Sulfur Release During Pyrolysis of Single Black Liquor Droplets, 1991 Forest Products Symposium Proceedings

An experimental method was developed to measure the rate of sulfur release from black liquor droplets during pyrolysis. The experiments were carried out in a single droplet pyrolysis reactor. The droplets were injected into a hot furnace environment where they heated rapidly to pyrolysis conditions. The sulfur gases released were oxidized in air and measured as SO2 by UV absorption. The temperature range for the experiments was 350°C to 1050°C, the droplets used were between 10 mg and 70 mg, and the initial solids content was 68%.

The results show that sulfur release does not occur during drying but begins when pyrolysis begins. The rate of sulfur release depends on the furnace temperature and is not greatly affected by initial droplet mass in the range studied (10-70 mg). It is influenced by both chemical kinetics and the rate of heat transfer to the black liquor droplet, with heat transfer becoming more important as furnace temperature increases. In most regions of a recovery boiler, therefore, the rate of sulfur release will be controlled by the rate of heat transfer.

The maximum in sulfur released during pyrolysis with temperature, reported earlier by others, can be explained qualitatively in terms of competing release and recapture mechanisms. However, this concept remains to be tested quantitatively.