BIOS - Bioenergy

Description of the biomass CHP technology based on a steam turbine process


The CHP technology based on a steam turbine process represents a field-tested large-scale application (>2 MWel) in the field of electricity production from solid biomass.

The process of generating electricity from steam comprises following parts: a firing subsystem (biomass combustion), a steam subsystem (boiler and steam delivery system), a steam turbine with electric generator, as well as a feed water and condensate system.

In terms of combustion technologies grate firing systems or over a firing thermal capacity of 20-30 MW fluidised bed combustion units are commonly implemented.
In the lower power range of steam turbine technologies either still fire tube boilers or already water tube boilers are applied as steam generators. Above a power range of 5 MWel, due to the higher attainable live steam parameters, steam is produced in water tube boilers.

Regarding steam turbine technology backpressure turbines and extraction condensing turbines have to be distinguished. If there is a constant heat demand in form of hot water or low pressure steam all over the year backpressure turbines are used. At projects with the need of uncoupling the electricity and heat production extraction condensing turbines are applied, using the steam which is not or only to a low part required for heat supply in the low pressure part of the turbine to increase electricity production.

Working principle and integration in a biomass CHP plant

The working principle is according to the classical Clausius-Rankine-Process. High temperature, high pressure steam is generated in the boiler and then enters the steam turbine. In the steam turbine, the thermal energy of the steam is converted to mechanical work. Low pressure steam exiting the turbine enters the condenser shell and is condensed on the condenser tubes. As the steam is cooled to condensate, the condensate is transported by the boiler feed-water system back to the boiler, where it is used again.

A simplified flow sheet illustrating a typical biomass CHP plant based on an extraction condensing turbine process is shown in Figure 1.

Fig. 1
Fig. 2

The steam boiler consists of the evaporator unit, the superheater and the economiser which are usually arranged in a four-pass design. In addition, some manufacturers implement an additional combustion air preheater in the flue gas downstream the economiser whereas others use steam or hot water for combustion air preheating.

The feed water of the water steam cycle is heated in the feed water economiser which is installed downstream the superheater, to a temperature close to the boiling point. The feed water economiser is the first of three possible heat recovery units (the others are combustion air preheater and condensate preheater) downstream the superheater.

In the combustion chamber, the chemically bounded energy of the fuel is released and transferred via boiler and surface of the heat-exchangers to the water steam cycle. The heated water is evaporated in the boiler evaporator and collected in the steam drum. Usually the vertically arranged evaporator tubes also constitute the upper part of the combustion chamber walls. The steam drum is located outside the flue gas flow. From the steam drum the saturated steam is transferred to the superheater.

The superheater uses flue-gas at a high temperature level to produce superheated steam. Attention should be paid to high temperature corrosion mechanisms which may require the implementation of a protective evaporator prior to the superheater in order to control the flue gas temperature.

After the boiler multi-cyclones and electrostatic precipitators or fabric filters are commonly used to remove dust from the flue-gas.

Superheated steam at high pressure and high temperature is ducted via pipes to the steam turbine where it is consumed and depressurised.

At the extraction condensing turbine steam is extracted from the turbine at a pressure state which is predetermined by the heat consumers. The main part of this extracted pressure steam goes to the heating condenser and a smaller part is used to transfer heat to the feed-water. The rest of the steam expands in the low pressure part of the turbine to the condenser pressure state and is then cooled at constant pressure. Depending on the conditions on site dry air-cooled condensers or water cooled condensers are installed.

In general the turbogenerator unit includes the modul

  • steam turbine
  • gearbox/generator unit
  • lubricating oil system
  • control oil system
  • measuring and control system

De-ionised water is used for the water steam circuit in order to keep an undisturbed operation. In the water treatment unit solved and dissolved impurities of the natural water must be removed.

Losses in the water-steam circuit caused by blow down and sampling are replenished by de-ionised water from the feed water treatment unit.

Relevant technical data and efficiencies of the steam turbine process

If only chemically untreated wood-like biomass is used, after present state of the art, live steam temperatures to approximately 540°C are achieved. Using waste wood the live steam temperatures must be lowered on approximately 450 °C to avoid increased deposition -und corrosion attack.
The attainable electric annual use efficiency (= annual electricity production / annual fuel input based on its net caloric value) depends on the live steam parameters (temperature, pressure) and on the other hand on the necessary temperature level for the process and/or district heat consumers. Electric annual use efficiencies are usually between 18 and 30 % for biomass CHP plants in the capacity range between 2 and 25 MWel.

Steam parameters and electric capacities at steam turbine processes:

  • Live steam temperature: 450 – 540 °C
  • Live steam pressure: 20 – 100 bar(a)
  • Live steam flow rate: 10 – 125 t/h
  • Back pressure or extraction steam pressure: 1 – 10 bar
  • Exhaust steam pressure: 0,05 – 0,60 bar(a)
  • Electric capacity: 2 – 25 MWel
  • Electric annual use efficiency: 18 – 30 %

Realised projects and proposals under design based on a steam turbine process

  • Biomass CHP plant based on a steam turbine process – Bionergie Kufstein (Tyrol, Austria)
  • Biomass CHP plant based on a steam turbine process – Linz Strom GmbH (Linz, Austria)
  • Biomass CHP plant based on a steam turbine process – Holzindustrie Stallinger (Frankenmarkt, Austria)
  • Biomass CHP plant based on a steam turbine process – EVN AG (Mödling, Lower Austria)
  • Biomass CHP plant based on a steam turbine process – EVN AG (Baden, Lower Austria)
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