(68b) Transient Reacting Flow Simulation of Biomass Combustion in a Pilot-Scale Circulating Fluidized Bed Combustor

Interest in circulating fluidized bed (CFB) combustors as a power generation technology has sky-rocketed in recent years because of several advantages this technology offers over conventional boilers such as increased gas-solid mixing resulting in higher efficiency and the ability to use lower quality fuels. CFB combustors are operated at lower temperatures than conventional boilers, which also mitigates NO_x and SO₂ emissions. A comprehensive computational fluid dynamics (CFD) model of a CFB combustor is developed employing the multiphase particle-in-cell (MP-PIC) module in the open-source MFiX Software Suite to understand and analyze the complex gas-solid hydrodynamics, chemical processes, energy conversion in such a system, and the scale-up considerations required to develop the industrial-scale CFB combustors of the future.

The experimental basis for the simulations is the 50 kW_th CFB combustor riser with a diameter of 0.15 m and a height of 5 m designed, built, and operated at CanmetENERGY in Ottawa, Canada. The MFiX-PIC model parameters for the simulation are tuned against cold flow experiments from Canmet using 9 kg of olivine sand as the inert bed material. It is shown that for the relatively coarse fluid meshes and large parcel sizes necessitated by the scale of the simulation, filter size dependent corrections to the drag law must be incorporated to ensure accuracy of the simulation results.

The validated cold-flow model is extended to simulate reacting flow with torrefied hardwood as the feedstock using an established reaction scheme from previous work. The effect of cooling air flow through heat exchanger tubes to maintain the bed temperature in the Canmet riser is represented by prescribing the heat flux across the tube surfaces in the simulation. The pressure and temperature profiles in the riser as well as the outlet gas compositions are compared against Canmet’s experiment and show satisfactory agreement. The simulations demonstrate the ability of MFiX-PIC to accurately capture the physics and chemistry of a circulating fluidized bed combustor at pilot plant scales, which can be further extended to industrial-scale systems. The detailed species information in the riser obtained from the simulation is also used to predict NO_x formation based on a sub-grid model.