European Bio-Energy Projects

BM SCREW
Objectives
The objective of this project is to obtain a
more efficient utilisation of biomass fuels
by using generated energy, not only for
heat supply but also for electricity
production. This will be done by
demonstrating the technical maturity and
economic competitiveness of an innovative
small-scale biomass CHP (combined heat
and power) technology based on a screwtype
steam engine. The steam engine, with
a nominal electric capacity of 800 kW
suitable for multi-fuel feed, will be
implemented into the steam cycle of a
district and process heating plant in
Hartberg (Austria). Technological targets
are to show that the overall efficiency of
the CHP plant is above 90 % and the
electric efficiency is about 13 %. Another
key aim is the improvement of the plant
availability and the reduction of operating
costs by using a new, automatic, steam
boiler cleaning system.
Demonstration of an 800 kW
screw-type steam engine
Challenges
This project covers the implementation of a CHP
module based on a screw-type steam engine
into an already existing biomass district and
process heating plant. At present the only useful
technologies from a technical and economical
point of view for CHP generation, based on
biomass fuels in the power range between 200
to 1,000 kWel, are the screw-type steam engine
(for steam applications) and the already
demonstrated ORC process (for thermal oil
applications).
Screw-type steam engines for small-scale
biomass CHP applications have a number of
advantages compared to steam turbines and
conventional steam engines. Screw-type engines
show a comparatively high electric efficiency for
small-scale CHP units (about 13 %), which only
slightly decreases at partial load operation. Due
to the high electric efficiency in a wide range of
load conditions, the whole process can be
operated heat controlled without a significant
reduction in electric efficiency.
The screw-type engine is a displacement rotary
engine. The main parts of a screw-type engine
are the male rotor, the female rotor and a casing,
which together form a V-shaped working chamber
whose volume increases during rotation. The
steam enters the casing through the intake port.
The intake is finished when the rotor faces pass
the guiding edges and the chamber is separated
from the intake port. At this stage steam
expansion starts and mechanical power is
produced at the output shaft. During expansion
the volume of the chamber increases, whereas
160
the energy content of the fluid decreases. This
process continues until the exhaust process starts
and the steam is extruded and leaves the machine
through the exhaust port. The expansion process
within a screw-type engine is shown in Figure 1.
The biomass district heating plant in Hartberg
(Austria) is equipped with a water tube steam
boiler producing saturated steam, which supplies
process and district heat consumers via a
hydraulic network. In order to optimise the CHP
plant energetically a superheater will be
implemented into the water tube steam boiler,
which generates superheated steam at a
pressure of 26 bar and a temperature of 260°C.
Based on the annual characteristic curve of the
heat demand of the district heat network (see
Figure 2), the biomass CHP plant is designed for
basic and medium load operation in heat
controlled mode. During the first year
approximately 5 000 operating hours will be
achieved and the electricity production will be
about 3,000 MWh/a. In Figure 3 the annual
energy flow of the heating plant is shown.
In addition, the demonstration of a new and
automatic boiler cleaning system, specially
developed for water tube steam boilers and
based on pressurised air, is foreseen. It focuses
on a boiler operation without the need for manual
cleaning. Thus maintenance and operating costs
can be reduced and the thermal boiler efficiency,
as well as the availability of the plant, will
increase.