Analytical and applied pyrolysis of challenging biomass feedstocks: Effect of pyrolysis conditions on product yield and composition

Pyrolysis of challenging Indian and Finnish feedstocks, viz., rice straw, empty oil palm fruit bunch, pine bark, and birch bark, has been studied using analytical and applied pyrolysis reactors. Round-robin characterization of the feedstocks, including elemental analysis, proximate analysis, bomb calorimetry, and thermogravimetric analysis (TGA) was performed to generate robust data. Pyrolysis studies were carried out using an analytical micro pyrolyzer coupled with a gas chromatograph/mass spectrometer (Py-GC/MS), a single particle reactor (SPR), and a batch microwave pyrolysis reactor (MWP) to shed light on the influence of heating rate and mechanism on pyrolysis product yields and composition. The Py-GC/MS and SPR experiments were performed at 400, 500, and 600 °C, while the MWP experiments were carried out at 500 °C. The TGA results showed that the decomposition temperature regimes for the hemicellulose, cellulose, and lignin components of the biomasses overlapped to a certain extent. The condensable pyrolysis vapor (“bio-oil”) yields obtained from pyrolysis of the samples in the SPR reached a maximum at 500 °C. However, the pyrolysis char yield decreased, and the CO and CO₂ gas yields increased with temperature in the temperature range of 400–600 °C. The bio-oil yields obtained from the MWP at 500 °C were low, and the char and CO/CO₂ gas yields were high compared to the corresponding yields obtained from the SPR at the same temperature. The low bio-oil yields, and high biochar and CO/CO₂ gas yields from the MWP were attributed to the slow heating rate in the MWP. The Py-GC/MS analysis results revealed that the pyrolysates were mainly composed of linear and cyclic oxygenated hydrocarbons and phenolics. However, the bio-oils obtained from pyrolysis of the samples in the MWP mainly contained phenolics. The results of the present work indicate that differences in the chemical composition of the biomass feedstocks (especially in the content of alkali and alkaline earth metals), the heating rates and the heating mechanisms played a major role in determining the pyrolysis product yields and compositions.