Assens Fjernvarme Amba, Denmark

Wood-fired CHP plant

Building the combined heat and power plant in Assens was part of the Danish political biomass agreement from 1993. The objective of this agreement is the conversion of all fossil-fired district heating plants larger than 1 MW into CHP plants based on biomass. The plant has the capacity to supply almost all of the 5,400 in habitants of Assens with heat and power. Assens Fjernvarme Amba is a co-operative society owned by the district heating consumers in Assens. The same company owns and operates the CHP plant.

In October 1997 Assens Fjernvarme Amba made a contract with us for the building of the new biomass-fired CHP plant. The plant was delivered as a turnkey plant in March 1999, only 15 months later.

Assens biomass fired plant

A tailor-made boiler

We designed the boiler specifically for firing with a wide range of biomass, which will consist mainly of forest wood chips with up to 55% moisture, sawdust, chipped clean wood from industry, and wood pellets.

The combustion technology

From the wood chip storage the fuel is carried via a conveyor and a buffer silo to two pneumatic throwers, which blow the fuel into the furnace. Here a partial drying and gasification takes place while the fuel is still in suspension. The actual combustion takes place on a water-cooled vibrating grate provided with three air zones.
Secondary combustion air and recycled flue gas are added to the furnace combustion zone through a large number of nozzles in the boiler front and rear walls. Slag and ash are gathered in containers and transported to a deposit site.

An electrostatic precipitator cleans the flue gas

After the boiler, the flue gases are carried through an electrostatic precipitator in order to clean the flue gas of fly ash, and subsequently through a flue condenser, if required.

The energy production

From the boiler the steam is sent through a turbine with a velocity of 12,500 rpm. Via a gear the turbine drives a generator with a velocity of 1,500 rpm. The portion of the steam energy which is not utilized in the turbine is converted into heat in two parallel district heat exchangers.

Control and instrumentation

The plant control system is based on a Simatic S7 PLC. The general decentralized control system consists of an IGSS32 operating system with several PC operator stations from which the plant is controlled and alarms and conditions of operation is monitored.

Plant design data

Process parameters Values Units
Energy input 17.3 MW
Electric output 4.7 MW
Electric efficiency 27 %
Thermal output, net excl. flue gas condenser 10.3 MW
Thermal efficiency, net excl. flue gas condenser 60 %
Thermal output, net incl. flue gas condenser 13.8 MW
Thermal efficiency, net incl. flue gas condenser 80 %
Steam temperature, turbine 525 °C
Steam pressure, turbine 77 bar
Forward flow temperature 70-90 °C
Return temperature 30-45 °C
Power generation, annual 26-28,000 MWh
Heat generation, annual 226,000 GJ
Local flue gas limits out the boiler
CO, max. 270 mg/Nm3
NOx 450 mg/Nm3
Dust 40 mg/Nm3

* Based on 10% O2 in dry flue gas