Comprehensive insights into the effects of acidogenic off-gas utilization on successive biogas production, microbial community structure and metabolite distribution during two-stage anaerobic digestion.

Although two-stage anaerobic digestion (TSAD) technology has been investigated, the mechanisms regarding the impact of acidogenic off-gas (AOG) on successive methane production have not been well addressed. In this study, a novel TSAD system was designed. Food waste, as the main substrate, was co-digested with chicken manure and corn straw. The acidogenic gas beyond atmospheric pressure was introduced into the bottom of the methanogenesis reactor through a stainless steel diffuser. Results showed the addition of AOG increased the methane yield from 435.2 to 597.1 mL/g VSin in successive methanogenesis stage, improved by 37.2 %, and increased the energy yield from 9.0 to 11.3 kJ/g VSsubstrate. However, the theoretical contribution of hydrogenotrophic methanogenesis using H2 contained in AOG was only 15.2 % of the increased methane yield. After the addition of AOG, the decreased levels of ammonia nitrogen and butyrate indicate that the stability of the AD system was improved. The electron transfer system and co-enzyme F420 activity were enhanced; however, the decrease in acetate kinase activity indicates aceticlastic methanogenesis may have been weakened. The microbial diversity and species richness were improved by the added AOG. Methanosarcina was more competitive than Methanothermobacter, enhancing the syntrophic effect. The relative abundance of protein degradation bacteria norank_f_Anaerolineaceae and lipid degradation bacteria Syntrophomonas was increased. Metabolite analysis confirmed that the addition of AOG promoted amino acid metabolism, the biosynthesis of other secondary metabolism and lipid metabolism. The improved degradation of recalcitrant organic components (lipids and proteins) in food waste was responsible for the increased methane yield. This study provides an in-depth understanding of the impact of AOG utilization on successive methane production and has practical implications for the treatment of food waste.

Investigation of product selectivity and kinetics of poplar sawdust catalytic pyrolysis over bi-metallic Iron-Nickel/ZSM-5 catalyst.

Py-GC/MS and thermogravimetric analysis were carried out to systematically explore product selectivity and kinetics of poplar sawdust catalytic pyrolysis over bi-metallic Fe-Ni/ZSM-5. The results showed that the Fe-Ni/ZSM-5 exhibited an additive effect on the production of monocyclic aromatic hydrocarbons compared to mono-metallic catalysts (Fe/ZSM-5 or Ni/ZSM-5). Fe-Ni/ZSM-5 further increased the yield of toluene (17.28 mg g-1), which was 41.4% and 80.9% higher than Fe/ZSM-5 and Ni/ZSM-5, respectively. According to the kinetic analysis, the average activation energy obtained from catalytic pyrolysis with Fe-Ni/ZSM-5 using the methods of Friedman, Starink, Flynn-Wall-Ozawa, and Kissinger-Akahira-Sunose was 156.19, 152.39, 154.30, and 152.11 kJ mol-1, respectively. Fe-Ni/ZSM-5 addition lowered the activation energy compared to non-catalytic pyrolysis at the conversion rate of 0.15-0.75. The overall catalytic pyrolysis process of poplar sawdust follows the diffusion and nucleation models. The thermodynamic parameters (enthalpy and entropy) showed positive and negative values, respectively, indicating non-spontaneous reactions during the catalytic pyrolysis process.

Effects of co-digestion of food waste, corn straw and chicken manure in two-stage anaerobic digestion on trace element bioavailability and microbial community composition.

Co-digestion is known to effectively alleviate trace elements (TEs) deficiency in mono-substrates; however, the bioavailability of TEs is crucial for the stability of anaerobic digestion. Therefore, this study investigated the effects of co-digestion of food waste (FW), corn straw (CS) and chicken manure (CM) in two-stage anaerobic digestion on TEs bioavailability and microbial community composition. Various VSFW:(VSCS:VSCM) ratios of 8:2, 7:3, 4:6, and 2:8 were evaluated in two-stage (group A, B, C, D) anaerobic digestion in which the VSCS:VSCM ratio was fixed at 3:1. Results showed that the highest hydrogen production of 106 mL/g VS and methane production co-efficiency of 125.3% was obtained in group A. Group A has a high close range of easily bioavailable TEs (32-64%) compared to other groups, especially the mono-substrate, where almost all TEs ranged between 10 and 36%. The increased relative abundance of the obligate hydrogenotrophic methanogens reflected a positive two-stage methane co-digestion efficiency.

Synergistic effect of the cotton stalk and high-ash coal on gas production during co-pyrolysis/gasification.

The synergistic effect of the cotton stalk (CS) and the high-ash coal (HAC) on the gas production in the co-pyrolysis/gasification processes was studied using the newly designed quartz boat in this work. The gas yield and the concentrations of main gas components were quantitatively compared between the co-pyrolysis/gasification and the individual pyrolysis/gasification. The results showed that the gas yield during the co-pyrolysis was promoted at 950℃. There was almost no interaction between CS and HAC, since the co-pyrolytic gas yield exhibited a linear relationship with CS mixing ratio of 20% to 60%. The catalytic effect of alkali metals and alkaline earth metals that existed in CS, was enhanced by the addition of steam, and the synergistic effect was reduced while gas yield was enhanced with CS blending ratio increasing during co-gasification. The results provided a method to enhance synergistic effect between biomass and coal during co-pyrolysis/gasification in this study.

Synergistic Effects of Anaerobic Co-Digestion of Pretreated Corn Stover with Chicken Manure and Its Kinetics.

The pretreatment effects and synergistic effects of anaerobic co-digestion of pretreated corn stover (CS) with chicken manure (CM) were studied. Results showed that the NaOH-H2O2 pretreatment effect on CS was better than urea pretreatment in terms of anaerobic digestion promotion. The highest cumulative methane yield of 332.7 mL/g VS added was obtained from the CS (NaOH-H2O2 pretreated)/CM ratio of 1:3, and the highest cumulative methane yield of 319.7 mL/g VS added was obtained from the CS (urea pretreated)/CM ratio of 1:2. Synergistic effects were found in CS (NaOH-H2O2 pretreated)/CM ratios of 2:1, 1:2, 1:3 and CS (urea pretreated)/CM ratios of 1:1, 1:2. Synergistic effect was not found at CS (unpretreated)/CM ratios of 1:2 and 1:3. Pretreatment of CS can produce synergistic effect on anaerobic co-digestion and increase cumulative methane yield by 6.54-24.65%. Among the four kinetic models, modified Gompertz model was best fitted in describing the methane production during anaerobic co-digestion (R2 = 0.9845-0.9988).

Essential regulators of iron chemical speciation distributions in anaerobic digestion of pretreated food waste: Organic volatile fatty acids or inorganic acid radicals?

In this work, a 30-days batched mesophilic assay on pretreated food waste (PFW) under different inoculum/substrate (I/S) ratios (1:5, 1:2, 1:1, 2:1, 4:1 and 1:0) was carried out, to target the most important parameters in AD matrix on regulating iron (Fe) chemical speciation. Correlation coefficients were calculated within four Fe chemical forms and AD parameters of pH, volatile fatty acids (VFAs), inorganic acid radicals (IARs), and alkalinity. Results showed that IARs were not key factors on regulating Fe speciation. Without acidification, IARs showed weak correlations (coefficients < 0.40) with Fe chemical dynamics while other parameters showed stronger correlations (coefficients ≥ 0.60). Under acidification, VFAs initiated the conversion of exchangeable Fe into water soluble fraction. Residual fraction might play important role in regulating Fe shifting to more bioavailable states.

Steam reforming of bio-oil from rice husks fast pyrolysis for hydrogen production.

Steam reforming of two kinds of bio-oil from rice husks fast pyrolysis was conducted for hydrogen production at three temperatures (650, 750 and 850 °C) with Ni-based catalyst in a fixed-bed reactor. The gas composition and organic compounds in liquid condensate were detected by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS), respectively. In addition, the carbon deposition was also investigated. The results showed that the mole fraction range of hydrogen was within 55.8-61.3% at all temperatures and more hydrogen was produced at the higher temperature. The highest H2 efficiency of bio-oil steam reforming was 45.33% when extra water was added. The bio-oil with lower content of chemical compounds has a higher H2 efficiency, but its hydrogen volume was less. Analysis of the liquid condensate showed that most of the organic compounds were circularity compounds. The carbon deposition can decrease the bio-oil conversion, and it was easier to form at the temperature of 750 °C.

Effect of hot vapor filtration on the characterization of bio-oil from rice husks with fast pyrolysis in a fluidized-bed reactor.

To produce high quality bio-oil from biomass using fast pyrolysis, rice husks were pyrolyzed in a 1-5 kg/h bench-scale fluidized-bed reactor. The effect of hot vapor filtration (HVF) was investigated to filter the solid particles and bio-char. The results showed that the total bio-oil yield decreased from 41.7% to 39.5% by weight and the bio-oil had a higher water content, higher pH, and lower alkali metal content when using HVF. One hundred and twelve different chemical compounds were detected by gas chromatography-mass spectrometry (GC-MS). The molecular weight of the chemical compounds from the condenser and the EP when the cyclone was coupled with HVF in the separation system decreased compared with those from the condenser and EP when only cyclone was used.