Reactor design for biomass fast pyrolysis
Introduction: The cost of the pyrolysis reactor amounts to approximately 10-15% of the total plant cost (Huber et al. 2006). However, the reactor is the central part of the process, having a key effect not only on the energy efficiency of the process but also the yields and characteristics of the resulting products. Reactors intended for the fast pyrolysis of biomass need to fulfill three basic requirements: first, very fast heat transfer in order to ensure rapid heating of the biomass particles at relatively low temperatures; second, very low residence time of the vapors inside the reaction chamber in order to minimize secondary reactions; and third, reduced purge gas flow in order to reduce dilution of volatile products and condensation requirements. Other aspects that need to be taken into consideration include attrition of the chars in order to avoid contamination of volatile products, and biomass particle size requirements in order to avoid excessive energy and economic costs in feedstock preparation. Various reviews have been dedicated to describing alternative reactor designs for fast pyrolysis (Meier and Foix, 1999; Bridgewater and Peacocked, 2000; Mohan et al., 2006).
These reactor designs can be classified in two main categories, fluidized bed and non-fluidized beds. In fluidized bed reactors, a fluidizing gas (recycled pyrolysis gas) is passed through a granular material (usually inert silica sand) at velocities sufficiently high to suspend the solid and cause it to behave like a fluid. Heat is transferred primarily by conduction between the hot sand and the biomass, allowing for very fast heating rates of the feedstock. Fluidized bed reactors are widely used in many industrial applications owing to its rapid heat transfer characteristics and uniform temperature profiles. Hence, the use of these conventional technologies involves lower technical and economic risks. A key drawback with fluidized bed reactors is the small particle size of the biomass feedstock required to ensure adequate fluidizing behavior of the bed and fast heat transfer conditions. Size reduction may involve significant economic costs. Figure 7 show a schematic representation of conventional (bubbling and circulating bed) and non conventional (conical spouted bed and spout fluid bed) fluidized bed reactors.
Conventional Fluidized Beds:
Bubbling Fluid Beds (BFB): The bubbling fluid bed is a reliable and well developed technology widely used incommercial processes like combustion, gasification and oil cracking. This type of reactor issimple to construct and operate, providing very effective heat and mass transfercharacteristics and thermal control. Sand is used as the solid phase of the bed and theresidence time of the vapors is controlled by the fluidizing gas flow rate. Owing to their costeffectiveness and high proportion of oil yields, bubbling fluidized beds are the most widelyused reactor design for the fast pyrolysis of biomass (Scott and Piskorz, 1984; Samolada etal., 1992; Gerdes et al., 2002; Wang et al., 2005).Fluidized bed reactors were first adapted for the fast pyrolysis of biomass in theDepartment of Chemical Engineering at University of Waterloo (Scott and Piskorz, 1982).The process, usually referred to as the Waterloo Flash Pyrolysis Process (WFPP), operates attemperatures between 450-500ºC, involves vapor residence times around 2 seconds and hasbeen claimed to produce liquid yields between 70-74 wt%.
Large scale WFPP reactorsusually employ lower reaction temperatures (430ºC) and longer residence times (10 s) in order to reduce gas recycling flow rates (Scott and Piskorz, 1982; Scott et al., 1999).Biomass particle size is a critical issue in bubbling fluidized beds. Small particle size isnecessary to ensure good bed mixing and avoid segregation. Typical biomass particle size arebetween 0.1-2.0 mm. Owing to the poor fluidization properties of the biomass, high sand tobiomass ratios (around 10:1) are usually required (Ramakers et al. 2004).
Circulating Fluid Beds (CFB): Circulating fluidizing bed reactors are also common in industrial chemical processes,providing higher throughputs than conventional BFB. CFB reactors require higher gasvelocities (3-7 m/s) in order to carry bed particles out of the main reactor. As a result, bedporosity is much higher than in conventional bubbling beds and the residence time of biomassparticles and volatile products is similar (Van de Velden and Baeyens 2006). The entrainedmixture of sand and carbonized biomass is usually driven to a secondary reactor where thechar is combusted and the sand is heated for process energy. At this point, ash content needsto be eliminated in order to reduce ash recirculation, which affects secondary reactions duringthe pyrolysis process. Standard sand to biomass ratios in circulating fluidized beds arebetween 10:1 and 20:1, and residence times for both gas and solids are lower than 2 seconds(Freel and Graham 1991, Boukis et al. 2007