Hydrochar Yield Research Articles

Improving green hydrogen production from Chlorella vulgaris via formic acid-mediated hydrothermal carbonisation and neural network modelling

This study investigates the formic acid-mediated hydrothermal carbonisation (HTC) of microalgae biomass to enhance green hydrogen production. The effects of combined severity factor (CSF) and feedstock-to-suspension ratio (FSR) are examined on HTC gas formation, hydrochar yield and quality, and composition of the liquid phase. The hydrothermal conversion of Chlorella vulgaris was investigated in a CSF and FSR range of −2.529 and 2.943; and 5.0 wt.% – 25.0 wt.%. Artificial neural networks (ANNs) were developed based on experimental data to model and analyse the HTC process. The results show that green hydrogen formation can be increased up to 3.04 mol kg−1 by applying CSF 2.433 and 12.5 wt.% FSR reaction conditions. The developed ANN model (BR-2-11-9-11) describes the hydrothermal process with high testing and training performance (MSEz = 1.71E−06 & 1.40E−06) and accuracy (R2 = 0.9974 & R2 = 0.9781). The enhanced H2 yield indicates an effective alternative green hydrogen production scenario at low temperatures using high-moisture-containing biomass feedstocks.

Simultaneous production of nutritional compounds and hydrochar from Chlorella pyrenoidosa via hydrothermal process

Hydrothermal process is an eco-friendly method for simultaneously extracting antioxidant compounds and producing hydrochar. This study examined the simultaneous production of nutritional compounds and hydrochar from Chlorella pyrenoidosa via a hydrothermal process. The experiments were conducted using a batch reactor at varying temperatures (160, 180, and 200 °C) and holding times (10, 20, and 30 min). The results revealed that phenolic contents, antioxidant activity, and hydrochar production were strongly affected by both temperature and time. The highest total phenolic content (1.70 ± 0.16 mg/g) and antioxidant activity (94.2 ± 1.8 %) were found in process water at 200 °C with a holding time of 20 min. Under these conditions, a hydrochar yield of 30.79 % was obtained. Intriguingly, the hydrochar obtained at 200 °C exhibited higher heating values (24.93–28.32 MJ kg−1) than typical sub-bituminous coals, giving it potential for use as a solid fuel.

Co-hydrothermal carbonization of corn stover and food waste: Characterization of hydrochar, synergistic effects, and combustion characteristic analysis

Hydrochar was prepared via the co-hydrothermal carbonization (Co-HTC) method using corn stover (CS) and food waste (FW) as materials, and the influences of the synergistic effect of Co-HTC on fuel performances and combustion behavior were analyzed. Characterization of hydrochar was conducted using elemental analysis, industrial analysis, SEM, BET, XRD, and FTIR. The outcomes indicated that the higher heating values (HHVs) of hydrochar increased, the atomic ratios of H/C and O/C decreased, the fuel ratios ranged from 0.33 to 0.57, and the average pore diameter decreased when the FW ratio was increased. The synergistic effect of Co-HTC of CS and FW improved hydrochar yield, energy yield, organics retention, and C capture efficiency. Thermogravimetric and kinetic analysis showed that Co-HTC improves the combustion characteristics of hydrochar. Thus, production of hydrochar via Co-HTC of CS and FW provides some guidance for the resourcing and reducing solid organic waste, and offers a promising direction for renewable energy production.

A Mini-Review: Biowaste-Derived Fuel Pellet by Hydrothermal Carbonization Followed by Pelletizing

This review article focuses on recent studies using hydrothermal carbonization (HTC) for producing hydrochar and its potential application as a solid fuel pellet. Due to the depletion of fossil fuels and increasing greenhouse gas (GHG) emissions, the need for carbon-neutral fuel sources has increased. Another environmental concern relates to the massive amount of industrial processing and municipal solid waste, which are often underutilized and end up in landfills to cause further environmental damage. HTC is an appealing approach to valorizing wet biomass into valuable bioproducts (e.g., hydrochar), with improved properties. In this review, the effects of the main HTC reaction parameters, including reaction temperature, residence time, and feedstock to water ratio on the properties and yield of hydrochar are described. Following this, the pelletizing of hydrochar to prepare fuel pellets is discussed by reviewing the influences of applied pressure, processing time, pellet aspect ratio, moisture content of the hydrochar, and the type and dosage of binder on the quality of the resulting fuel pellet. Overall, this review can provide research updates and useful insights regarding the preparation of biowaste-derived solid fuel pellets.

Hydrothermal carbonization of sawdust with the bio-oil of same origin impacts evolution of structures of hydrochar

The high tendency towards polymerization is a bottle-neck issue in upgrading of bio-oil, which, however, is highly desirable in co-hydrothermal carbonization (HTC) of biomass with bio-oil for enhancing production of hydrochar. In this study, co-HTC of bio-oil of the same origin with poplar sawdust was conducted at 180 to 260 °C. The results showed that the HTC in a water medium containing 10% bio-oil could enhanced the yield of hydrochar by ca. 36% at 220 or 260 °C through the cross-polymerization of the sugar-derived furans and ketones/aldehydes as well as the phenolics in bio-oil with the sawdust-derived organics. Furans/sugars in bio-oil showed much more contribution to the increase of hydrochar yield than the phenolics like guaiacol. The highly reactive organics in bio-oil even could react with the crystal region of cellulose at 180 °C, and affected property of hydrochar at especially the higher temperatures by promoting deoxygenation, enhancing carbonization, eliminating polar functionalities like -OH, retaining C=O for continued polymerization, etc. The co-existence of aliphatic organics and phenolics in bio-oil renders the simultaneous formation of highly volatile fraction and aromatic structures in the hydrochar. These created the carbon-rich hydrochar with higher crystallinity, lower hydrophilicity and improved comprehensive combustion index.

Effects of product recovery methods on the yields and properties of hydrochars from hydrothermal carbonization of algal biomass

This contribution reports the effects of product recovery methods on the yields and properties of hydrochars produced from hydrothermal carbonization (HTC) of algal biomass. A slurry of Chlorella vulgaris with 10 wt% of solid loading was hydrothermally carbonized at 180 – 220 °C with holding time of 15 and 60 min. The resulting hydrochars were recovered by two prevailing methods, namely direct filtration and dichloromethane (DCM)-aided filtration. The results indicate that, under identical HTC conditions, compared with DCM-aided filtration, direct filtration always results in higher hydrochar yield because of the biocrude retained on its surface. The presence of residue biocrude in the hydrochars from direct filtration further leads to considerable differences in their properties, as compared with their counterparts from DCM-aided filtration. Specifically, under identical HTC temperature and holding time, DCM-aided filtration yields hydrochars of lower volatile matter contents but higher fixed carbon and ash contents, as compared with the hydrochars from direct filtration. Direct filtration generally recovers hydrochars of greater higher heating values and energy-based yields as compared to DCM-aided filtration. The product recovery methods show considerable impacts on the concentrations of Na, K, Mg, and Ca and their retentions in the hydrochars at 220 °C for 60 min, with higher concentrations and retentions of these elements being observed in the hydrochars from DCM-aided filtration. The hydrochars at 220 °C for 15 and 60 min from DCM-aided filtration show higher specific reactivity of 0.040 – 0.058 min−1 and 0.066 – 0.096 min−1 at hydrochar conversions of ≤ 82 % and ≤ 78 %, respectively, as compared with their direct-filtration counterparts, due to their higher concentrations of catalytic alkali and alkaline earth metals. These findings highlight the importance of considering product recovery methods when comparing the yields and properties of hydrochars from HTC of algae reported in the literature.

Process simulation of hydrothermal carbonization of digestate from energetic perspectives in Aspen Plus

Digestate, a nutrient-rich substance, is a potential resource of income in biogas plants. It can be utilized as a soil amendment and solid biofuel due to containing inorganic and organic compounds. However, concerning the environmental regulations, there is a requirement for further processes, including hydrothermal carbonization (HTC). Regarding the selection of optimum conditions from the experimental data, this study proposes the energetic yield as the performance indicator from the techno-economic viewpoint, taking solid load and relative equipment size comparison into account as well as the heating value of hydrochar. The energetic yield is the energy content in hydrochar as the heating value per unit mass of reactor inlet, MJ/kg reactor inlet. Among the investigated data for various digestates, the optimum feedstock and conditions were HTC agricultural residue (with 5.02 Energetic yield MJ/kg reactor inlet) at 200 °C and the residence time of 1 h with 30 % solid load based on the energetic yields, i.e. selected for process simulation. This study also investigates required experimental data for enabling mass balance and simulation models. In addition to yield and proximate analysis of feedstock and hydrochar, the characterization of process water is important for representing the dissolved organics. The available data influences the accuracy and closure of elemental mass balances for process simulation. In addition, this study also investigates simulation aspects for producing hydrochar with 20 % moisture content as well: the impact of property method on energy balance, the heat of reaction compared to the literature values, and heat integration concepts. SRK, PSRK, NRTL, and IDEAL methods reported the same value, −122 kW, as the heat duty of the reactor. This value corresponds to −1.46 MJ/kg dry solid and −1.74 MJ/kg dry-ash-free solid inlet as the heat of reaction.

Ammonia-assisted thermal hydrolysis of sewage sludge: Solid and liquid phases characterization

This work investigated the effect of ammonia solution on hydrothermal carbonization (HTC) of sewage sludge (SS). Ammonia addition promoted hydrolysis of the SS, and the hydrochar (HC) yield increased with increasing temperature but was still relatively lower compared to the reaction in water alone. The total organic carbon (TOC) of the aqueous phase was significantly improved at all temperatures relative to those observed in water alone but decreased with increasing temperature. N-heterocyclics were promoted as the main compounds of the aqueous phase from ammonia addition, while O-containing compounds were predominant from HTC in water alone. Pyrazine derivatives were the most represented in the aqueous phase, followed by pyrrole derivatives. The formed amines and the ammonia would promote reverse aldol condensation reaction products that would further promote various pyrazine derivatives. A fraction of these N-compounds would undergo polymerization to increase the N content in the HC with increasing temperature. The basic environment created by ammonia addition promoted insoluble phosphorus in the HC. The improved P content and reduced C/N ratio make this HC a potential material for soil amendment, and the high TOC and N-heterocyclic content in the aqueous phase make it suitable as an N precursor for value-added material synthesis.

Hydrothermal carbonisation of paper sludge: Effect of process conditions on hydrochar fuel characteristics and energy recycling efficiency

Current management of solid waste from pulp and paper activities represents an environmental and economic burden worldwide due to pollution emissions. This study investigates the potential of hydrothermal carbonisation (HTC) treatment as a sustainable alternative for producing cleaner and energy-dense solid fuel from paper mill sludge. The effect of process parameters (temperature, reaction time and solid load) on hydrochar fuel formation from paper sludge was evaluated and, for the first time, the paper sludge-derived hydrochar was optimised to maximise the mass yield and calorific value using response surface methodology (RSM). The physicochemical characteristics, thermal fuel behaviour, energy recycling efficiency and electricity generation potential were assessed by proximate and ultimate analysis, thermogravimetry, bomb calorimeter, scanning electron microscopy and process energy assessment. Results showed that hydrochar fuel formation and properties were mainly influenced by the process temperature and residence time, and governed by dehydration and decarboxylation reactions which reduced the atomic H/C and O/C ratios by 35.5% and 64%, respectively. The produced hydrochars presented low sulphur, nitrogen and ash content with a maximum calorific value (HHV) of 22.9 MJ/kg, equivalent to the HHV of coal for commercial utility in South Africa. The HHV of the hydrochar corresponded to a 49.80% increase over the HHV of the initial feedstock. The optimum operating conditions were 231 ± 1 °C and 1.99 h for a hydrochar yield of 74.4% and calorific value of 18.5 MJ/kg. The energy assessment showed that up to 58.34% of the energy produced by hydrochar fuel combustion may be recycled as heat or power, while the remaining 41.66% of the combustion energy could be utilised to sustain the HTC treatment of paper sludge. The substantial water demand was concluded to be a drawback. Thus, water recirculation and the potential to catalyse the HTC reactions to increase overall process efficiency will constitute a future study to make the process more environmentally friendly for industrial-scale application.

Iron valence state evolution and hydrochar properties under hydrothermal carbonization of dyeing sludge

Iron (Fe) migration mechanisms and hydrochar properties in dyeing sludge hydrothermal carbonization (HTC) are important topics in wastewater treatment. HTC treatment of sludge produces wastewater containing Fe so it is necessary to study the migration behavior of Fe during HTC treatment. This study investigated the basic properties and Fe migration behavior of hydrochar during HTC treatment supplemented with nitric acid (HNO3). The results showed that the carbonization degree and yield of hydrochar treated with the HNO3 solution (HHC) were much lower than those of hydrochar treated with ultrapure water (WHC). The variation of total Fe (TF) concentration indicated that the decomposition of organic material and dissolution of minerals in the aqueous release of Fe during the liquid phase, led to much lower TF concentrations compared to the original dyeing sludge. Fe release was further enhanced with the addition of HNO3 and increase of temperature, rendering a much lower TF concentration of the HHC compared to the WHC. The variations of Fe3+ and Fe2+ concentrations indicated that the HTC-treated hydrochar contained more Fe2+, caused by Fe3+ reduction with hydroxyl methyl-furfural and glucose in the liquid and subsequent Fe2+/Fe3+ transferral to the solid hydrochar phase. X-ray diffraction (XRD) showed that the main Fe content in WHC was FeO(OH), while HHC contained mainly Fe(SO4)(OH)•2H2O and Fe3O4. XPS and XRF showed that Fe could more easily enter the internal pores of the hydrochar instead of being deposited on the surface. This study provided more insights on Fe migration behavior during HTC treatment.

Evaluation of the production of hydrochar from spent coffee grounds under different operating conditions

An increasing amount of spent coffee grounds (SCG) has resulted in a huge burden on landfills; thus, alternative viable technologies are needed. The hydrothermal carbonization of SCG was studied to determine to produce high-quality hydrochar. The influence of three independent variables (temperature, reaction duration, and solid to liquid ratio) on the yield of hydrochar and conversion was investigated through 15 experimental runs using central composite design (CCD). The most optimized conversion and hydrochar yield results were 89.8 % and 10.1 %, respectively. Higher heating value (HHV) progressed considerably from 19.1 MJ/kg of SCG to 21.2 MJ/kg of hydrochar. The ultimate analysis results for hydrochar revealed that the quantity of carbon (C) upgraded from 50.1 to 55.9 %, whereas; oxygen (O) content decreased from 40.5 to 38.7 %, hence improving the energy and combustion properties. The Scanning electron microscopy (SEM) analysis portrayed the alteration in the structure of SCG and improved porosity after hydrothermal processing, which was further investigated via Brunauer-Emmett-Teller (BET) analysis. BET surface area improved from 4.4 m2/g (for SCG) to 16.5 m2/g (for hydrochar). The thermal stability of hydrochar was upgraded from 330 °C for SCG to 375 °C for hydrochar.

Impact of temperature and residence time on the hydrothermal carbonization of organosolv lignin

Herein, we have investigated how pure lignin extracted from birch and spruce via a hybrid organosolv/steam explosion method reacts under hydrothermal carbonization (HTC) to produce hydrochar, a product that has found applications in environmental remediation, energy storage and catalysis. We subjected thirteen lignin samples obtained from birch and spruce under different extraction conditions to HTC at 260 ℃ for four hours. The yield of hydrochar varied between the different extraction conditions and source, although no clear correlation between extraction conditions and yield could be observed. For instance, lignin from birch pretreated in 60%v/v ethanol for 15 min resulted in a hydrochar yield of 39 wt%. Increasing the time to 30 and 60 resulted in a hydrochar yield of 27 wt% and 23 wt%, respectively. This suggested that small changes in the organosolv reaction conditions might produce highly structurally different lignin, resulting in the difference in HTC yield. Thus, we chose a subset of four lignin samples to investigate in-depth, subjecting these samples to a range of hydrothermal reaction temperatures and residence times. Solid State NMR and FTIR analysis indicated that the most significant structural changes occurred below 230 ℃ resulting in the breaking of C-O- linkages. Increasing the temperature or time had minimal impact, with no further C-O- linkages broken and no changes to the ring structure of C-C groups. Size exclusion chromatography indicated that the degree of micro and macromolecules in the liquid product varied significantly with lignin source and HTC reaction conditions. Overall, this study demonstrated that lignin has a large reaction range where it produces a very chemically similar solid product, with the only major difference being the yield of material. This is important for industry, as it indicates that a similar solid product can be easily achieved independently of extraction conditions allowing the HTC reaction to be tuned towards extracting the maximum benefit from products contained in the liquid.

Effects of process water recirculation on yields and quality of hydrochar from hydrothermal carbonization process of rice husk

Hydrothermal carbonization (HTC) is promising to convert moist lignocellulosic biomass into advanced fuels. However, it also generates a large amount of process wastewater (PW) which can’t be discharged directly due to its protentional pollution to the environment. This study aims to investigate the effects of PW recycling on the hydrochar performance. Rice husk (RH) was hydrothermally carbonized under the temperature of 220 ℃ for 60 min and PW was recycled 5 times. In the first 3 times recycle, the yield and high heating value (HHV) of hydrochar was improved from 68.51% to 76.62%, 19.71 MJ/kg to 21.97MJ/kg, respectively, and the energetic recovery efficiency (ERE) was highly increased from 72.17% to 89.72%. Functional group analysis by titration shows that PW recycling promoted the formation of oxygen-containing functional groups. The decrease of H/C and O/C ratio shows that PW recirculation promoted dehydration and decarboxylation reactions, thus improving the thermal stability of generated hydrochars. The adsorption performance of hydrochar was evaluated by removal of malachite green (MG). The results imply that the PW recirculation could also improve adsorption capacity of hydrochar for MG. This study provides a new insight for the treatment of PW from HTC of lignocellulosic biomasses.

Hydrothermal carbonization (HTC) of dairy waste: effect of temperature and initial acidity on the composition and quality of solid and liquid products.

Background: Hydrothermal carbonization (HTC) of dairy processing waste was performed to investigate the effect of temperature and initial pH on the yield and composition of the solid (hydrochar) and liquor produced. All hydrochars met the EU requirements of organo-mineral solid fertilizers defined in the Fertilizing Products Regulation in terms of phosphorus (P) and mineral content. Methods: Laboratory scale HTC was performed using pressurized reactors, and the products (solid and liquid) were collected, stored and analyzed for elemental composition and nutrient content using Inductively coupled plasma optical emission spectroscopy (ICP-OES), ultraviolet-visible spectrophotometry (UV-Vis) and other analytic techniques. Results: Maximum hydrochar yield (60.67%) was observed at T=180℃ and pH=2.25, whereas the maximum P-recovery was 80.38% at T=220℃ and pH=4.6. The heavy metal content of the hydrochars was mostly compliant with EU limitations, except for Ni at T=220℃ and pH=8.32. Meanwhile, further study of Chromium (Cr) species is essential to assess the fertilizer quality of the hydrochars. For the liquid product, the increase in temperature beyond 200℃, coupled with an increase in initial acidity (pH=2.25) drove P into the liquor. Simultaneously, increasing HTC temperature and acidity increased the concentration of NO 3 - and NH 4 + in the liquid products to a maximum of 278 and 148 mg/L, respectively, at T=180℃ and pH=4.6. Furthermore, no direct relation between final pH of liquor and NH 4 + concentration was observed. Conclusions: HTC allows for the production of hydrochar as a potential fertilizer material that requires further processing. Adjusting HTC conditions enhanced P-recovery in the hydrochar, while retrieving higher nitrate concentrations in the liquid product. Optimizing HTC for the production of qualified hydrochars requires further treatment of Cr content, studying the availability of P in the products and enhancing the hydrochar yield for economic feasibility.

Solid fuel production from co-hydrothermal carbonization of polyvinyl chloride and corncob: Higher dechlorination efficiency and process water recycling

The hydrothermal carbonization (HTC) of polyvinyl chloride (PVC) and wet herbal agricultural wastes for solid fuel production remains bleak economics and sustainability because of high chloride residual, wastewater burden and low production capacity. In this study, the HTC dechlorination was investigated using the first-order reaction kinetic analysis. We found that the co-hydrothermal carbonization (co-HTC) of PVC and the typical biomass (corncob) achieved a staggering drop of dechlorination activation energy from 189.95 kJ/mol to 110.04 kJ/mol. The co-HTC process achieved rapid dechlorination and carbonization due to synergistic effect, to suppress the chlorine content in bituminous-coal-like hydrochar less than 0.05 %. The process wastewater (process water) from co-HTC was recycled four times to evaluate the reusability and chemical evolution. The organics in co-HTC environment enhanced the carbonization which was confirmed by the improved heating value (30.06 to 32.42 MJ·kg−1), hydrochar yield (33.33 % to 36.47 %) and energy recovery efficiency (57.73 % to 68.13 %). The Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) evidenced the process water recirculation maintained high chloride removal. Moreover, the possible formation pathways of two kinds of hydrochars were discussed through the chemical composition of the aqueous phase and the characteristic structures of hydrochar. The co-HTC and process water recycling strategies provide a more promising prospect to convert PVC and biomass wastes into solid fuels.

EFFECT OF HYDROTHERMAL TEMPERATURE ON SAWDUSTHYDROCHAR PRODUCTION PRODUCED THROUGHMICROWAVE-ASSISTED ARTIFICIAL COALIFICATION

A microwave mode of heating was utilized to produce sawdust hydrochar through the artificial coalification process. Hydrochar was synthesized at temperatures 180, 200 and 220 °C with a 1:10 solid to water ratio and residence time of 40 minutes. Hydrochar was characterized for proximate properties and energy quality. With the increase in the temperature from 180 to 220 °C, hydrochar yield decreased from 76.69 to 58.35 %. The volatile matter and ash content depicted a decreasing trend and the energy content of hydrochar showed an increasing trend with the increase in the process temperature. The ratio of energy densification of hydrochar was in the range of 1.04 to 1.26.

Influence mechanism of aqueous organic components on the hydrochar formation reaction during the biomass hydrothermal carbonization wastewater recycling

Reusing the wastewater byproduct of the hydrothermal carbonization (HTC) process as a water source to prepare biomass slurry feedstock for the next HTC run provides a feasible wastewater disposal method. However, the influence of various organic components contained in the hydrothermal wastewater (HW) on the formation routes and characteristics of hydrochar is yet to be clarified. In this study, five representative organic components, phenol, acetic acid, furfural, 5-hydroxymethylfurfural (5-HMF), and 2,5-hexanedione, were selected to study their effects on the hydrochar fuel characteristics and HW components, based on which the hydrochar formation pathways in an organic wastewater environment were proposed. The phenol, furfural, and 5-HMF contained in the wastewater source could polymerize with the active intermediates from biomass degradation, thereby resulting in an increase in the hydrochar mass yield (HCY) by 1.13%–7.07% and an increase in the higher heating value of hydrochar to 27.36 MJ/kg. In contrast, the acetic acid and 2,5-hexanedione contained in the wastewater source resulted in a decrease in HCY, probably because of the catalytic effects of volatile fatty acids on biomass hydrolysis. Moreover, wastewater sources with all five organic components increased the energy recovery efficiency from biomass to a high level of 97.28% under the two effects of deep polymerization and catalytic carbonization.

Energy yield optimization of co-hydrothermal carbonization of sewage sludge and pinewood sawdust coupled with anaerobic digestion of the wastewater byproduct

High energy recovery efficiency is expected to be obtained from biomass waste by coupling hydrothermal carbonization (HTC) with anaerobic digestion (AD). This study focused on optimizing the total energy yield (TEY) from sewage sludge (SS) and pinewood sawdust (PS) in a HTC-AD coupling process, which has attracted little attention. Response surface methodology (RSM) was used to gain the optimal operation parameters including PS proportion, liquid–solid mass ratio (L/S) of raw feedstock (SS-PS mixture slurry), HTC temperature, and residence time and discuss how the parameters influenced the TEY. The results showed that the PS proportion, HTC temperature, and residence time had significant effects on the TEY (p < 0.05), while the influence of the L/S of raw feedstock on TEY was limited. The hydrochar mass yield (HCY), higher heating value (HHV) of hydrochar, and the cumulative methane production from the AD of HTC wastewater (HTCWW) increased with PS proportion increasing. By contrast, when HTC temperature increased, HHV increased while both HCY and cumulative methane production decreased. Prolonging the HTC residence time decreased the TEY of hydrochar and methane, because the HCY decreased considerably although the carbonization degree of the fed organics increased, and the AD of HTCWW was inhibited due to the increasingly generated toxic substances in a long-term HTC process. The optimal TEY, with a value of 89.61%, was obtained at the following conditions: PS proportion of 75%, original L/S of feedstock of 7:1, HTC temperature of 190 °C, and residence time of 3 h. Among which, the energy yields of hydrochar and methane were 86.46% and 3.15%, respectively. HTCWW showed good AD performance with a cumulative methane yield of 255.64 mL-CH4/g-COD in a 16-day AD process.

Hydrothermal carbonization (HTC) of dairy waste: effect of temperature and initial acidity on the composition and quality of solid and liquid products

Background: Hydrothermal carbonization (HTC) of dairy processing waste was performed to investigate the effect of temperature and initial pH on the yield and composition of the solid (hydrochar) and liquor produced. All hydrochars met the EU requirements of organo-mineral solid fertilizers defined in the Fertilizing Products Regulation in terms of phosphorus (P) and mineral content. Methods: Laboratory scale HTC was performed using pressurized reactors, and the products (solid and liquid) were collected, stored and analyzed for elemental composition and nutrient content using Inductively coupled plasma optical emission spectroscopy (ICP-OES), ultraviolet-visible spectrophotometry (UV-Vis) and other analytic techniques. Results: Maximum hydrochar yield (60.67%) was observed at T=180℃ and pH=2.25, whereas the maximum P-recovery was 80.38% at T=220℃ and pH=4.6. The heavy metal content of the hydrochars was mostly compliant with EU limitations, except for Ni at T=220℃ and pH=8.32. Meanwhile, further study of Chromium (Cr) species is essential to assess the fertilizer quality of the hydrochars. For the liquid product, the increase in temperature beyond 200℃, coupled with an increase in initial acidity (pH=2.25) drove P into the liquor. Simultaneously, increasing HTC temperature and acidity increased the concentration of NO3- and NH4+ in the liquid products to a maximum of 278 and 148 mg/L, respectively, at T=180℃ and pH=4.6. Furthermore, no direct relation between final pH of liquor and NH4+ concentration was observed. Conclusions: HTC improved the nutrient content of dairy waste, allowing for the production of potential solid organo-mineral fertilizers requiring additional treatment to ensure safe fertilizer application.

A parametric experimental validation of a biorefinery concept based on anaerobic digestion of fruit and vegetable waste

AbstractThe goal of this paper was to assess the possibilities for one‐stage reactor anaerobic digestion processing of fruit and vegetable biomass within a biorefinery concept for optimum products. Co‐digestion was the simplest strategy to implement to increase product yield among the variables tested (pH, ultrasonification, and co‐digestion). Methane yields from co‐digestion of fruit and vegetable waste (FVW) and pig manure (PM) ranged from 318 to 434 NmL /g VS, with the highest yield (434 NmL /g VS) achieved at a co‐digestion ratio of FVW75:PM25, representing a 20% increase over the methane production from FVW alone. The pH value was determined to be the most effective variable for enhancing methane production, with pH = 8 producing the maximum cumulative methane yield of 497 NmL/g VS. There was no significant change in the composition of volatile fatty acids (VFA) with pH in terms of dominant product type; valeric acid was the predominant VFA synthesized at all starting pH values except pH = 12, where formic acid was the predominant acid formed. As a digestate post‐treatment procedure, hydrothermal carbonization was combined with anaerobic digestion. At 200 °C, the maximum hydrochar yield production was 58.2%, with a higher heating value of 10.75 MJ/kg. © 2022 The Authors. Biofuels, Bioproducts and Biorefining published by Society of Industrial Chemistry and John Wiley &amp; Sons Ltd.