A decentralized biomass torrefaction reactor concept. Part II: Mathematical model and scaling law

Highlights: • A mathematical description for a small-scale, decentralized, autothermal biomass torrefaction reactor was proposed. • Mathematical model was shown to have a reasonable fit with various physical measurements from our experimental set-up. • Axial thermal conduction through the reactor wall is a non-trivial mechanism of heat loss for a small-scale test reactor. • As the reactor is scaled up, the model predicts the reactor performance (solid mass yield) will increase by 10–20%. • This study describes and quantifies some of the limitations for testing reactor prototypes at a small scale. -- Abstract: In Part I of the study, we proposed a simplified biomass torrefaction moving bed reactor design capable of decentralized, small-scale, and mobile deployment operated under an oxygen-lean condition. We built and validated a laboratory-scale test reactor. In the present study, we develop a mathematical description of the reactor and show that it produces reasonable fit to our experimental data. Contrary to many existing biomass gasifier studies, we demonstrate that at the small test-reactor scale, heat loss mechanism through the side wall is significant and cannot be ignored in the modeling. We further demonstrated that at the small test-reactor scale, the rapid axial thermal conduction plays a role in the heat transfer within the moving bed. Furthermore, by interrogating the scaling behaviors of the reactor, we show that as we scale up our current laboratory-scale reactor, at the same torrefaction severity, the mass yield of the torrefied biomass is predicted to increase by 10–20%, due to the decrease in relative heat losses at a larger scale. This study, therefore, seeks to understand and quantify some of the limitations of testing a scaled-down reactor prototype. The understanding gained in this study can both inform scaling laws for at-scale reactor designs, as well as point out areas of future work in order to develop a higher-fidelity description.