Effect of mineral oxides on slag formation tendency of mae moh lignites

Slagging is one of major ash deposition problems experienced in the boilers of coal-fired power plants especially the plants that use lignite, like Mae Moh lignites. The occurrence of slag is a complex phenomenon depending on several factors such as ash properties, furnace operating conditions, and coal properties. The main objective of this work is to study the effect of mineral components in Mae Moh lignite on ash fusion temperatures (AFTs), which is commonly used as a key indicator for slag formation tendency under pulverized combustion conditions. Two Mae Moh lignites from the coal seams planned to be used in the future were selected for the study to represent low CaO and high CaO lignite. The two lignites, namely K1 and K3, have 3.6 and 40.4 wt% CaO in ash, respectively. The AFT characterization shows that their initial deformation temperatures (ITs) were almost identical and considered as low for the typical flue gas temperature in the radiation section of Mae Moh boilers, i.e. 1050-1100°C. These observed similar ITs were rather unexpected, especially for K1 considering its sufficiently low base to acid (B/A) ratios. The X-ray diffraction analyses evidently show the presence of illite, pyrite and anhydrite in K1, which explains the observed lower IT of the sample. Anhydrite, which is known to lower the ITs, is also the most abundant mineral in K3. Washing the lignite samples with HCl can significantly reduce CaO, MgO, and SO₃ content in the ash but not Fe₂O₃ as it is present in the form of pyrite. The addition of Al₂O₃, SiO₂ and Fe₂O₃ can help increase AFTs of the studied samples. The Al₂O₃ addition gives the strongest effect on increasing AFTs, especially for the sample with low Al₂O₃ content. When the CaO is added to the low CaO samples, the fluxing effect will initially occur. However, when the CaO content is higher than a critical value (i.e. CaO > 38%), the effect of its high melting point will dominate hence the AFTs increased. Ternary phase diagrams of CaO-SiO₂-Al₂O₃ and Fe2O3-SiO₂-Al₂O₃ system were constructed based on the obtained ash composition and IT data to investigate the effect of SiO₂/Al₂O₃ ratio and basic oxide composition (CaO or Fe₂O₃). The upper limit of the ITs measured from lignites supplied to the power plant during the severe slagging period, i.e. 1220°C, was used as a rough indication of slag formation propensity.