Molten Phase Chemistry Relevant to Black Liquor Combustion under Oxyfuel Conditions in Recovery Boilers

Sammanfattning

Throughout the process of producing paper pulp, a waste stream called black liquor is generated that mainly consists of dissolved inorganics and organics from the feedstock, along with used cooking chemicals (white liquor). To recover these chemicals, black liquor is concentrated in evaporators and subsequently combusted in the recovery boiler. This recovers the inorganic chemicals as a molten smelt which undergoes a series of steps known as the lime cycle to regenerate the white liquor, where a critical step includes the conversion sodium carbonate (Na2CO3) into sodium hydroxide (NaOH) using lime. The incineration of black liquor leads to biogenic CO2 emissions. To mitigate these emissions and potentially achieve negative CO2 net output, oxyfuel combustion has been proposed. This technique replaces traditional combustion air with oxygen, and recirculated flue gases, to increase the CO2 partial pressure and simplify carbon capture. However, this shift in combustion conditions may alter the melt chemistry and increase the carbonate content, thereby increasing the demand for lime during the chemical recovery. This thesis aim to investigate the melt chemistry of black liquor when subjected to different atmospheres. To address the aim, synthetic mixtures of Na2CO3, sodium sulfate (Na2SO4) and sodium sulfide (Na2S) were investigated regarding chemical equilibrium, behavior when exposed to temperature increase in different atmospheres with thermogravimetric analysis (TGA) and crystal structure with X-ray diffraction (XRD). The results were also compared to black liquor combusted in various atmospheres. Equilibrium simulations using FactSage indicated that CO2-rich environments stabilize Na2CO3 to higher temperatures and suppress decomposition. In oxidizing atmosphere, all Na2S convert to Na2SO4, except for lower temperatures in CO2 atmosphere where Na2S forms COS (g). The TGA analysis supported these simulation by showing sample weight change corresponding to the chemical reactions. Sample containing Na2S exhibited weight increase in air, consistent with oxidation to Na2SO4, while the increase was less pronounced in CO2 due to COS formation. Notably, no significant reactions were observed for samples without Na2S. XRD analysis demonstrated that melting and slow cooling of Na2CO3 and Na2SO4 result in crystallization of double salt such as Na6CO3(SO4)2, Na4CO3SO4 and Na6(CO3)2SO4 depending on the ingoing material. These crystals were not detected for unmelted samples, indicating that thermal treatment is necessary for double salt formation. Furthermore, peak shifting and intensity variation correlated with changes in Na2CO3:Na2SO4 molar ratio, with higher carbonate content resulting in peak shifting to higher angles. When comparing the spectra of combusted black liquor to the synthetic samples, similar peak positions were observed, suggesting the presence of the same double salts. The black liquor combusted in oxyfuel conditions showed slightly shifted diffraction peaks towards higher 2θ values than for air or pure CO2, suggesting that oxyfuel conditions result in higher carbonate content in remaining material. No distinct signatures of Na2S were observed in the black liquor, reinforcing that it is not present in solid phase post combustion. Some additional peaks were observed in the black liquor sample, suggesting the presence of other elements which are not found in the synthetic systems.