Lecture No.

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Biomass-Importance of biomass, classification of energy production- principles of
combustion, pyrolysis and gasification

2.0 Biomass
Plant matter created by the process of photosynthesis is called biomass
(or) all organic materials such as plants, trees and crops are potential
sources of energy and are collectively called biomass. Photosynthesis is a
naturally occurring process which derives its energy requirement from
solar radiation. The plants may be grown on land (terrestrial plants) or
grown on water (aquatic plants). Biomass also includes forest crops and
residues after processing. The residues include crop residues (such as
straw, stalks, leaves, roots etc.,) and agro-processing residues (such as
oilseed shells, groundnut shells, husk, bagasse, molasses, coconut shells,
saw dust, wood chips etc.,). The term biomass is also generally
understood to include human waste, and organic fractions of sewage
sludge, industrial effluents and household wastes. The biomass sources
are highly dispersed and bulky and contain large amounts of water (50 to
90%). Thus, it is not economical to transport them over long distances,
and conversion into usable energy must takes place close to source,
which is limited to particular regions.

2.1 Availability of biomass

The total terrestrial crop alone is about 2 × 1012 metric tones. These
include sugar crops, herbaceous crops and silviculture plants. The
terrestrial crops have an energy potential of 3 × 1022 joules. At present
only 1% of world biomass is used for energy conversion. The estimated

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production of agricultural residue in India is 200 million tones per year
and that of wood is 130 million tones. At an average heating value of 18
MJ / kg db, a total potential of energy from agricultural residue is 6×
1012 MJ/ Year. At a power conversion rate of 35%, total useful potential
is about 75,000 MW. This can supply all our villages with power at a rate
of 30,000 kWh per day per village against the present meager
consumption of only 150 kWh per day per village.
The cattle production in India is nearly 237 million. Assuming the
average wet dung obtained per animal per day to be 10 kg and a
collection rate of 66%, the total availability of wet dung in the country
would be 575 million tones per annum. This itself would enable to
produce 22,425 million m3 of biogas, which can replace kerosene oil to
an extent of 13, 904 million litres per year. In a biogas plant, apart from
the gas that is produced, enriched manure is also obtained as a by-
product. It is estimated that, 206 million tones of organic manure per
annum would be produced in biogas plants, which would replace 1.4
million tones of nitrogen, 1.3 million tones of phosphate and 0.9 million
tones of potash.

2.3 Biomass Conversion

Biomass can either be utilized directly as a fuel, or can be converted into
liquid or gaseous fuels, which can also be as feedstock for industries.
Most biomass in dry state can be burned directly to produce heat, steam
or electricity. On the other hand biological conversion technologies
utilize natural anaerobic decay processes to produce high quality fuels
from biomass. Various possible conversion technologies for getting
different products from biomass is broadly classified into three groups,

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viz. (i) thermo-chemical conversion, (ii) bio-chemical conversion and
(iii) oil extraction.

These alternative technologies for biomass conversion offer sound and

alternative options for meeting the future fuels, chemicals, food and feed
requirements. Three main approaches can be adopted for generation and
utilization of biomass:
(i) Collection of urban and industrial wastes as supplementary fuel in
boilers and as a feed stock for producing methane and some liquid fuels.
(ii) Collection of agricultural and forest residues to produce fuels, organic
manures and chemical feed stock.
(iii) Growth of some specific energy plants for use as energy feedstock
and cultivation of commercial forestry, aquatic and marine plants for
different products.

Thermo-chemical conversion includes processes like combustion,

gasification and pyrolysis. Combustion refers to the conversion of
biomass to heat and power by directly burning it, as occurs in boilers.
Gasification is the process of converting solid biomass with a limited
quantity of air into producer gas, while pyrolysis is the thermal
decomposition of biomass in the absence of oxygen. The products of
pyrolysis are charcoal, condensable liquid and gaseous products.
Biochemical conversion includes anaerobic digestion to produce biogas
and fermentation to obtain alcohol fuels, The third approach is oil
extraction. Edible and non-edible oils can be extracted from a variety of
grains and seeds. They can be directly used as fuels by transesterification

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process to produce bio-diesel, which is a good substitute for conventional
diesel oil.
Thermal conversion processes for biomass involve some or all of the
following processes:
Pyrolysis: Biomass +heat charcoal , gas and oil
Gasification: Biomass +limited oxygen fuel gas
Combustion: Biomass +stoichiometric O2 hot combustion products

2.4 Principles of combustion

In general, the term combustion refers to the process of release of heat by
the exothermic heat of reaction for the oxidation of the combustible
constituents of the fuel. Practically the combustion process is an
interaction amongst fuel, energy and the environment.

Fuel may be defined as a combustible substance available in bulk,

which on burning in presence of atmospheric air generates heat that can
be economically utilized for domestic and industrial purposes. The
common fuels are compounds of carbon and hydrogen; in addition
variable percentages of oxygen and small percentages of sulphur and
nitrogen are also present. Biomass fuels are normally thermally
degradable solids. Combustion of organic materials not only generates
natural components of air such as carbon dioxide and water but also
produces carbonaceous residues, smoke and tar and gases of carbonyl
derivatives, and carbon monoxide. The important parameters affecting
combustion are moisture, organic compounds and minerals (ash).

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2.4.1 Combustion Process
The pyrolysis and subsequent combustion of biomass fuels proceed by
two alternative pathways. In the first pathway, which operates at higher
temperatures, pyrolysis or thermal decomposition of the biomass
provides a mixture of combustible gases. These gases mix with air to fuel
the flaming combustion that could rapidly spread in the gas phase. In the
second pathway, which dominates at lower temperatures, pyrolysis
produces mainly carbonaceous char and a gas mixture containing water
and carbon dioxide. That is not flammable. Oxidation of the resulting
active char then provides glowing or smouldering combustion. This type
of combustion proceeds as a front in the solid phase at a lower rate.

2.4.2 Principles of pyrolysis

The pyrolysis of solid wastes strictly refers to the thermal decomposition
of the wastes in an inert atmosphere. In this process, a mixture of gaseous
products, tars, water insoluble oils, and an aqueous solution of acetic
acid, methanol and other organic compounds is evolved and a solid
residue composed of the inert content of the waste and a char is
produced. The amounts of the various products generated are dependent
upon the rate of heating and the final temperature to which the wastes are
subjected. In general, the higher the heating rate, and higher the final
temperature, greater the fraction of the initial wastes that is converted
into the gaseous and liquid products. The yield of gaseous products is
highly variable but is about 25% of the refuse on dry ash-free basis. The
yield of char is about 15 to 25% by weight of the refuse.
Pyrolysis or charring of a biomass fuel has three main objectives: (i)
production of a less smoky, clean burning fuel without generation of any

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tar; (ii) production of a fuel with a high calorific value than that of the
initial feed material; (iii) production of a more reactive fuel.
As the biomass is subjected to thermal treatment, it decomposes and
volatilizes some of the volatile matters, leaving a carbonaceous residue
containing the mineral components. The volatile products consists of a
gaseous fraction containing CO, CO2, some hydrocarbons and H2; a
considerable fraction containing water and organic compounds of lower
molecular weights such as acids, alcohols, aldehydes, and ketones, and a
tar fraction. Fine airborne particles of tar and charred materials constitute
smoke. The amounts of volatiles matters formed, the residue left, and the
weight loss occurred can be determined by the thermogravimetric
analysis (TGA) and its derivative is called differential thermogravimetry
(DTG).The change in enthalpy, ∆H canbe measured by differential
thermal analysis (DTA). All these analysis are called thermal analysis.
The energy released during pyrolysis and combustion can be measured as
a function of time or temperature by thermal evolution analysis (TEA).

2.4.3 Principles of gasification

The equivalence ratio, φ is defined as the ratio of the actual air supplied
to the theoretical air required.
The efficiency, η of a gasifier is defined as the ratio of chemical energy
output in the dry producer gas at 150C to the energy input from the
biomass.The total energy in the gaseous phase increases with the increase
of equivalence ratio, Ø. The important parameters affecting the fixed bed
gasification are:
(a) Shape and size of the biomass fuel and fuel bed structure,
(b) Moisture content

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(c) Volatile matter content
(d) Ash content
(e) Ash composition, its moisture content and
(f) Energy content.
In the course of gasification, a number of thermochemical reactions
take place. The quality of the fuel gas is dependent upon the equilibrium
constants of the reactions. In gasification, the quantity of air that is
supplied to the gasifier is always sub-stoichiometric. A gasification
process that produces pyrolytic oil and char can achieve an overall
thermal efficiency in excess of 70%. It may be noted that the products of
combustion are generally CO2,H2O,N2 and excess O2 and those of
gasification are CO2, CO, H2, CH4, C2H4, C3H6, NH3, H2S, N2, H2O and tar
vapours and low molecular weight organic liquids.

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