comparative study of conventional biogas plant and biogas plant with gas recirculation
TRANSCRIPT
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CHAPTER 1
INTRODUCTION
Due to scarcity of petroleum and coal it threatens supply of fuel throughout the world also
problem of their combustion leds to research in different corners to get access the new
sources of energy, like renewable energy resources. Solar energy, wind energy, different
thermal and hydro sources of energy, biogas are all renewable energy resources. But, biogas
is distinct from other renewable energies because of its characteristics of using, controlling
and collecting organic wastes and at the same time producing fertilizer and water for use in
agricultural irrigation. Biogas does not have any geographical limitations nor does it requires
advanced technology for producing energy, also it is very simple to use and apply.
Kitchen waste is organic material having the high calorific value and nutritive value to
microbes, that’s why efficiency of methane production can be increased by several order of
magnitude as said earlier .It means higher efficiency and size of reactor and cost of biogas
production is reduced. Also in most of cities and places, kitchen waste is disposed in landfill
or discarded which causes the public health hazards and diseases like malaria, cholera,
typhoid. Inadequate management of wastes like uncontrolled dumping bears several adverse
consequences: It not only leads to polluting surface and groundwater through leachate and
further promotes the breeding of flies , mosquitoes, rats and other disease bearing vectors.
Also, it emits unpleasant odour & methane which is a major greenhouse gas contributing to
global warming.
Mankind can tackle this problem(threat) successfully with the help of methane , however till
now we have not been benefited, because of ignorance of basic sciences – like output of work
is dependent on energy available for doing that work. This fact can be seen in current
practices of using low calorific inputs like cattle dung, distillery effluent, municipal solid
waste (MSW) or sewage, in biogas plants, making methane generation highly inefficient. We
can make this system extremely efficient by using kitchen waste/food wastes
1.1 OBJECTIVES OF STUDY
How the yield of biogas is increased by introducing gas recycling to the reactor
How the reaction rate effected while increasing the amount of CO2 in the system
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How much cost effective is the system
How the reaction rate is enhanced by providing feed in the form of slurry instead
of solid feed
1.2 BIOGAS
BIOGAS is produced by bacteria through the bio-degradation of organic material under
anaerobic conditions. Natural generation of biogas is an important part of bio-geochemical
carbon cycle. It can be used both in rural and urban areas.
Composition of biogas depends upon feed material . Biogas is about 20% lighter than air has
an ignition temperature in range of 650 to 750 0C.An odorless & colorless gas that burns with
blue flame similar to LPG gas. Its caloric value is 20 Mega Joules (MJ) /m3 and it usually
burns with 60 % efficiency in a conventional biogas stove.
This gas is useful as fuel to substitute firewood, cow-dung, petrol, LPG, diesel, & electricity,
depending on the nature of the task, and local supply conditions and constraints.
Biogas digestor systems provides a residue organic waste, after its anaerobic digestion(AD)
that has superior nutrient qualities over normal organic fertilizer, as it is in the form of
ammonia and can be used as manure. Anaerobic biogas digesters also function as waste
disposal systems, particularly for human wastes, and can, therefore, prevent potential sources
of environmental contamination and the spread of pathogens and disease causing bacteria.
Biogas technology is particularly valuable in agricultural residual treatment of animal excreta
and kitchen refuse(residuals).
Many factors affecting the fermentation process of organic substances under anaerobic
condition are,
The quantity and nature of organic matter
The temperature
Acidity and alkanity (PH value) of substrate
The flow and dilution of material
Reducing usage of LPG for cooking
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CHAPTER 2
LITERATURE REVIEW
ARTI – appropriate rural technology of India, pune (2003) has developed a compact biogas
plant which uses waste food rather than any cow dung as feedstock, to supply biogas for
cooking. The plant is sufficiently compact to be used by urban households, and about 2000
are currently in use – both in urban and rural households in Maharashtra. The design and
development of this simple, yet powerful technology for the people.
Dr. Anand Karve (ARTI) developed a compact biogas system that uses starchy or sugary
feedstock (waste grain flour, spoilt grain, overripe or misshapen fruit, non edible seeds, fruits
and rhizomes, green leaves, kitchen waste, leftover food, etc). Just 2 kg of such feedstock
produces about 500 g of methane, and the reaction is completed with 24 hours. The
conventional biogas systems, using cattle dung ,Sewerage, etc. use about 40 kg feedstock to
produce the same quantity of methane, and require about 40 days to complete the reaction.
Thus, from the point of view of conversion of feedstock into methane, the system developed
by Dr. Anand Karve is 20 times as efficient as the conventional system, and from the point of
view of reaction time, it is 40 times as efficient. Thus, overall, the new system is 800 times as
efficient as the conventional biogas system.
Hilkiah Igoni (2008) studied the Effect of Total Solids Concentration of Municipal
SolidWaste on the Biogas Produced in an Anaerobic Continuous Digester. The total
solids (TS) concentration of the waste influences the pH, temperature and effectiveness of the
microorganisms in the decomposition process. They investigated various concentrations of
the TS of MSW in an anaerobic continuously stirred tank reactor (CSTR) and the
corresponding amounts of biogas produced, in order to determine conditions for optimum gas
production. The results show that when the percentage total solids (PTS) of municipal sold
waste in an anaerobic continuous digestion process increases, there is a corresponding
geometric increase for biogas produced. A statistical analysis of the relationship between the
volume of biogas produced and the percentage total solids concentration established that the
former is a power function of the latter, indicating that at some point in the increase of the
TS, no further rise in the volume of the biogas would be obtained.
Kumar et al., (2004) investigated the reactivity of methane. They concluded that it has more
than 20 times the global warming potential of carbon dioxide and that the concentration of it
in the atmosphere is increasing with one to two per cent per year. The article continues by
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highlighting that about 3 to 19% of anthropogenic sources of methane originate from
landfills.
Shalini Singh et al. (2000) studied the increased biogas production using microbial
stimulants. They studied the effect of microbial stimulant aquasan and teresan on biogas
yield from cattle dung and combined residue of cattle dung and kitchen waste respectively.
The result shows that dual addition of aquasan to cattle dung on day 1 and day 15 increased
the gas production by 55% over unamended cattle dung and addition of teresan to cattle
dung: kitchen waste (1:1) mixed residue 15% increased gas production.
Lissens et al. (2004) completed a study on a biogas operation to increase the total biogas
yield from 50% available biogas to 90% using several treatments including: a mesophilic
laboratory scale continuously stirred tank reactor, an up flow biofilm reactor, a fiber
liquefaction reactor releasing the bacteria Fibrobacter succinogenes and a system that add
water during the process. These methods were sufficient in bringing about large increases to
the total yield; however, the study was under a very controlled method, which leaves room
for error when used under varying conditions. However, Bouallagui et al. (2004) did
determine that minor influxes in temperature do not severely impact the anaerobic digestion
for biogas production.
As Taleghani and Kia (2005) observed, the resource limitation of fossil fuels and the
problems arising from their combustion has led to widespread research on the accessibility of
new and renewable energy resources. Solar, wind, thermal and hydro sources, and biogas are
all renewable energy resources. But what makes biogas distinct from other renewable
energies is its importance in controlling and collecting organic waste material and at the same
time producing fertilizer and water for use in agricultural irrigation. Biogas does not have any
geographical limitations or requires advanced technology for producing energy, nor is it
complex or monopolistic.
Murphy, McKeog, and Kiely (2004) completed a study in Ireland analyzing the usages of
biogas and biofuels. This study provides a detailed summary of comparisons with other fuel
sources with regards to its effect on the environment, finical dependence, and functioning of
the plant. One of the conclusions the study found was a greater economic advantage with
utilizing biofuels for transport rather than power production; however, power generation was
more permanent and has less maintenance demands.
Thomsen et al. (2004) found that increasing oxygen pressure during wet oxidation on the
digested biowaste increased the total amount of methane yield. Specifically, the yield which
is normally 50 to 60% increased by 35 to 40% demonstrating the increased ability to retrieve
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methane to produce economic benefits.
Carrasco et al. ( 2004) studied the feasibility for dairy cow waste to be used in anaerobic
digestive systems. Because the animal’s wastes are more reactive than other cow wastes, the
study suggests dairy cow wastes should be chosen over other animal wastes .
Jantsch and Mattiasson (2004) discuss how anaerobic digestion is a suitable method for the
treatment of wastewater and organic wastes, yielding biogas as a useful by-product.
However, due to instabilities in start-up and operation it is often not considered. A common
way of preventing instability problems and avoiding acidification in anaerobic digesters is to
keep the organic load of the digester far below its maximum capacity. There are a large
number of factors which affect biogas production efficiency including: environmental
conditions such as pH, temperature, type and quality of substrate; mixing; high organic
loading; formation of high volatile fatty acids; and inadequate alkalinity.
Jong Won Kang et al (2010) studied the On-site Removal of H2S from Biogas Produced
by Food Waste using an Aerobic Sludge Biofilter for Steam Reforming Processing. They
show that A biofilter containing immobilized aerobic sludge was successfully adapted for the
removal of H2S and CO2 from the biogas produced using food waste. The biofilter
efficiently removed 99% of 1,058 ppmv H2S from biogas produced by food waste treatment
system at a retention time of 400 sec. The maximum observed removal rate was 359 g-
H2S/m3/h with an average mass loading rate of 14.7 g-H2S/m3/h for the large-scale biofilter.
The large-scale biofilter using a mixed culture system showed better H2S removal capability
than biofilters using specific bacteria strains. In the kinetic analysis, the maximum H2S
removal rate (Vm) and half saturation constant (Ks) were calculated to be 842.6 g-H2S/m3/h
and 2.2 mg/L, respectively. Syngas was generated by the catalytic steam reforming of
purified biogas, which indicates the possibility of high efficiency electricity generation by
SOFCs and methanol manufacturing.
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2.1 PRODUCTION PROCESS
In biogas plants, organic substrates are converted in the liquid phase to methane and carbon
dioxide which accumulate in the gas phase. The degradation of organic matter to biogas
follows a complex reaction network and it is grouped into five degradation steps, which are
(1) disintegration of complex substrates to macromolecules, (2) hydrolysis of these to smaller
molecules, (3) acidogenesis and (4) acetogenesis, where acids are produced, and finally (5)
methanogenesis. In the last degradation step, methane is produced within two different
pathways. In the acetoclastic path, acetic acid is degraded to methane and carbon dioxide.
The latter can be consumed by hydrogenotrophic microorganisms and converted to methane,
as well. Both pathways are present in a microbial community of a biogas plant.
Organic substances exist in wide variety from living beings to dead organisms . Organic
matters are composed of Carbon (C), combined with elements such as Hydrogen (H), Oxygen
(O), Nitrogen (N), Sulphur (S) to form variety of organic compounds such as carbohydrates,
proteins & lipids. In nature MOs (microorganisms), through digestion process breaks the
complex carbon into smaller substances. There are 2 types of digestion process :
Aerobic digestion.
Anaerobic digestion.
The digestion process occurring in presence of Oxygen is called Aerobic digestion and
produces mixtures of gases having carbon dioxide (CO2), one of the main “green houses”
responsible for global warming.
The digestion process occurring without (absence) oxygen is called Anaerobic digestion
which generates mixtures of gases. The gas produced which is mainly methane produces
5200-5800 KJ/m3 which when burned at normal room temperature and presents a viable
environmentally friendly energy source to replace fossil fuels (non-renewable).
2.1.1 Anaerobic digestion
It is also referred to as biomethanization, is a natural process that takes place in absence of air
(oxygen). It involves biochemical decomposition of complex organic material by various
biochemical processes with release of energy rich biogas and production of nutrious
effluents.
Biological processes involved,
Hydrolysis: In the first step the organic matter is enzymolysed externally by extracellular
enzymes, cellulose, amylase, protease &lipase, of microorganisms. Bacteria decompose long
chains of complex carbohydrates, proteins, & lipids into small chains. For example,
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Polysaccharides are converted into monosaccharide. Proteins are split into peptides and
amino acids.
Acidification: Acid-producing bacteria, involved this step, convert the intermediates of
fermenting bacteria into acetic acid, hydrogen and carbon dioxide. These bacteria are
anaerobic and can grow under acidic conditions. To produce acetic acid, they need oxygen
and carbon. For this, they use dissolved O2 or bounded-oxygen. Hereby, the acid-producing
bacteria creates anaerobic condition which is essential for the methane producing
microorganisms. Also, they reduce the compounds with low molecular weights into alcohols,
organic acids, amino acids, carbon dioxide, hydrogen sulphide and traces of methane. From a
chemical point, this process is partially endergonic (i.e. only possible with energy input),
since bacteria alone are not capable of sustaining that type of reaction.
Methanogenesis: (Methane formation) Methane-producing bacteria, which were involved in
the third step, decompose compounds having low molecular weight. They utilize hydrogen,
carbon dioxide and acetic acid to form methane and carbon dioxide. Under natural
conditions, CH4 producing microorganisms occur to the extent that anaerobic conditions are
provided, e.g. under water (for example in marine sediments),and in marshes. They are
basically anaerobic and very sensitive to environmental changes, if any occurs. The
methanogenic bacteria belongs to the archaebacter genus, i.e. to a group of bacteria with
heterogeneous morphology and lot of common biochemical and molecular-biological
properties that distinguishes them from other bacterias. The main difference lies in the
makeup of the bacteria’s cell walls.
Symbiosis of bacteria:
Methane and acid-producing bacteria act in a symbiotical way. Acid producing bacteria
create an atmosphere with ideal parameters for methane producing bacteria (anaerobic
conditions, compounds with a low molecular weight). On the other hand, methane-producing
microorganisms use the intermediates of the acid producing bacteria. Without consuming
them, toxic conditions for the acid-producing microorganisms would develop. In real time
fermentation processes the metabolic actions of various bacteria acts in a design. No single
bacteria is able to produce fermentation products alone as it requires others too.
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2.2 CONVENTIONAL BIOGAS PLANT
A typical biogas system consists of the following components:
(1) Waste collection
(2) Anaerobic digester
(3) Effluent storage
(4) Gas handling
(5) Gas use
For the methanation process, carbon sources are required .
4H2+CO2→ CH4 + 2H2O
Since biogas consists of up to 50% carbon dioxide, it is commonly used for this purpose as a
carbon source. In most discussed processes the carbon dioxide of the biogas is separated from
the methane. In a Sabatier reactor, it can be methanized chemically under high temperatures
and pressures . Another option is the biological methanation, called methanogenesis. Its mild
conditions regarding temperature and pressure are advantageous, but these processes suffer
from the complexity involved in the use of a biological system, and insufficient process
knowledge
Biogas typically refers to a mixture of different gases produced by the breakdown of organic
matter in the absence of oxygen. Biogas can be produced from raw materials such as
agricultural wastes, manure municipal waste, plant material, sewage, green waste or food
waste. It is a renewable energy source and in many cases exerts a very small carbon footprint.
Biogas is primarily methane and carbon dioxide and many have small amounts of hydrogen
sulphide, moisture and siloxanes. The gases methane, hydrogen and carbon monoxide can be
combusted or oxidized with oxygen. This energy release allows biogas to be used as fuel; it
can be used for any heating purpose, such as cooking. It can also be used in a gas engine to
convert the energy in the gas into electricity and heat.
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Compound Formula %
Methane CH4 50-75
Carbon
dioxide
CO2 25-50
Nitrogen N2 0-10
Hydrogen H2 0-1
Hydrogen
sulphide
H2S 0-3
Oxygen O2 0-0.5
A biogas plant is the name often given to an anaerobic digester that treats farm wastes or
energy crops. It can be produced using anaerobic digesters. These plants can be fed with
energy crops such as maize, silage or biodegradable waste including sewage sludge and food
waste. During the process, the microorganisms transform biomass waste into biogas and
digestate.
Anaerobic digesters can be designed and engineered to operate using a number of different
configurations and can be categorized into batch vs. continuous process mode, mesophilic vs.
thermophilic temperature conditions, high vs. low portion of solids, and single stage vs.
multistage processes.
Batch or continuous
In a batch system, biomass is added to the reactor at the start of the process. The reactor is
then sealed for the duration of the process. In its simplest form batch processing needs
inoculation with already processed material to start the anaerobic digestion. As the batch
digestion is simple and requires less equipment and lower levels of digestion work, it is
typically a cheaper form of digestion.
In continuous digestion processes, organic matter is constantly added or added in stages to
the reactor. Here, the end products are constantly or periodically removed, resulting in
constant production of biogas. A single or multiple digesters in sequence may be used.
Examples of this form of anaerobic digestion include continuous stirred-tank reactors, up
flow anaerobic sludge blankets, expanded granular sludge beds and internal circulation
reactions.
Temperature
The two conventional operational temperature levels for anaerobic digesters determine the
species of methanogens in the digesters:
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Mesophilic digestion takes place optimally around 30 to 380C, or at ambient
temperature between 20 to 450C, where mesophiles are the primary microorganisms
present.
Thermophilic digestion takes place optimally around 49 to 570C, or at elevated
temperatures upto 700C, where thermophiles are the primary microorganisms present.
Solids content
In a typical scenario, three different operational parameters are associated with the solids
content of the feedstock to the dogesters:
High solids (dry-stackable substrate)
High solids (wet-pumpable substrate)
Low solids (wet- pumpable substrate)
Complexity
Digestion system can be configured with different levels of complexity. In a single stage
digestion system, all of the biological reactions occur within a single, sealed reactor or
holding tank. Using a single stage reduces construction costs, but results in less control of the
reactions occurring within the systems. Here the anaerobic reactions are contained within the
natural anaerobic sludge contained in the pool.
In a two stage digestion system, different digestion vessels are optimized to bring maximum
control over the bacterial communities living within the digesters. Acidogenic bacteria
produce organic acids and more quickly grow and reproduce than methanogenic bacteria.
Methanogenic bacteria require stable pH and temperature to optimize their performance
2.3 APPLICATIONS
Using anaerobic digestion technologies can help to reduce the emission of green house gases
in a number of key ways:
Replacement of fossil fuels
Reducing or eliminating the energy footprint of waste treatment plants
Reducing methane emission from landfills
Displacing industrially produced chemical fertilizers
Reducing vehicle movements
Reducing electrical grid transportation losses
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CHAPTER 3
REACTOR OVERVIEW
3.1 REACTOR DESIGN
Fig 3.1.1.Design of reactor
This biogas system consist of two tank with gas recirculation and each tank contain 40L
volume. The first tank is the one in which feed is admitted. Initially the feed is grinded using
a pulverizer and fed through an inlet pipe of 10 cm diameter. The first tank consist of a dome
at the top, at which biogas get collected after the process. There are also connections
provided for transferring the partially reacted feed to the second tank and for gas
recirculation. The second tank consist of a hand operated mixer at the top. The gas is
recirculated to first tank from second tank using a small pipe of 1.2 cm diameter. The outlet
pipe level designed as little below than the inlet to ensure there will not be any back mixing.
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Fig3.1.2.Reactor
Fig 3.1.3. Inlet
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Fig 3.1.4.Reactor tank 1
Fig 3.1.5. inner view of reactor tank 1
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Fig 3.1.6. The gas holder
Fig 3.1.7. The gas outlet
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Fig 3.1.8. The reactor tank 2
Fig 3.1.9.The inner view of reactor tank 2
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Fig 3.1.10. The hand operated agitator unit
3.2 WORKING
As described in the design, feed which is mainly kitchen waste is provided in the grinded
form using a grinding motor fitted outside the reactor, through the inlet to the first tank. The
grinding of the feed can enhance reaction rate by increasing the surface area exposed for
microbial action. Feed is always maintained to a particular level in tank and is in a semisolid
form and small amount of water is added in order to avoid foul smell. In the first tank mainly
acidogenesis takes place due to the presence of archeal population that is formed from the
kitchen waste itself. Along with CO2 some amount of methane is also is produced. When the
feed reaches a particular level in the tank the slurry moves to second tank through the
connection provided as in figure. The connection is made in such a way that there will not be
any back-mixing of the feed or slurry. And in the second tank methanogenisis is the main
process taking place. To accelerate this process an initial feed of cow dung is provided in the
second tank which acts as an inoculum. Most of the co2 is converted to methane in this tank
and this tank has also provision for the removal of feed or addition of cow dung as per the
requirement. Also a hand operated motor is provided at the top of the second tank which
provides adequate mixing of the slurry. Now most of the gases along with co2 and methane
produced in the second tank is re -circulated back to the first tank through a connection pipe.
This occurs naturally when the pressure reaches a particular level. This pipe is connected
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such that it reaches the bottom of the first tank and the gas is bubbled into the slurry. This in
turn help in the overall mixing of the feed and increases the rate of the reaction. The
propionic acid production and the anaerobic condition in turn enhances the rate of reaction
which would reduce the initial time taken for the production of gas. The gas formed is
collected in a dome and can be taken out through the outlet provided.
3.3 MICROBIAL COMMUNITY ANALYSIS
The reactor was initially fed with an inoculum enriched in methanogenic bacteria before
adding the sludge. The quantitative changes of the bacterial and archaeal population in the
raw sludge and in the inoculum enriched in methanogens. A significant difference was found
for the bacterial and archeal populations between raw sludge and inoculum enriched
methanogens. The bacterial population dominated the Archaea in raw sludge. In contrast, the
archaeal population was drastically increased in the inoculum enriched in methanogens to
4.64 _ 1013 rRNA gene copies mL_1 while the bacterial population dropped to 1.08 _ 109
rRNA gene copies mL_1. The increase in the archaeal population after enrichment shows that
the culture was dominated with methanogens which were ready to use the supplemented
gases as a substrate for CH4 production. Thus, CH4 evolution from the supplemented CO2
was from the assimilation of CO2 by the enrichment of WAS under anaerobic conditions
.The archaeal population was dominated by members of the Methanobacteriaceae,
Methanospirillaceae and Methanosarcinaceae family.
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CHAPTER 4
DISCUSSION
Biological methane production from CO2 sequestration by enriched methanogens from waste
activated sludge (WAS) was demonstrated in this study. This approach appears to be
reasonable to mitigate global warming. The WAS that was dominated by an active bacterial
community was changed to an active archaeal community after the enrichment for
methanogens Methane was not produced without CO2 supplementation to the enriched
methanogens due to the lack of available carbon for the archaeal community, and the carbon
in the methane was shown to come from utilization of the CO2 based on the use of 13CO2.
Therefore, application of this research can contribute to improving the environment by
reducing the greenhouse gas CO2.
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CHAPTER 5
CONCLUSION
Biogas systems are those that take organic material (feedstock) into an air-tight tank, where
bacteria break down the material and release biogas, a mixture of mainly methane with some
carbon dioxide. The biogas can be burned as a fuel, for cooking or other purposes, and the
solid residue can be used as organic compost. Through this compact system, it has been
demonstrated that by using feedstock having high calorific and nutritive value to microbes,
the efficiency of methane generation can be increased by several orders of magnitude. It is an
extremely user friendly system.
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