Parametric Studies of Coal Gasification in an Entrained-Flow Gasifier

Although alternative energy sources, such as nuclear, wind, and solar, are showing great potential, hydrocarbon fuels are expected to continue to play an important role in the near future. There is an increasing interest in developing technologies to use hydrocarbon fuels cleanly and efficiently. The gasification technology that converts hydrocarbon fuels into syngas is one of these promising technologies. Entrained-flow gasifiers are the preferred gasifier design for future deployment due to their high carbon conversion, high efficiency and high syngas purity. Current designs of entrained-flow gasifiers still have serious problems such as injector failure, refractory failure, slag blockages, downstream fouling and poisoning, poor space efficiency, and lack of dynamic feedstock flexibility. To better understand the entrained-flow gasification process, we performed parametric studies of coal gasification in the laboratory-scale gasifier developed at Brigham Young University (BYU) using ANSYS FLUENT. An Eulerian approach was used to describe the gas phase, and a Lagrangian approach was used to describe the particle phase. The interactions between the gas phase and particle phase was modeled using the particle-source-in-cell approach. Turbulence was modeled using the standard k-ε model. Turbulent particle dispersion was taken into account by using the discrete random walk model. Devolatilization was modeled using a version of the chemical percolation devolatilization (CPD) model, and char consumption was described with a shrinking core model. Turbulent combustion in the gas phase was modeled using a finite-rate/eddy-dissipation model. Radiation was considered by solving the radiative transport equation with the discrete ordinates model. Second-order upwind scheme was used to solve all gas phase equations. First, the numerical model was validated by using experimental data for the mole fractions of the major species (CO, CO₂, H₂, and H₂O) along the gasifier centerline. Then, the effects of concentrations of steam and oxygen at the inlets, and steam preheat temperature were studied. Model predictions found that increasing the steam concentration or steam preheat temperature in the secondary inlet generally decreases CO concentration, while increasing CO₂ and H₂ concentrations. Increasing the steam concentration in the secondary inlet showed no significant effects on predicted gas temperature in the gasifier. Increasing the oxygen concentration in the primary inlet generally increases gas temperature, CO and CO₂ concentrations, while decreasing H₂ concentration.