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28 New Trends in Eco-efficient and Recycled Concrete

2.1.4 Technologies for converting biomass into useful energy

Biomass conversion may be conducted on two broad pathways: chemical decompo-
sition and biological digestion. The most-used conversion technologies for utilising
biomass can be divided into three basic categories [summarised in Fig. 2.4
(Demirba¸s, 2001)]:

Direct combustion. This is the burning of biomass in the presence of oxygen. It is a
proven technology widely used to convert biomass energy into heat and/or electricity with
the help of a steam cycle (stoves, boilers and power plants). These processes are applied
from a very small scale, for domestic heating, up to a scale in higher ranges to produce
electricity. Biomass could be used as only fuel, but it is also possible to apply co-firing
that has important advantages, particularly when electricity is an output. Biomass can be
burnt mixed with a fossil fuel, such as coal; this process is named co-firing. In the case
that the biomass is used for simultaneous production of heat and electricity the process is
named a co-generation process or combined heat and power; in this case power plants
produce heat used in district heating and ‘waste heat’ is recovered as a by-product for
electricity production. Co-generation processes convert about 85% of the potential energy
of biomass into useful energy.
Thermochemical processes. These entail the application of heat and chemical processes
into the production of energy products from biomass and can be subdivided into pyrolysis
and gasification.
Pyrolysis. Pyrolysis is a thermal degradation of a substance in the absence of oxygen.
In this process, biomass undergoes partial combustion and is decomposed by heat,
without producing the combustion reactions. The basic characteristics of this process
are the following: the only oxygen present is the content of the waste to be treated, at
working temperatures ranging between 300 and 800 C, and as a result of this process,

liquid fuels (including tars, oils, methanol, acetone, etc.) and a solid residue rich in car-
bon (called biochar) are obtained.

Direct combustion

Pyrolysis
Thermochemical processes
Conversion processes Gasification
biomass-energy

Anaerobic digestion
Biochemical processes
Alcoholic fermentation

Figure 2.4 Conversion processes of biomass to energy.