Biogas Production from Anaerobic Digestion of Manure
Biogas Production
from Anaerobic Digestion of Manure
INTRODUCTION
Biogas can be produced by
using anaerobic digesters (air-tight tanks with different configurations).
These plants can be fed with energy crops such as maize silage or biodegradable
wastes including sewage sludge and food waste. Animal/cattle waste, human
excreta, crop residues, etc may also be used as feedstock in biogas
plants.
It is a mixture of colorless, flammable
gases obtained by the anaerobic digestion of plant-based organic waste
materials. Biogas is typically made up of methane (50-70%) , carbon dioxide
(30-40%) and other trace gases such as
Nitrogen, Hydrogen Sulfide, Carbon monoxide, Oxygen, water vapor and Hydrogen
gas. It is generally accepted that fuel consumption of a nation is an index of its
development and standard of living. There have been
increases in the use of and demand for fuel in terms of transportation and
power generation in many nations in the world. The raw materials used in
commercial methane generation include plant residues, animal waste like cow
dung and various urban wastes which are available in Kenya. Biogas technology has
advantages which include the following: generation of storable energy sources,
production of a stabilized residue that can be used as a fertilizer, an
energy-efficient means of manufacturing nitrogen containing fertilizer, a
process having the potential for sterilization which can reduce public health
hazards from faecal pathogens, and if applied to agricultural residues, a
reduction in the transfer of fungal and plant pathogens from one year’s crop to
the next.
The two enormous problems that
are increasingly threatening the good life of many nations include the task of
waste management and inadequacy of energy supply. A nation’s inability to
dispose waste and to find enough energy greatly affects living conditions. The
problem of fuel scarcity and sewage disposal in Kenya and many developing
countries is alarming. Energy generated from waste is therefore needful as it
will serve the dual purpose of cleaning the environment and providing a cheaper
source of energy. The aim of this report is to give guidelines for biogas
production from a cheap raw material (cow dung) using locally made materials.
Biogas Production
The main components of the digester
consisted of:
·
a 1,000-liter water tank
to store the gas
·
a 4-inch PVC inlet pipe
where the cow dung would be added
·
a 4-inch PVC outlet pipe
for the overflow of fermented slurry
·
a 1/2-inch plastic pipe
on the top of the tank for the gas outlet
Materials
a)
80kg Cow dung
b)
250 grams of country
made Jaggery (helps in multiplication of microorganisms at a faster rate)
c)
Unchlorinated water to
dilute the cow dung to make slurry
Simple Representation of
Biogas Plant Diagram
After two days when the gas has formed, the
following organic wastes commonly found in households can be added to the
digester;
- Rotten vegetables,
Vegetable peels, Fruit skins, Left-over spoiled food which are unfit for
consumption Very sour curd unfits for consumption, Over-fermented Dosa batter, Left-over
vegetable oil used for frying, Grass and weed clippings, Dried flowers, Tender
banana stems, Cattle waste like cow dung, goat's dung and poultry waste
·
Dry
skins of Onion and Garlic
·
Egg
shells
·
Fibrous
materials like coconut husk
·
Bones,
raw or cooked
·
Soaps
or cleaners
·
Generally
anything that floats
The
biogas prototype consists of the following
·
The
Biogas plant consists of a digester tank, where the organic material is stored
and the microorganisms work on them and release gas.
·
The
gas thus produced is collected in a tank known as gas collector. In a floating
type model, this tank is floating in the slurry and moves up-and-down based on
the amount of gas stored in it
·
A
guide pipe helps the gas collector tank to move up-and-down inside the digester
tank.
·
Waste
is fed through feed pipe inside the digester tank.
·
The
fully digested slurry drains out through the outlet pipe. This can be
collected, diluted and used as fertilizer for plants.
·
A
gas pipe line from the Gas collector tank helps in utilizing the gas for
cooking and lighting
Five
Main Steps to Making Homemade Biogas
The
following table outlines the five steps to creating flammable biogas and I will
get into further detail with each one. Biogas is reproduced in a special
airtight tank called an anaerobic digester. The design of the anaerobic
digester determines the first three steps.
5 steps to make Biogas
|
||
step
|
Conditions
|
Controlled by
|
1
|
Airtight
environment
|
Digester
Design
|
2
|
Water
content
|
|
3
|
Heat
|
|
4
|
Neutral
pH
|
Digester
loading
|
5
|
Carbon-to-Nitrogen
Ratio
|
|
Step
1:
Airtight Environment. A Ziploc baggie can be
used for an anaerobic digester. The difficulty arises from trying to add fresh
material without allowing oxygen into the system. The most common method of
creating a continuous flow digester is the “teapot” or “P-trap” shape. Most
biogas digesters are some variation of this teapot shape. Three main processes
take place inside the digester;
·
Acidification
·
Acetogenesis
·
Methanogenesis
Step
2:
Archaea
love water. When loading a digester, the water content in the material put in
it could be taken into consideration. A head of lettuce, for example, looks
very solid to us, however, it is 98% water. Dried rice is only 14% water.
Regardless of the size of your digester, the “40-50-10 Rule” is simple rule of
thumb to follow to get the correct volume: Forty percent material, fill the
rest of the digester with water except for 10% headspace. The gas will be
formed after 48 hours.
Step 3:
The Optimum temperature
for biogas production is between 32ºC
and 35ºC. Temperatures above and
below this optimum can result in less biogas being produced. Temperatures below
optimum slow the respiration rate of bacteria resulting in slower biogas
production. Temperatures above optimum begin to denature bacterial
enzymes, resulting in slower biogas production
Step
4:
Neutral
pH is
an important parameter in anaerobic digestion, just as it is for aerobic
composting. If pH is measured at the inlet, it will be slightly lower than
neutral —
usually
around 5.5 — as fresh material is converted into acids. The pH will neutralize
as these acids are converted into methane gas. By the time the liquid bio
fertilizer comes out the digester, it should be 7. If the pH of the bio fertilizer is lower than this, it is an
indicator the digester has been over-fed and is at risk to “sour,” or stop working
due to low pH. If the pH at the inlet goes below 5.5, it is necessary to add
some wood ashes or lime to buffer the digester. A soured digester has no bubble
activity and instead of producing gas, instead it draws air into it. The top
will be sucked in tightly against the surface of the liquid and if a brewer’s
airlock is being used, the water in the airlock will be sucked into the
digester. Restarting a soured digester is time consuming, and in most cases it
is simpler to dump it out and start over
Step 5:
Nature of
the material: Biogas
production is best at the same 25:1 C:N
ratio as aerobic composting. The reason cattle manure is far and away the
most common feedstock for biogas is cattle manure is naturally the perfect 25:1
carbon-to-nitrogen ratio. Cattle manure makes an excellent feedstock to begin
experimenting with biogas with. Other wastes need to be combined as a composite
pile;
Type of the material
|
Carbon-Nitrogen Ratio
|
Alfalfa hay
|
18 : 1
|
Bagasse from
sugarcane or sorghum stalks
|
150 : 1
|
Chicken manure
|
25 : 1
|
Clover
|
2.7 : 1
|
Cow dung
|
25 : 1
|
Cow urine
|
0.8 : 1
|
Grass clippings
|
12 : 1
|
Kitchen refuse
|
6 - 10 : 1
|
Pig
droppings
|
20 : 1
|
Pig urine
|
6 : 1
|
Potato tops
|
25 : 1
|
Sawdust
|
200 - 500 : 1
|
Seaweed
|
80 : 1
|
Straw
|
60 - 200 : 1
|
Sewage sludge
|
13 : 1
|
Slaughterhouse wastes
|
3 - 4 : 1
|
Biogas
is composed of methane (CH4) and carbon-dioxide (CO2)
along with some trace gases such as
·
water
vapor,
·
hydrogen sulphide (H2S),
·
nitrogen(N),
hydrogen and
·
oxygen.
Carbon
dioxide and trace gases such as water vapor and H2S must be removed before the
biogas can be used because:
a) the hydrogen sulphide
gas is corrosive.
b) water vapor may cause
corrosion when combined with H2S on metal surfaces and reduce the heating
value.
Removal
of Carbon dioxide (CO2)
I.
Carbon
molecular sieves: The
carbon molecular sieve method uses differential adsorption characteristics to
separate CH4 and CO2. This adsorption is carried out
at high pressure and is also known as pressure swing adsorption. For this
process to be successful, H2S should be removed before the
adsorption process.
II.
Water
scrubbing: Carbon dioxide is soluble in water. Water scrubbing uses
the higher solubility of CO2 in water to separate the CO2
from biogas. This process is done under high pressure and removes H2S
as well as CO2. The main disadvantage of this process is that it
requires a large volume of water that must be purified and recycled.
III.
Membrane separation: There are two membrane separation techniques:
·
high pressure gas separation
·
gas-liquid adsorption
IV.
Polyethylene
glycol scrubbing: This
process is similar to water scrubbing; however, it is more efficient. It also
requires the regeneration of a large volume of polyethylene glycol.
The
high pressure separation process selectively separates H2S and CO2 from
CH4. Usually, this separation is performed in three stages and
produces 96 per cent pure CH4. Gas liquid adsorption is a new development and uses
microporous hydrophobic membranes as an interface between gas and liquids. The
CO2 and H2S dissolve while the methane (in the gas)
is collected for use.
Hydrogen
Sulphide (H2S) Removal
a)
Membrane purification: Some components of the raw gas are transported
through
a thin membrane while others are retained. The permeability is a direct
function of the chemical solubility of the target component in the membrane.
Solid membranes can be constructed as hollow fiber modules which give a large
membrane surface per volume and thus very compact units. Operating pressures
are in the range of 25-40 bars. There are 2 membrane separation techniques
(high pressure gas separation and gas-liquid adsorption). The high pressure
separation process selectively
separates
H2S and CO2 from CH4. Usually, it is performed
in three stages and produces 96% pure CH4. Gas liquid adsorption is
a newly developed process that
uses
micro-porous hydrophobic membranes as an interface between gas and liquids. The
CO2 and H2S dissolve into the liquid while the methane
(which remains a gas) is collected for use.
b) Biological
desulphurization: Natural bacteria can convert H2S into elemental Sulphur
in the presence of oxygen and iron. This can be done by introducing a small
amount (two to five per cent) of air into the head space of the digester. As a
result, deposits of elemental Sulphur will be formed in the digester. Even
though this situation will reduce the H2S level, it will not lower it below
that recommended for pipeline-quality gas. This process may be optimized by a
more sophisticated design where air is bubbled through the digester feed
material. It is critical that the introduction of the air be carefully
controlled to avoid reducing the amount of biogas that is produced.
Water
vapor removal
can
also compromise the process considerably during the conversion of biogas to
electricity or biomethane because the biogas is saturated with steam inside the
digester. In order to avoid corrosion and other negative effects during
subsequent gas treatment, it is necessary to dry the biogas. Various methods are
available for drying biogas:
i.
Condensation drying: the biogas is cooled in gas coolers
(refrigeration units) or underground pipes so that the water vapor condenses;
ii.
Adsorption dryer: silica gel, aluminium oxides or molecular
sieves;
iii.
Drying by increasing the pressure. Using this method, the water is not removed but
the relative humidity reduced.
N.B
Traces
of ammonia can also be found in biogas. Because it is highly water-soluble, it
can be reduced by water removal. Biomethane must additionally be free from
impurities
such
as dust, oil and aerosols. Filters used in gas technology are installed for
this purpose.
PACKAGING OF BIOGAS
Storage of Biogas by Adsorption Method
the
natural gas or methane itself cannot be liquefied by simply increasing the pressure;
it is also necessary to provide subcritical temperatures, and these two
requirements make more expensive the transportation and storage of methane.
Gas component
|
Critical
Temperature,
|
Critical Pressure, k
|
CH4
|
- 82.1
|
47.3
|
C02
|
31.0
|
75.3
|
H2S
|
100.4
|
91.9
|
NH3
|
132.5
|
116.3
|
The
critical temperature of methane is -82.1oC and critical pressure is 47.3k/cm3
The critical temperature of a substance is the
temperature at and above which vapor of the substance cannot be liquefied, no
matter how much pressure is applied while The critical pressure of
a substance is the pressure required to liquefy a gas at its critical
temperature.
Thus,
adsorption on solid adsorbents allowed to expand storage capacity in comparison
with compressed natural gas or pure methane at the same conditions of
temperature and pressure. Adsorption is an exothermic process, i.e. it loses
energy to the environment; the reverse process, called desorption, is
endothermic. The adsorption process is governed by the nature of solid adsorbents
and forces distributed along the active surface and pores, so that the
interactions involved are dependent on the adsorbent and adsorbate structures,
crystal and pore size, purity of adsorbent and adsorbate and adsorbate size.
The
occurrence of adsorbate–adsorbate and adsorbate– adsorbent interactions are the
main properties observed in adsorption systems, and the study of these
properties is made by constructing sorption isotherms. These isotherms indicate
whether the system under the conditions at the time of observation favors the
adsorption or the adsorbate–
adsorbate
interactions are stronger than the force of attraction of the solid adsorbent,
however, they will not be addressed in the present study. Gas–solid adsorption
occurs in two ways, depending on the chemical activity of the gas employed and
the solid material: by physical adsorption or physisorption (electrostatic and
van der Waals forces) and by chemical adsorption or chemisorption (chemical
bonds). The first involves the same forces responsible for the condensation of
gaseous vapors, while the second involves the chemical interactions capable of
forming a chemical intermediate
Methane
storage is commonly accomplished through the use of microporous adsorbents
depending on the temperature and pressure conditions employed. For the
adsorption
process at low pressures (approximately ambient pressure), microporous
materials with a pore diameter above 7.6Å (diameter greater than two molecules
of methane) are recommended.
CONCLUSSION
By
the increasing demand of energy biogas demand has also increased. Biogas soon
will replace fossil fuels as a source of energy in addition landfills can be
used for other good purposes like developing research Centre for biogas
production, building biogas plant. Even after the biogas production is
completed, the slurry, which cannot produce more biogas, can be converted to
natural fertilizers. Since we are leaving the garbage on landfills, produce
biogas which mainly consist methane and carbon dioxide which are the major
greenhouse gases, can increase the environmental temperature leading to global
warming. Using the house hold waste for biogas production can eliminate the use
of landfills for garbage disposal and reduce the emission of biogas to the atmosphere,
which leads to GO GREEN.
References
[2] Brown, N. (1987).Biogas
systems in development. Appropriate Technology 4(3),5-7.
[3] Silayo, V.C. (1992). Small biogas plants. Design, management and use.
Agrotec
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