Furnaces and Refractories

$Page 1: Furnaces and Refractories$
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Training Session on Energy Training Session on Energy EquipmentEquipment

Furnaces and Furnaces and RefractoriesRefractories

Presentation from the

“Energy Efficiency Guide for Industry in Asia”

www.energyefficiencyasia.org

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Training Agenda: SteamTraining Agenda: Steam

Introduction

Type of furnaces and refractory materials

Assessment of furnaces

Energy efficiency opportunities

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IntroductionIntroduction

• 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

What is a Furnace?

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Furnace Components

(The Carbon Trust)

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

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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

What are Refractories:

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Refractories

Refractory lining of a furnace arc

Refractory walls of a furnace interior with burner blocks(BEE India, 2005)

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• 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

Properties of Refractories

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• 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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• Pyrometric cones• Used in ceramic industries

to test ‘refractoriness’ of refractory bricks

• Each cone is mix of oxidesthat melt at specific temperatures


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

the cone bend

• Refractory cannot be used above this temp

(BEE India, 2004)

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• 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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• 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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Introduction




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Type of Furnaces and RefractoriesType 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 Combustion FurnacesClassification method Types and examples

1. 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

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• 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

Forging Furnace

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• 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

Re-rolling Mill Furnace – Batch type

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• 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

Re-rolling Mill Furnace – Continuous pusher type

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• 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

Continuous Reheating Furnaces

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1. Pusher Furnace• Pushers on ‘skids’ (rails) with water-cooled

support push the stock

• Hearth sloping towards discharge end

• Burners at discharge end or top and/orbottom

• Chimney with recuperator forwaste heat recovery

(The Carbon Trust, 1993)


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2. Walking Beam Furnace• Stock placed on stationary ridges

• Walking beams raise the stock and move forwards

• Walking beams lower stock onto stationary ridges at exit

• Stock is removed

• Walking beams return to furnaceentrance



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3. Walking Hearth Furnace• Refractory blocks extend through hearth

openings

• Stock rests on fixed refractory blocks

• Stock transportedin small steps‘walking the hearth’

• Stock removedat discharge end



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4. Continuous Recirculating Bogie Furnace• Shape of long and narrow tunnel

• Stock placed on bogie (cart with wheels) with refractory hearth

• Several bogies move like train

• Stock removedat discharge end

• Bogie returnedto entrance



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5. Rotary Hearth Furnace• Walls and roof remain stationary

• Hearth moves in circle on rollers

• Stock placed on hearth

• Heat moves in opposite directionof hearth

• Temp 1300oC



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Classification of Refractories

Classification method Examples

Chemical composition

ACID, 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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• Common in industry: materials available and inexpensive

• Consist of aluminium silicates

• Decreasing melting point (PCE) with increasing impurity and decreasing AL2O3

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

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• >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

Silica Brick

• Chemically basic: >85% magnesium oxide

• Properties depend on silicate bond concentration

• High slag resistance, especially lime and iron

Magnesite

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• 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

Chromite Refractories

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• Zirconium dioxide ZrO2

• Stabilized with calcium, magnesium, etc.

• High strength, low thermal conductivity, not reactive, low thermal loss

• Used in glass furnaces, insulating refractory

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)

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• 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

Monolithics

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• 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)

Insulating Materials Classification

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• 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

Castables and Concretes

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• 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

Ceramic Fibers

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• 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

Ceramic Fibers

Remarkable 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

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• 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%

High Emissivity Coatings

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High Emissivity Coatings

(BEE India, 2005)

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Introduction




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Assessment of FurnacesAssessment of Furnaces

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

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Instruments to Assess Furnace Performance

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

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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)

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

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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%


m = 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

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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 furnaces


Furnace 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

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Introduction




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Energy Efficiency OpportunitiesEnergy 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

1. Complete Combustion with Minimum Excess Air

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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 furnace

2. Proper Heat Distribution

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

• Automatic controls eliminate human error

3. Operate at Optimum Furnace Temperature

Slab Reheating furnaces 1200oC

Rolling Mill furnaces 1200oC

Bar furnace for Sheet Mill 800oC

Bogie type annealing furnaces 650oC –750oC

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• 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

4. Reduce Heat Loss from Furnace Openings

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• 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!

5. Correct Amount of Furnace Draft

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• 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

6. Optimum Capacity Utilization

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• 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!)

7. Waste Heat Recovery from Flue Gases

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• 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

8. Minimum Furnace Skin Loss

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• 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%

9. Use of Ceramic Coatings

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Selection criteria

• Type of furnace

• Type of metal charge

• Presence of slag

• Area of application

• Working temperatures

• Extent of abrasion and impact

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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Training Session on Energy Training Session on Energy EquipmentEquipment

Furnaces and Furnaces and RefractoriesRefractories

THANK YOUTHANK YOU

FOR YOUR ATTENTIONFOR YOUR ATTENTION

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Disclaimers and ReferencesDisclaimers and References

• 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

Documents

Furnaces and Refractories