Carbon distribution in char residue from gasification of kraft black liquor

The char residue yields and the total carbon and carbonate content were measured for dry black liquor solids after pyrolysis or gasification in a laminar entrained-flow reactor. The experimental conditions were 700–1000°C in N2,CO2/N2 or water vapor/N2 at 1 bar total pressure, for residence times from 0.3 to 1.7 s. Fixed carbon yields, when measured at the same particle residence time, decreased with increasing reactor temperature. CO2 and water vapor diminished the char carbon significantly at temperatures above 800°C, compared with pyrolysis in N2. Water vapor oxidized the char carbon more rapidly than did CO2. At 1000°C, the reactions of carbon with sulfate and carbonate became faster, resulting in a smaller difference between carbon conversion rates in the different gas environments. By the end of devolatilization, the amount of carbonate in the char had changed very little at 700–800°C. After devolatilization, carbonate was formed more rapidly at higher temperatures. The presence of CO2 or water vapor increased the formation of carbonate. In the presence of these gases, more carbonate was measured at all temperatures and residence times. The maximum carbonate measured in the char was 16% of the carbon in the black liquor solids, as compared to 4.4% in the original dry liquor solids. Under most conditions, the carbonate, as a fraction of carbon input, first increased to a constant, temperature-independent value and then decreased.