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Western Regional
Boiler Association
Design and Operation
Spreader Grate Systems
Bob Morrow – Detroit Stoker
Red Lion Hotel
Port Angeles, WA
March 11-13, 2014
©2013 Detroit Stoker Company. All Rights Reserved
Detroit Stoker Company
• Administration, Sales, Engineering &
Manufacturing in Monroe, Michigan
• 85 Employees
• 19 North American Manufacturer Sale Reps
• 12 International Manufacturer Sales Reps
• Privately Owned
Products & Services
•
•
•
•
•
•
Solid Fuel Combustion Systems
Solid Fuel Feeding/Metering Systems
Rotary Seal Feeders/Double Flap Airlocks
Low NOx Gas/Oil Burners
Aftermarket Parts & Services
Engineering Studies
– CFD Analysis for Air Systems and Furnace Design
– Pilot Scale Testing
Solid Fuel Combustion Systems
• Spreader Fired Combustion Systems
• Mass Fired Combustion Systems
Detroit’s Supply for Nippon
Secondary Air
Fuel Distributors
Grate System
Nippon – Grate System Design
• MCR Steam Output = 225K lbs/hr
– 945 psig, 900°F, 250°F
• Thermal Input = 383 MBtu/hr
– Fuel = 95K lbs/hr
– HHV = 4050 Btu/lb @ 51% H2O
• 2 module Roto-Stoker type VCG
– Air-cooled, Vibrating grate
– 19’-4” wide X 24’-0” long
– Grate thermal load = 830 Kbtu/ft2/hr
• Primary Air & Secondary Air Temp. = 500°F
Basic Principal of Grate Operation
• Eccentric design drive arrangement
• Operation initially lifts fuel and ash ahead & forward. As
the grate surface moves back, fuel/ash particles deposited
farther ahead.
• Low speed operation (385 rpm nominal)
• Small amplitude (1/4” total)
• Intermittent operation
• Run cycle 3-5 seconds, Dwell for 10-20 minutes
• Grate surface supported on specifically designed spring (flex)
strap arrangement
VCG Grate System
Stationary Frame
Vibrating Frame
Grate Assembly
Grate Elements
Flex Straps
Vibrating Frame
Stationary Frame
Grate Drive
Grate Drive
7 ½ HP motor
Eccentric Bearing
Eccentric shaft
Speed reduction
sheaves
Pilot Bearings
Grate Ash Discharge Capacity
Feed Rate Factor Vs Eccentric rpm
0.25
Feed rate factor
0.2
0.15
Structural Design
Point
0.1
Nominal Operating
Range
0.05
0
360
365
370
375
380
385
390
395
400
405
410
415
Eccentric Speed (rpm)
Grate Speed
Too Slow
Too Fast
Spreader Type Combustion Systems
Smaller Fuel
Larger Fuel
Initial
Combustion
Thoughts
Combustion
Biomass Fuels
0.1 - 0.3 Seconds
Thermal Decomposition
Products:
Light Gases, Liquid HC’s, Tars
Combustion Products:
CO2, CO, H2O, PICs
Air
Glowing Particles
“Sparklers”
Distribution
between these
is fuel
dependent, but
typically more
volatile material
with biomass
Biomass particle
Charcoal
Air
1 - 2 Seconds
CO
Flyash or bottom ash
with unburned C
Spreader Stoker Combustion -101
Spreader Combustion
Theory ‘Fine Fuel”
– Suspension Firing
• Smaller particles with low terminal velocities dry/volatilize in
suspension over the bed flame.
• Smaller particles are less dense with lower mass of moisture.
• Smaller particles have higher surface area
• Distribution across entire grate area is constant with small particles
in front zone and larger particles in the rear.
• Immediate reduction of particle mass (H2O & VM), lesser fuel height
on grate surface. Grate designed for higher P for primary air
distribution.
– Enhanced volitazation by pre-heated primary air
Spreader Combustion
Theory ‘Larger Fuel”
– Grate Firing
• Large particles with high terminal velocity deposit on the grate.
• Larger particles include higher mass of moisture, therefore need
more time to dry and volatilize.
• Larger particle trajectories travel through the hottest zone above the
bed therefore radiant heat transfer increases dry / volatilize rates.
• Under grate air is pre-heated to assist and increase drying rates in
bed zone.
• As larger particles dry / volatilize, size and density decrease and
terminal velocity is reduced which re-introduces particles into the
suspension fired combustion zone.
Fuel Particle Entrainment
Maximum Entrained
Particle Size for Dry and
Devolitilized, Dry, 30% and
50% Moisture Particles
Mass Distribution of Particles
Douglas Fir
15 fps (4.5m/s)
1830ºF (1000ºC)
2% O2
20% MC
100%
Mass Distribution, %
80%
60%
Fraction Solid
Carryover
40%
Fraction Entrained
and Burned
20%
Fraction to Grade
0%
0.0 - 2.8
2.8 - 4.0
4.0 - 6.4
Particle Width (mm)
Mass Distribution of Douglas Fir Particles Which are Caught Overhead, Burned, or
Falling to the Grate as a Function of Particle Size
Anticipated Thermal Release Locations
• Design fuel (<25%
H2O & fine size)
• 60-70%
Suspension
• 10-20% grate
• Remaining output
fractioned from
grate area
• Design fuel (>45%
H2O & course size)
• 40-50%
Suspension
• 30-50% grate
• Remaining output
fractioned from
grate area
CFD Illustration of
Fractioning Rate from Grate
Fuel Particle Size
Fine Size
Course Size
Emission Comments
NOx & CO Emissions Influences
• NOx
–
–
–
–
Fuel sizing (direct effect)
OFA (direct effect)
Excess air (direct effect)
Fuel Distribution (direct
effect)
• CO
–
–
–
–
Fuel sizing (direct effect)
OFA (direct effect)
Excess air (direct effect)
Fuel Distribution (direct
effect)
Inverse Relationships with Excess Air
1.2
BIOMASS
1
Emissions (lbs/MMBtu)
0.8
NOx
CO
Linear (NOx)
Linear (CO)
0.6
0.4
0.2
0
0
1
2
3
4
5
6
Oxygen (blr wet)
Secondary Air Design
OFA Theory of Operation
• Utilize lower nozzle elevations at all steam loads
– Used to assist grate level fuel combustion
• Evaporate fuel moisture
• Volatize fuel
• Begin free burning of fuel carbon
• Utilize middle nozzle elevations based on flame height
– Decrease nozzle flow for lower steam loads
– Increase nozzle flow for higher steam loads
• Utilize upper nozzle elevations at higher steam loads.
– Control of thermal NOx
– Provide additional O2 for burning volatile gases (CO)
Thank You !!!

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