Black liquor gasification for chemical recovery and hydrogen gas production in supercritical water

Abstract

This study investigates the thermal conversion of industrial black liquors from the different pulping processes, including kraft process (KBL) and soda process (SBL). In order to study lignin content, kraft lignin (KL) and soda lignin (SL) were isolated from black liquor as the technical lignin. The research was divided into three parts. The first part was to study the effect of the pulping process on black liquors and technical lignins properties. The results showed the effect of the different pulping processes on black liquors and technical lignins composition. SBL mainly contained Na2CO3, which was the spent chemical while KBL contained both Na2CO3 and Na2SO4. The FTIR results presented the OH and OCH3 group as the primary constituents in black liquors and technical lignins. Aromatic compounds were the main component of KL while SL contained more aliphatic HCs. TGA technique revealed the similarity in thermal decomposition pattern of KL and lignin model compound, and SL and cellulose model compound. These finding indicated the unique structure of the technical lignins, especially the cellulose residue in SL. The second part investigated the thermal conversion of lignin via Py-GC/MS technique at 400°C-600°C. The results showed that KL was slightly responded to increasing temperature with majority of phenolic compound production. With increasing temperature, SL was highly degraded and mainly produced the oxygenated HCs compounds. These results demonstrated high thermal stability of KL, corresponding to lignin nature while the large portion of oxygen content in SL exhibited the mixture of cellulose and lignin in structure. The catalytic of Ni/ZrO2 can promote p-Hydroxyphenyl, catechol, and aliphatic HCs compounds from KL and enhance ether, aldehyde, ester, and aliphatic HCs compounds from SL. The third part investigated the SCWG of KBL and SBL at 400°C-600°C, 250-400 bar. The results showed that the most appropriate condition for H2-rich syngas was 600°C at 250 bar. Different lignin structure strongly effected gas yield, 38 and 58 mol H2 yielded per kg KBL and SBL, respectively. Nickel catalyst is the most promising option with regard to efficient and cost-effective provision.