Pyrolysis of kraft pulp and black liquor precipitates derived from spruce: Thermal and kinetic analysis

Abstract

The potential of seven kraft cook materials to become functional char materials and fuels is investigated. Thermogravimetric analysis was used to study the thermal properties while a model-free isoconversional method was used to derive kinetic rate expressions. Black liquor precipitates had lower thermal stability (20–60K lower) than pulps and spruce wood and the precipitates decomposed in a wider temperature range, producing chars with similar or higher thermal stability than char from pulps, but lower than those from spruce wood. Samples suitable to produce char were identified based on char yield, devolatilization rate and charring temperature. The highest char yield (46%), achieved from a precipitate, was more than twice as high as that from spruce powder. Under the studied conditions none of the materials had a pyrolysis process that for the whole conversion range could be described with a single set of kinetic parameters. The apparent activation energy varied between 170–260kJ/mol for the pulps and 50–650kJ/mol for the precipitates. The derived kinetic parameters were validated by predicting the conversion at a heating rate outside the range used for its derivation and at quasi-isothermal conditions. Both these tests gave satisfactory results in good agreement with experimental data.