Ignition and Combustion of Single Black Liquor Droplets in Reduced Gravity, 2004 International Chemical Recovery Conference

Black liquor, one of the most important industrial fuels, is the major by-product of the kraft process for production of pulp (Maèek, 1999). Black liquor is burned in Tomlinson recovery boilers in the form of large spray droplets, whose diameter varies between 0.5 mm and 6 mm (Kankkunen et al., 2001). The combustion of black liquor produces more than 1% of the total annual energy production in the United States (Maèek, 1999). Therefore, a small improvement in recovery boiler efficiency can equate to a substantial gain in energy economy.

Previous studies by Hupa and colleagues (Hupa et al., 1987), and Clay and colleagues (Clay et al., 1985; Clay et al., 1987; Clay et al., 1989) set up the basic understanding of black liquor combustion. Black liquor combustion occurs in four main stages: drying, devolatilization, char burning, and smelt oxidation. These stages are not strictly consecutive, as the physical processes significantly overlap during combustion due to large radial gradients in temperature and composition within the droplet. The definition of each stage varies greatly in the literature because of the overlap between the stages.

For this research the drying period is defined as the time period from insertion into the oven to the time momentarily prior to rapid expansion. During the drying stage, the droplet undergoes violent expansions and contractions, from evaporation of water within the droplet. A single droplet typically swells between 1.2-2.0 times its initial diameter, with an approximate swelling average of 1.5 times its original diameter by the end of drying (Frederick et al., 1991). Expansion of the droplet occurs because the hot ambient of the recovery boiler vaporizes the water in the droplet accompanied by rapid release of volatile gases (Maèek, 1999).

For this research devolatilization is defined as the period from initial rapid expansion to maximum swollen volume. During devolatilization (also known as pyrolysis), the droplet continues to expand in size and the particle internal temperature continues to increase. As internal droplet temperatures reach 200OC, the organic material in the black liquor begins to decompose releasing gas-phase volatiles, which include CO2, CO, H2, other light hydrocarbons, H2S, NO, and NH3. If the temperature of the gaseous environment is high enough (550OC), along with an oxygen mole fraction of at least 0.10, the released volatiles ignite and form a gaseous flame close to the droplet (Adams et al., 1997). In environments where the oxygen content and ambient temperature are not great enough, no gas-phase flame can exist.