Effects of Upgrading Treatment on the Physicochemical Structure, Moisture Re-Adsorption Ability, and NOx Emission Characteristic of Lignite Particles

A typical Chinese lignite (HLH) was used for determining the correlation among upgrading temperature, moisture re-adsorption behaviors, and nitrogen evolution characteristics during the combustion process. The upgraded samples were obtained from four drying temperatures: 393, 423, 453, and 483 K in the ComDry (counter flow multi baffle dryer) system. By means of the utilization of the N-2 adsorption method and scanning electron microscope, the effects of drying temperature on the surface morphology and pore structural characteristics of lignite particles were determined. With the increase of drying temperature (T-d), big pores collapsed into small ones leading to the enhancement of porosity, pore volume, and specific surface area, as well as the reduction of the average pore diameter. The distribution of oxygen-containing functional groups (C(O)) on the surface of the samples dried at different temperatures,was identified by Fourier transform infrared spectroscopy. More carboxyl and hydroxyl converted into ether and anhydride, which were thermally stable in the process of drying treatment performed at higher temperatures, reducing the possibility of spontaneous combustion in lignite storage. Both the physical and chemical structural characteristics could be regarded as indicators reflecting the moisture re-adsorption behavior. The moisture content re-adsorbed by the upgraded coal particles expressed negative effects on inhibiting NO emission during the combustion process, and increasing T-d could effectively inhibit the occurrence of moisture re-adsorption. The optimum T-d for the upgrading treatment of HLH lignite by the ComDry system is 483 K.