Black liquor, a byproduct from pulp production, is burned in a recovery boiler to generate electricity and recover inorganic materials. Black-liquor-droplet combustion is fundamental to understanding and potentially improving recovery boiler performance. This investigation reports comprehensive experimental data collected for a suite of five liquors. All the data were collected with newly-designed laboratory equipment, several aspects of which are unique. Single droplets were suspended on a thermocouple and heated in a furnace. Simultaneous diameter (size and shape), internal and surface temperatures, surface emissivity, and mass were measured under various furnace temperatures and gas compositions. The images recorded by the cameras show the droplet swelling characteristics during combustion. Droplets swell more in inert environments and with low heating-rates than in oxidizing environments and with high heating-rates. Softwood liquors show greater swelling than softwood/hardwood mixed liquors. The internal temperature measured by the thermocouple is lower than the surface temperature measured with the optical pyrometer. This temperature gradient between the droplet interior and surface can be larger than 300 K. Molecular-beam mass spectrometry data were used to determine droplet mass loss and off-gas composition. These mass-loss data differ by about 10% from the balance mass data. These data provide a self-consistent and reasonably comprehensive description of black-liquor-droplet reactivity. A one-dimensional transient droplet model predicts droplet temperatures and mass loss comparable to the data. The combustion of volatiles, droplet swelling, and enthalpy of devolatilization were proved to be significant. Intermediate-sized particle (ISP) formation was investigated with a suspended-droplet experimental technique. The images indicate that a small amount of particles are ejected during drying. Insignificant ISP formation during devolatilization occurs. Char burning and smelt oxidation produce the largest amount of ISP. Quantitative analyses indicate that both droplet solids-content and liquor type impact ISP formation. Longer burning times produce more ISP, with total ISP formation being 0.2-2% of dry solids. Sampled ISPs show consistent trends in surface structure and particle sizes under a scanning-electron-microscope, independent of the experimental conditions.



College and Department

Ira A. Fulton College of Engineering and Technology; Chemical Engineering



Date Submitted


Document Type





black liquor, combustion, droplet, ISP, recovery boiler