Leaching mechanisms of ash-forming elements during water washing of corn straw

One of the challenges for large-scale biomass gasification is inevitable ash-related problems such as ash deposition, corrosion, fouling, acid gas emission, and others, mainly caused by the volatile ash-forming elements in biomass. Water washing is an efficient, low-cost, and manageable way to alleviate these ash-related problems by reducing the concentrations of ash-forming elements in biomass. The leaching characteristics of ash-forming elements such as K, Na, Ca, Mg, Al, Fe, S, Cl, and P of corn straw (CS) were studied by inductively coupled plasma mass spectrometry (ICP-MS), ion chromatography (IC), and ultraviolet–visible spectroscopy (UV–Vis) during water washing at different time and temperatures. It was found that the water washing process removes almost all of K, Cl, and P with a removal efficiency higher than 90% within the first 10 min; large proportions of S, Na, and Mg with a removal efficiency of more than 70% within 120 min; and small amounts of Ca, Al, and Fe with a removal efficiency less than 63% within 120 min even at 50 ºC. The kinetic analysis indicated that the leaching of ash-forming elements was a two-step process consisting of an initial fast step and a second slow step. The leaching of ash-forming elements might be controlled by the first-order kinetic model, namely, homogeneous model and shrinking core model. Still, the second-order reaction model presents high regression coefficients, which is better suitable to fit the leaching kinetics of ash-forming elements from CS than the first-order kinetic leaching model. The reaction rate for the second-order reaction is faster than the first-order reaction during the water leaching of CS. The water washing could reduce the slagging tendency in the gasifier and diminish the emission of acid gases during corn straw gasification. Graphical abstract