Characterization of Cynara cardunculus L. stalks and their suitability for biogas production

Industrial Crops and Products 40 (2012) 318–323

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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop

Characterization of Cynara cardunculus L. stalks and their suitability for biogas production Ivo Oliveira a , Jorge Gominho b,∗ , Santino Diberardino c , Elizabeth Duarte a a b c

Unidade de Investigac¸ão de Química Ambiental, Instituto Superior de Agronomia, Universidade Técnica de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal Centros de Estudos Florestais, Instituto Superior de Agronomia, Universidade Técnica de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal Unidade de Bioenergia, Laboratório Nacional de Energia e Geologia, I.P. Estrada do Pac¸o do Lumiar, 22, 1649-038 Lisboa, Portugal

a r t i c l e

i n f o

Article history: Received 11 November 2011 Received in revised form 6 March 2012 Accepted 23 March 2012 Keywords: Biomass Biogas Energy crops Cynara cardunculus L

a b s t r a c t There is emerging interest in the production of biomethane as a biocombustible either through anaerobic digestion of biomass and/or energy crops. Cynara cardunculus L. is a crop with high biomass yields used in the production of bioenergy, with the seeds being used for biodiesel production and the remaining biomass used as solid fuel in biomass plants. This work aims to present results concerning the obtainable methane yield of Cynara stalks when submitted to anaerobic digestion, and the effects of selected pretreatments (mechanical, thermal and thermochemical). For this purpose, two batch anaerobic digestion experiments (Trial I and Trial II) were performed. Minimum cumulative methane was achieved for the untreated substrate yielding 0.3 L CH4 /g VS0 . A maximum methane yield of 0.5–0.6 L CH4 /g VS0 was achieved depending on the selected pre-treatment. Thermochemical pre-treatment using NaOH was revealed to be a very efficient hydrolysis method. © 2012 Elsevier B.V. All rights reserved.

1. Introduction There is emerging interest in finding new energy crops for the production of biomethane through anaerobic digestion (AD). Presently, Cynara cardunculus L. is cultivated as an industrial crop with high biomass yields, and requires less irrigation and fertilizer for cultivation than other crops grown in Mediterranean countries (Fernández et al., 2006; Gominho et al., 2011). The average annual biomass production varies from 15 to 20 tonne of biomass per hectare depending on soil and rainfall conditions. Cynara has roughly 11% moisture content and the following biomass partitioning: 40% stalks, 25% leaves and 35% capitula (Gominho et al., 2008, 2001). Cynara has been proposed as a favourable feedstock for the production of biofuel and bioenergy. As its lipid-rich seeds have a desirable composition for biodiesel production (Sengo et al., 2010). The remaining component is usually burned as solid fuel in biomass power plants for energy and heat production (Fernández et al., 2006; Gominho et al., 2011). Cynara stalks may represent a new suitable feedstock for AD, increasing the biofuel potential of this crop. Anaerobic digestion is an industrial process applied to organic waste treatment with several environmental and energetic advantages over other forms of processing and especially when it is integrated within the agricultural sector (Chynoweth and Isaacson, 1987; Möller and Stinner, 2009; Prochnow et al., 2009). In addition

∗ Corresponding author. E-mail address: [email protected] (J. Gominho). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.03.029

to the organic waste treatment, there is an emerging interest in the production of biomethane as a biocombustible through anaerobic digestion of biomass and/or energy crops (Chanakya et al., 2009; Chynoweth and Isaacson, 1987; CONCAWE, 2008; Gunaseelan, 1997; IEA, 2010; Tilche and Galatola, 2008; Yadvika et al., 2004). Different studies have shown that the addition of energy crops to the anaerobic digestion of cattle dung, or the anaerobic digestion of energy crop residues with the addition of partially digested cattle dung or digested sewage sludge, enhanced both biogas production and methane yield (Chanakya et al., 2009; Yadvika et al., 2004). However, several problems have been reported related to the use of energy crops (Lindorfer et al., 2007; Prochnow et al., 2009). Due to the lignocellulosic structure, these substrates are not fully hydrolyzed. For years, hydrolysis of lignocellulosic material has been regarded as the rate limiting step in anaerobic digestion of lignocellulosic biomass (Alvarez et al., 2000; Boone et al., 1993; Chynoweth and Isaacson, 1987; Hendriks and Zeeman, 2009; Lehtomäki et al., 2006; Mata-Alvarez, 1987; Mladenovska et al., 2006; Mosier et al., 2005; Palmowski and Müller, 2000; Siegert and Banks, 2005; Ward et al., 2008; Wyman et al., 2005). As a result, the material tends to float upon the fluid surface in the digester, leading to increased stirring expenses. For instance, the wrapping of long grass particles around moving devices can cause failures in the operation of the biogas plant (Prochnow et al., 2009). Previous studies have suggested that a pre-treatment step prior to feeding the digester increases hydrolysis rate (Angelidaki and Ahring, 2000; Chynoweth and Isaacson, 1987; Hendriks and Zeeman, 2009; Lehtomäki et al., 2006; Palmowski and Müller,

I. Oliveira et al. / Industrial Crops and Products 40 (2012) 318–323

2000). Recently, Cynara stalks were tested for ethanol production (Ballesteros et al., 2008) pairing a low dose sulphuric acid (0.1–0.2%, w/v) thermal–chemical pre-treatment with enzymatic hydrolyses. The achieved ethanol yields were relatively low for industrial applications, thus requiring further improvements. An interesting perspective concerning the growing of Cynara, is the integration with biodiesel and biogas production plants in Mediterranean countries. The goal of the research performed on C. cardunculus L. is to increase the knowledge concerning the use of this promising industrial crop for biogas production in Mediterranean countries or countries with similar edapho-climate conditions. For this purpose of this study, stalks were chemically characterised and two batch anaerobic digestion experiments were performed (Trial I and Trial II). The effect of mechanical, hydrothermal and thermochemical (with sodium hydroxide) pre-treatments are compared, making possible an evaluation regarding the energetic potential of this feedstock. 2. Materials and methods 2.1. Substrates and inoculums The C. cardunculus L. used in this study was collected from a pedagogic field (BioEnergISA) built at the ISA campus (Instituto Superior de Agronomia, Lisboa) in October 2008. The leaves and the capitula were removed and only the stalks were used. The stalks were milled to a particle size of 10 mm. Further mill processing reached fractions lower than 40 mesh. After sieving, the 40–60 mesh fractions and the 10 mm material was stored in a dry place at environment temperature until required. During the experiment the required raw-material was stored in an oven at 100 ◦ C, to keep the material dry. This material is specified as substrate (S) in the anaerobic digestion experiments. Two anaerobic digestion experiments were performed using two different substrate particle sizes: 40–60 mesh (Trial I) and 10 mm (Trial II). The sludge which provided the inoculums (I) for the experiments was collected freshly from a waste water treatment plant digester (WWTP, ETAR de Chelas, Lisboa), and stored in 5 L capacity plastic containers. 2.2. Experimental procedure Anaerobic digestion experiments were carried out using batch reactors with a working volume of 2.0 L and were operated under a mesophilic temperature of 37 ± 2 ◦ C. The temperature was maintained using a water-heating jacket, in which the water was circulated through an external heater. The reactors were connected to graduated cylindrical gas holders for measuring biogas production, with an incubation time of 32 days. In these experiments three reactors were used: B0 containing inoculum, B1 containing inoculum and pre-treated substrate and B2 containing inoculum and untreated substrate. Reactor contents were mixed daily for 10 min using a mechanical stirrer. The experiment was conducted using the protocol described in Table 1. The high volume of inoculum was adopted to maintain a constant pH during the anaerobic digestion trials and its characterisation is shown in Table 2. 2.3. Trial I In this experiment the substrate, Cynara stalk with a particle size of 40–60 mesh, was submitted to thermal pre-treatment using distilled water (autohydrolysis). The thermal pre-treatment was carried out in a set of inox steel reactors with a working volume of 300 ml using a fixed temperature of 160 ◦ C, a total solids content (TS) of 12.5% and a reaction time of 30 min. The reactors were tightly closed and placed together in a rotating structure within

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an oil heated bath (160 ◦ C). Following pre-treatment the pH was measured. The resulting pre-treated material was mixed with the inoculum and fed directly into the reactors. 2.4. Trial II A thermochemical pre-treatment was applied over Cynara stalks with a particle size of 10 mm. In this case, a fixed temperature of 160 ◦ C, 12.5% of TS, and a reaction time of 20 min were used. In addition, NaOH (1.4%, w/v) solution was used instead of distilled water. Following pre-treatment the pH was measured. The resulting pretreated material was mixed with the inoculum and fed directly into the reactors. 2.5. Analytical methods The chemical composition of the whole stalks, pith and depicted material was determined using Tappi standard methods in relation to ash, moisture content, extractives, lignin and polysaccharides as described in Gominho et al. (2001). Cynara stalks were also characterised in terms of total solids (TS), volatile solids (VS), total organic carbon (TOC), minerals (Ca2+ , Na+ , Mg2+ , K+ , Mn2+ , Cu2+ and Zn2+ ), nitrogen (N-NH4 + , NK and Ntotal ) and pH according to Standard Methods for Examination of Water and Wastewater. The determination of TS, VS and pH in the reactors (B0 , B1 , B2 ) was measured at the beginning and at the end of the experiments. Total carbon content was evaluated as suggested by Zucconi and De Bertoldi (1987). Pre-treated substrate was analysed in terms of holocellulose and ␣-cellulose according to the method described by Rowell et al. (2005). Daily biogas production was measured using 5000 ml and 2000 ml graduated cylinders with reversible gas–liquid displacement holders. Methane, carbon dioxide and oxygen determinations were obtained by a Varian 3800 gas chromatograph (carrier gas nitrogen) equipped with a packed column (Porapak-s 80–100 mesh, T = 50 ◦ C), a TCD detector (T = 150 ◦ C), and maintaining the injection chamber at 60 ◦ C. The methane yield was calculated by subtracting the amount of methane produced by the control from the methane production of each reactor and dividing the difference by the mass of VS in the substrate fed to the reactor. The following formula was used to estimate the VS reduction: VS reduction (%) = 100

 M − (M − M )  n 0 i M0

,

where M0 is the mass of VS in Cynara added to the reactors (g); Mn is the mass of VS in the reactor at the end of fermentation (g); and Mi is the mass of VS in the inoculums reactor B0 (control) at the end of fermentation (g). 3. Results and discussion 3.1. Chemical characterisation of C. cardunculus L. stalks Among the constituents of Cynara stalks, polysaccharides comprise the major fraction (57.4%). Cellulose and hemicelluloses represent 36.3% and 16.3% of the total mass, respectively (Table 3). In terms of extractives compounds, the value measured in our sample (6.4%) is 7% lower than the values already published with the same raw-material (Fernández et al., 2006; Gominho et al., 2001, 2008). This difference is possibly explained by heterogeneity present in lignocelulosic materials. The stalks presented a lower nitrogen content (NK = 0.3%) resulting in a high C/N ratio value which is unfavourable for the AD process. This factor may limit bacterial growth, however the substrate can be co-digested with another component to balance this ratio (Mshandete et al., 2004). Therefore it was decided to use a high proportion of the digested

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Table 1 Protocol used for the experimental trials. Reactors

B0 (control) B1 (pre-treated substrate) B2 (untreated substrate)

H2 O addition

Substrate concentration (S)

Inoculum (I)

I/S ratio

(mL)

(g VS L−1 )

(mL)

(mL I/g VS S)

462 462

NA 28.7

1538 1538

NA 116.1

Note: NA: not applicable.

Table 2 Inoculums characterisation used in anaerobic digestion experimental trials. Trial I

Inoculums

Trial II

pH

TS (%)

VS (%TS)

pH

TS (%)

VS (%TS)

6.8

2.4

70.8

6.9

2.4

41.7

Table 3 Chemical composition (% of oven dry mass) of the C. cardunculus L. stalks. Constituent

% of TS

Volatile solids (VS) Total organic carbon (TOC) Total nitrogen (Ntotal ) Kjeldahl nitrogen (NK) N-NH4 +

95.7 36.6 0.3 0.3 0.0

Minerals Ca2+ Na+ Mg2+ K+ Mn2+ Cu2+ Zn2+

2.3 0.2 0.4 0.0 0.0 0.1 0.1

Extractives Total lignin Total polysaccharides:

6.4 19.8 57.4

Monosaccharide’s composition Glucose Xylose Mannose Arabinose Galactose Rhamnose

% of Sugars 36.3 16.3 1.4 1.3 1.4 0.6

Table 5 Crude sugar contents of pre-treated and untreated substrate samples. Untreated substrate

Holocellulose (%TS) ␣-Cellulose (%TS) Hemicellulose (%TS)

nitrogen-rich sludge (inoculum), specifically: I/S = 116.1, which provided a favourable C/N ratio (C/N = 14). 3.2. Anaerobic digestion trials The results achieved in Trial I in terms of cumulative methane yield were: 0.62 L/g VS0 for B1 (pre-treated substrate) and 0.50 L/g VS0 for B2 (untreated substrate) (Fig. 1 and Table 4). These figures are within the range of values previously reported in the literature by other energy crops used as substrate (without pretreatment) in AD, i.e. 0.63 L/g VS0 for ryegrass, 0.56 L/g VS0 for timothy 0.33 L/g VS0 for wheat straw, 0.55 L/g VS0 for cocksfoot

Thermal pre-treated substrate

85 15 70

With H2 O

With NaOH

79 19 60

77 24 53

grass and 0.44 L/g VS0 for sorghum (Chynoweth and Isaacson, 1987; Gunaseelan, 1997, 2008; Prochnow et al., 2009). The thermochemical pre-treatment (Trial II) applied to the substrate in reactor B1 doubled the cumulative methane yield in comparison with the untreated substrate in reactor B2 (Fig. 2 and Table 4). A cumulative methane yield of 0.59 L/g VS0 and 0.31 L/g VS0 was achieved for the pre-treated and untreated substrates respectively. The enhancement achieved is also shown in terms of VS reduction, 44% in B1 and 21% in B2 reactors. The thermal pre-treatments (B1 ) enhanced the hydrolysis of hemicelluloses because the methyl groups released reacted to form acetic acid, resulting in a decreased pH value and promotion of the hydrolysis reaction of the sugars as described by different researchers (Caparros et al., 2007; Donghai et al., 2006; Garrote et al., 2007; Liu and Wyman, 2005; Yang and Wyman, 2008). In Trial II the decrease in pH was not realised due to the NaOH buffer solution. The holocellulose content for the untreated substrate was 85% (TS) while for the thermal pre-treated substrates with NaOH and H2 O the values decreased to 77% and 79% respectively (Table 5). Despite the high cumulative methane yields achieved, there is still an amount of organic matter expressed as volatile solids in the residual substrate (VSn ). Assuming that lignin is not degraded in anaerobic conditions, and that in Cynara stalks it represents approximately 19.8% (%TS) of total lignin, there is still approximately 44% (B1 ) and 58% (B2 ) of VSn in Trial I, and 45% (B1 ) and 74% (B2 ) of VSn in Trial II, of the original substrate remaining for further degradation.

Table 4 Results of the anaerobic digestion experiment. Reactors

pH0 (substrate)

pHi (mixture)

pHf

VS reduction (%)

Specific methane yield (L/g VS0 )

Trial I (40–60 mesh; 160 ◦ C; 30 min)

B1(H2 O) B2

4.4 6.3

6.9 7.1

7.2 7.3

56.2 41.7

0.62 0.50

Trial II (10 mm; 160 ◦ C; 20 min)

B1(NaOH) B 2

7.9 6.5

6.9 6.7

7.1 6.8

43.9 20.8

0.59 0.31

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Fig. 1. Cumulative methane yield during the anaerobic digestion experiment Trial I.

Fig. 2. Cumulative methane yield during the anaerobic digestion experiment Trial II.

Fig. 3. Volatile solids reduction in Trial I and Trial II.

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Fig. 4. Schematic of energy flow for methane production from C. cardunculus L. stalks.

Table 6 Gross energy content of C. cardunculus L. stalks calculated from their chemical composition. Sample

Total proteina (%TS)

Total carbohydrates (%TS)

Calculated energy (MJ kg−1 )b

Cynara stalks

1.9

56.6

9.4

a b

Calculated by multiplying the Kjeldahl nitrogen content by 6.25. Calculation based on 16.7 MJ kg−1 for carbohydrates and proteins.

3.3. Energy conversion balance Comparing the results achieved in terms of VS reduction (Fig. 3), energy conversion (Fig. 4) and cumulative methane yield (Figs. 1 and 2) it is possible to estimate the efficiency of the pre-treatments used in the anaerobic experimental trials. Granulometric reduction of Cynara stalks increased VS reduction by 20%, resulting in an increment in methane yield from 0.5 L CH4 /g VS0 (Trial I – B2 ) compared to 0.3 L CH4 /g VS0 (Trial II – B2 ). Table 6 shows the estimated energy in the form of carbohydrates and protein contained in the Cynara stalks and in Fig. 4 the conversion of that energy into methane is outlined. As is evident, in Trial I, B1 energy conversion was 24% higher than in B2 and in Trial II this value is 92% higher. This is supported by the VS reduction and cumulative methane yield achieved after anaerobic digestion. Thermal pre-treatment improved the conversion of carbohydrates and proteins to methane. The methane yields achieved show that Cynara stalks can be a suitable substrate for AD. Assuming a cumulative methane yield between 0.5 and 0.6 L CH4 /g VS0 achieved in trial I and a crop productivity of 15 tonne/ha (40% as stalks) (Gominho et al., 2008, 2001), Cynara substrate could contribute to a potential energy production in the range of 29.9–35.9 MWh/ha, assuming a biogas heating value of 9.97 kWh/m3 (Ahrens and Weiland, 2004). 4. Conclusions C. cardunculus L. is already a promising energy crop for the bioenergy field in Mediterranean countries. The results show that Cynara stalks are a good substrate for Mediterranean biogas and methane production. Cynara stalks have a specific methane

yield that is higher than 0.3 L CH4 /g VS0 depending on the pre-treatment applied. Mechanical and thermal pre-treatment with NaOH showed a positive effect in terms of carbon solubilisation, VS reduction, biogas yield and methane yield increment. However, there is still work to be done in terms of choosing the most suitable pre-treatment. There is also an interest in using green stalks instead of dried ones allowing two annual harvests, as well as using the entire crop material (stalks, leaves and seeds) for methane production (Gunaseelan, 2008). Additionally, there is potential for combining the two processes of biodiesel and biogas production through co-digestion of glycerol and de-oiled cake with other substrates. Finally, the application of the digestate as fertilizer to grow C. cardunculus L. should be also considered to reduce costs and improve Cynara productivity.

Acknowledgements This work was supported by Strategic Project (PEstOE/AGR/UI0239/2011) of Centro de Estudos Florestais, and Strategic Project (PEst-OE/AGR/UI0528/2011) of Unidade de Investigac¸ão de Química Ambiental a research units supported by the national funding of FCT – Fundac¸ão para a Ciência e a Tecnologia.

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