Producing, Characterizing and Quantifying Biochar in the Woods Using Portable Flame Cap Kilns

In many world regions, including the western US, climate change, drought, and alien invasive species have created a wildfire crisis that threatens ecosystems and communities. As forests and woodlands burn uncontrollably, large amounts of particulates and greenhouse gasses are emitted into the atmosphere, with devastating consequences for human health and the climate. For instance, wildfires in California in 2020 are estimated to have released around 127 million megatons of greenhouse gas emissions, approximately two times the amount of California's total GHG emission reductions from 2003 to 2019¹. Increasingly, scientists and land managers are investigating human actions that can help restore these forests and woodlands and their ecosystem services. The manual thinning and removal of excess biomass is one of the most important actions that must be taken². Removal of biomass includes its disposal, and where the biomass is situated in remote and difficult-to-access locations, there are few options other than incineration onsite in unmanaged slash piles. Unmanaged burn piles do the job of removing fuels from the landscape, but they damage forest soils as the concentrated heat under the piles incinerates the soil's organic horizon, leaving behind bare soil that is vulnerable to erosion and colonization by invasive species. It can take decades to regenerate the organic soil horizon in a burn pile scar³. Unmanaged burn piles are also a source of particulate and greenhouse gas emissions. Smoke from slash pile burning also restricts the burning window in air-quality-limited watersheds, making it more difficult to accomplish the work.

Researchers for the USDA Forest Service have examined the alternative of producing biochar from slash materials, and have identified several promising techniques, including the option of using small, mobile biochar kilns in the forest⁴. Converting forest slash to biochar onsite has many ecological advantages over the current practice of slash disposal by incineration in burn piles, including reduced soil heating and particulate emissions. Biochar produced onsite can be removed and utilized in agriculture, or it can be left in place where it serves several functions in restoring forest health and improving adaptation to climate change and drought. Because up to 50% of the total carbon in many forest soils is charcoal from historic, natural fires⁵, leaving biochar on the site where it is made can restore forest soil charcoal that is often missing from recent soil horizons due to fire suppression, with unknown impacts on ecosystem processes⁶. Biochar left in place on forest soils can mimic the effects of charcoal produced by natural fire and produce similar effects on soil carbon content and soil physical, chemical and biological properties⁷.

In recent years, an international network of forestry workers, woodland owners, researchers, and biochar consultants has developed a suite of carbonization methods for converting forest slash into biochar onsite as an alternative to slash pile incineration. These methods are based on the principle of flame carbonization, first developed and commercialized in Japan as the "smokeless carbonizing kiln" offered by the Moki company⁸. This steel ring kiln makes well-carbonized biochar with reported biomass-to-biochar conversion efficiency of 13% to 20%, depending on the feedstock used⁹.

The process of producing biochar or charcoal is often referred to as pyrolysis, the separation of biomass components by heat in the absence of oxygen. This is usually conceived of as retort pyrolysis, where biomass is physically isolated from air in an externally heated vessel. However, pyrolysis can also take place in the presence of limited air, as in gasification and flame carbonization, because solid fuels like wood burn in stages. When heat is applied to biomass, the first stage of combustion is dehydration, as water is evaporated from the material. This is followed by devolatilization and simultaneous char formation, also known as pyrolysis. Volatile gas containing hydrogen and oxygen is released and burned in a flame, continually adding heat to the process. As the gas is released, the remaining carbon is converted to aromatic carbon, or char. The final stage of combustion is the oxidation of the char to mineral ash¹⁰.

Because these are discrete phases that occur in an open combustion process, we have the opportunity to stop the process after char formation by removing air or heat. This is accomplished during the biochar production process by continually adding new material to the burn pile so that the hot char is buried by new material that cuts off the flow of oxygen. Hot charcoal accumulates in the bottom of the pile and is prevented from burning to ash as long as flame is present, because the flame consumes most of the available oxygen. When all of the fuel has been added to the pile, the flame begins to die down. At that point, the hot charcoal can be preserved by removing oxygen and heat, usually by spraying the coals with water and raking them thin to cool¹¹.

The basic principle of operation is that of counter-flow combustion. Counter-flow combustion air keeps the flame low and prevents the emission of embers or sparks. The flame also burns most of the smoke, reducing emissions. In summary, the following principles explain the operation of counter-flow combustion in a flame cap kiln: (1) Gas flows upward while combustion air flows downward, (2) Counter-current flow is established as burning fuel draws air downward, (3) Flames stay low and close to fuel, minimizing ember escape, (4) Smoke burns in the hot zone, (5) Because all the combustion air comes from above, it is consumed by the flames (6) Very little air is able to reach the unburned coals that fall to the bottom of the kiln, (7) The coals are preserved until the end of the process when they are quenched or snuffed.

In addition to its benefits to soil, biochar is also a leading method of carbon removal for climate change mitigation. Up to half the carbon in woody biomass can be converted to stable, aromatic carbon in the form of biochar¹². However, not all pyrolysis technologies produce the same amount of recalcitrant carbon that remains stable in soils for 100 years or more (the key metric for determining carbon removal value). Biochar stability is closely correlated with the temperature of production. The adiabatic flame temperature of burning wood is estimated to be close to that of propane, 1,977 °C¹³. Biochar production in a flame cap kiln is closely coupled with the flame, with no heat transfer losses by conduction through a metal wall, as in retort pyrolysis. Therefore, we would expect that the temperature of production would be high as long as a flame is maintained during the process. A survey of chars using Raman spectroscopy¹⁴ reported that a biochar sample from a flame cap kiln (provided by lead author Kelpie Wilson) was among the three samples with the highest apparent temperature of char formation, in the range of 900 °C.

Thermocouples are required to access the interior of the burn and accurately measure the production temperature of biochar in a flame cap kiln or burn pile, and these are expensive and not available to low-tech producers. Therefore, we have used a method described by researchers working in the Brazilian Amazon that uses heat crayons (used by welders to check the temperature of metal parts) that melt at a calibrated temperature¹⁵. Bricks are marked with crayons, wrapped in aluminum foil, and placed in various places in the kiln during production. We used this method several times and determined that the kiln temperatures exceeded 650° C, as the crayon marks were completely melted. This will be a useful method to confirm production temperatures where needed; however, the main verification point will be documenting the presence of flame throughout.

There is not much published data on the characteristics of biochar made by low-tech flame carbonizing methods. However, biochar samples made by flame carbonizing methods in several kiln types were analyzed by Cornellissen et al. and found to meet European Biochar Certificate (EBC) standards for biochar, including low PAH content and high biochar stability. Furthermore, the biochar produced from both woody and herbaceous feedstocks had an average carbon content of 76 percent¹¹. The US Forest Service Rocky Mountain Research Station¹⁶ analyzed five biochar samples from flame cap kilns and burn piles made at a field day in California in 2022. The average carbon content of the samples was 85 percent. Given these results, we can conclude that it is likely that biochar made from woody residues in flame cap kilns will meet the basic requirements for verified carbon removal: high carbon content and high biochar stability.

Two carbon removal protocols for low-tech, place-based biochar production have now been released by Verra¹⁷ and the European Biochar Consortium Global Artisan C-Sink protocol¹⁸. These newly developed protocols are promising; however, they have some limitations when applied to forests, woodland, and other landscapes under threat from drought and wildfire. Accordingly, this paper will describe a new methodology, the Methodology CM002 V1.0, from AD Tech¹⁹, that is being developed specifically for flame carbonization of woody debris as part of vegetation management and fuel load reduction activities. Life cycle analysis confirms that biochar carbon sequestration using onsite biochar production from woody biomass in flame cap kilns produces a net carbon removal benefit²⁰. Successful implementation of carbon removal protocols can help support financially the vital fuels reduction work that needs to take place to protect communities and ecosystems from wildfires and ecosystem degradation. In order to access carbon removal payments, field measurements and digital monitoring, reporting and verification (D-MRV) methods are incorporated as routine practices into the biochar production methodology described here. Details of the platform are discussed in the Supplemental Information (Supplementary File 1).

While several open-source designs of flame cap kilns are being manufactured by individuals for their own use²¹, to our knowledge, at this time, there is only one flame cap kiln with a capacity of greater than one cubic meter that is being mass produced for sale in North America, the Ring of Fire Kiln²², a lightweight, portable flame cap kiln that is designed for easy mobility using hand crews. The kiln consists of an inner ring comprising six sheets of mild steel secured together. An outer ring composed of lighter gauge steel bolts onto the brackets that hold the inner ring together. The outer ring serves as a heat shield that holds in heat for better efficiency. The top of the kiln is open to the air, and this is where the flame cap forms. Air flowing up through the annular gap between the main kiln body and the heat shield provides pre-heated combustion air to the kiln, further increasing combustion efficiency (Figure 1)

Figure 1: Schematic showing air flow, flame characteristics, and char accumulation in the Ring of Fire Kiln. Counter-flow combustion air pulls the smoke into the hot zone, where it burns up. Air flowing up through the annular gap between the main kiln body and the heat shield provides pre-heated combustion air to the kiln, further increasing combustion efficiency. Please click here to view a larger version of this figure.

The kiln diameter is 2.35 m, forming a cylinder that is one meter tall for a total volume of 4.3 m³. In practice, the kiln is never filled completely to the top, so a typical production batch will fill the kiln from between ½ to ¾ full for a volume of biochar that is between 2 and 3 cubic meters.

Because the Ring of Fire Kiln is a standardized design, it is being adopted as the first certified technology for use in the CM002 Component Methodology that provides standardized procedures for the quantification of greenhouse gas (GHG) benefits. Measurement and data collection steps meeting the requirements of the CM002 are incorporated into the method. Reporting is done through a smartphone application by answering short questionnaires throughout the process and uploading photos and video clips to the mobile app.