At its core, hydrochar is a carbon-rich solid material produced from biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More. But what makes it distinct from other chars, like biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More? The key lies in its production process: hydrothermal carbonization (HTC) or hydrothermal gasificationGasification is a high-temperature, thermochemical process that converts carbon-based materials into a gaseous fuel called syngas and solid by-products. It takes place in an oxygen-deficient environment at temperatures typically above 750°C. Unlike combustion, which fully burns material to produce heat and carbon dioxide (CO2​), gasification More. Unlike biochar, which is typically derived from thermochemical processes like pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More in the absence of water, hydrochar is created in the presence of subcritical or supercritical water. This wet conversion technology eliminates the energy-intensive drying step often required for wet biomass, making it a promising and environmentally friendly alternative.

Biomass, composed of cellulose, hemicellulose, and lignin, undergoes a series of complex chemical reactions during HTC, including hydrolysis, isomerization, dehydration, and finally polymerization, to form hydrochar. This process occurs at relatively lower temperatures, typically 180^∘C to 240^∘C, under subcritical water pressures.

Hydrochar and its preperation (Petrović et al., 2024)

Characteristics of hydrochar

The distinctive physicochemical properties that make hydrochar unique and valuable:

• Production Temperature: Hydrochar is produced at lower temperatures (180−240∘C) compared to biochar, which typically requires temperatures between 300−650∘C.
• Moisture Content of FeedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More: HTC is particularly well-suited for wet biomass (moisture content >30 wt.%), as it bypasses the need for a drying step, making it economically viable for such feedstocks.
• Specific Surface Area and PorosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More: Generally, hydrochar exhibits a very low specific surface area and porosity. This is often attributed to the formation of hydrocarbons on its surface, which hinders its application as a contaminant adsorbent and catalyst support. However, this can be significantly improved through activation and functionalization processes.
• Morphology: The surface of hydrochar samples is composed of spherical particles, including more homogeneous particle sizes. In contrast, biochar possesses graphite-like layers, including particles with different size ranges.
• Elemental Composition:
  • Higher H/C and O/C Ratios: As the HTC process occurs at a lower temperature, the carbon conversion is lower than in pyrolysis, resulting in higher atomic H/C and O/C ratios in hydrochar compared to biochar.
  • Total Carbon Content: Hydrochar typically has a total carbon content ranging from 58-64 wt.%, which is generally lower than biochar’s 60-80 wt.%.
• pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More: Hydrochar compared to biochar is slightly acidic, as hydrochar contains more oxygenated functional groups. This is also due to the demineralization of inorganic compositions of biomass in the water media during HTC. Biochar, conversely, is alkaline due to the loss of carboxyl and hydroxyl groups during pyrolysis and the presence of inorganic and metal compounds like Ca and Mg.
• Aromaticity: Hydrochar from HTC produced at a lower temperature (200−250∘C) contains more alkyl moieties, while biochar from pyrolysis produced at higher temperatures (500−600∘C) contains aromatic groups.
• Higher Heating Value (HHV): Hydrochar generally has a higher HHV compared to biochar. This is because hemicellulose, which contains lower energy density, starts decomposing faster during hydrothermal carbonization, leaving lignin with higher energy density in the solid product.
• AshAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More Content: Compared to biochar, which is produced through pyrolysis, hydrochar contains less ash content because inorganic compositions of biomass are demineralized in the water media during the HTC process.

Applications of Hydrochar

Hydrochar, especially activated hydrochar, has a wide variety of applications:

• Agriculture and Crop Improvement: Hydrochar can enhance soil activity by improving aeration, water supply, and mineral availability. While freshly produced hydrochar is hydrophobic, it becomes more hydrophilic over time in soil by interacting with atmospheric oxygen, forming phenolic and carboxylic functional groups, which increases its water holding capacityWater holding capacity is the amount of water that soil can retain. Biochar can significantly increase the water holding capacity of soil, improving its ability to withstand drought conditions and support plant growth.   More, cation exchange capacity, and nutrient retention.
• Pollutant AdsorptionBiochar has a remarkable ability to attract and hold onto pollutants, like heavy metals and organic chemicals. This makes it a valuable tool for cleaning up contaminated soil and water.   More (from Wastewater and Flue Gases): Due to its abundance of oxygen-rich functionality and chemically active functional groups (ketones, COOH, hydroxyl), hydrochar exhibits a higher adsorption capability than biochar for mineral and organic pollutants in aqueous solutions. It has been used to remove methylene blue dye, Congo red dye, copper, and cadmium. Activated hydrochar has also shown promise in adsorbing CO2 from simulated flue gas.
• Catalyst Support: Hydrochar can be modified to add charged surface functionalities, making it suitable as a catalyst material for enzymatic and heterogeneous catalytic reactions. It can serve as a scaffold for enzyme immobilization and a base for sulfonated catalysts and biodiesel production. Composites of hydrochar with materials like zeolite and nickel nanoparticles have also been explored for deoxygenation reactions and hydrogen-rich syngasSyngas, or synthesis gas, is a fuel gas mixture consisting primarily of hydrogen and carbon monoxide. It is produced during gasification and can be used as a fuel source or as a feedstock for producing other chemicals and fuels.   More production.
• Energy Production: Hydrochar is a promising solid fuel that can complement coal. The HTC process increases the lignin content and the C/O ratio in the solid product, leading to an increased higher heating value and hydrophobicity, which enhances its shelf life without biodegradation.
• Carbon Sequestration: Storing biomass-derived hydrochar in soil, also known as carbon capture and storage or carbon sequestration, can lead to a carbon-negative/neutral environment by removing anthropogenic CO2​ from the atmosphere.
• Electrochemical Devices: Hydrochar can be utilized in electrochemical devices such as supercapacitors and batteries. Hydrochar-derived supercapacitors can exhibit high cycle stability and power density. Activated hydrochar and its composites have shown improved specific capacitance and electrochemical performance, making them suitable electrode materials.

Hydrochar represents a versatile and sustainable bioresource with unique properties influenced by its watery production environment. Its distinct characteristics make it a compelling material for various energy, environmental, industrial, and commercial applications.

Reference

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Pavkov, I., Radojčin, M., Stamenković, Z., Bikić, S., Tomić, M., Bukurov, M., & Despotović, B. (2022). Hydrothermal carbonization of agricultural biomass: characterization of hydrochar for energy production. Solid Fuel Chemistry, 56(3), 225-235.http://dx.doi.org/10.3103/S0361521922030077

Kambo, H. S., & Dutta, A. (2015). A comparative review of biochar and hydrochar in terms of production, physico-chemical properties and applications. Renewable and Sustainable Energy Reviews, 45, 359-378.

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