Thermodynamic and Hydrodynamic Stabilization of Load Changes in a 1 MWth CFB Combustion Pilot Plant via Partial Flue Gas Recirculation

The increasing integration of renewable energy sources necessitates enhanced operational flexibility in thermal power plants. Circulating Fluidized Bed (CFB) combustion systems, known for their high thermal inertia, face considerable challenges in maintaining thermodynamic and hydrodynamic stability during load changes. This study investigates the application of Partial Flue Gas Recirculation (PFGR) as an innovative approach to stabilizing combustion conditions during load changes in a 1 MWth CFB pilot plant operating exclusively on Solid Recovered Fuel (SRF). Load step tests were conducted to compare conventional strategies – where total fluidization was reduced proportionally with fuel input – against PFGR, which maintains total fluidization by replacing a portion of fresh air with recirculated flue gas. Without PFGR, load reductions to 86% and 80% resulted in significant hydrodynamic changes, including decreased particle entrainment from the bed to the freeboard zone and the loop seal. This effect was confirmed using the Grace-Diagram, which indicated that conventional load reduction approaches push the system to the operational limits of the circulating fluidized bed regime. The altered hydrodynamics led to increased bed temperatures and reduced freeboard temperatures, causing their temperature difference to rise by 71 K. In contrast, PFGR effectively mitigated these instabilities by maintaining particle entrainment and subsequently pressure conditions in the freeboard and the loop seal, thereby limiting temperature difference increase to just 4 K. While PFGR entails a slight efficiency penalty due to increased heating demands for the recirculated gas, it enables long-term stable operation at reduced loads – an essential factor for flexible power plant operation. This study provides experimental validation of PFGR as a viable strategy for stabilizing CFB combustion under variable load conditions, particularly in 100% waste-fueled systems, contributing to the broader goal of enhancing the flexibility of low-carbon thermal power generation.