Big Box Biochar Kiln Operation and Best Practices

Typically, a portion of the biochar produced on the site is collected by workshop participants in buckets or bags and applied to people's gardens or agricultural projects. Biochar is friable and can be broken into small pieces to more readily incorporate into the soil by driving over it with a vehicle, stepping on it with a hard surface beneath, or mashing it with the bucket of the mini-excavator. This material can also be referred to as charcoal and has been collected for outdoor charcoal cooking, potentially providing a locally sourced material to add to the culinary features of a meal.

Comparing Big Box flame cap biochar kilns to other biochar production methods¹², mobile carbonizers can process 63,502 kg per day (70 tons), compared with 12,500 kg per day with a Big Box kiln. The cost of mobile carbonizers is much higher than a Big Box kiln, starting at $500,000 to purchase, as opposed to less than $10,000 to have a Big Box kiln manufactured. Although a single Big Box kiln can process only 20% of the material that a mobile carbonizer can, it will cost only 2% of the purchase price of a mobile carbonizer.

Heated auger kilns can process up to 5,443 kg of biomass per day, as another example, which is much lower than the 12,500 kg per day capacity of Big Box kilns. Additionally, the cost of preprocessing (chipping) the material can be more than actually pyrolyzing the material. Furthermore, refined machines such as the heated auger will not tolerate soiled feedstock that is common in forestry operations; a shovelful of soil can shut down an auger kiln while a Big Box kiln can tolerate several shovelfuls of soil without significantly impacting the operation. Finally, the cost of an auger kiln can easily be 10 times that of a Big Box kiln.

The first Big Box kiln constructed is referred to as the BB16 as it measures 4.9 m (16 ft) long by 2.4 m (8 ft) wide and is a single-walled construction. It was originally 1.8m (6 ft) high and weighed close to 1,360 kg (3,000 lbs.), which required a larger excavator, a qualified operator, and a low-boy trailer, which led to scheduling challenges. This approach was oversized to deal with the typical Utah fuel loads, and at 1.8 m (6 ft) tall, it was very difficult to light or see what was going on down inside the kiln. To address these issues, to better scale this approach to Utah fuel loads, and to make it more accessible to the average forest manager, the height was reduced to 1.2 m (4 ft) tall. This makes it easier to see into and to ignite. It also got it down to 1,043 kg (2,300 lbs.), which made it manageable to transport with a more available pickup truck and trailer, and to move and operate with a mini-excavator that requires no previous experience and can be rented from most equipment rental shops.

The second kiln built by the UBRG is a double-walled construction, which allows for better heat protection to the operators and equipment near the kiln and allows for more even heating inside the kiln¹³. Part of this modification was to go from 12-guage steel to 14-gauge steel, which is thinner and lighter. The UBRG has done dozens of burns in these kilns and while they do get a little bent up in spots, they are not yet showing any obvious signs of heat-related metal fatigue. Certainly, additional learning is likely to occur and there is ample room for continued innovation.

The double-walled BB12 is the design that has garnered the most attention and is perhaps the most accessible/practical for fuels in the Intermountain West. Larger kilns will be more appropriate with more/larger fuels, such as the Northwest US. This method has been proven up to a 4.9 m (16-ft-long kiln). To date, Big Box kilns have been built by other parties in Utah, Colorado, Montana, Texas, and New York.

The kilns can be operated to maximize biochar production or maximize hazardous fuels reduction, or somewhere between. If hazardous fuel reduction is the primary goal, then the kilns can be loaded randomly and quenched only when the kiln is full of coals. If surrounding fire danger is low, such as when the ground is covered in several centimeters of snow, the kilns can be stacked high with fuels in the evening before the end of the shift and left to burn all night; thus, consuming fuels in a controlled space. If biochar production is the primary goal, the feedstock can be sorted into like sizes and the kilns loaded with similar size class material and quenched frequently to preserve the coals. Typically, it is a mixture of these opposing goals and the kilns are operated between these two extremes. The species of the feedstock is less important unless a biochar with specific properties is the goal.

A limited amount of smoke comes out of these kilns; the idea is that the flame cap consumes the combustibles as they rise through the column of heat. In 2019 and 2020, the Utah Smoke Management System Coordinator Paul Corrigan brought his emissions testing equipment to Big Box biochar kiln demonstrations near Logan, in northern Utah, and Moab, in southern Utah. In both cases, the equipment registered no increase in emissions from the kilns because the flame-cap consumes the combustibles as they rise through the heat column. In April of 2023, the USDA Forest Service Fire Lab's emissions testing team conduct emissions testing on the kilns in Tooele, Utah; these results are not yet available.

The workers tending the kiln will need firefighting hand tools such as shovels, rakes, Pulaskis, and chainsaws. Best practices include having all people present wear safety gear such as leather gloves, eye protection, fire-resistant clothing or at least natural fiber clothing; synthetic clothing must be avoided. Hard hats and leather boots, long sleeves, and pants help to protect operators.

Emergency communication; contingency planning: The location (often remote) of the operation must be considered in addition to the possibility of an emergency and the communication needs around that. It is crucial to know where local cell phone reception might work best; a satellite phone or emergency locator beacon such as a Garmin InReach would be highly recommended. It is important not to work alone.

In case of an escaped ember/spot fire, the lid must be placed on the kiln to prevent further sparks from exiting the kiln. The machinery must be used to quickly dig a fire line around the spot fire, and the burning fuels must be separated from the unburned fuels. The water source must be used to extinguish the fire. If extinguishment cannot be obtained immediately, call 911.

Biochar from Big Box kilns has been characterized by Control Laboratories in Watsonville, CA using the International Biochar Initiative (IBI) Laboratory Tests for Certification Program and the results show 85% Organic Carbon and 8% ash; these are the characteristics of a moderately high-quality biochar. Collaborators are experimenting with adding roll-off coasters on the bottom, similar to large garbage dumpsters, as well as a door that is one of the end-walls to aid in removing the finished biochar. It remains to be seen if these features remain operable after extreme heat exposure.