Energy Notes1

7/29/2019 Energy Notes1

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Module 1: Fuels characterization and energy balance

Lecture 1: Energy and Environment

Introduction

Energy is a prime mover of economic growth and is vital to sustain the economy.Energy consumption is an indicator of economic growth of a nation

Economic growth depends, among other factors, on the long term availability ofresources that are affordable, accessible and their use do not pollute the environment.

Industrialization contributes to economic growth and requires energy. Major energyintensive industries consumed 68% of the total energy available in 2005. A similar

trend continues in the subsequent years also.

Energy consumption is strongly related to environment cleanliness, when fossil fuel isthe main source of energy.Fossil fuels are natural substances made deep within the earthfrom the remaims of ancient plants and animals.

Fossil fuel accounts for more than 70% of the total energy requirement of India andother countries



Energy resources

Classification of energy resources is given in the following flow sheet:
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Fossil fuels are formed by anaerobic decomposition of buried dead organisms. The age of the

organisms and their resulting fossil fuel is typically millions of years. Fosil fuel which containhigh percentage of carbon, include, coal, petroleum and natural gas.

Module 1: Fuels characterization and energy balanceLecture 1: Energy and Environment

Types of Non-Renewable sources of energy

Coal :

Coal is formed by the prolonged action of geological forces on the plant and vegetalmatter accumulated below the earth crust. The process is called COALIFICATION.Coalification is both time and force dependent. Coalification brings following changes to

the accumulated plant:

The formation of peat beds involves a combination of woody growth in wet, swampyplaces with favourable biochemical conditions. As coalification proceeds in the sequence
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given anove, both physical and chemical changes occur. The chemical changes are given

below:

Oxygen decreases from 40% for wood to 30% for peat, 20% for lignite, 5% forbituminous and 2% for anthracite coal.

Volatile matter decreases from about 70% for wood to 5% or less for anthracite coal. Increase in carbon from about 30% for wood and peat to 90-95% for anthracite coal.

Petroleum :

Petroleum is formed in the earth's crust from the accumulated vegetal and animal matter

metamorphic processes similar to coalification.

From crude petroleum; gasoline, lubricating oil, fuel oils etc. are obtained.

Natural gas :

It is used directly



Renewable sources of energy

Geothermal: energy obtained by tapping the heat of the earth below its surface. Hotunderground water or steam is used to produce electricity.

Biogas: produced from wastes of paper and sugar industries, animal and so on. CH4 isthe product.

Bio fuel: Biodiesel, ethanol etc. are derived from plants. Solid Biomass: Wood fuel, biogenic portion of municipal waste, certain plants.

Biomass may be used in a number of ways to produce energy. The common methods

are gasification, combustion, fermentation and anaerobic digestion. India is very rich in

biomass

Hydro-thermal: Energy in water in the form of KE, temperature difference Solar Energy: Energy collected from sunlight. It can be used in many ways:

Generate electricity using photovoltaic cells. Generate electricity using concentrating solar power. Photovoltaic cells have a low efficiency factor.


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Energy Scenario under Indian Condition

India ranks 6th in the world in total energy consumption and needs to accelerate thedevelopment of energy sector to meet 8-9% economic growth in the country.

India though rich in coal and abundantly endowed with renewable energy has verysmall hydrocarbon reserves (0.4% of the world's reserve).

India is a net importer of energy, more than 25% of primary energy needs being metthrough imports in the form of crude oil and natural gas.

In energy production, coal and oil account for 54% and 34% respectively with naturalgas, hydro and nuclear contributing to the rest. Industrial sector in India consumes 52%energy. Consumption of primary energy in India is 530 Kg of oil equivalent/person in

2004 compares to 1240 Kg oil equivalent/person in China and the world average of

1770 Kg of oil equivalent/person.

Primary energy consumption per person will grow with the growth in economy becauseenergy consumption is an index of country's economic growth and prosperity.

s



Issues related to Fossil Fuel Usage

Fossil fuel contains potential energy/chemical energy and is obtained by combustion. The

figure shows:
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What should be done????

Energy consumption is an indicator of the economic growth of a nation. A developing nation

like ours has the needs of energy for industrial growth. Unless renewable sources of energy areexploited on a massive scale, fossil fuel is the only source of energy. Limited reserves of fossil

fuels and the way in which fossil fuel energy is available (combustion and unutilized energy in

Products of combustion) need ways to search means to optimize energy consumption. Energy

Thus usage of fossil fuel energy source relates to environment sustainability (increaseduse of fossil fuel increases the C emission) and energy security (limited fossil fuel

reserves).
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saving will automatically reduce the carbon emission.

We should think in terms of the following concept

Switch: Can we switch over to renewable energy source? If yes, to what extent?

Reduce: Can we reduce the present level of energy consumption? If yes, then how? Capture: Can we capture the heat of POC which is exiting the industrial furnace? If

yes, how?

References:

(1) R. Schuhmann: Metallurgical Engineering, Vol.1 Engineering Principles

(2) O.P.Gupta: elements of fuels, furnaces and Refractories, Khanna Publishers

Module 2: Combustion and heat utilizationLecture16: Furnace:Type and classification

What is a furnace?

A furnace is essentially a thermal enclosure and is employed to process raw materials at high

temperatures both in solid state and liquid state. Several industries like iron and steel making,non ferrous metals production, glass making, manufacturing, ceramic processing, calcination in

cement production etc. employ furnace. The principle objectives are

a) To utilize heat efficiently so that losses are minimum, and

b) To handle the different phases (solid, liquid or gaseous) moving at different velocities fordifferent times and temperatures such that erosion and corrosion of the refractory are minimum.

Module 2: Combustion and heat utilizationLecture16: Furnace:Type and classification


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Module 2: Combustion and heat utilizationLecture 16: Furnace:Type and classification

What are the components of a furnace?

The principle components are

1.Source of energy

a. Fossil fuel: For fossil fuel one requires burner for efficient mixing of fuel and air.

Arrangement of burner is important.

b. Electric energy: Resistance heating, induction heating or arc heating.

c. Chemical energy: Exothermic reactions

ii. Suitable refractory material: Refractory design is important. Thermal enclosure of the

furnace is designed and constructed keeping in view the requirements. For example refractoryfacing the thermal enclosure must have high refractoriness, chemically inert etc. Whereas

refractory facing the surrounding must have low thermal conductivity to minimize heat losses.

iii. Heat exchanger: Heat exchanger is becoming now as part of the fossil fuel fired furnacesin order to recover and reuse the heat of POC. Heat of POC can be used either external to

furnace by installing a heat exchanger or internally by recirculation the POC within the

furnace.

iii. Instrumentation and control: Furnaces are equipped with POC analyzer and temperature

control.

Module 2: Combustion and heat utilization

Lecture 16: Furnace:Type and classification

Furnaces and their applications in high temperature industries:

Furnaces are used for wide variety of processing of raw materials to finished products in

several industries. Broadly they are used either for physical processing or for chemical


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processing of raw materials. In the physical processing the state of the reactants remains

unchanged, whereas in the chemical processing state of the reactants changes either to liquid ofgas. In the table given below some applications of furnaces for physical and chemical

processing are given ( the reader may go through detailed description in order to appreciate the

requirement of the design of thermal enclosure, i.e. furnace):

PHYSICAL PROCESSING

Unit process Purpose Energy

sourceTemperature inoC

Type of

furnace

CarbonizationConversion

of coal to

coke

Indirect

heating by

burning

fuel

1000 to 1200 Coke oven

Calcination Removal of

CO2 fromCaCO3 for

cement

production

Production of

anhydrous

alumina for

electrolysis

Fossil fuel

Fossil fuel

1200

1300

Rotary

kiln

Rotary

kiln

Roasting To convert

sulphide into

oxide

partially or

completely

Chemical

+ Fossil

fuel

900 Multiple

hearth

furnace,

Fluid bed

roaster,

etc

Heating To eliminate

segregation

To perform

hot working

To perform

heattreatment

Mostly oil

and gas

fired

Below the melting

points of

materials

Batch type

or

continuous

type

Sintering To produce

compacts of

particles

Fossil or

electricBelow the melting

pointSintering

furnaces


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Module 2: Combustion and heat utilizationLecture 16: Furnace:Type and classification

CHEMICAL PROCESSING

Unit process Purpose Energy

sourceTemperature in

oCType of furnace

Electrolysis

of molten

salt

To

produce

Al,Mg

and Na

Electric

energy

700 to 900 Hall-Heroult cell,

Refining To

produce

steel

Chemical

and

electric

1600 LD Converter

Electric furnace

Melting Toproduce

castings of

metals

and alloys

Electric

and fossil

fuel

Above the

melting points

of respective

metal and alloy

Induction

furnace,

reverberatory

furnace and

melting furnace

Matte

smeltingTo

produce

matte

Chemical

and fossil

fuel

1200 Flash smelter,

Reverberatory

smelter

Reduction

smelting

i) To

producehot metal

ii) To

produce

lead and

iii) To

produce

Zinc

Chemical

and fossilfuel

Chemical

and fossil

fuel

1700 to 1800at

the tuyere in allthe cases

Iron blast furnace

Lead blastfurnace and

Zn blast furnace

Converting To

produce

copper

from

matte

Chemical

energy1100 to 1200 Side blown

converter



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Lecture 16: Furnace:Type and classification

Issues in Furnace design

1) Source of energy in processing of raw materials is fossil fuel in most cases. Even if electricenergy is used, it is also derived from fossil fuels. Thus energy efficient design of thermal

enclosure is important; particularly heat losses should be as minimal as possible.

2) In chemical processing, fluid flow is important. Liquid and gases are flowing at hightemperature so erosion and corrosion of the refractory is important. In addition, fluid flow also

influences the rates of heat and mass transfer. The dead zones (dead zones are those areas in

which no movement of solid and liquid takes place) should be avoided while designing the

furnace chamber

3) Atmosphere in the furnace is also important to avoid oxidation of the material being heated

4) Control of furnace temperature is also an important issue. Overheating and under-heating

lead to inefficient utilization of fuel and also overheating or under-heating of material. Furnace

should be equipped with the temperature measurement and control devices

5) Furnaces are both batch and continuous type. In the continuous type for example in heatingof ferrous material for hot working, the furnace chamber consists of preheating, heating and

soaking zones. The material enters through the preheating zone and exits the soaking zone for

rolling. But the flow of products of combustion is in the reverse direction. Furnace design is

recuperative type in that material exits at the desired temperature from the soaking zone and theproducts of combustion discharge the preheating zone at the lowest possible temperature.

Different types of continuous furnaces are in use, like walking beam type, pusher type, rollerhearth type, screw conveyor type etc.

6) In the batch furnaces, the load is heated for the fixed time and then discharged from the

furnace. There are different types of batch furnaces like box type, integral quench type, pit typeand car bottom type

7) In many cases the furnace is equipped with either external heat recovery system or internalheat recovery system. In the external heat recovery system a heat exchanger like recuperator is

installed outside the furnace. Here heat exchanger must be integrated with the furnace

operation. In the internal heat recovery the products of combustion are recirculated in the

furnace itself so that flame temperature is somewhat lowered. The objective is to reduce theNOx formation.

8) The products of combustion are moving at high speeds in the furnace. The flow of products

of combustion is important to obtain rapid heat transfer and minimum thermal gradient.

Source: George E.Totten and M.A.H.Howes: Steel heat treatment handbook


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P.Mullinger and B. Jenkins: Industrial and process furnaces

W. Trinks: Industrial furnaces

W. Trinks and M.M. Mawhinney: Industrial furnaces


Lecture 17: Heat Utilization in Furnaces

Heat BalanceComplete heat balance of a furnace shows the flow of heat in a furnace. Heat balance contains

the information regarding the sources of heat input like sensible heat of reactants, calorific

value of fuel, heat of exothermic reactions etc. Heat balance also shows the heat output like

wall losses, sensible heat in POC, opening losses, heat carried by the charge etc.

We have to differentiate between the quantities of heat directly related to combustion from thequantities of heat that relate to the process in order to control combustion or to study the factors

affecting the fuel utilization. Sensible heats of air and fuel and amount of air relate to the

quantities affecting combustion, whereas heat carried by the charge relate to the process, that isif it is required to heat the material at 900

oC, the heat carried-away by the charge would be

corresponding to 900oC.Losses could also be considered as the parameters relating to the

process.



Fuel Utilization In analyzing utilization of heat from fuel combustion, both amount of heat andthe temperature are important since furnace is heated by the heat transfer from POC. Heat

transfer rate is proportional to the temperature difference between temperature of POC and the

furnace. In this connection adiabatic flame temperature is a very important parameter for fuelutilization (Fuel utilization and heat utilization are essentially similar since heat is derived from

combustion of fuel.

An obvious requirement is that the flame temperature must be greater than furnace temperature

so that POC is able to transfer heat for heating. Rapid heating of the furnace is achieved bygreater temperature difference, which means higher flame temperature. Higher flame

temperature, though increases the heat transfer rate but at the same time it may causeoverheating and destroy the lining. We have seen in lecture 13 that adiabatic flame temperature

(AFT) decreases with increase in excess air. Control of excess air is important to utilize fuel

effectively.

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Temperature of POCIn heat utilization, it must be born in mind that heat carried by POC is not available. Heat

carried by POC, i.e. HPOC

where TPOCis the temperature of POC leaving the furnace. The temperature of POC can berelated with the process critical temperature. The process critical temperature is the temperature

at which a process can be carried out. POC must exit at the critical process temperature. A POC

temperature lower than critical process temperature means that some portion of the furnace is

cooler than the rest, whereas POC temperature greater than critical process temperature meansoverheating of some portion in the furnace. Overheating will cause increase in fuel

consumption.

Module 2: Combustion and heat utilizationLecture 17: Heat Utilization in Furnaces

Available heatThe sensible heat in POC at the critical process temperature is not available to the furnace. The

higher the process critical temperature higher would be the sensible heat in POC. This sensibleheat in POC is very important from the point of view of fuel utilization. We define gross

available heat (GAH) as

(1)

GAH may also be considered as the heat given by POC in cooling from its flame temperature

(flame temperature is AFT in the following which is TAFT) to the process critical temperature(TCRIT). If we assume that specific heat capacity of POC does not vary significantly with

temperature and then

(2)

(3)

If for example is


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A combustion process generating can not be used. A combustion process

generating would be 25% and that generating would be 33%efficient according to eq.3

GAH represents the heat available at the critical process temperature; it may not represent heat

available to perform a given function due to the various types of losses. GAH may be used as acriterion for comparing different fuel-combustion systems.

Once the furnace is designed and built, the heat losses are not within the control of the

operator; it is governed by the process critical temperature, refractory lining thickness andthermal conductivity of the refractory. Defining net available heat (NAH) as

NAH = GAH Heat losses (4)

NAH can be used as a criterion for comparing the smelting/melting/heating efficieny of

different furnaces.


Variables affecting heat utilization

For a given furnace design and the daily heat requirements, GAH is fixed and it is required to

supply this much of heat on per day basis, we can calculate

(5)

If heat supply is the critical factor in determining the process throughput then GAH can not

determine the throughput, we have to consider the NAH

(6)

Heat utilization or fuel utilization according to equation 5 is inversely proportional to GAH/kg


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of fuel. We can derive the factors affecting heat utilization by considering eq.1

Air adjustment: Calorific value (CV) of fuel is the energy obtained on complete combustion of

fuel with theoretical amount of air. Excess air, air leakage, furnace draft, fuel/air ratio will

control the fuel consumption

Sensible heat of reactant; this heat directly adds to the furnace, fuel consumption will decrease.

POC temperature: an increase in POC temperature will increase fuel consumption

Incomplete combustion or un-burnt fuel; corresponding to incomplete combustion part of theCV of fuel is lost in POC.

Reference: P.Mullinger and B. Jenkins: Industrial and process furnacesO.P. Gupta: elements of fuels. furnaces and refractories.

R. Schumann: Metallurgical Engineering Ptinciples

Module 4: Heat transfer calculations, miscellaneous topics and carbon credit

Lecture 32: Steady state heat flow in furnaces and heat exchangers

Estimation of heat losses in furnaces

In furnaces operating at high temperatures, heat losses from the outer wall of the shell are

important to estimate, when the furnace operates at steady state. These losses correspond toloss in energy. In order to estimate the heat losses, wall temperature should be known. Shelltemperature can either be calculated or measured. In the following lecture a method is

discussed to calculate the shell temperature of the furnace.

Consider wall of the furnace at temperature which is lined with refractory material of

thickness , thermal conductivity as shown in the figure.
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Figure 32.1: Furnace wall showing heat balance



Surrounding temperature is . Let the shell temperature facing the surrounding is isunknown. Heat balance of the furnace is

[Heat flow by conduction to the outer shell = Heat loss from the

shell to the surrounding by convection andradiation]

(1)

(2)

(3)

is heat transfer coefficient for natural convection. is view factor, is emissivity of the
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shell and is the area of the furnace. Heat transfer coefficient can be evaluated by

(4)

Module 4: Heat transfer calculations, miscellaneous topics and carbon creditLecture 32: Steady state heat flow in furnaces and heat exchangers

Heat exchanger

Heat exchanger, as the name indicates is an equipment used to capture the heat of products of

combustion and to preheat the air simultaneously. Recuperators and regenerators are commonlyused to capture and reuse the heat.

A recuperator is a continuous type of heat exchanger in which both hot and cold streams flow

continuously. Both streams are separated by a wall. Transfer of heat from hot stream to coldstream is through the separating wall. Both streams may flow parallel flow as shown in figure

(a) or counter -current (as shown in b) or cross flow (as in c). Metallic heat exchangers are used

at low temperatures whereas ceramic heat exchangers can be used at high temperatures
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Figure 32.2:Types of recuperator (a) parallel flow, (b) counter-current

and (c) cross flow



Another type of heat exchanger for high temperature purposes is the regenerator. A regenerator

contains heat storage elements which alternately absorb heat from hot products of combustion andpreheat the incoming air. Two types of regenerators are in use:

(a) Continuous gas flow, moving element for heat storage and

(b) Intermittent gas flow, stationary heat storage element.

In the continuous gas flow type the two gas streams flow continuously through own compartments

and the heat storage elements move from hot stream to cold stream. They are normally constructedof metal and are primarily used for low temperature like boiler.

For high temperature applications, the regenerator contains stationary heat storage elements. It

consists of a chamber filled with brick chequework to give a multiple vertical gas passage. The hotproducts of combustion and cold air flow alternately through the same chamber and same passage in

a cyclic fashion.

In all the above types of heat exchangers, the residence of the stream is important for the heat

transfer efficiency which is in turn controlled by the flow rate of the stream, cross section area of thevessel and thermal conductivity of the material



Performance of a heat exchanger
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A heat exchanger captures and uses the heat of flue gases simultaneously. Performance of aheat exchanger can be evaluated in terms of its ability to capture and to preheat the air to the

maximum possible temperature.

Consider a coaxial type heat exchanger in which hot stream enters at and exits at .

Cold stream say air enters at temperature and pre heated stream exists at as shown inthe figure.

Figure 32.3:Heat exchanger under consideration for

macroscopic heat balance


Assumptions:

i) Flow of flue gases and air are at steady state.

ii) Flow is adiabatic which means no loss of heat, which means heat lost by flue gas iscompletely absorbed by air

(5)

Heat lost by flue gas (6)
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Heat taken by air (7)

and are heat content in hot and cold stream, and mass flow rate of hot and cold

stream and is the enthalpy.


There is no heat loss to surrounding, so

For ideal gases and in compressible fluids

(8)

(9)

Heat balance over a length of heat exchanger

(10)

is an overall coefficient for heat flow path consisting of a series of thermal resistances suchtha

Here and are convective thermal resistance and is thermal resistance of the wall of

thickness and of thermal conductivity due to conduction. Re-arrangement of equations 9and 10 gives

(12)

And

(13)
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Adding equations 12 and 3

(14)

By assuming as independent of l and integrating over the length l we get.

(15)

Expression relates terminal temperatures of the heat exchanger to stream rates and heat

exchanger dimensions.

It can be used to describe the performance of the exchanger By equation 8, 9 and 15

(16)

(17)

The equations 16 and 17 describe the rate of heat flow as a function of the terminal

temperatures of the heat exchanger and do not contain the manner in which streams areflowing. Therefore, the equations 16 or 17 are general equations to evaluate the performance of

the heat exchanger



Application to regenerator

Regenerators are unsteady heat flow system to which steady state heat, flow is not strictly

applicable. For most engineering applications, a regenerator can be considered in terms of heat


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flow analogous to a continuous recuperator as

and to deal with average temperature to eliminate time. By this analogy brick surfaces are at

higher temperature during flue gas cycle than during cold gas cycle and temperature differencecorresponds to that across the separating wall in a recuperator. Also heat flow in and out from

brick is equivalent to resistance to heat flow across the separating wall in a recuperator. With

this analogy we can define the overall heat transfer coefficient as applicable to regenerator as

(18)

In the equation 18

and are heat transfer coefficient from flue gas to brick surface and from brick

surface to air respectively. is the thermal resistance of the separating wall which is

analogous to that of brick in the regenerator.

Estimation of is relatively difficult to estimate in unsteady state flow. The equivalent

thermal resistance varies with the thickness of the brick and the time of contact. Its contribution

is 15 to of the total resistance to flow of heat from hot to cold stream. Equation 17 can beused for regenerators keeping in mind the above limitations.

References:

D. R. Poirier and G.H.Geiger: Transport: Transport Phenomena is materials processing1994.

R. Schuhmann: metallurgical Engineering, vol .1 Engineering principles
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