Substantial differences in the temperature of the biomass resource and the reactor temperature may affect the heat rate. A number of different kinds of pyrolysis reactors are available. Pyrolysis is a precursor to gasification and combustion, and the same reactors used for gasification (i.e., fixed bed and fluidized bed reactors) can be used for pyrolysis.
Bubbling fluidized bed reactors are simpler to design and construct than other reactor designs, and have good gas-to-solids contact, good heat transfer, good temperature control, and a large heat storage capacity. High liquid yields (60 to 75% weight of wood on a dry basis) can be typically achieved. Small fuel particle sizes are needed (< 2 -3 mm) to ensure high heat rates. The rate of particle heating is the rate limiting factor. Figure 1 shows a fluidized bed pyrolysis reactor developed by Daugaard (2003).
In the BioTherm reactor (figure 2), fluidized sand in a zero-oxygen environment quickly heats the fuel to 450oC, where the fuel is decomposed into solid char, gas, vapors and aerosols. After exiting the reactor zone, these products pass through two sequential cyclones where most of the solid char particles are removed and collected. The scrubbed gases, vapors and aerosols enter a direct quenching system where they are rapidly cooled (< 50oC) with a liquid immiscible in bio-oil. The bio-oil is condensed and collected and the quench liquid is recovered (in a heat exchanger) and recycled. Non-condensable gas and residual bio-oil aerosol droplets enter a precipitator that electrostatically removes particulates and aerosols. The clean, inert gas is then recycled back to the bubbling fluidized bed reactor. The excess non-condensable gas (a medium Btu gas) is combusted to provide heat to the reactor sand.
Circulating fluidized bed pyrolysis reactors are similar to bubbling fluidized bed reactors - but have shorter residence times for chars and vapors, which results in higher gas velocities, faster vapor and char escape, and higher char content in the bio-oil. They have higher processing capacity, better gas-solid contact, and improved ability to handle solids that are more difficult to fluidize than bubbling fluidized bed reactors, but are less commonly used. The heat supply typically comes from a secondary char combustor.
Ablative pyrolysis reactors function on the premise that, while under pressure, heat transferred from a hot reactor wall will soften feedstock in contact with it - allowing the pyrolysis reaction to move through the biomass in one direction. The feedstock is mechanically pushed through the reactor. High rates of pressure significantly affect the rate of the reaction and the velocity of the feedstock on the heat exchange surface. Rather than being limited by the rate of heat transfer through the biomass particle, the reaction rate is limited by the rate of heat supply to the reactor, and thus larger particles can be pyrolyzed. Inert gases are not required, resulting in smaller processing equipment and more intense reactions. However, the process is dependent on surface area, so scaling to larger facilities is costly and the use of mechanical drivers is more complex.
In a rotating cone pyrolysis reactor, room temperature biomass particles and hot sand are introduced near the bottom of the cone, mixed, and transported upwards by the rotation of the cone. Pressures are slightly above atmospheric levels. Rapid heating and short gas phase residence times can be achieved. Figure 3 presents a schematic of a rotating cone pyrolysis reactor.
