High-temperature pyrolysis of solid products obtained from rapid hydrothermal pre-processing of pinewood sawdust

A sample of pinewood sawdust was rapidly pre-processed in a torrefaction-type procedure, separately in subcritical water (neutral) and with added Na2CO3 (alkaline compound) and Nb2O5 (solid acid) in a batch reactor. The original sawdust and the three friable solid recovered products from the hydrothermal procedure were characterized in detail. The solid recovered products (SRPs) gave higher C/O and C/H ratios, higher calorific values and reduced moisture contents compared to the original sawdust. The four solid samples were then subjected to rapid high temperature pyrolysis in a fixed-bed reactor to investigate the effect of the pre-processing routes on the yields and compositions of the pyrolysis products. With increasing pyrolysis temperature, the pre-processed samples produced more CO and H2, far more char and less tar than the original sawdust. The trends in the composition of gases and the yields of char suggested a combination of Boudouard reaction and CO2 dry reforming as the predominant reactions during pyrolysis. For all samples, increased temperature led to reduced tar production with an increase in the aromatic oxygenates and aromatic hydrocarbon contents of the tar. At 800 °C, the ratio of aromatic hydrocarbons increased dramatically particularly from the sample pre-processed with Nb2O5 indicating possible deoxygenation catalysis.