Technical Analysis of Co-pyrolysis: Why Combining Rice Husk and Wood Produces Better Biochar

Against the backdrop of energy shortages and increasingly severe environmental pollution, the search for renewable energy solutions derived from agricultural waste is attracting significant attention from the scientific community and the business sector. In particular, the technology thermal decomposition (co-pyrolysis), which combines rice husks and wood to produce biochar, is yielding superior results compared to traditional pyrolysis methods.

Vietnam, with over 7 million tons of rice husks and millions of tons of wood waste generated annually, faces a significant opportunity to transform "waste" into a valuable resource. This article will provide an in-depth analysis of co-pyrolysis technology, explain why combining rice husks and wood produces higher-quality biochar, and offer specific technical specifications for researchers and businesses interested in the field.

An Introduction to Pyrolysis and the Potential of Agricultural Feedstocks

The context of energy shortages and the issue of agricultural waste management in Vietnam

Vietnam is an agricultural country with more than 4 million hectares of rice fields, producing approximately 7–8 million tons of rice husks per yearMeanwhile, the wood processing industry also generates millions of tons of sawdust, bark, and other waste products. Most of this waste is burned directly in the fields or at processing facilities, causing severe air pollution and wasting a potential resource.

According to data from the Ministry of Agriculture and Rural Development, only about 30% of rice husks are utilized as animal feed or simple fuel. The remainder becomes an environmental burden. Similarly, wood waste has not yet been effectively utilized, while the demand for clean fuel and soil-improving materials continues to rise.

The concept of co-pyrolysis and its differences from single pyrolysis

Pyrolysis (pyrolysis) is the thermal decomposition of organic compounds in an oxygen-free or oxygen-deficient environment at temperatures ranging from 300 to 700°C. This process produces three main products: biochar, bio-oil, and syngas.

Co-coking is an advanced pyrolysis technique in which two or more different types of biomass feedstocks are pyrolyzed simultaneously in the same reactor. What sets this method apart is that these feedstocks are not merely mixed together but also produce synergy effect (synergistic effect) — that is, the chemical interactions between them improve product quality and yield.

The key difference lies in the fact that, in simple pyrolysis, each type of feedstock decomposes independently according to its own characteristics. However, in co-pyrolysis, the chemical components from one feedstock can catalyze or modulate the decomposition process of the other feedstock, resulting in a final product with properties superior to the sum of its individual parts.

Why are rice husks and wood a promising combination of raw materials for biochar?

Rice husks and wood have properties that complement each other perfectly:

Rice husks contains high silica (SiO₂) (15–20%), creating a unique porous structure for biochar, but it has a low carbon content and high ash content.
Wood rich lignin and cellulose, provides an abundant source of carbon, resulting in high-strength charcoal with good carbon retention, but the raw material costs are higher.

When combined, rice husks provide a porous structure and catalytic minerals, while wood contributes carbon and thermal energy. This combination not only reduces raw material costs but also produces biochar that large surface area, high adsorption capacity, and optimal fixed carbon content - properties that are ideal for both soil remediation and pollution treatment.

Analysis of Raw Material Properties: Rice Husk and Wood in Pyrolysis

Chemical composition of rice husks: High silica content and characteristic ash content

Rice husks have a unique chemical composition that differs from most other types of biomass:

Elemental analysis (% dry weight):

Carbon (C): 35–40%
Water (H): 4–5%
Oxygen (O): 35–40%
Nitrogen (N): 0.3–0.8%
Sulfur (S): <0.2%
Ash: 15–25%

The most distinctive feature of rice husks is extremely high ash content (15–25%), of which silica accounts for 90–95% of the total ash content. This silica exists in an amorphous form, creating a unique cellular structure in the rice husk. During pyrolysis, the silica does not volatilize but retains its structure, forming a skeletal framework for biochar with a large surface area.

However, the high ash content is also a major limitation of simple rice husk pyrolysis. Biochar from rice husks has low fixed carbon content (typically <50%), reducing both soil carbon sequestration and fuel efficiency.

Characteristics of wood: Lignin, cellulose, and the ability to sequester carbon

Wood and wood waste have a chemical composition that is much more suitable for the production of biochar:

Elemental analysis (% dry weight):

Carbon (C): 48–52%
Water (H): 5.5–6.5%
Oxygen (O): 40–44%
Nitrogen (N): 0.1–0.5%
Sulfur (S): <0.1%
Ash: 0.5–3%

Structural components:

Cellulose: 40–50%
Hemicellulose: 20–30%
Lignin: 20–30%
Extract content: 2–5%

Lignin is the most important component in biochar production. It is a complex aromatic polymer with a stable benzene ring structure that is resistant to decomposition at high temperatures. Upon pyrolysis, lignin produces high carbon content (typically >70% in coal), providing durability and good carbon retention.

Cellulose and hemicellulose, although more easily degradable, also contribute to charcoal yield and create a porous structure through the evaporation of volatile compounds.

Comparison of the thermodynamic properties of the two types of materials

Thermogravimetric analysis (TGA) reveals distinct differences in pyrolysis behavior:

Rice husks:

Decomposition temperature: 200–250°C
Maximum decomposition temperature: 300–350°C (primarily cellulose and hemicellulose)
Rate of weight loss: Rapid in the 250–400°C range
Residue at 600°C: 35–45%

Wood:

Decomposition temperature: 220–280°C
Maximum decomposition temperature: 340–380°C (cellulose) and 400–500°C (lignin)
Rate of weight loss: Moderate, extending up to 500°C
Residue at 600°C: 25–35%

These differences in temperature and decomposition rates form the basis for the synergistic effect in pyrolysis. Intermediate products from the decomposition of rice husk cellulose can interact with the lignin in wood, while silica from rice husks can catalyze the carbonization process.

Limitations of pyrolyzing each type of raw material individually

Single-step rice husk pyrolysis:

Low fixed carbon content (<50%)
Ash content is too high (>40% in coal)
Poor carbon sequestration capacity in soil
Low coal yield (20–30%)
Low thermal value

Simple wood pyrolysis:

High raw material costs
The BET surface area is lower than expected
Lack of trace minerals necessary for soil improvement
The ability to adsorb heavy metals is limited due to the lack of oxygen-containing functional groups

These limitations mean that neither material achieves optimal performance when used alone, creating an urgent need for a co-pyrolysis solution.

Synergistic mechanisms in rice husk-wood co-pyrolysis

Interactions between cellulose and lignin at high temperatures

During the pyrolysis process, complex interactions occur between the components of rice husks and wood:

The 200–300°C range: Hemicellulose from both feedstocks begins to decompose, producing intermediate compounds such as furfural, acetic acid, and oligomers. These compounds do not evaporate immediately but interact with the wood’s lignin, forming more complex carbon structures.

300–400°C range: Cellulose undergoes extensive decomposition, producing levoglucosan and anhydrosugars. Under simple pyrolysis conditions, these compounds typically volatilize into gases and oils. However, under co-pyrolysis, Silica derived from rice husks acts as a catalyst, promoting the depolymerization and carbonization reactions, thereby retaining more carbon in the solid phase.

400–600°C range: Lignin continues to carbonize. Free radicals generated from the decomposition of lignin interact with the silica-carbon structure of rice husks, forming a high-strength hybrid carbon-silica network with a unique porous structure.

The role of silica in rice husks on the structure of biochar

Silica is not only an inert component during pyrolysis but also plays several important roles:

Catalytic role: Amorphous silica has a surface rich in silanol groups (Si-OH), which act as weak acid catalyst. They promote dehydration, repolymerization, and carbonization reactions, thereby increasing coal yield and fixed carbon content.

Structural role: The silica framework from rice husks remains intact after pyrolysis, resulting in microvascular and medium-sized capillary structures (2–50 nm). When carbon from wood is deposited onto this structure, it forms a silica-carbon composite material with a BET surface area that can reach 200–400 m²/g, which is significantly higher than that achieved by simple pyrolysis (typically <150 m²/g).

Thermal stabilization role: Silica enhances the thermal stability of biochar, preventing oxidation and secondary decomposition at high temperatures.

Cross-catalytic effects and the ash-carbon balance

The most important synergy effect in rice husk-wood co-pyrolysis is the balance between ash content and carbon content:

Pure rice husk: Excessively high ash content (>40%), low carbon content (<50%)
Pure wood: High carbon content (>75%), but ash content is too low (<5%), lacking in minerals
Co-pyrolysis (40:60 rice husk to wood ratio): Ash 15–25%, carbon 60–70% - the optimal range for both agricultural and environmental applications

A moderate ash content offers many benefits:

Supplies essential minerals (K, P, Ca, Mg) to the soil
Raise the pH, improve acidic soil
Create adsorption sites for heavy metals and organic compounds
Increase the stability of carbon in the soil (carbon sequestration lasting up to hundreds of years)

The TGA/DTG thermal analysis chart illustrates the synergy

TGA/DTG (differential thermal analysis) studies have clearly demonstrated a synergistic effect:

Key observation:

Maximum decomposition temperature range: In pyrolysis, the DTG peak (maximum mass loss rate) typically shifts to a temperature 10–20°C higher than the average value calculated from the two individual feedstocks. This indicates increased thermal stability.
Coal production is higher than expected: The residual mass at 600°C in isothermal pyrolysis is typically 5–15% higher than the theoretical value calculated based on the mixing ratio. For example, with a 50:50 rice husk to wood ratio, the theoretical charcoal yield is 35%, but the actual yield ranges from 38% to 42%.
Changing the shape of the curve: The DTG curve of the mixture is not a simple sum of the two individual curves; instead, it exhibits additional peaks or changes in peak intensity, indicating a chemical interaction.

This evidence from thermal analysis provides solid scientific proof of the synergistic effect in the co-pyrolysis of rice husks and wood.

Optimal blending ratio: Quantitative analysis

An experimental study on common rice husk-to-wood ratios

Studies both in Vietnam and abroad have tested various mixing ratios. The three most commonly studied ratios are:

30:70 ratio (rice husk:wood) - "Wood-rich":

Coal yield: 28–32%
Fixed carbon content: 65–72%
Ash content: 12–18%
BET surface area: 180–250 m²/g
Advantages: High carbon content, suitable for use as fuel and for carbon sequestration
Disadvantages: Higher raw material costs, lower mineral content

50:50 ratio - "Balance":

Coal yield: 32–38%
Fixed carbon content: 58–65%
Ash content: 18–25%
BET surface area: 220–320 m²/g
Advantages: Good balance between carbon and minerals, highest surface area
Weakness: Does not excel in any single category

A 70:30 ratio - "Rich in husks":

Coal yield: 35–42%
Fixed carbon content: 52–58%
Ash content: 25–32%
BET surface area: 200–280 m²/g
Advantages: Lowest cost, high mineral content, high coal yield
Disadvantages: Lower carbon content, lower calorific value

The Effect of Blending Ratios on Coal Performance and BET Surface Area

A detailed analysis reveals a nonlinear relationship between the blending ratio and the properties of the coal:

BET surface area: The optimal ratio is between 40:60 and 50:50 (rice husk:wood). At this ratio, the silica structure from the rice husk and the carbon from the wood form an optimal combination, resulting in the best-developed pore system. Increasing or decreasing the rice husk ratio reduces the surface area.

Coal production: Yield increases linearly with rice husk content, due to the high ash content of rice husks. However, actual yields are always 3–8% higher than theoretical calculations within the ratio range of 30:70 to 60:40, indicating that the synergistic effect is strongest in this range.

Iodine adsorption capacity: This index reflects the surface area of the micropores and the adsorption capacity for small molecules. The best values are achieved at a ratio of 45:55 to 55:45, with an iodine adsorption capacity of 600–800 mg/g, equivalent to that of low-grade commercial activated carbon.

Coal quality assessment: Fixed carbon content, adsorption capacity

The quality of biochar is assessed based on several criteria:

Approximate analysis:

Moisture content: <5% (all grades)
Volatile matter: 15–30% (decreases as the wood content increases)
Fixed carbon: 52–72% (increases with higher wood content)
Ash: 12–32% (increases as the rice husk content increases)

Elemental analysis:

H/C ratio: 0.3–0.6 (lower values indicate higher aromatic content and more durable coal)
O/C ratio: 0.15–0.35 (lower values are better for carbon retention; higher values are better for adsorption)
Ratio (O+N)/C: <0.4 (soil stability index)

Activated carbon produced by pyrolysis with a ratio ranging from 40:60 to 50:50 exhibits optimal values for these parameters: low enough to ensure durability, yet high enough to maintain the surface functional groups necessary for adsorption and cation exchange.

Heavy metal adsorption capacity:

Pb²⁺: 80–150 mg/g
Ca²⁺: 40–80 mg/g
Cu²⁺: 50–100 mg/g

This adsorption capacity is 50–100% higher than that of carbon from simple pyrolysis, thanks to the combination of a large surface area, oxygen-containing functional groups, and minerals from the ash.

Recommended ratio based on the intended use of biochar

For agricultural land reclamation: Ratio 50:50 to 60:40 (rice husk: wood)

Reason: A good balance between carbon (for durability) and minerals (for plant nutrition)
Recommended application rate: 5–20 tons per hectare
Benefits: Increases water-holding capacity by 20–40%, increases CEC (cation exchange capacity) by 30–50%, and improves soil structure

For wastewater treatment and pollutant adsorption: Ratio 40:60 to 50:50

Reason: Highest BET surface area, numerous surface functional groups
It can be further activated using steam or CO₂ to increase efficiency
High adsorption capacity for dyes, pharmaceuticals, and heavy metals

For biofuel production: Ratio 30:70 to 40:60 (rice husk: wood)

Reason: High fixed carbon content, high calorific value (20–25 MJ/kg)
Moderate ash content, reducing slag formation during combustion
Tablets can be compressed to increase energy density

For long-term carbon storage: Ratio 30:70 (rice husk: wood)

Reason: Lowest H/C and O/C ratios, highest durability
Carbon sequestration duration in soil: >500 years
Suitable for carbon credit projects

Performance Comparison: Co-pyrolysis vs. Single Pyrolysis

Biochar Yield: Specific Comparative Data

Comparison table of coal yield at 500°C with a residence time of 2 hours:

Ingredients	Coal yield (%)	Synergy effect
Single-layer rice husk	38–42%	-
Solid wood	26–30%	-
Theory (50:50)	32–36%	-
Equal parts (50:50)	36–40%	+8–12%

The synergy effect increases coal yield by 2–4 percentage points in absolute terms, equivalent to an 8–12% relative increase. With an annual production capacity of 1,000 tons of raw material, this translates to an additional 30–40 tons of biochar, equivalent to an additional profit of 60–120 million VND per year (at a biochar price of 2–3 million VND per ton).

Product quality: Durability, ability to sequester carbon in the soil

Chemical resistance: Pyrolytic carbon exhibits significantly higher durability:

H/C ratio: 0.35–0.45 (compared to 0.5–0.7 for rice husks alone and 0.3–0.4 for wood alone)
O/C ratio: 0.20–0.28 (good balance between durability and surface activity)
Aromatic carbon content: 65–75% (compared to 45–55% for rice husks alone and 70–80% for wood alone)

Carbon sequestration capacity: Mean residence time (MRT) of carbon in soil:

Single-stemmed rice straw: 50–150 years
Solid wood: 200–500 years
Thermal decomposition (40:60): 300–800 years

Pyrolytic charcoal combines the chemical durability of wood with the physical protection provided by the silica structure of rice husks, resulting in a material with superior carbon sequestration capabilities.

Mechanical strength: Pyrolytic charcoal has 40–60% greater hardness and fracture resistance than plain rice husks, making it easier to transport and store. Silica acts as a "cement" that binds the carbon structures together.

Economic efficiency: Raw material and energy costs

Analysis of raw material costs (prices in Vietnam):

Rice husks: 300–500 VND/kg (very cheap; some places give them away for free)
Wood scraps: 800–1,200 VND/kg
50:50 blend: 550–850 VND/kg - 30–40% cheaper than solid wood

Energy costs: Cogeneration is truly energy-efficient:

Pyrolysis of rice husks: 1.2–1.5 MJ/kg of feedstock
Pyrolysis of raw wood: 1.5–1.8 MJ/kg
Catalytic cracking: 1.3–1.6 MJ/kg - Save 5–10%

Reason: Rice husks have a lower moisture content than wood (8–10% vs. 12–15%), and the exothermic reaction offsets some of the input energy.

Economic feasibility analysis (scale: 1,000 tons per year):

Data for a 50:50 ratio of rice husks to wood:

Material costs: 550–850 million VND
Energy costs: 150–200 million
Other expenses (labor, maintenance): 100–150 million
Total cost: 800–1,200 million VND
Coal production: 360–400 tons
Revenue (at a price of 2.5 million per ton): 900–1,000 million
Other revenue (biofuel, gas): 100–150 million
Total revenue: 1,000–1,150 million
Estimated profit: 0–200 million VND per year (depending on management effectiveness)

Compared to pyrolysis of wood alone (which typically results in a loss due to high raw material costs) or rice husks alone (which yield low-value products), co-pyrolysis offers significantly better profitability.

Environmental Impact: Reducing Emissions and Reusing Waste Materials

Reducing greenhouse gas emissions: Each ton of biochar from pyrolysis can:

Sequestration of 0.6–0.7 metric tons of CO₂ equivalent (based on fixed carbon)
Replaces 0.3–0.5 tons of coal (if used as fuel), preventing the emission of 0.9–1.5 tons of CO₂
Improve soil quality, reduce the need for chemical fertilizers, and avoid emissions of 0.2–0.4 metric tons of CO₂ equivalent (from fertilizer production)
Total: 1.7–2.6 metric tons of CO₂ equivalent per metric ton of coal

With an annual coal production of 1,000 tons, it is possible to reduce CO₂ emissions by 1,700–2,600 tons per year, which is equivalent to planting and caring for 80,000–120,000 trees.

Processing of agricultural byproducts:

Reduce the burning of straw in fields to improve air quality
Creating economic value from agricultural byproducts and increasing farmers' income
Reduce landfill pressure and save landfill space

Improving soil and water quality:

Increase soil water-holding capacity by 20–40% and reduce irrigation needs
Reduces fertilizer runoff by 30–50%, protecting water sources
Adsorption of heavy metals and pesticides in soil
Increase beneficial microbial activity

These environmental impacts are not only ecologically significant but also generate substantial indirect economic value, particularly in the context of the growing carbon credit market.

Technical specifications and optimal operating conditions

Ideal pyrolysis temperature and heating rate

Pyrolysis temperature: The optimal temperature for co-pyrolysis of rice husks and wood depends on the intended use:

400–450°C: Charcoal for agricultural use
- Highest coal yield (40–45%)
- Retains many surface functional groups (carboxyl, hydroxyl)
- High O/C ratio, good for CEC and water retention
- pH: 7–9
500–550°C: Versatile, best balance
- Coal yield: 35–40%
- Highest BET surface area (250–350 m²/g)
- Balancing durability and surface activity
- pH: 8–10
- This is the recommended temperature for most applications
600–650°C: High-strength coal that retains carbon
- Coal yield: 30–35%
- Highest fixed carbon content (>70%)
- A long-lasting fragrance
- pH: 9–11
- Suitable for carbon credits and pollution control

Heating rate:

Slow heating (5–10°C/min): Improves coal yield and allows synergistic reactions to proceed fully
Moderate heating (10–20°C/min): Balancing coal production and biofuel production
Rapid heating (>50°C/min): Prioritize biofuels and natural gas; reduce coal production

For the production of biochar via pyrolysis, the following recommendations are made heating rate of 10–15°C per minute to optimize both coal production and quality.

Residence time and reaction atmosphere

Time at peak temperature:

30–60 minutes: Sufficient for the main thermal decomposition reaction, but the synergistic effect is not yet fully realized
60–120 minutes: Optimized for isothermal decomposition, allowing secondary reactions to proceed fully
>120 minutes: Does not significantly improve quality; wastes energy

Recommendation: 90–120 minutes at the peak temperature for best results.

Reactive atmosphere:

Nitrogen (N₂): Standard inert gas, easy to control, producing high-quality coal. Reasonably priced (3,000–5,000 VND per kilogram of coal).
Carbon dioxide (CO₂): It has a mild activating effect, increasing the surface area by 10–20%. It is less expensive than N₂.
Water vapor (H₂O): Highly activated, increasing surface area by 50–100%, but reducing coal yield by 15–25%. Used for the production of activated carbon.
Self-generated: Use the gas generated from the pyrolysis process to maintain a reducing atmosphere. This is the most cost-effective option and is suitable for small-scale operations.

For medium- and large-scale production, we recommend using autotrophic system supplemented with N₂ to strike a balance between cost and quality.

Feed particle size and initial moisture content

Particle size:

<2 mm: Large surface area, rapid heat transfer, but prone to being carried away by the gas, making it difficult to control
2–5 mm: Optimized for pyrolysis, ensuring good contact between the two materials
5–10 mm: Acceptable, but it needs to be stored for a longer period of time
>10 mm: Poor heat transfer, uneven reaction, and the core may not be fully carbonized

Ingredient preparation process:

Dry until the moisture content is less than 10%
Grind or chop to a size of 2–5 mm
Mix the rice husks and wood in the desired ratio
It can be pelletized to increase density and facilitate transportation

Initial moisture content:

<5%: Ideal, but drying costs are high
5–10%: Optimal, striking a balance between efficiency and cost
10–15%: Acceptable, but coal production will decrease by 3–5%
>15%: Not recommended; consumes a lot of energy; lowers the oven temperature

Rice husks typically have a natural moisture content of 8–10%, while green wood can have a moisture content as high as 40–50%. The wood must be dried to a moisture content of 10–15% before being mixed with rice husks.

Pyrolysis equipment suitable for small-scale and industrial applications

Small-scale (100–500 kg per batch) – Suitable for households and cooperatives:

Drum kiln:

Construction: 200-liter steel drum with a tight-fitting lid and a vent pipe
Advantages: Simple, low cost (5–10 million), easy to operate
Disadvantages: Difficult to control the temperature; inconsistent coal quality
Capacity: 30–50 kg of coal per batch (8–10 hours)

Simple retort autoclave:

Construction: Stainless steel pyrolysis chamber housed within the combustion chamber
Advantages: Better temperature control, consistent quality
Disadvantages: Higher cost (15–30 million)
Capacity: 50–100 kg of coal per batch (6–8 hours)

Medium-sized (0.5–2 tons per batch) – Suitable for small businesses:

Rotary drum kiln:

Construction: Slow-rotating steel drum, externally heated
Advantages: Even mixing, uniform pyrolysis, continuous or semi-continuous
Disadvantages: Requires a rotating motor; more complex maintenance
Cost: 100–300 million
Capacity: 200–500 kg of coal per batch (4–6 hours)

Fixed-bed reactor:

Construction: Cylindrical pyrolysis chamber, layered feedstock
Advantages: Simple, cost-effective, and scalable
Disadvantage: Requires a long cooling time
Cost: 80–200 million
Capacity: 300–800 kg of coal per batch (6–8 hours)

Industrial scale (>2 tons per batch or continuous):

Continuous rotary kiln:

Construction: A 5–15-meter-long steel drum, inclined, rotating slowly, with material moving from one end to the other
Advantages: Continuous production, high productivity, automation
Disadvantages: High initial investment costs, requires advanced operational expertise
Cost: 2–10 billion VND
Capacity: 500–2,000 kg of coal per hour

Fluidized bed reactor:

Construction: The material is "stirred" by an upward flow of gas
Advantages: Extremely fast and even heat transfer, high efficiency
Disadvantages: Complex, high cost, difficult to process materials of uneven size
Cost: 5–20 billion VND
Capacity: 1,000–5,000 kg of coal per hour
Suitable for very large-scale production

Recommendations for the Vietnamese context:

Households, cooperatives: Open-air kilns or simple retorts
Small businesses: Fixed-batch kilns or drum-type rotary kilns
Medium and large enterprises: Continuous rotary kilns
Only invest in a fluidized-bed furnace if the capacity exceeds 10 tons per day and you have a strong technical team

Practical applications of biochar from pyrolysis

Agricultural Land Improvement: Enhancing Fertility and Water Retention

Biochar produced through the pyrolysis of rice husks and wood is an excellent soil amendment:

Improving physical properties:

Increases soil porosity by 15–30%, improves drainage in clay soils and water retention in sandy soils
Reduces soil density by 10–20%, promoting root growth
Increases water retention by 25–45% and reduces watering frequency by 30–40%
Improve soil structure and increase the proportion of crumb-like soil

Improving chemical properties:

Increases the pH of acidic soil by 0.5–1.5 units and reduces aluminum toxicity
Increases CEC (cation exchange capacity) by 20–60%, helping to retain nutrients
Provides minerals: K, P, Ca, Mg, Si from ash
Reduce nitrogen leaching by 30–50% and improve fertilizer use efficiency

Improving biological properties:

Increase microbial biomass by 40–80%
Increase enzyme activity in the soil
Provides a habitat for beneficial microorganisms within the porous structure
Enhance biological nitrogen fixation

Experimental results in Vietnam: In the Mekong Delta, trials on rice using a dosage of 10 tons of biochar per hectare:

Increase productivity by 12–18%
Reduce nitrogen fertilizer by 20–25% while maintaining equivalent yields
Increase net income by 8–12 million VND per hectare per crop season
The results last for at least 3–5 years

Pollution Control: Adsorption of Heavy Metals and Toxic Organic Compounds

With a high surface area and a highly developed porous structure, biochar from pyrolysis is an effective adsorbent:

Treatment of heavy metals in soil:

Adsorption and immobilization of Pb, Cd, Cu, Zn, and Cr
Reduces bioavailability by 50–80%
Preventing heavy metals from entering crops
Applications: Remediation of soil contaminated by mining, industrial activities, and irrigation with wastewater

Wastewater treatment:

Dye adsorption: 80–95% efficiency with methylene blue and rhodamine B
Excluded medications: Antibiotics, hormones
Treatment of heavy metals in water: Pb²⁺, Cd²⁺, Cu²⁺
Removing color and odor from water

Exhaust gas treatment:

Adsorption of VOCs (volatile organic compounds)
Removing H₂S and NH₃ from livestock operations
CO₂ adsorption (potential for carbon capture)

Advantages over commercial activated carbon:

Costs are 60–80% lower (200,000–500,000 VND per ton vs. 2–5 million VND per ton)
Environmentally friendly, made from agricultural byproducts
After adsorption, it can be used as fertilizer (for certain pollutants)
Suitable for large-scale, low-cost processing

Renewable energy and biofuel production

Biochar from pyrolysis has a calorific value of 18–25 MJ/kg, equivalent to 60–80% of that of coal:

For use as a direct-burn fuel:

Replacing coal and firewood in small industrial furnaces
Co-firing with coal in power plants (5–20% biomass)
Fuel for brick kilns, lime kilns, and agricultural product drying

Pellet production:

Tablets measuring 6–8 mm in diameter and 10–30 mm in length
High energy density (15–20 GJ/m³)
Easy to transport, store, and use automatically
Selling price: 3–4 million per ton
Exports to Japan, South Korea, and the EU

Production of activated carbon:

Activation using steam or CO₂ at 700–900°C
Increase the surface area to 800–1,500 m²/g
Value increases 5- to 10-fold (10–30 million per ton)
Applications: Water filtration, air filtration, food industry, pharmaceutical industry

Valuable by-products:

Biofuel: 20–35% yield, which can be refined into fuel or chemicals
Synthetic gas: 25–40% of the output is used for combustion to provide heat for the pyrolysis process (self-sufficient energy supply)

Case Study: A Successful Implementation Model in Vietnam or the Region

Case Study 1: Vĩnh Long Agricultural Cooperative

Scale: 50 farming households, 200 hectares of rice
Investment: 1 fixed-bed pyrolysis furnace with a capacity of 500 kg per batch (150 million VND)
Ingredients: Rice husks from the cooperative’s mill + waste acacia wood (50:50 ratio)
Production: 5 tons of biochar per month
Usage: 70% is used for soil improvement for cooperative members, and 30% is sold on the market
Results after 2 years:
- Reduce fertilizer costs by 15–20%
- Increase rice yields by 10–15%
- Revenue from coal sales: 60–80 million per year
- Reduce the burning of straw and rice husks, improve the environment
- The model has been replicated in three other cooperatives in the province

Case Study 2: Dong Nai Biochar Co., Ltd.

Capacity: A plant with a capacity of 10 tons of raw material per day
Investment: Continuous rotary kiln + exhaust gas treatment system (3.5 billion VND)
Ingredients: Rice husks + sawdust from woodworking shops (40:60 ratio)
Product:
- Biochar for soil improvement: 2.5 tons per day
- Activated carbon for wastewater treatment: 1 ton per day
- Biodiesel: 1.5 tons per day (sold to a chemical company)
Revenue: 4–5 billion per year
Profit: 800–1,000 million per year
Job creation: 15 stable jobs
Processing capacity: 3,000 tons of waste per year, reducing CO₂ emissions by 1,500 tons per year

These two case studies demonstrate the feasibility of pyrolysis technology at both the small-scale (community) and commercial levels, with clear economic and environmental benefits.

Conclusions and Future Directions

Summary of the Key Advantages of Rice Husk-Wood Co-pyrolysis

Through a comprehensive analysis, the co-pyrolysis technology using rice husks and wood has demonstrated significant advantages:

Technically:

The synergy effect increases coal productivity by 8–12% compared to theoretical levels
Superior coal quality: 50–100% higher surface area, optimal balance between carbon and minerals
A unique silica-carbon composite structure that combines the advantages of both materials
Adsorption capacity and soil improvement are 40–80% better than with simple pyrolysis

Economically:

Reduces raw material costs by 30–40% compared to using wood alone
5–10% energy savings
Create products with higher market value
Good profitability, payback period of 2–4 years

From an environmental perspective:

Effective Management of the Two Main Types of Agricultural Byproducts
Reduces CO₂ emissions by 1.7–2.6 metric tons per metric ton of biochar produced
Storing carbon in the soil for hundreds of years
Reducing air pollution from straw burning
Improving soil and water quality

Socially:

Creating economic value from agricultural byproducts and increasing farmers' income
Creating new jobs in the biochar value chain
Contributing to the development of sustainable agriculture and a circular economy

Challenges to be addressed: Technology, policy, and the market

Although it has many advantages, this technology still faces a number of challenges:

Technological challenges:

There is a shortage of small- and medium-scale pyrolysis equipment suitable for conditions in Vietnam
There are currently no clear quality standards for biochar
Lack of a database on optimal mixing ratios for each type of local raw material
Further research is needed on the persistence and long-term effects of coal in soil

Policy challenges:

There are currently no clear policies to encourage the production and use of biochar
Lack of a carbon credit mechanism for biochar projects
The manufacturing licensing process remains complex
There is currently no financial or technological support for small businesses

Market challenges:

Farmers' awareness of the benefits of biochar remains limited
Biochar is more expensive than traditional organic fertilizer, making it difficult to adopt initially
The raw material supply chain remains unstable
Lack of effective distribution and marketing channels

Recommendations for stakeholders

For researchers:

Continue researching the optimal blending ratios for other types of local raw materials (bagasse, corn cobs, coffee husks, etc.)
Developing small-scale, low-cost pyrolysis equipment suitable for rural areas in Vietnam
Research on the application of biochar in new fields: batteries, construction materials, and catalysts
Develop a database on long-term environmental and economic impacts
International cooperation to access advanced technology

For businesses:

Start small and test the market before expanding
Establishing a stable raw material supply chain and long-term contracts with farmers
Product diversification: soil-remediation charcoal, pollution-control charcoal, activated charcoal, fuel pellets
Invest in marketing and educate customers about the benefits of biochar
Seeking international certification (IBI Biochar Certification) to access export markets
Participate in the carbon credit market to generate additional revenue

For policymakers:

Issue clear quality standards for biochar (TCVN)
Include biochar in the list of recommended fertilizers
Providing preferential loans and tax breaks for biochar-producing businesses
Establishing a carbon credit mechanism for biochar projects
Incorporating biochar technology into the national agricultural extension program
Support for research and technology transfer
Facilitating connections between producers, researchers, and farmers

For farmers:

Participate in training programs and biochar trials
Start with a small area to assess effectiveness
Forming a cooperative to invest in coal production equipment
Use biochar in combination with organic fertilizer and microbial fertilizer to maximize effectiveness
Keep records and track results to provide a basis for adjustments

Prospects for the Development of Biochar Technology in Vietnam

Vietnam has great potential for the development of the biochar industry:

Abundant raw materials: With over 7 million tons of rice husks and millions of tons of other agricultural and forestry byproducts each year, Vietnam could produce 2–3 million tons of biochar annually, creating an industry worth 5–10 trillion dong.

Rising market demand: With 9.5 million hectares of agricultural land, much of which is degraded, acidic, or saline, there is a significant need for land reclamation. The market for pollution treatment is also growing rapidly.

In line with global trends: Sustainable agriculture, a circular economy, and reducing greenhouse gas emissions are inevitable trends. Biochar is a solution that addresses all three of these goals.

Export opportunities: The global biochar market is growing at a rate of 15–20% per year and is projected to reach $3–5 billion by 2030. Vietnam has the potential to become a major supplier of biochar to the region.

Contributing to the Net Zero goal: The Vietnamese government is committed to achieving net-zero emissions by 2050. Biochar is a key tool for carbon sequestration, reducing emissions from agriculture, and generating carbon credits.

To turn this potential into reality, close collaboration is needed among the government, businesses, researchers, and farmers. Given its clear economic, social, and environmental benefits, the technology for co-pyrolysis of rice husks and wood to produce biochar has the potential to become a major green industry, contributing to the sustainable development of Vietnam’s agriculture and rural areas.

Take action today to create a greener future! If you are a researcher, keep digging deeper and sharing your knowledge. If you are a business, consider investing in this technology. If you are a farmer, try using biochar on your land. And if you are a policy maker, create an enabling environment for this green technology to thrive. Together, we can turn waste into a resource and build a sustainable agriculture system for future generations.