Organic waste as a biomass resource - Biomass as Energy Source: Resources, Systems and Applications - page 120

achieved by immobilized cells as biocatalysts. By this strategy, the reactor productivity has been
increased to 6.5-15.8 g/L/h, instead of 0.5 g/L/h achieved by the traditional batch fermentation
system. If immobilized cells or enzyme systems are used, also improved biocatalyst half-lives are
obtained in biochemical engineering (Dunnill, 1980).
In practical biotechnology, one remarkable advantage is the possibility of combining various
raw materials into same processes. Therefore, such a waste treatment plant exploiting several
bio-catalytic processes could effectively exploit different biomass sources in the same reactor
without causing overwhelming problems for the entire process run. This allows the combination
of dry waste and sewage, or industrial wastes with the municipal residues, as well as seasonal
variations. This ideal situation could be achieved by proper management of the entire process,
selection ofmicroorganisms and fermentation conditions, and process control and adjustment. The
microbiological method could be used in sequence with other techniques such as a pretreatment
for combustion where the overall environmental performance as well as the energy gain could
then improve.
6.5.2 Hydrothermal carbonization of organic waste fractions
The hydrothermal carbonization was put forward in 1913 by the scientist Friedrich Bergius who
explained the elementary process. Experiments involving the hydrothermal carbonization (HTC)
started at the beginning of the 20th century as an attempt to understand the natural process of
formation of coal. In the last years, the process of HTC has been rediscovered due to its likelihood
of having a positive impact on future problems of energy use and agriculture. This process is very
similar to the natural process of production of coal, but instead of thousands of years, it is
done within a couple of hours. It can be said, that in a very simple way, the process consists
in the separation of the water in the biomass with a parallel increasing of carbon concentration
(Morrondo-Martin, 2011).
6.5.2.1 HTC reactions
In a very simplified way, the HTC process takes place in four steps. Firstly, the activation energy
must be given to start the process and its reactions. This energy is reduced by a catalysator and
given in the form of temperature (180-200 ◦ C) and pressure (up to 20 bar). After the reaction
has started, the temperature reaches the expected stable value. The reactions continue as long
as energy is given, since it is an endothermic process. At a constant temperature and pressure
level, the water is separated from the biomass and the time of reaction determines the level of
carbonization. The fourth step is the cooling phase.
Depending on the wanted products, the process of HTC can take between 4 to 24 hours. Sub-
products besides bio coal are substances that remain in the water phase such as furfural, organic
acids and aldehydes, and gas products such as CO 2 ,CH 4 , etc. (Sevilla and Fuertes, 2009a).
There are three types of this process, which give different products, depending on the reaction
time. The first option is to make an incomplete carbonization, which gives a product rich in
nutrients that could be used, for example, to improve soil qualities. The second type is the
complete carbonization where the product is carbon slurry that can be dried to bio-coal and used
as an energy source. The third possibility is the so named “short carbonization”. It gives a liquid,
very reactive intermediate and hydrogen, which can perhaps be used for the production of fuel.
6.5.2.2 Substrates
A large amount of substrates seem to be suitable for the process of HTC (such as food industry
wastes, waste from wood and paper industry, garbage coming from biotopes and parks, sports
facilities, agricultural by-products, etc.). To the present, the potential of the HTC is still somehow
uncertain since aspects like total energy balance, effects of the HTC coal in the soil and the life
cycle assessment of the whole process are still unanswered.
Concerning the properties of the hydro char, analysis and laboratory results show that the
carbon content increases from approximately 40-44% in the substrates to approximately 64-66%