Biochar
Introduction
Biochar is a stable, carbon-rich material originating from biomass that is thermally treated under low or no oxygen conditions, typically through pyrolysis or gasification. As such, it forms a key link between raw biomass and a wide range of carbon-based products and applications.
This module focuses on understanding how biochar is formed, how its properties emerge, and how these properties can be influenced and assessed. Biochar is not defined by a single characteristic; instead, its behaviour and performance are shaped by a sequence of decisions along the production chain—from feedstock selection to conversion conditions and further processing steps.
The module is structured to build this understanding step by step:
- You begin by exploring the key properties of biochar, such as carbon stability, surface characteristics, mineral content and safety-related parameters.
- You then examine conversion, with a focus on pyrolysis, to understand how process conditions influence product yields and material properties.
- The module continues with pre-treatment, using IFBB as an example of how residual biomasses can be upgraded into more suitable feedstocks for biochar production.
- In post-treatment, you explore how biochar properties can be further modified, for example through steam activation, and what this means for material performance and process outcomes.
- Finally, the learning unit on quality integrates all previous steps and introduces frameworks for assessing biochar along the dimensions of safety, functionality and sustainability.
Together, these learning units provide a coherent view of biochar as a carbon material shaped by feedstock, processing and quality requirements. This understanding forms the foundation for the next module, Bioproducts, where the application and use of biochar in different product systems are addressed.
Shaped by Input and Process: The Properties of Biochar
Biochar is not a uniform material. Its properties vary widely depending on how it is produced and what it is produced from. Understanding these properties is essential, because they determine how biochar behaves, how safely it can be used, and which applications it is suitable for.
Some key properties of biochar are largely feedstock-dependent. The type of biomass used influences factors such as ash content, mineral composition and the potential presence of contaminants. Other properties are mainly process-dependent. Conversion conditions—especially temperature and residence time—shape carbon stability, pH, surface characteristics and the overall structure of the biochar. In practice, biochar properties always reflect a combination of input material and production parameters.
These properties are not assessed in isolation. They only gain meaning in relation to the intended application. Different use cases—such as soil improvement, water treatment or material applications—place different demands on biochar quality. What is an advantage in one context may be a limitation in another. The demand side therefore plays a key role in defining which properties are required and how “good” a biochar actually is.
At the same time, biochar production must meet safety and sustainability requirements. The European Biochar Certificate (EBC) provides a widely used framework that defines quality criteria, limit values and documentation requirements. Its focus is on ensuring that biochar is safe for people and the environment, while supporting climate and sustainability goals.
Finally, it is important to recognise that biochar production always involves trade-offs and constraints. Feedstock availability, technical limitations of conversion technologies and economic realities set boundaries on what can be achieved. There is no universally “perfect” biochar—only biochars that are well adapted to their purpose within these limits.
This learning unit explores the key properties of biochar, how they are shaped by feedstock and process choices, and how they relate to application requirements and quality standards.
Delve into the essential physical and chemical properties of biochar, which play a crucial role in its production and application across various fields.
Conversion to Biochar — Understanding Pyrolysis
Biochar is produced by converting biomass through thermochemical processes. Several conversion pathways exist, including pyrolysis, gasification and hydrothermal carbonisation. Among these, pyrolysis is the most widely used process for biochar production and is therefore the focus of this learning unit.
Pyrolysis is the thermal decomposition of biomass in the absence of oxygen. During this process, the feedstock is transformed into three product streams: a solid fraction (biochar and ash), a liquid fraction (bio-oil and tar) and a gaseous fraction (syngas, mainly composed of hydrogen, carbon monoxide, carbon dioxide, water vapour and methane). The relative proportions of these products depend on how the process is operated.
Along with the characteristics of the feedstock, key parameters such as temperature, residence time, heating rate and particle size determine the outcome of pyrolysis. In general, higher temperatures and more intensive process conditions shift conversion towards gases and liquids, while lower temperatures and longer residence times favour the formation of solid biochar. As a result, biochar yield and biochar properties are directly linked to conversion conditions.
In practice, different pyrolysis approaches are used depending on the desired outcome. While some processes prioritise liquid or gaseous products, biochar production typically relies on operating conditions that maximise the solid fraction and ensure stable, high-quality biochar.
This learning unit introduces the principles of pyrolysis and explains how conversion parameters shape both product distribution and biochar properties, forming the basis for later discussions on biochar quality and applications.
Post-treatment of Biochar — Physical Activation with Steam
Following pyrolysis, biochar properties can be further refined through post-treatment. A key method is physical activation, which significantly expands the internal surface area and enhances adsorption capacity. This makes the material ideal for diverse applications, such as removing impurities, contaminants, and odors from both liquids and gases. This learning unit focuses on steam activation as a key example.
Conventional activated carbon is often produced from hard coal or coconut shells. While these materials can achieve very high adsorption performance, their production is associated with high greenhouse gas emissions. Biochar-based activated carbon offers an alternative pathway, linking adsorption performance with renewable feedstocks and the potential for climate benefits.
Steam activation is a thermochemical post-treatment carried out at high temperatures, typically around 900 °C. During this process, steam is introduced into the reactor, where it reacts with the carbon surface of the biochar. This leads to a controlled partial oxidation of the carbon matrix. As carbon is selectively gasified, new pores are opened and existing pores are widened, significantly increasing surface area and adsorption capacity.
This activation comes with trade-offs. Part of the solid carbon is consumed during activation, resulting in a lower conversion rate, typically around 10–15 wt. % of the original dry biomass when considering the full conversion chain. At the same time, the partial oxidation increases the formation of hydrogen, raising the energy content of the pyrolysis gas.
Steam activation therefore illustrates how post-treatment can strongly enhance biochar functionality while influencing yields and energy balances. In this learning unit, you will explore how physical activation works, what changes it induces in biochar properties, and how these benefits and limitations must be balanced when designing biochar-based alternatives to conventional activated carbon.
Biochar Quality – From Production to Purpose
Biochar quality takes shape at the intersection of material properties, production choices and intended use. It cannot be judged in isolation, but only in relation to what the biochar is designed to do and how it is produced. This learning unit shifts the perspective from individual process steps to the overall outcome and asks a guiding question: What defines a high-quality biochar for a given application?
Biochar quality is defined along three closely connected dimensions: safety, functionality and sustainability.
- Safety is the first and non-negotiable dimension. A biochar must be safe to produce, handle and apply. This includes the absence of harmful substances such as toxic organic compounds or excessive heavy metals, as well as an assessment of risks related to dust formation, fine particles and worker exposure. Safety criteria ensure that biochar does not introduce new environmental or health risks while being used as a climate or environmental solution.
- Functionality is the second dimension. Biochar is produced for a purpose, and its quality depends on how well it fulfils that purpose. A biochar intended to improve soil fertility follows different criteria than one designed for long-term carbon storage or for use as a filtration or adsorption material. Properties such as surface area, pH, ash content, fixed carbon or pore structure only gain meaning when viewed in relation to the intended application.
- Sustainability forms the third dimension and extends quality beyond the material itself. It includes responsible feedstock selection (for example avoiding virgin wood or deforestation), environmentally sound harvesting and pre-treatment, greenhouse gas performance of the production process, and the fate of biochar-based products at the end of their life. A technically functional biochar cannot be considered high quality if it is produced or used in an unsustainable way.
To support transparent and comparable assessments, voluntary quality frameworks have been developed. The European Biochar Certificate (EBC) and the International Biochar Initiative (IBI) define widely used safety and product quality criteria for biochar. Building on these, the THREE C Quality approach extends the quality perspective across the entire value chain by explicitly addressing feedstock quality, biochar quality and carbon product quality, and by integrating ecological and sustainability considerations alongside material performance.
This learning unit brings all previous learning units together. The properties of biochar are shaped by feedstock choice, pre-treatment, conversion parameters and post-treatment steps. Assessing quality now means looking across this entire chain and matching the resulting biochar with the requirements of a specific application. Quality, in this sense, is not about producing the “best” biochar in general, but about producing the right biochar for the right use, in a way that is safe, functional and sustainable.