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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
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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.
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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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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
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140,000
120,000
= 100,000"̂ 80,000>
E' 60,000,,= 40,000Ul
20,000
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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
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962
300
Steam rate (kg/}IT)
3 1,884
Outlet pressure (bar)
33
30
Armualworking hours(lit)
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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
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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
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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