🌊⚡ Reaction Mechanism and Kinetics of Hydrothermal Liquefaction at Sub- and Supercritical Conditions: A Review

🔍 Introduction

The rapid depletion of fossil fuels, rising energy demand, and urgent need to mitigate greenhouse gas emissions have placed biomass conversion technologies in the spotlight 🌱. Among these, Hydrothermal Liquefaction (HTL) has emerged as a promising thermochemical pathway to convert wet biomass (such as algae, agricultural residues, sewage sludge, and food waste) into bio-crude oil—a sustainable alternative to petroleum-derived fuels ⛽.

Unlike pyrolysis or gasification, which require energy-intensive drying of biomass, HTL operates in hot compressed water—often under subcritical (200–374 °C, <22.1 MPa) and supercritical (374–600 °C, >22.1 MPa) conditions. The unique solvent properties of water under these conditions drive a series of complex reaction mechanisms including hydrolysis, decarboxylation, dehydration, condensation, and polymerization.

This review explores in depth:

⚛️ Reaction pathways in HTL
📈 Kinetics and rate-controlling steps
🔄 Differences between subcritical and supercritical conditions
🛠️ Factors influencing yield and selectivity
🌍 Future perspectives in scaling and optimization

🌡️ Hydrothermal Liquefaction: An Overview

HTL is often called “hydrothermal upgrading” because it mimics natural geological processes that convert biomass into fossil fuels—but on a much shorter timescale ⏱️.

⚙️ Process Conditions:

Subcritical water (200–350 °C, 5–20 MPa) → water remains liquid but exhibits enhanced ionic product (Kw), favoring acid/base catalyzed reactions.
Supercritical water (>374 °C, >22.1 MPa) → water loses polarity, becomes gas-like, and favors radical and free-radical chain reactions.

🛢️ Products:

Bio-crude oil (liquid fuel)
Aqueous phase (rich in organics like acids, sugars, alcohols)
Gas phase (mainly CO₂, with minor CH₄, H₂, CO)
Solid char (carbon-rich residue)

⚛️ Reaction Mechanisms in HTL

The HTL process is governed by multiple parallel and consecutive reactions. The mechanism depends strongly on whether the process occurs in subcritical or supercritical regimes.

1️⃣ Hydrolysis (Initial Breakdown) 💧

At the first stage, biopolymers such as cellulose, hemicellulose, proteins, and lipids undergo hydrolysis into smaller oligomers and monomers.

Carbohydrates (cellulose, starch):
→ Hydrolyzed into glucose, xylose, arabinose.
Proteins:
→ Hydrolyzed into amino acids.
Lipids:
→ Hydrolyzed into glycerol and fatty acids.

🧪 Example:

Cellulose + H₂O → Glucose

2️⃣ Dehydration and Decarboxylation 🚰💨

After hydrolysis, monomers undergo secondary reactions:

Dehydration (removal of water) → leads to furans, aldehydes, and ketones.
Decarboxylation (removal of CO₂) → reduces oxygen content, increasing energy density of bio-crude.

🧪 Example:

Glucose → Hydroxymethylfurfural (HMF) + H₂O

3️⃣ Fragmentation and Recombination 🔄

Proteins → Amino acids → Ammonia + Organic acids.
Lipids → Fatty acids → Long-chain hydrocarbons via decarboxylation and hydrogenation.
Sugars → Small organic acids (lactic, acetic, formic), alcohols, aldehydes.

4️⃣ Polymerization and Condensation 🧩

Reactive intermediates (furans, phenols, aldehydes) can recombine, leading to polymeric compounds.
This pathway is responsible for char and tar formation—undesirable for liquid fuel but important in carbon sequestration.

5️⃣ Radical and Free-Radical Reactions ⚡

In supercritical water, ionic species are suppressed, and radical chemistry dominates.

Formation of free radicals from hydrocarbons drives cracking and recombination.
This leads to shorter hydrocarbon chains, enhancing fuel quality.

📈 Kinetics of HTL

Understanding HTL kinetics is crucial for reactor design, scale-up, and optimization.

⏳ Reaction Order

Hydrolysis: Often modeled as first-order with respect to biomass.
Secondary reactions: Can be pseudo-first-order or complex parallel-consecutive kinetics.

🔥 Activation Energy (Ea)

Reported activation energies vary widely depending on feedstock:

Carbohydrates: ~120–200 kJ/mol
Proteins: ~100–150 kJ/mol
Lipids: ~50–100 kJ/mol

Lipids are the most reactive, while carbohydrates require higher activation energy.

🧮 Kinetic Models

Parallel First-Order Model:
Biomass → Oil + Gas + Char
Consecutive Model:
Biomass → Intermediates → Oil → Gas/Char
Distributed Activation Energy Model (DAEM):
Accounts for heterogeneous biomass composition.

⏱️ Rate-Controlling Steps

At subcritical conditions, hydrolysis is rate-limiting.
At supercritical conditions, radical chain termination and recombination dominate.

🌡️ Subcritical vs. Supercritical Conditions

🔵 Subcritical HTL (200–350 °C, 5–20 MPa)

Dominant reactions: Hydrolysis, ionic reactions.
Products: Higher yields of oxygenated intermediates (acids, alcohols).
Pros: Mild conditions, selective conversion.
Cons: Higher oxygen content in oil (requires upgrading).

🔴 Supercritical HTL (>374 °C, >22.1 MPa)

Dominant reactions: Radical and free-radical chain reactions.
Products: Deoxygenated hydrocarbons, better-quality oil.
Pros: Higher heating value, lower oxygen content.
Cons: Energy-intensive, higher risk of char formation.

🛠️ Factors Influencing HTL Kinetics

Feedstock Composition 🌱
- Lipids → higher oil yield.
- Proteins → nitrogen-rich compounds (undesirable for fuel).
- Lignin → phenolics, char.
Temperature and Pressure 🌡️
- Higher T → faster reactions, but risk of gas/char formation.
- Pressure ensures water remains liquid.
Catalysts ⚗️
- Alkali salts (Na₂CO₃, K₂CO₃) → enhance deoxygenation.
- Transition metals (Ni, Ru, Pd) → hydrogenation, hydrodeoxygenation.
Residence Time ⏱️
- Short → incomplete conversion.
- Long → secondary cracking, gas/char.
Water-to-Biomass Ratio 💧
- Too low → poor solubilization.
- Too high → energy penalty for heating excess water.

🌍 Applications of HTL

Bio-Crude for Refining 🛢️
- Can be upgraded via hydrotreating to drop-in fuels.
Waste Valorization ♻️
- Municipal waste, sewage sludge, food waste → energy recovery.
Carbon Recycling 🌱
- Captures CO₂ in solid char, contributing to carbon-negative pathways.
Platform Chemicals ⚗️
- Lactic acid, acetic acid, phenols → precursors for bioplastics.

🔮 Future Perspectives

Despite progress, HTL still faces challenges:

Energy efficiency: Supercritical conditions are energy-intensive.
Nitrogen/sulfur content: Needs upgrading for fuel standards.
Corrosion and scaling: Harsh conditions damage reactors.
Scale-up: Most studies are at lab/pilot scale.

Future research must focus on:

Catalyst development (bifunctional, recyclable, selective).
Process intensification (continuous reactors, co-solvents, microwave-assisted HTL).
Life-cycle assessment to evaluate sustainability.
Integration with carbon capture for net-zero fuels.

📝 Conclusion

Hydrothermal Liquefaction is a game-changing pathway for producing sustainable liquid fuels and value-added chemicals from wet biomass 🌱.

In subcritical water, ionic hydrolysis and dehydration dominate.
In supercritical water, radical-driven cracking and recombination occur.
Kinetics are complex but generally involve first-order or parallel-consecutive models.
Future progress lies in catalyst innovation, energy integration, and industrial-scale deployment.