Stirling

Final Report

November 1996

Scientific DirectionDipl.-Ing. J. Kern, Schlaich Bergermann und PartnerDipl.-Ing. J. Ployer, Mannesmann Anlagenbau Austria AG

Contract No. JOU2-CT92-o184

Production of electrical energy from biomass by a Stirling engine with

simultaneous integration into district heating system.

Schlaich Bergermann und Partner

Beratende Ingenieure im Bauwesen

SBP / MAB – Production of electrical energy from biomass by a Stirling engine with simultaneous integration into district heating system 32 SBP / MAB – Production of electrical energy from biomass by a Stirling engine with simultaneous integration into district heating system





Contents

1. Abstract (of interim reports)_________________________ 51.1 First interim report 01.04.1994 _____________________________ 51.2 Second interim report 01.10.1994 __________________________ 51.3 Third interim report 01.04.1995 _____________________________ 51.4 Forth interim report 01.10.1995 _____________________________ 61.5 Fifth interim report ________________________________________ 6

2. Introduction ____________________________________ 9

3. Objective _____________________________________ 11

4. Participants and Role___________________________ 15

5. Tasks Stirling (SBP) ____________________________ 175.1 Basic Investigations _____________________________________ 175.2 Design of the hot flue air heat exchanger ___________________ 17

5.2.1 Design of the heat exchanger ___________________ 175.2.2 Final design of the heat exchanger _______________ 195.2.3 Manufacture of the flue heat exchanger___________ 19

5.3 Design of the control system ______________________________ 215.3.1 General _______________________________________ 215.3.2 Principle of control _____________________________ 215.3.3 Modified electronic control ______________________ 225.3.4 Manufacture of the control system _______________ 22

5.4 Cavity and packaging ____________________________________ 225.4.1 Design of the cavity and packaging ______________ 225.4.2 Manufacture of the cavity and packaging _________ 22

5.5 Acceptance of the complete system _______________________ 23

6. Tasks Plant Installation (MAB) ___________________ 25

7. Start-up and Operation _________________________ 317.1 1st Trial Run 26.6.1996 ___________________________________ 317.2 Consequences from the 1st Trial Run ______________________ 337.3 2nd Trial Run 1.8.1996 ___________________________________ 347.4 Consequences from the 2nd Trial Run _____________________ 367.5 3rd Trial Run 8.8.1996 ____________________________________ 37

7.6 Consequences from the 3rd Trial Run ______________________ 387.7 4th Trial Run 12.8.1996 ___________________________________ 397.8 5th Trial Run 13.8.1996 ___________________________________ 407.9 6th Trial Run 14.8.1996 ___________________________________ 417.10 Summary of the Test Results ______________________________ 42

8. Results and Discussion _________________________ 45

9. Conclusions and Recommendations _____________ 47

10. Reference ____________________________________ 49






1. Abstract (of interim reports)

1.1 First interim report01.04.1994

In autumn 1993, at the beginning of the project, the original conceptions as wellas the conceptions necessitated by the change of partner were executed. Theessence of the project was to integrate a Stirling engine by Schlaich Bergermannund Partner into a biomass heating plant. In order to achieve this, wide-rangingdevelopment activities had to be performed on the heat exchanger. The startingpoint for this development work was a material study for the high-temperatureheat exchanger. This was followed by a thermal analysis, a stress analysis,consultation of literature on hot gas flows, an optimation computation for thecooling fins, and, for the time being, concepts for flue gas purification. At thesame time, assembly of the the components of the Stirling engine was commenced.

The first of two Striling motors to be deliveres is ready assembles and preparedfor mounting of the new developed Heat-Exchanger. The control system of thestirling unit was optimized for this special application with flue gas heating.

The Heat-Exchanger design is finished and the most important parameters: heat-exchanger surface and shape as well as the deal volume in the heat-exchangercould both be optimized. The heat exchanging surface with 68 tubes in two rowsans heat-exchanger fins is ready fabricates. The heat-exchanger will be completedin a final branzing cycle at the end of November 94. After this the hole system canbe completes and shipped around the end of the year and is reald for installation.

The first of two Striling motors are ready and also the control system is also readyinstalled.

The combustion unit has been changed becuase the amount of the dust was tohigh. We will try to take another combustion unit with less dust.The first of two Striling motors are ready and also the control system is also readyinstalled.

1.2 Second interimreport 01.10.1994

1.3 Third interim report01.04.1995






The combustion unit has changed. In the last report it was told that we aresearching for a new combustion unit. The unit is in the Johanneum Resarchinstitut in Graz.

Preparatory work for the installation of a Stirling engine at Joanneum Research,Institute for Energy Research, Graz, is currently under way. Following studies ofvarious methods of flue gas purification for their technical feasibility in considerationof economic aspects, we have selected an arrangement where the flue gas is ledto the heat exchangers via wire screens (mesh size 0.16 mm). We expect theconversion work to be concluded in May 1996. Immediately afterwards, a test runof 500 hours will be started. From today’s point of view, the end of the project willbe postponed and is not to be expected before week 45, 1996.

On 1st March 1996, project management at Mannesmann Anlagenbau AustriaAG was transfered from Dr. Dipl.-Ing. A. Oberhammer to Dipl.-Ing. J. Ployer.

1.4 Forth interim report01.10.1995

1.5 Fifth interim report






2. Introduction

At the beginning of the decade, our involvement with biological heating stationsand our contacts with operators of such plants showed that there was a keeninterest in parallel production of electrical energy and heat. MAB-Austria considereddevelopment of the appropriate technology an opportunity to stimulate businessin the biological heating station sector. We were looking for a process that permitsproduction of electricity at low operational expenditure. One such process is theStirling process which converts the energy contained in hot gases into mechanicalenergy without requiring expensive intermediate stages that would be required inother processes such as the steam process.






The driving force behind the project was the idea to search for a way of operatingbiomass heating stations as independently as possible of external power sources.Or, in more abstract terms:The aim is to find a process and develop it ready for operation that will permiteconomical production of electrical energy from biomass in future. This meansthat the process must be suitable for producing electricity at low investment andoperating costs.

On principle, there is general agreement that the production of electricity frombiomass hardly makes sense in high-capacity power stations due to the lowefficiency and high transport costs. Decentralized combined heat and power is asuitable application.

The following processes are possible for producing electricity from biomass byway of mechanical energy:

• Combustion• Steam process• Heat transfer oil• ORC-Process (Organic Rankine Cycle, similar to the steam process

on the basis of organic hydrocarbons, time-tested but has not beenexecuted for biomass so far. Temperature level 250°C.

• Stirling engine or hot-air turbine (Problems: flue gas/air-heat exchanger)• Gasification

• thermal gasification• Hot air turbine• Gas engine• Fuel cell

• anaerobic gasification (Biogas)• Gas engine

From the large number of possible processes the steam process stands out asthe one whose usefulness has been proved in innumerable installations. Itsapplication in small, decentralized biomass plants, however, was mostly preventedby the high investment costs and the need for continuous supervision by qualifiedpersonnel.

The starting point for the extension of the process aimed at – erection of biomassheating stations – led to a clear preference for combustion technologies. Thepreliminary decision was made easier by the fact that the biogenic substancesthat can be used for anaerobic gasification – mainly wood, but also corncobs,shells of sunflower seeds etc. – cannot be taken into serious consideration and

3. Objective






because thermal gasification of these substances entails a number of unsolvedproblems which are still waiting to be solved.

Among the combustion processes, the Stirling process offers some positiveaspects. It is well known that the Stirling process permits the conversion of theenergy contained in hot gases into mechanical energy without requiring costlyintermediate stages as are required by other processes such as the steamproces. We considered a combination of a mechanical unit derived from theseries-production of two-stroke engines and a heat exchanger to be newlydeveloped the most promising arrangement for fulfilling the need for a simple,robust device requiring little maintenance.

On the basis of these considerations, the title of the development project wasdetermined as:

Production of Electrical Energy from Biomass by a Stirling Engine withsimultaneous Integration into District Heating System

Some elements of the title already imply elements of our aim:

1. Production of electrical energy from biomass. The specific problems causedby residues from biomass are the centre of attention.

2. Economical in combination with a district heating system – electricityproduction serves to cover station-auxiliary power requirements (circulationpumps, drives for heating station assemblies) only. This implies the followinglimitations:• The problem of supplying power to the grid at marginal costs need not

be considered. The marginal value for an economically efficientelectricity price is therefore defined in accordance with costs forelectricity provided by public utilities.

• The electrical output to be achieved is low as compared with thethermal output of the heating station.

3. Operational reliability. Operational reliability is a precondition for commercialapplication. At first, we aimed at a 5000-hour operation. In the course of theproject, these expectations were reduced to an operating time of 500 hoursafter the original aims had proved unrealistic with regard to the time available.






4. Participants and Role

The initiative for the project came from a plant construction company -Mannesmann Anlagenbau. It was natural for a plant construction company to lookfor a partner who is experienced in the production and operation of Stirlingengines.

The partner we found originally was Stirling Motors Europe. We intended to erecta pilot plant at the biological heating station Göttlesbrunn / Österreich which wasequipped with a new type of combustor and which was under construction at thattime. Stirling Motors Europe were to provide a stable Stirling engine. MAB-Austriawas responsible for cleaning the flue gas to such a degree that the function of theheat exchanger operating directly in the flue gas flow would not be impaired.

Implementation of the project was seriously delayed - Stirling Motors Europeexperienced problems with the engine’s service life and finally stopped activitiesaltogether.

A new partner was found in Schlaich Bergermann und Partner (SBP), who havegained experience with their Stirling engines based on Solo standard engines inconnection with solar power stations.

SBP’s responsibility was to develop a heat exchanger designed for flue gas forthe Stirling engine that was mechanically fully developed and had been time-tested in solar energy plants.

Mannesmann Anlagenbau was responsible for erecting and operating the pilotinstallation. This included the development of flue gas purification facilities.






Basic investigation at the ‘Institut für Technische Wärmelehre’ of the TechnicalUniversity of Vienna were made in the field of cleaning of the hot air flue and dustseparation deAssembly of the Stirling engines

There are two alternative positions of the heater in a biomass combustion. Oneposition is in the biomass itself, the other position is in the flow of the flue gases.The temperature is nearly the same, but there is a difference in the heat transferrate. If the heater position is in the biomass bed, there is, additional to convectiveand radiant heat transfer, another part, namely heat transfer by pushing andchemical reactive particles. On the other hand, the wear and tear is much higher.For this reason, the heater was designed for the use in the flow of the flue gases.The calculation of the necessary heat transfer surface was based on the formulasof cross flow heat exchangers. It turned out that it was necessary to use fins.

A huge number of varieties were investigated, using the expected power andperformance with empirical and analytical procedures. For the finally design itwas also necessary to make a compromise between many contrary requirements:• The tube diameter should be very small, to cause a good heat transfer (high

working gas flow speed in the tube)• The dead volume of the heater should be very small (needs small heat

transfer area and a small tube diameter)• The surface of the heat exchanger should be very big in the interest of the

best possible utilisation of the flue gas.• The losses of flow friction should be minimised (need huge tube diameters)• The manufacturing effort should be reduce (need small numbers of similar

tubes)The geometry of the heat exchanger has to fulfil the uses of the flue air inlet.The following Tab. 1 shows the possible heater-designs for 3 and 4 rows of tubes,with different numbers of fins, depending on the used thickness.

5. Tasks Stirling (SBP)

5.2 Design of the hot flue air heat exchanger

5.1 Basic Investigations

5.2.1 Design of the heatexchanger






three rows are different, the heat stretching also is. That’s why only tubes of thesame row are connected with fins. To get economical fins, each time 6 tubes arefixed together with one fin. Depending on the used thickness of fins, there are upto 110, the middle distance between the fins is 2mm. Fig. 5.6-1 shows a sketch ofthe heater with 72 tubes in 3 rows.

From the way things stood at our last design, a heater with three rows of tubeswas favoured. After consultations with an external adviser, the heater design haschanged to a more bendend heater with only two rows (see Fig. 5.6-2). This is dueto the adviser’s opinion, the tubes of the old heater design are not enoughbended to stand low cycle fatigue stress.

Cause now the tubes are longer two rows of 34 tubes each are enough to get thesame heat transfer surface. The thinner manifolds are an advantage to reducedead volume, also that now only two different lengths of tubes are necessary ispositive for the costs of the heater. The distance between the tubes in one row isnow 10mm,. , the distance between the rows is 8 mm. Each time two tubes of thesame row are connected with a fin. The fins of the second row in the smoke gasdirection are connected with a fin. The fins of the fins of the second row in flue gasflow direction are longer because the temperature of the flue gases is lower. Thethickness of the fins is 0.mm, the distance between the fins is 0.6, so there are 309fins on each tube.

The total heat exchanger surface (tubes & fins) was extended to the 4times size ofthe regular heat exchanger of the Stirlingmotor

The heat exchanger surface with 68 tubes in two rows and heat exchanger finsare already fabricated. The heat exchanger has been completed in a final brazingcycle at the end of November 94. The following ig. 5.10-1 to Fig. 5.10-8 areillustrating very detailed the steps of the assembling procedure of the hightemperature heat exchanger

5.2.2 Final design of the heatexchanger

Because of the thinner manifolds, decision was made for the heater with 3 rows.Which thickness of fins finally will beused, depends on the productionengineering.The distance between two neighbouredtubes is 8 mm, between the rows 3 mm.Because of this free cross-section forthe flue gases is relatively high andthere is enough space between thetubes for the fins.

To minimise the heat stress the tubesare bended. Because an s-like bendingof the tubes will result in an unevenflowing through the finned heater, themanifolds are turned similar to thecircle-angle the tubes are describing.Because the length of the tubes in the

j}{}��}�|}�� L�� K��flue gas temperature [°C] 850

cooling [°C] 750

mass flow [kg/s] 0,236

volume flow [m³/h] 2536

log. Temperature fall [K] 130

tube diameter inside [mm] 2

tube diameter outside [mm] 3

number of tubes 4x18=72 3x24=72

tube division a=4,7 ; b=2 a=3,67 ; b=2

length of manifold [mm] 252 264

length of tubes [mm] 280

height of fins [mm] 3

distance between fins [mm] 2

thickness of fins [mm] 0,5 0,8 1,0 0,5 0,8 1,0

number of fins 110 98 90 110 98 90

free cross-section [m²] 0,0372 0,03255 0,03025 0,0354 0,0309 0,0287

receiver surface [m²] 0,8 0,77 0,72 0,8 0,77 0,72

max. flue gas speed [m/s] 18,9 21,6 23,3 19,9 22,8 24,5

transferred heat flow [kW] 30,1 31,7 30,4 29,7 31,2 29,9

Tab. 1 possibleheater designs

Fig. 5.2-2 Final receiver design

Fig. 5.1-2 Receiver design

5.2.3 Manufacture of the flueheat exchanger






Fig. 5.2.3-5 Heater unit assembled inthe hot gas inlet duct

Integrated in the Stirling engine unit is an electronic control that monitors engineoperation by means of sensors. The sensors feed information back to amicroprocessor. If any of the sensors should detect an abnormal function, the unitis switched off automatically by the stop program. The Stirling engine will beautomatically started after turning the working switch to “start” position. The fluegas flap opens and the generator will be connected to the power supply system.On “stop”-position the procedure will run the other way round. Therefor theelectronic control is switching the relay for the flue gas flap, the coolant pump andthe switch relay of the generator.

The control of the engine occurs by controlling the temperature of the workinggas. If the supplied heat is larger than the sum of the leaded heat (in the coolingwater) and the mechanical work, the temperature of the working gas rises. In thiscase the mass of working gas in the engine has to be increased. In the contrastingcase the mass of the working gas has to be reduced. For that the engine isconnected with an external Helium bottle.

Fig. 5.2.3-1 Bent tubes, bent in aspecial tool. Stamped fins before and afterassembling on tubes

Fig. 5.2.3-2 Tubes assemblies withmanifold assembled and EB (electronbeam) welded. A special EB weldingprocedure is machined and prepared forTIP welding (Tungsten inert gas)

Fig. 5.2.3-3 Finished heater unit withwelded cylinders, vacuum brazed fins andpressure tested

Fig. 5.2.3-4 Detail for the hot gas duct

Fig. 5.2.3-7 Complete unitFig. 5.2.3-6 Inner hot gas ductassembled in other housing

5.3.2 Principle of control

5.3 Design of the control system

5.3.1 General






5.3.4 Manufacture of thecontrol system

5.4.2 Manufacture of the cavityand packaging

5.5 Acceptance of the complete system

5.4.1 Design of the cavity andpackaging

Cause the dynamic response of the valves and the gas volume in the engine is anintegral response, the reaction of the engine is a proportional plus integralcontroller. That means that if the temperature of the working gas falls below therequired temperature and working gas is discharged through the discharge valvethe external Helium bottle, the temperature of the working gas rises above therequired temperature. It is not possible to prevent this oscillation, it is onlypossible to reduce the amplitude. Therefore the required temperate is set in threesteps:• 2 minutes on level 1 (580°C)• 1 minutes on level 2 (620°C)• required temperature (650°C)

If the temperature of the working gas falls below 400°C, the controller is chargedback to level2, if the temperature falls below 300°C, the controller is changedback to level 1. The regulation procedure continues at this level.

The control system of the Stirling unit was optimised for this special applicationwith flue gas heating.

To make sure that the flue gas flows through the heater, it is necessary to install afade with a bypass, that allows to regulate the flow through the heater.

To integrate the heat exchanger into the flue gas flow, an insulating cavity wasmanufactured.

5.3.3 Modified electroniccontrol

5.4 Cavity and packaging

The whole system has been completed and was shipped around the end of theyear 1995.

Cause the engines have still prototype character, it was necessary to run the unitfor a longer time on a test stand. If there is a Helium leak, it usually appears after2-4 weeks with interrupted operation.

For the purpose of a secure operation of the unit a new box with displays, whichmonitors the most important data (working gas temperature, working gas pressure,Helium tank pressure) was build.






6. Tasks Plant Installation (MAB)

0

200

400

600

800

1000

1200

100 150 200 250 300 350 400

n��}��s�äG�u

m

�

�

}

�

|

�

�

{

�

�

�

�

�

�

n

}

�

�

�

�

y

�

�

�

�

s

�

y

u

teilweise offen

voll offen

Fig. 6-1 Characteristic curve exhaust fan

As the original site at Göttlesbrunn could no longer be used and the search forsuitable heat sources pruved difficult, the beginning of the tests, which hadoriginally been scheduled for 1995, was deöayed. Finally, we found a combustorwhich fulfilled the requirements at the IEF, Joanneum Institute for Energy Rese-arch, Graz. Furthermore, the institute had run a project for the development ofStirling engines before and therefore possessed perfectly suitable infrastructureand was well experienced in the subject.

As time was now limited, the intended duration of the test was reduced to 500hours of operation as opposed to 5000 hours planned originally.

Examination of exhaust gas purification facilities which are both technologicallysuitable and ecologically justifiably had to include investigations concerning theeffects of the comparatively high flue gas temperatures of approximately 1100°Con the materials. For this reason we seriously considered filters made of ceramicsand dry powdered metal. The high costs and long delivery periods of severalmonths, however, caused us to decide against applying these materials.

It was then decided to use wire meshes of mesh size 0.16 mm. This decision wasbased on the following arguments:

• The fins of the heat exchanger are situated at a distance of approx. 0.3 mm.A close-meshed screens reduces probability of clogging the free cross-section with larger particles considerably.

• The risk of caking arising on the heat exchanger’s fins was rated very low. AStirling engine had been connected to the combustion unit at Graz before -however, with smooth heat exchanger surfaces - and caking did not occur.

• It was planned to provide a compressed-air cleaning facility at the screensat a later stage.

After the decision on the type of filter had been taken in principle, the componentswer installed according to the following pattern:

The flue gas flows from the combustor (shown on the right) burning waste wood toa duct. This duct leads directly to a heat exchanger in which the thermal energy ofthe flue gas is transferred to cooling water. An exhaust fan, type Heinisch LMA 28(Pv 1,8 kW, Pm 2,2 kW, 3000 U/min, pt=9,5 mbar) is situated downstream of theheat exchanger and leads the gas, that has been cooled down to 150°C to achimney.

A mechanically driven damper is situated in the duct and operated alternatelywith another damper situated immediately upstream of the Stirling engine.Furthermore, the duct can be sealed off by way of a gat valve operated manually.






Fig. 6-5 Fuel

Fig. 6--6 Flame image during firing. At anair condition of λ = 1.5 a flue gas mass flowof 0.03 - 0.05 kg/s arises, the dust contentis 50 – 300 mg/Nm³, the thermal output 40 –

Fig. 6-3 Flue gas screen with internalsupporting structure

Fig. 6-4 Flue gas screen installed ininternally insulated sheet metal duct

Fig. 6-2 General view of the test arrange-ment: The flue gas flows from the com-bustion chamber (shown on the far right)via the main duct to a heat exchanger inwhich the remaining heat is fed to thecooling water. Alternatively, the flue gas isled via a branch containing a screen, viathe Stirling engine and back to the mainduct. The Stirling engine is connected tothe duct by way of 2 steel compensators. Agenerator which feeds the excess energyinto the public power network is coupled tothe engine. The helium interim storage tankcan be seen situated vertically in front ofthe generator.

A branch is situated between combustor and damper, which leads the combustiongases via a steel compensator and the damper mentioned to the engine’s heatexchanger. The engine itself is flexibly mounted on the floor. The engine iscoupled to a generator which feeds the excess energy into the public grid.Having passed the heat exchanger, the flue gas is cooled down and flows viaanother compensator and another duct branch back to the main duct and,therfore, to the heat exchanger.

The flue gas ducts are rectangular ducts of galvanized sheet metal of an internalsection of 300 x 300 mm. Such ducts are usually installed in ventilation plants. Theducts are fitted with internal installations in order to protect them from excessivetemperature loads.

The screen situated directly at the branch of this parallel duct serves to protectthe Stirling engines from large particles contained in the flue gas. As time waslimited, we decided to insert a sieve mesh of customary material (1.4301) via thesupporting structure made of material 1.4841 in order to gain initial experience.

Various measuring devices enable us to read and record the flue gas temperaturesat the intake to the flue gas duct as well as immediately upstream and downstreamof the Stirling engine. Furthermore, the cooling water temperatures before andafter admission to the cooler as well as the coolign water flow rate are measured.Based on these three data, the cooling water rating can be computed. In addition,

the helium temperature and pressuresinside the bottle and the engine arerecorded. Finally, the engine rating isregistered. Readings of absolutepressures at various points inside theflue gas ducts and pressure differencescan be taken from purely opticaldisplays.






Fig. 6-7 The test stand permits display andregistration of the individual measurementdata on computer






The first trial runs during which measurements were not recorded served to checkthe control system and the function of the individual components. These trial runsshowed that it was not possible to recirculate helium from the machine to theinterim storage tank at the planned rate. When the filters and non-return valves inthe helium recirculating pumping system were cleaned some rust deposits werefound and removed. This improved the situation slightly but did not lead to asatisfactory solution. Exchange of individual subassemblies between the engineinstalled and the standby unit helped to narrow down the fault. Nevertheless, itwas impossible to remove the fault completely. As this malfunction did notseriously impair operation but merely resulted in the helium pressure requiringmanual adjustment during start-up, fault diagnosis was interrupted temporarily.

Afterwards, a trial run of approx. two hours was performed in order to check thefunction of the flue gas screen and take initial measurements.

• The output measurement showed that the rated output of 10 kW was notachieved. Possible causes:• The induced draft creates too little underpressure to overcome the

flow resistances in the ducts, in the screen and in the engine at themass flow required.

• Thermal stresses inside the ducts cause distortions which permit theadmission of leakage air, resulting in a temperature drop of approx.150 K.

• Pressure measurements showed that the underpressure before admissionto the heat exchanger was 6.7 mbar. At the beginning of the trial runs thefilter contributed to the overall loss with 4 mbar and the engine’s heatexchanger with 2.7 mbar. Later this ratio reversed. The filter loss wasreduced to 1.4 mbar, the engine loss increased to 4.7 mbar.

Additional sealing measures at the ducts as well as an increase of the flue gastemperature up to 1150 °C resulted in a brief increase in output up to 5 kW.Towards the end of the trial period, however, the output dropped to approx. 2 kW.

After a registered running time of approx. 2 hours (approx. 2.5 h running timealtogether) and approx. 4 hrs. burning time, the trials were interrupted. The filterhousing as well as the engine were opened in order to gain information on anydeposits.

It was found that the wire screen was completely scaled and had practically lostall firmness. More than a quarter of the sectional area was completely destroyed.The remaining parts of the filter showed no recognisable coating. Particles couldtherefore pass this free section of the filter and travel unhindered to the Stirlingengine’s receiver where they formed a layer of 1 – 2 mm. These deposits were

7. Start-up and Operation

Fig. 7.1-1 The high flue gas temperatures atthe inlet to the Stirling engine’s heatexchanger make the material glow brightred

7.1 1st Trial Run26.6.1996






7.2 Consequences fromthe 1st Trial Run

The trial run showed that the method chosen is certainly promising. The pressureloss, which is high compared to the fan rating, is not a fundamental problem as itcan easily be reduced by increasing the duct cross sections and filter dimensions.The deposits on the filter itself are loose so that we have reason to assume thatthey will easily be removed during operation by pressurized air surges.

In future, the emphasis will be on optimising the wire screen. The followingmeasures have been planned:• Selection of material• Variation of mesh size and wire size• Mounting of flow baffles

The material envisaged for future trial runs is high-temperature-resistant steel1.4841. The criteria for determination is loss of scale. Loss of scale increasesdistinctly with the increase of temperature. In the relevant temperature range,erosion doubles with every 100°C temperature increase. It is planned to limit theflue gas temperature to 1,000°C.

By reducing the wire size, the mass-surface-ratio varies linearly in favour of thesurface. Therefore, if the wire diameter is reduced, the screen life is reduced inproportion to the modification of diameter.

We shall therefore try to find a compromise between the screen life – which isessentially determined by the wire size – and the extraction capacity. For thispurpose, we have ordered screens of different mesh sizes. In order to keep themechanical stress on the screen material low, a coarse-meshed screen with amesh size of 2 mm and a wire size of 1 mm will be placed over the supportingstructure to serve as a support for the fine-meshed grid. Thus, the bending stressof the 0.1 mm wire subjected to a differencial pressure of 10 mbar will be reducedto approx. 1 N/mm² – a value distinctly below the 1000 h-1% creep limit of 4 N/mm². Nevertheless, we have to bear in mind that in normal operation the grid will

be bent inwards by the differencial pressure applied to thescreen and outwards by the exhaust process. It is thus subjectedto alternating stress.

The worst damage to the screen was noted on the reverse.Whilst the leading edge was protected by a baffle plate, thepart on the far side of the burner chamber was directly exposedto the turbulent flue gas flow. Flow baffles in this area couldreduce the stress.

700800

9001000

11001200

0,1

0,2

0,3

0,4

-

500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

d

}

z

}

�

�

|

y

�

}

�

�

s

�

u

l�sv[u

\

�

y

�

�

|

�

�

{

�

�

}

�

�

}

�

�

s

�

�

u

Fig. 7.2-1 Limitation of the the screenmaterial’s life due to scaling of material1.4841. Life expectancy is reduced inrelationship to smaller wire sizes and higher

temperatures.messer und mit zunehmenderTemperatur.

0

200

400

600

800

1000

1200

10:2

2:20

10:3

2:21

10:4

2:21

10:5

2:21

11:0

2:21

11:1

2:21

11:2

2:21

11:3

2:21

11:4

2:21

11:5

2:21

12:0

2:21

12:1

2:21

0

1

2

3

4

5

6

T vor WT

T nach WT

kWh

Fig. 7.1-2 Temperature curves upstreamand downstream of the engine as well asthe power output curve. A maximum outputis shown at approx. 11:10 hrs which isfollowed by a continuous decrease. It canbe assumed that larger filter sections werefirst cleared at the time of the maximum,increasing the flue gas volume.The ensuingdecrease is explained by the continuousincrease of deposits on the engine’s heatexchanger.

easily removed. The situation established was a perfect explanation for thereversal of pressure drops registered at the filter and the heat exchanger as wellas for the shape of the output curve.






particles of insulating material which had been loosened during erection or bymeasuring probes. These deposits were removed by compressed air withoutproblems.

Fig. 7.3-3 Deposits of very fine dust, whichcan easily be removed, have accumulatedaround the edges of the heat exchanger. Acircular core zone subjected to direct flowis virtually clear of deposits.

7.3 2nd Trial Run1.8.1996

Following the exchange of the fine meshed screen which had been destroyed,the Stirling engine was put into operation again on 1.8.1996 with a screen made ofmaterial 1.4841 with a mesh size of 0.16 mm and a wire size of 0.125 mm. Apartfrom minor problems concerning helium pressurization during start-up, the engineoperated without problems. We tried to maintain a flue gas temperature of 100 °C.As the control system was not quite perfect, peak temperatures of up to 1080°Coccurred.

We took special interest in the pressure losses at the filter and the heat exchanger.Pressure drop at the filter started at 0.4 bar and increased rapidly to 1.5 mbar atfirst. From then on, increase was distinctly slower. At the end of the test period ofapprox. 5 hours the filter resistance was 4.4 mbar. In parallel, output was reducedfrom 5.3 kW to 3.4 kW.

No increase in resistance was noted at the engine’s heat exchanger. In fact, thepressure loss dropped slightly from the original 2.6 mbar to 1.9 mbar. This wasexclusively due to the decrease in flow rate caused by the increase in the totalresistance of filter and heat exchanger.

Examination of the screen and the heat exchanger showed as follows:As had been expected, the screen was heavily coated with dust deposits whichwere most pronounced on the far side of the combustor. It was, however,surprising to find that the material of the fine mesh had scaled just like the materialof the first screen made of 1.4301 during the first test when temperatures weredistinctly higher.The screen had endured the test period undamaged but this wasonly due to the supporting screen attached inside and outside. Later examinationof the screen material by the producer of the screen Haver & Boecker confirmedthat the material declared to be 1.4841 by the supplier was in fact 1.4401 and1.4404, respectively. An annealing test performed by IEF in parallel also ruled out

that the screen was made of 1.4841. The problem concerningthe screen’s service life can therefore be attributed to falsedelivery.

Attempts to remove the dust deposit from the screen withcompressed air at this stage did not lead to are of littleinformational value as blowing down not only removed thedeposit but also the screen material, which lacked firmness.It was, however, established for certain that simple blowingdown with compressed air is not sufficient to remove thedeposit. It was therefore assumed that concentration ofcompressed air on individual sections of the screen doesnot lead to the desired results. Considerable amounts of ash

and dust were found in the duct around the screen.

Esamination of the heat exchanger showed that a circular core zone had almostremained clear of deposist whilst deposits were found outside this zone. Thesedeposits were only found on the leading edge and did not penetrate the channelsformed by the fins. They consisted mainly of very fine dust and some larger

Fig. 7.3-2 Admission of compressed air tothe screen caused considerable dustemissions but did not have a satisfactorycleaning effect.

Fig. 7.3-1 After 5 hours of operation, thefilter’s leading edge shows a distinctcoating. On the side facing the duct wallwhich is exposed to higher flow velocitiesthe supporting screen structure can berecognized at least.

0

200

400

600

800

1000

01:55:12 03:07:12 04:19:12 05:31:12 06:43:12 07:55:12

Zeit

T [°C]

0

1

2

3

4

5

6

N [kW]

T vor WTT nach WT

kWh

Fig. 7.3-4 Temperatures upstream anddownstream of the engine as well as thecourse of power output. The outputmaximum is followed by a continuousdecrease. Output drop from more than 5kWel down to 2 kWel is now slower that inthe first trial run. It is mainly due to theformation of deposits on the screen whichentail an increase in flow resistance and,consequently, a decrease in gas flow.






The attempted trial run on 8.8.1996 with the conversions mentioned was of shortduration. The resistance of the close-meshed Hitherm filter fabric proved to be 6mbar even in its original condition. Under these circumstances, the induced draftcould not draw off the fuel gas sufficiently and operation of the engine was notpossible.

Fig. 7.5-1 The screen as seen through theinspection hole. The screen is no longervisible as it is covered by deposits. Somelarge particles - in the order of 1/2 mm - reston a layer of fine deposits.

7.5 3rd Trial Run8.8.1996

7.4 Consequences fromthe 2nd Trial Run

The following measures for improvement were derived from this test:

• As the problems concerning the screen material were not completely settledat the time and in order to provide an alternative offering longer service life,particularly in cases of small mesh and wire sizes, we used the materialHitherm by manufacturers Haver & Boecker.

• A close-meshed screen made of material Hitherm - mesh and wire sizeapprox. 0.1 mm – was applied for the next test in order to study theinstallation’s behaviour under these conditions. We decided to do without asupporting screen, thus providing a smaller contact surface for the flue gasparticles.

• Furthermore, the duct around the screen was provided with a cover, mainlyto facilitate removal of accumulating ash. The cover, however, wasdimensioned large enough to permit mechanical cleaning of the screenwhen it has cooled down without requiring disassembly.

• A sight glas in the screen casing permits permanent optical checks of thedeposits on the screen.

• The outer supporting screen was removed since we assumed that itscoarse structure favours deposits.

• The deposits on the heat exchangers showed that the oncoming flow wasnot optimized. Apparently, flow separations occured at the intake reducer.A flow baffle in concentrical arrangement inside the reducer was to causethe flow to stay close to the intake reducer. This should entail steadieradmission to the heat exchanger, and, therefore, improvement of the output.At the same time, it was to be expected that deposits would now beconsiderably reduced or would no longer occur. The reducer could besupported by three slotted tubes which could also effect compressed-airsupply for cleaning. For the time being, this potential improvement was notimplemented.

Fig. 7.4-1 Schematic representation of theconcentrical flow baffles which apparentlyare necessary for avoiding flow sepa-rations, kept in position by slotted tubes,which are used for cleaning simultaneously.Alternatively, a reducer with a wall slopebelow 7° could be installed.






Another trial run could be performed on 12.8.1996. Pressure loss at the filter was0.6 mbar at the beginning and increased to 4.3 mm in the course of 3 hours.Obviously, the outer supporting screen, which had now been omitted, had notincreased the service life as expected. Pressure loss at the heat exchanger wasconstant at 2.4 mbar.

At reduced heating, compressed air was applied to the filter by way of themovable nozzle. This showed that the filter deposit was not loosened evenly, ashad been expected, but only locally so that the pressure drop did not revert to theoriginal value but only dropped to 1.9 mbar. After another hour of trial run thepressure drop at the screen was back to 4.3 mbar. The test was stopped at ameter reading of 18,755 kWh.

Examination of the cold filter showed what had already been seen when the filterwas hot: 100 l compressed air at 10 bar removed no more than 20 % of the filtercake.

Manual cleaning showed that the screen had resisted the trial run unharmed.

7.7 4th Trial Run12.8.1996

7.6 Consequences fromthe 3rd Trial Run

The following conversions were made:• The screen was replaced by a screen made of 1.4841, mesh size 0.15 mm

and wire size 0.12 mm. The existing clamping bolts that did not withstand thetemperature load were replaced by bolts made of 1.4841.

• In order to permit more effective cleaning of the screen with compressed air,a nozzle was installed excentrically inside the screen, which was movable by360° and acting along the longitudinal line of a generatrix of the screenbody.

• The spare parts provided by SBP were installed in the engine in order toeliminate the helium loss which was of secondary importance but had to beattended to. The following runs showed that this did not solve the problem.






NF�l}��y�~R�\��{�yz~y��y��k�}z

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

0 1 2 3�

�

z

y

�

During the 6th trial run filter resistance increased from 0.8 to 4.2 mbar withinapprox. 3 hours. Again, output dropped from 2.8 to 0.8 mbar.

A slight increase of the heat exchanger’s pressure drop was also experiencedduring this test. A sight check showed that the heat exchanger was covered invery fine dust of scale and soot. Due to a distortion of the supporting screen, thefine screen had torn in one place, which is supposed to have caused the soiling tosome degree.

7.9 6th Trial Run14.8.1996

During the 5th trial run on 13.8.1996 filter resistance increased from 0.9 mbar to4.1 mbar after approx. 3 hours of combustor and 1.6 hours of engine operation. Atthe same time, output decreased from 2.4 to 0.9 kW, whereas a maximum of 2.8kW had been achieved before. Contrary to expectations, the change from EPROM625 to EPROM 575 showed no measurable effects on the output.

When cooled down, the filter was cleaned pneumatically at first and mechanicallylater.

7.8 5th Trial Run13.8.1996

MF�l}��y�~R�\��{�yz~y��y��k�}z

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

0 1 2 3 4�

�

z

y

�

Fig. 7.9-1 Pressure loss at filterFig. 7.8-1 Pressure loss at filter






deposits in the outer zones. Further plans concern installation of a pneumaticcleaning device to remove deposits formed by fine particles. Without thesemeasures, the heat exchanger needs to be cleaned mechanically afterapprox. 8 hours of continuous operation, which is too costly in the long run.Furthermore, the function of the helium intermediate tank remainsunsatisfactory and causes some additional expense.

Testing was stopped at this stage as the short service life of the filter and the needfor extensive cleaning involved gave reason to assume that continuation of thetests without further improvements would not have the desired effects.

As a consequence we examined a variant: cleaning the screen by way of a brushthat can be moved longitudinally around the screen. A producer was found. Itwas, however, established that the deviations from the circular cylinder shapewere in the order of 10 mm due to manufacturing defects and distortions. For thisreason, we could not expect the brush to work reliably when heated.

Another variant is possible: arrangement of several screens of which one at a timecan be removed for cleaning. In cold state we expect that an appropriatelymanufactured brushing system can achieve the desired effect in spite of theinaccuracies. The conversions required for this variant, however, would haveexceeded the budget by far.

As an alternative, we examined dust separation by way of a cyclone connected inseries. A problem was posed by the low density of the fuels - bulk density 80 kg/m³ – which makes separation by the cyclone difficult. Calculations according toVDI-Wärmeatlas showed that a cyclone with a diameter of 0.4 m and a height ofapprox. 1 m can separate particles down to 0.15 mm diameter (density 120 g/m³).Pressure loss, however, is approximately 1,000 Pa (10 mbar). Therefore, exchangeof the exhaust fan is absolutely necessary.

The existing Heinisch LMA 28 fan, which is specified as having an output of3000 m³/h at 9,5 mbar, would have to be replaced by a fan of approximately twicethe delivery head.

The following problems can be expected:• The fan’s engine rating is increased accordingly. On the basis of available

documents it will increase from 2.2 kW at present to 3,5 kWmech, which is 2/3of the Stirling engine’s maximum output measured so far- not a very favourablestarting point for the energy balance. Delivery time would be about 7 weeks.

• Increase of differential pressure will also increase admission of leakage airinto the duct system caused by leaks. Temperature at the engine will drop.This problem is specific to the testing arrangement. The duct system of aproduction plant would have to be made of ducts in a gas-tight finish.

• In case a stronger fan is installed - which would be very costly in any case –an examination must be conducted prior to construction and installation ofthe cyclone in order to determine how far the service life can be extendeddue to the grater scope for pressure loss at the screen. The increase in thefan’s output may be sufficient for operation of more than one day so that acyclone can be omitted. The advantage for a production plant would be thatthe pressure loss can be kept low by dimensioning the screen generously.

• With this low solids density, however, a cyclone requires a high intakespeed which will entail a distinct pressure drop in any case.

• Improvements to the engines are yet outstanding. These include flow bafflesto achieve an even flow onto the heat exchanger and reduce formation of

7.10 Summary of the TestResults

Druckverlust und Abscheidegrad in Zyklonabscheidern(aus VDI-Wärmeatlas 1991 - Kap. Lj)

Stoffwerteη_gas 5,08E-05 Pas ve 36,60 m/s Eintrittsgeschwindigkeitρ_gas 0,2734 kg/m³ µe 0,00023529 - Staubbeladungν_gas 1,86E-04 m²/s α 0,67 - Einschnürungsbeiwert

ρ_fest 120 kg/m³ ua 45,72 m/s äußere Umfangsgeschwindigkeitum 38,74 m/s mittlere Umfangsgeschwindigkeitui 32,82 m/s innere Umfangsgeschwindigkeit

V_gas 0,183 m³/sm_fest 300 mg/Nm³ Re_r 229,625 - Reynoldszahl des Zyklons

m_gas 0,050 kg/s aus Diagramm: siehe ---->m_fest 1,18E-05 kg/s λ0 9,00E-03 - Reibbeiwert Gas (aus VDI-W.atl. Bild4)

λs 9,28E-03 - Reibungsbeiwert mit Staubk 0,1 mm

vax 4,19 m/s axial Geschwindigkeit (abwärts)vi 10,36 m/s Tauchrohr Geschwindigkeit

alle größeren Partikel werden abgeschiedend* 15,24 µm Grenzkorngröße Primärströmung

Da 300 mm Aw 0,56549 m² dT* 15,24 µm Grenzkorngröße Sekundärströmunght 200 mm Ar 1,08974 m²hz 200 mmhk 800 mm hg 1000 mm dp_e 487,7 Pa Druckverlust im Zyklomb 50 mm hi 800 mm dp_i 381,4 Pa Druckverlust im Tauchrohrh 100 mm dp_g 869,1 Pa gesamter DruckverlustDi 150 mmDu 300 mm

be 50 mmhe 100 mmAe 0,005 m²

ws0 0,146 m/sra 150 mmre 125 re_q 125 mmß 0,33333333 - ue1 30,52 m/s

rm 100 mm ze 6459,88 m/s²k/ra 6,67E-04

de* 1,31E-05 mhT 200 mm de* 13,12 µmhi 800 mmri 75 mm Re_s 0,01028186 -ru 150 mmr2 150 mm

u2 26,46 m/s

∆ρ 119,7266 kg/m³ µg

d* 1,52E-05 m

dT* 1,5244E-05 m

Fig. 7.10-1 One of many cyclone dimen-sioning computations which were per-formed in order to find an alternative tolabour-intensive flue gas purification by wayof screens. The high flow losses to beexpected, however, speak against thisvariant.






8. Results and Discussion

Operation of the engine at the test stand is possible in principle. Electrical poweroutput reached peak values of more than 5 kW. Selection of materials and minorproblems concerning the mesh size of the screen could be solved to such adegree that operation can be continued for several hours. Problems are stillcaused by the formation of deposits on the screen which require manual cleaningafter approx. 3 hours. Efforts to clean the screen mesh during operation either byway of compressed air or mechanically did not lead to the desired result. Underthese circumstances, it is hardly conceivable that the trial operation can betranslated into practical application. It is, however, conceivable that filters arecleaned during operation by subjecting one of several filters at a time to acleaning process performed by an expensive, fully automatic installation.

One useful improvement for increasing the service life of the screen is theinstallation of a powerful fan. A cyclone connected in series, too, which separateslarger particles, could possibly achieve noticeable improvements - on its own orin combination with the existing screen. The fan’s high power demand for all thesevariants, however, reduces the plant’s net power output substantially, namely by2/3 of the maximum output. Thus, the economic efficiency can hardly beestablished.

A concept for removing the unavoidable deposits from the heat exchanger hasbeen elaborated but not tested so far.

On the whole, progress has been very slow. Due to servicing and improvementmeasures constantly required, expenses for operation rose significantly abovethe level planned. Moreover, these measures entailed considerable delays. Fun-damental changes in this respect cannot be expected, either. After all, theeffectiveness of planned measures can hardly be predicted as experience fromcomparable plants is not available.

For these reasons, the time and cost schedule originally planned cannot beadhered to if 500 hours of operation at the test stand are to be achieved.Therefore, the project is stopped at the state achieved.

Based on the assumption that measures, which are considered promising withregard to stable operation from today’s point of view, can be installed withoutgreat difficulty, costs for a 500 hour-operation are expected to be approximately1.5m ATS.






9. Conclusions and Recommendations

The tests have shown on principle a Stirling engine’s functionality in combinationwith biomass combustion. The V160 Stirling Engine have achieved a goodperformance in several operating hours.

The design of the heat exchanger used in this project with numerous fins arrangedat a close distance makes the heat exchanger vulnerable to soiling and placesheavy demands on exhaust gas purification. These demands could not befullfilled satisfactorily within the scope of this project.

According to this, the main effort in research & development of biomass applicationhas be introduce effective solutions for the flue gas cleaning in order to preventfouling of the heat exchanger and the associated decrease of the heat transfer.An alternative to the investigated low-cost metal mesh could be the use on a totalgas cleaning using ceramic filters.

On the other hand, if the heat exchanger used had a coarser structure and wouldnot be so vulnerable to clogging, efforts could probably be concentrated onseparating larger particles. A hot cyclone is likely to suit this purpose. On theother hand, a cyclone separator requires very high flow velocities if permissiblenear-mesh material sizes are as low as in the current project. This entailsconsiderable power losses.

If an optimum could be found between the heat exchanger’s theoretical efficiencyand its resistance to soiling during operation, the Stirling engine could be suitablyapplied in biological heating stations. The experience available gives us reasonto believe that this aim can only be achieved by gradual activities involving time-consuming and costly conversions.






10. Reference [1] J. E. Bennethum, T. D. Laymac, L. N. Johansson, T. M. GodettCommercial Stirling Engine Development ans ApplicationsFuture Transportation Technologie Conference and ExpositionPortland, Oregon August 5-7, 1991

[2] H. Flaig, H. MohrEnergie aus Biomasse - Eine Chance für die LandwirtschaftSpringer-Verlag 1993

[3] H. Flaig, E. von Lüneberg, E. Ortmaier, Ch. SeegerEnergiegewinnung aus Biomasse - agrarische, technische und wirtschaftli-che AspekteAkademie für Technikfolgenabschätzung in Baden-Württemberg, Nr. 431995

[4] H. Flaig, G. Linckh, H. MohrDie energetische Nutzung von Biomasse aus der Land- und ForstwirtschaftAkademie für Technikfolgenabschätzung in Baden-Württemberg, Nr. 16,1995

[5] F. Gossen, A. Baumüller, H. SchellDer Stirlingmotor V160 – leistungsfähiges Herz von kleinen Blockheizkraft-werkenGaswärme International – 44(1995) Heft 3 – März

[6] F. J. LegererWärme-Kraft-Kopplung für Heizkraftwerke auf Strohbasis

[5] NNManche mögen’s heißEnergiespektrum September 1992

[8] M. NovyBiomass Fuelled Stirling Engines - Application and Development Programsfor Combustion SystemsFifth International Stirling Conference, Dubrovnik, May 8 - 10, 1991

[9] M. Novy, M. Lauer, J. Spitzer, W. StanzelKraft-Wärme-Kopplung mit Holz über den Stirlingmotor im Vergleich zumDampf- und VergaserprozeßBericht Nr. ief-B-13-88Joanneum Research Forschungsgesellschaft m.b.H., Graz, 1988

[10] M. Novy, M. Lauer, J. Spitzer, W. StanzelHeat and Power Cogeneration with BiomassInternational Congress Energy and Environment, Opatija, Yugoslavia, 18 -21.April 1990

[11] E. PodesserDezentrale Wärme-Kraft-Kopplung mit Stirlingmotoren an Biomassefeue-rungenJoanneum Research Forschungsgesellschaft m.b.H., Graz

[12] A. ReichlÖkologische Bewertung der Fernwärme aus Kraft-Wärme-Kopplung –Ecological aspects of District Heating produced by CHP25. UNICHAL-Congress, 4.-6.6.1991

[13] A. ReichlEnergetische und ökologische Bewertung der Fernwärme aus Kraft-Wär-






me-KopplungBWK Bd. 42 (1990) Nr. 6 – Juni

[14] B.V. Stirling Motors EuropeStirling engine – a clear source of power

[15] Schlaich Bergermann und PartnerService and Operator’s Manual for the V160 Biomass Solid Fuel StirlingEngine,Version 1.0, Jan. 1995

[16] Stirling Power SystemsService and Operator’s Manual for the V160F Cogeneration System ThreePhase,Version 2.0, Jan. 1989

[17] TheisenHeißgasreinigung mit Kerzenfiltern

Stirling

Documents

Transcript of Stirling