A new study published in Biochar has found that water naturally present in plant biomass may play a far more important role in biochar production than previously understood, influencing how biomass breaks down during pyrolysis and helping increase biochar yield.
Pyrolysis, an oxygen-limited heating process used to produce biochar, bio-oil and gases, has traditionally relied on drying biomass before processing because freshly harvested plant materials contain significant moisture. However, the new findings suggest that water should not be viewed only as a barrier to efficient production, EurekaAlert reported.
Researchers examined cellulose, lignin and rice straw samples with varying moisture levels and found that both free water and bound water reduced the intensity of pyrolysis reactions while increasing the amount of biochar produced.
According to corresponding author Bo Pan, different forms of water interact differently with biomass components and understanding these interactions could help improve control over biochar formation.
The study separated water into two categories—free water, which evaporates more easily, and bound water, which remains attached to plant polymers through hydrogen bonding.
To observe these interactions, researchers used thermogravimetric analysis, differential scanning calorimetry, mass spectrometry and in situ infrared spectroscopy to monitor how water influenced biomass breakdown during pyrolysis.
One of the key findings was that bound water reduced the activation energy required for hemicellulose decomposition, making this biomass component easier to break down. Researchers found that bound water formed hydrogen bonds with O-acetyl groups in hemicellulose, accelerating decomposition and supporting earlier release of acetic acid.
At the same time, bound water had the opposite effect on cellulose by increasing activation energy and strengthening hydrogen bond networks, resulting in greater thermal stability during heating.
The study also identified a distinct sequence in how water affected functional groups during rice straw pyrolysis. Hydroxyl groups reacted first, followed by carboxyl C=O, aliphatic C-H, carbohydrate C-O-C and aromatic ring structures.
According to the researchers, this reaction pattern may support the formation of more condensed aromatic carbon structures, which are considered important for producing stable biochar.
Across all tested materials—including cellulose, lignin and rice straw—higher water content consistently increased biochar yield. Biochar produced from lignin delivered the highest yield, reaching up to 78 per cent under the experimental conditions.
However, researchers noted a trade-off, as higher moisture levels also increased energy requirements due to the additional heat needed to remove water.
Balancing improved biochar production with energy use, the study suggested that maintaining biomass moisture content at around 30 per cent could offer a practical operating point for pyrolysis systems.
The researchers said the findings provide a molecular-level understanding of how water influences biomass conversion and offer producers a scientific basis for adjusting feedstock moisture to improve product outcomes.
The study also points to broader opportunities for more sustainable use of agricultural residues and other lignocellulosic biomass resources by treating moisture not simply as a processing challenge but as a factor that can help shape biochar quality and yield.
