Slagging and Fouling of Large-Scale Pulverized Fuel Boilers Firing Rhenish and Central German Lignites

This paper presents the results of the investigation of the ash deposition behavior of lignite coals sourced from two mining regions, Central German and Rhenish, fired at two modern large-scale power plants, namely, Lippendorf (LEAG) and Niederaussem (RWE). Lignite from the respective mines is blended/mixed and supplied to the units. To understand the mechanisms involved in the thermochemical conversion process and validate the current blending/mixing strategies, assessment of the impact of fired lignite on boiler operation is essential. Two large-scale measurement campaigns were performed at the different units by using the same sample probing and analytic procedures. For detailed assessment of the impact on furnace slagging and fouling, an online and uncooled ash deposition rate measurement as well as a cooled deposition monitoring device was utilized. The subsequent analyses were supported by SEM/EDX images. Phase equilibrium calculations are performed to further shed light on the boundary conditions necessary for such ash buildup chemistries. It was found that for the Central German lignite, the main cause of increased slagging was a melt formation and deposition of partially oxidized pyrites/marcasite, followed by deposition of sticky calcium aluminosilicates. In the case of the optimized Rhenish lignite blend, the collected deposition samples were less sintered due to the high quartz content, resulting in turn to the dilution of alkali and alkaline earth components. The study serves as an opportunity to understand the behaviors of the current fuel blends/mixtures of both facilities and provides room for optimization.