Improving anaerobic digestion of sugarcane straw for methane production: Combined benefits of mechanical and sodium hydroxide pretreatment for process designing

Energy Conversion and Management xxx (2016) xxx–xxx

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Improving anaerobic digestion of sugarcane straw for methane production: Combined benefits of mechanical and sodium hydroxide pretreatment for process designing Leandro Janke a,b,⇑, Sören Weinrich a, Athaydes F. Leite c, Filippi K. Terzariol a, Marcell Nikolausz c, Michael Nelles a,b, Walter Stinner a a

Department of Biochemical Conversion, Deutsches Biomasseforschungszentrum gemeinnützige GmbH, Torgauer Straße 116, 04347 Leipzig, Germany Faculty of Agricultural and Environmental Sciences, Chair of Waste Management, University of Rostock, Justus-von-Liebig-Weg 6, 18059 Rostock, Germany c Department of Environmental Microbiology, Helmholtz Centre for Environmental Research – UFZ, Permoserstraße 15, 04318 Leipzig, Germany b

a r t i c l e

i n f o

Article history: Available online xxxx Keywords: Sugarcane straw Sodium hydroxide pretreatment Methane yield Process designing Economic assessment

a b s t r a c t Sodium hydroxide (NaOH) as an alkaline pretreatment method to enhance the degradation kinetics of sugarcane straw (SCS) for methane production was investigated with a special focus on the benefits for designing the anaerobic digestion process. For that, SCS was previously homogenized by milling in 2 mm particle size and pretreated in NaOH solutions at various concentrations (0, 3, 6 and 12 g NaOH/100 g SCS) and the methane yields were determined in biochemical methane potential (BMP) tests. The obtained experimental data were used to simulate a large-scale semi-continuous process (100 ton SCS day
1) according to a first-order reaction model and the main economic indicators were calculated based on cash flows of each pretreatment condition. The BMP tests showed that by increasing the NaOH concentration the conversion of the fibrous fraction of the substrate to methane was not only accelerated (higher a value), but also increased by 11.9% (from 260 to 291 mL CH4 gVS
1). By using the experimental data to simulate the large-scale process these benefits were translated to a reduction of up to 58% in the size of the anaerobic reactor (and consequently in electricity consumption for stirring), while the methane yield increased up to 28%, if the liquid fraction derived from the pretreatment process is also used for methane production. Although the use of NaOH for substrate pretreatment has considerably increased the operational expenditures (from 0.97 up to 1.97 €  106 year
1), the pretreatment method was able to increase the profitability of methane production from SCS since a sensitivity analysis by varying the prices of anaerobic reactor, methane and NaOH showed a less attractive payback, net present value and internal rate of return for the control condition (0 g NaOH/100 g SCS) in all analyzed scenarios. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Sugarcane is the most produced agricultural commodity in the world. It is mainly cultivated in tropical and sub-tropical regions of Brazil, India, Pakistan, Thailand and China for sugar, bioethanol and bioelectricity production [1]. Sugarcane straw (SCS), also known as tops and trash, is generated by the mechanized harvest system which has been gradually implemented in substitution to the pre-harvest burning system due to different environmental, agronomic and economic reasons [2]. According to the typical ⇑ Corresponding author at: Department of Biochemical Conversion, Deutsches Biomasseforschungszentrum gemeinnützige GmbH, Torgauer Straße 116, 04347 Leipzig, Germany. E-mail address: [email protected] (L. Janke).

Brazilian conditions SCS corresponds to approximately one third of the total primary energy of the cane [3]. Therefore, nowadays new developments are aiming to add value to such type of biomass fraction with the intention to diversify the product portfolio of the sugarcane plants in a biorefinery concept [4,5]. Anaerobic digestion (AD) is a promising strategy to treat such type of biomass, since methane and/or platform chemicals for value-added products could be produced as a result of different biochemical phases: hydrolysis, acidogenesis, acetogenesis and methanogenesis [6–8]. However, it is well known that hydrolysis is often the rate-limiting step during AD when fibrous material such as SCS is used as feedstock due to the presence of lignin, which prevents the action of microorganisms and enzymes by its hydrophobic and recalcitrance nature [9–11]. Thus, resulting in lower gas yields and longer hydraulic retention times (HRT),

http://dx.doi.org/10.1016/j.enconman.2016.09.083 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Janke L et al. Improving anaerobic digestion of sugarcane straw for methane production: Combined benefits of mechanical and sodium hydroxide pretreatment for process designing. Energy Convers Manage (2016), http://dx.doi.org/10.1016/j.enconman.2016.09.083

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Nomenclature List of abbreviations AD anaerobic digestion ADL acid detergent lignin BMP biochemical methane potential CAPEX capital expenditures COD chemical oxygen demand CSTR continuous stirred-tank reactor FM fresh matter IRR internal rate of return NaOH sodium hydroxide NCF net cash flow NDF neutral detergent fiber NFC non-fiber carbohydrates NPV net present value OGR operational gross revenues OLR organic loading rate OPEX operational expenditures PB payback SCS sugarcane straw VFA volatile fatty acids List of model parameters and symbols D dilution rate (d
1) DQ digestibility quotient (gDS gVS
1) DS digestible solids (ton) HRT hydraulic retention time (d) kF first-order reaction constant of rapidly degradable substrate components (d
1)

directly affecting the profitability of biogas plants based on conventional continuous stirred-tank reactors (CSTR). In this case, different types of substrate pretreatment (e.g. physical, chemical, biological, etc.) are often suggested as alternatives to enhance the AD process by increasing the accessible surface area, modifying the crystalline structure or partially depolymerize cellulose, solubilize hemicellulose and/or lignin, or to modify lignin structure [12,13]. The pretreated substrate is intended to make AD faster, potentially increase biogas yields and prevent operational constraints, such as excessive electricity requirements for mixing or the formation of floating layers [14]. Although the benefits of substrate pretreatment for methane production has been frequently reported in the literature [15,16], most part of its economic benefits for large-scale applications are still unknown. To our knowledge, not only few studies have focused on this aspect [17,18], but they also neglect the benefits of AD process acceleration and reduction the size of anaerobic reactors. Moreover, a previous study from our group assessed the main characteristics of SCS from different sugarcane plants [19]. The sodium concentration of SCS was found below (37.1 mg L
1) the optimum values (100–200 mg L
1) recommended for AD by early studies [20]. In this case, the use of sodium hydroxide (NaOH) as an alkaline reagent for substrate pretreatment appears as an interesting strategy to overcome these possible draw-backs, since at the same time cellulose and hemicellulose would become more accessible to hydrolytic enzymes by breaking down the lignocellulosic structure of SCS, and sodium as an important macronutrient for microbial growth would be provided [21]. Therefore, in this study four different NaOH concentrations were used for SCS pretreatment while the methane yields were determined in biochemical methane potential (BMP) tests. To allow a more comprehensive assessment of the possible

kL kVFA mF mL mVFA _ DS m _ FM m qin SBMP

SCOD

TS Vch4 Vgas Vliq VS V_ ch4 V_ gas Ych4 Ygas

a

first-order reaction constant of slowly degradable substrate components (d
1) first-order reaction constant of volatile fatty acids (d
1) mass of slowly degradable substrate components (ton) mass of rapidly degradable substrate components (ton) mass of volatile fatty acids (ton) mass flow of digestible solids (feed) (ton d
1) mass flow of fresh matter (feed) (ton d
1) total input volume flow, including water and NaOH (m3 d
1) calculated total methane potential, based on the simulation of the experimental biochemical methane potential test (mL CH4 gVS
1) calculated methane potential, based on the chemical oxygen demand of the volatile substrate components (mL CH4 gVS
1) total solids (% FM or g gFM
1) produced methane volume (m3) produced biogas volume (m3) reaction volume (m3) volatile solids (% TS or g gTS
1) produced biogas volume flow (m3 d
1) produced biogas volume flow (m3 d
1) stoichiometric methane yield (mL Biogas gDS
1) stoichiometric biogas yield (mL CH4 gDS
1) ratio of rapidly degradable substrate to total degradable substrate (–)

pretreatment benefits, a first-order modelling tool was used to simulate a large-scale AD process (100 ton SCSFM day
1) and an economical assessment based on cash flows of each pretreatment condition was performed to evaluate the profitability of the pretreatment method. Such approach could provide important inputs for an optimal process design leading to a more efficient utilization of this agricultural residue and encouraging the development of a sustainable biofuel production from it. 2. Material and methods 2.1. Substrate and inoculum Raw SCS was obtained from a sugarcane plant in the state of São Paulo (Brazil) during the 2014/2015 season, transported to Germany in sealed plastic bags, homogenized by using a milling machine SM 200 (Retsch, Germany) with a 2 mm sieve and stored at 4 °C until its use. A large-scale biogas plant that uses maize silage and cattle manure as substrate provided fresh digestate, which was used as inoculum for the BMP tests. 2.2. Alkaline pretreatment The alkaline pretreatment was carried out in 500 mL glass flasks applying different NaOH concentrations (0, 3, 6 and 12 g NaOH/100 g SCSFM), hereafter referred as to control, low, mild and high NaOH concentrations. The solids concentration was adjusted to 86 g TS L
1 by adding distilled water. SCS, NaOH and water mixture was stirred for 30 min (100 rpm) at 75 °C using a magnetic stirrer (Heidolph Instruments). After pretreatment, the SCS was neutralized with hydrochloric acid and the fiber fraction separated with a 0.5 mm sieve for subsequent BMP tests.

Please cite this article in press as: Janke L et al. Improving anaerobic digestion of sugarcane straw for methane production: Combined benefits of mechanical and sodium hydroxide pretreatment for process designing. Energy Convers Manage (2016), http://dx.doi.org/10.1016/j.enconman.2016.09.083

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2.3. Analytical methods Total solids (TS) and volatile solids (VS) of the raw SCS, fibrous and liquid fractions were determined by drying the samples for 24 h at 105 °C in a drying oven (Binder, Germany) and further reducing the organic content to ashes for 3.5 h at 550 °C in a high temperature oven (Carbolite, UK). Nutritional values of the raw SCS were determined according to Weender followed by Van Soest methods. By the Weender method raw protein, raw fat, non-fiber carbohydrates (NFC) and raw fiber are determined. Van Soest method allows the determination of the remaining fractions from the neutral detergent fiber (NDF), which represents hemicellulose and cellulose, and the lignin content is depicted by the acid detergent lignin (ADL). Detailed description of the method was previously published by Van Soest & Wine [22]. To determine the macro elements composition of the raw substrate (C, H, N, S), about 30 mg of SCS and 30 mg of WO3 were weighted and pressed in an aluminum foil for subsequent burning at 1150 °C catalytically with oxygen. The combustion gases were directed through a reduction tube where NOx is reduced to N2. The remaining gases (CO2, H2O, SO2) passed through three different adsorption columns and detected with thermal conductivity detector (C, H, N) and infrared spectroscopy detector (S) by using a Vario Macro Cube (Elementar, Germany) [23]. The chemical oxygen demand (COD) of the liquid fraction was analyzed using LCK 014 COD kit (Hach-Lange, Germany) according to the manufacturer’s protocol. Volatile fatty acids (VFA) and esters, including acetic, n-butyric and phenylacetic acids, furfural and 5-hydroxylmethylfurfural (5-HMF), were determined using a 5890 series II gas chromatograph (Hewlett Packard, USA) equipped with an HS40 automatic headspace sampler (Perkin Elmer, USA) and an Agilent HP-FFAP column (30 m  0.32 mm  0.25 lm) according to previously described methods [24,25]. 2.4. BMP tests The methane yields of the fibrous fraction of SCS obtained after pretreatment with different NaOH concentrations were obtained through BMP tests using an Automatic Methane Potential Test System II (Bioprocess Control, Sweden) under mesophilic temperature (38 °C) during 30 days. Prior to the tests, the inoculum was degassed at 38 °C during approximately 7 days to reduce the non-specific biogas generation. To prevent inhibition, the ratio of substrate/inoculum (g VS/g VS) was set to 0.31 ± 0.02 (i.e. 3.2 ± 0.2 higher amount of inoculum than substrate based on VS) and the pH value in each batch reactor was measured before and after the BMP tests as described earlier [26]. 2.5. Kinetics modelling

Fig. 1. Components and parameters of the utilized model structure (according to [27]).

was realized by using the software Matlab as described by Brulé et al. [27]. Additionally, in order to provide a more realistic behavior based on the experimental data, the kinetics and biogas potential (k and S) were adjusted to accept only positive values, as well as the ratio between rapidly and slowly degradable substrate components (a) varying between 0 and 1. 2.6. Simulation of CSTR process To predict the gas production in continuous operation mode by using a CSTR the basic model structure needed to be translated into a general set of differential equations. Considering a constant reaction volume the change of each component in time can be described by the respective mass added and removed during substrate feeding (input and output) as well as the amount involved in anaerobic degradation (first-order kinetics), according to the following equations.

dmL _ DS
D  mL
mL  kL ¼ ð1
aÞ  m dt

ð1Þ

dmF _ DS
D  mF
mF  kF ¼am dt

ð2Þ

dmVFA ¼
D  mVFA þ mL  kL þ mF  kF
mVFA  kVFA dt

ð3Þ

dV gas ¼ Y gas  mVFA  kVFA dt

ð4Þ

Thus, the calculation of the resulting biogas (methane) volume Based on the different model derivations presented by Brulé et al. [27] an exponential dual-pool two-step model (Model D) was used to evaluate the methane production kinetics of the BMP tests. As shown in Fig. 1 this modelling approach differentiates between rapidly and slowly degradable fractions (dual-pool) of the available substrate. Furthermore, it includes the acidification of the two fractions to VFA as well as the degradation of the resulting intermediate VFA concentration to methane (two-step). Thus, five model parameters and constants need to be adjusted to depict the respective measurement results: the total methane potential SBMP (mL gVS
1), the ratio of rapidly degradable substrate to total degradable substrate a (–) and the three first-order reaction constant for the degradation of rapidly degradable substrate kF (d
1), slowly degradable substrate kL (d
1) and the degradation of VFA kVFA (d
1). The model implementation as well as the numeric parameter identification (Levenberg-Marquard algorithm)

flow V_ gas (V_ ch4 ) depends on the specific rate of VFA degradation and the stoichiometric biogas (methane) yield Ygas (Ych4) as described in Eq. (4). Generally, the theoretical biogas potential of any substrate can be calculated based on the stoichiometric equations of Buswell and Müller [28] or Boyle [29] and the respective sum formulas of the digestible nutrients (detailed composition of degradable carbohydrates, proteins and lipids). In the current investigation the theoretical biogas potential of forage and cereal crops presented by Weibbach [30] is utilized for the stoichiometric description of SCS. Therefore, the stoichiometric biogas and methane yield Ygas and Ych4 for SCS is approximated by 809 mL biogas (STP) and 420 mL methane (STP) per g digestible solids DS (including mvfa), considering 5% of DS for microbial growth and maintenance. Mass balance (and recirculation) of dead biomass is neglected in the presented modelling approach.

Please cite this article in press as: Janke L et al. Improving anaerobic digestion of sugarcane straw for methane production: Combined benefits of mechanical and sodium hydroxide pretreatment for process designing. Energy Convers Manage (2016), http://dx.doi.org/10.1016/j.enconman.2016.09.083

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_ DS can be calculated The input mass flow of digestible solids m based on the daily mass of fresh matter content of total and volatile solids (TS in kg kg
1 FM and VS in kg kg
1 TS) as well as the respective digestibility quotient DQ.

2.7. Preliminary economic assessment

To simulate steady state process conditions the change of mass in time equals zero.

In order to provide a profitability evaluation of the different pretreatment conditions in a continuous large-scale application, a preliminary economic assessment delimited by the NaOH pretreatment and biogas production steps of a hypothetical biogas plant with capacity to process 100 ton SCSFM day
1 was performed. Therefore, the capital expenditures (CAPEX) of the anaerobic reactor, including civil work, stirrers, insulation, rooftop and installation, as well as the electricity consumption for stirring the anaerobic reactors were obtained from the Brazilian-based company Ecometano (personal communication). Considering a previous study on different methane utilizations applied to the sugarcane industry [32], the displacement of diesel consumption by the methane production was considered to estimate the operational gross revenues (OGR) for each pretreatment condition. For the operating expenditures (OPEX) the costs of NaOH, electricity consumption for stirring and anaerobic reactor depreciation were considered. The net present value (NPV), internal rate of return (IRR) and discounted payback (PB) were chosen as indicators for the economic assessment. The NPV represented by the difference between the present value of OGR and OPEX for a project life cycle of 20 years was calculated according to the equation below:

dmF dmL dmVFA ¼ ¼ ¼0 dt dt dt

NPV ¼
CAPEX þ

_ DS ¼ m _ FM  TS  VS  DQ m

ð5Þ

The DQ can be approximated based on the total methane potential obtained from the BMP test (Table 2) of the fibrous fraction SBMP and the theoretical methane potential of the liquid fraction (SCOD) of the investigated SCS as shown in Eq. (6).

DQ ¼

SBMP þ SCOD Y ch4

ð6Þ

The SCOD is calculated based on the product of the respective COD concentration and the standard methane potential of 320 mL (STP) per g COD according to VDI [26] and Hermann et al. [31]. Furthermore, the dilution rate D is defined as the quotient of the total input volume flow and reaction volume or the inverse of the respective HRT.

D¼

qin 1 ¼ V liq HRT

ð7Þ

ð8Þ

Thus, the differential Eqs. (1)–(3) can be explicitly solved for the unknown masses of each component in steady state, depending on known parameters or previously calculated masses.

_ DS ð1
aÞ  m D þ kL

mL ¼ mF ¼

mVFA

ð9Þ

a  m_ DS

ð10Þ

D þ kF

ð11Þ

Finally, the respective biogas production rate can be gained by inserting the results of Eqs. (9) and (10) into Eqs. (11) and (4).

dV gas ðmL  kL þ mF  kF Þ  kVFA ¼Y dt D þ kVFA

ð12Þ

The resulting balancing scheme has been implemented as simple spreadsheet calculation in MS-Excel 2016 (Microsoft, USA). All kinetic parameters and substrate characteristics are shown in Tables 1 and 2. Table 1 Main characteristics of the raw SCS. SCSa

Units

Total solids (TS) Volatile solids (VS) Carbon (C) Nitrogen (N) Sulfur (S) Raw protein Raw fat Carbohydrates

78.19 ± 0.17 87.45 ± 1.75 46.22 ± 0.79 0.40 ± 0.01 0.21 ± 0.01 30.82 ± 0.24 18.00 ± 1.21 35.24 ± 7.89 342.3 ± 3.85 263.98 ± 21.56 210.36 ± 8.37 118.17 ± 2.37

% FMb % TS % TS % TS % TS g kg
1 TS g kg
1 TS g kg
1 TS g kg
1 TS g kg
1 TS g kg
1 TS –

NFCc Cellulose Hemi-cellulose

Note: Values are presented in mean (n = 3); ± represents the standard deviation. a Sugarcane straw. b Fresh matter. c Non-fiber carbohydrates.

ð14Þ

where – P is the value of the methane generated based on diesel displacement; – CH4y is the yearly methane generated; – OPEXy is the yearly operational expenditures based on NaOH cost, electricity consumption for stirring and depreciation of the anaerobic reactor. The PB is defined as the number of years necessary to recover the investment made. Its calculation is described by the following equation:

CAPEX ¼

Parameters

ð13Þ

where – CAPEX is the capital expenditures based on anaerobic reactor costs; – k is the cost of capital or discount rate, estimated based on historical prices of the Brazilian CELIC rate of 11.81% per year [33]; – NCFy is the net cash flow in the year ‘‘y” according to the following equation:

NCF y ¼ ðP  CH4y
OPEX y Þ

mL  kL þ mF  kF ¼ D þ kVFA

Lignin C:N ratio

n X NCF y y ð1 þ kÞ y¼1

PB X NCF y

ð15Þ

y

The IRR was calculated as the inverse of the discounted PB as follows:

IRR ¼

1 PB

ð16Þ

2.8. Statistical analysis An analysis of variance (on-way ANOVA) was performed for each day on the cumulative methane yields obtained from the BMP tests to verify whether statistical differences could be observed as an effect of the NaOH pretreatment with 95% of confidence level. Additionally, a Tukey pairwise comparison on the cumulative methane yields after 5, 10, 15, 20, 25 and 30 days of

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BMP test was performed. Both analyses were run with the software MS-Excel 2016 (Microsoft, USA).

3. Results and discussion 3.1. Substrate composition 3.1.1. Raw SCS The main characteristics of the raw SCS used in the present study are presented in Table 1. The TS and VS contents (TS: 78.19% and VS: 87.45% of TS) were similar to the average values found in a recent study analyzing SCS samples from different sugarcane plants [19]. However, in terms of balance of macronutrients (N:P:S) in relation to the carbon content, it was found an even higher C:N ratio (118:1) than observed in the earlier study (C:N ratio of 83:1) which could lead to an incomplete conversion of the carbon contained in SCS (if nitrogen is not supplemented), resulting in lower methane yield. Additionally, the 29% higher lignin content (210 g kgTS
1) found in the sample used for the present study could also hamper the substrate conversion to methane since less cellulose and hemicellulose would be available for microbial degradation. Thus, reinforcing the necessity of substrate pretreatment for an optimized methane production process [14]. 3.1.2. Pretreated SCS The COD and VFA contents found in the liquid fraction of the homogenized SCS (milling in 2 mm) and pretreated with different NaOH concentrations are presented in Fig. 2. The low NaOH concentration was responsible for solubilize additional 42% of COD

Fig. 2. Characteristics of the liquid fraction after NaOH pretreatment. Control (0 g NaOH/100 g SCSFM), low (3 g NaOH/100 g SCSFM), mild (6 g NaOH/100 g SCSFM) and high (12 g NaOH/100 g SCSFM).

in comparison to the control sample. In the meantime, a much stronger saponification effect could be noted by applying the mild and high concentrations. In this case, approximately 1.5 and 3 times higher COD release were found in mild and higher NaOH concentrations than in control sample. Interestingly, the major VFA found (acetic acid) did not show the same trend as observed in COD analysis. The low NaOH pretreatment condition was able to increase in 10 times the acetic acid concentration (1727 mg L
1) in comparison to the control sample (160 mg L
1), while nearly no difference was observed in acetic acid concentration between mild (2261 mg L
1) and high (2288 mg L
1) NaOH concentrations. This fact demonstrates the effectiveness of NaOH as a reagent for chemical hydrolysis of the substrate but with limited effect on VFA production. Additionally, it is important to note that furfural and 5-HMF, which could potentially inhibit the AD process [34], were not detected for any pretreatment condition.

3.2. BMP tests The methane yields of the fibrous fraction obtained from the BMP tests and fitted to the to the dual-pool two-step model are shown in Fig. 3 and Table 2. The one-way ANOVA showed significant differences (P < 0.05) among the treatments from the beginning to the end of the experiment (Fig. 4). The control sample reached a total methane potential of 260 mL CH4 gVS
1 which is similar to values previously reported for SCS also milled in 2 mm [19], and 13% higher than the methane yield (229 mL CH4 gVS
1) earlier reported for SCS pretreated by steam-explosion technique at 190 °C, 20 bar and 15 min of exposure [15]. Interestingly, the low NaOH concentration did not improve the total methane potential in comparison to the control sample, but was able to accelerate the AD process, since the ratio of rapidly degradable substrate to total degradable substrate (a value) increased from 0.20 to 0.62. In the meantime, the mild NaOH concentration was not only able to achieve 5.8% higher methane potential (275 mL CH4 gVS
1), but also considerably increase the a value to 0.93. By applying the highest NaOH dosage in this experiment, the effect of substrate pretreatment on methane yield was even higher, improving in 11.9% the total methane potential (291 mL CH4 gVS
1) in comparison to the control sample and reaching the highest a value (1.00) of all tested pretreated conditions. Whereas, the additional COD solubilized as an effect of the alkali reagent seems to be the reason of such higher performance. In fact, the chemical hydrolysis provided by the NaOH pretreatment was able to improve the conversion of digestible solids as rapidly degradable substrate (dual-pool), displaying the highest kF values of 0.78 and 0.83 for mild and high pretreatment conditions, respectively.

Table 2 Results of the BMP test fitted to the dual-pool two-step model. Condition

SBMP (mL CH4 gVS
1)

a (–)

kF (d
1)

kL (d
1)

kVFA (d
1)

SBMPa (mL CH4 gVS
1)

aa (–)

Increase Sa (%)

Control Low Mild High

267 ± 18 256 ± 07 277 ± 10 291 ± 01

0.57 ± 0.03 0.59 ± 0.04 0.81 ± 0.03 0.92 ± 0.01

0.41 ± 0.01 0.64 ± 0.18 0.78 ± 0.01 0.83 ± 0.01

0.08 ± 0.01 0.17 ± 0.11 0.14 ± 0.01 0.13 ± 0.01

0.42 ± 0.03 0.63 ± 0.19 0.80 ± 0.01 0.80 ± 0.01

260 ± 17 260 ± 04 275 ± 09 291 ± 01

0.20 ± 0.01 0.62 ± 0.07 0.93 ± 0.03 1.00 ± 0.01

– 0.1 5.8 11.9

Note: Values are presented in mean (n = 3); ± represents the standard deviation. SBMP – total methane potential. a – ratio of rapidly degradable substrate to total degradable substrate. kF – rapidly degradable substrate. kL – slowly degradable substrate. kVFA – degradation of VFA. a Optimized parameter values (curve fitting) for constant kF = 0.66 (d
1), kL = 0.13 (d
1), kVFA = 0.66 (d
1), respectively.

Please cite this article in press as: Janke L et al. Improving anaerobic digestion of sugarcane straw for methane production: Combined benefits of mechanical and sodium hydroxide pretreatment for process designing. Energy Convers Manage (2016), http://dx.doi.org/10.1016/j.enconman.2016.09.083

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Fig. 3. Cumulative methane yields obtained from the BMP test and fitted to the dual-pool two-step model. (a) Methane from rapidly degradable fraction; (b) methane from slowly degradable fraction; (c) intermediate VFA production and (d) total methane potential of control, low, mild and high SCS pretreatment conditions.

3.3. Simulation of continuous process

Fig. 4. One-way ANOVA on each day of the cumulative methane yields obtained from the BMP test.

The Tukey’s pairwise comparisons after 5, 10, 15, 20, 25 and 30 days of experiment (Fig. 5) confirmed the effects of NaOH pretreatment on process acceleration, since in the beginning of the test 4 groups statistically different (P < 0.05) to each other were observed, while towards to the end of the experiment a gradual reduction to 2 different groups (P < 0.05) was displayed. Such statistical results from one-way ANOVA followed by Tukey pairwise comparisons demonstrate with 95% confidence level the both benefits of process acceleration and increase of total methane potential provided by NaOH pretreatment during AD of SCS.

In order to provide a more comprehensive understanding of the pretreatment effects for large-scale applications, each pretreatment condition was simulated for a biogas plant with capacity to process 100 ton SCSFM d
1. The results in terms of anaerobic reactor size, total methane yield (fibrous and liquid fractions) and their respective process design indicators are shown in Fig. 6. Similarly to the results found for the BMP tests, the differences in methane yield between the control and low NaOH condition were negligible (±1.25%). But due to the acceleration of the AD process the HRT needed to produce such methane yield could be reduced from 100 d (control) to 67 d (low NaOH concentration), which means a 32% reduction in the anaerobic reactor volume (and consequently in electricity consumption for stirring). Such improvement represents a better utilization of the anaerobic reactor structure since the volumetric methane yield would be increased by 50% from 0.38 (control) to 0.57 m3 CH4 m3 d
1 (low NaOH concentration). As expected, by using the mild and high NaOH concentration for SCS pretreatment a considerably higher performance in comparison to the control sample was found. In this case the methane yield was increased by 12% and 28%, respectively, for mild and high NaOH condition. Such improvement is higher than observed during the BMP test since for the simulation of the continuous process also the liquid fraction derived from the pretreatment step (rich in COD) is considered as substrate together with the fibrous fraction. Nevertheless, the effects of acceleration the AD process were also pronounced, allowing the reduction of the HRT to 46 d (mild NaOH condition) and 41 d (high NaOH condition), which means a

Please cite this article in press as: Janke L et al. Improving anaerobic digestion of sugarcane straw for methane production: Combined benefits of mechanical and sodium hydroxide pretreatment for process designing. Energy Convers Manage (2016), http://dx.doi.org/10.1016/j.enconman.2016.09.083

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Fig. 5. Tukey’s pairwise comparison on the cumulative methane yields after 5, 10, 15, 20, 25 and 30 days of BMP test.

reduction of up to 58% in the size of the anaerobic reactor. Finally, by using the mild and high pretreatment conditions the utilization of the anaerobic reactor structure could be improved, respectively, by the factors 2.4 and 3, although the operational costs of the methane production would increase by the acquisition of the alkaline reagent.

3.4. Preliminary economic assessment The cash flows of each pretreatment condition (Table 3) was used to perform a preliminary economic assessment to analyze whether the benefits provided by the NaOH pretreatment would result in a more profitable methane production process. The Fig. 7 shows that the mild and high NaOH concentrations displayed a shorter PB (3 years) in comparison to the low and control, respectively 5 and 8 years. This is mainly explained by the lower anaerobic reactor investment and higher methane production provided by the mild and high pretreatment conditions which have overcompensated the costs of NaOH. Additionally, considering the higher variation of CAPEX (±41.2%) in comparison to the NCF (±4.7%) among the different pretreatment conditions, it is possible to assume that the CAPEX was the most decisive factor influencing the higher profitability of the mild and high NaOH dosages. Therefore, a sensitivity analysis varying the price of anaerobic reactor, as well as methane and NaOH was performed (Table 4).

By increasing the price of anaerobic reactor by 50% (from 354 to 531 EUR/m3) the profitability is drastically reduced, especially for the control pretreatment with an IRR slightly lower (11.35%) than the Brazilian interest rate (11.81%) which makes financially unattractive to produce methane without substrate pretreatment under this scenario (PB > 20 years). On the other hand, by reducing the price of anaerobic reactor by 50% (from 354 to 177 EUR/m3) the advantage of substrate pretreatment is reduced, even though the mild and high condition still showed a higher IRR than the control option. Nevertheless, the use of low-cost anaerobic reactors for large-scale applications would make that the low NaOH dosage displayed a relatively similar PB in comparison to the mild and high pretreatment conditions. The variation of methane price from 0.71 to 1.06 EUR/m3 didn’t show any change in terms of relative profitability among the analyzed pretreatment conditions. However, by reducing the methane price by 50% (from 0.71 to 0.35 EUR/m3) a trend of a better profitability for the mild NaOH dosage in comparison to the other pretreatment conditions is observed, since for this scenario only the mild condition was able to cover the costs of NaOH. The NaOH price was the less sensitive parameter. For the different scenarios covered by this analysis (NaOH price from 182 to 547 EUR/ton) all NaOH concentrations showed better economic indicators than the control condition which reinforces the profitability of using NaOH as a pretreatment method to enhance the AD of SCS. Moreover, it is seen that by increasing the NaOH price from 364 to 547 EUR/ton the most profitable pretreatment condi-

Table 3 Cash flows for the different plant configurations. Parameters

Control

Low

Mild

High

Unit

15.9

10.7

7.4

6.7

€  106

Operating gross revenues (OGR) Diesel displacement by methane

4.43

4.51

4.96

5.71

€  106 year
1

Operating expenditures (OPEX) Cost of NaOHc Cost of electricity for stirringd Cost of depreciatione Total OPEX Net cash flow (NCF)

– 0.17 0.79 0.97 3.46

0.39 0.12 0.53 1.05 3.46

0.72 0.08 0.37 1.18 3.77

1.56 0.07 0.33 1.97 3.74

€  106 year
1 €  106 year
1 €  106 year
1 €  106 year
1 €  106 year
1

Capital expenditures (CAPEX) Anaerobic reactor investmenta b

a

Civil work, stirrers, insulation, rooftop and costs of installation (354 EUR/m3). Calculated based on methane production of each pretreatment condition, diesel price of 0.71 EUR/L [41], methane LHV of 8578 kcal/m3 and diesel LHV value 8605 kcal/L [42]. c NaOH cost of 364 EUR/ton [43]. d Electricity cost of 0.04 EUR/kW h [44]. e Calculated based on 5% per year of depreciation [45]. b

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L. Janke et al. / Energy Conversion and Management xxx (2016) xxx–xxx

Fig. 6. Process flow diagram for the different plant configurations. (a) Control (0 g NaOH/100 g SCSFM); (b) low (3 g NaOH/100 g SCSFM); (c) mild (6 g NaOH/100 g SCSFM) and (d) high (12 g NaOH/100 g SCSFM).

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L. Janke et al. / Energy Conversion and Management xxx (2016) xxx–xxx

9

Fig. 7. Net present value (NPV) and discounted payback of the different pretreatment conditions. (a) Control (0 g NaOH/100 g SCSFM); (b) low (3 g NaOH/100 g SCSFM); (c) mild (6 g NaOH/100 g SCSFM) and (d) high (12 g NaOH/100 g SCSFM).

Please cite this article in press as: Janke L et al. Improving anaerobic digestion of sugarcane straw for methane production: Combined benefits of mechanical and sodium hydroxide pretreatment for process designing. Energy Convers Manage (2016), http://dx.doi.org/10.1016/j.enconman.2016.09.083

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Table 4 Sensitivity analysis of the CH4, tank and NaOH prices variation on internal rate of return (IRR) and discounted payback (PB). Reactor price variation (EUR/m3)

177 265 354 442 531

(
50%) (
25%) (0%) (+25%) (+50%)

CH4 price variation (EUR/m3)

0.35 0.53 0.71 0.88 1.06

(
50%) (
25%) (0%) (+25%) (+50%)

NaOH price variation (EUR/ton)

182 273 364 456 547

(
50%) (
25%) (0%) (+25%) (+50%)

Control

Low

Mild

High

IRR (% year)

PB (year)

IRR (% year)

PB (year)

IRR (% year)

PB (year)

IRR (% year)

PB (year)

48.51 30.54 21.31 15.49 11.35

3 5 8 12 >20

69.28 44.49 32.02 24.40 19.17

2 3 5 6 9

106.74 69.49 50.86 39.65 32.13

2 2 3 4 5

116.43 75.95 55.71 43.54 35.39

1 2 3 3 4

Control

Low

Mild

High

IRR (% year)

PB (year)

IRR (% year)

PB (year)

IRR (% year)

PB (year)

IRR (% year)

PB (year)

4.70 13.65 21.31 28.55 35.63

>20 15 8 5 5

9.32 21.21 32.02 42.57 53.07

>20 8 5 3 3

16.68 34.08 50.86 67.57 84.27

11 4 3 2 2

11.77 34.36 55.71 76.97 98.24

>20 4 3 2 2

Control

Low

Mild

High

IRR (% year)

PB (year)

IRR (% year)

PB (year)

IRR (% year)

PB (year)

IRR (% year)

PB (year)

21.31 21.31 21.31 21.31 21.31

8 8 8 8 8

33.85 32.93 32.02 31.10 30.18

4 4 5 5 5

55.76 53.31 50.86 48.40 45.95

2 3 3 3 3

67.32 61.51 55.71 49.90 44.08

2 2 3 3 3

IRR – internal rate of return. PB – discounted payback.

tion is changed from high to mild NaOH dosage. Indeed, by increasing the NaOH price, the higher revenues provided by the high pretreatment condition is surpassed by the higher NaOH cost in comparison to the mild NaOH dosage.

3.5. Final remarks A combined mechanical and NaOH pretreatment strategy to improve the degradation kinetics of SCS for methane production was tested in lab-scale (batch mode) and the semi-continuous feeding process was simulated to assess the profitability of the substrate pretreatment for large-scale applications. For that, it was considered the utilization of a CSTR system with lateral stirrers, which are often found in agricultural biogas plants in different locations [35]. The utilization of others suitable technologies for digestion of solid substrates, such as CSTR system with central stirrers, plug flow reactors, sprinkler recirculation, among others, could provide different results since each one have their own specific cost of anaerobic reactor (EUR/m3) and electricity consumption, leading to a possible different profitability of the pretreatment technique. Nevertheless, by varying the cost of anaerobic reactor by ±50% during the sensitivity analysis, the present study was able to cover a wide range of anaerobic reactor possibilities. Furthermore, although an inhibition of the AD process by toxic Na+ concentration was not observed during the lab-scale experiment, a different behavior could occur in a continuous feeding system since usually in batch experiments a higher proportion of inoculum in comparison to the substrate is used. However, in a recent study based on continuous digestion of ensiled sorghum forage it was not observed Na+ inhibition for similar pretreatment concentration (10 g NaOH/100 g TS) used in our study [21]. Additionally, in case the digestate is used as fertilizer, the long-term salinization effect on agricultural soil is still unknown. Nevertheless, the NaOH concentration for SCS pretreatment could

possibly be reduced, since the severity of thermo-chemical pretreatment techniques follows a first-order kinetics influenced by the residence time, chemical concentration and temperature of the reaction [13]. Thus, suggesting that the chemical concentration could be reduced without reducing its effectiveness, if the residence time and temperature of the reaction would be higher than used in our experiment. Alternatively, another possibility to reduce such negative effects could be to separate the substrate in different chemical pretreatments, as for example, by also using calcium hydroxide (CaOH) or ammonia fiber explosion (AFEX). In this case, not only the concentration of Na+ would be reduced in the reactor and agricultural soil, but also calcium and nitrogen in the digestate would be provided for agricultural purposes. Finally, BMP tests are dependent on several factors [36]. Specially the source, activity and adaptation of the inoculum have been proven to have a large influence of the resulting degradation kinetics [37–39]. In the present study, no functional inhibition or physico-chemical reactions (gas transfer or dissociation equilibria) are considered during the modelling approach. As shown by Batstone et al. [40] the kinetics parameters and DQ in batch operation could differ from the actual process characteristics in longterm continuous operation. Thus, the individual process conditions and the resulting methane production of the substrate pretreated with different NaOH concentrations could also differ from the predicted simulated results. Specially the effect of high NaOH concentration on process stability in continuous operation mode is recommended to be investigated in future studies.

4. Conclusions The present study demonstrated how methane production from SCS could be optimized by a NaOH pretreatment in a profitable way. Although the alkali reagent was able to significantly increase (P < 0.05) the total methane potential of SCS, most part of the

Please cite this article in press as: Janke L et al. Improving anaerobic digestion of sugarcane straw for methane production: Combined benefits of mechanical and sodium hydroxide pretreatment for process designing. Energy Convers Manage (2016), http://dx.doi.org/10.1016/j.enconman.2016.09.083

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pretreatment effects were observed on the AD process acceleration. In a simulated semi-continuous feeding large-scale application, those effects were translated in a HRT reduction from 100 d up to 41 d and in an increase of the methane yield from 251 up to 324 m3 CH4 tonVS
1. The use of NaOH for substrate pretreatment has considerably increased the operational expenditures of the AD process, but the economic benefits in terms of reduced capital expenditures (lower anaerobic reactor investment) and higher revenues (higher methane production) provided by the NaOH pretreatment were able to improve the profitability of methane production. The most influencing parameters observed in a sensitivity analyses were reactor and methane prices. Thus, the profitability of the proposed pretreatment method was highly dependent on the choice of anaerobic reactor system and local incentives for biogas production. Such results provide important inputs for designing the AD process using SCS as substrate encouraging the efficient utilization of one of the major residual biomass fractions from the sugarcane industry for a sustainable biofuel production in a biorefinery concept.

[16]

Acknowledgements

[17]

The authors would like to acknowledge the support of the Brazilian National Scientific Counsel (CNPq) under the Program Science without Borders for the financial support of the PhD students Leandro Janke (237938/2012-0) and Athaydes Francisco Leite (202024/2012-1). The present research was partially financed by the i-NOPA Project ‘‘Sustainable bioeconomy in Brazil: Bioenergy from biogas using various types of waste substrates from the Brazilian bioethanol industry”.

[10]

[11]

[12]

[13]

[14] [15]

[18]

[19]

[20] [21]

Appendix A. Supplementary material [22]

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.enconman.2016. 09.083.

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