Prof. Vilas shamrao Patil

1. 13.4 : Biomass. Conversion Processes :-
Conversion of biomass into usable energy is the main aim of
biomass-energy-technology. Such a bioconversion process has
several routes depending upon the conditions like temperature,
pressure, micro-organisms utilised etc. The routes are classified
in the following three broad categories, namely :
(1) Direct combustion (Incineration),
(2) Thermochemical conversion and
(3) Biochemical conversion.
1. Direct Combustion of biomass (Incineration) : Combustion is
the process of burning which is rapid oxidation accompanied by
heat and light. It may be slow or rapid. In the slow combustion,
oxidation is accompanied by heat, but no light.
Incineration is the process of burning completely to ashes. It is
applicable to solid fuels including cultivated and waste biomass.
It is convenient and economical to burn the solid, semisolid
biomass at the location of the biomass source only. For example,
trees can be burnt at their sites in the forest, sugar cane baggase
can be burnt near the factory site, etc.

2. The usable energy obtained is heat which can be directly used
for cooking, heating water, generation of steam and indirectly
for generation of electrical power from steam. Thus, incineration
process is :
Dry biomass---- Burning in air ^(o2)- Heat Direct use for
cooking etc.
Indirect use for
generation of
electrical power
Incineration of urban waste is of prime importance. Urban
waste, if not Dilution, obstruction properly handled or disposed
off, creates problems of pollution, obstruction to transport,
spreading of diseases etc. If this waste biomass is properly
processed, then its incineration removes tonnes of waste-
material per day and can generate 15 to 20 MW of electrical
power and supply cooking gas supplying heat energy upto 56
MJ/kg. Moreover incineration can be done at sites near the
urban areas, which reduces cost.

3. Thus, incineration is a non-conventional, renewable, economical
and eco and environment friendly process of harnessing biomass
energy.
2. Thermochemical conversion of biomass : It involves a
number of the combinations of temperatures and pressures by
which biomass is decomposed in thermochemical processes.
This is done by gasification of biomass. The biomass is converted
into gas/gases by
(i) heating it with limited air or oxygen and
(ii) heating it at high temperature and high pressure in presence
of steam and oxygen.
Pyrolysis : But the most important and commonly used process
is the pyrolysis. It is a biothermal process in which the biomass is
heated at high temperature in the range of 500°C to 900°C at
high pressure in a closed vessel in the absence of oxygen. The
biomass by pyrolysis is converted into gases, liquids and solids.
Pyrolysis can process all forms of organic materials, including
rubber and plastics, which are otherwise very difficult to handle
in other processes

4. Fig. 13.2 : Pyrolysis process plant
A schematic diagram of pyrolysis plant for biomass processing
and converting it into gases and other hydrocarbons, oil-like
materials and charcoal like material is shown in Fig. [(13.2)].
The figure itself makes the process clear. The gas obtained is a
mixture of nitrogen, methane, carbon monoxide, carbon dioxide.
They are separated to pure forms.

5. 3. Biochemical conversion : There are two main conversion
processes in biochemical conversion :
(i) Anaerobic digestion and
(ii) Fermentation.
1) Anaerobic digestion : A anaerobe is a microscopic Organism
which can live and grow without external oxygen or air. When it
is added to a biomass, it extracts oxygen by decomposing the
biomass at low temperatures up to 65°C in presence of large
proportion of moisture (about 80%). Thus, anaerobe digests the
biomass, hence, the biochemical conversion involving digestion
of biomass is called anaerobic digestion.
In the presence of moisture and the absence of oxygen most of
the organic materials (from the biomass) undergo fermentation
in which 60 to 80% of the carbon in the organic material is
converted into a mixture of CO2, CH4 (Methane), traces of H2S
and N2. The conversion process takes place at 15 to 65°C and at
nearly atmospheric pressure. The CO2 and H2S can be removed
and pure methane gas is obtained. It has a heating value of
about 56 MJ/kg.

6. Methane gas obtained is called biogas. It is mainly used for
heating water, cooking food and sometimes is used for
generating electrical power; sometimes it is converted into
liquid. Simple plant for producing domestic gas (methane) by
anaerobic digestion is shown in Fig. (13.3). It is a schematic
diagram and the plant is known as Biogas plant or Gobar gas
plant, since cow dung (gobar) is used as biomass in rural areas.
Fig. 13.3: Biogas generation plant by Anaerobic digestion .
It consists mainly of three components : (i) Digester (ii) Gas
holder and (iii) Distribution line and gas appliances.
(i) Digester is a slurry tank (constructed below ground level). It is
leak-proof. Digester chamber or tank has two slanted cement

7. pipes A and B. 'A' is the inlet pipe which allows the biomass
(mixture of cow dung and water or other suitable biomass) to
fall into the digester. The outlet pipe B drains out the used up
slurry into the side tank.
(ii) Gas holder is an inverted steel U vessel kept inverted over
the digester and its walls float in water seal between the brick
walls of the digester, It makes the digester air tight.
The biomass in the digester undergoes anaerobic digestion at
about 35 to 50°C at almost atmospheric pressure. The methane
and carbondioxide gases generated increase the pressure inside
the gas holder and it moves up.
(iii) Distribution line-Gas is allowed to pass through the
galvanised iron pipe or high density PVC pipe fitted at the top of
the gas holder. The pipe is passed through a U trap which
absorbs moisture from the gas and the gas is supplied for
domestic use.
A number of precautions have to be taken : e.g.
(i) The pipes A and B should not block,
(ii) There should be no floating material like grass-leaves,

8. (iii) The digester and gas holder unit should be leak proof.
(iv) The digester has to be washed, cleaned frequently to remove
any solid matter accumulated at the bottom, to avoid blockage
of the pipes etc.
But the plant is simple, cheap and small. Hence, in India its use
is encouraged by Indian Government for cooking gas or gas light.
It saves the wastage of fire wood and also supplies waste slurry
as fertiliser. It reduces pollution. It is renewable.
For generating gas on large scale, the plant used involves a
number of other steps and stages, though the principle is the
same.
Anaerobic digestion technologies use urban waste, agricultural
biomass, forest biomass, aquatic biomass and human and animal
excreta. It is obvious that it helps to dispose off waste material,
minimise pollution and acts as a cheap renewable source of
energy.
(II) Fermentation : Fermentation is one of the oldest known
chemical processes used by man to obtain liquor (ethyl alcohol)

9. from grains, molasses, fruits. It can be used to obtain other
products also.
Fermentation is a process of decomposition of organic matter
by microorganisms, especially bacteria and yeast. Sugar cane,
sugar beets, molasses, fruit juices, cereals and potatoes (starch),
cellulose, organic wastes are the substances used for
fermentation and products like ethenol, C02, n-butane,
isopropanol, acetine, acetic acid, propionic acid, aldehydes, H2S,
methane, acids from hydrogen are formed.
Table (13.1) gives the various types of fermentation, the organic
materials required (Infeeds) and the products obtained by
fermentation.
Table 13.1: Types of Fermentation
Sr.
No
Name of
process
In-feed Products
1. Ethenol
Fermentation
Sugar cane, sweet
beets, fruit juices,
molasses, starch
(cereals,
Ethanol and co2

10. potatoes)
cellulose
2. Butenol-Iso-
propanol
Fermentation
Carbohydrates
mixed solvents
n-butane,
isopropanol, acetin,
ethanol
3. Methane
Fermentation
Organic wastes,
carbohydrates,
fats, proteins,
sugars.
Acids : Acetic,
Propionic, Formic,
Aldehydes, lower
alcohols, Ammonia,
H2S, Methane + CO2
4. Hydrogen
Fermentation
Hydrogen mixed
with acids.
Ethenol (Ethyl alcohol) can be mixed with petrol (gasoline) to
produce gasohol(10 to 25%) of ethenol is mixed with petrol in
volume) and it has proved to be the best alternative to using
pure petrol. Pure petrol is costly and indications are that it will
be very costly in near future and it is non-renewable too. Ethenol

11. is comparatively very cheap and is renewable source. The
consumption of pure petrol can be reduced by mixing it with
ethenol. Gasohol gives equally good results like pure petrol.
Therefore, all over the world, production of ethenol is being
encouraged. Brazil has increased its capacity to produce ethenol
to about 3 x 109
Litre (3000 million litres) per year to substitute
20% of petrol consumption.
Ethenol (Ethyl alcohol) fermentation
(i) From molasses : Molasses is a dark coloured fluid left as a
viscous residue after crystalisation of cane sugar from sugar-
cane juice. It contains about 50% glucose, fructose and sugars.
When it is combined with yeast, which contains enzymes
invetase and zymass, fermentation takes place at about 20°C to
30°C, the process completes in about 50 hours and ethenol as
obtained as 90% yield. (ii) If starch is used, mixed with water and
enzymes are added, dextrose (sugar) results as
(C2H10O5)5+nH20Enzymen C6H12O6
i.e. Starch + Water Dextrose
Dextrose with action by yeast on fermentation yields ethenol as

12. C6H12O6Yeast2C2H5OH+2CO2
Dextrose -Ethenol +Carbondioxide
Ethenol is also used to prepare many organic chemicals, liquors,
perfumes drugs, plastic. It is used in liquid detergents sprays,
montrivaters. It is also used as a fuel.