Strängnäs Energi AB in Sweden

Flexible multi-fuel (wood and RDF) CHP plant

In January, 2001 we were awarded the contract to supply a multi-biofuel steam boiler for Strängnäs Energi AB. 

The plant is a combined heat and power (CPH) plant and energy produced is used for high-efficiency electricity production, process steam for neighbouring industries, and district heat for the town of Strängnäs. The plant went into operation in November 2008.

Strängnäs multi-fuel fired plant

Fuel flexibility 

In order to benefit from green certificates for the initial 10 years of plant operation, virgin wood (GROT) and waste wood chips are the main fuels. The plant has also been designed to burn refuse-derived fuel (RDF) based on wood, paper and plastic and a fraction of peat.  

2 types of fuels - 2 different sets of steam parameters

The Strängnäs plant is designed for two different sets of steam parameters in order to improve the electrical efficiency when burning clean biomass.

The fuel feeding and combustion grate 

The fuel arrives at the plant in trucks which offload the fuel into the bunker. Two overhead cranes then mix the fuel and feed it into the boiler hopper. The robust fuel-feed pusher system has proven its worth in numerous waste-to-energy plants and makes it unnecessary to screen the fuel for metals or oversized material before feeding it in.

The plant is equipped with a water-cooled vibration grate due to its ability to handle low melting point metals. The benefits include low maintenance, smooth operation, and well-proven performance from biomass.

The boiler

The boiler has two empty radiation passes which cool the flue gas sufficiently before the convection part to avoid clogging the convection pass. The superheaters and economizer are in a vertical arrangement. The steam parameters have been adjusted to avoid corrosion in the boiler when burning waste fuels. 

PDMS modelling of Strängnäs plant

NOx removal and flue gas cleaning

NOx is removed in the boiler with an SNCR plant based on NH3 water solution injected into the first boiler pass.

Flue gas cleaning consists of a bag filter with lime injection followed by scrubber/condenser for final polishing of the flue gas.

The bottom ash is removed on a submerged chain conveyor with rubber belts, and is moved to closed containers.

Heat and power for the industry and inhabitants of Strängnäs

The energy is used for high-efficiency electricity production, process steam for neighbouring industries, and district heat for the town of Strängnäs. The steam is expanded through a steam turbine with a 13 bara extraction of 7 MW process steam and the condensation heat is used for district heating production.

CFD illustration of the Strängnäs plant

Plant design data

Process parameters Values Units
Nominal capacity 36.7 MW steam
Heating values 7-15 MJ/kg
Gross calorific value, incl. flue gas condensation 24 MWt
Total thermal efficiency 90 %
Steam from waste
- flow 48 t/h
- temperature 427 °C
- steam pressure 72 bara
- electricity 8.35 MWe
Steam from wood
- flow 46 t/h
- temperature 482 °C
- steam pressure 72 bara
- electricity 8.7 MWe
Flue gas guarantees out of boiler:
CO 50 mg/Nm3 *)
NOx 80 mg/Nm3 *)
Dust 10 mg/Nm3 *)
N20 20 MJ fuel
HCL 10 mg/Nm3 *)
SO2 50 mg/Nm3 *)
TOC 10 mg/Nm3 *)

Strängnäs illustration

*) The emission values refer to 11 % dry O2

The plant limit values comply with the waste incineration directive regarding co-combustion. The values are before flue gas cleaning. Naturally the Strängnäs plant is equipped with a modern flue gas cleaning system.