What if fire could do more than destroy—what if it could protect? In the ‘Spilling the Char’ series, flame curtain kilns reveal how a simple wall of flame uses the laws of heat and airflow to turn burning into preservation. By shielding carbon, cutting smoke, and transforming waste into lasting value, this elegant technology shows that innovation does not always come from complexity. Sometimes, it comes from seeing familiar things—like fire—in a smarter way.

From Open Fires to Controlled Carbon: Why Flame Curtain Kilns Matter

For most of human history, turning biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More into charcoalCharcoal is a black, brittle, and porous material produced by heating wood or other organic substances in a low-oxygen environment. It is primarily used as a fuel source for cooking and heating.   More meant letting fire run its course and accepting whatever remained. While this approach provided fuel, it was inefficient, smoky, and wasteful of carbon. As biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More has emerged as a tool for soil health, pollution control, and climate mitigation, the need for cleaner and more controlled production methods has become urgent. Flame curtain kilns represent one of the most important responses to this need. They are not high-tech in appearance, yet they embody a sophisticated understanding of combustion science, enabling communities around the world to produce high-quality biochar with dramatically lower emissions and higher carbon efficiency than traditional methods.

Fire as a Shield: The Physics Behind the Flame Curtain

At the heart of flame curtain technology is a clever reversal of how fire is usually used. Instead of suppressing flames to protect the biomass, the system uses flames themselves as a protective barrier. When biomass is added in thin layers to an open-top kiln, it heats up and releases combustible gases. These gases rise and ignite above the solid material, forming a continuous layer of fire known as the flame curtain. This curtain consumes most of the available oxygen, preventing it from reaching the hot carbon below. As a result, the biomass undergoes pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More rather than full combustion, transforming into stable biochar instead of burning away into ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More. This process reflects well-established principles of combustion chemistry, where separating the oxidation of gases from the stabilization of solids allows carbon to be conserved rather than lost.

Turning Smoke into Heat: Clean Combustion and Gas Oxidation

One of the defining advantages of flame curtain kilns is how they deal with smoke. In conventional open burning and traditional charcoal pits, volatile organic compounds escape into the air as thick plumes of smoke rich in methane, carbon monoxide, and fine particles. Flame curtain kilns instead channel these gases directly into the flame zone, where they are oxidized at high temperatures. This turns what would have been pollution into useful heat that sustains the pyrolysis process. Scientific measurements have shown that, under good operating conditions, methane emissions from flame curtain kilns can be extremely low compared to older technologies, significantly improving the overall climate balance of biochar production. In this way, the system transforms smoke from a liability into a resource.

Engineering Simplicity: How Kiln Design Shapes Performance

Although the principle is simple, the physical design of flame curtain kilns plays a critical role in performance. Many modern versions, including the well-known Kon-Tiki design, use a conical shape that concentrates heat and promotes smooth airflow. The geometry helps hot gases rise evenly, maintaining a stable flame curtain while minimizing the intrusion of oxygen into the char bed. This design also allows operators to add feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More progressively, keeping the process under control without complex machinery. Research comparing different kiln geometries shows that these structural features significantly influence temperature stability, char yield, and emission levels, demonstrating that even low-tech systems benefit from thoughtful engineering rooted in fluid dynamics and heat transfer science.

Carbon Yields and Climate Balance: What the Data Shows

Beyond visual simplicity, flame curtain kilns perform impressively when judged by scientific metrics. Multiple field and laboratory studies report biochar yields of roughly one-fifth to one-quarter of the dry biomass input, with carbon retention efficiencies often approaching or exceeding 40 percent. These numbers place flame curtain kilns on par with many industrial batch pyrolysis systems, especially when lifecycle impacts are considered. Because they require no external fuel and can operate close to feedstock sources, their overall carbon footprint is often lower than that of centralized facilities that depend on long-distance transport and fossil-based energy inputs. This makes them particularly relevant in discussions about decentralized carbon removal and community-scale climate solutions.

Skill as Technology: Why Operation Matters as Much as Design

While the kiln provides the structure, human skill provides the control. The performance of flame curtain systems depends heavily on how they are operated. Feedstock moisture, particle size, and layering rhythm all influence whether the flame curtain remains stable. Wet material can cool the flame zone and allow oxygen to penetrate, leading to increased smoke and methane emissions. Adding biomass too quickly can smother the flame curtain, while adding it too slowly can allow excessive oxidation of the char. Quenching at the end of the process must be timely, because once the flame curtain disappears, the newly formed biochar becomes vulnerable to burning. Scientific studies repeatedly emphasize that best practices in operation are as important as kiln design in determining environmental performance and product quality.

Advantages in Practice: Accessibility, Scalability, and Quality

The greatest strength of flame curtain kilns lies in their accessibility. They can be built from locally available materials, operated with minimal infrastructure, and scaled to suit everything from small farms to community cooperatives. This makes them uniquely suited to regions where industrial pyrolysis plants are impractical. At the same time, the biochar they produce often meets international quality benchmarks, including low contaminant levels and favorable physical and chemical properties for soil amendmentA soil amendment is any material added to the soil to enhance its physical or chemical properties, improving its suitability for plant growth. Biochar is considered a soil amendment as it can improve soil structure, water retention, nutrient availability, and microbial activity.   More, water filtration, and pollution control. By bridging the gap between artisanal methods and industrial standards, flame curtain kilns democratize access to high-quality biochar production.

Where the Limits Appear: Scale, Emissions Variability, and Certification

Despite their many strengths, flame curtain kilns are not a universal solution. Their batch-based nature limits throughput, making them less suitable for large-scale industrial supply chains that demand continuous operation and uniform product specifications. Emissions, although far lower than in traditional kilns, can vary depending on feedstock and operator skill, which complicates monitoring and standardization. These factors have also posed challenges for integrating flame-curtain-produced biochar into formal carbon credit schemes, which require consistent, verifiable performance. Ongoing research and improved operational guidelines are gradually addressing these issues, but they highlight the importance of viewing flame curtain kilns as part of a broader technological ecosystem rather than a one-size-fits-all answer.

Appropriate Technology for Carbon Removal

In many ways, flame curtain kilns exemplify the concept of appropriate technology. They apply advanced scientific understanding without demanding advanced infrastructure. Instead of concentrating biochar production in a few industrial hubs, they distribute it across landscapes where biomass residues already exist. This decentralization reduces transport emissions, creates local value chains, and empowers communities to participate directly in climate solutions. From a systems perspective, this makes flame curtain kilns not just tools for producing biochar, but platforms for integrating carbon management into everyday land-use practices.

Join the Conversation
Are you producing biochar using flame curtain or Kon-Tiki kilns? We’d love to hear from you. If you have observations, field experiences, or results—from emissions to yields to real-world challenges—share them with us at hello@biochartoday.com. Selected stories and insights may be featured in future editions of Biochar Today, helping build a global, practice-driven understanding of what works on the ground.

Recommended Reading

Namaswa, T., Burslem, D. F., & Smith, J. (2023). Emerging trends in appropriate kiln designs for small-scale biochar production in low to middle income countries. Bioresource Technology Reports, 24, 101641.

Cornelissen, G., Pandit, N. R., Taylor, P., Pandit, B. H., Sparrevik, M., & Schmidt, H. P. (2016). Emissions and char quality of flame-curtain” Kon Tiki” Kilns for Farmer-Scale charcoal/biochar production. PloS one, 11(5), e0154617.

Cornelissen, G., Makate, C., Mulder, J., Janssen, J., Trimarco, J., Obia, A., … & Sørmo, E. (2024). Emission Factors for Biochar Production from Various Biomass Types in Flame Curtain Kilns. Applied Sciences, 14(21), 9649.

Cornelissen, G., Sørmo, E., de la Rosa, R. K. A., & Ladd, B. (2023). Flame curtain kilns produce biochar from dry biomass with minimal methane emissions. Science of the Total Environment, 903, 166547.