Thermochemical conversion characteristics of a single wood pellet in a convective low‐temperature air environment

Abstract

Summary In this study, the thermochemical conversion process of a single wood pellet under low temperature (350°C‐650°C) isothermal heating is investigated to identify the outstanding low‐temperature ignition and combustion characteristics for heat generation, particularly during the preheating stages in the furnace. The thermal behavior of wood powder was analyzed using thermogravimetric/Fourier transform infrared spectroscopy. The suspended single pellet combustion phenomenon under a convective hot air stream was also explored. The results showed that the wood primarily decomposed in a range from 240°C to 520°C, and low‐temperature ignition was found at 293°C due to heterogeneous oxidation. For pellet combustion, an outstanding heterogeneous‐induced homogeneous combustion phenomenon occurred when the stream temperature was higher than 350°C. In contrast, the ignition mechanism shifted to volatile auto‐ignite mode at 650°C due to the interplay of fuel devolatilization and oxygen diffusion. The pellet temperature evolution was measured, and the results revealed a maximum heating rate of 6.61‐16.95 K/s under a stream temperature ranging from 350°C to 650°C. The weight loss evolution showed three stages during the entire heating process, and the average activation energy (13.12 kJ/mol) was obtained. Char combustion was found favorable to CO production for gaseous emissions, but the burning of volatiles tended to convert fuel into CO2. Finally, a simplified reaction scheme based on different onset temperatures, including pyrolysis, heterogeneous/homogeneous oxidation, and heterogeneous gasification was proposed to illustrate a potential thermochemical conversion route for further utilization.