Numerous factors influence biochar making, directly determining its yield and quality, as well as the economic and environmental benefits of the entire production process. These factors can be categorized into three main types: raw material characteristics, production process conditions, and reactor design.
Raw Material Characteristics
Raw materials are the material basis for biochar production, and their properties are crucial.
Raw materials with high lignin content (such as wood and nut shells) generally produce more and more stable biochar. Cellulose and hemicellulose tend to produce more volatile matter and tar.
Raw materials with high ash content (such as straw, livestock manure, and sludge) may have a higher biochar yield. However, the stability and pore structure of the biochar may be poor, and heavy metals in the ash may cause pollution. Generally, the order of ash content is: sludge > livestock manure > crop residue > woody plants.
Pre-treating raw materials into smaller particles facilitates heat and mass transfer, resulting in more uniform and faster pyrolysis. However, excessively fine powder may cause difficulties or blockages in airflow.
High moisture content (>30%) consumes a large amount of energy for moisture evaporation, reducing pyrolysis efficiency. It also increases operating costs and may affect biochar quality.
Biochar Making Process Conditions
Pyrolysis temperature is the most important influencing factor. Temperature directly determines the physicochemical properties of biochar.
Increased temperature leads to increased volatile matter release and decreased biochar yield. Higher temperatures (typically >500°C) result in more stable aromatic ring structures, longer carbon persistence, and generally larger specific surface areas.
Low-temperature biochar (<500°C) contains more oxygen-containing functional groups on its surface, making it more reactive and capable of cation exchange. High-temperature biochar has fewer of these functional groups and becomes more hydrophobic.
Heating rate and residence time also affect biochar production.
Slow pyrolysis involves slow heating and long residence time. The main purpose is to maximize biochar yield.
Fast pyrolysis involves extremely rapid heating and short gas-phase residence time. The main purpose is to maximize bio-oil yield, with biochar being a byproduct.
The longer the feedstock resides in the carbonization reactor, the more complete the pyrolysis, but the yield will further decrease. The longer residence time of volatiles in the high-temperature zone is beneficial for the secondary cracking of large tar molecules into smaller gas molecules. This is beneficial for improving the quality of syngas, but it will also affect the gas yield and composition.
Pyrolysis Reactor Design
The pyrolysis reactor is the physical carrier for biochar production.
Batch pyrolysis reactors are simple and require low investment. However, they have low production efficiency, inconsistent product quality, and are difficult to control pollution.
Continuous carbonization reactors are highly automated, have high output, stable product quality, and are easy to recover energy. However, they have high investment and operation and maintenance costs. They are suitable for large-scale industrial production.
Advanced biochar production equipment can recover and utilize the syngas and heat generated during pyrolysis. This significantly reduces external energy input and improves the economic and environmental benefits of biochar projects. 
