Authors: Adams, Bradley R; Fry, Andrew R; Tree, Dale R
Brigham Young University, Provo, UT
Abstract: Researchers at Brigham Young University (3 faculty, 9 graduate students, and 35 undergraduate students) completed this 5-year research program with the main objective to build and test a 100 kWth pressurized dry-feed oxy-coal combustor (POC). This objective was achieved through the development and testing of a unique pressurized dry coal fluidization feed system that operates in conjunction with a pressurized oxy-coal combustor. The project included a series of R&D tasks, including design, modeling, bench-scale testing, construction, safety reviews, and reactor testing. BYU researchers were supported in some modeling tasks by project partners Reaction Engineering International (REI) and CPFD Software. The heart of the constructed POC system is a 1.83-m long vertically oriented refractory lined pressure vessel with a 0.762 m OD and 203 mm ID. It is capable of operation up to a pressure of 27.5 bar and wetted wall temp of 2033 K. Optical access is included at five elevations. The POC is down fired, and the burner consists of cylindrical refractory block with a center protrusion of three annular tubes and an outer array of eight additional tertiary tubes. The center tube has a 4.92 mm ID and introduces coal and CO2. The first annulus has an ID of 6.35 mm and a cross sectional area of 19.07 mm¬2 and injects a variable mixture of O2 and CO2. The third annulus is only used at startup. Eight tertiary tubes each with an ID of 3.86 mm oriented in an equally spaced array around the center annulus at a diameter of 162 mm introduce additional O2 and CO2. CFD modeling was used to optimize the burner dimensions and operation for desired heat release profile and particle entrainment. The hot effluent exits the reactor horizontally near the bottom with gas temperature up to 1755 K and then is first quenched by water spray and then cooled in a vertically oriented shell and tube heat exchanger (HX). A trap and lock hopper remove condensate and ash from the HX. At the outlet of the HX a manifold of parallel valves 1) allow atmospheric operation, 2) perform fine pressure control when operating at pressure, and 3) allow emergency depressurization. Subsequent valves reduce the combustion gases to atmospheric pressure and introduce dilution air before the gases pass through a cyclone separator to remove any remaining particulates. Coal is fed to the reactor using a pressurized fluidized bed system developed for pulverized coal. Proof of concept was performed using a bench-scale apparatus which measured coal flow rate as loss-in-mass at a nominal 13.5 kg/s. The flow rate was correlated to fluidization, dilution, and exhaust flows of CO2 and it was determined that a steady rate of coal could be delivered at the nominal rate. The full-scale feeder consists of a 6 in. SCH 80 pipe, 4.57 m (15 ft) in height which acts as a coal storage hopper coupled to a 2 in. SCH 80 pipe acting as a fluidized bed with ID of 4.93 cm. The full-scale feeder is designed to operate in batch mode and can provide 13.5 kg/s pulverized coal for approximately 6 hours at pressure. The POC is controlled using a OPTO 22 PLC with 120 I/O points. Before operation, a full Hazard and Operability Analysis (HAZOP) review was conducted on the reactor by the project team, BYU internal safety and risk management personnel, and personnel from an external company that specializes in oxygen-fuel combustion and equipment. The reactor has been operated on coal at pressures of 13.8 Bar. Operation at steady state coal flow has proven to be challenging. Additional factors that were determined to impact the coal feed rate were feeder/reactor pressure differential and coal bridging. These variables were identified and controlled near the end of the project. It was shown that a pressurized oxy-coal flame was measured by the radiometers and showed good response to flame shape and natural fluctuations in flame luminosity with a highly radiating flame extending out past the first port at 305 mm beyond the burner face. Several process models and CFD packages were used to support POC design and operation. The Barracuda CFD software from CPFD was used to model dense phase flow in the coal feed system and guide design of the fluidization system. The GLACIER reacting CFD code from REI was used to guide design of the reactor firing system and benchmark the FENICS enhanced process model. The FENICS process model developed for this project at BYU utilizes a one-dimensional zonal model for gas-phase equilibrium calculations, a prescribed axial distribution of coal devolatilization and char oxidation, a three-dimensional radiation model, and a heat conduction model for the refractory layers. FENICS was shown to produce a reasonable representation of data and full CFD predictions at a fraction of the computation time, which is useful for scoping and optimization tasks.