A new study has found that moisture content in biomass plays an important role in determining biochar yield and the thermal decomposition process. The findings provide a scientific basis for improving biochar production by regulating the initial moisture levels of biomass feedstocks, Bioengineering.org reported.
The study, led by Wenmei Tao and Bo Pan, examined the pyrolysis behaviour of cellulose, lignin and rice straw using thermogravimetric analysis, differential scanning calorimetry, mass spectrometry and infrared spectroscopy. The researchers also used two-dimensional correlation spectroscopy to understand how water interacts with different biomass components during heating.
The study identified two forms of water in biomass: free water, which is not chemically bonded to the polymer structure, and bound water, which forms hydrogen bonds with components such as cellulose and hemicellulose.
The researchers found that cellulose retained more free water than lignin at similar moisture levels, mainly because of the hydrophilic nature of its hydroxyl groups.
Thermal analysis showed that both free and bound water reduced the maximum rate of biomass decomposition. This was attributed to the heat consumed during water evaporation, which lowered the actual reaction temperature within the biomass samples.
As moisture content increased from 10% to 60%, biochar yield also increased. Lignin recorded the highest biochar yield at 78%, while cellulose and rice straw showed lower yields. The researchers attributed this trend to water slowing biomass decomposition and restricting the release of volatile compounds.
The kinetic analysis, conducted using the Coats-Redfern method, also revealed that bound water influenced the activation energy of different biomass components. In hemicellulose, activation energy decreased as bound water content increased, with a correlation coefficient of -0.90.
The researchers linked this reduction to the disruption of hydrogen bonds within the hemicellulose structure, which increased molecular mobility and facilitated hydrolysis. This enabled hemicellulose to decompose at lower temperatures and promoted the release of acetic acid.
Overall, the findings highlight the importance of controlling biomass moisture content to influence pyrolysis reactions and improve biochar production.
