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Kinetics of methane production and hydrolysis in anaerobic digestion of corn stover
Energy 102 (2016) 1e9
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Kinetics of methane production and hydrolysis in anaerobic digestion of corn stover Dong Li a, b, Xianbo Huang c, Qingjing Wang a, Yuexiang Yuan a, Zhiying Yan a, Zhidong Li a, Yajun Huang c, Xiaofeng Liu a, * a
Key Laboratory of Environmental and Applied Microbiology, Environmental Microbiology Key Laboratory of Sichuan Province, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China Jiangsu Key Laboratory for Biomass Energy and Material, Jiangsu Province, Nanjing 210042, China c Chengdu Organic Chemicals Co., LTD., Chinese Academy of Sciences, Chengdu 610041, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 December 2015 Received in revised form 8 February 2016 Accepted 12 February 2016 Available online xxx
In order to develop a time-saving method for determination of ultimate methane production, obtain the hydrolysis kinetic constant, and identify a determination method for the nonbiodegradable organic fraction of substrate (VSNB) of green and air-dried corn stover, the kinetics of methane production and hydrolysis were studied using batch tests. The results showed that the conventional first-order hydrolysis kinetic model was not suitable for describing the entire hydrolysis process of corn stover, because there were two first-order decay periods for hydrolysis of corn stover. The hydrolysis kinetic constants kH,1 and kH,2 of the first and second periods were 0.1701 and 0.0415 1/d for green stover and 0.1052 and 0.0360 1/ d for air-dried stover. The value of VSNB could be obtained by the graphical method rather than by the hydrolysis kinetic model. The obtained VSNB contents were 12.9% and 24.7% of VS (volatile solid) for green and air-dried stover, respectively. The ultimate methane production and corresponding digestion time could be understood through the methane production kinetic model by digestion experiments within a short time. The ultimate methane productions were 347.1 and 319.4 mL/g based on VS and the corresponding digestion times were 69.2 and 182.3 days for green and air-dried stover, respectively. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Anaerobic processes Biogas Modelling Kinetic parameters Hydrolysis Nonbiodegradable
1. Introduction The Methane yield potential and methane production rate of substrates are two important indices for anaerobic biogas production. Correspondingly, the biodegradable organic fractions and hydrolysis rates of substrates are two important parameters for anaerobic biodegradation. In fact, these parameters determine, to a certain extent, both the design and economic details of biogas plants. Anaerobic batch tests are the standard method for the determination of methane yield potential of substrates [1]. When performing a batch test, the decision of when to terminate the experiment is crucial. In 2006, the Association of German Engineers (VDI) published the guideline VDI 4630, “Fermentation of organic materials, characterization of the substrate, sampling, collection of
* Corresponding author. No. 9 Section 4, Renmin Nan Road, Chengdu, Sichuan, China. Tel.: þ86 2882890229; fax: þ86 2882890233. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.energy.2016.02.074 0360-5442/© 2016 Elsevier Ltd. All rights reserved.
material data, fermentation tests” [2]. This guideline introduces the so-called 1% criterion, in which the experiment is continued until the daily gas production is less than 1% of the total gas production. Many experiments have been performed according to this guideline [3e6]. The ultimate methane production, known as the BMP (biochemical methane potential) of the substrate, is theoretically reached after infinite incubation. In 2009, a guideline for BMP assays was proposed by the ‘Task Group for the Anaerobic Biodegradation, Activity and Inhibition of the Anaerobic Digestion’ of the IWA (International Water Association) [7]. In this protocol, batch experiments were not considered complete until no methane was produced. This protocol is now the BMP test basis for most of experiments performed. The methane potential of wheat straw stillage was determined in 118 mL serum bottles with working volume of 40 mL [8]. The small and large BMP assays of grass silage were carried out in small serum bottles and 1.5 L reactor, respectively [9]. BMP test was conducted in 240 mL serum bottle with working volume 100 mL using beef feedlot manure as substrate [10]. BMP tests were carried out at different scales (5 L and 0.5 L) for both wet
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D. Li et al. / Energy 102 (2016) 1e9
and dried food waste [11]. However, BMP assays require long experimental times, and the method proposed by VDI 4630 does not yield the true ultimate methane production. Anaerobic degradation of particulate organic matter involves a complex network of reactions in series and in parallel involving several key groups of bacteria. Hydrolysis of particulate matter to soluble substrates is often assumed to be the rate-limiting step in anaerobic digestion. Substrate hydrolysis and biogas production typically follow first-order kinetics when macro- and micronutrients are available and there is no inhibition [12e14]. To date, first-order kinetics have been used to model the hydrolysis process of anaerobic digestion of organic fractions of municipal solid waste [14], sunflower oil cake [15], and mixtures of manure, animal feed, slaughterhouse waste, and municipal solid waste [13] in batch mode. However, there are no first-order hydrolysis kinetics studies of corn stover. Moreover, the values of the hydrolysis kinetic constants (kH) of these models were calculated indirectly from data of methane production rather than substrate reduction. Notably, the nonbiodegradable organic fraction of the substrate cannot be obtained by using traditional first-order hydrolysis kinetics. The aims of this study were to develop a time-saving method for determination of ultimate methane production and corresponding digestion time, to identify an appropriate determination method for the nonbiodegradable organic fraction of substrate, and to obtain the hydrolysis kinetic constant of corn stover. 2. Material and methods
parallel. Prior to operation, the reactors were flushed with nitrogen for 5 min to ensure anaerobic conditions. The digesters were placed in a water bath at 35 ± 1 C. Each reactor was manually mixed twice a day. The experiment was carried out for 20 days. Two reactors were randomly removed for measurement of the TS and VS (volatile solid) contents of the digested residues at different times (Table 2). 2.3. Analytical methods The TS and VS contents were determined using standard techniques [16]. Analyses for C and N were conducted using a Vario EL element analyzer (Elementar Analysensysteme GmbH, Germany). Cellulose, hemicellulose, and lignin contents were determined as previously described [17]. Biogas production was estimated by measuring the water displacement. Biogas analysis was performed using an Agilent 6890 GC (gas chromatography) system (Agilent Technologies, USA) with a TCD (thermal conductivity detector) and a 2-m stainless steel column packed with Porapak Q (50/80 mesh). The operating temperatures at the injection port, column oven, and detector were 200, 80, and 200 C, respectively. Argon was used as the carrier gas at a flow rate of 30 mL/min. 2.4. Process fitting and first-order kinetic model 2.4.1. Methane production fitting model A modified Gompertz model (Eq. (1)) was used to fit the data of methane production [18]:
2.1. Substrates and inoculums Green and air-dried corn stover with harvest time of 6 and 8 months, respectively, were obtained from rural Chengdu, China. The collected stover was chopped and sieved to particles measuring less than 5 mm in size. The sieved green corn stover was stored at 4 C. The residue left on a 1-mm sieve of material taken from a mesophilic (37 C) anaerobic digester fed with pig manure was used as the inoculum. The anaerobic digestion of the inoculum was conducted untill no biogas production in order to ignore the methane production of the inoculum itself. The characteristics of the substrates and inoculum are listed in Table 1. The lignin and cellulose contents of air-dried corn stover were higher than that of green corn stover, since the serious lignification was occurred for air-dried corn stover. 2.2. Experimental setup and operation
Rm ,e ,ðl tÞ þ 1 MðtÞ ¼ P,exp exp P
where M is the cumulative methane production (L/kgVS) at a fermentation time t (d), P is the maximum cumulative methane production (L/kgVS) for the entire experimental digestion time, Rm is the maximum methane production rate (L/(kgVS$d)), and l is the lag-phase time (d). The values of P, Rm, and l were obtained by data fitting. The methane production rate (Eq. (2)) was obtained by differentiating the modified Gompertz equation,
Table 2 Substrate concentrations during the digestion of corn stover. Time (d)
A 250-mL bottle with working volume 200 mL was used for the batch anaerobic digestion tests. The bottle was sealed using a rubber stopper with a pipe for extracting the biogas. The reactor was connected to a gas collection system consisting of a saturated brine displacement bottle and a brine gathering bottle. The initial substrate concentration was about 30 g/L based on TS (total solid). The batch reactors were filled with 30 g of green corn stover or 6.5 g of air-dried corn stover before adding inoculum to a total weight of 200 g. The VS (volatile solid) ratio of substrate to inoculum was 1.44 and 1.29 for batch digestions of green and air-dried corn stover. Each test of green and air-dried corn stover had 20 reactors in
(1)
0 1 2 3 5 7 9 10 12 15 18 20
Green corn stover
Air-dried corn stover
TS (g/L)
VS (g/L)
TS (g/L)
26.9 ± 0.6 21.2 ± 0.5 18.7 ± 0.6 16.3 ± 0.5 14.9 ± 0.4 13.8 ± 0.3 13.7 ± 0.4 e 12.2 ± 0.2 9.6 ± 0.3 8.7 ± 0.2 8.1 ± 0.2
31.0 30.9 26.3 24.4 22.4 21.5 e 19.1 17.9 16.8 15.9 14.1
30.3 24.2 20.7 18.9 17.3 16.6 16.0 e 14.0 13.1 12.3 11.4
± ± ± ± ± ± ±
0.8 0.6 0.6 0.7 0.5 0.4 0.3
± ± ± ±
0.5 0.4 0.3 0.3
± ± ± ± ± ±
0.9 0.8 0.7 0.8 0.6 0.6
± ± ± ± ±
0.5 0.4 0.5 0.4 0.4
VS (g/L) 28.6 26.4 23.2 20.9 19.3 18.2 e 16.1 14.6 13.6 12.5 11.1
± ± ± ± ± ±
0.8 0.8 0.7 0.7 0.5 0.6
± ± ± ± ±
0.5 0.4 0.5 0.4 0.3
Table 1 Main characteristics of the substrates and inoculum. Materials
Total solid (%)
Volatile solid (%)
C/N
Hemicellulose (%VS)
Cellulose (%VS)
Lignin (%VS)
Green corn stover Air-dried corn stover Inoculum
20.2 ± 0.7 91.4 ± 3.8 3.6 ± 0.1
17.9 ± 0.7 84.2 ± 2.9 2.2 ± 0.1
28.5 ± 1.1 26.0 ± 0.9 11.8 ± 0.3
31.8 ± 0.9 24.0 ± 0.8 e
39.8 ± 1.1 44.0 ± 1.3 e
11.0 ± 0.3 14.1 ± 0.5 e
D. Li et al. / Energy 102 (2016) 1e9
nðtÞ ¼
3
dMðtÞ dt
Rm ,e Rm ,e ¼ Rm ,exp 2 þ ,ðl tÞ exp ,ðl tÞ þ 1 P P (2)
where v is the methane production rate (L/(kgVS$d)). The time at which v achieves vmax (tmax) can be calculated as:
tmax ¼ l þ
P Rm ,e
(3) Fig. 1. Relationship between substrate degradation and methane production.
2.4.2. Substrate degradation fitting model Assuming that the total VS was comprised of biodegradable and nonbiodegradable fractions and that biodegradable VS could be divided into water-soluble and non-water-soluble fractions, the following equations were obtained:
VST ¼ VSB þ VSNB
(4)
VSB ¼ VSB;S þ VSB;NS
(5)
where VST,t and VSB,t are the total VS and biodegradable VS content at time t, and VST,0 and VSB,0 are the initial total VS and initial biodegradable VS content at time zero, respectively, calculated using the following equation:
VSB;0 ¼ VST;0 VSNB
(11)
By substituting Eqs. (10) and (11) into Eq. (4), the following equation is obtained:
where VST is the total VS content (g/L), VSNB is the nonbiodegradable VS content (g/L, or % VST), VSB is the biodegradable VS content (g/L, or % VST), VSB,S is the biodegradable and water-soluble VS content (g/L, or % VST), and VSB,NS is degradable and non-watersoluble VS content (g/L, or % VST). The substrate degradation model (Eq. (6)), which was proposed by Ørskov and McDonald to model the digestion of biomass in the rumen [19], was adopted for degradation in an anaerobic reactor.
Adjustment of the pairs of experimental data (t, VST,t) by nonlinear regression allowed the calculation of the fraction of VSNB and kH,B. The half-life of hydrolysis of biodegradable VS was calculated using the following equation:
DðtÞ ¼ VSB;S þ VSB;NS ,ð1 expð a,tÞÞ
t1=2 ¼ ln 2=kH
(6)
where D is the degradation proportion (%) at a fermentation time t (d), VSB,S is the biodegradable and water-soluble VS content (% VST), VSB,NS is the degradable and non-water-soluble VS content (% VST), and a is the model constant (1/d). The values of VSB,S, VSB,NS, and a can be obtained by data fitting. 2.4.3. First-order kinetics of hydrolysis and methane production In order to describe the evolution of the VS content with time, the following differential equations, describing hydrolysis as a firstorder reaction not directly coupled to bacterial growth, were considered:
dVST ¼ kH;T ,VST dt
(7)
(8)
where kH,T is the conventional apparent hydrolysis kinetic constant (1/d) for hydrolysis of substrate including nonbiodegradable fractions, and kH,B is the apparent hydrolysis kinetic constant (1/d) for hydrolysis of substrate without consideration of nonbiodegradable fractions. Eqs. (7) and (8) can be integrated to obtain the following equations:
VSB;t ¼ VSB;0 ,exp kH;B ,t
(9) (10)
(12)
(13)
where t1/2 is the time required for hydrolysis of half of the VSB. It was assumed that methane generation was directly proportional to the degraded VS, and the following equation could be formulated:
VSB;0 VSB;t ¼ Yp$G
(14)
where Yp is the methane yield coefficient, and G the volume of methane accumulated at time t.
Table 3 Model parameters of methane production and substrate degradation. Model parameters
dVSB ¼ kH;B ,VSB dt
VST;t ¼ VST;0 ,exp kH;T ,t
VST;t ¼ VSNB þ VST;0 VSNB ,exp kH;B ,t
Green corn stover
Methane production process P (mL/g VS) 320.1 Rm (mL/(g VS$d)) 38.17 € e (d) 0.22 tmax (d) 3.3 2 R 0.9951 Substrate degradation process VSNB (g/L)a 3.47 VSNB (%VS) 12.9 VSNB (g/L)b 4.67 VSNB (%VS) 17.4 VSB,S (g/L) 5.51 VSB,S (%VS) 20.5 VSB,NS (g/L) 15.33 VSB,NS (%VS) 57.0 a (1/d) 0.0930 R2 0.9804 a b
Air-dried corn stover 234.8 15.77 0.54 6.0 0.9967 7.04 24.7 6.82 23.9 1.51 5.3 16.83 59.4 0.1128 0.9894
VSNB was obtained using the graphical method (Fig. 3b). VSNB was obtained through simple arithmetic calculations (Eq. (18)).
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Fig. 2. (a) Cumulative methane production and (b) methane production rate during the anaerobic digestion of corn stover.
When t / ∞, VSB,t / 0 and G / G∞, Eq. (14) could be transformed into the following equation [15]:
VSB;0 ¼ Yp$G∞
(15)
By substituting Eq. (15) into Eq. (14), Eq. (16) was obtained to express the relationship between substrate degradation and methane production, as illustrated in Fig. 1:
VSB;0 VSB;t VSB;t G G∞ G ¼ or ¼ G∞ G∞ VSB;0 VSB;0
3. Results and discussion 3.1. Methane production and substrate degradation
(16)
Similar to the hydrolysis kinetic model of Eq. (10), the methane production kinetic model of Eq. (17) could be obtained:
Gt ¼ G∞ ð1 expð kM ,tÞÞ
2.4.4. Data processing The experimental data were analyzed and plotted using Origin software (version 8.0). Regression analysis was carried out using Curve Expert (version 1.4).
(17)
where G∞ is the ultimate methane production obtained at an infinite digestion time, and kM is the apparent kinetic constant for methane production. The kinetic constant kM is different from kH because hydrolysis and methanogenesis are carried out by different groups of microorganisms. The values of G∞ and kM were calculated by nonlinear regression of the pairs of experimental data (t, Gt).
Cumulative methane production is shown in Fig. 2a. The modified Gompertz equation could not perfectly fit with cumulative methane production, despite the high determination coefficients (R2 values) of 0.9951 and 0.9967 for green and air-dried corn stover, respectively. The values of P, Rm, l, and tmax are listed in Table 3. Maximum methane production amounts and maximum methane production rates were higher for green corn stover than for air-dried corn stover owing to the occurrence of the serious lignification and loss of organic matter, particularly soluble carbohydrates. The methane yields in this study were higher than that of previous studies. A methane yield of 265.1 mL/g VS was obtained by mesophilic anaerobic digestion of fungal-pretreated corn stover silage [20]. A methane yield of 194.8 mL/g VS was obtained by
D. Li et al. / Energy 102 (2016) 1e9
5
Fig. 3. (a) Degradation rate during anaerobic digestion and (b) solving graph for the nonbiodegradable fraction of corn stover.
thermophilic solid-state anaerobic digestion of alkaline-pretreated corn stover [21]. The differential modified Gompertz equation did not accurately reflect the methane production rate (Fig. 2b). The values of Rm were much lower than the experimental values for both green and airdried corn stover. Additionally, the value of tmax was greater than the experimental value for air-dried corn stover. For green corn stover, the methane production process was divided into two main stages. The first stage, occurring during days 0e3, was a microorganism-limited process, while the second stage, occurring during days 3e20, was a substrate-limited process. For the second stage, the methane production rate during days 3e7 exhibited a sharp decline, with a slow decline after day 7. This may be because easily degradable organic matter, including VSB,S, such as monosaccharides and partial VSB,NS such as hemicellulose, were almost completely consumed for production of methane before day 7. Microorganisms can only use refractory organic matter, such as cellulose, to produce methane after day 7. Thus, in this study, we defined day 7 as the methane production inflection point. Similar results were obtained for air-dried corn stover. Table 2 summarizes the substrate concentrations during the digestion of corn stover. The VST degradation proportions after 20 days of digestion were 69.9% and 61.3% for green and air-dried corn
stover, respectively. They were similar to the results of previous studies, where the VS reduction was 57.5e70.5% for anaerobic digestion of NaOH pretreated corn stover with initial substrate of 35e80 g/L based on TS. Fig. 3a shows both the observed and modeled substrate degradation proportion (Eq. (6)) for the investigated samples. The model fit well with the observed data, yielding determination coefficients of 0.9804 and 0.9894 for green and airdried corn stover, respectively. The values of VSB,S, VSB,NS, and a are listed in Table 3. The VSB,S content of green corn stover was 3.9-fold higher than that of air-dried corn stover. In order to obtain the VSNB content, the graphical method (Fig. 3b) was employed using the reciprocal of time and the observed substrate concentrations as horizontal and vertical coordinates, respectively. The obtained VSNB contents are listed in Table 3. 3.2. First-order kinetics of hydrolysis and methane production Fig. 4 displays both the experimental and simulated VST,t and G t according to Eqs. (12) and (17). Kinetic parameters are summarized in Table 4. The hydrolysis kinetic model of Eq. (12) did not fit the experimental data for both the green and air-dried corn stover. Unfortunately, the experimental value of VST,t continued to fall,
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D. Li et al. / Energy 102 (2016) 1e9
methane production kinetic model of Eq. (17) could perfectly fit the experimental data for both the green and air-dried corn stover. Taking green corn stover as example, the experimental data curve continued to rise, whereas the modified Gompertz fitted curve levels off (Fig. 2a). The experimental data curve and the methane production kinetic model curve both to continued to rise (Fig. 4a). Occasionally, the modified Gompertz is mistakenly considered to be a methane production kinetic model [23]. In fact, the modified Gompertz Eq. (1) is only used to fit the experimental data, whereas the methane production kinetic model of Eq. (17) can be used to predict methane production at different digestion times. The parameter G∞ of the first-order kinetic equation was different from the modified Gompertz equation parameter P. G∞ represents the theoretical ultimate methane production when all VSB is degraded over an infinite digestion time t∞ (69.2 and 182.3 days for green and air-dried corn stover, respectively). P is the maximum methane production over the experimental digestion period (20 days). The theoretical ultimate methane production and corresponding digestion time could be obtained through short digestion experiments using a first-order methane production kinetic model. In this study, P was 92.2% of G∞ for green corn stover and 73.5% for airdried corn stover. For engineering applications, anaerobic digesters were designed to obtain at least 90% of the theoretical ultimate methane production [24]. The recommended retention times would be 15 and 37 days for green and air-dried corn stover, respectively, which were calculated from the first-order methane production kinetic Eq. (17). 3.3. Estimate of VSNB content
Fig. 4. Kinetic model curve for methane production and hydrolysis during the anaerobic digestion of corn stover.
whereas the fitted curve leveled off. The values of VSNB obtained using Eq. (12) were much greater than the experimental final VST. We concluded that Eq. (12) was not a good description of the entire hydrolysis process. The lag-phase should not be included in the fermentation period when the apparent kinetic constant (kM) is obtained using Eq. (17) as the kinetic model for methane production [22]. Therefore, the zero times used in the kinetic model for methane production were experimental days 0.22 and 0.54 for green and air-dried corn stover. Different from the modified Gompertz model of Eq. (1), the
Table 4 Kinetic parameters of hydrolysis and methane production during the digestion of corn stover. Kinetic parameters
Green corn stover
Hydrolysis kinetic model VSNB(g/L)a 9.84 VSNB(% VS) 36.6 kH,B (1/d) 0.2661 R2 0.9667 t1/2(d) 2.6 Methane production kinetic model G∞ (mL/gVS) 347.1 kM (1/d) 0.1585 R2 0.9974 t∞ (d) 69.2 t90 (d)b 15 a b
Air-dried corn stover 11.33 39.7 0.1489 0.9907 4.7 319.4 0.0624 0.9994 182.3 37
VSNB was obtained using hydrolysis kinetic Eq. (12). The time for methane production to reach 90% of ultimate methane production.
In addition to the graphical method shown in Fig. 3b and fitting of the model (Eq. (12)) to the data, VSNB could be obtained through simple arithmetic calculations. VSNB (% VST) was obtained by calculation based on COD (chemical oxygen demand) using Eq. (18):
VSNB ð%VST Þ ¼
CODVST;0 CODG∞ CODVSNB 100% ¼ CODVST;0 CODVST;0
(18)
The CODs of VST,0 and G∞ were obtained from Eqs. (19) and (20), respectively:
C6 H10 O5 þ 6O2 /6CO2 þ 5H2 O 162ðVSÞ 192ðCODÞ
(19)
CH4 þ 2O2 /CO2 þ 2H2 O 22:4ðLÞ 64ðg CODÞ
(20)
The nonbiodegradable fractions of VS were 17.4% and 23.9% for green and air-dried corn stover, respectively. The VSNB obtained using the graphical method (Fig. 3b) was similar to that obtained using Eq. (18) (Table 3). This suggested that the graphical method was an effective approach for obtaining the VSNB of substrates. For determination of VSNB using Eq. (12), adding VSNB to the values of VSB,S and VSB,NS listed in Table 3 would result in total values greater than 100% for both green and air-dried corn stover. Thus, we concluded that the hydrolysis kinetic model of Eq. (12) was not suitable for anaerobic hydrolysis of corn stover, and that the value of VSNB could not be obtained using the hydrolysis kinetic model. 3.4. Sectionalized hydrolysis kinetics As described above, the hydrolysis kinetic model described in Eq. (12) did not fit with the substrate hydrolysis of the entire 20-day process. As shown in Fig. 4, there were systematic deviations
D. Li et al. / Energy 102 (2016) 1e9
7
Fig. 5. Sectionalized hydrolysis kinetic model curve for two stages of anaerobic digestion of corn stover.
Table 5 The anaerobic hydrolysis kinetic constant of corn stover. Substrate
Green corn stover Air-dried corn stover
First stage
Second stage
Intersection point
kH,1 (1/d)
R2
kH,2 (1/d)
R2
t(d)
VST (gVS/L)
0.1701 0.1052
0.9871 0.9952
0.0415 0.0360
0.9813 0.9955
2.6 3.1
16.9 20.7
Fig. 6. Inflection point of the hydrolysis rate during anaerobic digestion of corn stover.
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Table 6 Lignocellulose components of solid residues at different digestion times. Component Green corn stover Hemicellulose (g) Cellulose (g) Lignin (g) Air-dried corn stover Hemicellulose (g) Cellulose (g) Lignin (g) a
Raw stalk
Residue at hydrolysis intersection point
Residue at methane production intersection point
Final residue
1.75 ± 0.07 2.19 ± 0.08 0.61 ± 0.03
0.62 ± 0.03 (64.35)a 1.64 ± 0.05 (25.11) 0.60 ± 0.03 (1.99)
0.53 ± 0.02 (69.78) 1.04 ± 0.04 (52.70) 0.60 ± 0.02 (1.99)
0.12 ± 0.01 (93.02) 0.74 ± 0.03 (66.36) 0.55 ± 0.02 (10.05)
1.33 ± 0.05 2.45 ± 0.07 0.78 ± 0.03
0.85 ± 0.02 (36.13) 1.96 ± 0.06 (19.97) 0.74 ± 0.03 (5.02)
0.63 ± 0.03 (52.77) 1.42 ± 0.06 (42.26) 0.69 ± 0.2 (12.21)
0.17 ± 0.1 (86.99) 0.92 ± 0.04 (62.55) 0.57 ± 0.02 (26.74)
Figures in brackets indicate the degradation rates (%).
between the fitted curve and the data. The value of kH,B reflected the methane production rate or substrate hydrolysis rate of the entire process and may be valid for any period for a simple substrate, such as glucose. However, the hydrolysis kinetic model of Eq. (12) was not expected to be reliable for any period for complex substrates, such as corn stover, which contain easily biodegradable soluble sugar and hemicellulose, refractory cellulose, and nonbiodegradable lignin. The entire degradation process of complex substrates containing both easily biodegradable and refractory fractions may not be the real first-order reaction. The rate of hydrolysis during the first 3 days was faster than that during subsequent days for both green and air-dried corn stover. There were two first-order decay periods, characterized by different values of kH. The sectionalized hydrolysis kinetics were analyzed based on Eq. (9) for the first stage (0e3 days) and second stage (5e20 days). The sectionalized hydrolysis kinetic model curves and the fitted values of the kinetic constants kH,1 and kH,2 are shown in Fig. 5 and Table 5. For green corn stover, the determination coefficients (R2) increased from 0.9667 for the entire process hydrolysis model of Eq. (12) to 0.9871 and 0.9813 for the sectionalized hydrolysis model of Eq. (9). Thus, the sectionalized hydrolysis model was able to fit to the data better than the entire process hydrolysis model for corn stover, particularly green corn stover. The intersection point of the hydrolysis rate could be obtained through extrapolating the first curve forwards and the second curve backwards and finding the intersection point (Fig. 6). The time and substrate concentration at the intersection point are shown in Table 5. These results can be explained by the following hypothesis. Soluble monosaccharides and oligosaccharides and easily hydrolyzed hemicellulose and cellulose were quickly dissolved and enzymatically hydrolyzed with a kH of 0.1701 1/d during the first 3 days, while refractory hemicellulose, cellulose and lignin were slowly hydrolyzed with a kH of 0.0415 1/d during subsequent days. Table 6 lists the lignocellulose components of solid residues at different digestion times. The easily hydrolyzed hemicellulose and cellulose, which accounted for 64.35% and 25.11% of total hemicellulose and cellulose, were degraded during the first phase. The refractory hemicellulose, cellulose and lignin, which accounted for 28.67%, 41.25% and 8.06% of total hemicellulose, cellulose and lignin, respectively, were degraded during the second phase. During the anaerobic digestion of corn stover, hemicellulose was primarily degraded before the hydrolysis intersection point, cellulose was mainly degraded before the methane production intersection point, and lignin was degraded after the methane production intersection point.
4. Conclusions There were two first-order decay periods for hydrolysis of corn stover. The conventional first-order hydrolysis kinetic model was
not suitable for describing the entire hydrolysis process of corn stover. The graphical method was an effective approach for obtaining the VSNB of substrates. The ultimate methane production and corresponding digestion time could be understood through the methane production kinetic model. Hemicellulose was primarily degraded before the hydrolysis intersection point, cellulose was mainly degraded before the methane production intersection point, and lignin was degraded after the methane production intersection point. Acknowledgments This study was supported by the National Natural Science Foundation of China (21106145), the Open Found of Jiangsu Key Laboratory for Biomass Energy and Material (JSBEM201604), the National Key Technology Support Program of China (2015BAD21B01), and the Sichuan Science and Technology Program (2014HH0036). References [1] Adl M, Sheng K, Gharibi A. Technical assessment of bioenergy recovery from cotton stalks through anaerobic digestion process and the effects of inexpensive pre-treatments. Appl Energ 2012;93:251e60. [2] 4630 V. Fermentation of organic materials e characterisation of the substrate, sampling, collection of material data, fermentation tests. Düsseldorf: Verein Deutscher Ingenieure; 2006. [3] Pham CH, Triolo JM, Cu TTT, Pedersen L, Sommer SG. Validation and recommendation of methods to measure biogas production potential of animal manure. Asian Australas J Anim Sci 2013;26(6):864e73. [4] Triolo JM, Pedersen L, Qu H, Sommer SG. Biochemical methane potential and anaerobic biodegradability of non-herbaceous and herbaceous phytomass in biogas production. Bioresour Technol 2012;125:226e32. [5] Menardo S, Balsari P. An analysis of the energy potential of anaerobic digestion of agricultural by-products and organic waste. Bioenerg Res 2012;5(3): 759e67. [6] Pastor L, Ruiz L, Pascual A, Ruiz B. Co-digestion of used oils and urban landfill leachates with sewage sludge and the effect on the biogas production. Appl Energ 2013;107:438e45. [7] Angelidaki I, Alves M, Bolzonella D, Borzacconi L, Campos JL, Guwy AJ, et al. Defining the biomethane potential (BMP) of solid organic wastes and energy crops: a proposed protocol for batch assays. Water Sci Technol 2009;59(5):927e34. [8] Kaparaju P, Serrano M, Angelidaki I. Optimization of biogas production from wheat straw stillage in UASB reactor. Appl Energ 2010;87(12):3779e83. [9] Nizami AS, Orozco A, Groom E, Dieterich B, Murphy JD. How much gas can we get from grass? Appl Energ 2012;92:783e90. [10] Gopalan P, Jensen PD, Batstone DJ. Biochemical methane potential of beef feedlot manure: impact of manure age and storage. J Environ Qual 2013;42(4):1205e12. [11] Browne JD, Murphy JD. Assessment of the resource associated with biomethane from food waste. Appl Energ 2013;104:170e7. [12] Vavilin VA, Fernandez B, Palatsi J, Flotats X. Hydrolysis kinetics in anaerobic degradation of particulate organic material: an overview. Waste Manag 2008;28(6):941e53. s Díaz J, Pereda Reyes I, Lundin M, S ri Horva th I. Co-digestion of [13] Page arva different waste mixtures from agro-industrial activities: kinetic evaluation and synergetic effects. Bioresour Technol 2011;102(23):10834e40. [14] Rao MS, Singh SP. Bioenergy conversion studies of organic fraction of MSW: kinetic studies and gas yield-organic loading relationships for process optimisation. Bioresour Technol 2004;95(2):173e85.
D. Li et al. / Energy 102 (2016) 1e9 [15] Raposo F, Borja R, Martin MA, Martin A, de la Rubia MA, Rincon B. Influence of inoculum-substrate ratio on the anaerobic digestion of sunflower oil cake in batch mode: process stability and kinetic evaluation. Chem Eng J 2009;149(1e3):70e7. [16] APHA. Standard methods for the examination of water and wastewater. 21st ed. American Public Health Association; 1998. [17] Vansoest J. Use of detergents in analysis of fibrous feeds. 2. A rapid method for determination of fiber and lignin. J Assoc Off Agric Chem 1963;46(5): 829e31. [18] Nopharatana A, Pullammanappallil PC, Clarke WP. Kinetics and dynamic modelling of batch anaerobic digestion of municipal solid waste in a stirred reactor. Waste Manag 2007;27(5):595e603. [19] Ørskov ER, McDonald I. The estimation of protein degradability in the rumen from incubation measurements weighted according to rate of passage. J Agric Sci 1979;92(02):499e503.
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[20] Liu S, Li X, Wu SB, He J, Pang CL, Deng Y, et al. Fungal pretreatment by phanerochaete chrysosporium for enhancement of biogas production from corn stover silage. Appl Biochem Biotech 2014;174(5):1907e18. [21] Li YQ, Zhang RH, He YF, Liu XY, Chen C, Liu GQ. Thermophilic solid-state anaerobic digestion of alkaline-pretreated corn stover. Energ Fuel 2014;28(6):3759e65. [22] Li D, Sun YM, Yuan ZH, Kong XY, Zhang Y. Kinetic study of the mesophilic anaerobic digestion of organic waste components. Acta Energ Solaris Sin 2010;31(3):385e90. [23] Li LH, Kong XY, Yang FY, Li D, Yuan ZH, Sun YM. Biogas production potential and kinetics of microwave and conventional thermal pretreatment of grass. Appl Biochem Biotech 2012;166(5):1183e91. [24] Li D, Liu S, Mi L, Li Z, Yuan Y, Yan Z, et al. Effects of feedstock ratio and organic loading rate on the anaerobic mesophilic co-digestion of rice straw and pig manure. Bioresour Technol 2015;187:120e7.
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Energy journal homepage: www.elsevier.com/locate/energy
Kinetics of methane production and hydrolysis in anaerobic digestion of corn stover Dong Li a, b, Xianbo Huang c, Qingjing Wang a, Yuexiang Yuan a, Zhiying Yan a, Zhidong Li a, Yajun Huang c, Xiaofeng Liu a, * a
Key Laboratory of Environmental and Applied Microbiology, Environmental Microbiology Key Laboratory of Sichuan Province, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China Jiangsu Key Laboratory for Biomass Energy and Material, Jiangsu Province, Nanjing 210042, China c Chengdu Organic Chemicals Co., LTD., Chinese Academy of Sciences, Chengdu 610041, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 December 2015 Received in revised form 8 February 2016 Accepted 12 February 2016 Available online xxx
In order to develop a time-saving method for determination of ultimate methane production, obtain the hydrolysis kinetic constant, and identify a determination method for the nonbiodegradable organic fraction of substrate (VSNB) of green and air-dried corn stover, the kinetics of methane production and hydrolysis were studied using batch tests. The results showed that the conventional first-order hydrolysis kinetic model was not suitable for describing the entire hydrolysis process of corn stover, because there were two first-order decay periods for hydrolysis of corn stover. The hydrolysis kinetic constants kH,1 and kH,2 of the first and second periods were 0.1701 and 0.0415 1/d for green stover and 0.1052 and 0.0360 1/ d for air-dried stover. The value of VSNB could be obtained by the graphical method rather than by the hydrolysis kinetic model. The obtained VSNB contents were 12.9% and 24.7% of VS (volatile solid) for green and air-dried stover, respectively. The ultimate methane production and corresponding digestion time could be understood through the methane production kinetic model by digestion experiments within a short time. The ultimate methane productions were 347.1 and 319.4 mL/g based on VS and the corresponding digestion times were 69.2 and 182.3 days for green and air-dried stover, respectively. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Anaerobic processes Biogas Modelling Kinetic parameters Hydrolysis Nonbiodegradable
1. Introduction The Methane yield potential and methane production rate of substrates are two important indices for anaerobic biogas production. Correspondingly, the biodegradable organic fractions and hydrolysis rates of substrates are two important parameters for anaerobic biodegradation. In fact, these parameters determine, to a certain extent, both the design and economic details of biogas plants. Anaerobic batch tests are the standard method for the determination of methane yield potential of substrates [1]. When performing a batch test, the decision of when to terminate the experiment is crucial. In 2006, the Association of German Engineers (VDI) published the guideline VDI 4630, “Fermentation of organic materials, characterization of the substrate, sampling, collection of
* Corresponding author. No. 9 Section 4, Renmin Nan Road, Chengdu, Sichuan, China. Tel.: þ86 2882890229; fax: þ86 2882890233. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.energy.2016.02.074 0360-5442/© 2016 Elsevier Ltd. All rights reserved.
material data, fermentation tests” [2]. This guideline introduces the so-called 1% criterion, in which the experiment is continued until the daily gas production is less than 1% of the total gas production. Many experiments have been performed according to this guideline [3e6]. The ultimate methane production, known as the BMP (biochemical methane potential) of the substrate, is theoretically reached after infinite incubation. In 2009, a guideline for BMP assays was proposed by the ‘Task Group for the Anaerobic Biodegradation, Activity and Inhibition of the Anaerobic Digestion’ of the IWA (International Water Association) [7]. In this protocol, batch experiments were not considered complete until no methane was produced. This protocol is now the BMP test basis for most of experiments performed. The methane potential of wheat straw stillage was determined in 118 mL serum bottles with working volume of 40 mL [8]. The small and large BMP assays of grass silage were carried out in small serum bottles and 1.5 L reactor, respectively [9]. BMP test was conducted in 240 mL serum bottle with working volume 100 mL using beef feedlot manure as substrate [10]. BMP tests were carried out at different scales (5 L and 0.5 L) for both wet
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D. Li et al. / Energy 102 (2016) 1e9
and dried food waste [11]. However, BMP assays require long experimental times, and the method proposed by VDI 4630 does not yield the true ultimate methane production. Anaerobic degradation of particulate organic matter involves a complex network of reactions in series and in parallel involving several key groups of bacteria. Hydrolysis of particulate matter to soluble substrates is often assumed to be the rate-limiting step in anaerobic digestion. Substrate hydrolysis and biogas production typically follow first-order kinetics when macro- and micronutrients are available and there is no inhibition [12e14]. To date, first-order kinetics have been used to model the hydrolysis process of anaerobic digestion of organic fractions of municipal solid waste [14], sunflower oil cake [15], and mixtures of manure, animal feed, slaughterhouse waste, and municipal solid waste [13] in batch mode. However, there are no first-order hydrolysis kinetics studies of corn stover. Moreover, the values of the hydrolysis kinetic constants (kH) of these models were calculated indirectly from data of methane production rather than substrate reduction. Notably, the nonbiodegradable organic fraction of the substrate cannot be obtained by using traditional first-order hydrolysis kinetics. The aims of this study were to develop a time-saving method for determination of ultimate methane production and corresponding digestion time, to identify an appropriate determination method for the nonbiodegradable organic fraction of substrate, and to obtain the hydrolysis kinetic constant of corn stover. 2. Material and methods
parallel. Prior to operation, the reactors were flushed with nitrogen for 5 min to ensure anaerobic conditions. The digesters were placed in a water bath at 35 ± 1 C. Each reactor was manually mixed twice a day. The experiment was carried out for 20 days. Two reactors were randomly removed for measurement of the TS and VS (volatile solid) contents of the digested residues at different times (Table 2). 2.3. Analytical methods The TS and VS contents were determined using standard techniques [16]. Analyses for C and N were conducted using a Vario EL element analyzer (Elementar Analysensysteme GmbH, Germany). Cellulose, hemicellulose, and lignin contents were determined as previously described [17]. Biogas production was estimated by measuring the water displacement. Biogas analysis was performed using an Agilent 6890 GC (gas chromatography) system (Agilent Technologies, USA) with a TCD (thermal conductivity detector) and a 2-m stainless steel column packed with Porapak Q (50/80 mesh). The operating temperatures at the injection port, column oven, and detector were 200, 80, and 200 C, respectively. Argon was used as the carrier gas at a flow rate of 30 mL/min. 2.4. Process fitting and first-order kinetic model 2.4.1. Methane production fitting model A modified Gompertz model (Eq. (1)) was used to fit the data of methane production [18]:
2.1. Substrates and inoculums Green and air-dried corn stover with harvest time of 6 and 8 months, respectively, were obtained from rural Chengdu, China. The collected stover was chopped and sieved to particles measuring less than 5 mm in size. The sieved green corn stover was stored at 4 C. The residue left on a 1-mm sieve of material taken from a mesophilic (37 C) anaerobic digester fed with pig manure was used as the inoculum. The anaerobic digestion of the inoculum was conducted untill no biogas production in order to ignore the methane production of the inoculum itself. The characteristics of the substrates and inoculum are listed in Table 1. The lignin and cellulose contents of air-dried corn stover were higher than that of green corn stover, since the serious lignification was occurred for air-dried corn stover. 2.2. Experimental setup and operation
Rm ,e ,ðl tÞ þ 1 MðtÞ ¼ P,exp exp P
where M is the cumulative methane production (L/kgVS) at a fermentation time t (d), P is the maximum cumulative methane production (L/kgVS) for the entire experimental digestion time, Rm is the maximum methane production rate (L/(kgVS$d)), and l is the lag-phase time (d). The values of P, Rm, and l were obtained by data fitting. The methane production rate (Eq. (2)) was obtained by differentiating the modified Gompertz equation,
Table 2 Substrate concentrations during the digestion of corn stover. Time (d)
A 250-mL bottle with working volume 200 mL was used for the batch anaerobic digestion tests. The bottle was sealed using a rubber stopper with a pipe for extracting the biogas. The reactor was connected to a gas collection system consisting of a saturated brine displacement bottle and a brine gathering bottle. The initial substrate concentration was about 30 g/L based on TS (total solid). The batch reactors were filled with 30 g of green corn stover or 6.5 g of air-dried corn stover before adding inoculum to a total weight of 200 g. The VS (volatile solid) ratio of substrate to inoculum was 1.44 and 1.29 for batch digestions of green and air-dried corn stover. Each test of green and air-dried corn stover had 20 reactors in
(1)
0 1 2 3 5 7 9 10 12 15 18 20
Green corn stover
Air-dried corn stover
TS (g/L)
VS (g/L)
TS (g/L)
26.9 ± 0.6 21.2 ± 0.5 18.7 ± 0.6 16.3 ± 0.5 14.9 ± 0.4 13.8 ± 0.3 13.7 ± 0.4 e 12.2 ± 0.2 9.6 ± 0.3 8.7 ± 0.2 8.1 ± 0.2
31.0 30.9 26.3 24.4 22.4 21.5 e 19.1 17.9 16.8 15.9 14.1
30.3 24.2 20.7 18.9 17.3 16.6 16.0 e 14.0 13.1 12.3 11.4
± ± ± ± ± ± ±
0.8 0.6 0.6 0.7 0.5 0.4 0.3
± ± ± ±
0.5 0.4 0.3 0.3
± ± ± ± ± ±
0.9 0.8 0.7 0.8 0.6 0.6
± ± ± ± ±
0.5 0.4 0.5 0.4 0.4
VS (g/L) 28.6 26.4 23.2 20.9 19.3 18.2 e 16.1 14.6 13.6 12.5 11.1
± ± ± ± ± ±
0.8 0.8 0.7 0.7 0.5 0.6
± ± ± ± ±
0.5 0.4 0.5 0.4 0.3
Table 1 Main characteristics of the substrates and inoculum. Materials
Total solid (%)
Volatile solid (%)
C/N
Hemicellulose (%VS)
Cellulose (%VS)
Lignin (%VS)
Green corn stover Air-dried corn stover Inoculum
20.2 ± 0.7 91.4 ± 3.8 3.6 ± 0.1
17.9 ± 0.7 84.2 ± 2.9 2.2 ± 0.1
28.5 ± 1.1 26.0 ± 0.9 11.8 ± 0.3
31.8 ± 0.9 24.0 ± 0.8 e
39.8 ± 1.1 44.0 ± 1.3 e
11.0 ± 0.3 14.1 ± 0.5 e
D. Li et al. / Energy 102 (2016) 1e9
nðtÞ ¼
3
dMðtÞ dt
Rm ,e Rm ,e ¼ Rm ,exp 2 þ ,ðl tÞ exp ,ðl tÞ þ 1 P P (2)
where v is the methane production rate (L/(kgVS$d)). The time at which v achieves vmax (tmax) can be calculated as:
tmax ¼ l þ
P Rm ,e
(3) Fig. 1. Relationship between substrate degradation and methane production.
2.4.2. Substrate degradation fitting model Assuming that the total VS was comprised of biodegradable and nonbiodegradable fractions and that biodegradable VS could be divided into water-soluble and non-water-soluble fractions, the following equations were obtained:
VST ¼ VSB þ VSNB
(4)
VSB ¼ VSB;S þ VSB;NS
(5)
where VST,t and VSB,t are the total VS and biodegradable VS content at time t, and VST,0 and VSB,0 are the initial total VS and initial biodegradable VS content at time zero, respectively, calculated using the following equation:
VSB;0 ¼ VST;0 VSNB
(11)
By substituting Eqs. (10) and (11) into Eq. (4), the following equation is obtained:
where VST is the total VS content (g/L), VSNB is the nonbiodegradable VS content (g/L, or % VST), VSB is the biodegradable VS content (g/L, or % VST), VSB,S is the biodegradable and water-soluble VS content (g/L, or % VST), and VSB,NS is degradable and non-watersoluble VS content (g/L, or % VST). The substrate degradation model (Eq. (6)), which was proposed by Ørskov and McDonald to model the digestion of biomass in the rumen [19], was adopted for degradation in an anaerobic reactor.
Adjustment of the pairs of experimental data (t, VST,t) by nonlinear regression allowed the calculation of the fraction of VSNB and kH,B. The half-life of hydrolysis of biodegradable VS was calculated using the following equation:
DðtÞ ¼ VSB;S þ VSB;NS ,ð1 expð a,tÞÞ
t1=2 ¼ ln 2=kH
(6)
where D is the degradation proportion (%) at a fermentation time t (d), VSB,S is the biodegradable and water-soluble VS content (% VST), VSB,NS is the degradable and non-water-soluble VS content (% VST), and a is the model constant (1/d). The values of VSB,S, VSB,NS, and a can be obtained by data fitting. 2.4.3. First-order kinetics of hydrolysis and methane production In order to describe the evolution of the VS content with time, the following differential equations, describing hydrolysis as a firstorder reaction not directly coupled to bacterial growth, were considered:
dVST ¼ kH;T ,VST dt
(7)
(8)
where kH,T is the conventional apparent hydrolysis kinetic constant (1/d) for hydrolysis of substrate including nonbiodegradable fractions, and kH,B is the apparent hydrolysis kinetic constant (1/d) for hydrolysis of substrate without consideration of nonbiodegradable fractions. Eqs. (7) and (8) can be integrated to obtain the following equations:
VSB;t ¼ VSB;0 ,exp kH;B ,t
(9) (10)
(12)
(13)
where t1/2 is the time required for hydrolysis of half of the VSB. It was assumed that methane generation was directly proportional to the degraded VS, and the following equation could be formulated:
VSB;0 VSB;t ¼ Yp$G
(14)
where Yp is the methane yield coefficient, and G the volume of methane accumulated at time t.
Table 3 Model parameters of methane production and substrate degradation. Model parameters
dVSB ¼ kH;B ,VSB dt
VST;t ¼ VST;0 ,exp kH;T ,t
VST;t ¼ VSNB þ VST;0 VSNB ,exp kH;B ,t
Green corn stover
Methane production process P (mL/g VS) 320.1 Rm (mL/(g VS$d)) 38.17 € e (d) 0.22 tmax (d) 3.3 2 R 0.9951 Substrate degradation process VSNB (g/L)a 3.47 VSNB (%VS) 12.9 VSNB (g/L)b 4.67 VSNB (%VS) 17.4 VSB,S (g/L) 5.51 VSB,S (%VS) 20.5 VSB,NS (g/L) 15.33 VSB,NS (%VS) 57.0 a (1/d) 0.0930 R2 0.9804 a b
Air-dried corn stover 234.8 15.77 0.54 6.0 0.9967 7.04 24.7 6.82 23.9 1.51 5.3 16.83 59.4 0.1128 0.9894
VSNB was obtained using the graphical method (Fig. 3b). VSNB was obtained through simple arithmetic calculations (Eq. (18)).
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D. Li et al. / Energy 102 (2016) 1e9
Fig. 2. (a) Cumulative methane production and (b) methane production rate during the anaerobic digestion of corn stover.
When t / ∞, VSB,t / 0 and G / G∞, Eq. (14) could be transformed into the following equation [15]:
VSB;0 ¼ Yp$G∞
(15)
By substituting Eq. (15) into Eq. (14), Eq. (16) was obtained to express the relationship between substrate degradation and methane production, as illustrated in Fig. 1:
VSB;0 VSB;t VSB;t G G∞ G ¼ or ¼ G∞ G∞ VSB;0 VSB;0
3. Results and discussion 3.1. Methane production and substrate degradation
(16)
Similar to the hydrolysis kinetic model of Eq. (10), the methane production kinetic model of Eq. (17) could be obtained:
Gt ¼ G∞ ð1 expð kM ,tÞÞ
2.4.4. Data processing The experimental data were analyzed and plotted using Origin software (version 8.0). Regression analysis was carried out using Curve Expert (version 1.4).
(17)
where G∞ is the ultimate methane production obtained at an infinite digestion time, and kM is the apparent kinetic constant for methane production. The kinetic constant kM is different from kH because hydrolysis and methanogenesis are carried out by different groups of microorganisms. The values of G∞ and kM were calculated by nonlinear regression of the pairs of experimental data (t, Gt).
Cumulative methane production is shown in Fig. 2a. The modified Gompertz equation could not perfectly fit with cumulative methane production, despite the high determination coefficients (R2 values) of 0.9951 and 0.9967 for green and air-dried corn stover, respectively. The values of P, Rm, l, and tmax are listed in Table 3. Maximum methane production amounts and maximum methane production rates were higher for green corn stover than for air-dried corn stover owing to the occurrence of the serious lignification and loss of organic matter, particularly soluble carbohydrates. The methane yields in this study were higher than that of previous studies. A methane yield of 265.1 mL/g VS was obtained by mesophilic anaerobic digestion of fungal-pretreated corn stover silage [20]. A methane yield of 194.8 mL/g VS was obtained by
D. Li et al. / Energy 102 (2016) 1e9
5
Fig. 3. (a) Degradation rate during anaerobic digestion and (b) solving graph for the nonbiodegradable fraction of corn stover.
thermophilic solid-state anaerobic digestion of alkaline-pretreated corn stover [21]. The differential modified Gompertz equation did not accurately reflect the methane production rate (Fig. 2b). The values of Rm were much lower than the experimental values for both green and airdried corn stover. Additionally, the value of tmax was greater than the experimental value for air-dried corn stover. For green corn stover, the methane production process was divided into two main stages. The first stage, occurring during days 0e3, was a microorganism-limited process, while the second stage, occurring during days 3e20, was a substrate-limited process. For the second stage, the methane production rate during days 3e7 exhibited a sharp decline, with a slow decline after day 7. This may be because easily degradable organic matter, including VSB,S, such as monosaccharides and partial VSB,NS such as hemicellulose, were almost completely consumed for production of methane before day 7. Microorganisms can only use refractory organic matter, such as cellulose, to produce methane after day 7. Thus, in this study, we defined day 7 as the methane production inflection point. Similar results were obtained for air-dried corn stover. Table 2 summarizes the substrate concentrations during the digestion of corn stover. The VST degradation proportions after 20 days of digestion were 69.9% and 61.3% for green and air-dried corn
stover, respectively. They were similar to the results of previous studies, where the VS reduction was 57.5e70.5% for anaerobic digestion of NaOH pretreated corn stover with initial substrate of 35e80 g/L based on TS. Fig. 3a shows both the observed and modeled substrate degradation proportion (Eq. (6)) for the investigated samples. The model fit well with the observed data, yielding determination coefficients of 0.9804 and 0.9894 for green and airdried corn stover, respectively. The values of VSB,S, VSB,NS, and a are listed in Table 3. The VSB,S content of green corn stover was 3.9-fold higher than that of air-dried corn stover. In order to obtain the VSNB content, the graphical method (Fig. 3b) was employed using the reciprocal of time and the observed substrate concentrations as horizontal and vertical coordinates, respectively. The obtained VSNB contents are listed in Table 3. 3.2. First-order kinetics of hydrolysis and methane production Fig. 4 displays both the experimental and simulated VST,t and G t according to Eqs. (12) and (17). Kinetic parameters are summarized in Table 4. The hydrolysis kinetic model of Eq. (12) did not fit the experimental data for both the green and air-dried corn stover. Unfortunately, the experimental value of VST,t continued to fall,
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D. Li et al. / Energy 102 (2016) 1e9
methane production kinetic model of Eq. (17) could perfectly fit the experimental data for both the green and air-dried corn stover. Taking green corn stover as example, the experimental data curve continued to rise, whereas the modified Gompertz fitted curve levels off (Fig. 2a). The experimental data curve and the methane production kinetic model curve both to continued to rise (Fig. 4a). Occasionally, the modified Gompertz is mistakenly considered to be a methane production kinetic model [23]. In fact, the modified Gompertz Eq. (1) is only used to fit the experimental data, whereas the methane production kinetic model of Eq. (17) can be used to predict methane production at different digestion times. The parameter G∞ of the first-order kinetic equation was different from the modified Gompertz equation parameter P. G∞ represents the theoretical ultimate methane production when all VSB is degraded over an infinite digestion time t∞ (69.2 and 182.3 days for green and air-dried corn stover, respectively). P is the maximum methane production over the experimental digestion period (20 days). The theoretical ultimate methane production and corresponding digestion time could be obtained through short digestion experiments using a first-order methane production kinetic model. In this study, P was 92.2% of G∞ for green corn stover and 73.5% for airdried corn stover. For engineering applications, anaerobic digesters were designed to obtain at least 90% of the theoretical ultimate methane production [24]. The recommended retention times would be 15 and 37 days for green and air-dried corn stover, respectively, which were calculated from the first-order methane production kinetic Eq. (17). 3.3. Estimate of VSNB content
Fig. 4. Kinetic model curve for methane production and hydrolysis during the anaerobic digestion of corn stover.
whereas the fitted curve leveled off. The values of VSNB obtained using Eq. (12) were much greater than the experimental final VST. We concluded that Eq. (12) was not a good description of the entire hydrolysis process. The lag-phase should not be included in the fermentation period when the apparent kinetic constant (kM) is obtained using Eq. (17) as the kinetic model for methane production [22]. Therefore, the zero times used in the kinetic model for methane production were experimental days 0.22 and 0.54 for green and air-dried corn stover. Different from the modified Gompertz model of Eq. (1), the
Table 4 Kinetic parameters of hydrolysis and methane production during the digestion of corn stover. Kinetic parameters
Green corn stover
Hydrolysis kinetic model VSNB(g/L)a 9.84 VSNB(% VS) 36.6 kH,B (1/d) 0.2661 R2 0.9667 t1/2(d) 2.6 Methane production kinetic model G∞ (mL/gVS) 347.1 kM (1/d) 0.1585 R2 0.9974 t∞ (d) 69.2 t90 (d)b 15 a b
Air-dried corn stover 11.33 39.7 0.1489 0.9907 4.7 319.4 0.0624 0.9994 182.3 37
VSNB was obtained using hydrolysis kinetic Eq. (12). The time for methane production to reach 90% of ultimate methane production.
In addition to the graphical method shown in Fig. 3b and fitting of the model (Eq. (12)) to the data, VSNB could be obtained through simple arithmetic calculations. VSNB (% VST) was obtained by calculation based on COD (chemical oxygen demand) using Eq. (18):
VSNB ð%VST Þ ¼
CODVST;0 CODG∞ CODVSNB 100% ¼ CODVST;0 CODVST;0
(18)
The CODs of VST,0 and G∞ were obtained from Eqs. (19) and (20), respectively:
C6 H10 O5 þ 6O2 /6CO2 þ 5H2 O 162ðVSÞ 192ðCODÞ
(19)
CH4 þ 2O2 /CO2 þ 2H2 O 22:4ðLÞ 64ðg CODÞ
(20)
The nonbiodegradable fractions of VS were 17.4% and 23.9% for green and air-dried corn stover, respectively. The VSNB obtained using the graphical method (Fig. 3b) was similar to that obtained using Eq. (18) (Table 3). This suggested that the graphical method was an effective approach for obtaining the VSNB of substrates. For determination of VSNB using Eq. (12), adding VSNB to the values of VSB,S and VSB,NS listed in Table 3 would result in total values greater than 100% for both green and air-dried corn stover. Thus, we concluded that the hydrolysis kinetic model of Eq. (12) was not suitable for anaerobic hydrolysis of corn stover, and that the value of VSNB could not be obtained using the hydrolysis kinetic model. 3.4. Sectionalized hydrolysis kinetics As described above, the hydrolysis kinetic model described in Eq. (12) did not fit with the substrate hydrolysis of the entire 20-day process. As shown in Fig. 4, there were systematic deviations
D. Li et al. / Energy 102 (2016) 1e9
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Fig. 5. Sectionalized hydrolysis kinetic model curve for two stages of anaerobic digestion of corn stover.
Table 5 The anaerobic hydrolysis kinetic constant of corn stover. Substrate
Green corn stover Air-dried corn stover
First stage
Second stage
Intersection point
kH,1 (1/d)
R2
kH,2 (1/d)
R2
t(d)
VST (gVS/L)
0.1701 0.1052
0.9871 0.9952
0.0415 0.0360
0.9813 0.9955
2.6 3.1
16.9 20.7
Fig. 6. Inflection point of the hydrolysis rate during anaerobic digestion of corn stover.
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D. Li et al. / Energy 102 (2016) 1e9
Table 6 Lignocellulose components of solid residues at different digestion times. Component Green corn stover Hemicellulose (g) Cellulose (g) Lignin (g) Air-dried corn stover Hemicellulose (g) Cellulose (g) Lignin (g) a
Raw stalk
Residue at hydrolysis intersection point
Residue at methane production intersection point
Final residue
1.75 ± 0.07 2.19 ± 0.08 0.61 ± 0.03
0.62 ± 0.03 (64.35)a 1.64 ± 0.05 (25.11) 0.60 ± 0.03 (1.99)
0.53 ± 0.02 (69.78) 1.04 ± 0.04 (52.70) 0.60 ± 0.02 (1.99)
0.12 ± 0.01 (93.02) 0.74 ± 0.03 (66.36) 0.55 ± 0.02 (10.05)
1.33 ± 0.05 2.45 ± 0.07 0.78 ± 0.03
0.85 ± 0.02 (36.13) 1.96 ± 0.06 (19.97) 0.74 ± 0.03 (5.02)
0.63 ± 0.03 (52.77) 1.42 ± 0.06 (42.26) 0.69 ± 0.2 (12.21)
0.17 ± 0.1 (86.99) 0.92 ± 0.04 (62.55) 0.57 ± 0.02 (26.74)
Figures in brackets indicate the degradation rates (%).
between the fitted curve and the data. The value of kH,B reflected the methane production rate or substrate hydrolysis rate of the entire process and may be valid for any period for a simple substrate, such as glucose. However, the hydrolysis kinetic model of Eq. (12) was not expected to be reliable for any period for complex substrates, such as corn stover, which contain easily biodegradable soluble sugar and hemicellulose, refractory cellulose, and nonbiodegradable lignin. The entire degradation process of complex substrates containing both easily biodegradable and refractory fractions may not be the real first-order reaction. The rate of hydrolysis during the first 3 days was faster than that during subsequent days for both green and air-dried corn stover. There were two first-order decay periods, characterized by different values of kH. The sectionalized hydrolysis kinetics were analyzed based on Eq. (9) for the first stage (0e3 days) and second stage (5e20 days). The sectionalized hydrolysis kinetic model curves and the fitted values of the kinetic constants kH,1 and kH,2 are shown in Fig. 5 and Table 5. For green corn stover, the determination coefficients (R2) increased from 0.9667 for the entire process hydrolysis model of Eq. (12) to 0.9871 and 0.9813 for the sectionalized hydrolysis model of Eq. (9). Thus, the sectionalized hydrolysis model was able to fit to the data better than the entire process hydrolysis model for corn stover, particularly green corn stover. The intersection point of the hydrolysis rate could be obtained through extrapolating the first curve forwards and the second curve backwards and finding the intersection point (Fig. 6). The time and substrate concentration at the intersection point are shown in Table 5. These results can be explained by the following hypothesis. Soluble monosaccharides and oligosaccharides and easily hydrolyzed hemicellulose and cellulose were quickly dissolved and enzymatically hydrolyzed with a kH of 0.1701 1/d during the first 3 days, while refractory hemicellulose, cellulose and lignin were slowly hydrolyzed with a kH of 0.0415 1/d during subsequent days. Table 6 lists the lignocellulose components of solid residues at different digestion times. The easily hydrolyzed hemicellulose and cellulose, which accounted for 64.35% and 25.11% of total hemicellulose and cellulose, were degraded during the first phase. The refractory hemicellulose, cellulose and lignin, which accounted for 28.67%, 41.25% and 8.06% of total hemicellulose, cellulose and lignin, respectively, were degraded during the second phase. During the anaerobic digestion of corn stover, hemicellulose was primarily degraded before the hydrolysis intersection point, cellulose was mainly degraded before the methane production intersection point, and lignin was degraded after the methane production intersection point.
4. Conclusions There were two first-order decay periods for hydrolysis of corn stover. The conventional first-order hydrolysis kinetic model was
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