New insights into the release of sodium, sulphur, potassium and chlorine during black-liquor combustion and gasification

New insights into the release of sodium, sulphur, potassium and chlorine during black-liquor combustion and gasification have been obtained through laboratory experiments and through theoretical studies. In the experiments employing a grid- heater, the effects of pyrolysis conditions on material release have been investigated over a wide range of conditions (temperature: 300-900 °C, time: 2-120 s, heating rate: 100-750 °C/s, pressure: 1-21 bar). Detailed analyses of gaseous products and char inorganic components have been performed. A study of sodium release, initiated prior to the LIEKKI 2 Program, was brought to a successful conclusion during the first year of the program. The sodium-release mechanisms were confirmed to be the same as those previously observed at low heating rates. There was no evidence of significant volatilisation of sodium during the primary phase of black-liquor pyrolysis (temperatures up to about 700 °C), as had been suggested by other researchers at that time. In experiments in which fume material was recovered and analysed, the closure of the sodium mass-balance exceeded 90%. During the LIEKKI 2 Program, sulphur release has been the main topic of the experimental research. Information of a new kind has been generated. Satisfactory sulphur balances have been obtained in experiments in which both gas and char have been analysed. On the basis of both the new and previously published data, mechanisms of sulphur release, consistent with nearly all of the available laboratory and field data, have been proposed. These mechanisms have, in turn, be used to explain the effects of furnace temperature, of liquor drop-size, and of liquor solids-content on sulphur release in the recovery furnace. The more recent work has focused on measuring the rates of the sulphur-release reactions, the aim being to generate the basic rate data necessary for the single-drop sulphur-release model currently under development. In the grid-heater experimental program, useful data concerning the release of potassium and chlorine during black-liquor pyrolysis have also been generated. In addition, to support the development work on black- liquor gasification, the effects of elevated pressure on the release of sodium and sulphur have been elucidated. In the theoretical part of the research work, thermodynamic constraints on the release of sodium, potassium and chlorine have been determined by equilibrium calculations carried out with a new computational procedure. The new procedure differs from previously used methods in that fume formation in the reaction zone is taken into account. In contrast with the prior methods, the new computational procedure yields results which are compatible with recovery-boiler experience. Overall, the research has led to an improved understanding of the effects of black-liquor composition and of furnace conditions on the release of sodium, sulphur, potassium and chlorine during black-liquor combustion.