Biorefineries using waste – production of energy and chemicals from biomasses by micro-organisms - Biomass as Energy Source: Resources, Systems and Applications - page 235

in biorefinery system, the provision of a renewable, consistent and regular source of feedstock is
a favorable condition. These biomass sources are replacing the fossil deposits.
On the other hand, many biomass substrates are available on seasonal basis after harvesting, for
example. Then it comes to the flexibility of the microbial bioprocesses. Since the heterotrophic
processing by the microbial cultures and communities is based on the exploitation of the energy
contents of the biomass substances, the overall economics of the process is dependent on the
energy density of the substrate. If reduced costs of transport, handling or storage are aimed for, the
energy densities need to be increased or concentrated. In a properly functioning biorefinery unit,
this could be achieved by combining the raw materials, and the above-mentioned unit operations.
If this flexibility is associated with the option for multiple uses of the bioreactor facilities, it
could be well stated that the establishment of a full-scale biorefinery system is requiring and also
bringing along a new kind of industrial logics. On the other hand, this revolution in our thinking
is needed for successful management of biological materials and processes, where ordinary skills
and knowledge of basic process chemistry often are inadequate. For example, with proper planning
we can move from one operational mode to another according to the availability or cost of the
raw materials, or based on the demand of the various product substances in the market.
Biomass can be defined as organic matter available on a renewable basis (Lucia et al. , 2006).
Renewable carbon-based raw materials for biorefinery purposes are provided from four different
sectors (Cherubini, 2010):

• agriculture (dedicated crops and residues),

• forestry,

• industries (process residues and leftovers) and households (municipal solid waste and

wastewaters),

• aquaculture (algae and seaweeds).

A further distinction can be made between those feedstocks, which come from dedicated crops
and residues from agricultural, forestry and industrial activities, which can be available without
deprivation from e.g. food production. The main biomass feed stocks can be grouped in three
wide categories: carbohydrates and lignin, triglycerides and mixed organic residues. In the plant
materials, the lignin is usually occurring together with cellulose and hemicellulose, forming some
extra challenges for the process construction (Gressel and Zilberstein, 2007).
The most important energy products, which can be produced in biorefineries, are, according
to Cherubini (2010):

• gaseous biofuels (biogas, syngas, hydrogen, biomethane),

• solid biofuels (pellets, lignin, charcoal),

• liquid biofuels for transportation (bioethanol, biodiesel, FT-fuels, bio-oil).

It is important to take into account that in the industrial combustion it is possible to combine the
solid fuels and waste materials with bioprocess fluids and suspensions. Ethanol from fermentation
is usually used as fuel in transportation. The gas from anaerobic digestion can also be used after
upgrading as fuel for transportation. It can also be burned directly in an oven, gas engine or gas
turbine for heat and power production (Thorin et al. , 2011).
Biobutanol is a promising alternative for vehicle fuels (www.butanol.com). Its feasible indus-
trial production has been so far restricted by the low product yield and productivity in the
bioprocesses. The main route for butanol is the acetone-butanol fermentation, which was found
about 100 years ago by ChaimWeizman (Jones andWoods, 1986). This reaction is carried out by
the anaerobic bacterium Clostridium acetobutylicum .
In some process alternatives, Clostridium beijerinckii is being explored as promising producer
organism for biobutanol production (Kumar and Gayen, 2011). Also genetically modified organ-
isms (GMO) have been suggested and attempted for that purpose. Their usage, however, includes
complications, which possibly could be avoided by the sophisticated use of mixed cultures,
integrated or consolidated bioprocess (CBP) etc. modern means.