international journal of mechanical engineering and technology (ijmet…€¦ · ·...

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 115-121, © IAEME

115

WASTE HEAT RECOVERY TO INCREASE BOILER EFFICIENCY USING

BAGASSE AS FUEL

Rajendra N. Todkar1, P.E.Chaudhari

2, U.M.Shirsat

3

1post graduate student of Mechanical Engineering, MIT College of Engineering, Pune, India,

2Dept. of Mechanical Engineering, MIT College of Engineering, Pune, India,

3Nav shayadri College of Engineering, Pune, India,

ABSTRACT

Many industrial heating processes generate waste energy in textile industry; especially

exhaust gas from the boiler at the same time reducing global warming. Waste heat found in the

exhaust gas can be used to preheat the incoming gas. This is one of the basic methods for recovery of

waste heat. Therefore, this article will present a study the way to recovery heat waste from boiler

exhaust gas by mean of shell and tube heat exchanger. The present investigation has been carried out

in order to increase the efficiency of the boiler, used in the sugar mills. Methods for recovering the

heat of flue gases from boilers were using Water preheater, Air preheater. With the help of real

example of sugar factory.

Keywords: Bagasse; Boiler Efficiency; Exhausts Gas; Heat Exchanger; Heat Recovery.

1. INTRODUCTION

Growing cost of fuel and the need to fulfill the requirements of the Kyoto Protocol force the

majority of countries, even with temperate climate, to revise their attitude to district heating systems.

Heat is generated in boiler by combustion process of fuel. The strategy of how to recover this

heat depends in part on the temperature of the waste heat gases and the economics involved. Large

quantity of hot flue gases is generated from Boilers, Kilns, Ovens and Furnaces. If some of this waste

heat could be recovered, a considerable amount of primary fuel could be saved. The energy lost in

waste gases cannot be fully recovered. However, much of the heat could be recovered and loss

minimized [1].

Depending upon the type of process, waste heat can be rejected at virtually any temperature

from that of chilled cooling water to high temperature waste gases from an industrial furnace or kiln.

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING

AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)

ISSN 0976 – 6359 (Online)

Volume 5, Issue 2, February (2014), pp. 115-121 © IAEME: www.iaeme.com/ijmet.asp Journal Impact Factor (2014): 3.8231 (Calculated by GISI) www.jifactor.com

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Usually higher the temperature, higher the quality and more cost effective is the heat recovery. In

waste heat recovery must have some use for the recovered heat. It can be use would be preheating of

combustion air, space heating, or pre-heating boiler feed water or process water. In any heat recovery

situation it is essential to know the amount of heat recoverable and also how it can be used.

Recovery of waste heat has a direct effect on the efficiency of the process. This is reflected

by reduction in the utility consumption & costs, and process cost. We have taken real example of one

boiler of sugar factory having flue gas temperature 200OC , heat loss in dry gases is 16.96%. Heat

loss due to moisture formed by burning is 11.55% and due to moisture in fuel is 14.22%. Boiler

efficiency is 53.31%.So aim is to minimize heat losses through flue gas [2].

These systems have many benefits which could be direct or indirect of waste heat recovery.

Direct benefits: The recovery process will add to the efficiency of the process and thus decrease the

costs of fuel and energy consumption needed for that process. Indirect benefits: Reduction in

Pollution: Thermal and air pollution will dramatically decrease since less flue gases of high

temperature are emitted from the plant since most of the energy is recycled. Reduction in the

equipment sizes: As Fuel consumption reduces so the control and security equipment for handling

the fuel decreases. Also, filtering equipment for the gas is no longer needed in large sizes. Reduction

in auxiliary energy consumption: Reduction in equipment sizes means another reduction in the

energy fed to those systems like pumps, filters, fans, etc.

Nomenclature:

m’w mass flow rate of water (kg/s).

m the moisture fraction of the fuel.

a the ash fraction of the fuel.

Cpw specific heat of water (kJ/ kgOC).

Tw1 initial temperature of water (OC).

Tw2 final temperature of water (OC).

m’f mass flow rate of flue gases (kg/s).

Cpf specific heat of flue gases (kJ/ kgOC).

Tf1 initial temperature of flue gases (OC).

Tf2 final temperature of flue gases (OC).

m’a mass flow rate of combustion air (kg/s).

Cpa specific heat of air (kJ/kg K).

Ta1 initial temperature of combustion air (OC).

Ta2 final temperature of combustion air (OC).

η Efficiency of boiler (%).



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Q quantity of steam generated (TPH).

q quantity of bagasse consumed (TPH).

hg enthalpy of steam (kJ/kgOC)

hf enthalpy of feed water (kJ/kgOC)

GCV gross calorific value of bagasse. (kJ/kg)

2. LITERATURE REVIEW

The paper [3] [4] explained about methods for recovering the heat of flue gases from boilers

using heat of vaporization are analyzed. High profitability of the developed thermal circuit involving

deep recovery of the heat of flue gases and its storage, as well as good prospects for using it, are

demonstrated by a real example in this paper. It conclude that Use of a comprehensive approach for

attacking the problem of deeply recovering the heat of boiler flue gases, including heat of

vaporization and involving consideration of all elements participating in the heat supply cycle, makes

it possible not only to solve this problem technically, but also to optimize the parameters of coolant.

J Barroso, H Amaveda, Antonio Lozano [5] carried out investigation in order to increase the

efficiency of the RETAL-type boiler, used in the Cuban sugar mills. A test method is used in the

evaluation process and further adjustment of the boilers operation was ASME and GOST. Pointing

the attention on the importance of the stoichiometric ratio and steam power on the overall efficiency.

They calculated gas temperature as well as the range of the optimal value for the excess air fraction.

Their influence on the efficiency was analyzed and the total costs determined.

Drying bagasse by using flue gas which comes from air preheater to chimney is an optimum solution

to enhance efficiency of boiler in sugar factory as bagasse has high calorific value but due to its

moisture about 50% not able to use its full heat is explain by Sankalp Shrivastav1, Ibrahim Hussain

[6]. The work suggest to place Cylindrical shell type dryer, in between the air preheater and

chimney, and flue gas pass from dryer’s one end and from another end bagasse by carriage, makes

dryer to act as a counter flow heat exchanger where flue gas gives its heat at 190°C to the bagasse at

45°C this reduce moisture of bagasse from 50% to 46%, increased CV of bagasse around 784 KJ/kg

which increases boiler efficiency from 79% to 81% in sugar industries.

3. METHODS TO INCREASE BOILER EFFICIENCY

a. Reduce Excess Air.

b. Preheat Combustion Air.

c. Blow down Heat Recovery.

d. Exhaust Heat Recovery.

e. Turbulators and Soot Blowers.

f. Replace Burners.

g. Condensate Return.

4. METHODS OF HEAT RECOVERY

The principal reason for attempting to recover waste heat is economic. All waste heat that is

successfully recovered directly substitutes for purchased energy and therefore reduces the

consumption of and the cost of that energy. A second potential benefit is realized when waste-heat



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substitution results in smaller capacity requirements for energy conversion equipment. Therefore, the

use of waste-heat recovery can reduce the requirement for space heating energy. This permits a

reduction in the capacity of furnaces or boilers used for heating the plant. In every case of waste-heat

recovery, a gratuitous benefit is derived: That of reducing thermal pollution of the environment by an

amount exactly equal to the energy recovered, at no direct cost to the recover.

4.1. Preheat Combustion Air Efficiency Improvement Up to 1 percentage point. If we have a large temperature difference

(20° to 40°) between your boiler intake air location and the ceiling of your boiler room and this hot

air is a result of boiler and stack losses, you can increase your boiler efficiency by either extending

the intake upwards or forcing the hot air down. Both options may require a fan and ductwork. If the

hot air is due to boiler wall losses, we may want to consider insulating the boiler.

4.2. Exhaust Heat Recovery A device like the one shown below can be attached to the flue to recover a portion of the

exhausted heat. This heat can be used to preheat boiler make-up water. Take care not to extract so

much heat that the flue gases condense (causing corrosion).

4.3. Feed water preheater A feed water heater is a power plant component used to pre-heat water delivered to a steam

generating boiler. Preheating the feed water reduces the irreversibility’s involved in steam generation

and therefore improves the thermodynamic efficiency of the system. This reduces plant operating

costs and also helps to avoid thermal shock to the boiler metal when the feed water is introduced

back into the steam cycle.

In a steam power plant (usually modeled as a modified Rankine cycle), feed water heaters

allow the feed water to be brought up to the saturation temperature very gradually. This minimizes

the inevitable irreversibility’s associated with heat transfer to the working fluid (water).

5. CALCULATIONS

Thermodynamic Analysis

1 st Law 2

nd Law

Input/output Input/output

Energy balance

Exergy balance

HHV LHV

Fig 1: Thermodynamic analysis applied to bagasse boiler [7][8]

By using shell and tube type heat exchanger: [9]. Methodology is used for calculations are

shown in fig.1; we have done thermodynamic energy balance of input and output energy.



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5.1. Feed water preheater The flue gases leaving a modern 3-pass shell boiler are at temperatures of 200 to 300 °C.

Thus, there is a potential to recover heat from these gases. The flue gas exit temperature from a

boiler is usually maintained at a minimum of 200 °C, so that the sulphur oxides in the flue gas do not

condense and cause corrosion in heat transfer surfaces. When a clean fuel such as natural gas, LPG

or gas oil is used, the economy of heat recovery must be worked out, as the flue gas temperature may

be well below 200 °C.[1]

“m’w x Cpw x (Tw2-Tw1) = m’f x Cpf x (Tf2-Tf1) (1)”

Flue gases (Cpf-1.097kJ/kgOC is when flue gas is at 200

OC ) come in to feed water heater at

200OC and water at 30

OC. In order to decreased exhaust flue gas temperature we passed it through

feed water heater. By heat balance (equation no.1), we get temperature after feed water heater, water

at 100OC with mass flow rate of water is 3kg/s and flue gas at 125

OC. Feed water preheater having

effectiveness of 0.411.

5.2. Preheat Combustion Air

Combustion air preheating is an alternative to feed water heating. In order to improve thermal

efficiency by 1%, the combustion air temperature must be raised by 20 °C. Most gas and oil burners

used in a boiler plant are not designed for high air preheats temperatures. Modern burners can with-

stand much higher combustion air preheat, so it is possible to consider such units as heat exchangers

in the exit flue as an alternative to an economizer, when either space or a high feed water return

temperature make it viable.

“m’a x Cpa x (Ta2-Ta1) = m’f x Cpf x (Tf2-Tf1) (2)”

Flue gases after passing through feed water heater, it comes in air preheater. Air having

temperature 25OC and flue gases enter at 175

OC. By heat balance (equation no.2), we get

temperature after air preheater, air is at 50OC with mass flow rate of air is 2.6kg/s and flue gas at

60OC. Air preheater having effectiveness of 0.169.

Boiler efficiency is calculated by direct method. This is also known as ‘input-output method’

due to the fact that it needs only the useful output (steam) and the heat input (i.e. fuel-bagases) for

evaluating the efficiency. The gross calorific value of a biomass fuel, as determined with a bomb

calorimeter, can be written in most cases in the form:

“GCV = k x (1 – m – a) (3)”

GCV is 9204.8 kJ/kg. The efficiency can be evaluated using the formula (4) [2].

“η = {Q x (hg-hf)} x 100 / (q x GCV) (4)”

At 20 bar pressure and 333OC enthalpy of superheated steam is 3093.16 kJ/kg

OC and

enthalpy of feed water is 419.1 kJ/kgOC. Quantity of steam generated is 17 TPH [2] and quantity of

bagasse consumed is 8TPH [2]. By using equation no.(4) we get boiler efficiency as 61.73%.



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6. RESULT TABLE

Table 1: Heat exchanger calculations

Temperature (O

C) Mass Temperature of flue

flow rate gases (O

C)

(kg/s)

In Out In Out

Water 30 100 3 200 125

preheater

Air preheater 25 50 2.6 125 60

Table 2: Efficiency

Efficiency

Before (%) After (%)

53.31 61.73

7. CONCLUSION

• The efficiency of bagasse fired boiler was calculated using flue gases temperature leaving the

boiler and on the basis of total heat values of steam. The boiler efficiency found was 61.73 %

and previously it was 53.31 % under prevailing conditions of the boiler as shown in Table.2.

Steam from bagasse was 17 TPH at a pressure of 21 kg/cm2 and at a temperature of 333

OC.

• Through recovery of waste heat by installation of an economizer. Waste heat found in the

exhaust gas of various processes is used to preheat feed water and combustion air. After

recovery of waste heat we get 8.42% more efficiency than previous one at mass flow rate of

water and combustion air 3kg/s and 2.6kg/s respectively as shown in Table.1.

• By using water preheater and air preheater we decreased flue gas temperature up to 60Oc.

• Both the methods of efficiency calculations Direct and indirect gave similar results.

REFERENCE

[1] Bureau of Energy Efficiency, 2009.

[2] Energy Audit Report of MSSK Ltd. Boiler Efficiency, 2006, Malegoan, Pune

[3] A.D.Kiosova, G.D.Avrutskiib, Deep Recovering and Storing of the Heat of Flue Gases from

Boilers, ISSN 0040-6015, Thermal Engineering., 58(2), 2011, 948-952.

[4] Prateep Pattanapunt, Kanokorn Hussaro, Tika Bunnakand, Waste Heat Recovery From Boiler

Of Large-Scale Textile Industry, American Journal of Environmental Science, 9(3), 2013,

231-239.



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[5] J Barroso, H Amaveda, Antonio Lozano, On the optimization of boiler Efficiency using

bagasse as fuel, Elsevier Science Ltd , 82, 2003, pp.1451– 1463.

[6] Sankalp Shrivastav1, Ibrahim Hussain, Design of Bagasse Dryer to Recover Energy of Water

Tube Boiler in a Sugar Factory, International Journal of Science and Research (IJSR), 2(8),

2013, 356-358.

[7] J. H. Sosa, Arnao,S. A. Nebra, First and Second Law to Analyze the Performance of Bagasse

Boilers. 14(2), 2011, 51-58.

[8] P. Regulagadda, I. Dincer, G.F. Naterer, Exergy analysis of a thermal power plant with

measured boiler and turbine losses, Applied Thermal Engineering, 30, 2010, 970–976.

[9] Javier Uche, Luis Serra, Antonio Valero, “Thermoeconomic optimization of a dual-purpose

power and desalination plant”, Desalination Elsevier Science Ltd, 2001, 136, 147-158.

[10] Isam H. Aljundi, Energy and exergy analysis of a steam power plant, Applied Thermal

Engineering, 29, 2009, 324–328.

[11] M. A. Khaustov, Experience Gained from Operation of an Individual Boiler House in a

District Heating System, Novosti Teplosnab., 12, 2008, 49–50.

[12] S. Bhanuteja and D.Azad, “Thermal Performance and Flow Analysis of Nanofluids in a Shell

and Tube Heat Exchanger”, International Journal of Mechanical Engineering & Technology

(IJMET), Volume 4, Issue 5, 2013, pp. 164 - 172, ISSN Print: 0976 – 6340, ISSN Online:

0976 – 6359.

[13] Sunil Jamra, Pravin Kumar Singh and Pankaj Dubey, “Experimental Analysis of Heat

Transfer Enhancement in Circular Double Tube Heat Exchanger using Inserts”, International

Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 3, 2012,

pp. 306 - 314, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.

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