Elemental migration and transformation during hydrothermal liquefaction of biomass.

Over the past few decades, energy and environmental crises have worsened due to the excessive consumption of fossil fuels. Hydrothermal liquefaction (HTL) is a promising technology for sustainable biocrude production from biomass. However, elemental migration and transformation during HTL of biomass have only received scant attention to date. Understanding the transformation mechanism is beneficial for downstream biocrude upgrading and by-products utilization for the future industrialization of HTL. In this paper, biomass is grouped into six categories: microalgae, macroalgae, lignocellulose, food waste, manure, and sludge. The biochemical composition and HTL product distribution of six kinds of biomass are compared. The conversion process of the biomacromolecules (including lipids, proteins, cellulose, hemicellulose, and lignin) and the interactions between them are also reported. Furthermore, the distribution of carbon, nitrogen, sulfur, and inorganic elements (Na, K, Ca, Mg, Al, Fe, Zn, Cu, Pb, Cd, etc.) in the HTL products is summarized, and the transformation of the organic and inorganic elements during HTL of biomass is explored. Finally, outlooks for the HTL of biomass are proposed.

Biocrude Oil from Algal Bloom Microalgae: A Novel Integration of Biological and Thermochemical Techniques.

Algal bloom microalgae are abundant in polluted water systems, but their biocrude oil production potential via hydrothermal liquefaction (HTL) is limited. This study proposed a novel process that combined biological (dark fermentation) and thermochemical (HTL) techniques aimed at changing the feedstock characteristics to be more suitable for thermochemical conversion, herein named integrated dark fermentation-hydrothermal liquefaction (DF-HTL). DF-HTL conversion of algae significantly enhanced the biocrude oil yield (wt %), carbon content (mol), energy content (MJ), and energy conversion ratios by 9.8, 29.7, 40.0, and 61.0%, respectively, in comparison to the control. Furthermore, DF-HTL processing significantly decreased the aqueous byproduct yield (wt %), carbon content (mol), nitrogen content (mol), and ammonia content (mol) by 19.0, 38.4, 25.0, and 13.2%, respectively, in comparison to the control. Therefore, DF-HTL reduced the environmental impact associated with disposing of the wastewater byproduct. However, DF-HTL also augmented the nitrogen content (mol) of the biocrude oil by 42.2% in comparison to the control. The benefits of DF-HTL were attributed to the increased acid content, the incorporation of H2 as a processing gas, and the enhancement of the Maillard reaction, which shifted the distribution of reaction products from the aqueous phase to the biocrude oil phase. This article provides insights into the efficacy of a novel integrated biological-thermochemical processing method with distinct environmental and energetic advantages over conventional HTL that heightens the biocrude oil yield for feedstocks with a high carbohydrate and a high protein content.

In Situ hydrochar regulates Cu fate and speciation: Insights into transformation mechanism.

Cu is one of the dominant heavy metals toxic to human health and environmental ecosystems. Understanding its fate and chemical speciation is of great importance for hydrothermal liquefaction (HTL) of Cu-rich hazardous streams. Herein, we investigated its evolution during the HTL of wastewater algae through ICP-MS, XRD, XANES, and EXAFS. Cu-cysteine complexes (51.5%) and Cu2S (40.4%) were the main components of Cu in algae, whereas the predominant form was CuS (70.9%) in 220 °C-hydrochar. Model compound experiments indicated that Cu-cysteine could be converted into CuS, while Cu2S was stable during HTL. However, Cu2S was partially converted into CuS in the hydrochar. Subsequently, the positive Gibbs free energy (36.8 KJ/mol) indicates that the oxidation from Cu+ to Cu2+ can't occur spontaneously. Furthermore, cyclic voltammograms demonstrated that hydrochar facilitated the oxidation of Cu2S due to its higher capability of electron acceptance. All these results prove that hydrochar serves as a catalyst for the conversion of Cu2S to CuS during HTL. This study firstly elucidated that Cu2S was oxidized into CuS in the presence of hydrochar, and Cu-cysteine was converted into CuS under HTL. This study provides a critical insight into the transformation mechanism of Cu during the HTL of hazardous streams.

Environment-enhancing process for algal wastewater treatment, heavy metal control and hydrothermal biofuel production: A critical review.

Coupling algae growth on wastewater with hydrothermal liquefaction (HTL) is regarded as an environment-enhancing pathway for wastewater management, biomass amplification, sustainable energy generation and value-added products generation. Through this integrated pathway, microalgae can not only recover nitrogen and phosphorus, but also absorb heavy metals from the wastewater. The migration and transformation of heavy metals need to be specifically assessed and considered due to the environmental concerns associated with metal toxicity. This work reviewed recent advances with respect to bioremediation mechanisms. Particular emphasis was placed on the heavy metal migration, transformation, and the key factors involved in algal wastewater treatment and biomass conversion. Additionally, the challenges of coupling algae wastewater treatment, hydrothermal conversion, and heavy metal control were addressed. Finally, a paradigm involving enhanced algal wastewater treatment and bioenergy production for field application was proposed.

An innovative multistage anaerobic hythane reactor (MAHR): Metabolic flux, thermodynamics and microbial functions.

Biohythane production from wastewater via anaerobic fermentation currently relies on two-stage physically separated biohydrogen and biomethane reactors, which requires closed monitoring, the implementation of a control system, and cost-intensive, complex operation. Herein, an innovative multistage anaerobic hythane reactor (MAHR) was reported via integrating two-stage fermentation into one reactor. MAHR was constructed using an internal down-flow packed bed reactor and an external up-flow sludge blanket to enhance microbial enrichment and thermodynamic feasibility of the associated bioreactions. The performance of MAHR was investigated for 160 d based on biogas production, metabolic flux and microbial structure in comparison to a typical anaerobic high-rate reactor (up-flow anaerobic sludge blanket (UASB)). A biohythane production with an optimized hydrogen volume ratio (10-20%) and a high methane content (75-80%) was achieved in the hythane zone (MH) and methane zone (MM) in MAHR, respectively. In addition, MAHR showed a stronger capability to accommodate a high organic loading rate (120 g COD/L/d), and it enhanced the conversion of organics leading to a methane production rate 66% higher than UASB. Thermodynamic analysis suggested that hydrogen extraction in MH significantly decreased the hydrogen partial pressure (<0.1% vol) which favored acetogenesis in MM. Metabolic flux and microbial function analysis further supported the superior performance of MAHR over UASB, which was primarily attributed to enhanced acetogenesis and acetoclastic methanogenesis.

Reduce recalcitrance of cornstalk using post-hydrothermal liquefaction wastewater pretreatment.

Hydrothermal pretreatment (HTP) using an acidic catalyst is known to be effective for reducing lignocellulosic biomass recalcitrance. Post-hydrothermal liquefaction wastewater (PHW) from hydrothermal liquefaction of swine manure contains a large fraction of organic acids and thus was introduced to improve the HTP of cornstalk in this study. The response surface methodology was performed to optimize operating parameters of HTP for preserving structural polysaccharides while removing the barrier substances. A remarkable co-extraction of cell wall polymers was observed during PHW-catalyzed HTP at 172 °C for 88 min. The analysis of particle size, crystalline cellulose, the degree of polymerization (DP), mole number (MN) and SEM suggested that the co-extraction effect could distinctively alter lignocellulosic structures associated with recalcitrance and thus accelerate biomass saccharification. Additionally, the biodegradability of PHW was improved after HTP as a result of balanced nutrients and increased acids and sugars suitable for biogas production via anaerobic fermentation.

Biohythane production of post-hydrothermal liquefaction wastewater: A comparison of two-stage fermentation and catalytic hydrothermal gasification.

Developing efficient methods to recover energy from post-hydrothermal liquefaction wastewater (PHW) is critical for scaling up hydrothermal liquefaction (HTL) technology. Here we evaluated two-stage fermentation (TF) and catalytic hydrothermal gasification (CHG) for biohythane production using PHW. A hydrogen yield of 29 mL·g-1 COD and methane yield of 254 mL·g-1 COD were achieved via TF. In comparison, a higher hydrogen yield (116 mL·g-1 COD) and lower methane yield (65 mL·g-1 COD) were achieved during CHG. Further, a techno-economic and sensitivity analysis was conducted. The capital cost and operating cost for TF varied with the different reactor systems. TF with high-rate reactors suggested its promising commercialized application as it had a lower minimum selling price (-0.71 to 2.59 USD per gallon of gasoline equivalent) compared with conventional fossil fuels under both the best and reference market conditions. Compared with TF, CHG was only likely to be profitable under the best case conditions.

Elemental migration and characterization of products during hydrothermal liquefaction of cornstalk.

Biofuel production from lignocellulosic biomass through hydrothermal liquefaction (HTL) is a promising direction. This study characterized the products and investigated the elemental migration during the HTL of cornstalk at seven different operation temperatures (210-375°C). The biocrude oil yield significantly increased from 7.04% (210°C) to 23.32% (290°C) as the temperature increased, and decreased to 21.07% when further increased to 375°C. A carbon recovery of 11.03-38.69%, and a hydrogen recovery of 7.77-25.61% were achieved in the biocrude oil. Hydrogen (27.87-70.94%) and nitrogen (74.56-81.76%) were effectively recovered in the aqueous phase. GC-MS, HPLC, TGA and FT-IR analysis indicated that major organic compounds in the biocrude oil were interestingly similar between 210°C and 270°C. The identified compounds included hydrocarbons, esters and carboxylic acid. The calculative yields of biocrude, hydrogen, methane and biochar reached 7.04-23.32, 0.07-0.29, 7.12-12.08 and 3.01-22.42t/100t cornstalks, respectively.

Improve the biodegradability of post-hydrothermal liquefaction wastewater with ozone: conversion of phenols and N-heterocyclic compounds.

Hydrothermal liquefaction is a promising technology to convert wet biomass into bio-oil. However, post-hydrothermal liquefaction wastewater (PHWW) is also produced during the process. This wastewater contains a high concentration of organic compounds, including phenols and N-heterocyclic compounds which are two main inhibitors for biological treatment. Thus, proper treatment is required. In this work, ozone was used to convert phenols and N-heterocyclic compounds with a dosage range of 0-4.64 mg O3/mL PHWW. After ozone treatment, the phenols were fully converted, and acids were produced. However, N-heterocyclic compounds were found to have a low conversion rate (21.7%). The kinetic analysis for the degradation of phenols and N-heterocyclic compounds showed that the substitute played an important role in determining the priority of ozone reactions. The OH moiety in the ring compounds (phenols and pyridinol) may form hydroxyl radical, which lead to an efficient reaction. A substantial improved biodegradability of PHWW was observed after ozone treatment. The ratio of BOD5/COD was increased by about 32.36%, and reached a maximum of 0.41. The improved biodegradability of PHWW was justified by the conversion of phenols and N-heterocyclic compounds.