operation of biochar-producing air curtain burner

Air Curtain Burners and Biochar &#8211; Advancing Forest Health and Carbon Capture

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&#39; 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.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/66716/66716fig01.jpg" /> 
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. <a href="https://www.jove.com/files/ftp_upload/66716/66716fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
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.

Author Spotlight: Advancing Place-Based Biochar Production for Ecosystem Restoration and Soil Health

Continuous In-woods Production of Biochar Using a Trailer-Mounted Air Curtain Burner

Our research focuses on implementing place-based biochar production methods as a strategy to reduce hazardous fuels in the forest while simultaneously improving forest and soil health. Currently, most biochar is created in large-scale bioenergy plants, but place-based technologies have a place in many areas where it's too expensive to ship low value woody residues or where infrastructure is lacking. We have learned the appropriate application rates, production methods, and additional uses for in-woods-produced biochar.We're beginning to understand its usefulness for mine site restoration, reduction of invasive weeds, and soil health benefits.

Operation of Biochar-Producing Air Curtain Burner

After placing the biochar-producing air curtain burner, load the firebox with dry feedstock using a mini excavator or by hand if a mini excavator is not available. Fill the firebox to approximately 2/3 full, alternating the direction of the materials to create air spaces that assist in burning. Allow the base fire in the firebox to burn until it is about 1/2 full of burning residues and coals.Then, start the engine and adjust the fan speed according to the fire conditions. Take care not to blow out the fire when advancing the air flow with the green rabbit button on the control panel. Once a solid base of coals is established, signal the mini excavator driver to add more feedstock without smothering the fire.Once the base fire has stabilized and additional feedstock has been added, set the fan speed to 2, 800 rpm. After that, switch the system power switch to biochar. Then, activate the conveyor and shaker, which will cycle every three minutes automatically.To maintain continuous biochar production, alternate the system power switch from biochar to firebox. Using the wide rake, pull the biochar to the end of the quench pan, then use the sifting shovel to transfer the biochar from the pan to a pile. Check the pressure gauges on the main control panel when responding to warning strobes.Turn off the shaker to burn down bound wood or manually advance the conveyor belt counterclockwise at the manual control point to clear a conveyor bind. Based on field trials, the biochar-producing air curtain burner can burn approximately one ton of feedstock per hour and produce carbon content ranging from 70 to 90%

Research

JoVE Journal

Environment

Fuel treatments and other forest restoration thinning practices aim to reduce wildfire risk while building forest resilience to drought, insects, and ...