Power generation and
For power production through biomass combustion, steam turbines and
|Example of small-scale biomass fired CHP system. In a rice drying and packaging plant in Malaysia, rice husk is burned in a water cooled step grate furnace to generate steam (17,5 Bar) for a single stage 225
kWe backpressure turbine. Remaining heat is used for paddy drying.
steam piston engines are available as proven technology. While steam
engines are available in the power range from approximately 50 kWe to 1
MWe, steam turbines cover the range from 0.5 MWe up to more than 500 MWe
with the largest biomass-fired plant around 50 MWe.
Small-scale steam turbines are usually built with a single expansion
stage or few expansion stages, and operated at quite low steam parameters
as a result of the application of firetube boilers. Plants smaller than 1
MWe are usually operated as backpressure CHP plants and aim for
electricity net efficiencies of typically 10% to 12%. The backpressure
heat can be used as process heat.
Steam piston engines can also be used for small-scale applications,
enabling efficiencies of 6% to 10% in single-stage and 12% to 20%
multi-stage mode. Steam engines are relatively robust – even saturated
steam can be used.
For large steam turbine plants, water tube boilers and superheaters are
employed, thus enabling high steam parameters and the use of multi-stage
turbines. Furthermore, process measures such as feed water preheating and
intermediate tapping are implemented for efficiency improvement. This
results in electricity efficiencies of around 25% in plants of 5 to 10 MWe.
In plants around 50 MWe and larger, up to more than 30% is possible in
cogeneration mode and up to more than 40% if operated as condensing plant.
|400 kWe ORC plant, fired
by a biomass grate furnace using sawdust and woodchips as fuel in
Admont, Austria; the remaining heat is used for drying purposes and
for district heating. (Courtesy of Turboden Srl, Italy)
As an alternative to conventional steam plants in the range 0.5 MW to 2
MW, Organic Rankine Cycles (ORC) using a thermal oil boiler instead of a
costly steam boiler are also available, enabling operation at lower
temperatures. A few plants are in operation with biomass combustion. ORC
plants can be operated without superheater due to the fact that the
expansion of the saturated steam of the organic medium leads to dry steam.
|35 kWe Stirling engine
for biomass combustion plants. (Courtesy of Henrik Carlson, Denmark)
Another interesting development for small-scale biomass power
production is the externally fired Stirling engine. A 30 kWe prototype
plant has reached approximately 20% electric efficiency in CHP operation.
Up to 28% efficiency is aimed at by improving the process and scaling up
to 150 kWe. It is expected that Stirling engines may enable economic
small-scale power production by biomass combustion in the future.
In spite of the high complexity, closed gas turbine cycles or hot air
turbines may become attractive for medium-scale applications. Before
market introduction, however, development of process and component design