What is the technical mechanism of the hydrothermal carbonization (HTC) process? Transform Waste into High-Value Hydrochar

The technical mechanism of hydrothermal carbonization (HTC) relies on subjecting waste mushroom substrates to a sealed, high-pressure liquid-phase environment at 180°C for one hour. This specific combination of heat and pressure triggers a sequence of thermochemical reactions—specifically hydrolysis, dehydration, decarboxylation, and polymerization—which converts raw lignocellulose into a structured, solid hydrochar.

Core Takeaway: By maintaining an aqueous environment under autogenous pressure, the HTC reactor forces the deep structural conversion of biomass without the need for pre-drying. This process triples the material's heavy metal adsorption capacity by developing a complex pore structure and enriching the surface with aromatic and oxygen-containing functional groups.

The Reactor Environment

Controlled Thermal Conditions

The core of the process is the high-pressure hydrothermal reactor, which maintains a constant temperature of approximately 180°C. This temperature is critical for initiating the breakdown of the mushroom substrate's biological structure.

Autogenous Pressure Generation

The reactor operates as a closed system. As the temperature rises, the water and volatile components inside generate "autogenous" (self-generated) pressure, typically ranging between 2 and 10 MPa.

The Subcritical Water State

This high pressure keeps the water in a liquid (subcritical) state, even well above its standard boiling point. This liquid medium allows for efficient heat transfer and acts as a solvent and reactant for the biomass conversion.

The Chemical Mechanism

Step 1: Hydrolysis

Under these conditions, the complex lignocellulose structures found in the mushroom substrate begin to break down. Water molecules cleave the chemical bonds of the biomass macromolecules, reducing them into smaller fragments.

Step 2: Dehydration and Decarboxylation

Following hydrolysis, the material undergoes dehydration (removal of water molecules) and decarboxylation (removal of carboxyl groups as CO2). These reactions remove oxygen and hydrogen from the biomass, effectively increasing its carbon density.

Step 3: Polymerization

The fragmented molecules then re-combine through polymerization. This step reconstructs the carbon skeleton into stable, spherical porous carbon materials known as hydrochar.

Material Transformation and Outcome

Surface Functionalization

The high-pressure environment promotes the formation of specific chemical groups on the hydrochar's surface. Specifically, it increases the presence of aromatic and oxygen-rich functional groups, which are chemically active and vital for binding contaminants.

Pore Structure Development

The process drastically alters the physical topography of the material. It converts the relatively non-porous raw substrate into a material with a highly developed pore structure, significantly increasing its surface area.

Enhanced Adsorption Capacity

These chemical and physical changes turn the waste into an efficient environmental adsorbent. The capacity to adsorb Cadmium ions (Cd2+) increases from 28 mg/L in the raw substrate to 92 mg/L in the resulting hydrochar.

Understanding the Trade-offs

Equipment Requirements

Unlike simple composting or open-air drying, HTC requires specialized high-pressure reactors capable of withstanding 180°C and pressures up to 10 MPa. This increases the initial capital investment and safety complexity of the operation.

Energy and Batch Constraints

While the process eliminates the need for energy-intensive pre-drying of the wet substrate, maintaining the reactor at temperature for one hour requires a consistent energy input. Furthermore, high-pressure reactors often operate as batch systems, which may limit continuous throughput compared to flow-through systems.

Making the Right Choice for Your Goal

This technology is versatile, but your intended outcome dictates how you should view the process data.

• If your primary focus is Environmental Remediation: Leverage the hydrochar's 92 mg/L adsorption capacity, specifically targeting the removal of heavy metals like Cadmium from wastewater.
• If your primary focus is Solid Fuel Production: Focus on the dehydration and decarboxylation phases, as these lower the combustion activation energy and create a more stable, energy-dense fuel source.

The HTC process effectively valorizes mushroom waste by engineering its molecular structure to triple its utility as a heavy metal adsorbent.

Summary Table:

Process Phase	Key Mechanism	Reaction Outcome
Hydrolysis	Subcritical water cleaves bonds	Breakdown of biomass macromolecules
Dehydration/Decarboxylation	Removal of H2O and CO2	Increased carbon density and stability
Polymerization	Recombination of fragments	Formation of stable, porous carbon skeleton
Functionalization	Surface chemistry enrichment	3x increase in heavy metal (Cd2+) adsorption

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References

1. Ivan Savić, Ivana Savić. Microwave-assisted extraction of antioxidants from black locust flowers (Robinia pseudoacacia flos). DOI: 10.21175/rad.abstr.book.2023.45.1

This article is also based on technical information from Kintek Solution Knowledge Base .