Combustion behavior of black liquors : droplet swelling and influence of liquor composition

Abstract

The combustion of black liquor (BL) in a recovery boiler is a central process in a kraft pulp mill for recovering the cooking chemicals and for producing heat and power. This work explored the most important combustion behavior of BL, the swelling of in-flight droplets, from the viewpoint of liquor composition. It also studied the combustion behavior of BL droplets obtained from two biorefining subprocesses (carbonation and hot-water pretreatment, HWP) and sulfur-free pulping alternatives (soda-anthraquinone (AQ) and oxygen-alkali pulping). The formation of a plastic state essential for the swelling of BL droplet was found to result from the melting of an array of carbohydrate-derived aliphatic carboxylic acids (sodium salts) and NaOH. Another two necessary conditions for swelling were: (i) the release of volatiles by the decomposition lignin and other organics for expanding the plastic mass and (ii) the formation of a rigid char surface with the organic carbons mainly from the pyrolysis of lignin. Biorefining subprocess using carbonation to partially separate lignin from the BL had a significant impact on the swelling behavior of the resulting BL droplets. The reduced lignin content and the increased Na2CO3 content led to lower maximum swelling of the droplet but did not influence the combustion times. The content of lignin seemed to affect the degree of swelling greater than its molar mass distribution. Further experiments indicated that the medium molar mass lignin (i.e., the 5-10 kDa fraction) pyrolyzed in mixture with organic acids swelled more than other fractions (i.e., the 1-5 kDa and >10 kDa fractions). The HWP of feedstock prior to chemical pulping was another biorefining subprocess that influenced the combustion behavior of the BLs by altering the delignification behavior and thus the liquor composition. Generally, the HWP reduced the amount of acetic acid but increased the amount of hydroxy acids and lignin for almost all BLs. Consequently, the maximum swelling of BL droplets from chips increased but for most of the BLs from sawdust it decreased after the HWP. The sulfur-free BL droplets, especially the soda-AQ droplets, typically swelled less and burned slower than the kraft droplets. The mass ratio lignin-to-organic acids was found to correlate with the swelling of BL droplets.