1

CFD Modeling For Entrained Flow Gasifiers

Michael J. Bockelie Martin K. Denison, Zumao Chen, Temi Linjewile, Constance L. Senior, Adel F. Sarofim

Reaction Engineering International

77 West 200 South, Suite 210 Salt Lake City, UT 84101

Ph: 801-364-6925

http://www.reaction-eng.com (bockelie@reaction-eng.com)

Neville Holt

Electric Power Research Institute 3412 Hillview Avenue

Palo Alto, CA 94304-1395 The gasification industry has identified improved performance of entrained flow gasifiers as a key item to reduce the technical and financial risk associated with Integrated Gasification Combined Cycle (IGCC) power plants. Recent review papers by [Steigel et al, 2001] and [Holt, 2001b] identified several items that could lead to improved Reliability, Availability and Maintainability of solid fuel gasifiers. Specific problem areas that were identified in these papers include fuel injector life, refractory wear, carbon conversion and slag management. A better understanding of these phenomena and how they are impacted by operational changes such as fuel type, slurry content and oxidant flow rate would be highly beneficial. As noted by Steigel and Holt, Computational Fluid Dynamics (CFD) modeling of gasifiers could assist in building the required knowledge base.

Over the last ten years, CFD modeling has played an important role in improving 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 that will lead to improved performance. Used correctly, a CFD model is a powerful tool that can be used to address many problems. Incorporated into the model is the gasifier geometry, operating conditions and gasification processes. The outputs, or predicted values, from the CFD model are quite extensive and can provide localized information at hundreds of thousands of points within the gasifier. An excellent review of recent CFD modeling studies for gasifier systems is available in [IEA, 2000].

As part of a DOE Vision 21 project, Reaction Engineering International (REI) is developing a CFD modeling capability for entrained flow gasifiers [Bockelie et al, 2002a]. Our modeling efforts are focused on two generic gasifier configurations:

single stage, down fired system and two stage system with multiple feed inlets that can be opposed or tangentially fired.

These systems are representative of the dominant, commercially available gasifier systems [NRC,1995], [Holt, 2001b]. The single stage gasifier configuration we are currently studying contains a single fuel injector, located along the gasifier centerline. However, configurations

2

with multiple injectors or an up fired orientation could also be studied. The two stage gasifier consists of two sections connected by a diffuser. Each section can have two or four feed injectors. The first stage is assumed to be a slagging combustor used to separate the ash from the slag and to provide hot gases to the second stage. For both classes of gasifiers hot mineral matter is deposited on the wall as slag. Proper slagging behavior is important for protecting the refractory-lined walls of the gasifier from the harsh environment within the gasifier. Examples of a commercial sized single stage, downfired gasifier would be those used at the Polk Power Station, Eastman Chemical plant (Kingngsport, Tennessee) or the gasifier test facility at the Freiberg R&D center in Germany. Examples of commercial sized two stage gasifiers are the opposed fired used at the Wabash River power plant and the air blown, tangentially fired systems being developed in Japan. Although our focus is on oxygen blown, slurry fed, pressurized systems, the same CFD modeling tool can be used to model air blown, dry feed or atmospheric systems. Our efforts in developing the gasifier models are being guided through collaborations with Neville Holt of the Electric Power Research Institute (EPRI), Prof. Terry Wall and his colleagues at the Collaborative Research Center for Coal and Sustainable Development (CCSD) in Australia and Prof. Klaus Hein from the Institute for Process Engineering and Power Plant Technology (IVD) at the University of Stuttgart, Germany.

The gasifier models are being developed with an eye toward addressing a broad range of problems related to Reliability, Availability and Maintainability. Of immediate interest is the ability to predict the impact on gasifier performance due to operational changes. The range of operational changes that can be investigated, includes: fuel switching: coal type, petcoke, wastes feed type: wet (slurry) versus dry (N2, CO2) fuel-oxidant ratio, fluxing materials to modify slagging properties char recycle and flue gas recycle Numerous criteria are available for measuring gasifier performance. At present, the metrics we use to evaluate gasifier performance include the gasifier exit conditions:

carbon conversion, cold gas efficiency syngas properties:

o flow rate, temperature, heating value o species composition

Major: CO2, CH4, H2, H2O, N2, O2, Minor: H2S, COS, NH3, HCN,

flyash properties o unburned carbon content, mass flow

and wall properties: o slag properties

temperature, viscosity, thickness, flux & ash composition including carbon content

o heat extraction. In addition to gross values, a wealth of localized information is available, including:

Flow patterns, velocities and pressure Gas and surface temperatures

3

Wall heat transfer (incident flux, net flux, backside cooling) Particle / Droplet trajectories and reactions

o Time temperature histories o Wall deposition (location, amount)

In addition to operational problems, the models are equally applicable for investigating research questions, design modifications and scale-up of pilot scale systems. Here, target problems could include the impact of alternative firing systems (e.g., multiple fuel injectors, oxidant or FGR injectors, injector orientation, alternative nozzle designs), system pressure scaling and altering the gasifier volume and shape (L/D ratio). In the following sections, we first present an overview of the CFD model used for the gasifier model, after which are highlighted some example calculations that have been performed to highlight the capabilities of the models.

GASIFIER MODEL DESCRIPTION In the following we briefly discuss the basic CFD solver, high pressure reaction kinetics and a flowing slag model. A more thorough description of the gasifier model is available in [Bockelie et al, 2002b].

Basic CFD Model The gasifier models have been constructed using GLACIER, an in-house comprehensive, coal combustion and gasification modeling tool that has been used to simulate a broad range of coal and fossil fuel fired systems [http://www.reaction-eng.com]. An important aspect of GLACIER is the tight coupling used between the dominant physics for gasifier applications:

turbulent fluid mechanics, radiation and convective heat transfer, wall / slag surface properties, chemical reactions and particle/droplet dynamics.

With GLACIER it is possible to model two-phase fuels for either gas-particle or gas-liquid applications. To establish the basic combustion flow field, full equilibrium chemistry is employed. Pollutants (e.g., NOx), vaporized metals and other trace species for which finite rate chemistry effects are important, but which do not have a large heat release that would impact the flow field, are computed in a post-processor mode. Gas properties are determined through local mixing calculations and are assumed to fluctuate randomly according to a statistical probability density function (PDF) which is characteristic of the turbulence. Turbulence is modeled with a two-equation non-linear k-e model that can capture secondary recirculation zones in corners. Gas-phase reactions are assumed to be limited by mixing rates for major species as opposed to chemical kinetic rates. Gaseous reactions are calculated assuming local instantaneous equilibrium. The radiative intensity field is solved based on properties of the surfaces and participating media and the resulting local flux divergence appears as a source term in the gas phase energy equation. Our models include the heat transfer for absorbing-emitting, anisotropically scattering, turbulent, sooting media. Particle mechanics are computed by following the mean path for a discretized group of particles, or particle cloud, through the

4

gasifier. Particle reaction processes include coal devolatization, char oxidation, particle energy (including particle radiation, convection and chemical reaction), particle liquid vaporization and gas-particle interchange. Particle reactions based on fuels other than coal can be modeled. The dispersion of the particle cloud is based on statistics gathered from the turbulent flow field. Heat, mass and momentum transfer effects are included for each particle cloud. (note: liquid droplets are modeled in a manner similar to that of solid particles) For applications with especially high particle loading, additional smoothing of the source terms can be applied. More rigorous descriptions of the models described here are available in previous publications, such as [Bockelie et al, 2002b], [REI_Models], [Bockelie et al, 1998], [Adams et al, 1995], [Smoot and Smith, 1985]. Reaction Kinetics There is an extensive literature on the kinetics of devolatilization and gasification. Much of it is directed at the early moving bed and fluidized bed gasifiers and therefore is not directly relevant to entrained flow gasifiers, that involve higher temperatures and shorter residence times than packed and f