Pilot tests at 1 MWth scale on retrofitting fluidized bed boilers for oxyfuel-combustion of solid recovered fuel

The transition from coal to waste-derived fuels and the implementation of CCS technologies are of significant interest for existing coal-fired fluidized bed power plants. This approach not only extends operational lifespans beyond the coal phase-out, but also facilitates negative CO2 emissions. Oxyfuel combustion is a well-established CO2 capture technology that has attracted considerable interest for its application in fluidized bed technology since the 1990s. Numerous large-scale test campaigns with a common emphasis on fossil fuels like bituminous coal, petroleum coke, lignite, and anthracite have been conducted by various organizations and companies with thermal inputs of up to 30 MW [1–3]. Smaller-scale studies with thermal inputs of up to 200 kW have examined oxyfuel combustion using coal or sewage sludge in combination with biomass [4,5] and with solid recovered fuel (SRF) [6]. Nevertheless, the relatively brief history of oxyfuel combustion with SRF underscores a significant research gap and highlights the need for MWth-scale combustion tests using SRF. The EU-funded REBECCA project addressed this knowledge gap following a three-step process: 1) transitioning from 100% coal to 100% SRF as fuel [7]; 2) replacing sand with ilmenite as the bed material to counteract challenges associated with higher SRF shares through oxygen carrier-aided combustion (OCAC); and 3) implementing oxyfuel combustion to examine CO2 separation in retrofitted CFBs at a pilot scale. The combination of OCAC and oxyfuel combustion represents an innovative technological approach that continues to be explored. So far, only small-scale experiments, with thermal inputs up to 84 kWth, have been reported, primarily using two types of coal [8] or woody forest residues [9]. This study presents oxyfuel combustion results from tests conducted at the 1 MWth pilot plant at the Technical University of Darmstadt, where 100% SRF served as feedstock. Two distinct test campaigns were carried out, utilizing sand and ilmenite as bed materials, respectively. This setup allowed for an evaluation of oxyfuel combustion with 100% SRF and demonstrated the benefits of using an active bed material. Figure 1 provides a schematic representation of the 1 MWth pilot plant used for the oxyfuel combustion tests. The existing unit was upgraded with two separate flue gas recirculation lines, along with control systems for injecting pure oxygen (99.999%). These modifications allow for independent adjustment of the oxygen content in both the primary and secondary fluidization lines. Gas analysis systems were installed in both fluidization lines to measure O2 and CO2 concentrations. Additionally, the loop seal fluidization and filter pulsing systems were modified to enable the use of CO2 in both locations. The CFB reactor and flue gas pathway, however, were left unaltered. The results show the feasibility of retrofitting existing CFB boilers for 100% SRF combustion in combination with CO2 capture technology. The transition from air to oxyfuel combustion can be achieved quickly, requiring less than 15 minutes to reach stable conditions in terms of hydrodynamics, thermodynamics, and flue gas composition. A comparison between oxyfuel combustion with sand versus ilmenite as bed materials highlights the benefits of using an active bed material, such as improved oxygen distribution within the reactor. The use of ilmenite leads to lower CO levels and higher CO2 concentrations in the flue gas when maintaining the same O2 inlet concentrations in the primary and secondary fluidization lines. For both bed materials, increasing the O2 inlet concentration results in a decrease in SO2 levels in the flue gas, while a reduction in NO levels is observed only when sand is used as the bed material.