Characterizing devolatilized wood pellets for fluidized bed applications

Abstract

We investigated devolatilized wood pellets to characterize their mechanical behavior and their microstructure. The work’s aim is to increase the understanding and modeling capabilities for the application in fluidized bed gasification as a sustainable alternative to generate synthesis gas. Our experiments showed that devolatilized wood pellets are a stable but highly porous and fragile structure. Computed tomographic images of the same pellets before and after devolatilization showed that the existing pore network in raw conditions characterizes the final structure. Along with the pores, the reaction rate likely increases and the pores massively enlarge, and internal cavities are formed. The resulting pore network dominates the mechanical behavior and leads to micro fragmentations already at low static loads or slow dynamic impacts. This results in the creation of fines or breakage already at low impact velocities. For fluidized bed devolatilization, the large-scale open pore network of the biochar pellets allows the penetration of bed material into the pellet leading to an estimated increase in the pellet’s mass of up to 45%. However, an increase in pore size caused by the penetration was not apparent. Due to the pellet’s porous structure, breakage and attrition induced by mechanical stresses are likely to be as or even more important than primary fragmentation caused by the devolatilization process itself in a reactor.

Modeling the devolatilization and fragmentation of biomass pellets with the bonded particle method for fluidized bed applications

“…As shown by [20], during devolatilization, the pellet is losing roughly 80% of its mass causing massive enlargement of internal pores and the shrinkage of the structure. The main changes found are shown in Fig.…”

“…After devolatilization, the wood chips are covered by small pores (dimension > 150 mm), which are connected to the global pore network. Images showing the structural changes can be found in [20] and in Sect. 3.3. Despite the large changes, the cylindrical shape of the pellet remains after devolatilization, but the strength is heavily reduced.…”

“…For generating the structural model at raw conditions, we will use the micro-computed tomography (microCT) data from [20]. Moreover, the images at devolatilized state will be used to evaluate if the model is capable of mimicking the true structural changes.…”

“…In the case of the biomass pellets, their attrition and breakage can result in fines leaving the reactor unreacted, carbon slipping into the air reactor, or a change in their mixing behavior. [20] have shown that after devolatilization in a fluidized bed reactor, the pellet shape typically remains, but large internal cavities and macro-scale pores are formed. As a result, the pellets' compressive strength is massively reduced, Fig.…”

“…As a result, the pellets' compressive strength is massively reduced, Fig. 1 Schematic representation reprinted from [20] of microstructural modification occurring in the wood pellet during devolatilization based on a micro-computed tomography image analysis resulting in their fragmentation under minimal load. In this work, the described changes of wood pellets during devolatilization are modeled to deepen the knowledge about the pellets' behavior.…”