New insights into the synergy and mechanisms during high-temperature co-gasification of biomass and carbon-rich fly ash

This study investigated the co-gasification characteristics of fly ash (FA) derived from an industrial circulating fluidized bed coal gasifier and soybean straw (SS) using a drop tube reactor (DTR) operating at 1400 degrees C. Carbon conversion was used as a key metric to explore the synergistic mechanisms of FA-SS co-gasification. The results revealed that an SS blending ratio of 50% produced the most significant synergistic effect. At this ratio, the experimentally measured carbon conversion was 5.6%-5.8% higher than the calculated values. Notably, after the acid de-ashing of SS, the synergistic effect of the co-gasification process was retained but was slightly weakened, as indicated by experimental carbon conversions being 3.3%-3.6% higher than calculated values. The synergistic effect of co-gasification was mainly attributed to the lower CO2 and water yields of SS50 compared with the average yields of FA and SS. The synergistic effects of high-temperature co-gasification resulted from two mechanisms. First, alkali and alkaline earth metals (AAEMs) in SS ash catalyzed the high-temperature gasification of FA char. Moreover, high temperatures increased the gasification reactivity of char with H2O and CO2. During co-gasification, the high volatile content of SS yielded H2O and CO2, which served as additional gasifying agents to react with char, leading to enhanced carbon conversion and cold gas efficiency (CGE). The H2 yields (9.4-15.5 mmol g-1) from FA gasification at steam-to-coal (S/C) ratios of 0.1-0.3 were comparable to those from FA-SS co-gasification at SS mass ratios of 20%-50%. Overall, FA and biomass co-gasification provided more technical benefit than FA gasification with added steam. These findings provide valuable insights into the synergistic mechanisms of high-temperature entrained-flow co-gasification.