Against the backdrop of increasingly severe climate change and growing pressure from agricultural and industrial waste, biochar has emerged as a promising, versatile solution. However, biochar produced from a single feedstock is often limited in terms of functionality and application effectiveness. That is precisely why the technology multifunctional pyrolytic biochar — the simultaneous use of multiple different feedstocks in a single pyrolysis process — is becoming a focal point of research and application worldwide, including in Vietnam.
An Introduction to Pyrolysis Technology and the Role of Biochar in the Green Economy
An Overview of Trends in Sustainable Biochar Production in Vietnam and Around the World
Biochar—a form of charcoal produced through the pyrolysis of biomass in an oxygen-deprived environment—is no longer a foreign concept in the scientific community. Since the 2000s, the number of research studies on biochar has grown exponentially. Today, the global biochar market is valued at over $400 million and is projected to grow significantly by 2030, driven by demand for soil remediation, environmental remediation, and carbon sequestration.
In Vietnam, where the agricultural sector produces tens of millions of tons of byproducts annually—such as straw, rice husks, and sugarcane bagasse—the potential for biochar production is immense. Numerous ministry- and national-level research projects have been launched to harness this source of byproducts. However, most studies remain focused on biochar derived from a single feedstock, failing to fully exploit the value of diverse combinations.
Why is multi-feedstock pyrolysis technology attracting the attention of the scientific community?
Co-pyrolysis technology raises an intriguing question: if we combine multiple types of materials with complementary chemical compositions in the same reactor, will the resulting product be superior to each individual material? The answer, based on a series of international studies, is has, thanks to the synergistic effect between the raw materials. As a result, the biochar produced has a better porous structure, a higher nutrient content, and greater adsorption capacity.
Objectives and scope of application of multifunctional biochar in practice
Multifunctional biochar derived from pyrolysis targets three main application areas: (1) the remediation and restoration of degraded agricultural land, (2) the treatment of wastewater and soil contaminated with heavy metals, and (3) use as a functional material in batteries, capacitors, or as a microbial carrier. This versatility makes pyrolytic biochar a product of high commercial value and aligns with the principles of a circular economy.
Basic Concepts and Principles of the Pyrolysis Process
What is pyrolysis, and what is the difference between simple pyrolysis and isothermal pyrolysis?
Pyrolysis is the thermal decomposition of organic materials at high temperatures (typically 300–700°C) under oxygen-deprived or oxygen-free conditions. The resulting products consist of three phases: biochar (solid phase), bio-oil (liquid) and syngas (gas mixture).
Single-feedstock pyrolysis uses only one type of feedstock, so the properties of the product depend entirely on the chemical composition of that feedstock. In contrast, thermal decomposition (co-pyrolysis) involves the simultaneous use of two or more types of feedstock, creating a more complex but potentially more productive reaction environment.
The chemical reaction mechanism in the simultaneous pyrolysis of multiple feedstocks
When multiple feedstocks are introduced into the reactor simultaneously, the reactions do not occur independently but interact with one another. A typical example: when cellulose- and lignin-rich biomass is combined with protein- and lipid-rich sewage sludge, the free radicals generated from protein degradation can catalyze the cellulose degradation reaction, thereby increasing biochar production efficiency. Additionally, mineral ions in the sludge (Fe, Ca, Mg) act as natural catalysts, promoting deeper carbonization and creating a more refined material structure.
Key technical specifications that affect product quality
The three key technical parameters in pyrolysis include:
- Pyrolysis temperature: Typically between 400–700°C. Higher temperatures produce biochar with higher carbon stability but lower recovery efficiency.
- Residence period: Affects the degree of carbonization. Longer retention times result in more uniform biochar.
- Heating rate: Slow pyrolysis prioritizes biochar production; fast pyrolysis prioritizes bio-oil production.
The synergistic interaction between different feedstocks in the reactor
Synergistic effects are the factor that makes co-pyrolysis superior to the sum of the individual processes. A study by Zhang et al. (2020) showed that co-pyrolysis of rice straw and sewage sludge at a 7:3 ratio yields biochar with a 15–20% higher yield than theoretical calculations from each source alone, while significantly improving the product’s BET specific surface area.
Common feedstocks used in pyrolysis technology
Agricultural biomass: straw, rice husks, bagasse, and crop byproducts
Agricultural biomass is the most abundant and inexpensive raw material source in Vietnam. Each year, the Mekong Delta produces millions of tons of rice straw and rice husks. These materials are rich in cellulose, hemicellulose, and lignin—the building blocks of biochar’s carbon framework. Rice husks are particularly rich in silica (SiO₂), which enhances the mechanical strength of the final biochar product.
Municipal and industrial sludge as a nutrient-rich raw material
Sewage sludge contains high levels of nitrogen, phosphorus, and trace minerals. However, it also poses a potential risk of containing heavy metals and pathogenic microorganisms. The high-temperature pyrolysis process addresses hygiene concerns while simultaneously immobilizing heavy metals within the biochar structure, reducing the risk of leaching into the environment. When combined with agricultural biomass, sewage sludge supplements the nutrients that the biomass lacks.
Plastic waste and scrap rubber: opportunities for recycling while enhancing the value of biochar
This is a particularly promising new area of research. When plastics and rubber are pyrolyzed together with biomass, they can provide reactive hydrogen, thereby increasing the hydrogen content in bio-oil and improving the porous structure of biochar. Some studies have shown that adding 10–20% PE plastic to the biomass mixture can increase the surface area of biochar by up to 30%. This also serves as an alternative solution for plastic waste management, replacing landfilling and open burning.
Livestock manure, poultry manure, and organic waste from livestock farming
Animal manure is rich in nitrogen, phosphorus, and potassium—the three primary nutrients in agriculture. Biochar produced through the pyrolysis of poultry manure and rice straw typically has a slightly alkaline pH, making it suitable for improving acidic soils in the Central Highlands and the northern mountainous regions of Vietnam.
Selection criteria and optimal ingredient blending ratios
The selection of raw materials should be based on the following criteria: C/N ratio (ideal ratio of 20–40:1), ash content (affecting catalytic properties), humidity (must be processed to reduce the content to less than 15%) and seasonal availability. The most commonly studied mixing ratios are biomass:sewage sludge = 7:3 or biomass:animal manure = 3:1.
Key characteristics of multifunctional biochar produced by pyrolysis
The porous structure and specific surface area have been significantly improved
One of the most notable advantages of pyrolytic biochar is BET specific surface area typically ranges from 200 to 500 m²/g, which is significantly higher than that of single-source biochar (typically 50–150 m²/g). The richer meso- and microporous structure results in superior adsorption capacity, which is particularly important in wastewater treatment applications.
Macro- and micronutrient content in pyrolytic biochar
Compared to biochar produced solely from biomass, pyrolytic biochar made with the addition of sewage sludge or animal manure typically contains:
- Total nitrogen: 2–4 times higher
- Available phosphorus: 3–5 times higher
- Exchangeable potassium: 1.5–2 times higher
This serves as the foundation for developing slow-release biochar-based biofertilizers.
Ability to adsorb heavy metals and organic pollutants
Thanks to its large surface area, negative surface charge, and abundant functional groups (–COOH, –OH, C=O), multifunctional biochar exhibits excellent adsorption capacity for heavy metals such as Pb²⁺, Cd²⁺, As³⁺, and organic pollutants such as tetracycline and pesticides. A study at Can Tho University found that biochar co-pyrolyzed from rice husks and sewage sludge removed up to 92% of Pb²⁺ from contaminated water at a dosage of 2 g/L.
High carbon stability and long-term carbon storage potential
The organic H/C ratio (typically < 0.7 in high-quality pyrolytic biochar) reflects the degree of carbonization and the stability of the aromatic structure. Biochar with a low H/C ratio can persist in the soil for hundreds to thousands of years, directly contributing to the goal biological carbon sequestration in international carbon credit mechanisms.
Reactor technology and equipment used in pyrolysis
Common types of reactors
The three most commonly used types of reactors:
- Fixed-bed reactor: Simple, low-cost, and suitable for laboratory and pilot-scale applications. The drawback is that it is difficult to maintain uniform temperature control when processing large quantities of material.
- Fluidized-bed reactor: More uniform heat transfer, suitable for industrial-scale applications, but requires a larger equipment investment.
- Continuous rotary kiln: Ideal for continuous biochar production on a commercial scale, it easily processes mixed feedstocks with varying moisture content and particle sizes.
Temperature and Atmospheric Control System
To ensure consistent biochar quality, the control system must maintain an inert gas environment (N₂ or CO₂) or strict anoxic conditions. Multi-point temperature sensors and automatic PID control systems are essential components of modern reactor designs.
Recovery and processing of byproducts: biodiesel and synthetic gas (syngas)
One of the economic advantages of pyrolysis is make full use of the products. Syngas (containing CO, H₂, and CH₄) can be reused to provide heat for the reactor itself, reducing operating energy costs by 30–50%. Bio-oil can be refined into fuel or chemical feedstock. This economic model significantly enhances the technology’s commercial competitiveness.
Practical Applications of Multifunctional Biochar in Agriculture and Environmental Management
Rehabilitation of degraded agricultural land
The Red River Delta and the Central Highlands regions have extensive areas of degraded land characterized by low pH and low organic matter content. Trials in Đắk Lắk showed that applying 5 tons of pyrolyzed biochar per hectare in combination with organic fertilizer increased coffee yields by 18% and reduced the amount of chemical fertilizer required by 25% after two consecutive growing seasons. Biochar improves the soil’s water-holding capacity and nutrient retention, making it particularly valuable in the context of increasingly frequent droughts.
The application of biochar in wastewater treatment
Multifunctional biochar can be used as a filtration medium in small-scale wastewater treatment tanks in craft villages, livestock farms, or rural communities. Its ability to simultaneously remove multiple types of pollutants (heavy metals, ammonia, phosphate, and antibiotics) in a single treatment step is a significant advantage over traditional filtration media.
Using biochar as a carrier for beneficial microorganisms
The porous structure of biochar creates an ideal environment for nitrogen-fixing bacteria, phosphate-solubilizing bacteria, and mycorrhizal fungi to colonize. When biochar is inoculated with beneficial microorganisms and applied to the soil, it increases soil microbial density by 3–5 times compared to the control group, laying the foundation for a new generation of biofertilizers.
Potential applications in the energy sector
Biochar with high electrical conductivity derived from pyrolysis (particularly when combined with feedstocks rich in transition metals) is currently being researched as an electrode material for supercapacitors and lithium-ion batteries. This is a highly economically viable application that opens up the energy materials market for Vietnamese biochar products.
Challenges, Barriers, and Development Directions for Pyrolysis Technology in Vietnam
Challenges in standardizing raw materials and quality control
One of the biggest challenges is the inconsistency of the raw material supply. The chemical composition of rice straw varies depending on the rice variety, growing season, and cultivation conditions. Sludge from different wastewater treatment plants contains varying levels of heavy metals. This necessitates a rigorous quality control system for incoming materials and flexibility in adjusting operating parameters.
Economic and policy barriers
The production cost of industrial-scale pyrolysis biochar remains higher than that of conventional chemical fertilizers, posing challenges for commercialization. In addition, Legal framework for biochar quality standards In Vietnam, the regulatory framework is not yet fully developed, and there is a lack of national standards regarding heavy metal content, pH, and biosafety criteria for biochar derived from sewage sludge.
Cutting-edge research direction: AI integration and modeling
The application artificial intelligence and machine learning Optimizing pyrolysis parameters based on the composition of the feedstock is a prominent trend. Machine learning models can predict biochar properties with high accuracy without the need for costly experiments, significantly shortening the time required for research and new product development.
The Role of Government Policy and Public-Private Partnerships
To promote Vietnam’s biochar industry, the following are needed: (1) policies supporting green credit for businesses investing in pyrolysis technology, (2) a carbon credit mechanism for certified biochar, (3) collaborative research programs between universities and businesses, and (4) the development of national biochar standards compatible with international standards (IBI, EBC).
Conclusions and Future Prospects of Pyrolysis Technology
Summary of the outstanding benefits of multifunctional biochar
Multifunctional biochar pyrolysis technology offers three distinct categories of benefits: addressing the waste issue (utilizing agricultural byproducts, sludge, and plastic waste), create high-value-added products (multifunctional biochar with superior properties) and contribute to climate goals (long-term carbon sequestration in soil). This represents the ideal convergence of environmental solutions and economic opportunities.
Assessing the potential contribution to carbon neutrality and the circular economy
Given Vietnam’s commitment to achieving net-zero by 2050, biochar from pyrolysis could make a significant contribution to this category biological carbon dioxide removal. It is estimated that converting just 30% of Vietnam’s agricultural byproducts into biochar using pyrolysis technology could sequester the equivalent of 20–30 million tons of CO₂ annually—a significant contribution to the country’s emissions reduction efforts.
Recommendations for researchers, businesses, and policymakers
- Researcher: Prioritize research into the synergistic mechanisms among specific pairs of raw materials unique to Vietnam; develop AI-based predictive models.
- Business: Invest in a pilot project with a capacity of 1–5 tons per day to gather data for commercialization; collaborate with the agricultural supply chain to ensure a stable supply of raw materials.
- Policy maker: Promptly issue national standards for biochar; establish a carbon credit mechanism for biochar; integrate biochar into the national sustainable agricultural development program.
Co-pyrolysis technology using multiple feedstocks is not just a technical solution—it is A bridge between waste management, agricultural revitalization, and climate change adaptation. On the path toward a green and circular economy, multifunctional biochar deserves to be at the heart of Vietnam’s sustainable development strategy.
