COW DUNG

Presented To: Dr. Nasir Uddin Shaikh 1
Assessment of cow dung as a supplementary fuel in
a downdraft biomass gasifier
Authors:
Prokash C. Roy
Department of Mechanical Engineering,
National Institute of Technology, Silchar,
Assam 788010, India
Amitava Datta
Department of Power Engineering, Jadavpur
University, LB-8, Sector-III, Salt Lake
Campus, Kolkata 700098, India
Niladri Chakraborty
Department of Power Engineering, Jadavpur
University, LB-8, Sector-III, Salt Lake
Campus, Kolkata 700098, India

CONTENTS :
• 1. Introduction
• 2. Literature Review
• 2.1: Previously Researched works
• 2.2: Present Research work
• 2.3: Model description
• 2.3-a: Modeling of Pyro-oxidation zone
• 2.3-b: Modeling of Reduction zone
• 3. Results and discussion
• 4. Conclusions

 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 directly burned in a furnace to generate
process heat or it can be fuel internal combustion engines,
gas turbines etc.
 The aim of 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 amount of oxygen from the available supply of
air.

 Biomass Gasifier is a technologically and commercially viable option
for the decentralized generation.
 Animal manure is one of the foremost biomass fuel
available.
Bio-Energy
Production
Routes
 First is biological and second is thermo-chemical
 Anaerobic digestion has the disadvantage of large
installation cost, long reaction time, huge amount of
water required and large area for installing plants.
 Gasification process is attractive for supplying fuel
to small capacity engines & micro-turbines
 The gasification of animal manure, such as cow dung, results in a low temperature
in the gasifier due to the low volatile matter content, high ash content and low
heating value of the fuel.
 The low temperature renders the gasification reactions ineffective.
 On the other hand, cow dung has a good binding property and can be used as a
supplement to a more conventional biomass fuel in the gasifier.
 This will reduce the load on the conventional woody biomass.

 Processes in Gasification
1. Drying
2. Pyrolysis
3. Combustion
4. Reduction

2. LITERATURE REVIEW
2.1 Previously Researched work
 Many researchers adopted the kinetic modelling in the reduction zone of the gasifiers
after considering chemical equilibrium of species in the pyrolysis/oxidation zone
 Giltrap et al. [20] considered the chemical kinetics of four important reactions in the
redction zone for a good prediction of the producer gas composition and temperature.
The model output was found to be sensitive at the initial temperature and composition
at the top of the reduction zone.
 Giltrap et al. further took into account a constant char reactivity factor, to take care of
the active reaction sites in the reduction zone
 Sharma [22] considered two distinct zones in the gasifier: pyro-oxidation zone and char
reduction zone. The gaseous products of the pyro-oxidation zone are considered to be
in equilibrium, while the char is considered as a non-equilibrium product from this zone.
Formation of methane is neglected in the pyro-oxidation zone. Kinetic controlled
reduction by char is considered in the reduction zone with the same equations as
suggested by Giltrap et al., but with linearly varying char reactivity factor.

2. LITERATURE REVIEW
2.2 Present Research work
 In the present work, a biomass fired downdraft gasifier model
has been presented considering two distinct zones: (i) oxidation
zone lumped with drying and pyrolysis, and (ii) reduction zone.
 Instead of considering the gasification of a single biomass, we have
considered the gasification of a mixture of cow dung with a
conventional biomass (sawdust) in different proportions.
This is accomplished to assess the performance of blended
fuels on the gasifier performance (namely heating value, gas
production rate and conversion efficiency) and fuel cost.

2.3 MODEL DESCRIPTION
 The oxidation zone has been solved based on the thermodynamic equilibrium
approach, while the reduction zone has been solved considering char chemical kinetics.
The reduction zone is divided into a large number of control volumes and the species
and energy balance equations have been solved across each control volume taking into
account the chemical reaction rates.

 The modelling of zone-1 has been performed assuming chemical equilibrium of the
species in that zone.
 On the other hand, the modelling of zone-2 considers finite rate chemical
 reactions following the reaction kinetics.
 The output from the zone-1 serves as the input data of the zone-2.
 The biomass feed to the gasifier is a mixture of two biomass stocks – a conventional
biomass (sawdust) and cow dung.
 Table 2 shows the analysis results of the biomass feedstocks used.
 The ash content in the sawdust is only very little, while the ash content in the cow dung
is considerable.
2.3 MODEL DESCRIPTION

 In the pyro-oxidation zone, the fuel is pyrolyzed and oxidized in a sub-stoichiometric
environment of air supplied from the atmosphere.
 The quantity of air is determined using the equivalence ratio, at which the gasifier is
operated.
 The conversion of biomass to the product mixture in zone-1 follows a global reaction
equation:
 Where, b depends on the proportion of the two fuels in the blended feed.
 w is number of moles of moisture.
 a is number of moles of oxygen from air.
2.3-a MODELLING OF PYRO-OXIDATION ZONE (ZONE-1)

 Chemical equilibrium of CO, H2O, CO2 and H2 following the equilibrium of water gas shift reaction
(COþ H2O5CO2 þ H2) has been considered in this zone.
 The equilibrium constant (k1) of this reaction:
 Furthermore, in zone-1, methane is assumed to form only through the methanation reaction
(Cþ 2H25CH4) at the char surface, which attains its equilibrium in the zone.
 The equilibrium constant (k2) of this reaction:
 Values of K1 and K2 calculated from Gibbs function at the temperature of that zone.
 The char yield from the biomass, obtained as the fixed carbon data of the proximate analysis of dry
biomass,
 The temperature in the zone-1, is determined from the energy balance across the zone considering
a heat loss from the gasifier.
 Simultaneous solution of all 7 Eqs. has been performed to evaluate the composition and
temperature of the product at the exit of the zone-1.
 The mole fractions of the individual gas species are evaluated from the total number of moles.
2.3-a MODELLING OF PYRO-OXIDATION ZONE (ZONE-1)

 In zone-2, reductions of the products formed in zone-1 take place following the kinetically
controlled chemical reactions to form the final producer gas. The reduction reactions and
rate of these reactions are considered in this zone are:
2.3-b MODELLING OF REDUCTION ZONE (ZONE-2)

 The flow rate of each species at the entry of the reduction zone is calculated based on the feed rate of
the total dried biomass to the gasifier (mF ) as follows:
 where xi is the number of moles of species i generated in the pyrooxidation zone. The mass flow rate
of ash flowing along with the product can be expressed as
 The entire reduction zone has been divided into a number of elemental control volumes, having
uniform temperature and concentrations. The species mass and energy balance have been performed
across each of the control volume.
 The balance for any of the species ‘i’ across the control volume k gives:

 The temperature within a control volume (k) of the reduction zone is calculated using the
energy balance across the control volume as,
 Simultaneous solution of second last eq., for all the seven species (i=1 through 7), and
above Eq. evaluate the species concentrations and temperature at the outlet of the
control volume k.
 The solution proceeds for all the control volumes into which the reduction zone is divided.
The species concentrations and temperature at the outlet of the last elemental control
volume of the reduction zone determine the values in the producer gas leaving the
gasifier.

 Comparison of Jayyah et al. experimental results with our model.

 Comparison of Pedroso experimental results with our model.
 Gasification behavior of Cow dung alone and with Saw dust.

 Based on the quality and quantity of the producer gas, a unified factor called
gasifier conversion efficiency (hg) has been defined as follows to specify the
overall performance of the gasifier.

 The on-site cost of sawdust may be taken as Rs. 1000/- per tonne [27], while that of
cow dung may be taken as Rs. 250/- per tonne.
 We cannot compare the fuel cost on the basis of unit quantity of gas produced as the
heating value of the producer gas also varies with the change in the constituent of the
blend.
 We have rather compared the fuel cost based on unit amount of energy that can be
obtained through the combustion of the producer gas.
 The results show that the cost is the minimum with a fuel blend containing 40–50% of
the cow dung by mass.

 It is found from the above study that the use of cow dung as a feedstock for biomass
gasifiers is not technologically viable.
 The low gasification temperature of cow dung fuel reduces the rates of the reduction
reactions. Therefore, the major portion of the char remains unreacted and leaves with the
ash.
 However, cow dung can be used as a supplement to the woody biomass fuels in the gasifier.
 Increased fraction of the cow dung in the blend worsens the quality of the producer gas as
its combustible content and heating value reduce.
 The quantity of producer gas evolved from a particular feed rate of the biomass fuel also
reduces with the increased fraction of cow dung in the sawdust and cow dung blend.
 Overall, the conversion efficiency of the gasifier monotonically decreases with the increased
fraction of cow dung in the blend.
 Economic analysis based on fuel cost shows that the cost per unit of energy available in the
producer gas is the minimum when between 40 and 50% cow dung is blended with the
sawdust by mass.
 Therefore, a blend of sawdust and cow dung having 40–50% of the latter is technologically
and economically most viable for the gasification purpose in the particular downdraft gasifier
under study.

COW DUNG

More Related Content

What's hot

Viewers also liked

Similar to COW DUNG

COW DUNG