PCW Biochar System: Flame-Curtain Pyrolysis Windrow Biochar Production and Carbon Sequestration

PCW Biochar System Agricultural Flame-Curtain Pyrolysis Using Pyrolytic Windrows (Flame-Curtain Pyrolysis Windrow) This article presents a conceptual and theoretical model based on existing knowledge of biomass pyrolysis and agroforestry productivity. Author: Médéric Grandjean Year: 2026 Abstract Biochar is a carbon-rich material produced through the pyrolysis of biomass under limited oxygen conditions. Due to its high chemical stability, this material can persist in soils for several centuries and represents a potential pathway for long-term atmospheric carbon sequestration. The PCW Biochar System (Flame-Curtain Pyrolysis Windrow) is an agricultural pyrolysis method that enables biochar production directly on agricultural land using locally available biomass. In this system, biomass is arranged in elongated windrows in which pyrolytic gases generated during the thermal decomposition of biomass burn at the surface, forming a curtain of flames. This combustion provides the heat required for pyrolysis while limiting oxygen penetration into the lower biomass layers, thereby promoting the formation of stable carbon. Integrating the PCW system into productive agricultural or agroforestry systems could contribute to soil restoration, increased biomass production, and long-term carbon sequestration. 1. Introduction The increase in atmospheric carbon dioxide concentration is one of the primary drivers of climate change. At the same time, many agricultural soils have lost a significant portion of their organic matter, reducing both fertility and carbon storage capacity. Biochar is a carbon-rich material obtained through the pyrolysis of plant biomass. Its aromatic structure provides high chemical stability, allowing it to remain in soils for centuries. Numerous scientific studies have demonstrated that biochar application can improve: soil water retention nutrient availability soil structure microbial activity. However, biochar production is generally carried out in centralized industrial facilities requiring significant infrastructure. The PCW Biochar System (Flame-Curtain Pyrolysis Windrow) offers an alternative approach based on decentralized biochar production directly on agricultural land using locally produced biomass. 2. Principle of Flame-Curtain Pyrolysis Pyrolysis is the thermal decomposition of biomass when heated at high temperatures under limited oxygen conditions. This process produces three main fractions: a solid carbon-rich residue known as biochar combustible gases volatile compounds. In a flame-curtain system, pyrolytic gases generated in the lower biomass layers rise toward the surface and burn when exposed to air. This combustion creates a curtain of flames that: provides the energy necessary for pyrolysis limits oxygen penetration into the lower layers protects the biochar from oxidation. 3. Construction of the Pyrolytic Windrow In the PCW system, biomass is arranged in windrows directly on the soil surface. Typical dimensions are: width: approximately 5 meters height: approximately 1.5 meters length: 20 to 30 meters. Biomass is arranged along the longitudinal axis of the windrow, which promotes the circulation of pyrolytic gases and stabilizes the combustion process. 4. Ignition and Thermal Dynamics The windrow is generally ignited from the top, creating an immediate combustion zone at the surface. In some cases, the windrow can also be ignited from both ends in order to accelerate the propagation of the thermal front. Three thermal zones typically appear within the windrow: combustion zone: 700–900 °C pyrolysis zone: 350–600 °C drying zone: 100–200 °C. Heat produced in the combustion zone gradually propagates downward into the biomass layers. 5. Optimal Biomass Composition The composition of biomass used in the windrow strongly influences both pyrolysis efficiency and biochar quality. An effective mixture may include: approximately 50 % lignin-rich woody biomass approximately 40 % cellulose-rich fibrous plants approximately 10 % mineral-rich plants. Woody biomass promotes the formation of stable carbon during pyrolysis. Fibrous plants generate large amounts of pyrolytic gases that sustain surface combustion. Mineral-accumulating plants enrich the biochar with nutrients beneficial for soil fertility. Additional optimization can be achieved by placing large tree leaves on the surface of the windrow, particularly those of Paulownia tomentosa, which are known for their large size. These leaves form a vegetal layer that: reduces oxygen inflow along the sides of the windrow stabilizes pyrolysis conditions protects the biochar from excessive combustion. 6. Theoretical Yield of a PCW Windrow A pyrolytic windrow may contain approximately 35–40 tonnes of dry biomass. With a pyrolysis yield ranging between 25 % and 35 %, this can produce approximately: 10–14 tonnes of biochar. Biochar generally contains 70–85 % stable carbon, allowing it to remain in soils for centuries. 7. Comparison With Other Biochar Production Methods Biochar can be produced through several thermal methods. Industrial pyrolysis systems use closed reactors with precise temperature control. Advantages include precise thermal control and consistent biochar quality, but disadvantages include high capital costs and the need for biomass transport. Traditional kilns are simple and inexpensive but often produce variable yields and higher emissions. Kon-Tiki systems use open conical reactors where pyrolytic gases burn at the surface, offering a relatively simple and cleaner combustion process but with limited production capacity. The PCW system differs by using elongated pyrolytic windrows. Its advantages include on-site production, elimination of biomass transport, and adaptability to agricultural systems. However, thermal control is less precise than in industrial systems, and fire management requires careful supervision. 8. Integration into Agroforestry Systems In intensive agroforestry systems, biomass production can reach very high levels due to the combined productivity of multiple plant species occupying different ecological niches. Several highly productive species are particularly suitable for this type of system, including: giant bamboo Eucalyptus species Gliricidia sepium Leucaena leucocephala elephant grass (Pennisetum purpureum). Under favorable tropical conditions, such systems may produce approximately 150 to 190 tonnes of dry biomass per hectare per year. With an average pyrolysis conversion efficiency of about 30%, this biomass could theoretically produce approximately 45 to 55 tonnes of biochar per hectare per year. This level of production illustrates the potential of integrated agroforestry–biochar systems to combine high biomass productivity with significant long-term carbon sequestration in soils. 9. Global Potential and Positive Biomass Feedback Global agricultural land covers approximately 5 billion hectares, including around 1.5 billion hectares of cultivated land. If approximately 5–10 % of these surfaces adopted biochar production and application systems such as the PCW Biochar System, this would represent 75–150 million hectares. With an estimated average production of 8–10 tonnes of biochar per hectare per year, global production could reach 800 million to 1 billion tonnes of biochar annually, corresponding to approximately 2.5–3.5 gigatonnes of CO₂ sequestered each year in stable soil carbon. However, the potential of this system does not rely solely on direct carbon stabilization. Numerous studies have shown that biochar can improve several physical, chemical, and biological soil properties, including water retention, soil structure, cation exchange capacity, microbial activity, and mycorrhizal development. These improvements can lead to increased plant productivity, particularly in degraded or nutrient-poor soils. Meta-analyses of global studies indicate that biochar application can increase crop yields by 5–25 % on average, depending on soil conditions and climate. In biomass-producing agricultural systems, this productivity increase may result in additional biomass available for pyrolysis. Consequently, biochar integration can create a positive feedback loop: atmospheric CO₂ ↓ photosynthesis ↓ biomass production ↓ PCW pyrolysis ↓ biochar ↓ soil improvement ↓ increased biomass production. This mechanism suggests that the global carbon sequestration potential could gradually increase over time in agricultural systems integrating sustainable biochar production and application. Fire Safety Considerations in Field Flame-Curtain Pyrolysis Systems The PCW (Flame-Curtain Pyrolysis Windrow) system relies on controlled combustion processes to convert biomass into biochar directly in agricultural fields. While this approach offers several advantages in terms of simplicity, decentralization, and reduced transportation of biomass, it also involves the intentional use of open fire. As a result, fire safety constitutes a critical aspect of the practical implementation of this technology. Field pyrolysis systems such as flame-curtain windrows operate through the combustion of pyrolysis gases generated by the thermal decomposition of biomass. These gases burn at the surface of the biomass pile, forming a flame curtain that both provides heat for the pyrolysis process and limits oxygen penetration into the underlying biomass layers. Although this mechanism enables efficient biochar production, it also creates high temperatures that may exceed 800–900 °C in the combustion zone. The use of open flame in agricultural environments introduces several potential risks. These include the unintended spread of fire to surrounding vegetation, the influence of wind conditions on flame stability, and the possibility of incomplete combustion leading to smoke emissions. For these reasons, the implementation of field pyrolysis systems