The following is an excerpt from Kelpie Wilson’s new book, “The Biochar Handbook” (Chelsea Green Publishing June 2024), and is printed with permission from the publisher.

With real-world examples and hands-on applications, this go-to guide aims to offer a comprehensive look at the principles and practices of biochar. It provides information that can help tree care companies, if not create a new revenue stream, manage disposal of woody debris and reduce operating costs. Based on decades of research, Wilson demonstrates that biochar is a low-tech but effective means of reducing wildfire risks, restoring soil carbon, managing manure, weaning farms off of toxic inputs and producing the best compost ever made. In this book, you’ll also find:
A pocket history of biochar.

• Step-by-step instructions for making biochar.
• Applications for soil water retention, pest deterrence, compost enhancement and more.
• Inspiring examples of ecosystem restoration and improved forest management.

The following excerpt looks at one method of making biochar applicable to various-sized operations.

Making biochar in existing furnaces

One way to make biochar in a flaming combustion process is to make it in a furnace. A furnace is simply an insulated burn chamber, with or without an integrated heat exchanger, that accepts biomass loaded manually or through automated systems. Wood stoves are common examples. Industrial wood-fired furnaces are often called “boilers,” and there are many different designs and sizes. Smaller furnaces mostly have fixed beds, while larger furnaces often have fluidized beds to allow clean combustion of large amounts of fuel.

Here’s a little secret about the biochar industry in the United States today: The biggest commercial biochar producers are retrofitted biomass power plants.

Biomass power plants use ground-up wood and operate much like a coal-fired power plant; fine-ground fuel is blown into a furnace with boiler tubes. The fuel burns and heats the water to make steam that drives a steam turbine to produce electric power. The exhaust gas continues up through a stack, but it first goes through a cyclone filter to drop out large particulates, and then a series of other pollution controls to filter out fine particulates.

With modern pollution controls, these wood-fired power plants are very clean. But the interesting thing about them is what happens to the large particulates. Because the transit time in the furnace is unavoidably fast, these particulates are not just made of mineral ash. They also are partly composed of carbon that charred but did not burn completely.

In the Pacific Northwest

In the Pacific Northwest, power plants and furnaces at sawmills that are used to dry lumber have been producing mountains of this “high-carbon ash” from their cyclone filters. Instead of paying to landfill it, they have quietly offered it to farmers. When I started reaching out to farmers in Oregon about the benefits of biochar, I soon ran into some who had used biochar for years, already in the form of this ash. We tested a few samples of it and found it was about 40% carbon.

In Canyonville, Oregon, Michaels Ranch produces grass-fed beef on pasture soils that are low pH (between 5.2 and 6.0) and somewhat low in potassium. Several years ago, Troy Michaels began using high-carbon boiler ash as a liming agent and as a supply of potassium. Boiler ash is an economical alternative to lime, which is very expensive in Oregon. Lime spreading costs $100 an acre or more for one ton of lime. While the high-carbon ash has only about one-seventh of the liming value of lime, it is free and the sawmills will deliver it to the farm. Michaels noticed that the fields where he had applied boiler ash greened up faster in the spring, so he was very interested in learning more about biochar.

Most users of biochar do not want to apply that much ash, so some of these boiler operators have figured out how to filter out the ash component to produce a biochar product that is close to 85% carbon.

A biomass power plant

One biomass power plant that has jumped into the biochar business is Biomass One, a 30-megawatt biomass power plant in White City, Oregon. They can produce 3,500 to 4,000 tons of biochar a year with no impact on the power output of the plant. It requires a very slight increase in the amount of biomass fuel. The largest share of the company’s revenue comes from electricity sales, followed by tipping fees from their green-waste drop-off service. Biochar and a soil-blending yard make up the last piece of the revenue stream.

Karl Strahl, the company’s vice president in charge of biochar sales, says that even though they make money selling electricity, it makes more sense to think of Biomass One as a wood-waste recycling plant that recovers value from waste in the form of electricity and biochar. Biomass One is a great example of how we can squeeze every last bit of value out of what we often consider waste. In Oregon, where woody debris is abundant and transportation costs will never support the export of woodchips, being able to fully use this resource locally is an important contribution to our self-sufficiency.

The power of smaller biomass furnaces

Smaller biomass furnaces also can make biochar while producing energy. The term for this technology is “combined heat and biochar,” or CHAB. While a plant that produces electricity would cost many millions of dollars to site and build, smaller units that provide heat can be had for much less. CHAB technologies can include retort systems, but gasifiers and furnaces are more common. One example of a simple, robust but powerful CHAB furnace is a unit from BioMass Energy Techniques (BET), a Mennonite-owned company.

Basic BET furnace applications

The basic BET furnace design exists in dozens of installations, mostly at sawmills, to provide heat for lumber-drying kilns. In its CHAB configuration, it uses an inclined grate with a char-removal auger below the grate. The long, highly insulated burn chamber and limited primary-combustion air produce gasification conditions that can promote water-gas reactions to liberate burnable hydrogen from water.

This allows the use of feedstocks with higher moisture content. In this furnace, green woodchips provide more heat energy than fully dried woodchips, and optimum moisture content is a minimum of 25%.
The open grate is designed so that as the woodchips char and shrink, they will fall through the grate to a char-removal auger before they burn completely to ash. An induction fan controls airflow. The furnace is designed for very low airflow, which means low transport of particulates and clean flue gas. The furnace can be supplied with an integrated boiler or a flue gas-to-air heat exchanger.

I was able to visit a sawmill in Oregon that has one of the BET furnaces. This furnace does not make biochar, but it was interesting to see how simple and robust it is. The furnace has very few moving parts, which is important for reliability and reduced maintenance needs. Adding a char-removal auger would add more moving parts, but the company has a track record of manufacturing reliable equipment that is in service for decades.

BET 49-S

The BET 49-S model that makes biochar supplies 3.5 million BTU (British thermal units) of heat per hour while producing 214 cubic yards (164 m3) of biochar per month of operation. The heat in one BTU equals roughly the heat supplied by one wooden kitchen match. To visualize this, think of how many million kitchen matches it would take to heat a giant warehouse in the middle of winter in Nebraska.

CHAB technologies

To see a comparison of different CHAB technologies, you can read a report I produced for the Nebraska Forest Service and the U.S. Biochar Initiative (USBI), with the help of Tom Miles, USBI executive director: “Combined Heat and Biochar Technology Assessment for a Composting Operation.” This report looked at technologies available from five different vendors that could provide heat and biochar for Big Red Worms, a worm farm and composting facility in Lincoln, Nebraska. The worms are in a 4,000-square-foot (372 m2) warehouse that is heated to protect them from harsh Nebraska winters. The peak heating requirement for the minimally insulated warehouse is in the range of 500,000 to 750,000 BTU per hour.

A CHAB system that provides heat for the worms and biochar to improve the composting operation could add a lot of value. Current heating bills to warm the worms run about $1,500 a month in the winter.

Woodchips are free and abundant, while the price of propane keeps going up. Add in the biochar and its value in producing more and better compost, and a CHAB unit could easily pay for itself. And there is no issue about providing enough heat while making biochar. The smallest CHAB unit I looked at produced one million BTU an hour. The largest could provide eight million BTU.

“The Biochar Handbook” is 288 pages with color images and illustrations throughout. It goes on sale June 27, 2024, on amazon.com, chelseagreen.com and bookshop.org.