Pyrolysis of Black Liquor in a Pressurized Free Fall Reactor in a Pressurised Grid Heater, 1998 International Chemical Recovery Conference Proceedings

The objective of the present study was to investigate rapid pyrolysis of black liquor at high temperatures and high pressures using a pressurized free fall reactor (PFFR) and a pressurized grid heater (PGH). Typical conditions for pressurized fluidized bed gasification and combustion were used with respect to pressure, temperature and heating rate. Char yield and composition as well as sodium release during pressurized pyrolysis were investigated. Char yield was determined both in the PFFR and in the PGH. Sodium release was examined only in the PGH. The differences between the two ways of heating the sample have also been studied. Since the PFFR required a solid feed, the black liquors used were first spray-dried. It was therefore also necessary to study the effect of the pretreatment of the black liquor. Three different types of samples were examined; original black liquor, spray-dried powder and dissolved powder. In the PGH, spray-dried powder resulted in higher char yields compared to the original black liquor and to the char yields obtained in the PFFR. This can be explained by uneven heat transfer through the spray-dried black liquor layer on the net. The char yield for a certain sample was found to be approximately constant in the temperature range between 700-850°C in the PGH, but at 1100°C, a drop in the char yield could be observed. Variation of the heating rate between 1000-2000 K/s showed only a marginal effect; a higher heating rate slightly decreased the char yield. At 700°C, the operating pressure had a negligible effect on the char yield for a certain sample. At 1100°C, the char yield increased with increasing pressure for the original black liquor. For spray-dried powder, however, the effect of pressure was found to be the opposite; the char yield decreased with increasing pressure. The contact between sample and net is probably better at higher pressures due to the decreased swelling of the black liquor. Sodium release was increased with increasing final temperature (700-1100°C) and with decreasing pyrolysis pressure (l-10 bar). However, at 850 and 1100°C no effect of pressure on the sodium release from the spray-dried powder could be observed. Sodium release from the original black liquor and from the dissolved powder was approximately the same. It was therefore concluded that spray-drying of the original black liquor had negligible effect on the sodium release. At the temperatures and pressures studied, the original black liquor showed the highest sodium release. The observed differences in the results from the PFFR and the PGH indicates that the use of the PGH is limited to wet (undried) black liquor applications, where the sample layer is very thin (5-10 µm), and the heat flux to the sample is uniform and sufficient.