a presentation on furnaces and refractories by stead fast engineers

$: A presentation on furnaces and refractories by stead fast engineers$
A PRESENTATION ON FURNACES AND REFRACTORIES BY STEAD FAST ENGINEERS

http://www.steadfastengg.com/

http://www.steadfastengg.com/

OUR OBJECTIVES

IntroductionType of furnaces and refractory materialsAssessment of furnacesEnergy efficiency opportunities

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INTRODUCTION

Equipment to melt metals Casting Change shape Change properties

Type of fuel important Mostly liquid/gaseous fuel or electricity

Low efficiencies due to High operating temperature Emission of hot exhaust gases

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What is a Furnace?

INTRODUCTION

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Furnace Components

Furnace chamber: constructed of

insulating materials

Hearth: support or carry the steel.

Consists of refractory materials

Burners: raise or maintain chamber

temperature

Chimney: remove

combustion gases

Charging & discharging doors for loading & unloading stockCharging & discharging doors for loading & unloading stock

INTRODUCTION

Materials that Withstand high temperatures and sudden

changes Withstand action of molten slag, glass, hot

gases etc Withstand load at service conditions Withstand abrasive forces Conserve heat Have low coefficient of thermal expansion Will not contaminate the load

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What are Refractories:

INTRODUCTION

Refractory lining of a furnace arc

Refractory walls of a furnace interior with burner blocks

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Refractories

INTRODUCTION

Melting point Temperature at which a ‘test pyramid’ (cone)

fails to support its own weight Size

Affects stability of furnace structure Bulk density

Amount of refractory material within a volume (kg/m3)

High bulk density = high volume stability, heat capacity and resistance

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Properties of Refractories

INTRODUCTION

Porosity Volume of open pores as % of total refractory

volume Low porosity = less penetration of molten

material Cold crushing strength

Resistance of refractory to crushing Creep at high temperature

Deformation of refractory material under stress at given time and temperature

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INTRODUCTION

Pyrometric cones Used in ceramic industries

to test ‘refractoriness’ of refractory bricks

Each cone is mix of oxidesthat melt at specific temperatures

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• Pyrometric Cone Equivalent (PCE)• Temperature at which the refractory brick and

the cone bend• Refractory cannot be used above this temp

(BEE India, 2004)

INTRODUCTION

Volume stability, expansion & shrinkage Permanent changes during refractory service

life Occurs at high temperatures

Reversible thermal expansion Phase transformations during heating and

cooling

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INTRODUCTION

Thermal conductivity Depends on composition and silica content Increases with rising temperature

High thermal conductivity: Heat transfer through brickwork required E.g. recuperators, regenerators

Low thermal conductivity: Heat conservation required (insulating

refractories) E.g. heat treatment furnaces

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OBJECTIVE : STEAM


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TYPE OF FURNACES AND REFRACTORIES

Type of Furnaces Forging furnaces Re-rolling mill furnaces Continuous reheating furnaces

Type of Refractories Type of Insulating Materials

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Classification method Types and examples1. Type of fuel used Oil-fired

Gas-fired

Coal-fired

2. Mode of charging materials Intermittent / Batch

Periodical Forging Re-rolling (batch/pusher) Pot

Continuous Pusher Walking beam Walking hearth Continuous recirculating bogie furnaces Rotary hearth furnaces

3. Mode of heat transfer Radiation (open fire place)

Convection (heated through medium)

4. Mode of waste heat recovery

Recuperative

Regenerative 14

Classification Combustion Furnaces


Used to preheat billets/ingots Use open fireplace system with radiation

heat transmission Temp 1200-1250 oC Operating cycle

Heat-up time Soaking time Forging time

Fuel use: depends on material and number of reheats

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Forging Furnace


Box type furnace Used for heating up scrap/ingots/billets Manual charge / discharge of batches Temp 1200 oC Operating cycle: heat-up, re-rolling Output 10 - 15 tons/day Fuel use: 180-280 kg coal/ton material

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Re-rolling Mill Furnace – Batch type


Not batch, but continuous charge and discharge

Temp 1250 oC Operating cycle: heat-up, re-rolling Output 20-25 tons/day Heat absorption by material is slow,

steady, uniform

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Re-rolling Mill Furnace – Continuous pusher type


Continuous material flow Material temp 900 – 1250 oC Door size minimal to avoid air infiltration Stock kept together and pushed

Pusher type furnaces Stock on moving hearth or structure

Walking beam, walking hearth, continuous recirculating bogie, rotary hearth furnaces

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Continuous Reheating Furnaces


1. Pusher FurnacePushers on ‘skids’ (rails) with water-cooled

support push the stockHearth sloping towards discharge endBurners at discharge

end or top and/orbottom

Chimney with recuperator forwaste heat recovery

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2. Walking Beam FurnaceStock placed on stationary ridgesWalking beams raise the stock and move

forwardsWalking beams lower stock onto stationary ridges

at exitStock is removedWalking beams

return to furnaceentrance

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3. Walking Hearth FurnaceRefractory blocks extend through hearth

openingsStock rests on fixed refractory blocksStock transported

in small steps‘walking the hearth’

Stock removedat discharge end

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4. Continuous Recirculating Bogie FurnaceShape of long and narrow tunnelStock placed on bogie (cart with wheels) with

refractory hearthSeveral bogies

move like trainStock removed

at discharge endBogie returned

to entrance 22



5. Rotary Hearth FurnaceWalls and roof remain stationaryHearth moves in circle on rollersStock placed on hearthHeat moves in

opposite directionof hearth

Temp 1300oC

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Classification method Examples

Chemical compositionACID, which readily combines with bases Silica, Semisilica, Aluminosilicate

BASIC, which consists mainly of metallic oxides that resist the action of bases

Magnesite, Chrome-magnesite, Magnesite-chromite, Dolomite

NEUTRAL, which does not combine with acids nor bases

Fireclay bricks, Chrome, Pure Alumina

Special Carbon, Silicon Carbide, Zirconia

End use Blast furnace casting pit

Method of manufacture Dry press process, fused cast, hand moulded, formed normal, fired or chemically bonded, unformed (monolithics, plastics, ramming mass, gunning castable, spraying)

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Classification of Refractories


Common in industry: materials available and inexpensive

Consist of aluminium silicates Decreasing melting point (PCE) with

increasing impurity and decreasing AL2O3

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Fireclay Refractories

• 45 - 100% alumina• High alumina % = high refractoriness• Applications: hearth and shaft of blast furnaces,

ceramic kilns, cement kilns, glass tanks

High Alumina Refractories


>93% SiO2 made from quality rocks Iron & steel, glass industry Advantages: no softening until fusion point is

reached; high refractoriness; high resistance to spalling, flux and slag, volume stability

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Silica Brick

• Chemically basic: >85% magnesium oxide

• Properties depend on silicate bond concentration

• High slag resistance, especially lime and iron

Magnesite


Chrome-magnesite 15-35% Cr2O3 and 42-50% MgO Used for critical parts of high temp furnaces Withstand corrosive slags High refractories

Magnesite-chromite >60% MgO and 8-18% Cr2O3 High temp resistance Basic slags in steel melting Better spalling resistance 27

Chromite Refractories


Zirconium dioxide ZrO2 Stabilized with calcium, magnesium, etc. High strength, low thermal conductivity, not

reactive, low thermal loss Used in glass furnaces, insulating refractory

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Zirconia Refractories

• Aluminium oxide + alumina impurities• Chemically stable, strong, insoluble, high

resistance in oxidizing and reducing atmosphere• Used in heat processing industry, crucible shaping

Oxide Refractories (Alumina)


Single piece casts in equipment shape Replacing conventional refractories Advantages

Elimination of joints Faster application Heat savings Better spalling resistance Volume stability Easy to transport, handle, install Reduced downtime for repairs

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Monolithics


Material with low heat conductivity: keeps furnace surface temperature low

Classification into five groups Insulating bricks Insulating castables and concrete Ceramic fiber Calcium silicate Ceramic coatings (high emissivity coatings)

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Insulating Materials Classification


Consist of Insulation materials used for making piece

refractories Concretes contain Portland or high-alumina

cement Application

Monolithic linings of furnace sections Bases of tunnel kiln cars in ceramics industry

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Castables and Concretes


Thermal mass insulation materials Manufactured by blending alumina and

silica Bulk wool to make insulation products

Blankets, strips, paper, ropes, wet felt etc Produced in two temperature grades

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Ceramic Fibers


Low thermal conductivity Light weight Lower heat storage Thermal shock resistant Chemical resistance Mechanical resilience Low installation costs Ease of maintenance Ease of handling Thermal efficiency

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Ceramic FibersRemarkable properties and benefits

• Lightweight furnace• Simple steel fabrication

work• Low down time• Increased productivity• Additional capacity• Low maintenance costs• Longer service life• High thermal efficiency• Faster response


Emissivity: ability to absorb and radiate heat

Coatings applied to interior furnace surface: emissivity stays constant Increase emissivity from 0.3 to 0.8 Uniform heating and extended refractory life Fuel reduction by up to 25-45%

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High Emissivity Coatings


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High Emissivity Coatings

TRAINING AGENDA: STEAM


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ASSESSMENT OF FURNACES

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Heat Losses Affecting Furnace Performance

FURNACE

Flue gas

Moisture in fuel

Openings in furnace

Furnace surface/skin

Other losses

Heat inputHeat in stock

Hydrogen in fuel

FURNACE

Flue gas

Moisture in fuel

Openings in furnace

Furnace surface/skin

Other losses

Heat inputHeat in stock

Hydrogen in fuel


Parameters to be measured

Location of measurement

Instrument required

Required Value

Furnace soaking zone temperature (reheating furnaces)

Soaking zone and side wall

Pt/Pt-Rh thermocouple with indicator and recorder

1200-1300oC

Flue gas temperature In duct near the discharge end, and entry to recuperator

Chromel Alummel Thermocouple with indicator

700oC max.

Flue gas temperature After recuperator Hg in steel thermometer 300oC (max)

Furnace hearth pressure in the heating zone

Near charging end and side wall over the hearth

Low pressure ring gauge +0.1 mm of Wc

Oxygen in flue gas In duct near the discharge end

Fuel efficiency monitor for oxygen and temperature

5% O2

Billet temperature Portable Infrared pyrometer or optical pyrometer

- 38

Instruments to Assess Furnace Performance


Direct Method Thermal efficiency of furnace

= Heat in the stock / Heat in fuel consumed for heating the stock

Heat in the stock Q: Q = m x Cp (t1 – t2)

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Calculating Furnace Performance

Q = Quantity of heat of stock in kCal m = Weight of the stock in kg Cp= Mean specific heat of stock in kCal/kg oC t1 = Final temperature of stock in oC t2 = Initial temperature of the stock before it enters the furnace in oC


Direct Method - example Heat in the stock Q =

m x Cp (t1 – t2) 6000 kg X 0.12 X (1340 – 40) 936000 kCal

Efficiency = (heat input / heat output) x 100 [936000 / (368 x 10000) x 100 =

25.43% Heat loss = 100% - 25% = 75%

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Calculating Furnace Performancem = Weight of the stock = 6000 kg Cp= Mean specific heat of stock = 0.12 kCal/kg oC t1 = Final temperature of stock = 1340 oC t2 = Initial temperature of the stock = 40 oCCalorific value of oil = 10000 kCal/kgFuel consumption = 368 kg/hr


Indirect Method Heat lossesa) Flue gas loss = 57.29 %b) Loss due to moisture in fuel = 1.36 %c) Loss due to H2 in fuel = 9.13 %d) Loss due to openings in furnace= 5.56 %e) Loss through furnace skin = 2.64 %Total losses = 75.98 %Furnace efficiency =

Heat supply minus total heat loss 100% – 76% = 24%

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Typical efficiencies for industrial furnacesFurnace type Thermal efficiencies (%)

1) Low Temperature furnaces a. 540 – 980 oC (Batch type) 20-30

b. 540 – 980 oC (Continous type) 15-25

c. Coil Anneal (Bell) radiant type 5-7

d. Strip Anneal Muffle 7-12

2) High temperature furnaces

a. Pusher, Rotary 7-15

b. Batch forge 5-10

3) Continuous Kiln

a. Hoffman 25-90

b. Tunnel 20-80

4) Ovens

a. Indirect fired ovens (20 oC –370 oC) 35-40

b. Direct fired ovens (20 oC –370 oC) 35-40 42


TRAINING AGENDA: STEAM


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ENERGY EFFICIENCY OPPORTUNITIES

1. Complete combustion with minimum excess air

2. Proper heat distribution 3. Operation at the optimum furnace

temperature 4. Reducing heat losses from furnace openings 5. Maintaining correct amount of furnace draft 6. Optimum capacity utilization 7. Waste heat recovery from the flue gases 8. Minimize furnace skin losses 9. Use of ceramic coatings 10.Selecting the right refractories

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Importance of excess air Too much: reduced flame temp, furnace

temp, heating rate Too little: unburnt in flue gases, scale losses

Indication of excess air: actual air / theoretical combustion air

Optimizing excess air Control air infiltration Maintain pressure of combustion air Ensure high fuel quality Monitor excess air

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1. Complete Combustion with Minimum Excess Air


When using burners Flame should not touch or be obstructed No intersecting flames from different burners Burner in small furnace should face upwards

but not hit roof More burners with less capacity (not one big

burner) in large furnaces Burner with long flame to improve uniform

heating in small furnace46

2. Proper Heat Distribution


Operating at too high temperature: heat loss, oxidation, decarbonization, refractory stress

Automatic controls eliminate human error

Slab Reheating furnaces 1200oC Rolling Mill furnaces 1200oC

Bar furnace for Sheet Mill 800oC

Bogie type annealing furnaces 650oC –750oC 47

3. Operate at Optimum Furnace Temperature


Heat loss through openings Direct radiation through openings Combustion gases leaking through the openings Biggest loss: air infiltration into the furnace

Energy saving measures Keep opening small Seal openings Open furnace doors less frequent and shorter

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4. Reduce Heat Loss from Furnace Openings


Negative pressure in furnace: air infiltration

Maintain slight positive pressure Not too high pressure difference: air ex-

filtration

Heat loss only about 1% if furnace pressure is controlled properly!

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5. Correct Amount of Furnace Draft


Optimum load Underloading: lower efficiency Overloading: load not heated to right temp

Optimum load arrangement Load receives maximum radiation Hot gases are efficiently circulated Stock not placed in burner path, blocking

flue system, close to openings Optimum residence time

Coordination between personnel Planning at design and installation stage

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6. Optimum Capacity Utilization


Charge/Load pre-heating Reduced fuel needed to heat them in furnace

Pre-heating of combustion air Applied to compact industrial furnaces Equipment used: recuperator, self-

recuperative burner Up to 30% energy savings

Heat source for other processes Install waste heat boiler to produce steam Heating in other equipment (with care!) 51

7. Waste Heat Recovery from Flue Gases


Choosing appropriate refractories Increasing wall thickness Installing insulation bricks (= lower

conductivity) Planning furnace operating times

24 hrs in 3 days: 100% heat in refractories lost

8 hrs/day for 3 days: 55% heat lost

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8. Minimum Furnace Skin Loss


High emissivity coatings Long life at temp up to 1350 oC Most important benefits

Rapid efficient heat transfer Uniform heating and extended refractory life Emissivity stays constant

Energy savings: 8 – 20%

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9. Use of Ceramic Coatings


Selection criteria Type of furnace Type of metal charge Presence of slag Area of application Working

temperatures Extent of abrasion

and impact54

10. Selecting the Right Refractory

• Structural load of furnace

• Stress due to temp gradient & fluctuations

• Chemical compatibility

• Heat transfer & fuel conservation

• Costs

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Furnaces and Furnaces and RefractoriesRefractories

THANK YOUTHANK YOU

DISCLAIMERS AND REFERENCES

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• This PowerPoint training session was prepared as part of the project “Greenhouse Gas Emission Reduction from Industry in Asia and the Pacific” (GERIAP). While reasonable efforts have been made to ensure that the contents of this publication are factually correct and properly referenced, UNEP does not accept responsibility for the accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this publication. © UNEP, 2006.

• The GERIAP project was funded by the Swedish International Development Cooperation Agency (Sida)

• Full references are included in the textbook chapter that is available on www.energyefficiencyasia.org

a presentation on furnaces and refractories by stead fast engineers

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