The Influence of Oxyfuel Combustion Conditions on the Behavior of Inorganic Cooking Chemicals during Black Liquor Conversion

In this experimental study, we investigated the influence of various gas atmospheres on the chemistry of the inorganics during black liquor conversion. The focus was placed on oxyfuel conditions, which are characterized by a mixture of O2 and recycled flue gases (e.g., CO2) as oxidation medium for combustion, to facilitate efficient CO2 capture. The conventional air-based conversion and pure CO2 and N2 atmospheres were also tested for comparison. The experiments were conducted in a drop-tube furnace (DTF) at 1000 °C. A solid and gas phase compositional analysis was carried out to be able to quantify the carbonate, sulfate, and sulfide content in the collected solids and mapping gaseous compositions. In addition, thermodynamic equilibrium calculations (TEC) were used to analyze the chemistry that governed the composition of the formed inorganics. We found that CO2 had an oxidative effect on the inorganics in the conversion of black liquor, contrary to inert N2. Moreover, in a CO2 atmosphere, significantly higher levels of SO2 were formed than in a pure N2 atmosphere. These findings have important implications for the Industrial chemical recovery of sodium and sulfur in the kraft pulp process. Predictions of the effect of oxyfuel mode at realistic conditions relevant to industrial recovery boiler operations were also made with TEC. We found support for the existence of fuel-to-gas ratios that can render similar levels of sulfur reduction to sulfide as for conventional air-based operation in the substoichiometric zone of the boiler. These results can guide future development toward retrofitted recovery boilers to oxyfuel mode.