Advanced Fossil Energy Separations and Analysis Research – Thermal Desorption Using Spouted Beds
Mercury and other contaminants in emissions from coal-fired power plants are of interest due to the cost associated with capture and disposal of toxic mercury species. It had recently been postulated in the fossil energy community that spouted beds may be a viable coal pre-treatment technique for thermal desorption of mercury. Spouted beds have the potential to be a near-ideal solid-gas contactor for desorption of mercury species, especially with a few proposed design changes specific for this application. Introduction of draft tubes for spouted beds has addressed some issues with scale-up to better handle the throughput and particle size found in coal-fired power plants. In Phase I, Mainstream Engineering invented and developed a Venturi draft tube spouted bed (VDTSB) to enable process scale-up with fine coal particles and to improve heat and mass transfer. The VDTSB concept increases suction at the bottom of the bed to better entrain material (and reduce the propensity for bridging) at the bottom of the conical vessel. The main advantages of this patent-pending approach are the ability to accept a wide range of coal particle sizes, good heat/mass transfer, process scalability, and robustness. The desorber design was optimized through bed flow visualization experiments; these experiments demonstrated the increased throughput and ease of process scale-up. A high-temperature pilot-scale VDTSB desorber system was designed, fabricated, commissioned, and tested. The heat and mass transfer rates with the VDTSB were measured and compared to a baseline spouted bed with and without a conventional straight draft tube. Pilot-scale VDTSB system performance tests were completed with the Energy and Environmental Research Center (EERC) to measure the rate of mercury desorption over a range of pulverized coal particle sizes, bed temperatures, and reactor geometry. Based on performance results, an initial technoeconomic analysis (TEA) was completed to estimate the cost reduction for emissions treatment compared to current flue-gas mercury capture systems. The following was accomplished in Phase I: Developed scalable design correlations for high-throughput spouted bed systems; Determined cycle time distribution (CTD) and turnover time for straight and Venturi draft tube spouted beds, and showed the VDTSB had lower cycle time (faster turnover) than the straight draft tube (SDT); Reduced weighted-average turnover time by 14% to 43% with Venturi draft tube; narrowed the CTD by up to 75%; Measured heat and mass transfer within the pilot-scale VDTSB desorber and showed that the highest heat transfer rate was 2.98 kW/m 2 ·s with 400 µm particles at a 10 L/min motive gas flowrate; Measured mercury (Hg) desorption rate for lignite, bituminous, and subbituminous coal at 200 °C, 225 °C, 250 °C and 275 °C; Achieved 5.6×10 -10 kg/m 2 -h, 2.6×10 -10 kg/m 2 -h, and 6.5×10 -11 kg/m 2 -h Hg removal rates for lignite, bituminous, and sub-bituminous coals, respectively with no loss in volatile matter and an increase in heating value due to a reduction in moisture content; The unoptimized VDTSB pilot desorber decreased the total mercury concentration of lignite and bituminous coal tests by 61% (dry basis). The subbituminous Powder River Basin (PRB) coal showed a reduction of 18%; Based on our TEA of coal-fired power plants, implementing the VDTSB for coal pretreatment could reduce the mercury capture capital and operating expenses by 36% and 38%, respectively, which exceeded the target goal of 25% reduction; A patent application was prepared, and the results were presented at the 2017 AIChE Annual Meeting (Minneapolis, MN).