Two kinds of chars were used in this research, the original rice straw char and the acid washed rice straw char. Their reactivity evolution during the char-NO reaction was studied in a fixed bed reactor at 900 degrees C and 1000 degrees C. The char samples obtained at different conversion were analyzed by BET and Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). The reactivity of the acid washed char increased first and then decreased with the turning point at the char conversion of about 70% at both 900 degrees C and 1000 degrees C. This was consistent with the development of specific surface area. The reactivity evolution of the original char was similar as acid washed char at 900 degrees C, but the reactivity decreased continually to zero at 1000 degrees C, which contradicted the specific surface area development of original char. The measurements of mineral concentrations in char showed that K, Ca and Mg were the primary minerals in the original char. During the char-NO reaction, the transformation of K, Mg and Ca occurred, the concentrations of acid soluble K and Mg decreased 26.5% and 2.7% respectively, while the concentration of Ca increased 20.4% from char conversion X-char = 0% to X-char = 44.6% at 1000 degrees C. The transformation of acid soluble K was consistent with the reactivity evolution of char during the char-NO reaction, which indicates that acid soluble K played an important role on char reactivity evolution during the char-NO reaction. The specific reactivity of char had a nearly linear relationship with the concentration of acid soluble potassium in char. (C) 2013 Elsevier Ltd. All rights reserved.