A relatively simple and ancient product shows promise in mitigating the negative effects of climate change, while also improving soil quality, producing energy, and reducing waste. It’s called biochar, a black, fine-grained, highly porous, and lightweight substance with a large surface area. With a carbon content of about 70 percent, it also contains oxygen, nitrogen, hydrogen, and other elements, depending on what the biochar is made from and how it’s heated.
Producing biochar actually reduces carbon dioxide during production, so it is considered a carbon-negative material, as it takes unstable carbon from rotting organic material and converts it into a stable form. Once it’s put into soil, it keeps carbon securely in the ground for hundreds, or even thousands, of years. Before looking at how to process and use biochar, let’s first answer these two questions: “What is biochar?” and “What is biochar used for?”
What is Biochar?
So, what is biochar? Used for improving and maintaining soil fertility for millennia by a number of cultures, biochar is made from organic waste. It results from heating biomass at temperatures between 300-700°C (572-1292°F) while depriving it of oxygen. Biochar is made via a process called slow pyrolysis, and it can be produced from a wide variety of biomass feedstocks.
What biochar is made from includes:
- agricultural waste
- animal manure
- green urban waste
- paper products
- rice husks
Biochar is sometimes known as agrichar, pyrochar or simply char. Occasionally referred to as black carbon, it excludes those types not made from biomass waste or that are produced from derivatives of fossil fuels. Sometimes biochar is just called charcoal, which is essentially what it is. However, unlike many charcoals used for cooking, it has none of the additives that help it burn.
A Brief History: Where Does Biochar Come From?
Although considered a modern strategy for sequestering carbon, biochar’s origins are ancient. Using charred biomass to improve the quality of soil goes back to a practice used in the Amazon basin over two thousand years ago. Here indigenous tribes used a blend of charcoal, bones, broken pottery, composted organic matter, and manure to add nutrients to the otherwise nutrient-poor forest soil of the Amazon basin, where rain and floods leach away much of the nutrients in the soil.
Known in Portuguese as “terra preta” and in English as “black (or dark) earth” due to charcoal content, it’s largely the result of soil management techniques and slash-and-char agriculture. Terra preta contains nutrients like calcium, manganese, nitrogen, phosphorus and zinc while also showing significant microorganic activity.
The origins of these soils go back to farming communities that existed in the Amazon between 450 BCE to 950 CE, which in some places reached depths of up to two meters (6.6 feet). Terra preta is now thought to have formed from kitchen waste that was cast out onto middens, though there’s evidence it was also used to farm larger areas around these settlements. Less fertile soil called “terra mulata,” or “mixed earth” was likely improved by adding terra preta to make the nutrient-poor soils on the forest floor more fertile.
Little other evidence of these ancient forest communities remains, as it’s thought much of their architecture was made from wood, which decomposes quickly in tropical conditions. However, evidence of manmade structures and geoglyphs similar to the Nazca lines in Peru has been uncovered in deforested areas within the Amazon basin. Yet it is the use of their sophisticated agricultural techniques that utilized biochar to trap carbon within the soil that has most piqued the interest of scientists worldwide.
What is Biochar Used For?
There are various methods for applying biochar to agricultural land. What biochar is used for largely depends on how and from what it’s made, along with the specific conditions of the earth to which it’s added? When used for agriculture, biochar is often mixed with compost or other materials that encourage the growth of beneficial organisms. Biochar is still largely used for its original purpose, supplementing nutrient-poor soils that have been degraded.
Applications that biochar is used for include:
- adding moisture to dirt
- augmenting soil structure
- bettering water quality and retention
- decreasing emissions of nitrous oxide
- enhancing microbial properties
- improving soil health and fertility
- increasing agricultural productivity
- lowering emissions of greenhouse gases
- promoting electrical conductivity
- raising pH and decreasing acidity in soil
- reducing pressure on old-growth forests
- regulating nitrogen leaching from earth
- remediation of polluted earth
Biochar helps with composting by reducing greenhouse gases released when material decomposes, along with promoting the activity of microbes to accelerate the process. It additionally assists with reducing ammonia loss, bulk density, and smell resulting from composting.
How is Biochar Good for the Environment?
More modern use for biochar involves sequestering carbon in the ground to mitigate climate change. Though it’s unknown how long biochar keeps carbon contained in soil, certain estimates suggest it could do so for up to five thousand years. In fact, biochar offers an immediate solution for reducing agricultural waste and the negative impact of largescale farming operations.
When organic matter decomposes naturally or is burnt, it releases large amounts of carbon dioxide. By storing carbon in the earth, biochar presents a means to significantly reduce greenhouse gases that contribute to climate change, essentially acting as a carbon sink. Due to the fact production varies considerably based on the composition of feedstocks, conditions under which it’s processed and different levels of technology used, biochar can’t be considered a commodity.
It's been suggested that subsistence farmers in central Africa that utilize slash-and-burn agricultural techniques would benefit greatly from biochar, contributing to sustainable development and helping to alleviate poverty. By adding nutrients to the soil, biochar can also address food insecurity by making nutrient-poor agricultural land more productive. Subsistence farmers would then no longer need to move and cut down trees to clear land for agriculture, decreasing deforestation and biodiversity loss.
Additionally, biochar can be utilized as a fuel for cooking, or even small-scale energy production. As a byproduct of processing, biochar production also produces energy in a clean and renewable manner, replacing fossil fuels that contribute to climate change. While it can be used to filter water directly, biochar also reduces pollution in groundwater and retains moisture in the soil, an advantage for farmers in drought-prone areas.
Alone or blended with other soil additives, it can also address numerous challenges involving agriculture, ecosystems, and forests. Beyond this, various US government agencies are researching how biochar can be used to remediate the environment by binding to chemicals and heavy metals in runoff coming from fields and roads.
How is Biochar Processed?
Now that we know what biochar is used for, let’s look at how it’s made. The most common modern method for producing biochar involves a process called pyrolysis. This essentially means that the biomass is broken down by heat under low oxygen conditions. The ideal feedstock for producing biochar has a moisture content between 10-20 percent, with high lignin content. These feedstocks might include residues from fields after harvest or woody biomass. Care must be taken not to use contaminated feedstocks, however, as it will introduce these toxins into the soil. Though currently biochar projects are only being done on a smaller scale, technology has developed sufficiently that it has the potential to make a significant impact on carbon sequestration.
For pyrolysis to take place properly, it must go through the following steps:
1. Drying & Conditioning
Though proportions differ depending on the feedstock, biomass generally has five core elements. These are cellulose, hemicellulose, lignin, minerals (basically ash), and water. First, most of the moisture in the biomass is removed by heating it above 100°C (212°F). Once it reaches 150°C (302°F), the biomass begins to break down and soften, a process known as conditioning. During this process, water that’s bound chemically is released by the biomass, along with minor amounts of carbon dioxide and volatile organic compounds (VOCs). For a high yield of quality biochar, it ideally has a moisture content of around 15 percent upon entering the pyrolysis kiln.
Biomass is heated further until it reaches between 200-280°C (392-536°F), which causes the chemical bonds within the biomass to break down. This process is referred to as endothermic, requiring heat input to increase the temperature and causing molecular bonds to break down. This process releases acetic acid, methanol, and VOCs that have been oxygenated. Carbon monoxide and carbon dioxide are also emitted during this stage as cellulose and hemicellulose break down. This torrefied biomass is more brittle at this point than it was initially, so grinding will be easier and use less energy. At this stage, the biomass can be stored for later processing, as it can resist water uptake and biological degradation. The vapors resulting from low-temperature pyrolysis condenses to become pyroligneous acid, known also as liquid smoke, smoke water, or wood vinegar. This substance can be used to help seeds germinate, promote plant growth, accelerate composting, or as a means to enhance biochar effectiveness.
3. Exothermic pyrolysis
Depending on the initial feedstock, the thermal decomposition of the biomass intensifies between 250-300°C (482-572°F), releasing a mixture of carbon dioxide, carbon monoxide, hydrogen, methane, and other hydrocarbons, along with tar. The process now becomes exothermic as large polymers in the biomass break apart and release energy. This liberates some oxygen within the biomass, which then reacts with the char and gases. By releasing energy, the chemical bonds are further broken and the process becomes self-sustaining until it reaches about 400°C (752°F), once all the oxygen is depleted, which leaves a charcoal-like residue rich in carbon. This achieves a maximum yield before the exothermic pyrolysis process ends, leaving variable levels of ash depending upon the initial feedstock. With wood biochar, about 1.5-5 percent is ash content, 25-35 percent is made up of VOCs and fixed carbon makes up the balance, falling between 60-70 percent.
4. Endothermic pyrolysis
Remaining biochar after exothermic pyrolysis still contains significant amounts of VOCs. To increase fixed carbon content, further heating is necessary, which also increases porosity and surface area by ridding it of VOCs. Typically it takes temperatures from 550-600°C (1022-1112°F) to achieve a carbon content of 80-85 percent, with a lower VOC content of around 12 percent. These temperatures usually produce 25-30 percent of what the original feedstock weighed.
5. Activation & gasification
As temperatures exceed 600°C (1112°F), introducing small amounts of air and steam will raise surface temperature to 700-800°C (1292-1472°F).
This causes two processes to occur:
- Activation: The combination of air, heat, and steam activates the biochar surface area to release more VOCs, which increases the surface area and decreases yield.
- Gasification: When large amounts of air and/or steam are introduced into this process, it produces a relatively clean gas that can generate electricity, though it results in lower biochar yields with high ash content.
Feedstocks with high ash content undergoing this process can cause minerals and other inorganic compounds to melt. The tradeoff with producing biochar via gasification biochar rather than the slow pyrolysis method is the higher likelihood of toxic compounds within the biochar, and its reaction may differ.
Prater Machinery for Biochar Production
Prater Industries builds industrial equipment and develops systems used in the production of biochar. The company’s equipment is state-of-the-art, durable, long-lasting, and reliable, offering solutions for biochar processors with high-capacity throughputs or smaller production requirements. Prater’s hammermills, fine grinders, and classifier mills provide functional efficiency under harsh conditions for any biochar processing system.
Prater has been selling hammermills since its founding nearly a century ago. During this time, the company has refined and innovated upon its original designs, customizing its hammermills to work optimally under various conditions and for specific applications. Utilizing a grinding chamber made with separate zones for grinding and releasing, enables the product to feed into it from the top of the unit. Afterward, high-speed hammers impact the material, forcing it against stationary cutting plates at the top of the chamber. Prater’s hammermills are designed to process biomass efficiently, screening out undesirably-sized particulates. They discharge into a hopper below the mill, where pneumatic systems or mechanical conveyors move material on to the next stage. Prater offers a full range of hammers, screens, and other implements to customize our hammermills for any biochar grinding application.
Fine grinders made by Prater operate via high-speed impact. Raw biomass can be fed through meters into the mill’s center, where rotor blades cause collisions between individual particles. Prater’s fine grinders accelerate material outwards to enhance shear and impact against the grinder’s screens and stationary jaws. The static jaws cause particles to decelerate, which maximizes impact speed and causes them to rebound back against the rotor blades. Once properly sized, processed biomass can be pulled through a screen and transported to the next stage of processing.
When it comes to air classification, Prater’s classifier mills work in three stages.
- First stage grinding: Conveys air and material from behind the rotor into grinding blades, which impact and accelerate particles outwards while facilitating additional collisions against jaws and screens.
- Classifying stage: After the first stage of grinding, particles gather outside the grinding chamber, with secondary intakes drafting pneumatic air to help fluidize and cool particles. Pulling material inwards to the classifying rotor, it separates particles by size, with right-sized particles passing through the rotor and then conveyed pneumatically to the next stage. Particles rejected during this process are recirculated.
- Second stage grinding: Rejected particles re-enter to the front of the rotor, with grinding blades again accelerating and impacting particles outwards. Grinding ring segments reduce size more aggressively, with the material combined with other right-sized particles to the classifier.