Hydrothermal biomass conversion: Quo vadis?

An Exploratory Study of Hydrochar as a Matrix for Biotechnological Applications

This paper explores the potentialities of hydrochar in protein separation and enzyme immobilization for non-energy biorefinery applications of hydrothermal carbonization. An innovative experimental procedure monitors soluble protein-hydrochar interactions and enzymatic reactions in a continuously stirred tank reactor. The hydrochar comes from hydrothermal carbonization of silver fir (200 °C, 30 min, 1/7 solid/water ratio) and standard activation (KOH, oven, 600 °C). Bovine serum albumin, a non-active, globular protein, was adsorbed at ≤3300 mg/g. Sip's isotherms fitted data well (R2 = 0.99999). The immobilization used a commercial β-glucosidase, which catalyzes the hydrolysis of cellobiose to glucose, a bottleneck of the cellulose to fermentable sugar bioconversion network due to the fast enzyme deactivation. The hydrochar adsorbed ≤26 w/w% of enzyme. The heterogeneous biocatalyst operational stability was 24 times that of the soluble one. The results encourage further investigations and foreshadow process schemes coupling hydrothermal carbonization and industrial bioconversions.

Hydrothermal carbonization coupling with liquid dimethyl ether extraction pretreatment of sewage sludge: Hydrochar performance improvement and low-nitrogen biocrude production

Hydrothermal carbonization of sewage sludge converts waste into hydrochar; however, the complex organic composition of feedstock limits the product value. A novel process that combines liquid dimethyl ether extraction and hydrothermal carbonization (DE-HTC) was proposed for improving the product value by simultaneously producing biocrude/hydrochar and improving feedstock suitability for thermochemical conversion. Biocrude and hydrochar with a product yield of 2.62% and 55.83% were produced via DE-HTC, respectively. The hydrochar yield increased by 12.65%-29.90% compared to traditional single-step hydrothermal carbonization. The hydrochar energy densification was decreased by 1.16%-10.28%, while hydrochar's energy yield increased by 47%-66%, and it had a more prominent porous structure. By avoiding the decomposition of proteins during thermochemical conversion, the nitrogen content of the biocrude obtained via DE-HTC was only 0.38%. The biocrude was also further qualitatively analyzed. This study provides insights into the efficacy of a novel hydrothermal method with distinct product value advantages over direct hydrothermal carbonization.

Co-Processing Lignocellulosic Biomass and Sewage Digestate by Hydrothermal Carbonisation: Influence of Blending on Product Quality

Hydrothermal carbonisation (HTC) can be integrated with anaerobic digestion (AD) for the treatment of digestate, resulting in a solid hydrochar or bio-coal and a process water, which can be recirculated back into AD to produce biogas. The properties of digestate-derived hydrochars do not lend themselves to producing high quality bio-coal and blending with lignocellulosic feedstocks can improve its properties. This study investigates the co-processing of sewage sludge (SS) digestate with three lignocellulosic biomass (grass, privet hedge, and woodchip). The calorific value of the resulting bio-coal is increased following co-processing, although feedstock interactions result in non-additive behaviour. The largest increase in calorific value was observed for co-processing with woodchip. There is evidence for non-additive partitioning of metals during co-processing resulting in only moderate improvements in ash chemistry during combustion. Co-processing also effects the composition of process waters, influencing the potential for biogas production. Experimental biomethane potential (BMP) tests indicate that grass clippings are the most suitable co-feedstock for maintaining both calorific value and biogas production. However, above 200 °C, BMP yields appear to decrease, suggesting the process water may become more inhibitory. Co-processing with wood waste and privet hedge produce the higher CV bio-coal but significantly reduced BMP.

Structural, inorganic, and adsorptive properties of hydrochars obtained by hydrothermal carbonization of coffee waste

The hydrothermal carbonization process is a suitable process for the conversion of potentially harmful lignocellulosic waste into hydrochars. Defective coffee beans were the precursor raw material for hydrochar synthesis. Reactions were performed in a high-pressure reactor at 150, 200, and 250 °C, in autogenous pressure, for 40 min. Hydrochars were recovered by filtration and characterized by energy dispersive X-ray fluorescence spectroscopy, UV-Vis spectrophotometry, attenuated total reflection Fourier-transform infrared spectroscopy, differential thermal analysis, and scanning electron microscopy. Methylene blue adsorption tests were performed and analyzed by Langmuir and Freundlich adsorption isotherms. Adsorption mechanisms were investigated by computational calculations at DFT level. Results suggest that hydrochars from defective coffee beans can be applied as technological resources in the agronomic and environmental fields due to their inorganic composition, mainly to high magnesium content, the structural characteristics of porosity, biodegradation control, soil carbon-fixation and adsorption capacity. Important adsorption processes are caused by the development of oxygenated functional groups on the hydrochar surface.

Influence of Reaction Conditions on Hydrothermal Carbonization of Fructose

Hydrothermal carbonization is a powerful way to convert cellulosic waste into valuable platform chemicals and carbonaceous materials. In this study, to optimize the process, fructose was chosen as the carbon precursor and the influence of reaction time, acid catalysis, feed gas and pressure on the conversion products is evaluated. 5-hydroxymethylfurfural (HMF) is produced in high amounts in relatively short time. Both strong and weak acids accelerate fructose conversion. Levulinic acid (LevA) formation is faster than that of hydrothermal (HT) carbon in acidic conditions. Strong acid catalysts should be considered to target preferentially LevA production, whereas milder conditions should be preferred for HMF production. Moreover, a slight initial overpressure of the reactor is always beneficial in terms of conversion. FT-IR and ¹³ C ss-NMR spectroscopy and SEM showed that HT carbon evolves through time from a furanic-based structure with alkylic linkers to an increasingly cross-linked condensed structure. MALDI-ToF mass spectrometry showed the existence of a series of oligomers in a mass range within 650 Da and 1500 Da formed by condensation of repeating units.

Biomass Hydrothermal Carbonization: Markov-Chain Monte Carlo Data Analysis and Modeling

This paper introduces Bayesian statistical methods for studying the kinetics of biomass hydrothermal carbonization. Two simple, specially developed computer programs implement Markov-chain Monte Carlo methods to illustrate these techniques' potential, long since established in other areas of chemical reaction engineering. A range of experimental data, both from this study and the literature, test the soundness of a Bayesian approach to modeling biomass hydrothermal carbonization kinetics. The first program carries out parameter estimations and performs better or equal than the traditional deterministic methods (R2 as high as 0.9998). For three out of the 22 datasets, the program detected the global minima of the parameter space, while the deterministic least-square found local values. The second program uses Gillespie's algorithm for the statistical simulation of the reactions occurring in hydrothermal carbonization. Comparing six basic kinetic models with literature data tested the stochastic simulation as a tool for assessing biomass conversion reaction networks rapidly. Among the simple models discussed, reaction scheme 3 fitted better to the experimental data (R2 > 0.999). The proposed approach is worth extending to more complex, time-consuming computer models and could support other techniques for studying hydrothermal conversions.

Char derived from sewage sludge of hydrothermal carbonization and supercritical water gasification: Comparison of the properties of two chars

Supercritical water gasification (SCWG) is considered a promising technology for sewage sludge (SS) treatment and utilization; however, char produced by a side reaction has become a bottleneck in SCWG. In this study, SS and its model compound (10% humic acid) were treated in an autoclave by SCWG at 400 °C for 30 min and by hydrothermal carbonization (HTC) at 250 °C for 300 min. The char yield was 15.4% in SCWG and 41.3% in HTC. The chars were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, Brunauer-Emmett-Teller (BET) analysis, and elemental analysis. By comparing the properties the char produced by SCWG and the hydrochar produced by HTC, which has been considered a valuable product, the feasibility of using char as an additional product in SCWG was explored. Compared with the char produced by HTC, the char generated in SCWG exhibits a lower BET specific surface area (8.257 and 15.782 m²/g) and combustion activity, a higher proportion of small pores (with pore sizes of 1-2 nm), and greater thermal stability. The formation pathway of the two types of chars is related to both dehydration and aromatization; decarboxylation also occurs in char formation during SCWG. Humus was proved to be related to char formation during the SCWG of SS based on experimental results obtained with the model compound. This work provides insights needed to guide follow-up treatments or utilization of the char produced during the SCWG of SS.

Modeling biomass hydrothermal carbonization by the maximum information entropy criterion

The kinetics of biomass hydrothermal carbonization is modeled by the MaxEnt principle, without assuming a reaction network. Modeling is in good accordance with the experimental data concerning a broad range of biomass and reaction conditions.

Olive mill wastewater: From a pollutant to green fuels, agricultural and water source and bio-fertilizer - Hydrothermal carbonization

Hydrothermal carbonization (HTC) is considered as a promising technique for wastes conversion into carbon rich materials for various energetic, environmental and agricultural applications. In this work, the HTC of olive mill wastewater (OMWW) was investigated at different temperatures (180-220 °C) and both, the solid (i.e., hydrochars) and the final process liquid derived from the thermal conversion process were deeply analyzed. Results showed that the solid yield was affected by the temperature, i.e., decrease from 57% to 25% for temperatures of 180 °C and 220 °C, respectively. Furthermore, the hydrochars presented an increasing fixed carbon percentage with the increase of the carbonization temperature, suggesting that decarboxylation is the main reaction driving the HTC process. The decrease in the O/C ratio promoted an increase of the high heating value (HHV) by 32% for hydrochar prepared at 220 °C. The process liquids were sampled and their organic contents were analyzed using GC-MS technique. Acids, alcohols, phenols and sugar derivatives were detected and their concentrations varied with carbonization temperatures. The assessment of the physico-chemical properties of the generated HTC by-products suggested the possible application of the hydrochars for energetic insights while the liquid fraction could be practical for in agricultural field.

Conductive Silk-Based Composites Using Biobased Carbon Materials

There is great interest in developing conductive biomaterials for the manufacturing of sensors or flexible electronics with applications in healthcare, tracking human motion, or in situ strain measurements. These biomaterials aim to overcome the mismatch in mechanical properties at the interface between typical rigid semiconductor sensors and soft, often uneven biological surfaces or tissues for in vivo and ex vivo applications. Here, the use of biobased carbons to fabricate conductive, highly stretchable, flexible, and biocompatible silk-based composite biomaterials is demonstrated. Biobased carbons are synthesized via hydrothermal processing, an aqueous thermochemical method that converts biomass into a carbonaceous material that can be applied upon activation as conductive filler in composite biomaterials. Experimental synthesis and full-atomistic molecular dynamics modeling are combined to synthesize and characterize these conductive composite biomaterials, made entirely from renewable sources and with promising applications in fields like biomedicine, energy, and electronics.

Hydrothermal and organic-chemical treatments of eucalyptus biomass for industrial purposes

This study aimed to evaluate the promising feasibility of the hydrothermal pre-processing of eucalyptus wood and eucalyptus bark under organosolv and organic acid conditions to produce a highly concentrated cellulose feedstock. For that, particulate samples of both biomasses were heated in water solutions containing from 0 to 50%_vol/vol of ethanol and from 0 to 50 mmol.L^-1 of oxalic acid at temperatures between 140 and 180 °C. Significant differences on the thermal degradation profiles were observed for both biomasses indicating the partial hydrolysis converted them into a more homogeneous solid fraction with higher contents of cellulose. It was also observed a significant variation of the glycan content from approximately 39 to 76% for wood particles, whereas the variation for bark was from 32 to 50%. In general, the proposed pre-processing route was considered potentially feasible to concentrate the cellulose/glycan contents of eucalyptus biomasses for subsequent industrial utilization.

Hydrochars from industrial macroalgae "Gelidium Sesquipedale" biomass wastes

Macroalgae wastes from the Agar-Agar industry were used as a feedstock to obtain hydrochars by means of hydrothermal carbonization. The effect of temperature (200 °C and 230 °C) and time (2 h and 6 h) on the yield, higher heating value (HHV) and chemical-morphological-textural properties of the hydrochars was studied. The carbon content and the higher heating value were observed to increase with the hydrothermal carbonization. The hydrochars yields (up to 60%) were much higher than yields obtained using conventional char (27.5-33.5%). The hydrochar obtained at 230 °C and after 6 h showed a HHV of 23.25 MJ/kg, which is similar to that of lignite HHV. The H/C and O/C atomic ratios decreased as a consequence of the dehydration and decarboxilation reactions. Hydrothermal carbonization barely changed the vegetal structure of the macroalgae waste. The hydrochars were found to be essentially meso-macroporous with average pore sizes of up to 110.5 nm.

Comparative Life Cycle Assessment of HTC Concepts Valorizing Sewage Sludge for Energetic and Agricultural Use

In many countries, sewage sludge is directly used for energy and agricultural purposes after dewatering or digestion and dewatering. In recent years, there has been a growing interest in additional upstream hydrothermal carbonization (HTC), which could lead to higher yields in the energetic and agricultural use. Twelve energetic and agricultural valorization concepts of sewage sludge are defined and assessed for Germany to investigate whether the integration of HTC will have a positive effect on the greenhouse gas (GHG) emissions. The study shows that the higher expenses within the HTC process cannot be compensated by additional energy production and agricultural yields. However, the optimization of the HTC process chain through integrated sewage sludge digestion and process water recirculation leads to significant reductions in GHG emissions of the HTC concepts. Subsequently, nearly the same results can be achieved when compared to the direct energetic use of sewage sludge; in the agricultural valorization, the optimized HTC concept would be even the best concept if the direct use of sewage sludge will no longer be permitted in Germany from 2029/2032. Nevertheless, the agricultural valorization concepts are not generally advantageous when compared to the energetic valorization concepts, as it is shown for two concepts.

Key Development Factors of Hydrothermal Processes in Germany by 2030: A Fuzzy Logic Analysis

To increase resource efficiency, it is necessary to use biogenic residues in the most efficient and value-enhancing manner. For high water-containing biomass, hydrothermal processes (HTP) are particularly promising as they require wet conditions for optimal processing anyway. In Germany, however, HTP have not yet reached the industrial level, although suitable substrates are available and technological progress has been made in previous years. This study aims to determine why this is by identifying key factors that need to occur HTP development in Germany until 2030. By using results of previous analyses within this context (i.e., literature review, SWOT analysis, expert survey, and focus group workshop) and combining them with the results of an expert workshop and Delphi-survey executed during this analysis, a comprehensive information basis on important development factors is created. Fuzzy logic is used to analyze these factors in terms of interconnections, relevance, and probability of occurrence by 2030. The results show that technological factors, such as a cost-efficient process water treatment and increased system integration of HTP into bio-waste and wastewater treatment plants, are given high relevance and probability of occurrence. The adaptation of the legal framework, for example, the approval of end products from HTP as standard fuels, has very high relevance but such adaptions are considered relatively unlikely.

Upgrading agricultural wastes using three different carbonization methods: Thermal, hydrothermal and vapothermal

In this study, three different methods for high quality solid fuel production were tested and compared experimentally. Oil palm empty fruit bunches, mesocarp fibers, palm kernel shells and rubber seeds shells were treated using thermal (TC), hydrothermal (HTC) and vapothermal (VTC) carbonization. All thermochemical methods were accomplished by using a custom made batch-type reactor. Utilization of novel single reactor equipped with suspended internal container provided efficient operation since both steam generator and raw materials were placed inside the same reactor. Highest energy densification was achieved by VTC process followed by TC and HTC processes. The heating value enhancement in VTC and TC was achieved by the increase in fixed carbon content and reduction in volatile matter. The formation of the spherical components in HTC hydrochar which gave a sharp peak at 340 °C in the DTG curves was suggested as the reason that led to the increment in energy content.

Microalgae hydrothermal liquefaction and derived biocrude upgrading with modified SBA-15 catalysts

In this study, a novel route was proposed for microalgae biofuel production by catalytic upgrading of Chlorella hydrothermal liquefaction (HTL) derived biocrude. Al-SBA-15, CuO/Al-SBA-15, ZuO/Al-SBA-15, and CuO-ZnO/Al-SBA-15 catalysts were synthesized in a facile, one-pot way, and tested for methyl palmitate decarboxylation and biocrude upgrading without H₂ addition. These modified SBA-15 catalysts enhanced alkane selectivity of methyl palmitate decarboxylation from 7.6 wt% up to 79.6 wt% at 340-350 °C. FT-IR, TG and GC-MS characterizations were employed to identify the composition and properties of the upgraded bio-oils. Compared with thermal upgrading, modified SBA-15 catalysts enriched the yield of low boiling point compounds, and the content of heavy bio-oil (>400 °C) declined from 9.57 wt% to 1.89 wt%. Hydrocarbon yield was greatly enriched on the catalysts, and aromatics predominant on Al-SBA-15 while aliphatics abundant on metal oxide(s) supported catalysts. The hydrocarbon yield was increased from 25.1 wt% (thermal) to 65.7 wt% on the CuO/Al-SBA-15.

Influence of Hydrothermal Carbonization on Composition, Formation and Elimination of Biphenyls, Dioxins and Furans in Sewage Sludge

In many areas of application, the influence of hydrothermal carbonization (HTC) on the composition of organic pollutants is still unexplored. In this study, sewage sludge (SS) was carbonized and the input as well as the hydrochar were examined for the organic pollutants: polychlorinated biphenyls (PCB), polychlorinated dibenzo-dioxins (PCDDs), and polychlorinated dibenzo-furans (PCDFs). The process temperatures of carbonization were 200 °C, 220 °C, and 240 °C and the holding time was 5 h for all tests. The total concentration of PCBs was relatively stable for all temperatures, whereas the toxicity equivalent (WHO-TEQ) at 200 °C and 220 °C increases compared to the input material. The strongest impact on toxicity was observed for PCDDs where concentrations were reduced for higher temperatures, whereas the toxicity increases by more than 16 times for temperatures of 240 °C. The concentrations and toxicity of PCDFs were reduced for all carbonization temperatures. In hydrochar from HTC at 240 °C, the limit values for the application of SS in German agriculture have been exceeded. The results indicate that the process conditions for HTC should be controlled also for SS with average contamination if the hydrochar is to be used as material, especially in agriculture.