Characterization of the Spent MgO-Based Refractory Lining from a Rotary Lime Kiln Cofired with Fossil Fuels and Potassium-Rich Biomass

Rotary kilns are widely used for various high temperature industrial applications, e.g., quicklime and cement production. Most of these operations rely on fossil fuels like coal; however, there is growing interest in increasing the share of biomass fuels. During the high-temperature combustion process, ash transport onto, and reactions with, refractory bricks can significantly influence refractory wear. Ash infiltration may impair refractory performance through chemical reactions and exacerbated cracking caused by structural changes in combination with thermal shock. Analysis of spent refractory materials provides valuable insights for understanding and predicting corrosion mechanisms. In the present study, three MgO-based spent refractory bricks were collected from different locations within the burn zone of a rotary lime kiln that was cofired with a mixture of coal, olive pomace (a potassium-rich biomass), and oil. Infiltrated ash and reaction products within the refractory bricks were sampled and characterized using SEM-EDX for elemental mapping and morphology analysis. EBSD analysis was employed to measure the grain size distributions. Micrographic images revealed that all three spent refractory bricks were more sintered and cracked on their hot sides compared to their middle sections and cold sides. Si-rich and K-rich ashes from the coal and biomass fuels, respectively, all infiltrated into the refractories as well as Ca-rich constituents from the limestone/quicklime. XRD analyses revealed the formation of phases such as Mg2SiO4, Ca12Al14O33, and Ca2SiO4. No potassium from the fuel ash was found on the hot side of the refractory bricks, but some were detected deeper within the middle section and cold side of the bricks. The combined use of analytical techniques enabled detailed mapping of ash-forming elements and identification of newly formed phases from the reactions between refractory bricks, ash, and quicklime. These findings provide critical insights into fuel-specific interactions and highlight potential risks for refractory degradation.