An Experimental Study of CO2 Separation Using a Bench-Scale Dual-Fluidized Bed Calcium Looping Process

Ca-Looping (CaL) process that utilizes CaO as absorbent has been developed as a low energy-penalty CO₂ separation process. In this work, a bench-scale dual-fluidized bed solid circulating system which comprised a bubbling bed carbonator (bed height: 30 cm) and a fast fluidized bed regenerator (riser height 1.93 m) was operated. Calcined limestone (CaO) was employed as bed material. Simulated flue gas from air-blown combustor (CO₂ 15%) was fed to the carbonator in which the temperature was maintained at 876 K. Most part of the fed CO₂ was captured by CaO in the carbonator. The produced CaCO₃ was transported to the regenerator and the decomposition of CaCO₃ to CaO and CO₂ was conducted at 1073 – 1223 K. The produced CaO was transported to the carbonator again. Material balance of CO₂ was evaluated using two different methods, gas concentration measurement and flow rate measurement using a thermal mass flow meter. Material balance of CO₂ under steady state conditions nearly closed. Transient change in CO₂ transportation rate after starting CO₂ feed to the carbonator was predicted using a complete-mixing vessel model. The solid circulation rate was measured by injecting tracer particles (cold limestone) into the stand pipe and by measuring the delay of the temperature change between two points in the stand pipe. The experimental results of the transient CO₂ transportation rate agreed well with the predicted results. In addition to CO₂ transportation, coal feed to the regenerator was conducted. It was found that the emissions of NOx from the regenerator using CaO as bed material was much higher than that for the case of inert silica sand bed. The high NOx emissions from the CaO bed was qualitatively explained by the literature results about the catalytic activity of CaO to oxidize volatile-N compounds (HCN and NH₃) to form NOx.