HEAT LOAD ANALYSIS FOR OPTIMAL USE OF CUPULA FURNACE … · 2019-03-03 · iron for cupola operations are metal scraps, wrought iron, steel, cast iron, and bronzes. Essentially the

European Journal of Mechanical Engineering Research

Vol.6, No.1, pp.32-43, April 2019

Published by European Centre for Research Training and Development UK (www.eajournals.org)

32

Print ISSN: ISSN 2055-6551(Print), Online ISSN: ISSN 2055-656X(Online)

HEAT LOAD ANALYSIS FOR OPTIMAL USE OF CUPULA FURNACE IN IRON

CASTINGS AND STEEL MANUFACTURING

Tobinson A. Briggs1 and Shadrack Mathew Uzoma2

1,2Department of Mechanical Engineering, University of Port Harcourt,

Port Harcourt, Rivers State

ABSTRACT: This study is gear towards generating optimization models culminating in

the improved thermal performance of the cupola furnace. The cupola furnace handles

almost 90% of pig iron produced from the blast furnace. Raw materials ordering than pig

iron for cupola operations are metal scraps, wrought iron, steel, cast iron, and bronzes.

Essentially the copula is a cylindrical steel shell lined with refractory materials. It is a

high point of emphasis that over the years a lot of innovative techniques had been

introduced geared towards optimal throughput delivery of cupola furnace systems —

cupola furnaces designed with performance efficiencies in the range of 30% to 50%. In

this paper attainment of improved optimal heat load, thermal performance would revolve

around the critical thickness of insulation, the nature of refractory material, shell

thickness, the calorific value of coke and the quantity and temperature of the hot air blast.

This consideration is to ensure maximum heat conservation and recovery for optimal

throughput delivery.

KEYWORDS: Heat Optimization, Cupola operation, Performance efficiencies, Critical

thickness, optimal throughput delivery, and heat load analysis

INTRODUCTION

The bedrock of any nation towards industrial and technological advancement hovers

around the well-articulated base for iron and steels technology. Nigeria as a country richly

endowed with vast reserves of iron ore. Broad and all-inclusive facilities built in the

Ajaokuta Steel Complex. The complex has integrated Blast Furnaces for production of

liquid iron. The molten liquid iron cast in moulds pig-like in shape, then the derivative of

the name pig iron. A cupola, regarding geometric structure, is a cylindrical vertical steel

shell lined at the inner wall with refractory materials. Over the years the cupola had been

the first furnace for melting scrap metals, cast iron, steel, bronzes and pig iron for the

production of iron and steel castings [1].

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The thermal performance of the cupola furnace is highly dependent on the available heat in

the heating space. Given this critical factor, the design and construction of cupola ensure

maximum heat conservation and recovery in the workspace. Optimization of furnace lining

thickness by determination of the essential radius of insulation and experimenting with

different grades of refractory lining materials are approaches to improving performance

efficiency of heating furnaces, even the cupola [2,3,4,5,6].

A typical cupola could melt 15 tons of pig iron per hour, of composition Carbon (3.5%),

Silicon (2.2%), Manganese (0.2%), Phosphorus (0.7%), Iron (remaining %) and 5 tons of

scraps containing Carbon (3%), Silicon (8%), Manganese (11%) and Phosphorus (0.2%).

During melting process, 20% of total Silicon charged, 15% of total Manganese charged,

1% of total iron charged and 5% of total Carbon charged are oxidized; while 19% of

carbon of the coke is absorbed [7]. The fluxing agent limestoneCaCO3) is supplied in

enough quantity to provide 30% CaO in the slag. The quality of coke is 92% Carbon and

8% Silicon dioxide (SiO2). The Weight of coke in the charge is 1/9 of the total weight of

pig iron and scraps [8].

The chief advantage of the cupola is that it melts iron with a lesser quantity of fuel

comparable with any other furnace(s). It can be run intermittently with no higher risk of

the problem of thermal expansion on heating and cooling. Low-grade fuels that could not

apply to other furnaces could be adaptable to cupola furnace.

RESEARCH SIGNIFICANCE

Recycling our abundant scraps resources in a manner amenable to the high rate of

production, while operating at optimal throughput condition called for the development of

mathematical models for improvement on the thermal performance efficiency of a cupola

furnace.



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34


MODEL FORMULATION

The following valid assumptions made in the process of formulating the combustion and

heat load analysis for the cupola furnace:

(i) The coke is completely consumed to produce carbon monoxide (CO) and carbon

dioxide (CO2) at a combustion temperature of 16000C.

(ii) The models for the heat loads derived at steady state. The implication being that no

system parameter is time-dependent.

(iii) The models developed subject to the fact that the cupola is cylindrical in structure.

Based on the concept of the cylindrical coordinate system, the temperature at the center of

the cupola, that is at r=0, is infinite; but it is assumed to be the temperature at which solid

iron changes to molten iron.

(v) The heat transfer at interfacial boundary zones is considered insignificant; hence the

heat content of the different zones should be constant.

The cupola furnace is a cylindrical piece of equipment for processing pig iron and scraps

iron to produce iron castings for structural purposes. The schematic of the cupola is as

depicted in Figure 1.

3

1 2

3

1 2 Flux

Coke Metal

Spark arrester

Charging door

Steel shell

Refractory lining

Hollow box

Air blast inlet

Tuyeres

Drum bottom

Sand bottom

Tapping hole

Slag hole

Stack Zone

Preheating Zone

Melting Zone

Combustion Zone

Well

Reducing Zone Coke

Melt

Fig. 1: Cupola Furnace Nomenclature

Schematically, the temperature distribution through the walls of the cupola is as in Figure

2 below.



Vol.6, No.1, pp.32-43, April 2019


35


r(m

Tm

T1

T2 T3

Ta

Tf1

T(K)

Tm T1 T2 T3

Q

Ambient Ambient

T1 T2 T3 Ta

Fig. 2: Temperature Distribution Curve

At steady state, the heat flux, rq̂ through the system remains constant. The flux remains

constant through the furnace workspace, lining, and the steel shell. Heat fluxr

q , flowing

radially through the system expressed as [9]:

rLA

Where

r

TAkq

r

rr

2

,

1ˆ1

The heat transfer rate through the furnace lining could be expressed as:



Vol.6, No.1, pp.32-43, April 2019


36


2

2

ln

2

21

21

2

1

1

2

212

1

2

1

1

1

1

2

1

2

1

Lk

r

rn

TTTT

r

r

Lkq

TLkrr

q

TLkrr

q

t

TrLkq

r

T

T

r

rr

r

r

The rate of transfer through the steel shell given as:

3

2

22ln

2

2

3

2323

3

2

Lk

r

rn

TTTT

r

r

Lkqss

Heat transfer through the refractory material is given as:

4

2

ln

2

2

1

23

1212

1

2

2

Lk

r

rn

TTTT

r

r

Lkqrm

Let the convective heat transfer coefficient of the melt and the surrounding air is hm and ha

respectively. Heat transfer by convection to the internal surface of the furnace lining

represented as:

5

2

12ˆ

1

111

Lhr

TTTTLrhq

m

mmmmr

The heat transfer rate from the surface of the steel shell to the surroundings by convection

expressed as:

6

2

1

32ˆ

3

333

Lhr

TTTTLrhq

a

aaasa

At steady state,



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37


71

2

1

2

1

2

2ln

2

ln3

3

1

1

2

3

23

1

1

2

12

Lhr

TT

Lhr

TT

Lk

r

r

TT

Lk

r

r

TTq

a

a

m

m

r

Generally,

8tan

ˆ

th

Overallr

R

T

cesresisthermalofSummation

differencepotentialThermalq

Re-arranging equations 3, 4, 5 and 6, the expression for the inter-boundaries temperatures

T1, T2, and T3 are derived:

11lnln

100/ln/ln/lnln

9

2/31322212/31

3122212223112/31

313311

mmm

aammam

TrrhrTkTkTrrhr

TrrkTrrkrrkTrrk

ThrThrThrThr

Where,

rq̂ -heat transfer rate per unit mass (Watts)

rA -the internal and external surface area of the lining and the steel shell (m2)

k1—thermal conductivity of the furnace lining W/mK)

k2—thermal conductivity of the steel shell K)

hm—convective heat transfer coefficient of the hot melt (W/mK)

ha—convective heat transfer coefficient of the surroundings (W/mK)

Rth—thermal resistance (K/W)

OverallT -thermal potential difference (K)

r—radial positions from the center of the furnace (m)

L—height of the furnace (m)

T1—the temperature at the inner surface of the lining (K)

T2—interfacial temperature between the lining and the steel shell K)

T3—temperature at the external surface of the steel shell (K)

Tm—temperature at the center of the furnace (K)

Ta—ambient temperature (K)

Thermal resistance analogy of the system depicted as follows:



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38


ahr12

1

Lrk

rr

2

23

2

/ln

Lrk

rr

1

12

2

/ln

mhr12

1

12

12lnln

1

2

2

1

2

2ln

2

ln

2

1

32

3

1

1

2

132

3

1

1

2

1 am

am

am

amr

hrk

r

r

k

r

r

hr

TTL

LhrLk

r

r

Lk

r

r

Lhr

TTq

The above expression represents the rate of heat flow from the surface of the steel shell to

the surrounding air.

The heat load of the system per day represented as:

13ˆˆexhaustpairaairhotairpairaafluxnpcokenpironp

exhaustHOTAIRRZrfluxercokeironr

TCVTCVTmcTmcTmc

QQQQQQQ

Where,

Q -the thermal load of the system (Watts)

ironQ -thermal load of iron (Watts)

cokeQ -thermal load of coke (Watts)

f luxQ -thermal load of flux (Watts)

air -the density of air (kg/m3)

rsV -volumetric of air required per heat (m3)

T -a temperature difference of 1ron, coke and the melt (K)

hotaTir -temperature difference of the hot air blast and the surroundings (K)

A cupola or cupolette with the capacity of refining one ton of iron per heat per day has

iron-coke –flux ratio of 0.906 : 0.091 : 0.00362. On mass basis of one ton, the ratio in

kilogram becomes 906 : 91 : 3.62 [10]. The heat load expression in Equation 13 re-

expressed as:

14ˆˆ62.391906 exhaustaairhotairaafluxnpcokenpironpr TVTVTCTCTCQ

Performing heat balance, Equation 8 is equal to Equation 10. If the value of the ambient

Tfa Tm T1 T2 T3

qr



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39


temperature at infinity was fixed, as well as the temperature at the center of the system at

r=0, being Tc cupola height, L, could be determined.

The different zones of the cupola being, the stack, the preheat zone, melting zone, reducing

zone, a combustion zone and the well represented as H1, H2, H3, H4, H5, and H6. The

dimensional relationships and temperature distribution in the different zones, while the

overall cupola height, H given as six meters represented as follows [10] :

CmHWELL

CmHmZPONECOMBUSTION

CmHmZONEREDUCING

CmHmZONEMELTING

CmHmZONEPREHEAT

CeTemperaturmHSTACK

0

0

0

0

0

0

1500",666.5

1850",6.552.5

1200",2.545

1600",534

110",421

600,110,

The reactions in the reducing zone are endothermic.

The average temperatures of the zones over their interfacial boundaries given as:

15

1472281500

1822281850

1578281600

1078281100

38028400

0

5

0

4

0

3

0

2

0

1

CT

CT

CT

CT

CT

H

H

H

H

H

Where,

H1—stack zone height (m)

H2—preheat zone height (m)

H3—melting zone height (m)

H4—reducing zone height (m)

H5—combustion zone height (m)

H6—well zone height (m)

1HT -the temperature difference of the hot effluent gases in the stack zone (K)

2HT -the temperature difference in the preheat zone (K)

3HT -the temperature difference in the melting zone (K)

4HT -the temperature difference in the reducing zone (K)

5HT -the temperature difference in the combustion zone (K)



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6HT -the temperature difference in the well region (K)

Performing heat balance in the different zones:

(i) Stack zone

161HpairrsairSZ TCVQ

(ii) Preheat zone

1762..391906 2HpfluxpironSZ TCCpcokeCQ

(iii) Melting zone

1862.391906 3HpfluxpironPHZ TCCpcokeCQ

(iv) Reducing zone

1962.391906 4HpfluxpironRZ TCCpcokeCQ

(iv) Combustion zone

2062.391906 4HpfluxpironCZ TCCpcokeCQ

The net amount of heat to melt a ton of the charge expressed as

21EXHAUSTHOTAIRRZCZMZPHZNET QQQQQQQ

Where,

SZQ --heat content per unit time of the flue gases in the stack zone (J)

PHZQ --heat content per unit time in the preheat zone (J)

MZQ --heat content per unit time in the melting zone (J)

RZQ --heat content per unit time in the reducing zone (J)

CZQ --heat content per unit time in the combustion or superheat zone (J)

Cp iron—specific heat capacity of iron (J/kg K)

Cp coke—specific heat capacity of coke (J/kg K)

Cp flux—specific heat capacity of flux (J/kg K)

Cupola thermal efficiency rated by the heat utilized in the preheating zone, melting zone, a

combustion zone and heat input to the system due to combustion of coke. Exothermic

oxidation of carbon, iron, silicon and manganese and the heat in the air blast are other

aspects of heat loads affecting the overall thermal efficiency of cupola as iron smelting

system.

If Ccoal (J/kg) is the calorific value of coal and the mass of coal consumed is m(kg), then

the amount of heat provided for smelting operation could be given as:

22coalcoal CmQ

The heat content of the hot air expressed as:



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41


23airpairairairair TCVQ

Where,

24exhaustblastair TTT

The heat of formation of carbon dioxide shown as:

25/94050)()()( 0

2522 0 moleCalHgCOgOsCC

Heat evolution due to the oxidation of iron to the highest oxidation state being Fe2O3 given

as:

26/196500 32

0

32OFeofmolecalH OFe

The heat of formation of MnO per mole gave as:

27/352090 moleCalH MnO

The heat of formation of SiO2 per mole expressed as:

28/886180

2molecalH SiO

From the over-riding stipulated conditions, the thermodynamic efficiency of a cupola

furnace system expressed as:

29

,,

sup

exhaustRZhotairoxicoke

QZMZPHZ

th

QQQQQ

QQQ

heatExhaustzonereducingtheinutilizedHeatblastairofcontentHeatMnandSiFeC

ofoxidationtodueevolvedHeatcokeofvaluecalorifictodueHeat

erheatingmeltingpreheatinginutilizedHeat

This efficiency influenced by type, nature, properties, and thickness of the refractory lining

and the steel shell. These properties engender the need for the determination of the critical

thickness of the refractory lining of the cupola furnace.

CONCLUSION

Optimization models had been developed for the heat load analysis of cupola furnace. This

analysis is to improve the thermal performance of the furnace which inadvertently would

significantly impact upon the furnace throughput delivery, hence, enhance performance

efficiencies of a cupola furnace.



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RECOMMENDATION FOR FUTURE RESEARCH

Computational analysis of the mathematical models should be the next area of the potent

research endeavor. The significant effects of the following parameters on the optimal

throughput and thermal performance of the furnace treated in greater details:

(i) The critical thickness of insulation for a given refractory material.

(ii) Employment of different refractory materials of varying thickness of insulation

(iii) Varying the flow rate and temperature of the hot blast injected into the cupola.

NOMENCLATURE

rsV -volumetric of air required per heat (m3)

Tm—the temperature at the center of the furnace (K)

Q -heat loss from the surface of the steel shell (J)

ironQ -thermal load of iron (J)

f luxQ -thermal load of flux (J)

cokeQ -thermal load of coke (J)

hotairTi -temperature difference of the hot air blast and the surroundings (K)

T -the temperature difference of 1ron, coke and the melt (K)

Ta—ambient temperature (K)

T3—the temperature at the external surface of the steel shell (K)

T2—interfacial temperature between the lining and the steel shell K)

T1—the temperature at the inner surface of the lining (K)

Rth—thermal resistance (K/W)

r—radial positions from the center of the furnace (m)

L—height of the furnace (m)

k2—thermal conductivity of the steel shell K)

k1—thermal conductivity of the furnace lining W/mK)

rA -the internal and external surface area of the lining and the steel shell (m2)

hm—convective heat transfer coefficient of the hot melt (W/mK)

rq̂ -heat transfer rate per unit mass (Watts)

SZQ --heat content per unit time of the flue gases in the stack zone (J)

RZQ --heat content per unit time in the reducing zone (J)

PHZQ --heat content in the preheat zone (J)

MZQ --heat content in the melting zone (J)

CZQ --heat content in the combustion or superheat zone (J)

ha—convective heat transfer coefficient of the surroundings (W/m K)

air -the density of air (kg/m3)

Cp iron—specific heat capacity of iron (J/kg K)

Cp flux—specific heat capacity of flux (J/kg K)



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Cp coke—specific heat capacity of coke (J/kg K)

OverallT -thermal potential difference (K)

H1—stack zone height (m)

H2—preheat zone height (m)

H3—melting zone height (m)

H4—reducing zone height (m)

H5—combustion zone height (m)

H6—well zone height (m)

1HT -a temperature difference of the hot effluent gases in the stack zone (K)

2HT -the temperature difference in the preheat zone (K)

3HT -the temperature difference in the melting zone (K)

4HT -the temperature difference in the reducing zone (K)

5HT -the temperature difference in the combustion zone (K)

6HT -the temperature difference in the wall region (K)

REFERENCES

1. Izumi Oishi, Hisato Nimomiya, Kenje Kawashima-Talkabutsu refractories 1980-No 8-

P8-P35-38.

2. Abbakumov V. G., Tsibin I. P. , Nivikov V. L. Designing efficient Industrial Unit //

Lining Refractories- 1987—52, No 5. P822-827.

3. Golle Daniel Resource-friendly refractories technologies for the cupola furnace // Get

and Technol. Int. - 2011.-27, No 3. –P14-16.

4. Energieeffiizieente Auskeldung von Bvennofe n// Stal.und Eisen-2013—133, No 8.-P

46.

5. Intelligent Control of Cupola Melting E. D., Larsen. Et.All. Lockheed Martin Idaho

Technologies Company, June 1997.

6. British Cast Iron Research Association : Cupola design, Operation and Control. (1st

edition). BCIRA 1979.

7. Fine A. H. , Geiger H., Handbook on Material and Energy balance calculation in

Metallurgical Process, 1980.

8. Kueke O., Kybaschenski O., Hessel mann K., Thermo-Chemical Properties of

Inorganic Substances. Second edition (Springer-Verlug) 1991.

9. J. P. Holman, “Heat transfer,” McGraw-HILL INTERNATIONAL BOOK

COMPANY, Fifth Edition, 1983. ISBN : 0-07-029618-9.

10. O.P. KHANNA, “A Text Book of Material Science and Metallurgy,“ Dhanpat Rai

Publication (P) Ltd, New Delhi-110002, 1999. ISBN Ph: 2327 4073 2324 6573


Documents

HEAT LOAD ANALYSIS FOR OPTIMAL USE OF CUPULA FURNACE … · 2019-03-03 · iron for cupola operations are metal scraps, wrought iron, steel, cast iron, and bronzes. Essentially the