Biomass gasification ash reutilization: Recirculation reusability and mechanism analysis

Abstract

A biomass gasification ash disposal method, recirculating it to gasification process, was proposed in this work. The recirculation reusability was analyzed via reusing corn straw (CS) gasification ash for six recirculation in a gasifier. Metals, especially K, got enriched in ash during ash recirculation. Under the 1st ash recirculation, CO content, H2 content and lower heating value of gas products increased from 16.02 to 19.97 vol%, from 11.30 to 15.72 vol%, from 5.38 to 6.47 MJ/Nm3, respectively. Moreover, it also exhibited an ability in tar decomposition, resulting in a tar reduction (4.63 g/Nm3 for raw CS gasification and 3.62 g/Nm3 for 1st ash recirculation). These positive effects gradually increased during the first four ash recirculation. The positive effects highly depended on the accumulation of metals (K, Na, Ca, Mg, and Fe) in ash. The positive effect did not increase as the fifth and sixth ash recirculation were conducted, resulting from the constant metals content in ash. Grey relational analysis showed that the total content of these metals was the most significant influencing parameter, followed by K content, water-soluble K content, Ca content, respectively. Moreover, pollutant SO2 could be captured by CaO during ash recirculation. This reaction competed with simultaneous CaO carbonation reaction, while it was inferior to the latter. Thus, only the first and second ash recirculation exhibited satisfactory ability in SO2 capture. In summary, ash recirculation in gasification process was feasible for upgrading gasification performance and its effect was persistent.