Thermal decomposition of pyrolytic lignin under inert conditions at low temperatures

- Chars prepared from the pyrolysis of pyrolytic lignin (PL) at 100-350 °C are characterised.
- Chars prepared at temperatures above 300 °C consist of mainly heavy aromatic oligomers.
- Light and heavy aromatic fractions interact and enhance char formation during PL pyrolysis.
- Polymerization of heavy aromatic oligomers in PL enhances char formation at T > 250 °C.

A pyrolytic lignin, separated from the bio-oil generated from biomass fast pyrolysis bio-oil at 500 °C via cold-water extraction, was thermally decomposed in a drop-tube/fixed-bed reactor under inert conditions at 100-350 °C. It is found that the char yield of the pyrolytic lignin under such fast pyrolysis conditions decreases rapidly from ∼82% at 100 °C to ∼23% at 350 °C. The majority of the weight loss is contributed by the significant reduction in the fraction of light aromatic oligomers (i.e., the CH2Cl2-soluble fraction) retained in the pyrolytic lignin chars, with such reductions increasing with temperature. On the contrary, there is insignificant change in the fraction of heavy aromatic oligomers (i.e., the CH2Cl2-insoluble fraction) retained in the pyrolytic lignin chars during pyrolysis at such low temperatures. It is evident that there are strong interactions between the light and heavy aromatic oligomers during thermal decomposition of the pyrolytic lignin, leading to the observed additional char formation. The main reactions during thermal decomposition of the pyrolytic lignin are the release of aliphatic and small phenolic compounds (i.e., via demethylation and decarbonylation reactions), mainly due to the breaking of ether bonds in the pyrolytic lignin structure. The char structure becomes more aromatic as temperature increases, due to the enhanced polymerization reactions at high temperatures (>250 °C).