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Models for Predicting Bed-Related CFB Performance Parameters Pete Rozelle U.S. Department of Energy ARIPPA, May 23, 2006

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Undersized Bottom Ash Equipment Some CFB Plant Issues Related to Solids Characteristics and Flows

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Undersized Bottom Ash Equipment Some CFB Plant Issues Related to Solids Characteristics and Flows Excessive Upper Combustor Temperature (Delta P Too Low)

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Undersized Bottom Ash Equipment Some CFB Plant Issues Related to Solids Characteristics and Flows Excessive Upper Combustor Temperature (Delta P Too Low) Excessive Limestone Consumption

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Two Items to be Covered Today Bottom Ash Flow Rate –Based on Readily Measured Fuel Properties Limestone Consumption –Based on Ash and Limestone properties Both of the Above –Based on Boiler Operating Parameters (DCS Data) –Boiler Solids Partition Function

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Solids Flows in Mineral Processing Physical Separation Equipment Feed Stream Product Stream Tailings Stream Separation Stimulus

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Quantifying the Separation by a Coal Preparation Circuit: the Partition Function k(y) is the mass fraction of particles of specific gravity y that reports to the coal stream The sum of each gravity interval times k(y) is the clean coal yield.

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Applying the Partition Function to a CFB Boiler k d (x) is the mass fraction of particles of ash size interval x that reports to bottom ash stream The sum of each size interval times k d (x) is the bottom ash flow rate.

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Example of a Generic Partition Function

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What is Needed to Develop a Partition Function for a Boiler Flyash and Bottom Ash Size Analyses Flyash and Bottom Ash Flow Rates 1.Major and Minor Oxide Analyses for Flyash, Bottom Ash, Limestone, and Fuel Ash 2.Flyash and Bottom Ash Flow Rates can be Calculated from a System of Simultaneous Material Balances (see Next Slide) Composite Ash Flow (by Particle Size) Calculated from Flyash and Bottom Ash Flow Rates and Size Analyses Partition Function Calculated from Composite Ash Flow (by Size) and Bottom Ash Flow (by Size)

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Calculating Ash Flow Rates from Simultaneous Material Balances CaO and SiO 2 Material Balances: DCS Quantities: F f : Fuel Feed Rate F s : Limestone Feed Rate Lab Analyses: CaO f : Fuel CaO Content CaO s : Limestone CaO Content CaO ba : Bottom Ash CaO Content CaO fa : Flyash CaO Content SiO 2f : Fuel SiO 2 Content SiO 2s : Limestone SiO 2 Content SiO 2ba : Bottom Ash SiO 2 Content SiO 2fa : Flyash SiO 2 Content Unknowns F ba : Bottom Ash Flow Rate F fa : Flyash Flow Rate

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Calculating Ash Flow Rates from Simultaneous Material Balances CaO and SiO 2 Material Balances: DCS Quantities: F f : Fuel Feed Rate F s : Limestone Feed Rate Lab Analyses: CaO f : Fuel CaO Content CaO s : Limestone CaO Content CaO ba : Bottom Ash CaO Content CaO fa : Flyash CaO Content SiO 2f : Fuel SiO 2 Content SiO 2s : Limestone SiO 2 Content SiO 2ba : Bottom Ash SiO 2 Content SiO 2fa : Flyash SiO 2 Content Unknowns F ba : Bottom Ash Flow Rate F fa : Flyash Flow Rate 2 Equations and 2 Unknowns

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Calculating the Composite Ash Flow Rate by Size- 1 Calculate the Flow Rate of Each Size with Each Ash Stream- Example: Bottom Ash Flow Rate = 35,000 pph, 12% of Bottom Ash is 80 Mesh by 100 Mesh, Flow Rate of 80 by 100 Mesh Size with Bottom Ash= 0.12 X 35,000 pph = 4,200 pph

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Calculating the Composite Ash Flow Rate by Size- 2 Flyash Ash Flow Rate = 45,000 pph, 7% of Flyash Ash is 80 Mesh by 100 Mesh, Flow Rate of 80 by 100 Mesh Size with Flyash Ash= 0.07 X 45,000 pph = 3,150 pph

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Calculating the Composite Ash Flow Rate by Size- 3 Flow Rate of 80 by 100 Mesh Size with Bottom Ash = 4,200 pph Flow rate of 80 by 100 Mesh Size with Flyash = 3,150 pph Flow Rate of 80 by 100 Mesh Size with Composite Ash = 4,200 pph + 3,150 pph = 7,350 pph

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Calculating the Partition Function Values f ba (x) is the flow rate of size interval x with the bottom ash (calculated) f ca (x) is the flow rate of size interval x with the composite ash (calculated)

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Calculating the Partition Function Values From our Example: k d (x) for x = 80 by 100 mesh is: (4,200 pph)/(7,350 pph) = 0.57

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Example of a Partition Function for a CFB Boiler

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Calculating Ash Flow Rates and the Partition Function Using Simultaneous Material Balances can reduce Effort Required for Measuring the Ash Split The Partition Function can be a Useful Determinant of Cyclone Performance.

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Prediction of Bottom Ash Flow Rate What is Needed: Partition Function DCS Data: –Solids, Air, and Main Steam Flow Rates Solids Analyses –Short Prox for Fuel –Major and Minor Oxides for Fuel and Limestone –Float Sinks for Fuel

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The Significance of Float Sink Analyses They Split the Fuel up by Specific Gravity and Ash Content. SinkFloatDirect Wt% Direct Ash% Cumulative Wt% Cumulative Ash%

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The Significance of Float Sink Analyses Combustion Behavior Low Ash Fuel Particle High Ash Fuel Particle

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The Attrition Index The Fraction of a Particle Originally Large enough to make Bottom Ash that Reports to the Flyash Stream Low Ash Fuel Particle= 1 High Ash Fuel Particle=0

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And Now Some Model Results: Quantifying Bottom Ash Flow in Terms of Fuel and Limestone Properties

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Laboratory Combustor Partition Curve

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The Bottom Ash Model: The Fuel Contribution From DCS: F f : The Fuel Feed Rate From Float Sink Table: M f (x,y): The Mass Fraction of Fuel with in Size and Specific Gravity Increment x,y A(x,y): The Ash Content of the Above Fuel Increment k d (x): The partition function calculated from boiler data K f (y) the fuel attrition index based on particle specific gravity (i.e. ash content) The Result: F ba,f : The flow rate of bottom ash derived from the fuel

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Float Sink Analyses SinkFloatDirect Wt% Direct Ash% Cumulative Wt% Cumulative Ash% M f (x,y) A(x,y)

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The Bottom Ash Model: The Limestone Contribution From DCS: F s : The Limestone Feed Rate From a Lab Analysis: M s (x): The Mass Fraction of Limestone in Size Increment x L s : The Limestone LOI S: fractional SO 3 Content of Limestone in Bottom Ash k d (x): The partition function calculated from boiler data K s,a the limestone attrition index The Result: F ba,s : The flow rate of bottom ash derived from the Limestone

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The Bottom Ash Model: The Result

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The Bottom Ash Model: some Test Results Analyses of Fuels Tested FuelIIIIIIIVVVI Wt% Ash* Wt% S* HHV, MJ/kg** Preparation Method(1) (2)(3) Parent FuelIIIIIIIIIIIII

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The Bottom Ash Model: some Test Results

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Conditions Used for Combustion Tests Firing Rate, MJ/hr253 Superficial Gas Velocity, m/s3.1 Sorbent Feed Rate, Molar Ca:S2.2 Bed Pressure Drop, Pa2,250 Bed Temperature, K1,230

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The Bottom Ash Model: some Test Results

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The Bottom Ash Model Reducing the Presence of Coarse, High Ash Content Fuel Particles in the Feed can Bias the Ash Split Toward Flyash Identifying the Species that Create Bottom Ash can Narrow the Search for Fuel Constituents that can Cause Ash Cooler Problems Updated Version is Being Developed

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Prediction of Limestone Consumption What is Needed: Partition Function DCS Data: –Solids, Air, and Main Steam Flow Rates Solids Analyses –Short Prox for Fuel –Major and Minor Oxides for Fuel and Limestone –Float Sinks for Fuel Limestone Attrition Index Limestone Sulfation Levels in Ash Streams

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The Limestone Model M s (x) is the mass fraction of limestone of size x I s,a is a CaO Attrition Index The Result: ba : The Fraction of Limestone CaO that will report to the Bottom Ash Stream A word on Limestone and Attrition: Sparry Stones Attrit a Lot more than Micritic Stones

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Limestone Attrition Products Micritic Stone Sparry Stone

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The Limestone Model fa : The Fraction of Limestone CaO that will report to the Flyash Stream

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The Limestone Model F f : Fuel Feed rate S f : Sulfur Content of Fuel : Fractional Sulfur Capture R ba : Sulfation Level Characteristic of Bottom Ash R fa : Sulfation Level Characteristic of Flyash MW CaO : The Molecular Weight of CaO MW S : The Molecular Weight of Sulfur Ca s : CaO Content of Limestone The Result: F’ s : The Predicted Limestone Consumption by the Boiler

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The Limestone Model: Comparison with Boiler Test Results Chemical Analyses of Sorbents Examined Sorbent Wt% CaO Wt% MgO Wt% Fe 2 O Wt% SiO Wt% Al 2 O Wt% LOI* Grain Size Micrite SparMicrite Spar

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The Limestone Model: Comparison with Boiler Test Results Particle Size Distributions of Sorbents Tested in the Boiler Particle Size, mmSorbent (wt% direct) PassingRetainedMean

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The Limestone Model: Comparison with Boiler Test Results

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Uses for the Limestone Model Predicting Limestone Consumption Based on Boiler Parameters and Limestone Lab Analyses Helping to Select the Lowest Cost Stone Whether Changes to a Grinding Circuit can Lower Costs

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What is required to use these Models Lab Analyses of Solids DCS Data Inexpensive Software –MathCad –Excel

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A Word on Limestone

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Limestones under Boiler Conditions have been shown to still Absorb SO hours after introduction to the System. The Mean Bed Residence Time in a Rock Burner is Determined by Fuel Ash and Sulfur Content and Delta P. It may be less than an Hour. Reducing Ash and Sulfur Content can Increase Mean Bed Residence Time

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