Type of CHP system: Grate fired boiler

Plant owner: Assens Fjernvarme A.m.b.a. is a co-operative society owned by its app. 2362 heat consumers.

Commissioned: 1999

Status of plant: Commercial

Contractor details:

Turnkey contract: Ansaldo Vølund (today Babcock & Wilcox Vølund)

Buildings: Højgård & Schultz

Boiler: Ansaldo Vølund

Turbine: Blomm & Voss

Generator: LDW

SCADA system: Dansk Automation og Analyse (DAA)

Power generation: Steam turbine

The flue gas condensation unit can increase the heat output significantly by cooling the flue gas down to app. 46°C.

Main fuel:

Wood chips:

The plant uses mostly wood chips, but a wide range of clean wood fuels, such as forest and industrial wood chips, saw dust, and pellets with a moisture content between 5-55 % can be used. Waste products from wood industries may be used depending on the market conditions.

Total fuel consumption is app. 30.000 tonnes/year.

Nominal capacities & efficiencies:

Thermal input, biomass & waste: 18,72 MW

Total thermal input, incl. fossil fuels: 17,72 MW

Gross electrical output: 5,15 MWe

Net electrical output: 4,85 MWe

Heat output: 15,1 MJ/s

Electrical efficiency, gross: 28 %

Electrical efficiency, net: 26 %

Overall efficiency, gross: 88 %

Power to heat ratio: 0,43

The outputs are given at 110 % load, which is the maximum load approved by the contractor. The plant is usually running at 110 % load.

Without the flue gas condenser the heat output is 11,34 MJ/s. With flue gas condensation the heat output is increased to 15,1 MJ/s and the overall efficiency is increased to 108%.

How is plant controlled: The plant is controlled by the heat demand.

Heat backup: Peak and back-up boiler using wood fuels.

Heat usage: Heating only

Energy usage:

The plant supplies app. 2362 customers in the town of Assens (households and industries) with district heating. The annual heat supply (heat sale) is of app. 54,000 MWh (2001/02). The electricity production was 25,237 MWh in 2001/02.

The heat storage equals 70 hours of high-summer load.

District heat coverage (% of total demand in DH network): 95 %

DH inlet temperature: 90 °C

DH return temperature: 30°C

Heat storage size: 2 x 2500 m3

Fuel storage:

An indoor fuel storage can contain 5,800 m3 corresponding to the consumption of app. 10 days and nights.

Furthermore there is an outdoor storage of 5000 m2 (app. 50 days and nights).

Waste water:

App. 3000 m3/year are led to the drain and 2200 m3/year are condensed in the flue gas condenser (2001/02).

Projects have been initiated to reduce the amount of waste water led to the drain. This is sought done by collecting the reject water from the water treatment plant and use it for the ash.

Ash discharge:

Slag and ash are landfilled due to the cadmium content. In 2001/02, 707 tonnes of ash and slags were landfilled  (of this 382 tonnes ash and 161 tonnes slags – the rest is water).

Efforts are made to reduce the amount of ash by reducing the content of unburned matter.

Plant description:

Assens district heating plant is owned by the district heating consumers in Assens. The plant is wood-fired combined heat and power plant from 1999. The plant is supplied by Ansaldo Vølund A/S (now Babcock & Wilcox Vølund) and consists of a spreader stoker, a grate fired boiler, a steam turbine and an exhaust gas condenser.

The plant supplies the major part of Assens with heat for space heating in domestic houses and industries and is operated according to the heat demand. The plant is designed to fit the heat demand in the town and includes a condenser, which provides for a very high fuel efficiency.

The fuel is dominantly wood chips, although it is possible to use a variety of pure wood fuel with a water content of between 5-55%. Both domestic and imported fuel is used and there are indoor as well as outdoor fuel storage facilities.

Electricity is sold to the grid and heat is sent to the DH-system or stored in the two heat accumulation tanks. which allow the plant’s production to be controlled in relation to electricity tariffs.

The exhaust gas is cleaned in an electrofilter in order to remove fly ash. It then passes through an exhaust gas condenser, and finally the gas is released via a smoke stack. Due to the cadmimum content, slag and ash are collected in containers and transported to a controlled disposal site.

Fuel feeding system:

The fuel is transported on a belt conveyor from the storage to the feeders. The transport unit is proportioned for 8.5 tons/hour. Two pneumatic feeders throw the fuel into the combustion chamber.


The boiler is the design of Ansaldo Vølund’s, designed particularly for various biomass fuels. Two pneumatic feeders throw the fuel into the combustion chamber, where the larger particles drop down at the grate (a water-cooled vibrating grate). Small easy ignitable particles like dry sawdust burn in suspension.

Turbine: The turbine runs with 12,500 r/min.

Steam data:

Steam pressure, turbine: 77 bar

Steam temperature, turbine: 525°C

Steam production: 21,2 tons/hour

Flue gas cleaning:

Large fly ash particles are removed from the flue gas in a hot cyclone and recycled to the combustion chamber due to the high content of unburned matter. Further cleaning takes place in an electrostatic precipitator. Finally the flue gas pass on either to the combined wet scrubber/condenser or to the chimney.

Cooling system:

Two district heat exchangers in parallel utilise the remaining energy in the outlet steam from the turbine.


4,5 persons operate the plant during daytime (operation is automatic and unmanned during nighttime).

3,5 persons in the administration (and 2 persons working with the district heating network and the customers).

Electricity sale info:

Three different tariffs are applied for the electricity sale. The tariffs are fixed and known in advance. The heat accumulation tanks enable, to a wide extent, the plant to optimise its production according to the electricity tariffs.

Total capital cost: € 17,000,000

Cost quoted in 1999 euros

Data collected and quality assured by: FORCE Technology.

Contact person: Anders Evald