Abstract
The oxidation of lignite and bituminous coal samples modified by 5 wt% (in terms of dry salt) addition of copper salts Cu(NO3)2, CuSO4, and Cu(CH3COO)2 was studied. The samples’ reactivity was studied by thermogravimetry within a temperature range of 45–600 °C at a heating rate of 2.5 °C/min in an oxidizing environment. The introduction of activating additives has resulted in a significant decrease in the temperature of intense oxidation onset (ΔTi = 20/94 °C), in a reduction in the sample residence time in the volatile matter release region (Δte = 2/22 min) and the total duration of the coal combustible mass oxidation (Δtf = 8/14 min). The Friedman method was used to calculate the activation energy values for the oxidation process of the modified samples. The maximum change in activation energy values was observed for the bituminous coal sample. The possible mechanism behind the action of the copper-salt additives, which activate the oxidation of lignite and bituminous coal, is discussed. According to the data of mass spectrometric analysis, the concentration of NOx in the reaction products decreases as the temperature of the activated oxidation process is shifted towards the low-temperature region.
Highlights
With the continuous growth of coal consumption for the needs of the power industry (IEA 2017), there is an actual challenge to improve technological approaches for designing new and modernizing existing fuel-burning equipment
The properties of the initial coal samples subjected to a milling procedure and dried at 105 °C are presented in Samples are different in terms of the particle size distribution (X50 = 13.9–20.4 lm) that could be explained by certain differences in their morphology (Fig. 2) (Sriramoju et al 2019)
One can see that the powders of coal consist of faceted irregular particles with a heterogeneous surface and contained many smaller fragments that were less than 5 lm in size
Summary
With the continuous growth of coal consumption for the needs of the power industry (IEA 2017), there is an actual challenge to improve technological approaches for designing new and modernizing existing fuel-burning equipment. The change in the initial ignition temperature of fuels of different ranks is explained by the nature of the oxidation reaction (homogeneous or heterogeneous) (Gong et al 2010a) It was suggested by Guo et al 2014 that metal oxides can be reduced to a metallic state or oxides with a lower valence as a result of the interaction with carbon; after the contact with oxygen, they may return to their initial state. In numerous papers, it is mentioned that the solid oxide precursors (salts and their bases) are characterized by strong oxidizing properties, low melting (decomposition) temperatures, and good solubility in water (Morozov et al 2003; Ryu et al 2004) The latter simplifies their introduction significantly by means of impregnation of the solid fuels. In the current paper we tried to study the extended set of different copper salt precursors—Cu(NO3), CuSO4 and Cu(CH3COO)2—which could promote the oxidation of coals with different level of coalification
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