We describe the use of a place-based mobile pyrolysis unit equipped with an air curtain to continuously create biochar. The technology reduces the need for open slash pile burning, which results in lower emissions and fewer soil impacts. The protocol includes guidelines for site selection, loading, and quenching.
Fuel treatments and other forest restoration thinning practices aim to reduce wildfire risk while building forest resilience to drought, insects, and diseases and increasing aboveground carbon (C) sequestration. However, fuel treatments generate large amounts of unmerchantable woody biomass residues that are often burned in open piles, releasing significant quantities of greenhouse gases and particulates, and potentially damaging the soil beneath the pile. Air curtain burners offer a solution to mitigate these issues, helping to reduce smoke and particulates from burning operations, more fully burn biomass residues compared to pile burning, and eliminate the direct and intense fire contact that can harm soil beneath the slash pile. In an air curtain burner, burning takes place in a controlled environment. Smoke is contained and recirculated by the air curtain, and therefore burning can be conducted under a variety of climatic conditions (e.g., wind, rain, snow), lengthening the burning season for disposal of slash material. The mobile pyrolysis unit that continuously creates biochar was specifically designed to dispose of residual woody biomass at log landings, green wood at landfills, or salvaged logged materials and create biochar in the process. This high-carbon biochar output can be used to enhance soil resilience by improving its chemical, physical, and biological properties and has potential applications in remediating contaminated soils, including those at abandoned mine sites. Here, we describe the general use of this equipment, appropriate siting, loading methods, quenching requirements, and lessons learned about operating this new technology.
Across the U.S.A., many forest stands have increased in standing tree volume in the absence of frequent low-intensity fires, historically set by indigenous people and suppressed in modern times1,2. From this fire exclusion, the resulting overstocked stands pose challenges for land managers striving to enhance forest resilience against wildfires, pests, diseases, and drought effects3. Standard management practices for reducing tree volume include prescribed fire, pre-commercial thinning, and mature stand harvesting. These operations generate substantial amounts of low- and no-value woody biomass, often called residues. For example, in the 15 states of the western U.S.A., it is estimated that harvest operations produce nearly 8 million dry tons of residues each year4. Further, the USDA Forest Service has implemented a wildfire crisis plan, which will treat an additional 50 million acres (20 million hectares) over a 10-year period. This will result in the need to dispose of additional unmerchantable material and likely require the use of a variety of place-based options. Although tops, limbs, and unmerchantable trees can be used for bioenergy or biofuels, limited market opportunities often lead to these residues being piled and burned. Building slash piles aims to reduce wildfire and insect risk, create growing space for understory plants, and address other land management objectives.
Open pile burning is a low cost, relatively fast method for reducing wood volume, but it also produces smoke and air pollutants, including greenhouse gases5. Further, it may also cause undesirable impacts on soil physical, chemical, and biological properties, leading to burn scars that can persist for decades6. To mitigate the detrimental side effects of open burning, alternative approaches are needed that reduce negative impacts on climate change, wildfire risk, and soil health7.
Here, we focus on the use of a novel method for continuous biochar production developed through a Cooperative Research and Development Agreement (CRADA) between the U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, and Air Burners, Inc. (Palm City, FL). The resulting technology, henceforth referred to as the biochar-producing air curtain burner (BACB; Figure 1), continuously produces biochar from woody residues while limiting smoke and particulate emissions. Compared with open burning, the functionality of the BACB reduces fire risk and the spread of smoke8, creating avenues for the safe extension of operational burn windows. Unlike traditional methods that leave residue piles on the landscape where they contribute to fuel accumulations9 or alternative combustion methods that primarily produce smoke and ash, the BACB effectively reduces fuel loads while protecting the soil and creating biochar, a consistent, high carbon charcoal, that can be used for soil restorations' on or near the site of processing. Further, the BACB is mobile and can easily be sited, for example, at a log landing, along a roadside, or in a campground. It can also burn wet or dry wood, mixed feedstock types, and sizes, and can be used in inclement weather and at times when weather conditions are too risky for open pile burning.
Biochar produced in the BACB is generally 70%-90% carbon, highly porous, and consistent in particle size distribution, making it suitable for remediating degraded soils, often found at abandoned mines, log landings, skid trails, riparian areas, or agricultural sites. Biochar can also be used as a mix with compost or in livestock feedlots to adsorb nutrients and reduce odor. In general, the best soil uses for woody residue biochar are to reduce erosion and nitrate leaching while enhancing soil aggregate stability and available water in coarse-textured, low organic matter soils10,11.
Figure 1: Mobile biochar-producing air curtain burner. The test site shows the general configuration of the slash and the biochar-producing air curtain burner. This figure has been modified from12. Please click here to view a larger version of this figure.
Overview of design and operation
Although there are several types of air curtain burners available, the BACB continuously creates biochar. The unit is approximately 7 m long x 2.5 m wide x 2 m high. It has an air manifold along the top of the firebox that provides a steady curtain of air to contain smoke, particulates, and embers within it and promotes complete oxidation of emissions. For optimal operation, the firebox is loaded to a level just below the manifold, ensuring uninterrupted airflow across the firebox. The BACB can be towed behind any vehicle with a standard hitch package and adequate towing capacity. A level, smooth site is prepared close to the piled biomass (also called feedstock), but with enough room around the site to allow equipment and staff to move freely and safely. Once positioned, the unit is secured by lowering the firebox sides onto the soil using the onboard hydraulic system. A blower motor pressurizes the air curtain manifold, which runs the length of the firebox on one side. Biomass is initially lit and burned to establish a bed of coals in the firebox. Then, additional biomass can be added periodically while keeping incoming material below the air curtain. As the material burns, coal is produced and falls through an opening in the bottom of the firebox, where they are cleared from the machine by the conveyor belt. One panel inside the firebox oscillates to facilitate this process. Ash or fine materials fall through the conveyor belt onto the soil below. Hot coals exit the machine through a slot in the bottom of the firebox and are deposited into a pan filled with water, which stops combustion and cools the biochar to a temperature where it can be handled safely. When ready to cease operation, the firebox is cleared by allowing any remaining material to burn down. Typically, the unit cools overnight and can be moved safely the next day, as needed. Do not attempt to move the BACB while it is burning wood or cooling unless there is an emergency. In case of emergency, sand or soil can be used to extinguish the fire and smother the coals. Water should never be sprayed directly on the ceramic tiles in the firebox.
NOTE: Additional details on the operation of the BACB can be found in the operator's manual that comes with the equipment.
1. Transportation to site
2. On-site preparation
Figure 2: Example of an excavator moving wood residues. Mini-excavators can be used to load the biochar-producing air curtain burner. This figure has been modified from12. Please click here to view a larger version of this figure.
Figure 3: Example of the quench basin placement. The quench basin is kept full of water and positioned approximately 2 inches (5 cm) in front of the conveyor belt. This figure has been modified from12. Please click here to view a larger version of this figure.
3. Hazard reduction
Figure 4: Control panel on the biochar-producing air curtain burner. The control panel is used to turn on the air curtain, conveyor, and system power and to raise and lower the firebox. This figure has been modified from12. Please click here to view a larger version of this figure.
4. Pre-operation checks
5. Initial loading and creating a base fire
6. Addition of wood to the firebox
7. Biochar production
8. Ceasing operations
Figure 5: Example of ash and biochar under the conveyor belt. During a burn, some ash and biochar will fall through the metal conveyor belt. This figure has been modified from12. Please click here to view a larger version of this figure.
9. Re-positioning the BACB
Figure 6: Example of ash buildup. By the end of the day, the ash accumulation under the conveyor belt can reach the conveyor belt. Trenching before placing the biochar-producing air curtain burner is one method to extend the burn time in one location. If trenching is not an option, then the equipment should be pulled forward and ash cleaned out each day. This figure has been modified from12. Please click here to view a larger version of this figure.
From January 2022 through October 2023, the BACB pyrolyzed various feedstocks into biochar (Table 1). Based on field trials, the BACB can burn approximately 1 ton (900 kg) of feedstock per h, and approximately 11%-25% of dry mass of the feedstock will be retained as biochar, with the balance going to gas (water vapor, CO2, etc.) and ash. Carbon content ranges from 70%-90%. Larger pieces of wood (>30 cm) take a longer time to pyrolyze than wood with a diameter ranging from 2-30 cm. The amount and quality of biochar is dependent on feedstock characteristics (e.g., size, species, moisture content) and loading rate. The size of the biochar produced is 1 inch (2.5 cm) and smaller, although larger pieces are possible especially at the beginning of a run. In addition, some unburned material can pass through the opening to the conveyor at startup, and these can be placed back in the firebox, set aside, or piled with the biochar. In field trials under a wide variety of operating conditions, the BACB has been shown to be an effective in-woods method to create biochar from the woody biomass produced from fuel treatments and other harvest operations and can reduce hazardous fuels.
Production location | Feedstock | Approximate green weight pryrolyed in 1 h | Biochar carbon | ||||
kg | % | ||||||
Flagstaff, Arizona | Pinus ponderosa | 1360 | 64 | ||||
Springfield, Oregon | Pseudotsuga menziesii and Thuja plicata | 910 | 94 | ||||
Princeton, Idaho | Abies grandis, Pinus contorta, and Pseudotsuga menziesii | 900 | 89 | ||||
Palm City, Florida | Quercus and Pinus | 950 | 82 | ||||
Tollgate, Oregon | Pinus ponderosa, Abies grandis | 800 | 87 |
Table 1: Examples of results. Location of production, representative feedstocks used, quantity pyrolyzed in 1 h, and biochar carbon concentration from a continuous biochar-producing air curtain burner. An elemental analyzer was used to determine the carbon content of oven-dried (60 °C until a constant weight) samples.
The best worker configuration is one mini-excavator driver and two people looking for ignition points, directing where and when to load feedstock, and removing the biochar from the quench pan. If logs are too long or large in diameter, then a fourth person may be needed to operate a chainsaw.
The BACB is mobile, which gives it the flexibility to work at a variety of sites. This, coupled with the concomitant benefits of no soil impacts and less smoke than pile burning, allows for additional ecosystem services (e.g., carbon sequestration, water holding, soil cover, and reduced fuels). In-woods mobile biochar production is an optimal method for restoring local degraded soils associated with log landings, skid trails, forest roads, erosion, and mining.
The first critical step in this method is to ensure the equipment is placed nearly level on both axes so that the firebox side panels block smoke from leaving and air from entering the firebox. Other critical steps are to conduct frequent conversations with the excavator operator to place the feedstock into the firebox where needed and brief all workers on the safety aspects of operating the air burner and excavator. There are a few modifications that can be made with this equipment, but running the conveyor belt and shaker panels continuously once biochar production is high is an option. The key limitation is the difficulty in determining production rates since this depends on the loading rate and feedstock type, size, or moisture content.
Workers operating the BACB should have access to firefighting tools such as rakes, chainsaws, and shovels. They should also wear cotton or flame-resistant clothing, eye and ear protection, and leather gloves. Hard hats, sturdy boots, long-sleeved shirts, and long pants will protect workers from embers, heat, and accidental wood releases from the excavator. Mobile BACB's are often sited in remote locations and therefore, emergency planning is critical. Crews should have a method for emergency communication, particularly if the location is out of cell phone service range. Often, a radio, satellite phone, or emergency beacon is used.
The BACB is one tool for reducing excess woody biomass in slash piles. It will not eliminate open pile or broadcast burning but is useful in sensitive areas. For example, it can reduce fuel loads at roadsides or log landings since a level surface is required for operation. In addition, it is a good tool for removing woody residues at campgrounds, other recreational sites, and at the wildland-urban interface or urban settings where smoke from open burning can be problematic13. Biochar can be used for local soil remediation projects, and the BACB can create it locally. In addition, large-scale, post-disturbance (e.g., hurricane, tornado) clean-up is another place where BACB technologies can reduce biomass and deliver a soil amendment. Because BACB's are clean burning with little smoke and few embers, their use when conditions limit open burning (e.g., burn bans) offers another option for disposing of woody residues.
Air curtain burners were developed in compliance with the U.S. Environmental Protection Agency (EPA) to provide a clean burn with less than 10% opacity, using the EPA Method 9 Testing, as compared with open burning, which usually has 80%-100% opacity14. The use of place-based biochar production was found to be 2-40 times lower in net CO2eq when compared to open burning15.
Catastrophic wildfires have been increasing across North America in the last decade due to increased environmental stresses that exacerbate drought, insect, and disease mortality in forest stands. Decreasing wildfire risk entails active forest management that results in unmerchantable material in need of disposal. Rather than open burning, a mobile BACB can be used at forest sites, campgrounds, urban parks, or in cities to reduce the amount of woody fuels. Air curtain burners have been used for decades as a method to reduce emissions from burning woody residues16, and this is the major benefit of using this technology. The added ability to continuously produce a consistent biochar product to use for soil restoration work responds to an urgent need to increase climate-smart forest practices17. Instead of open pile burning, air curtain burners limit effects on the soil18, reduce air pollutants and greenhouse gas emissions, and the technology can engage a new workforce by developing conservation crews that work near roadsides, log landings, rural communities, or campgrounds where hazardous fuel reduction treatments are on-going.
The authors have nothing to disclose.
We thank the numerous University personnel, National Forest, and Bureau of Land Management personnel who helped conduct demonstrations and gather data. The field work for this method was supported by the U.S.D.A. Forest Service Rocky Mountain Research Station, Washington D.C., and Pacific Northwest Region offices. The findings and conclusions in this publication are those of the authors and should not be construed to represent any official U.S.D.A. or U.S. Government determination or policy.
CharBoss air curtain burner | Air Burners, Inc. | T26 | Comes with 36" landscape rake, sifting shovel, 1/2" drive standard ratchet with 1 1/8" socket, grease gun, and quench pan |
Diesel fuel (Ultra-low sulfur) | Purchased locally | ||
Diesel fuel tanks | Uline | H-1849Y | or similar |
Engine oil (diesel grade) | Any diesel grade oil 15W40 or 10W40. | ||
Excavator | Local rental company. Smaller sizes require less fuel. | ||
High temperature anti-seize lubricant (16 oz cartridge) | McMaster-Carr | 1288K97 | lubricating hydraulic fittings |
Hydraulic Oil | Amsoil | HVH05-EA (ISO32) /HVG05-EA (ISO22) | Any ISO32 synthetic hydraulic oil, ISO22 option for cold weather. Amsoil ISO32 is factory installed. |
Large buckets | Uline | 5495 | or similar |
Lighting torch (propane) | Grainger | 9RCF3 | or similar |
1-ton pickup (or larger) | Rent locally | for transporting CharBoss to site | |
Viewing step | Gorilla | GLP=WP | Stable step to allow viewing into firebox or other bench-style step |
Water truck | Any available water truck with minimum 300 gallon capacity; gravity feed of water to the quench pan can be used. | ||
Wheel chocks | Blocks of 4"x 4" lumber or commercially available chocks while hitching/unhitching unit | ||
Personal protective equipment | |||
Ear protection | Uline | S-22141 | or similar |
Eye protection | Amazon | or similar | |
Fire shirt | Grainger | 12R487 | or similar |
Fire pants | Grainger | 39EM96 | or similar |
Hard hat | Discount Safety Gear | SFTSCHH1000038126 | or similar |
Leather gloves | Uline | S-6777M | or similar |
Sturdy boots | any thick soled, leather boot. | ||
Emergency gear | |||
Garmin InReach | Cabelas | 100195666 | or similar |
Pulaski axe | Forestry suppliers | 85274 | or similar |
Fire rake | Forestry suppliers | 85210 | or similar |