0.5, and 7 wt % for solid, gas, and liq-oil. As in fast pyrolysis, the maximum error appears for the liquid mass yield, indicating a partial recovery. In the literature, mass balance closures are generally poor or not reported.17,18,23 Mass yields calculated by difference21,36 can be overevaluated. Some mass balances are presented22 within D


Energy & Fuels

Article

composition corresponds to the one obtained in previous work.27 Even if quantification of biomass conversion yield is difficult, partial conversion seems obvious if the wall temperature is below 900 K. In direct liquefaction, between 483 and 528 K, solid compositions varies from CH1.54O0.70 to CH1.46O0.66 and is not significantly different from biomass composition (CH1.48O0.72). Between 553 and 603 K, solid composition varies from CH0.84O0.28 to CH0.70O0.24. The last composition is similar to that of solid recovered from direct liquefaction of sawdust at 573 K (CH0.77O0.35).38 Combined with mass yields, it can be concluded that up to 528 K, biomass is only partially converted in our conditions. Above 553 K, solid composition is similar to charcoal recovered in fast pyrolysis. It could however be noticed that solid recovered in direct liquefaction is more hydrogenated and oxygenated than that from fast pyrolysis. Gas Composition. Figure 8 shows gas composition from both processes. Gas obtained by fast pyrolysis (Figure 8(a)) is a mixture of CO, CO2, H2, CH4, C2H4, and other C2−C3. With the increase of wall temperature, CO2 fraction decreases in favor of H2, C2H4, and other C2−C3 (C2H2, C2H6 and C3H8) fractions. Meanwhile, CO and CH4 fractions seem to be fairly constant. Such fractions are very similar to those previously measured with the same cyclone reactor14,26,27 and also with those obtained in fluidized bed reactors.12,33 Gases from direct liquefaction (Figure 8(b)) are essentially composed of CO2. As in fast pyrolysis, H2 fraction increases with temperature. Some traces of CO and light hydrocarbons are found by IR-analysis, but their concentrations are too low to be detected by GC. The large majority of CO2 has also been reported in other studies.20,21,39 Bio-Oil Composition. In this section, average values refer to runs P3 and P6 for fast pyrolysis and L5 and L6 for direct liquefaction, for which the highest bio-oils mass yields are obtained. Ultimate Analysis. Table 3 displays elemental composition of pyro-oil and liq-oil fractions, obtained from ultimate analysis. On the basis of three determinations of each analysis, errors are respectively 0.3 and 0.1 wt % for C and H mass contents. O content is calculated by difference (no N nor S are detected). Compositions of pyro-oils fractions are quite different from those reported in Lédé et al.14 with composition of heavy oil, light oil and aerosols respectively of CH1.46O0.56N0.005, CH5.06O3.03N0.03, and CH1.50O0.62N0.005. For the liq-oil, the

Figure 6. Temperature influence on mass yields for direct liquefaction.

the range 62 wt % to 90 wt %, that are similar to the values of this study. As in fast pyrolysis runs, the solid mass yields strongly decreases with increasing temperature, from 46.4 wt % at 483 K to 13.7 wt % at 603 K. At the opposite, the gas mass yield increases with increasing temperature from 2 wt % at 503 K to 7.3 wt % at 603 K. Heavy oil and water-soluble organics mass yields show similar trends until 528 K. Then, the LWSO mass yield decreases slightly, whereas the LHO mass yield reaches a maximum (30.4 wt %) at 573 K. In addition, total liq-oil mass yield reaches a maximum between 528 and 573 K. For this study, this maximum appears at 573 K with 47.0 wt % liquid mass yield, 17.8 wt % of solid, and 5.5 wt % of gas. Comparison with the literature is difficult because of differences in the liq-oil fraction definitions and the type of biomass used. Nevertheless, gas mass yields below 10 wt % are also observed21,36,37 as well as solid mass yields near 20 wt % at 573 K21,37 and higher for lower temperature.36 Figure 7 compares our results with those of Sun et al.29 who used the same type of recovery protocol. Product mass yields values and evolutions are similar to this study, except gas yields which are not presented. Solid Composition. Ultimate analysis of the solid has been repeated 3 times for each sample: errors are respectively 0.2 and 0.05 wt % for C and H mass contents. O content is calculated by difference (no N nor S are detected). In fast pyrolysis, solid recovered at 870 K (CH1.26O0.57) is close to initial dry biomass (CH1.48O0.72), whereas solid recovered at 970 K (CH 0.54 O 0.24 ) and at 1040 K (CH0.37O0.18) can be considered as ”charcoal”. This last

Figure 7. Direct liquefaction mass yields of (a) beech (present study) and (b) paulownia wood.29 E


Energy & Fuels

Article

Figure 8. Variations of gas composition (N2 free) with temperature.

On a water free basis, the elemental compositions of pyro-oil and liq-oil fractions are similar indicating that the oxygen content difference between these two bio-oils is mainly explained by the water content. Atomic Balances and Reconciliation. Atomic balances can be computed for both processes with the preceding results. For fast pyrolysis, average atomic balances (C, H, and O) are respectively 90%, 80%, and 95%. For direct liquefaction, average atomic balances are respectively 76%, 64%, and 65%. For both processes, misclosure of atomic balances mainly results from mass balances misclosures. For fast pyrolysis, the relatively bad mass balances misclosure could be explained by an incomplete recovery of pyro-oils. After calculating liquid mass yield by difference from solid and gas mass yields (eq 7), various hypothesis could be taken: recalculate the mass yield of each fractions on the basis of global liquid fraction or only some of them. Atomic balances (C, H, and O) are respectively 100%, 89%, and 99% if aerosols mass yield is recalculated (eq 10). Since aerosols correspond to the fraction the most difficult to recover, this recalculation matches with experimental observations.

Table 3. Elemental Compositions of Bio-Oils Fractions process

fraction

composition

θi (%)

”water free”

fast pyrolysis

PHO PLO PAE LHO LWSO

CH3.34O1.73 CH4.08O1.98 CH1.32O0.46 CH1.11O0.43 CH1.65O0.78

40 36.5 6.5 (−) (−)

CH1.42O0.78 CH2.15O1.01 CH1.16O0.39 (−) (−)

direct liquefaction

comparison with the literature is once again difficult because of different fraction definitions and varieties of biomass used. Moreover, during evaporation steps, which are different from one study to another, some light compounds can be eliminated (cf. section ”GC-MS/FID analysis”). However, heavy oil and water-soluble organics are usually less oxygenated than those of the present work.23,40,41 Water content (θi) of pyro-oil fractions are determined by Karl Fischer analysis and are presented in Table 3. Liq-oil fractions are free of water since it is evaporated through the recovery protocol. By subtraction of water content, ”water free” elemental compositions of pyro-oil fractions can be calculated (3). They are quite similar to those calculated in14 CH1.23O0.45N0.005, CH1.78O1.39N0.03, and CH1.32O0.53N0.005. Moreover these ”water free” compositions are coherent with those reported in Pollard et al.,13 in which a staged condensation allows the recovery of fractions with water free compositions between CH1.09O0.34 and CH1.44O0.96.

Y L*, Pyro = 1 − YS , Pyro − YG , Pyro

(7)

On the basis of the known water content fraction, mass yields are calculated using eqs 8−10. Then, by the difference between total liquid mass yield (calculated on dry basis, eq 7) and pyro-

Figure 9. Comparison of bio-oil fraction mass yields. F


Energy & Fuels

Article

Table 4. Identification of Compounds in Bio-Oils

oils fractions mass yields, the mass yield of water formed during fast pyrolysis is calculated thanks to eq 11. YPHO =

YPLO

mPHO ·(1 − θPHO) mB ·(1 − θB)

m ·(1 − θPLO) = PLO mB ·(1 − θB)

YPAE =

mPAE ·(1 − θPAE) + (Y L*, Pyro − YL , Pyro) ·(1 − θPAE) mB ·(1 − θB) (10)

(8)

YWater = Y L*, Pyro − (YPHO + YPLO + YPAE)

(11)

Hereafter in this article, liquid fractions mass yields used are those calculated using eqs 7−11.

(9) G


Energy & Fuels

Article

For direct liquefaction experiments, an hypothesis often encountered37,42 consists in closing mass balances by addition of a water mass yield. In our case, this hypothesis leads to an overestimation of H (120%) and O (122%) without any change for C. Another hypothesis may be to calculate liquid mass yield by difference from solid and gas mass yields,21 but this leads to an overestimation of C (108%). That is the reason why, for this study, no recalculation is made for direct liquefaction. Figure 9 displays mass yield of all fractions of pyro-oil, including water formed by reaction, and fractions of liq-oil. First of all, global water mass yield formed during fast pyrolysis ranges from 3.7 wt % to 9.9 wt %, which agree with the average yield of 12 wt % obtained by Westerhof et al.43 and the value of 2 wt % at 900 K with the same cyclone.27 Aerosols (from 36.0% to 51.4%) and heavy oil (from 34.7% to 46.4%) are the main fractions of pyro-oils. By considering water free bio-oils, their mass yields are similar for both processes, taking into account that there may be some losses of liq-oil during evaporation steps. The maximum of pyro-oil−water free mass yield is 52.8% at 970 K, whereas up to 47.0% of liq-oil are obtained at 573 K. Moreover, global “water free” pyro-oil composition (CH 1.32 O 0.57 ) is close to global liq-oil composition (CH1.27O0.53). GC-MS/FID Analysis. Table 4 lists major compounds of biooils identified by GC-MS and quantified by GC-FID, ranked according to their retention time, with mass content in each fraction. Most identified compounds like acetic acid, dimethylfuran, furanone, phenol, guaiacol, cresol, dimethoxyphenol, and levoglucosan are well-known bio-oils products.44−46 3-Methyldihydro-2,5-furanedione and 4-methyl-4-hepten-3-one are not identified in the literature, but similar compounds are 2,3dihydro-5-methylfuran-2-one45 and 2-octanone.46 For pyro-oil, on average, 26.5, 18.5, and 20.5 wt % of, respectively, heavy oil, light oil, and aerosols are quantified. For liq-oils, on average, 1.2 and 14.7 wt % of, respectively, heavy oil and water-soluble organics are quantified. Moreover, in watersoluble organics fractions, several peaks are not clearly identified because of a high number of similar specific masses. They likely correspond to polyphenolic compounds as shown in Sun et al.29 Several pyro-oil compounds are not found in liq-oils. Acetone, used as solvent, is eliminated during the first step. So, even if some acetone is produced, it cannot be detected in liq-oil. In order to know if other compounds are eliminated during these steps, evaporated acetone and water were analyzed. No compounds are eliminated with acetone. However, in water phase, acetic acid, phenol, and two others unidentified compounds were found but not quantified. These eliminated compounds could partly explain the misclosure of mass balances. The main difference between the two bio-oils is the absence of levoglucosan (major pyro-oil compound) in liq-oil. Conversely, there is no levulinic acid nor etheric compounds (major liq-oil compounds) in pyro-oil. These properties might indicate the existence of hydrolysis reactions, resulting in cycle opening of carbohydrates like levoglucosan. 1 H NMR. Figure 10 shows molar distribution of hydrogen atoms, for all fractions of pyro-oils and liq-oils, after peaks integration. It has to be noticed that peaks corresponding to deuterated acetone and water are not taken into account.

Figure 10. Molar repartition of hydrogen atoms in bio-oils, according to proton assignment.

Results shown therefore correspond to bio-oils free of water and acetone. First of all, pyro-oil hydrogen distribution agrees with results reported by Mullen et al.47 with a majority of ”aliphatics α-to heteroatom or unsaturation”, ”alcohols, methylene-dibenzene”, ”methoxy, carbohydrates”, and ”(hetero-) aromatics”. Liq-oil and pyro-oil show, once again, some differences in their compositions. In particular, heavy oil and light oil from fast pyrolysis have a similar composition, whereas aerosols are closer to both fractions of liq-oil. Moreover, no ”aldehyde” or ”methoxy, carbohydrates” are present in liq-oils, contrary to pyro-oils. These observations match with GC-MS/FID analysis in which no levoglucosan (carbohydrate) are found in liq-oil, while is a major compound of pyro-oil. Fractions of hydrogen from ”alkanes” and ”aliphatics α-to heteroatom or unsaturation” are more important in liq-oil than in pyro-oil, which can confirm the presence of etheric compounds. The majority of hydrogen from ”aliphatics α-to heteroatom or unsaturation”, for both bio-oils, can be explained by polyphenolic compounds. The higher proportion of hydrogen from ”alkanes” and ”(hetero-) aromatics” in liq-oils may also indicate that liq-oils contains heavier compounds that are not identified by GC-MS/ FID.

■

CONCLUSIONS The production of bio-oil by fast pyrolysis and direct liquefaction of beech sawdust has been studied through the determination of products yields and compositions. For both processes, a maximum of bio-oil mass yields is observed. For fast pyrolysis, 62.6 wt % of pyro-oils are recovered with 25.7 wt % of gas and 11.7 wt % with a wall temperature of 970 K and a ratio between nitrogen and biomass mass flow rates of 7.3. For direct liquefaction, 47.0 wt % of liqoils are recovered with 5.5 wt % of gas and 17.8 wt % of solids at 573 K. Ultimate analysis shows that pyro-oils are more oxygenated than liq-oil. Fractions of pyro-oil, recovered by vapors condensation, contain up to 40 wt % of water, whereas heavy oil and water-soluble organics from direct liquefaction are considered free of water. This difference of water content has a direct impact on bio-oil oxygen content. By subtraction of water content, pyro-oil elemental composition (CH1.32O0.57) is very close to that of liq-oil (CH1.27O0.53). Moreover, on water free basis, the maximum of pyro-oil and liq-oil mass yields are respectively 52.8 and 47.0 wt %. In conclusion, the differences H


Energy & Fuels

Article

(17) Yang, Y. F.; Feng, C. P.; Inamori, Y.; Maekawa, T. Resour., Conserv. Recycl. 2004, 43, 21−33. (18) Yin, S.; Dolan, R.; Harris, M.; Tan, Z. Bioresour. Technol. 2010, 101, 3657−3664. (19) Demirbas, A. Energy Convers. Manage. 2000, 41, 1601−1607. (20) Watanabe, M.; Bayer, F.; Kruse, A. Carbohydr. Res. 2006, 341, 2891−2900. (21) Zhang, B.; von Keitz, M.; Valentas, K. J. Anal. Appl. Pyrolysis 2009, 84, 18−24. (22) Xu, C.; Lancaster, J. Water Res. 2008, 42, 1571−1582. (23) Qu, Y.; Wei, X.; Zhong, C. Energy 2003, 28, 597−606. (24) Elliott, D. C. Energy Fuels 2007, 21, 1792−1815. (25) Demirbas, M. F. Appl. Energy 2009, 86, S151−S161 Bio-fuels in Asia.. (26) Broust, F. Le cyclone: un réacteur multifonctionnel. Application à la pyrogazéification et à la pyroliquéfaction de la biomasse. Ph.D. thesis, Institut National Polytechnique de Lorraine, Nancy, France, 2003. (27) NDiaye, F. T. Pyrolyse de la biomasse en réacteur cyclone Recherche des conditions optimales de fonctionnement. Ph.D. thesis, Institut National Polytechnique de Lorraine, Nancy, France, 2008. (28) Lédé, J. Ind. Eng. Chem. Res. 2000, 39, 893−903. (29) Sun, P.; Heng, M.; Sun, S.-H.; Chen, J. Energy Convers. Manage. 2011, 52, 924−933. (30) Olcese, R.; Bettahar, M.; Petitjean, D.; Malaman, B.; Giovanella, F.; Dufour, A. Appl. Catal., B 2012, 115−116, 63−73. (31) Lédé, J. Energies 2010, 3, 886−898. (32) Thermal Biomass Conversion; Bridgwater, A. V., Hofbauer, H., van Loo, S., Eds.; CPL Press: 2009; Vol. 1, pp 37−78. (33) Wang, X.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 2005, 44, 8786−8795. (34) Scott, D. S.; Majerski, P.; Piskorz, J.; Radlein, D. J. Anal. Appl. Pyrolysis 1999, 51, 23−37. (35) Demirbas, A. Fuel Process. Technol. 2007, 88, 591−597. (36) Karagoz, S.; Bhaskar, T.; Muto, A.; Sakata, Y. Bioresour. Technol. 2006, 97, 90−98. (37) Cheng, S.; D’cruz, I.; Wang, M.; Leitch, M.; Xu, C. C. Energy Fuels 2010, 24, 4659−4667. (38) Sinag, A.; Gülbay, S.; Uskan, B.; Güllü, M. J. Supercrti. Fluids 2009, 50, 121−127. (39) Wang, C.; Du, Z.; Pan, J.; Li, J.; Yang, Z. J. Anal. Appl. Pyrolysis 2007, 78, 438−444. (40) Minowa, T.; Kondo, T.; Sudirjo, S. T. Biomass Bioenergy 1998, 14, 517−524. (41) Xu, C.; Lad, N. Energy Fuels 2008, 22, 635−642. (42) Xu, C.; Etcheverry, T. Fuel 2008, 87, 335−345. (43) Westerhof, R. J. M.; Kuipers, N. J. M.; Kersten, S. R. A.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 2007, 46, 9238−9247. (44) Scholze, B.; Hanser, C.; Meier, D. J. Anal. Appl. Pyrolysis 2001, 58−59, 387−400. (45) Oasmaa, A.; Kuoppala, E.; Solantausta, Y. Energy Fuels 2003, 17, 433−443. (46) Marsman, J. H.; Wildschut, J.; Mahfud, F.; Heeres, H. J. J. Chromatogr., A 2007, 1150, 21−27. (47) Mullen, C. A.; Strahan, G. D.; Boateng, A. A. Energy Fuels 2009, 23, 2707−2718.

of mass yields and oxygen content between liq-oil and pyro-oil is mainly explained by the water content of pyro-oil. Differences in molecular composition between pyro-oil and liq-oil have been shown by GC-MS/FID and 1H NMR analysis. Carbohydrates (like levoglucosan) are major compounds identified in pyro-oil but are not detected in liq-oil. Conversely, levulinic acid and etheric compounds are identified in liq-oil and are not detected in pyro-oil. This is the main difference between these two types of bio-oils, indicating possible cycle opening reactions of carbohydrates during direct liquefaction. For both bio-oils, there are high fractions of hydrogen from ”aliphatics α-to heteroatom or unsaturation” and ”(hetero-) aromatics”, indicating heavy compounds not identified by GC/ MS, especially for liq-oils. These compounds can mainly be attributed to polyphenolics from lignin. Finally, gas formed during direct liquefaction is mainly composed of CO2 (more than 99 wt %), whereas gas from fast pyrolysis is a mixture of CO, CO2, H2, CH4, and light hydrocarbons.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial support provided by the Fondation Tuck, through the ENERBIO project, is gratefully acknowledged. R. N. Olcese (LRGP) and Olivier Fabre (LCPM) are kindly acknowledged for technical support and advice on respectively GC-MS/FID and 1H NMR analysis. J. P. Nisteron (M2P2) is also acknowledged for his technical help on direct liquefaction setup.

■

REFERENCES

(1) Yu, Y.; Lou, X.; Wu, H. Energy Fuels 2007, 22, 46−60. (2) Rogalinski, T.; Ingram, T.; Brunner, G. J. Supercrit. Fluids 2008, 47, 54−63. (3) Di Blasi, C. Prog. Energy Combust. Sci. 2008, 34, 47−90. (4) Grahn, M.; Azar, C.; Lindgren, K.; Berndes, G.; Gielen, D. Biomass Bioenergy 2007, 31, 747−758. (5) Bridgwater, A. Biomass Bioenergy 2012, 38, 68−94. (6) Toor, S. S.; Rosendahl, L.; Rudolf, A. Energy 2011, 36, 2328− 2342. (7) Lédé, J.; Authier, O. Biomass Convers. Biorefin. 2011, 1, 133−147, DOI: 10.1007/s13399-011-0014-2. (8) Kumar, S.; Gupta, R. B. Energy Fuels 2009, 23, 5151−5159. (9) Akhtar, J.; Amin, N. A. S. Renewable Sustainable Energy Rev. 2011, 15, 1615−1624. (10) Garcia-Perez, M.; Chaala, A.; Pakdel, H.; Kretschmer, D.; Roy, C. Biomass Bioenergy 2007, 31, 222−242. (11) Westerhof, R. J. M.; Brilman, D. W. F.; Garcia-Perez, M.; Wang, Z.; Oudenhoven, S. R. G.; van Swaaij, W. P. M.; Kersten, S. R. A. Energy Fuels 2011, 25, 1817−1829. (12) Jendoubi, N.; Broust, F.; Commandre, J.; Mauviel, G.; Sardin, M.; Lédé, J. J. Anal. Appl. Pyrolysis 2011, 92, 59−67. (13) Pollard, A.; Rover, M.; Brown, R. J. Anal. Appl. Pyrolysis 2012, 93, 129−138. (14) Lédé, J.; Broust, F.; Ndiaye, F.-T.; Ferrer, M. Fuel 2007, 86, 1800−1810. (15) Karagoz, S.; Bhaskar, T.; Muto, A.; Sakata, Y.; Oshiki, T.; Kishimoto, T. Chem. Eng. J. 2005, 108, 127−137. (16) Mazaheri, H.; Lee, K. T.; Bhatia, S.; Mohamed, A. R. Bioresour. Technol. 2010, 101, 745−751. I


Comparative Study of Biomass Fast Pyrolysis and Direct Liquefaction

Recommend Documents