furnaces and refractories.ppt

7/15/2019 Furnaces and refractories.ppt

1/68

1

Imbaba Aviation Institute

Mechanical Power Department, 4th YEAR

Fall Semester 2010/2011

- Introduction to Combustion systems

- Definition of combustion efficiency andfactors affecting it.

- Methods of energy conservation incombustion systems.

- Control systems in combustion.

- Waste heat recovery.

- Performance control of various systems.


2/68

2

Energy Management

What Is Energy Management?

The use of Engineering and Economicprinciples to CONTROL the cost of energy

to provide needed services in buildings andindustries.


3/68

3

Energy Management

NEED FOR ENERGY MANAGEMENT

IMPORTANT REASONS:

1. ENVIRONMENTAL QUALITY

2. ECONOMIC COMPETITIVENESS3. REDUCE COSTSAND CREATE JOBS

4. ENERGY SECURITY

5. CORPORATE REQUIREMENT


4/68

4

DEFINITIONS

ENERGY: the capacity of doing work

Thermal, Electromagnetic, Nuclear,

Mechanical, Chemical, etc.ENERGY CONSERVATION LAW

Energy is transformed from one form to

another and the total amount of energy

remains the same.


5/68

5

DEFINITIONS

EFFICIENCY is the ratio of the output of asystem in relation with its input.

MOTORS a device that converts electrical

energy into mechanical energy.

GENERATOR converts mechanical energy into

electrical energy.

TRANSFORMER- Is a device that converts AC

electric energy at one voltage level to an AC electric energyat another voltage level. They are classified as step-up or

step-down transformers depending of the function theyare being used for.


6/68

6

DEFINITIONS

POWER FACTOR : is the ratio of the total

power produced between the power used.

PF = COS

KVA

KW

KVAR


7/68

7

DEFINITIONS

COGENERATION is the sequentialproduction of thermal and electric energyfrom a single fuel source.

Heat, that would normally be lost, is recoveredin the production of one form of energy. Theheat is then used to generate the secondform of energy.


8/68

8

HOW& WHY ENERGY CONSERVATION

HOW?

Energy Audits

Fuel Switching

Electric Rate Structures

Electrical System Utilization

PF Correction

Lighting Improvements

Motors And Applications

Insulation HVAC Improvements

Waste Heat Recovery; Cogeneration, ETC.


9/68

9

ENERGY AUDITS

ENERGY AUDITS

An Energy Audit (or Energy Survey) is a study of how energy isused in a facility and an analysis of what alternatives could beused to reduce energy costs.

This process starts by collecting information of the facilitysoperation and about its past record of utility bills. This data isthen analyzed to get a picture of how the facility uses ( andpossibly wastes) energy, and identify

ECOs (Energy Conservation Opportunities).


10/68

10

COMBUSTION EFFICIENCY

In any closed combustion system suchas a boiler, we can measure preciselywhat occurred at the burner bycarefully measuring the exhaust.

The goal is to be able to carefully

control the fuel and airflow to ensurethe complete and efficient combustion.

We will see why excess air is importantand why too much excess air isexpensive.


11/68

11

SAVINGS

% SAVINGS IN FUEL= (New Eff. Old Eff.)/New Eff.

Savings = (% Savings)(Fuel consumption)


12/68

12

SAVINGS

Example 1 Last year a 20 x 106 BTU/HR boiler consumed

19000 MCF of natural gas at $4.00/MCF. The

boiler operates at 6% O2 and 700 F STR.What is the saving to correcting that to

3% O2 ?


13/68

13

SAVINGS

As can be seen later

Eff 1. = 74.5% Eff. 2 = 77%

% Savings = (77 74.5)/77 = 3.2 %

$ Savings = (3.2%) [ 19,000 MCF][$ 4.00]

= $ 2,500 / YR


14/68

14

Introduction to Furnaces

Introduction

Type of furnaces and refractory

materials

Assessment of furnaces

Energy efficiency opportunities


15/68

15

MAIN COMPONENTS OF COMBUSTION SYSTEM

There are six components that may be important in industrialcombustion processes load itself, a combustor, heat recovery

device flow control system air pollution control system.

Schematic of an industrial combustion process.


16/68

16


17/68

17

FURNACES


18/68

18

COMBUSTION PRINCIPLES

Combustion chemistry

In practice, since combustion conditions are never ideal.

The actual quantitydepends on many factors, such as fueltype and composition, furnace design, firing rate, and thedesign and adjustment of the burners stoichiometricrequirement industrial processes.


19/68

19

Theother speciesdepends on what oxidizer is used andwhat is the ratio of the fuel to oxidizer is air nearly 79% N2

by volume. If the combustion is fuel rich, If the combustionis fuel lean.

Figure 18. Stoichiometric Air

Requirements for Combustion


20/68

20

Unburned hydrocarbons

Fuel was not fully combusted

Fuel properties: Heating value of the fuel either the higherheating value (HHV) lower heating value (LHV)excludes the heat ofvaporization.

The stoichiometry or mixture ratio in industry is as follows:

The mixture ratio :


21/68

21

Where SP is the stoichiometric ratio for theoretically perfect

combustion . fuel-rich combustion of CH4, S2 < 9.52. For thefuel-lean combustion of CH4, S2 > 9.52. Using the above definition forthe mixture ratio,

1.0 for fuel-lean flames.


22/68

22

Combustion properties

Combustion products

The oxidizer composition, mixture ratio, air and fuel preheattemperatures, and fuel composition.


23/68

23

An adiabatic process means that no heat is lost during the reaction,or that the reaction occurs in a perfectly insulated chamber.

An equilibrium process means that there is an infinite amount of timefor the chemical reactions to take place.

FLAME TEMPERATURE


24/68

24

FLAME TEMPERATURE

The actual flame temperature is lower than the adiabaticequilibrium flame temperature due to imperfect combustion andradiation from the flame.

A highly luminous flame generally has a lower flametemperature than a highly non-luminous flame. The actual flametemperature will also be lower when the load and the walls are

more radiatively absorptive.

The flame temperature is a critical variable in determining theheat transfer from the flame to the load.


25/68

25

Oxidizer and fuel composition

The flame temperature increases significantly when air isreplaced with oxygen


26/68

26


27/68

27

Nearly all industrial combustion applications are run at fuel-leanconditions to ensure that the CO emissions are low.

NOx emissions are also maximized since NOx increasesapproximately exponentially with gas temperature.


28/68

28

1. Point A:2C + O2 2 CO + heat

2. Point B:2CO + O2 2 CO2 + heat

3. Point C CO to have reached a low level.

A small amount of oxygen

4. To achieve .complete. combustion, a small amount of air

must be added over. Point D. At this point, the CO 2

level reaches a peak (typically around 15- 16 percent foroil fuels, and 11-12 percent for natural gas).

5. Point E, oxygen level builds towards 20.9 percent.

STACK GAS COMPOSITION


29/68

29

BURNER TESTING:

Operating parameters, pollutant

emissions, flame dimensions,

heat flux profile, safetylimitations, and noise data heat

release range of the burner.

Turndown is defined as the ratio

of maximum heat release to

minimum heat release:


30/68

30

An operator also needs to know the point at which a burnerwill become unstable if fired below the minimum heat release

absolute minimum the combustion air settings can bedetermined through testing to ensure the efficient operation.

The emissions of pollutants such asNOx ,CO, and unburnedhydrocarbons (UHC).When firing burners on a wide varietyof fuels, flame dimensions can change, depending on the fuel

fired.

Another valuable piece of data that can be collected is noise.

API 535 gives some good guidelines for specifications anddata required for burners used in fired


31/68

31


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:


32/68

32


Refractories

Refractory lining of a

furnace arc

Refractory walls of afurnace interior with

burner blocks

(BEE India, 2005)


33/68

33


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


34/68

34


Porosity Volume of open pores as % of total refractory

volume

Low porosity = less penetration of moltenmaterial

Cold crushing strength Resistance of refractory to crushing

Creep at high temperature Deformation of refractory material under

stress at given time and temperature



35/68

35


Pyrometric cones Used in ceramic industries

to test refractoriness ofrefractory bricks

Each cone is mix of oxidesthat melt at specifictemperatures


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

the cone bend

Refractory cannot be used above this temp

(BEE India, 2004)


36/68

36


Volume stability, expansion &

shrinkage

Permanent changes during refractory service

life

Occurs at high temperatures

Reversible thermal expansion

Phase transformations during heating andcooling



37/68

37


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


T f F d R f t i


38/68

38

Type of Furnaces and Refractories

Classification of Refractories

Classification method Examples

Chemica l compos i t ion

ACID, which readily combines with bases Silica, Semisilica, Aluminosilicate

BASIC, which consists mainly of metallicoxides that resist the action of bases

Magnesite, Chrome-magnesite, Magnesite-chromite, Dolomite

NEUTRAL, which does not combine withacids nor bases

Fireclay bricks, Chrome, Pure Alumina

Special Carbon, Silicon Carbide, Zirconia

End use Blast furnace casting pit

Method o f manufacture Dry press process, fused cast, handmoulded, formed normal, fired or chemicallybonded, unformed (monolithics, plastics,ramming mass, gunning castable, spraying)

T f F d R f t i


39/68

39


Common in industry: materials available andinexpensive

Consist of aluminium silicates

Decreasing melting point (PCE) with increasingimpurity 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

T f F d R f t i


40/68

40


>93% SiO2 made from quality rocks

Iron & steel, glass industry

Advantages: no softening until fusion point isreached; 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

T f F d R f t i


41/68

41


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

T f F d R f t i


42/68

42


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, highresistance in oxidizing and reducing atmosphere

Used in heat processing industry, crucible shaping

Oxide Refractories (Alumina)

T f F d R f t i


43/68

43


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

T f F d R f t i


44/68

44


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

T pe of F rnaces and Refractories


45/68

45


Consist of

Insulation materials used for making piece

refractories

Concretes contain Portland or high-aluminacement

Application

Monolithic linings of furnace sections

Bases of tunnel kiln cars in ceramics

industry

Castables and Concretes



46/68

46


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



47/68

47


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



48/68

48


Emissivity: ability to absorb and

radiate heat

Coatings applied to interior furnacesurface:

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



49/68

49


High Emissivity Coatings

Assessment of Furnaces


50/68

50


IntroductionType of furnaces and refractory

materials





51/68

51


Heat Losses Affecting Furnace Performance

FURNACE

Flueg

as

Moist

ureinfuel

Openingsinfurnace

Furna

cesurface/skin

Other

losses

Heat inputHeat in stock

Hydro

geninfuel

FURNACE

Flueg

as

Moist

ureinfuel

Openingsinfurnace

Furna

cesurface/skin

Other

losses

Heat inputHeat in stock

Hydro

geninfuel



52/68

52


Instruments to Assess Furnace Performance

Parameters

to be measured

Location of

measurement

Instrument

required

Required

Value

Furnace soaking zonetemperature (reheatingfurnaces)

Soaking zone and side wall Pt/Pt-Rh thermocouple withindicator and recorder

1200-1300oC

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

Chromel AlummelThermocouple with indicator

700oC max.

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

Furnace hearth pressure inthe heating zone

Near charging end and sidewall over the hearth

Low pressure ring gauge +0.1 mm of Wc

Oxygen in flue gas In duct near the dischargeend

Fuel efficiency monitor foroxygen and temperature

5% O2

Billet temperature Portable Infrared pyrometer or opticalpyrometer

-



53/68

53


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 kCalm = Weight of the stock in kgCp= Mean specific heat of stock in kCal/kg oCt1 = Final temperature of stock in oCt2 = Initial temperature of the stock before it enters thefurnace in oC



54/68

54


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 thestock = 6000 kgCp= Mean specificheat of stock = 0.12kCal/kg oCt1 = Final temperatureof stock = 1340 oC

t2 = Initial temperatureof the stock = 40 oCCalorific value of oil =10000 kCal/kgFuel consumption =368 kg/hr



55/68

55


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%




56/68

56


Typical efficiencies for industrial furnaces


Furnace type Thermal efficiencies (%)

1)Low Temperature furnaces

a. 540 980 oC (Batch type) 20-30

b. 540 980o

C (Continous type) 15-25c. 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 oC370 oC) 35-40

b. Direct fired ovens (20o

C370o

C) 35-40

Energy Efficiency Opportunities


57/68

57


IntroductionType of furnaces and refractory

materials





58/68

58


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 coatings10.Selecting the right refractories



59/68

59


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



60/68

60


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



61/68

61


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 650oC750oC



62/68

62


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



63/68

63


Negative pressure in furnace: air

infiltration

Maintain slight positive pressure

Not too high pressure difference: air

ex-filtration

Heat loss on ly abou t 1% if furnace

pressure is con tro l led proper ly !

5. Correct Amount of Furnace Draft



64/68

64


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



65/68

65


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



66/68

66


Choosing appropriate refractories

Increasing wall thickness

Installing insulation bricks (= lowerconductivity)

Planning furnace operating times

24 hrs in 3 days: 100% heat in refractorieslost

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

8. Minimum Furnace Skin Loss



67/68

67

gy y pp

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



68/68

gy y pp

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 & fuelconservation

Costs

furnaces and refractories.ppt

Documents

energy survey

energy managementneed

energy managementwhat

acelectric energy

energy costs

secondform of energy

energy security5

energy audits fuel