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

Summary

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.

Abstract

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.

Introduction

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 times^1,2. 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 soils^10,11.

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.

Protocol

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 Using a Class 5 commercial duty hitch (ball size 2-5/16 inch or 58.75 cm), hitch the BACB onto a 1-ton or larger pickup truck, adjusting the hitch height to ensure the frame rails are level for transport. Before towing, check the brake controller, break-away system, brakes, wheels, and trailer lighting. M…

Representative Results

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…

Discussion

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 sha…

Materials

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