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©2013 Detroit Stoker Company. All Rights Reserved

Design and Operation

Spreader Grate SystemsBob Morrow – Detroit Stoker

Red Lion Hotel

Port Angeles, WA

March 11-13, 2014

Western Regional

Boiler Association


• Administration, Sales, Engineering &

Manufacturing in Monroe, Michigan

• 85 Employees

• 19 North American Manufacturer Sale Reps

• 12 International Manufacturer Sales Reps

• Privately Owned

Detroit Stoker Company


• 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

Products & Services


• Spreader Fired Combustion Systems

• Mass Fired Combustion Systems

Solid Fuel Combustion Systems


Detroit’s Supply for Nippon

Fuel Distributors

Secondary Air

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


• 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

Basic Principal of Grate Operation


VCG Grate System


Stationary Frame


Vibrating Frame


Stationary Frame

Vibrating Frame

Grate Elements

Flex Straps

Grate Assembly


Grate Drive


Grate Drive7 ½ HP motor

Speed reduction

sheaves

Pilot Bearings

Eccentric shaft

Eccentric Bearing


Feed Rate Factor Vs Eccentric rpm

0

0.05

0.1

0.15

0.2

0.25

360 365 370 375 380 385 390 395 400 405 410 415

Eccentric Speed (rpm)

Feed

rate

facto

r

Nominal Operating

Range

Structural Design

Point

Grate Ash Discharge Capacity


Grate Speed

Too Slow Too Fast


Spreader Type Combustion Systems

Larger FuelSmaller Fuel


Initial

Combustion

Thoughts


Combustion


Biomass particle

Thermal Decomposition

Products:

Light Gases, Liquid HC’s, Tars

Charcoal

CO

Combustion Products:

CO2, CO, H2O, PICs

Air Air

Flyash or bottom ash

with unburned C

Distribution

between these

is fuel

dependent, but

typically more

volatile material

with biomass

Glowing Particles

“Sparklers”1 - 2 Seconds

0.1 - 0.3 Seconds

Biomass Fuels


Spreader Stoker Combustion -101


– 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 ‘Fine 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.

Spreader Combustion

Theory ‘Larger Fuel”


Maximum Entrained

Particle Size for Dry and

Devolitilized, Dry, 30% and

50% Moisture Particles

Fuel Particle Entrainment


0%

20%

40%

60%

80%

100%

0.0 - 2.8 2.8 - 4.0 4.0 - 6.4

Particle Width (mm)

Mass D

istr

ibu

tio

n,

%

Fraction Solid

Carryover

Fraction Entrained

and Burned

Fraction to Grade

Douglas Fir

15 fps (4.5m/s)

1830ºF (1000ºC)

2% O2

20% MC

Mass Distribution of Douglas Fir Particles Which are Caught Overhead, Burned, or

Falling to the Grate as a Function of Particle Size

Mass Distribution of Particles


• 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

Anticipated Thermal Release Locations


CFD Illustration of Fractioning Rate from Grate


Course Size

Fine Size

Fuel Particle Size


Emission Comments


• 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)

NOx & CO Emissions Influences


0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6

Oxygen (blr wet)

Em

issio

ns (

lbs/M

MB

tu)

NOx

CO

Linear (NOx)

Linear (CO)

Inverse Relationships with Excess Air

BIOMASS


Secondary Air Design


• 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)

OFA Theory of Operation


Thank You !!!

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