Review on Generation of Thermoelectricity From Flue Gases

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1. Introduction

Heat is utilized and generated in the process industries at various places like evaporator, condenser, fractionation column etc.

Equipments and processes those are using large amount of heat are heaving furnaces or boilers as a source of heat

Boilers provide required heat by combustion of appropriate fuel.

Combustion is a chemical reaction that will provide large amount of heat and gases like CO2, SO2, H2O etc. based on the type of fuel

Our main target in process industry is to supply essential heat required in the processes

Real question is what about those high temperature flue gases???

Can those high temperature gases be thrown into open atmosphere???

Can those gases directly be used into some other process???

Providing cooling for the flue gases before throwing them into open atmosphere is economical at all???

Answer of all the above question is NO!!!

A famous quote is “Necessity is the mother of invention”

Something similar happened to the research on those flue gases.

Need of reducing the release of high temperature flue gases invented different paths to utilize the waste temperature and made something best out of it.

The recovered temperature is utilized in different ways like for drying operations, for low temperature processes in the plant itself, to generate thermoelectricity etc.

Target of our discussion today is the way of using waste heat from flue gases to generate thermoelectricity

2. Scope of Production

Production of thermoelectricity require high temperature from the stack.

But only high temperature is not enough for the desired application.

Application must be based on QUANTITY as well as QUALITY of the flue gases.

Only high flow rate of flue gases with low temperature gradient is not sufficient and if high temperature is available at very low flow rate then also the heat recovery can not be sufficient for targeted application.

Different heat producing equipments and temperature range at which flue gases are available.

Types of Device Temperature, oC

Nickel refining furnace 1370 –1650

Aluminium refining furnace 650-760

Zinc refining furnace 760-1100

Copper refining furnace 760- 815

Steel heating furnaces 925-1050

Copper reverberatory furnace 900-1100

Open hearth furnace 650-700

Cement kiln (Dry process) 620- 730

Glass melting furnace 1000-1550

Hydrogen plants 650-1000

Solid waste incinerators 650-1000

Fume incinerators 650-1450

Typical flow diagram for generation of thermoelectricity from waste flue gases

3. Techniques for production

1.Production of electricity from Seeback effect

2.Production of electricity from Peltier effect

3.Production of electricity from Thomson effect

4.Production of electricity from conventional steam turbine

1. Seeback effect:

Working diagram of Seeback effect

Voltage ‘V’ generated by the dissimilar metal junction made from metal A and metal B placed at different temperature ‘T1’ and ‘T2’ as shown in figure

In Seeback effect the electromotive force generated is directly

proportional to the temperature difference ΔT that is;

EAB = SAB X ΔT

Where;EAB = Electromotive force generated with temperature difference SAB = Seeback constant

ΔT = Temperature difference

2. Peltier Effect:

Current ‘I’ flows in the circuit because of heat source and sink applied to different ends of junction comprising of different metals ‘A’ and ‘B’. This effect also gives direction with heat that is generated at from flowing of current.

P = PAB X I

Rate of heat absorbed or produced depends on the direction of electric current flow and amount of heat liberation or production is directly proportional to ‘I’

Where,P = Rate of heat liberated or absorbedPAB = Peltier constantI = electric current from ‘A’ to ‘B’

3. Thomson effect:

This effect shows the effect of temperature difference of ΔT in single conductor. If conductor heaving different temperature at both end then electric current is flowing in direction shown in the figure heat is liberated to the direction shown in the figure.

If a homogeneous conductor under Thomson effect supplied at a rate of q per unit volume, current density J is formed by the given equation;

q = -k J ΔT

Where,q = Rate of heat applied-k= Thomson’s constantJ = Density of electric current ΔT= Temperature difference

4. Using conventional steam turbine:

If the recovered heat is at high temperature then conventional method is widely useful for production of steam and generating electricity

As water can be transferred through pipe, and electricity is transferred through electric wires similarly some equipments must be provided to transfer recovered waste heat into sink for further use

Mainly conventional heat exchangers are used for this purpose but some advanced equipments are also used and serve better for transferring waste heat.

Steps in generation of thermoelectricity from flue gases using conventional steam turbine

1. Recovering waste heat from flue gases using suitable heat exchangers

2. Transfer this waste heat to water for generating high pressure steam

3. Using this HP steam to operate conventional steam turbine

4. Provide a condenser to recover water from the process and continue the cycle

4. A Success Story

• Information of industry: 

• Type of industry: Cement industry• Name of industry: KCP Ltd.• Location of industry: Macherla, Andhra Pradesh, 100 M above sea level• Source of heat: 1. Flue gases from rotary kiln & 2. Clinker cooler• Temperature available: 1. Rotary kiln gases at 300-350o C

2. Clinker cooler gases at 200-300oC• Capacity of plant for cement production: 1500 TPD• Working days for plant per annum: 7920 hours

A typical cement process plant showing targets of heat recovery equipments

Rotary kiln gases Clinker cooler gases

Flue gases quantity 175300 kg/hr 113550 kg/hr

Flue gases temperature 295oC 350oC

Outlet Flue gases temperature

130oC 225oC

Total heat recovery 1,12,50,000 kcal/hr

Gross power output 2.5 MW

Auxiliary power required 230 KW

Net power output 2.4 MW

•Technical specification of heat recovery system working successfully:

5. Detailed steps for production of electricity from flue gases

1. Transfer waste heat from flue gases to make up water Equipments used for transfer of heat

• Recuperators• Heat wheel• Heat Pipe• Shell and Tube Heat Exchanger

Steps (contd..)

Recuperators

Equipments (contd..)

Heat wheel

Equipments (contd..)

Heat Pipe

Equipments (contd..)

Shell and tube heat exchangers

Equipments (contd..)

2.Add heat to make up water incase heat recovery is low and convert steam from water

3. Transfer high temperature steam to the turbine for producing electricity

4. Transfer the steam to cooling tower and collect the waste water

Steps (contd..)

6. Cost economics of the system

• Capital cost is only Rs.50 – 60 Lacs / MW as against 4 – 6 crores / MW• The operating cost for above cogen is only 20 – 30 paise / unit (Kwh) as

against Rs.4 – 6 per Kwh

• Power generated per hour -2.2 (mw)• Total working hours of the year- 8000 hr• Cost of power considered - 0.35 (rs./ kwh) • Total savings per annum – 72,00,000 rs• Total initial investment – 1,64,00,000 rs • Estimated payback period 27.33 (months)

Economic aspects that are given by M/s.Transparent, Pune

SUMMARY:

1. Scope of production is larger where quality and quantity is available in ample amount

2. Waste heat transfer from flue gases to water and make up steam for conventional steam turbine is most preferable as well as economic option

3. Success story of currently working plant of KCP ltd, Macherla, A.P operating successfully at 2.4 MW

4. We studied the steps or production of thermoelectricity

5. As per the details given by Ms/ Transplant we knew the economic aspects related to electricity generation

6. Generating thermoelectricity from flue waste heat is not only eco friendly but healthy for wallet also

[1] www.eere.energy.gov/industry, 6-09-2013, 15:35[2] Bureau of Energy Efficiency, issued by government of India, published in 2003, chapter 8

waste heat recovery, page 1-15[3] Jaydeep. V. Joshi, N. M. Patel, Thermoelectric system to generate electricity from waste

heat of the flue gases, first edition, published in 2008, page 2-5[4] http://en.wikipedia.org/wiki/Thermoelectric_effect, 3-10-2013, 19:45[5] Plant manual, M/s.Transparent Energy System Pvt.Ltd, Pune.[6] Plant manual, KCP Limited (Macherla) - 2.5 MW.[7] Plant manual, Raasi Cement Ltd, (now India Cement) Co-generation.[8] C. Elanchezhian, L. Saravanakumar, B. Vijaya Ramnath, Power Plant Engineering, I.K.

International Publishing House Pvt. Limited, Issued on November 2007, Page-138-175[9] S. Lecompte, H. Huisseune, M. van den Broek, S. De Schampheleire, M. De Paepe, Partload based thermo-economic optimization of the Organic rankine cycle (ORC) applied toa combined heat and power (CHP) system, Applied Energy, Volume 111, November2013, Pages 871-881[10] Paisarn Naphon, Study on the heat transfer characteristics of an evaporative coolingtower, International Communications in Heat and Mass Transfer, Volume 32, Issue8, August 2005, Page 328-352

REFERENCES