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Industrial Crops and Products 40 (2012) 318– 323

Contents lists available at SciVerse ScienceDirect

Industrial Crops and Products

journa l h o me pag e: www.elsev ier .com/ locate / indcrop

haracterization of Cynara cardunculus L. stalks and their suitability for biogasroduction

vo Oliveiraa, Jorge Gominhob,∗, Santino Diberardinoc, Elizabeth Duartea

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

r t i c l e i n f o

rticle history:eceived 11 November 2011eceived in revised form 6 March 2012ccepted 23 March 2012

a b s t r a c t

There is emerging interest in the production of biomethane as a biocombustible either through anaerobicdigestion of biomass and/or energy crops. Cynara cardunculus L. is a crop with high biomass yields usedin the production of bioenergy, with the seeds being used for biodiesel production and the remainingbiomass used as solid fuel in biomass plants. This work aims to present results concerning the obtainable

eywords:iomassiogasnergy cropsynara cardunculus L

methane yield of Cynara stalks when submitted to anaerobic digestion, and the effects of selected pre-treatments (mechanical, thermal and thermochemical). For this purpose, two batch anaerobic digestionexperiments (Trial I and Trial II) were performed. Minimum cumulative methane was achieved for theuntreated substrate yielding 0.3 L CH4/g VS0. A maximum methane yield of 0.5–0.6 L CH4/g VS0 wasachieved depending on the selected pre-treatment. Thermochemical pre-treatment using NaOH was

ient

revealed to be a very effic

. Introduction

There is emerging interest in finding new energy crops forhe production of biomethane through anaerobic digestion (AD).resently, Cynara cardunculus L. is cultivated as an industrial cropith high biomass yields, and requires less irrigation and fertilizer

or cultivation than other crops grown in Mediterranean coun-ries (Fernández et al., 2006; Gominho et al., 2011). The averagennual biomass production varies from 15 to 20 tonne of biomasser hectare depending on soil and rainfall conditions. Cynara hasoughly 11% moisture content and the following biomass parti-ioning: 40% stalks, 25% leaves and 35% capitula (Gominho et al.,008, 2001). Cynara has been proposed as a favourable feedstockor the production of biofuel and bioenergy. As its lipid-rich seedsave a desirable composition for biodiesel production (Sengo et al.,010). The remaining component is usually burned as solid fuel iniomass power plants for energy and heat production (Fernándezt al., 2006; Gominho et al., 2011). Cynara stalks may represent

new suitable feedstock for AD, increasing the biofuel potentialf this crop. Anaerobic digestion is an industrial process applied torganic waste treatment with several environmental and energetic

dvantages over other forms of processing and especially when it isntegrated within the agricultural sector (Chynoweth and Isaacson,987; Möller and Stinner, 2009; Prochnow et al., 2009). In addition

∗ Corresponding author.E-mail address: jgominho@isa.utl.pt (J. Gominho).

926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.indcrop.2012.03.029

hydrolysis method.© 2012 Elsevier B.V. All rights reserved.

to the organic waste treatment, there is an emerging interest in theproduction of biomethane as a biocombustible through anaerobicdigestion 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 cropsto the anaerobic digestion of cattle dung, or the anaerobic diges-tion of energy crop residues with the addition of partially digestedcattle dung or digested sewage sludge, enhanced both biogas pro-duction and methane yield (Chanakya et al., 2009; Yadvika et al.,2004). However, several problems have been reported related tothe use of energy crops (Lindorfer et al., 2007; Prochnow et al.,2009). Due to the lignocellulosic structure, these substrates are notfully hydrolyzed. For years, hydrolysis of lignocellulosic materialhas been regarded as the rate limiting step in anaerobic diges-tion 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 andBanks, 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 wrappingof long grass particles around moving devices can cause failures inthe operation of the biogas plant (Prochnow et al., 2009).

Previous studies have suggested that a pre-treatment stepprior to feeding the digester increases hydrolysis rate (Angelidakiand Ahring, 2000; Chynoweth and Isaacson, 1987; Hendriks andZeeman, 2009; Lehtomäki et al., 2006; Palmowski and Müller,

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000). Recently, Cynara stalks were tested for ethanol produc-ion (Ballesteros et al., 2008) pairing a low dose sulphuric acid0.1–0.2%, w/v) thermal–chemical pre-treatment with enzymaticydrolyses. The achieved ethanol yields were relatively low for

ndustrial applications, thus requiring further improvements. Annteresting perspective concerning the growing of Cynara, is thentegration with biodiesel and biogas production plants in Mediter-anean countries.

The goal of the research performed on C. cardunculus L. is toncrease the knowledge concerning the use of this promising indus-rial crop for biogas production in Mediterranean countries orountries with similar edapho-climate conditions. For this purposef this study, stalks were chemically characterised and two batchnaerobic digestion experiments were performed (Trial I and TrialI). The effect of mechanical, hydrothermal and thermochemicalwith sodium hydroxide) pre-treatments are compared, makingossible an evaluation regarding the energetic potential of thiseedstock.

. Materials and methods

.1. Substrates and inoculums

The C. cardunculus L. used in this study was collected from aedagogic field (BioEnergISA) built at the ISA campus (Institutouperior de Agronomia, Lisboa) in October 2008. The leaves andhe capitula were removed and only the stalks were used. Thetalks were milled to a particle size of 10 mm. Further mill pro-essing reached fractions lower than 40 mesh. After sieving, the0–60 mesh fractions and the 10 mm material was stored in a drylace at environment temperature until required. During the exper-

ment the required raw-material was stored in an oven at 100 ◦C,o keep the material dry. This material is specified as substrate (S)n the anaerobic digestion experiments. Two anaerobic digestionxperiments were performed using two different substrate par-icle sizes: 40–60 mesh (Trial I) and 10 mm (Trial II). The sludgehich provided the inoculums (I) for the experiments was collected

reshly from a waste water treatment plant digester (WWTP, ETARe Chelas, Lisboa), and stored in 5 L capacity plastic containers.

.2. Experimental procedure

Anaerobic digestion experiments were carried out using batcheactors with a working volume of 2.0 L and were operated under aesophilic temperature of 37 ± 2 ◦C. The temperature was main-

ained using a water-heating jacket, in which the water wasirculated through an external heater. The reactors were connectedo graduated cylindrical gas holders for measuring biogas produc-ion, with an incubation time of 32 days. In these experiments threeeactors were used: B0 containing inoculum, B1 containing inocu-um and pre-treated substrate and B2 containing inoculum andntreated substrate. Reactor contents were mixed daily for 10 minsing a mechanical stirrer. The experiment was conducted usinghe protocol described in Table 1. The high volume of inoculum wasdopted to maintain a constant pH during the anaerobic digestionrials and its characterisation is shown in Table 2.

.3. Trial I

In this experiment the substrate, Cynara stalk with a particleize of 40–60 mesh, was submitted to thermal pre-treatment usingistilled water (autohydrolysis). The thermal pre-treatment was

arried out in a set of inox steel reactors with a working volumef 300 ml using a fixed temperature of 160 ◦C, a total solids con-ent (TS) of 12.5% and a reaction time of 30 min. The reactors wereightly closed and placed together in a rotating structure within

Products 40 (2012) 318– 323 319

an oil heated bath (160 ◦C). Following pre-treatment the pH wasmeasured. The resulting pre-treated material was mixed with theinoculum and fed directly into the reactors.

2.4. Trial II

A thermochemical pre-treatment was applied over Cynara stalkswith a particle size of 10 mm. In this case, a fixed temperature of160 ◦C, 12.5% of TS, and a reaction time of 20 min were used. In addi-tion, NaOH (1.4%, w/v) solution was used instead of distilled water.Following pre-treatment the pH was measured. The resulting pre-treated material was mixed with the inoculum and fed directly intothe reactors.

2.5. Analytical methods

The chemical composition of the whole stalks, pith and depictedmaterial was determined using Tappi standard methods in relationto ash, moisture content, extractives, lignin and polysaccharides asdescribed in Gominho et al. (2001). Cynara stalks were also charac-terised in terms of total solids (TS), volatile solids (VS), total organiccarbon (TOC), minerals (Ca2+, Na+, Mg2+, K+, Mn2+, Cu2+ and Zn2+),nitrogen (N-NH4

+, NK and Ntotal) and pH according to StandardMethods for Examination of Water and Wastewater. The determi-nation of TS, VS and pH in the reactors (B0, B1, B2) was measuredat the beginning and at the end of the experiments.

Total carbon content was evaluated as suggested by Zucconi andDe Bertoldi (1987). Pre-treated substrate was analysed in terms ofholocellulose and �-cellulose according to the method describedby Rowell et al. (2005). Daily biogas production was measuredusing 5000 ml and 2000 ml graduated cylinders with reversiblegas–liquid displacement holders. Methane, carbon dioxide andoxygen determinations were obtained by a Varian 3800 gas chro-matograph (carrier gas nitrogen) equipped with a packed column(Porapak-s 80–100 mesh, T = 50 ◦C), a TCD detector (T = 150 ◦C), andmaintaining the injection chamber at 60 ◦C.

The methane yield was calculated by subtracting the amountof methane produced by the control from the methane productionof each reactor and dividing the difference by the mass of VS inthe substrate fed to the reactor. The following formula was used toestimate the VS reduction:

VS reduction (%) = 100(

M0 − (Mn − Mi)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); andMi is the mass of VS in the inoculums reactor B0 (control) at the endof fermentation (g).

3. Results and discussion

3.1. Chemical characterisation of C. cardunculus L. stalks

Among the constituents of Cynara stalks, polysaccharides com-prise the major fraction (57.4%). Cellulose and hemicellulosesrepresent 36.3% and 16.3% of the total mass, respectively (Table 3).In terms of extractives compounds, the value measured in oursample (6.4%) is 7% lower than the values already published withthe same raw-material (Fernández et al., 2006; Gominho et al.,2001, 2008). This difference is possibly explained by heterogeneitypresent in lignocelulosic materials. The stalks presented a lowernitrogen content (NK = 0.3%) resulting in a high C/N ratio value

which is unfavourable for the AD process. This factor may limitbacterial growth, however the substrate can be co-digested withanother component to balance this ratio (Mshandete et al., 2004).Therefore it was decided to use a high proportion of the digested

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

Table 1Protocol used for the experimental trials.

Reactors H2O addition Substrate concentration (S) Inoculum (I) I/S ratio

(mL) (g VS L−1) (mL) (mL I/g VS S)

B0 (control) 462 NA 1538 NAB1 (pre-treatedsubstrate)B2 (untreatedsubstrate)

462 28.7 1538 116.1

Note: NA: not applicable.

Table 2Inoculums characterisation used in anaerobic digestion experimental trials.

Trial I Trial II

pH TS (%) VS (%TS) pH TS (%) VS (%TS)

Inoculums 6.8 2.4 70.8 6.9 2.4 41.7

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

Constituent % of TS

Volatile solids (VS) 95.7Total organic carbon (TOC) 36.6Total nitrogen (Ntotal) 0.3Kjeldahl nitrogen (NK) 0.3N-NH4

+ 0.0

MineralsCa2+ 2.3Na+ 0.2Mg2+ 0.4K+ 0.0Mn2+ 0.0Cu2+ 0.1Zn2+ 0.1

Extractives 6.4Total lignin 19.8Total polysaccharides: 57.4

Monosaccharide’s composition % of SugarsGlucose 36.3Xylose 16.3Mannose 1.4

np

3

yVultt

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

Untreatedsubstrate

Thermal pre-treatedsubstrate

With H2O With NaOH

Holocellulose (%TS) 85 79 77�-Cellulose (%TS) 15 19 24

TR

Arabinose 1.3Galactose 1.4Rhamnose 0.6

itrogen-rich sludge (inoculum), specifically: I/S = 116.1, whichrovided a favourable C/N ratio (C/N = 14).

.2. Anaerobic digestion trials

The results achieved in Trial I in terms of cumulative methaneield were: 0.62 L/g VS0 for B1 (pre-treated substrate) and 0.50 L/gS0 for B2 (untreated substrate) (Fig. 1 and Table 4). These fig-

res are within the range of values previously reported in the

iterature by other energy crops used as substrate (without pre-reatment) in AD, i.e. 0.63 L/g VS0 for ryegrass, 0.56 L/g VS0 forimothy 0.33 L/g VS0 for wheat straw, 0.55 L/g VS0 for cocksfoot

able 4esults of the anaerobic digestion experiment.

Reactors pH0 (substrate)

Trial I (40–60 mesh; 160 ◦C; 30 min) B1(H2O) 4.4

B2 6.3

Trial II (10 mm; 160 ◦C; 20 min) B′1(NaOH) 7.9

B′2 6.5

Hemicellulose (%TS) 70 60 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 sub-strate in reactor B1 doubled the cumulative methane yield incomparison with the untreated substrate in reactor B2 (Fig. 2 andTable 4). A cumulative methane yield of 0.59 L/g VS0 and 0.31 L/gVS0 was achieved for the pre-treated and untreated substratesrespectively. The enhancement achieved is also shown in terms ofVS reduction, 44% in B1 and 21% in B2 reactors.

The thermal pre-treatments (B1) enhanced the hydrolysis ofhemicelluloses because the methyl groups released reacted to formacetic acid, resulting in a decreased pH value and promotion ofthe hydrolysis reaction of the sugars as described by differentresearchers (Caparros et al., 2007; Donghai et al., 2006; Garroteet al., 2007; Liu and Wyman, 2005; Yang and Wyman, 2008). InTrial II the decrease in pH was not realised due to the NaOH buffersolution. The holocellulose content for the untreated substrate was85% (TS) while for the thermal pre-treated substrates with NaOHand H2O the values decreased to 77% and 79% respectively (Table 5).

Despite the high cumulative methane yields achieved, there isstill an amount of organic matter expressed as volatile solids in theresidual substrate (VSn). Assuming that lignin is not degraded inanaerobic conditions, and that in Cynara stalks it represents approx-imately 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 inTrial II, of the original substrate remaining for further degradation.

pHi (mixture) pHf VS reduction (%) Specific methaneyield (L/g VS0)

6.9 7.2 56.2 0.627.1 7.3 41.7 0.50

6.9 7.1 43.9 0.596.7 6.8 20.8 0.31

I. Oliveira et al. / Industrial Crops and Products 40 (2012) 318– 323 321

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

Fig. 2. Cumulative methane yield during the

Fig. 3. Volatile solids reductio

anaerobic digestion experiment Trial II.

n in Trial I and Trial II.

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

Fig. 4. Schematic of energy flow for methane

Table 6Gross energy content of C. cardunculus L. stalks calculated from their chemicalcomposition.

Sample Total proteina

(%TS)Total carbohydrates(%TS)

Calculatedenergy (MJ kg−1)b

Cynara stalks 1.9 56.6 9.4

3

((plr(sttevcmp

sbdCio

4

bta

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

.3. Energy conversion balance

Comparing the results achieved in terms of VS reductionFig. 3), energy conversion (Fig. 4) and cumulative methane yieldFigs. 1 and 2) it is possible to estimate the efficiency of there-treatments used in the anaerobic experimental trials. Granu-

ometric reduction of Cynara stalks increased VS reduction by 20%,esulting in an increment in methane yield from 0.5 L CH4/g VS0Trial I – B2) compared to 0.3 L CH4/g VS0 (Trial II – B2). Table 6hows the estimated energy in the form of carbohydrates and pro-ein contained in the Cynara stalks and in Fig. 4 the conversion ofhat energy into methane is outlined. As is evident, in Trial I, B1nergy conversion was 24% higher than in B2 and in Trial II thisalue is 92% higher. This is supported by the VS reduction andumulative methane yield achieved after anaerobic digestion. Ther-al pre-treatment improved the conversion of carbohydrates and

roteins to methane.The methane yields achieved show that Cynara stalks can be a

uitable substrate for AD. Assuming a cumulative methane yieldetween 0.5 and 0.6 L CH4/g VS0 achieved in trial I and a crop pro-uctivity of 15 tonne/ha (40% as stalks) (Gominho et al., 2008, 2001),ynara substrate could contribute to a potential energy production

n the range of 29.9–35.9 MWh/ha, assuming a biogas heating valuef 9.97 kWh/m3 (Ahrens and Weiland, 2004).

. Conclusions

C. cardunculus L. is already a promising energy crop for theioenergy field in Mediterranean countries. The results showhat Cynara stalks are a good substrate for Mediterranean biogasnd methane production. Cynara stalks have a specific methane

production from C. cardunculus L. stalks.

yield that is higher than 0.3 L CH4/g VS0 depending on thepre-treatment applied. Mechanical and thermal pre-treatmentwith NaOH showed a positive effect in terms of carbon solubili-sation, VS reduction, biogas yield and methane yield increment.However, there is still work to be done in terms of choosing themost suitable pre-treatment. There is also an interest in using greenstalks instead of dried ones allowing two annual harvests, as well asusing the entire crop material (stalks, leaves and seeds) for methaneproduction (Gunaseelan, 2008). Additionally, there is potential forcombining the two processes of biodiesel and biogas productionthrough co-digestion of glycerol and de-oiled cake with other sub-strates. Finally, the application of the digestate as fertilizer to growC. cardunculus L. should be also considered to reduce costs andimprove Cynara productivity.

Acknowledgements

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

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