title: Integration of Two-Stage Anaerobic Digestion Process for Production of Hydrogen and Methane from First-Generation Sugar Mill Substrates
Creators: name: Mukherjee, Tirthankar
Publisher: name: Griffith University
Year: 2024
Subjects: subject: hydrogen
subject: methane
subject: anaerobic digestion
subject: sugar mill substrates

description: The Australian sugar industry is worth $2.5 billion. However, low sugar prices by new global competitors, and increased production costs are leading to unprofitable businesses and potential closure of sugar mills in the future. Thus, it is imperative to diversify sugar mill activities through valorisation of different mill by-products into high value economic biofuels and bio commodities and increase profitability. Various substrates like sugarcane trash, bagasse, mill mud and molasses from sugar mills have been used to produce biohydrogen (BioH2) and methane (CH4) via anaerobic digestion (AD). These substrates can be classified into two major feedstocks i.e., 1G (1st Generation) consisting of molasses and 2G (2nd Generation) comprising of trash, bagasse and mill mud. Besides the existing substrates (both 1G and 2G) which have been used to produce BioH2 and CH4 via AD process, an intermediary product in a sugarcane processing industry is sugar syrup (1G). Both molasses and sugar syrup are rich in carbohydrates and can be an ideal substrate for BioH2 and CH4 production. The effect of temperature, inoculum-to-substrate ratio (ISR), and reactor configuration is essential to maximize the BioH2 and CH4 yields from 1G sugar mill substrates. Thus, the aim of this study is to evaluate the effect of temperature (37°C, 55°C and 70°C) and ISR (0.25, 0.5, 1, 2, 4 and 6) on BioH2 and CH4 yields. Reactor configurations including one-stage and two-stage batch AD along with thermophilic (55°C) reactor studies were investigated to optimize the BioH2 and CH4 yields. AspenPlusV11 simulation software was used to simulate a biogas plant using molasses and sugar syrup as the feed for three different scenarios. - Scenario 1: CHP-Biogas is used for electricity and heat generation in a combined heat and power (CHP) plant. - Scenario 2: CHP + BioCNG-A portion of biogas is used for CHP to generate electricity and heat to meet the parasitic demand of the biogas plant and the remaining biogas is upgraded and compressed (BioCNG) for vehicle fuel or distribution via virtual pipeline (road transport). - Scenario 3: CHP+ BioRNG-Similar to Scenario 1 but the biogas is upgraded to renewable natural gas (BioRNG) for grid injection. Results from one-stage batch studies of sugar syrup showed that highest cumulative H2 production of 188.85 NL-H2/kg-VSadded was obtained at 37°C (ISR of 1) followed by 149.97 NL-H2/kg-VSadded at 55°C (ISR of 0.5). On the other hand, lowest H2 production was obtained at 70°C with the maximum yield of 20.84 NL-H2/kg-VSadded at ISR 1. The highest CH4 yield was obtained at 37°C (349.21 NL-CH4/kg-VSadded) followed by at 55°C (317.89 NL-CH4/kg-VSadded). Interestingly, both of these methane yields were obtained at ISR 2. Least cumulative CH4 production was obtained at 70°C with the maximum yield of 154.05 NL-CH4/kg-VSadded at ISR 6. The effect of temperature (37°C, and 55°C) and ISR (0.25, 0.5, 1, 2, 4 and 6) on H2 and CH4 yield of 1G feedstock, sugar syrup through two-stage batch AD using mixed anaerobic consortia, showed that both the parameters had profound effect on the biogas yields. At 37°C, the first-stage at ISR 1 witnessed the highest cumulative H2 yield of 82.58 NL-H2/kg-VSadded and the second-stage at ISR 6 observed the highest cumulative CH4 yield of 1,152.41 NL-CH4/kg-VSadded. However, at 55°C, the first-stage at ISR 1 observed the highest cumulative H2 yield of 24.28 NLH2/kg-VSadded and the second-stage at ISR 2 observed the highest cumulative CH4 yield of 499.88 NL-CH4/kg-VSadded. The reactor studies investigate on the effect of organic loading rate (OLR) on BioH2 and CH4 production rate (mL/Day) from sugar syrup via two-stage AD in a thermophilic reactor (55°C CSTR). The OLRs studied in the first-stage were 30, 40, 50, 60, 70, and 80 gCOD/l/day with an hydraulic retention time (HRT) of 12 hours. OLR 50 witnessed the highest BioH2 production rate of 1,200 mL/Day (approximately) and the maximum sCOD (soluble chemical oxygen demand) removal% of 14.52. BioH2 was produced in first-stage via butyricacetic acid pathway. The digestate of 50 OLR from the first-stage was used as the feed for the second-stage. The OLRs studied in the second-stage were 5, 10, and 15 gCOD/l/day and the HRT was maintained at 10 days. Highest CH4 production rate of 2,112 mL/Day (approximately) was obtained at OLR 10 with maximum sCOD removal% of 85. The study also investigates the impact of temperature (37°C and 55°C) and ISR on H2 and CH4 yields during two-stage batch AD of molasses using mixed anaerobic consortia. Results indicate significant effects of both temperature and ISR on biogas production, highlighting the importance of optimizing these parameters for maximizing biogas yields from molasses. At 37°C, the first-stage at ISR 1 witnessed the highest cumulative H2 yield of 63.61 NL-H2/kg-VSadded and the second-stage at ISR 4 observed the highest cumulative CH4 yield of 1,092.70 NL-CH4/kg-VSadded. However, at 55°C, the first-stage at ISR 0.50 witnessed the highest cumulative H2 yield of 25.03 NL-H2/kg-VSadded and the second-stage at ISR 4 observed the highest cumulative CH4 yield of 806.54 NL-CH4/kg-VSadded. The reactor studies examine on the effect of OLR on BioH2 and CH4 production rate (mL/Day) from molasses via two-stage AD in a thermophilic reactor (55°C CSTR). The OLRs studied in the first-stage were 30, 40, and 50 gCOD/l/day with an HRT of 12 hours. OLR 30 witnessed the highest BioH2 production rate of 1,025 mL/Day (approximately) and the maximum sCOD removal% of 13.97. H2 was produced in first-stage via butyric-acetic acid pathway. The digestate of 30 OLR from the first-stage was used as the feed for the second-stage. The OLRs studied in the second-stage were 8, 9, and 10 gCOD/l/day and the HRT was maintained at 10 days. Highest CH4 production rate of 1,903 mL/Day (approximately) was obtained at OLR 8 with maximum sCOD removal% of 71.5. From the AspenPlus simulations it can be concluded that, it can be used to simulate an AD plant with the 3 scenarios. Mossman sugar mill located in Queensland, produces around 150,000 t/year of sugar syrup and 20,400 t/year of molasses which has been used as the substrate for the Aspen Model simulation for AD process. The biogas produced from the simulation of sugar syrup was 59.27 x 106m3/year while from molasses was 8.96 x 106m3/year. The electricity produced from Scenario 1 using sugar syrup was 5,00,485.08 GJ/year while heat obtained was 491,874 GJ/year. From Scenario 2 and 3 using sugar syrup as the feedstock the upgraded biomethane produced was 20.07 x 106 m3/year and 21.50 x 106 m3/year, respectively. On the other hand, from Scenario 1 using molasses as the feedstock, the electricity produced was 62,791.68 GJ/year and heat obtained was 95,571.6 GJ/year. From Scenario 2 and 3 using molasses as the substrate for AD, the upgraded biomethane produced was 3.23 x 106m3/year and 3.49 x 106m3/year, respectively. Thus, from this study, it can be firmly concluded that two-stage AD is more efficient with 1G sugar mill substrates as its feedstock. Low pH around 5.5 and ISR 1 is suitable for BioH2 production while high pH around 7.5 with ISR 2 is appropriate for CH4 production. Similarly, from the reactor studies it can be inferred that high OLR is promising for BioH2 production and low OLR for CH4 production. AspenPlus can be used to simulate an AD plant with the biogas generated been used for heat/electricity generation and upgraded biomethane for grid injection.