Effect of Operating Parameters on Ablative Pyrolysis of Wood

One of the main requirements for the production of bio-oil from fast pyrolysis of wood is the use of small feedstock particle sizes (1-3 mm), in order to minimize temperature gradients within the particles and therefore avoid undesired reactions that take place in the range 200-400°C. The only exception to the rule is ablative pyrolysis. In ablation, heat transfer to the wood is carried out via direct contact with a hot, moving metallic surface. This situation creates a steep temperature gradient within the particle, such that pyrolysis takes place within a thin liquid layer contacting the solid as opposed to the entire wood particle. For this reason, in principle there is no limit to the particle size in ablative pyrolysis, and this feature is of great value, since grinding costs represent 7-9 % of the total process costs in pyrolysis. Though this characteristic has been speculated in the past, there were no experimental data to back up this claim. Recently, our group developed an ablative pyrolysis unit that is able to convert entire wood chips (about 1’’ size) and even entire wood rods directly into bio-oil in one single step, with yields similar to those in a fluidized bed. In this work, we evaluated the effects of the main operating parameters involved in ablative pyrolysis of whole wood chips: the temperature of the hot plate, the relative speed between the wood chips and the hot plate, the pressure applied by the plate on the chips, and the initial thickness of the assembly of wood chips. Comparisons involve the bio-oil yield, LHV, water content, composition as detected by GC-MS, and functional groups detected by FTIR. In addition, differences in the char produced is evaluated by elemental composition and FTIR.