CO2 emissions; power plant; pilot scale study; fly ash; mineralization
The mineral carbonation, a process of converting CO2 into stable minerals (mineralization), has been studied extensively to capture and store CO2. However, most of the mineral carbonation studies have been largely investigated at lab scale. Preliminary and pilot scale studies for accelerated mineral carbonation (AMC) were conducted at one of the largest coal-fired power plants (2120 MW) in the USA by reacting flue gas with fly ash particles in a fluidized bed reactor. In the preliminary experiments, flue gas CO2 and SO2 concentrations decreased from 13.0 to 9.6% and from 107.8 to 15.1 ppmv, respectively during the first 2 min. of reaction. The flue gas treatment increased mercury (Hg) concentration in fly ash (0.1 to 0.22 mg/kg) suggesting that fly ash particles also mineralized flue gas Hg. From these results, we designed and developed pilot scale process skid consisting - a moisture reducing drum (MRD) (0.9 m Φ × 1.8 m), a heater/humidifier (0.9 m Φ × 1.8 m), and a fluidized bed reactor (FBR) (0.9-1.2 m Φ × 3.7 m) to capture and mineralize flue gas CO2. Flue gas was withdrawn from the stack and was fed to the MRD at about 0.094m3/s. The MRD and the heater/humidifier pretreat flue gas before it enters the FBR. The MRD captures droplets of water entrained in the flue gas to protect the blower placed between the MRD and the heater/humidifier. The heater/humidifier enables control of flue gas moisture and temperature. Approximately 100-300 kg of fresh fly ash was collected from the electrostatic precipitator through ash hopper and placed in the fluidized-bed reactor. The fly ash particles were fluidized by flow of flue gas through a distributor plate in the FBR. The pilot scale studies were conducted at a controlled pressure (115.1 kPa) by controlling the flue gas moisture content. The flue gas was continuously monitored to measure flue gas CO2, SO2, and NOX concentrations by an industrial grade gas analyzer, while the fresh and spent fly ashes were analyzed for calcium carbonate (CaCO3), sulfur (S), and mercury (Hg) content. The pilot scale study results suggest that an appreciable amount of flue gas CO2 and significant amounts of SO2 and Hg can be directly captured (without separation) and mineralized by the fly ash particles.