Insight into the structural feature, ash transformation behavior, and alkali metal heat release characteristic of industrial lignin via experimental and equilibrium evaluation

[Display omitted] •The chemical structures of three lignin samples were investigated by 13C NMR.•The CL and DL structures are S-type lignin and the SL structure is GS-type lignin.•Na+ and Ca2+ promoted and inhibited the pyrolysis of the lignin, respectively.•Two major temperature intervals exist for the release of Na+ metal during pyrolysis. The release characteristics of inorganic minerals during the thermochemical transformation of organic solid wastes are one of the key issues for their resource utilization. This study concentrated on the chemical structure of three kinds of industrial lignin and the release pattern and mechanism of alkali metal elements during heat treatment by means of 13C NMR, FTIR, XRD, SEM, and chemical thermodynamic equilibrium simulation. The microstructures of the three lignin samples were studied using 13C NMR and FTIR. The three lignin samples contained much more aliphatic and aromatic carbon than carbonyl carbon, and the aliphatic carbon moieties contained the most oxygenated carbon. The lignin samples mainly had aromatic molecular structures with aromaticity (fa) of 55.76% (sodium lignin sulfonate, SL), 52.62% (calcium lignin sulfonate, CL), and 63.93% (de-alkalized lignin, DL), respectively. Carbonyl content in DL was lower than lignin sulfonate SL and CL. The benzene ring content of SL was lower than the other two lignin samples. Na+ metal salts promoted the pyrolysis of lignin samples, while Ca2+ metal salts inhibited the pyrolysis of lignin samples. Microstructural studies showed that the structure of SL evolved from columnar and granular coexistence breaking into filamentous protrusions and lamellar structures with the increase of pyrolysis temperature. The CL and DL samples became ash presented in granular state and the particle size continued to become smaller with increasing temperature as the distribution gap became larger. The chemical thermodynamic equilibrium calculations showed that the variation in the initial concentrations of Na+ and Ca2+ in the three lignin samples led to different release characteristics of the three lignin samples. Na+ release rate was higher than that of Ca2+, and the valence state of the metal ions was an important reason for this phenomenon. This study provides data support and theoretical basis for the migration and release pattern of alkali metal during the thermal conversion reaction of organic solid waste applications.