Pressurized entrained-flow high temperature black liquor gasification : CFD based reactor scale-up method and spray burner characterization

Abstract: Black liquor is an important by-product in the chemical kraft pulping process, which provides the paper mill with pulp. Due to both economical and environmental protection driving forces it is important to recover the pulping chemicals and energy contained in the black liquor. In the conventional recovery cycle, the energy is recovered as steam by combustion of the black liquor in a recovery boiler and the pulping chemicals are initially recovered as a smelt formed in the boiler. The recovery boiler has been serving the kraft pulping industry for many decades and continuous efforts have been made to improve its efficiency. However, despite these efforts the recovery boiler cycle still has a relatively low overall efficiency. A promising alternative recovery technology currently under development is Pressurized Entrained-flow High Temperature Black Liquor Gasification (PEHT-BLG). By introducing PEHT-BLG in the mill’s recovery cycle the flexibility in the energy and chemical recovery is greatly enhanced. The syngas obtained from the process can be used to generate electricity or be reformed into chemicals, e.g. bio-fuels for transportation. The sulfur species in the syngas can be converted to pure sulfur and thereby open up possibilities for alternative pulping methodologies Lack of demonstration of the reliability of the PEHT-BLG technology has delayed a large scale industrial implementation of the process. In order to remedy this situation and to obtain proof on reliability and deepen the insight in the PEHT-BLG process, a development plant (DP-1) has since late 2005 been operating periodically by the technology vendor Chemrec AB at the laboratory of Energy Technology Centre in Piteå, Sweden. In order to aid in future scale-up needs, the main objective of the applied research work presented in this thesis has been to develop a comprehensive CFD model of the black liquor conversion process for the PEHT-BLG reactor. In addition, work has also been performed in order to develop a suitable methodology for spray burner characterization. Different modelling approaches at different levels of complexity have been covered within the project. As a result, a comprehensive self-consistent CFD model for the PEHT-BLG reactor has been developed and compared against experimental data from the DP-1 plant. The model makes use of eight key assumptions in a ‘proximate analysis model’ for the translation of elements in the black liquor into a model black liquor composition. The model ensures elemental conservation and a consistent handling of the available chemical energy in the black liquor. Regarding the validation of the PEHT-BLG CFD reactor model, comparisons against experimental data obtained from the DP-1 have been made. Most of the obtained experimental and numerical simulation results had to be corrected and adjusted in order to make any comparisons since direct measurements are only available for temperatures via three thermocouples inside the reactor. The features of the PEHT-BLG CFD reactor model currently considered to be reasonably accurate are; the temperature predictions in the lower part of the reactor, reduction efficiency, carbon conversion for trend predictions, and the inorganic smelt composition. However, before a definite conclusion about the validity of the model can be drawn, more in-situ measurements from the inside of the reactor are necessary. When the current model is sufficiently validated it can be used as a tool in scale-up of the reactor. Regarding the spray characterization, an air-assisted T-jet nozzle was tested with water and a syrup-water mixture to investigate the effect of viscosity on the nozzle performance. The number distribution and the lower order diameter statistics for the viscous syrup-water mixture shows that the small droplets actually decrease in size compared to water. This behavior may be explained by that a high viscosity allows ligaments and sheets to stretch further before break-up, thus yielding relatively small droplets. Furthermore, work was also performed in order to quantitatively investigate the spray characteristics from a coaxial gas-assisted atomizer under different operational conditions.