1

TECHNICAL NOTES TN287

Modeling Coal Gasification with CFD and the Discrete Phase Method

Shaoping Shi1, Stephen E. Zitney2, Mehrdad Shahnam1, Madhava Syamlal2, William A. Rogers2

1Fluent, Inc., 3647 Collins Ferry Road, Suite A, Morgantown, WV 26505, USA 2U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Rd,

Morgantown, WV 26505, USA

Software: FLUENT

Presented at the 4th International Conference on Heat and Mass Transfer, Paris-Cachan, France, May 2005

ICCHMT-05-273

Abstract

In this paper we describe a computational fluid dynamics (CFD) model of a two-stage, oxygen-blown, entrained-flow, coal slurry gasifier for use in an advanced power plant simulation. The discrete phase model (DPM) is used to simulate the coal slurry flow. The physical and chemical processing of coal slurry gasification is implemented by using user-defined functions (UDFs) in which the coal particles undergo moisture release, vaporization, devolatilization, char oxidation, and gasification processes. Using specified plant boundary conditions, the gasification model predicts a synthesis gas composition which is very close to the values calculated by an Aspen Plus® restricted equilibrium reactor model tuned to represent typical experimental data. The char conversions are 100% and 86% for the first stage and second stage, respectively. Keywords: CFD, DPM, Coal Gasification, Chemical Reaction, Kinetics

2

INTRODUCTION Because of deregulation, rapidly changing market demands, fluctuations in natural gas prices, and increased environmental concerns, gasification will become the centerpiece of tomorrow’s advanced power plants. Large improvements in the efficiency, reliability, and feedstock flexibility of gasification systems are necessary for the success of gasification-based power plants. To address these challenges, the U.S. Department of Energy (DOE) is sponsoring a broad spectrum of gasification research and demonstration projects. For example, the DOE’s $1 billion, 10-year, FutureGen project is aimed at creating the world’s first coal-fired, gasification-based, near-zero emissions electricity and hydrogen production power plant [1]. Gasifiers involve complex physical and chemical phenomena including fluid flow, heat and mass transfer, and chemical reactions. Combined with data from existing gasifiers, CFD models offer a powerful method for understanding and improving gasification systems. Over the past decade, CFD modeling has played an important role in optimizing the performance of the current fleet of pulverized coal-fired electric utility boilers. Likewise, CFD modeling can provide insights into the flow field within the gasifier, which can be used to enhance its design, analysis, and operation. Coal gasification takes place when coal reacts with an oxidizing agent such as air, oxygen, steam, or carbon dioxide (CO2) to form a carbon monoxide (CO)/hydrogen (H2) rich synthesis gas that is sent to downstream plant sections such as gas cleaning and CO2 separation before entering gas turbines, steam turbines, or fuel cells for power production. Developments in coal gasification have been reviewed by Vamvuka [2] and recently by Niksa et al. [3, 4]. Due to the simplicity of the geometry, lower pollutant generation, and wide fuel compatibility, the entrained-flow gasification technique is very attractive and will be the focus of this paper. The entrained-flow gasifier considered here is modeled using the commercial finite-volume FLUENT CFD software [5]. COAL GASIFICATION MODELING The coal gasification model used in this study evolved from earlier models developed at NETL for fixed bed gasifiers [6], and dilute [7] and dense [8-10] transport gasifiers. Coal contains four pseudo-components: ash, moisture, volatile matter, and fixed carbon. Ash does not take part in any reaction. Moisture is released in the initial stage of reaction drying. Volatile matter in the coal produces several gas phase species through devolatilization. Fixed carbon takes part in combustion and gasification reactions. In entrained-flow gasification, the coal particle flow mainly follows the gas flow and the gasifier is typically in a dilute flow regime, in which the volume occupied by the particles and the particle-particle interactions are negligible. A criterion often used in CFD for dilute flow is that the particle volume fraction is less than 10%. In this case, a discrete phase method (DPM) can be applied to model the particle flow. Also the ratio of the mass flow rate of solids to that of the gas is less than or equal to one, which is required for ensuring the stability of DPM calculations [7]. Using DPM, the particle trajectories, along with mass and energy transfer to/from the particles, are computed with a Lagrangian formulation. The coupling between the continuous phase (gas) and the discrete phase (particle) is solved by tracking the exchange of mass, momentum, and energy. In the following sub-sections, the detailed physical and chemical processes of the coal slurry and the continuous gas phase will be described.

3

Coal Slurry: In this study, we are using coal slurry as the feeding fuel. The coal slurry is modeled as two separate particle types: water droplets and coal particles. This assumption is reasonable because the water evaporates quickly after the slurry enters the gasifier. The slag in the slurry is ignored. Water droplets and coal particles proceed in the reactor controlled by different laws. Water Droplets: The water droplet processes include injection and mass/heat transfer. Droplet injection: The droplets are injected in the gasifier through the carrying gas with a particle diameter distribution of Rosin-Rammler:

nddd eY )/(−= (1)

where d is the mean diameter and n is the spread parameter. Both are defined in the injection input panel. dY is the mass fraction of particles with diameter greater than d . Mass/Heat transfer: The mass/heat transfer process for a water droplet is modeled with standard Laws that are widely used and can be found in most heat and mass transfer literatures, e.g. [5]. Coal Particles: The coal particle processes include injection, inert heating, moisture release, devolatilization, combustion and gasification, and ash heating or cooling. Coal particle injection: Like the water droplets, the coal particles are injected in the gasifier through the carrying gas with a particle diameter distribution of Rosin-Rammler [5]. Inert heating: The coal particles are heated up until the vaporization/ devolatilization temperature. There is no mass transfer or chemical reaction during this stage. Moisture release: When the coal particles reach a certain temperature, for example, the vaporization temperature, moisture is released. Accordingly, the moisture behaves as source for the gas phase and is added to the gas continuity equation and species (H2O) transport equation. Meanwhile, energy is taken out from the gas phase to supply the latent heat of vaporization. The reaction rate for the moisture release is calculated using an Arrhenius rate equation:

)/(1

RTEeAk −= (2) The kinetic constants A1 and E have the values suggested by Syamlal and Bissett [6]. Devolatilization: The stoichiometry is determined from a phenomenological model that predicts the yields of some major gas components and preserves a strict elemental balance. The phenomenological model is based on data from certain lab-scale experiments that characterize the coal [6]. The main species included in the devolatilization model are CH4, CO2, CO, O2, H2, H2S, N2, and H2O. The devolatilization rate is determined from the two-competing-rates Kobayashi model [11]. Char Combustion and Gasification: This stage takes place after the volatiles have been released and until all the char is consumed or the particle flows out of the reactor. The chemical reactions include char combustion (oxidation) and char gasification (with water and steam). The reaction rate is modeled as a kinetics/diffusion controlled model and is written as [12]:

4

(3)

where diffk is the gas film diffusion coefficient, rk is the chemical reaction constant, ashk is the ash

diffusion constant, Y is the char conversion factor, and *ii PP − is the effective partial pressure of i -

component taking account of the reverse reaction • Char Combustion

The shrinking core model by Wen and Chaung [12] is applied to model the char combustion: C + xO2 (2 – 2x) CO + (2x – 1) CO2 (4) where x = (2+a)/(2a+2) and a is CO/CO2 ratio that is computed in the model. The empirical formulations for computing diffk , rK , ashk , and Y in Equation 3 can be found in [12, 13].

• Char Steam Gasification The reaction of carbon with steam is:

C + H2O CO + H2 (5)

The empirical formulations of computing diffk , rK , ashk , and Y in Equation 3 can be found in [12, 13].

• Char CO2 Gasification

The reaction of carbon with carbon dioxide is:

C + CO2 2CO (6)

The empirical formulations for computing diffk , rK , ashk , and Y in Equation 3 can be found in [12, 13]. Gas Phase Reactions: Simple global reactions are used to describe the gas phase chemistry. The reaction paths taken into account are given as following:

CH4 + 2O2 CO2 + 2H2O (7) CO + 1/2O2 CO2 (8) CO + H2O CO2 + H2 (9) CO2 + H2 CO + H2O (10)

The interaction between chemistry and turbulence is modeled by using a finite rate/eddy dissipation model in which the reaction rate is defined by taking the minimum of the chemical reaction rate and the turbulent mixing rate [14]; that is:

)()11(111

1 *

2

ii

dashrdiff

PP

YkYkk

rate −−++

=

5

)),,,(,min( PRmixch XXkRRR ε= (11)

where chR is the chemical reaction rate, mixR is the turbulent mixing rate, k is turbulent kinetic energy, and ε is the dissipation rate, RX is the reactant mole concentration, and PX is the product mole concentration. Radiative Heat Transfer Model: The Discrete Ordinates (DO) radiation model [15] is used. The DO model allows one to solve problems ranging from surface-to-surface radiation to participating radiation in combustion problems. It spans the entire range of optical thickness.

Coupling Between Discrete Phase and Continuous Phase: The impact of coal particles on the continuous phase are computed by adding an appropriate source term to the conservation equations (Figure 1).

The particle trajectory is calculated by using a Lagrangian formulation. The dispersion of particles due to turbulence in the fluid phase is predicted using the stochastic tracking model in which random velocity fluctuations based on turbulence intensity and eddy lifetime are added on top of the particle instantaneous velocities.

Figure 1. Heat, mass, and momentum transfer between the discrete and continue phase

The mass transfer from the discrete phase to the continuous phase is computed by examining the change in mass of a particle as it passes through each control volume. This mass exchange appears as a source of mass in the continuous phase continuity equation and as a source of a chemical species. The heat transfer from the continuous phase to the discrete phase is computed by examining the change in thermal energy of a particle as it passes through each control volume. This heat exchange appears as a source or sink of energy in the continuous phase energy balance during any subsequent calculations of the continuous phase flow field. The continuous phase and discrete phase equations are calculated alternatively until a converged coupled solution is achieved. During iterations on the continuous phase flow, the accumulated sources from the particles remain unchanged, and vice versa.

6

VALIDATION AND RESULTS The two-stage, up-flow gasifier considered here consists of a horizontal first stage and a vertical second stage (Figure 2). Coal slurry and air are injected into the two side inlets of the first stage. The first stage is mainly a coal combustor and provides hot gases through the connection to the second stage in which only coal slurry is injected. Most of the coal gasification process occurs in the second stage. The total coal slurry mass flux is 39.7 kg/s, which consists of 30% water and rest coal. The second stage is fed with 22% of the slurry. The remaining 78% is evenly divided between the left- and right-hand inlets on the first stage. The oxygen mass flux is 22.9 kg/s with 50% going to each first stage inlet. Assuming that the water in the slurry evaporates quickly, the ratio of solids to gas mass flow rate is 0.8. Illinois #6 coal is used in the feed. The proximate analysis and ultimate analysis are listed in Table 1. The total volume of the gasifier is 45.5 m3. From the calculations the particle volume fraction is estimated to be around 4% and the average particle residence time is estimate to be 10 seconds. The operating pressure is 2.8 MPa. The coal slurry and the oxygen are fed into the gasifier at temperatures of 450K and 280.8K, respectively. The geometry shown in Figure 2 is meshed with 12,256 hexahedral computational cells. We should state that this is a prototype gasifier design which is not intended to represent any existing gasifier designs, commercial or otherwise. Table 1. Coal Properties

Proximate Analysis Ultimate Analysis Fixed Carbon 44.19 Carbon 63.75 Volatile 34.99 Hydrogen 4.50 Ash 9.70 Nitrogen 1.25 Moisture 11.12 Chlorine 0.29 Sulfur 2.51 Oxygen 6.88

Surface injection model was used for all three inlets. In this model, coal particles or water droplets were injected from each cell on the surface with different diameter. With “Stochastic Model” being used to model the influence of turbulent dispersion on the particles, a number of tries were performed for each injector. In this simulation, a total of 4030 particles were tracked. This number is sufficient and the results will not change with more particles being tracked. Reflect wall boundary condition was used for the particle. The particle will be rebounded whenever it hits the wall. The particle reaching the outlet or the bottom will escape from the gasifier. A temperature of 2500K was patched in the gasifier to initialize the combustion reaction. DPM calculations were performed at every 50th iteration of the fluid phase calculation. A length scale of 0.01 m was used to control the integration time step size used to integrate the equations of motion for the particle. A typical run took 50,000 gas-phase iterations for convergence, which was judged by the criteria that the residuals were less than the specified values and the DPM mass and energy are balanced.

7

Figure 2. A two-stage entrained-flow gasifier The temperature contours for the entrained-flow gasifier are provided in Figure 3. The hot gas generated from combustion of the volatiles in the first stage provides the necessary energy for the second-stage coal gasification. The char conversion is 100% for the first stage (coal injected in the first stage) and 86% for the second stage. Such results are typical for a large-scale coal gasifier. In Figure 4 the mole fraction contours of some major chemical species are presented. Note here that the dark red represents the highest level while the dark blue represents the lowest level. The mole fractions of the species at the outlet are shown in Table 2. The CFD results are compared with the results from an Aspen Plus restricted equilibrium reactor simulation tuned to represent experimental data. As can be seen, the CFD simulation results agree well with the experiments. CONCLUSIONS In this paper, we described a CFD model using DPM for coal gasification. The details of the underlying chemistry, physics, and numerical approach were discussed. We applied the CFD model to a two-stage, upright, coal-slurry gasifier and presented some key results including the synthesis gas composition.

Figure 3. Temperature contours at the center plane.

Coal Slurry

Coal slurry & O2

8

.

CO H2 H2O Figure 4. Species contours at the center plane

The high-fidelity CFD gasifier model was recently coupled with an Aspen Plus process simulation of a FutureGen power plant [16]. Process simulations are used to perform overall material and energy balances on the tightly integrated power plant flowsheet. The coupled CFD and process simulations ensure that the analysis of the coal gasifier using CFD is not done in isolation but within the context of the whole power plant system, so that a system-wide improvement can be achieved, not a local one at the expense of another part of the power plant system.

Table 2. Species mole fraction at gasifier exit.

ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of the U.S. Department of Energy, Fossil Energy Advanced Research Program. We also gratefully acknowledge the help of Dr. Dinesh Gera of Fluent Incorporated and Dr. Walter Shelton of EG&G for valuable discussions on gasification modeling.

Aspen Plus CFD

0.230

0.008

0.008

0.007

0.015

0.103

0.237

0.392

0.264H2O

0.008N2

0.008AR

0.007H2S

0.019CH4

0.090CO2

0.226H2

0.378CO

9

REFERENCES 1.Department of Energy (DOE), 2004, “FutureGen Report to Congress, FutureGen: Integrated

Hydrogen Electric Power Production and Carbon Sequestration Research Initiative,” www.fossil.energy.gov/programs/powersystems/futuregen/futuregen_report_march_04.pdf.

2.Vamvuka, D., 1999, “Gasification of Coal”, Energy Exploration and Exloitation, Vol. 17, No. 6, 515-581.

3.Niksa, S., Liu, G., and Hurt, R., 2003, “Coal conversion submodels for design applications at elevated pressure. Part I. devolatilization and char oxidation”, Progress in Energy and Combustion Science, Vol. 29, 425-477.

4.Liu, G. and Niksa, S., 2004, “Coal conversion submodels for design applications at elevated pressure. Part II. Char gasification”, Progress in Energy and Combustion Science, Vol. 30, 679- 717.

5.Fluent, 2004, Fluent User Manual, Fluent Inc. Lebanon, NH. 6.Syamlal, M., and Bissett, L.A., 1992, "METC Gasifier Advanced Simulation (MGAS) Model,"

Technical Note, NTIS report No. DOE/METC-92/4108 (DE92001111). 7.Shahnam, M., Syamlal, M., and Cicero, D., 2000, “Numerical Modeling of Combustion and

Gasification Processes using the Discrete Particle Method,” ASME Fuels & Combustion Technologies Division, 2000 International Joint Power Generation Conference, Miami, FL.

8.Syamlal, M., S. Venkatesan, S.M. Cho, 1996, “Modeling of Coal Conversion in a Carbonizer”, Proceedings of Thirteenth Annual International Pittsburgh Coal Conference, Vol. 2, Ed. S.-H. Ciang, University of Pittsburgh, Pittsburgh, PA, 1309-1314, September 3-7.

9.Guenther, C., M. Shahnam, M. Syamlal, J. Longanbach, D. Cicero, and P. Smith, 2002, “CFD Modeling of a Transport Gasifier," Proceedings of the 19th Annual Pittsburgh Coal Conference, Pittsburgh, PA, September 23-27.

10. Guenther, C., M. Syamlal, P.V. Smith, J. Longanbach, 2003, “Two-fluid model of an industrial scale transport gasifier,” Presented at the AIChE Annual Meeting, November 16-21, San Francisco, CA.

11. Kobayashi, H., Howard, J., and Sarofim, A., 1976, “Coal devolatilization at high temperature”, In 16th Symp. (Int'l.) on Combustion, The Combustion Institute.

12. Wen, C. Y. and Chaung, T.Z., 1979, “Entrainment Coal Gasification Modeling”, Ind. Eng. Chem. Process. Dev., Vol. 18, No. 4, 684-695.

13. Wen, C.Y., Chen, H., and Onozaki, M., 1982, "User's Manual for Computer Simulation and Design of the Moving Bed Coal Gasifier," DOE/MC/16474-1390, NTIS/DE83009533, National Technical Information Service, Springfield, VA.

14. Magnussen B.F. and Hjertager, B.H., 1976, On mathematical models of turbulent combustion with special emphasis on soot formation and combustion, In 16th Symp. (Int'l.) on Combustion. The Combustion Institute.

15. Chui, E.H. and Raithby, G.D., 1993, Computation of radiant heat transfer on non-orthogonal mesh using the finite volume method, Numerical Heat Transfer, Part B, Vol. 23, 269-288.

16. Zitney, S. E., Rogers, W.A., Syamlal, M., Osawe, M., Madsen, J. and Shi, S., 2005, “Advanced Process Engineering Co-Simulation of Power Generation Systems,” Accepted for publication in Proc. of the 30th International Technical Conference on Coal Utilization & Fuel Systems, April 17-21, Clearwater, FL.