A salt-induced mechanism for the swelling of black liquor droplet during devolatilization

- A salt-induced mechanism for the swelling of black liquor droplet during combustion and gasification is proposed.
- The formation of a plastic droplet by the melting of salts is crucial for swelling.
- Lignin forms carbon materials for building a swollen char surface.
- The mechanism explains relevant droplet swelling phenomena of black liquor.

Black liquor (BL) from the pulp and paper industry is a significant bioenergy source. Swelling of BL droplet during the pyrolysis stage of combustion and gasification is a profound phenomenon affecting various boiler processes. However, the mechanism for droplet swelling has remains a puzzle due to an insufficient understanding of the formation of the plastic droplet at the pyrolysis stage of the thermal conversion. Thermogravimetric studies with model aliphatic carboxylates unraveled the melting and decomposition behavior of the complex mixture of BL salts during pyrolysis, indicating that the melting of salts forming a liquid state may be the primary cause for the development of a plastic droplet, which is confirmed by the significant swelling of the salt-polymer mixtures pyrolyzed at 500 °C. It is revealed that the consecutive and concurrent melting of an array of aliphatic carboxylates and NaOH over a temperature range of 230-500 °C brings about a plastic droplet, while lignin forms carbon materials ideal for building a rigid char surface. Both lignin and aliphatic carboxylates are major sources of volatiles providing the momentum for swelling. In conclusion, three basic aspects account for the swelling of BL droplet: the creation of a plastic state, the production of volatiles, and the formation of a rigid char surface.

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