(624c) CFD Simulation of Coal Gasification in An Entrained-Flow Gasifier

The current work makes use of computational fluid dynamics (CFD) to analyze the physical and chemical processes occurring within a gasifier. We focus here on CFD simulations using different grades of coal as feedstock (fuel) with comparison of mean values and space-resolved properties from experimental studies. This improved approach follows on the heels of work using comparisons with gross data such as maximum temperature and outlet composition which was used initially to gain confidence in the CFD modeling technique. The results obtained are to be compared with profile data from experimental studies by Brown et al (1988) for one type of coal.

During the process of gasification, a feedstock (fuel) such as coal undergoes partial combustion in the presence of oxygen usually at elevated temperature and pressure. The distinguishing feature of the gasification process (as compared to combustion) is the amount of oxygen required, for this reason it is also known as partial oxidation. The gasification process takes place between the temperatures of 760 and 1540ºC in an oxygen lean environment and a pressure range of 1 to 60 atmospheres. The product of gasification is a combustible mixture of primarily carbon monoxide and hydrogen which is known as ?synthesis gas? (or ?syngas?, in short). Entrained-flow gasifiers comprise the largest category of commercial gasifier designs used for power applications and in the chemical synthesis industry.

Gasification can be broken down into a number of sub-processes for ease of modeling (Wen and Chaung, 1979). The coal particles are heated till the vaporization temperature is reached. There is no mass transfer or chemical reaction during this stage known as ?inert heating?. When the coal particles reach the vaporization temperature, moisture is released. Meanwhile, energy is taken out from the gas phase to supply the latent heat of vaporization. This is known as the ?moisture release? stage. A phenomenological model is constructed based on data from experiments used to characterize the coal. This model can then be used to predict the yields of some major gas components while preserving strict elemental balance to determine stoichiometry. Further heating beyond the moisture release stage results in a stage known as ?devolatilization? characterized by the release of volatiles from the coal. The main species included in the devolatilization model are CO, CO2, O2, H2, N2, H2S, SO2 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 Brown et al (1988) and Chen et al (2000), while the gas-phase reaction kinetic parameters have been obtained from Watanabe and Otaka (2006).

Entrained-flow gasification is difficult to test and model due to the low residence times and the high heating rates involved, which often results in very large gradients of temperature, composition and reaction rates. In the study by Brown et al (1988), the heterogeneous reaction rate coefficient, extent of heat loss and oxygen-carbon ratio were found to be important factors influencing the results of their computer model. The current work builds up on CFD models based on experimental work and simulation studies on a pilot-scale gasifier by Chen et al (2000) and a lab-scale gasifier by Watanabe and Otaka (2006). A discrete particle model (DPM) based on the Euler-Lagrange description of multiphase flow has been developed using ANSYS FLUENT 12.1.

This study would provide detailed predictions which will be compared with the radial and axial profiles of species concentrations and temperatures for one type of coal analyzed in experimental studies by Brown et al (1988). This simulation would later be extended to cover all four types of coal studied in the same experimental work. This detailed comparison would form a solid basis for validation of the CFD model employed for this work, and pave the way for more detailed analysis in the future.

References

1. Brown, B.W., Smoot, L.D., Smith, P.J., and Hedmen, P.O. (March 1988). 'Measurement and Prediction of Entrained-Flow Gasification Processes'. AIChE Journal 34(3), 435-446. 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. Watanabe, H., and Otaka, M. (2006). 'Numerical simulation of coal gasification in entrained flow coal gasifier'. Fuel 85, 1935-1943. 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.