Anuwat Luxsanayotin, Suneerat Pipatmanomai, and Sankar Bhattacharya

pp. 403 - 412

Abstract

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.