Experimental and modeling investigation on char combustion and char-nitrogen evolution characteristics under different conditions

Abstract

The proper description of char combustion and char-nitrogen (char-N) evolution characteristics are imperative to analyze the operating performance of decoupling combustion systems. The char-O2 combustion and char-CO2 gasification reactivity of three typical coal chars, including lignite, bituminous, and anthracite, was tested in a lab-scale fixed-bed reactor in the temperature range of 1023 K–1223 K. The relevant chemical kinetic parameters were determined by applying a one-dimensional packed-bed reactor model embedded with a lumped particle model. Meanwhile, a 1-dimensional spherical particle model is necessary for describing the char-N conversion behaviors. Experimental results show that the conversion rate of char-N in the entire bed to NO (XNO, bed) gradually decreases with increasing temperature, and the decrease of coal rank leads to a decline in the NOx emission. Further simulation results indicate that for a single char particle in the initial combustion stage, if the char particle size increases (0–1000 μm), the external gas mass transfer coefficient decreases (0–5 m·s−1), or the oxygen content decreases (0–20%), the char-N conversion to NO (XNO, p) will gradually decrease. However, due to the complicated effects of temperature on multiple factors, involving reactivity, CO/CO2 ratio in products, and dual nature of CO influence on char-NO reaction, the relationship between char-N conversion and temperature for a single particle is not fixed. When the oxygen concentration is low, the XNO, p increases as temperature rising; while, an opposite trend may be formed under high oxygen conditions.