Modelling of reaction kinetics during black liquor pyrolysis

Abstract

With this work the modelling of a single black liquor droplet has been investigated with focus on pyrolysis kinetics. An existing program has been extended to handle several different gas species, in particular CO, CO2, CH4 and SO2. In addition, the heat transfer modelling in the program has been improved by implementing a relation that takes into account the thermal radiation. Experiments have been conducted, where a single black liquor droplet has been exposed to high temperatures in a nitrogen atmosphere. Online measurements of the release of these four gases in a temperature range of 275 to 400ºC, where the onset of pyrolysis reactions is to be expected, were recorded. Mass balances were set up to get more information about the dry mass loss during pyrolysis. The experimental data has been converted by the deconvolution method in order to have representative data for the gases directly at the droplet location. This data was then used to adjust kinetic parameters in the model used for the simulation. A set of two parallel reactions gave good fit for the evaluated temperatures of 375 and 400ºC. The kinetic parameters used for these two reactions are in the range of devolatilization parameters for coal but resulting in slower reaction kinetics for the observed temperature range. It was found that it is also possible to represent the gas release with one reaction only in the investigated temperature range.