biomass based combined heat and power generation

20

COMBINED HEAT AND POWER

At the beginning of thismillenium Denmark had installedbiomass fueled combined heatand power plants with a totalpower capacity of 200 MWe.Biomass CHP generation ac-counts for 1.5 percent of the totalcapacity for electricity productionin Denmark. Danish energy pro-duction is to a great extend basedon combined production of heatand power.

Why co-production?There is a very good technicaland environmental argument forchoosing this technology. In atraditional steam based power-plant with condensing operation,only 40-45 percent of the energy

input is converted into electricalpower. The remaining part is lostwith the cooling water into thesea and with the hot flue gas upthrough the stack. In a CHP plantthe electrical power is generatedin the same way as in a powerplant, but instead of disposing ofthe cooling water, the steam iscooled by the return water in adistrict heating system and thusused for generation of heat.

As the society needs bothheat and power, combined heatand power generation is environ-mentally and economicallyadvantageuos. The exploitationof the fuel input is much better,which is the main reason that thedevelopment within the CHP sec-

tor has a high priority in the Dan-ish energy policy.

The structural possibilitiesfor utilising the heat produced byCHP plants are good in Denmarkas a large part of the heat supplyis covered by district heating.This means that the CHP plantsand thus the electricity produc-tion in Denmark is to a large ex-tent decentralised as opposed to astructure with large central powerplants with a demand for seawa-ter cooling.

Active politicalengagementThree fundamental factors haveinfluenced the developmentwithin biomass based CHP gen-

Biomass based combined heat

and power generation

By Morten Tony Hansen

Electrical powergeneration 40%

Loss 60%Po

wer

pla

nt

Electrical powergeneration 25%

Loss 15%

De

ce

ntr

ali

se

dC

HP

pla

nt

Generationof heat 60%

Loss 15%

Dis

tric

th

eati

ng

pla

nt

Generationof heat 85%

Figure 8: By separate electrical power generation and generation of heat at a power plant and at a district

heating plant, total losses are much larger than by combined heat and power production at a CHP plant.

Biomass fueled CHP plants have now for

many years been a common part of the

Danish electricity and district heating

supply. The Danish CHP systems are

designed to handle a variety of fuels and

display a very constant operation. The

installation of new biomass CHP plants

continues steadily and the CHP systems

are constantly undergoing development

towards even higher efficiency. Recently

new plants have been put in operation in

Assens on Funen and in Maribo-

Sakskøbing on Lolland.

As the exploitation of the fuel is much better in combined heat and

power.


21

Co

mm

issio

ned

1

Co

ntr

acto

r

Fu

el

Tech

no

log

y

Ste

am

pre

ssu

re

Ste

am

tem

pera

ture

Max.ste

am

flo

w

Po

wer

ou

tpu

t,g

ross

Heat

ou

tpu

t

Ele

ctr

icaleff

icie

ncy

(gro

ss)

Overa

lleff

icie

ncy

(gro

ss)

Sto

rag

eta

nk

year bar °C tons/h MW MJ/s % % m3

Assens 1999 Vølund Wood chips,

sawdust

Steam

turbine

77 525 19 4.67 10.33

27% 87%3

2 ×

2,500

Avedøre 2 2001 Vølund Straw and wood

chips

Steam

turbine

300 582 - - - 43 - -

Ensted EV3 1998 FLS Miljø A/S Straw and

wood chips

Steam

generation

200 542 120 39.7 - - - -

Grenå 1992 Aalborg Boilers

Ahlstrøm

Straw and coal Steam

turbine

92 505 104 18.62

60 18 - 4,000

Harboøre 1993/00 Vølund Wood chips Updraught

gasifier

- - - 1.3-1.5 6-8 32-35 105 1,050

Haslev 1989/99 Vølund Straw Steam

turbine

67 450 26 5.02

13 25 86 3,200

Hjordkær 1997 Sønderjyllands

Maskinfabrik

Wood chips,

bio waste

Steam

turbine

30 396 4,4 0.6 2.7 16 86 1,000

Høgild 1994/98/00 Hollensen Wood Downdraught

gasifier

- - - 0.13 0.16 22 573

-

Junckers-7 1987 B&W Energi Sawdust, chips,

bark, shavings

Steam

turbine

93 525 55 9.6 - - - -

Junckers-8 1998 Vølund Sawdust, chips,

shavings, dust

Steam

turbine

93 525 64 16.42

- - - -

Maribo-

Sakskøbing

2000 FLS Miljø Straw Steam

turbine

90 540 43.2 10.2 20 29 87.5 6,000

Masnedø 1996 B&W Energi Straw, wood chips Steam

turbine

92 522 43 8.32

20.8 282

91 5,000

Måbjerg 1993 Vølund Straw, wood chips,

natural gas, waste

Steam

turbine

65 520 123 282

67 27 88 5,000

Novopan 1980 Vølund Various wood

wastes

Steam

turbine

71 450 35 4.2 - 19 88 -

Rudkøbing 1990 B&W Energi Straw Steam

turbine

60 450 12,8 2.3 7.0 22 87 2,500

Skarp Salling 1999 Reka Wood chips Stirling

engine

- - - 0.03 0.09 18 87 8

Slagelse 1990 Aalborg Ciserv,

BWE, Vølund

Straw Steam

turbine

67 450 40 11.72

28 29 - 3,500

1. New plant or renovation of existing plant.2. Power output, net3. With flue gas condensation the heat output increases to 13,8 MJ/s and the overall efficiency increases to 106 %..

Table 2: Main data for the combined heat and power plants in Denmark.

Reference: Danish Follow-up Programme for Small Scale Solid Biomass CHP.

22


eration in Denmark during thelast twenty years. Steeply risingprices on fossil fuels, the clearDanish vote against nuclear en-ergy supply and the positive tech-nological development supportedby political incentives, require-ments and agreements.

In 1986 the Danish Govern-ment made an energy policyagreement on the construction ofdecentralised CHP plants with atotal power output of 450 MW tobe completed by the year 1995.All these plants should be fueledwith domestic fuels such asstraw, wood, waste, biogas andnatural gas. In 1990 the Govern-ment further agreed on an in-creased utilisation of natural gasand solid biofuels in the districtheating plants. As a result of thisagreement a large number ofplants were converted to naturalgas based CHP generation, whileother district heating plants wereconverted to biomass based heatproduction.

Later, in 1993, the DanishParliament agreed on theso-called Biomass Plan, whichdirects the utilities to utilize 1.4million tonnes of biomass peryear. Of this a minimum of 1 mil-lion tonnes must be straw. Theutilities were supposed to honourthe agreement by the year 2000,but as this has not been possible,the agreement has now been pro-longed until the year 2005.

During the early nineties theutility sector constructed the firststeam based CHP plants for bio-mass. Steam based CHP genera-tion is a suitable technology forlarge plants, but it is more ques-tionable for small plants, as the

efficiency of small steam basedplants will be low.

The Danish CHP plantsIn order to promote the develop-ment - especially of technologiesfor small scale CHP generation -the Danish Energy Agency since1995 has managed the “DanishFollow-up Programme for SmallScale Solid Biomass CHPPlants”. In 1999 this programmeincluded twelwe demonstrationplants, which are divided amongdifferent fuels and different tech-nologies:

• Two gasification plants

• Four steam turbine plants forwood fuel

• Three steam turbine plants forstraw

• Two steam turbine plants forwaste and a variety of biofuels

• One Stirling engine for woodchips

These plants present differentlevels of development. While thesteam turbine part of the steam

Måbjerg

Skarp Salling

Høgild

Assens

NovopanGrenå

Hjordkær

Ensted EV3

Steam turbine Gasification system Stirling engine

Haslev

Masnedø

Maribo-Sakskøbing

Slagelse

Rudkøbing

Junckers

Avedøre 2

Harboøre

based technologies are wellknown and tested, the boiler partsconstantly go through further de-velopment. The Stirling and thegasification technologies are stillunder development. The fol-low-up programme includesplants owned by private compa-nies as well as utilities and dis-trict heating companies.

In addition to the twelweplants covered by the follow-upprogramme, there exist a numberof other biomass fueled CHPplants in Denmark. These plantsuse well known technology andare no longer undergoing any fur-ther development or experiments,but simply work as a reliable sup-ply of heat and power.

All the Danish CHP plantsare included in the table, whichshows the main plant data.

M.sc. in Engineering Morten

Tony Hansen is employed with

dk-TEKNIK ENERGY & ENVI-

RONMENT, one of the four part-

ners of the Danish Centre for

Biomass Technology.

Figure 9: The location of the

Danish CHP plants. The plants

are owned by private companies

as well as utilities and district

heating companies. Both the

steam based systems, the gasifi-

cation systems and the Stirling

engine system are constantly sub-

jected to development towards a

higher efficiency.

On 29 May 1999 Denmark’slargest privately owned bio-mass-fired combined heat andpower (CHP) plant was inaugu-rated at the small town of Assenson the island of Funen.

The plant was erected bydistrict heating operator AssensFjernvarme, which has long beenknown for its ability to supplydistrict heating to the town’s10,000 inhabitants at very favour-able prices.

The operator expect to beable to maintain this positionwhen the new DKK 127m CHPplant has been put into operation.The plant constitutes the largestinvestment ever in the local area,and it meets the entire electricityand heating demand of the townthrough “eco-friendly energy”.26,000 MWh of electrical powerand 50,000 MWh of heat is gen-erated each year.

The original district heatingplant was coal-fired, but by theend of the 1980’s the boardagreed that this was not a viablepath, considering the increase inenergy and environmental taxes.Consequently, the coals were re-placed by wood pellets, and lateron other types of wood fuel wereintroduced.

The decision to build thenew CHP plant was made almostfour years ago, when the DanishEnergy Agency ordered the plantto produce electrical power aswell as heat in the future.

The two alternatives at thetime were natural gas and bio-mass. A natural gas-fired plant

would cost DKK 55m plus an ad-ditional DKK 17m to deliver thegas to the plant. As mentionedabove, the biomass-fired plantwas more expensive, but as thestate was willing to provide aDKK 25m grant to help coverconstruction costs and an addi-tional subsidy amounting to DKK0.27 per kWh generated, the bio-mass plant was the most attrac-tive solution.

150 tonnes of woodper dayThe new CHP plant uses woodonly, and there is a good reasonfor this: At Assens they are quiteexperienced in buying wood atthe right price. Apart from usingDanish wood chips and pelletsthey import tree-trunks from theBaltic countries, and they knowwhere to buy sawdust, woodshavings and other sorts of wastewood from the industry.

“The most expensive fuelsare wood pellets and Danish

wood chips”, says plant managerJohn Jessen. Sawdust is ourcheapest fuel, but the boiler does-n’t allow us to use more than 30per cent sawdust.

The plant has an indoor stor-age capacity that covers approx.10 days’ consumption at fullload. Approx. 150 tonnes ofwood products are used per day,and the plant generates 4.68 MWof electrical power and 10.3 MWof heat.

Several innovativefeaturesA large part of the plant, such asthe boiler and the steam turbine,is based on proven technology,but it also includes several inno-vative features. The handling ofthe different sorts of wood fuelshas required a good deal of inno-vation, and the boiler has beenequipped with an entirely newfeeding system.

The vast majority of thefunctions are automatic, so a

24

Denmark’s largest privately owned

biomass-fired combined heat and power

plant is situated at the small town of

Assens on the island of Funen. The plant,

which has cost DKK 127m, covers almost

the entire electrical power and heating

demand of the town and its 10,000

inhabitants. In the winter, when the plant

is operating at full load, 150 tonnes of

wood is converted per day.

By Torben Skøtt

Wood-firedCHP plant covers

local energy demand



mere four employees manage tooperate the plant.

Two fully automatic cranesand a conveyor transport the fuelfrom the storage area to thefeeder silo in front of the boiler.From here it is led to theso-called “air spouts” that throwthe fuel onto an oscillating gratein the boiler. Much of the fuel iscombusted while flying throughthe chamber, and the large partsburn on the grate.

The heat from the boiler isused to produce steam at a tem-perature of 525°C and a 75 barpressure. This output is similar tothat achieved at far larger plants,and it has made it possible toreach an electrical power effi-ciency of 27 per cent.

The smoke is cleaned in a70,000 volt DC electrostaticprecipitator and led to the chim-ney at a temperature of approx.110°C. The fly ash, approx. 90per cent of the ash, is kept in acontainer and subsequently dis-posed of at a controlled refusedump.

The steam leaves the turbineat a temperature of 85°C and a0.6 bar pressure. This allows thesteam to heat the district heatingsystem return water from approx.37°C to 75°C.

In the winter, when the useof wet fuels such as wood chipstypically increases, it is possibleto incorporate a flue gas con-denser capable of generating afurther 3.5 MW of heat. Natu-rally, the condenser is not intro-duced until the heat demand ex-ceeds the 10.3 MW that the plantcan produce without the con-denser.

Three-period rateThe plant charges the electricalpower produced by a so-calledthree-period rate. This means thatthe highest price is obtained dur-ing the peak load periods in themorning and in the evening, whilethe lowest price is obtained duringthe low load periods at night.

In the summer months,when the heat demand is low, theplant only operates 6-8 hours a

day. It is normally started upearly in the morning to be readyfor the first peak load period,when the price is high. Surplusheat is stored in two accumula-tion tanks, each of which holds2,500 m3 of hot water.

“The problem is that it takesa couple of hours to start up theplant, and during that period weonly produce heat”, says JohnJessen.

“We therefore look forwardto the replacement of thethree-period rate with a standardrate, which hopefully will be in-

troduced soon. This will be aclear advantage, as it will allowus to run the plant for longer con-tinuous periods. Consequently,we can produce more electricalpower and avoid the manystart-up processes, which wear onthe plant.

The new CHP plant wassupplied by Vølund with COWIConsulting Engineers acting asconsultant.

Torben Skøtt is a journalist and

editor of the magazine Dansk

BioEnergi (Danish Bioenergy).

Figure 10: Flow dia-

gram showing the

new wood- fired CHP

plant at Assens.

25

MULTI-HEAT biomass boiler- the boiler for the future

Smedevej • DK-6880 Tarm • Tlf. 97 37 15 11 • e-mail [email protected] • www.baxi.dk

It all started in the late 1980’swith research at the TechnicalUniversity of Denmark, the Dan-ish Technological Institute, in in-dustry and within the utility sec-tor. In 1993, as a result of thiswork, subsidies were granted forthe setting up of an actual dem-onstration project at the HarboøreFjernvarme district heating plant.Since then, the Danish EnergyAgency and the utility companyElkraft have provided grants for atotal of six different demonstra-tion projects.

The background to these ini-tiatives was very simple. The en-gineers were convinced that thetechnology of converting biofuelinto gas was the ideal solution forcombined heat and power genera-tion purposes. Especially at rela-tively small plants, where theproven steam turbine techniqueappeared to be costly as well asinefficient.

As regards environmentalimpact, there are also advantagesin converting solid fuels into gas.The gas mainly consists of hydro-gen and carbon monoxide, whichafter conversion turn into fluegases that only contain water,

carbon dioxide and nitrogen com-pounds.

Harboøre running steadyAfter several years of develop-ment, the first gasification plantwas put into commercial use atthe Harboøre Fjernvarme districtheating plant by the end of 1993.Supplied by Vølund, the plant isbased on a so-called updraftgasifier capable of firing fuelwith a water content of up to 50per cent.

Until 1996 the plant wasconverted and optimised severaltimes. In this connection a num-ber of tests were carried out,which showed that the plant out-put was twice as high as esti-mated. In addition, the load couldquickly be changed from 10 to100 per cent, which is not possi-ble in a conventional wood chipboiler.

At the time when the plantwas built, no efficient gas cleaningtechniques were available. There-fore, the gas was combusted in aboiler for many years, but todayan efficient gas cleaning tech-nique has been found, so now thegas is used in two engine/genera-

tor plants supplied by Jenbacher.The overall electrical output is 1.5MW, and the cogenerated amountof heat meets the demand of the600 households connected to theplant.

In recent years the gasifica-tion plant has been reliableenough to be left unmanned forperiods of two to three weeks. Ithas covered approx. 95 per centof the heat demand, and for thelast two years there have beenpractically no production inter-ruptions due to the gasifier.

Høgild is too smallIn 1994 a small-scale gasificationplant was set up in the village ofHøgild just south of Herning tosupply heat and electricity to theapprox. 100 households of thevillage. The plant was built bythe public utility company inHerning, who bought a gasifica-tion plant from the French com-pany Martezo. The gasifier wasbased on the so-called downdraftprinciple, which was used onmany vehicles during the SecondWorld War.

The principle of the plantwas viable, but the technical de-

By Henrik Flyver

Bio-gasification breakthroughThanks to ten years of

research in biofuel gasi-

fication, Denmark is to-

day one of the leading

countries in the area of

small-scale combined

heat and power plants

based on bio-gasifica-

tion. The oldest plant

has been operating

since 1993 and pro-

duces electricity as well

as heating to approx.

600 consumers.

On the international arena Denmark takes a prominent position in the

area of straw gasification. One of the centres of expertise is this com-

bined heat and power plant at Haslev, where a pyrolysis plant capable

of converting straw into gas was in operation for several years.

Christiansen


26

sign was so poor that it was notsafe to continue operating theplant. Consequently, in 1997 itwas decided to have Danish engi-neers Hollensen ApS carry outcomprehensive refurbishing andreconstruction of the plant.

This work was completed bythe end of January 1998, andfrom mid-April it has been possi-ble to monitor the plant from ahome-based PC outside normalworking hours.

The plant is now undergoingreconstruction to allow it to useordinary wood chips as fuel. Sofar it has only been operational ifthe fuel consisted of driedwooden blocks of a certain size -a fuel source that has often beenhard to provide in sufficientquantities.

The experience from theHøgild plant shows that it is diffi-cult to make plants economicallyviable with only about 100 con-sumers connected. It seems thatat least 2-300 consumers are re-quired in order for such a projectto be economically viable.

Other projectsThe other demonstration projectshave not yet come this far, butthey have all been developedover the past years.

In the village of Blære nearAars the machinery manufacturerREKA has co-operated with theTechnical University of Denmarkto set up a two-stage gasifier witha gas cleaning and engine/genera-tor system. Having dealt withmost of the initial teething trou-bles, the operators succeeded inrunning the engine for 100 hours.

After this, certain problems oc-curred causing the plant to standidle for 18 months so far. How-ever, further development hastaken place at the Technical Uni-versity of Denmark, and now thetechnology is considered to befully developed.

The company dk-TEKNIKENERGY & ENVIRONMENThas co-operated with the machinemanufacturer Butina to furtherdevelop the so-called “open core”principle. The principle hasproven to be viable for ordinary

The gasification plant

at Høgild, which has

been running since

1994. The plant is

currently being con-

verted to use ordi-

nary wood chips as

fuel.


27

Plant Built Plant owner Supplier Principle Output Status

Harboøre 1993 HarboøreFjernvarme

Vølund Updraught gasi-fier for wood

4 MW Steady operation. In March 2000 an engi-ne/generator system was installed at theplant, which now produces both electricityand heat for 600 consumers.

Høgild 1994 HerningKommunaleVærker

Martezo/Hollesen ApS

Downdraughtgasifier fordried wood

700 kW Steady operation, but the plant is too smallto make it economically viable. Convertedto wood chip-firing in the spring of 2000.

Blære 1994 Ove Olsen MaskinfabrikenReka/DTU

Two-stage gasi-fier for wood

400 kW The plant has been running for 100 hours.Technology fully developed.

Haslev 1996-1999

Elkraft COWI/Elkraft/Vølund

Pyrolysis gasifi-cation of straw

3 MW This project proved that it is possible togasify straw. The plant has now been dis-mantled.

Holbæk 1996 dk-TEKNIK dk-TEKNIK/Maskinfabrik-ken Butina

“Open Core”principle

200 kW 400 operating hours, including 100 hoursof continuous operation for electricity pro-duction. The construction of a larger pilotplant is being considered.

Table 3: Outline of

gasification plants in

Denmark.

wood chips, provided that the wa-ter content is below 25 per centbefore gasification. In particular,the dust and tar levels measured inthe cleaned gas justify optimisticexpectations for the future of theplant. It has been operating forapprox. 400 hours, including 100hours of continuous operationwith a 20 kWe engine/generatorsystem connected. Now the aim isto achieve at least 3,000 hours offully automatic operation of thepilot plant. The natural next step isa demonstration plant supplying amajor institution or a small-scaleheating plant.

In 1988, TK-Energi inRoskilde and its French partnersobtained grants from the EU-programme Joule and the DanishEnergy Agency. Based on multi-stage straw gasification, the pro-ject was launched in the autumnof 1998.

Straw gasificationIn recent years the main efforthas been concentrated on gasifi-

cation of wood, as experienceshows that gasification of straw ismuch more problematic.

At international level, how-ever, Denmark takes a prominentposition in this area, as we areone of the few countries that haveworked seriously with straw gasi-fication. In this context the re-search department at Elkraft hasmade a particularly fine long-term effort in the area, whichnow appears to bear fruit.

For instance, the Kyndbyplant has carried out severalstraw gasification tests, theTechnical University of Den-mark has had a straw gasifica-tion plant, and at the straw-firedcombined heat and power plantin Haslev they have a so-calledpyrolysis plant at which some ofthe straw can be converted intogas and coke. The gas can becombusted in the super-heater atthe plant, whereas the coke canbe exploited in the straw-firedboiler. This means that the elec-tricity efficiency is increased,

while the risk of corrosion is re-duced.

Follow-up programmeThe various gasification technol-ogies form part of the Danish En-ergy Agency’s follow-upprogramme for decentralisedcombined heat and power genera-tion from solid biomass fuel. Inaddition, work is being carriedout on various steam technolo-gies and stirling-engines.

The purpose of the fol-low-up programme is to make anassessment of the economic as-pects as well as the energy andenvironmental aspects of plantswith an output of up to 10 MW.

Thanks to the positive expe-rience, especially from the plantsat Høgild and Harboøre, Den-mark has managed to place itselfat the forefront of internationaldevelopment. We have come par-ticularly far in the developmentof small-scale plants, e.g. minordistrict heating plants.

Things are now moving sofast that the Danish EnergyAgency has already received re-quests for support for the firstsecond-generation plants. Andwe expect that there will be morein the years to come, as 6 - 8 pilotprojects concerning the installa-tion of new plants or conversioninto combined heat and powerplants are ready.

M.sc. in Engineering Henrik

Flyver Christiansen is employed

with the Biomass section of the

Danish Energy Agency.

A glance into one of

the first versions of

the “Open Core”

gasifier from

dk-TEKNIK.


28

Gasification prepares the heating

plant for

the futureWhen setting up new

heating plants or re-

placing existing boilers,

operators should seri-

ously consider investing

in a gasification plant

instead. The costs are

approximately the

same, and the gasifica-

tion plant is prepared

for the future.

At the latest seminar of the Dan-ish Energy Agency on decentral-ised combined heat and powergeneration, the company Vølundstated that today they can offergasification plants for wood un-der the same guarantees as con-ventional grate-fired heatingplants. Vølund base their guaran-tees on the experience gainedfrom the district heating plant atHarboøre, which has been operat-ing a gasification plant since1992/93. For quite some time theplant has only been producingheat, but in March 2000 it wasfitted with two gas engines, sothat it now produces electricity aswell as heat.

The seminar also revealedthat Vølund is no longer the onlyDanish supplier of gasificationplants operating according to theupdraft principle. Today, bothDanish Shell and FLS Miljø haveembraced the technology.

As this is a considerabletechnological breakthrough, theDanish Energy Agency has as-sessed the possibility of existingand future district heating plantsmaking use of the technology.

In many of the oldest straw-and wood-fired heating plants theboilers are now worn out and in

need of replacement. In this con-nection many of the plant opera-tors have to look at the possibili-ties of converting the plant into acombined heat and power plant.

This is where the gasifica-tion technology should be consid-ered. Instead of investing in anew boiler, which normally can-not be converted to CHP genera-tion, the operators now have thepossibility of investing in a gasi-fication plant that can subse-quently be extended to includeCHP generation.

Two solutionsFigure 11 shows the two alterna-tives that heating plant operatorscan choose from today. The up-per one is the conventional heat-ing plant with a wood chip-firedboiler, and the lower one is thegasification system, where thechip-fired boiler has been re-placed by a combination of agasifier and a gas/oil boiler con-verted from the existing oilboiler. The fuel handling and fluegas cleaning installations aremaintained. Later on, the gasifi-cation plant can be converted toCHP generation by adding a gascleaning system and a gas engine,as shown in the box.

Table 4 shows a relativecomparison of the initial invest-ments for the gasification plant

and the conventional boiler plant,respectively. The costs relating tothe district heating network havebeen excluded, as they are thesame in the two cases.

The boiler plant figures arebased on experience from a num-ber of established chip-fired heat-ing plants. As regards the gasifi-cation plant, the prices are basedon the costs at Harboøre and vari-ous pilot projects as well as anestimate of the technical mea-sures required.

Higher outputAs you can see from the table,the gasification plant is morecostly than the conventionalboiler plant, but this is largelydue to the fact that the outputfrom the gasification plant is30-40 per cent higher, as it is pre-pared to produce electricity aswell. Thus, it is not only thegasifier itself that is more expen-sive. The costs in connectionwith fuel handling, flue gas con-denser, oil/gas boiler and thebuilding are also higher due tothe higher output.

In the case of two plants ofthe same size, the gasificationplant is currently deemed to beslightly more expensive, but assoon as gasification plant numberfive is set up it is expected thatinvestments are at the same level

On the basis of seven years of experience with the Harboøre gasifica-

tion plant, Vølund now offers the same guarantees for this type of plant

as for conventional boiler plants.

photo

:to

rben

skøtt/b

iopre

ss

By Martin W. Fock and

Henrik Flyver Christiansen


29

as or lower than those for aboiler.

As is the case with the initialinvestments, it is possible tocompare the operating costs forthe two plant types.

As appears from table 5, theoverall operating costs are identi-cal for the two plants. The differ-ence is that in the case of thegasifier the electricity and lyeconsumption is higher, while theoil consumption is lower, as theheat production of the gasifiercan more easily be adjusted to theconsumption.

Advantages anddisadvantagesAt a conventional boiler plant theoutput can normally be controlledbetween 30 and 100 per cent. Ifthe boiler is forced to produce ahigher output, the amount of un-burned particles in the ash in-creases, which may necessitatelandfilling of the ash. At theHarboøre plant it is possible tocontrol the output from thegasifier between 10 and 200 percent, and a further advantage ofthis technology is that there arepractically no unburned particlesin the ash - the ash is all white.

Normally, the wood chip-fired boiler can convert all sortsof wood chips, wood pellets andin several cases also sawdust.

The gasification plant, on theother hand, is not expected to becapable of converting wood pel-lets and sawdust.

In conventional boiler plantsthe grate must constantly be re-paired, whereas the costs incurredby gasification plants particularlyconcern maintenance of the gasburner.

The futureWhen setting up new plants or re-placing existing boilers, it ishighly recommendable to checkthe possibility of installing a gasi-fication system instead. The gasi-fication plant may be slightlymore expensive, but it provides amuch more flexible solution thatcan quickly be adapted to theheat consumption and convertedinto a combined heat and powerplant.

The Danish Energy Agencyis currently processing applica-tions from all three plant supplierswho have opted for the same gasi-fication technology but apply dif-ferent gas cleaning methods anduse the gas for different purposes.

M.sc. in Engineering Martin

W. Fock is employed with dk-

TEKNIK ENERGY & ENVIRON-

MENT, one of the four partners

at the Danish Centre for Biomass

Technology.

M.sc. in Engineering Henrik

Flyver Christiansen is employed

with the Biomass section of the

Danish Energy Agency.

Fuelhandling

Chip-firedboiler

Oilboiler

Gasificationplant

Gascleaning

Oil/gasboiler

Gasengine

Flue gascondensation

Fuelhandling

Flue gascondensation Heating

Heating

Wood-fired boiler plant

Wood-fired gasification plant

Electricity

New plant Replacement

Boiler Gasifier Boiler Gasifier

Fuel handling 13 15

Boiler/gasifier 30 38 30 38

Flue gas condenser 10 12

Flue gas cleaning 3 0

Ash and slag system 4 3

Oil/gas boiler 5 8 3

Building 35 39

Total 100 115 30 41

Table 4: Relative comparison of the initial investments in a conven-

tional boiler plant and a gasification plant.

Boiler Gasifier

Revenue from sale of heat 100 100

Costs:

Operation and maintenance 10 15

Fuel, wood chips 45 48

Fuel, oil 10 2

Loan instalment and interest 20 20

Labour, administration 15 15

Total costs 100 100

Table 5: Relative comparison of the operating costs of a conventional

boiler plant and a gasification plant.

Figure 11: The oper-

ating principle of a

conventional chip-

fired heating plant

and a gasification

plant, where the

boiler has been re-

placed by a combina-

tion of a gasifier and

a converted oil boiler.

The gasification plant

can subsequently be

converted into a com-

bined heat and power

plant by adding a gas

cleaning system and a

gas engine, as shown

in the box.


30

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Smedemester farm biogas plantat Hemmet. The plant has a re-actor of 800 m

3and a 400 kW

dual-fuel engine.

400 kW dual-fuel engine. Smedemester farm biogas plantat Hadsund. The plant has a re-actor of 600 m

3and a 400 kW

dual-fuel engine.

Smedemester Biogas PlantDanish Biogas Technology A/S, a sister company of Bio-Energy Lab, under-takes planning, installation, operation and marketing of Smedemester farmand industrial biogas plants. The Smedemester biogas plant is a unique typeof biogas plant that uses animal manure together with small amounts of or-ganic waste, resulting in the production of 2 - 4 times more biogas comparedto traditional plants and concepts. The produced biogas is used in a CHP unitto produce “green” electricity and heat. The Smedemester biogas plant isprofitable and has several environmental benefits such as reduction in theemissions of greenhouse gases, reduction of the smell inconvenience from

animal manure, reduction in water and air pollution.

For more information contact:

Danish Biogas Technology A/SSoenderhoej 46DK-8260 Viby J.Tlf.: +45 87345603Fax: +45 87345602E-mail: [email protected]


32

A major Danish effort in

the development of Stir-

ling engines for decen-

tralised combined heat

and power (CHP) pro-

duction now appears to

bear fruit. The Stirling

engine has now been

operating on wood chips

for just over 1,000

hours, and all major

problems finally appear

to have been solved.

So far, CHP production based onbiomass-fuels has been problem-atic at minor plants. The largeplants owned by the electricalpower corporations apply theproven technique involving steamturbines, but this solution is toocostly for small plants, and theelectrical efficiency is far toolow.

By using a Stirling enginethat powers an electrical genera-tor it is possible to achieve a rela-tively high electrical efficiency,and as the technology is fairlysimple, capital costs can be keptat a reasonable level. The engineis particularly suitable for fuelsthat are hard to process, as com-bustion does not take place insidethe cylinders of the engine but inan external boiler.

The development of woodchip-fired Stirling engines startedback in 1990. The utility com-pany ELKRAFT saw the possi-bilities of the technology and ac-cepted to participate in the pro-ject and bear a significant part ofthe financing burden. Since then,the Danish Energy Agency hasactively contributed to the projectby providing subsidies to theplant now being developed.

The first test results showthat it is possible to operate awood chip-fired Stirling engine

for more than a few hours. Previ-ous results from abroad have notbeen promising, because the testshad to be interrupted after a fewhours due to clogging in theboiler section of the engine.

The Department of EnergyEngineering at the TechnicalUniversity of Denmark has beenin charge of the development ofthe engine in co-operation withVølund and Danstoker. The ma-chinery manufacturer REKA hasdeveloped the wood chip-firingsystem. The company PlanEnergihas co-ordinated the project andhandled the contact with the Dan-ish Energy Agency. Tests are car-ried out at farmer Poul Munk’sfarm at Salling in North Jutland.

The new Stirling engineThe Stirling engine is designed asa hermetically sealed unit like thecompressor in a refrigerator. Allmechanical parts of the engineare enclosed in a housing to-gether with the generator, and theonly connection to the surround-ing environment is the cable con-necting it to the electrical powergrid.

The engine is based on theprinciple that gas expands when

heated and contracts when cooleddown. The Danish Stirling engineuses helium, which is heated to650-700°C by means of achip-fired boiler. The pressure inthe cylinder increases, and thepiston is forced downwards. Inthe next phase the gas is cooleddown, and the piston returns to itsinitial position.

Due to the high temperatureson the engine’s boiler surfacesthe flue gas is very hot, and if leddirectly to the chimney the effi-ciency drops significantly. There-fore, the flue gases are used topre-heat the combustion air.

The use of wood chips andstraw involves the risk of hightemperatures making the ash meltand stick to the heat conductingsurfaces. Consequently, an im-portant part of the Danish activi-ties has been concentrated on de-veloping an efficient combustionsystem.

The other components of theengine have been adapted to thestrong tubes in the boiler and thelarge internal volume. Thismeans that the four cylinders ofthe engine have a large cylindervolume and a low mean pressureof only 40 bar.

Stirling engine breakthrough

By Henrik Carlsen

The development of the Stirling engine makes it possible to generate

electricity and heat from biomass, even at very small plants. The elec-

trical power output of the engine is 28 kW, and the entire plant is auto-

mated, eliminating the need for manual operation monitoring.


33

The piston rings and seals aremade of non-lubricated materials.An entirely new crank mechanismensures low piston ring wear. Oilin the engine is avoided com-pletely by using grease-lubricatedbearings. The bearings have beendesigned for a service life of50,000 hours. No oil or spark plugchange is required, and conse-quently the need for service hasbeen reduced to a minimum.

The engine’s combustionsystem is designed for operationwith ordinary wood chips. A con-siderable effort has been put intodeveloping a control system toensure that the temperature in theengine boiler can be maintainedat the required level. This is vitalin order to achieve sufficientelectrical power output.

The entire plant is fully au-tomated, eliminating the need formonitoring of operations. Evenday-to-day production start-upand stop is carried out automati-cally. However, manual firing isrequired if the plant has been outof operation for several days.

Promising resultsAfter several rounds of plant re-construction, testing was finallystarted in the beginning of July1998. Since then, the plant hasbeen operating for just over 1,000hours.

The host, farmer Poul Munk,has put the plant back into opera-tion whenever an alarm from themonitoring system stopped it, butapart from this the plant has beenin unmanned operation.

In the laboratory, where theengine has been tested with natu-ral gas, the generator has yieldeda 35 kW output. When firing withwood chips, however, it has beennecessary to lower the tempera-ture in the Stirling engine boilerby approx. 50 degrees and in-crease the inlet temperature of thecooling water from 50°C to 80°C.Consequently, the electricalpower output is only approx. 28kW at max. operating pressure inthe engine. The electrical powerefficiency has been measured at19 per cent for wood chips with a40 per cent water content.

The boiler tubes of the Stirlingengine were cleaned after 500hours’ operation. Deposits onthe tubes and fins could easilybe removed by means of a brushand compressed air. The fact thatthe boiler is not clogged in slagin a matter of hours and that thedeposits can easily be removedis one of the major results of theproject. Tube cleaning at 500hour intervals, i.e. approxi-mately once a month, is fully ac-ceptable. The inspection of thetubes did not reveal any signs ofcorrosion.

Backed by the Danish En-ergy Agency, the team behind theStirling engine is now fully en-gaged in exploiting the experiencefrom the first engine with a viewto building a new version incorpo-rating a number of improvementsin the construction. One of themain concerns has been to ensurethat, as originally required, thenew engine yields 35 kW of elec-trical power output, also whenrunning on wood chips.

The new plant will have anupdraft gasifier developed byVølund. The possibility of settingup yet another plant with a woodchip-fired boiler from REKA isbeing considered.

Henrik Carlsen is a lecturer at

the Department of Energy Engi-

neering at the Technical Univer-

sity of Denmark.

Figure 12: The Stir-

ling engine is placed

on top of a boiler,

which is automati-

cally fuelled with

wood chips. The elec-

trical power genera-

tor at the top of the

picture yields a 28

kW output.

biomass based combined heat and power generation

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