Biogas Production Performance of Different Components from Banana

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Biogas Production Performance of Different Components from Banana Stems Chengming Zhang,†,‡ Shenglei Bi,§ Mingxing Zhao,∥ Sandra Chang,†,‡ Yanfei Li,⊥ Pei Pei,†,‡ Xiang Gao,# Lei Zhang,†,‡ Jihong Li,†,‡ and Shizhong Li*,†,‡ †

Institute of Nuclear and New Energy Technology, Tsinghua University, Tsinghua Garden, Beijing 100084, People’s Republic of China ‡ Beijing Engineering Research Center for Biofuels, Beijing 100084, People’s Republic of China § State Key Laboratory of Motor Vehicle Biofuel Technology, Henan Tianguan Group Company, Limited, Nanyang, Henan 473000, People’s Republic of China ∥ School of Environment and Civil Engineering, and #School of Biotechnology, Jiangnan University, Wuxi, Jiangsu 214122, People’s Republic of China ⊥ Qingxian Office of New Energy, Qingxian, Hebei 062650, People’s Republic of China ABSTRACT: The object of this paper was to provide data for process design and economic feasible analysis of biogas production from banana stems. Fresh banana stems were separated into fiber cells (FC), parenchyma cells (PC), and liquid, and their biochemical methane potential (BMP) was analyzed. The BMP of FC, PC, and liquid were 225.0 ± 17.0 mL/g of total solids (TS), 187.5 ± 12.1 mL/g of TS, and 299.6 ± 12.6 mL/g of chemical oxygen demand (COD), accounting for 56.18, 40.48, and 3.34% of the total methane production of banana stems, respectively. Smashing before squeezing increased the COD of liquid from 1939 to 7464 mg/L, and the percentage of methane production from liquid would increase to 11.96%. Therefore, squeezing before transportation directly or after smashing can be used to both reduce transportation cost and, thus, increase the economic feasibility of biogas production from banana stems.

1. INTRODUCTION Bananas are one of the most important economic fruits and crops in subtropical and tropical zones. Large amounts of banana stems, up to 100 t per hectare annually, are produced after banana harvest.1 Currently, banana stems are directly crashed and left to rot in the soil to replenish the nutrients for growth of the next banana generation.1,2 However, this treatment causes serious environmental and ecological issues, such as greenhouse gas emissions and outbreak of banana Fusarium wilt.1 Researchers have made efforts to study feasible utilization of banana stems, such as paper making, production of animal feed, and generation of fuels, natural fiber, and biogas.3−6 Among these options, using banana stems for biogas production by anaerobic digestion (AD) and natural fiber production has the most potential. Although several related reports have been published,1,4,7−12 there are not enough data for the preliminary economic feasibility analysis for biogas or natural fiber production. Different from other agriculture stems, such as corn, rice, and wheat stems, fresh banana stem contains a very high water content, sometimes up to 94%, which would increase transportation cost and, thus, reduce economic feasibility. Squeezing the fresh banana stems before transportation was believed to significantly reduce the transportation cost and enlarged transportation radius, which would also increase the supply of raw materials. For example, when the water content of banana stem decreased from 94 to 70%, the banana stem weight will decrease by 80%. However, this treatment could cause methane production loss because the liquid generated © 2016 American Chemical Society

after squeezing contains some organic matter and may be converted to methane during AD. Consequently, the amount of methane loss from squeezing should be assessed to judge whether squeezing would be beneficial. For the nature fiber production, after the extraction of fiber from the banana stems, the residues, mainly parenchyma cells, can be used for biogas production. However, biogas production from parenchyma cells has not been reported. In this paper, fresh banana stems were separated into three components, fiber cells, parenchyma cells, and liquid, and their biochemical methane potential (BMP) and methane production performance (MPP) were investigated. Data obtained can be used for the preliminary economic feasibility analysis of biogas or natural fiber production from banana stem.

2. MATERIALS AND METHODS 2.1. Materials. Banana stems used in this study were collected from Hainan province, China. Banana stems were divided into five kinds of samples in this study. Banana stems were cut and separated into fiber cells (FC, sample 1), parenchyma cells (PC, sample 2), and liquid (liquid 1, sample 3) by a disc mill (BR30-300CB KRK, Kogyo).10 FC and PC accounted for 49.78 and 43.13%, by weight, of the banana stem, respectively. A second kind of liquid sample (liquid 2, sample 4) was obtained by direct squeezing without smashing. Two liquid samples were collected because of the two possible banana stem processing methods tested. One method is squeezing the fresh banana Received: March 25, 2016 Revised: June 14, 2016 Published: July 20, 2016 6425

DOI: 10.1021/acs.energyfuels.6b00657 Energy Fuels 2016, 30, 6425−6429

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Energy & Fuels Table 1. Characteristics of PC, FC, and MC item

VS/TS (%)

ash

cellulose

hemicellulose

lignin

C/N

PC FC MC

95.29 ± 0.06 98.40 ± 0.05 95.13 ± 0.05

4.71 ± 0.06 1.60 ± 0.05 4.50 ± 0.05

49.64 ± 2.44 55.06 ± 3.23 52.52 ± 3.23

21.17 ± 1.26 16.28 ± 0.57 18.28 ± 0.57

12.70 ± 0.55 12.29 ± 0.33 12.40 ± 0.33

53 174 72

Figure 1. (Left) CMP and (right) DMP from different components of banana stems.

Figure 2. (Left) CMP and (right) DMP of liquid samples from banana stems. stems after smashing (sample 1), and the other method is directly squeezing without smashing (sample 2). Smashed fresh banana stems without separation were used as sample 5 [mixed cells (MC)]. Samples were stored at −20 °C before biogas fermentation. Anaerobic granular sludge was bought from Beijing Byewaste Biological Technology Co., Ltd., Beijing, China. After separation, the granular sludge (>20 mesh) was used as inoculum. Total solids (TS) and volatile solids (VS) contents of inoculum were 15.6 ± 0.2 and 60.0 ± 0.6%, respectively. The total nitrogen (TN) and total phosphorus (TP) of inoculum were 2.4 ± 0.1 and 0.25 ± 0.03% on a dry basis, respectively. 2.2. Biogas Fermentation Conditions. The BMP tests were conducted according to ref 13. Methane production was measured by the Automatic Methane Potential Test System (AMPTS II), Bioprocess Control Sweden AB, Sweden. Batch experiments were conducted at 37 ± 1 °C and automatically shaken. The total volume of AD reactor was 500 mL, and the working volume was 400 mL. A total of 3 g each based on TS of FC, PC, and MC were used for AD. The inoculum/substrate ratio (both expressed in grams of VS) was about 2:1. A total of 150 mL of liquid 1 and 300 mL of liquid 2 were used for AD, and the same amount of inoculum was used. NH4Cl and NaHCO3 were added to adjust the C/N ratio and alkalinity of the system to 25:1 and 2500 mg of CaCO3/L, respectively. Nitrogen was

used before fermentation to ensure an anaerobic environment. The methane generation from inoculum was considered during the calculation of methane production. All samples were run in three parallel replicates. 2.3. Analysis Methods and Sampling. TS, VS, hemicellulose, cellulose, lignin, and ash contents of samples were analyzed according to ref 10. Chemical oxygen demand (COD), TP, TN, and NH4+ N were analyzed according to ref 14. All samples were collected before AD for characteristic analysis. CH4 production potential (Pmax), CH4 production rate (Rmax), and lag phase (λ) were modeled using the modified Gompertz equation14

⎧ ⎡R e ⎤⎫ P = Pmax ⎨− exp⎢ max (λ − t ) + 1⎥⎬ ⎣ Pmax ⎦⎭ ⎩ ⎪

⎪

⎪

⎪

where P is the cumulative specific methane yield (mL/g of VS) for a given time t, Pmax is the maximum CH4 potential (mL/g of TS) at the end of AD, Rmax is the CH4 production rate (mL g−1 of TS day−1), λ is the lag phase (days), t is the time (days), and e is the exponent (1), i.e., 2.71828. 6426

DOI: 10.1021/acs.energyfuels.6b00657 Energy Fuels 2016, 30, 6425−6429

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Energy & Fuels

3. RESULTS AND DISCUSSION 3.1. Characteristics of the Different Components of Banana Stems. Basic characteristics of solid samples were shown in Table 1. All of the substrates contained very high contents of VS (all above 95%) and similar low contents of lignin (around 12.5%), which would benefit biogas production because lignin could not be converted to biogas during AD.15−17 Cellulose and hemicellulose contents of FC were 10.9% higher and 23.1% lower than those of PC, respectively. However, the combined sum of lignocellulose and hemicellulose was almost the same for PC and FC. The C/N ratio of each substrates was significantly different. The C/N ratio of PC was 53, which meant that it could be directly used for biogas production, while a nitrogen source should be added when using FC, which had a much higher C/N ratio of 174. 3.2. Methane Production Performance of Different Components of Banana Stems. Cumulative and daily methane production of solid substrates were investigated and shown in Figure 1. The cumulative methane productions (CMPs), from the highest to the lowest, were FC (675.0 mL), PC (562.4 mL), and MC (508.3 mL). The higher CMPs of FC and PC could be attributed to the mechanical treatment during sample preparation, which may have destroyed the recalcitrant structure of the banana stems.3 Daily methane productions (DMPs) of the substrates were also different. Two gas production peaks (GPPs) were observed for all of the substrates. The higher daily methane production of MC resulted from the more easily degradable liquid component. GPPs of PC were between 5 and 10 days, while those of FC appeared on the 5th and 15th days. Differences of GPPs between PC and FC suggested that their physical structures were different from the view of biogas production. For the industrial-scale biogas production, PC was a better substrate than FC because it was fully digested in 2 weeks. CMP and DMP of two liquid samples were investigated and shown in Figure 2. Liquid 1 contained more COD (7464 mg/ L), TP (0.079 mg/L), and NH4+ N (0.455 mg/L) than liquid 2 (1939 mg/L of COD, 0.02 mg/L of TP, and 0.339 mg/L of NH4+ N). CMP of liquid 1 was significantly higher than liquid 2 because liquid 1 contained more COD (1.12 g) than liquid 2 (0.58 g) and more nutrients. However, both liquid samples showed a similar trend of DMP and a good digestion performance. According to DMP, the hydraulic retention time (HRT) can be set as 7 days or shorter when using these liquids for biogas production on the industrial scale. 3.3. BMP of Different Components of Banana Stems. On the basis of CMP and weight of samples added to the digestion reaction, methane yields of substrates were calculated and shown in Figure 3. The BMP was 306.2 ± 15.3 mL/g of COD for liquid 1, 299.6 ± 12.6 mL/g of COD for liquid 2, 225.0 ± 17.0 mL/g of TS for FC, 187.5 ± 12.1 mL/g of TS for PC, and 169.4 ± 7.6 mL/g of TS for MC. The higher BMPs of the liquid samples suggested that they were more biodegradable than solid samples and were better candidates for biogas production. Although the CMPs of liquid samples were significantly different (Figure 2), their BMPs were almost the same. Among the solid samples, FC showed a higher BMP, which was 20.0 and 32.8% higher than PC and MC, respectively. The differences of BMPs of solid samples also suggested that their physical structures were different. Although BMPs of PC and MC were lower, their BMPs were similar to

Figure 3. Methane yields of different substrates from banana stems.

other agriculture wastes.18,19 Furthermore, in recent research, Han et al. compared the BMP between banana stems and other typical agriculture crop stems under the same AD condition and found that the BMP of banana stems (186.1 mL/g of VS) was the highest, followed by rice stems (142.68 mL/g of VS), wheat stems (137.71 mL/g of VS), and corn stems (126.55 mL/g of VS).20 In other words, all of the solid substrates from banana stems were also good raw materials for biogas production. To further evaluate the methane production efficiency, the experimental data of CMP from different components of banana stems were fitted to the modified Gompertz equation, with R2 > 0.95 (Table 2). Results showed that the AD performance from solid samples was different. The correlation coefficient (R2) in each group was more than 0.95, suggesting that the modified Gompertz equation was able to describe the methane generation. The Pmax value predicted from the modified Gompertz equation was very close to the experimental BMP. The Rmax values from PC and FC were similar but were lower than that of 20.5 mL g−1 of TS day−1 from MC. The higher Rmax value from MC could be contributed to the more easily degradable liquid component contained in it (Figure 1). The λ value showed there was almost no lag phase for the AD from PC and liquid samples, but there was a 2.49 day lag phase for FC, which indicated that the FC was harder to be degraded than PC and MC in the early stage of AD. Results further showed that the physical structures of PC and FC were different. The percentage of methane production from different components of banana stems was calculated according to their BMP and weight percentage of the total banana stems (49.87% of FC and 43.13% of PC, by weight; section 2.1), and the result was shown in Figure 4. It was found that methane production from FC was better than PC, and its proportion of the whole banana stem was more than 50% (Figure 4). However, using FC for fiber production may be even better. In recent years, with the growing shortage of wood from the forest, searching for an alternative fiber-producing plant has been initiated worldwide. Using FC as a natural fiber for making cellulosic products, such as tablecloths and clothes, can be a potential option and may have more benefit. Panels A and B of Figure 4 represented two potential process options (section 2.1). To compare the two options, methane loss was 6427

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Energy & Fuels Table 2. Estimated Parameters from the Modified Gompertz Equation from Different Components of Banana Stems sample PC FC MC liquid 1 liquid 2 a

Pmax (mL/g of TS)

Rmax (mL g−1 of TS day−1)

192.2 231.7 168.0 306.6a 329.4a

λ (day) −6

2.52 × 10 2.49 0.89 1.97 × 10−13 4.95 × 10−13

12.9 13.3 20.5 73.6b 56.4b

R2 0.9841 0.9930 0.9918 0.9531 0.9906

The unit was mL/g of COD. bThe unit was mL g−1 of COD day−1.

Figure 4. Percentage of methane production from different components of banana stems.

banana stems, squeezing equipment should be selected and its energy consumption should be investigated.

negligible when directly squeezing the fresh banana stems (option 2, Figure 4B), but there was an 11.96% loss when the banana stems were smashed before squeezing (option 1, Figure 4A). 3.4. Suggestions for Utilization of Banana Stems. Banana stems are valuable biomass in subtropics and tropics, but they have not been used well. On the basis of previous reports, among the byproducts of banana production and processing, only waste bananas were successfully used for biogas production on the commercial scale.21 A high water content of banana stems significantly increased the transport costs and, thus, severely hinders the utilization of banana stems wherever it may be used for production. Squeezing before transportation can significantly reduce the weight and volume of fresh banana stems and, thus, reduce the transportation costs and storage space. On the basis of the industrial production experience, the water content of banana stems should be reduce to 55−65% and the total raw material cost, including squeezing, transportation, and purchase costs, should be about 130 yuan (about 19.7 dollars) per tonne. Moreover, co-digestion of banana stems and animal manures is a practical choice. The liquid generated after squeezing can be used for biogas production because of its high BMP and short digestion time (7 days). For the biogas production, the methane production loss caused by squeezing could be accepted because of its relatively low percentage (3.34−11.96%). Furthermore, the methane loss could be compensated by building small-scale biogas fermenters on the banana stem collection sites and using the liquids left over from the squeezing for AD. Separating FC from banana stems for natural fire or textile production and using PC and other residues for biogas production may be a potential process strategy. Data obtained in this study could be used for preliminary economic feasibility analysis of this process along with other data from natural fire or textile production from banana stems. Before the industrial-scale utilization of

4. CONCLUSION Banana stem utilization is hindered by its high water content. A total of 80% weight and volume of fresh banana stems could be reduced by squeezing. For biogas production, the methane loss caused by squeezing is between 3.34 and 11.96%, which is acceptable and can be compensated using the squeezed liquid for biogas production. All of the components of banana stems, FC, PC, and liquid, were good candidates for biogas production because of their high BMP and short digestion time. The HRT can be set as 20 and 7 days for the solid and liquid components of banana stems, respectively.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-62772123. Fax: +86-10-80194050. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Science and Technology Program of Beijing (Z141100000614005), the National Natural Science Foundation of China (NSFC, 31470229), the Ministry of Science and Technology of China, the National Key Technologies R&D Program (2012BAC18B01), the Science a n d T e c h n o l o g y P r o g r a m f o r P u b l i c W e l lb e i n g (2013GS460202-X), and the International S&T Cooperation Program (2010DFB64040).

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