Cogeneration plants
Cogeneration plants are typically designed to cover the base heat load of an industrial site.
In
New Zealand sawmills, drying kilns usually operate continuously and so
provide a suitable heat load for a cogeneration plant. However,
electricity demand tends not to coincide with heat demand, especially
for small to medium mills running only one shift a day. This makes
generalisations regarding sizing and optimising cogeneration plants
inappropriate, and each site should be considered on a case-by-case
basis.
Possible considerations:
- Do I have enough wood waste to supply cogeneration plant?
- Do I have suitable base heat load to make a cogeneration plant economic?
- If I have surplus electricity, am I able to export electricity to the grid and what price can I get for this electricity?
For a quick preliminary assessment use our Biomass Cogeneration Investment Tool.
Due to the complexity of cogeneration plants, professional advice
from consultants should be sought. Contact the call centre to talk to a
bienergy expert 0800 BIOENERGY (0800 243636).
A possible cogeneration setup is shown in the following diagram
The standard cogeneration technology is the Steam Turbine, however, there are new technologies, such as Gasification and the Organic Rankine Cycle,
that enable the production of electricity from wood with increased
efficiency. The option of using both of these technologies for
cogeneration at a New Zealand sawmill is currently being assessed in a
FIDA funded study by Scion.
Steam turbine
High
pressure steam from a boiler is passed through a single or multistage
turbine to generate electricity. The exhaust steam is piped to the
kilns for timber drying, the steam is then condensed and fed back into
the boiler for reuse. Steam turbines are a reliable, proven means of
producing electricity. Of the fuel used they convert around 10-20% to
electricity.
Gasification
Gasification
halts the combustion process and captures uncombusted volatiles. The
captured gas, called syngas, is mainly carbon monoxide, nitrogen,
methane and hydrogen.
Syngas burns more efficiently and cleanly than the solid biomass
from which it was made. Biomass gasification can thus improve the
efficiency of cogeneration plants. Syngas can be burnt in an internal
combustion engine and in a gas turbine, which generate electricity more
efficiently than steam turbines.
Note that the energy density of Syngas is about one fifth that of
natural gas or LPG, and so is more appropriate for on-site use, than
for transporting to another site for use.
For more information on gasification see the following links:
Read a fact sheet on pyrolysis and gasification
US Department of Energy Energy Efficiency and Renewable Energy: Biomass Gasification
IEA Bioenergy: Thermal Gasification of Biomass
An international company selling gasifiers commercially is Ankur Scientific.
Organic Rankine Cycle
The
Clausius-Rankine Cycle is a thermodynamic cycle which describes the
process of converting heat to electricity by using water and steam as
working fluid.
The main components of the process are the boiler, the steam turbine, the condenser and the feed water pump.
The thermal efficiency of this process significantly depends on the
temperature of the source. It decreases with lower temperature which
results in an unsuitable use for renewable sources as solar energy,
geothermal and biomass.
With using an organic working fluid (e.g. NH3) the cycle can be
optimized for a certain temperature level due to different properties
of the working fluid (e.g. boiling temperature). This cycle is called
Organic Rankine Cycle (OCR).