Influences of graphite on the fusibility and phosphorus release characteristics of phosphorus-rich biomass ash under different atmospheres

In gasification applications of phosphorus-rich biomass, phosphates in the feedstock may be reduced by the residual carbon and thus alter the melting behavior of the ash and lead to phosphorus release into the gas phase. In this study, the fusion characteristic temperatures of two phosphorus-rich biomass ashes mixed with 0–10 wt% graphite were determined under argon (Ar) or carbon dioxide (CO₂), respectively. Results showed that the ash fusion temperatures (AFTs) of sludge ash decreased and then increased with the increase of graphite content under Ar atmosphere, while it increased and then decreased under CO₂ atmosphere. AFTs of distillers’ grains ash increased due to the graphite addition in both atmospheres, and the increase was more.

significant under Ar atmosphere. The presence of CO₂ in the atmosphere changed the magnitude of the variation in AFTs caused by graphite. According to XRD analysis, equilibrium calculations and reaction thermodynamic parameters, metallic iron, free SiO₂ and Al₂O₃ facilitated the lowering of the thermodynamic temperature threshold for the carbothermic reaction of calcium-bearing phosphates. The iron phosphide compounds, which formed after the capture of phosphorus by metallic iron, had good thermal stability and prevented the release of phosphorus from the ash slag into the gas phase. Under CO₂ atmosphere, the carbothermic reaction of phosphates was limited by the preferential reaction between graphite and CO₂. Measurements of the phosphorus content in the 1400°C quenched slag obtained by digestion and ICP-OES indicated that a graphite content of 5 wt% or more in the ash along with a low iron oxide content, a low CO₂ concentration in the atmosphere, and the attainment of a temperature threshold for the carbothermic reaction are necessary conditions for the release of phosphorus to the gas phase under gasification conditions.