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The effect of mechanical pretreatment on the anaerobic digestion of Hybrid Pennisetum
Fuel 252 (2019) 469–474
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Full Length Article
The effect of mechanical pretreatment on the anaerobic digestion of Hybrid Pennisetum
T
Xihui Kanga,d, Yi Zhanga,b,c, Bing Songe, Yongming Suna,b, , Lianhua Lia,b,d, , Yu Heb, Xiaoying Konga,c, Xinjian Luoa,d, Zhenhong Yuana,b ⁎
⁎
a
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China CAS Key Laboratory of Renewable Energy, Guangzhou 510640, PR China c Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China d University of Chinese Academy of Sciences, Beijing 100049, PR China e Institute of Bioresource and Agriculture, Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, PR China b
GRAPHICAL ABSTRACT
Flow diagram of mechanical pretreatment and anaerobic digestion of Hybrid pennisetum.
ARTICLE INFO
ABSTRACT
Keywords: Hybrid Pennisetum Biomass Anaerobic digestion Methane Mechanical pretreatment Particle size
This study investigated the effect of particle size on the performance of anaerobic digestion of Hybrid Pennisetum. Hybrid Pennisetum was ground and sieved to provide different particle sizes between 0.180 and 1.000 mm. Characterization of the different particle sizes suggested that the composition of ground Hybrid Pennisetum altered-carbohydrate content decreased and lignin content increased with a decrease in particle size. The highest specific methane yield was 291.9 ± 4.7 mL CH4·g−1 VS at a particle size of 0.250–0.380 mm and this value plateaued as the particle size was reduced to below 0.250 mm. Excessive size reduction did not improve the methane yield but did result in a reduction of digestion time by 28.6–35.7%. The net energy output from the process was calculated at 300 kWh/t VS. Therefore, grinding was proved to enhance the anaerobic fermentation efficiency and energy output of Hybrid Pennisetum.
1. Introduction The depletion of fossil fuels, such as petroleum and natural gas, is accentuating the need for alternative renewable energy sources. ⁎
Biomass, including animal manures, lignocellulosic materials and municipal solid waste, are key resources for renewable energy production. Such biomass can be converted into biofuels via multiple biorefinery technologies [1,2].
Corresponding authors at: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China. E-mail addresses: [email protected] (Y. Sun), [email protected] (L. Li).
https://doi.org/10.1016/j.fuel.2019.04.134 Received 3 January 2019; Received in revised form 25 March 2019; Accepted 24 April 2019 Available online 01 May 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
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Among the sources of lignocellulosic biomass, Hybrid Pennisetum, the cross of Pennisetum americanum and P. purpureum, is regarded as a promising feedstock for a biorefinery due to its high biomass yields, wide distribution and low plantation expenditure [3]. Previous studies have investigated various applications and treatment methods for Hybrid Pennisetum to produce biofuels. For instance, Wang et al. [4] reported that 96.7% glucose was recovered after treating Hybrid Pennisetum with ionic liquid; Yu et al. [5] treated Hybrid Pennisetum with combined dehydration pretreatments and torrefaction, upgrading the fuel properties with elevated carbon content, higher heating value and increased oxygen content. Among the various treatment methods used for Hybrid Pennisetum, anaerobic digestion (AD) has been proved to be an effective method for biogas production [6–9]. However, the recalcitrance of the feedstock can hinder the AD process of Hybrid Pennisetum, leading to a low bioconversion efficiency [10]. To enhance the anaerobic digestion performance of lignocellulosic biomass, various methods have been investigated, including physical, chemical and biological pretreatments [11–13]. Mechanical pretreatment enhances the AD performance of biomass by reducing the particle size, increasing the accessible surface area and decreasing cellulose crystallinity [14,15]. Larger biomass particle sizes have been known to cause low biogas production and mechanical problems in continuous anaerobic reactors [16], however, by reducing the particle size, the bulk density of the biomass increases and facilitates a more successful operation [17,18]. Izumi et al. [19] found that a particle size reduction from 0.888 to 0.718 mm enhanced the methane yield by 28% from food waste. After testing the effects of particle size on the anaerobic digestion of seven agricultural and forest residues, Sharma et al. [20] conclude that it was not economic for reducing the particles below 0.40 mm due to non-significant improvement of methane production compared to 0.088 mm. In addition, excessive size reduction could lead to the build-up of VFAs [21,22]. Furthermore, previous studies have focused on the effects on the method of milling/crushing lignocellulosic biomass or the texture of beads in ball-milling on the methane production/enzyme hydrolysis efficiency, and investigated the effect of mechanical pretreatment on the crystalline changes of biomass [23–25]. Ferreria et al. [26] suggested that the composition of lignocellulosic biomass should be assessed to compare fine and coarse sizes, as the carbohydrate and protein content was inconsistent in the different fractions and had different methane potentials. Thus, this study advances beyond the state of the art by investigating the effect of a grinding mechanical pretreatment on the feedstock Hybrid Pennisetum to indicate the change in composition at different particle sizes, and the effect of particle size on AD performance. The objective of the study was to improve the methane production and determine the optimal particle size for Hybrid Pennisetum.
in a grinder (XB-HP, Xiaobao, China) at different times (30, 60 and 90 s, respectively). The ground samples were dried at 60 °C for 3 days using a drying oven and subsequently screened to obtain mean particle sizes of between 0.15 and 1 mm using a series of sieve sizes: 20 (0.830 mm), 40 (0.380 mm), 60 (0.250 mm) and 80 (0.180 mm). Thus, the sieved particles used in the experiments were not uniform but a mixture of diametrical particles (dp) in the size range: dp > 0.830 mm, 0.380 < dp < 0.830, 0.250 < dp < 0.380, 0.180 < dp < 0.250 and dp < 0.180, respectively. For convenience, the sizes of the sieved particles are termed as > 0.830, 0.380–0.830, 0.250–0.380, 0.180–0.250 and < 0.180 mm. Experiments were conducted in duplicate. The TS, VS, and ash in dried samples were 89.04 ± 0.06%, 81.53 ± 0.04% and 7.53 ± 0.02%, respectively. 2.3. Biomethane potential assay The biomethane potential (BMP) assays of samples of different particle size were investigated using an automatic methane potential test system (Bioprocess Control Sweden AB, AMPTS II). The BMP assays were set according to the previous research [27]. All experiments were performed at 37 ± 0.5 °C in triplicate and lasted for 30 days. Herrmann [28] reported that the ratio of feedstock/inoculum (F/I) was a crucial parameter to obtain the highest methane yields and a stable process. Therefore, in order to investigate the effects of particle size on the methane yields in the BMP, F/I ratio was set to 1 in terms of VS [20,29]. 2.4. Analytical methods The TS, VS, and ash content of all samples were determined through drying at 105 °C for 24 h and subsequent heating at 550 °C for 2 h, according to the standard method [7]. The cellulose, hemicellulose and lignin content of all samples were determined according to the standard procedure provided by NREL [30]. The contents of C, N and H were analyzed by an elemental analyzer (Vario EL cube, Germany), and the O content was calculated by the subtraction rule. 2.5. Calculation methods Using the Buswell equation as shown in Eq.(1) [31], the theoretical biomethane potential (TMP) of samples was calculated using analyzed element contents.
CnHaOb + n
a 4
b H 2O 2
n a b + CH 4 2 8 4 n a b + + CO2 2 8 4
2. Material and methods
(1)
Kinetic studies were conducted to assess the biodegradability, biodegradability efficiency of the substrate, and the fermentation time to reach 80% of the methane volume (T80), with DataFit 9.0(Oakdale Engineering, USA). The biodegradability index was defined as the ratio of BMP yield to the TMP yield. Based on the following modified Gompertz Equation (Eq. (2)) [32], the bespoke Datafit programme generates a list of variables to describe the decay course of organic matter in the batch process.
2.1. Raw material and inoculum Hybrid Pennisetum (Pennisetum americanum × P. purpureum) was grown in Zengcheng District, Guangdong Province, China, and harvested in July 2017. The contents of total solids (TS), volatile solids (VS), and ash in fresh sample were 29.06 ± 0.02%, 26.01 ± 0.03% and 3.01 ± 0.06%, respectively. The inoculum (TS = 6.0%; VS = 1.0%; pH = 7.61) used for the experiments was derived from a mesophilic continuously stirred tank reactor(CSTR)fed with cellulose and grass. The inoculum was starved for 7 days to remove any residual gas.
Y = Ym exp
exp
Rme ( Ym
t) + 1
(2)
where, Y, Ym and Rm correspond to the cumulative CH4 production (mL CH4/g VS), the maximum CH4 production (mL/g VS), and the maximum CH4 production rate (mL/(g VS·d)) respectively. λ is the lag time (d); t represents the incubation time (d), and e is 2.72. R2 is a measure (in %) of how well the kinetic model fits the methane production curve. Energy balance was calculated using the equations as below:
2.2. Mechanical pretreatment Fresh samples of Hybrid Pennisetum were initially cut to a small size of 2–3 cm, manually. To investigate the effect of grinding time on size distribution, the same quality of partitioned samples were then ground 470
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40
Proportion (%)
the C content was stable at 40.09% to 42.42% as the particle size reduced from 0.830 mm to 0.180 mm, whilst there was a minor increase in N content from 0.54% to 0.88%. Thus the corresponding C/N ratio decreased from 78.7 for the largest particles (> 0.830 mm) to 45.6 for the smallest particles (< 0.180 mm), which is consistent with Tamaki et al. [34] who observed an increase in protein in finer particles (under 0.180 mm). The optimal C/N ratio is between 20 and 30 for anaerobic digestion [9]. The decrease in C/N ratio indicated that feedstock with smaller particle size would be more suitable for anaerobic digestion [35]. Further efforts to analyze the chemical composition of different particle sizes lead to several findings. Firstly, the xylan and araban contents were shown to be stable at the range of 21.0–22.4% and 2.8–3.6%, respectively, suggesting there was no significant difference (p > 0.05) between different particles sizes for hemicellulose. Secondly, the lignin content increased with the decrease in particle size. Specifically, the lignin content for the largest particles (> 0.830 mm) was 17.0%, which increased to 19% and 21% with the decreasing particle sizes of 0.380–0.830 mm and 0.180–0.250 mm. The highest lignin content was 23.3% with particle size below 0.180 mm. Finally, the cellulose (glucan) content decreased with reductions in particle size. The results showed that the cellulose content of particle sizes over 0.830 mm was 37.5%, which decreased to 34.4% and further to 31.1% as the particle size reduced to < 0.180 mm.
> 0.830 mm 0.380-0.830 mm 0.250-0.380 mm 0.180-0.250 mm < 0.180 mm
30
20
10
0
30 s
60 s
90 s
Grinding time
Fig. 1. Effects of grinding time on the distribution of particle sizes.
Enet = Eout
(3.1)
Ein
(3.2)
Ein = P × t
3.2. Anaerobic digestion of Hybrid Pennisetum of different particle size
where, Ein is the specific energy consumption. P is the rated power (1400 W) of the device used, with a capacity of 500 g of substrate. t is the treating time. The data of chemical compositions and specific methane yield were statistically analyzed using SPSS 17.0 software with one-way analysis of variance (ANOVA) at a probability level of 0.05.
To understand the effect of particle size on the anaerobic digestion performances of Hybrid Pennisetum, the specific quantity of the different particle sizes were collected for methane production. Fig. 2(A) presents the daily methane yield of Hybrid Pennisetum at different particle sizes. The daily methane yield of different particle sizes followed a similar trend as the incubation time progressed. For example, the samples at > 0.830 mm were shown to have the lowest methane yield of 13.7 mL/g VS on the first day, followed by a sizeable increase to the maximal yield of 36.6 mL/g VS on day 2. Subsequent to that, the yield decreased continuously to 3.0 mL/(g VS·d) and the lowest yield of 0.1 mL/(g VS·d) was reported at the end of the incubation. Further observation suggests that the methane yields at an early incubation stage (day 1–5) were higher for the smaller particle sizes. For example, the highest daily methane yield of particles at > 0.830 mm of 36.6 mL/g VS, was surpassed by the particle size of 0.380–0.830 mm (yielding 44.3 mL/g VS), and the particle size of below 0.180 mm (yielding 58.3 mL/d/g VS). After day 5, the daily methane yields for different particles became similar and deceased with the same trend over the incubation period. The results suggested that the particle size reduction significantly influenced the methane yields obtainable from the anaerobic digestion of Hybrid Pennisetum. Fig. 2(B) reports the final cumulative methane yields for samples at different particle sizes. It can be seen that the specific methane yields for all samples became stable at around 270 to 292 mL/g VS after 30 days of digestion. The cumulative yields increased with decreasing particle size. For example, with the decrease of particle size from over 0.830 mm to 0.380–0.830 mm and 0.180–0.250 mm, the cumulated methane yields in day 10 increased from 190.2 to 201.9 and 209.0 mL/
3. Results and discussion 3.1. Composition of Hybrid Pennisetum at different particle sizes As indicated, Hybrid Pennisetum was ground in a grinder for 30–90 s with the particles were sieved into 5 different particle ranges (> 0.830, 0.380–0.830, 0.250–0.380, 0.180–0.250 and < 0.180 mm). As shown in Fig. 1, the main particle size when grinding for 30 s was over 0.380 mm, accounting for 61.7%, with particle size of below 0.380 mm constituting the remaining 38.3%. When the grinding time was prolonged to 60 s, a significant change was observed as the particle size below 0.380 mm accounted for 48.3% (an increase of 26.3%). Thus, grinding time was found to have a significant effect on the distribution of particles, which is in agreement with previous studies in literature for other biomass sources [33]. However, a further increase of grinding time to 90 s did not cause considerable changes to the particle distribution; this is likely due to the limitations of the grinding device used. Thus, a grinding time of 60 s was chosen for the Hybrid Pennisetum pretreatment to obtain the relevant particle sizes. After grinding for 60 s, the samples were then collected for characterization to identify the compositions of feedstock at different particle sizes. As presented in Table 1, elemental analysis suggested that
Table 1 Chemical compositions and element analysis of Hybrid Pennisetum in different particle sizes. Mesh < 20 20–40 40–60 60–80 > 80 abcd
Particle size(mm) > 0.83 0.38–0.83 0.25–0.38 0.18–0.25 < 0.18
C (%) 42.42 41.59 41.06 41.22 40.09
H (%) ± ± ± ± ±
0.18 0.21 0.37 0.09 0.04
6.03 5.98 5.92 5.91 5.84
± ± ± ± ±
O (%) 0.01 0.02 0.04 0.01 0.03
51.00 51.70 52.06 51.83 52.62
N (%) ± ± ± ± ±
0.13 0.18 0.28 0.11 0.03
0.54 0.65 0.76 0.75 0.88
Mean with different superscripts in row (P < 0.05). 471
± ± ± ± ±
C/N 0.03 0.04 0.00 0.01 0.01
78.7 64.6 54.0 55.3 45.6
Glucan (%) 37.5 34.4 34.5 34.3 31.1
± ± ± ± ±
Xylan (%) a
0.8 0.1b 1.7b 1.6b 0.0c
21.6 21.0 22.4 22.2 21.0
± ± ± ± ±
Araban (%) a
0.6 0.3a 1.2a 0.9a 0.1a
2.8 2.8 3.2 3.2 3.6
± ± ± ± ±
a
0.3 0.0a 0.2ab 0.2ab 0.1b
Lignin (%) 17.0 19.0 20.0 21.0 23.3
± ± ± ± ±
0.5a 1.4b 0.0bc 0.5c 0.0d
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was 69.5%, which increased to 80.9% with the particle size decreased to < 0.180 mm. The increase of BI suggested that the AD efficiency could be significantly enhanced with the reduction of particle size. Furthermore, the digestion time to achieve the 80% of cumulative methane yield also decreased with smaller particle sizes; a value of 14 days was apparent for particle sizes over 0.830 mm, which decreased to 9 days for particles below 0.180 mm. Fig. 2(B) shows that the cumulative methane yields from all the samples had very high coefficient as the R2 was higher than 0.977, which demonstrated a good fit of the experimental data to the model [36]. 3.3. Discussion on the effects of grinding on the composition and the anaerobic digestion of Hybrid Pennisetum at different particle sizes As presented above, the grinding of Hybrid Pennisetum (as a means of mechanical pretreatment) altered both the particle composition and the anaerobic digestion performance. Post-grinding, pretreated Hybrid Pennisetum separated into different particle sizes, showed a change in the distribution of some major components (i.e. glucan and lignin). Subsequently the AD performance of Hybrid Pennisetum at different particle sizes was also influenced. These effects will now be further discussed. 3.3.1. Discussion on the compositional changes of different particle sizes and the impact on the anaerobic digestion of Hybrid Pennisetum As shown in Fig. 1 and Table 1, after grinding, the distribution of some major composition components, primarily glucan and lignin, were altered. This is emphasized in Fig. 3 which presents the total carbohydrate content (namely cellulose and hemicellulose) and lignin content at the different particle sizes. As shown, reducing the particle size resulted in an increase in lignin and a reduction in carbohydrates for Hybrid Pennisetum. Specifically, as the particle size reduced from > 0.830 mm to < 0.180 mm, the carbohydrate content decreased from 62.0% to 55.7% (P < 0.05); while lignin content increased from 17.0% to 23.3% (P < 0.05). This is in agreement with much of previous literatures. Zadrazil et al. [37] reported that the lignin content in sugarcane bagasse increased from 23.0% to 25.7% as the particle size decreased from 3 to 5 mm to < 1 mm. Dumas et al. [38] and Tamaki et al. [34] found that the quantity of hemicellulose and cellulose of wheat straw and triticale in fine particles (under 0.05 and 0.150 mm, respectively) were smaller than that in the larger particles. Motte et al. [39] observed that the cellulose content decreased as the particle size of wheat straw reduced from 1.45 to 0.11 mm. Dumas et al. [38] reported that for particle size reductions in milled wheat straw from 0.759 to 0.880 mm, both the lignin/cellulose and lignin/xylan ratio increased by 16.7% and 13.6%, respectively, suggesting increased lignin quantities and/or decreased cellulose content. In essence, these results imply that different compositional components of lignocellulosic biomass react differently to the grinding [40]. Ash + proteins, lignin + hemicellulose, cellulose could be classified from the most to the least grindability [38], which may also explain the decrease of C/N ratio presented in Table 1. Bridgeman et, al [41] also found that the C/N ratio of two energy crops (swithgrass and reed canary grass) was lower for finer particle sizes (< 0.090 mm), compared to coarse particles (0.090–0.600 mm). The rind (outer) of plants has a high number of
Fig. 2. Daily (A) and cumulative (B) methane yield of samples.
g VS respectively. The cumulative methane yields for particles below 0.180 mm were the highest on day 1–15, followed by those with a size of 0.180–0.250 and 0.250–0.380 mm. To further understand the differences caused by the particle size reduction, the specific methane yields (SMY), theoretical methane yields and the kinetics were calculated; the results are presented in Table 2 and Fig. 2(B). Table 2 shows that the SMY increased with the smaller particle size from 269.9 ± 6.3 mL CH4/g VS (particle size at > 0.830 mm) to 276.7 ± 4.4 mL CH4/g VS (0.380–0.830 mm) and 291.9 ± 4.7 mL CH4 /g VS (0.250–0.380 mm). However, further reduction of particle sizes to 0.180–0.250 and < 0.180 mm demonstrated no improvement in SMY. In essence the results indicate that decreasing the particle size enhances the methane yield when the particle sizes are > 0.380 mm; while there is no enhancement of the methane yield for particle sizes below 0.380 mm. The theoretical methane yield values can be seen to decrease with the reduction of particle size from 388.3 to 358.7 mL CH4 /g VS; this can be explained by the decrease in percentage carbohydrates experienced in the particle size reduction. Interestingly, the biodegradability index (BI) was significantly increased with the decrease of particle sizes. The BI for particles over 0.830 mm
Table 2 Biochemical methane production based on results of BMP analysis, kinetic and theoretical analysis. Particle size (mm)
BMP yield (mL CH4/g VS)
> 0.83 0.38–0.83 0.25–0.38 0.18–0.25 < 0.18
269.9 276.7 291.9 290.0 290.2
± ± ± ± ±
6.3a 4.4a 4.7b 4.7b 4.3b
Theoretical composition (CH4 %)
Theoretical yield (mL CH4/g VS)
BI (%) (BMP/ theoretical)
Ym (mL/g VS)
Rm (mL/d/g VS)
λ (d)
R2
T80
48.8 48.2 47.8 47.9 47.2
388.3 377.3 370.2 372.3 358.7
69.5 73.3 78.9 77.9 80.9
264.7 271.2 288.7 284.9 283.7
36.6 44.3 53.4 55.5 58.3
0 0 0 0 0
0.979 0.977 0.978 0.976 0.977
14 13 11 10 9
472
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However, the methane yield results presented in this study clearly exhibit the presence of a threshold particle size (approximately 0.380 mm). This is consistent with a study by Sharma et al. [20] who also reported that the methane yield was similar for particle sizes between 0.088 and 0.40 mm for seven agricultural and forest residues. Dumas et al. [38] found that there were no differences in methane yield between 0.200 and 0.048 mm for wheat straw. Based on investigating the effects of different mechanical pretreatment conditions (200–1200 rpm) on the anaerobic digestion of meadow grass and wheat straw, Tsapekos et al. [45] also reported that an intensive pretreatment operation (1200 rpm) did not enhance the methane yield compared with an operation of 600 rpm. Such results indicate that going below a specific particle size, where the accessibility of cellulose to bacteria and enzymes is optimized, it is the cellulose content that influences the final methane yield [46]. Fig. 4 illustrates that a significant decrease in T80 was observed with the decrease of particle size. There was a linear relationship between digestion time and the reduction of particle size (R2 = 0.983); the T80 decreased from 14 to 9 days when the particle sizes were reduced from 0.830 to 0.180 mm. This represented a shorter digestion time of 35.7%, which was in the range of the 23–59% reported by Delgenes et al. [47]. Dumas et al. [38] also reported that the biogas production kinetic constants of wheat straw were increased by 43% when the particle size decreased from 0.759 to 0.048 mm. This effect on the digestion efficiency was likely due to the increase in available specific surface area caused by grinding pretreatment. Furthermore, the reduction in the degree of polymerization (DP) during the reduction of particle size could also be responsible for the shorter T80 times as suggested in previous literature [48,49]. Both the increase of available specific surface area and the decrease of DP could result in the improvement of the accessibility of carbohydrates, thereby enhancing the hydrolysis efficiency and cumulative methane yield.
Fig. 3. The correlation of particle sizes with carbohydrate and lignin.
vascular bundles, which mainly consist of lignin [42,43]. Due to the difference in tissue hardness and anatomical, physical and chemical properties, it would be plausible to suggest that in fresh samples, lignin was more easily fractionated during grinding and thus further accumulated in the fine particles (e.g., < 0.180 mm); whilst the carbohydrate fractions had a tendency to remain in the larger particles [44]. 3.3.2. Effect of grinding on the anaerobic digestion of Hybrid Pennisetum The above analysis indicated that the amount of carbohydrate decreased while lignin increased with reduction in particle size, which may negatively influence the AD performance (and methane production) of Hybrid Pennisetum. Ferreira et al. [26] argued that the methane potential varied for different particle sizes due to the different compositions, which was also confirmed in this study. The theoretical methane yield decreased from 388.3 to 358.7 mL/g VS as the particles decreased from 0.830 to 0.180 mm, respectively (Table 2). Interestingly, as presented in Fig. 4, with the decrease in particle size from 0.830 mm to 0.380 mm, the specific methane yield increased from around 270 to 292 mL/g VS, corresponding to an increase of 8%.
3.4. Energy balance Calculations for the energy balance were based on the optimal particle size identified in this study (below 0.380 mm) with the methane production of 291.9 L/kg VS, corresponding to an increase of 48 L CH4/kg VS compared to the methane production of 244 L/kg VS from untreated sample [27]. The energy content of the methane was calculated based on a value of 10 kWh/m3 (approximately 36 MJ/m3) [50]. Thus, the increased energy was 480 kWh/t VS. The specific energy consumption of the grinding pretreatment in this study was 180 kWh/t VS. The calculation showed a net energy output of 300 kWh/t VS, corresponding to 1.7 times the energy input. In general, the energy consumption of mechanical pretreatment will depend on the specific machine characteristics [51]. Lindmark et al. [25] reported the energy output from Grubben deflaker and Krima disperser treated ley crop silage was 21–26 times and 2–3 times the energy input. Therefore, mechanical particle size reduction requires cost-effective approaches and the optimal particles should be assessed for different lignocellulosic biomass in a biorefinery approach. 4. Conclusion The effect of grinding of Hybrid Pennisetum was investigated in terms of its effect on the methane yield generated from anaerobic digestion. Particle size ranges from 830 to 180 mm were investigated. Biomethane potential assays revealed that grinding to smaller particle sizes had a positive effect on the total methane yields obtainable and the methane production kinetics. Particle sizes of 0.380, 0.250 and 0.180 mm produced similar methane quantities, thus grinding below 0.250 mm would seem to be uneconomical. The conclusions achieved in this study were derived from small-scale batch analysis, thus further investigations need to be conducted to relate these findings to the operation of full-scale continuous digestion processes.
Fig. 4. The relationship of particle size reduction with specific methane yield and T80. 473
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Acknowledgements [24]
This work was supported financially by the National Natural Science Foundation of China [grant number 51776208], the Strategic Priority Research Program of Chinese Academy of Sciences [grand number XDA21050400], Bureau of International Cooperation, Chinese Academy of Sciences (182344KYSB20170009), the Science and Technology Planning Project of Guangdong Province [grant number 2017A050501049] and the Science and Technology Program of Guangzhou [grant number 201707010201].
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Full Length Article
The effect of mechanical pretreatment on the anaerobic digestion of Hybrid Pennisetum
T
Xihui Kanga,d, Yi Zhanga,b,c, Bing Songe, Yongming Suna,b, , Lianhua Lia,b,d, , Yu Heb, Xiaoying Konga,c, Xinjian Luoa,d, Zhenhong Yuana,b ⁎
⁎
a
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China CAS Key Laboratory of Renewable Energy, Guangzhou 510640, PR China c Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China d University of Chinese Academy of Sciences, Beijing 100049, PR China e Institute of Bioresource and Agriculture, Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, PR China b
GRAPHICAL ABSTRACT
Flow diagram of mechanical pretreatment and anaerobic digestion of Hybrid pennisetum.
ARTICLE INFO
ABSTRACT
Keywords: Hybrid Pennisetum Biomass Anaerobic digestion Methane Mechanical pretreatment Particle size
This study investigated the effect of particle size on the performance of anaerobic digestion of Hybrid Pennisetum. Hybrid Pennisetum was ground and sieved to provide different particle sizes between 0.180 and 1.000 mm. Characterization of the different particle sizes suggested that the composition of ground Hybrid Pennisetum altered-carbohydrate content decreased and lignin content increased with a decrease in particle size. The highest specific methane yield was 291.9 ± 4.7 mL CH4·g−1 VS at a particle size of 0.250–0.380 mm and this value plateaued as the particle size was reduced to below 0.250 mm. Excessive size reduction did not improve the methane yield but did result in a reduction of digestion time by 28.6–35.7%. The net energy output from the process was calculated at 300 kWh/t VS. Therefore, grinding was proved to enhance the anaerobic fermentation efficiency and energy output of Hybrid Pennisetum.
1. Introduction The depletion of fossil fuels, such as petroleum and natural gas, is accentuating the need for alternative renewable energy sources. ⁎
Biomass, including animal manures, lignocellulosic materials and municipal solid waste, are key resources for renewable energy production. Such biomass can be converted into biofuels via multiple biorefinery technologies [1,2].
Corresponding authors at: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China. E-mail addresses: [email protected] (Y. Sun), [email protected] (L. Li).
https://doi.org/10.1016/j.fuel.2019.04.134 Received 3 January 2019; Received in revised form 25 March 2019; Accepted 24 April 2019 Available online 01 May 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
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Among the sources of lignocellulosic biomass, Hybrid Pennisetum, the cross of Pennisetum americanum and P. purpureum, is regarded as a promising feedstock for a biorefinery due to its high biomass yields, wide distribution and low plantation expenditure [3]. Previous studies have investigated various applications and treatment methods for Hybrid Pennisetum to produce biofuels. For instance, Wang et al. [4] reported that 96.7% glucose was recovered after treating Hybrid Pennisetum with ionic liquid; Yu et al. [5] treated Hybrid Pennisetum with combined dehydration pretreatments and torrefaction, upgrading the fuel properties with elevated carbon content, higher heating value and increased oxygen content. Among the various treatment methods used for Hybrid Pennisetum, anaerobic digestion (AD) has been proved to be an effective method for biogas production [6–9]. However, the recalcitrance of the feedstock can hinder the AD process of Hybrid Pennisetum, leading to a low bioconversion efficiency [10]. To enhance the anaerobic digestion performance of lignocellulosic biomass, various methods have been investigated, including physical, chemical and biological pretreatments [11–13]. Mechanical pretreatment enhances the AD performance of biomass by reducing the particle size, increasing the accessible surface area and decreasing cellulose crystallinity [14,15]. Larger biomass particle sizes have been known to cause low biogas production and mechanical problems in continuous anaerobic reactors [16], however, by reducing the particle size, the bulk density of the biomass increases and facilitates a more successful operation [17,18]. Izumi et al. [19] found that a particle size reduction from 0.888 to 0.718 mm enhanced the methane yield by 28% from food waste. After testing the effects of particle size on the anaerobic digestion of seven agricultural and forest residues, Sharma et al. [20] conclude that it was not economic for reducing the particles below 0.40 mm due to non-significant improvement of methane production compared to 0.088 mm. In addition, excessive size reduction could lead to the build-up of VFAs [21,22]. Furthermore, previous studies have focused on the effects on the method of milling/crushing lignocellulosic biomass or the texture of beads in ball-milling on the methane production/enzyme hydrolysis efficiency, and investigated the effect of mechanical pretreatment on the crystalline changes of biomass [23–25]. Ferreria et al. [26] suggested that the composition of lignocellulosic biomass should be assessed to compare fine and coarse sizes, as the carbohydrate and protein content was inconsistent in the different fractions and had different methane potentials. Thus, this study advances beyond the state of the art by investigating the effect of a grinding mechanical pretreatment on the feedstock Hybrid Pennisetum to indicate the change in composition at different particle sizes, and the effect of particle size on AD performance. The objective of the study was to improve the methane production and determine the optimal particle size for Hybrid Pennisetum.
in a grinder (XB-HP, Xiaobao, China) at different times (30, 60 and 90 s, respectively). The ground samples were dried at 60 °C for 3 days using a drying oven and subsequently screened to obtain mean particle sizes of between 0.15 and 1 mm using a series of sieve sizes: 20 (0.830 mm), 40 (0.380 mm), 60 (0.250 mm) and 80 (0.180 mm). Thus, the sieved particles used in the experiments were not uniform but a mixture of diametrical particles (dp) in the size range: dp > 0.830 mm, 0.380 < dp < 0.830, 0.250 < dp < 0.380, 0.180 < dp < 0.250 and dp < 0.180, respectively. For convenience, the sizes of the sieved particles are termed as > 0.830, 0.380–0.830, 0.250–0.380, 0.180–0.250 and < 0.180 mm. Experiments were conducted in duplicate. The TS, VS, and ash in dried samples were 89.04 ± 0.06%, 81.53 ± 0.04% and 7.53 ± 0.02%, respectively. 2.3. Biomethane potential assay The biomethane potential (BMP) assays of samples of different particle size were investigated using an automatic methane potential test system (Bioprocess Control Sweden AB, AMPTS II). The BMP assays were set according to the previous research [27]. All experiments were performed at 37 ± 0.5 °C in triplicate and lasted for 30 days. Herrmann [28] reported that the ratio of feedstock/inoculum (F/I) was a crucial parameter to obtain the highest methane yields and a stable process. Therefore, in order to investigate the effects of particle size on the methane yields in the BMP, F/I ratio was set to 1 in terms of VS [20,29]. 2.4. Analytical methods The TS, VS, and ash content of all samples were determined through drying at 105 °C for 24 h and subsequent heating at 550 °C for 2 h, according to the standard method [7]. The cellulose, hemicellulose and lignin content of all samples were determined according to the standard procedure provided by NREL [30]. The contents of C, N and H were analyzed by an elemental analyzer (Vario EL cube, Germany), and the O content was calculated by the subtraction rule. 2.5. Calculation methods Using the Buswell equation as shown in Eq.(1) [31], the theoretical biomethane potential (TMP) of samples was calculated using analyzed element contents.
CnHaOb + n
a 4
b H 2O 2
n a b + CH 4 2 8 4 n a b + + CO2 2 8 4
2. Material and methods
(1)
Kinetic studies were conducted to assess the biodegradability, biodegradability efficiency of the substrate, and the fermentation time to reach 80% of the methane volume (T80), with DataFit 9.0(Oakdale Engineering, USA). The biodegradability index was defined as the ratio of BMP yield to the TMP yield. Based on the following modified Gompertz Equation (Eq. (2)) [32], the bespoke Datafit programme generates a list of variables to describe the decay course of organic matter in the batch process.
2.1. Raw material and inoculum Hybrid Pennisetum (Pennisetum americanum × P. purpureum) was grown in Zengcheng District, Guangdong Province, China, and harvested in July 2017. The contents of total solids (TS), volatile solids (VS), and ash in fresh sample were 29.06 ± 0.02%, 26.01 ± 0.03% and 3.01 ± 0.06%, respectively. The inoculum (TS = 6.0%; VS = 1.0%; pH = 7.61) used for the experiments was derived from a mesophilic continuously stirred tank reactor(CSTR)fed with cellulose and grass. The inoculum was starved for 7 days to remove any residual gas.
Y = Ym exp
exp
Rme ( Ym
t) + 1
(2)
where, Y, Ym and Rm correspond to the cumulative CH4 production (mL CH4/g VS), the maximum CH4 production (mL/g VS), and the maximum CH4 production rate (mL/(g VS·d)) respectively. λ is the lag time (d); t represents the incubation time (d), and e is 2.72. R2 is a measure (in %) of how well the kinetic model fits the methane production curve. Energy balance was calculated using the equations as below:
2.2. Mechanical pretreatment Fresh samples of Hybrid Pennisetum were initially cut to a small size of 2–3 cm, manually. To investigate the effect of grinding time on size distribution, the same quality of partitioned samples were then ground 470
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40
Proportion (%)
the C content was stable at 40.09% to 42.42% as the particle size reduced from 0.830 mm to 0.180 mm, whilst there was a minor increase in N content from 0.54% to 0.88%. Thus the corresponding C/N ratio decreased from 78.7 for the largest particles (> 0.830 mm) to 45.6 for the smallest particles (< 0.180 mm), which is consistent with Tamaki et al. [34] who observed an increase in protein in finer particles (under 0.180 mm). The optimal C/N ratio is between 20 and 30 for anaerobic digestion [9]. The decrease in C/N ratio indicated that feedstock with smaller particle size would be more suitable for anaerobic digestion [35]. Further efforts to analyze the chemical composition of different particle sizes lead to several findings. Firstly, the xylan and araban contents were shown to be stable at the range of 21.0–22.4% and 2.8–3.6%, respectively, suggesting there was no significant difference (p > 0.05) between different particles sizes for hemicellulose. Secondly, the lignin content increased with the decrease in particle size. Specifically, the lignin content for the largest particles (> 0.830 mm) was 17.0%, which increased to 19% and 21% with the decreasing particle sizes of 0.380–0.830 mm and 0.180–0.250 mm. The highest lignin content was 23.3% with particle size below 0.180 mm. Finally, the cellulose (glucan) content decreased with reductions in particle size. The results showed that the cellulose content of particle sizes over 0.830 mm was 37.5%, which decreased to 34.4% and further to 31.1% as the particle size reduced to < 0.180 mm.
> 0.830 mm 0.380-0.830 mm 0.250-0.380 mm 0.180-0.250 mm < 0.180 mm
30
20
10
0
30 s
60 s
90 s
Grinding time
Fig. 1. Effects of grinding time on the distribution of particle sizes.
Enet = Eout
(3.1)
Ein
(3.2)
Ein = P × t
3.2. Anaerobic digestion of Hybrid Pennisetum of different particle size
where, Ein is the specific energy consumption. P is the rated power (1400 W) of the device used, with a capacity of 500 g of substrate. t is the treating time. The data of chemical compositions and specific methane yield were statistically analyzed using SPSS 17.0 software with one-way analysis of variance (ANOVA) at a probability level of 0.05.
To understand the effect of particle size on the anaerobic digestion performances of Hybrid Pennisetum, the specific quantity of the different particle sizes were collected for methane production. Fig. 2(A) presents the daily methane yield of Hybrid Pennisetum at different particle sizes. The daily methane yield of different particle sizes followed a similar trend as the incubation time progressed. For example, the samples at > 0.830 mm were shown to have the lowest methane yield of 13.7 mL/g VS on the first day, followed by a sizeable increase to the maximal yield of 36.6 mL/g VS on day 2. Subsequent to that, the yield decreased continuously to 3.0 mL/(g VS·d) and the lowest yield of 0.1 mL/(g VS·d) was reported at the end of the incubation. Further observation suggests that the methane yields at an early incubation stage (day 1–5) were higher for the smaller particle sizes. For example, the highest daily methane yield of particles at > 0.830 mm of 36.6 mL/g VS, was surpassed by the particle size of 0.380–0.830 mm (yielding 44.3 mL/g VS), and the particle size of below 0.180 mm (yielding 58.3 mL/d/g VS). After day 5, the daily methane yields for different particles became similar and deceased with the same trend over the incubation period. The results suggested that the particle size reduction significantly influenced the methane yields obtainable from the anaerobic digestion of Hybrid Pennisetum. Fig. 2(B) reports the final cumulative methane yields for samples at different particle sizes. It can be seen that the specific methane yields for all samples became stable at around 270 to 292 mL/g VS after 30 days of digestion. The cumulative yields increased with decreasing particle size. For example, with the decrease of particle size from over 0.830 mm to 0.380–0.830 mm and 0.180–0.250 mm, the cumulated methane yields in day 10 increased from 190.2 to 201.9 and 209.0 mL/
3. Results and discussion 3.1. Composition of Hybrid Pennisetum at different particle sizes As indicated, Hybrid Pennisetum was ground in a grinder for 30–90 s with the particles were sieved into 5 different particle ranges (> 0.830, 0.380–0.830, 0.250–0.380, 0.180–0.250 and < 0.180 mm). As shown in Fig. 1, the main particle size when grinding for 30 s was over 0.380 mm, accounting for 61.7%, with particle size of below 0.380 mm constituting the remaining 38.3%. When the grinding time was prolonged to 60 s, a significant change was observed as the particle size below 0.380 mm accounted for 48.3% (an increase of 26.3%). Thus, grinding time was found to have a significant effect on the distribution of particles, which is in agreement with previous studies in literature for other biomass sources [33]. However, a further increase of grinding time to 90 s did not cause considerable changes to the particle distribution; this is likely due to the limitations of the grinding device used. Thus, a grinding time of 60 s was chosen for the Hybrid Pennisetum pretreatment to obtain the relevant particle sizes. After grinding for 60 s, the samples were then collected for characterization to identify the compositions of feedstock at different particle sizes. As presented in Table 1, elemental analysis suggested that
Table 1 Chemical compositions and element analysis of Hybrid Pennisetum in different particle sizes. Mesh < 20 20–40 40–60 60–80 > 80 abcd
Particle size(mm) > 0.83 0.38–0.83 0.25–0.38 0.18–0.25 < 0.18
C (%) 42.42 41.59 41.06 41.22 40.09
H (%) ± ± ± ± ±
0.18 0.21 0.37 0.09 0.04
6.03 5.98 5.92 5.91 5.84
± ± ± ± ±
O (%) 0.01 0.02 0.04 0.01 0.03
51.00 51.70 52.06 51.83 52.62
N (%) ± ± ± ± ±
0.13 0.18 0.28 0.11 0.03
0.54 0.65 0.76 0.75 0.88
Mean with different superscripts in row (P < 0.05). 471
± ± ± ± ±
C/N 0.03 0.04 0.00 0.01 0.01
78.7 64.6 54.0 55.3 45.6
Glucan (%) 37.5 34.4 34.5 34.3 31.1
± ± ± ± ±
Xylan (%) a
0.8 0.1b 1.7b 1.6b 0.0c
21.6 21.0 22.4 22.2 21.0
± ± ± ± ±
Araban (%) a
0.6 0.3a 1.2a 0.9a 0.1a
2.8 2.8 3.2 3.2 3.6
± ± ± ± ±
a
0.3 0.0a 0.2ab 0.2ab 0.1b
Lignin (%) 17.0 19.0 20.0 21.0 23.3
± ± ± ± ±
0.5a 1.4b 0.0bc 0.5c 0.0d
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was 69.5%, which increased to 80.9% with the particle size decreased to < 0.180 mm. The increase of BI suggested that the AD efficiency could be significantly enhanced with the reduction of particle size. Furthermore, the digestion time to achieve the 80% of cumulative methane yield also decreased with smaller particle sizes; a value of 14 days was apparent for particle sizes over 0.830 mm, which decreased to 9 days for particles below 0.180 mm. Fig. 2(B) shows that the cumulative methane yields from all the samples had very high coefficient as the R2 was higher than 0.977, which demonstrated a good fit of the experimental data to the model [36]. 3.3. Discussion on the effects of grinding on the composition and the anaerobic digestion of Hybrid Pennisetum at different particle sizes As presented above, the grinding of Hybrid Pennisetum (as a means of mechanical pretreatment) altered both the particle composition and the anaerobic digestion performance. Post-grinding, pretreated Hybrid Pennisetum separated into different particle sizes, showed a change in the distribution of some major components (i.e. glucan and lignin). Subsequently the AD performance of Hybrid Pennisetum at different particle sizes was also influenced. These effects will now be further discussed. 3.3.1. Discussion on the compositional changes of different particle sizes and the impact on the anaerobic digestion of Hybrid Pennisetum As shown in Fig. 1 and Table 1, after grinding, the distribution of some major composition components, primarily glucan and lignin, were altered. This is emphasized in Fig. 3 which presents the total carbohydrate content (namely cellulose and hemicellulose) and lignin content at the different particle sizes. As shown, reducing the particle size resulted in an increase in lignin and a reduction in carbohydrates for Hybrid Pennisetum. Specifically, as the particle size reduced from > 0.830 mm to < 0.180 mm, the carbohydrate content decreased from 62.0% to 55.7% (P < 0.05); while lignin content increased from 17.0% to 23.3% (P < 0.05). This is in agreement with much of previous literatures. Zadrazil et al. [37] reported that the lignin content in sugarcane bagasse increased from 23.0% to 25.7% as the particle size decreased from 3 to 5 mm to < 1 mm. Dumas et al. [38] and Tamaki et al. [34] found that the quantity of hemicellulose and cellulose of wheat straw and triticale in fine particles (under 0.05 and 0.150 mm, respectively) were smaller than that in the larger particles. Motte et al. [39] observed that the cellulose content decreased as the particle size of wheat straw reduced from 1.45 to 0.11 mm. Dumas et al. [38] reported that for particle size reductions in milled wheat straw from 0.759 to 0.880 mm, both the lignin/cellulose and lignin/xylan ratio increased by 16.7% and 13.6%, respectively, suggesting increased lignin quantities and/or decreased cellulose content. In essence, these results imply that different compositional components of lignocellulosic biomass react differently to the grinding [40]. Ash + proteins, lignin + hemicellulose, cellulose could be classified from the most to the least grindability [38], which may also explain the decrease of C/N ratio presented in Table 1. Bridgeman et, al [41] also found that the C/N ratio of two energy crops (swithgrass and reed canary grass) was lower for finer particle sizes (< 0.090 mm), compared to coarse particles (0.090–0.600 mm). The rind (outer) of plants has a high number of
Fig. 2. Daily (A) and cumulative (B) methane yield of samples.
g VS respectively. The cumulative methane yields for particles below 0.180 mm were the highest on day 1–15, followed by those with a size of 0.180–0.250 and 0.250–0.380 mm. To further understand the differences caused by the particle size reduction, the specific methane yields (SMY), theoretical methane yields and the kinetics were calculated; the results are presented in Table 2 and Fig. 2(B). Table 2 shows that the SMY increased with the smaller particle size from 269.9 ± 6.3 mL CH4/g VS (particle size at > 0.830 mm) to 276.7 ± 4.4 mL CH4/g VS (0.380–0.830 mm) and 291.9 ± 4.7 mL CH4 /g VS (0.250–0.380 mm). However, further reduction of particle sizes to 0.180–0.250 and < 0.180 mm demonstrated no improvement in SMY. In essence the results indicate that decreasing the particle size enhances the methane yield when the particle sizes are > 0.380 mm; while there is no enhancement of the methane yield for particle sizes below 0.380 mm. The theoretical methane yield values can be seen to decrease with the reduction of particle size from 388.3 to 358.7 mL CH4 /g VS; this can be explained by the decrease in percentage carbohydrates experienced in the particle size reduction. Interestingly, the biodegradability index (BI) was significantly increased with the decrease of particle sizes. The BI for particles over 0.830 mm
Table 2 Biochemical methane production based on results of BMP analysis, kinetic and theoretical analysis. Particle size (mm)
BMP yield (mL CH4/g VS)
> 0.83 0.38–0.83 0.25–0.38 0.18–0.25 < 0.18
269.9 276.7 291.9 290.0 290.2
± ± ± ± ±
6.3a 4.4a 4.7b 4.7b 4.3b
Theoretical composition (CH4 %)
Theoretical yield (mL CH4/g VS)
BI (%) (BMP/ theoretical)
Ym (mL/g VS)
Rm (mL/d/g VS)
λ (d)
R2
T80
48.8 48.2 47.8 47.9 47.2
388.3 377.3 370.2 372.3 358.7
69.5 73.3 78.9 77.9 80.9
264.7 271.2 288.7 284.9 283.7
36.6 44.3 53.4 55.5 58.3
0 0 0 0 0
0.979 0.977 0.978 0.976 0.977
14 13 11 10 9
472
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However, the methane yield results presented in this study clearly exhibit the presence of a threshold particle size (approximately 0.380 mm). This is consistent with a study by Sharma et al. [20] who also reported that the methane yield was similar for particle sizes between 0.088 and 0.40 mm for seven agricultural and forest residues. Dumas et al. [38] found that there were no differences in methane yield between 0.200 and 0.048 mm for wheat straw. Based on investigating the effects of different mechanical pretreatment conditions (200–1200 rpm) on the anaerobic digestion of meadow grass and wheat straw, Tsapekos et al. [45] also reported that an intensive pretreatment operation (1200 rpm) did not enhance the methane yield compared with an operation of 600 rpm. Such results indicate that going below a specific particle size, where the accessibility of cellulose to bacteria and enzymes is optimized, it is the cellulose content that influences the final methane yield [46]. Fig. 4 illustrates that a significant decrease in T80 was observed with the decrease of particle size. There was a linear relationship between digestion time and the reduction of particle size (R2 = 0.983); the T80 decreased from 14 to 9 days when the particle sizes were reduced from 0.830 to 0.180 mm. This represented a shorter digestion time of 35.7%, which was in the range of the 23–59% reported by Delgenes et al. [47]. Dumas et al. [38] also reported that the biogas production kinetic constants of wheat straw were increased by 43% when the particle size decreased from 0.759 to 0.048 mm. This effect on the digestion efficiency was likely due to the increase in available specific surface area caused by grinding pretreatment. Furthermore, the reduction in the degree of polymerization (DP) during the reduction of particle size could also be responsible for the shorter T80 times as suggested in previous literature [48,49]. Both the increase of available specific surface area and the decrease of DP could result in the improvement of the accessibility of carbohydrates, thereby enhancing the hydrolysis efficiency and cumulative methane yield.
Fig. 3. The correlation of particle sizes with carbohydrate and lignin.
vascular bundles, which mainly consist of lignin [42,43]. Due to the difference in tissue hardness and anatomical, physical and chemical properties, it would be plausible to suggest that in fresh samples, lignin was more easily fractionated during grinding and thus further accumulated in the fine particles (e.g., < 0.180 mm); whilst the carbohydrate fractions had a tendency to remain in the larger particles [44]. 3.3.2. Effect of grinding on the anaerobic digestion of Hybrid Pennisetum The above analysis indicated that the amount of carbohydrate decreased while lignin increased with reduction in particle size, which may negatively influence the AD performance (and methane production) of Hybrid Pennisetum. Ferreira et al. [26] argued that the methane potential varied for different particle sizes due to the different compositions, which was also confirmed in this study. The theoretical methane yield decreased from 388.3 to 358.7 mL/g VS as the particles decreased from 0.830 to 0.180 mm, respectively (Table 2). Interestingly, as presented in Fig. 4, with the decrease in particle size from 0.830 mm to 0.380 mm, the specific methane yield increased from around 270 to 292 mL/g VS, corresponding to an increase of 8%.
3.4. Energy balance Calculations for the energy balance were based on the optimal particle size identified in this study (below 0.380 mm) with the methane production of 291.9 L/kg VS, corresponding to an increase of 48 L CH4/kg VS compared to the methane production of 244 L/kg VS from untreated sample [27]. The energy content of the methane was calculated based on a value of 10 kWh/m3 (approximately 36 MJ/m3) [50]. Thus, the increased energy was 480 kWh/t VS. The specific energy consumption of the grinding pretreatment in this study was 180 kWh/t VS. The calculation showed a net energy output of 300 kWh/t VS, corresponding to 1.7 times the energy input. In general, the energy consumption of mechanical pretreatment will depend on the specific machine characteristics [51]. Lindmark et al. [25] reported the energy output from Grubben deflaker and Krima disperser treated ley crop silage was 21–26 times and 2–3 times the energy input. Therefore, mechanical particle size reduction requires cost-effective approaches and the optimal particles should be assessed for different lignocellulosic biomass in a biorefinery approach. 4. Conclusion The effect of grinding of Hybrid Pennisetum was investigated in terms of its effect on the methane yield generated from anaerobic digestion. Particle size ranges from 830 to 180 mm were investigated. Biomethane potential assays revealed that grinding to smaller particle sizes had a positive effect on the total methane yields obtainable and the methane production kinetics. Particle sizes of 0.380, 0.250 and 0.180 mm produced similar methane quantities, thus grinding below 0.250 mm would seem to be uneconomical. The conclusions achieved in this study were derived from small-scale batch analysis, thus further investigations need to be conducted to relate these findings to the operation of full-scale continuous digestion processes.
Fig. 4. The relationship of particle size reduction with specific methane yield and T80. 473
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Acknowledgements [24]
This work was supported financially by the National Natural Science Foundation of China [grant number 51776208], the Strategic Priority Research Program of Chinese Academy of Sciences [grand number XDA21050400], Bureau of International Cooperation, Chinese Academy of Sciences (182344KYSB20170009), the Science and Technology Planning Project of Guangdong Province [grant number 2017A050501049] and the Science and Technology Program of Guangzhou [grant number 201707010201].
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