EXERGY for control age and has high potential

EXERGY
ANALYSIS OF BOILER IN COGENERATION BAGASSE PLANT

Lakshmi
Naga Swetha Yanamandra & Sethupathy Rethinasamy

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.

Co-generation is the concept of
producing two forms of energy from one fuel. One of the forms of energy must
always be heat and the other may be electrical or mechanical energy. In a
conventional power plant, fuel is burnt in a boiler to generate high-pressure
steam which is used to drive a turbine, which in turn drives an alternator
through a steam turbine to produce electrical power. The exhaust steam is
generally condensed to water which goes back to the boiler. As the low-pressure
steam has a large quantum of heat which is lost in the process of condensing,
the efficiency of conventional power plants is only around 35%. In a
cogeneration plant, high productivity levels, in the scope of 75%– 90%.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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