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 degrees C. 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.