Energy efficiency and environmental aspects of the combustion of coal-water slurries with and without petrochemicals

Abstract

The paper presents the results of the experimental research into maximum concentrations of anthropogenic emissions from typical brown low-rank coal combustion in the traditional pulverized state as well as from the combustion of coal-water slurries with or without petrochemicals. The temperature of fuel heating in the model combustion chamber varied in the range from 500 °C to 1000 °C. The relative values of concentrations of anthropogenic emissions are calculated with due consideration of key energy characteristics of the fuel: combustion heat, specific mass of the fuel component in a mixture, specific energy obtained per fuel cost. The findings illustrate considerable differences not only between the concentrations of the analyzed anthropogenic emissions, but also between the times, during which these concentrations remained high. In addition, we compare the boundary conditions (i.e. borderline or minimum sustainable ignition temperatures) necessary and sufficient for the combustion of the fuel compositions under study and analyze fuel heating and ignition delay times. Special aspects are determined of the environmentally friendly combustion of coals as part of coal-water slurries with or without petrochemicals at different temperatures: from minimum temperatures usually observed at the beginning of sustainable ignition to those typical of small- or medium-sized power plants. Concentrations of the most hazardous oxides of sulfur and nitrogen produced during coal combustion are 3–5 times higher than during coal-water slurry combustion. The differences between these parameters for coals and coal-water slurries containing petrochemicals are less noticeable. Nevertheless, the emissions of nitrogen and sulfur oxides from the combustion of coal-water slurries containing petrochemicals are at least 30–60% lower than from the combustion of coals. Taking into account the cost of the components of coal-water slurries containing petrochemicals, as well as the combustion heat and the concentrations of anthropogenic emissions, specific indicators show that coal-water slurries containing petrochemicals are more efficient than coals or the coal-water slurries without petrochemicals. If filter cakes are used as the main component of coal-water slurries containing petrochemicals, the specific indicators of such fuels grow 5–6 times higher (or even 10 times higher in special cases) versus pure coals. The research findings show that the temperature monitoring in the combustion chamber is obligatory to provide high ecological indicators. At temperatures below 1000 °C, it is possible to provide stable combustion of fuels and significantly reduce emissions as compared to the traditional high-temperature regimes (above 1000 °C). Thus, temperatures lower than 1000 °C are more rational. Finally, the conditions for the optimal coal fuel combustion are presented in terms of the environmental friendliness, energy and cost efficiency, as well as fire and explosion safety.