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Effect of mineral oxides on slag formation tendency of Mae Moh lignites

Author(s): Anuwat Luxsanayotin | Suneerat Pipatmanomai | Sankar Bhattacharya

Journal: Songklanakarin Journal of Science and Technology
ISSN 0125-3395

Volume: 32;
Issue: 4;
Start page: 403;
Date: 2010;
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Keywords: lignite | combustion | pf boiler | slag | ash fusion temperature

Slagging is one of major ash deposition problems experienced in the boilers of coal–fired power plants especially theplants that use lignite, like Mae Moh lignites. The occurrence of slag is a complex phenomenon depending on several factorssuch as ash properties, furnace operating conditions, and coal properties. The main objective of this work is to study theeffect of mineral components in Mae Moh lignite on ash fusion temperatures (AFTs), which is commonly used as a keyindicator for slag formation tendency under pulverized combustion conditions. Two Mae Moh lignites from the coal seamsplanned 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 deformationtemperatures (ITs) were almost identical and considered as low for the typical flue gas temperature in the radiationsection of Mae Moh boilers, i.e. 1050-1100°C. These observed similar ITs were rather unexpected, especially for K1 consideringits sufficiently low base to acid (B/A) ratios. The X-ray diffraction analyses evidently show the presence of illite, pyriteand anhydrite in K1, which explains the observed lower IT of the sample. Anhydrite, which is known to lower the ITs, is alsothe most abundant mineral in K3. Washing the lignite samples with HCl can significantly reduce CaO, MgO, and SO3 contentin 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 ofthe studied samples. The Al2O3 addition gives the strongest effect on increasing AFTs, especially for the sample with lowAl2O3 content. When the CaO is added to the low CaO samples, the fluxing effect will initially occur. However, when the CaOcontent is higher than a critical value (i.e. CaO > 38%), the effect of its high melting point will dominate hence the AFTsincreased. Ternary phase diagrams of CaO-SiO2-Al2O3 and Fe2O3-SiO2-Al2O3 system were constructed based on the obtainedash composition and IT data to investigate the effect of SiO2/Al2O3 ratio and basic oxide composition (CaO or Fe2O3). Theupper limit of the ITs measured from lignites supplied to the power plant during the severe slagging period, i.e. 1220°C, wasused as a rough indication of slag formation propensity.
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