Hydrothermal carbonization as an all-inclusive process for food-waste conversion

Hydrothermal pretreatment for enhanced thermochemical or biochemical conversion of pharmaceutical biowastes into fuels, fertilizers, and carbon materials

Pharmaceutical biowastes, rich in organic matter and high in moisture, are typical light industry byproducts with waste and renewable attributes. Thermochemical and biochemical conversion technologies transform these residues into value-added bioproducts, including biofuels, biofertilizers, and bio-carbon materials. Hydrothermal pretreatment effectively removes toxic substances and enhances feedstock for these processes. This review comprehensively examines its role in improving the formation of bioproducts from pharmaceutical biowastes, focusing on (i) upgrading and denitrogenating solid biofuels with better combustion performance; (ii) enhancing biodegradability and gaseous biofuel production via organic matter decomposition; (iii) enriching soluble carbon and nitrogen for liquid biofertilizer; (iv) eliminating antibiotic residues and reducing antibiotic resistance in solid biofertilizers; and (v) stabilizing carbon and nitrogen structures and optimizing pore characteristics for functionalized carbon materials. The review recommends a potential staged thermochemical approach to co-produce nitrogen-enriched liquid biofertilizers and porous carbon materials from pharmaceutical biowastes. Hydrothermal pretreatment emerges as a key technique for facilitating the migration and conversion of essential elements like carbon and nitrogen. This study reveals the potential of hydrothermal pretreatment to address the limitations of pharmaceutical biowastes and offers insights into their valorization.

Enhanced energy and nutrient recovery via hydrothermal carbonisation of sewage sludge: Effect of process parameters

Integration of waste management with energy and resource recovery is being widely explored to achieve sustainability. To achieve this, sewage sludge was treated with hydrothermal carbonisation (HTC) at temperatures ranging from 180 °C-260 °C with an increment of 20 °C for three different duration of 1 h, 3 h, and 5 h. The energy and resource recovery potential of the HTC treatment was evaluated through of hydrochar (HC) and process water (PW) properties. Dehydration and decarboxylation reactions resulted in reduced H/C and O/C atomic ratios of 1.35 and 0.45 respectively in HC-260-3, exhibiting peat-like propertied. The calorific value of HC-260-5 was enhanced to 5.9 MJ/kg (increase of 25.8 %) due to the combined effect of H/C and O/C atomic ratios, increased volatile organics and fixed carbon. A maximum energy recovery efficiency of 82.44 % was realised at 240 °C for 3 h rendering it the optimal process condition to ensure energy enrichment. Thermogravimetric analysis (TGA) of HC samples indicated an enhanced combustion behaviour with an increased HTC severity. The elevated levels of volatile fatty acids (VFAs) in PW (maximum 2296 mg/L) made it viable for energy recovery in anaerobic digestion units. Additionally, the PW contains significant concentrations of N and P (2091.68 mg/L and 40.51 mg/L, respectively), indicating enhanced resource/nutrient recovery potential.

Hydrothermal carbonization of food waste for sustainable biofuel production: Advancements, challenges, and future prospects

With the improvement of living standards, food waste (FW) has become one of the most important organic solid wastes worldwide. Owing to the high moisture content of FW, hydrothermal carbonization (HTC) technology that can directly utilize the moisture in FW as the reaction medium, is widely used. Under mild reaction conditions and short treatment cycle, this technology can effectively and stably convert high-moisture FW into environmentally friendly hydrochar fuel. In view of the importance of this topic, this study comprehensively reviews the research progress of HTC of FW for biofuel synthesis, and critically summarizes the process parameters, carbonization mechanism, and clean applications. Physicochemical properties and micromorphological evolution of hydrochar, hydrothermal chemical reactions of each model component, and potential risks of hydrochar as a fuel are highlighted. Furthermore, carbonization mechanism of the HTC treatment process of FW and the granulation mechanism of hydrochar are systematically reviewed. Finally, potential risks and knowledge gaps in the synthesis of hydrochar from FW are presented and new coupling technologies are pointed out, highlighting the challenges and prospects of this study.

A Recent Review of Electrospun Porous Carbon Nanofiber Mats for Energy Storage and Generation Applications

Electrospun porous carbon nanofiber mats have excellent properties, such as a large surface area, tunable porosity, and excellent electrical conductivity, and have attracted great attention in energy storage and power generation applications. Moreover, due to their exceptional properties, they can be used in dye-sensitized solar cells (DSSCs), membrane electrodes for fuel cells, catalytic applications such as oxygen reduction reactions (ORRs), hydrogen evolution reactions (HERs), and oxygen evolution reactions (OERs), and sensing applications such as biosensors, electrochemical sensors, and chemical sensors, providing a comprehensive insight into energy storage development and applications. This study focuses on the role of electrospun porous carbon nanofiber mats in improving energy storage and generation and contributes to a better understanding of the fabrication process of electrospun porous carbon nanofiber mats. In addition, a comprehensive review of various alternative preparation methods covering a wide range from natural polymers to synthetic carbon-rich materials is provided, along with insights into the current literature.

Hydrothermal carbonization of food waste as sustainable energy conversion path

Every day, a large amount of food waste (FW) is released into the environment, causing financial loss and unpredictable consequences in the world, highlighting the urgency of finding a suitable approach to treating FW. As moisture content makes up 75% of the FW, hydrothermal carbonization (HTC) is a beneficial process for the treatment of FW since it does not require extensive drying. Moreover, the process is considered favorable for carbon sequestration to mitigate climate change in comparison with other processes because the majority of the carbon in FW is integrated into hydrochar. In this work, the reaction mechanism and factors affecting the HTC of FW are scrutinized. Moreover, the physicochemical properties of products after the HTC of FW are critically presented. In general, HTC of FW is considered a promising approach aiming to attain simultaneously-two core benefits on economy and energy in the sustainable development strategy.

Hydrothermal treatment: An efficient food waste disposal technology

The quantities of food waste (FW) are increasing yearly. Proper disposal of FW is essential for reusing value-added products, environmental protection, and human health. Based on the typical characteristics of high moisture content and high organic content of FW, hydrothermal treatment (HTT), as a novel thermochemical treatment technology, plays unique effects in the disposal and utilization of FW. The HTT of FW has attracted more and more attention in recent years, however, there are few conclusive reviews about the progress of the HTT of FW. HTT is an excellent approach to converting energy-rich materials into energy-dense fuels and valuable chemicals. This process can handle biomass with relatively high moisture content and allows efficient heat integration. This mini-review presents the current knowledge of recent advances in HTT of FW. The effects of HTT temperature and duration on organic nutritional compositions (including carbohydrates, starch, lipids, protein, cellulose, hemicellulose, lignin, etc.) and physicochemical properties (including pH, elemental composition, functional groups, fuel properties, etc.) and structural properties of FW are evaluated. The compositions of FW can degrade during HTT so that the physical and chemical properties of FW can be changed. The application and economic analyses of HTT in FW are summarized. Finally, the analyses of challenges and future perspectives on HTT of FW have shown that industrial reactors should be built effectively, and techno-economic analysis, overall energy balance, and life cycle assessment of the HTT process are necessary. The mini-review offers new approaches and perspectives for the efficient reuse of food waste.

Multi-Variate and Multi-Response Analysis of Hydrothermal Carbonization of Food Waste: Hydrochar Composition and Solid Fuel Characteristics

To maximize food waste utilization, it is necessary to understand the effect of process variables on product distribution. To this day, there is a lack of studies evaluating the effects of the multiple variables of HTC on food waste. A Design of Experiment (DoE) approach has been used to investigate the influence of three process variables on the product distribution and composition of process streams from the HTC of food waste. This work evaluates the effect of hydrothermal carbonization process conditions on the composition and utilization capabilities of hydrochar from food waste. Parametric analysis was carried out with a design of experiments of central composite rotatable design (CCRD) and response surface methodology (RSM). Derringer’s desirability function was employed to perform a multi-response evaluation. The optimized process conditions were 260.4 °C, 29.5 min reaction time, and 19.6% solid load. The predicted optimized responses were EMC = 2.7%, SY = 57.1%, EY = 84.7%, ED = 1.5, and HHV of 31.8 MJ/Kg, with a composite desirability of 0.68. Temperature and solid load had a significant effect on all evaluated responses, while reaction time was non-significant.

An environmental assessment for municipal organic waste and sludge treated by hydrothermal carbonization

Climate change is the world's greatest challenge today, the reason why it is urgent to optimize industrial processes and find new renewable energy sources. Hydrothermal carbonization (HTC) is one of the Waste-to-Energy technologies with greater projections due to its operative advantages. However, for its large-scale implementation, there are challenges related to the variability of the composition of the waste biomass and the seasonal and geographical availability. This research applied the Life Cycle Analysis methodology to evaluate the environmental impacts caused by three biomasses blends as raw material in the HTC process at laboratory scale. The blends analyzed considered different organic fractions of municipal solid waste (food and pruning) and sewage sludge. The results showed that blend 1 had a lower environmental impact for the case of production in the experimental laboratory level, compared with blends 2 and 3. This is mainly due to its greater calorific value and mass yield, which allows obtaining more hydrochar compared with the other blends, increasing the energy efficiency of the process. Also, between 87.94% and 98.00% of the energy reduction is required to obtain neutral impacts regarding the energy requirements in the experimental laboratory level scenario and the Chilean energy matrix. The processing of blends in HTC has excellent potential in a context where municipal solid wastes have been disposed in sanitary landfills or dumps, as in most emerging countries. Since this study incorporated data from the literature, future studies should perform an elemental analysis to provide experimental and differentiated data.

Emerging waste valorisation techniques to moderate the hazardous impacts, and their path towards sustainability

Due to the recent boom in urbanisation, economy, and global population, the amount of waste generated worldwide has increased tremendously. The World Bank estimates that global waste generation is expected to increase 70% by 2050. Disposal of waste is already a major concern as it poses risks to the environment, human health, and economy. To tackle this issue and maximise potential environmental, economic, and social benefits, waste valorisation - a value-adding process for waste materials - has emerged as a sustainable and efficient strategy. The major objective of waste valorisation is to transit to a circular economy and maximally alleviate hazardous impacts of waste. This review conducts bibliometric analysis to construct a co-occurrence network of research themes related to management of five major waste streams (i.e., food, agricultural, textile, plastics, and electronics). Modern valorisation technologies and their efficiencies are highlighted. Moreover, insights into improvement of waste valorisation technologies are presented in terms of sustainable environmental, social, and economic performances. This review summarises highlighting factors that impede widespread adoption of waste valorisation, such as technology lock-in, optimisation for local conditions, unfavourable regulations, and low investments, with the aim of devising solutions that explore practical, feasible, and sustainable means of waste valorisation.

Hydrothermal carbonization of simulated food waste for recovery of fatty acids and nutrients

We conducted Hydrothermal carbonization (HTC) of simulated food waste under different reaction conditions (180 to 220 °C, 15 and 30 min), with the aim of recovering both fatty acids from the hydrochar and nutrients from the aqueous-phase products. HTC of the simulated food waste produced hydrochar that retained up to 78% of the original fatty acids. These retained fatty acids were extracted from the hydrochar using ethanol, a food-grade solvent, and gave a net recovery of fatty acid of ∼ 50%. The HTC process partitioned more than 50 wt% of the phosphorus and around 38 wt% of the nitrogen into the aqueous-phase products. A reaction path consistent with decarboxylation predominated during HTC under all of the reaction conditions investigated. A path consistent with dehydration was also observed, but only for the more severe reaction conditions. This work illustrates the potential that HTC has for valorization of food waste.

Upgrading the Organic Fraction of Municipal Solid Waste by Low Temperature Hydrothermal Processes

In comparison to lignocellulosic biomass, which is suitable for thermo-chemical valorization, the organic fraction of municipal solid waste (OFMSW) is mainly treated via composting or anaerobic digestion (AD). An efficient utilization of OFMSW is difficult due to variations in its composition. Based on the characteristics of OFMSW, hydrothermal treatment (HTT) experiments at temperatures < 200 °C as an alternative OFMSW-processing were evaluated in this study. The raw OFMSW was characterized with a dry matter (DM)-based organic dry matter (oDM) content of 77.88 ± 1.37 %DM and a higher heating value (HHV) of 15,417 ± 1258 J/gDM. Through HTT at 150, 170 and 185 °C, the oDM contents as well as H/C and O/C ratios were lowered while the HHV increased up to 16,716 ± 257 J/gDM. HTT led to improved fuel properties concerning ash melting, corrosion stress and emission behavior. Negative consequences of the HTT process were higher contents of ash in the biochar as well as accumulated heavy metals. In the sense of a bioeconomy, it could be beneficial to first convert raw OFMSW into CH4 through AD followed by HTT of the AD-digestate for the generation of solid fuels and liquid products. This could increase the overall utilization efficiency of OFMSW.

Co-hydrothermal carbonization of food waste with yard waste for solid biofuel production: Hydrochar characterization and its pelletization

In this study, Co-HTC of food waste with yard waste was conducted for biofuel pellets production, and also to understand any possible synergy between two feedstock types. The calorific value of blended raw feedstock was 13.5 MJ/kg which increased to 27.6 MJ/kg after Co-HTC at 220 °C for 1 h. Energy yield and fuel ratio calculated was 45% and 0.65 respectively. Hydrochar produced demonstrated a stable combustion profile as compared to reactive combustion profile for raw samples. The blend of food and yard waste hydrochar was easily pelletized, and its pellets showed improvement in mechanical properties as compared to pellets made from mono-substrate((food waste) hydrochar. Pellets produced from the blend of food and yard waste hydrochar showed higher energy (46.4 MJ/m³) and mass density (1679 kg/m³) as compare to the pellet produced from food waste hydrochar alone. Tensile strength obtained for the blended hydrochar pellet was 2.64 MPa while same for the pellets produced from food waste hydrochar alone was 1.30 MPa. In addition to improving hydrophobicity, soften lignin from yard waste also helped in binding the food waste hydrochar particles together within the pellets matrix during heated pelletization. The results presented in the study indicated that in the presence of all favorable conditions, there is a potential that approximately 11% of the global coal consumption could be replaced by the combustion of hydrochar produced from food and yard waste globally.

Hydrochars as Emerging Biofuels: Recent Advances and Application of Artificial Neural Networks for the Prediction of Heating Values

In this study, the growing scientific field of alternative biofuels was examined, with respect to hydrochars produced from renewable biomasses. Hydrochars are the solid products of hydrothermal carbonization (HTC) and their properties depend on the initial biomass and the temperature and duration of treatment. The basic (Scopus) and advanced (Citespace) analysis of literature showed that this is a dynamic research area, with several sub-fields of intense activity. The focus of researchers on sewage sludge and food waste as hydrochar precursors was highlighted and reviewed. It was established that hydrochars have improved behavior as fuels compared to these feedstocks. Food waste can be particularly useful in co-hydrothermal carbonization with ash-rich materials. In the case of sewage sludge, simultaneous P recovery from the HTC wastewater may add more value to the process. For both feedstocks, results from large-scale HTC are practically non-existent. Following the review, related data from the years 2014–2020 were retrieved and fitted into four different artificial neural networks (ANNs). Based on the elemental content, HTC temperature and time (as inputs), the higher heating values (HHVs) and yields (as outputs) could be successfully predicted, regardless of original biomass used for hydrochar production. ANN3 (based on C, O, H content, and HTC temperature) showed the optimum HHV predicting performance (R2 0.917, root mean square error 1.124), however, hydrochars’ HHVs could also be satisfactorily predicted by the C content alone (ANN1, R2 0.897, root mean square error 1.289).

Porous Carbon Materials Obtained by the Hydrothermal Carbonization of Orange Juice

Porous carbon materials are currently subjected to strong research efforts mainly due to their excellent performances in energy storage devices. A sustainable process to obtain them is hydrothermal carbonization (HTC), in which the decomposition of biomass precursors generates solid products called hydrochars, together with liquid and gaseous products. Hydrochars have a high C content and are rich with oxygen-containing functional groups, which is important for subsequent activation. Orange pomace and orange peels are considered wastes and then have been investigated as possible feedstocks for hydrochars production. On the contrary, orange juice was treated by HTC only to obtain carbon quantum dots. In the present study, pure orange juice was hydrothermally carbonized and the resulting hydrochar was filtered and washed, and graphitized/activated by KOH in nitrogen atmosphere at 800 °C. The resulting material was studied by transmission and scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and nitrogen sorption isotherms. We found porous microspheres with some degree of graphitization and high nitrogen content, a specific surface of 1725 m²/g, and a pore size distribution that make them good candidates for supercapacitor electrodes.

Dye Adsorption and Electrical Property of Oxide-Loaded Carbon Fiber Made by Electrospinning and Hydrothermal Treatment

Our current study deals with the dye adsorption and electrical property of a partially carbonized composite fiber containing transition metal oxides including, iron oxide, nickel oxide, and titanium oxide. The fiber was made by electrospinning, carbonization, and hydrothermal treatment. During the electrospinning, titanium oxide particles were dispersed in polyacrylonitrile (PAN) polymer-dimethylformamide (DMF) solution. Nickel chloride and iron nitrate were added into the solution to generate nickel oxide and iron oxide in the subsequent heat treatment processes. The polymer fiber was oxidized first at an elevated temperature of 250 °C to stabilize the structure of PAN. Then, we performed higher temperature heat treatment at 500 °C in a furnace with hydrogen gas protection to partially carbonize the polymer fiber. After that, the oxide-containing fiber was coated with activated carbon in a diluted sugar solution via hydrothermal carbonization at 200 °C for 8 h. The pressure reached 1.45 MPa in the reaction chamber. The obtained product was tested in view of the dye, Rhodamine B, adsorption using a Vis-UV spectrometer. Electrical property characterization was performed using an electrochemical work station. It was found that the hydrothermally treated oxide-containing fiber demonstrated obvious dye adsorption behavior. The visible light absorption intensity of the Rhodamine B dye decreased with the increase in the soaking time of the fiber in the dye solution. The impedance of the fiber was increased due to the hydrothermal carbonization treatment. We also found that charge build-up was faster at the surface of the specimen without the hydrothermally treated carbon layer. Electricity generation under visible light excitation is more intensive at the hydrothermally treated fiber than at the one without the hydrothermal treatment. This result is consistent with that obtained from the dye adsorption/decomposition test because the charge generation is more efficient at the surface of the hydrothermally treated fiber, which allows the dye to be decomposed faster by the treated fibers with activated carbon.

Pretreatment Processes of Biomass for Biorefineries: Current Status and Prospects

This article seeks to be a handy document for the academy and the industry to get quickly up to speed on the current status and prospects of biomass pretreatment for biorefineries. It is divided into two biomass sources: vegetal and animal. Vegetal biomass is the material produced by plants on land or in water (algae), consuming sunlight, CO2, water, and soil nutrients. This includes residues or main products from, for example, intensive grass crops, forestry, and industrial and agricultural activities. Animal biomass is the residual biomass generated from the production of food from animals (e.g., manure and whey). This review does not mean to include every technology in the area, but it does evaluate physical pretreatments, microwave-assisted extraction, and water treatments for vegetal biomass. A general review is given for animal biomass based in physical, chemical, and biological pretreatments.

An overview of the recent trends on the waste valorization techniques for food wastes

A critical and up-to-date review has been conducted on the latest individual valorization technologies aimed at the generation of value-added by-products from food wastes in the form of bio-fuels, bio-materials, value added components and bio-based adsorbents. The aim is to examine the associated advantages and drawbacks of each technique separately along with the assessment of process parameters affecting the efficiency of the generation of the bio-based products. Challenges faced during the processing of the wastes to each of the bio-products have been explained and future scopes stated. Among the many hurdles encountered in the successful and high yield generation of the bio-products is the complexity and variability in the composition of the food wastes along with the high inherent moisture content. Also, individual technologies have their own process configurations and operating parameters which may affect the yield and composition of the desired end product. All these require extensive study of the composition of the food wastes followed by their effective pre-treatments, judicial selection of the technological parameters and finally optimization of not only the process configurations but also in relation to the input food waste material. Attempt has also been made to address the hurdles faced during the implementation of such technologies on an industrial scale.