Black liquor gasification characteristics. 2. Measurement of condensable organic matter (tar) at rapid heating conditions

Experiments were performed to investigate tar formation during devolatilization of black liquor at high heating rates, at temperatures from 700 to 1000 degreesC and 1 bar of total pressure. The tar compounds detected were grouped into two categories, semivolatiles and nonvolatiles, based on their molecular weights and boiling points. The semivolatile tar collected ranged from 0.1% to 5% of the carbon in black liquor, while the nonvolatile tar collected ranged from 0.02 to 1%. However, carbon balances suggested that tar may have accounted for 20% or more of the carbon in black liquor at 900 degreesC and below. Tar characterization revealed a similarity between the lignin substructure and some of the tar compounds produced. This indicates that kraft lignin was the source of these tar compounds. Tar yields were controlled by a number of competitive formation and decomposition reactions, which are influenced by the residence time, temperature, and gas composition. Low temperatures favored the formation of more aromatic tar species with diversified substituted groups such as toluene and xylenes. These compounds were formed at a finite rate, and their concentrations were increased with gas/particle residence time. Tar species formed more rapidly and decomposed more rapidly at higher temperatures. Nonsubstituted aromatics such as benzene were more stable at higher temperatures and were formed by decomposition of substituted aromatics. Oxidizing gases enhanced both the formation and destruction of tar species, depending on the temperature and residence time.