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Mahesh Dasar et. al/ Design Analysis of Bagasse Drier

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DESIGN ANALYSIS OF BAGASSE DRIER

Mahesh Dasar

Assistant Professor,

Department of Mechanical

Engineering

Annasaheb Dange College of

Engineering & Technology, Ashta,

Maharashtra, India

D R Wadkar, R B Sutar,

C H Pujari, A M Toraskar, &

A J Patil

Students, Department of

Mechanical Engineering,

Annasaheb Dange College of

Engineering & Technology

Ashta, Maharashtra, India

ABSTRACT

The prices of sugar cane, sugar produced and molasses are fixed by the government

authorities, hence the only method for generating profits for sugar mills is by reducing

manufacturing cost where steam and fuel economy plays an important role. The aim of the

present research work is to reduce the moisture content of the bagasse by designing the counter

flow heat exchanger configuration to increase the dryness fraction of the bagasse. The proposed

design of Bagasse Drier consists of a device wherein the hot flue gases are indirectly mixed with

the wet bagasse falling on the conveyer plate from the crushing section. Є-NTU method is used

for analysis of counter flow heat exchanger and 1-D conductive heat transfer is considered

across a thin plate. Reduction of dryness fraction of bagasse has increased its CV from 2295

KJ/kg to 2232 KJ/kg which enhanced boiler efficiency by 60% to 65%. The wet bagasse dried up

from 49% to 48%.

Keywords: bagasse, Є-NTU, boiler efficiency, dryer, moisture contains.

I. Introduction

India has been known as the original home of sugar and sugarcane. Indian mythology supports

the above fact as it contains legends showing the origin of sugarcane. India is the second largest

producer of sugarcane next to Brazil. Apart from sugar, the sugar industry produces certain by-

products, which can be used for production of other industrial products. The most important by-

product is molasses, which is utilized for production of chemicals and alcohol. In addition, the

other important by product is bagasse. It is mainly utilized as a captive fuel in the boilers but it is

also used as a raw material in the paper industry.

International Journal of Mechanical Engineering & Computer Sciences, Vol.1, Issue 1,

Oct-Dec, 2015, pp 12-26 ISSN: 2455 –WYGY (Online)


Sugarcane is a tropical, perennial grass that forms lateral shoots at the base to produce multiple

stems, typically three to four meters high and about five centimeter in diameter. The stems grow

into cane stalk, which when mature constitutes approximately 75% of the entire plant. A mature

stalk is typically composed of 11–16% fiber, 12–16% soluble sugars, 2–3% non-sugars, and 63–

73% water. A sugarcane crop is sensitive to the climate, soil type, irrigation, fertilizers, insects,

disease control, varieties, and the harvest period. The average yield of cane stalk is 60–70 tons per

acre per year [1]

.

In addition to molasses, the other important by product is bagasse. Bagasse is the fibrous

residue remaining after sugarcane or sorghum stalks are crushed to extract their juice.

Traditionally bagasse has been a waste by product of the sugarcane production process. More

recently is has been used as a fuel source for sugar mills, a fiber for paper production and as

annually renewable resource in the production of sustainable materials and packaging.

Once sugarcane is harvested it is brought to a milling plant where it is crushed – typically with

a series of large rollers. These rollers crush the sugarcane stalks and thus extract the juice from the

sugarcane. The juice is collected and removed to be processed into sugar. The remaining fibrous

stalk (which has been crushed, squeezed, and removed of its juice) is bagasse.

Typically the mill wet bagasse contains around 48% to 52% moisture with a gross calorific

value (GCV) of around 2270 Kcal/kg (~9500 kj/kg). Normally the bagasse is directly fed to the

boiler to generate steam and surplus bagasse is stored in the bagasse yard. The boilers installed in

the plant are designed to burn bagasse with this moisture.

It is a known fact that GCV of bagasse is largely dependent upon its moisture content. Higher

moisture content in bagasse reduces its GCV and also results in higher energy loss because the

fuel moisture carries that latent heat of vaporization up the stack.

The GCV of bagasse can be determined can be determine by the following equation –

GCV = 196.05 x (100-Ww % - WA %) – 31.14 x WRDS

(KJ/Kg)



Where,

Ww - is the moisture content

WA - is the ash content

WRDS - is the Brix

As can be seen from the above equation, the GCV of bagasse shows a decrease of 196KJ/kg

(47Kcal/kg) for every 1% increase in moisture. We can analyze that the variation of GCV with

bagasse moisture for a typical bagasse sample with 2.75% ash on air dried basis [1]

.

The above results indicate that more than 90% of losses from the boiler are stack losses and out

of these losses the moisture loss is the most significant. Therefore by reducing the moisture

content in the bagasse, the efficiency of the boiler can be improved and extra bagasse saved will

be available for other use.

II. BENEFITS OF DRYING FUEL FOR COMBUSTION BOILERS

Using dry fuel in a direct combustion boiler results in improved efficiency, increased steam

production, reduced ancillary power requirements, reduced fuel use, lower emissions, and

improved boiler operation.

One of the main reasons for these benefits is an increased flame temperature. With wet fuel,

some heat of combustion is used to evaporate the water in the fuel. With dry fuel, all the heat of

combustion goes into heating the air and products of combustion. As a result, dry fuels have a

flame temperature of about 2,300°-2,500°F (1,260°-1,370°C ), while green wood has a

combustion temperature of about 1,800°F (982°C) . In cold climates, the heat of fusion of any ice

that may be mixed with the fuel will also have a significant effect on the flame temperature.

This increased flame temperature is beneficial in a number of ways. First, the higher flame

temperature means there is a larger temperature gradient in the boiler for radiant heat transfer.

More heat transfer takes place for the same boiler tube area, increasing steam production. In new

boilers designed for dried fuel, the boiler can be smaller because less heat transfer area is needed.




With the higher flame temperature there will be more complete combustion of the fuel,

resulting in lower carbon monoxide (CO) levels and less fly ash leaving the boiler. More complete

combustion also means more heat is released from the fuel. In a new boiler, the fire box can be

smaller and the downstream ash handling system can be smaller.

With better combustion the excess air can be reduced and acceptable opacity and CO

levels maintained. For moist fuels, approximately 80% excess air is required to prevent smoke

formation, but for dry fuels, only 30% excess air is required. This reduction in excess air means

less heat of combustion goes into heating air. Using less excess air also reduces sensible heat

losses with the flue gases, increasing boiler efficiency. Less air flow through the boiler increases

the residence time in the boiler and lowers the gas velocities, aiding in more complete combustion

and reducing the amount of light fuel blown out of the fire box before it completely burns.

The forced draft (FD) fan, which provides the combustion air for the boiler, will consume less

power with less excess air. Likewise, the induced draft (ID) fan, which draws the flue gas out of

the boiler and through the pollution control equipment, will require less power because of the

lower air flow and the reduced water vapor from the fuel. For boilers that are limited by the ID

fan, this can result in increased capacity. For new boilers, using drier fuel allows the FD fan, ID

fan, and downstream pollution control equipment to be smaller.

Another reason for a higher overall boiler efficiency is the lower flue gas temperature to the

stack. In a boiler without fuel drying, the flue gas temperature might be 350°F (177°C) or higher,

but with a dryer this temperature will be closer to 220°F (104°C) coming out of a dryer, This heat

that would otherwise be lost goes instead into drying the fuel. Overall thermal efficiency increases

can amount to 5%-15%, with steam production increases of 50%-60% [2]

.

III. DRAWBACKS OF USING DRIED FUEL

As mentioned before, burning dried fuel results in higher combustion temperatures in the

boiler, which for the most part provides overall benefits to the boiler. However, as the flame

temperature increases, it approaches the fusion temperature of the ash. If the ash starts to flow and

form slag, this can be very detrimental to boiler operation. Usually the flowing temperature of the



ash is safely above the flame temperature, but when contaminants from construction debris or

salts are mixed with the fuel, the flowing temperature can be lower.

A second concern is what to do if a boiler is designed to use dry fuel and there is a problem

with the dryer, because the boiler will be undersized for burning wet fuel. One solution is to use a

fossil fuel backup to allow the boiler to operate at full capacity until the dryer can be repaired.

The final concern is the materials of construction. When the hot flue gases from the boiler are

cooled below the dew point of the flue gas, sulfur trioxide (SO) can condense, resulting in sulfuric

acid formation. This can seriously corrode downstream equipment and duct work. Depending on

the configuration of the dryer and boiler, and whether the dryer is a new installation or a retrofit,

this may require expensive materials of construction or result in higher maintenance costs.

Nitrous oxide (NO) emissions may increase or decrease depending on the boiler design.

Lower excess air tends to decrease NO emissions, but high flame temperatures can increase NO.

IV. DRYER DESIGN & METHODOLOGY

According to physical contact between bagasse & heating media, there are two types of dryers,

namely.

Direct or contact Dryers

Indirect or non-contact Dryers

Here we used Indirect or non-contact dryer for removing the dryness fraction of bagasse. The

advantage of using non-contact dryer is to avoid the chances of burning of bagasse since the

initial temperature of bagasse & flue gas is 64oC & 180

oC respectively.




A counter flow heat exchanger principle is employed transfer the heat energy from a hot fluid

(Flue Gas) to a

cold fluid (Bagasse), with maximum rate & minimum investment & running costs. In heat

exchangers the temperature of each fluid changes as it passes through the exchangers, & hence the

temperature of the dividing wall between the fluids also changes along the length of heat

exchanger.

E- NTU METHOD

The Number of Transfer Units (NTU) Method is used to calculate the rate of heat transfer in

heat exchangers (especially counter current exchangers) when there is insufficient information to

calculate the Log-Mean Temperature Difference (LMTD). In heat exchanger analysis, if the fluid

inlet and outlet temperatures are specified or can be determined by simple energy balance, the

LMTD method can be used; but when these temperatures are not available The NTU or The

Effectiveness method is used.

To define the effectiveness of a heat exchanger we need to find the maximum possible heat

transfer that can be hypothetically achieved in a counter-flow heat exchanger of infinite length.

Therefore one fluid will experience the maximum possible temperature difference, which is the

difference of (The temperature difference between the inlet temperature of the hot

stream and the inlet temperature of the cold stream). The method proceeds by calculating the heat

capacity rates (i.e. mass flow rate multiplied by specific heat) and for the hot and cold

fluids respectively, and denoting the smaller one as . The reason for selecting smaller heat

capacity rate is to include maximum feasible heat transfer among the working fluids during

calculation[3]

.



A quantity:

is then found, where is the maximum heat that could be transferred between the fluids.

According to the above equation, to experience the maximum heat transfer the heat capacity

should be minimized since we are using the maximum possible temperature difference. This

justifies the use of in the equation.

The effectiveness (E), is the ratio between the actual heat transfer rate and the maximum

possible heat transfer rate:

Where:

Effectiveness is dimensionless quantity between 0 and 1. If we know E for a particular heat

exchanger, and we know the inlet conditions of the two flow streams we can calculate the amount

of heat being transferred between the fluids by:

For any heat exchanger it can be shown that:

For a given geometry, can be calculated using correlations in terms of the "heat capacity ratio"




and the number of transfer units,

Where is the overall heat transfer coefficient and is the heat transfer area.

For example, the effectiveness of a parallel flow heat exchanger is calculated with:

Or the effectiveness of a counter-current flow heat exchanger is calculated with:

For

Similar effectiveness relationships can be derived for concentric tube heat exchangers and shell

and tube heat exchangers. These relationships are differentiated from one another depending on

the type of the flow (counter-current, concurrent, or cross flow), the number of passes (in shell

and tube exchangers) and whether a flow stream is mixed or unmixed.

Note that is a special case in which phase change condensation or evaporation is

occurring in the heat exchanger. Hence in this special case the heat exchanger behavior is

independent of the flow arrangement. Therefore the effectiveness is given by:

[3]



V. DESIGN ANALYSIS

Experimental setup[4]

The data collected from Rajarambapu Patil Sahakari Sakhar Karkhana, Karandwadi Unit.

Mass flow rate of bagasse, mb = 5.63kg/s

Specific heat at constant pressure of bagasse, Cpb = 1.018KJ/kg

Initial temperature of bagasse = 640C.

Specific Heat of flue gas, Cpg = 0.2808 KJ/Kg

Initial temperature of flue gases = 180 oC

Velocity of flue gas = 12 m/s

Ambient temperature of air = 31 0C

Characteristic length for inner flue gas flow is 0.5 m

Characteristic length for outer ambient air flow is 0.16 m

Velocity of ambient air = 3.5 m/s

.

DESIGN ANALYSIS OF BAGASSE DRYER

Coefficient of Convective Heat Transfer for Internal Flow

hi = Nu x K /L

Reynolds Number for internal flow,

Re = V x L / Ʋ

= 12 X 0.5 / 0.000030

Re = 200000

Hence the flow is Turbulent.




Nusselt Number, Nu = 0.02 Re 0.8

= 349

Nu = h x L / k

Thermal Conductivity of Flue Gas @ 180o,

K = 0.02442 + 0.6992 x 10-4

x Tgi

= 0.02442 + 0.6992 x 10-4

x 180 = 0.0370 W/mk

Hence, hi = Nu x K /L

= (349 x 0.0370) / 0.5

= 25.83 W/m2k

Coefficient of Convective Heat Transfer for External Flow

ho = Nu x K / L

Reynolds Number for External flow,

Re = V x L / Ʋ

= 3.5 X 0.16 / 0.000016

Re = 34530

Hence the flow is Turbulent.

Nusselt Number, Nu = 0.24 Re 0.6

= 252

Nu = h x L / k

Thermal Conductivity of Ambient Air at 31o,

K = 0.02442 + 0.6992 x 10-4

x Tgi

= 0.02442 + 0.6992 x 10-4

x 31 = 0.0265 W/mk

Hence, hi = Nu x K /L

= (252 x 0.0265) / 0.16 = 41.60 W/m2k

Internal coefficient of heat transfer (hi)

hi= 25.83 W/m2K

External coefficient of heat transfer (ho)

ho = 41.60 W/m2K

Thermal conductivity of mild steel, Kms = 45 W/Mk

Overall coefficient of heat transfer (U)



U=1/[1/hi +L/K+1/ho]

L=0.008 m

U=1 / [1/25.83+0.008/45+1/41.60]

U = 15.90 W/m2K

Number of transfer unit (NTU)

NTU = UA/Cmin

Where, A = Surface area of heat transfer

A = [(13230 x 890) x 2]+[13230 x 1700] mm2

= 32.81 m2

Heat Capacity (C)

Heat Capacity of bagasse(Cb)

Cb = mb x cpb

= 5.63 x 1.018

= 5.7313 KJ/Sec

Heat capacity of flue gas (Cg)

Here mass flow rate of flue gas (mg)

mg = ƍ A V

= 0.4246 x 0.78 x 12

= 3.97 kg/s

Cg = mg x cpg

= 3.97 x 0.280

= 1.1144 KJ/Sec

Hence Cb > Cg

So, Cmin= Cg = 1.1144 KJ/Sec

Now,

NTU =UA/Cmin

= 15.90 x 32.81/ (3.97 x 280.8)

= 0.2886




Effectiveness of heat exchanger

ɛ = [1-e(-NTU(1-R))

/(1-R(-NTU(1-R))

]

Where,

R=Cmin/Cmax

= (3.97 x 280.8) / (5.63 x 1018)

= 0.1945

ɛ = [1-e(-0.4679(1-0.1945

)] / [1-0.1945(e(-0.4679)(1-0.1945)

]

= 0.36

Now,

Heat transfer in bagasse dryer

Q = ɛ x Cmin x (Tgi-Tbi)

= 0.36 x 1.114 x (180-64)

=47.01 Kw

Final Temperature of bagasse

Q = mb x Cpb x (Tbo-Tbi)

47.01= 5.63 x 1.018 x (Tbo-64)

Tbo = 72.200C

Final Temperature of flue gases

Q = mg x Cpg x (Tgi-Tgo)

47.01 = 3.97 x 0.2808 x (180-Tgo)

Tgo= 137.700C

Moisture reduction in bagasse

Enthalpy decrease in gas



= mg x Cpg x (Tgi-Tgo)

Enthalpy increase in moisture

= mb x cpb x (Tbo-Tbi)

Enthalpy increase in moisture

= MFh2+ [M-MF] x (hgo-Mh1)

Where,

M = Initial moisture content in the bagasse

MF = Final moisture content in the bagasse

h1 = Enthalpy of moisture at temperature Tbo

h2 = Enthalpy of moisture in bagasse at Tbi

h = Enthalpy of gas at Tgi

From steam tables [6]

h1 = 302.19 KJ/kg

h2 = 267.8 KJ/kg

hgo = 2630.66 KJ/kg

The energy balance equation for the bagasse dryer

Mg x Cpg x [Tgi-Tgo] = mb x Cpb x [Tbo-Tbi] + MFh2+ [M-MF] x h0

Final moisture of bagasse (MF),

MF = {[mg x Cpg x [Tgi-Tgo]-mb x Cpb x [Tbo-tbi] + [M x h1]-[M x hgo]}/[h2-hgo]

= {[3.97 x 0.2808 x (180-137.70)] - [5.63 x 1.018 x (72.20-64)] + [0.49 x 302.19] – [0.49 x

2630.66]} / [267.8- 2630.66]

MF = 0.4828

% of moisture in bagasse after drying is 48.28 %




VI. RESULT AND DISCUSSION

The design of heat exchanger as bagasse dryer seems successful to reduce the moisture content

of the bagasse. The final temperature of bagasse and flue gases from the bagasse dryer is given as

follows. The temperature of bagasse after drying is increases. And the moisture content in the

bagasse is decreases. The calorific value of bagasse is also is increases with reduction in moisture.

Total heat transfer in heat exchanger

Q = 47.01 Kw

Final Temperature of bagasse

Tbo=72.200C

Final Temperature of flue gases

Tgo=137.700C

Final moisture content in bagasse

MF =0.4820

% of moisture after drying is 48.20%

VII. CONCLUSION

The aim of work was to reduce the dryness fraction of bagasse and it is achieved by 1% by just

utilizing the flue gas heat, the wet bagasse dried up from 49 to 48.2%. Reduction in the moisture

content of bagasse has increased its CV from 2270.62 KJ/kg to 2308.48 KJ/kg which enhanced

boiler efficiency by 60% to 63%. The introduction of the dryer was to reduce the biomass

moisture content in order to improve boiler efficiency and reduce device costs. The results

obtained show clearly that these aims were succeeded. The boiler efficiency was improved.

2200

2300

2400

2500

44 46 48 50 52

C

V

(

K

J

/

K

g)

Moisture Content in %age



VIII. ACKNOWLEDGMENT

IT'S A QUEER TO THINK OF A CLAP WITHOUT THE STRIKING OF TWO

HUMAN HANDS, SIMILARLY THIS WORK OWES A LOT OF CREDIT TO MANY

PEOPLE WHO HAVE HELPED OR INFLUENCED FOR THIS PROJECT. IT IS WITH

UTMOST GRATITUDE THAT I EXPRESS MY SINCERE THANKS TO MY INSTITUTE

FOR ASSIGNING ME THIS PROJECT AND RAJARAMBAPU PATIL SAHAKARI

SAKHAR KARKHANA, KARANDWADI UNIT, FOR MAKING US PART OF THEIR REAL

TIME PROJECT AND EXTENDING TO US ALL RESEARCH AND ESSENTIAL

RESOURCES TO CARRY OUT THE PROJECT WORK. WE EXTEND OUR THANKS TO

OUR HOD DR. S S AHANKARI, DEPT OF MECHANICAL ENGG, ADCET, ASHTA FOR

ALLOWING US TO DO THIS WORK. WE EXPRESS OUR DEEP SENSE OF GRATITUDE

AND INDEBTEDNESS TO VIJAY MORE, CHIEF ENGINEER, RAJARAMBAPU PATIL

SAHAKARI SAKHAR KARKHANA, KARANDWADI UNIT FOR THEIR INVALUABLE

GUIDANCE, ADVICE, AND ENCOURAGEMENT EXUDED BY THEM AT EVERY PHASE

OF THE WORK.

IX. References

[1]. J SUDHAKAR, P VIJAY (2013). International Journal of Engineering Trends and

Technology (IJETT) – Volume 4 Issue5.

[2]. Wade A Amos (1998). National Renewable Energy Laboratory, 1617 Cole Boulevard.

[3]. Fundamentals of Heat Exchanger Design; By Ramesh K Shah and Dusan P Sekulic.

Copyright © 2003 John Wiley & Sons, Inc.

[4]. Rajarambapu Patil Sahakari Sakhar Karkhan, Karandwadi Unit, Maharashtra-India-

416301.

[5]. Heat and Mass Transfer; By Dr. R K Hegde, Niranjan Murthy. Copyright © 2013 Sapna

Book House (P) Ltd.

[6]. Steam Table; By R S Kurmi. Copyright © 2011 S Chand.

[7]. Heat and Mass Transfer Data Book (S.I.Units); By Domkundwar and Domkundwar.

Copyright © 2010 Dhanpat rai & Co. (P) Ltd.

[8]. E Hugot (1986). Handbook of Cane Sugar Engineering

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