Abstract
Non-isothermal oxidation of brown coal with 5 wt% of Cu(NO3)2, 5 wt% of Ce(NO3)3 and {2.5 wt% Cu(NO3)2 + 2.5 wt% Ce(NO3)3} additives was studied. The introduction of additives was carried out by an incipient wet impregnation method to ensure uniform distribution of cerium and copper nitrates within the structure of coal powdery samples (according to SEM and EDX mapping). The samples reactivity was studied in an isothermal oxidation regime at 200 °C (1 h) and by DSC/TGA at 2.5 °C/min heating rate. The additives implementation was found to reduce significantly the oxidation onset temperature (ΔTi = 20–55 °C), the samples oxidation delay time (Δti = 2–22 min) and overall duration of the oxidation process (Δtc = 8–16 min). The additives efficiency could be graded in accordance with the activation on the coal oxidation in the following row: Cu(NO3)2 > {Cu(NO3)2 + Ce(NO3)3} > Ce(NO3)3. According to the mass spectroscopy, the obtained row of activation correlates well with the initial temperature of the studied nitrate’s decomposition (from 190 to 223 °C). A presence of nitrates was found to change significantly the trend of heat release taking place during the oxidation of coal samples (according to DSC/TGA data). The influence of coal morphology and volatiles content in initial sample on the parameters of the oxidation process was studied as well. Activation energy (Ea) of the coal oxidation was calculated using Coats–Redfern method. Maximum decrease in Ea from 69 to 58 kJ/mol was observed for the samples with Cu(NO3)2.Graphical abstract
Highlights
A coal is one of the most popular types of fuel for heat and electricity production along with natural gas and oil (International Energy Agency 2017)
The cost of heat produced by coal burning is still approximately 2.6 times lower compared to natural gas, and this value is the lowest in the history
To study the effect of cerium oxide and copper oxide on the coal oxidation process the impregnated coal samples were heated in air at 200 °C for 1 h to convert cerium and copper nitrates into their corresponding oxides
Summary
A coal is one of the most popular types of fuel for heat and electricity production along with natural gas and oil (International Energy Agency 2017). According to Chang et al (2016), current studies in this area are dedicated to a search for new technologies aimed to increase the efficiency of combustion, reduce capital costs for new equipment and improve its environmental performance. One of such approaches is focused on intensification of the coal combustion process by using of activation additives in order to improve the efficiency of coal combustion (Tikhov et al 2013). Many nitrates are known to have strong oxidizing properties, low melting (decomposition) point and excellent solubility in water (Morozov et al 2003) The latter significantly simplifies their application using the incipient wetness impregnation of the solid fuel. CuO and CeO2 oxides are known to perform high catalytic activity in coal oxidation processes (Sheng et al 2017)
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