Fuel Residence Times for Clean Combustion of Coal in a Pressurized Fluidized Bed - Cold Flow Study

Abstract

Anthropogenic Climate Change is amongst the greatest challenges of human civilization. A key area that will play a large role in mitigating its effects are clean fossil fuel applications. Clean coal combustion is one such application with an urgent timeline. This can be achieved with an oxygen-fired pressurized fluidized bed combustor with downstream carbon capture and sequestration. In relation to pressurized fluidization processes, understanding the influence of pressure on bed hydrodynamics and in turn their effect on parameters including fuel residence time is essential. For the proposed combustor, the heat exchanger boiler tubes are submerged in the fluidized bed such that the effect of a horizontal tube bank on the fuel residence time is also of great importance. The main focus of present work was to evaluate the impact of gas velocity, pressure, presence of a tube bank and fuel feed rate on the average fuel residence time. Experiments were conducted under cold flow conditions in a pilot-scale pressurized fluidized bed with an inner diameter of 0.15 m. The fluidization material was relatively large glass beads (1.0 mm in diameter) while the fuel particles were simulated with smaller glass beads (40 to 138 μm in diameter), susceptible to entrainment. Operating pressures and superficial gas velocities tested were between 101.3 and 1200 kPa and 0.4 and 1.1 m/s respectively. To simulate coal combustors, experiments were then conducted in a continuous mode where the fuel particles were continuously fed to the fluidized bed of large particles over a desired period of time. Downstream, entrained particles were continuously captured to determine the entrainment rate and mass of fuel particles inside the fluidized bed at steady state, which yielded the average fuel residence time. The combination of elevated pressure with the tube bank present was found to enhance gas bubble break up and reduce the average gas bubble size. In turn, this increased the average fuel residence time of 83 μm particles by nearly 3 fold to a value of 77 s in comparison to 27 s at atmospheric pressure. The effect of gas velocity was not found to be statistically significant under the range tested. Similarly the effect of increased fuel feed rate by 50% neither had a statistically significant impact.