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In this work, the sulfur transformations during thermal conversion of two straw samples have been experimentally investigated. Sulfur was found to be associated partly as inorganic sulfate (40-50% of the total S) and partly as organic sulfur (50-40%) in typical Danish wheat straw samples. Batch pyrolysis and combustion experiments were conducted in a lab-scale tubular reactor in order to obtain quantitative information on the sulfur transformations during devolatilization and char burnout. The lab-scale experiments indicated that 35-50% of the total sulfur was released to the gas phase during the devolatilization. The release was predominantly caused by decomposition of organically associated sulfur. During char burnout at low temperature (<500 °C), no sulfur was released to the gas phase, but was instead completely retained in the straw ash. As the combustion temperature was increased, sulfur was gradually released to the gas phase; approximately 85% of the total S was released at 950 °C. Pyrolysis experiments with SO 2 addition indicated that additional sulfur may be fixed in char. Combustion of the sulfurenriched char samples resulted in significantly higher sulfur concentrations in the residual ashes. On the basis of the experimental results, the transformation and release to the gas phase of biomass sulfur is discussed.

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Initial reaction mechanisms of lignin pyrolysis were studied by large-scale ReaxFF molecular dynamics simulations (ReaxFF MD) facilitated by the first GPU-enabled code (GMD-Reax) and the unique reaction analysis tool (VARxMD). Simulations were performed over wide temperature ranges both for heat up at 300–2100 K and for NVT at 500–2100 K with a large lignin model, which contained 15920 atoms and was constructed based on Adler’s softwood lignin model. By utilizing the relatively continuous observation for pyrolysate evolution in slow heat up simulations, three stages for lignin pyrolysis are proposed by pyrolysate fractions. The underlying mechanisms for the three stages are revealed by analyzing the species structure evolution and the reactions of linkages, aryl units, propyl chains, and methoxy substituents. Stage I is characterized with the complete decomposition of source lignin molecules at low temperatures dominated by breaking of α-O-4 and β-O-4 linkages. The temperature in stage II is relatively high where cracking of all the linkages occurs, accompanied by conversion of propyl chains and methoxy substituents. Stage III mapping to high temperature shows the formation of heavy pyrolysates by recombination reactions of five-, six-, or seven-membered aliphatic rings. The heterocyclic oxygen-containing rings are revealed as important intermediates for the aryl monomer ring opening into aliphatic rings of five-membered, seven-membered, or even larger. The pathways for small molecule formation observed in this work are broadly in agreement with the literature. This work demonstrates a new methodology for investigating the overall behaviors and the underlying complex mechanisms of lignin pyrolysis.