From the mass transfer point of view, the diffusion resistance of the gas phase is larger for larger particles (e. g., wood chips). This implies that less heat is brought by saturated steam to diffuse into the inner core of a single biomass particle. A longer time is required for saturated steam to diffuse into the core of the large particle to initiate the hydrolysis. Hydrolysis usually starts to take place when the reaction temperature is higher than 160 °C, excluding the mass transfer limitation.

Saturated steam is a better heat transfer medium thanNitrogen. Its high-energy state (a higher latent heat of vaporization) allows them to reach the core of the particle with a smaller diffusion resistance. Hydrolysis is preferentially taking place inside the polymeric system instead of pyrolysis reaction using high-pressure saturated steam. In general, the development of chemical reaction kinetics models usually work with the particles with a <8 mm diameter by assuming no diffusion limitation [49, 50]. Since sawdust has a smaller particle size and a higher specific surface area than wood chip, more glycosidic bonds are accessible for cleavage by hydronium ions (H3O+) resulting in higher degree of hemicelluloses solubilisation. Hydronium ions have a typical size of 0.4 nm in wood chips [49]. Its small size allows them to penetrate into the wood pores to hydrolyze the glycosidic bonds of xylan molecules (hemicellu- loses) as well as to promote lignin solubilization and restructuring. Restructuring of lignin helps to activate and enhance the accessible lignin as a self-binding agent for durable pellet production.

Reactor Types

Steam explosion pretreatment is a commercial available process. It has been used as a pretreatment for bioethanol production. In Europe, Andritz Sprout developed a screw type horizontal continuous scale steam explosion unit. This reactor is similar to the one used by Mascoma Corporation. The feed material can be in a wide particle range in ground particles (diameter: 6 mm) or chips (diameter: 25-75 mm) form depending on the size of the hopper and the blow valve. There is an opening on the top of the horizontal chamber that can be connected to a hopper for material feeding. A steam chamber in horizontal direction is used for steam treatment of biomass. Saturated steam is supplied from the other steam generator via the nozzles from the side wall of the chamber. At the end of the chamber, there is a rapid opening blow valve controlled by a controller to release the high-pressure steam-treated biomass to the ambient condition and further conveying to the other downstream process. Discharged steam is separated by a flow discharger and recovered steam can be recycled for the pretreatment process. Both horizontal reactor and vertical reactor can be equipped with a dry discharger. The resulted material can be ready for pellet production without post-drying.