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Palm oil mill is among the few industries which are self-sustainable as they are able to generate their own

utilities; electricity and steam.

The steam system in the palm oil mill consists of a highpressure boiler, a turbine and a back pressure vessel

(BPV).

The fuel to the boiler usually consists of palm oil waste;empty fruit bunches (EFB), shell and fiber.

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The high pressure boiler supplies high pressure steam at

35 bar, which is mainly passed through three streams; i.e.stream 201 A, 201 B and 201 C, which forms 7%, 10% and

84% of the boiler output respectively.

Steam was supplied to the vacuum dryer in stream 201 Aand make up valve in stream 201 B.

At stream 201 C, steam was supplied to the turbine to

generate electricity.

The exhaust steam from the turbine, which is now at 3

bar is channelled to the BPV.

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The BPV functions as a temporary storage of

low pressure steam, distributing the steam

mainly to sterilizers and other processes in themill.

Nevertheless, some of the steam ends upgetting lost to the atmosphere.

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The main process that dominates the pattern of steam

demand and distribution is the sterilization process.

This process involves the extraction of crude palm oil by

freeing the fruits from the fresh fruit bunch (FFB) and

breaking the oil cells in the mesocarp.

This operation uses the largest amount of steam in the

palm oil mill.

However sterilisation, tend to cause the steam demand

to fluctuate severely as it is operates in batches.

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Boiler

TurbineSteam

Accumulator

Loss to

Atmosphere

Other

Processes

Sterilizer

Vacuum Dryer

Fibers

Shell

7% (Sivasothy Kandiah et al., 1992)



STEAM DISTRIBUTION IN A

TYPICAL PALM OIL MILL

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In palm oil mill, the steam system plays a vital role as

energy or steam supplier. It not only produces steam forprocesses such as sterilisers and other processing stations

but also distributes steam to the turbine for the

generation of electricity.

This production of steam as a source of heating and

power generation has a great advantage.

The cost of fuel used for the boiler combustion processis low due to the availability of waste materials such as

fibre and shell generated during the production of crude

oil and kernel.

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In addition, the consumption of diesel oil used assupplementary fuel for the generation of electricity is also

reduced as a result of the above available sources of

energy.

Heat from combustion of fibre and shell is transferred

through tubes or drums to heat the water in the boiler

which then evaporates as steam.

A typical arrangement of the main components of a

steam station in palm oil mills is shown in next figure.

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BOILER

STERILISERSBACK PRESSURE VESSEL

FIBRE/SHELL

P : 40-45 psig

P : 250-300 psigPOWER

PROCESS

HIGH

PRESSURE

PROCESS

TURBINE

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The steam leaves the boiler through the boiler outlet or

main steam line (high pressure main):

This main steam line branches out into three sub-

lines, where one of the lines carries steam to the high

pressure processing stations.

The second sub-line channels steam from the boiler

into the turbine.

The third sub-line conveys steam directly from theboiler to the back pressure vessel (BPV) and the flow of

this sub-line is controlled by a pneumatic make-up

valve.

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In the turbine, part of the high pressure steam istransformed into electrical energy and the remainder is

exhausted as low pressure steam before entering the

BPV, which acts as low pressure steam storage vessel

(low pressure main).

From the BPV, the low pressure steam is distributed

to sterilisers and other heating stations such as press

station, clarification station and nut and kernel silo.

The low pressure steam at about 40 psig is used to

cook fresh fruit bunches (FFB) for a certain period of

time in the steriliser.

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One of the most difficult units to control in a palm oilmill is back pressure vessel (BPV).

Also known as steam accumulator, BPV is critical and has

to be kept stable all the time.

The size of the BPV used in the mill is 1.0 meter in

diameter and 5.0 meters in length.

BPV control is difficult as it has to balance the steam

supply and demand to several processes, such as boiler

combustion, turbine steam consumption, sterilization and

other mill heating processes.

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If the steam demand is high but the storage is

insufficient to be supplied, steam is bypassed directly

from main stream and the flow is controlled by a make-up

valve, located in stream 201 B.

This valve only functions when the BPV pressure is less

than 3 bar. For low steam demands, excess steam is blown

off to the atmosphere through a relief valve to maintain

the BPV pressure.

This relief valve functions when the BPV is more than 4

bar.

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A major problem associated with back pressure vessel is

imbalances of steam demand.

A steam deficit or low back pressure arises fromexcessive steam demand during sterilizer peaking

operation.

While low steam demand occurs rapidly during sterilizerexhaust and holding periods resulting in excessive steam

blow off.

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This causes a steam surplus in the BPV.

If this problem is not tackled, the operations of steam

turbine and mill processing will be affected.

This could result in operation down time thus affecting

mill throughput.Besides, the design of BPV also influences

its operations to a certain extent.

The specific details for this design are closely guarded

by industrial designers as trade secrets.

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A steam accumulator is a pressure vessel, partially filled

with hot water that allows the operation of boiler at a

constant output equal to the average steam demand.

To use a steam accumulator, there must be some

demand on steam at a pressure significantly lower than

boiler and the maximum high pressure demand cannot

exceed the boiler operating rate.

The fluctuating load can be either at low pressure main

or high pressure main.

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However, when the steam consumption of a factory or

mill is fairly constant and there are no sharp peak

demands, then an accumulator is not required

Steam stored in a steam accumulator is immediately

available to the consumers and can be used to supplement

various types of steam demand.

For short periods, the accumulator can discharge steam

at very high rates and reduce the size of the boiler plant

required.

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When steam is subject to regular cycles or predictablefluctuations, the load can be balanced for hours or even

days.

In every case, there will be a more steady load on the

boiler plant, whereby losses are reduced and fuel savings

achieved.

Thus, steam storage acts as an additional tool to ensure

that steam supply is adequate and efficient at any time.

Furthermore, it can convert the most unfavourable type

of steam demand fluctuation to the most favourable type

with steady consumption

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Goldstern (1970) has reported that an adequate size of

accumulator installation is capable to introduce an elastic

connection between the steam generator and the user.

The steam accumulator can take the sensitive loadfluctuations.

Consequently, the generation of steam from the boiler can

be made uniform and held constant for long intervals.

The effect of steam accumulator on balancing pressure

fluctuations in the steam demand is shown in next figures

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0

5

10

15

6 8 10 12 14 16

Time of day (hr)

P

ressure

Pressure variation without steam accumulator (Goldstern, 1970)

02

4

68

10

12

14

6 8 10 12 14 16

Time of day (hr)

Pressure

Figure 2.3 : Pressure variation with steam accumulator of adequate size

(Goldstern, 1970)

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The actual pressure required for most of the processes in

palm oil mills is low (40 to 45 psig) compared to the steam

main from boiler output (250 to 300 psig).

This system allows for the utilisation of the exhaust low

pressure from a turbine or steam engine.

Fibre and shell are used as fuel to produce the electricalpower, steam supply and meet all processing demands.

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The steam demand for sterilisation operation is notregular, depending on the size and the number of steriliser

units used in mill operation.

On the contrary, the steam demand for other process

requirement tends to be steadier.

The steam distribution to various stations of a typical

palm oil mill for a triple peak sterilisation of 25 tonnes/hr

of FFB under normal operation is shown in next table

In a triple peak sterilisation, steam is admitted into and

removed from sterilisers for a series of three continuous

cycles.

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The purposes of this steam admission and removal cycles

are to remove the air in the sterilisers and ease the build-

up of cooking pressure for proper sterilisation.

The figure for steam utilisation shown in this table is

only an average overall hourly distribution and does not

reflect instantaneous value especially during sterilisation.

In the initial pressure build-up stage of sterilisation,steam demand to sterilisers may be varied accordingly to

the size and the number of steriliser units, as well as

sterilisation method.

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Measurement Steam Flowrate(kg/hr) Percentage of BoilerOutput (%)

Boiler output 14744 100

Before back pressure vesselTurbine

Make-up

Vacuum dryer

12389

1540

1068

83

10

7

Before back pressure vessel

Sterilisers

Other processes

Loss to atmosphere

5219

3302

5433

37

24

39

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This can easily exceed the boiler capacity if the steam

distribution is not accurately regulated, and if the

sequencing cycles are not properly controlled.

Utilization of existing energy resources is crucial not

only for large industrial processes but also for small

production plant and in particular oil palm mills where the

balance between heat and power are required for

production process which are pre-condition for a

combined heat and power (CHP) scheme. Or commonly

referred to as CO-GENERATION SYSTEM.

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Solid waste fuel in the form of shell, fibre and empty

bunches which are by-products of the process are utilized

as fuel for the boiler.

Steam is required for processing at the approximate rate

of 500kg per hour per ton FFB.

This steam can be easily raised in a reasonably efficient

water tube boiler with fuel available from the fibre, shell

and empty bunch.

S lid F l Ch i i &

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Solid Fuel Characteristic &

Combustion

Existing Boiler System For Palm Oil Mill.

A conventional palm oil mill: 20 - 40 ton

biomass/hour

Electrical energy required to process 1 ton of

FFB is about 20 kWh.

In Malaysia the most widely used boiler is

of D type which is either in the form of fire

tube or water tube.

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S lid F l Ch t i ti &

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CombustionFire tube boiler is normally used for small millto process about 20 ton to 30 ton per hour and

water tube boiler is used for 50 ton to 60 ton

per hour.

In this boiler, combustion of palm oil biomass

generates steam and this steam is used for

power generation, sterilization and digestion.

Flue gases resulted from combustion process

are transported to the chimney.


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The Existing Boiler System at Palm Oil Mill


Combustion


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Properties/Chemical of Solid Fuel

Four standard tests are common:

The ultimate chemical analysis determines the mass

percentage of C, H, O, N, S, ash and water of thefuel.

The proximate analysis, determines the mass

percentage of Volatiles,Ash, and Fixed Carbon.

A test to determine the moisture content of the fuelon a wet or dry basis.

A bomb calorimeter test to determine the Higher

Heating Value (HHV) of the fuel in MJ/kg, as received.


Combustion

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ONE SHOULD BE AWARE OF THE FOLLOWING: The HHV of the fuel is determine by a laboratory. Most

laboratories report the HHV on a moisture free basis

(mf). That may cause the misconception that the

HHV is the energy content of a fuel in a totally drystate.


Combustion

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The LHV is not measured, but rather calculated from

the HHV. The amount of water vapor generated during

the combustion of the fuel must be known. The water

vapor refers to physically and chemically boundwater in the fuel. Calculating the chemically boundwater in the fuel, one must know the Hydrogencontent of the fuel.

The LHV is certainly not the energy content of the fuel

in a wet state. It is the HHV minus the energy storedin the physically and chemically bound water of thefuel.


Combustion

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ONE SHOULD BE AWARE OF THE FOLLOWING:

Any ultimate chemical analysis (C, N, H, O, S) of a fuel

where the percentage of the species does not add up to100 % is incomplete. Whoever has done the testforgot to report the results of the proximateanalysis (Moisture, Volatile Matter, Fixed Caron,

Ash).

You are mostly interested in four numbers:

HHV, ASH, MOISTURE, and SULFUR CONTENT of

the fuel.


Combustion

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ONE SHOULD BE AWARE OF THE FOLLOWING:

All biomass fuels, except coal, no matter whether wetalk about rice hulls, water lilies, wood, bagasse or

coconut shells have on the average the following

ultimate chemical composition on a moisture and ash

freebasis:

C H O N S

50 % 6 % 44 % 0 % 0 %


Combustion

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S lid l Ch i i &

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It is reported that in a typical palm oilmill, for 1 ton of FFB the mill

produces 140 kg of fibre and 60 kg of

shell are produced.


Combustion

S lid

F l Ch i i &

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Component % By Mass

Proximate Analysis

Fixed carbon 18.56

Volatile matter 72.47

Moisture 7.96

Ash 1.01

Ultimate Analysis

Carbon 45.61

Hydrogen 6.23

Oxygen 37.51

Nitrogen 1.73

Sulphur 0

Ash 1.01

Moisture 7.96

Higher heating value (MJ/kg) 18

Typical Proximate & Ultimate Analysis Of Oil Palm Solid Waste


Combustion

S lid F l Ch i i &

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Annual Production of Oil Palm Biomass In A Typical Oil Palm Mill


Combustion

S lid F l Ch i i &

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Properties/Chemical of Solid Fuel -Moisture Contents

The major problem in the use of oil palm biomasses asfuels.

Moisture content for oil palm fibre and shell is 30 to 35%.

Since for the gasification system, the moisture content of

approximately 20% is desired.


Combustion

S lid

F l Ch t i ti &

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Properties/Chemical of Solid Fuel - Calorific values

The calorific values are determined based on the

moisture and oil contents of the oil palm fibres and shells.

The net calorific values of oil palm shells and fibres are

given in this table

The Calorific Values of Oil Palm Shells & Fibres


Combustion


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Oil Palm Fibre

Oil Palm Shell

Oil Palm Empty Fruit Bunch


Combustion

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Power is required at the approximate rate of 15 to 25 kwper ton FFB.

This can be easily be provided by placing a back-

pressure single stage steam turbine between the boilerand the header of the mill processing system.

Steam is generated from the boiler at a pressure of say

20 Bar.g and into the steam turbo alternator at 18.5 Bar.gat 260C with back pressure of 3.16 Bar.g for the mill

process which is convenient and effective for process

heating.

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Every Tonne of FFB Can Produce

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Every Tonne of FFB Can Produce

733 kg Steam & 30 kw Power

Steam is produced by water tube boilers at pressures and temperatures higher

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p y p p g

(20 bar.g 207 deg. C) than required for the process. First it is expanded in steam

turbines, and then led into the process where the latent heat contained in the

exhaust steam (3.16 bar.g) is utilized for sterilisation of FFB and heating systems

in the process. The diagram below show a typical CHP (Combined Heat & Power)

scheme of a modern oil palm mill.

The energy released during the expansion of steam is converted by the turbine into mechanical power

to drive an alternator

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There is a direct relationship between the number of

palms cultivated and the corresponding harvest yield of a

given plantation area processed by the mill, the primary

energy available in the by product fuel, and power / heat

requirement of the mill

A properly design Oil Palm Mill will not only providesufficient steam and electrical power for its operation

requirement but will provide an additional 17 to 33 % more

power for other planned integrated down stream

processes, domestic use or sold to other consumers ofpower.

Power Plant Operation In A Typical

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Power Plant Operation In A Typical

Palm Oil MillThe operation of power plants within a palm oil mill is

not so complex. These plants are normally staffed by local

steam drivers and engineers.

A typical 60 tons FFB per hour mill operating about 20 h

a day. A total of 23% by weight EFB (empty fruit bunches)or 13.8 tons of EFB per hour is sent back to the estate to

be used as mulch in the fields.

The fuel produced from the waste comes from:Shell amounting to 6%, out of which about 30% is dry

enough to be used as boiler fuel, or 1 ton/h and Fibre

amounting to 14% or 8.4 ton/h

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The power requirement of the mill is 15e17kW per ton

FFB or 1,020 kW for a 60 tons FFB per hour mill.

This is typically met by a non-condensing turbine usingsteam with a pressure of 21-bar gauge and exhausting at 3-

bar gauge.

The size of the generator is about 1.2 MW. When the millis not in service, a diesel generator takes over to supply

security lighting and domestic supply.

Two units are usually installed: one of 800 kW andanother of 250 kW. In a mill break down which may last a

while, the large diesel generator will be operated to supply

power to some plants in the nut station, effluent plant,

water works, lighting etc.

Th di l ti t t i d if th ill i

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The diesel generating sets are not required if the mill is

close to the national electricity board grid and is connected

to it.

New schemes for grid-connected power plants under the

small renewable energy programme involve the

construction of boilers burning EFB and producing up to 10

MW of power, which can be sold to the national electricityboard. It is predicted that such a plant will serve several

mills.

Another concept is to produce methane gas from POME,

and burning the gas in boilers, gas engines or gas turbines.

It has been shown that such plants can meet the power

and steam needs of the mills and still allow power export.

Documents

03. EPF 4707 - POWER [02.04.2013]