EXERGY bagasse plant at Ratchasima Sugar Mill, Thailand

EXERGY ANALYSIS OF BOILER IN COGENERATION BAGASSE PLANT

Lakshmi Naga Swetha Yanamandra & Sethupathy Rethinasamy

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Halmstad University, Sweden

 

ABSTRACT

Bagasse, the
loss from sugar handling, is one of the biomass build-ups utilized as fuel. It
would thus be able to be a valuable inexhaustible asset for vitality which
likewise guarantees to keep away from the natural discharges of energy age from
petroleum derivatives. The bagasse plant at Ratchasima Sugar Mill, Thailand is
considered for our project. The sugar process forms 30,000 tons of sugarcane
for each day. The power plant, joined to the sugar process, is of cogeneration
compose. The vitality and water adjust demonstrate that 272 tons of bagasse can
create 342 tons of steam at 420C utilized as a part of the sugar procedure and
around 25.5 MWh of power. Consequently, bagasse can be a naturally neighbourly
crude material for control age and has high potential as another, sustainable
source of energy. This work is mainly energy and exergy investigation of sugarcane
bagasse boilers. The technique depends on the standard and genuine responses
which permits the count of the enthalpies. The strategy is given utilizing a
case genuine information from a sugarcane bagasse plant.

Keywords:
Cogeneration plant, Boiler, Exergy, Energy, Bagasse

 

INTRODUCTION

Exhausting
conventional energy resources and mounting pressure to reduce carbon emissions
together make the conservation of energy and the identification of new and/or
renewable energy sources the prime challenge for industry and the public at
large today. Resourceful usage of energy becomes more important in
our world where the fossil fuels are limited. Industrial
consumers of energy resources must implement energy saving techniques as a
means for achieving cost reductions and competitive advantage.

The cogeneration of power and steam (combined heat and
power – CHP) is a proven method of energy cycle optimization. The main process
industry segments that benefit from CHP are textiles, paper, food, district
heating and sugar production. The last decade witnessed a huge momentum in
sugar cogeneration, with many opting for CHP plants. The primary model
envisioned power as a by-product to sugar production and a shield against the
cyclical nature of sugar price between the production season and the off
season.

The advantages obtained
by using cogeneration system are:

1. Due to usage of
electricity where it is generated, there is no loss of transfer and by
regaining the heat energy, the cost of energy is kept low. 

2. Saving from the
primary fuel in proportion to regained heat energy, the limited sources are
used efficiently furthermore emission harm is reduced to minimum. 

3. National electricity
generation is supported.

 

 

WHY BAGASSE USED IN POWER
GENERATION PLANT?

Bagasse is the fibrous
material that comes after sugarcanes are crushed to extracting the juice for
sugar production process. This sugarcane juice is a product of the process of manufacturing sugar. The
bagasse can either be a sale or captively
consume for generating the steam as a fuel in a boiler. So literally, it’s a biofuel. Apart from power
generation, the sugarcane trash(Bagasse) used for manufacturing of pulp and
paper products. Every 10 tons of
sugarcane can give around 3 tons of natural bagasse (50% of wet). Bagasse is
the by-product of sugar production industry. Moreover, the main use of bagasse
is to avoid the use of fossil fuels like coal and gasoline and reduce the
carbon dioxide emissions from the stack. Globally, the bagasse has already begun
to use in the cogeneration power plant
which has boiler, steam, and turbine. So, the carbon dioxide emission will be achieved by the help of bagasse.

Over the years, as the sugar mills have tended towards
cogeneration, combustion technology has also advanced. Today supplementary
fuels are used along with bagasse, namely coal, biomass and biogas (a
by-product in an integrated sugar mill). Using a by-product – bagasse – as an
energy source and producing combined heat and power (CHP) to meet their sugar
mills’ demand makes immense sense for customers. In addition, surplus
electrical power from the cogeneration process can be exported to the state
grid.

 

Fig
1: Schematic diagram of a bagasse plant

 

EXERGY:  It’s known that energy can’t be created nor
destroyed due to the first law of thermodynamics, only converted. In an
irreversible process, the quality of energy decreases according to the second
law of thermodynamics. This is described by the concept of entropy. If the
entropy increases in a system, then the quality of the energy decreases. Exergy
(E) is defined as the maximum theoretical work (W) that can be extracted from a
system, meaning that entropy and exergy pretty much explain the same thing.

 

 

 

 

SYSTEM
DESCRIPTION:

Based on the information
accessible for the Ratchazima Sugar Factory in Nakorn Ratchazima, Thailand, the
plant limit is 30,000 tons of sugarcane every day consistently amid the crushing
season. The power plant is cogeneration sort with 2 water-tube boilers and 2 steam
turbines.

Demineralized water is
used in the boilers at start-up operation and condensate water from sugar
processes is reused from there on. Steam created in the boilers is used in the
sugar processes and steam turbines. This control plant by and large operates at
24 MW, using 16 MW for the sugar production line and power plant.

For the project work we
have taken single boiler for analysis.

 

 

                                           
Flue gas

 

                     Feed water                                      

                                                                            
Steam   Steam

                      Bagasse

                                            

                                                  Ash

 

Fig:
Schematic representation for Boiler

 

BOILER:

                        TYPES:

                                         
I.        
Fire-tube boiler

                                       
II.        
Water-tube boiler

Boilers
are classified as a high-pressure boiler
(or) low pressure and steam-boiler (or) hot water boiler. The high-pressure boiler can operate higher than 160psig.
High-temperature hot water boiler has a minimum temperature of above 120
celsius or pressure minimum of  160psig.

Low-temperature hot water boiler has not greater than 120
Celsius or pressure higher than 160psig. The casting of the boiler is usually
iron, bronze (or) brass during their construction. Those that are insulated
uses of steel, brass, and copper with steel is the most common material.

 

The following definitions
are acknowledged for analysis calculations of the cogeneration system: 

• Equipment are systems
having continuous flow.

 • Boiler, pipes and components of other
installations are considered with heat losses. 

• The fuel enters boiler
under environmental conditions.

 • Reference state T0 = 298 K and P0 = 1 bar

•      
Feed water= 637t/h @ 108oc

•      
Bagasse=272t/h

•      
Steam=605t/h @420oc

•      
Flue gas=833981.1m3/hr @141.5oc

•      
Ash=39.82 GJ/hr

Energy
analysis of Boiler:

Energy balance: EW+Eb=Eflue+Esteam+Eash+Eloss

Energy
of Feed water:

h= 452.84 KJ/Kg

? (feed water) = 637 t/h

Ew= h*?=
288.45 GJ/hr

Energy
for bagasse:

Calorific value= 1750
Kcal/Kg= 7.32 GJ/ton

?(bagasse)= 272t/h

Eb=
1991.04GJ/hr

Energy
of steam:

h= 3.26GJ/t

?(steam)= 605 t/h

Esteam=1972.3GJ/hr

Energy
of flue gas:

?(flue)= 833981.1 m3/hr

flue gas temp= 141.5°c

Mean gas specific heat=
0.00137 J/m3°c

Energy
of flue gas = 133.10 GJ/hr

Energy
of Ash:

2% of total heat=
39.82GJ/hr

Energy loss:

6.7% of total heat=
134.29 GJ/hr

Energy
Efficiency calculation:

 

 

Exergy
analysis of Boiler:

Exergy
of Bagasse:

Analysis of bagasse
composition of Ratchasima Sugar Factory:

COMPONENT

WEIGHT VALUE
(%)

MASS FLOWRATE
(Kg/s)

MOLAR MASS
(Kmol/s)

Carbon

41.54

28.4

2.36

Hydrogen

5.40

3.69

3.66

Oxygen

33.14

22.7

1.42

Nitrogen

1.83

1.23

0.088

Sulphur

1

0.685

0.021

 

 

 

Total
(?)  = 7.5

 

 

Ex, fuel=

 

Chemical exergy of
fuel:

Exch= = 2377.53Kcal/Kg

h,c,o,s are the mass
fractions of hydrogen, carbon, oxygen and sulphur respectively.

 

Molar mass of bagasse=
7.5 Kmol/s

Reference temperature
and pressure:

To= 25oc=
298k

Po= 1 bar

ho=104.89KJ/kg

So=0.3674KJ/kg.k

Exergy
of feed water:

T= 108oc

h=452.89

s=1.3964

?=160.52kg/s

Ex,feed water=?(h-ho-To(s-so))=
6639.10KJ/s

Exergy
of steam:

T=420oc

h=3263.73KJ/Kg

s=6.864KJ/Kg.k

?=152.45kg/s

Ex,steam=?(h-ho-To(s-s0))=
186426.97KJ/s

Exergy
of flue gas:

Ex,flue gas=?(h-ho-To(S-So)+exch)

 

Composition of Flue gas
in the stack:

PARAMETER

FLOW RATE
(Kg/s)

MOLAR MASS
(Mol/hr)

SO2

0.22

3.43

NO2

1.86

40.4

CO

255.7

9128.71

 

 

Total
(?)  = 9172.54

 

*co2
emission from the boiler is not considered, it doesn’t contribute global
warming.

Considering each case,

1)   
SO2:

SO2=
0.22Kg/h

Molar
mass=64.0628 kg/kmol

Exergy
of SO2=0.298KJ/s

 

2)   
NO2:

NO2=
1.86 kg/h

Molar
mass= 46.0055kg/kmol

Exergy
of NO2= 0.62KJ/s

 

3)   
CO:

CO=255.7kg/h

Molar
mass= 28.0105kg/kmol

Exergy
of CO=697.58KJ/s

 

Exch=exSO2+
exNO2+ exCO= 698.50KJ/s

 

Exergy
of flue gas @141.5oc

T=
141.5oc

h=2735.81KJ/kg

s=6.916KJ/Kg.k

Ex,flue
gas=?(h-ho-To(S-So)+exch)=3513.72

 

Ash:

Chemical analysis of the
sugarcane bagasse ash:

COMPONENT

WEIGHT VALUE
(%)

MASS FLOWRATE
(Kg/s)

SiO2

85.5

1.17

Al2O3

2.29

0.03

Fe2O3

1.21

0.016

TiO2

0.20

0.00274

CaO

4.05

0.0554

P2O5

3.01

0.041

SO3

2.28

0.031

K2O

1.33

0.018

MnO

0.8

0.010

 

Convert
into molar fraction:

 

1)    SiO2=1.17kg/s

Molar mass=60.085kg/kmol

Exergy of SiO2=7.9*0.0194=0.1532*103KJ/s    

2)    Al2O3=
0.03/101.96=0.0589*103KJ/s

3)    FeO3=
0.016/159/69=1.65KJ/s

4)    TiO2=2.74*10-3/79.90=0.731KJ/s

5)    Cao=0.0554/56.08=0.1087*103KJ/s

6)    P2O5=0.041/283.8=0.1188103KJ/s

7)    SO3=0.031/80.06=0.0964103KJ/s

8)    K2O=0.018/94.20=0.0789103KJ/s

9)    MnO=0.010/70.93=0.0167103KJ/s

 

Total molar mass of ash=?=25.06mol/s

 

Total chemical exergy of ash=exch=633.98KJ/s

 

Exergy of ash=Ex,ash=?*exch=15887.56.

 

Exergy Efficiency:

 

 

 

Where, To=25oc=298k

 

E/Q of steam=( =0.11

 

E/Q of water=

 

Substituting the values obtained,

 

 

Exergy of boiler= 21%

 

Exergy destruction:

 

 

 

RESULTS
AND DISCUSSION:

In this study, exergy analysis is done by
taking the values of measurements of a cogeneration system located in Thailand
into consideration.  In the table below,
the energy and exergy accordingly are given.

 

Parameter

Energy

Exergy

Feed water

288.45

6639.10

Bagasse

1991.04

17947.97

Steam

1972.3

186426.97

Flue gas

133.10

3513.72

Ash

39.86

15887.56

Efficiency

84%

21%

 

Considering different temperatures

 

Exergy Efficiency:

When  To=25oc,

            To=20oc,

            To=15oc,

            To=10oc,

 

 

CONCLUSION:

An
exergy and energy efficiencies are determined and calculated on the data basis
of a sugarcane cogeneration plant in Thailand.Analysis of calculations shows
values of exergetic efficiency essentially less then corresponding energetic efficiency.
Calculated value of energy is 84%, at the same time value of exergy is equal to
21%. That shows quite low degree of thermal energy potential use in given
technological scheme and working conditions.  

 

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