FEDERAL UNIVERSITY OF TECHNOLOGY
P.M.B 1560 OWERRI
A
SEMINAR REPORT
WRITTEN BY
NNADI AZUKA WALTER
20091640803
SUPERVISED BY
ENGR OKWARA
SUBMITTED TO
THE DEPARTMENT OF CHEMICAL ENGINEERING
FEDERAL UNIVERSITY OF TECHNOLOGY, OWERRI
IN PARTIAL FUFILLMENT OF THE REQUIREMENT FOR THE AWARD OF
BACHELOR OF ENGINEERING (B.ENG) IN CHEMICAL ENGINEERING
JULY, 2014
SUMMARY
Due to energy challenges in Nigeria today and finite nature of our petroleum
resources, I decided to research on other energy substitutes to the available
ones in the country and that was the reason for my research on the
production and purification of biogas. Biogas is a renewable energy source
like solar energy it is a combination of different elements and compounds
like methane, carbon dioxide, hydrogen sulphide and maybe hydrogen and
nitrogen or siloxanes but these constituents are referred to as impurities.
Biogas is produced from landfills (biodegradable substances) like manure,
crops, wastes etc. The production of biogas occurs when these landfill
subtances are enclosed in an Anaerobic digestion under the influence of
Anaerobic bacterias, the interaction of these Anaerobic bacteria on the
landfill substances inside the Anaerobic digester is what causes the
production of biogas, this fresh biogas produced is referred to as impure
biogas because of the inclusion of trace elements like hydrogen sulphide,
carbon dioxide, siloxanes etc. This was the reason for the biogas upgrading
or purification techniques stated in this report. Although there are many
methods to be used for the purification, my interest was based on the use of
water washing or rather water scrubbing method, due to the availability of
water for scrubbing and economical approach of this method. In this method
water is used to absorb these traces of impurities in the biogas using a
packed bed in a counter current approach.
1.0 INTRODUCTION
WHAT IS BIOGAS?
Biogas typically refers to a gas produced by the breakdown of organic
matter in the absence of oxygen. It is a renewable energy source, like solar
and wind energy. Furthermore, biogas can be produced from regionally
available raw materials such as recycled waste and is environmentally
friendly. Biogas is produced by anaerobic digestion with anaerobic bacteria
or fermentation of biodegradable materials such as manure, sewage,
municipal waste, green waste, plant material, and crops.
1.1 PRIMARY COMPOSITION OF BIOGAS
Table 1. Typical Composition of Biogas
Compound
Molecular 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
Biogas is primarily methane (CH4) and carbon dioxide (CO2) and may have
small amounts of hydrogen sulphide (H2S), moisture and siloxanes. The
gases methane, hydrogen, and carbon monoxide (CO) can be combusted or
oxidized with oxygen. This energy release allows biogas to be used as a 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.
Biogas can be compressed, the same way natural gas is compressed to CNG,
and used to power motor vehicles. In the UK, for example, biogas is
estimated to have the potential to replace around 17% of vehicle fuel. It
qualifies for renewable energy subsidies in some parts of the world. Biogas
can be cleaned and upgraded to natural gas standards when it becomes bio
methane.
1.2 BIOGAS PRODUCTION
Biogas is practically produced as landfill gas (LFG) or digested gas. 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
wastes including sewage sludge and food waste. During the process, an airtight tank transforms biomass waste into methane producing renewable
energy that can be used for heating, electricity, and many other operations
that use an internal combustion engine, such as GE Jenbacher gas engines.
There are two key processes: mesophilic and thermophilic digestion. In
experimental work at University of Alaska Fairbanks, a 1000-litre digester
using psychrophiles harvested from "mud from a frozen lake in Alaska" has
produced 200–300 liters of methane per day, about 20%–30% of the output
from digesters in warmer climates. Landfill gas is produced by wet organic
waste decomposing under anaerobic conditions in a landfill.
The waste is covered and mechanically compressed by the weight of the
material that is deposited above. This material prevents oxygen exposure
thus allowing anaerobic microbes to thrive. This gas builds up and is slowly
released into the atmosphere if the site has not been engineered to capture
the gas. Landfill gas is hazardous for three key reasons: It becomes
explosive when it escapes from the landfill and mixes with oxygen. The
lower explosive limit is 5% methane and the upper is 15% methane. The
methane in biogas is 20 times more potent a greenhouse gas than carbon
dioxide. Therefore, uncontained landfill gas, which escapes into the
atmosphere may significantly contribute to the effects of global warming. In
addition, volatile organic compounds (VOCs) in landfill gas contribute to the
formation of photochemical smog.
1.3 ANAEROBIC DIGESTION
Figure 1. Anaerobic Digester Plant
Anaerobic digestion is a collection of processes by which microorganisms
break down biodegradable material in the absence of oxygen. The process is
used for industrial or domestic purposes to manage waste and/or to produce
fuels. Much of the fermentation used industrially to produce food and drink
products, as well as home fermentation, uses anaerobic digestion. Silage is
produced by anaerobic digestion. Anaerobic digestion also occurs naturally
in some soils and in lake and oceanic basin sediments, where it is usually
referred to as "anaerobic activity". This is the source of marsh gas methane
as discovered by Volta in 1776. The digestion process begins with bacterial
hydrolysis of the input materials. Insoluble organic polymers, such as
carbohydrates, are broken down to soluble derivatives that become available
for other bacteria. Acidogenic bacteria then convert the sugars and amino
acids into carbon dioxide, hydrogen, ammonia, and organic acids. These
bacteria convert these resulting organic acids into acetic acid, along with
additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens
convert these products to methane and carbon dioxide. The methanogenic
archaea populations play an indispensable role in anaerobic wastewater
treatments. It is used as part of the process to treat biodegradable waste and
sewage sludge. As part of an integrated waste management system,
anaerobic digestion reduces the emission of landfill gas into the atmosphere.
Anaerobic digesters can also be fed with purpose-grown energy crops, such
as maize. Anaerobic digestion is widely used as a source of renewable
energy. The process produces a biogas, consisting of methane, carbon
dioxide and traces of other ‘contaminant’ gases. This biogas can be used
directly as fuel, in combined heat and power gas engines or upgraded to
natural gas-quality biomethane. The nutrient-rich digestate also produced
can be used as fertilizer.
1.4 ANAEROBIC DIGESTION FLOW CHART AND STEPS OF
REACTION
Figure 1.1 Steps of reaction in anaerobic digestion
Anaerobic digestion is a multistep biological and chemical process that is
beneficial in not only waste management but also energy creation. There are
four fundamental steps of anaerobic digestion that include hydrolysis,
acidogenesis, acetogenesis, and methanogenesis. Throughout this entire
process, large organic polymers that make up Biomass are broken down into
smaller molecules by chemicals and microorganisms. Upon completion of
the anaerobic digestion process, the Biomass is converted into Biogas,
namely carbon dioxide and methane, as well as digestate and wastewater.
Figure 1.2 Anaerobic Digestion Flow Chart
Dig. 1.3 Anaerobic Digester—Fundamental Steps
I. HYDROLYSIS
In general, hydrolysis is a chemical reaction in which the breakdown of
water occurs to form H+ cations and OH- anions. Hydrolysis is often used to
break down larger polymers, often in the presence of an acidic catalyst. In
anaerobic digestion, hydrolysis is the essential first step, as Biomass is
normally comprised of very large organic polymers, which are otherwise
unusable. Through hydrolysis, these large polymers, namely proteins, fats
and carbohydrates, are broken down into smaller molecules such as amino
acids, fatty acids, and simple sugars. While some of the products of
hydrolysis, including hydrogen and acetate, may be used by methanogens
later in the anaerobic digestion process, the majority of the molecules, which
are still relatively large, must be further broken down in the process of
acidogenesis so that they may be used to create methane.
II. ACIDOGENESIS
Acidogenesis is the next step of anaerobic digestion in which acidogenic
microorganisms further break down the Biomass products after hydrolysis.
These fermentative bacteria produce an acidic environment in the digestive
tank while creating ammonia, H2, CO2, H2S, shorter volatile fatty acids,
carbonic acids, alcohols, as well as trace amounts of other byproducts. While
acidogenic bacteria further breaks down the organic matter, it is still too
large and unusable for the ultimate goal of methane production, so the
biomass must next undergo the process of acetogenesis.
III. ACETOGENESIS
In general, acetogenesis is the creation of acetate, a derivative of acetic acid,
from carbon and energy sources by acetogens. These microorganisms
catabolize many of the products created in acidogenesis into acetic acid, CO2
and H2. Acetogens break down the Biomass to a point to which
Methanogens can utilize much of the remaining material to create Methane
as a Biofuel.
VI. METHANOGENESIS
Methanogenesis constitutes the final stage of anaerobic digestion in which
methanogens create methane from the final products of acetogenesis as well
as from some of the intermediate products from hydrolysis and acidogenesis.
There are two general pathways involving the use of acetic acid and carbon
dioxide, the two main products of the first three steps of anaerobic digestion,
to create methane in methanogenesis:
CO2 + 4 H2 → CH4 + 2H2O
CH3COOH → CH4 + CO2
While CO2 can be converted into methane and water through the reaction,
the main mechanism to create methane in methanogenesis is the path
involving acetic acid. This path creates methane and CO2, the two main
products of anaerobic digestion.
1.4 CONFIGURATION OF ANAEROBIC DIGESTER
Anaerobic digesters can be designed and engineered to operate using a
number of different process configurations:
I. BATCH AND CONTINUOUS
Anaerobic digestion can be performed as a batch process or a continuous
process. 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. In a typical scenario, biogas
production will be formed with a normal distribution pattern over time.
Operators can use this fact to determine when they believe the process of
digestion of the organic matter has completed. There can be severe odour
issues if a batch reactor is opened and emptied before the process is well
completed. A more advanced type of batch approach has limited the odour
issues by integrating anaerobic digestion with in-vessel composting. In this
approach inoculation takes place through the use of recirculated degasified
percolate. After anaerobic digestion has completed, the biomass is kept in
the reactor which is then used for in-vessel composting before it is opened.
As the batch digestion is simple and requires less equipment and lower
levels of design work, it is typically a cheaper form of digestion. Using more
than one batch reactor at a plant can ensure constant production of biogas. In
continuous digestion processes, organic matter is constantly added
(continuous complete mixed) or added in stages to the reactor (continuous
plug flow; first in – first out). 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, upflow
anaerobic sludge blankets, expanded granular sludge beds and internal
circulation reactors.
1.5 BIOGAS CYCLE
In a generic biogas production plant, biogas is essentially produced from
carbon that is fixed by photosynthetic organisms, which capture solar
energy, using water, atmospheric CO2 and soil nutrients. These crops are
harvested for use in human and animal foods and in industrial processing.
Residue from crop production and processing, manure from animal
production, and wastewater from industrial and municipal sources all
contain waste organic matter, which can be converted to biogas. In addition,
energy crops can be grown that are used directly as a biogas feedstock. The
biomass, waste, or wastewater feedstocks are conveyed into the anaerobic
digester where a consortium of natural bacteria feed on the organic matter
producing simpler intermediate compounds that are eventually converted to
mineralized nutrients and biogas. The biogas is insoluble and separates into
the gas phase and is removed from the digester through piping that conveys
it for storage or final use. The remaining liquids contain plant nutrients,
which are best used by returning them to crop production. The biogas is
burned for energy production converting the methane into the same amount
of CO2 that was fixed during photosynthesis.
Figure 1.4 Biogas cycle
2.0 BIOGAS PURIFICATION OR UPGRADING
Raw biogas produced in anaerobic digestion is roughly 60% methane and
29% carbon dioxide with trace elements of hydrogen sulphide, nitrogen and
hydrogen. It is not high quality enough to be used as fuel gas for machinery
because the corrosive nature of hydrogen sulphide alone is enough to destroy
the internals of a plant, the solution to the low quality of biogas is to use
biogas upgrading or purification process whereby contaminants in the raw
biogas stream are absorbed or scrubbed, leaving more methane per unit
volume of gas, there are some methods used for this upgrading. The various
methods used for the purification of biogas, although follow different
patterns or approach but have the same principle and as well does one thing
which is reduction of carbon dioxide and trace elements of hydrogen
sulphide and others per unit volume of the biogas, thereby increasing
percentage of methane per unit volume of the gas
2.1 WATER SCRUBBING METHOD OF BIOGAS PURIFICATION
Figure 2. Packed Tower for Scrubbing of Impure Biogas
The most prevalent method is water scrubbing where high pressure gas
flows into a column where the carbon dioxide and other trace elements are
scrubbed by cascading water running counter-flow to the gas. This
arrangement could deliver 98% methane with manufacturers guaranteeing
maximum 2% methane loss in the system. It takes roughly between 3% and
6% of the total energy output in gas to run a biogas upgrading system.
Out of different methods used for the purification of biogas water scrubbing
method is known to be the cheapest and easiest method for the purification
of impure Biogas in West Africa especially in Nigeria, due to availability of
raw materials like water etc
2.2 HOW WATER SCRUBBING METHOD IS USED
Normally fresh biogas from the anaerobic digester contains traces of
impurities like carbon dioxide, hydrogen sulphide, and very small amount of
nitrogen or hydrogen. Here our aim is to eliminate these impurities and
increase the amount of methane per volume of the total biogas and to do this
we employ water scrubbing method of purification. The inpure biogas is
channeled into the packed tower from below (being gas) the tower where on
moving up the tower get scrubbed by water which is channeled into the
tower from the top which (being liquid) reacts with some of the carbon
dioxide and hydrogen sulphide present in the biogas thereby increasing the
amount of biogas per unit volume in the biogas, the scrubbed biogas leaves
from the top as a clean gas while the water leaves from below as the Drain
3.0 BENEFITS OF BIOGAS
When biogas is used, many advantages arise. In North America, use of
biogas would generate enough electricity to meet up to 3% of the continent's
electricity expenditure. In addition, biogas could potentially help reduce
global climate change. Normally, manure that is left to decompose releases
two main gases that cause global climate change: nitrous oxide and methane.
Nitrous oxide (N2O) warms the atmosphere 310 times more than carbon
dioxide and methane 21 times more than carbon dioxide. By converting cow
manure into methane biogas via anaerobic digestion, the millions of cattle in
the United States would be able to produce 100 billion kilowatt hours of
electricity, enough to power millions of homes across the United States. In
fact, one cow can produce enough manure in one day to generate 3 kilowatt
hours of electricity; only 2.4 kilowatt hours of electricity are needed to
power a single 100-watt light bulb for one day. Furthermore, by converting
cattle manure into methane biogas instead of letting it decompose, global
warming gases could be reduced by 99 million metric tons or 4%. In Nepal
biogas is being used as a reliable source of rural energy, says Bikash Haddi
of Biogas promotion center.
3.1 APPLICATION OF BIOGAS
Biogas can be used for electricity production on sewage works, in a CHP gas
engine, where the waste heat from the engine is conveniently used for
heating the digester; cooking; space heating; water heating; and process
heating. If compressed, it can replace compressed natural gas for use in
vehicles, where it can fuel an internal combustion engine or fuel cells and is
a much more effective displacer of carbon dioxide than the normal use in onsite CHP plants. Methane in biogas can be concentrated via a biogas
upgrader to the same standards as fossil natural gas, which itself has had to
go through a cleaning process, and becomes biomethane. If the local gas
network allows, the producer of the biogas may use their distribution
networks. Gas must be very clean to reach pipeline quality and must be of
the correct composition for the distribution network to accept. Carbon
dioxide, water, hydrogen sulfide, and particulates must be removed if
present.
3.2 BIOGAS IN TRANSPORT
If concentrated and compressed, it can be used in vehicle transportation.
Compressed biogas is becoming widely used in Sweden, Switzerland, and
Germany. A biogas-powered train, named Biogaståget Amanda, has been in
service in Sweden since 2005. Biogas powers automobiles. In 1974, a
British documentary film titled Sweet as a Nut detailed the biogas
production process from pig manure and showed how it fueled a customadapted combustion engine. In 2007, an estimated 12,000 vehicles were
being fueled with upgraded biogas worldwide, mostly in Europe
CONCLUSION AND RECOMMENDATION
Biogas as a renewable energy source fits in very well as an alternative
energy supply due to its capability to combust and generate useful heat for
cooking, electricity generation and vehicle fuel. In the developing countries
like Nigeria biogas production through anaerobic procedures can be very
economical and as well useful because of the availability of cow dung
produced by millions of cows in Nigeria. These dung are left out to decay
contributing to global warming, this uneconomical approach of leaving the
dung to decay not only pose environmental risk to our community due to the
generation of gases like Nitrous Oxide and Carbon dioxide which are green
house gases but also referred to as a waste of resource. It is essential that
these dung are properly used for biogas production instead of leaving over to
degenerate into these non-environmental friendly gases and as well generate
competitive amount of energy.
RECOMMENDATION
Biogas generation is very essential for a developing country like ours
because of the energy challenges faced generally in Nigeria by virtually
every sector both private and public sectors, including personal individuals
who make use of the power generated by the power sectors in Nigeria. This
is because biogas as an alternative energy supply can be used for almost all
energy requirement like cooking, electricity generation and as well as
vehicle fuel and is economical because of the availability of the needed raw
materials for the production process.