Hydrochar Formation Research Articles

Co-hydrothermal carbonization of digested sewage sludge and cow dung biogas residue: Investigation of the reaction characteristics

In order to recovery energy from digested sewage sludge (DSS) and cow dung biogas residue (CDBR), co-hydrothermal carbonization (co-HTC) was employed to obtain the hydrochar for the use as solid fuel. Carbon content and higher heating value of hydrochars were increased, while the O/C and H/C atomic ratios reduced, revealing the dominance of the dehydration reaction during the co-HTC of DSS and CDBR. The interaction between DSS and CDBR or their derivate intermediates took place during the co-HTC for the TOC value exceeded far more than that in the raw materials and the intensity of mixture was lower than pure material. The CDBR can notably promote hydrochar formation during the co-HTC with DSS, resulting in a high fraction of nitrogen content in hydrochar of DSS. The combustion properties of hydrochar were improved notably when CDBR was added, evidencing by the increasing ignition temperature and the comprehensive devolatilization index. The activation energy of the hydrochar from co-HTC increased notably especially the hydrochar with 75% CDBR. This study suggested that co-HTC of the mixed feedstocks was a promising strategy to prepare a high-quality hydrochar for recovery energy.

Understanding the influence of biomass particle size and reaction medium on the formation pathways of hydrochar

The chemical-physical processes controlling hydrothermal carbonization (HTC) are still not completely understood. This paper focuses on two aspects: the influence on the hydrochar formation of the particle size of the feedstock and the presence of solved compounds in the feedwater. To address these, brewer’s spent grains were crushed to 250 μm proved by HPLC analysis of liquid byproducts, particularly when rPW, containing readily condensable/polymerizable intermediates, is added. This has a positive effect on the yield and carbon content of the hydrochars caused mainly by an increase in its secondary char fraction. The reaction pathways involved are discussed in detail.

Co-hydrothermal carbonization of polyvinyl chloride and moist biomass to remove chlorine and inorganics for clean fuel production

This work proposes an innovative integrated process to produce clean solid biofuel from chlorinated wastes and polyvinyl chloride (PVC). The PVC and pinewood sawdust were used as parent materials. We studied the effect of parameters such as the hydrothermal temperature, the residence time and the particle size (PS) of the sawdust on the dechlorination efficiency (DE), the inorganics removal efficiency (RE), and the HHV of hydrochar. The co-hydrothermal carbonization (Co-HTC) process was performed by mixing the PVC and pinewood sawdust at a mass ratio of 1:9. For the DE, the most important factor was the hydrothermal temperature, followed by residence time and particle size of pine sawdust. The DE could reach about 84% by the co-HTC at a temperature of 260 °C for 120 min. The particle size of pine sawdust has noticeable effect on DE because of heat and mass transfer. The DE was decreased from 79.17% to 71.12% when the PS was increased from 0.22–0.49 to 0.6–0.9 mm. The RE of inorganics from pine sawdust was significantly promoted because the addition of PVC enhanced the acidity of the reaction system, regardless of the co-HTC operating parameters investigated in this work. The temperature increase is conducive for the removal of K and Na. The maximal RE of Al, Ca, and Mg increased significantly from 49.39%, 49.19% and 41.86 to 97.61% (Da-260-30), 98.59% (Da-260-90) and 97.66% (Dc-260-30), respectively. The maximal RE of Fe, K and Na increased from 49.79%, 50.80% and 47.44% to 92.82% (Da-220-30), 92.32% (Dc-260-30) and 87.43% (Dc-260-30), respectively. The oxygen-containing functional groups decreased with the increase of HTC temperature (220–260 °C), residence time (30–90 min) and particle size (0.22–0.49 to 0.6–0.9 mm), resulting in the weakening absorption ability of hydrochar for inorganics. The addition of PVC and the temperature increase are not conducive to the formation of porous hydrochar. Nevertheless, the residence time extension and particle size growth could increase the porosity of hydrochar. The hydrochar with low chlorine, low inorganics content and improved higher heating value (HHV) of 24–30 MJ/kg was similar to bituminous coal, which could be utilized as clean solid biofuels. A high-energy yield of 74–81% was achieved by this co-HTC process. These results show that the co-HTC of PVC with biomass was feasible for clean biofuel production, because the chlorine and inorganics could be removed effectively by the positive synergistic effect. This work provides a new viewpoint for the development of WtE and biomass upgrading technologies.

Correlations between the physicochemical properties of hydrochar and specific components of waste lettuce: Influence of moisture, carbohydrates, proteins and lipids

This study aims to figure out the influence of moisture content and chemical constitution, i.e. carbohydrates, proteins and lipids, of waste lettuce on the physicochemical structure of hydrochar produced via hydrothermal carbonization. The experimental results showed that homogenized carbon material can be obtained by hydrothermal treatment, regardless of the moisture content of feedstock. During the hydrothermal carbonization process of waste lettuce, carbohydrates were the most active reactants contributing to hydrochar formation. Meanwhile, Maillard reaction between proteins and carbohydrates occurred, which promoted the aromatization of the organic intermediates and increased the relative content of nitrogenous heterocyclic functional groups on the surface of hydrochar from 10.7 to 18.7%. Different from these two constitution, lipids did not participate in the carbonization reaction, the main hydrolyzates of lipids were adsorbed to the surface of hydrochar, leading to an increase in the mass of solid products.

Hydrothermal Carbonization of Fructose: Growth Mechanism and Kinetic Model

Hydrothermal carbonization experiments were performed with fructose solutions as feedstock. A kinetic model is proposed that models hydrochar formation based on molar concentrations. On the basis of SEM pictures, the mean particles size of the hydrochar was determined, and it could be demonstrated that the growth of the particles is not directly related to the formation rate of hydrochar or the decreasing rate of the intermediates concentration in the aqueous solution. Four additives (salts) have been tested, and the results point out that the ionic strength is an important factor to control the particle size. Together, these observations lead to the conclusion that coalescence is an important driver for the growth of the particles and that a LaMer-like nucleation followed by diffusion-controlled growth is unlikely to be the formation and growth mechanism of hydrochar.

Novel kinetic studies on biomass hydrothermal carbonization

This work aims to study kinetic data on biomass hydrothermal carbonization by a new point of view. The time course of essential properties signaling hydrochar evolution is found to be sigmoidal ubiquitously. The logistic curve here proposed, attributable to the Hill equation, furnished near-perfect regressions (R2 as high as 0.999) in all cases studied. The broad applicability and the excellent correlations stimulated the research heuristically and routed process optimization. The evidence is reported that the success of fittings bases on profound mechanistic reasons. Arguments are discussed supporting the hypothesis that hydrochar formation is a stochastic phenomenon. The method deduces the Hill equation by applying statistical methods to the hydrothermal carbonization, modeled as a discrete Markov-chain process. The role of the model equation parameters is also discussed. The potentiality of the method proposed in this pioneering work could pave the way for a new paradigm in the modeling of hydrothermal carbonization.

Comprehensive characterization of hydrothermal liquefaction products obtained from woody biomass under various alkali catalyst concentrations

ABSTRACTHydrothermal liquefaction (HTL) of lignocellulosic biomass has been widely investigated for the production of renewable and alternative bio-crude oil. In this study, catalytic hydrothermal processing of two biomasses (larch and Mongolian oak) was performed using different K2CO3 concentrations (0, 0.1, 0.5, 1.0 wt% of solvent) to improve fuel yield and properties. HTL oil, hydrochar, water-soluble fraction (WSF) and gas were characterized, and carbon balance was investigated. As a result, the maximum yield of HTL oil, 27.7 wt% (Mongolian oak) and 25.7 wt% (larch), and the highest carbon conversion ratio was obtained with 0.5 wt% of catalyst. The high catalyst concentration also resulted in an increase in higher heating values up to 31.9 MJ/kg. In addition, the amount of organic compounds in HTL oil also increased, specifically for lignin-derived compounds including catechol and hydroquinone which can be derived from secondary hydrolysis of lignin. On the other hand, formation of hydrochar was suppressed with the addition of alkali catalyst and the yield dramatically decreased from 30.7–40.8 wt.% to 20.0–21.8 wt.%. Furthermore, it was revealed that WSF had low organic carbon content less than 3.4% and high potassium content mostly derived from alkali catalyst, indicating that it may be reusable with simple purification. This work suggests that the addition of the proper amount of alkali catalyst can improve the production efficiency and quality of bio-crude oil, and another potential of WSF to be recyclable in further work.

Opal promotes hydrothermal carbonization of hydroxypropyl methyl cellulose and formation of carbon nanospheres.

Hydrothermal carbon nanospheres were prepared by introducing opal into the hydrothermal carbonization system of hydroxypropyl methyl cellulose (HPMC). Then the effects of opal on hydrothermal carbonization of HPMC were investigated after different reaction durations (105–240 min). The reaction products were characterized by elemental analysis, gas chromatography-mass spectrometry (GC-MS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR) and N2 adsorption–desorption. Results of elemental analysis indicated that the H (hydrogen) and O (oxygen) content of HPMC decreased through dehydration, demethylation, decarbonylation and hydrolysis reactions, forming hydrochar with higher carbon content. The addition of opal was confirmed to accelerate the hydrolysis of HPMC. N2 adsorption–desorption tests and SEM analysis showed that opal with a large specific surface area adsorbed HPMC hydrolysis products, such as furans, and facilitated furan cyclodehydration on its surfaces to form cross-linked carbons, which contributed to the quick formation of hydrochar. Moreover, the adsorption by opal also inhibited hydrochar aggregation, so the final hydrothermal carbon spheres had sizes of 20–100 nm.

Hydrothermal Carbonization of Glucose, Fructose, and Xylose—Identification of Organic Products with Medium Molecular Masses

The monosaccharides glucose, fructose, and xylose were subjected to hydrothermal carbonization in aqueous solution at temperatures of 180, 220, and 250 °C for different operating times (30 min to 16 h). Here, 68% to 78% of the organic carbon was converted into hydrochar at 220 °C with glucose and fructose as feedstock, whereas hydrothermal treatment of xylose did not result in significant hydrochar formation under these conditions. The main topic of this contribution was the identification of stable organic products in the process water in the molecular mass range between 120 and 300 Da by means of GC-MS analysis using several derivatization agents. Special attention was paid to polar OH- and COOH-functionalized compounds. The overwhelming majority of identified organic compounds had cyclic structures of which a prominent group included hydroxylated benzofurans. However, the combined yield of products, which might be potential substrates for liquid biofuels, turned out very low.

Noncatalyzed Liquefaction of Celluloses in Hydrothermal Conditions: Influence of Reactant Physicochemical Characteristics and Modeling Studies

This paper reports a fundamental study on the hydrothermal liquefaction of cellulose into light products. First it aimed at comparing the noncatalyzed intrinsic reactivity of model celluloses such as Avicel and Sigmacell with various physicochemical characteristics (degree of polymerization, crystallinity, particle size, morphology). This information is of importance to dissociate the chemophysical phenomenon from the catalytic one in the case of catalytic liquefaction to better evaluate the added-value of a catalyst. Under various conditions (3–70 g·L–1, 175–200 °C, 0–120 h) despite a higher degree of polymerization Sigmacell reacted faster compared to Avicel and formed a higher amount of light products. The reactivity was preferentially influenced by the initial morphology of cellulose. The liquefaction in the absence of catalyst was never complete and led to insoluble hydrochar formation (20–30 wt %). Then a model was built based of three reaction pathways with kinetics and thermodynamic investigation....

Insight into formation of montmorillonite-hydrochar nanocomposite under hydrothermal conditions

Montmorillonite-hydrochar nanocomposite is of great significance in biofuel production, energy storage and conversion, catalysis, environmental protection, and biology and medicine. To understand how montmorillonite facilitates the conversion of cellulose into hydrochar, it is essential to accurately control the properties and structure of hydrochar and its nanocomposites for technological applications. In this work, hydrochar and montmorillonite-hydrochar nanocomposites were produced by hydrothermal carbonization of cellulose at 200°C for 2–24h in the absence and the presence of montmorillonite. The roles of montmorillonite in the formation of hydrochar were explored in detail. Results showed that montmorillonite acted as a catalyst, an adsorbent, and an inorganic template for hydrochar formation. The formation process of montmorillonite-hydrochar nanocomposite can be divided into two stages: first, the hydrolysis of cellulose yielding degraded liquid products (e.g. glucose, fructose, and organic acids); second, the formed liquid products were adsorbed by montmorillonite, which catalyzed condensation and aromatization to form hydrochar particles on the surface of montmorillonite.

Chemical, Energetic, and Structural Characteristics of Hydrothermal Carbonization Solid Products for Lawn Grass

Hydrothermal carbonization (HTC) of lawn grass was carried out at 200 °C and 240 °C for 30 to 180 min. The chemical, energetic, and structural characteristics of HTC solid residues were investigated. Results from HTC experiments indicate that solid mass yield of all solid residues was 31 to 50%. The hydrogen/carbon (H/C) and oxygen/carbon (O/C) atomic ratios of all solid residues were 1.17 to 1.64 and 0.45 to 0.65, respectively. The higher heating value (HHV) increased up to 20.54 MJ/kg with increasing HTC residence time at 240 °C for 180 min. Both XRD patterns and FTIR spectra show that differences occur with samples treated as compared to the raw material. Solid hydrochar exhibited higher ordered structure characteristics and was mainly derived from amorphous components degradation when the residence time was increased from 30 to 180 min at 200 °C, while hydrochar formed from cellulose components degradation with increased residence time at 240 °C. According to the results studied, it was found that prolonged residence time was favorable to the formation of hydrochar from lawn grass.

Kinetics of hydrothermal carbonization (HTC) of soft rush

Hydrothermal carbonization of soft rush (Juncus effusus) has been studied in the temperature range of 453–513 K. Citric acid at the concentration of 0.2 kg m−3 did not have a significant catalytic effect. A kinetic model is suggested for substrate hydrolysis and the formation of hydrochar as well as byproducts remaining dissolved. Specifically, increasing hydrochar yield at higher substrate concentration can be described with a reaction order of 1.53 for hydrochar formation from intermediate products.