Physicochemical, Geomineralogical, and Evolved Gas Analyses of Newly Discovered Nigerian Lignite Coals

Abstract

This paper examines and highlights the energy recovery potential and emissions profiles of three (3) newly discovered lignite coals from Obomkpa (BMK), Ihioma (IHM), and Ogboligbo (OGB) in Nigeria. Therefore, the physicochemical, geo-mineralogical, and evolved gas characteristics of the lignite coals from Nigeria’s sedimentary basins were examined and highlighted. Physicochemical analyses revealed that BMK contains the highest carbon and hydrogen but the lowest oxygen content. This accounted for its higher heating value (HHV = 19.66 MJ/kg) compared to IHM (HHV = 19.40 MJ/kg) and OGB (HHV = 15.55 MJ/kg). Proximate analysis revealed that BMK has the highest ash compared to OGB which has the lowest volatiles, moisture, and ash. Geomineralogical and microstructural analyses showed the lignites contain layers of fine to coarse particles with a distinct lustre which may be due to quartz, kaolinite or metal elements. Thermal analysis revealed that BMK, IHM, and OGB experienced high loss of ranging from 55.1 to 78.1%, resulting in 20.9 to 44.6% residual mass (R_M). Therefore, IHM is the most reactive whereas OGB is the most thermally stable due to its thermal resistance during degradation. Thermal analysis also revealed that the oxidative degradation occurred in three stages namely; drying, devolatilization, and char oxidation. Lastly, evolved gas analysis revealed low to high intensity bands indicating the evolution of H₂O, CO₂, CO, SO₂ and HCN in six (6) stages from; 4000–3400, 2400–2200, 2200–2000, 2000–1600, 1600–1300, and from 800–600 cm^–1 during TG-FTIR. In conclusion, the paper successfully presented insights into the physicochemical, geo-mineralogical, evolved gas characteristics and potential applications of lignite coals from Nigeria.