Commercial torrefaction technology suppliers require a strict/narrow particle size distribution as feedstock for producing fuels with a homogenous quality. Many re­actor types require a maximum chip size of 2.5 cm (1"). In particular, the reactors with a shorter residence time (in the order of seconds or a few minutes) require a more stringent particle size limitation (less than 0.5 cm), for example, Torbed and moving bed reactor. If the feedstock particles are with a larger proportion of fines, fines will block the gas to limit the heat transfer and also become difficult to fluidize or transport pneumatically.

Some reactors allow using large particles as feedstock, for example, multiple hearth reactor and rotary drum reactor. The rotary drum reactor can accommodate a mixture of large and small particles. Both groups of particle will be treated at the same temperature and reside within the same chamber at the same time. Meanwhile, the attrition of wood chips or particles occurs and generates lots of fines due to the rotational motion. However, large particles would undergo with non-homogenous reactions within a single particle and result in non-homogenous product quality. Feedstock with a wider particle size range can be used if the torrefaction temperature range is within a mild region (i. e., 260-290 °C). Besides, a pack bed of large particles are usually with lower bulk density. This limits the throughput as well as introducing a limited load capability for some reactor types including rotary drum reactor, screw reactor, and belt reactor.

A moving bed scale reactor setup for studying complex gas-solid reactions has been designed in order to obtain kinetic data for scale-up purpose. In the bench scale reactor setup, gas and solid reactants can be contacted in a co-current and counter-current manner at high temperatures. Gas and solid sampling can be per­formed through the reactor bed with their composition profiles determined at steady state. The reactor setup can be used to evaluate and corroborate model parameters accounting for intrinsic reaction rates in both simple and complex gas-solid reaction systems. The moving bed design allows experimentation over a variety of gas and solid compositions in a single experiment unlike differential bed reactors where the gas composition is usually fixed. The data obtained from the reactor can also be used for direct scale-up of designs for moving bed reactors. The recommended particle size range: 5 mm < diameter < 20 mm; length <70 mm.