Multi-biofuel heating plant parallels 2022 emission targets

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A world-class 30MW multi-biofuel plant in Hjørring, northern Jutland, Denmark achieves exceptionally low emission levels, and at the same time is one of the most efficient plants of its kind in the world. Its competitive status as one of the most internationally efficient biofuel plants ever built is achieved by high demands placed on all equipment. These demands include maximal implementation of the newest IE4 synchronous reluctance (SynRM) motor technology, driving the numerous pumps and fans throughout the plant. Performance of each SynRM motor is enabled by Danfoss frequency converters.

The plant has the capacity to drive a future geothermic plant on the same site. Despite heavy investment in new technologies, the plant has a payback period of only 7 years. The result for the consumer is a planned reduction of heating rates by 5.5 per cent for the coming financial year, 2014–2015. The goals set by Operations Manager Niels-Peter Heje at tender phase in 2011 specified the newest technologies:

1. Flexible fuel utilisation

The primary fuel is wood chips, readily available in the local area. However the boiler must be able to burn many different non-explosive fuels such as straw, tree roots or tree trunks in the form of wood chips. director Per Sorensen explains: “It’s a major economic advantage for us that the plant can use all types of biofuels. We have a surplus of wood chips in the local area, so we also save on transportation costs because fuel is so close. Furthermore, it’s an advantage in CO2 accounting”.

To achieve multi-fuel operation, the plant tolerates a range of 35–55 per cent moisture in the fuel. The optimum is 45 per cent. Therefore if the fuel is too wet, the combustion air is preheated. If it is too dry, the fuel is dampened by spraying with water. Water spraying is one of the several novel technologies used in this plant. The alternative is energy-costly flue gas recirculation.

For loading and blending purposes there are two wood chip loading areas, connected to a common conveyer feeder. Wood chip storage and blending is computer managed by creating multiple virtual stocks, keeping track of wood chips based on type, moisture, and a number of other properties.

2. Vertical boiler for optimal efficiency

3. An electrostatic precipitator, instead of traditional multicyclones, removes particles from flue gas

4. Synchronous reluctance (SynRM) motors and IE4 induction (IM) motors

All motors over 0.75kW were specified as synchronous reluctance type (SynRM), 1500rpm 4 pole motors, rated IE3, or IE4 if available. Niels-Peter prefers SynRM over permanent magnet (PM) motors, due to PM being difficult and expensive to repair. The construction of the SynRM motor is very simple by comparison, and he therefore expects lower maintenance, and greater motor uptime.

SynRM motors have superior efficiency over IM motors. For ratings above 75kW, IE4 SynRM surpasses IE4 IM for partial speed and load efficiency performance. For ratings below 75kW, IM motors only meet IE3. SynRM losses are substantially lower for partial load – this is where SynRM has a distinct advantage over IM motors.

At the time of ordering in April 2013, the project paid a 20 per cent higher price to obtain IE4 motor rating, by comparison with IE3. This is no longer the case. Prices are decreasing, and in Denmark an IE4 SynRM motor is now available for the same price as IM motors rated IE3.

5. Radial flow pumps, no inline pumps

6. High-efficiency ventilation fans

7. Transformer located closer to plant to minimise cable losses

8. ABB 800 XA control system, set up for PROFIBUS communication to frequency converters, control valves and instrumentation

An OPC system is used for communication with the cranes, absorber and emergency generator. Operators control the plant using iPads, a new approach for district heating, although already well known in other fields.

The absorber enables optimum energy utilisation. The absorber heat pump cools flue gas down to a record low 9.5degC, far exceeding the initial target of 12degC. The low temperature expresses the very high utilisation of energy transferred from the boiler output. The absorber consumes extra power compared with a traditional biofuel plant without absorber, around 1kW electricity for 20kW extra heat. However, due to the use of modern IE4 motors, energy consumption is still competitive with a traditional biofuel plant.

Hjørring plant’s entire power consumption, including district heating pumps, is 12kW per 1MW heat produced. Traditional power ratio for a boiler alone is 10kW per 1MW heat produced.

Flue gas emission levels are very low; levels of gas and dust emission are a fraction of the current permissible limits, and parallel the planned compliance levels for the year 2022.

SynRM motor compatibility now standard

As a dedicated frequency converter supplier, Danfoss is committed to manufacturing frequency converters compatible with all motor types. Software development is always ongoing, to include algorithms to comply with the emerging motor types.

In this case, the control algorithm was customised to the SynRM motors at Hjørring District Heating. Danfoss laboratories tested two motors, a 55kW and a 22kW, to prove SynRM compatibility and confirm IE4 efficiency levels.

Triggered by the development performed for this project, SynRM compatibility is now implemented as standard feature for VLT AutomationDrive. During commissioning, the electrician sets four motor parameters: current, RPM, frequency and torque, and enables the new automatic motor adaptation (AMA). The AMA for the SynRM motor takes 2.7 seconds, measuring the remaining required motor parameters for optimal performance and energy efficiency.

The project chose a Danfoss customised frequency converter development system over an off-the-shelf drive-motor package from a competing supplier. This Danfoss system is now available as standard for VLT AutomationDrive. Some of the reasons:

  • Danfoss offered extensive application support
  • Danfoss frequency converters are compatible with all the different motor types installed in the plant
  • Hjørring District Heating already had long experience using Danfoss frequency converters

The contractor Weiss is convinced this is the right option, and that the result is an excellent multi-biofuel plant. Gorm Gade Knudsen from Weiss explains: “We like Danfoss products and were not hesitant about committing to a development process. At the beginning of the project, it was quite difficult to find an IE4 motor. However nowadays, it’s already realistic to specify IE4 motors.”

Based on required performance, Danfoss dimensioned the frequency converters to match the specified motors. Each frequency converter was selected for the individual load and application – pump or fan.

To learn more about IE4 synchronous reluctance (SynRM) motor technology and Danfoss frequency converters, please go to

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