Sawdust drying process in a large-scale pellets facility: An energy and exergy analysis

Biomass is an important renewable resource that can be directly used as a raw material in energy converting processes, in wood-based industries, or it can be transformed into other types of biofuels. Its thermochemical properties, such as moisture content and chemical composition, directly influence the lower heating value and, consequently, systems' efficiency. To overcome these problems, drying processes are being optimized. This study aims to perform an energy and exergy analysis of a facility's sawdust drying process performance with an installed capacity to produce 1.8 tons per hour of high-quality pellets over two different seasons of the year. In this regard, energy and exergy analyses were carried out for the nominal operating conditions, and temperature measurements of the flue gases flow rate and moisture content were considered. Sawdust samples were collected before the drying process to determine the moisture content, corresponding to 53.49% and 50.23% for the winter and summer seasons, respectively. The analysis was conducted considering that the drying process should guarantee a reduction of the sawdust moisture content to a value of 10%, so the pellets produced at the facility could be certified following EN14961:2010 European certification standard. Consequently, energy and exergy efficiency values of the rotary dryer were found to be 1.5% and 2.2% higher in the summer season since the woodchips and sawdust moisture content are lower on that season and, therefore, less heat is necessary to dry the sawdust to 10% of moisture content. These results demonstrated that wood chips with higher moisture content require more effective fuel mass flow rates to the system to guarantee constant production. The ambient temperature does not seem to affect the efficiency of the dryer significantly. As a recommendation, using the waste enthalpy of flue gases for biomass pre-heating purposes is proposed.