Biochar: Properties and Potential

Introduction

The production and use of Biochar from a variety of organic materials presents an opportunity to capture and sequester carbon from the atmosphere while taking advantage of properties of the material in a range of applications, displacing the use of fossil-sourced alternatives. Biochar has a long history of use in helping cultures flourish while living on lands without which it is likely that the culture would not have survived.

In particular, the Amazon basin is noted for its poor soil conditions, yet archeologists have found evidence of large societies that were able to thrive there. By learning to produce biochar and incorporate it with soil, they created "Terra Preta" (TP), or black earth. Common characteristics of TP that enabled these ancient populations to feed themselves include its ability to absorb and store water and nutrients, nurture soil microbial growth, and bolster soil organic carbon content. With these qualities enriching the naturally poor soil conditions of the Amazonian Basin, human populations were able to thrive for over a thousand years, until the arrival of explorers from Europe in the 16th century.

Still found in pockets today, TP or black earth is still being studied to learn the basics of its makeup and the methods Amazonian tribes used to produce it many centuries ago. Converting biomass materials to biochar, much like the ancient tribes on the Amazon, isolates and stores atmospheric carbon in solid form. Absorbed in the form of carbon dioxide by plants during photosynthesis, this carbon can now displace the demand for carbon found in fossil form, either as coal or petroleum. Fossil carbon is believed to have been absorbed in the same way, but occurred millions of years ago.

There are strong arguments in favor of leaving ancient carbon (i.e., fossil fuels) sequestered in the earth as coal or petroleum and using the abundance of atmospheric carbon to grow plants and produce biochar that, in turn, can be put to use for a wide variety of applications. Soil quality enhancement, such as was done in the Amazon Basin many years ago, is only one of the many opportunities for its use.

What is Biochar?

Many definitions are floating around as to what is or isn't biochar, but for the purposes of this fact sheet, we will define biochar as follows:

Biochar: Thermally treated biomass that consists primarily of carbon

The first thing to note from this definition is that biochar must be made from biomass, which means that biochar can be a renewable resource when biomass production is done in a long-term, sustainable manner. Second, note that biochar is thermally treated. The application of heat is what transforms biomass into biochar. Lastly, biochar consists primarily of carbon. While raw biomass is about half oxygen by mass (when measured on a dry basis), the thermal treatment process that creates biochar drives off much of the oxygen, leaving a blackened material that is mostly composed of carbon.

It is important to note that not all biochar is equivalent, and the properties of biochar can vary dramatically. Most people are familiar with "charcoal" from wood, which is, in fact, one form of biochar that remains popular throughout the world today as a renewable cooking and heating fuel. However, that is only one of the many possible types of biochar that can be produced.

One of the main factors that impacts biochar's performance is the "severity" of the thermal treatment. In a simplified way, we can think of biochar processing as being either mild, moderate, or severe:

The properties of biochar can also vary depending on the feedstock type. For example, biochar from wood is likely to have different properties than grass-based biochar. This is, in part, due to chemical differences between the feedstocks, but is also due to the different physical structure (at the microscopic scale) of different plant materials.

Biochar properties can also be impacted by the characteristics of the processing parameters and equipment. The control of temperature, heating rates, holding time, off-gases, use of steam or other carrier gases, and physical handling systems can all impact the quality and behavior of the final product. For example, biochar from fast pyrolysis reactors tends to be very finely grained, as a result of the physical and thermal processes used by the equipment.

Biochar typically contains 50 to 80 percent carbon, depending on the parent substrate and processing parameters. It can be made from a wide variety of organic materials, including wood chips and tree cuttings, bagasse, distiller's grains, press cake from vegetable oil and juice production, rice and nut hulls, crop residues (corn cobs and stover, straw), sewage sludge, poultry litter, dairy manure, and others. Biochar is considered one of the best ways to sequester carbon because biochar is very stable in the environment, with an estimated life of at least 1000 years, but some studies estimate an average of 5000 years. Many cave drawings (petroglyphs) were done using char or charcoal, which is good evidence of its longevity.

As a result of all these variables, biochar's properties can vary widely. It can be hydrophobic or hydrophilic and can have varying amounts of surface area, different reactivities, different strength characteristics, and varying electrical properties. In addition, further processing of the biochar (often in the form of either "activation" or "graphitization" can change those properties even more, giving it valuable characteristics for different end uses. Because of this, it is important to know what "kind" of biochar you are either producing or seeking to purchase. The following list outlines some of the "specifications" you may want to consider when selecting a biochar:

Benefits of Biochar

Biochar is produced from organic material that either grows naturally, is grown as a result of agricultural activity, is manure from farm animals, or is waste material such as cardboard, waste wood material, or other organic material that cannot be recycled. The carbon content of the given material was absorbed from the atmosphere through the process of photosynthesis. So, first and foremost, Biochar in its basic form is a carbon-neutral material. It can even be thought of as "carbon negative" if its end use involves a long lifetime in the form of elemental carbon rather than CO₂.

Since biochar can be produced from such a wide array of low-value materials, there can be little to no competition for the variety of materials from which it can be produced. In fact, production of biochar can often help reduce amounts of unwanted waste by utilizing materials that do not otherwise have a marketable value. Furthermore, since biochar can be produced from invasive plants, "weed trees" and similar materials, it is possible to use biochar as a management tool to improve the local ecosystem through strategic harvest of overgrown or undesirable species.

How is Biochar Made?

As we mentioned above, biochar is made using a "thermal process." This essentially means that biochar is biomass that has been baked at an elevated temperature in an oven-like device called a "retort." At its heart, this is not an overly complex process, and most of us have created biochar at some point by, for example, leaving bread in the toaster too long.

Biochar manufacture can be either a dry process (roasting) or a wet process (hydrothermal processing). Usually, dry processing is less expensive, but wet processing can be worth considering, especially if your biomass feedstock has very high moisture content. During the thermal process, much of the dry mass is lost from the biomass, and yields can range from 90% down to 10%. The temperature of the process can vary from 200 to over 1000°C, and the duration of the treatment may vary from a fraction of a second to several hours, depending on the desired result. In all cases, the amount of oxygen available to the feedstock must be drastically limited - otherwise, the feedstock has a tendency to burst into flames. Occasionally, elevated pressure is also used to affect the thermal processing of the biomass (especially when wet processing is used).

Another way to think about biochar manufacturing is that biochar can be manufactured either as a primary product or as a byproduct of another process. In terms of primary (or "dedicated") biochar production, a wide range of equipment can be used, ranging from something as simple as a 55-gallon steel drum to something as sophisticated as a continuous flow, computer-controlled industrial retort designed to manufacture tons of biochar per hour. The elevated temperature used to convert biomass into biochar can come from an externally applied source of heat, but is often generated by burning a portion of the feedstock.

Biochar as a byproduct is char that is left after some other material is created from biomass. The most likely source of byproduct biochar is from either fast pyrolysis (creation of "biocrude oil") or from gasification (creation of "synthetic natural gas"). In both cases, the biochar is a small portion of the original mass of the feedstock.

Uses for Biochar

Biochar remains a material under development, a thousand years or more after it was first used by tribes in the Amazonian basin as a soil amendment. Comprised primarily of carbon, absorbed from the atmosphere in the process of photosynthesis, biochar is very porous, with surfaces pocked with thousands of pores. Each of these pores contributes to the total internal surface area. These pores and the large surface area of even a small amount of biochar enable the ability to attract, absorb, and retain water and dissolved compounds such as phosphorus and nitrogen, as well as metals such as lead and cadmium.

Biochar can be used as a soil amendment to potentially increase yields, improve soil structure, increase water holding capacity, decrease soil penetration resistance, increase pH, increase nutrient absorption, and reduce nitrous oxide emissions. When the proper amount and type is applied to the soil, it can remediate the runoff of excess water and nutrients while retaining absorption for plants during dry conditions. With the plentiful pore openings and ample internal surface area, biochar provides a substrate for microbial growth as well as for anchoring root systems, as a "reservoir" to store moisture and nutrients. Selection of the type of biochar is very important. A high specific surface area (SSA) biochar applied to soils at high rates can bind up herbicides and pesticides, making them ineffective. One study (Graber, 2011) found that using a high specific surface area biochar (EUC-800) at high rates affected the efficacy of sulfentrazone herbicide, while a low specific surface area biochar at the same rate did not. In some cases, when a high specific surface area biochar was applied at high rates, the herbicides were not effective at all, even at the highest label rates. Since biochar has a half-life of hundreds and possibly thousands of years, using the wrong product or the wrong rate could have long-term effects on the agronomic practices that work.

Biochar has a high pH, typically ranging between 10–11 pH, but there is variation depending on the feedstock. It will increase the pH of soils it is applied to and some have suggested that 3 tons of biochar is about the equivalent of 1 ton of lime, but other research (Collins, 2009) found it was a 31:1 ratio in sandy soil. The length of time biochar will buffer the soils is unknown.

Crop yield increases are often cited as a reason to apply biochar. A meta-analysis found lots of variation in reported crop yield increases, but an average increase of 10% is typical. Currently, there are no publications with a recommended application rate of biochar for any of the Mid-Atlantic or surrounding states. Utah Extension has published a factsheet that recommends a maximum application rate of 10 tons per acre and can be considered a one-time cost, as its benefits last many years. The publication cites that a supplemental nitrogen application may be required to provide enough available for the crop, as some of the nitrogen will bond to the biochar. Wood biochar costs about $350/ton plus $8/ton (2022 $) to apply, so the total cost would be about $3580 per acre. Applying a lower amount over 2 to 3 years can help spread the cost over multiple cropping seasons.

There are many abandoned mine lands in the Mid-Atlantic region that could benefit just as cropland does from biochar to increase the water-holding capacity of reclaimed soils and increase soil water percolation rates, soil flora activity, and nutrient levels. The improved soil properties would allow vegetation to start growing, which reduces soil erosion from rain and wind and ultimately improves surface water quality.

Biochar can be tailored for used as a remediation agent to capture organic and inorganic contamination in the soil or water. It will bind to heavy metals, toxic chemicals, pesticides, and nutrients to keep them from translocating in the soil profile. This could be used at abandoned mine sites or on roadside runoff, in tile line effluent, or in stormwater basins to capture pollutants and water-soluble nutrients. This trait can pose a problem if the wrong type of biochar is used, as it can bind up soil-applied herbicides used to control weeds.

In other applications, biochar can be used as a filtration medium in much the same manner as activated carbon. It is useful in filtering both liquids and gases as it absorbs various contaminants as well as odor-producing constituents related to manures and municipal wastewater treatment operations.

Biochar also has potential for use in the formulation of polymer products, the development of electronic components, or as a catalyst for industrial processes. It is also being tested as a filler in concrete and asphalt.

Researchers have also found that biochar may have benefits as a feed supplement for livestock, in that it can help to absorb ammonia within the digestive tract, thus enhancing the digestive process.

Finally (and perhaps most obviously), biochar has potential as a renewable substitute for coal, or "biocoal." Biomass that is subjected to a very mild thermal treatment (torrefaction) functions well as a renewable replacement for coal in power plants and other applications. In addition to the above, new applications for biochar are the topic of research, and we can expect additional markets to be developing in the coming years.

Economics

Biochar could be a value-added enterprise for loggers and forest managers by turning slash and non-marketable wood into biochar. An air-dry ton of wood slash processed using fast pyrolysis would yield 120 gallons of bio-oil and 500 pounds of bio-char. Based on $2.50 per gallon (2021-22) for wholesale heating oil, the bio-oil value is approximately $300, and the biochar value in bulk is about $115 for a total of $415 per ton of air-dried wood slash.

It could be left in the forest or sold as a soil amendment, and selling voluntary carbon credits could be used to offset the production costs. Biochar could potentially be a market for forests that are in need of thinning to prevent major forest fires or be managed more sustainably. There are mobile or portable biochar units that could be used in-forest to reduce transport costs of bulky materials; however, if the forest slash could be transported to a permanent site, the syngas and oil produced from the pyrolysis process could be captured. Potential uses for the syn-gas and oil might be an energy source for kiln-drying of lumber, to process heat for manufacturing. or to clean and sell as renewable natural gas or renewable oil.

The economics of biochar will dictate its success. A life cycle analysis in Ontario, Canada, indicated that to break even, transportation distances needed to be less than 200 km (125 miles) with a carbon offset of about $44 USD (Homeagain, 2016).

Conclusions

Biochar is a fascinating product with many possible uses and great potential to not only replace fossil fuel carbon products but also to be the basis for a variety of new materials and products. The market for biochar is still developing, and as such, there are opportunities that exist for the entrepreneur who is willing to learn about the technology and manage the risks of a dynamic, emerging commercial sector.