Two typical types of Chinese lignite samples were employed to investigate the coal drying characteristics of an innovative COMBDry lignite drying system. The drying rate increased significantly with an increase in drying temperature and ratio of the flue gas to the lignite. Using a nitrogen adsorption instrument, it was found that the drying process promoted generation and enlargement of the surface pore structures of the particles, which can enhance coal combustion rate. A horizontally fixed bed furnace and a SIGNAL S4i pulsar NDIR (infrared (IR)) gas analyzers were used to investigate the combustion characteristics of coal samples after the drying treatment. In an attempt to analyze the variation in the sample surface chemical structure during the drying process, Fourier transform infrared (FT-IR) spectrometry and Raman spectroscopy were employed for the investigation. The results showed that the concentration of aliphatic hydrogen decreased with an increase in drying temperature and resulted from the decomposition of oxygen-containing complexes (released CO and CO 2 ) and reaction with hydroxyl groups. The number of functional groups decreased when the drying temperature exceeded 210 °C. The amount of carbonyl and carboxylic esters initially increased and then (210 °C) decreased with an increase in flue gas temperature, and the content of aromatic carbon was unchanged with the treatment. The ordered crystalline carbon changed into the crystal defect structure and amorphous carbon and the degree of graphitization decreased and thus the combustion reactivity of the dried lignite was improved.

No abstract

The effect of preoxidative treatments on the reducing characteristics of ShenHua (SH) coal char and the evolution of surface nitrogen-containing complexes during the reducing process under different temperature conditions were investigated using temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The SH samples were oxidized to different conversion degrees (0.15, 0.22, 0.32, 0.42, 0.52, 0.73, and 0.89) under an O2 atmosphere, and the reductivity of each char sample was considerably enhanced after the preoxidative treatment. This phenomenon could be attributed to the generation of active sites and oxygen-containing complexes on the SH particle surface during the oxidation process. The preoxidized samples were obviously more reactive than the raw char, and per unit mass almost all the preoxidized samples could consume more NO than SH raw char under the same experimental conditions. The entire TPR process could be divided into the following four sections: (a) the reversible physical adsorption stage, (b) the heterogeneous reaction stage, (c) the multireaction stage, and (d) the equilibrium reaction stage. The primary reaction path at each stage could be summarized using the TPR and XPS results. The evolution of the C­(N) and the variation in the elemental distribution (C, O, and N) during the TPR process were investigated by XPS. The results showed that N-Q was the most stable organic structure of the nitrogen-containing complexes on the particle, and the decomposition of N-6, N-5, and N-Q occurred when the reaction temperature reached 1173 K. The total amounts of N-Q, N-6, and N-5 decreased when the reaction temperature exceeded 1173 K, indicating interaction between the nitrogen-containing complexes occurred. Meanwhile, prior to the attachment of NO molecules to the char particle surface and forming C­(N), more NO molecules were consumed by CO at this temperature. The results in this research clarified the effect of the conversion degree on the char reductivity and the primary reduction reaction path under different temperature conditions, providing a technical framework for the reducing process of air-staged combustion technology.