Reactivity and Structural Changes of Coal during Its Combustion in a Low-Oxygen Environment

The aim of this study is to improve the understanding of phenomena that occur when a solid carbonaceous material is burned in a high temperature with a low-oxygen-content (<10%, v/v) environment similar to that found in the moderate and intense low-oxygen dilution (MILD) combustion. The morphology, reactivity, and physicochemical properties of partially reacted coal samples extracted from an environment emulating MILD combustion conditions were investigated through different analytical techniques, including elemental and thermogravimetric analyses, scanning electron microscopy, surface area, and Raman spectroscopy. The early stage of coal burnout was characterized by some changes in the organic constituents of coal because the low-molecular-weight compounds react quickly and are the first to be removed. After the devolatilization process under low oxygen combustion, an increase in surface area and porosity was observed simultaneously with a reduction in carbonaceous material reactivity as a result of the gradual increase of crystalline order and structural rearrangement of the carbonaceous network, making it more resistant to oxidation, as evidenced by Raman spectroscopy. The low reactivity of the carbonaceous material during the last stage of heterogeneous oxidation could explain the high level of unburned carbon that can be found in combustion systems that use flue gas recirculation, as occurs in MILD combustion.