For the biogas production, sludge discharged from shrimp culture [(S1) shrimp pond sludge] was used as a source of mesophilic methanogenic bacteria, and (S2) bagasse and (S3) molasses discharged during sugar production were used as fermentation substrates. A volume of 350 mL of S1 collected from a shrimp farm in Ben Tre province, Vietnam was sterilized in an autoclave at 121°C for 20 min, followed by drying at 105°C. The necessary quantities of S2 and S3 collected from a sugar factory located in Ben Tre were dried at 105°C.

Raw and dry weights of S1, S2, and S3 were measured to calculate their moisture. Dry samples of S1, S2, and S3 were pulverized in a vibration mill, subsequently fired at 600°C to measure the content of volatile solids (VS) and ash corresponding to organic and inorganic substances, respectively, followed by elemental analysis using energy dispersive X-ray analysis (EDX7000, Shimadzu).

Figure 2 shows the setup for lab-scale methane fermentation (wet-type mesophilic fermentation) composed of a fermenter (4 L), pH meter and gas holder. First of all, acclimatization of bacteria was conducted using 1 kg seed sludge. One kilogram of S1 (VS: 1.86%, Ash: 24.5%, Moisture: 73.6%), 3 L of tap water and 27 g of S2 was added into the fermenter which was thermostated at 35°C, and stirred at 300 rpm. This state was maintained for 120 days for the acclimation of methanogenic bacteria during which 80 g of S2 and designated amounts of minerals (nutrient salts) were added under pH monitoring. The minimum requirement for the nutrient salts to maintain an acetic acid consumption rate of 30 g L⁻¹ d⁻¹ (Takashima and Speece, 1989), and the salts used in this study are summarized in Table 1.

In the acclimation process, aiming at multiplication of bacteria that accelerate the decomposition of bagasse, sufficiently moist bagasse that is expected to include cellulase and lignase active bacteria was selected and used for the biogas production experiments.

After the acclimation process mentioned above, molasses (VS 10 g) was added to the digested liquid to start methane fermentation. After 1 week, a mixture of S2 and S3 with a weight ratio of 4:1 (Mixture-I, VS 20 g) was added to the fermenter, and the biogas production amount and pH of the digested liquid were monitored. The composition of the produced biogas was measured with a gas chromatograph (GC-8AT, Shimadzu) equipped with a stainless-steel column (3 mm diameter, 2 m length) packed with ShinCarbon ST (80–100 mesh) and using Ar as a carrier gas (50 ml min⁻¹).

A dense 10ScSZ (10 mol% Sc₂O₃–1 mol% CeO₂-ZrO₂) plate with 160 μm thickness and 2 × 2 cm² area was made from 10ScSZ powder provided by Daiichi Kigenso Kagaku Kogyo, Japan, and used as the electrolyte to fabricate an electrolyte-supported cell as shown in Figure 3A. The paste for the anode functional layer (AFL) [Ni-10ScSZ (NiO:10ScSZ = 56:44 wt%)] was deposited on the 10ScSZ plate via screen printing following by sintering at 1,300°C for 3 h to form a porous anode material with an area of 1.4 × 1.4 cm². Prior to fabrication of the cathode, paste for the barrier layer [Gd_0.10Ce_0.90O₂ (GDC)] and anode current collector layer (ACCL) (NiO:10ScSZ = 80:20 wt%) was screen printed on the surface of the electrolyte and AFL, respectively, followed by co-sintering at 1,200°C for 5 h. The paste for the cathode functional layer (CFL), which consists of a mixture of (La_0.60Sr_0.40)_0.95(Co_0.20Fe_0.80)O_3−x and Gd_0.10Ce_0.90O_1.95 (LSCF-GDC) (fuelcellmaterials, USA), was screen printed on the sintered GDC layer. Then, the LSCF paste was printed on the dried LSCF-GDC layer as a cathode current collector layer (CCCL). Finally, the deposited pastes were co-sintered at 900°C for 5 h to obtain a porous cathode with an area of 1.4 × 1.4 cm². Pt wires welded with Pt mesh were applied to the anode and cathode as voltage and current-collecting probes.

As shown in Figure 3B, 9 wt% Ni-loaded hydrotalcite ([Mg₆Al₂(OH)₁₆CO₃]4H₂O, HT)-dispersed PSC (9Ni/HT-PSC) was prepared by a papermaking technique using a dual polyelectrolyte retention system and subsequent impregnation and sintering processes (Shiratori et al., 2015; Shiratori and Sakamoto, 2016). The microstructure of the prepared 9Ni/HT-PSC is shown elsewhere (Shiratori and Sakamoto, 2016). Ceramic fiber (SiO₂: 52 wt%, Al₂O₃: 48 wt%) was welded using a yttria stabilized zirconia (YSZ) sol to form an inorganic fiber network with an average size of macropores and porosity of 16 μm and 89%, respectively. Ni loading was performed via the impregnation process, during which Ni was selectively loaded on the HT-derived oxide phase to form very fine Ni particles (<10 nm).

Three pieces of 9Ni/HT-PSCs with an area of 2 × 2 cm² and a thickness of 1.1 mm were stacked and mounted in an alumina cell housing as shown in Figure 3B. Then, the electrolyte-supported SOFC was placed on the PSC to form a PSC/SOFC stack. A porous Pt electrode was attached to the electrolyte surface of the cathode side as a reference electrode. Ceramic adhesive (Aron Ceramic, Toagosei Co., Ltd., Japan) was pasted around the outer perimeter of the cell for sealing, followed by heating to 800°C in 5 h. Then, 50 ml min⁻¹ of pure N₂ was fed to the anode side in-plane direction to completely purge the anode compartment, after which the anode gas was switched from pure N₂ to 50 ml min⁻¹ of pure H₂ (dry) for reduction treatment of the anode and 9Ni/HT-PSC. After 15 h, testing of the DIR-SOFC was started at 800°C.

The anode gas was switched from pure H₂ to 40 ml min⁻¹ of simulated biogas [equimolar mixture of CH₄ and CO₂ (CH₄/CO₂ = 1)]. For the cathode side, 1 L min⁻¹ of air was supplied. Electrochemical measurements were performed using an automatic SOFC testing system (Toyo Corporation, Japan). H₂S poisoning test was conducted during a galvanostatic test at 0.2 A cm⁻² in the direct feed of CH₄/CO₂ = 1 (CH₄: 20 ml min⁻¹, CO₂: 20 ml min⁻¹) by adding 5 or 10 ppm H₂S to the fuel simulating a real biogas produced from the biomass feedstock in Mekong Delta. Electrochemical impedance spectroscopy (EIS) was carried out before and during H₂S poisoning under open-circuit conditions using a frequency response analyzer (VersaSTAT 3, Princeton Applied Research, USA) in a frequency range between 0.1 and 100 kHz with a voltage amplitude of 10 mV.

For the test with real biogas derived from the mixture of S1, S2, and S3, as shown in Figure 3C, the gas stored in a polymer bag (Smart Bag PA, GL Sciences, Japan) was supplied to the PSC/SOFC stack using an aspiration pump at 20 ml min⁻¹ without any fuel processing such as desulfurization and reforming.

Mass content of VS and ash (dry basis) and moisture, and elemental composition of the biomass feedstock collected in the Mekong Delta are listed in Table 2. Shrimp pond sludge contained only 7.4% VS. The high amount of Si and Al indicates that clay is a main component of the sludge. When sludge is collected from unlined ponds like those in Ben Tre, materials that are unsuitable for methane fermentation such as inorganic substances must be separated using a precipitation technique.

When only molasses was added, as shown in Figure 4, a rather high gas production rate was measured; however, this was accompanied by a large drop in pH down to 5.5, and the generation of CO₂ and H₂ was pronounced (see Figure 5A). The CH₄/CO₂ ratio was 0.66 at 3 days, which was considerably lower than the normal ratio of 1.5. For Mixture-I subsequently added to the fermenter, CH₄ production was steady at pH 6.5 without H₂ production, and biogas with CH₄/CO₂ = 1.7 was obtained (see Figure 5B).

After the termination of gas production for Mixture-I in the experiment of Figure 5B, 1-week methane fermentation experiments were performed for the different mixtures of bagasse and molasses (total VS 20 g) with the weight ratios of (II) 4:1, (III) 3:2, and (IV) 2:3 in series. When a fermentation substrate was supplied to the fermenter, 500 mL of digested liquid was discharged, and then double the necessary quantities of the nutrient salts listed in Table 1 were added with 400 mL of tap water. Finally, 100 mL of tap water was poured into the fermenter while washing the supply port so that the total volume of slurry became 4 L. The results of the fermentation experiments are summarized in Table 3.

In the lab-scale methane fermentation experiments with the mixtures of biomass feedstock collected in the Mekong Delta, biogas production yields of 165, 404 and 461 L kgVS⁻¹ were recorded for the Mixture-II, III and IV, respectively. As shown in Figure 4 and Figure 5A, molasses leads to a considerable drop in slurry pH accompanied by H₂ production, and therefore, pH of the digested liquid decreased and the production amount of H₂ increased in the order of Mixture-II, III and IV. The addition of the minerals listed in Table 1 promoted fermentation of the biomass feedstock of the Mekong Delta. Furthermore, in this study, partially fermented bagasse collected from Ben Tre was used as a substrate, and therefore, cellulose- and lignin-assimilating bacteria are considered to be activated to form more acetic acid for assimilation by methanogenic bacteria, resulting in a rather high biogas production rate comparable to that of alkali-treated bagasse (540 L kgVS⁻¹) (Ishihara et al., 1988).

Stable biogas production from the biomass feedstock of the Mekong Delta, bagasse and molasses, was achieved by using the shrimp pond sludge as the source of methanogenic bacteria. An attractive feature of the present biogas is its very low H₂S concentration of 9.0, 14 and 51 ppm for Mixture-II, III and IV, respectively, in comparison to general biogas containing 500–4,000 ppm (Alves et al., 2013). Using SOFC technology, the real biogas produced in this study is considered to be converted directly into electricity without any fuel processing, provided that PSC is applied on the anode material (Shiratori and Sakamoto, 2016).

Figure 6 shows the voltage of the SOFCs in the direct feed of simulated biogas mixture (CH₄/CO₂ = 1) at 800°C with and without 9Ni/HT-PSC monitored during H₂S poisoning under galvanostatic conditions at 0.2 A cm⁻². The result without 9Ni/HT-PSC (Shiratori and Sakamoto, 2016) supports the previous reports (Shiratori et al., 2008; Hagen et al., 2014; Papurello et al., 2016) that DIR-SOFCs are quite susceptible to fuel impurities (H₂S), and their electrochemical performances rapidly deteriorate due to the chemisorption of sulfur species derived from H₂S on the surface of the Ni particles used in the anode material, which blocks the active sites for methane reforming and electrochemical oxidation, leading to a large increase in anode overvoltage. The drastic cell voltage drop by the addition of 5 ppm H₂S indicates the strong deactivation of dry reforming of methane (CH₄ + CO₂ → 2H₂ + 2CO) within the porous anode volume but not by the deactivation of electrochemical oxidation of H₂ and CO. Because the real biogas produced from Mixture-III had almost identical composition to that of the simulated biogas mixture (CH₄/CO₂ = 1) except for the presence of 14 ppm H₂S, in this study, 10 ppm H₂S poisoning test was performed during the galvanostatic measurement at 0.2 A cm⁻² in the direct feed of the simulated biogas. In this 10 ppm poisoning test, 9Ni/HT-PSC was applied on the anode material. Although DIR-SOFC fueled by biogas was intolerant to the 5 ppm H₂S contamination of the fuel, as shown in Figure 6, significant improvement of sulfur tolerance could be achieved by the application of 9Ni/HT-PSC even in the presence of H₂S at twice the concentration (10 ppm).

Electrochemical behavior at the anode/electrolyte interface was investigated with EIS. Figure 7 shows the anode-side impedance spectra for the direct feed of the simulated biogas mixture (CH₄/CO₂ = 1) with and without 9Ni/HT-PSC measured under open-circuit voltage (OCV) conditions (Shiratori and Sakamoto, 2016), showing the influence of the addition of 5 ppm H₂S to the fuel. For both cases with and without 9Ni/HT-PSC, the trace H₂S retarded the electrochemical processes. In the case without 9Ni/HT-PSC, impedances corresponding to charge transfer processes (>500 Hz) and mass transfer processes (<100 Hz) were both increased; especially the diffusion of reactant gases in the porous anode (100–10 Hz) and the gas conversion related to fuel reforming (<5 Hz) were strongly affected (Hagen et al., 2014). Application of 9Ni/HT-PSC on the anode material was confirmed to be highly effective for suppressing the increase in mass transfer resistance by sulfur introduction, indicating that H₂ transport to the anode functional layer adjacent to the electrolyte/anode interface was uninterrupted even under 5 ppm H₂S contamination of the CH₄-CO₂ mixture.

Judging from the results of the simulated biogas (CH₄/CO₂ = 1), if 9Ni/HT-PSC is applied, SOFC operation with direct feed of the real biogas derived from Mixture-III (CH₄/CO₂ ≈ 1, H₂S concentration of 14 ppm) is considered to be possible. Figure 8 shows the result of the galvanostatic test of DIR-SOFC fueled by the real biogas without any fuel processing such as desulfurization, CH₄ enrichment, and pre-reforming. Stable voltage over 0.9 V was obtained until the biogas stored in the polymer bag was used up. The duration of the test with the real biogas was limited within 4 h due to the capacity of the polymer bag, however the performance stability obtained with the simulated biogas under the same testing condition shown in Figure 6 supports that direct conversion of the biogas derived from the biomass feedstock in Mekong Delta into electricity is feasible by the SOFC-PSC combined technology. In future, this technology can contribute to cost reduction and downsizing of the SOFC system because fuel purification unit and external reformer can be simplified or removed.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer MG and handling Editor declared their shared affiliation, and the handling Editor states that the process nevertheless met the standards of a fair and objective review.