(362f) CFD Study of Heavy Oil Gasification In An Entrained-Flow Gasifier

Gasification of heavy oil is investigated by employing computational fluid dynamics (CFD) to model the physical and chemical processes taking place within an entrained-flow gasifier. This model needs to account for a number of complex processes such as turbulence, multiphase flow, heat transfer, mass transfer and chemical reactions. Here, the experimental results of Ashizawa et al (2005) have been compared with the predictions obtained from the CFD model.

Among the different types of gasifiers in widespread use, entrained-flow gasifiers are popular for the gasification of coal and petcoke, in particular, for power generation and production of synthetic fuels. This type has also been utilized for the gasification of heavy oil, for instance in the Shell gasification process (SGP) and in Texaco gasification studies. The entrained-flow gasifier is characterized by high operating temperatures (in excess of 1400°C) and an operating pressure ranging from 20 to 70bar. These high temperatures are necessary to ensure high carbon conversion despite the short residence time of the feedstock within the reactor.

 With the aim of promoting fuel diversification and an increase in thermal efficiency, feeds such as Orimulsion^TM (heavy oil) are being considered for use in combined-cycle power plants. Despite a number of studies focusing on the investigation of gasification of solid fuels, only a limited number of studies are found in the literature for the gasification of liquid fuels, most of which deal with the gasification of black liquor.

When the flow is dilute, that is, the volume fraction of the disperse (discrete) phase is less than 10% of that of the continuous phase, an Euler-Lagrange description of the flow known as the discrete phase model (DPM) is applicable. Using DPM, the particle trajectories are estimated along with mass and energy transfer to/from the particles using a Lagrange formulation, whereas an Euler description is used for the continuous phase. The coupling between the continuous phase and the discrete phase is solved by tracking the exchange of mass, momentum and energy. To simplify the modelling process, gasification can be broken down into a number of sub-processes (Wen and Chaung, 1979). The oil droplets which are injected within the gasifier are heated till the devolatilization temperature is reached. There is no mass transfer or chemical reaction during this stage known as “inert heating”. Based on data from experiments used to characterize the heavy oil, a phenomenological model is constructed. This model is then used to predict the yields of some major gas components while preserving strict elemental balance to determine stoichiometry. Further heating results in a stage known as “devolatilization” characterized by the release of volatiles from the heavy oil. The main species included in the devolatilization model are CO, CO₂, O₂, H₂, CH₄, H₂S and H₂O. After all the volatiles have been released, char combustion and gasification takes place until all the char is consumed or the particles flow out of the reactor. The heterogeneous reaction kinetic data have been obtained from Chen et al (2000), while the gas-phase reaction kinetic parameters have been obtained from Maki and Miura (1997).

This study would provide predictions which will be compared with the axial temperature and species mole fraction profiles for different oxygen ratios in experimental studies by Ashizawa et al (2005). This detailed comparison would form a basis for validation of the CFD model employed for this work, and pave the way for more detailed analysis in the future using a more comprehensive reaction chemistry, turbulence, radiation and chemical kinetics models.

References

1. Ashizawa, M., Hara, S., Kidoguchi, K., and Inumaru, J. (2005). “Gasification characteristics of extra-heavy oil in a research-scale gasifier”. Energy 30, 2194-2205.

2. Chen, C., Horio, M., and Kojima, T. (2000). “Numerical simulation of entrained flow coal gasifiers. Part I: modeling of coal gasification in an entrained flow gasifier”. Chemical Engineering Science 55, 3861-3874.

3. Maki, T., and Miura, K. (1997). “A Simulation Model for the Pyrolysis of Orimulsion”. Energy & Fuels 11, 819-824.

4. Wen, C. Y., and Chaung, T. Z. (1979). “Entrainment coal-gasification modeling”. Industrial and Engineering Chemistry Process Design and Development 18(4), 684-695.