Biochar is a type of charcoal, sometimes altered, that is used in soil. It is the lightweight black material left after biomass is heated without oxygen, made mostly of carbon and ash. Although its name includes "bio," biochar has no living organisms right after it is made. It only gains life when exposed to living things. According to the International Biochar Initiative, biochar is defined as "a solid material created by heating biomass in an environment with little oxygen."

Biochar is used in soil to improve air flow, reduce greenhouse gas emissions, increase soil nutrients, prevent nutrients from washing away, lower soil acidity, and possibly increase water storage in sandy soils. Adding biochar to soil can improve soil quality and crop production. However, using too much biochar or using it on soil that is not suitable for it can cause problems, such as harming soil life, reducing water availability, changing soil pH, and increasing salt levels.

Biochar is also used in farming methods like slash-and-char, to help soil hold water, and as an ingredient in animal feed. Scientists are studying how biochar might help reduce the effects of climate change. Because biochar is very stable, it can remain in soil or other environments for thousands of years. This has led to the idea of using biochar to remove carbon from the atmosphere, a process called carbon sequestration. Carbon removal can happen when high-quality biochar is added to soil or used in materials like concrete and tar.

Etymology

The word "biochar" is a new word created in the late 20th century. It comes from the Greek word "bios," which means "life," and the word "char," which means charcoal made by burning plant material without oxygen. Biochar is a type of charcoal that helps support living processes in soil, water environments, and the digestive systems of animals.

History

Pre-Columbian Amazonians made biochar by burning agricultural waste, such as plant material, slowly in pits or trenches covered with soil. It is unknown whether they used biochar to improve soil fertility. European settlers named this type of soil "terra preta de Indio." After studying the soil, a research team in French Guiana suggested that a type of earthworm, called Pontoscolex corethrurus, was likely responsible for breaking down charcoal into small pieces and mixing them into the soil.

Production

Biochar is a high-carbon, fine-grained material made through a process called pyrolysis. This process involves heating biomass without oxygen, which stops it from burning. It creates three types of products: solid biochar, liquid bio-oil, and gas syngas.

Gasifiers are the main source of biochar sold in the United States. Gasification has four main steps: oxidation, drying, pyrolysis, and reduction. During pyrolysis, temperatures range from 250–550 °C (523–823 K). In the reduction zone, temperatures are 600–800 °C (873–1,073 K), and in the combustion zone, they reach 800–1,000 °C (1,070–1,270 K).

The amount of biochar produced during pyrolysis depends on factors like temperature, heating speed, and how long the material is heated. Lower temperatures (400–500 °C) create more biochar, while higher temperatures (above 700 °C) produce more liquid and gas fuels. Pyrolysis happens quickly at high temperatures, usually in seconds. Higher heating speeds reduce biochar yields when temperatures are between 350–600 °C. Typical results include 60% bio-oil, 20% biochar, and 20% syngas. Slow pyrolysis can make more biochar (about 35%), which helps improve soil. Both fast and slow pyrolysis create more energy than they use. For example, fast pyrolysis uses about 15% of the energy it produces. Plants that make biochar can use syngas to generate 3–9 times more energy than they need.

The Amazonian pit/trench method does not collect bio-oil or syngas. Instead, it releases carbon dioxide, black carbon, and other greenhouse gases into the air. However, it produces less greenhouse gas than is captured during plant growth. Commercial systems process waste like agricultural materials, paper, and city trash. These systems avoid releasing harmful gases by capturing and using liquid and gas products. In 2018, a winner of the X Prize Foundation used a gasification process to collect drinking water from the drying stage. Biochar production is usually not the main goal in these systems.

Small farmers in developing countries can make biochar without special tools. They pile crop waste (like corn stalks, rice straw, or wheat straw), light the top, and cover the embers with dirt or water. This method reduces smoke compared to burning waste directly. This technique is called a top-down burn or conservation burn.

Industrial methods can also be used on small scales. In some systems, unused biomass is sent to a central plant for processing. Alternatively, farmers can use a mobile pyrolyzer that travels between locations. The truck’s power comes from syngas, and biochar stays on the farm. Biofuel is sent to refineries or storage. Choices about system types depend on transportation costs, the amount of material to process, and the ability to supply energy to the grid.

Companies in North America, Australia, and England sell biochar or equipment to make it. In Sweden, the "Stockholm Solution" uses 30% biochar to help urban trees grow. At the 2009 International Biochar Conference, a mobile pyrolyzer that processes 1,000 pounds (450 kg) of material was introduced for farming.

Common crops for biochar include trees and energy plants like Napier grass, which store carbon more quickly than trees. For crops not used mainly for biochar, the residue-to-product ratio (RPR) and collection factor (CF) help estimate how much material is available. For example, Brazil harvests 460 million tons of sugarcane yearly, with 30% of the tops (RPR = 0.30, CF = 0.70) and 29% of bagasse (RPR = 0.29, CF = 1.0) available for pyrolysis. This provides about 230 million tons of material for energy and soil improvement. Some residue must stay on the soil to avoid extra costs and emissions from fertilizers.

Besides pyrolysis, torrefaction and hydrothermal carbonization can also break down biomass into solid material. However, these products are not strictly called biochar. Torrefaction creates a material with some volatile organic components, so its properties are between biomass and biochar. Hydrothermal carbonization makes a carbon-rich solid called "hydrochar," not biochar, because the process differs from pyrolysis.

Thermo-catalytic depolymerization is another method that uses microwaves to make biochar. It has been used on an industrial scale to convert organic matter into biochar, producing about 50% char.

Properties

The physical and chemical features of biochars depend on the materials used and the production methods. Information about these features helps explain how biochars work in certain applications. For example, the International Biochar Initiative has published guidelines that describe standard ways to test and evaluate biochars. These features can be grouped into categories such as basic makeup, elemental content, pH level, and porosity. The ratios of atoms in biochar, like hydrogen to carbon (H/C) and oxygen to carbon (O/C), are linked to characteristics related to organic material, such as polarity and aromaticity. A van-Krevelen diagram can display how these atom ratios change during the production process. During carbonization, both H/C and O/C ratios decrease because hydrogen and oxygen-containing groups are released.

Production temperatures affect biochar properties in multiple ways. The structure of carbon molecules in the solid biochar material is especially influenced. Initial pyrolysis at 450–550 °C creates an amorphous carbon structure. Temperatures above this range cause the gradual transformation of amorphous carbon into turbostratic graphene sheets. Biochar conductivity also increases with higher production temperatures. For carbon capture, aromaticity and natural resistance to breakdown increase as temperature rises.

Applications

Biochar is a material made from organic matter that is heated without oxygen. It is very stable and does not break down easily, which makes it useful for storing carbon in the soil. This process, called biochar carbon removal, helps reduce the amount of carbon dioxide and methane in the atmosphere. These gases are released when plants and other organic materials burn or decompose naturally. However, biochar production also releases some carbon dioxide, about 50% of the carbon in the original material. The remaining carbon stays in the soil for a very long time, sometimes hundreds or thousands of years, which helps slow the increase of greenhouse gases in the air. Biochar can also improve soil quality, help plants grow better, and reduce the need to cut down forests for farming.

Biochar can store carbon in the soil for hundreds to thousands of years, similar to coal. Studies show that biochar can keep 10% to 70% of the carbon from the original material in the soil, and it slows the breakdown of carbon by a large amount, which can take centuries or even millennia. Scientists first suggested using biochar to remove carbon dioxide from the air in the early 2000s. Experts like James Hansen and James Lovelock have supported this idea.

A 2010 report said that using biochar in a sustainable way could reduce global carbon emissions by up to 1.8 billion tonnes each year, without harming food supplies or the environment. However, a 2018 study questioned whether enough organic material would be available to make this happen. A 2021 review estimated that biochar could remove between 1.6 and 3.2 billion tonnes of carbon dioxide per year. By 2023, biochar had become a business linked to carbon credits, which are payments for reducing emissions.

As of 2023, biochar is widely recognized as a way to store carbon. On average, it could store 7% of global carbon emissions, with some countries storing over 20% of their emissions. Bhutan leads this, storing about 68% of its emissions, followed by India at 53%.

In 2021, biochar cost about the same as carbon prices in Europe, but it was not part of the EU or UK emissions trading system. The ability of biochar to absorb carbon dioxide depends on its surface area, which can be increased using special mixing techniques.

In developing countries, biochar made from improved cookstoves can reduce emissions compared to traditional stoves, while also helping with other environmental goals. Biochar is considered a technology that can remove more carbon from the air than it produces, which could help fight climate change. However, recent studies suggest that the amount of carbon biochar can remove may be smaller than earlier estimates, especially if carbon prices are low.

Currently, biochar production is small, with only a few hundred thousand tonnes made each year. This results in less than 1 million tonnes of carbon being stored annually, though interest in biochar has grown. The cost of biochar depends on factors like where it is made and what it is used for. In some areas, biochar is only practical if it helps improve soil or if it can be sold for carbon removal credits.

Biochar improves soil health in tropical areas but is less helpful in temperate regions. Its porous structure helps hold water and nutrients in the soil. Soil scientist Elaine Ingham noted that biochar can support helpful microorganisms in the soil, which improve plant health when added with these organisms.

Biochar can reduce the spread of harmful bacteria like E. coli through sandy soil, depending on how much is used, the type of material it is made from, and other conditions. For plants that need more potassium and higher soil pH, biochar can help increase crop yields.

Biochar improves water quality, reduces greenhouse gas emissions from soil, prevents nutrients from washing away, and reduces the need for water and fertilizers. Its pores help keep water and minerals near the soil surface, which supports plant growth. In some cases, biochar helps plants fight diseases caused by fungi or bacteria in the soil. It can also remove heavy metals from the ground.

The effects of biochar depend on its properties and how much is used. However, scientists still have much to learn about how it works in different environments. Small amounts of biochar can reduce emissions of nitrous oxide and methane, which are more harmful than carbon dioxide.

Studies show that biochar helps crops grow better in poor soil. A project in Europe found that adding biochar and compost improved soil quality and crop production in several countries. Biochar can be tailored to address specific soil issues. In Colombia, biochar reduced the loss of important nutrients and increased plant nutrient uptake. At 10% application rates, biochar reduced harmful substances in plants by up to 80%. However, biochar can sometimes reduce the effectiveness of pesticides, especially with high-surface-area types.

Biochar can be mixed into soil in fields or added to gardens to improve fertility and store carbon long-term. It works well when spread on top of the soil. In Europe, biochar has helped improve soil health and plant resistance to diseases. Some gardeners use biochar to grow more plants, which helps remove more carbon from the air. Biochar can also be added to animal feed to reduce methane emissions from livestock.

To improve plant growth significantly, biochar is usually applied at rates between 2.5 and 20 tonnes per hectare. In developed countries, biochar costs between $300 and $7,000 per tonne, which is expensive for farmers. In developing countries, the main challenges are finding enough organic material and time to produce biochar. A cost-effective solution is to mix small amounts of biochar with fertilizers.

Using too much biochar or combining it with unsuitable soil types or materials can harm soil life, reduce water availability, or change soil conditions.

Research

Research on pyrolysis and biochar is being conducted worldwide, but as of 2018, it was still in its early stages. Between 2005 and 2012, 1,038 articles in the ISI Web of Science included the words "biochar" or "bio-char" in their topic descriptions. Research is being carried out by institutions such as the University of Edinburgh, the University of Georgia, the Volcani Center, and the Swedish University of Agricultural Sciences.

Studies are also being conducted on using biochar in coarse soils within semi-arid and degraded ecosystems. In Namibia, biochar is being tested as part of efforts to help communities adapt to climate change, improving their ability to withstand droughts and ensuring food security by producing and using biochar from locally available encroacher biomass. Similar approaches have been explored in Australia for rangelands affected by the spread of woody plants.

In recent years, biochar has drawn attention for its ability to filter wastewater and trap pollutants such as pharmaceuticals, personal care products, and per- and polyfluoroalkyl substances.

In some regions, public interest and support for biochar have encouraged government research into its potential uses.

Long-term effects of biochar on carbon storage have been studied using soil samples from arable fields in Belgium. These samples contained charcoal-enriched black spots from before 1870, created by charcoal production. The study found that soil treated with charcoal over many years had a higher amount of carbon from maize plants and reduced respiration, likely due to physical protection, microbial communities becoming saturated with carbon, and possibly increased annual plant growth. This research shows that biochar can help store carbon in soil by slowing its release.

Biochar stores carbon in soil because it remains in the soil for a long time, from years to thousands of years. It can also indirectly store carbon by increasing crop yields while possibly reducing the breakdown of carbon in the soil. Laboratory studies have shown how biochar affects carbon breakdown using carbon signatures.

Fluorescence analysis of organic matter in soil treated with biochar revealed an increase in a humic-like fluorescent component, which may be linked to biochar-derived carbon in solution. Using spectroscopy and microscopy together, researchers found that aromatic carbon accumulated in specific spots within soil microaggregates and combined with clay minerals in soil treated with raw materials or biochar. Biochar use consistently reduced the co-location of aromatic carbon and polysaccharide carbon. These findings suggest that reduced carbon metabolism is a key way biochar helps stabilize carbon in soil.