Efficiency and Design
Improvements in
Multiple Hearth & Fluid Bed
Incinerators
CBE Chavond-Barry Engineering
400 Rt. 518, Blawenburg, NJ 08504
Outline
• Fluid Bed Incinerators (FBI’s)
- Reversible Bed Resizing
- Air Preheating
• Multiple Hearth Incinerators (MHF’s)
- Reheat and Oxidize (RHOX) Process
- Flue Gas Recirculation (FGR)
- Center Shaft Air
• General
- Improved Dewatering
- Grease
Fluid Bed Incinerator
• Preheated Air
& Burner
• Windbox
• Dome
• Tuyeres
EXISTINGOIL GUNS EXISTINGSLUDGE NOZZLES
• Bed
TUYERE PIPES
BELLY BAND
• Freeboard
SKEWBACK BRICKS
REFRACTORY DOME
Fluid Bed Incinerator
• Preheated Air
& Burner
• Windbox
• Dome
• Tuyeres
EXISTINGOIL GUNS EXISTINGSLUDGE NOZZLES
• Bed
TUYERE PIPES
BELLY BAND
• Freeboard
SKEWBACK BRICKS
REFRACTORY DOME
Fluid Bed Incinerator
• Preheated Air
& Burner
• Windbox
• Dome
• Tuyeres
EXISTINGOIL GUNS EXISTINGSLUDGE NOZZLES
• Bed
TUYERE PIPES
BELLY BAND
• Freeboard
SKEWBACK BRICKS
REFRACTORY DOME
Fluid Bed Incinerator
• Preheated Air
& Burner
• Windbox
• Dome
• Tuyeres
EXISTINGOIL GUNS EXISTINGSLUDGE NOZZLES
• Bed
TUYERE PIPES
BELLY BAND
• Freeboard
SKEWBACK BRICKS
REFRACTORY DOME
Fluid Bed Incinerator
• Preheated Air
& Burner
• Windbox
• Dome
• Tuyeres
EXISTINGOIL GUNS EXISTINGSLUDGE NOZZLES
• Bed
TUYERE PIPES
BELLY BAND
• Freeboard
SKEWBACK BRICKS
REFRACTORY DOME
Fluid Bed Incinerator
• Preheated Air
& Burner
• Windbox
• Dome
• Tuyeres
EXISTINGOIL GUNS EXISTINGSLUDGE NOZZLES
• Bed
BED
TUYERE PIPES
BELLY BAND
• Freeboard
SKEWBACK BRICKS
REFRACTORY DOME
Fluid Bed Incinerator
• Preheated Air
& Burner
• Windbox
• Dome
• Tuyeres
EXISTINGOIL GUNS EXISTINGSLUDGE NOZZLES
• Bed
TUYERE PIPES
BELLY BAND
• Freeboard
SKEWBACK BRICKS
REFRACTORY DOME
Fluid Bed Incinerator
• Air enters
through bottom
• Sludge and fuel
introduced in the
bed
• Ash and exhaust
EXISTINGOIL GUNS
TUYERE PIPES
BED
EXISTINGSLUDGE NOZZLES
gasses exit
BELLY BAND
through the top
SKEWBACK BRICKS
REFRACTORY DOME
Oversized FBI
• High minimum
fluidizing air
required
• Inefficient at low
sludge feed rates
EXISTINGOIL GUNS EXISTINGSLUDGE NOZZLES
TUYERE PIPES
BELLY BAND
SKEWBACK BRICKS
REFRACTORY DOME
Bed Resizing
• Remove dome
• Shift bed downward
– Oil guns
– Sludge guns
– Instrumentation
• Shrink diameter of
bed
SLUDGE AND OIL NOZZLES
Bed Resizing
• Add over bed
burner
– Provides live flame
in freeboard
– Directed at top of
sand
– Can be used to heat
freeboard directly
SLUDGE AND OIL NOZZLES
Bed Resizing
• Pipe Tuyeres
– Set of parallel
pipes
– Holes in pipes to
distribute air
• External air
OVERBED BURNER
TUYERE MANIFOLD SLUDGE AND OIL NOZZLES
manifold
TUYERE PIPE SAND
TUYERE PIPE
CLEANOUT
• Pipe clean out
manifold
• Parallel pipe
tuyeres receive
through external
NEW 11'-3" BED DIAMETER
manifold
Reversible Bed
Resizing
• Bed can be
expanded
incrementally
back to original
size if needed
– Remove row of
OVERBED BURNER
bricks
TUYERE MANIFOLD SLUDGE AND OIL NOZZLES
TUYERE PIPE
TUYERE PIPE SAND
CLEANOUT
– Drill additional
holes in pipe
tuyeres
Pipe-Tuyere Design Considerations
• Fluidizing air
– Blower efficiency at lower air flow
– Heat exchanger bypass
• Pipe durability
– No shutdowns from lost tuyeres
– Can clean-out sand from pipes while operating
– Can still operate with broken pipe tuyere
• Over-bed burner
– Improved freeboard temperature control
– Live flame can reduce CO
– Better freeboard mixing
Fluid Bed Incinerator
• Preheated Air
& Burner
• Windbox
• Dome
• Tuyeres
EXISTINGOIL GUNS EXISTINGSLUDGE NOZZLES
• Bed
BED
TUYERE PIPES
• Freeboard
BELLY BAND
SKEWBACK BRICKS
REFRACTORY DOME
Preheating the Combustion Air
• Fluidizing air
– Older FBI designs incorporate no or very
low temperature, air preheating
– Preheating combustion air reduces fuel
required during operation
– Higher preheat temperatures = less
auxiliary fuel
– Often accomplished with a flue gas heat
exchanger
FGTT Heat Exchanger
Furnace exhaust
flows through
the inner tubes
of the heat
exchanger
preheating
fluidizing air
Preheating the Combustion Air
• Example:
- 5,000 SCFM, Preheated +1000°F
.25BTU/lbm°F*.075lbm/ft³*5000SCFM*1000°F
- 93,750 BTU/min, or < 40 gal/hr fuel oil
40 gal/hr*3$/gal*24hrs/day = $2880/day
Multiple Hearth Furnace
Multiple Hearth Furnace
• Refractory lined cylindrical steel shell
• Separated into a series of combustion
chambers - refractory hearths one above
the other
• Temperature and reaction environment
well controlled on each hearth
Multiple Hearth Furnace
• Dewatered sludge cake enters the
furnace at the top
• Inject air and fuel where needed to
maintain temperature and supplement
the combustion process
• Ash product exits the bottom
• Furnace exhaust gases exit at the
top and head to downstream air
processing
Multiple Hearth Furnace
• Dewatered sludge Cake enters the
furnace at the top
Heating & Drying Zone
• Inject air and fuel where needed to
maintain temperature and supplement
the combustion process
Combustion Zone • Ash product exits the bottom
• Furnace exhaust gases exit at the
top and head to downstream air
Ash & Cooling Zone processing
•Generalized to three processing
“zones”
Multiple Hearth Furnace
• Counter current flow of rising exhaust
gases and good mixing of descending
sludge ensure complete combustion
Multiple Hearth Furnace
Poor distribution of sludge
across top hearth:
- Under utilization of furnace
area, less efficient operation
- Uneven hearth temperatures
- Burning in lower hearths
Adding rabble improves
sludge distribution
RHOX – Reheat & Oxidize Process
• In NJ, all MHF are required to maintain an
afterburner at >1500°F
• Typical afterburner designs include:
– Top Hearth
– Top heath with Jumper Flue
– External Chamber
• Afterburners located directly after the
incinerator (before APC equipment)
RHOX – Reheat & Oxidize Process
• Traditional afterburner designs require 1 or
more burners
• Require high fuel usage to maintain
afterburner temperature
• Additional burners can produce NOx
RHOX – Reheat & Oxidize Process
• RHOX Process differs in that:
– Occurs after the APC equipment
– Recovers heat from exiting exhaust gasses
– Requires 1 burner (less potential Nox
production)
• Common RHOX process application is the
Regenerative Thermal Oxidizer (RTO)
Regenerative thermal Oxidizer
(http://www.thecmmgroup.com/custom-designed-regenerative-thermal-oxidizer-rto)
Regenerative Thermal Oxidizer
• RTO:
– Utilizes 2 or more heat recovery chambers
– Cold inlet gas passes through a heated chamber,
preheating the gas
– Hot exhaust exits through and heats another
chamber
– A single burner maintains gas temperature
within the RTO
– Periodically, a valve switches the inlet/outlet
chambers
Regenerative Thermal Oxidizer
• RTO benefits:
– More efficient that traditional afterburners
- The use of waste heat recovery decreases the
fuel requirements
– Provides more control than traditional
afterburners
- Less affected by furnace upsets / changes
Flue Gas Recirculation (FGR)
•Another efficiency
improvement for
MHF’s is Flue Gas
Recirculation
•FGR moves exhaust
gas from the feed (top)
hearth to a hearth below
the volatile burning
zone
Flue Gas Recirculation (FGR)
• FGR
– Injection of cooler mostly inert gas:
• Reduces fuel usage
• Increases operational stability
• Reduces slag formation
• Lowers hearth peak temps
lowering NOx production
• Reduces oxygen content
• Increases operational stability
• Reduces flare-up during feed stoppage
• Promotes complete ash burnout
• Better solids gas phase mixing
Flue Gas Recirculation (FGR)
• Hearth without FGR (Left) and with (Right)
Center Shaft Air
• Another way to reduce fuel usage in MHF’s
is by utilizing heated Center Shaft Air
• The Center shafts and rabble arms are air
cooled
• Heated center shaft air can be:
– Injected into the stack for steam plume
suppression & increased dispersion
– Utilized as burner air supply or furnace
combustion air to decrease fuel usage
Improved Dewatering
• Typical Sludge Cake
– Belt filter press: <21-25% solids
– Centrifuge: 27-30% solids
~75% water
• Why does this matter?
– Heating Value of Water = 0
– Water requires a large heat input to vaporize
∆Hvap = -1059 BTU/lb
• More water = More auxiliary fuel
Improved Dewatering
• At low moisture content, sludge can burn
without the addition of fuel oil (Autogenous).
• Typically at >26% for a Fluid Bed
• Super-Autogenous conditions limit operations
Aux. Fuel Usage
Max. Feed Rate
% Solids % Solids
Fat, Oil & Grease
• Fat, Oil, and Grease are waste-products
from the restaurant industry
• Consists of some food debris, mostly
cooking oils & fats, and ~96% water
• Often concentrated to <50% water before
added to an incinerator
• Grease can be used to supplement auxiliary
fuel (sometimes up to 100% during
operation)
Fat, Oil & Grease
• No petroleum products or other hazardous
materials found in grease
• Non-processed fuel (concentrating aside)
• Restaurants typically pay a tipping fee for
removal and disposal
• With current fuel prices, R.O.I. for a grease
receiving/handling facility can be less than
3 years with tipping fees or 6 years without
Questions?
Chavond-Barry Engineering Corp.
400 County Route 518
Blawenburg, NJ 08504
Tel: (609) 466-4900
CBE Fax: (609) 466-1231
