Modeling Pressurized Dense Phase Coal Fluidization and Transport

A transient gas-solid model based on CPFD Software’s Barracuda Virtual Reactor was developed for a feed system to a pilot-scale pressurized oxy-coal (POC) reactor. A simplified geometry with a vertical coal hopper feeding into a 0.635-cm diameter horizontal pipe was used to represent key elements of the feed system. Coal particles were transported with 20-atm CO2 gas. The feed system was required to maintain a steady flow of gas and solids at a coal flow rate of approximately 3.8 g/s and a CO2 to coal mass ratio in the range 1–2. Sensitivity of model results to mesh size and particle interaction sub-model settings was assessed. Two design concepts were evaluated. A gravity-fed concept was found to be infeasible due to inadequate coal flow rates even at very high CO2 to coal flow ratios. This was due to gravitational forces being insufficient to move the pressurized coal from the hopper into the CO2 stream at the desired rate. A fluidized bed concept was found to provide the desired coal flow rate and CO2 to coal flow ratio. CO2 injected at the hopper base first fluidized the vertical coal bed before transporting it through a horizontal exit pipe. A second CO2 inlet downstream of the hopper exit pipe was used to dilute the fluidized coal and increase pipe velocities to minimize coal drop out. The amount of coal transported from the hopper was dependent on the net CO2 hopper flow but independent of the CO2 dilution flow. This meant that the coal flow rate and CO2 to coal flow ratio could be controlled independently. Pipe exit coal flow rates were found to fluctuate at levels acceptable for steady burner operation.