A new, simplified biomass torrefaction reactor concept that operates under oxygen-lean conditions is proposed as a potential way to downscale torrefaction reactors for small- and medium-scale applications. To verify the feasibility of the concept, a multi-scale analysis was conducted to understand the design requirements, underlying chemistry, infra-particle effects, and overall reactor-scale heat transfer. We demonstrate that the heat transfer within the reactor and the appropriate reactor height is largely determined by gas-phase advection. Finally, by implementing a laboratory-scale reactor and operating it under diverse conditions, we show that such a design can indeed satisfy the requirements for torrefaction. This lays the basis for the second part of this two-part paper, where we develop a detailed mathematical model for this concept. In future studies, we will also systematically define and map the performance metrics and reaction conditions in order to understand the scaling laws for potential commercialization of this concept.