Renewable Energy

Lecture No.3
Types of biogas plants, constructional details, biogas production and
utilization-problems

3.1 Biogas
Most organic materials undergo a natural anaerobic digestion in the presence of moisture and
absence of oxygen and produce biogas.The biogas so obtained is a mixture of methane (CH4):
55-65% and Carbon dioxide (CO2) : 30-40%. The biogas contains traces of H2, H2S and N2. The
calorific value of biogas ranges from 5000 to 5500 Kcal/Kg (18.8 to 26.4 MJ /m3).
Digestion is biological process that occurs in the absence of oxygen and in the presence of
anaerobic organisms at temperatures (35-70ºC) and atmospheric pressure. The container in
which, this process takes place is known as digester.
3.2 Types of biogas plants
Biogas plants basically are two types
3.2.1 Floating dome type
o The floating-drum plant with a cylindrical digester (KVIC model)
3.2.2 Fixed dome type
o The fixed-dome plant with a brick reinforced, moulded dome (Janata model)
o The fixed-dome plant with a hemisphere digester (Deenbandhu model)
3.2.1 Floating dome type
Floating-drum plants consist of an underground digester and a moving gas-holder. The gas-
holder floats either directly on the fermentation slurry or in a water jacket of its own. The gas is
collected in the gas drum, which rises or moves down, according to the amount of gas stored.
The gas drum is prevented from tilting by a guiding frame. If the drum floats in a water jacket, it
cannot get stuck, even in substrate with high solid content.

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Fig 3.1 Floating dome type plant

Drum:-In the past, floating-drum plants were mainly built in India. A floating-drum plant
consists of a cylindrical or dome-shaped digester and a moving, floating gas-holder, or drum.
The gas-holder floats either directly in the fermenting slurry or in a separate water jacket. The
drum in which the biogascollects has an internal and/or external guide frame that provides
stability and keeps the drum upright. If biogas is produced, the drum moves up, if gas is
consumed, the gas-holder sinks back.
Size:-Floating-drum plants are used chiefly for digesting animal and human feces on a
continuous feed mode of operation, i.e. with daily input. They are used most frequently by small-
to middle-sized farms (digester size: 5-15m3) or in institutions and larger agro-industrial estates
(digester size: 20-100m3).
3.2.1.1 KVIC type biogas plant
This mainly consists of a digester or pit for fermentation and a floating drum for the
collection of gas. Digester is 3.5-6.5 m in depth and 1.2 to 1.6 m in diameter. There is a partition
wall in the center, which divides the digester vertically and submerges in the slurry when it is
full. The digester is connected to the inlet and outlet by two pipes. Through the inlet, the dung is
mixed with water (4:5) and loaded into the digester. The fermented material will flow out
through outlet pipe. The outlet is generally connected to a compost pit. The gas generation takes
place slowly and in two stages. In the first stage, the complex, organic substances contained in
the waste are acted upon by a certain kind of bacteria, called acid formers and broken up into

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small-chain simple acids. In the second stage, these acids are acted upon by another kind of
bacteria, called methane formers and produce methane and carbon dioxide.

Fig 3.2 KVIC model biogas plant

Gas holder :-The gas holder is a drum constructed of mild steel sheets. This is cylindrical in
shape with concave. The top is supported radically with angular iron. The holder fits into the
digester like a stopper. It sinks into the slurry due to its own weight and rests upon the ring
constructed for this purpose. When gas is generated the holder rises and floats freely on the
surface of slurry. A central guide pipe is provided to prevent the holder from tilting. The holder
also acts as a seal for the gas. The gas pressure varies between 7 and 9 cm of water column.
Under shallow water table conditions, the adopted diameter of digester is more and depth is
reduced. The cost of drum is about 40% of total cost of plant. It requires periodical maintenance.

The unit cost of KVIC model with a capacity of 2 m3/day costs approximately Rs.14, 000.
3.2.1.2 Advantages and Disadvantages of floating dome plants

Advantages Disadvantages
 Simple, easily understood operation  High material costs of the steel drum
 Volume of stored gas is directly visible  Susceptibility of steel parts to corrosion
 The gas pressure is constant, floating drum plants have a shorter life
determined by the weight of the gas span than fixed-dome plants
holder
 The construction is relatively easy,  Regular maintenance costs for the
construction mistakes do not lead to painting of the drum

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major problems in operation and gas
yield.

3.2.2 Fixed-dome type plants

A fixed-dome plant consists of a digester with a fixed, non-movable gas holder, which sits on
top of the digester. When gas production starts, the slurry is displaced into the compensation
tank. Gas pressure increases with the volume of gas stored and the height difference between the
slurry level in the digester and the slurry level in the compensation tank.

Fig 3.3 Fixed-dome type plants

1 Mixing tank with inlet pipe and sand trap. 6 Entry hatch, with gastight seal
2 Digester 7 Accumulation of thick sludge.
3 Compensation and removal tank 8 Outlet pipe
4 Gasholder 9 Reference level
5 Gaspipe 10 Supernatant scum, broken up by varying level

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Fig 3.4 Basic function of a fixed-dome biogas plant

1. Mixing pit, 2. Digester, 3. Gasholder, 4. Displacement pit, 5. Gas pipe

a) Function - A fixed-dome plant comprises of a closed, dome-shaped digester with an

immovable, rigid gas-holder and a displacement pit, also named 'compensation tank'. The gas is
stored in the upper part of the digester. When gas production commences, the slurry is displaced
into the compensating tank. Gas pressure increases with the volume of gas stored, i.e. with the
height difference between the two slurry levels. If there is little gas in the gas-holder, the gas
pressure is low.
b) Digester - The digesters of fixed-dome plants are usually masonry structures, structures of
cement and ferro-cementexist. Main parameters for the choice of material are:
o Technical suitability (stability, gas- and liquid tightness)
o Cost-effectiveness
o Availability in the region and transport costs
o Availability of local skills for working with the particular building material.
Fixed dome plants produce just as much gas as floating-drum plants, if they are gas-tight.
However, utilization of the gas is less effective as the gas pressure fluctuates substantially.
Burners and other simple appliances cannot be set in an optimal way. If the gas is required at
constant pressure (e.g., for engines), a gas pressure regulator or a floating gas-holder is
necessary.

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c) Gas Holder - The top part of a fixed-dome plant (the gas space) must be gas-tight. Concrete,
masonry and cement rendering are not gas-tight. The gas space must therefore be painted with a
gas-tight layer (e.g. 'Water-proofer', Latex or synthetic paints). A possibility to reduce the risk of
cracking of the gas-holder consists in the construction of a weak-ring in the masonry of the
digester. This "ring" is a flexible joint between the lower (water-proof) and the upper (gas-proof)
part of the hemispherical structure. It prevents cracks that develop due to the hydrostatic pressure
in the lower parts to move into the upper parts of the gas-holder.

3.2.2.1 Advantages and Disadvantages of fixed dome plants

Advantages Disadvantages

 Low initial costs and long useful life-  Masonry gas-holders require special
span sealants and high technical skills for
 No moving or rusting parts involved gas-tight construction
 Basic design is compact, saves space  Gas leaks occur quite frequently;
and is well insulated fluctuating gas pressure complicates
 Construction creates local gas utilization
employment.  Amount of gas produced is not
 The underground construction saves immediately visible, plant operation
space and protects the digester from not readily understandable
temperature changes  Fixed dome plants need exact planning
of levels; excavation can be difficult
and expensive in bedrock.

3.2.3 Types of Fixed Dome Plants

3.2.3.1 Janata model
The design of this plant is of Chinese origin but it has been introduced under the name
“Janata biogas plant” by Gobar Gas Research Station, Ajitmal in view of its reduced cost. This is
a plant where no steel is used, there is no moving part in it and maintenance cost is low. The
plant can be constructed by village mason taking some pre-explained precautions and using all
the indigenously available building materials. Good quality of bricks and cement should be used
to avoid the afterward structural problems like cracking of the dome and leakage of gas.

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Substrates other than cattle dung such as municipal waste and plant residues can also be used in
janata type plants.
The plant consists of an underground well sort of digester made of bricks and cement having
a dome shaped roof which remains below the ground level is shown in Fig 3.5.At almost middle
of the digester, there are two rectangular openings facing each other and coming up to a little
above the ground level, act as an inlet and outlet of the plant. Dome shaped roof is fitted with a
pipe at its top which is the gas outlet of the plant. The principle of gas production is same as that
of KVIC model. The biogas is collected in the restricted space of the fixed dome, hence the
pressure of gas is much higher, which is around 90 cm of water column.

Fig 3.5Janta model biogas plant

3.2.3.2 Deenbandhu Model

Deenbandhu model biogas plant was developed by AFPRO (Action for Food Production,
New Delhi) in 1984. The world Deenbandhu is meant as the friend of the poor. This plant is
designed on the principle that the surface area of biogas plants is reduced (minimized) to reduce
their installation cost without sacrificing the efficiency of the plant. The design consists of
segments of two spheres of different diameters, joined at their bases. The structure thus formed
act as the digester as fermentation chamber as well as the gas storage chamber. The higher
compressive strength of the brick masonry and concrete makes it preferable to go in for a

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structure which could always be kept under compression. A spherical structure loaded from the
convex side will be under compression and therefore, the internal load will not have any residual
effect on the structure.
The digester is connected with the inlet pipe and the outlet tank. The upper part above the
normal slurry level of the outlet tank is designed to accommodate the slurry to be displaced out
of the digester with the generation and accumulation of biogas and is called outlet displacement
chamber. The size of these plants is recommended up to 6 m3 per day. The different components
of Deenbandhu model biogas plant are show in Fig. 3.6.

Fig 3.6 Deenbandhu model biogas plant

3.3 Comparison among KVIC, Janta and Deenbandhu biogas plants

Comparison among the above mentioned biogas plants is explained in Table 3.1
Table 3.1 Comparison between KVIC, Janata and Deenbandhu biogas plants
KVIC Janata Deenbandhu

The digester of this plant is a Digester of this plant is a Digester is made of segments
deep well shaped masonry shallow well shaped masonry of two spheres: one for the
structure. In plants of above structure, No partition wall is bottom and other for the top
3m3capacity a partition wall provided
is provided in middle of the
digester

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Gas holder is generally made Gas holder is an integral part of The structure described
of mild steel. It is inverted into the masonry structure of the above includes digester and
the digester and goes up and plant. Slurry from the gas the gas storage chamber. Gas
down with formation and storage portion is displaced out is stored in the same way as
utilization of gas with the formation of gas and in the case of Janata plants
comes back when it is used

The gas is available at a Gas pressure varies from 0 to Gas pressure varies from 0 to
constant pressure of about 10 90 cm of water column 75 cm of water column
cm of water column

Inlet and outlet connections Inlet and outlet tanks are large Inlet connection is through
are provided through A.C masonry structures designed to A.C pipe. Outlet tank is a
Pipes store the slurry displaced out of large masonry tank designed
the digester with the formation to store slurry displaced out
of gas of the digester with the
formation of gas

Gas storage capacity of the It is the combined volume of It is the volume of outlet
plant is governed by the inlet and outlet displacement displacement chamber and is
volume of gas holder and is chambers and is 50% of gas 33% of gas produced per day
50% of gas produced per day produced per day

The floating mild steel gas There is no moving part and There is no moving part and
holder needs regular care and hence no recurring expenditure. hence no recurring
maintenance to prevent the gas It also has long working life expenditure. It also has a
holder from getting worn out long working life
because of corrosion. It also
has short life span.

Installation cost is very high It is cheaper than the KVIC It is much cheaper than
type plants KVIC and Janata type plants

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Digester can be constructed Entire plant can be built by a Entire plant can be built by a
locally but the gasholder needs trained mason using locally trained mason using locally
sophisticated workshop available materials available material
facilities

3.4 Design procedure for a biogas digester

A biogas plant may be designed based on the energy requirement or on the feed material
available. Generally standard designs are available for the common biogas plant designs.
However, it is advisable to understand the basic procedure.
The following example shows the design for cooking requirement for a medium family.

= 6 X 0.30

Gas requirement Gas required for cooking for 6 people (@ 0.30 m3 / day /
person)
= 1.8 m3/day
= ~ 2 m3/day
Cow dung requirement
1 kg of wet cow dung yields = 0.035 m3
= = 2.0/0.035
= 57 kg
= 60 kg
Average cow dung yield from 1 cattle = 12 kg (wet)
So, number of animals required = 60/12
= 5 animals
Digester dimensions
Amount of slurry fed (1:1 ratio of slurry: water) = 60+60 litre/day
= 120 litres/day
(Sp. Gravity of slurry is assumed to be 1.0) = 0.12 m3/day
The plant can also be designed as per the dung availability. If 5
cows are available, Then,

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Dung production per cow = 12 kg/day

Total dung production = 12 * 5 = 60 kg

Assuming Sp. Gravity as 1, volume = 60 litre
Daily slurry volume by mixing with water = 120 litres/day
Retention time = 35 days
Volume of the digester required = 0.12 * 35
Using a ratio of 1 to 1.1 for height to diameter, = 4.2 m3
π D2 * (1.1D) / 4 = 4.2 m3

Diameter D can be found from the above relation

Design of gas holder
Gas produced daily = 2 m3
It is assumed that the gas produced during the night is used up for cooking the breakfast and
lunch in the morning. The gas produced in the day time after the morning cooking is used for
cooking dinner in the evening. So the storage requirement is only 50% of daily gas production.
Gas to be stored = 1 m3
Diameter of he gas holder will be slightly less than the diameter of he digester so as to ensure its
free movement.
π D2 * H / 4 = 1 m3
We can find the height of gas holder, H from the above relation ship
3.5 Utilization of biogas
Biogas generated from anaerobic digestion processes is a clean and environmentally
friendly renewable fuel. But it is important to clean, or upgrade, biogas before using it to
increase its heating value and to make it useable in some gas appliances such as engines and
boilers. Biogas can potentially be used in many types of equipment, including:
 Internal Combustion (Piston) Engine – Electrical Power Generation, Shaft Power
 Gas Turbine Engine (Large) – Electrical Power Generation, Shaft Power
 Microturbine Engine (Small) – Electrical Power Generation
 Stirling Heat Engine – Electrical Power Generation
 Boiler (Steam) Systems

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 Hot Water Systems
 Process Heaters (Furnaces)
 Space or Air Heaters
 Gas Fired Chiller - Refrigeration
 Absorption Chiller - Refrigeration
 Combined Heat and Power (CHP) - Large and Small Scale – Electrical Power and Heat
 Fuel Cells – Electrical Power, Some Heat
There are a variety of end uses for biogas. Except for the simplest thermal uses such as odor
flaring or some types of heating, biogas needs to be cleaned or processed prior to use. With
appropriate cleaning or upgrade, biogas can be used in all applications that were developed for
natural gas. The three basic end uses for biogas are:
a. Production of heat or steam
The most straightforward use of biogas is for thermal (heat) energy. In areas where fuels
are scarce, small biogas systems can provide the heat energy for basic cooking and water
heating. Gas lighting systems can also use biogas for illumination. Conventional gas burners are
easily adjusted for biogas by simply changing the air-to-gas ratio. The demand for biogas quality
in gas burners is low, only requiring a gas pressure of 8 to 25 mbar and maintaining H2S levels
to below 100 ppm to achieve a dew point of 150 degrees C.
b. Electricity Generation or Combined Heat and Power (CHP)
Combined heat and power systems use both the power producing ability of a fuel and the
inevitable waste heat. Some CHP systems produce primarily heat, and electrical power is
secondary (bottoming cycle). Other CHP systems produce primarily electrical power and the
waste heat is used to heat process water (topping cycle). In either case, the overall (combined)
efficiency of the power and heat produced and used gives a much higher efficiency than using
the fuel (biogas) to produce only power or heat. Other than high initial investments, gas turbines
(micro-turbines, 25-100 kW; large turbines, >100 kW) with comparable efficiencies to spark-
ignition engines and low maintenance can be used for production of both heat and power.
However, internal combustion engines are most cmmonly used in CHP applications. The use of
biogas in these systems requires removal of both H2S (to below 100 ppm) and water vapor.
c. Vehicle fuel

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Gasoline vehicles can use biogas as a fuel provided the biogas is upgraded to natural gas
quality in vehicles that have been adjusted to using natural gas. Most vehicles in this category
have been retro-fitted with a gas tank and a gas supply system in addition to the normal petrol
fuel system. However, dedicated vehicles (using only biogas) are more efficient than these retro-
fits.

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Lecture No.4

Agricultural wastes, principles of combustion, pyrolysis and gasification

4.1 Biomass
Plant matter created by the process of photosynthesis is called biomass (or) all organic
materials such as plants, trees and crops are potential sources of energy and are collectively
called biomass.The term biomass is also generally understood to include human waste, and
organic fractions of sewage sludge, industrial effluents and household wastes. The biomass
sources are highly dispersed and bulky and contain large amounts of water (50 to 90%). Thus, it
is not economical to transport them over long distances, and conversion into usable energy must
takes place close to source, which is limited to particular regions.

Fig. 4.1 Schematic diagram of utilization of biomass

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4.1.2 Biomass Conversion
Biomass can either be utilized directly as a fuel, or can be converted into liquid or
gaseous fuels, which can also be as feedstock for industries. Most biomass in dry state can be
burned directly to produce heat, steam or electricity. On the other hand biological conversion
technologies utilize natural anaerobic decay processes to produce high quality fuels from
biomass. Various possible conversion technologies for getting different products from biomass is
broadly classified into three groupsviz. (i) thermo-chemical conversion, (ii) bio-chemical
conversion and (iii) oil extraction.
Thermo-chemical conversion includes processes like combustion, gasification and
pyrolysis. Combustion refers to the conversion of biomass to heat and power by directly burning
it, as occurs in boilers. Gasification is the process of converting solid biomass with a limited
quantity of air into producer gas, while pyrolysis is the thermal decomposition of biomass in the
absence of oxygen. The products of pyrolysis are charcoal, condensable liquid and gaseous
products.
Combustion, gasification and pyrolysis are all thermochemical processes to convert
biomass into energy. In all of them, the biomass is heated to evaporate water and then to cause
pyrolysis to occur and to produce volatiles.
Thermal conversion processes for biomass involve some or all of the following
processes:
Pyrolysis: Biomass +heat charcoal , gas and oil
Gasification: Biomass +limited oxygen fuel gas
Combustion: Biomass +stoichiometric O2 hot combustion products
4.1.3 Combustion
Combustion is a process whereby the total or partial oxidation of carbon and hydrogen
converts the chemical energy of biomass into heat. This complex chemical reaction can be
briefly described as follows:
Burning fuel = Products from reaction + heat
During the combustion process, organic matter decomposes in phases, i.e. drying,
pyrolysis/gasification, ignition of volatile substances and charcoal combustion. Generally
speaking, these phases correspond to two reaction times: release of volatile substances and
respective combustion, followed by charcoal combustion.

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Wood, agricultural residues, wood pulping liquor, municipal solid waste (MSW) and
refuse derived fuel are examples of feed stocks for combustion. Combustion requires high
temperatures for ignition, sufficient turbulence to mix all of the components with the oxidant,
and time to complete all of the oxidation reactions. The moisture content of the feedstock should
be low and pre-drying may be necessary in some cases.
Biomass combustion starts by heating and drying the feedstock. After all of the moisture
has been removed, temperature rises for pyrolysis to occur in the absence of oxygen. The major
products are hydrogen, CO, CO2, CH4 and other hydrocarbons. In the end, char and volatile
gases are formed and they continue to react independently. The volatile gases need oxygen in
order to achieve a complete flame combustion. Mostly CO2 and H2O result from complete
combustion. When combusting biomass in a furnace, hot gases are released. They contain about
85% of the fuel‟s potential energy. The heat can be used either directly or indirectly through a
heat exchanger, in the form of hot air or water. Boiler used for biomass combusting transfers the
produced heat into steam. The steam can be used for producing electricity, mechanical energy or
heat.
4.1.4 Gasification
Gasification is a process whereby organic matter decomposes through thermal reactions,
in the presence of stoichiometric amounts of oxidising agents. The process generates a
combustible gas mix, essentially composed of carbon monoxide, hydrogen, carbon dioxide,
methane, steam and, though in smaller proportions, other heavier hydrocarbons and tars. The
process is aimed at converting the energy potential of a solid fuel into a gas product, whose
energy content has the form of chemical energy with the capacity to generate work.
Gasification is carried out in two steps. First, the biomass is heated to around 600
degrees. The volatile components, such as hydrocarbon gases, hydrogen, CO, CO2, H2O and tar,
vaporize by various reactions. The remaining by-products are char and ash. For this first
endothermic step, oxygen is not required. In the second step, char is gasified by reactions with
oxygen, steam and hydrogen in high temperatures. The endothermic reactions require heat,
which is applied by combusting some of the unburned char. Main products of gasification are
synthesis gas, char and tars. The content depends on the feedstock, oxidizing agent and the
conditions of the process. The gas mainly consists of CO, CO4, H2O, CH4 and other

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hydrocarbons. The synthesis gas can be utilized for heating or electricity production. It can also
be used for the production of ethanol, diesel and chemical feed stocks.
4.1.5 Pyrolysis
In pyrolysis, biomass is heated in the absence of air. The process results liquid, solid and
gaseous fractions, mainly gases, bio-oil and char. The gases and the bio-oil are from the volatile
fraction of biomass, while the char is mostly the fixed carbon component. In the first step,
temperature is increased to start the primary pyrolysis reactions. As a result, volatiles are
released and char is formed. Finally, after various reactions, pyrolysis gas is formed. The main
product of slow pyrolysis, a thousands of years old process, is char or charcoal. In slow pyrolysis
biomass is heated to around 500 degrees for 5 to 30min.Fast pyrolysis results mainly in bio-oil.
The biomass is heated in the absence of oxygen and the residence time is 0, 5 to 5s. Vapours,
aerosols and char are generated through decomposition. After cooling, bio-oil is formed. The
remaining non condensable gases can be used as a source of energy for the pyrolysis reactor.
Calculated by weight, fast pyrolysis results in 60%-75% liquid bio-oil, 15%-25% solid char, and
10%-20% non-condensable gases.
Table 4.1 Comparison between pyrolysis, combustion and gasification
Process Pyrolysis Combustion Gasification

oil, tar (liquid/vapour), heat, flue gas and gases as: CO2, H2O
CO2,H2O, combustible gases as: CO2, H2O, and N2 ) in case air
Main products gas(es)as:CO,H2, CH4 N2. was thegasifying
and char agent),heat, tar and
combustiblegas as:
CO, H2 and CH4
Carbon
conversion, % ≈75 >99 80-95
Oxygen nil >1, typically 1.3 for 0-1, typically 0.2-0.4.
stoichiometry solid fuels.
Chemical
reactivity of main reactive, combustible non-reactive stable, combustible.
product

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Physical existence Solid, liquid and gas gas gas

High heating
value (HHV), 16-19 nil 5-20
MJ/kg
air, pure oxygen,
Oxidant non air steam or their
combinations
550-900 with air
Operating 500-800 850-1200 gasification. 1000-
temperature, °C 1600 with other
gasifying agents
Operating higher than or atmospheric atmospheric
pressure atmospheric
particulates, tars and particulates and particulates, tars and
Pollutants compounds of chloride, compounds of compounds of
nitrogen and sulfur chloride, nitrogen chloride, nitrogen and
and sulfur sulfur

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