Spray visualisations of gas-assisted atomisation of black liquor

Black liquor gasification at high temperature is a promising alternative to the conventional recovery boiler process used in chemical pulping today. In the black liquor gasification process black liquor is atomised by a gas- assisted nozzle and sprayed into a reactor where it is directly gasified by an reaction with oxygen steam and carbon dioxide. A proper atomisation of black liquor is very important for the performance of the black liquor gasification process. The work presented in this report was performed to gain understanding and aid in optimisation of the atomisation process in the current black liquor gasification process. The method used was spray visualisations by shadowgraphy using intensive lighting and high speed photography. The experiments were carried out in the spray test rig at the Energy Technology Centre (ETC) in Piteå and performed at elevated ambient pressures and different operational conditions. The first part of the work was to improve the current spray test rig at ETC considering the window purging system to avoid droplets on the sight windows, the nitrogen delivering system used for atomisation and a black liquor heating and delivering system. The final setup of the spray test rig made it possible to keep the optical access windows clean, control the temperature of the atomisation gas and pump and heat the black liquor up to 115°C. The second part of the current work was to perform visualisations of the black liquor atomisation process in the improved spray test rig. Variables like fluid temperatures, ambient pressure, nitrogen gas flow and black liquor flow were changed in order to see how these influence the gas- assisted atomisation process of black liquor. The nozzle used in this work was a standard coaxial gas-assisted nozzle, which is not an optimised nozzle for the black liquor gasification process. The visualisations of the resulting sprays from the current nozzle showed that black liquor does not behave like water when it is atomised. The black liquor mainly form thin ligament-shaped droplets and not spherical droplets. Based on the few result with varying black liquor temperature and dry solids content, the qualitative results showed only minor dependence from these variables in the atomisation process. Under the influence of elevated ambient pressure the black liquor spray looks more like a dense cloud of small droplets than larger more coarsely distributed droplets found in the atmospheric case.Validerat; 20101217 (root