Renewable Energy – Solutions for application in the communal energy infrastructure

BIOENERGY |
21
Functional principle and design
Wood pellets are produced by compressing wood shavings
and sawdust without the use of any other additives.
Pellet plants are often affiliated with sawmills.
The cylindrical pellets are typically about 6 millimetres
thick and 1 to 4 centimetres long. The pellets are characterised
by a higher heating value than wood chips and
contain little moisture.
Pellet boilers are installed in the boiler room like other
central heating systems. The fuel supply from the
store to the burner is managed automatically, as is the
control system. The combustion heat is transferred to
the boiler. From here, the heat is distributed throughout
the building via radiators, as with any other central
heating system. A heat accumulator compensates for
any deviations between heat generation and demand.
In contrast to gas and oil heating systems, the ash in
a pellet heating system must occasionally be removed
from the collection container. In the case of pellet
stoves to be installed in the living room, fuel is manually
refilled into a storage container, which is then automatically
transported into the combustion chamber.
There are also special pellet stoves that can deliver up
to four-fifths of their heat to a central heating system
via a water bag.
Wood pellet heating: Pellets can be stored in aboveground or
underground tanks. The fuel feed to the heater works automatically.
Possible applications and benefits for municipalities:
Woodchips and pellets are locally produced energy
sources that are easy to store and, therefore, not only
reduce emissions but also increase value creation at a
local level. These are used for electricity and heat generation:
• Generation of electricity and heat from wood chips in
cogeneration plants, e.g. for schools or community
centres
• Generation of district heating with woodchips (if
possible, in cogeneration)
• Operation of central heating systems and stoves with
wood pellets
Woodchips are often produced in mobile chipping
plants directly in forested areas or during landscape
maintenance work. Production from untreated waste
wood is also possible. Wood chips are typically used
for larger heating and cogeneration plants up to the
three-digit megawatt range, as they are more cost-effective
than wood pellets. Compared to wood pellets,
however, they require about three times as much storage
space for the same heating value.
In wood chip systems, the fuel and air supply are also
automatically regulated according to energy demand.
Here, the hot flue gases from the furnace also heat the
water in the boiler first. In order to generate electricity,
steam is produced from this, which is used to drive a
turbine. The residual heat left over after the turbine can
be used for heating (combined heat and power). When
only being used for heat, the boiler transfers the heat
directly to the heating circuit or storage unit.