Biomass

Fuels consist primarily of hydrogen and carbon - called hydrocarbon fuels;

general formula CnHm.
Hydrocarbon fuels exist in all phases - coal, gasoline and natural gas.

The term biomass generally refers to renewable organic matter generated by

plants through photosynthesis in which the solar energy combines the
carbondioxide and water to form carbohydrate and oxygen. Materials having
combustible organic matter is also referred to as biomass. It contains C, H and O
and is the oxygenated hydrocarbon.

Biomass includes forest and mill residues, agricultural crops and wastes,
wood and wood wastes, animal wastes, livestock operation residues,
aquatic plants, municipal and industrial wastes.

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 1

Biomass (cont’d)
Wood - primary biomass for most pre-
twentieth century societies.
As biomass is a solid fuel, it is at an
enormous disadvantage to petroleum
and natural gas.
The goal of many biomass
conversion process is to convert
solid fuel into more useful forms:
gaseous or liquid fuels.

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Biomass (cont’d)
Examples of conversion of biomass to
gaseous fuels include anaerobic digestion of
wet biomass to produce methane gas and high
temperature gasification of dry biomass to
produce flammable gas mixture of hydrogen,
carbon monoxide and methane.

Examples of converting biomass to liquid

fuels include fermentation of sugars to ethanol,
thermochemical conversion of biomass to
pyrolysis oils and processing of vegetable oils
to biodiesel.
The resulting liquid and gaseous fuels can then
be used in machinery to produce heat and
power.

Biomass can also be converted to heat and

power by burning it as occurs in boilers and
steam power plants. Dr Mubashir Ali Siddiqui, Mechanical Department, NED University
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Photosynthesis
Photosynthesis refers to a chemical reaction occurring in the earth between
sunlight and green plants (Chlorophyll).

In this process, the organic compound formed, is stored within the plants in the
form of chemical energy.

Photo means light and synthesis means the making. Photosynthesis is a

process in which the light energy i.e. solar energy is used by green plants and
some microorganisms to synthesize energy rich organic compounds from low
energy carbondioxide and water.

The organic compounds formed are nothing but the biomass, which is a
renewable energy source due to its natural and repeated occurrence in the
environment in the presence of sunlight.

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 4

Photosynthesis (cont’d)
The energy of the sun that reaches the earth is of electromagnetic radiation with
a spectrum that ranges from about 0.3 to 3 µm in wavelength. This corresponds
to radiation from the near ultra-violet through the visible to the infrared. The
energy association with a quantum of light is hν where h is the Planck’s constant
and ν is the frequency of the light. The energy of a quantum or photon in the
ultraviolet portion of solar spectrum is sufficient to break a chemical bond.
However, this is not generally so, in the middle portion of the spectrum where
solar intensity is at its peak and where most of the photosynthesis takes place. A
photon in the visible portion of the spectrum, although not capable of breaking a
chemical bond, has sufficient energy to raise an atom to an excited state. The
excited atomic state may make it possible for bonding to take place with a
neighbouring atom. This is generally the way that photochemical reactions
proceed. The photosynthetic process is a very special photochemical
reaction sequence whereby light interacts with the molecules of water and
carbon dioxide to form carbohydrates. The following generalized chemical
equation expresses these ideas for the simplest carbohydrate:
CO2 + 2H2O Light CH2O + H2O + O2
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Dr Mubashir Ali Siddiqui, Mechanical Department, NED University
Energy Farming
Energy farming is defined as the growing of crops specifically for production of
fuels or energy as a main or subsidiary product of agriculture (fields), silviculture
(forests) and aquaculture (fresh and sea water).
The crops grown in the energy farming system are called energy crops. It is
important to note that firewood obtained from cutting down an old-growth forest
doest not constitute an energy crop.
An energy crop is planted and harvested periodically. Harvesting may occur on
an annual basis, as with sugar beets or switchgrass, or on a 5-7 year cycle, as
with certain species of fast-growing trees such as hybrid poplar or willow. The
cycle of planting and harvesting over a relatively short time period assures that
the resource is used in a sustainable fashion.

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Advantages and Disadvantages of Energy Farming
Advantages
 Storage of solar energy in chemical energy form, which is available to a certain
degree whenever desired
 Large potential supply (including transport fuel and electricity generation)
 Linked with established agriculture and forestry
 Efficient uses of byproducts, residues and wastes
 Environmental improvement by reducing the level of carbon dioxide
 Establishes agro-industry that may include full range of technical tasks and
processes, including the need for skilled and trained personnel

Disadvantages
 May lead to soil infertility and erosion
 May compete with food production
 Low energy density, large requirements, transport and storage problems
become uneconomic
 Large scale agro-industry may be too complex for efficient operation
 Poorly designed and incompletely integrated systems may produce water and
air pollution Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 7
Biofuel Production Processes
Biomass is the primary source to produce biofuels. Biomass materials are either burnt
directly or upgraded physically or chemically to produce better fuels, giving a higher
calorific value.
Purpose of Biomass Conversion to Biofuel Production
In comparison with conventional fossil fuels, fresh biomass has the following relative
inferior characteristics
 They have only a modest thermal content with fossil fuels
 They have a high moisture content resulting in the inhibition of their ready combustion,
causing significant energy loss on combustion.
 They usually have a low-bulk density which necessitates the use of relatively large
equipment for handling, storage and burning.
 The physical form is often not homogeneous which poses difficulties in vehicular
transportation and feeding to end-use equipment.
The key objective of biomass conversion prior to its use is to improve the relatively poor
characteristics of the material as fuel as indicated above. The conversion processes of
biomass usually involve the following:
 The reduction of the water content of the material, resulting in the simultaneous
increase in its thermal value and ensure its preservation.
 Improving the handling characteristics of the material, for example converting them into
fluid which may be either gas or liquid. Dr Mubashir Ali Siddiqui, Mechanical Department, NED University
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Biomass Conversion Routes
 Physical
 Densification of biomass into solid briquettes

 Thermochemical
 Combustion
 Carbonization
 Pyrolysis
 Gasification
 Liquefaction
 Anaerobic digestion to methane

 Biochemical
 Enthanol fermentation
 Hydrogen formation for fuel cell

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University

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Physical Methods of Bioconversion
Compression of combustible material.
As bulkiness of the biomass adds to its transportation cost, it is densified by
compression through the processes called briquetting and pelletization.

Briquetting
This is brought about by compression bailing i.e. by sequeezing out moisture and
breaking down the elasticity of the wood and bark. Densification is carried out by
compression under a die at high temperature and pressure.

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Physical Methods of Bioconversion
Pelletization
Pelletization is a process in which wood is
compressed and extracted in the form of rods
(5-12mm dia and 12mm long) facilitating its
use in steam power plants and gasification
systems. The purpose of pelletization is to
reduce the moisture contents and increase
the energy density of wood, making it more
feasible for long transportation haulage.
Pelletizing reduces the moisture content to
about 7 to 10 percent and increases the heat
values of the biomass. Pallets are more
uniform fuel and have better and more
efficient combustion characteristic than
directly burned chips.
Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 11
 Thermochemical Methods of Bioconversion

Direct Combustion for Heat

The simplest route of utilization ;
the oldest thermochemical method
of bioconversion
The main biomass which has been
used over the years for
combustion is wood. It is burnt to
provide heat for cooking, comfort
heat (space heat), crop drying,
factory processes and forming
steam for electricity production
and transport.
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Carbonization
Carbonization is a process whereby wood is heated
with restricted air flow to form a high carbon product
by removing volatile materials from it. The product is
known as charcoal which is extensively used as
a domestic fuel. Charcoal contains 20-25% volatiles
and 75-80% fixed carbon. Charcoal stoves have a
higher overall efficiency than wood burning stoves.

Pyrolysis (Pyro:Fire, Lysis:Separating)

Pyrolysis is similar to carbonization, but the process temperatures are higher and unlike
carbonization, the energy-rich gaseous products of the process are restored in addition to the
charcoal. The products of pyrolysis of wood are nearly charcoal (25%), wood gas (20%)
pyroligneous acid (40%) and tar of or wood oil (15%) excluding the moisture content, the last two
liquid products being obtained by condensation of the volatiles from the wood. Both the liquid and
gaseous products of pyrolysis are combustible and are potential fuel feedstocks.
The process is used heavily in the chemical industry, for example, to
produce charcoal, activated carbon, methanol, and other chemicals from wood, to
convert ethylene dichloride into vinyl chloride to make PVC, to produce coke from coal, to
convert biomass into syngas and biochar, to turn waste into safely disposable substances,
and for transforming medium-weight hydrocarbons from oil into lighter ones like gasoline.
These specialized uses of pyrolysis may be called various names, such as dry
distillation, destructive distillation, or cracking. Dr Mubashir Ali Siddiqui, Mechanical Department, NED University
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 Gasification
Gasification of biomass involves thermal decomposition in the presence of
controlled air. It is the conversion process of solid, carbonaceous fuels into
combustible gas mixtures, normally known as producer gas (or wood gas,
water gas, synthesis gas). This gas can be burned directly in a furnace to
generate process heat or it can fuel internal combustion engines, gas turbines
etc. It can also serve as feedstock for production of liquid fuels, as described in a
subsequent section.
The aim of the gasification is the almost complete transformation of these
constituents into gaseous form so that only ashes and inert materials remain. In a
sense, gasification is a form of incomplete combustion; heat from the burning
solid fuel creates gases which are unable to burn completely due to insufficient
amounts of oxygen from the available supply of air.

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 14

Gasification (cont’d)

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 15

Gasification (cont’d)
Chemical Reactions in Gasification
The gasifier is essentially a chemical reactor where various complex and
physical as well as chemical processes take place. Biomass gets dried,
heated, pyrolysed, partially oxidized and reduced in the reactor as it
flows thorough it. The essence of gasification process is the conversion
of solid carbon fuels into carbon monoxide and hydrogen by
thermochemical process. The gasification of solid fuel is accomplished in air
sealed, closed chamber under slight suction or pressure relative to the
atmospheric pressure. A general layout of gasification of biomass has been
shown in figure:

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 16

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 17
Gasification is a complex thermochemical process. Splitting of gasifier into strictly
separate zones is not realistic, but nevertheless conceptually essential. Four distinct
processes take place in a gasifier; drying of the fuel, pyrolysis, combustion and
reduction. Heat is supplied during drying, pyrolysis and reduction processes where
as restricted amount of air is supplied during oxidation process. Although there is the
considerable overlap in the above processes, each can be considered as occupying
a separate zone in which fundamentally different chemical and thermal reactions
take place. The fuel must pass through all of these zones to be completely classified.

The combustion zone is generally situated near the base of the gasifier as shown in
figure. Also it is called as the oxidation or hearth zone, where air is fed into the
gasifier allowing combustion of the fuel to take place. Depending on the application,
the necessary air draught may be created by the suction of an engine or by an
appropriate arrangement of fans. The key feature is that the air supply is restricted
so that the burning does not spread to the whole full load. If this is occurred, the
gasifier would simply become a stove producing heat and incombustible gases. The
fuel column is ignited at one point exposed to the air blast and the gas is drawn off at
another location in the reactor. When combustion starts in the oxidation zone, the
following processes begins with the addition of heat to raise the temperature of the
fuel particles. Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 18
Drying
Biomass fuels consist of moisture ranging from 5 to 35%. At the
temperature above 120 oC, the moisture is removed and converted to steam.
In the drying, fuels do not experience any kind of decomposition. Depending on
the kind of reactor, the fuel composition and the size of fuel, drying may require
several minutes to accomplish or may occur almost instantaneously.

Pyrolysis
At about 400oC, the complex structure of biomass begins to breakdown
with the release of gases, vapours and liquids. Many of these released
components are combustible and contribute significantly to the heating value of
the product gas from the gasifier. The ratio of products is influenced by the
chemical composition of biomass fuels and their operating conditions. Of the
original solid, there remains only char, a porous solid consisting mostly of
elemental carbon and ash.

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 19

Oxidation
As the temperature approaches 700oC, the char begins to react with oxygen.
Introduced air in the oxidation zone contains inert gases such as nitrogen and argon
besides oxygen and water vapours. The oxidation takes place at the temperature 700-
300oC. Heterogeneous reaction takes place between oxygen in the air and solid
carbonized (char) fuel, producing carbon dioxide (CO2) and water vapour
C + O2 = CO2 + 393800 kJ/kg mol (Exothermic reaction)
Hydrogen in the fuel reacts with oxygen in the air blast, producing steam
H2 + O2 = H2O + 242000 kJ/kg mol (Exothermic reaction)
Reduction
In reduction zone, a number of high temperature chemical reactions take place in
the absence of air (or oxygen). Most of the reactions are endothermic and the
heat released during exothermic reactions in oxidation is also utilized in reaction
zone. Hence temperature of gas goes down in this zone. The temperature in this zone
ranges from 800 – 1000oC. The principal reactions that take place is reduction are as
follows:
C + CO2 = 2CO - 172600 (Boundouard reaction)
C + H2O = H2 + CO – 131000 (Water gas reaction)
CO + H2O = CO2 + H2 + 42000 (Water-shift-reaction)
C + 2H2 = CH4 + 75000 ( Hydrogenation reaction)
Hence final gas produced in the gasifier is composed of mainly CO and H2 .
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Dr Mubashir Ali Siddiqui, Mechanical Department, NED University
 21
Dr Mubashir Ali Siddiqui, Mechanical Department, NED University
Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 22
Types of Gasifiers
 Fixed Bed Gasifier
• Up Draft
• Down Draft
• Cross Draft

Fluidized Bed Gasifier

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 23

Air enters at the bottom and the gas is drawn
off at the top.

Highest efficiency as the hot gas passes

through the fuel bed (due to more heat transfer
time) and leaves the gasifier at low
temperatures (more energy per unit volume of
product gas).

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 24

 Air enters at the combustion zone
and the gas produced leaves near
the bottom of the gasifier.
 Purpose is to convert the tar
produced in the pyrolysis to
gaseous product by complete
thermal cracking which is not
possible in updraft type.
 This type of gasifier is most
commonly used for engine
applications because of its ability to
produce a relatively clean gas.
 A disadvantage of this type of
gasifier is that slagging of ash may
occur due to the concentrated
oxidation (combustion) zone.
 Gasifier efficiency is less than
updraft because gas leaves at
higher temperature.
 Not suitable for high ash fuels and
high moisture fuels. Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 25
26
The flow of air and gas is across the
gasifier.

Operates at very high temperatures.

Water cooling is required.

Responds rapidly to changes in gas

production as the path length is
shorter for gasification reactions

High exit temperatures of the gases

and low CO2 reduction results in
poor quality of the gas and low
efficiency; thus fewer applications.

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 27

 Anaerobic Digestion
Anaerobic digestion is the decomposition of organic waste to gaseous fuel by
bacteria in an oxygen-free environment. The process occurs in stages to
successively break down the organic matter into simpler organic compounds. The final
product, known as biogas is a mixture of methane (CH4), carbon dioxide (CO2)
and some trace gases. Hence biogas can be produced by fermenting organic materials
in the absence of air (or oxygen) with the help of bacteria (micro-organisms) to
breakdown materials to intermediate such as alcohols and fatty acids and finally to
methane, carbon dioxide and water. The process is called anaerobic fermentation.
Fermentation is the process of chemical change in Organic matter brought about by
living organisms like bacteria. Biogas is also known as the swamp gas, sewer gas,
fuel gas, marsh gas, wet gas etc. Natural gas is also produced by the action of
anaerobic bacteria on plants that were buried thousands of years ago and is one
of the fossil fuels, often found in association with oil and coal. Biogas and natural
gas are therefore very much akin to one another. The main fuel component of both is
methane gas. Natural gas was created naturally, in contrast, biogas is produced in
a digester by anaerobic fermentation. Digester is a sealed tank or container in which
the biological requirements of anaerobic digestion are controlled to achieve
fermentation and to produce biogas.
Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 28
Anaerobic Digestion (cont’d)
Anaerobic digestion is a simple and low cost process which can be
economically carried out in rural areas where organic wastes are generated
in plenty which otherwise pollute environment and pose health hazards.
Biogas is a renewable and non-fossil fuel that is created as a by-product of
plant and animal materials which are available plentifully all over. Wastes in
large quantities on renewable basis are also available from agricultural crops
and residues, fruit and vegetable plants and municipal refuse.

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 29

Floating gasholder digester type
plants

Dr Mubashir Ali Siddiqui, Mechanical Department, NED University 30

 Energy available from a biogas digester
E = ηHbVb [MJ/day]
where η = Burner efficiency (~60%)
Hb = Heat of combustion per unit volume of biogas (~20MJ/m3)
Vb = Volume of biogas (m3)

E = ηHmfmVb [MJ/day]
where fm = Fraction of methane in biogas (~70%)
Hm = Heat of combustion per unit volume of methane (~56MJ/kg or 28MJ/m3)

Thus H b = H m fm

 Volume of biogas
Vb = cmo [m3/day]
where c = Biogas yield/Dry mass input (0.2-0.4m3/kg)
mo = Dry mass input/day (e.g. 2 kg/day)

 Volume of fluid per day or flow-rate of digester

Vf = mo / ρ [m3/day]
where ρ = Density of dry matter in fluid

 Volume of digester
Vd = Vf tr [m3]
where tr = Retention time (10-50 days) 31
Calculate volume and power available from a biogas digester given that,
number of cows = 8, retention period = 20 days, temperature of fermentation
= 30⁰C, dry matter consumed per cow per day = 2kg, burner efficiency = 0.7
and methane proportion = 0.7. Density of dry matter is 50. Heat of
combustion for methane is 28 MJ/m3.

Total mass of dry input (mo) = 2 x 8 = 16 kg/day

Vf = 16/50 = 0.32 m3/day
Vd = Vf x tr = 0.32 x 20 = 6.4 m3
Vb = 0.2 x 16 = 3.2 m3/day
E = 0.7 x 28 x 0.7 x 3.2 = 43.90 MJ/day = 510 W
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