INTERNATIONALTROFICALTIMBERORGANIZTION

(ITTO)

EXECUTINGAGENCY: FORESTRYRESEARCHINSTITUTEOFGHANA

(FORIG)

FINALTECHNICALREFORTN0.3

I~TITLEOlRFRE-PROJECT:

SIZINGCONGENERATIONPLANT

USINGWOODRESIDUEASFUEL

SERIALNl. IMEER:

DEVELOPMENTOFENERGYALTERNATIVES

FORTHEEFFICIENT UTILIZATIONOF WOOD

PROCESSINGRESIDUE: CO-GENERATION

ANDBRIQUETTEPRODUCTION.

PLACE OF ISSUE:

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DATEOFISSl. IE: MARCH2005

ITTO PROJECTPPD 53/02REV. I(I)

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KUMASl, GHANA

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Abstract ........................................................................................ it

1.0 Introduction. ............................,.................................,.,,,,,,,,,,,,,, I

2.0 Co-generation in wood processing mills in Ghana. ................................. I

3.0 Techntcal options for biomass co- generation ........................................ 2

3.1 Direct combustion boiler/ steam technology. ............................... 2

3.2 Gasificationtechaology. ...................................................... 4

3.3 The Stirlingengines. .......................................................... 5

4.0 Identification of sites for good co-generation potential. ........................... 5

4.1 Methodology .................................................................... 5

4.2 Selection of potential sites. .................................................. 6

Selected potential sites. .................................................. 64.2. I

4.2.1.1 Asuo Bornosadu Timbers and Sawmills (ABTS Ltd)............ 6

4.2.1.2 Logs and Lumber Limited (LLL)................................... 7

4.2.1.3 Omega Wood Processing Limited (OWPL)........................ 7

TABLEOlr'CONTENTS

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5.0 Deterrnination of thennal energy demand profiles. ................................. 7

5.1 Methodology. .................................................................... 8

5.2 Results. ........................................................................... 8

6.0 Detennination of electrical demand profiles. ........................................ 8

6.1 Methodology .................................................................... 9

6.2 Results ......................-------..........-~~~~~~~~"""""""""""""' '

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7.0 Determination of the specification for co-generation plants ....................... 12

7.1 Introduction ..................................................................... I 2

7.2 Electrical loadmatching. ...................................................... 13

,--

7.3 ThemIalloadmatching. ........................................................ 13

7.4 Methodology. .................................................................... 13

7.5 Results ........................................................................... I 5

8.0 Conclusion. ............................................................................... I 7

9.0 Recoinmendation ........................................................................ 18

References. ............................................................................... I 9

Appendix I ....................................................................,......... 20

Appendix 2 ............................................................................. 22

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Currently growing awareness of biomass as a potentially environmentally friendly sourceof energy is leading to an increased number of initiatives and projects in the wood energyfield. In Ghana the existing wood processing mills generate wood residue, a portion ofwhich is used to fire on-site boilers to generate steam or hot water fortheirprocess heatneeds. The remaining residue is burntin the open air causing environmental pollution.To determine cogeneration potential at the Ghanaian wood processing mills, certaincriteria were used to selectthree woodprocessing mills as case study. The mills selectedwereAsuo BornosaduTimbers and Sawmills Limited (ABTS LTD), Log andLumberLimited (LLL) and Omega WoodProcessing Limited (OWPL) which have annual woodresidue of about 27,360 in', 32,610 in' and 19,230 in' respectively.

The power requirements forthe mills, necessary for sizing cogeneration wits, werederived from their monthly electricity bills. A bill for a particular month indicated themaximum demand of the month andthetotalenergy consumed forthe month. Asthemills do not have instr^Gritationto monitorthennalenergy consumption, their peakprocessrequirement was assumed to be the installed thennal capacity at the site. Themills have high steam consumption compared to availability of wood residue.Consequently, the backpressure steam turbine was selected. Its size was detenninedtomeet anthe thermal demand at the site.

ABSTRt^. CT

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The size of the cogeneration plant was calculated for typical inlet pressures of 20, 30, and40 bars and at superheated temperatures of 300'C and 400'C. The calculations weredone assuming typical boiler, generator and turbine efficiency of 76, 96 and 60%respectively and a boiler feed-water temperature of 90'C.

From the results of the calculations the power ratings for cogeneration units at OWPL,LLL and ABTS were specified as 2,000 kWe, 1,200 kWe and 400 kWe respectively.These gave reasonable heatto power ratios of 19, 21 and 19 respectively. Thecorresponding fuel consuinptions were about 80,000 in'/year, 32,000 in'/year and17,000 in lyear.

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SIZING CONGENERATIONPLANTl. ISINGWOODRESIDl. IEAS

FUEL

I. INTRODUCTION

Cogeneration is the simultaneous production of heat and electricity from a single primaryenergy source. This mode of energy conversion is also known as combined heat andpower generation (CHP)(Prasertson at a1, 2001). More electricity and heat are generatedfor a lesser amount of fuel by a cogeneration urnt than by electricity-only and thennal-only units. The overall efficiency of energy conversion by means of cogeneration can beup to 80 percent and above with certain technologies(Energy Tips-Steam, 2004;ESDD, 2000; Sims & Gigler, 2002).

According to the sequence of energy use, a cogeneration system can be classified aseither atopping or bottoming cycle (ESDD, 2000):

(a) A topping cycle: the primary fuelis used to first produce electricity and then thermalenergy is obtained as by-product. A topping cycle cogeneration system is used wherelow pressure steam or hot water is required for process heat. A typical area ofapplication is the wood products industries.

(b) A bottoming cycle: the primary fuelis used to produce high temperature thermalenergy and the heat rejected from the process is used to generate electricity.Industries using bottoming cycle cogeneration include cement, steel andpetrochemical industries.

A cogeneration system can incorporate a vapour absorption chiller to also producecooling (Introduction to CHP Catalog of Technologies ;.... ESDD, 2000). A low qualityheat exhausted from the cogeneration plant can drive these absorption chiners. Thisconcept of deriving three different fonns of energy from the primary energy source isternied tri-generation or combined heating, cooling and power generation (CHCP). Thisis particularly of interest in Ghana where buildings require comfort cooling throughoutthe year.

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I~ 2. COGENERATIONINWOODPROCESSINGMILLSINGHANA

The wood processing mills in Ghana generates large volume of wood residue annuallyduring production process. The wood residue includes sawdust, shavings, trillrrnings,slabs, edgings and off-cuts. A substantial portion of the residue is reprocessed into usefulproducts such as flooring parquets, flooring strips and triangular mouldings.

The existing mills obtain electricity supply from the national grid and use a small fractionof their unused residue to fire on-site boilers to generate steam or hot water to meet alltheirprocess heat demand. The remainder of the unused residue is disposed of as waste in

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open burning and dumping sites creating environmental problems in the surroundingcommunities.

Using the unused residue for on-site cogeneration will not only offer both environmentaland economic benefits but also higher energy conversion efficiency. Notwithstanding,cogeneration technology has not been adopted in Ghanaian wood processing mills. Thehindering factors include the following:

(a) Lack of successful references.(b) Lack of technical and economic inforrnation to make a decision.(c) Uncertainty of sustainable fuelsupply.(d) High investment cost as a result of the fact that all system components must be

imported.(e) Inability to operate the cogeneration plant as grid-connected system.

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I~~' 3. TECHNICALOPTIONSlF'ORBIOMASSCOGENERATION

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The well established technology for converting biomass to heat and electricity is theAdditionaldirect-combustion boiler/steam technology (Sims & Gigler, 2002).

technologies involving gasification of biomass fuels are either at very near to coriumercialavailability or at demonstration orresearch and development stage (Sims & Gigler, 2002,Biopower).

,-3.1. Direct-combustion boiler/steam technology

This technology has matured over years and it is wellproven in industries having demandfor both electricity and large quantity of steam at high and low pressures. In thistechnology, the biomass is burntto produce steam in boilers. The steam is then used toproduce electricity by means of steam turbines and to also provide process heat. The twotypes of steam turbines most widely used are the backpressure and the extractioncondensing types(CECA, 2002-2003).

In the backpressure turbine, incoming high pressure steam is reduced to low pressuresteam which provides thennal energy for the plant process heat. In the process, shaftpower is produced which turns a generator coupled to the shaft to produce electricity.Where process heat is required at two different pressure or temperature levels someamount of steam can be extracted from the turbine after being expanded to a certainpressure level.

In the extraction-condensing turbine, a portion of the steam is extracted at one of thestages of the steaniturbine for process heat and the remainder goes from the turbine intoa condenser to ensure that the maximum amount of heatis converted into electricity. Theextraction-condensing turbine plant has higher power to heat ratio. It requires auxiliaryequipment such as the condenser and cooling towers. However it provides a bettermatching of electric power and heat demand where electricity demand is much higher

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I~'than the steam demand and the load patterns are highly fluctuating (ESDD, 2000). Thebackpressure turbine cogeneration plant on the other hand has higher heatto power ratioand higher overall efficiency. Since it needs less auxiliary equipment, the initialinvestment costs are low.

The choice between backpressure turbine and extraction-condensing turbine depends onthe quantities of power and heat, quality of heat and economic factors. Heat to-powerratios and other parameters of the two systems are given in Table I(ESDD, 2000).

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Table I: Heatto power ratios and other parameters of steam turbine cogenerationsystems

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Cogenerationsystem

Backpressure steamturbine

Extraction-

condensing steamturbine

A complete steam turbine cogeneration system consists of several components: fuelpreparation; handling and storage; fuel combustion, heat conversion, electrical energygeneration; and automation and controls for the entire system. There are suppliers whichcan provide the complete tumultey supply of cogeneration mitts packaged as "of-the-shelf'products and guarantee their themIal and electrical perlonnance. The package units canbe installed in a few days with very little structural or engineering work at the site.

Biomass-fired steam turbine cogeneration plants are available in the power range from0.5 MW, up to around 50 MW, . Plants smaller than I MW, , are usually operated asbackpressure CHP plants and aim at electrical net efficiencies of typically 10 % to 12 %.For large steam turbines, higher efficiencies can be attained. It is around 25 % in plantsof5 to 10 MW, and up to more than 30 % in plants around 50 MW, (Power Generationand Cogeneration, 2003).

Heat-to-powerratio (kWth/kWe)

4.0 - 14.3

2.0 - 10.0

Power output(aspercent offuel

in ut

14 -28

22 - 40

Overallefficiemcy(percent)

84-92

60 - 80

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3.2. Gasification technology

This is another potentially attractive technology for producing bio-power. In thistechnology, the wood residue is converted to medium- or low-calorific gas and the gasused to produce heat and electricity by any of the prime movers or technology optionsdescribed below. Even though some of the technologies are mature, for example, internalcombustion engines, problems occur when using biomass fuels due to the quality of thefuel gas produced by a gasification system. Research to improve gasification technologyis ongoing (Sims and Gigler, 2002).

Gas IIJrbines. . The gas is used to heat air which passes through a turbine to createelectricity and the energy released at high temperature in the exhaust stack is recoveredfor process heat. If more power is required at the site, it is obtained by using the gasturbine in a combined cycle with a backpressure or extraction condensing steam turbinebottoming cycle. The exhaust or extracted steam from the steam turbine then providesthe process heat. With this system the overall efficiency of the cogeneration plant canexceed 80 %.

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Gas turbines are a mature technology, but biomass-fired gas turbines coupled togasification systemsremain in research stage. They have potential in 5 to 20 MW, rangeor above (Sims and Gigler, 2002).

Intornol combustion engines. . The gas from the gasification plant provides fuel for thegas engine. The process heatis harnessed from the exhaust gas at high temperatures andengine jacket cooling water system at low temperatures. Power produced can beincreased here also using it in a combined cycle with a steam turbine. One of the majoradvantages of internal combustion engines over the other prime movers is their higherelectrical efficiency. They can achieve electrical efficiencies of around 25-30 % (Simsand Gigler, 2002).

The system finds application in smaller energy consuming facilities having greater needfor electricity than thennal energy and where the quality of heat required is not high e. g.low pressure steam or hot water. This system has low initial invesiment cost but highoperating and maintenance costs. The units under development are in the range of 5 to25 kWe (Sims and Gigler, 2002).

Fuelce/Is. . Biomass gasification system can provide fuelto fuel cell. Several fuel cellsdesigns are under development. They have such advantages as high system efficiencies,low noise levels, low emissions and good reliability. This technology needs several moreyears of research and development before they become economicalIy and technicalIyfeasible at powerscales of 50 kW* to 5 MW, .

Microt"Ibines. . These are relatively new development. Microturbines and internalcombustion engines integrated with gasification technologies are promising cost-effectiveand small-scale systems but further research into gas cleaning is still needed in order toimprove system perfonnance.r'

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Different biomass-fired microturbines systems are under development in the range 2 -250 kWe. Microturbines are compact and lightinweight and have low noise levels.

3.3. The Stirling Engines

These are externalIy fired engines suitable for small-scale biomass power production. Ina Stirling engine, a gas in a sealed system expands and contracts as it is subjected toheating and cooling cycles. The resulting pressure cycles are then used to drive a pistonand crankshaft, which, in turn, power an electrical generator (Hislop).

The Stirling engines are just reaching the coriumercialtechnology phase. They havepotential in the power range of 10 to 150 kW, . They have many advantages at the smallscale such as reasonable efficiencies (up to 30 %), low noise levels, low maintenance,and expected long engine lifetime (Sims and Gigler, 2002). They can be coupled tocombustion or gasification system.

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4. IDENTIFICATIONOFSITESFORGOODCOGENERATIONPOTENTIAL

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Wood processing mills which are potential candidates for biomass-firedcogeneration should have the following characteristics ( ESDD):

(a) adequate thermal energy demand, matching with the electrical energy demand(b) reasonably highelectricalloadfactor(c) reasonably high annual operating hours(d) fairly constant and matching electrical and themIal demand profiles(e) availability of unused woodresidue

The availability of unused wood residue and guarantee of its long-tenn supply are amajor factor detennining the potential site for cogeneration. Since stoppages forscheduled maintenance or unscheduled breakdown are inevitable, the site should alsohave back-up power to ensure continuity of essential activities at the site.

Most of the existing wood processing mills in Ghana fulfilthe requirements for goodcogeneration potential. For the purposes of this study, numerous mills were visited andthree of them, finally selected for further technical and economic feasibility studies.

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4.1. Methodology

Identification of potential sites for cogeneration was achieved by visits to a number ofimportant and well-known wood processing mills in the country. During on-site visits tothese mills structured questionnaires were filled out and direct discussions held with themill management.

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4.2. Selection of Potential Sites

Most of the wood processing mills fulfil the technical requirements for goodcogeneration. To selecttliree out of the lot visited, the following criteria were used:

(a) minisimportant consumer of electricity and heat(b) millhas higher thermal energy demand than electricity(c) management is willing to implement the cogeneration technology(d) millhaslarge annual operating hours(6) unused wood residue is available in large quantity and there is a guarantee of its

long-term supply

4.2. I. Selected Potential Sites

The three wood processing mills selected are presented below:

4.2.1.1. Asuo Bornosad" Timbers & Sawmills Limited (Abts Ltd)

Asuo Bornosadu Timbers & Sawmill Limited is located at Berekum in Brong AhafoRegion of Ghana. The mill has two main sections, namely sawmill and moulding millsection; and ply mill section. It produces lumber, veneer, plywood, flooring parquet andflooring strips for both local and international markets. The Sawmill and Moulding millsection operates 16 hours a day and 305 days a year and the ply mill section operates 24hours a day and 305 days ayear.

Its monthly log input is approximately 6000 in , with 2500 in and 3,500 in going intosawing and veneer production respectively.

WoodResidue

The wood residue generated consists principalIy of sawdust, bark, veneer chippings,slabs, edgings, peeler cores and off-cuts.

Used Residue

The sawdust and dry veneer chippings are used for boiler fuel, peeler cores arereprocessed into boards using woodmizer and about 20% of the slabs and edgingsreprocessed into flooring parquets, flooring strips and triangular mouldings.

Unused Residue

This includes slabs, bark, sapwood edgings and wet veneer chippings. Theresidue constitutes about 38% of the total log input which is about 6000 in .

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unused

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4.2.1.2. Log And Lumber Limited (LLL)

This is one of the largest wood processing mills in Kuniasi. The mill has lumberproduction, slice veneer and rotary veneer sections. Its main products are lumber, veneer,plywood and T&G for export. It operates 24 hours a day in two shifts and a minimum of7512 hours per amIrun. Its log input for January 2004 was approximately 7,152,024 inwith 27.03%, 23.40% and 49.56% going into production of sliced veneer, ply mill andlumber respectively.

4.2.1.3. Omega Wood Processing Limited (OWPL)

This minis located in Kmnasi. It has four sections: sawmill, moulding, rotary veneer, plywood and slice veneer. Its products are lumber, veneer and plywood for domestic andinternational markets. The mill operates 24 hours a day and 6 days a week which resultsin annual working hours of 7512.

The avera e monthly log input for the plywood and rotary veneer sections was about4007.57 in .

WoodResidue

The main fomis of residue generated by the mill are sawdust, slabs, edgings, trimmings,bark, off-cuts and veneer core. Its secondary process, which is practiced by few mills inGhana, produces mainly sawdust and wood shavings.

The total monthly average volume of bark, sawdust and off-cuts for the period was966.73 in'. That of veneer, core, trimmings and defective veneer and plywood was about16/7.98 in3.

5. DETERMINETHERMALENERGYDEMANlDIPROFILES

Thermal energy requirements of selected mills are as follows:

A.

The mill has two thermal oil boilers and one hot water boiler providing process heat forits kiln dryers. The supply and return temperatore of the hot water boiler are 90'C and80'C respectively. It has maximum operating temperature and pressure of 95'C and 3bars and apedonnance of 1600 kW.

Each thennal oil boiler has maximum operating temperature of 300'C, an output pressureof 3.5 to 4 bars, a capacity of 2,000,000 kg/}I, volume of 2,278 in and a themIal capacityof 3,000,000 kcal/h (3488 kW)

ABTSLIMITED

The fuel for the boilers consists of all the sawdust generated on site, dry veneerchippings, branches harvested in the forest and forest residues.

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The sawdustproduced is about 480.92 in'/month. This represents about 25% of the totalfuelrequirement forthe boiler.

B.

The millhastwo boilers which provide steam for kiln dryers with a third one to be addedin the near future. Each boiler has a maximum steam rate capacity of 10,000 kg/}I. Theboilers have an outlet pressure of 15 bars and temperature of 201'C. About 50 % of thesawdust generated on site is used as fuel for the too boilers.

LLL

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The millhas four saw-dust fired boilers which provide steam for kiln dryers. Each boilerproduces steam at a pressure of 10 bars and a temperature of 150'C. Maximum steamflow rate for each boiler is 10,000 kg/h.

OMEGA

5.1. Methodology

The three mills do not have any record of thennal energy consumption due to lack ofinstrumentation. Since infonnation on themIal energy usage patterns was lacking, thepeak process heat requirement was assumed to be the installed thennal capacity at thesites.

5.2. Results

The process heat requirement was assumed to be the installed thennal capacity givenabove. The themIal energy needs for each mill was supposed to be constantthroughoutthe operation of the mill.

6. Determination of the Electrical Energy demand Profiles

The three selected mills obtain their electricity supply from the national grid and havediesel generator sets to provide power for their essential loads in the event of poweroutage. Their electrical energy requirements are swimiarized as follows:-

ABTS

It obtains its electric power from the national grid via two transformers, each rated IMVA. One of the transfomiers services the sawmill and moulding millsection and theother the ply mill section. The mill owns three standby diesel generator sets, each rated630 kVA.

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LLL

Electric power is supplied from the national grid through fourtransfonners having a totalcapacity of 3.8 MVA. It owns one diesel-generatorsetrated 975 kW.

OMEGA

Three 800-kVA transfonners supply power to the mill from the national grid. Threediesel-generator sets of total capacity of 1350 kVA provide partial backup in case ofpower outage.

6.1. Methodology

In the absence of instrornentation for regular monitoring of electrical energy consumed,the electricity consumption patterns were derived from the monthly electricity bills over aperiod of one year. A bill for a particular month indicates the maximum demand forthemonth and the total energy consumed for the month.

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6.2. Results

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The annual load curves plotted from monthly demand derived from the electricity billsare shown in Figure I. Monthly energy consumptions for a period of one year are alsoshown in Figure 2. analysis of the monthly electricity consumption of the mills in aperiod of one year gives the data in Table 2 relevant forthe study

Table 2: the total installed thermal capacities the applied factors and estimatedthermal energy consumptions for 3 wood processing mills

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Maximum monthly electricityconsumption (i?20nih/energy in kWh)Minimum monthly electricityconsumption (month"energy in kWh)Maximum monthly demandOrionth4?ower in kll?

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Minimum monthly demandOrionth470wer in kll?

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Total electricity consumption07eriod/energy in kWh)

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Aug/ 210,400Ply mill

ABTS

Jan I 74,155

June I 510

Nov I 576,500

Sawmill

Jan I 186

Mar/ 70,970

Jan-Dec, 2003/1,970,340

Oct/ 318

LLL

Aug/758,736

July I 269

Jan/ 596,985

Jan-Dec, 2003/1,508,465

OMEGA

Jan I 1,550

May/ 315,650

June I 1440

July/ 212,840

Oct 2003- Sept2004/ 8065512

May/846

9

Mar/ 607

Oct 2003- Sept2004/2959040

140,000

120,000

= 100,000"̂ 80,000>

E' 60,000,,= 40,000Ul

20,000

o

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Figure ,: Electricity consumption at ABTS sawmills

Mar

330

320

^ 310" 300^

, 290^ 280o 270o 260

250

240

May

Figure2: Annual Load curve of ABTS sawmills

JulJun

year2003

D Energy(kWh)

Feb

ALU

Mar

Sep

^ 200,000250,000

^. 150,000>

P 100,000o=

50,000IU

o

Od

May

Nov

Figure3: Electricity consumption at ABTS Plymill

JulJun

Year2003

~- Demarxi (kW)

600

^ 500^. 400

^ 300"

E 200oo 100

o

Figure4: Annual Load curve of ABTS Plumill

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>,

2' 150,000a,

^5 100,00050,000

o

S'*

Figure5: Electricity consumption at Omega

Year2003

~-Demarid(kW)

^\ ,.* ^? *

^.

0,143 buts

,.,Q

900

800

_ 700

, 600, 50015 400o 300Q

D. e-03

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^

J. n44

^$,0'

F. bC4

200

100

o

Figure6: Annual Load curve of Omega

Apr. 04". r. 04

Months

Energy (kWh)

Od43

". y44

Nov43

J""-04

Deco3

Jul. 04

J. n44

Aug. 04

Feb44

Sep-04

Apr-04MarC4

Month

-- Demarxi (kW)

May^4 Jun44 Jul4, Aug44 Sep44

50 . ...

.. ..,

... ...

.. ...

.. ..,

.. ...

., .,.

.. ...

.

7. DETERMINETHESPECIFICATIONF'ORCOGENERATIONPLANTS

7.1. Introduction

In general, cogeneration plant may be sized in four ways leading to four operatingschemes described below (ESDD, 2000):

Base electrical load matching

In this operating scheme, the plantis sized to meetthe minimum electric power demandof the site and the extra power required is imported from the Utility grid. Ifthe thennalenergy generated is not enough, additional boilers are used to generate the deficit.

Base thermal load matching

Here, the plant is sized to supply the minimum thennal energy need of the site. Excessthermal demand overthe base is met by stand-by boilers. Electricity is imported from or

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exported to the Utility grid depending on whether the power produced by the plant fallsshort of or exceedsthe power requirement of the site.

7.2. Electrical load matching

In this scheme, the plant is sized to meet anthe power requirement of the site thusmaking it independent of Utility grid. If there is deficit of thermal energy, additionalboilers are used. Ifon the other hand there is excess thennal energy, it is either exportedto neighbouring facilities or wasted.

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7.3. Thermal load matching

In this case, the plant is sized to meetthe thermal energy requirement of the site at anytime. In the event of the power generated by the cogeneration plant not matching theelectricity demand of the site, the excess or deficitis sold to or purchased from the Utilityasthe case may be.

7.4. Methodology

Wood processing mills in Ghana obtain electric power from the national grid and use on-site boilers to meet the thennal energy need. The introduction of cogeneration in themills is meant to achieve higher utilization efficiency of the wood residue by convertingpart of the wasted primary energy associated with the existing energy conversion systeminto electricity. Consequently the cogeneration plant is sized for thermal load matching.The mills, having access to the national electricity grid, can either import or export powerat any instantifthe power produced by the cogeneration plant does notmatch its demand.Since infonnation on thermal energy usage patterns was lacking, the peak process heatrequirement was assumed to be the installed themIalcapacity at the sites.

The backpressure steam turbine was selected due to the high steam consumption in themills compared to availability of wood residues. The size of a single-stage backpressuresteam turbine topping cycle cogeneration plant was determined for typical inlet pressuresof 20, 30 and 40 bars and at superheated temperatures of 300'C and 400'C. The actualwork done by the turbine was calculated by multiplying the isentropic efficiency by thework done by an ideal turbine under the same conditions. For smallturbines, the turbineefficiency is generally 60 to 80 %, for large turbines, it is generally about 90 %(Engineers Edge 2002. ). The calculations were done assuming typical boiler, generatorand tarbine efficiencies of 78, 96 and 50 % respectively (Steam Tip Sheet # 20, 2004,Steam Tip Sheet # 22, 2002) and a boiler feed-water temperature and pressure of 90'Cand 2.5 bar gauge respectively. Anthe relevant fomiulas are as follows:

. Electric Power

p = IfZ(hin ~ h, ",,, }7,677g, " (kW. )

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Where,

in =massflowrateofsteam, kg/s

h, =specificenthalpyofsteamenteringtheturbine, kJ/kg

h, ,r, , = specificenthalpyofsteamleavinganidealturbine, kJ/I, g771b =turbineefficiency

77, ,, =generatorefficiency

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. Outlet Enthalpy of Real Turbine

h, ,,,, = hi, ~ ty, b (fom ~ ham, , )(kJ/ICg)

Where,

h, ,l, , = specificenthalpyofsteamleavingtheacttialturbine, kJ/kg

. Thermal Power

S = Ifzh, ,,,, (kWth )

Where,

S = heatgenerated, kW

In cases where thennal power rather than steam rate is specified the above fomiula isstillrequired for the calculation of the corresponding steam rate.

. Power to Heat Ratio

.

pHR=PISFuel Consumption in kW

Fk\Ifz(hin - hf)in _ I (kW)

Where,

'ib

h =specificenthalpyoffeedwater, kill:g

tyb = boiler efficiency

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. Fuel Consumption intolines/year

Fk\ 3.6F*, HR (,,, ines/ ear)C

Where,

HR = actual working hoursperyearC = calorificvalueoffuel, kJ/kg

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7.5. Results

The results are given in Tables 3 to 5

Table 3. Results for OmegaSteam rate (kg/lit)

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Outlet pressure (bar)Armual working hours (lit)Turbine inlettemperature ('C)Turbine inlet pressure (bar)Electric power(kW)Thermal power(kW)Heatto power ratioFuelconsumption (kW)Fuel consumption (tomies/year)Fuelconsumption (in lyear)

40,00010

7512

L

20

L_

962

300

Steam rate (kg/}IT)

3 1,884

Outlet pressure (bar)

33

30

Armualworking hours(lit)

L . _

38,635

1,433

Turbine inlettemperatore ('C)

58,045

31,723

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Turbine inlet pressure (bar)

73,850

22

20

38,190

400

1,162

Electric power(kW)

57,377

34,854

ThemIalpower(kW)

75,900

30

30

Heat to power ratio

41,940

1,759

Fuelconsumption (kW)

Table 4. Results for LLL

41,940

34,045

Fuelconsumption (tonnes/year)

80,160

19

Fuelconsumption (in lyear)

30,000

41,694

15

41,694

7512

79,700

20

316

300

16,557

30

78

28,976

715

43,533

31,723

40

66

55.39

28,643

957

43,033

23,638

20

25

54.75

28,279

382

400

42,486

26,650

30

54,05

70

31,455

878

15

47,258

25,994

40

60.12

30

31,270

1,194

46,980

25,521

59.77

21

31,08046,69559.41

I~

L_

L .

Thennalpower(kW)Outlet pressure (bar)Annual working hours (lit)Turbine inlettemperature ('C)Turbine inlet pressure (bar)

C'~

Electric powerThemIalpowerHeat to power ratio

,-

Steam rate

Table 5. Results for ABTS

Corresponding electric power(kW)Corresponding fuelconsumption (kW)

r~

Corresponding fuelconsumption (tonnes/year)

steam rate of 1000kg/}I(kW)

Corresponding fuelconsumption (in

steam rate of 1000kg/h (kW)

thermal power of 8,600 kW

r~

8,60010

7320

20

24,050

300

797.1

L_

33

30

lyear)

r

8,903

35.83

214

793.08

8,599

22

20

r'~

9,008

12,589

29.05

L

400

322.76

16,020

871.35

8,600

30

30

I_

12,590

8,202

43.98

238.27

16,018

851.13

8,600

19

12,590

8,251

L__

16,018

362.87

r-

8,60012,59016,018

r~

L.

I~

16

I.

I~

8. Conclusion

The wood processing mills in Ghana use on-site boilersto generate steam or hot water tomeet antheirthennaldemand. Hence the cogeneration plants forthe three selected millswere sized forthennalload matching. As the steam consumption in the mills was high,the backpressure steam turbine was selected.

Several turbine inlet pressures and temperatures were considered forthe sizing of thecogeneration plants and from the results of the study, the inlettemperature was specifiedas 400'C at anthe three sites and the inlet pressure taken as 30 bars at both OWPL andABTS and 40 bars at LLL. These led to power ratings of 2,000 kWe, 1,200 kWe and 400kWe and heat to power ratios of about19, 21 and 19 at OWPL, LLL and ABTSrespectively. The corresponding fuel consumptions were about 80,000 in lyear, 32,000in'/year and 17,000 in'/year. The fuelrequirement forthe plants at LLL and ABTScompares favourably with the annual volume of residue generated which is estimated as32,610 in' and 27,360 in' at LLL and ABTS respectively.

r

I~

17

I~9. Recommendations

I. Carry out a detailed demonstration co-generation project studies at one of thethree selected sites, namely, ABTS, LLL AND OMEGA.

2. Undertake studies to monitorthe thermal and power demand patterns for correctsizing of the cogeneration unit.

\ .

r~.

r~-

I_

18

Reference

I) Prasertsan Suteera; Krukanont Pongsak; NgamsritragulPanyarak andKirirat Pairoj: Strategy for optimal operation of a biomass- firedcogeneration power plant;Institution of Mechanical Engineers Part A :Power and Energy, 215(41): 13- 26, 2001 imp://WWW.clib. psu. an. th/00ad-441psuteel. html

2) Energy Tips - Steam: Consider installing high- pressure boilers with backpressure tarbine generators;Industrial Technologies Programme. SteamTip Sheetn0. 22. Us Department of energy; Energy Efficiency andRenewable Energy, September, 2004

3) Sims Ralph and Gigler JOTg: The brilliance of bioenergy- smallprojectsusing biomass. Renewable Energy World ; James and James (sciencepublishers) ; January-February, 2002.

I_

htt ://WWW. 'x'. conyma sand'/rew/2002-011sims/himI

4) Biopower: Gasificationtechnology for clean cost-effective biomasselectricity generation. Department of energy, Library .htt://WWW. eere. ener . ov. /bio ower/b lib/libra In- asification. htm

5) Hislop Drummond: Development of abiomass fuelled gasifier/ Stirlinggenerator for developing countries: Reliable, low-cost power generation,Innovation in Europe: Research and Results; Energy, Sustainable EnergySystems Ltd. Ref. Jou2-CT 92 -0160, Prograrnme Joule 11

6) Debra Jenkins: Researchand development of markets and supplystrategies for short rotation forestry, forest residue and conversiontechnology for CHP- electricity from wood: Integrated Production andprocessing chain - Electricity from wood: Fair co-operative Research forSMEs. July, 2000. htt ://mayw. incaws. ov. bc. call d/pol-research/ubcnV1993/B47-Co-generation-potential. html

7) Engineers Edge : Power plant components- Thennodynamics, 2000

8) Introduction to CHP Catalog of Technologies:http:// WWW. e a. ov/ch I dinntr0%20 to%20cat%200f% 200f%20tech. pdf

19

Year

2003

2003

Month

2003

January

APPENDIXl

2003

February

2003

March

2003

April

2003

May

2003

Energy (kWh)

June

ABTS Ply mill

2003

July

2003

August

2003

September

2003

74,155

October

120,690

November

115,015

Demand(KVA)

December

210,240

Year

163,240163,000

2003

163,400210,400

2003

Month

200

2003

192,400

January

Demand(kW)

251

2003

153,600

February

216

2003

193,800

March

224

210,400

2003

April

251

2003

May

548

2003

Energy (kWh)

186

ABTS Sawmill

June

436

2003

233

July

504

2003

201

August

506

2003

208

September

352

2003

81,470

233

October

352

71,220

November

510

432

Demand(KVA)

70,970

405

December

82,305

469

72,600

471

72,600

327

76,600

327

100,200

402

302

108,000

Demand(kW)

302

116,200

315

115,500

320

79,800

315

286

296

274

296

300

309

304

314

324

309

318

280

304

269

294

298

3 18

312

20

298

L .

Year

r-

2003

2003

Month

2003

October

2004

November

2004

December

2004

January

2004

February

2004

Energy (kWh)

March

2004

April

OMEGA

2004

May

2004

June

226,610

2004

July

Year

250,690

August

179,330

Demand(KVA)

September

223,540

2003

241,230

2003

Month

246,770

2003

October

292,320

2004

November

315,650

2004

December

690

215,880

2004

January

740

Demand(kW)

212,840

2004

February

700

277,720

2004

Energy (kWh)

March

720

276,460

2004

April

680

2004

LLL

May

690

2004

656

June

870

688,521

2004

688

July

910

652,635

637

August

880

527,943

662

Demand(KVA)

September

870

596,985

632

880

619,182

607

870

702,009

774

662,283

846

693,111

2,388

818

688,137

1,788

809

Demand(kW)

749,295

1,560

827

758,736

1,632

818

726,675

1,5121,584

2,101

1,560

1,699

1,584

1,529

1,500

1,550

1,608

1,467

1,572

1,521

1,632

1,4981,5211,4401,5281,4931,518

21

Site Information:

I.

2.

3.

4.

5.

mestiomnaire for Co emeration in a Sawmill

APPENDIX2

Main Activity:Hours of Operation:Working Days:Total Armual Operating Hours:Period and Duration of Annualshutdown:

Einer

I. Electricity Data (at least for last 12 months)

Demand and FuelReso"rce Data

Year Month

2. Transfonner Capacity (kVA)3. amualPeakDemand (kW/ICVA)4. Any changes in the future demand patterns expected:5. Ally alternative source of power apart from ECG:6. Ifyes, please specify hypo of plant):7. Whatisthe capacity of the alternative:8. What do you when currentsource fails:

Whatisthe capacity of back-up system:At what cost:

ConsumptionMWh

I.

11.

PeakHours

Wh

I. Whatis current source of thennan}IeatrequirementSteam from boiler .Electrical heating .

2. Ifsteamfrom boiler

Off-peakHo"rs

MWh

i. Boiler outletpressure(Bar)andtemperature('C)ii. Processheatrequirementpressure (Bar)andtemperature ('C):

3. Hotwater:

r

Thermal/Heat Data

Yes . No.

22

I .I~

i. Supplytemperature('C)ii. RetumTemperature(')

Thermal Requirement(for last12 months)

4.

Year Month

5. Any alternative should currentsource fail:6. Any changes in the future demand patterns expected:

Boiler fuelcons"inption (for last 12 months)

7.

I~ ~

Year

Steam

Ton

Month

8. Capacity of Boiler facility:9. Does the Boiler require retrofitting/replacement?

Wood Waste/Sawdust data

I.

HotWater

G

Whattype of sawdustis produced (eg. Weti'dry)(List by type of wood ifpossible)

................................................

Fuel

...............................................

2.

I, _

...............................................

What other type of wood waste is produced (eg. Bark, wood chippings, etc. )...............................................

..................................................

3.

..............................................,.....

How much volume of each do you produce (monthly/daily estimated)

Other Fuel

T Gofwoodwaste

........................................

...............................................

.......................................

Age:

.................................................

...................................

...................................

...................................

...................................

...................................

...................................

Volume

...................................

23

...................................

...................................

...................................

4. Ally uses forthe wood waste:Solid/Given out(B)Used (A)Recycled (E)Burnt'Disposed off(D)

i. Sawdust: ........................................

ii. Bark: ............................................

I __

111.

IV.

V.

5.

...................................................

...................................................

Ifsold or given out, what does buyer/recipient use it for:UseIL:{^

...................................................

...................................

...................................

...................................

6.

...................................

Any problem(s) being caused by wood waste:ProblemI^P^

...................................

...................................

7.

...................................

Any changes in wood waste handling expected.

Fuel(C)

...............................

...............................

...............................

...............................

...............................

...............................

...............................

24

I_

Project Coordinator

DTDanielSekyere, FORIG, Box 63 ,KNUST, Kumasi, Ghana

Email:dsek ere fori. or

Project Technical and Scientific staffs:

r~~

Dr. P. Y. Okyere

Department of Electrical and Electronic Engineering, Kl\IUST, Kumasi, Ghana

Dr. N. A. Darkwah

Institute of Renewable Natural Resources, KNUST , Kuinasi, Ghana

Mr. K. S. Nketiah

Up 982, KNUST, Kmnasi, Ghana

Email: ksnketiah ahoo. coin

25