Efficient Demineralization and Desulfurization of Lignite Coal using Alkali and Alkali-Acid Follow up Treatments for Reduced Flue Gas Emissions and Enhanced Calorific Value: A Comparative Study

In world the largest reserves of low-grade coal or lignite coal are available. A low-ranking coal with high mineral content, sulfur, moisture and low calorific value greatly contribute to environmental pollution via emission of greenhouse gases, SOx and NOx. To resolve this, the coal must be extracted or reduced of its minerals and sulfur through the processes called demineralization and desulfurization. In this, the researchers have been unveiling new and efficient ways to treat coal for reduced air pollution during combustion. Herein, demineralization and desulfurization of coal were carried out using alkali and alkali-acid hybrid treatments. In the first method, the samples were treated solely with alkali while in the second, they were treated with acid followed by alkali. The coal samples were analyzed via SEM, TGA, XRD and FTIR; their chemical composition determined through ONH and SC analyzer. As per results, alkali-acid hybrid treatment was more efficient than just alkali treatment. The hybrid treatment of 100 g/L NaOH followed by 10% HCl exhibited higher porosity, thermal stability, demineralization of 90.9%, desulfurization of 63.46% and calorific value of 26585.924 KJ/Kg in the respective coal sample. On the other hand, alkali treatment with 100 g/L NaOH resulted in comparatively lower porosity, thermal stability, demineralization of 63.86%, desulfurization of 49.42% and calorific value of 23770.905 KJ/Kg respectively. When compared with 6 relevant studies, the coal sample with the least sulfur and mineral content in the present work indicated lowest impurities.