Analytical pyrolysis of fir sawdust, olive stone and sewage sludge in molten carbonate salts

Py-GC-MS was applied to study the chemical composition of semivolatile pyrolysates evolved from fir wood sawdust, olive stone and sewage sludge, in the presence of molten Li2CO3-Na2CO3-K2CO3 salt. The effects of pyrolysis temperature from 400 to 800 ◦C and salt content (2.5, 5 and 10 times the biomass) were investigated utilising a semi-quantitative approach. Carbonate molten salts inhibited the generation of pyrolysis products from cellulose(anhydrosugars, pyranes, hydroxyacetaldehyde) with the exception of pentacyclic ketones (e.g. 2- cyclopenten-1-ones). The molten salts did not affect considerably the pyrolytic pattern of alkylated phenols, guaiacols and syringols from lignin, but lowered significantly the occurrence of monomers with oxygenated side chains (e.g. vanillin, syringaldehyde) and dimers. The production of catechols was inhibited by molten carbonates at 400–500 ◦C, but increased at higher pyrolysis temperatures. The marked decrease of pyrolysis products evolved from fir wood and olive stone in the presence of salts was not observed for sewage sludge. The pyrograms of sewage sludge were characterised by amines from proteins (indoles, pyrroles, diketopiperazines) even at high salt content. Steroids, dominated by thiocholestane and thiostigmastane, featured the lipid fraction of sewage sludge without and with molten salts. The production of aromatic hydrocarbons increased significantly with increasing pyrolysis temperature at expenses of oxygenated constituents.