Morphological Characteristics of Chars Obtained from Low-Temperature Pyrolysis of Pulverized Lignite

Abstract

Staged coal conversion, with pyrolysis as its foundation, is one clean and effective coal usage technology. In this study, the structural and morphological characteristics of Ximeng lignite chars obtained from low-temperature pyrolysis during preheating in staged coal conversion are studied. The pulverized lignite was pyrolyzed under N2 in a temperature range of 3–7°C. Sieve analysis was used to investigate changes in char particle size. N2 adsorption/desorption, and a scanning electron microscope (SEM) and Fourier transform infrared (FTIR) spectroscopy were used to investigate changes in pore structure, surface morphology, and chemical structure of the char particles. The mechanisms of changes in particle size are discussed. Results suggest that the char particles have the trend of being smaller with increasing pyrolysis temperature. Changes in particle size, pore structure, and chemical structure at temperatures above 5°C are more significant than those below 5°C as a result of greater extent of pyrolysis and more vigorous condensation reactions at high temperatures. Specific surface area and pore volume increase with increasing pyrolysis temperature. More micropores are developed, and the number of mesopores with an average size smaller than 1 nm decreases as the temperature ranges up to 5°C. Moreover, when the pyrolysis temperature increases above 5°C, the specific surface area and pore volume increase sharply as a result of large number of pores and wide pore-size distribution. FTIR analysis shows that the aromaticity of char increases oxygen-containing functional groups, and aliphatic structures decrease as the pyrolysis temperature increases.