APPLICATION OF AN LES BASED CFD CODE TO SIMULATE COAL AND BIOMASS - - PowerPoint PPT Presentation

APPLICATION OF AN LES BASED CFD CODE TO SIMULATE COAL AND BIOMASS COMBUSTION IN GENERAL REACTOR CONFIGURATIONS

• A. Suo-Anttila, J. D. Smith, and L.D. Berg, SAS, Inc.

and J.P. Goldring, RJM International

9th European Conference on Industrial Furnaces and Boilers Estoril, Portugal www.cenertec.pt/infub April 26-29, 2011

OUTLINE

• Introduction
• LES Based Code Background
• Code Validation and Application
• Plant Burner Simulation
• Observations and Conclusions

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SAS/RJM Capabilities

• Site survey and testing to quantify plant performance
• Analyze and Recommend “Best” NOx abatement strategies

– Over Fire Air (OFA) – Burners Out of Service (BOOS) – Low NOx Burners – SCR and SNCR systems – CFD modeling to identify “minimum cost” strategy

• Combustion air supply

– CFD model complex ductwork to balance flow – Physical modeling to “fix” flow/particle mal-distribution

• Coal blending analysis
• Fuel conversion analysis
• Multi-fuel modeling of combustion systems

– natural gas, refinery gases, landfill gases, #2 & #6 fuel oil, bio-oil, sub- bituminous/bituminous/lignite coals, bio-mass

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Code Background: General Comments

• Transient conservation equations with radiative heat transfer and

combustion chemistry

• Considers soot formation and other multi-phase systems using

Eulerian/Eulerian formulation

• Accurately assess different operation scenarios (wind, flow rate, fuel

type, surroundings)

• Reasonable CPU time requirements on “standard” workstation
• Trade offs for “Engineering” Approach

– Sacrifice generality (large fires only) in favor of quick turnaround with quantitative accuracy – Reaction rates and radiation heat transfer models apply best to optically thick fires – Models intended to make predictions “good-enough” for industrial use – Model validation for each application to establish accuracy of results

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Gas Combustion Model*

• Variant of Said et al. (1997) turbulent flame model
• Relevant Species (model includes relevant reactions)
• F = Fuel Vapor (from evaporation or flare tip)
• O2 = Oxygen
• PC = H20(v) + CO2
• C = Radiating Carbon Soot
• IS = Radiating Intermediate Species
• Eddy dissipation effects and local equivalence ratio effects
• Reactions based on Arrhenius kinetics
• C and TA determined for all reactions

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*Coal combustion Model follows same outline but includes more reaction parameters

EBU-Finite Rate Combustion Formulation

• Arrhenius rate model

– Consumption of primary reactant increases on reactants mass fraction fRi and temperature T in volume – Coefficients C and Activation Temperatures (TA) determined for all reactions

Where:

Ak = Pre-exponential Factor X1 = Natural Gas Mol Frac X2 = O2 Mol Frac Ea = Activation Temperature T = Local Gas Temperature b, c, d = Global Exponents

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Radiation Inside Large Fires

• High soot volume fractions make large fires non-transparent (optically thick)

which causes flame to radiate as a cloud (radiatively diffuse)

• Fire volume defined where product-of-combustion (fpc) mass fraction is greater

than user specified minimum fraction (fmin), usually 6% or (fpc > fmin)

• Flame edge (fflame edge) where products of combustion volume fraction =

minimum volume fraction

Calculated flame surfaces from 3 time steps from validation against test

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Code Validation - Test Facility Details

• 18.9 m diameter JP8 fuel pool
• Pipe suspended 0.6m above leeside
• f pool
• Pipe axis perpendicular to

predominant wind

• Wind speed and direction measured

2 and 10 m above the ground on two upwind poles

x z θ

18.9-m diameter fuel pool

Culvert Pipe Thermocouple Ring Locations

3.66 m

18.28 m

6.09 m

1 2 3 4 5 6 7 8 3.66 m

30 m 30 m Southwest Anemometer Pole South Anemometer Pole

Predominant Wind Direction

LE L C R RE

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Model Validation: Computational Domain

• 60 m square base; 15 m high
• 16,500 elements, highly refined near pipe
• Each pipe grid linked to 1-D conduction module (see next

slide)

X Y Z

LY = 15 m NY = 19 LZ = 60 m NZ = 28 LX = 60 m NX = 31

Pool Location Pipe

Wind direction

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Code Validation –Predicted vs Measured

• Coolest near the top, hottest on leeside: caused by flame tilt and

downstream recirculation zone

• Nearly uniform at each axial position
• Predicted max temperature closer to ground than measured data

300 500 700 900 1100 1300 1500 1700 Top Lee Top Leeward Lee Bot Bottom Wind Bot Wind Wind Top Left End Left Center Right Right End 300 500 700 900 1100 1300 1500 1700 Top Lee Top Leeward Lee Bot Bottom Wind Bot Wind Wind Top left end left middle right right end

Predicted Data

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Slide 11

Applications of C3d: Elevated Flare Ignition

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• Nominal Firing Rate = 350 Tons Per

Hour (TPH)

• Max Firing Rate – 1350 TPH
• Mostly Natural Gas (Mwt = 18)
• Experienced Pressure Wave during

ignition

• Conducted Tests to quantify ignition

phenomena:

− Microphones used to measure pressure wave − High Speed Video used to capture flame during ignition

• Test results reported elsewhere

(summarized below)

• Test video shows ignition behavior

Coal/Biomass Validation Test

• Primary Inlet

– Coal flow = 2.835e-3 kg/sec – Air flow = 6.228e-3 kg/sec – Gas Temperature = 300 K

• Secondary Inlet (co-annulus)

– Air flow = 0.019 kg/sec – Gas Temperature = 589 K

• Reactor

– Refractory lined – Flanged sections (20 cm ID x 30 cm long

• Outlet

– Constricted/cooled exit – Assume Constant pressure in model to allow unrestricted exit of particles/gases

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Test Reactor Geometry and Mesh

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Model assumptions:

Computational grid size & cell number Turbulence model (zero equation and

• ne-equation LES)

Numerical upwind and central differencing using TVD scheme Hydrostatic pressure on outlet boundary (allow free exit of particles/gases) Initial Primary gas temperature and composition set to 300 K air

Coal Combustion Reaction Model

• Reaction Coefficient selected as providing “best” fit to Combustion

Data from ASAY Case

• 8 Reactions involving 11 Species Considered:

– Fine Particles (FCP) – Coarse Particles (CCP) – Volatile Organic Hydrocarbons from the devolatilization process (VOC) – Partially Devolatilized Fine Particles (PDFP) – Partially Devolatilized Coarse Particles (PDCP) – Soot Particles (Soot) – Fine Char Particles (FPchar) – Coarse Char Particles (CPchar) – Ash (Ash) – Oxygen (O2) – Products of Combustion (PC)

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Coal/Biomass Combustion Mechanism (1)

1-Fine Coal Particles (FCP) undergo fast devolatilization:

1.0 FCP  0.5 VOC + 0.5 PDFP Activation temperature = 12,581 K; Pre-exponential coefficient = 1.0e11 sec-1

2-Partially Devolatilized Fine Coal Particles (PDFP) undergo slow devolatilization:

0.5 PDFP  0.2 VOC + 0.3 FPchar Activation temperature = 12,581 K; Pre-exponential coefficient = 1.0e7 sec-1

3-Fine Char Particle (FPchar) combustion uses global combustion form:

0.3 FPchar + 0.72 O2  0.7 PC + 0.05 Ash Activation temperature = 7,337 K; Pre-exponential coefficient = 1.4e7 sec-1; Oxygen exponent = 0.5, Char particle exponent = 0.33; Temperature exponent = 0.6

4-Coarse Coal Particles (CCP) undergo fast devolatilization:

1.0 CCP 0.2 VOC + 0.8 PDCP Activation temperature = 12,581 K; Pre-exponential coefficient = 1.0e8 sec-1.

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Coal/Biomass Combustion Mechanism (2)

5-Partially Devolatilized Coarse Particles (PDCP) undergo slow devolatilization :

0.8 PDCP  0.25 VOC + 0.55 CPchar Activation temperature = 581 K; Pre-exponential coefficient = 2.0e7 sec-1.

6-Coarse Char Particle (CPchar) combustion uses global combustion form:

0.55 CPchar + 1.32 O2  PC + 0.143 Ash Activation temperature = 7,337 K; Pre-exponential coefficient = 0.43e8 sec-1, oxygen exponent = 0.5, Char particles exponent = 0.33; Temperature exponent = 0.6.

7-Volatile Organic Hydrocarbon (VOC) combustion and soot formation follow 2nd order Arrhenius kinetics:

1.0 VOC + 2.5 O2  3.29 PC + 0.21 Soot Activation temperature = 15,500 K; Pre-exponential coefficient = 1.0e15 sec-1.

8-Soot combustion uses global combustion form:

1.0 Soot + 2.66 O2  3.66 PC Activation temperature = 25,500 K; Pre-exponential coefficient = 2.0e10 sec-1, oxygen exponent = 1.0, Char particles exponent = 0.33; Temperature exponent = 0.6.

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O2 Profile in Reactor: Predicted (lines) vs Measured (solid dots)

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Predicted Coal Combustion Results for Asay Case

(snap shot from 2 sec simulation)

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Ash Concentration Volatile (VOC) Concentration Coarse Char Particle Concentration Oxygen Concentration

4/28/2011 Eggbrough Coal Combustion Analysis – CONFIDENTIAL – PROTECTED INFORMATION Slide 19

Process Conditions: Plant Case w/ Low NOx burner fired into a cylindrical reactor

Inlet Process Conditions Value Fuel flow (kg/s) 2.16 Prim Air (kg/s) 2.98 Prim Air temp (°C) 300.00 Prim Air/Prim Fuel ratio (-) 1.38 Sec Air (kg/s) 19.11 Sec Air bypass thru back plate (kg/s) 1.14 Total Sec Air flow (kg/s) 20.25 Sec Air Temp (°C) 218.00 Total Air Flow (kg/s) 22.49 Calculated burner stoich (-) 1.33 Furnace Pressure assumed (mbar)

• 1.00

Results

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Results:

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Results: Burner Flame showing Particles at 5 sequential time step 50 ms apart

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Conclusions

• Transient LES based Coal Combustion Code:

– LES mixing captures devolatilization/oxidation for different particle sizes – Coal Combustion model based on EBU/finite rate reactions for coal, char, volatiles, ash, and soot

• Asay Test Case used to develop reaction model (results partially

validated)

• Results compare well to ASAY experimental

– Identify importance of fine particle devolatilization in flame stabilization

• Applied LES Coal Combustion Model to Full Plant Case

– Plant uses “unstable” burner (difficult to model with standard RANS code)

• Future Work

– Additional Validation needs to be performed to establish reaction set – Need to add additional particle sizes to better distribute energy between gas/particle phases – Need to add fouling/slagging mechanism to collect ash on walls and determine temporal nature of slag build up

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Questions from Audience

Questions following presentation

• How would you handle ash deposition given limitations on LES at walls?

– Use Law of wall models to tie bulk phase to near wall phenomena same as RANS models do

• How do you apply devolatilization reactions to Bio-mass combustion?

– Need biomass devolatilization data to correlate devolatilization reactions in combustion model (current work is correlated to coal devolatilization for Sub- Bitumnious C coal at atomspheric conditions

• Using RANS codes one can not predict deposition on backside of boiler

tubes and LES may be able to do so but not certain?

– In general the mesh refinement near the backside of the tubes would be important to allow LES to transport ash to this region. In principle it should work better than RANS

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Backup Slides

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SAS/RJM Capabilities (2)

• Operational issues

– Trouble shooting experience for process and utility equipment to quickly identify root cause and identify cost effective solutions – Analyze cooling water intakes to protect aquatic life – Use CFD analysis with slag models to analyze complex ash/refractory issues and recommend corrective actions

• Support/trouble shooting for Coal Gasification Processes

– CFD modeling of coal injectors, fluid bed systems, and entrained flow reactors – trouble shoot operational problems related to erosion, slagging, and thermal/mechanical fatigue

• Development and Application of Catalytic Systems

– Catalytic systems to reduce CO/NOx emissions – Develop/apply detailed kinetics models for catalytic systems

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• When fpc < fflameedge then outside “flame” (participating medium

considered)

• View factors from fire to surrounding surfaces calculated at each time

step (includes attenuation by gas and soot media for flames)

• Re-radiation from surroundings also calculated at each time
• Fire considered black body radiator (εfiresurface = 1)
• Radiation from flame to surroundings assumes Tsurround = constant

Radiation Outside of Large Fires

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Reactions Involving Fuel

• Incomplete Fuel Combustion (soot producing)

1 kg F + (2.87-2.6S1) kg O2 → S1 kg C + (3.87-3.6S1) kg PC + (50- 32S1) MJ – Combustion Soot Mass Parameter, S1 = 0.005

• Endothermic Fuel Pyrolysis (soot producing)

1 kg F + 0.3 MJ → S2 kg C + (1-S2) kg IS – Cracking Parameter, S2 = 0.15

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Reactions Not Involving Fuel

• Soot Combustion

1 kg C + 2.6 kg O2 → 3.6 kg CO2 + 32 MJ

• Combustion of Intermediate Species

– Coefficients chosen so that complete combustion of C and IS produce same species and thermal energy as direct combustion of fuel

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Diffuse Radiation Within Fire

• Calculated indirectly using a Rossland effective thermal

conductivity

• σ = Stefan-Boltzman Constant
• T = local temperature
• βR= local extinction coefficient. Dependent on local species concentrations
• Radiation transport model:

– Predicts radiant flux on external (and internal) surfaces – Provides source/sinks terms to overall energy equation

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Air R R

k T k >> = β σ 3 16

3

Computational Techniques

• Fully compressible predictor-corrector pressure-based algorithm
• 1st and 2nd order Finite Volume Using Total Variation Diminishing

(TVD) Methods

• Porosity method for curved surfaces (i.e., structured grid with body

fitted computational cells)

• Uses acceleration factor (TF) to model long burn times

Portion of cell occupied by computational domain Portion of cell outside computational domain Code accounts for cell fractional flow areas on all cell faces to approximate complex geometry

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Slide 32

Code Validation - Test Setup

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Code Validation: Object Engulfed in Fire

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Slide 34

Applications of C3d: Multi-Tip ground flare

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• Propane, Ethylene, and Mixed Gas

injected as mass, momentum and species sources

• Fuel Mol wt ~ 18 - 44
• Tip elevation – 2.0 m
• Test Conditions = 0.46 kg/s
• Flame height determined by soot
• Air inflow calculated at boundaries
• Radiation Flux calibrated from

measured data at two locations

• Correctly predicted flame

height/shape, flame radiation, soot formation, and wind effect