Venturi Draft Tube Spouted Bed Reactor for Mercury Removal from Coal

As legislation continues to tighten regulations on emissions for coal-fired power plants, new technologies and control strategies are required. Mercury and other coal contaminants are of interest due to the cost associated with capture and disposal of toxic mercury species. More recently, the Fossil Energy program at DOE has identified spouted bed systems for thermal desorption of mercury as a viable coal pre-treatment technique. In addition to studying the effects of operational temperature and particle size of coal powders on desorption, Mainstream proposes to develop a technology to enhance desorber performance and reduce capital and operational costs. Mercury capture and emissions control systems have capital costs between $5–15/kW and operational costs of 0.2¢/kWh (5% of the total operational cost) for coal-fired power plants. These costs have been a point of contention as the mercury standards were adopted in 2011 and upheld in 2016. Over 50% of mercury emissions in the U.S. come from coal-powered utilities with an estimated public health cost of $37 to $90 billion annually. 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 in coal-fired power plants. Mainstream Engineering proposes a new Venturi draft rube spouted bed (VDTSB) to enable scale-up with fine 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 reactor. The main advantages of this approach are the ability to accept a broad range of coal feedstock particle diameter, good heat/mass transfer, process scalability, and robustness. In the Phase I, we will optimize the desorber design and measure the heat and mass transfer rates with a bench-scale demonstrator. Mainstream will leverage our commercial partnership with the Energy and Environmental Research Center (EERC) to demonstrate a pilot-scale system and measure the rate of mercury desorption over a range of pulverized coal particle sizes, bed temperatures, and reactor geometry. The Phase II will focus on developing a larger demonstration-scale system for performance testing and validation of cost models. Commercial Applications and Other Benefits The proposed technology addresses critical emissions control of mercury to deliver clean coal energy. Reducing the costs of emissions control and mercury capture ultimately lowers delivered energy costs and environmental impact. Improving domestic production of coal energy has significant impacts on the energy security and sustainability of the U.S. economy. This proposal outlines a plan to develop a technology that will reduce emissions of mercury and other contaminants from coal. The ultimate goal is to reduce the capital, operational, and maintenance costs of coal-fired power plants and clean-up equipment required to meet regulated air quality standards. Improving emissions control for clean coal technology benefits the public and environment by limiting mercury emissions. Our system will be designed to handle a variety of crushed/pulverized coal feedstocks with the added benefit of being readily scalable to full-scale systems. This technology is also applicable to other chemical processes that use spouted beds for material drying and processing, such as grain drying. Our approach would lead to a scalable and adaptable spouted bed suitable for existing and future power plants.