The Influence of Composition on the Swelling of Kraft Black Liquor During Pyrolysis

Abstract

During the drying and pyrolysis phases of kraft black liquor combustion, significant swelling of individual liquor particles occurs. Swollen volumes can reach 20 to 30 times the original volume during combustion. The swelling process can conceivably affect the combustibility of black liquor and the amount of carryover in a recovery furnace.The composition of black liquor was found to have a large influence on swelling. A combination of sugar acids and kraft lignin swelled to a larger extent than when either component was pyrolyzed separately. A 1:1 ratio of these two components resulted in maximum swelling for the various ratios tested. The molecular weight of kraft lignin had an effect on swollen volume with higher molecular weight fractions producing lower swelling chars.Other components were found to reduce the swelling of black liquors. Extractives interfered with the swelling by appearing to change the deformable properties of the pyrolyzing material. Inorganic salts acted as a diluent.Analysis of the surface characteristics of chars revealed that good swelling chars were composed of small bubbles 50 to 150 microns in diameter. Poor swelling liquors did not exhibit this phenomenon. The formation of bubbles was found to be initiated at 240°C, which closely corresponded to the thermal decomposition temperature of a sugar acid. Kraft lignin appeared to have a major influence on the viscous properties of the pyrolyzing particle. The composition of black liquor determines to a large extent the surface active and viscous forces present in black liquor; these forces are believed to be responsible for the extent to which kraft black liquors deform and swell during pyrolysis.

Combustion Properties of Birch (Betula pendula) Black Liquors From Sulfur-Free Pulping

“…[17,27] The combustion properties of BL are usually described in terms of the characteristics of a single liquor droplet during combustion, including the duration of the different combustion stages (i.e., drying, pyrolysis, char burning, and inorganic coalescence) and the swelling behavior. [28][29][30][31][32][33][34][35][36][37][38][39] Upon rapid heating, the droplet undergoes drying, after which the organic material partially breaks down into combustible gases and light hydrocarbons, producing a luminous flame around the droplet.During the pyrolysis stage, the droplet swells significantly. The organic carbon remaining in the char after pyrolysis is then burned off in the char burning stage, producing a small molten bead consisting of various inorganics (mainly Na 2 SO 4 and Na 2 CO 3 ), which need to be recovered.…”

The Thermal Swelling Properties of Plant Chemical Alcohol Waste Liquid

“…However, when the droplet size increased, the specific surface area decreased, the heat transfer and mass transfer became slower, and the oxidation of the air formed a dense shell on the surface of the droplet, which hindered the volume expansion of the droplet. When the furnace temperature was 700-1000 • C, the droplet evaporation and small molecule evaporation analysis time was very short-that is, by entering the macromolecular pyrolysis stage, a large amount of gas was released and accompanied by combustion, and the droplet expansion index reached the maximum value [22]. When the mass of the droplet increased, the precipitation inside the droplet was weaker than the combustion reaction on the surface, and the droplet expansion index became smaller.…”

Method of Black Liquor Combustion to Remove Silicon from Wheat Straw Pulping

“…The isothermal expansion volume index was related to viscosity, silicon content, and ash of black liquor and other factors. The isothermal expansion volume index of straw pulp black liquor was low due to its high ash content and viscosity (Miller and Clay 1989). VIE values were used in the furnace operation, which represented the dryness of the black liquor and cushion looseness degree in black liquor combustion indirectly.…”