BIOS - Bioenergy

Description of the biomass CHP technology based on Stirling engines


Within the scope of a R+D&D (research, development and demonstration) cooperation of the BIOS BIOENERGIESYSTEME GmbH, MAWERA Holzfeuerungsanlagen GesmbH and the TECHNICAL UNIVERSITY OF DENMARK, a small-scale CHP technology with a 35 kWel and a 70 kWel Stirling engine for biomass fuels has been designed. A 35 kWel pilot plant was put into operation in 2002 and was successfully tested for more than 10,000 hours. A second pilot plant with a 70 kWel engine is in operation since autumn 2003.
The newly developed CHP technology can be considered as a breakthrough in the application of Stirling engines for small-scale CHP plants burning solid biomass fuels. A field test with seven 35 kWel engines has been initiated and first demonstration plants were put into operation in summer 2005. The Biomass CHP technology based on Stirling engine is in demonstration phase and not available on the market at the moment. It is expected that this technology will be commercially available within the next few years.

Working principle and integration in a biomass CHP plant

Stirling engines are based on a closed cycle, where the working gas is alternately compressed in a cold cylinder volume and expanded in a hot cylinder volume. The advantage of the Stirling engine in comparison to internal combustion engines is that the heat is not supplied to the cycle by combustion of the fuel inside the cylinder, but transferred from the outside through a heat exchanger in the same way as in a steam boiler. Consequently, the combustion system for a Stirling engine can be based on proven furnace technology, thus reducing combustion related problems. The heat input from fuel combustion is transferred to the working gas through a hot heat exchanger at high temperatures. The heat that is not converted into work on the shaft is rejected to the cooling water in a cold heat exchanger.
Additional information concerning the working principle of a Stirling engine

Design and development of the Stirling engine

Fig. 1

The Stirling engine developed at the TECHNICAL UNIVERSITY OF DENMARK uses Helium as working gas and is designed as a hermetically sealed unit. The use of Helium is very efficient concerning the electric efficiency of the engine, but utilisation of these low molecular weight gas makes it difficult to design a piston rod seal, which keeps the working gas inside the cylinder and prevents the lubrication oil from entering the cylinder. Many solutions have been tested, but it is still a delicate component in the engine. An attractive possibility is to bypass the problem by designing the engine as a hermetically sealed unit with the generator incorporated in the pressurised crankcase, just like the electric motor in a hermetically sealed compressor for refrigeration. Only static seals are necessary and the only connections from the inside to the outside of the hermetically sealed crankcase are the cable connections between the generator and the grid (see Figure 1).

The problems concerning utilization of biomass fuels in connection with a Stirling engine are concentrated on transferring the heat from the combustion of the fuel into the working gas. The temperature must be high in order to obtain an acceptable specific power output and efficiency, and the heat exchanger must be designed so that problems with fouling are minimised. Because of the high temperatures in the combustion chamber and the risk of fouling, it is not possible to use a Stirling engine designed for natural gas, as narrow passages in the Stirling heat exchanger are blocked after less than an hour of operation with biomass fuels. The risk of fouling in biomass combustion processes is mainly due to aerosol formation and condensation of ash vapours when the flue gas gets cooled. BIOS BIOENERGIESYSTEME GmbH has developed a programme calculating the heat transfer from the flue gas to the internal working gas (Helium). Based on this development comprehensive design studies were performed and the performance of the Stirling heat exchanger was improved and optimised.
Furthermore, BIOS BIOENERGIESYSTEME GmbH has developed and designed an automatic cleaning system for the Stirling heat exchanger which was subsequently optimised during plant operation. The system comprises a pressurised air tank and air nozzles at each heat exchanger panel. The nozzles are equipped with magnetic valves. The valves are opened at regular intervals (only one valve at a time, all other valves remain closed) and the air is blown into the heat exchanger sector and cleans the tubes from ash deposits. The ash is then entrained with the flue gas and subsequently collected in the fly-ash precipitators. In addition, a vibration reduction system for the 35 kWel Stirling engine was developed in order to efficiently reduce vibrations occuring in a 4 cylinder engine (see Figure 1).

Design and development of the high-temperature furnace

In order to obtain a high overall electric efficiency of the CHP plant, the temperature in the hot heat exchanger and consequently the temperature of the flue gas should be as high as possible. Therefore, it is necessary to preheat the combustion air with the flue gas leaving the hot heat exchanger by means of an air preheater. Typically the temperature of the combustion air is raised to 500 °C – 600 °C, resulting in very high temperatures in the combustion chamber. This can cause ash slagging and fouling problems in biomass combustion systems and in the hot heat exchanger. Consequently, the design of the furnace and its adaptation to the special requirements of a CHP plant with a 35 kWel Stirling engine is an important and difficult task. The plant should operate at a high temperature level to gain a high electric efficiency from the Stirling engine but temperature peeks in the furnace should be impeded in order to reduce ash slagging and fouling. The plant is designed for temperature levels in the furnace of about 1,300 °C (the typical flue gas temperatures in conventional biomass furnaces are in the range of approx. 1,000 °C).

An appropriate combustion system was developed and optimised using CFD simulations which have been performed by BIOS BIOENERGIESYSTEME GmbH. The results achieved showed that it is a very important task to optimise the design of the furnace geometry, of the secondary air nozzles and the nozzles for flue gas recirculation in order to reduce temperature peaks in the furnace as well as CO emissions. In addition, the CFD simulations performed improved an equal distribution of the flue gas flow into the different sections of the hot gas heat exchanger and thus ensured an equal heat transfer to the cylinders of the Stirling engines.

Fig. 2
Fig. 3

Figure 2 shows the geometry of the furnace with conventional nozzle design and placement. The secondary air nozzles are placed at the inlet of the secondary combustion chamber. The results of the CFD simulations performed for this geometry show that the flue gas burn out in the secondary combustion chamber is not efficient (see Figure 4). The CO emissions at outlet according to CFD simulations are about 100 mg/Nm3 (dry flue gas, 13 vol% O2).

Figure 3 shows a furnace geometry with optimised nozzle design and placement. The secondary air nozzles are arranged horizontally in the transition zone between primary and secondary combustion chamber. With this configuration the combustion air is efficiently mixed with the flue gas and a swirl flow is established in the secondary combustion chamber. Consequently, the resulting CO emissions are low. Figure 5 shows the contours of CO in mg/Nm3 calculated for the geometry with optimised air nozzles. For the optimised geometry, CFD simulations predict CO emission of approx. 15 mg/Nm3 (dry flue gas, 13 vol% O2). The results demonstrate that an efficient turbulent flow enhances the combustion process and reduces CO emissions, which stresses the relevance of an optimisation of the combustion system supported by CFD analyses.

Fig. 4
Fig. 5

Description of the integrated CHP plant

Fig. 6

Figure 6 shows a schematic illustration of the small-scale CHP technology developed for biomass fuels based on Stirling engines. The furnace is equipped with underfeed stoker technology. In the combustion chamber the flue gas reaches temperatures of approx. 1,300 °C. Heat is then transferred to the Stirling hot heat exchanger and the temperature of the flue gas is reduced to about 800 °C at heat exchanger outlet. Subsequently, the flue gas passes through an air preheater and an economiser mounted downstream the hot heat exchanger.

Figure 7 and Figure 8 show pictures of the CHP pilot plants based on a 35 kWel and a 70 kWel Stirling engine. The furnace of the CHP plants is equipped with underfeed stoker technology. The Stirling engine is mounted in a horizontal position downstream of the secondary combustion chamber for convenient maintenance. The air pre-heater and the economiser are placed on top of the furnace in order to achieve a compact design of the plant. The CHP plant should not require substantially more space than a normal biomass combustion plant with the same heat output. Consequently, it is easy to replace existing biomass hot-water boilers by CHP modules based on Stirling engines. To remove fly ash particles from the hot gas heat exchanger, a pneumatic and fully automatic cleaning system was developed and installed.

Fig. 7
Fig. 8

Relevant technical data and efficiencies of the CHP technology based on the Stirling process

Fig. 9

Figure 9 shows an energy flow sheet of the CHP plant based on a 35 kWel Stirling engine. The electric plant efficiency amounts to approx. 12% and the overall plant efficiency to about 85 to 92%. The thermal heat output is normally in the range of approx. 230 kW and the fuel capacity (based on net calorific value) amounts to 300 kW.

The following table shows relevant technical data and efficiencies of the CHP technology based on the 35 kWel- and the 70 kWel-Stirling engine:

Electric power output - Stirling engine kW 35 70
Thermal power output - Stirling engine kW 105 210
Thermal power output - CHP plant kW 230 460
Fuel power input (based on NCV) kW 300 600
Electric efficiency - Stirling engine % 25,0 25,0
Overall electric efficiency - CHP plant % 11,7 11,7
Overall efficiency - CHP plant % 88,3 88,3
Working gas   Helium Helium
Mean pressure MPa 4,5 4,5
Temperature of hot heat exchanger °C 750 750
Revolution speed rpm 1.010 1.010
Engine weight kg 1.600 3.500

Technological evaluation of the CHP technology based on the Stirling process


  • Compact design
  • Fully automatic operation
  • Low noise emissions
  • Small-scale applications possible

Weak points/optimisation potential:

  • Automatic cleaning system for the hot heat exchanger necessary to reduce ash deposition problems and to achieve a high availability of the system
  • Only limited long-term experience in biomass fired boilers exists
  • Application limited to non-contaminated wood fuels with low ash and chlorine contents (wood chips, sawdust, wood pellets)

Realised projects and proposals under design based on the Stirling engine process

  • Biomass CHP plant based on Stirling engine technology located at the TDZ Ennstal (Raichraming, Upper Austria)
  • Biomass CHP plant based on an ORC cycle / Allendorf (Hessen, Germany)

More information about the Stirling process


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


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

*BIOS BIOENERGIESYSTEME GmbH, Inffeldgasse 21b, A-8010 Graz, Austria
**Technical University of Denmark, Denmark
***MAWERA Holzfeuerungsanlagen GesmbH, Austria

Small-scale CHP Plant based on a 75 kWel Hermetic Eight Cylinder Stirling Engine for Biomass Fuels - Development, Technology and Operating Experiences – PDF#0007

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

*BIOS BIOENERGIESYSTEME GmbH, Inffeldgasse 21b, A-8010 Graz, Austria
**Technical University of Denmark, Denmark
***MAWERA Holzfeuerungsanlagen GesmbH, Austria

Biomasse-Verstromung mittels Stirlingmotor - Grundlagen und praktische Erfahrungen – PDF#0003

Ingwald Obernberger*

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

Biomasse-Kraft-Wärme-Kopplung auf Basis Stirlingmotor – Technologiebeschreibung und Entwicklungsstand – PDF#0061

Development of a hot gas heat exchanger and a cleaning system for a 35kWel hermetic four cylinder Stirling engine for solid biomass fuels – PDF#0062