Article pubs.acs.org/EF
Sequential Generation of Fermentative Hydrogen and Methane from Swine Manure with Physicochemical Characterization Jun Cheng,* Richen Lin, Ao Xia, Yaqiong Liu, Junhu Zhou, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: Swine manure, a typical livestock waste, has great potential for biohydrogen production. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) analyses were employed to study the physicochemical characteristics of swine manure. SEM and TEM reveal that swine manure has a significantly damaged lignocellulosic matrix with cracks and debris on the surface. XRD and FTIR demonstrated that the cellulose crystallinity index of swine manure was higher than that of raw lignocellulosic biomass. A threestage fermentation process comprising dark hydrogen, photo-hydrogen, and dark methane was performed using swine manure. Through the combined dark and photo-hydrogen production, the hydrogen yield was dramatically increased from 71.8 (dark fermentation only) to 247.7 mL of H2/g of total volatile solids (TVS). The subsequent methane yield was 87.2 mL of CH4/g of TVS using the residue of photofermentation, which increased the heat value conversion efficiency to 29.76%.
1. INTRODUCTION The rapid development and growth of the livestock industry in China and the world over the past few decades has highlighted the need for a sustainable and advanced livestock waste treatment technology. According to the National Bureau of Statistics of China, in 2012, China produced approximately 0.7 billion heads of pigs. The large amounts of manure produced would contaminate the environment if improperly handled but would potentially provide an abundant renewable source for biofuel production. Traditionally, swine manure is used to produce biogas through anaerobic fermentation, and the associated technologies for its production have been studied extensively and are well-developed around the world.1 Recently, hydrogen has drawn more wide attention as a renewable, carbon-neutral, and environmentally friendly alternative energy source. Biological hydrogen production (e.g., dark and photohydrogen fermentation) from biomass feedstock is considered an important strategy for hydrogen production.2,3 Water hyacinth has caused serious environmental and economic problems worldwide, because of its high reproduction rate and dispersion.4,5 The moisture content in raw water hyacinth is about 95%, and the residue is lignocellulosic biomass. The dried biomass of water hyacinth mainly contains cellulose (∼29%), hemicellulose (∼31%), lignin (∼5%), and protein (∼21%).6 Therefore, water hyacinth has been used as animal feed for decades in China.5 Literature has reported that water hyacinth has great potential to produce biofuels (e.g., hydrogen, methane, and ethanol) in recent years.6−8 Experiments showed that the hydrogen yield from swine manure was much lower than that from water hyacinth. It is interesting to compare hydrogen fermentation characteristics between swine manure and water hyacinth. It is valuable to further reveal the reasons for hydrogen production differences based on their physicochemical microstructures. Therefore, water hyacinth as a typical lignocellulosic biomass was used for a comparative study. Various studies have investigated dark fermentative hydrogen production using livestock manure (e.g., swine and cow © 2013 American Chemical Society
manure) as feedstock. The biohydrogen yield of dark fermentation varies from 4 to 53 mL/g of total volatile solids (TVS) using livestock manure.3 Typically, these yields are much lower than those observed from lignocellulosic biomass (e.g., water hyacinth), reaching 112.3 mL of H2/g of TVS reported in our previous study.8 Studies on biohydrogen production using livestock manure have mostly focused on pretreatment, substrate concentration, operating pH, and temperature, which proved that each of these parameters may significantly influence biohydrogen production.1,9−11 Hydrogen production from cow manure was examined at temperatures ranging from 37 to 85 °C using the natural bacterial flora. A biohydrogen yield of 29 mL/g of TVS has been observed at 60 °C.12 Biohydrogen production under hyperthermophilic conditions (70 °C) inhibits methanogenic bacteria activity and creates better thermodynamic conditions for biohydrogen production.9 However, a high temperature necessitates high energy consumption and lowers the feasibility for industrial applications. Prior to hydrogen fermentation, the manure must be pretreated chemically or thermally to prevent methanogenic activity and degrade lignocellulosic materials. Acidification pretreatment is effective for cow manure, yielding 18 mL of H2/g of TVS.13 In comparison to the alkali and acid treatments, heat treatment of swine manure at 80 °C for 30 min achieves the highest H2 yield of 36.6 mL/g of TVS.14 Many pretreatment methods (e.g., acid, alkaline, and heat treatments) can be used to improve dark fermentative hydrogen production using manure as feedstock. Nevertheless, the hydrogen yield of dark fermentation is generally lower than that using water hyacinth, which lowers the energy conversion efficiency. Moreover, dark fermentation could cause environmental pollution if the resulting residue solution is improperly treated. A two-stage fermentation process for H2 and CH4 cogeneration has been performed to eliminate the environmental Received: September 6, 2013 Published: December 5, 2013 563
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120 keV electron energy emission. XRD (X’Pert PRO, Netherlands) was used for cellulose crystallinity analysis of swine manure and water hyacinth. The changes in chemical composition and crystallinity were examined via a FTIR spectrometer (Nicolet 5700, U.S.A.). The H2 and CH4 concentrations were detected by a gas chromatography system (Agilent 7820A, U.S.A.). The SMP compositions were determined by another gas chromatography system (Thermo Finigan TRACE 2000, U.S.A.). The dynamic parameters of H2 and CH4 production were simulated by the modified Gompertz equation using Origin 8.5. All of the tests were carried out in triplicate. Further, the experimental results were analyzed using analysis of variance (ANOVA) at the 95% confidence level (p < 0.05) to determine the statistical significance.
contamination caused by dark hydrogen fermentation residue and, subsequently, improve the energy conversion efficiency.6,15 However, dark fermentation has not been combined with photofermentation for hydrogen production using livestock manure, which could dramatically enhance the hydrogen production. In addition, the lower hydrogen yield of manure compared to water hyacinth remains unexplained. The microscopic physicochemical properties of animal manure likely differ from those of lignocellulosic biomass, resulting in different fermentation characteristics. In the present study, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) analyses were employed to study the differences in the chemical components and microstructure of swine manure and water hyacinth. A three-stage fermentation process was performed to maximize the hydrogen production and utilization efficiency of swine manure.
3. RESULTS AND DISCUSSION 3.1. Physical and Chemical Properties of Swine Manure and Water Hyacinth. The lignocellulosic components in untreated swine manure and untreated water hyacinth can hardly be used for fermentation because they have a compact structure that strongly resists biodegradation. In our laboratory, water hyacinth has been extensively used as feedstock for hydrogen production.6,7 Comparative results show that the hydrogen yield using swine manure is much lower than that using water hyacinth. We characterized the chemical and physical features of swine manure and water hyacinth using SEM, TEM, XRD, and FTIR analyses. The SEM micrographs of swine manure and water hyacinth in Figure 1 show that the surface of swine manure is less integrated and rougher compared to that of water hyacinth. Digestion in the stomach of the pig causes considerable damage to the lignocellulosic structure of swine manure, producing cracks, debris (2−4 μm), and micropores on the surface. In contrast, water hyacinth clearly has an unbroken and smooth lignocellulosic structure with smaller bits of debris (0.5−1 μm). The TEM analyses of swine manure and water hyacinth were used to further characterize differences in physical structure. The micrographs in Figure 1 show that the thickness of the lignocellulosic structure of swine manure ranged from 3 to 7 μm, whereas that of water hyacinth ranged from 0.6 to 1.3 μm. Water hyacinth has a more complete and unbroken cell wall, with cellular structures that are thinner and longer. The cell wall in the swine manure has numerous cracks and fractures, which is consistent with the SEM micrographs displaying the remarkably deconstructed structure. The cell wall surfaces in swine manure are more uneven and coarser, indicating that the digestion process and alkaline-rich property effectively degrade this raw material. However, swine manure has a denser cell wall structure, which leads to poor bioaccessibility for further degradation. Consequently, although the lignocellulosic components in swine manure have been digested, the internal cell wall structures are thicker and denser than those of water hyacinth, thereby complicating the subsequent pretreatment and fermentation and producing a lower hydrogen yield. The XRD patterns (Figure 2a) of swine manure and water hyacinth were analyzed to recognize the cellulose structure. Previous studies 20−22 indicated that the peak at 23° demonstrates the highly organized crystalline region of cellulose. While the peak at 18° represents the amorphous region of cellulose. The crystallinity coefficient was calculated according to the Segal empirical method. The cellulose crystalline structure of swine manure is more easily discernible than that of water hyacinth, which suggests that digestion does not disrupt the crystalline structure of cellulose. The crystallinity coefficient of swine manure was 25.6, and the crystallinity coefficient of water hyacinth was 23.6. The higher
2. MATERIALS AND METHODS 2.1. Substrates and Inocula. Swine manure was sampled from Huzhou pig farm in Zhejiang province, China. Water hyacinth was collected from the Fuchun River in Hangzhou. The swine manure and water hyacinth were first heated at a temperature of 105 °C for 20 h and pulverized to 200 μm. The organic compositions of the swine manure and water hyacinth were analyzed on the basis of Chinese standard analysis methods.6 Dark-hydrogen-producing bacteria, photo-hydrogen-producing bacteria, and methane-producing bacteria were obtained from anaerobicactivated sludge, as described in previous stydies.16,17 2.2. Hydrolysis and Fermentation Processes. Hydrolysis was conducted in 10 polytetrafluoroethylene reactors containing 1.35 g of dry swine manure with 27 mL of diluted H2SO4 (0−2%, v/v) solution. The reactors were sealed and then heated inside a microwave system at 140 °C for 15 min. The reducing sugars were subsequently tested according to the 3,5-dinitrosalicylic acid method.7 2.2.1. Dark Hydrogen Fermentation. The optimal pretreatment method for dark hydrogen fermentation was chosen according to the yield of reducing sugars. Dark fermentative hydrogen fermentation was conducted in 350 mL bottles. Each bottle was filled with 135 mL of microwave-pretreated solution (equivalent to 6.75 g of dry swine manure) and 25 mL of hydrogen-producing bacteria. The total liquor volume was adjusted to 250 mL with deionized water. The pH value of dark hydrogen fermentation solution was adjusted to 6.0 ± 0.1 initially and then was readjusted to 6.0 ± 0.1 every 10 h. The fermentation temperature was kept at 35.0 ± 1.0 °C. 2.2.2. Photo-hydrogen Fermentation. To eliminate NH4+ in the supernatant residue of dark hydrogen fermentation, which is a potential inhibitor for photo-hydrogen fermentation, zeolite was employed for NH4+ removal.17 Then, the soluble metabolite product (SMP) concentration in the supernatant solution was diluted to 15 mM. A total of 30 mL of photosynthetic bacteria was then inoculated in a 250 mL bottle.18 The initial pH value for photofermentative hydrogen fermentation was adjusted to 7.0 ± 0.1. The experiments were performed in an illuminated incubator at a temperature of 30.0 ± 1.0 °C.18 2.2.3. Methane Fermentation. The residue of photo-hydrogen fermentation was inoculated with 10 mL of methane-producing bacteria. The initial pH value for methane fermentation was adjusted to 8.0 ± 0.1. The temperature was kept at 35.0 ± 1.0 °C.6,19 All bottles in experiments were purged with N2 gas for 20 min to keep the initial anaerobic environment. 2.3. Analytical Methods. Samples of pulverized swine manure and water hyacinth were photographed under field emission SEM (Hitachi S3700, Japan) after the samples were sputtered with a thick layer of gold. The structural differences between swine manure and water hyacinth were examined with TEM (Hitach H-7650, Japan) at 564
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Figure 2. X-ray diffractograms and FTIR spectra of swine manure and water hyacinth: (a) X-ray diffractograms and (b) FTIR spectra.
Table 1. Characteristics of Bands in FTIR Spectra of Swine Manure and Water Hyacinth wavenumber (cm−1) 3413
Figure 1. SEM and TEM images of swine manure and water hyacinth. (a) SEM, swine manure at 2000×; (b) SEM, swine manure at 10000×; (c) SEM, water hyacinth at 2000×; (d) SEM, water hyacinth at 10000×; (e) TEM, swine manure at 8000×; (f) TEM, swine manure at 40000×; (g) TEM, water hyacinth at 8000×; and (h) TEM, water hyacinth at 40000×.
2854 1638 1558 1430 898
crystallinity of swine manure indicates it has a more organized crystalline region. FTIR analysis has been widely used for analyzing structural and chemical changes.22,23 Table 1 and Figure 2b show the results of FTIR analysis of swine manure and water hyacinth. Swine manure contains the same functional group as water hyacinth. The characteristic peaks at wavenumbers of 898, 1030, 1064, 1106, and 1168 cm−1 are easily identified, which are typical wavenumbers of cellulose.24 Swine manure exhibited no peaks at 1712 cm−1 and a less prominent peak at 1043 cm−1 that is assigned to hemicellulose. Swine manure appears to have a lower hemicellulose content than water hyacinth. The total crystallinity of the lignocellulosic matrix was calculated as the absorbance ratio A1375/A2900, and the crystallinity index was calculated as the absorbance ratio A1427/A898, with the 1430 cm−1 band denoting cellulose I and the 898 cm−1 band denoting cellulose II.25 The total crystallinity index was 2.02 for water hyacinth and 1.51 for swine manure. The crystallinity index was 0.754 for water hyacinth and 0.761 for swine manure.
functional group
assigment
−OH stretching intramolecular cellulose II hydrogen bonds C−H stretching cellulose CO stretching of acetyl or hemicellulose and carboxylic acid lignin CC stretching of the aromatic ring lignin CH2 symmetric bending cellulose asymmetric, out of phase ring cellulose stretching swine water manure hyacinth
crystallinity index (A1430/A898) of cellulose particles total crystallinity (A1375/A2900) of the lignocellulosic matrix
0.761
0.754
1.51
2.02
The higher cellulose crystallinity index of swine manure indicates its stronger recalcitrance during hydrolysis and fermentation, which results in a lower hydrogen yield. The total crystallinity of swine manure was lower than that of hyacinth, likely because some of the crystal lignocellulosic matrix was damaged to a certain extent by bacteria during digestion, leaving behind inert and stable crystalline structures. 3.2. Effects of Acid Concentrations on the Reducing Sugar Yield. The swine manure sample was primarily composed of 22.65% cellulose, 17.92% crude protein, 12.65% hemicellulose, and 6.23% lignin. The disruption of the lignocellulosic structure to produce reducing sugars plays a 565
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1.1 ± 0.2 0.2 ± 0.03 0.8 ± 0.1 0.9895 59.4 5.67 a
SMPs = acetate + propionate + butyrate + valerate + caproate + ethanol.
86.1 0.17 ± 0.0004 (CH4) 87.2 ± 0.1 (CH4)
0.59
5.4 ± 0.3 2.0 ± 0.3 2.1 ± 0.2 0.9925 26.1 11.9 184.8 5.75 ± 0.5 (H2) 175.9 ± 7.9 (H2)
residuals from dark hydrogen after NH4+ removal residuals from photo-hydrogen
4.8
71.7 ± 3.8 24.1 ± 1.9 34 ± 1.2 0.986 21.8 3.4 1.5 75 1.76 ± 0.2 (H2) 71.8 ± 5.19 (H2) swine manure hydrolysate
dark hydrogen production photo-hydrogen production dark methane production
butyrate (mM) acetate (mM) Tm (h) λ (h) Rm (mL g−1 of TVS h−1) Hm (mL/g of TVS) biogas production peak rate (mL g−1 of TVS h−1) biogas yield (mL/g of TVS) feedstock stage
kinetic model parameters
Table 2. Dynamic Parameters of Hydrogen, Methane, and SMP Production from Swine Manure in Three-Stage Fermentation
R2
soluble metabolite products
key role in the fermentation of lignocelluloses.26 Calculations of the theoretical value estimate that the yield of reducing sugars is 36.534 g/100 g of dry manure.6 The effect of pretreatments with microwave-assisted dilute acid at different concentrations on saccharification was tested. The yield of reducing sugars at different acid concentrations of 0.5, 1, 1.5, and 2% was 9.08, 34.93, 28.87, and 29.43 g/100 g of dry manure, respectively. Microwave-assisted pretreatment with 1% H2SO4 achieved the highest reducing sugar yield of 34.93 g/100 g of dry manure (95.6% of the theoretical value). The statistical difference was found to be significant [analysis of variation (ANOVA); p < 0.05], indicating that different acid concentrations cause a significant effect on the reducing sugar yield. Dilute acids are effective for cellulose hydrolysis, and they have been successfully applied as a pretreatment for lignocellulosic materials.22,27 For swine manure with endogenous alkalinity,28 most cellulose and hemicellulose can be degraded to small molecules, such as glucose and xylose, respectively, through microwave-assisted dilute H2SO4 pretreatment. 3.3. Hydrogen Production through Dark Fermentation after Microwave-Assisted Dilute Acid Hydrolysis. Untreated swine manure is hard to produce hydrogen ( 0.05). The dynamic dark hydrogen production parameters of swine manure were compared to those of water hyacinth. It was worth noting that the lag time of dark hydrogen fermentation using swine manure was 3.4 h, whereas the lag time using water hyacinth was only 2.4 h.8 The total dark fermentation time was 70 h for swine manure and 24 h for hyacinth. The difference could be attributed to the stronger biodegradation resistance of the lignocellulosic materials in swine manure compared to water hyacinth, which is consistent with the results obtained from the FTIR and TEM analyses. 3.4. Photo-hydrogen Production from Dark Fermentation Effluent. SMPs and the NH4+ concentrations in the residue of dark hydrogen production are shown in Table 2. The dark hydrogen fermentation effluent mainly contained acetate (34 ± 1.2 mM) and butyrate (24.1 ± 1.9 mM), which could be used in photofermentation. The NH4+ concentration in the residual solution was 16.9 mM, which significantly inhibits the enzyme activity.17,30 The effluent solution from dark hydrogen
SMPsa (mM)
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efficiency). While the SMP concentrations decreased insignificantly (ANOVA; p > 0.05), which can be attributed to the low adsorption of zeolite for organics and low affinity for anions.32 Dynamic photofermentative hydrogen production parameters are presented in Table 2 and Figure 4b. The peak photohydrogen production rate was 5.74 mL g−1 of TVS h−1 at 40 h. After 60 h of fermentation, the hydrogen yield reached 175.9 mL of H2/g of TVS. An increase of fermentation time from 0 to 60 h caused an significant increase of the hydrogen yield from 0 to 175.86 mL/g of TVS (ANOVA; p < 0.05); however, the further increase of fermentation time from 60 to 80 h caused an insignificant increase of the hydrogen yield from 175.86 to 175.92 mL/g of TVS (ANOVA; p > 0.05). The photo-hydrogen yield of swine manure is lower than that of hyacinth,8 which is 639.2 mL of H2/g of TVS. This result is likely attributed to the lower amount of SMPs produced from dark hydrogen fermentation using swine manure than that using water hyacinth. The SMP concentration of swine manure is 71.7 mM, while that of water hyacinth is 119.4 mM.8 Moreover, the remaining NH4+ and some unknown inhibitors in the swine manure residual solution have an inhibitory effect on photosynthetic bacteria and lower the hydrogen yield. Through combined dark and photo-hydrogen production, the hydrogen yield from swine manure dramatically increased from 71.8 to 247.7 mL of H2/g of TVS. 3.5. Methane Fermentation from the Residue of Photo-hydrogen Fermentation. To improve the utilization efficiency and the energy conversion efficiency of swine manure, methane fermentation was performed using the residue of photo-hydrogen fermentation. The methane production yield and rate are shown in Figure 5. After 3 days
Figure 3. Dark fermentative hydrogen production from microwaveassisted 1% H2SO4-treated swine manure.
fermentation cannot yield biohydrogen when its NH4 + concentration is higher than the inhibitory concentration for photosynthetic bacteria (5 mM).31 Hence, NH4+ must be removed prior to photofermentation. In our previous study, modified zeolite treatment was proven effective for NH4+ removal.16,17 Table 2 and Figure 4a show that the NH4+ and SMP concentrations change after NH4+ removal. After zeolite treatment, the NH4+ concentrations in dark hydrogen fermentation effluent significantly (ANOVA; p < 0.05) decreased from 17.15 to 1.62 mM (90.55% NH4+ removal
Figure 5. Dark methane production from the residue of photofermentative hydrogen production.
of methane fermentation, the peak methane production rate was 4.09 mL g−1 of TVS day−1. As the methane fermentation time increased to 18 days, the methane yield increased to 87.2 mL/g of TVS. An increase of fermentation time from 0 to 15 days caused an significant increase of the methane yield from 0 to 87.16 mL/g of TVS (ANOVA; p < 0.05); however, the further increase of fermentation time from 15 to 18 days caused an insignificant increase of the methane yield from 87.16 to 87.18 mL/g of TVS (ANOVA; p > 0.05). Dynamic parameters of methane fermentation are presented in Table 2. The lagphase time of methane fermentation was 5.67 h, and the peak time was 59.4 h. The energy conversion efficiency reached
Figure 4. Photofermentative hydrogen production from the residue of dark fermentative hydrogen production with NH4+ removal. (a) Concentrations change of SMPs and NH4+ after zeolite treatment. (b) Photofermentative hydrogen production from the residue of dark fermentative hydrogen production. 567
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Figure 6. Rough analysis of the mass balance of swine manure during the pretreatment and three-stage fermentation. DW = dry weight.
Table 3. Biohydrogen Production from Various Livestock Manure by Fermentation feedstock
pretreatment method
temperature (°C)
initial pH
swine manure
no pretreatment
70
5.3
cow manure
boiled with 0.2% NaOH for 30 min irradiated by infrared for 2 h boiled with 0.2% HCl for 30 min no pretreatment no pretreatment heated at 80 °C for 30 min no pretreatment
36
cow manure cow manure
cow manure cow manure swine manure 30% cow manure with 70% waste milk (w/w) 65% swine manure with 35% fruit and vegetable waste (w/w) swine manure swine manure
a
reactor configuration
hydrogen yield (mL/g of TVS)
fermentation type
references
3.65
dark fermentation
9
5
continuously stirred tank reactor batch
14
dark fermentation
13
36
5
batch
14
dark fermentation
13
36
5
batch
18
dark fermentation
13
60 45 35
7 5.5 6.2
batch batch batch
29a 53a 68.5a
dark fermentation dark fermentation dark fermentation
12 11 14
55
6.4
batch
59.5
dark fermentation
33
no pretreatment
55
5.45
semi-continuously fed reactor
126
dark fermentation
28
no pretreatment
35
5
209a
dark fermentation
10
microwaved with 1% H2SO4 for 15 min
35
semi-continuously fed reactor batch
247.7
dark + photofermentation
in this study
6 in dark and 7 in photofermentation
Calculated from literature data.
29.76% through the three-stage fermentation process compared to 13.74% through hydrogen fermentation alone. 3.6. Rough Analysis of the Mass Balance and Hydrogen Production Efficiency. The rough analysis of the mass balance of swine manure (6.75 g) during the pretreatment and three-stage fermentation was summarized in Figure 6. After microwave acid pretreatment, a large amount of swine manure was degraded to reducing sugar (2.36 g). In the following fermentation processes, most reducing sugar was further degraded into H2, CH4, and SMPs. The hydrogen production efficiency was defined as the ratio of the actual hydrogen yield to the theoretical hydrogen yield as follows:
pretreatment process (C6H10O5)n + nH 2O → nC6H12O6 (C5H8O4 )n + nH 2O → nC5H10O5
combined dark and photo-hydrogen fermentation from glucose and xylose C6H12O6 + 6H 2O → 12H 2 + 6CO2 C5H10O5 + 5H 2O → 10H 2 + 5CO2
The contents of cellulose and hemicellulose in swine manure are 22.65 and 12.65%, respectively. The TVS content is 72.12%. In theory, 1 g of TVS of swine manure can produce [(1 × 22.65% ÷ 162 × 12 + 1 × 12.65% ÷ 132 × 10) ÷ 72.12% × 22 400 = 818.8] mL of hydrogen, where 162 and 132 are the molar masses of cellulose and hemicellulose and 22 400 mL/ mol is the gas molar volume at standard pressure and temperature. The actual hydrogen yield is 247.7 mL/g of TVS. Therefore, the hydrogen production efficiency is 247.7 ÷ 818.8 = 30.3%. This is probably because of the fact that (1) the hydrolysis efficiency is 95.6%, (2) inhibitory byproducts (e.g., furfural, acetic acid, and succinic acid) were produced during pretreatment, (3) reducing sugars are not fully used during
hydrogen production efficiency =
actual hydrogen production (mL/g of TVS) theoretical hydrogen production (mL/g of TVS)
The fermentable compositions of swine manure mainly comprise cellulose (C6H10O5)n and hemicellulose (C5H8O4)n, which can be hydrolyzed into glucose (C6H12O6) and xylose (C5H10O5), respectively. The theoretical hydrogen production was calculated by the following equations: 568
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Energy & Fuels fermentation, and (4) part of fermentable feedstock was consumed for bacteria growth, and (5) H2 is not the only metabolic product. Lactic and propionic acids are also produced during dark fermentation. 3.7. Comparison of Hydrogen Production from Various Livestock Manure. Numerous studies have investigated the influence of pretreatment methods, fermentation temperature, initial pH, and reactor configuration on dark hydrogen production using various livestock manures as feedstock. The hydrogen production characteristics from various livestock manures through fermentation are presented in Table 3. The dark hydrogen production values using only livestock manures ranged from 4 to 68.5 mL of H2/g of TVS. The highest yield (68.5 mL of H2/g of TVS) was obtained in research using heat-treated swine manure (Table 3). Although many pretreatment methods were applied, the hydrogen yield from pure manure feedstock is still much lower than that from water hyacinth, up to 112.3 mL of H2/g of TVS.8 Co-fermentation of livestock manures and other carbohydrate-rich feed proved to be a viable method for increasing the hydrogen yield. A study focusing on the co-fermentation of cow manure and waste milk achieved a hydrogen yield of 59.5 mL of H2/g of TVS.33 Co-digestion of swine manure waste and fruit waste achieved a hydrogen yield of 126 ± 22 mL of H2/g of TVS.28 A hydrogen yield of 209 mL of H2/g of TVS was obtained using swine manure after adding glucose as an additional feedstock.10 Evaluating the hydrogen yield contributed from manure feedstock is difficult when glucose supplementation occurs, because of the great potential of hydrogen production from glucose. Moreover, the addition of glucose apparently increases the economic cost of biofuel production for industrial applications. In the present study, pure swine manure was used for dark H2 production. Through combined dark and photo-hydrogen production, the H2 yield dramatically increased to 247.7 mL/g of TVS, which is the highest value ever reported. On the basis of the calculation method proposed in our previous study,17 the subsequent methane fermentation (87.2 mL of CH4/g of TVS) further increased the energy conversion efficiency to 29.67%.
ACKNOWLEDGMENTS
■
REFERENCES
This research was supported by the National High Technology Research and Development Program of China (2012AA050101), the National Natural Science Foundation of China (51176163), the Specialized Research Fund for the Doctoral Program of Higher Education (20110101110021), the International Science and Technology Cooperation Program of China (2012DFG61770 and 2010DFA72730), the National Key Technology Research and Development Program of China (2011BAD14B02), the Program for New Century Excellent Talents in University (NCET-11-0446), and the Science and Technology Project of Guangxi Province (1346011-1).
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4. CONCLUSION H2 and CH4 co-generation from swine manure has great potential in future industrial applications. The SEM, TEM, XRD, and FTIR results show that swine manure has a higher cellulose crystallinity than water hyacinth and, therefore, is strongly resistant to biodegradation. The hydrogen yield was dramatically increased from 71.8 (dark fermentation only) to 247.7 mL of H2/g of TVS through combined dark and photofermentation using swine manure as feedstock. The heat value conversion efficiency was enhanced to 29.76% with a subsequent methane yield of 87.2 mL of CH4/g of TVS.
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dx.doi.org/10.1021/ef402388j | Energy Fuels 2014, 28, 563−570
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