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Enhancement of methane production from sunflower oil cakes by dilute acid pretreatment
Applied Energy 102 (2013) 1105–1113
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Enhancement of methane production from sunflower oil cakes by dilute acid pretreatment Florian Monlau a,b, Eric Latrille a, Aline Carvalho Da Costa b, Jean-Philippe Steyer a, Hélène Carrère a,⇑ a b
INRA, UR050, Laboratoire de Biotechnologie de l’Environnement, Avenue des Etangs, 11100 Narbonne, France School of Chemical Engineering, University of Campinas, Av. Albert Einstein, 500, Campinas SP 13 083-852, Brazil
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Dilute acid pretreatment increased
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methane potential of sunflower oil cakes by up to 50%. Optimal conditions were 170 °C, 1% (weight) sulphuric acid concentration. Methane produced by the soluble fraction represented more than 60% of total volume. At 130 6 T 6 170 °C, increase in methane production was linked to solubilisation of organic carbon. At T > 170 °C, recalcitrant compounds were formed in the liquid phase.
a r t i c l e
i n f o
Article history: Received 9 March 2012 Received in revised form 14 May 2012 Accepted 11 June 2012 Available online 26 July 2012 Keywords: Anaerobic digestion Biogas Lignocellulosic biomass Acid pretreatment Methane potential Solubilisation
a b s t r a c t The conversion of sunflower oil cake (SOC) into methane by mesophilic anaerobic digestion was the object of this study. The effect of a combined dilute acid-thermal pretreatment (acid concentration and temperature) on solubilisation and methane potential was investigated using a central composite design (CCD). For temperatures up to 170 °C, solubilisation of each parameter (total organic carbon, sugars and proteins) increased with the severity of the pretreatment (high temperature and high acid concentration). Methane production was higher for pre-treated samples than for the untreated samples (195 mL CH4/ g VS). The highest yield (302 ± 10 mL CH4/g VS) was obtained after acid pretreatment at 170 °C. At this temperature, acid concentrations lower than 1% had no significant impact on methane production in comparison to thermal treatment alone. The volume of methane produced by the soluble fraction reached more than 60% of total methane production. An increase in methane production was correlated to the concentration of organic carbon in the liquid phase of samples pretreated at 130–170 °C with acid. At temperatures higher than 170 °C, some recalcitrant compounds were formed in the liquid phase. Ó 2012 Elsevier Ltd. All rights reserved.
Abbreviations: AD, anaerobic digestion; BMP, biochemical methane potential; Carb, carbohydrates; CDD, central composite design; Pro, proteins; RSM, response surface methodology; sCarb, soluble carbohydrates; SEM, scanning electron microscopy; SOC, sunflower oil cakes; sPro, soluble proteins; sTOC, soluble total organic carbon; TOC, total organic carbon; TS, total solids; VS, volatile solids; X1, coded acid concentration; X2, coded temperature. ⇑ Corresponding author. Tel.: +33 4 68 42 51 68. E-mail address: [email protected] (H. Carrère). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.06.042
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1. Introduction In 1997, the Kyoto Protocol fixed the objective to reduce by 5.2% the world greenhouse effect gas emissions from the 1990 level over the 2008–2012 period. Various alternatives have been studied (wind, solar, hydraulic, etc.) to achieve this goal. One such alternative is the use of biomass for the production of biofuels or bioenergy [1]. However, first-generation biofuels that use the edible part of a plant would be responsible for a sharp rise in the price of food. Consequently, one major challenge for the future is the production of energy from renewable sources, especially from agricultural waste and woody waste, which will not compete with food or feed uses in a context of higher demand for food. Conversion of lignocellulosic materials through anaerobic digestion (AD) technology is one of the promising alternatives [2]. The extraction of oil from various biomasses such as rapeseed, Jatropha or sunflowers generates solid waste (oil cakes) with a high organic content; and for example, biogas installations to digest seed cakes have been reported to boost the energy efficiency of Jatropha biodiesel production [3]. Sunflower oil cake (SOC) production is the third highest after soy-bean and rapeseed cakes, its European level reaching about 12 million tons per year [4]. The interest of producing biomethane from these residues has been underlined but the methane potential of sunflower oil cakes has been shown to be quite low: from 186 [5] to 215 [6] or 227 N mL CH4 g volatile solids (VS) [7], which corresponds to the anaerobic degradation of approximately 40% of the volatile solids or organic matter [8]. This low conversion yield may be explained by the low accessibility of biodegradable organic compounds (hemicellulose, cellulose) within the lignocellulosic network. Lignocellulosic biomass is composed mainly of cellulose, hemicellulose and lignin which are strongly linked to each other and form complex three-dimensional structures which are in fact less accessible to micro-organisms [9,10]. The use of pretreatment has been recommended to overcome the poor conversion yield of sunflower oil cakes and to improve their overall substrate properties prior to anaerobic digestion [11]. An efficient pretreatment of lignocellulosic biomass should make hollocelluloses more accessible to enzymatic or bacterial attack in order to increase the overall biodegradability of lignocellulosic materials. It should break down the linkage between polysaccharides and lignin to make cellulose and hemicelluloses more accessible to bacteria [10,12,13]. Dilute acid pretreatment of lignocellulosic biomass has been extensively studied for second-generation bioethanol production [14–17]. It is performed by soaking the material in a dilute acid solution and then heating to temperatures between 140 and 200 °C for a certain time (from several minutes up to an hour). Sulphuric acid, at concentrations usually below 4 wt%, has been of most interest in such studies as it is both inexpensive and effective [10,18]. Several studies using acidic pretreatment have been carried out with the aim of enhancing anaerobic digestion of such lignocellulosic materials as bagasse, coconut fibres [19], cassava residues [20], whole plant maize [21], greenhouses residues [22], etc. Pretreatment of bagasse and coconut fibres with 1 M HCl at 25 °C for 30 days improved the formation of biogas from these materials by 31% and 74%, respectively [19]. On the other hand, HCl pretreatment (2% HCl/g total solids (TS) at 20 °C for 24 h) of whole plant maize had no impact on methane production [21]. The production of biogas from newsprint pretreated with 30% acetic acid and 2% nitric acid during 30 min in a boiling bath increased by three times during the 60-day incubation period [23]. Combined thermal–dilute sulphuric acid pretreatment (158 °C, 2.99% (w/w TS) H2SO4 for 20 min) led to a 57% increase in the methane yield of cassava residues [20].
In this study, dilute sulphuric acid pretreatment was applied to sunflower oil cakes anaerobic digestion in the same conditions range as for bioethanol production. The effect of H2SO4-thermal pretreatment on structural composition (hemicelluloses, cellulose and lignin) was first studied. Then, central composite design (CDD) coupled with response surface methodology (RSM) was used to investigate the effect of the H2SO4-thermal pretreatment parameters (acid concentration and temperature) on the solubilisation of chemical compounds (soluble total organic carbon (sTOC), soluble carbohydrates (sCarb) and soluble proteins (sPro)) and on the methane potential of sunflower oil cake. In addition, the methane production originating from the soluble and the solid fractions was investigated. 2. Materials and methods 2.1. Raw material The substrate used in this study was sunflower oil cakes resulting from the extraction of oil with hexane. They were produced by the Grandes Huileries Médaco (Béziers, France), a company producing crude oil. The main characteristics and composition of the sample used in the experiments were (average values of three determinations with standard deviations): total solids (TS) 0.90 ± 0.01 g/g SOC, volatile solids (VS) 0.83 ± 0.02 g/gTS, lipids: 0.0070 ± 0.0003 g/gTS; Van Soest fractionation (% VSinitial) soluble fraction 12%; hemicellulose 30%; cellulose 39%; lignin 19%. 2.2. Combined dilute acid-thermal pretreatment of sunflower oil cakes Sunflower residues were treated in a Zipperclave autoclave series 02-0378-1 (Autoclave France) with a solids load of 50 gSOC/L for 5 min after the operational temperature has been reached. This autoclave, with a stainless steel body, has a capacity of 1 L and can reach a temperature of 250 °C and a pressure of 79 bars. The reactor was agitated at 400 rpm by a shaft with two propellers and heated by a ceramic furnace. Pretreated samples were immediately separated into solid and liquid fractions by centrifugation (10,000 rpm for 10 min). A first series of pretreatment experiments was carried out based on the experimental design described in Section 2.3. In a second experiment series, the temperature was set at 170 °C and different acid concentrations (0; 0.3; 0.5; 0.7; 1; 1.5; 2.5% w/wSOC) were tested. 2.3. Experimental design The effects of experimental variables (temperature and acid concentration) on responses (solubilisation and methane production) were investigated using a central composite design where the central point was performed in triplicate. In this study, the effects of two main parameters (H2SO4 concentration in w/wSOC and temperature) were investigated. The coded and real values used in the experiment design are presented in Table 1. The effect of these
Table 1 Coded and real values used in the experimental composite design. Coded unit
1.21 1 0 1 1.21
True value X1: temperature (°C)
X2: acid concentration (g/gSOC)
130 142 170 198 210
0 0.74 2.5 4.26 5
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experimental variables on the responses was represented by response surface methodology (RSM) using the Statistica (Statsoft, v.7.0) software. Responses were correlated to experimental variables in accordance with the following equation:
Y ¼ a0 þ a1 X 1 þ a2 X 2 þ a12 X 1 x2 þ a11 X 21 þ a22 X 22
ð1Þ
where Y is the response, X1 and X2 are the coded experimental variables, a0, a1, a2, a12, a11, a22 are the model coefficients calculated from the experimental data. The partial least squares method was used to estimate the coefficients. In addition, statistical analysis was performed using the analysis of variance (ANOVA) with a confidence level of 95%. 2.4. Anaerobic digestion The methane potential of untreated and pretreated sunflower oil cakes was determined in batch anaerobic flasks. The volume of each flask was 0.5 L, with a working volume of 0.4 L and 0.1 L volume of head space. Each flask contained solutions of: macroelements (NH4Cl, 26 g/L; KH2PO4, 10 g/L; MgCl2, 6 g/L; CaCl2, 3 g/L), oligoelements (FeCl2, 2 g/L; CoCl2, 0.5 g/L; MnCl2, 0.1 g/L; NiCl2, 0.1 g/L; ZnCl2, 0.05 g/L; H3BO3, 0.05 g/L; Na2SeO3, 0.05 g/L; CuCl2, 0.04 g/L; Na2MoO4, 0.01 g/L), bicarbonate buffer (NaHCO3, 50 g/L), an anaerobic inoculum (5 g VS/L) and raw or treated SOC (0.5 g/g VSinoculum). The inoculum was granular sludge from a mesophilic anaerobic digester of a sugar factory. The sludge contained 89.9 g/L total solids (TS) and 83.1 g/L volatile solids (VS). Once the flasks were prepared, nitrogen bubbling was carried out to obtain anaerobic conditions and the bottles were closed with airtight butyl rubber septum-type stoppers. Duplicate bottles were incubated at 35 °C for 37 days. The initial pH was adjusted to 7 using NaOH (1 M). For three pretreatment conditions (142 °C–0.74%/170 °C–5%/ 198 °C–4.26%), the methane potential assays were done on the soluble and solid fractions in addition to the whole sample. The solid and liquid phases were separated by centrifugation at 15,000 g and 5 °C for 15 min.
determined using a gas chromatograph (Varian GC-CP4900) equipped with two columns: the first (CP914509 molecular sieve, 10 m) was used at 110 °C to separate O2, N2 and CH4; the second (CP914510 Poraplot Q, 10 m) at 70 °C to determine CO2. The detection of gaseous compounds was done using a thermal conductivity detector. The temperatures were 110 °C for the injector and 55 °C for the detector. The calibration was carried out with a standard gas composed of 25% CO2, 2% O2, 10% N2 and 63% CH4. 2.5.2. Substrate analysis The untreated and pretreated sunflower oil cakes were analysed for TS and VS in accordance with the APHA standard method [24]. The total organic carbon (TOC) was analysed with the TC analyser (TOC-V) with a runner sample (ASI-V). The Lowry and anthrone methods were used for, respectively, protein and sugar determination in the soluble phases. The content of the soluble fraction, hemicelluloses, cellulose and lignin was measured using the Van Soest procedure [25] with the FIBERBAG system (Gerhardt). This procedure is based on sequential extraction with neutral and acid detergents, followed by strong acid extraction. The different fractions were: (i) the fraction soluble in neutral detergent (soluble part), (ii) hemicelluloses, extracted by acid detergent, (iii) cellulose, extracted by 76% sulphuric acid, and (iv) lignin. Lipids were determined after extraction by 40–60 °C-petroleum ether at 67 bar 105 °C, using an accelerated solvent extractor ASE 200 (Dionex). All the analyses of chemical composition were done in duplicate. 2.5.3. Microscope observation The scanning electron microscopy (SEM) was done using a high resolution Leica microscope model LEO440i at the Laboratory of Analysis and Calibration (LRAC), School of Chemical Engineering – UNICAMP, with detectors for secondary electrons. The dry material was metalized with a thin layer of gold in a Sputter Coater polaron, model SC 7620, to ensure the conductivity of its observation. 3. Results and discussion 3.1. Impact of pretreatment on sunflower oil cake composition
2.5. Analytical methods 2.5.1. Biogas analysis Biogas volume was monitored by the water displacement method and the corresponding cumulative biogas volume then calculated. Acidified water (pH = 2) was used to minimise the dissolution of carbon dioxide in water. Biogas composition was
Soluble
Hemicellulose
The composition of raw sunflower oil cake (Van Soest soluble fraction, hemicellulose, cellulose and lignin) and of the residual solid fractions after pretreatment is shown in Fig. 1, expressed in %VS initial. A significant increase of the soluble fraction was observed for all pretreated samples due to the solubilisation of cellulose and hemicelluloses into soluble carbohydrates. The percentage of
Cellulose
Lignin
100% 80% 60% 40% 20%
R 13 aw 0° C ;2 14 .5 % 2° C ;0 .7 14 4% 2° C ;4 .2 6% 17 0° C ;0 17 % 0° C ;2 .5 17 % 0° C ;2 .5 17 % 0° C ;2 .5 % 17 0° C ;5 19 % 8° C ;0 .7 19 4% 8° C ;4 .2 6 21 % 0° C ;2 .5 %
0%
Fig. 1. Van Soest fractionation of raw sunflower oil cake and of the solid fraction of sunflower oil cake after acid pretreatment.
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Fig. 2. Microscope picture of (A) raw sunflower oil cake and (B) sunflower oil cake after pretreatment at 170 °C 2.5% H2SO4.
the Van Soest soluble fraction varied according to the pretreatment conditions, reaching a maximum after the most severe pretreatment conditions (210 °C–2.5%). At 170 °C, the increase in acid concentration led to an increase of the Van Soest soluble fraction. However, the percentage of lignin remained constant in treated and untreated samples (around 20%). Also, the reduction of hemicelluloses was greater than that cellulose: the hemicellulose fraction decreased from 30% in the raw substrate to 4% after 210 °C– 2.5% pretreatment whereas the cellulose fraction decreased from 39% to 19% in the same conditions. These results confirm conclusion: acid pretreatment is effective in removing hemicelluloses by breaking the ether bonds in lignin/phenolic carbohydrates complexes without dissolving the lignin [26]. Structural modification of
sunflower oil cake during thermal acid pretreatment is supported by microscope observations as shown in Fig. 2. 3.2. Impact of pretreatment on sunflower oil cake solubilisation Changes in sunflower oil cake composition during dilute-acid pretreatment were measured by the solubilisation of organic matter (sTOC, sCarb and sPro). The effect of dilute acid pretreatment parameters (acid concentration and temperature) on the organic matter solubilisation was investigated using CDD. By applying multiple regression analysis to the experimental data, second-order polynomial equations were obtained in terms of coded value to describe the correlation between the variables (X1 coded acid
Table 2 ANOVA for the model regression representing sCOT. sCarb. sPro and BMP. Source
Sum of square
Soluble COT (g/L) (R2 = 0.9509) X1 19.24 3.45 X 21
Degrees of freedom
Mean square
F-value
Prob
1 1
19.24 3.45
665.89 119.26
0.0015 0.0083
5.13 0.07
1 1
5.13 0.07
177.59 2.42
0.0055 0.2602
0.57 1.44 0.06 30.61
1 3 2 10
19.72 16.67
0.0471 0.0571
Soluble sugars (g/L) (R2 = 0.8778) X1 0.64 1.98 X2
1 1
0.64 1.98
49.51 153.17
0.0196 0.0065
1.67 0.05
1 1
1.67 0.05
129.13 3.92
0.0076 0.1861
0.03 0.58 0.026 4.96
1 3 2 10
1.98 14.96
0.2947 0.0633
X2 X 22 X1 X2 Lack of fit Pure error Total
1
X2 X 22 X1 X2 Lack of fit Pure error Total
0.57 0.48 0.029
0.026 0.19 0.01
Soluble protein (g/L) (R2 = 0.9519) X1 57.81 18.94 X2
1 1
57.81 18.94
419.94 137.58
0.0024 0.0072
12.79 0.10
1 1
12.79 0.10
92.91 0.76
0.0106 0.4753
2.54 4.52 0.27 99.70
1 3 2 10
2.54 1.51 0.14
18.48 10.95
0.0501 0.0849
BMP (mL CH4/g VS) (R2 = 0.8814) X1 1047.43 4520.01 X 21
1 1
1047.43 4520.01
8.56 36.95
0.0996 0.0260
2426.89 541.42
1 1
2426.89 541.42
19.84 4.43
0.0469 0.1701
100.00 846.19 244.67 9198.54
1 3 2 10
100.00 282.06 122.33
0.82 2.31
0.4614 0.3168
1
X2 X 22 X1 X2 Lack of fit Pure error Total
X2 X 22 X1 X2 Lack of fit Pure error Total
F. Monlau et al. / Applied Energy 102 (2013) 1105–1113
concentration and X2 coded temperature) and the responses sTOC, sCarb and sPro (Eqs. )(2)–(4)).
sTOC ðg=LÞ ¼ 5:23 þ 1:55 X 1 0:78 X 21 þ 0:8 X2 þ 0:11 X 22 0:38 X 1 X 2
ð2Þ
sCarb ðg=LÞ ¼ 2:19 þ 0:28 X 1 0:59 X 21 þ 0:46 X 2 0:09 X 22 0:08 X 1 X 2
ð3Þ
sPro ðg=LÞ ¼ 10:78 þ 2:69 X 1 1:83 X 21 þ 1:26 X 2 þ 0:13 X 22 0:79 X 1 X 2
ð4Þ
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ANOVA was conducted to test the significance of the fit of the second-order polynomial equations for the sTOC, sCarb and sPro, as shown in Table 2. The accuracy of the fit of polynomial model was expressed by the coefficient of determination R2. Coefficients of determination R2 of 0.95, 0.88 and 0.95 were obtained for, respectively, sTOC, sCarb and sPro, showing that the models were significant. Values of probability (Prob) lower than 0.05 indicate that model terms are significant at the 95% confidence level. None of the models presented evidence of lack of fit. Significant model terms are represented in bold in Table 2. The linear effect of temperature (X1) and acid concentration (X2), the interactive term (X1 X2) and the quadratic effect of temperature (X 22 ) were
Fig. 3. 3D response surface plots: effect of acid concentration and reaction temperature on sTOC (A), sCarb (B), sPro (C).
F. Monlau et al. / Applied Energy 102 (2013) 1105–1113
significant for the sTOC. The linear effect of temperature (X1) and acid concentration (X2) and the quadratic effect of temperature (X 22 ) were significant for the sPro and sCarb. The regression model developed can be represented using response surface methodology (RSM) to show the interaction. Response surface plots with all the terms are presented in Fig. 3. For temperatures up to 170 °C, the solubilisation of each soluble response increased as more severe pretreatment conditions (high temperature and high acid concentration). Raising the temperature from 170 °C to 200 °C led to a slight increase in soluble organic carbon and protein concentration and, at 200 °C, there was no significant impact of acid concentration between 0.74% and 4.26%. At 210 °C, a decrease was observed in solubilised organic carbon and protein concentrations which may have been due to the degradation of organic matter possibly lost in the gas phase when the reactor was open. A decrease in soluble carbohydrate concentration was observed from 170 °C, due probably to reactions of some carbohydrates with others (‘‘caramel’’ reactions) or with proteins or amino acids (Maillard reactions). As carbohydrate measurement dosage was based on spectrophotometry which quantified the carbonyl function (C@O), the carbohydrates which had reacted via their carbonyl functions were not measured, in contrast to proteins which were quantified even when they had formed Maillard compounds [27]. In conclusion, dilute acid pretreatment was effective in protein solubilisation and permitted the release of carbohydrates by solubilising hemicelluloses and to a lesser extent, cellulose. On the basis of organic carbon measurements, maximum solubilisation of organic matter was obtained after pretreatment at 170 °C with 5% H2SO4 or at 200 °C, at which temperature 0.74% H2SO4 was enough. 3.3. Methane potentials 3.3.1. Impact of pretreatment on the methane potential of sunflower oil cake The methane potential of raw sunflower oil cake after 37 days of batch anaerobic digestion was 195 mL CH4/g VS, which is in the same range as previous data: 215 mL CH4/g VS [6] and 227 mL CH4/g VS [8]. CDD was performed to investigate the interaction of the parameters of dilute acid pretreatment (acid concentration X1 and temperature X2) on the methane potential of sunflower oil cakes. By applying multiple regression analysis to the experimental data, a second-order polynomial equation was obtained to describe in terms of coded values the correlation between the variables (X1 and X2) and the methane potential response (Eq. (5)):
BMPðmL CH4 =g VSÞ ¼ 293:33 þ 11:44 X 1 28:29 X 21 þ 17:41 X 2 9:79 X 22 þ 5 X 1 X 2
ð5Þ
ANOVA was conducted to test the significance of the fit of the second-order polynomial equation for the methane potential as shown in Table 2. A coefficient of determination R2 of 0.88 was obtained. Values of Prob less than 0.05 indicate significance of the model term, which is represented in bold in Table 2. The quadratic coefficient of temperature (X 21 ) and the linear acid coefficient concentration (X2) significantly affected the methane potential. The regression model developed can be represented using response surface methodology (RSM) with all the terms (Fig. 4). Except for the pretreatment at the lowest temperature (130 °C, 2.5%), which led to a methane potential of 206 mL CH4/g VS corresponding only to a 5% increase in comparison to the untreated substrate, all the various dilute acid pretreatment conditions led to significant enhancement of sunflower oil cakes methane potential. This increase ranged from 20% (treatment at 170 °C with no acid addition) to around 50% (treatment at 170 °C, 2.5% or 5% H2SO4). Temperatures higher than 170 °C led to a reduction of methane potential in comparison to 170 °C using 2.5% or 5% of acid. This decrease in
Fig. 4. Response surface plot: effect of temperature and acid concentration on methane potential.
methane potential was probably due to the combined effect of high temperature and acid addition. Indeed, Chandra et al. observed a high increase in methane potential of rice straw after hydrothermal pretreatment at 200 °C [28]. Although the calculated optimal conditions were 172 °C and 4.19% for temperature and acid concentration, respectively, this acid concentration seems to be excessive, as the difference of methane potential with acid concentrations of 1% and 5% was slight. The reduction of sulphuric acid concentration during pretreatment is an important point as sulphate is a stronger acceptor compared to CO2 and the reduction of sulphate to H2S competes with biogas production [29]. Also, the process is less expensive if less acid is used. Consequently, in a further experiment series, the temperature was set at 170 °C and different acid concentrations (0; 0.3; 0.5; 0.7; 1; 1.5; 2.5% w/wSOC) were tested considering 5 min pretreatment time. 3.3.2. Effect of H2SO4 concentration at 170°C on methane production The results of varying acid concentration from 0% to 5% are shown in Fig. 5. Results show that at 170 °C, acid concentrations below 1% had no significant effect on the methane yield of sunflower oil cakes. When the acid concentration increased from 0.7% to 1%, an increase of 15% of the methane production was observed. Moreover, there was no significant difference within an acid concentration range of 1–2.5%. Moreover, if previous results (Fig. 5) are pretreated 170°C
BMP (mL CH4 / g VS)
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raw
350 300 250 200 150
0
1
2
3
4
5
6
Acid concentration (%) Fig. 5. Methane potential of sunflower oil cake before and after pretreatment at 170 °C: impact of sulfuric acid concentration.
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130 to 170°C
T> 170°C
0% H2SO4
linear (130 to 170°C)
BMP increase (%)
70 60
y = 9.1454x - 0.1331 2 R = 0.8702
50
Table 3 Ratio of methane volume produced by the liquid and solid fractions divided by the volume produced by the total fraction of pretreated sunflower oil cake. The sum of the volumes produced by the liquid and solid fractions was slightly higher than the volume produced by the total fraction. Pretreatment conditions
40 30
142 °C; 0.74% 170 °C; 5% 198 °C; 4.26%
20 10
Fraction of methane originating in Liquid fraction (%)
Solid fraction (%)
37 61 58
65 46 51
0 0
1
2
3
4
5
6
7
8
TOC soluble (g/L) Fig. 6. Correlation between BMP increase ((BMPtreated-BMPraw)/BMPraw100) and concentration of organic carbon in the liquid phase of pretreated sample. Linear correlation was plotted for samples pretreated at temperatures ranging from 130 °C to 170 °C and with acid addition from 0.74% to 5%.
considered, acid concentration from 1% to 5% at 170 °C led to the same result, methane production ranging from 289 to 302 mL CH4/g VS. From economic and environmental points of view, it is better to limit the use of acid. Consequently, the optimal conditions for the pretreatment of sunflower oil cake are a temperature of 170 °C with an acid concentration of 1%. 3.3.3. Relationship between increased methane yield and sunflower oil cake solubilisation An increase in methane production by substrate pretreatment is often linked to the solubilisation of organic matter [27,30]. The increase in methane potential was thus plotted versus the organic carbon concentration in the liquid phase of pretreated sunflower oil cakes (Fig. 6). A satisfactory linear correlation was observed between methane potential enhancement and soluble organic carbon concentration only for samples pretreated at temperatures ranged from 130 to 170 °C along with an addition of sulphuric acid between 0.74% and 5%. Indeed, the sample pretreated at 170 °C with no addition of acid, resulted in less methane potential enhancement as compared to acid samples. This can easily be explained by the different mechanisms occurring in thermal and thermo-acid pretreatment and confirms the importance of using acid. However, too severe pretreatment conditions (T = 198 °C–0.74% or 4.26%) or (T = 210 °C–2.5%) led to less enhancement of methane production than could have been expected from high levels of solubilisation. These results revealed the formation of recalcitrant or hard-to-degrade compounds in the liquid phase. Such compounds may be melanoidins resulting from Maillard reactions between proteins and carbohydrates at high temperature in acid media. Such reactions are highly probable in the case of sunflower oil cake which is rich in proteins and carbohydrates [11]. Moreover, the decrease in methane yield after excessively high temperature treatment has already been ascribed to the formation of Maillard compounds in the case of sewage sludge [27]. Beside, in the case of lignocellulosic biomass and high temperature pretreatment such as liquid hot water, steam explosion and acid or organosolv pretreatments, byproducts such as furfural, hydroxymethylfurfural and soluble lignin compounds can be formed [31]. These compounds cause problems in the fermentation stage during bioethanol production because they can inhibit or even stop the fermentation [32]. But in contrast to bioethanol process, anaerobic digestion can convert these compounds to methane [33,34]. 3.3.4. Origin of methane production: solid and liquid phases of pretreated samples The anaerobic digestion of the liquid and solid phases of a number of pretreated samples (142 °C–0.74%/170 °C–5%/198 °C–4.26%)
was carried out in order to determine the relative part of each phase (soluble and solid) in the production of methane. Results are shown in Table 3. The sum of the methane produced from the solid and soluble fractions was equal to the volume produced by the total fraction within 5–10%. This result is a justifiable basis for making assumptions about the origin of the methane produced. Whereas for pretreatment at 142 °C–0.74% the production of biogas from the soluble part was only 36%, pretreatment at 170 °C–5% led to the highest volume of methane produced by the soluble fraction (61%) and although presenting an equivalent organic carbon solubilisation, the fraction of methane produced by the liquid phase at 198 °C–4.26% was slightly lower (58%). Thus, with efficient pretreatment (170 °C–5%) the major part of the methane produced originated from the liquid phase of pretreated biomass. These results are also interesting in the context of the second-generation bioethanol process. Indeed as bioethanol is generally produced from the solid phase of pretreated biomass, the liquid phase can be used to produce methane, which increases the overall energy yield of the process [35]. 3.4. Cost aspects It is necessary to assess if the implementation of dilute acid pretreatment could be economically viable. However, BMP tests are absolutely not designed for process scale up or for costs assessment. Indeed, the impact of any pretreatment is highly dependent on the anaerobic digestion process. Generally, methane production enhancement is higher in continuous process with low hydraulic retention time because pretreatments of complex substrates increase both rate and extent of anaerobic digestion [36]. In BMP tests which are designed to achieve the maximum methane production yields, the effect of pretreatment on digestion rate is generally hindered. On the other hand, the increase in digestion rate can lead to high increase in methane production in continuous digesters when the retention time is too low to achieve the complete conversion of the untreated substrate. Therefore, following results are only rough indications. Biogas can be converted either to heat, to heat and electricity by combined heat and power (CHP), or to gaseous fuels (mainly biomethane or compressed biogas). It can also been injected to the natural gas grid. Considering that dilute acid pretreatment requires a high level of heat energy, only conversions into heat and heat and electricity will be considered. This point is consistent with the present development of farm anaerobic digestion in France where biogas is converted by CHP in most of the cases. Heat production generally exceeds the local needs and may be used for substrate pretreatment. Table 4 shows the assessment of the methane conversion to heat and by CHP for sunflower oil cakes with thermal (170 °C) and dilute acid (170 °C, 1% H2SO4) pretreatments. Methane conversion into heat was considered as 9.95 kW h m3. CHP yields were considered to be 50% for heat and 35% for electricity. Heat requirements were assessed by the energy needs to raise the temperature of the substrate from 25 °C to 170 °C, assuming
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Table 4 Energy balance of thermal and dilute acid pretreatments of sunflower oil cakes. Energy produced Pretreatment Methane (m3/tonTS) Methane increase1 (m3/tonTS) Conversion into heat Conversion into heat and electricity
170 °C 194 32 1934 322 966 161 659 110
Heat produced (kW h/tTS) Heat increase1 (kW h/tTS) Heat produced (kW h/tTS) Heat increase1 (kW h/tTS) Electricity produced (kW h/tTS) Electricity increase1 (kW h/tTS)
170 °C + 1%H2SO4 240 78 2388 778 1194 389 815 265
Heat energy requirements for pretreatment Solid loading (kg/m3) 2
Heat requirement (kW h/tonTS) Heat requirement2 with 80% heat recovery (kW h/tTS) 1 2
50
100
200
300
3536 530
1852 278
1010 152
730 109
Compared to anaerobic digestion without pretreatment. To increase temperature from 25 to 170 °C.
that specific heat of the substrate suspension in water can be evaluated by the water specific heat (4.18 kJ kg1 °C1). This energy need to treat 1 ton of total solids is thus highly dependent on the solid loading during pretreatment. In our study, solid loading was 50 kg m3. Working on dilute acid pretreatment for the conversion of corn stover into bioethanol, Aden and Foust reported regular operation at 30% solid loading (i.e. 300 kg m3) [37]. Table 4 shows that in the case of low solid loading, heat requirements can be higher than the heat that can be recovered by burning all the methane produced. However, with 300 kg/m3 solid loading, pretreatment heat can be provided by the increase of methane due to dilute acid pretreatment. Nevertheless, thermal energy integration has to be carried out in a full scale implementation of thermal or dilute acid pretreatment. This can be achieved by exploiting the high temperature and enthalpy of streams like vapour, pretreated biomass, exhaust gases and hot water from the gas engine [38]. The heat energy of pretreated substrate suspension can be recovered to heat the digester [39] or to preheat raw substrate suspension [40]. Nevertheless, in the case of recovering thermal energy from flue gas during dilute acid pretreatment, Budzianowski recommended to avoid sulphuric acid condensation by avoiding cooling the gas below 180 °C [41]. Dhar et al. [42] reported 80% heat recovery from thermally pretreated sludge. Thus assuming 80% heat recovery from pretreated SOC, Table 4 shows that, in the case of full conversion of methane into heat, increase in heat production by dilute acid pretreatment is higher than the energy requirements for pretreatment whatever the solid loadings. In the case of CHP conversion, increase in heat production by dilute acid pretreatment is higher than the energy requirements for pretreatment when solid loading is higher than 100 kg/m3. Thus it can be assumed that dilute acid pretreatment process can be designed in such a way that the increase in methane production will provide enough heat for pretreatment requirements, even in the case of CHP conversion. It is thus interesting to consider the increase in electricity production that can be achieved thanks to thermal (110 kW h/tTS) or dilute acid (265 kW h/tTS) pretreatments. If price of electricity is assumed to be the maximum that can be achieved in France for this kind of substrate (0.1737 c€/ kW h), the gain can be 19 and 46 €/tTS after thermal and dilute acid treatments, respectively. Within a technoecomic analysis of dilute sulphuric acid and enzymatic hydrolysis of corn stover, Aden and Foust reported a cost of 17 €/tTS for dilute acid pretreatment [37]. Even if this cost cannot be directly applied in the present study, it is worth noting that it can support the economic feasibility of combining dilute acid pretreatment and anaerobic digestion of lignocellulosic biomass such as sunflower oil cakes. Nevertheless this energy and cost assessments have to be confirmed by pilot scale continuous anaerobic digestion experiments.
4. Conclusions The anaerobic digestion of sunflower oil cakes is possible even without any pretreatment. However, combined thermal–dilute acid pretreatment was beneficial for methane production from sunflower oil cakes. Yield was increased from 195 mLCH4/g VS to 289 mLCH4/g VS by pretreatment at 170 °C, 1% H2SO4. This can be attributed to the enhanced accessibility of cellulose and hemicelluloses to microbial attack. Indeed, dilute acid pretreatments led to the solubilisation of proteins and carbohydrates. However at temperature higher than 170 °C, soluble carbohydrate concentration decreased due to Maillard or ‘‘caramel’’ reactions. Furthermore, it was shown that pretreatments at 170 °C with acid concentrations ranging from 0.3% to 0.7% had no significant effect in comparison to thermal treatment alone and concentrations higher than 1% did not lead to further improvement in the methane yield. In the same way, excessively high temperatures were shown to lead to less methane production than pretreatments at 170 °C due, probably, to the formation of recalcitrant melonoidins. Moreover, it was shown that the volume of biogas produced by the soluble fraction could reach more than 60% of total methane yield after severe pretreatment.
Acknowledgments We gratefully acknowledge the financial support from the ‘Institut National de la Recherche Agronomique’ (INRA) and from the Universidade Estadual of Campinas (UNICAMP) through the Biomass Pretreatment Project (FAPESP).
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Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Enhancement of methane production from sunflower oil cakes by dilute acid pretreatment Florian Monlau a,b, Eric Latrille a, Aline Carvalho Da Costa b, Jean-Philippe Steyer a, Hélène Carrère a,⇑ a b
INRA, UR050, Laboratoire de Biotechnologie de l’Environnement, Avenue des Etangs, 11100 Narbonne, France School of Chemical Engineering, University of Campinas, Av. Albert Einstein, 500, Campinas SP 13 083-852, Brazil
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Dilute acid pretreatment increased
"
"
"
"
methane potential of sunflower oil cakes by up to 50%. Optimal conditions were 170 °C, 1% (weight) sulphuric acid concentration. Methane produced by the soluble fraction represented more than 60% of total volume. At 130 6 T 6 170 °C, increase in methane production was linked to solubilisation of organic carbon. At T > 170 °C, recalcitrant compounds were formed in the liquid phase.
a r t i c l e
i n f o
Article history: Received 9 March 2012 Received in revised form 14 May 2012 Accepted 11 June 2012 Available online 26 July 2012 Keywords: Anaerobic digestion Biogas Lignocellulosic biomass Acid pretreatment Methane potential Solubilisation
a b s t r a c t The conversion of sunflower oil cake (SOC) into methane by mesophilic anaerobic digestion was the object of this study. The effect of a combined dilute acid-thermal pretreatment (acid concentration and temperature) on solubilisation and methane potential was investigated using a central composite design (CCD). For temperatures up to 170 °C, solubilisation of each parameter (total organic carbon, sugars and proteins) increased with the severity of the pretreatment (high temperature and high acid concentration). Methane production was higher for pre-treated samples than for the untreated samples (195 mL CH4/ g VS). The highest yield (302 ± 10 mL CH4/g VS) was obtained after acid pretreatment at 170 °C. At this temperature, acid concentrations lower than 1% had no significant impact on methane production in comparison to thermal treatment alone. The volume of methane produced by the soluble fraction reached more than 60% of total methane production. An increase in methane production was correlated to the concentration of organic carbon in the liquid phase of samples pretreated at 130–170 °C with acid. At temperatures higher than 170 °C, some recalcitrant compounds were formed in the liquid phase. Ó 2012 Elsevier Ltd. All rights reserved.
Abbreviations: AD, anaerobic digestion; BMP, biochemical methane potential; Carb, carbohydrates; CDD, central composite design; Pro, proteins; RSM, response surface methodology; sCarb, soluble carbohydrates; SEM, scanning electron microscopy; SOC, sunflower oil cakes; sPro, soluble proteins; sTOC, soluble total organic carbon; TOC, total organic carbon; TS, total solids; VS, volatile solids; X1, coded acid concentration; X2, coded temperature. ⇑ Corresponding author. Tel.: +33 4 68 42 51 68. E-mail address: [email protected] (H. Carrère). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.06.042
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1. Introduction In 1997, the Kyoto Protocol fixed the objective to reduce by 5.2% the world greenhouse effect gas emissions from the 1990 level over the 2008–2012 period. Various alternatives have been studied (wind, solar, hydraulic, etc.) to achieve this goal. One such alternative is the use of biomass for the production of biofuels or bioenergy [1]. However, first-generation biofuels that use the edible part of a plant would be responsible for a sharp rise in the price of food. Consequently, one major challenge for the future is the production of energy from renewable sources, especially from agricultural waste and woody waste, which will not compete with food or feed uses in a context of higher demand for food. Conversion of lignocellulosic materials through anaerobic digestion (AD) technology is one of the promising alternatives [2]. The extraction of oil from various biomasses such as rapeseed, Jatropha or sunflowers generates solid waste (oil cakes) with a high organic content; and for example, biogas installations to digest seed cakes have been reported to boost the energy efficiency of Jatropha biodiesel production [3]. Sunflower oil cake (SOC) production is the third highest after soy-bean and rapeseed cakes, its European level reaching about 12 million tons per year [4]. The interest of producing biomethane from these residues has been underlined but the methane potential of sunflower oil cakes has been shown to be quite low: from 186 [5] to 215 [6] or 227 N mL CH4 g volatile solids (VS) [7], which corresponds to the anaerobic degradation of approximately 40% of the volatile solids or organic matter [8]. This low conversion yield may be explained by the low accessibility of biodegradable organic compounds (hemicellulose, cellulose) within the lignocellulosic network. Lignocellulosic biomass is composed mainly of cellulose, hemicellulose and lignin which are strongly linked to each other and form complex three-dimensional structures which are in fact less accessible to micro-organisms [9,10]. The use of pretreatment has been recommended to overcome the poor conversion yield of sunflower oil cakes and to improve their overall substrate properties prior to anaerobic digestion [11]. An efficient pretreatment of lignocellulosic biomass should make hollocelluloses more accessible to enzymatic or bacterial attack in order to increase the overall biodegradability of lignocellulosic materials. It should break down the linkage between polysaccharides and lignin to make cellulose and hemicelluloses more accessible to bacteria [10,12,13]. Dilute acid pretreatment of lignocellulosic biomass has been extensively studied for second-generation bioethanol production [14–17]. It is performed by soaking the material in a dilute acid solution and then heating to temperatures between 140 and 200 °C for a certain time (from several minutes up to an hour). Sulphuric acid, at concentrations usually below 4 wt%, has been of most interest in such studies as it is both inexpensive and effective [10,18]. Several studies using acidic pretreatment have been carried out with the aim of enhancing anaerobic digestion of such lignocellulosic materials as bagasse, coconut fibres [19], cassava residues [20], whole plant maize [21], greenhouses residues [22], etc. Pretreatment of bagasse and coconut fibres with 1 M HCl at 25 °C for 30 days improved the formation of biogas from these materials by 31% and 74%, respectively [19]. On the other hand, HCl pretreatment (2% HCl/g total solids (TS) at 20 °C for 24 h) of whole plant maize had no impact on methane production [21]. The production of biogas from newsprint pretreated with 30% acetic acid and 2% nitric acid during 30 min in a boiling bath increased by three times during the 60-day incubation period [23]. Combined thermal–dilute sulphuric acid pretreatment (158 °C, 2.99% (w/w TS) H2SO4 for 20 min) led to a 57% increase in the methane yield of cassava residues [20].
In this study, dilute sulphuric acid pretreatment was applied to sunflower oil cakes anaerobic digestion in the same conditions range as for bioethanol production. The effect of H2SO4-thermal pretreatment on structural composition (hemicelluloses, cellulose and lignin) was first studied. Then, central composite design (CDD) coupled with response surface methodology (RSM) was used to investigate the effect of the H2SO4-thermal pretreatment parameters (acid concentration and temperature) on the solubilisation of chemical compounds (soluble total organic carbon (sTOC), soluble carbohydrates (sCarb) and soluble proteins (sPro)) and on the methane potential of sunflower oil cake. In addition, the methane production originating from the soluble and the solid fractions was investigated. 2. Materials and methods 2.1. Raw material The substrate used in this study was sunflower oil cakes resulting from the extraction of oil with hexane. They were produced by the Grandes Huileries Médaco (Béziers, France), a company producing crude oil. The main characteristics and composition of the sample used in the experiments were (average values of three determinations with standard deviations): total solids (TS) 0.90 ± 0.01 g/g SOC, volatile solids (VS) 0.83 ± 0.02 g/gTS, lipids: 0.0070 ± 0.0003 g/gTS; Van Soest fractionation (% VSinitial) soluble fraction 12%; hemicellulose 30%; cellulose 39%; lignin 19%. 2.2. Combined dilute acid-thermal pretreatment of sunflower oil cakes Sunflower residues were treated in a Zipperclave autoclave series 02-0378-1 (Autoclave France) with a solids load of 50 gSOC/L for 5 min after the operational temperature has been reached. This autoclave, with a stainless steel body, has a capacity of 1 L and can reach a temperature of 250 °C and a pressure of 79 bars. The reactor was agitated at 400 rpm by a shaft with two propellers and heated by a ceramic furnace. Pretreated samples were immediately separated into solid and liquid fractions by centrifugation (10,000 rpm for 10 min). A first series of pretreatment experiments was carried out based on the experimental design described in Section 2.3. In a second experiment series, the temperature was set at 170 °C and different acid concentrations (0; 0.3; 0.5; 0.7; 1; 1.5; 2.5% w/wSOC) were tested. 2.3. Experimental design The effects of experimental variables (temperature and acid concentration) on responses (solubilisation and methane production) were investigated using a central composite design where the central point was performed in triplicate. In this study, the effects of two main parameters (H2SO4 concentration in w/wSOC and temperature) were investigated. The coded and real values used in the experiment design are presented in Table 1. The effect of these
Table 1 Coded and real values used in the experimental composite design. Coded unit
1.21 1 0 1 1.21
True value X1: temperature (°C)
X2: acid concentration (g/gSOC)
130 142 170 198 210
0 0.74 2.5 4.26 5
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experimental variables on the responses was represented by response surface methodology (RSM) using the Statistica (Statsoft, v.7.0) software. Responses were correlated to experimental variables in accordance with the following equation:
Y ¼ a0 þ a1 X 1 þ a2 X 2 þ a12 X 1 x2 þ a11 X 21 þ a22 X 22
ð1Þ
where Y is the response, X1 and X2 are the coded experimental variables, a0, a1, a2, a12, a11, a22 are the model coefficients calculated from the experimental data. The partial least squares method was used to estimate the coefficients. In addition, statistical analysis was performed using the analysis of variance (ANOVA) with a confidence level of 95%. 2.4. Anaerobic digestion The methane potential of untreated and pretreated sunflower oil cakes was determined in batch anaerobic flasks. The volume of each flask was 0.5 L, with a working volume of 0.4 L and 0.1 L volume of head space. Each flask contained solutions of: macroelements (NH4Cl, 26 g/L; KH2PO4, 10 g/L; MgCl2, 6 g/L; CaCl2, 3 g/L), oligoelements (FeCl2, 2 g/L; CoCl2, 0.5 g/L; MnCl2, 0.1 g/L; NiCl2, 0.1 g/L; ZnCl2, 0.05 g/L; H3BO3, 0.05 g/L; Na2SeO3, 0.05 g/L; CuCl2, 0.04 g/L; Na2MoO4, 0.01 g/L), bicarbonate buffer (NaHCO3, 50 g/L), an anaerobic inoculum (5 g VS/L) and raw or treated SOC (0.5 g/g VSinoculum). The inoculum was granular sludge from a mesophilic anaerobic digester of a sugar factory. The sludge contained 89.9 g/L total solids (TS) and 83.1 g/L volatile solids (VS). Once the flasks were prepared, nitrogen bubbling was carried out to obtain anaerobic conditions and the bottles were closed with airtight butyl rubber septum-type stoppers. Duplicate bottles were incubated at 35 °C for 37 days. The initial pH was adjusted to 7 using NaOH (1 M). For three pretreatment conditions (142 °C–0.74%/170 °C–5%/ 198 °C–4.26%), the methane potential assays were done on the soluble and solid fractions in addition to the whole sample. The solid and liquid phases were separated by centrifugation at 15,000 g and 5 °C for 15 min.
determined using a gas chromatograph (Varian GC-CP4900) equipped with two columns: the first (CP914509 molecular sieve, 10 m) was used at 110 °C to separate O2, N2 and CH4; the second (CP914510 Poraplot Q, 10 m) at 70 °C to determine CO2. The detection of gaseous compounds was done using a thermal conductivity detector. The temperatures were 110 °C for the injector and 55 °C for the detector. The calibration was carried out with a standard gas composed of 25% CO2, 2% O2, 10% N2 and 63% CH4. 2.5.2. Substrate analysis The untreated and pretreated sunflower oil cakes were analysed for TS and VS in accordance with the APHA standard method [24]. The total organic carbon (TOC) was analysed with the TC analyser (TOC-V) with a runner sample (ASI-V). The Lowry and anthrone methods were used for, respectively, protein and sugar determination in the soluble phases. The content of the soluble fraction, hemicelluloses, cellulose and lignin was measured using the Van Soest procedure [25] with the FIBERBAG system (Gerhardt). This procedure is based on sequential extraction with neutral and acid detergents, followed by strong acid extraction. The different fractions were: (i) the fraction soluble in neutral detergent (soluble part), (ii) hemicelluloses, extracted by acid detergent, (iii) cellulose, extracted by 76% sulphuric acid, and (iv) lignin. Lipids were determined after extraction by 40–60 °C-petroleum ether at 67 bar 105 °C, using an accelerated solvent extractor ASE 200 (Dionex). All the analyses of chemical composition were done in duplicate. 2.5.3. Microscope observation The scanning electron microscopy (SEM) was done using a high resolution Leica microscope model LEO440i at the Laboratory of Analysis and Calibration (LRAC), School of Chemical Engineering – UNICAMP, with detectors for secondary electrons. The dry material was metalized with a thin layer of gold in a Sputter Coater polaron, model SC 7620, to ensure the conductivity of its observation. 3. Results and discussion 3.1. Impact of pretreatment on sunflower oil cake composition
2.5. Analytical methods 2.5.1. Biogas analysis Biogas volume was monitored by the water displacement method and the corresponding cumulative biogas volume then calculated. Acidified water (pH = 2) was used to minimise the dissolution of carbon dioxide in water. Biogas composition was
Soluble
Hemicellulose
The composition of raw sunflower oil cake (Van Soest soluble fraction, hemicellulose, cellulose and lignin) and of the residual solid fractions after pretreatment is shown in Fig. 1, expressed in %VS initial. A significant increase of the soluble fraction was observed for all pretreated samples due to the solubilisation of cellulose and hemicelluloses into soluble carbohydrates. The percentage of
Cellulose
Lignin
100% 80% 60% 40% 20%
R 13 aw 0° C ;2 14 .5 % 2° C ;0 .7 14 4% 2° C ;4 .2 6% 17 0° C ;0 17 % 0° C ;2 .5 17 % 0° C ;2 .5 17 % 0° C ;2 .5 % 17 0° C ;5 19 % 8° C ;0 .7 19 4% 8° C ;4 .2 6 21 % 0° C ;2 .5 %
0%
Fig. 1. Van Soest fractionation of raw sunflower oil cake and of the solid fraction of sunflower oil cake after acid pretreatment.
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Fig. 2. Microscope picture of (A) raw sunflower oil cake and (B) sunflower oil cake after pretreatment at 170 °C 2.5% H2SO4.
the Van Soest soluble fraction varied according to the pretreatment conditions, reaching a maximum after the most severe pretreatment conditions (210 °C–2.5%). At 170 °C, the increase in acid concentration led to an increase of the Van Soest soluble fraction. However, the percentage of lignin remained constant in treated and untreated samples (around 20%). Also, the reduction of hemicelluloses was greater than that cellulose: the hemicellulose fraction decreased from 30% in the raw substrate to 4% after 210 °C– 2.5% pretreatment whereas the cellulose fraction decreased from 39% to 19% in the same conditions. These results confirm conclusion: acid pretreatment is effective in removing hemicelluloses by breaking the ether bonds in lignin/phenolic carbohydrates complexes without dissolving the lignin [26]. Structural modification of
sunflower oil cake during thermal acid pretreatment is supported by microscope observations as shown in Fig. 2. 3.2. Impact of pretreatment on sunflower oil cake solubilisation Changes in sunflower oil cake composition during dilute-acid pretreatment were measured by the solubilisation of organic matter (sTOC, sCarb and sPro). The effect of dilute acid pretreatment parameters (acid concentration and temperature) on the organic matter solubilisation was investigated using CDD. By applying multiple regression analysis to the experimental data, second-order polynomial equations were obtained in terms of coded value to describe the correlation between the variables (X1 coded acid
Table 2 ANOVA for the model regression representing sCOT. sCarb. sPro and BMP. Source
Sum of square
Soluble COT (g/L) (R2 = 0.9509) X1 19.24 3.45 X 21
Degrees of freedom
Mean square
F-value
Prob
1 1
19.24 3.45
665.89 119.26
0.0015 0.0083
5.13 0.07
1 1
5.13 0.07
177.59 2.42
0.0055 0.2602
0.57 1.44 0.06 30.61
1 3 2 10
19.72 16.67
0.0471 0.0571
Soluble sugars (g/L) (R2 = 0.8778) X1 0.64 1.98 X2
1 1
0.64 1.98
49.51 153.17
0.0196 0.0065
1.67 0.05
1 1
1.67 0.05
129.13 3.92
0.0076 0.1861
0.03 0.58 0.026 4.96
1 3 2 10
1.98 14.96
0.2947 0.0633
X2 X 22 X1 X2 Lack of fit Pure error Total
1
X2 X 22 X1 X2 Lack of fit Pure error Total
0.57 0.48 0.029
0.026 0.19 0.01
Soluble protein (g/L) (R2 = 0.9519) X1 57.81 18.94 X2
1 1
57.81 18.94
419.94 137.58
0.0024 0.0072
12.79 0.10
1 1
12.79 0.10
92.91 0.76
0.0106 0.4753
2.54 4.52 0.27 99.70
1 3 2 10
2.54 1.51 0.14
18.48 10.95
0.0501 0.0849
BMP (mL CH4/g VS) (R2 = 0.8814) X1 1047.43 4520.01 X 21
1 1
1047.43 4520.01
8.56 36.95
0.0996 0.0260
2426.89 541.42
1 1
2426.89 541.42
19.84 4.43
0.0469 0.1701
100.00 846.19 244.67 9198.54
1 3 2 10
100.00 282.06 122.33
0.82 2.31
0.4614 0.3168
1
X2 X 22 X1 X2 Lack of fit Pure error Total
X2 X 22 X1 X2 Lack of fit Pure error Total
F. Monlau et al. / Applied Energy 102 (2013) 1105–1113
concentration and X2 coded temperature) and the responses sTOC, sCarb and sPro (Eqs. )(2)–(4)).
sTOC ðg=LÞ ¼ 5:23 þ 1:55 X 1 0:78 X 21 þ 0:8 X2 þ 0:11 X 22 0:38 X 1 X 2
ð2Þ
sCarb ðg=LÞ ¼ 2:19 þ 0:28 X 1 0:59 X 21 þ 0:46 X 2 0:09 X 22 0:08 X 1 X 2
ð3Þ
sPro ðg=LÞ ¼ 10:78 þ 2:69 X 1 1:83 X 21 þ 1:26 X 2 þ 0:13 X 22 0:79 X 1 X 2
ð4Þ
1109
ANOVA was conducted to test the significance of the fit of the second-order polynomial equations for the sTOC, sCarb and sPro, as shown in Table 2. The accuracy of the fit of polynomial model was expressed by the coefficient of determination R2. Coefficients of determination R2 of 0.95, 0.88 and 0.95 were obtained for, respectively, sTOC, sCarb and sPro, showing that the models were significant. Values of probability (Prob) lower than 0.05 indicate that model terms are significant at the 95% confidence level. None of the models presented evidence of lack of fit. Significant model terms are represented in bold in Table 2. The linear effect of temperature (X1) and acid concentration (X2), the interactive term (X1 X2) and the quadratic effect of temperature (X 22 ) were
Fig. 3. 3D response surface plots: effect of acid concentration and reaction temperature on sTOC (A), sCarb (B), sPro (C).
F. Monlau et al. / Applied Energy 102 (2013) 1105–1113
significant for the sTOC. The linear effect of temperature (X1) and acid concentration (X2) and the quadratic effect of temperature (X 22 ) were significant for the sPro and sCarb. The regression model developed can be represented using response surface methodology (RSM) to show the interaction. Response surface plots with all the terms are presented in Fig. 3. For temperatures up to 170 °C, the solubilisation of each soluble response increased as more severe pretreatment conditions (high temperature and high acid concentration). Raising the temperature from 170 °C to 200 °C led to a slight increase in soluble organic carbon and protein concentration and, at 200 °C, there was no significant impact of acid concentration between 0.74% and 4.26%. At 210 °C, a decrease was observed in solubilised organic carbon and protein concentrations which may have been due to the degradation of organic matter possibly lost in the gas phase when the reactor was open. A decrease in soluble carbohydrate concentration was observed from 170 °C, due probably to reactions of some carbohydrates with others (‘‘caramel’’ reactions) or with proteins or amino acids (Maillard reactions). As carbohydrate measurement dosage was based on spectrophotometry which quantified the carbonyl function (C@O), the carbohydrates which had reacted via their carbonyl functions were not measured, in contrast to proteins which were quantified even when they had formed Maillard compounds [27]. In conclusion, dilute acid pretreatment was effective in protein solubilisation and permitted the release of carbohydrates by solubilising hemicelluloses and to a lesser extent, cellulose. On the basis of organic carbon measurements, maximum solubilisation of organic matter was obtained after pretreatment at 170 °C with 5% H2SO4 or at 200 °C, at which temperature 0.74% H2SO4 was enough. 3.3. Methane potentials 3.3.1. Impact of pretreatment on the methane potential of sunflower oil cake The methane potential of raw sunflower oil cake after 37 days of batch anaerobic digestion was 195 mL CH4/g VS, which is in the same range as previous data: 215 mL CH4/g VS [6] and 227 mL CH4/g VS [8]. CDD was performed to investigate the interaction of the parameters of dilute acid pretreatment (acid concentration X1 and temperature X2) on the methane potential of sunflower oil cakes. By applying multiple regression analysis to the experimental data, a second-order polynomial equation was obtained to describe in terms of coded values the correlation between the variables (X1 and X2) and the methane potential response (Eq. (5)):
BMPðmL CH4 =g VSÞ ¼ 293:33 þ 11:44 X 1 28:29 X 21 þ 17:41 X 2 9:79 X 22 þ 5 X 1 X 2
ð5Þ
ANOVA was conducted to test the significance of the fit of the second-order polynomial equation for the methane potential as shown in Table 2. A coefficient of determination R2 of 0.88 was obtained. Values of Prob less than 0.05 indicate significance of the model term, which is represented in bold in Table 2. The quadratic coefficient of temperature (X 21 ) and the linear acid coefficient concentration (X2) significantly affected the methane potential. The regression model developed can be represented using response surface methodology (RSM) with all the terms (Fig. 4). Except for the pretreatment at the lowest temperature (130 °C, 2.5%), which led to a methane potential of 206 mL CH4/g VS corresponding only to a 5% increase in comparison to the untreated substrate, all the various dilute acid pretreatment conditions led to significant enhancement of sunflower oil cakes methane potential. This increase ranged from 20% (treatment at 170 °C with no acid addition) to around 50% (treatment at 170 °C, 2.5% or 5% H2SO4). Temperatures higher than 170 °C led to a reduction of methane potential in comparison to 170 °C using 2.5% or 5% of acid. This decrease in
Fig. 4. Response surface plot: effect of temperature and acid concentration on methane potential.
methane potential was probably due to the combined effect of high temperature and acid addition. Indeed, Chandra et al. observed a high increase in methane potential of rice straw after hydrothermal pretreatment at 200 °C [28]. Although the calculated optimal conditions were 172 °C and 4.19% for temperature and acid concentration, respectively, this acid concentration seems to be excessive, as the difference of methane potential with acid concentrations of 1% and 5% was slight. The reduction of sulphuric acid concentration during pretreatment is an important point as sulphate is a stronger acceptor compared to CO2 and the reduction of sulphate to H2S competes with biogas production [29]. Also, the process is less expensive if less acid is used. Consequently, in a further experiment series, the temperature was set at 170 °C and different acid concentrations (0; 0.3; 0.5; 0.7; 1; 1.5; 2.5% w/wSOC) were tested considering 5 min pretreatment time. 3.3.2. Effect of H2SO4 concentration at 170°C on methane production The results of varying acid concentration from 0% to 5% are shown in Fig. 5. Results show that at 170 °C, acid concentrations below 1% had no significant effect on the methane yield of sunflower oil cakes. When the acid concentration increased from 0.7% to 1%, an increase of 15% of the methane production was observed. Moreover, there was no significant difference within an acid concentration range of 1–2.5%. Moreover, if previous results (Fig. 5) are pretreated 170°C
BMP (mL CH4 / g VS)
1110
raw
350 300 250 200 150
0
1
2
3
4
5
6
Acid concentration (%) Fig. 5. Methane potential of sunflower oil cake before and after pretreatment at 170 °C: impact of sulfuric acid concentration.
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F. Monlau et al. / Applied Energy 102 (2013) 1105–1113
130 to 170°C
T> 170°C
0% H2SO4
linear (130 to 170°C)
BMP increase (%)
70 60
y = 9.1454x - 0.1331 2 R = 0.8702
50
Table 3 Ratio of methane volume produced by the liquid and solid fractions divided by the volume produced by the total fraction of pretreated sunflower oil cake. The sum of the volumes produced by the liquid and solid fractions was slightly higher than the volume produced by the total fraction. Pretreatment conditions
40 30
142 °C; 0.74% 170 °C; 5% 198 °C; 4.26%
20 10
Fraction of methane originating in Liquid fraction (%)
Solid fraction (%)
37 61 58
65 46 51
0 0
1
2
3
4
5
6
7
8
TOC soluble (g/L) Fig. 6. Correlation between BMP increase ((BMPtreated-BMPraw)/BMPraw100) and concentration of organic carbon in the liquid phase of pretreated sample. Linear correlation was plotted for samples pretreated at temperatures ranging from 130 °C to 170 °C and with acid addition from 0.74% to 5%.
considered, acid concentration from 1% to 5% at 170 °C led to the same result, methane production ranging from 289 to 302 mL CH4/g VS. From economic and environmental points of view, it is better to limit the use of acid. Consequently, the optimal conditions for the pretreatment of sunflower oil cake are a temperature of 170 °C with an acid concentration of 1%. 3.3.3. Relationship between increased methane yield and sunflower oil cake solubilisation An increase in methane production by substrate pretreatment is often linked to the solubilisation of organic matter [27,30]. The increase in methane potential was thus plotted versus the organic carbon concentration in the liquid phase of pretreated sunflower oil cakes (Fig. 6). A satisfactory linear correlation was observed between methane potential enhancement and soluble organic carbon concentration only for samples pretreated at temperatures ranged from 130 to 170 °C along with an addition of sulphuric acid between 0.74% and 5%. Indeed, the sample pretreated at 170 °C with no addition of acid, resulted in less methane potential enhancement as compared to acid samples. This can easily be explained by the different mechanisms occurring in thermal and thermo-acid pretreatment and confirms the importance of using acid. However, too severe pretreatment conditions (T = 198 °C–0.74% or 4.26%) or (T = 210 °C–2.5%) led to less enhancement of methane production than could have been expected from high levels of solubilisation. These results revealed the formation of recalcitrant or hard-to-degrade compounds in the liquid phase. Such compounds may be melanoidins resulting from Maillard reactions between proteins and carbohydrates at high temperature in acid media. Such reactions are highly probable in the case of sunflower oil cake which is rich in proteins and carbohydrates [11]. Moreover, the decrease in methane yield after excessively high temperature treatment has already been ascribed to the formation of Maillard compounds in the case of sewage sludge [27]. Beside, in the case of lignocellulosic biomass and high temperature pretreatment such as liquid hot water, steam explosion and acid or organosolv pretreatments, byproducts such as furfural, hydroxymethylfurfural and soluble lignin compounds can be formed [31]. These compounds cause problems in the fermentation stage during bioethanol production because they can inhibit or even stop the fermentation [32]. But in contrast to bioethanol process, anaerobic digestion can convert these compounds to methane [33,34]. 3.3.4. Origin of methane production: solid and liquid phases of pretreated samples The anaerobic digestion of the liquid and solid phases of a number of pretreated samples (142 °C–0.74%/170 °C–5%/198 °C–4.26%)
was carried out in order to determine the relative part of each phase (soluble and solid) in the production of methane. Results are shown in Table 3. The sum of the methane produced from the solid and soluble fractions was equal to the volume produced by the total fraction within 5–10%. This result is a justifiable basis for making assumptions about the origin of the methane produced. Whereas for pretreatment at 142 °C–0.74% the production of biogas from the soluble part was only 36%, pretreatment at 170 °C–5% led to the highest volume of methane produced by the soluble fraction (61%) and although presenting an equivalent organic carbon solubilisation, the fraction of methane produced by the liquid phase at 198 °C–4.26% was slightly lower (58%). Thus, with efficient pretreatment (170 °C–5%) the major part of the methane produced originated from the liquid phase of pretreated biomass. These results are also interesting in the context of the second-generation bioethanol process. Indeed as bioethanol is generally produced from the solid phase of pretreated biomass, the liquid phase can be used to produce methane, which increases the overall energy yield of the process [35]. 3.4. Cost aspects It is necessary to assess if the implementation of dilute acid pretreatment could be economically viable. However, BMP tests are absolutely not designed for process scale up or for costs assessment. Indeed, the impact of any pretreatment is highly dependent on the anaerobic digestion process. Generally, methane production enhancement is higher in continuous process with low hydraulic retention time because pretreatments of complex substrates increase both rate and extent of anaerobic digestion [36]. In BMP tests which are designed to achieve the maximum methane production yields, the effect of pretreatment on digestion rate is generally hindered. On the other hand, the increase in digestion rate can lead to high increase in methane production in continuous digesters when the retention time is too low to achieve the complete conversion of the untreated substrate. Therefore, following results are only rough indications. Biogas can be converted either to heat, to heat and electricity by combined heat and power (CHP), or to gaseous fuels (mainly biomethane or compressed biogas). It can also been injected to the natural gas grid. Considering that dilute acid pretreatment requires a high level of heat energy, only conversions into heat and heat and electricity will be considered. This point is consistent with the present development of farm anaerobic digestion in France where biogas is converted by CHP in most of the cases. Heat production generally exceeds the local needs and may be used for substrate pretreatment. Table 4 shows the assessment of the methane conversion to heat and by CHP for sunflower oil cakes with thermal (170 °C) and dilute acid (170 °C, 1% H2SO4) pretreatments. Methane conversion into heat was considered as 9.95 kW h m3. CHP yields were considered to be 50% for heat and 35% for electricity. Heat requirements were assessed by the energy needs to raise the temperature of the substrate from 25 °C to 170 °C, assuming
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F. Monlau et al. / Applied Energy 102 (2013) 1105–1113
Table 4 Energy balance of thermal and dilute acid pretreatments of sunflower oil cakes. Energy produced Pretreatment Methane (m3/tonTS) Methane increase1 (m3/tonTS) Conversion into heat Conversion into heat and electricity
170 °C 194 32 1934 322 966 161 659 110
Heat produced (kW h/tTS) Heat increase1 (kW h/tTS) Heat produced (kW h/tTS) Heat increase1 (kW h/tTS) Electricity produced (kW h/tTS) Electricity increase1 (kW h/tTS)
170 °C + 1%H2SO4 240 78 2388 778 1194 389 815 265
Heat energy requirements for pretreatment Solid loading (kg/m3) 2
Heat requirement (kW h/tonTS) Heat requirement2 with 80% heat recovery (kW h/tTS) 1 2
50
100
200
300
3536 530
1852 278
1010 152
730 109
Compared to anaerobic digestion without pretreatment. To increase temperature from 25 to 170 °C.
that specific heat of the substrate suspension in water can be evaluated by the water specific heat (4.18 kJ kg1 °C1). This energy need to treat 1 ton of total solids is thus highly dependent on the solid loading during pretreatment. In our study, solid loading was 50 kg m3. Working on dilute acid pretreatment for the conversion of corn stover into bioethanol, Aden and Foust reported regular operation at 30% solid loading (i.e. 300 kg m3) [37]. Table 4 shows that in the case of low solid loading, heat requirements can be higher than the heat that can be recovered by burning all the methane produced. However, with 300 kg/m3 solid loading, pretreatment heat can be provided by the increase of methane due to dilute acid pretreatment. Nevertheless, thermal energy integration has to be carried out in a full scale implementation of thermal or dilute acid pretreatment. This can be achieved by exploiting the high temperature and enthalpy of streams like vapour, pretreated biomass, exhaust gases and hot water from the gas engine [38]. The heat energy of pretreated substrate suspension can be recovered to heat the digester [39] or to preheat raw substrate suspension [40]. Nevertheless, in the case of recovering thermal energy from flue gas during dilute acid pretreatment, Budzianowski recommended to avoid sulphuric acid condensation by avoiding cooling the gas below 180 °C [41]. Dhar et al. [42] reported 80% heat recovery from thermally pretreated sludge. Thus assuming 80% heat recovery from pretreated SOC, Table 4 shows that, in the case of full conversion of methane into heat, increase in heat production by dilute acid pretreatment is higher than the energy requirements for pretreatment whatever the solid loadings. In the case of CHP conversion, increase in heat production by dilute acid pretreatment is higher than the energy requirements for pretreatment when solid loading is higher than 100 kg/m3. Thus it can be assumed that dilute acid pretreatment process can be designed in such a way that the increase in methane production will provide enough heat for pretreatment requirements, even in the case of CHP conversion. It is thus interesting to consider the increase in electricity production that can be achieved thanks to thermal (110 kW h/tTS) or dilute acid (265 kW h/tTS) pretreatments. If price of electricity is assumed to be the maximum that can be achieved in France for this kind of substrate (0.1737 c€/ kW h), the gain can be 19 and 46 €/tTS after thermal and dilute acid treatments, respectively. Within a technoecomic analysis of dilute sulphuric acid and enzymatic hydrolysis of corn stover, Aden and Foust reported a cost of 17 €/tTS for dilute acid pretreatment [37]. Even if this cost cannot be directly applied in the present study, it is worth noting that it can support the economic feasibility of combining dilute acid pretreatment and anaerobic digestion of lignocellulosic biomass such as sunflower oil cakes. Nevertheless this energy and cost assessments have to be confirmed by pilot scale continuous anaerobic digestion experiments.
4. Conclusions The anaerobic digestion of sunflower oil cakes is possible even without any pretreatment. However, combined thermal–dilute acid pretreatment was beneficial for methane production from sunflower oil cakes. Yield was increased from 195 mLCH4/g VS to 289 mLCH4/g VS by pretreatment at 170 °C, 1% H2SO4. This can be attributed to the enhanced accessibility of cellulose and hemicelluloses to microbial attack. Indeed, dilute acid pretreatments led to the solubilisation of proteins and carbohydrates. However at temperature higher than 170 °C, soluble carbohydrate concentration decreased due to Maillard or ‘‘caramel’’ reactions. Furthermore, it was shown that pretreatments at 170 °C with acid concentrations ranging from 0.3% to 0.7% had no significant effect in comparison to thermal treatment alone and concentrations higher than 1% did not lead to further improvement in the methane yield. In the same way, excessively high temperatures were shown to lead to less methane production than pretreatments at 170 °C due, probably, to the formation of recalcitrant melonoidins. Moreover, it was shown that the volume of biogas produced by the soluble fraction could reach more than 60% of total methane yield after severe pretreatment.
Acknowledgments We gratefully acknowledge the financial support from the ‘Institut National de la Recherche Agronomique’ (INRA) and from the Universidade Estadual of Campinas (UNICAMP) through the Biomass Pretreatment Project (FAPESP).
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