What is biochar and how is it made?

Biochar is a carbon-rich material that is made from biomass through a thermochemical conversion process known as pyrolysis. Don’t worry if that all sounds like a mouthful—read on for an introduction to thermochemical conversion with a focus on biochar. You’ll learn how biochar is made and the role it could play in building a sustainable, circular economy.

The challenge of organic waste

When it comes to building a circular economy, every kind of waste is either eliminated or recycled into new valuable materials. But waste comes in all shapes and sizes—there’s no single “magic bullet” for addressing every different kind. That’s why scientists and policymakers call for a diversity of tools and solutions for achieving sustainable economies and societies. To understand biochar, it’s worth appreciating a specific form of waste—organic waste—and the problems it poses from a sustainability perspective.

The most common types of organic waste that we encounter in our day-to-day lives are food waste, yard trimmings and clippings, and—hold your nose—sewage. While these are each different in terms of material composition and life cycle (how they are made and disposed of), they tend to present similar challenges (and, as we’ll see, opportunities).

Loosely understood, any material that is immediately derived from plants and animals is organic. Another common term for this is biomass. Sometimes the term “biodegradable” is used to describe biomass as waste because it can be broken down into organic molecules by microscopic living things like bacteria, fungi, and microbes.

Most everything, from an old cookie to a car axle, eventually breaks down under the right conditions and with enough time. But organic materials break down much faster than inorganic ones do. Because of the fast pace with which organic matter decomposes, it presents unique challenges when it comes to mitigating its environmental impacts as waste.

When biomass degrades in a landfill or a treatment center, the potent greenhouse gases (GHGs) carbon dioxide and methane are released, among others. Methane is twenty times more potent than carbon dioxide as a greenhouse gas, though carbon dioxide remains in the atmosphere for much longer.  

The value of organic waste

One of the most interesting things about biochar is that it can turn what a lot of people think is useless into something valuable. Sustainability experts call this valorization.

“Valorization” might sound like an overly technical word, but what it describes is actually fairly simple: returning value to wasted materials. That value might be as an industrial additive, a new product, or even as a form of clean energy. The concept of valorization redefines the very idea of waste, applying instead a more dynamic understanding of how material changes over the course of its life cycle as a product. Using different methodologies and technologies, the properties and qualities of wasted biomass can be exploited to keep materials in circulation, rather than going into a landfill or a conventional treatment facility, both of which levy a heavy toll on our resources and ecosystems. Waste valorization is an application of the principles that underpin the concept of the circular economy.

Thermochemical conversion is a basket term for many different technologies and methodologies. In addition to biochar production, it offers many promising pathways for valorizing different kinds of organic (and inorganic) waste for different purposes.

Thermochemically converting organic waste

Organic waste can be converted into energy or new kinds of material in highly controlled environments. By varying heat, air pressure, or oxygen levels, the properties of biomass can be transformed, resulting in liquid and gas fuels as well as solid materials with new properties. The main types of thermochemical conversion are listed below.

  • pyrolysis
  • gasification
  • combustion


Each process requires different levels of oxygen to occur. Pyrolysis occurs when there is none. In gasification, there’s a limited amount, while combustion can’t happen without it.

Sustainable alternatives to incineration

Thermochemical conversion is a scientific discipline with a history that long predates its consideration as a sustainable pathway. Thermochemical processes, like gasification, have been applied to produce energy for more than two centuries. Coal and peat were “gasified” to fuel the first gas lamps in Victorian London, for example. Wood was gasified in Germany during both world wars to power vehicles when petroleum was unavailable. Yet, of these, the most well-known is probably incineration.

The incineration of waste—combusting or burning municipal solid waste (MSW)—remains a common practice throughout the world. Incinerating waste like MSW (which usually contains a mix of organic and inorganic materials) not only contributes high volumes of GHGs to the atmosphere, but it has been proven to release toxic gases and particles. The scientists, business innovators, and policymakers who are working to develop thermochemical conversion as a sustainable strategy for mitigating organic waste take great care to distinguish their novel work from conventional methods like incineration to achieve sustainable ends.  

Common feedstocks and products

Thermochemical conversion can be applied to one or more kinds of waste, individually called “inputs” or “feedstocks.” Some common feedstocks include the following:

  • food waste
  • municipal solid waste (MSW)
  • plastics
  • sewage sludge (also known as “bio-solids”)
  • agricultural by-products
  • cuttings and trimmings from parks and residences


As might be imagined, every thermochemical process results in a different final product (or “output”) when applied to a different feedstock. Three basic categories of outputs are possible, depending on the combination of feedstocks and methodologies: gas, liquid, and solid products. One methodology can lead to a combination of all three product types, depending on factors like temperature, air content, and pressure. 

Gas products: The gasification of wasted biomass can result in industrially valuable gas-phase products like light alkalines and olefins, typically derived from petroleum, a fossil fuel. These gases can be used directly for heat, power generation, electricity, transportation, as well as chemical and plastic production. It’s also a potential source for pure hydrogen that can be used to generate green hydrogen energy.

Liquid products: Liquid products can also be made through thermochemical conversion. Pyrolysis and catalytic upgrading are two methods that can be used to create bio-oil or bio-diesel from biomass. Pure hydrogen can also be produced in this way, which can be added to fossil fuels like gasoline or liquid natural gas to improve efficiency and lower overall GHG emissions. Other bio-fuels like mixed alcohols, ethanol, and methanol can also be made using this method.

Solid products: Thermochemically treating organic or inorganic waste usually leaves behind a solid material or residue. When biomass is subjected to full pyrolysis, the result is biochar. However, combinations of partial or slow pyrolysis and gasification can be applied to organic and inorganic materials to create other materials like charcoal and carbon black.

What is biochar?

A fine-grained, highly porous type of charcoal made from biomass, biochar (despite the futuristic name) has been used by humans for over two thousand years as a soil enhancer. It helped to increase crop yields while sustaining essential soil biodiversity. One of the most well-known instances of naturally occurring biochar is in the Amazon, where native peoples there used (and still use) “terra preta” in their agricultural practices.

Natural biochar occurs when vegetation is left to smolder in layers on the forest floor following a forest fire. Plant and animal matter bakes slowly in a nearly oxygen-free environment. Today, biochar can be made in much the same way using a kiln, which allows for the precise control of its internal atmosphere and temperature.

There are no roaring flames in a biochar kiln. Instead, biomass of different kinds is slowly baked until it becomes a carbon-rich char. This process is pyrolysis, which refers to the chemical decomposition of organic material when exposed to elevated temperatures in an atmosphere with restricted levels of oxygen.

 

Sewage, yard waste, food waste, and other types of feedstock can be used to make biochar.

Biochar as carbon capture

Combustion—when materials burn in the presence of an oxygen-rich atmosphere—releases GHGs into the air, most notably carbon dioxide. In contrast, pyrolysis leaves most of the carbon in the original biomass trapped in a solid form. If, as an example, someone were to chop up a fallen tree and put it into a kiln, most of the carbon that the tree absorbed from the atmosphere over the course of its life would stay in the resulting biochar (which would be much smaller in volume than the original amount of wood).

One ton of biochar sequesters (stores) carbon that would have otherwise generated 3.6 tons of carbon dioxide if left to degrade by natural processes. As a form of thermochemical conversion, biochar not only valorizes waste, but it’s a very effective method for capturing carbon and storing it in a solid state that can remain stable for centuries.

Biochar as part of the circular economy

As an industrial material

A growing number of scientists and policymakers have turned to biochar as a powerful yet simple solution for addressing the climate challenges that follow from organic wastes like sewage, food, and agricultural by-products. A recent paper indicated that converting waste produced by China’s massive corn-growing industry into biochar could reduce the sector’s overall GHG emissions by 20 percent or more.

Kathleen Draper, a biochar researcher and board member of the International Biochar Intitiative (IBI), is a long-time advocate for new uses of biochar beyond soil enrichment. She wants to see biochar applied in many more ways, whether as an additive for construction materials like cement and concrete or as a manufacturing material that can be used to make plastics. Ultimately, Draper’s research seeks to unlock biochar’s full potential as a carbonate material that can be combined with others to make strong, durable composites for industrial use. If successful, such applications would valorize unsustainable waste streams while sequestering carbon.

The majority of biochar today is made from plant and animal biomass like residential plant trimmings, food processing residues, or forestry cuttings because it’s used to improve soils. Researchers like Draper believe that widening what feedstocks can be used to make biochar will, in turn, open new applications. Namely, they have in mind problematic sources of waste like sewage from treatment plants. These types of biomass could be pyrolized to make bitumen, carbon fibers, and other industrial materials currently made from fossil fuels.

Researchers at the Golisano Institute for Sustainability (GIS) at Rochester Institute of Technology (RIT) have explored how biochar can be made from specific types of waste to create specific products. For example, a cross-disciplinary research team led by GIS faculty member Thomas Trabold and Scott Williams, a professor at RIT’s School of Chemistry and Materials Science, successfully created a carbon-black ink using biochar made from cardboard. Another project saw GIS researchers and a team from RIT’s Department of Packaging Science, led by Carlos Diaz, find a way to make plastic coffee cup lids using biochar made from coffee grinds. Along with colleagues at RIT and across the Rochester region, Trabold continues to investigate how activated carbon from biochar could be used in to reduce the environmental impacts of materials and products like asphalt, concrete, and tires.

Sustainable energy through pyrolysis

Much of Sweden’s capital city’s heating comes from something most people don’t think about more than once or twice a year: yard waste. The Stockholm municipality collects sticks, leaves, and other trimmings from residences and parks to not only make biochar, but to capture a gas by-product of pyrolysis that works just the same as natural gas. The only difference is that it’s not a fossil-based fuel. The biochar itself is then delivered to gardeners and farmers to help them grow healthy plants.

In addition to clean energy and a circular soil amendment, the Stockholm Biochar Project is achieving a third, critical goal in support of the city’s plan to completely decarbonize: It is sequestering carbon from the atmosphere.

Integration with existing valorization technologies

There are already many methods for valorizing organic waste that are widely used. A major focus of current biochar research is to discover how pyrolysis and gasification can be paired with one of the most common technologies, anaerobic digestion.

Anaerobic digestion, a biochemical process used to convert food waste into energy, takes advantage of fermentation, the natural process whereby bacteria breaks down organic matter. It is used to turn waste from the food and agricultural industries into fuels like bio-gas. Effluent (or digestate) is what the bacteria cannot use. Today, effluent has limited economic value because it offers inconsistent results as an agricultural product, earning it the nickname “the waste of waste.”  

GIS’s Trabold was curious to see if thermochemical conversion could be used to help further valorize the effluent stream. His team applied pyrolysis to it to see what happened. What they found was surprising: The pyrolized effluent was magnetic. With these findings, Trabold and his colleagues at GIS are now exploring potential uses for the new material within the electronics industry. If successful, this would offer a sustainable, renewable pathway to replace raw-ore mining. 

Challenges

The path ahead for biochar is technically feasible and promising, but the road to full technological and market maturity is far from clear. The biggest challenges can be grouped into three areas: policy, logistics, and public perception.

Policy  

The viability of any technology depends heavily on there being supportive policies in place that will allow it to develop and mature. This is especially true when it comes to thermochemical-conversion technology, a field of sustainability that is still largely in its infancy. Thermochemical processing has gained the most traction among supporters of bioenergy with carbon capture and storage (BECCS). A catch-all for “negative-emission” processes and technologies, many policymakers are turning to BECCS as an alternative to carbon-credit trading (“cap and trade”) or carbon taxes when it comes to incentivizing climate-friendly economic growth.

Advocates of sustainable thermochemical conversion argue that biochar is an ideal application of BECCS because it both sequesters carbon and converts otherwise problematic waste into economic value. Other thermochemical conversion methods, like gasification, can also be combined with biochemical processes, like anaerobic digestion, as part of a BECCS strategy.

Despite this potential, biochar remains a rare feature of existing climate legislation, even those that include BECCS as a strategy. For example, New York State’s ambitious Climate Leadership and Community Protection Act of 2019 (CLCPA) stipulates that the state’s efforts must include bioenergy and BECCS technologies. While gasification and anaerobic digestion can be used to create low- or zero-carbon fuels like biogas and biodiesel, CLCPA does not recognize pyrolysis as a BECCS process. Biochar champions worry that this will have a knock-on effect that will leave biochar out of any policies following from the bill when it could offer substantial benefits.

New York State has the third largest number of dairy cows in the United States and offers a strong opportunity for what some call a circular “bioeconomy.” The large volume of cow manure that dairy farmers end up with—a well-known source of stress on the environment—could serve as a biochar feedstock, according to Johannes Lehmann, a professor of soil science at Cornell University who has collaborated with Trabold to draw the attention of New York’s policymakers to biochar. Lehmann has estimated that the potential value of the economy would be $272 million for farmers and $1.3 billion for retailers. In addition, it would cut transport costs by as much $114 million while lowering GHG emissions. 

Policy can also help grow markets for biochar-based products. Through targeted taxes and other incentives, governments can encourage startups and established businesses to innovate biochar products and applications.

Business, supply, and operational logistics

Today there are few facilities in operation that are designed for large-scale, sustainable thermochemical conversion. Those that do exist tend to be focused on a single method, whether it’s a biorefinery for the production of biofuels or a kiln for making biochar.

Siting a thermochemical conversion facility, whatever its size and purpose, is complex. Coordinating a specific feedstock to make a type of biochar that can be used to make a particular product needs to take into account many different variables. Is the feedstock available all year? Is it heavy or difficult to transport? Where is the market demand for the output? Will that be consistent in the long term? These are just a few of the many questions planners and businesses need to ask to evaluate all the contingencies that could determine whether a site will be successful or not.

Regional geography is especially important when considering where to site facilities. Poultry manure, for example, has a high phosphorous content that can be sustainably recovered through thermochemical conversion (rather than mined). There’s a market for phosphorous: Large-scale farmers rely on it for their crops. However, in a state like New York, a logistical challenge arises. The region where most industrial chicken raising happens is far from where most crop-heavy farming is done. That means a planner would need to carefully consider where a facility should be built that is practical and cost-effective, but that doesn’t offset intended sustainability goals.

In the end, the potential value of biochar-based products needs to match production capacity and market demand. It’s a balancing act all businesses know, but it can be especially challenging at the innovation stage of new, sustainable technologies.

A team of researchers based at Cornell’s College of Agriculture and Life Sciences is working to solve this problem. They have found that flexibility and education are essential. Thermochemical conversion facilities need to be equipped with technology that allows them to be flexible in what it produces in order to offer value to consumers and businesses up and down the supply chain. In practice, this might mean producing biochar for farmers to use in their soil on the one hand, while also being able to “upgrade” it into activated carbon for more industrial purposes. The team points to an opportunity for establishing standards and best practices that will support growth of the sector and the quality of its products. Along with these developments, stringent efforts are needed to increase the public’s awareness of biochar and sustainable thermochemical conversion more generally so that they can appreciate the economic and ecological benefits they offer.

Public perception of biomass

Thermochemical conversion offers unique pathways for turning otherwise impactful forms of organic waste like uneaten food, sewage sludge, and plant debris from agriculture and industry into fuel or economically valuable products. However, a barrier facing this corridor of innovation within sustainability has nothing to do with technology or science: It’s the use of the word “biomass.”

The use of biomass in waste-to-energy systems is by no means a new concept—and it’s not necessarily a green one, either.

Biomass is a scientific term for describing how energy from the sun is captured within plant and animal matter. Radiant solar energy is stored by plants through photosynthesis, where it is converted into chemical energy as glucose. From there, the carbon in those sugars finds its way throughout our ecosystem until, eventually, it’s released into the atmosphere as carbon dioxide. This natural cycling of carbon in the earth’s atmosphere is not itself a problem. It contributes to climate change only as a fraction of the many additional megatons of carbon that go into the air through the combustion of fossil fuels (which are extracted from fossilized biomass).  

Direct combustion—a thermochemical conversion process—is when biomass is burned, something that humans have done for thousands of years to provide heat and to cook food. In many developing countries, burning plant material remains the principle form of energy usage. Such practices have been linked to a high incidence of respiratory illnesses due to constant exposure to fire smoke. Elsewhere, wooden biomass fuels what are called wood-to-energy power plants, where wood pellets are burned to produce power as a replacement to coal. Biomass provided about 5 percent of the total amount of primary energy used in the United State in 2019, the equivalent of nearly 5 quadrillion British thermal units (BTUs).

But many question the sustainability of such wood-to-energy strategies. England’s Drax power plant, for example, uses pellets made in the southeastern United States, adding a considerable carbon footprint to the total life cycle of the fuel. In some cases, wood-burning furnaces can actually have higher net carbon emissions than coal or natural gas plants per unit of electricity. Many experts and activists have also raised concerns over the impact of wood-burning plants on human health.

Biomass encompasses a lot of different materials, from felled trees and sticks to leftover pizzas and New York City’s sewage. It’s clear that a life cycle assessment (LCA) of biomass used to create biochar would produce very different results from an LCA of a wood-pellet power plant. What matters most when talking about biomass in the context of sustainability is not so much what it is, but what’s being done with it.

One thing is clear: Biochar has a clear role to play as part of the circular economy. It’s an ideal solution for turning emission-heavy organic wastes into value that can drive an economy locally, nationally, and globally.

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About the author

Golisano Institute for Sustainability (GIS) is a global leader in sustainability education and research. Drawing upon the skills of more than 100 full-time engineers, technicians, research faculty, and sponsored students, it operates six dynamic research centers and over 84,000 square feet of industrial infrastructure for sustainability modeling, testing, and prototyping. Graduate-level degree programs are also offered that convey the institute's knowledge to the next generation of industry professionals.

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