Upgrading Biomass via Hydrothermal Carbonization and its Comparison with Conventional Thermal Pre-Treatments

Biomass is an attractive fuel source of energy and a potential feed stock for further energy conversions. However, its application is very limited due to poor physicochemical properties such as low heating value, high moisture content, low grind ability, low bulk density, and high alkali content in ash content. A thermal pre-treatment process referred as “wet torrefaction or hydrothermal carbonization (HTC)” during which biomass is treated with water under subcritical conditions. The process not only overcomes the limitations of conventional conversion pre-treatments (like dry-torrefaction and Slow-Pyrolysis) but also results into a product (biochar) having combustion properties similar to that of coal. Furthermore densification of biomass in the form of wood pellets is an attractive process for efficient handling and transportation. Pellets produced from dry-torrefied biomass have higher bulk energy density and significantly improved hydrophobicity i.e. resistance to water damage and microbial growth. However these pellets are weak in strength thus break easily and generating dust upon handling. Moreover production of these pellets required much higher compaction pressure and sometimes may also need additional binders. Where on the other hand pellets produced from HTC-biochar have high bonding-strength and significantly improved energy density without any addition of separate binder. To evaluate the performance of HTC, series of experiments with different combinations of residence time and reaction temperatures were performed on woody and agricultural biomass. The result shows that HTC narrows the differences in fuel qualities and has potential to replace coal in existing coal-fired power plants without any further modifications