Search

This research investigated the emission and evolution characteristics of fuel-N during the O2/H2O combustion process of a typical bituminous coal (Shenhua). To avoid the catalytic interference of alkali metal salts, the demineralized coal obtained from the raw coal sample was employed for the investigation in this research. The effects of reaction temperature (Tr) and H2O concentration, which were two vital factors affecting coal combustion characteristics, on the emission characteristics of fuel-N during the combustion process were also taken into consideration. Isothermal combustion tests performed under different O2/H2O conditions (Tr: 1073 and 1473 K; O2: 30%; H2O: 0, 3.5, 8.5, 15, 20, and 30 vol %) indicated that relatively high concentrations of N2O, HCN, and NH3were measured in the O2/H2O combustion of the coal sample. With the increase of Trand H2O concentration, more fuel-N would be released in the devolatilization/volatile oxidation stage of the reaction. The X-ray photoelectron spectroscopy results of chars after devolatilization indicated that most of the N-5 decomposed into HCN or converted to N-6 or N-Q in the early reaction stage at high temperatures and that N-Q was the dominated form of a nitrogen-containing functional group (C(N)) existing on the char particle surface. The reducibility of the chars after O2/H2O devolatilization played vital roles in reducing the emission of NO during the whole O2/H2O combustion process.

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 N2adsorption 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 (Td), 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 Tdcould effectively inhibit the occurrence of moisture re-adsorption. The optimum Tdfor the upgrading treatment of HLH lignite by the ComDry system is 483 K.

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 CO2) 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.