Process Simulation and Optimization

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R jeas Research Journal in Engineering and Applied Sciences 1(5) 266-273 R jeas© Emerging Academy Resources (2012) (ISSN: 2276-8467)

www.emergingresource.org

266

PROCESS SIMULATION AND OPTIMIZATION OF PALM OIL WASTE

COMBUSTION USING ASPEN PLUS

Mohd Halim Shah Ismail, Zahra Haddadian, and Mohammad Amin ShavandiDepartment of Chemical and Environmental Engineering,

Faculty of Engineering, Universiti Putra Malaysia,43400 UPM Serdang, Selangor, Malaysia.

Corresponding Author: M ohd H alimS hah I smail

___________________________________________________________________________

ABSTRACT

The difficulty of controlling the required air during the incineration of the fiber and shell; the low heating value

of solid fuel due to the excessive moisture content of fiber and shell and further formation of slagging or clinkerin the reactor as a result of high ash content, are some problems raised when incinerating both fiber and shell. in

this simulation work, the effects of air flow rate; moisture content of shell, moisture content of fiber, and

moisture content of both shell and fiber; the ash content of fiber and shell and temperature were investigated and

optimized on flue gas emissions and the combustion behavior using steady state simulation by ASPEN PLUS

(Version 7.1). From the results obtained, the fiber-shell type solid fuel is preferable and the air flow rate should be controlled and suggested maintained at 30% excess air to regulate NOX emission. Besides, the moisture

content and ash shows negative effect to the combustion efficiency and the moisture content is suggested in therange of 6%-19.5% for fiber and 5%-13% for shell. Last but not least, the operating temperature is suggested donot exceed 972ºC to regulate the NOX emission.

©Emerging Academy Resources

KEYWORDS: Simulation; Modeling; Combustion; Aspen Plus; Fiber; Shell.

________________________________________________________________________________________

INTRODUCTION

World demand for energy sources is increasing, and

thus, renewable energy sources have become analternative to the depleting fossil fuel. One type of

renewable energy sources comes from combustion of

biomass waste, which is also called solid fuel, to produce heat and energy. This is a promising

technology to reduce waste and moreover provide aclean and renewable energy source by applying

waste-to-wealth concept. Malaysia has become thelargest exporter of oil palm product in the world (Foo

& Hameed, 2010). While generating huge income

from oil palm business, there are abundant of oil

palm biomass waste (generally fiber and shell)

generated at the same time. This biomass waste has been utilized to generate energy and electricity to

support the mill process. In addition, the fiber andshell are also burnt to generate steam for downstream

processes that required steam such as sterilization. As

such, a lot of savings can be done because this energyis considered free for the palm milling process. At the

same time, using the fiber and shell as boiler fuel canhelp to dispose these bulky materials which can

contribute to environmental pollution. The energycontent varies depending on the moisture, residual oil

contents and its high specific energy content. in 2003

a simulation work has been done by Mahlia et al .,(Mahlia, Abdulmuin, Alamsyah, & Mukhlishien,

2003) to develop a steady-space dynamic model for a

palm waste boiler. The solid fuel used was also fiberand shell from palm oil processing. However, in

Mahlia et al. study, moisture content and calorificvalue of fuel, and air-fuel ratio are assumed to be

constant, while temperature of the boiler is assumed

to be proportional to fuel rate. This is not the caseapply for the current simulation study because the

ultimate aim in this simulation work is to study theeffect of moisture content, ash content, air flow rate

and temperature on combustion process and flue gasemission. Bignal et al . (Bignal, Langridge, & Zhou,

2008) has investigated the effect of moisture content

of fuel and boiler operating conditions on pollutant

concentrations, and it is suggested that solid fuel

should have low moisture content in order to reduceair pollutants. In another study, Yang et al . (Yang,

Sharifi, & Swithenbank, 2004) has carried outmathematical simulations and experiments to studythe effect of primary air flow rate and moisture level

in the fuel on the combustion process of wood chipsand the incineration of simulated municipal solid

wastes. From Yang et al . (Yang, et al., 2004) study, itis found that volatile release and char burning has

been intensified with increasing in the primary airflow rate until a critical point is reached, and also

increase in moisture level in the fuel produces a

higher flame front temperature at low air flow rates.However, in the current simulation, the effect of

moisture level in the fuel on calorific value or mass



Research Journal in Engineering and Applied Sciences ( ISSN: 2276-8467) 1(5):266-273Process Simulation and Optimization of Palm Oil Waste Combustion Using Aspen Plus

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enthalpy is highlighted, but not on the flame fronttemperature. While for air flow rate, its effect on flue

gas emission is studied on current simulation work,which can be compared with study from Yang et al .

(Yang, et al., 2004). The typical values for proximate

and ultimate analysis on the dry basis of shell andfiber biomass feedstock are shown in Table 1.

Fiber Shell

Proximate

analysis,

wt%

• Moisture: 31.84

• Fixed carbon: 48.61

• Volatile matters: 13.2

•

Ash: 6.35

• Moisture: 12

• Fixed carbon:

68.2

• Volatile

matters: 16.3

• Ash: 3.5

Ultimate

analysis,

wt%

• Ash: 8.4

• Carbon: 47.2

• Hydrogen: 6

• Nitrogen: 1 .4

•

Chlorine: 0

•

Sulfur: 0.3

• Oxygen: 36.7

• Ash: 3.2

• Carbon: 52.4

• Hydrogen: 6.3

• Nitrogen: 0 .6

•

Chlorine: 0

•

Sulfur: 0.2

• Oxygen: 37.3

Table 1. Properties of the solid fuel used.

THEORY

Heating value is an important indication for solid

fuel, and it can be reported as high heating value

(HHV) or low heating value (LHV). The difference between HHV and LHV is equal to the heat of

vaporization of water formed by combustion of thefuel. The potential energy from fiber and shell can be

obtained by Dulong formula based on ultimateanalysis:

338.2 1442.8( )8( / )

1000

OC H

HHV M J k g

+ −

= (1)

20.16 2.24( / )

1000

HHV H M LHV MJ kg

− −=

(2)

From the steam generated amount, the amount ofsolid fuel consumed in the boiler can be known. With

a known palm oil milling capacity, the amount ofsteam needed to generate electricity can be calculated

based on the following equation:

Steam required = Energy required to process 1 ton of

FFB ! Milling capacity ! Amount of steam required

to produce 1kWh electric(

Theoretically, the potential energy conversion from

fiber and shell for palm oil mill can be represented bythe following formula:

= +P f f S S

E M LHV M LHV (3)

where Mf and Ms is the mass composition of fiberand shell in total solid fuel.

MATERIAL AND METHODAn Overview to SimulationThe software used in the simulation was ASPEN

PLUS (Version 7.1). The layout of simulationmethod was shown in Figure 1, which was from

reactor selection until gaining the final result ofsimulation.

Figure 1. Layout of simulation methods

Reactor Selection

The unit operation selected in ASPEN PLUS to run

the simulation was shown in Figure 2. Reactor B1

was RYield which represented Reactor Yield, and it

was used as non stoichiometric reactor based onknown yield distribution. Reactor B2 was RGibbswhich represented Reactor Gibbs was used for

rigorous reaction and multiphase equilibrium basedon Gibbs free energy minimization. As a result of the

reaction occurred between the fuel and air, flue gasthat composed of carbon, hydrogen, oxygen, carbon

monoxide, carbon dioxide, water vapor, nitrogen,sulfur, nitrogen dioxide, nitrogen trioxide, and sulfur

dioxide were produced. Unit operation B3 was thetwo streams heat exchanger which modeled co-

current or counter current shell and tube heat

exchanger. Flue gas with temperature at 800°C and20.27bar was exchanged heat with water at 70°C to

produce steam with temperature 260.69°C. This high

pressure and temperature steam was the final product

to generate electricity or for downstream processes

usage.

FIBER

SHELL

FUEL

AIR

FLUEGAS

WATER

STEAM

B1

B2

B3

Figure 2. Process flow diagram.

Data Specification and Aspen Simulation

In the simulation work, data and specifications ofinput material such as mass flow rate, temperature,

pressure, and component composition were entered.

During the simulation, several conditions were

studied by varying air flow rate, moisture content of

fiber and also moisture content of shell, ash contentof fiber and shell, and also temperature of

combustion reaction. Thus, data specifications and

properties for these conditions were explainedseparately. Some assumptions were made in the

simulation which may affect the accuracy of the finalresult. One of them was assuming coal properties for

fiber and shell density and enthalpy because both






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Table 5. Summary of each simulationSet 1 Set 2 Set 3

Solid fuel

Composition

70% fiber

and 30%

shell

100% fiber 100% shell

Mass of fiber, kg/hr 2360 3490 -Mass of shell, kg/hr 1011 - 3120

Total Solid fuel,

kg/hr

3370 3490 3120

Steam generated,kg/hr

18000 18000 18000

Moisture content infiber, %

31.84 31.84 -

Moisture content in

shell, %

12 - 12

Calorific Value,kJ/kg

27,573 27,167 28,415

Potential energy ,

kJ/hr

92,949,270 94,842,979 88,623,277

Mass flow rate of

CO2, kg/hr

6460 6591 6159

Mass flow rate of

SO2, kg/hr

20.2 23.01 12.46

Mass flow rate of

NO, kg/hr

3.2 3.17 3.22

Mass flow rate of NO2, kg/hr

0.5 0.49 0.51

Mass flow rate of

NO3, kg/hr

Trace

(3.21x10-7

)

Trace

(3.13x10-7

)

Trace

(3.27x10-7

)

From the calculation, it shows that solid fuelconsumed in simulation set 2 is higher than set 1 and

set 3. This is because the fiber has lower heating

value/calorific value compared to the shell, and thiscan be proven from the simulation results.

Figure 3. Calorific value for different fuel.

Figure 3 shows the calorific value of the solid fueldetermined by computer simulation. From the results

obtained, it is obviously shown that shell has the

highest calorific value, and the fiber has lowestheating value. Meanwhile, the simulation set 1, which

comprised of solid fuel mixture, has moderate heatingvalue. The high calorific value of shell can be due to

the low moisture content. Besides, there are many

similar researches, which were proven that moisturecontent in the shell is usually much lower than fiber

and eventually has higher calorific value (Li, Yin,Zhang, Liu, & Yan, 2009; Olufayo, 1989; Werther,

Saenger, Hartge, Ogada, & Siagi, 2000).

Experimentally, the calorific value for palm waste is

in the range of 18MJ/hr to 20MJ/hr (Yusoff, 2004).The results obtained from the simulation are

obviously higher than the experimental value. Thismay due to the limitation of software Aspen Plusused in this report. In the nutshell, it is suggested that

the shell which has the highest calorific value is the

most suitable solid fuel for combustion. However, theamount of shell produced for a typical palm oil mill is

60kg/hr for every 1ton FFB/hr. This amount is notenough to generate steam for whole plant. Thus, it is

suggested that solid fuel with the fiber-shell mixture

is the better solution. The composition of flue gas isanalyzed in this report with the aid of computer

simulation. Figure 4 shows the major gas constituentin the flue gas for different type of solid fuel. From

the results obtained, the CO2 is the major component

in the flue gas and followed by SO2, NOX.Meanwhile, the simulation results show that there is

no CO gas emission. This is due to the completecombustion of solid fuel in the excess air, and all the

carbon are converted to CO2. Besides, the resultsshow that the amount of NO3 is extremely small and

can be negligible and the NO is the majority of NOX.According to the research, the NOX are

predominantly NO and NO2, in which NO is the 95%

of total NOX (Ganapathy, 2003).

Figure 4. Mass flow rate of major gases in flue gas

When compare the gas emission among the solidfuel, solid fuel with fiber solely has the highest CO2

and SO2 emission. On the other hand, the solid fuelwith shell solely has the highest NOX emission.

Meanwhile, the solid fuel consists of fiber-shell

mixture has moderate emission for all gases. This

obviously showed that shell which has the highest

calorific value is not suitable to become the onlysolid fuel in the combustion. In order to reduce theemission, the solid fuel should comprise of both fiber

and shell. A table that summarized the results of gasemission for all types of solid fuel showed as below.

Table 6. Summary of emission in excess air.Gaseous Fiber-shell

mixture Fiber Shell

CO2 Moderate High Low

SO2 Moderate High Low

NO Moderate Low High

NO2 Moderate Low High NO3 Moderate Low High

Effect of Air Flow Rate to the Composition of Flue

Gas

In order to avoid emission of CO, the solid fuel must

be combusted in the condition of excess air. In thisreport, the air flow rate is varying in the range from




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20000 kg/hr to 40000 kg/hr to determine theminimum air required and investigate the effect of air

flow rate to the emission. Theoretically, the minimumair required for complete combustion can be

determined by observing the emission of CO and

CO2. Figure 5 shows the amount of CO and CO2 released when burning of fiber-shell mixture at

different air flow rate. From the results obtained, theamount of CO released is decreasing with the

increase of air flow rate while the amount of CO2 is

directly proportional to the air flow rate. The presentof CO at the front part of the graph shows the

insufficiency of oxygen in reaction. However, bothgraphs are become flat when the air flow rate

increases to 24827 kg/hr. This is because completecombustion has occurred at the air flow rate of 24827

kg/hr. At this point, all carbons in the solid fuel areconverted into CO2 and further increment in air flowrate will not increase the amount of CO2 released.

Hence, the air flow rate required for fiber-shell solidfuel with the mill capacity 30 tons FFB/hr is

24827kg/hr. The effect of combustion in the excess

air will be analyzed by observing the emission of SO2

and NOX.

Figure 5. Effect of Air flow rate to the emission of

CO and CO2

From the results showed in Figure 6, the emission ofSO2 has the similar trend with CO2. The amount of

SO2 is increasing sharply until the point of minimumair flow rate, and it becomes constant under the

condition of complete combustion. For the emission

of NOX, the trend was slightly different with thegases previously and the amount of NOX released is

relatively small, especially the emission of NO3.According to the results, the NOX starts to release

when there is sufficient for oxygen present, and its

amount is increasing in the condition of excess air.

Figure 6. Effect of air flow rate to the emission of

SO2 and NO2

Figure 7. Effect of air flow rate to the emission of

NOX

The trend of NOX emission can be explained throughthe theory of NOX formation in literature. According

to Ganapathy, (Ganapathy, 2003) NOX are produced

during the combustion of solid fuel through theoxidation of atmospheric nitrogen and fuel-boundnitrogen. These sources produce three kinds of NOX,

which is fuel NOX, prompt NOX, and thermal NOX.The fuel NOX is generated when nitrogen in fuelcombines with oxygen in combustion. This type of

NOX is insensitive to flame temperature but isinfluenced by oxygen. Thus, when there is sufficient

oxygen present, the nitrogen bound in solid fuel will

turn into NOX and the amount of fuel NOX eventually becomes constant at certain air flow rate. On the

other hand, the thermal NOX produced will not become constant when the air flow rate increasing.

This is because the thermal NOX is produced when

atmospheric nitrogen combines with oxygen underintensive heat. When there is a rise in air flow rate,the amount of NOX generated will be increasedexponentially. Hence, the mass flow rate of NO, NO2,

and NO3 in the figure 7 increase exponentially when

the air flow rate is increasing.

According to the industry practice, the 30% excess

air is applied to the solid fuel combustion. Thus,

when 30% excess air is applied to the simulation, thecomposition of each gas is shown as table 7.




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Table 7. Composition of gases in 30% excess air.

Comparing the reading in Table 7 and 8, the amount

of CO2 and SO2 released are same. However, theamount of NOX released in 30% excess air is

significantly reduced. In short, the solid fuel whichconsists of solid only has the best performance in

30% excess air and the solid fuel which comprises

fiber-shell mixture has moderate gases emission.Hence, it is suggested that the solid fuel in palm oil

mill should be comprised of fiber-shell mixture orshell only.

Table 8. Summary of emission in 30% excess airGaseous Fiber-shell mixture Fiber Shell

CO2 Moderate High LowSO2 Moderate High Low

NO Moderate High Low

NO2 Moderate High Low

NO3 Moderate High Low

Effect of Moisture Content to the Heating Value

and Heat Duty of Boiler

The proximate analysis of fiber and shell issignificantly different. The fiber has the typical

moisture content in the range of 6% to 33% while itis 5% to 13% for the shell (Li, et al., 2009). As a

result, the effect of moisture content to the heat of

combustion is investigated in this report. Table 9

shows the summarized of each trial in the simulation.

Table 9. Summary of each trial in the simulation

Trial

Moisture

Content of

Fiber, %

Moisture

Content of

Shell, %

Calorific

Value of

Fuel, kJ/kg

Heat duty

of Boiler,

kJ/hr

1 6 5 27573.2 111507488

2 6 9 27573.2 111950207

3 6 13 27573.2 112392926

4 19.3 5 27573.2 115014102

5 19.3 9 27573.2 115456821

6 19.3 13 27573.2 115899540

7 33 5 27573.2 118520716

8 33 9 27573.2 118963435

9 33 13 27573.2 119406154

Figure 8. Effect of moisture content to the calorificvalue

From results in Figure 8, the mass enthalpy is directly proportional to the moisture content. As mention

previously, the present of moisture content in solidfuel eventually decreases the heating value. The

computer simulation failed to perform the effect of

moisture content to heating value can be due to thelimitation of Aspen Plus. The computer simulation is

suggested alter other properties on solid fuel in orderto get the constant heating value for the solid fuel.

However, the effect of moisture content to low

combustion efficient can be observed from the Figure9 which shows the relationship between the moisture

content and heat duty of reactor.

Figure 9. Effect of moisture content to heat duty

From the graph plotted, the heat duty of reactor is

directly proportional to the moisture content. Whenthe moisture content is high, the reactor has to work

more to provide sufficient energy for combustion.

Again, the results show that solid fuel with fiber only

is not environmental and economically friendly. Thesolid fuel used in the boiler should be shell or fiber-

shell mixture. Figure 10 shows the heat duty ofreactor by using fiber-shell mixture as solid fuel.

Figure 10. Effect of different moisture content

combination to heat duty

From the graph plotted in Figure 10, it shows that the

effect of fiber moisture is more significant comparedto the shell. This may be due to the high moisturecontent of fiber which required extra energy to

vaporized water in the solid fuel. In short, the fiber-shell mixtures are better than fiber only. Therefore,

the fiber-shell solid fuel, which has the moisturecontent similar with trial 4 to 6 is suggested. The

Fiber-shell Fiber Shell

CO2 6460 6591 6159

SO2 20.2 23.01 12.46

NO 1.186 1.231 1.133

NO2 0.12 0.125 0.112

NO3 4.932x10-8 5.211x10

-8 4.575x10

-8

Air flow rate,

kg/hr

32,275 33,172 31,379




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results may slightly deviate from the experimentalvalue due to the assumption Aspen Plus.

Effect of Ash Content to the Heating Value

Ash content is referring to the mass fraction of

incombustible material in solid fuel. It was foundthat the ash is the heat sink in the same way as

moisture, lowering combustion efficiency (Ciolkosz,2010). Besides, the similar literature showed that

systems that are designed to combust wood can be

overwhelmed by the volume of ash if other biofuelsare used, which can reduce the combustion efficiency

or clog the ash handling mechanisms (Pritchard,2002). Thus, the relationship between heating value

of solid fuel and ash content is the study to predictthe effect to combustion efficiency. Table 10 shows

the summarized of each trial in the simulation.

Table 10. Summary of each trial in the simulation ash

sensitivityTrial Ash

content

ofFiber,

%

Ash

Content

ofShell,

%

Heating

Value of

Fiber,kJ/kg

Heating

Value of

Shell,kJ/kg

Heat duty

of Boiler,

kJ/hr

1 2 2 -8833.66 -

6346.94

120213475

2 2 4 -8833.66 -

6158.07

120022532

3 2 6 -8833.66 -

5969.21

119831589

4 5 2 -8614.24 -

6346.94

119695626

5 5 4 -8614.24 -

6158074

119504683

6 5 6 -8614.24 -

5969208

119313740

7 8 2 -8394.81 -

6346939

119177778

8 8 4 -8394.81 -6158074

118986835

9 8 6 -8394.81 -

5969208

118795892

Figure 11. Effect of ash to the mass enthalpy of solid

fuel

From the results present in Figure 11, the heating

value of solid fuel is inversely proportional to the ashcontent. When there is high ash in the solid fuel, the

heat released from the combustion will decrease. This

is because the heat of combustion for a given fuel is

mostly a function of the fuel’s chemical composition.

Thus, more incombustible ash contained in a specificamount of solid fuel is eventually decreasing the heat

of combustion and yield low combustion efficiency.In terms of heat duty, the heat duty for the furnace

will be lower at high ash content (Figure 12). This is

because less work is done by the furnace to combustthe solid fuel which has the high volume of

incombustible material.

Figure 12. Effect of ash content to heat duty

Effect of Temperature to Gas Emission

As discussed in section 4.2, the NOX emission will

not be constant with the increasing of air flow ratedue to the thermal NOX. However, Ganapathy

(Ganapathy, 2003) proposed that the formation ofthermal NOX increase exponentially with theincrease in operating temperature because it is a

function of flame temperature. For the natural gas in15% excess air, it was found that each 37.78ºC

increase in combustion temperature will increase theflame temperature by 18.33ºC. So, the operating

temperature is the study in this section to investigatethe effect of temperature to NOX and determine the

optimal operating temperature. The temperature in

the furnace is varying from 600ºC to 1200ºC and the

NOX released is shown in Figure 13 and 14.

Figure 13. Effect of operating temperature to NOx emission.




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Figure 14: Effect of operating temperature to NO3

formation.

From the results in Figure 13 and 14, the amount of

NOX released is exponentially increased to theincrease of operating temperature. The emission is

increasing sharply at higher temperature (1000ºC).

According to the information provided by Ciolkosz(Ciolkosz, 2010), the operating temperature of

furnace should not over 972ºC in order to regulate to

NOX emission. Therefore, the assumption of 800ºCoperating temperature in this report is acceptable.

CONCLUSION

From the results obtained from computer simulation,

the mixture of fiber-shell solid fuel is better

compared to the solid fuel with fiber only. The shellwhich has the highest heating value and lower

emission cannot be used as the only fuel in

combustion because the typical amount of shell

produced is unable to sustain whole mill. Moreover,

the air flow rate should be controlled and theemission at 30% excess air is acceptable.

Furthermore, the moisture content is determined

lower the combustion efficiency, and the suggestedmoisture content of fiber and shell is 6%-19.5% and5%-13%, respectively. Last but not least, the

operating temperature should not exceed 972ºC as itwill promote the NOX emission.

Due to the limitation of Aspen Plus, the effect ofmoisture content on heating value and the tolerable

ash content is unable to identify. However, it is

suggested that heating value is inversely proportionalto moisture content and the ash content. In order to

improve the findings in the future, other simulationsoftware, which is purposely designed for

combustion or design of boiler, can replace the Aspen

Plus in order to get an accurate result. For futureresearch purpose, ASPEN PLUS is suggested to be

replaced by other computer simulation softwarewhich is design for boiler.

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