Energetic and exergetic evaluation of residual biomass in a torrefaction process

• Any biomass can be improved by thermal process of torrefaction.
• Sawdust and coffee wastes have an increased HHV of 14.5% and 14.1% respectively.
• Less energy supply was requiring for the thermal process of sawdust and rice husk.
• An optimal operation was simulated through of available energy and cost ratio.

A torrefaction process in a TGA (Termo-gravimeter analyzer) for six different types of residual biomass (sugarcane bagasse, banana rachis, rice husk, palm oil fiber, sawdust and coffee waste) was developed in this paper. These six materials were evaluated before and after the torrefaction process through HHV (High Heating Value) and energetic and exergetic balances in order to find a promising solid fuel biomass in a torrefaction process. Torrefaction is a thermal process performed in an inert atmosphere at temperatures between 200 and 300 °C, with residence times lower than 60 min and heating rates lower than 20 °C/min. Its aim is to improve biomass as a solid fuel. In this processing, the lignocellulosic components are degraded (hemicellulose and cellulose are more degraded than lignin), having as result a biomass with a predominant amount of lignin. In this work, the torrefaction process was carried out at a temperature of 250 °C in an inert atmosphere with 10 °C/min of heating rate and a residence time of 30 min. As a result, it was found that the biggest and lowest increases in HHV for torrefied biomass were14.5% and 5.2% for sawdust and palm oil fiber, respectively. Sawdust was found to have the best performance in the torrefaction process evaluated from the energy yield parameter but rice husk was the best biomass in the energetic balances of the process. Energy and exergy balances show that palm oil fiber and banana rachis are the least efficient biomass in the torrefaction process.