Future directions and research challenges in biomass pyrolysis

Recent interest in conversion of lignocellulosic biomass has increased focus on the engineering of pyrolysis of wood fibers and grass for subsequent refining to fuels and chemicals. Pyrolysis of biomass can occur within numerous types of reactors utilizing several different heating, mixing, and quenching methods. Additionally, biomass feedstocks exhibit significant variability in both composition of biopolymers, extractives, and inorganic components (natural catalysts). The ordered combination of these components comprises a microstructure developed from plant cells which also varies with different species and growing conditions. The variability within lignocellulosic materials and reactor types requires a more detailed understanding of the reaction chemistry and transport phenomena within thermally degrading biomass particles. Existing descriptions of reacting wood fibers couple heat transfer and lumped kinetic models capable of describing up to six chemical groups. This approach successfully describes approximate yield of product vapors and particle conversion which is useful for optimizing reactors for bio-oil yield. The next generation of biomass pyrolysis models must be capable of predicting the molecular-level chemistry occurring during pyrolysis and the impact of transport phenomena on specific chemical reactions. Recent experimental evidence has indicated that the reactive intermediates for carbohydrates, lignin, and lignocellulose exist within a short-lived intermediate liquid which subsequently evaporates to vapors and gases. The entire pyrolysis process occurs through a combination of solid, liquid, and gaseous chemistries which must be independently described. Additionally, mass, heat and momentum transfer between the three reacting phases dictates the residence time distribution for reactions within each phase and ultimately the distribution of products from pyrolysis. These chemistries are also affected by the microstructure of lignocellulosic materials (e.g. wood fibers) which are known to exhibit significant changes in geometry as well as heat and mass transfer throughout the lifetime of the biomass particle. The ability to examine each of these reaction/transport phenomena independently and also the effect of their coupling within particle models will ultimately determine our ability to design pyrolysis reactors which select for optimal fuel precursors.