Understanding the primary and secondary slow pyrolysis mechanisms of holocellulose, lignin and wood with laser-induced fluorescence

To understand the complex reaction mechanisms involved in biomass pyrolysis, volatile products are characterized on-line by laser-induced fluorescence (LIF), together with on-line measurements of permanent gases by GC-TCD (Gas Chromatograph-Thermal Conductivity Detector) and temperature evolutions in the bed. The focus is to determine the components that emit fluorescence and reactions involved in producing them from wood and from its two main macromolecular components, holocellulose and lignin. A technical-scale fixed-bed reactor is used to identify primary and secondary reactions involved in pyrolysis. The excitation wavelength used for the LIF measurements is 266 nm and the detected species are aromatic compounds (including one-ring phenolics and two-, three- or four-ring polycyclic aromatic hydrocarbons (PAHs)) and species containing carbonyl groups. Holocellulose volatiles show fluorescence that is attributed to the formation of carbonyl compounds and two-ring PAHs during heterogeneous secondary char-forming reactions, which also enhance the production of CO2. Volatiles from lignin show first fluorescence typical of one-ring phenolics and small (two-three rings) PAHs. Then, due to the enhancement of heterogeneous secondary reactions, fluorescence signal typical of bigger PAHs (three-four rings) is detected. These aromatic species are produced in parallel to gas species like CH4. The fluorescence that can be observed in pyrolysis of wood comes mainly from the lignin fraction, undergoing also heterogeneous secondary reactions resulting in the formation of bigger PAHs, although a contribution from cellulose is also present.