A Novel Approach to the Production of Biochar with Improved Fuel Characteristics from Biomass Waste

This paper examines the potential for using forward self-sustaining smouldering for the oxidative pyrolysis/torrefaction of lignocellulosic biomass, aiming to produce biochar with fuel characteristics. A vertical, fixed-bed reactor was used for this purpose, where an upwards air flow acted as oxidant. Once smouldering is initiated, the reaction is sustained without the need of supplementary energy. The biochar production was studied at three different airflow rates. In order to perform a systematic study, commercial wood pellets were used for this proof-of-concept. The wood pellets and the biochar obtained were characterized by means of elemental, proximate and thermogravimetric analyses. Results showed that the airflow rate can be used to control the temperature of the process and therefore, the characteristics of the final product. Energy densifications of up to 70% were obtained, and it has shown to increase with the airflow rate. The resulting biochar in the best case scenario has a HHV of 31.3 kJ/g, along with an atomic O/C and H/C ratios of 0.009, and 0.338 respectively. The Van Krevelen diagrams of the biochar samples suggested that energy densification is due to the loss of compounds containing hydrogen and oxygen. The increase of the airflow used during the process showed a reduction in the content of alkali and alkaline earth metallic species in the products, which is desired for its application as solid fuel. This proof-of-concept showed promising results towards the development of an on-site, sustainable, cost effective and continuous process for the torrefaction/pyrolysis of lignocellulosic residue from harvesting operations.