Effect of chemical composition of ash on sintering of lignites in circulating fluid bed combustion and successful operation of large CFBC boilers

• Heating microscope, CCSEM and HTXRD used in sintering characterization.
• Kaolinite-halloysite intrusion and bottom ash morphology provide sintering evidence.
• Influence of ash chemical composition on sintering established.
• Useful thumb rule correlation derived for predictive purposes and operational ease.
• Novel operating process modification to overcome sintering established.

The combustion behavior of lignite is strongly influenced by its mineralogy. Agglomeration and clogging/blockage are experienced due to sintering of lignite at CFB boilers operating regime of 800 °C–900 °C. The heating microscope is used in lignite characterization for understanding the sintering phenomenon. Mineral clays kaolinite that cause sintering in the lignite, are confirmed by CCSEM and mineral transformations are seen through the HTXRD. The silica-alumina ratio (SiO2/Al2O3) of lignite almost tending 1.0 and negligible presence of the other mineral oxides are identified as indicators for the sintering behavior. The first shrinkage phase begins due to the collapse of kaolinite structure caused by de-hydroxylation above 600 °C. The sintering tendency of the coal ashes below 1000 °C is due to the amount and type of clay minerals present in the coal ashes and the extent of de-hydroxylation. Using conventional chemical ash composition, a thumb rule correlation derived for sintering behavior of this lignite, is validated for regional lignites. This correlation based on commonly used coal ash chemical composition in the assessment of sintering would save considerable time, effort and costs in operation. This novel technical research was demonstrated on utility scale lignite fired 2 × 125 MWe CFB steam generators at Barsingsar/Rajasthan/India, wherein the sintering issue and the blocking of dense phase at cyclone outlet was faced. Operational issues, investigation carried out, process modification and successful establishment of operating regime in overcoming the difficult characteristics of the fuel used in the boiler are summarized.