Giant cane (Arundo donax L.) can substitute traditional energy crops in producing energy by anaerobic digestion, reducing surface area and costs: A full-scale approach

Bioresource Technology 218 (2016) 826–832

Contents lists available at ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Giant cane (Arundo donax L.) can substitute traditional energy crops in producing energy by anaerobic digestion, reducing surface area and costs: A full-scale approach Luca Corno a, Samuele Lonati a, Carlo Riva a, Roberto Pilu b, Fabrizio Adani a,⇑ a b

Gruppo Ricicla, Biomass and Bioenergy Laboratory, DiSAA, University of Milan, Via Celoria 2, 20133 Milan, Italy Genetic Laboratory, DiSAA, University of Milan, Via Celoria 2, 20133 Milan, Italy

h i g h l i g h t s
Arundo donax was used in partial substitution of corn to produce biogas.
Substitution did not led to modification in biogas and electricity production.
The use of A. donax reduced both area to produce biomass and cost.
The use of only A. donax reduced cost for the electricity production of 22%.

a r t i c l e

i n f o

Article history: Received 7 June 2016 Received in revised form 10 July 2016 Accepted 11 July 2016 Available online 12 July 2016 Keywords: Anaerobic digestion Arundo donax (Giant cane) Electric energy cost Energy crop Renewable energy

a b s t r a c t Arundo donax L. (Giant cane) was used in a full-scale anaerobic digester (AD) plant (power of 380 kW hEE) in partial substitution for corn to produce biogas and electricity. Corn substitution was made on a biomethane potential (BMP) basis so that A. donax L. after substitution accounted for 15.6% of the total mix-BMP (BMPmix) and corn for 66.6% BMPmix. Results obtained indicated that Giant cane was able to substitute for corn, reducing both biomass and electricity production costs, because of both higher biomass productivity (Mg total solid Ha1) and lower biomass cost (€ Ha1). Total electricity biogas costs were reduced by 5.5%. The total biomass cost, the total surface area needed to produce the energy crop and the total cost of producing electricity can be reduced by 75.5%, 36.6% and 22%, by substituting corn completely with Giant cane in the mix fed to the full-scale plant. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Anaerobic digestion is a well-known and widely diffused biotechnology used to convert biomasses and wastes into biogas (Marcato et al., 2008; Weiland, 2010), contributing to the substitution of fossil fuels and to the production of renewable energy (Sheldon, 2014; Capodaglio et al., 2016). The main products obtainable from anaerobic digestion processes are electricity and heat by the direct combustion of biogas (Weiland, 2010) and/or bio-methane after biogas upgrading (Corneli et al., 2016; Weiland, 2010). Anaerobic digestion plants are widely diffused in Europe, producing 155,591 GWh of total energy; most of this energy is produced in Germany, the United Kingdom and Italy, i.e. 77% of the total (Capodaglio et al., 2016; Raboni and Urbini, 2014). Italy in ⇑ Corresponding author. E-mail address: [email protected] (F. Adani). http://dx.doi.org/10.1016/j.biortech.2016.07.050 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.

particular, is the third ranking European producer of biogas with a total amount of energy produced of 21,113 GWh. In the past, large financial benefits provided by governments to the bioenergy sector, pushed for the biogas industry to develop very fast, above all in agricultural contexts (Schievano et al., 2015). More recently, the reduction of benefits is slowing down the construction of new biogas plants, posing uncertainty for the future of biogas (Schievano et al., 2015). Moreover, the low price currently characterizing the fossil fuel market (Oil Price, 2016) makes bioenergy less competitive and of less appeal, since fossil fuel prices dropped by about 70% from 2004 to 2016 (IBTimes, 2016). As consequence of that, biogas cost reduction has become a necessity for the sector to survive. Energy crops are nowadays the most widely employed biomass for biogas generation, above all in Germany (Weiland, 2010). In other countries, such as Italy, energy crops are much less used although they represent about 30–40% of the total feed mix (on a wet weight basis) (Riva et al., 2014). Crop energy accounts for

L. Corno et al. / Bioresource Technology 218 (2016) 826–832

about 33% of the total biogas production cost (Schievano et al., 2015) making its use non-sustainable without the earlier large subsidies. On the other hand, energy crops contribute largely (more than 70%) to the total biogas produced in biogas plants, allowing large-scale plants (1 MW) to be proposed. Large scale plants reduce total biogas costs (Riva et al., 2014), contributing to the economic sustainability of AD. Therefore, the development of cheap energy crops needs to be encouraged (Capodaglio et al., 2016) taking into consideration, also, that such a crop could be cultivated on marginal lands in order to reduce the conflict with food production (Cherubini, 2010; Sheldon, 2014). Arundo donax L. or Giant cane is a suitable feedstock for biogas production because it is cheap (production cost of 700– 1000 € Ha1 y1) (Corno et al., 2015) and highly productive (40– 70 Mg total solid Ha1) (Corno et al., 2014). Low inputs are required for its cultivation (Corno et al., 2014), which suggests that Giant cane can be cultivated on marginal lands or on lands normally not dedicated to food-crops (e.g. contaminated land to be remediated) (Lewandowski et al., 2003). Although Giant cane has been reported to be less productive (Khudamrongsawat et al., 2004) than traditional energy crops in term of bio-methane production per total solids (TS) unit (Corno et al., 2014; Ragaglini et al., 2014), available data indicate a potential bio-methane production (BMP) of 251 ± 29 Nm3 CH4 Mg1 TS1 (Corneli et al., 2016; Corno et al., 2015; Ragaglini et al., 2014), comparable to those of traditional energy crops. As consequence, Arundo donax has been proposed to substitute for traditional energy crops in producing biogas. Unfortunately, not many data about Giant cane performance during biogas production exist and the few available data refer to batch studies (Ragaglini et al., 2014). Only recently, Corno et al. (2015) published in this scientific journal a paper in which Giant cane silage was compared to corn silage in a continuously stirred tank reactor (CSTR) at lab-scale. The results obtained indicated that the biogas production was lower for Giant cane than for corn because the former did not contain starch. Nevertheless, taking into consideration the high biomass yields per hectare for A. donax, the total producible biomethane was much higher for this crop than for corn, i.e. 12,292 Nm3 CH4 Ha1 and 4549 Nm3 CH4 Ha1. The consequence of that was that the calculated costs to produce gas and electric energy with Giant cane were definitely much lower than those obtainable with corn and other energy crops. Data obtained were on a lab-scale and they needed to be confirmed by a full scale plant approach. The aim of this work was to monitor at a full-scale anaerobic digester plant (380 kW hEE of power) the ability of A. donax to substitute for corn silage in daily AD feeding.

2. Materials and methods 2.1. Biomass supply Corn silage and Giant cane silage were produced in a farm sited in northern Italy (CR Province, Italy, 45° 150 N, 9° 580 E). Corn employed in this work came from two different trenches filled up respectively with 1st crop corn and 2nd crop corn. Both biomasses were sown and cultivated in 2014 with standard agronomic practices normally used in this area (Corno et al., 2015). Phytosanitary interventions against weeds were performed before and after corn sowing; during the summer season (July) an ad hoc intervention against European Corn Borer (Ostrinia nubilalis) was carried out. Corn was fertilized with digestate (50 m3 Ha1) and urea (450 kg Ha1 divided into two interventions: before corn sowing and during corn growth). Because of climate trends in the summer of 2014, which was particularly rainy and cold, the harvest of

827

the 2nd crop of corn was done at the end of August with a low amount of grain in the silage. Corn yields were of 60 Mg wet weight (ww) Ha1 production for both the harvests, corresponding to 19 Mg total solids (TS) Ha1 and 19.8 Mg TS Ha1, for 1st crop and 2nd crop, respectively. After the harvest, corn biomass was ensiled in trenches without starters or inoculum following the standard procedure (Corno et al., 2016). Giant cane biomass was obtained from a full field crop established on April 2013 by transplantation of micro-propagated plant (Arundo Italia, Pescara, Italy). The soil was ploughed and harrowed; plant density for transplantation was of 10,000 plants Ha1. During the first year, irrigation was applied during the growing season to supplement rainfall, in order to facilitate plant establishment. One fertilization with digestate (150 kg N Ha1) was applied after the transplantation while no phytosanitary interventions were needed. At the end of the growing season (end of October) Giant cane was harvested. During the second year of cultivation (2014) no relevant agronomic interventions were applied, except for crop fertilization with digestate and for crop harvesting at the end of October. Giant cane production in the second year was of 110 Mg ww Ha1, corresponding to 33 Mg TS Ha1. Biomass was ensiled in a trench with the same technique used for corn (Corno et al., submitted). The other biomasses considered in this work were provided by the farm activities; bovine manure was stored in an open collection tank while chicken manure was collected and stored in a sheltered structure. Corn flour was also employed, to integrate the feeding mixture used in the anaerobic digestion plant. The solid fraction of digestate, deriving from centrifugal separation of digestate, was accumulated in a concrete pit before being used in the feeding mix.

2.2. Anaerobic digestion plant A single continuously stirred tank reactor (CSTR) with a volume of 2660 m3 composed the anaerobic digestion plant (380 kW hEE of Power), working at a temperature of 38–40 °C and with a hydraulic retention time (HRT) of 66 days. Solid biomasses of feeding mixture were loaded directly into the digester by using a rotary spiral feeder while the bovine slurry, stored in an open tank, was pumped separately; no mixing tank was provided in the plant design. The effluent (digestate) from the digester was separated into liquid and solid fractions by using a centrifugal separator: the liquid fraction was stored in a storage tank and then employed as fertilizer on fields while the solid fraction was used as biomass in the feeding mixture as described above. Giant cane silage was used in partial substitution for corn silage, while keeping constant all the other biomasses. Because biomethane production can vary a lot for the same kind of biomass (e.g. corn) depending on moisture content and biomass quality, energy crop substitution was calculated taking into consideration the potential bio-methane production (BMP) (Nm3 d1) instead of fresh weight or total solid weight (Mg d1). To do so, all biomasses characterizing feeding mixtures before and after Giant cane introduction into the mix were characterized for BMP. Taking into consideration HRT data, a washout period, during which Giant cane was used in the mix, of 66 days was considered. After this period, the anaerobic process was considered stable with the new mix. Consequently, full-scale plant monitoring was divided into three phases: i. anaerobic stable phase of 15 d characterized by feeding mix containing only corn as energy crop (Mix 1); ii. washout phase of 66 d (HRT) during which corn was partially substituted with Giant cane; iii. anaerobic stable phase of 16 d,

828

L. Corno et al. / Bioresource Technology 218 (2016) 826–832

characterized by feeding mix with both Giant cane and corn (Mix 2). During monitoring, twice per week (one per week for the wash out phase) samples of both feeding mixtures and effluents were taken and analysed. Moreover, data on power generation, biogas production and methane percentage in biogas were directly measured in the full-scale plant. All biomass and digestate samples were stored at 4 °C for further analysis.

gas plants of 0.5 MW, located in the area in which this work was done.

2.3. Chemical analysis on raw materials, feeding mixtures and digestates

The chemical characteristics of biomasses used to prepare the feeding mixture, i.e. bovine slurry, chicken manure, corn flour, digestate solid fraction, corn silage and Giant cane silage are shown in Table 1. Corn silage was characterized by low pH and high content of VFA (Corno et al., 2015). Unfortunately, corn used in the mix before Giant cane addition (Mix 1) (Tables 1 and 2) was different from that used in the mix using Giant cane (Mix 2), because different trench silos were used depending on availability on the farm. These factors led to different potential bio-methane production (Table 2), as later discussed. Giant cane was characterized by higher pH, VFA content and alkalinity than corn silages due to the different ensilage processes which occurred (Corno et al., 2016). Chemical differences in crop energies, i.e. corn vs. Giant cane, determined changes in the mixes, i.e. pH value was higher for Mix 2 than Mix 1; on the other hand, VFA and alkalinity contents were lower for Mix 2 than Mix 1. Total solids content was higher for Mix 2 than Mix 1 because of both the higher amount of biomass employed in this Mix and higher TS content of corn used in Mix 2 (Table 2). More interesting were the data regarding potential biogas and bio-methane production (Table 2). Corn silage used in the two mixes differed each other for ABP and BMP. In particular, corn used in Mix 2 showed a much lower ABP and BMP than those for corn used in the starting mix, reflecting differences in chemical composition (Table 1). Corn used in Mix 2 showed lower ABP/BMP (ABP of 449 ± 9 Nm3 Mg1 TS1 and BMP of 242 ± 6 Nm3 Mg1 TS1) than those generally reported in the literature for corn, i.e. ABP of 614 ± 54 Nm3 Mg1 TS1 and BMP of 344 ± 27 Nm3 Mg1 TS1 (Corno et al., 2015; Schievano et al., 2015), unlike the data reported for the corn of Mix 1 (ABP of 577 ± 0 Nm3 Mg1 TS1 and BMP of 312 ± 1 Nm3 Mg1 TS1). This was due to low grain presence in the corn silage of Mix 2, both as reported by the farmer and detected directly in the field. The high TS content of corn used in Mix 2 seems to confirm this trend. Giant cane ABP/BMP (ABP of 501 ± 17 Nm3 Mg1 TS1 and BMP of 281 ± 10 Nm3 Mg1 TS1) was consistent with previous findings that reported an average BMP of 251 ± 29 Nm3 Mg1 TS1 (Corneli et al., 2016; Corno et al., 2015; Ragaglini et al., 2014). A lower ABP/ BMP for Giant cane than that for corn silage has often been described in the literature and is ascribed to the fact that this energy crop is composed entirely of lignocellulose structures and there is no grain content because of the plants’ sterility (Ragaglini et al., 2014; Corno et al., 2015). Giant cane ABP/BMP were similar to those of the corn in Mix 2, confirming that this latter was characterized, above all, by lignocellulose material and by the almost complete absence of grains containing starch. All the other organic matrices showed ABP/BMP in line with those expected, reflecting chemical composition (Tables 1 and 2). In particular, biomasses used for both Mix 1 and Mix 2, did not show appreciable differences as regards ABP/BMP (Table 2). Despite the different composition of Mix 1 and Mix 2, the ABP (and BMP) of both mixtures showed similar and not statistically different (p < 0.05) data, that were of 557 ± 43 Nm3 Mg1 TS1 and of 518 ± 33 Nm3 Mg1 TS1 (BMP of 289 ± 29 Nm3 Mg1 TS1 and of 274 ± 17 Nm3 Mg1 TS1), respectively.

The determination of total solids (TS), volatile solids (VS), total nitrogen (TN) and total ammonia (TAN), Volatile fatty acids (VFA) and alkalinity (TA) were conducted according to standard procedures (APHA, 1998). The anaerobic biogas potential and the biomethane potential (ABP and BMP) tests were determined by batch test as previously described by Schievano et al. (2009). Briefly, biomass samples were incubated in batch at 37 °C with a stable inoculum coming from an anaerobic digestion plant fed with energy crops; constant measurements of biogas production were performed by sampling it with 60 mL syringes. Qualitative analyses of biogas were performed on sampled gas determining CH4 percentage (v/v) by gas-chromatograph (Micro GC 3000, Agilent Technology, Les Ulis Cedex, France). All chemical analysis and ABP/BMP tests were performed in triplicate. Anaerobic digestion performance was evaluated in terms of biomethane yield (BMY) calculated as reported by Schievano et al. (2011) with the following equation:

BMYð%Þ ¼

SMP  100 BMPin

ð1Þ

in which BMPin represents the bio-methane potentials of inputs mixtures (Nm3 CH4 Mg1 TS1) and SMP is the specific methane production obtained during CSTR trials (Nm3 CH4 Mg1 TS1). 2.4. Total costs of producing biomass, biogas and electricity Total costs for producing biogas and electricity were calculated by considering the costs of producing biomasses and costs of transforming biomasses into products, i.e. biogas and electricity. By doing so, biomasses’ costs were evaluated directly in the field, taking into consideration costs for agronomic operations: ploughing, harrowing, sowing or plant transplanting, weeding, chemical treatments, irrigations, fertilizations, harvesting, silage processing and digestate use when necessary, and costs to buy materials, i.e. seeds or plants, herbicides, insecticides, chemical fertilizers and irrigation. For the Giant cane some of these costs were necessary only for the first year (ploughing, harrowing, sowing or plant transplanting, irrigation, weed control and plant buying) and so they were spread over 12 years (Angelini et al., 2009). The data referring to the biomass yields were corrected with respect to mass losses occurred during the silage process (Corno et al., 2015) getting the real data about the available biomass Ha1 producing biogas or electricity. The costs to produce biomass were of 1800 € Ha1 and 700 € Ha1 for corn and Giant cane respectively, while for corn flour the price was of 190 € Mg1 TS1. Bovine slurry, chicken manure and bovine solids (separated) were considered to have nil cost, as they were already produced by the farm or delivered free. A lower methane heating value of 31.6 MJ Nm3 (8.79 kW h Nm3 CH1 4 ) was employed for energy content determination (Schievano et al., 2015). The biogas and electric energy unit costs (costs due to biomass production) were calculated as reported by Schievano et al. (2015). Costs to transform biomasses into electricity were those reported by Riva et al. (2014) for a bio-

3. Results and discussion 3.1. Chemical characterization of biomasses and feeding mixtures

829

L. Corno et al. / Bioresource Technology 218 (2016) 826–832 Table 1 Chemical characterization of biomasses composing feeding mixtures. pH

a

TS % (w/w)

VS % TS

TAN g kg1

TKN g kg1

TS

TS

VFA g CH3COOH kg 1 WW

Alk g CaCO3 kg 1 WW

VFA/Alk

Corn silage Bovine slurry Chicken manure Corn flour Digestate solid fraction Mix 1a

3.36 ± 0.42 7.54 ± 0.66 7.19 ± 0.05 – 8.99 ± 0.01 4.87 ± 0.13

31.6 ± 2.4 2.8 ± 0.02 28.9 ± 1.4 88.8 ± 2.5 22 ± 0.6 19 ± 0.5

95.2 ± 0.6 76.7 ± 1 69.3 ± 3.4 98.5 ± 0.9 87.7 ± 0.6 90.9 ± 1.5

3.35 ± 0 38.7 ± 0.2 16.7 ± 0.6 – 10.2 ± 0.2 5.98 ± 0.42

10.5 ± 0.5 68.1 ± 0.2 50.5 ± 4.6 1.44 ± 0.14 27 ± 0.5 18.8 ± 0.4

10.8 ± 0.1 6.55 ± 3.66 15.1 ± 0.2 – 0.21 ± 0.03 8.41 ± 0.16

10.7 ± 3.82 10.4 ± 3.1 16.6 ± 0.1 – 7.96 ± 1.08 8.45 ± 0.38

1 0.63 0.91 – 0.03 0.99

Corn silage Arundo silage Bovine slurry Chicken manure Corn flour Digestate solid fraction Mix 2a

3.74 ± 0.39 4.66 ± 0.41 7.39 ± 0.03 8.12 ± 0.68 – 8.62 ± 0.02 5.81 ± 0.03

34.7 ± 1.1 30.4 ± 2.1 3.3 ± 0.87 29.1 ± 1.9 88.9 ± 2.6 21.5 ± 1.4 22.7 ± 0.8

96.9 ± 0.1 93.3 ± 0.8 83.6 ± 0.4 67.8 ± 2.2 98.7 ± 0.4 86.8 ± 0.5 89.4 ± 1.1

1.5 ± 0.5 – 26 ± 0.8 21 ± 1.1 – 10.1 ± 0.5 8.33 ± 0.62

11.2 ± 0.4 8.87 ± 0.12 54.4 ± 3.2 45.9 ± 2.7 1.51 ± 0.23 27.1 ± 0.4 21.3 ± 0.13

18.5 ± 0.2 11.8 ± 5.1 6.92 ± 1 46.8 ± 1.2 – 0.22 ± 0.04 11.5 ± 0.4

29.3 ± 0.4 11 ± 3.4 11.4 ± 1.6 48.5 ± 0.9 – 9.76 ± 0.7 11.1 ± 1.4

0.63 1.07 0.6 0.96 – 0.02 1.04

Data reported where calculated taking into consideration biomasses data and mix composition.

Table 2 Biomasses compositions, biogas potentials (ABP), biomethane potentials (BMP) and, daily biogas and biomethane productions. Input TS MgTS d1

ABP Nm3 Mg1 TS

CH4 %

Corn silage Bovine slurry Chicken manure Digestate solid fraction Corn flour Mix 1a

15 18 2 2.5 0.09 37.6

4.74 0.5 0.58 0.55 0.08 6.45

577 ± 0 246 ± 7 290 ± 8 91 ± 1 578 ± 2 557 ± 43

54 ± 0.2 71.3 ± 1 65.3 ± 2.1 59.8 ± 0.9 53 ± 0 51.9 ± 0.8

Corn silage Arundo silage Bovine slurry Chicken manure Digestate solid fraction Corn flour Mix 2a

13 3 18 2 2.5 0.09 38.6

4.51 0.91 0.59 0.58 0.54 0.08 7.22

448 ± 9 501 ± 17 267 ± 98 307 ± 7 99 ± 0 581 ± 4 518 ± 33

54 ± 0.6 56 ± 0.4 77.8 ± 0.9 62 ± 0.2 61.2 ± 0.7 53 ± 0 53 ± 0.2

BMP Nm3 Mg1 TS

ABP Nm3 d1

ABP %

BMP Nm3 d1

BMP %

312 ± 1 175 ± 6 189 ± 8 54.4 ± 1 306 ± 1 289 ± 23

2735 124 168 50.1 46.2 3123

87.6 3.97 5.37 1.60 1.48

1477 88.4 109 29.9 24.5 1729

85.4 5.11 6.33 1.73 1.42

242 ± 6 281 ± 10 208 ± 76 190 ± 4 60.6 ± 0.7 308 ± 2 274 ± 17

2021 457 159 179 53.2 46.5 2915

69.3 15.7 5.44 6.13 1.83 1.59

1091 256 123 111 32.6 24.6 1639

66.6 15.6 7.53 6.76 1.99 1.5

Data were determined on feeding mixtures recreated taking into consideration the ratio of every biomass.

10000

60

9000

55 50

8000

45 7000

40

6000

35

5000

30

4000

25

%

Nm3 d-1; Nm3 CH4 d-1; kWhEE d-1

a

Input WW Mg WW d1

20

3000

15 2000

10

1000

Mix 1

Wash-out phase

Mix2

0

5 0

1

6

11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 Days Biogas

Bio-methane

Electric Energy

CH4

Fig. 1. Biogas, bio-methane, power generation and CH4 percentage recorded in the plant during the monitoring phases.

Taking into consideration BMPs data (Nm3 Mg1 TS1) and total dry weights (Mg TS d1) used daily to prepare Mix 1 and Mix 2, daily BMPs for each biomass and for both Mixes were calculated (Nm3 d1) (Table 2). Data indicated that BMP from energy crops contributed greatly and equally to the total BMP of the Mixture,

even if corn of the Mix 2 was partially substituted with Giant cane, i.e. BMPenergy crop of 85.4% BMPtotal for Mix 1 (corn) and 82.2% BMPtotal for Mix 2 (corn plus Giant cane). The contribution to the total BMP of the other biomasses was very similar for the two mixes studied (Table 2). These results were a consequence of the

168 ± 11 60 ± 8 66 ± 9 129 ± 8 44 ± 6 57 ± 8 61 59 58 275 ± 17 103 ± 14 114 ± 16 212 ± 42 74 ± 10 99 ± 14 56.1 ± 0.7 83.4 ± 1 27.1 ± 0.4 32 ± 1.2 49.2 ± 3.4 10.1 ± 0.5 0.05 0.05 0.02 15.5 ± 2.2 13.1 ± 2.5 9.76 ± 0.7 0.85 ± 0.21 0.72 ± 0.04 0.23 ± 0.04 77 ± 1.3 72.2 ± 0.8 86.8 ± 0.5 9.25 ± 0.68 5.89 ± 1.42 21.5 ± 1.4 Digestate Digestate liquid fraction Digestate solid fraction Mix 2

7.76 ± 0.39 7.94 ± 0.04 8.62 ± 0.02

– – – – – – – – – – – – – – – 60.4 ± 0.7 90.5 ± 0.6 29.1 ± 1.2 34.3 ± 0.5 52.4 ± 0.2 10.3 ± 0.2 0.01 0.09 0.07 13.5 ± 1.1 14.7 ± 0.2 10.2 ± 1 0.16 ± 0.07 1.38 ± 0.02 0.72 ± 0.05 78.9 ± 0.9 68.8 ± 1.5 85.3 ± 0.6 8.3 ± 0.03 5.38 ± 0.01 18.8 ± 0.27 Digestate Digestate liquid fraction Digestate solid fraction Wash-out phase

8.07 ± 0.02 8 ± 0.01 8.17 ± 0.03

133 ± 6 90 ± 11 53 ± 8 60 59 58 288 ± 13 213 ± 27 104 ± 16 222 ± 10 152 ± 19 91 ± 14 55.8 ± 0.2 91.9 ± 1.1 27 ± 0.5 30.7 ± 0.2 55.2 ± 1.7 10.2 ± 0.2 0.03 0.06 0.03 14.7 ± 0.4 16 ± 0.05 7.96 ± 1.08 0.47 ± 0.03 1.02 ± 0.05 0.21 ± 0.03 77.2 ± 1.2 71.2 ± 0.2 87.7 ± 0.6 8.94 ± 0.52 5.18 ± 0.49 22 ± 0.6 7.96 ± 0.08 7.85 ± 0.03 8.00 ± 0.01

TAN g kg1 VFA/Alk TA g CaCO3 kg1 WW VFA g CH3COOH kg1 WW VS % TS TS % (w/w) pH

Table 3 Chemical characterization, biogas, and biomethane potential production for digestates.

After about 15 days of monitoring of the AD-plant fed with Mix 1 corn silage was partially substituted with A. donax silage, as discussed in the previous section (Table 2). No problems were reported about mixing and/or technical operations in the plant and no reduction of biogas and power production were detected (Fig. 1). Biogas/bio-methane production was almost constant during all of the monitored period, with an average recorded biogas of 3759 ± 246 Nm3 d1 and bio-methane of 1905 ± 124 Nm3 d1. The CH4 percentage in the biogas and electric energy generation were quite constant for all the monitored period with an average values of 50.7 ± 0.3% v/v and 9003 ± 161 kW hEE d1, respectively. During the stable anaerobic phase with Mix 1 (1st–15th day) (Fig. 1), the bio-methane and power produced were of 1873 ± 205 Nm3 d1 and 8999 ± 140 kW hEE d1 respectively, data that were not statistically different from those obtained during the stable anaerobic phase operated with the Mix 2 (82th–96th day) in which A. donax partially substituted corn, i.e. biomethane production of 1963 ± 24 Nm3 d1 and power generation of 9055 ± 97 kW hEE d1 (Fig. 1) (p < 0.05; n = 16). During the wash-out phase (period 16th-81th d) the productions of biomethane and electric energy, and the percentage of methane in biogas, i.e. 1899 ± 106 m3 d1, 8990 ± 178 kW hEE d1 and 50.7 ± 0.3% v/v respectively, were comparable with those of the stable phases operated with Mix 1 and Mix 2 (p < 0.05; n = 96), indicating that the changes in the feeding mixtures did not affect the correct functioning of the plant. Potential bio-methane figures (BMP) calculated for Mix 1 (1729 Nm3 d1) and Mix 2 (1639 Nm3 d1) (Table 2) were actually lower than those obtained in the full-scale plant, i.e. Mix 1 of 1873 ± 205 Nm3 d1 and Mix 2 of 1963 ± 24 Nm3 d1, indicating that the full-scale plant performed better than the lab-tests. Fullscale plant ability to produce more bio-methane in comparison to lab tests (biomethane yield – BMY) (Schievano et al., 2011) were of 108% and of 120% for Mix 1 and Mix 2. Differences which were found could be due to the longer HRT adopted in the full-scale plant (HRT of 66 d) compared with lab-scale (HRT of 60 d). Schievano et al. (2011) reported for full-scale plants bio-methane yields (BMY) of 84–93% (Schievano et al., 2011). In particular, full-scale plants fed with animal slurry and crop energy (HRT of 57 d, lower than HRT adopted for the lab-scale test, i.e. 60 d), registered a biogas yield of 93%. Organic Loading Rate (OLR) reported for that plant was of 2.8 VS m3 d1, in line with those adopted in

TS

3.2. Biogas production and anaerobic digestion monitoring

Digestate Digestate liquid fraction Digestate solid fraction

TKN g kg1 TS

ABP Nm3 Mg1 TS

ABP Nm3 Mg1 VS

CH4 %

BMP Nm3 Mg1 TS

BMP Nm3 Mg1 VS

fact that the total amounts of energy crops composing Mixes were adapted in order to allow the maximum substitution of corn with Giant cane (depending on farm availability of this biomass) while keeping constant the total BMP in terms of methane generated per day (Nm3 d1) (Table 2). By doing so, in Mix 2 the corn contribution to the total BMP (Nm3 d1) was of 66.6% instead of the 85.4% of Mix 1 (Table 2). This reduction was replaced by Giant cane (15.6% of total Mix 2 BMP) (Table 2). Unfortunately, the impossibility to control all variables on a full-scale approach, did not allow substituting more corn with A. donax. However, the Giant cane contribution to the total BMP production above reported, was sufficient to get the first information about the ability of Giant cane to replace corn in producing biogas in a full-scale plant. The calculation of total BMP (Nm3 d1) taking into consideration BMP for each biomass and total amount of biomass used daily, gave BMP (Nm3 d1) for Mix 1 and Mix 2 of 1729 Nm3 d1 and 1639 Nm3 d1, respectively. These data represent the potential bio-methane production, and need to be confirmed at a full-scale plant.

173 ± 8 126 ± 16 60 ± 9

L. Corno et al. / Bioresource Technology 218 (2016) 826–832

Mix 1

830

831

L. Corno et al. / Bioresource Technology 218 (2016) 826–832

the present work, i.e. 2.2 and 2.42 kg SV m3 d1 for Mix 1 and Mix 2, respectively. All these data seem to suggest that full-scale plants are able to produce all the potential bio-methane (BMP) if enough HRT is provided and the biological process is well performed. The proper functioning of the anaerobic digestion at the monitored full-scale plant was confirmed by the chemical analyses of digestates (Table 3). The pH, VFA and ammonia contents, and VFA/TA were all in the optimal range (Table 3) in agreement with those reported in the literature for full-scale studies using energy crops as feedstock (Lindorfer et al., 2008; Cavinato et al., 2010; Schievano et al., 2011), and far from the values reported to inhibit the AD process (Pind et al., 2003; Chen et al., 2008). Again the digestate coming from the washout phase (Table 3) was comparable with those registered for the two stable phases monitored, suggesting that the change in feeding mixtures did not affect the chemistry and the biology of the AD process.

electricity costs (Table 4) have been calculated for the two mixes to understand the effect of substitution of corn with A. donax. The data were calculated making two assumptions: i) economic evaluations were performed considering cost and performance of corn used for Mix 1, as corn used in Mix 2 was not representative; (ii) biogas and electric energy production for the two periods considered were of 3784 ± 128 Nm3 d1 and of 9027 ± 39 kW hEE d1 respectively, calculated as average of data measured for the two monitoring periods (no statistical differences were detected between them). The first effect of the partial substitution of corn with Giant cane was the slight reduction of the total surface area required to produce the energy crop, i.e. from 91 to 89 Ha. This was due to the higher productivity (Mg TS Ha1) of Giant cane compared with corn (Corno et al., 2015) i.e. 33 Mg TS Ha1 versus 19 Mg TS Ha1, respectively. The lower surface area needed to produce the energy crop in addition to the lower costs to produce A. donax (Angelini et al., 2009; Corno et al., 2014) reduced by about 9% the total cost to produce biomass (corn plus Giant cane) in Mix 2 (Table 4). The lower costs for biomass production also influenced the cost for producing biogas and electricity. The biogas and electric energy unit costs of Mix 1 (Table 4) were consistent with those reported by

3.3. Economic aspects The substitution of corn silage with Giant cane silage was analysed from an economic point of view. Energy crop, biogas, and

Table 4 Economic evaluations of biogas and electric energy obtainable from the two feeding mixtures.

1

Hectares Biomass cost per hectare Total cost for biomass supply Biogas unit cost (energy crops)a Biogas unit cost (other biomasses) Total cost of biogasb Electric Energy unit cost (energy crops)a,c Electric Energy unit cost (other biomasses)b,c Total cost of electric energyc

Ha year € Ha1 € year1 € Nm3 € Nm3 € Nm3 € kW h1 EE € kW h1 EE € kW h1 EE

Mix 1

Mix 2

91 1800 164,250 0.12 0 0.36 0.05 0 0.18

89 1667 149,318 0.10 0 0.34 0.04 0 0.17

a

Costs referred to only energy crops supply. Total cost to produce biogas and electric energy calculated taking into consideration management/maintenance costs and depreciation costs referred to a 550 kW hEE plant (Riva et al., 2014). c Electric energy calculated with a CH4 lower heating value of 8.79 kW hEE Nm3 CH1 4 (Schievano et al., 2015). b

180000

100

160000

90

140000

80

b

0.40

0.20

0.35

0.1 0.16

0.30

0.14

50 80000

40

60000

€ m-3

60

100000

Ha year-1

70

120000

0.25

0.12

0.20

0.10

0.15

0.0

30

0.10

40000

20

0.05

20000

10

0

0 0

20

40 60 % kWhEE from Arundo

80

100

0.06 0.04 0.02 0.00

0.00 0

20

40 60 % kWhEE from Arundo

80

100

Energy crops production cost

Total biogas production cost

Energy crops production cost; experimental data

Total biogas production cost; experimental data

Surface area need

Total electric energy production cost

Surface area; experimental data

Total electric energy production; experimental data

Fig. 2. Corn substitution with A. donax in mix fed to full scale plant studied expressed as percentage of total electric energy produced by energy crops (X axis). Y axis: (a) total annual cost to produce energy crop and annual surface need for energy crop supply; (c) biogas and electric energy total costs.

832

L. Corno et al. / Bioresource Technology 218 (2016) 826–832

Schievano et al. (2015) and they were lower by 16.6% and 20% respectively for Mix 2 compared with Mix 1(data refer to the cost of producing biogas/energy coming from the energy crop only). The total costs, considering all biomasses used, management/maintenance and depreciation costs, were complexly reduced by 5.5% by passing from Mix 1 to Mix 2. Considering the results obtained in this work and assuming the possibility to progressively substitute completely the corn in the Mix 1 with Giant cane (based on BMP values), total annual costs due to energy crop production, total surface area needed for energy crop supply, total biogas and electrical energy costs for the plant studied, were calculated and reported in Fig. 2. The results obtained indicate that the total substitution of corn with Giant cane had a significant effect on energy crop cost that was cut by about 75.5% (Fig. 2a). At the same time, the reduction of the surface area for biomass supply with the use of Giant cane was of 36%, i.e. from 91 Ha (100% corn) to 58 Ha (100% Giant cane) (Fig. 2b). This reduction was probably underestimated, as for the calculation we simply considered the productivity of Giant cane in its second year (33 Mg TS Ha1), which is lower than the annual productivity reported, on average over 12 years, of 37.7 Mg TS Ha1 (Angelini et al., 2009). The reduction of biomass cost led to a marked decrease in the total cost to produce biogas, that was reduced by 25% in the case in which corn was substituted completely by Giant cane (Fig. 2c). Considering the electrical energy production, again a reduction in total costs was found to be of 22.2% (Fig. 2d). Sgroi et al. (2015) using Giant cane silage in mixes with animal slurry at different rates reported total cost to produce electricity in the range of 0.13–0.17 € kW h1, with 0.14 € kW h1 the cost indicated for plants of 400 kW of power. These data agreed with those calculated in this work for the plant studied (380 kW of power) when corn was completely substituted with Giant cane, i.e. 0.14 € kW h1 (Fig. 2b). Unfortunately, nowadays there are no operating full-scale plants fed with only Giant cane so that there are not yet any available data to confirm our economic data. Nevertheless, Corno et al. (2015) studying the complete substitution of corn with Giant cane at lab-scale AD, did not report any problem with biological processes, concluding that Arundo donax could replace corn with success during anaerobic digestion. 4. Conclusion A donax was able to replace partially corn silage in a full-scale anaerobic digestion plant to produce biogas. Total bio-methane and electricity production remained stable before, during and after Giant cane introduction. Corn substitution with Giant cane was able to reduce both the total area needed to produce the energy crop and the total cost of producing electrical energy. In conclusion, the full-scale approach confirms previous findings that indicated that using A. donax was capable of reducing both the cost to produce biogas and the total surface area to produce the energy crop, therefore making biogas production more sustainable. Acknowledgements Gruppo Ricicla Labs, Found number 14-8-30-14000-2, financed project.

Author are grateful to Arundo Italia (Pescara, Italy) for their help in this study. References Angelini, L.G., Ceccarini, L., Nassi o di Nasso, N., Bonari, E., 2009. Comparison of Arundo donax L. and Miscanthus x giganteus in a long-term field experiment in Central Italy: analysis of productive characteristics and energy balance. Biomass Bioenergy 33, 635–643. APHA-American Public Health Association, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. APHA, Washington, DC. Capodaglio, A.G., Callegari, A., Lopez, M.V., 2016. European framework for the diffusion of biogas uses: emerging technologies, acceptance, incentive strategies, and institutional-regulatory support. Sustainability 8, 298. Cavinato, C., Fatone, F., Bolzonella, D., Pavan, P., 2010. Thermophilic anaerobic codigestion of cattle manure with agro-wastes and energy crops: comparison of pilot and full scale experiences. Bioresour. Technol. 101, 545–550. Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: a review. Bioresour. Technol. 99, 4044–4064. Cherubini, F., 2010. The biorefinery concept: using biomass instead of oil for producing energy and chemicals. Energy Convers. Manage. 51, 1412–1421. Corneli, E., Dragoni, F., Adessi, A., De Philippis, R., Bonari, E., Ragaglini, G., 2016. Energy conversion of biomass crops and agroindustrial residues by combined biohydrogen/biomethane system and anaerobic digestion. Bioresour. Technol. 211, 509–518. Corno, L., Pilu, R., Adani, F., 2014. Arundo donax L.: a no-food energy crop for bioenergy and bio-compound production. Biotechnol. Adv. 32, 1535–1549. Corno, L., Pilu, R., Tambone, F., Scaglia, B., Adani, F., 2015. New energy crop giant cane (Arundo donax L.) can substitute traditional energy crops increasing biogas yield and reducing costs. Bioresour. Technol. 191, 197–204. Corno, L., Pilu, R., Cantaluppi, E., Adani, F., 2016. Giant cane (Arundo donax L.) for biogas production: the effect of two ensilage methods on biomass characteristics and biogas potential. Biomass Bioenergy, submitted. IBTimes, http://www.ibtimes.com/low-oil-prices-potential-obstacle-us-cleanenergy-sector-renewables-growth-remains-2292874, visited on July 5th 2016. Khudamrongsawat, J., Tayyar, R., Holt, J.S., 2004. Genetic diversity of giant reed (Arundo donax) in the Santa Ana River, California. Weed Sci. 52, 395–405. Lewandowski, I., Scurlock, J.M., Lindvall, E., Christou, M., 2003. The development and current status of perennial rhizomatous grasses as energy crops in the US and Europe. Biomass Bioenergy 25, 335–361. Lindorfer, H., Corcoba, A., Vasilieva, V., Braun, R., Kirchmayr, R., 2008. Doubling the organic loading rate in the co-digestion of energy crops and manure – a full scale case study. Bioresour. Technol. 99, 1148–1156. Marcato, C.E., Pinelli, E., Pouech, P., Winterton, P., Guiresse, M., 2008. Particle size and metal distribution in anaerobically digested pig slurry. Bioresour. Technol. 9, 2340–2348. Oil Price, http://www.oil-price.net/, visited on July 5th 2016. Pind, P.F., Angelidaki, I., Ahring, B.K., 2003. Dynamic of anaerobic process: effects of volatile fatty acids. Biotechnol. Bioenerg. 82 (7), 791–801. Raboni, M., Urbini, G., 2014. Production and use of biogas in Europe: a survey of current status and perspectives. Ambient. Água 9, 191–202. Ragaglini, G., Dragoni, F., Simone, M., Bonari, E., 2014. Suitability of giant reed (Arundo donax) for anaerobic digestion: Effect of harvest time and frequency on biomethane yield potential. Bioresour. Technol. 152, 107–115. Riva, C., Schievano, A., D’Imporzano, G., Adani, F., 2014. Production costs and operative margins in electric energy generation from biogas. Full-scale case studies in Italy. Waste Manage. 34, 1429–1435. Schievano, M., D’Imporzano, G., Adani, F., 2009. Substituting energy crops with organic wastes and agro-industrial residues for biogas production. J. Environ. Manage. 90, 2537–2541. Schievano, A., D’Imporzano, G., Orzi, V., Adani, F., 2011. On-field study of anaerobic digestion full-scale plants (Part II): new approaches in monitoring and evaluating process efficiency. Bioresour. Technol. 102, 8814–8819. Schievano, A., D’Imporzano, G., Orzi, V., Colombo, G., Maggiore, T., Adani, F., 2015. Biogas from dedicated energy crops in Northern Italy: electric energy generation costs. Glob. Change Biol. 7, 899–908. Sgroi, F., Di Trapani, A.M., Foderà, M., Testa, R., Tudisca, S., 2015. Economic performance of biogas plants using giant reed silage biomass feedstock. Ecol. Eng. 81, 481–487. Sheldon, R.A., 2014. Green and sustainable manufacture of chemicals from biomass: state of the art. Green Chem. 16, 950–963. Weiland, P., 2010. Biogas production: current state and perspectives. Appl. Microbiol. Biot. 85 (4), 849–860.