Experimental study of the influence of temperature and particle’s aspect ratio on the products of single beech wood cylinder pyrolysis

Introduction: The fundamentals of biomass pyrolysis kinetics had been established for fine powders (pure kinetic regime). With an increase in particle size, heat and mass transfer limitations start having an influence on the particle’s thermal behaviour and its degradation kinetics. The process is additionally complicated by secondary reactions, which shift the composition of primary degradation products. An in-depth investigation of single-particle pyrolysis requires the use of a special reactor with coupled online flue gas analysis. Due to the need for the use of such a complex measurement system, the amount of available results are too low to allow for simultaneous assessment and quantification of the influence of factors. Aim: The aim was to conduct single particle pyrolysis, which would allow for a quantitative assessment of the influence of temperature and particle’s aspect ratio on the composition of evolved pyrolysis products and relating this to process kinetics. Methods: For the assessment, beech wood cylinders in 2 different sizes (Ø8x10 mm and Ø8x16 mm) had been pyrolysed individually (in triplicate) at 5 different temperatures (300, 400, 500, 700, 900 °C) in a single-particle reactor (SPR). Next to having 2 thermocouples at the particle, the particle’s mass was recorded during the pyrolysis process. The SPR’s outlet was connected to 2 gas analysers: a conventional flue gas analyser (ABB EL 3020) and a FT-IR (Ansyco Gasmet DX 4000). Such configuration allowed for the real-time measurement of the particle’s centre and surface temperature, mass loss and the composition of 14 evolved pyrolysis volatiles (CO2, CO, H2O, CH4, C2H4, formaldehyde, acetic acid and furfural, among others). Produced solids and bio-oils had been analysed with an elemental analyser and their result combined with gas composition were used to close the mass and the elemental balance. Results: The difference in thermal behaviour, mass loss rate and gas composition during pyrolysis, was initially negligible, but grew in significance above 400 °C. The 6 mm difference in particle’s length resulted in an increase in char yield, on average by 1 wt.% above 400 oC. For the longer (16 mm) beech wood cylinder, the average between 500 oC and 900 oC yield of furfural was lower by 13,0% and the yield of methane in the same range was higher by 52,8% in comparison to the shorter (10 mm ) cylinder, on a relative basis. Conclusion: Obtained results indicate that the effect of the secondary reactions initially induced by the higher temperature is enhanced by the increase in length (longitudinal direction) of the wood cylinders. With the conversion, the escape path of evolved gaseous compounds is getting longer, and, at the same time, the thickness of the highly-heated char layer increases. Due to the higher permeability in the longitudinal than in the radial direction, the path of the evolved gases is longitudinally oriented. An increase of the particle’s size in that direction, leads to lengthening the escape path, so an increase of its residence time in the high-temperature zone and in consequence an enhancement of the secondary reactions