Thermal Degradation and Fuel Characteristics of Douglas Fir Sapwood and Heartwood for Bioenergy Applications

This study investigates the thermal degradation behavior and fuel characteristics of sapwood and heartwood portions of Douglas fir softwood using thermogravimetric analysis (TGA) and proximate analysis. The samples were subjected to non-isothermal torrefaction heating from room temperature (25 °C) to 350 °C at a heating rate of 10 °C/min, followed by combustion from 350 °C to 750 °C at 20 °C/min. The TGA results showed that both sapwood and heartwood exhibited similar thermal degradation behavior during torrefaction, with total weight losses of approximately 62.86 wt% and 60.90 wt%, respectively. During the isothermal holding period at 350 °C for 60 min, only slight additional degradation was observed for both samples, indicating that most devolatilization occurred during the heating stage prior to the isothermal condition. Distinct differences were identified in the derivative thermogravimetric (DTG) curves. Heartwood exhibited two distinguishable degradation peaks at approximately 325 °C and 347 °C, suggesting multiple decomposition stages associated with variations in chemical composition. In contrast, sapwood showed one dominant degradation peak at approximately 353 °C. Proximate analysis revealed that sapwood contained 10.164% moisture, 85.897% volatile matter, 0.927% ash, and 3.012% fixed carbon, while heartwood contained 10.302% moisture, 85.062% volatile matter, 0.935% ash, and 3.700% fixed carbon. The slightly higher fixed carbon content in heartwood contributed to a higher higher heating value (HHV) of 18.468 MJ/kg compared to 18.267 MJ/kg for sapwood. The HHV values of Douglas fir are comparable to other woody biomass fuels such as pine and eucalyptus, which generally range from 18–21 MJ/kg. Although lower than bituminous coal (24–35 MJ/kg) and charcoal (28–33 MJ/kg), the values are within the lower range of lignite coal (15–25 MJ/kg). The low ash content and renewable nature of Douglas fir indicate its strong potential as an environmentally friendly solid biofuel for thermochemical conversion applications.