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Description of the ORC technology for biomass Combined Heat and Power plants as well as further possibilities for process integration

BIOS BIOENERGIESYSTEME GmbH, Graz

The ORC technology is based on a long term development with the aim to efficiently use solar energy, geothermal energy as well as energy from biomass in decentralised units. The principle of electricity generation by means of an ORC process corresponds to the conventional Rankine process. The substantial difference is that an organic working medium (hydrocarbons such as iso-pentane, iso-octane, toluene or silicon oil) with favourable thermodynamic properties at lower temperatures and pressures is used instead of water - hence the name Organic Rankine Cycle (ORC). The right choice of the organic working medium used is very important for an optimised operation of the ORC process. Considering the framework conditions given for biomass Combined Heat and Power applications (CHP plants), silicon oil is the most appropriate working fluid.

Working principle and implementation in the biomass CHP plant

Figure 1 illustrates a possible implementation of the ORC process based on the already successfully realised EU demonstration project “Biomass CHP plant Lienz”. The energy produced by biomass combustion is transferred from a thermal oil boiler (inclusive thermal oil economiser) via a thermal oil cycle to the ORC process. Thermal oil is used as a heat transfer medium because the temperature required for operating the ORC process (thermal oil feed temperature 300°C) can be achieved while operating the thermal oil boiler practically at atmospheric pressure (therefore, no constant boiler supervision is needed). The pressurised organic working fluid is vaporised and slightly superheated in the evaporator by the energy supplied from the thermal oil cycle. The steam is expanded in an axial turbine which is directly connected to an asynchronous generator (no intermediate gear box is necessary). Subsequently, the expanded silicon oil passes through a regenerator (where in-cycle heat recuperation takes place which increases the electric efficiency) before it enters the condenser. The condensation of the working medium takes place at a temperature level which allows the heat recovered to be utilised as district or process heat (hot water feed temperature about 80 to 100°C). The liquid working medium then passes the feed pumps in order to regain the appropriate pressure level of the hot end of the cycle, passes the regenerator and returns to the evaporator.

The flue gas from the outlet of the thermal oil boiler (respectively from the thermal oil economiser) is cooled down from about 280°C to about 160°C by an efficient heat recovery system (for example combustion air pre-heater and hot water economiser). Subsequently, the flue gas is cleaned in a multi-cyclone (precipitation of larger dust particles) followed by a respective flue gas cleaning unit (in many cases an electro-static precipitator or a flue gas condensation unit is installed). After the flue gas cleaning unit the flue gas enters – cleaned according to the local regulations – the stack.

Fig. 1

The ORC process can be designed in such a way that hot water feed temperatures between 80 and 120°C as well as a temperature differential between feed and return in a range of 15 to 50°C are possible. Therefore, the return temperatures vary between 50 and 100°C. On this basis the exact level of the hot water feed temperature required can be perfectly adjusted to the design requirements of the heat or cooling energy customers. For the hydronic implementation of the ORC unit, the hot water economiser should be installed after the ORC process (see Figure 1), in order to keep the level of the hot water feed temperature from the ORC as low as possible. The lower the hot water feed temperature at the condenser outlet, the higher is the electric efficiency.

Figure 2 shows some selected components of the ORC unit Lienz (nominal electric power 1,000 kW) completely mounted and insulated. The modular design as well as the description of the main components of the ORC unit is given in Figures 3 and 4. It is important to outline, that the scheme given in Figure 4 is suitable for a module size corresponding to a nominal electric power of 1,500 kW and that this design concept (configuration, space needed) is different to the one which is given in Figure 3. The size measures and draft conception drawings can be downloaded from the homepages of the relevant ORC-manufacturing companies.

Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6

Efficiencies and relevant technical data of the ORC process

The implementation of the ORC process into the overall plant should always be carried out in consideration of a possibly high production of electric energy at a simultaneously secure supply of hot water feed temperatures required from the heat consumers.

Based on the requirements from the heat consumers and the plant design chosen, the resulting hot water temperatures at the ORC condenser are determined for the nominal design point (for example: feed temperature 80°C; return temperature 60°C) and allow a net electric efficiency of about 15% (based on the primary energy input, Hu biomass). In Figure 5, the energy flow chart of the ORC unit of the biomass CHP plant Lienz at nominal load conditions is given.

Since decentralised biomass Combined Heat and Power plants are usually operated in a heat controlled mode - for economic as well as energetic reasons – the partial load behaviour and the partial load efficiency of the ORC process is very important. Due to the slowly rotating axial turbine installed and because of the thermodynamic properties of the organic working medium used the partial load behaviour of the ORC process is excellent At 40% of the net electric power of the ORC unit, the net electric efficiency still amounts to 85% of the nominal value, which was also confirmed by data evaluation, gained amongst others from the biomass CHP plant Lienz (see Figure 6). This circumstance is a substantial advantage of the ORC process in comparison to steam turbines and also steam engines, which show a stronger efficiency decrease at partial load.

The axial turbine, which is installed in the ORC process (for example see Figure 7), is optimised for small-scale CHP plants and works with a low circumferential speed and rotational frequency, hence a lower mechanical stress. The turbine also allows a direct drive of the generator without an intermediate gear box, which increases the electric efficiency. Because of the smaller specific enthalpy difference within the turbine in comparison to conventional water-steam-processes, a basic and robust turbine construction is possible. The factors mentioned above lead to improved life-cycle periods of the turbine as well as a high availability of the ORC unit.

Security aspects, process control, human resources needed

Fig. 7

Particularly important are the high safety aspects of the ORC process. There is the possibility to have all welding seams of the pressure vessels of the ORC unit X-ray tested as well as tested at maximum pressure level. This way the time periods between recurring internal inspections by a notified body can be extended.

The process control of the ORC unit is installed over a storage-programmable logic controller (PLC), which allows the fully automated start-up and shut-down as well as the synchronisation to the public electric grid of the local utility. Load alternations of the ORC unit are also controlled fully automated over the hot water feed temperature at the outlet of the ORC condenser. It is not necessary that operation staff is at the site permanently, because also a shut-down procedure is carried out of the process control system alone. The same is true for the start-up procedures. In a pre-heated or still hot condition of the ORC unit, the ORC process can be coupled to the public electric grid within 15 minutes (after processing the continuous security tests required). A continuous operation of the ORC unit is possible between 10% and 100% of the nominal load.

As already mentioned the ORC process is connected with the thermal oil boiler (inclusive the thermal oil economiser) via a thermal oil cycle. The heat transfer medium (thermal oil) allows an operation of the thermal oil boiler practically at atmospheric pressure ranges despite high operation temperatures required. Therefore, no constant boiler supervision is needed, which results in lower personal costs in comparison to the conventional steam boiler operation. Furthermore, a water treatment is not necessary for an ORC unit, which would be the case for water / steam as a heat transfer medium. The operation of the ORC process (as mentioned above) is not under the regulations of the steam boiler operation law.

ORC processes are characterised by a high reliability and low numbers of breakdowns, which is confirmed through the experiences gained from the applications in the area of geothermal energy production. Since the ORC process is operated as a closed cycle and therefore marginal losses of the working fluid occur, the operating costs are relatively low. There are only moderate costs for lubricants, maintenance and personnel. Because of the fully automated process control, a practically unmanned operation of the ORC process is possible. Regarding maintenance required, it is common practice to have an inspection once a year from the manufacturing company, which lasts 1 to 2 days. Eventually occurring alarms are easily traceable by the process visualisation system and the operation data acquisition system via the user interface of the process control system (work station with monitor and printer) and can be forwarded in time to the operating staff over a GSM system.

Further development of the ORC-Process

The following paragraphs give a short overview of the ongoing development of the ORC process which occurred within the last years and which is still in process: The focus of the development concentrates on improving the electric efficiency of the ORC unit. For detailed information about these improvement potentials, please see the download section at the bottom of this page.

In this respect, the newly designed ORC process with a branched condensate cycle (also known as split system) is an interesting option to the conventional system described above. This new technological approach enables due to an enhanced interconnection of the thermal oil circuit as well as the condensate circuit of the ORC process an additional utilisation of heat from the flue gas in a second thermal oil economiser (see Figure 8). In comparison with the conventional system a higher portion of energy of the flue gas can be transferred to the ORC process which leads to a significant higher electrical efficiency of the total installation in comparison to state-of-the-art systems. However, the additional head exchangers and hydraulic components required for the split system lead to higher investment costs.

A further possibility to improve the electric plant efficiency of the ORC process is given by an increase in the thermal oil temperatures. A higher temperature difference between feed and return in the thermal oil system leads to a reduction of operating costs because of lower costs for a reduced operation of the thermal oil pumps. Typical feed and return temperatures in the thermal oil system are e.g. 315/250 °C in comparison with 300/250 °C at state-of-the-art systems. As consequence of the higher operation temperatures it has to be stated, that the life-cycle period of the thermal oil used will decrease.

The increase of the thermal oil temperatures in combination with the implementation of the split system mentioned above enables an improvement in the electric plant efficiency of about 10% in comparison to state-of-the-art systems. The improvement in efficiency is depending on the real conditions like the moisture content of the biomass fuel or the respective effort in the plant design and can be calculated.

Fig. 8

Power production based on waste heat and further utilisation possibilities of the ORC-Process

The ORC process is especially suited for the power production based on industrial waste heat. In order to produce electricity waste heat is transferred to a thermal oil cycle and further on to the ORC unit by heat exchangers. The ORC unit produces electric energy which can be used to cover the auxiliary power demand of the manufacturing plant or for the feed-in into the public grid. Additionally, the ORC unit produces low temperature heat. Depending on the site constraints of the manufacturing plant the low temperature heat can be used as low temperature process heat (e.g. drying), for the internal space heat supply or external heat consumers (e.g. via a district heating system).
An innovative process implementation is given by the possibility to implement the ORC process in a fluidised-bed-steam-gasification process (see Figure 9). This seems to be meaningful for CHP plants in a range of 2.5 MWel from an energetic as well as economic point of view. By coupling these different CHP technologies, an improvement of the electric plant efficiency of about 20% can be expected.

Abb. 9

Strengths of the ORC technology:

  • Excellent partial load behaviour
  • Ability for quick load alternations (in particular an advantage for heat controlled operation and achieving high annual utilisation rates)
  • Mature and reliable technology
  • No danger of droplet erosion on turbine blades (because of the favourable thermodynamic properties at lower of the working medium used)
  • No constant steam boiler supervision is needed
  • High degree of automation
  • Low maintenance costs
  • The implementation of ORC units in existing biomass combustion plants is relatively easy
  • Various possibilities for process integration

Already realised projects with an ORC process

  • Biomass CHP plant based on an ORC cycle / STIA Holzindustrie, Admont - EU-THERMIE demonstration project (Styria, Austria)
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  • Biomass CHP plant based on an ORC cycle and a newly developed fuzzy logic control system / Stadtwärme Lienz - EU-THERMIE demonstration project (Tyrol, Austria)
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  • Waste wood-fired combined heat, cooling and power (CHCP) plant based on an ORC cycle and an absorption chiller / BIOSTROM, Fussach - national demonstration project (Vorarlberg, Austria)
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  • Biomass CHP plant based on an ORC cycle / Längenfeld (Tyrol, Austria)
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  • Biomass CHP plant based on an ORC cycle / Dobbiaco (South Tyrol, Italy)
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  • Biomass CHP plant based on an ORC cycle / Theurl sawmill (Tyrol, Austria)
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  • Extension of the existing biomass district heating plant in Lofer with a biomass CHP plant based on an ORC cycle / Lofer (Salzburg, Austria)
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  • Extension of the existing biomass district heating plant in Grossarl with a biomass CHP plant based on an ORC cycle / Grossarl (Salzburg, Austria)
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  • Biomass CHP plant based on an ORC cycle – extension Stadtwärme Lienz (Tyrol, Austria)
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  • Biomass CHP plant based on three ORC units / Biomasse-KWK-Leoben Betriebsgesellschaft m.b.H. (Styria, Austria)
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  • Biomass CHP plant based on an ORC cycle / TILLY Holzindustrie Ges.m.b.H., Treibach/Althofen (Carinthia, Austria)
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  • Biomass district heating plant / Tamsweg - EU-THERMIE demonstration project (Salzburg, Austria)
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  • Extension of the existing biomass district heating plant St. Walburg in the Ulten Valley with a biomass CHP plant based on an ORC cycle / St. Walburg in the Ulten Valley (South Tyrol, Italy)
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  • Biomass CHP plant based on an ORC cycle - enlargement of existing district heating plant / Olang (South Tyrol, Italy)
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  • Biomass CHP plant based on an ORC cycle / Josko Fenster und Türen GmbH, Kopfing (Upper Austria, Austria)
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  • Biomass CHP plant based on an ORC cycle / Allendorf (Hessen, Germany)
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  • Heat and power production by waste heat recovery of industrial flue gas streams based on an ORC cycle – RHI AG, Radenthein (Carinthia, Austria)
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More information to the ORC process

FUZZY LOGIC CONTROLLED CHP PLANT FOR BIOMASS FUELS BASED ON A HIGHLY EFFICIENT ORC PROCESS - EU DEMONSTRATION PROJECT LIENZ – PDF#0001

Heinz Reisinger*, Gerold Pointner*, Ingwald Obernberger**, Peter Thonhofer**, Erwin Reisenhofer**

*STADTWAERME LIENZ, Schulgasse 1, A-9900 Lienz, Österreich
**BIOS BIOENERGIESYSTEME GmbH, Inffeldgasse 21b, A-8010 Graz, Österreich

Biomasse-KWK auf Basis des ORC-Prozesses - Vorstellung der EU-Demonstrationsprojekte Holzindustrie STIA/Admont und Fernheizwerk Lienz (Österreich) – PDF#0004

Ingwald Obernberger*

*BIOS BIOENERGIESYSTEME GmbH, Inffeldgasse 21b, A-8010 Graz, Austria

STATE-OF-THE-ART AND FUTURE DEVELOPMENTS REGARDING SMALL-SCALE BIOMASS CHP SYSTEMS WITH A SPECIAL FOCUS ON ORC AND STIRLING ENGINE TECHNOLOGIES – PDF#0005

Ingwald Obernberger*, Henrik Carlsen**, Friedrich Biedermann*

*BIOS BIOENERGIESYSTEME GmbH, Inffeldgasse 21b, A-8010 Graz, Austria
**Technical University of Denmark, Section Energy Engineering, Dept. of Mechanical Engineering, Denmark

Description and evaluation of the new 1,000 kWel Organic Rankine Cycle process integrated in the biomass CHP plant in Lienz, Austria – PDF#0008

Ingwald Obernberger*, Peter Thonhofer*, Erwin Reisenhofer*

*BIOS BIOENERGIESYSTEME GmbH, Inffeldgasse 21b, A-8010 Graz, Österreich

Biomasse-Kraft-Wärme-Kopplungen auf Basis des ORC-Prozesses - EU-THERMIE-Projekt Admont (A) – PDF#0016

Ingwald Obernberger*, Alfred Hammerschmid*, Roberto Bini**

* BIOS - BIOENERGIESYSTEME GmbH, Inffeldgasse 21b, A-8010 Graz, Austria
** Turboden, Viale Stazione 23, I-25122 Brescia, Italien

Biomasse-Kraft-Wärme-Kopplung auf Basis des ORC-Prozesses – Stand der Technik und Möglichkeiten der Prozessoptimierung – PDF#0045

Obernberger Ingwald *, GAIA Mario **

*BIOS BIOENERGIESYSTEME GmbH, Inffeldgasse 21b, A-8010 Graz, Austria
** Turboden, Viale Stazione 23, I-25122 Brescia, Italien