Thermal loss analysis and improvements for biomass conversion reactors

•We propose rigorous approach to quantify thermal losses for biomass reactors.•Thermal loss is experimentally analyzed for a torrefaction reactor as an example.•Side wall losses are reduced more by minimizing residence time than by insulating.•Char-cooling segment length is sized by safe char discharge temperature.•Significant heat recovery occurs for a narrower air jacket around the cooling tube. The published torrefaction design analysis either ignore thermal loss or assume a generic value in their characterization of biomass thermochemical reactors. This study, using a small-scale biomass reactor prototype for torrefaction as an example, demonstrates a low-cost but scientifically rigorous way to measure thermal losses, and proposes a mathematical description to fit with the measurements. Losses from reactor side walls and losses from char-cooling segment were characterized and analyzed separately using total solid energy flux as a basis. The thermal dissipation for both reactor side walls and char-cooling segment depended primarily on solid residence time, and hence minimizing the solid residence time was proposed as a process improvement. The other improvement tested was using heat from the char-cooling segment to preheat the air fed to the reactor. For the considered torrefaction reactor, the char-cooling segment length of 0.5 m combined with an outer jacket width of 0.01 m, ensured faster cooling of char product and lower temperature gradient in the bed, therefore, resulting in 75% savings in energy loss compared to the default design case without air pre-heating in place. The presented mathematical analysis with some experimental measurements not only quantified the thermal losses but also helped to identify improvements to the torrefaction reactor design to optimize their key parameters for scaling.