Operation of a 1 MWth calcium looping pilot plant firing waste-derived fuels in the calciner

The calcium looping (CaL) process is a promising second-generation carbon capture technology for post-combustion CO₂ capture from power and industrial plants. The CO₂ in a flue gas stream is captured by means of the reversible carbonation reaction. The CaL process is based on two coupled circulating fluidized bed reactors (carbonator and calciner) using natural limestone as sorbent. Until now, remarkable progress has been made in terms of long-term CaL pilot testing and evaluation in various test rigs. Mainly fossil fuels have been applied as supplementary heat source in the calciner. This work presents first results from long-term pilot testing of the CaL process in the 1 MWₜₕ CaL pilot plant at Technische Universität Darmstadt, firing solid recovered fuels under real oxy-fuel conditions in the calciner. The flue gas to be decarbonized in the carbonator was supplied by on-site combustion of pulverized lignite or natural gas. The CO₂ concentration in the resulting flue gas stream to be decarbonized in the carbonator was adjusted according to typical values of waste-to-energy plants. During the experimental campaigns, two types of solid recovered fuel were investigated. The lower heating value of the SRF was approximately 21 MJ/kg for the first and approximately 16 MJ/kg for the second type. Overall, the plant was operated over 230 h in interconnected CFB mode, while firing SRF. The process evaluation is based on a simplified reactor model, which is typically applied in the field of CaL technology. It was experimentally proven that carbonator CO₂ absorption rates close to the theoretical maximum are feasible while firing SRF in the calciner. Furthermore, it was found that accumulation and distribution of coarse SRF ash fraction in the solid looping system are a key challenge to be addressed in the design stage and operation of a SRF-fired CaL system.