Hydrothermal carbonization of rape straw: Effect of reaction parameters on hydrochar and migration of AAEMs

Speciation, leachability, and phytoaccessibility of heavy metals during thermochemical liquefaction of contaminated peanut straw

In this study, the speciation, leachability, phytoaccessibility, and environmental risks of heavy metals (Cd, Zn, and Cu) during liquefaction of contaminated peanut straw in ethanol at different temperatures (220, 260, 300, 340, and 380 °C) were comprehensively investigated. The results showed that elevated temperatures facilitated heavy metal accumulation in the biochar. The acid-soluble/exchangeable and reducible fraction percentages of heavy metals were substantially reduced in the biochar after liquefaction as the temperature increased, and the oxidizable fraction became the dominant heavy metal fraction, accounting for 44.14-78.67%. Furthermore, although an excessively high liquefaction temperature (380 °C) increased the residual fraction percentages of Zn and Cu, it was detrimental to Cd immobilization. The acid-soluble/exchangeable Cd in the contaminated peanut straw readily migrates to the bio-oil during liquefaction, with the highest concentration of 1.60 mg/kg at 260 °C liquefaction temperature, whereas Zn and Cu are predominantly bound to the unexchangeable fraction in the bio-oil. Liquefaction inhibited heavy metal leachability and phytoaccessibility in biochar, the lowest extraction rates of Cd, Zn, and Cu were 0.71%, 1.66% and 0.95% by diethylenetriamine pentaacetic acid, respectively. However, the leaching and extraction concentrations increased when the temperature was raised to 380 °C. Additionally, heavy metal risk was reduced from medium and high risk to no and low risk. In summary, liquefaction reduces heavy metal toxicity and the risks associated with contaminated peanut straw, and a temperature range of 300-340 °C for ethanol liquefaction can be considered optimal for stabilizing heavy metals.

Energy recovery from agro-forest wastes through hydrothermal carbonization coupled with hydrothermal Co-gasification: Effects of succinic acid on hydrochars and H2 production

Aiming to upgrade agro-forest wastes into value-added solid and gaseous fuels in the present investigation, hydrothermal carbonization (HTC) of spruce (SP), canola hull (CH), and canola meal (CM) was optimized in terms of operating conditions, maximizing the higher heating value of hydrochars. The optimal operating conditions were achieved at HTC temperature, reaction time, and solid-to-liquid ratio of 260 ^°C, 60 min, and 0.2 g mL^-1, respectively. At the optimum condition, succinic acid (0.05-0.1 M) was used as HTC reaction medium to investigate the effects of acidic medium on the fuel characteristics of hydrochars. The succinic acid assisted HTC was found to eliminate ash-forming minerals e.g., K, Mg, and Ca from hydrochar backbones. The calorific values, H/C and O/C atomic ratios of hydrochars were in the range of 27.6-29.8 MJ kg^-1, 0.8-1.1, and 0.1-0.2, respectively, indicating the biomass upgrading into coal-like solid fuels. Finally, hydrothermal gasification of hydrochars with their corresponding HTC aqueous phase (HTC-AP) was assessed. Gasification of CM resulted in a relatively high H₂ yield of 4.9-5.5 mol kg^-1 followed by that for SP with 4.0-4.6 mol H₂ per kg of hydrochars. Results suggest that hydrochars and HTC-AP have a great potential for H₂ production via hydrothermal co-gasification, while suggesting HTC-AP reuse.

Reduction and valorization of dairy manure by organic chelating acid-assisted hydrothermal process: Dewatering performance, energy recovery, and effluent toxicity

Livestock manure with high moisture content is a challenge for management and further disposal. In this study, the organic chelating acid(EDTA)-assisted hydrothermal (EAHT) process was used to achieve dewatering, dry mass minimization, and volume reduction of dairy manure (DM). The hydrophobic modification of DM resulted in a 55% reduction in dry mass, and the specific resistance to filtration (SRF) showed a shift in dewatering performance from unfilterable to highly filterable. An investigation of the reaction mechanisms suggests that proteins and polysaccharides were released from the damaged extracellular polymeric substances (EPS) of the DM into effluent. The surface functional groups of the hydrochar were changed from hydrophilic to hydrophobic, which promotes the transformation of bound water to free water in the DM with enhanced dewatering performance. The obtained hydrochar at 17.5 mg/g EDTA dosage exhibited the highest calorific value (HHV_daf = 29.25 MJ/kg). The HHV_dry of samples have little difference and approach that of anthracite coal (19.2-21.1 MJ/kg)After EAHT, the combustion safety of the hydrochar was improved, which is highly significant for its use as biofuel. The by-product effluent showed lower biological toxicity after EAHT than after HT. The findings of this study demonstrated that EAHT can be efficient in achieving DM reduction and energy recovery, which provides widespread agricultural and environmental application prospects.

Directional regulation and mechanism analysis of the surface properties of hydrothermal carbon by circulating liquid in the hydrothermal carbonization procedure

The complexity of the chemistry behind the hydrothermal conversion is enormous. Components interact with their own physical and chemical structure, making it harsh to understand the conversion as a whole. Herein, the six-water recirculation and loading nano SiO₂ experiment in a one-pot hydrothermal carbonization procedure was designed to elucidate the mechanism of regulating the functional groups and microporous structure of the hydrochar surface. The hydrochar prepared by the second circulating liquid and loading nano-SiO₂ (HBC-R2/Si) was equipped most enriched functional groups (carboxyl = 11.48 μmol/g, phenolic hydroxyl = 52.98 μmol/g, lactone groups = 46.52 μmol/g) and suitable pore size (1.90 nm-1.93 nm) as a sorbent riched in hemicellulose. The sorption kinetics (equilibrium reached ≈ 480 min) are approximately evenly fitted by the pseudo-second-order, Weber and Morris, and Elovich models, indicating that membranes and particles diffusion, pore diffusion, and surface sorption coexisted in the sorption of methylene blue (MB) on the hydrochar materials. Simultaneously, all hydrochar materials achieved over 25% MB removal within 90 min (liquid membrane diffusion) and over 40% for HBC-R2 and HBC-R2/Si, suggesting that liquid membrane diffusion is the predominant rate-limiting step. Pearson's correlation analysis and Mantel's analysis announced that the cation exchange capacity (CEC), pore size, and carboxyl groups on the hemicellulose affect the sorption capacity by limiting the pore diffusion procedure. However, the CEC and the phenolic hydroxyl groups on the cellulose and hemicellulose affect the sorption rate by limiting membrane diffusion. Three consecutive sorption/desorption cycles confirmed the high stability and reusability of HBC-R2/Si composites.

Microwave-Assisted Hydrothermal Carbonisation of Waste Biomass: The Effect of Process Conditions on Hydrochar Properties

Hydrochars are an alternative form of biochar produced by hydrothermal carbonisation (HTC), a potentially cheaper and greener method. In this paper, the effect of multiple variables on hydrochar properties was investigated. Waste biomass was converted to hydrochar via microwave-assisted hydrothermal carbonisation. The variables were temperature, solution ratio (water-biomass ratio), time, particle size, pH and acetone washing. The measured properties were yield, carbon, oxygen and ash content, higher heating value (HHV), carbon and energy recovery and dye and water adsorption. Feedstock significance was investigated using apple, wheat, barley, oat and pea straw. The investigation into this specific combination of variables and feedstock has not been done before. HTC increased carbon content (~60%), HHV (~24 MJ/kg) and water adsorption and reduced oxygen content and dye adsorption. Thermal analysis suggested hydrochars were not suitable for sequestration. Decreasing the solution ratio was the most significant factor in increasing yield, carbon recovery and energy yield. Increasing the temperature was the most significant factor in increasing carbon and decreasing oxygen content. This affected HHV, with higher temperatures producing a higher energy material, surpassing brown coal. Hydrochars produced at a high solution ratio, temperature and times showed the best carbonisation. Smaller particle size increased yield and carbonisation but increased ash content. Low solution pH increased carbon content, HHV and water adsorption but lowered yield, carbon recovery, energy yield, dye adsorption and oxygen and ash content. High pH increased ash content and dye adsorption but lowered yield, carbon recovery, energy yield and dye adsorption. Acetone decreased yield, carbon recovery, energy yield, carbon content and HHV but increased oxygen, ash content and dye and water adsorption. Barley biomass showed the highest yield and carbon recovery, and pea showed the highest energy yield and HHV. Apple showed the highest carbon content. All the hydrochars showed promise as solid fuels, a soil additive and a precursor for activated carbon but lacked high adsorption for pollutant adsorbents and stability for carbon sequestration.

Hydrothermal Carbonization of Spent Coffee Grounds for Producing Solid Fuel

Spent coffee grounds (SCG) are industrial biowaste resulting from the coffee-brewing process, and they are often underutilized and end up in landfills, thereby leading to the emission of toxic gases and environmental damage. Hydrothermal carbonization (HTC) is an attractive approach to valorize wet biomass such as SCG to valuable bioproducts (i.e., hydrochar). Thus, in this work, the HTC of SCG was carried out in a 500 L stainless steel vessel at 150, 170, 190, 210, and 230 °C for 30 min, 60 min, 90 min, and 120 min and a feedstock to water weight ratio of 1:5, 1:10, and 1:15, and the use of the resulting hydrochar as a solid fuel was evaluated. The results showed that a high energy recovery (83.93%) and HHV (23.54 MJ/kg) of hydrochar was obtained at moderate conditions (150 °C, 30 min, and feedstock to water weight ratio of 1:5) when compared with conventional approaches such as torrefaction. Following this, the surface morphology, functionality, and combustion behavior of this hydrochar were characterized by SEM, FTIR, and TGA, respectively. In short, it can be concluded that HTC is an effective approach for producing solid fuel from SCG and the resulting hydrochar has the potential to be applied either in domestic heating or large-scale co-firing plants.