03. EPF 4707 - POWER [02.04.2013]

EPF 4707

Palm

oil mill is among the few industries which are selfsustainable as they are able to generate their own utilities; electricity and steam. The

steam system in the palm oil mill consists of a high pressure 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.

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 A and 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.

The

BPV functions as a temporary storage of low pressure steam, distributing the steam mainly to sterilizers and other processes in the mill. Nevertheless,

some of the steam ends up getting lost to the atmosphere.

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.

7% (Sivasothy Kandiah et al., 1992)

Vacuum Dryer 10% (Sivasothy Kandiah et al., 1992)

Sterilizer 84% (Sivasothy Kandiah et al., 1992)

Steam Accumulator

Turbine

Other Processes

Loss to Atmosphere Boiler

Fibers

STEAM DISTRIBUTION IN A TYPICAL PALM OIL MILL

In

palm oil mill, the steam system plays a vital role as energy or steam supplier. It not only produces steam for processes 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 process is low due to the availability of waste materials such as fibre and shell generated during the production of crude oil and kernel.

In

addition, the consumption of diesel oil used as supplementary 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.

HIGH PRESSURE PROCESS

BOILER 

FIBRE/SHELL TURBINE

POWER 

P : 250-300 psig

PROCESS

BACK PRESSURE VESSEL

STERILISERS P : 40-45 psig

The

steam leaves the boiler through the boiler outlet or main steam line (high pressure main): This

main steam line branches out into three sublines, 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 the boiler to the back pressure vessel (BPV) and the flow of this sub-line is controlled by a pneumatic make-up valve.

In

the turbine, part of the high pressure steam is transformed 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.

One

of the most difficult units to control in a palm oil mill 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.

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

bar.

relief valve functions when the BPV is more than 4

A

major problem associated with back pressure vessel is imbalances of steam demand. A

steam deficit or low back pressure arises from excessive steam demand during sterilizer peaking operation. While

low steam demand occurs rapidly during sterilizer exhaust and holding periods resulting in excessive steam blow off.

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.

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.

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.

When

steam is subject to regular cycles or predictable fluctuations, 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

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 load fluctuations. 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

15      e      r      u      s      s      e      r        P

10 5 0 6

8

10

12

14

16

Time of day (hr)

Pressure variation without steam accumulator (Goldstern, 1970) 14 12      e 10      r      u 8      s      s      e 6      r        P 4 2 0 6

8

10

12

14

16

Time of day (hr)

Figure 2.3 : Pressure variation with steam accumulator of adequate size (Goldstern, 1970)

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 electrical power, steam supply and meet all processing demands.

The

steam demand for sterilisation operation is not regular, 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.

The

purposes of this steam admission and removal cycles are to remove the air in the sterilisers and ease the buildup 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.

Measurement

Boiler output

Steam Flowrate (kg/hr)

Percentage of Boiler Output (%)

14744

100

12389 1540 1068

83 10 7

5219 3302 5433

37 24 39

Before

back pressure vessel  Turbine Make-up  Vacuum dryer

Before

back pressure vessel Sterilisers Other processes Loss to atmosphere

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.

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.

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.

Solid Fuel Characteristic & Combustion Fire

tube boiler is normally used for small mill to 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.

Solid Fuel Characteristic & Combustion

The Existing Boiler System at Palm Oil Mill

Solid Fuel Characteristic & Combustion  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 the fuel.  The proximate analysis, determines the mass percentage of Volatiles, Ash, and Fixed Carbon.  A test to determine the moisture content of the fuel on a wet or dry basis.  A bomb calorimeter test to determine the Higher Heating Value (HHV) of the fuel in MJ/kg, as received.

Solid Fuel Characteristic & Combustion  Properties/Chemical

of Solid Fuel

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 dry state.

Solid Fuel Characteristic & Combustion  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 bound water in the fuel. Calculating the chemically bound water in the fuel, one must know the Hydrogen content 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 stored in the physically and chemically bound water of the  fuel.

Solid Fuel Characteristic & Combustion  Properties/Chemical

of Solid Fuel

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 to 100 % is incomplete. Whoever has done the test  forgot to report the results of the proximate analysis (Moisture, Volatile Matter, Fixed Caron,  Ash).  You

are mostly interested in four numbers: • HHV, ASH, MOISTURE, and SULFUR CONTENT of the fuel.

Solid Fuel Characteristic & Combustion  Properties/Chemical

of Solid Fuel

ONE SHOULD BE AWARE OF THE FOLLOWING:  All biomass fuels, except coal, no matter whether we talk about rice hulls, water lilies, wood, bagasse or coconut shells have on the average the following ultimate chemical composition on a moisture and ash free basis: C 50 %

H

O

6 % 44 %

N

S

0%

0%

Solid Fuel Characteristic & Combustion A

Higher Heating Value of about 20 MJ/kg on a moisture and ash free basis.

 The

above generic ultimate chemical composition of biomass combined with more specific data about the moisture and ash content of the fuel is sufficient as data input for thermal efficiency calculations, of biomass fired combustion systems.

Solid Fuel Characteristic & Combustion It

is reported that in a typical palm oil mill, for 1 ton of FFB the mill produces 140 kg of fibre and 60 kg of shell are produced.

Solid Fuel Characteristic & Combustion Typical Proximate & Ultimate Analysis Of Oil Palm Solid Waste 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

Solid Fuel Characteristic & Combustion Annual Production of Oil Palm Biomass In A Typical Oil Palm Mill

Solid Fuel Characteristic & Combustion Properties/Chemical

of Solid Fuel - Moisture Contents

The major problem in the use of oil palm biomasses as fuels. •

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. •

Solid Fuel Characteristic & Combustion 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

Solid Fuel Characteristic & Combustion

Oil Palm Shell Oil Palm Fibre

Oil Palm Empty Fruit Bunch

Power

is required at the approximate rate of 15 to 25 kw per ton FFB. This

can be easily be provided by placing a backpressure single stage steam turbine between the boiler and 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.g at 260ºC with back pressure of 3.16 Bar.g for the mill process which is convenient and effective for process heating.

The

additional power generated in this system is made possible by burning of the empty bunches as shown in the enclosed Fuel /Steam /Power balance and Steam Production from 1 Ton Solid Waste Fuel for a Oil Palm Mill. Every

ton of FFB can produce 733 kg steam and 30kw power shown, in the diagram below : A

system has been introduced for the treatment and disposal of empty bunches and recovery of palm oil and at the same instance reduces the moisture contents of the empty bunches to approx. 45 % so that they can be used as solid waste fuel for the boiler and production of additional steam and electrical power.

14 12     e     r     u     s     s     e     r       P

10

8

Pressure variation without steam accumulator (Goldstern, 1970)

6 4

2 0 6

8

10

12

14

16

Time of day (hr)

14 12

Pressure variation with steam accumulator of adequate size (Goldstern, 1970)

     e      r     u      s      s      e      r       P

10

8 6 4

2 0 6

8

10

12

Time of day (hr)

14

16

Every Tonne of FFB Can Produce 733 kg Steam & 30 kw Power

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

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 provide sufficient 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 of power.

Power Plant Operation In A Typical Palm Oil Mill The

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

power requirement of the mill is 15e17 kW per ton FFB or 1,020 kW for a 60 tons FFB per hour mill. The

This

is typically met by a non-condensing turbine using steam with a pressure of 21-bar gauge and exhausting at 3bar gauge. The

size of the generator is about 1.2 MW. When the mill is not in service, a diesel generator takes over to supply security lighting and domestic supply. Two

units are usually installed: one of 800 kW and another 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.